Visible-to-NIR-Light Activated Release: From Small Molecules to Nanomaterials

Abstract

Photoactivatable (alternatively, photoremovable, photoreleasable, or photocleavable) protecting groups (PPGs), also known as caged or photocaged compounds, are used to enable non-invasive spatiotemporal photochemical control over the release of species of interest. Recent years have seen the development of PPGs activatable by biologically and chemically benign visible and near-infrared (NIR) light. These long-wavelength-absorbing moieties expand the applicability of this powerful method and its accessibility to non-specialist users. This review comprehensively covers organic and transition metal-containing photoactivatable compounds (complexes) that absorb in the visible- and NIR-range to release various leaving groups and gasotransmitters (carbon monoxide, nitric oxide, and hydrogen sulfide). The text also covers visible- and NIR-light-induced photosensitized release using molecular sensitizers, quantum dots, and upconversion and second-harmonic nanoparticles, as well as release via photodynamic (photooxygenation by singlet oxygen) and photothermal effects. Release from photoactivatable polymers, micelles, vesicles, and photoswitches, along with the related emerging field of photopharmacology, is discussed at the end of the review.

1. Introduction

Photoactivatable (alternatively, photoremovable, photoreleasable, or photocleavable) protecting groups (PPGs) or caged compounds are used to achieve non-invasive spatiotemporal control over the release of molecules of interest including biologically active compounds, synthetic precursors, fluorescent probes, initiators of polymerization reactions, fragrances, and gasotransmitters. As such, they constitute one of the most important current applications of photochemistry in diverse research areas. The first PPGs were reported in the early works of Barltrop,¹ Barton,^2,3 Woodward,⁴ and Sheehan,⁵ and their first biological applications were presented by Engels and Schlaeger⁶ and Kaplan⁷ and co-workers. Since then, tens of photoactivatable molecules and systems have been developed. Several reviews and perspectives covering the applications of organic⁸⁻⁵⁵ and (transition) metal-containing⁵⁶⁻⁷⁶ PPGs have been published in the past two decades. Special attention has been paid to compounds that release gasotransmitters such as nitric oxide (NO; photoactivatable NO-releasing moieties or photoNORMs), carbon monoxide (photoactivatable CO-releasing moieties or photoCORMs), and hydrogen sulfide (photoactivatable H₂S-releasing molecules).⁷⁷⁻¹¹⁴

Key criteria for the design and use of PPGs, as discussed at length in previous works,^10,115−118 are often specific to individual applications. In general, however, a PPG (a) must exhibit sufficient absorption of the irradiated light, which must either not be absorbed by other molecules or not trigger unwanted photochemical transformations in the system of interest, (b) should release protected species within a time-frame compatible with the application, (c) must be soluble and stable in the targeted medium/environment (an aqueous solution in typical biological/medical applications), (d) should not produce reactive or toxic side-products upon irradiation, and (e) should be detectable in the medium, for example, by light emission. The overall efficiency of species release is evaluated using the quantity Φ_rε(λ_irr), sometimes called the uncaging cross section, which takes units of M^–1 cm^–1, where Φ_r is the reaction quantum yield and ε is the decadic molar absorption coefficient.¹⁰

Short-wavelength UV photons have sufficient energy to induce bond cleavage, isomerization, or rearrangement reactions in many organic and inorganic molecules. For example, the energy of a photon with a wavelength of λ ≈ 300 nm (N_Ahν = 95.6 kcal mol^–1) is sufficient to induce homolytic cleavage of most single bonds in organic molecules. Most PPGs absorb light in the 300–400 nm region.¹⁰ However, excitation in the UV region presents several challenges, especially in biological settings; high-energy UV light has very limited tissue penetration due to high optical scattering and strong absorbance by endogenous chromophores (e.g., hemoglobin or melanin),^119−121 can lead to sample overheating, and can cause phototoxic or photoallergic reactions resulting from its interactions with endogenous molecules such as DNA, RNA, and lipids.^122−124 Visible and especially NIR light can penetrate deeper into tissues^{119,120,125−128} and is considerably less harmful to biological matter, opening the door to new applications in areas such as drug delivery.^{20,103,129,130} Encouragingly, some photoresponsive approaches are already used routinely in clinical applications.^131−135 In addition, visible/NIR light sources, both coherent and non-coherent, are often cheaper, more common, and more accessible to non-specialist end-users than UV-light sources.

The desire to exploit these advantages has motivated several recent efforts to develop PPGs activated by visible/NIR light. Until recently, only a few PPGs activated directly by light of wavelengths above ∼600 nm were known, and the design of PPGs that undergo efficient photorelease upon irradiation at wavelengths above 500 nm was considered challenging.^10,11 According to the gap law,¹³⁶ nonradiative transition rate constants increase approximately exponentially as the associated energy gap contracts, which is one reason why π-extended organic PPGs absorbing visible or NIR light generally undergo inefficient photoreactions. However, while the quantum yields for release from such PPGs can be very small, their chromophores can have very large molar absorption coefficients, making their Φ_rε(λ_irr) values large enough for practical use.¹¹ Alternatively, PPG activation by one (1P)-photon direct excitation using short-wavelength radiation can be replaced by alternative methods using substantially less energetic photons such as two (2P)-photon excitation or sensitization via photoinduced energy- or electron-transfer.

The applications of PPGs are not restricted to the release of a single species of interest. Careful selection of complementary photoactivatable moieties that undergo specific phototransformations can enable wavelength-selective release, which is often called chromatic orthogonality. Photochemical reactions are also in principle orthogonal to reagent- or thermally-initiated chemical processes. A unique and elegant approach exploiting this orthogonality was introduced by Bochet and co-workers,^137,138 but the general concept remains somewhat underexplored. Multiple chromatically orthogonal systems including (among others) a monochromophoric system,¹³⁹ a single multichromophoric entity,¹³⁸ and mixtures of independent photoactivatable compounds^140−144 have been reported. The latter approach is uniquely well-placed to benefit from the expansion of the photoexcitation window resulting from the development of visible- and NIR-light excitable PPGs. We discuss several orthogonal systems here, and further examples can be found in recent reviews.^{10,16,145−147}

This review follows up on an earlier article that provided a comprehensive overview of the photochemistry and applications of PPGs known and used before 2013.¹⁰ We present a comprehensive list of PPGs absorbing in the visible and near-infrared (NIR) range including organic (section 2) and (transition) metal-containing molecular PPGs (section 3) that absorb photons directly (via 1P and (in several examples) 2P^30,31,148 excitation) to release various leaving groups (LG) (Table 1), organic and metal-containing photoCORMs, photoNORMs, and photoactivatable H₂S-releasing molecules (section 4, Table 2), and photoacids and photobases (section 5). These sections are followed by an overview of PPGs that use indirect methods of photoactivation, including photosensitization by molecular photosensitizers, quantum dots, upconversion, and second-harmonic nanoparticles, as well as photorelease by the photodynamic effect and photothermally-controlled release (section 6). The final sections discuss the chemistry of photoactivatable polymers, micelles, vesicles (section 7), and photoswitches (section 8), concluding with a brief discussion of the new concept of photopharmacology (section 9) (Table 3).

Table 1. Organic and Metal-Containing PPGs Covered in This Review^ab.

^aValues in parentheses indicate the longest wavelength that can be used for PPG activation.

^bLeaving groups (LG) are depicted in red.

Table 2. Organic and Transition Metal-Containing CO-, NO-, and H₂S-Releasing Molecules Covered in This Review^ab.

^aValues in parentheses indicate the longest wavelength that can be used for PPG activation.

^bLeaving groups/moieties are depicted in red.

Table 3. Other Photoactivatable Systems Covered in This Review.

2. Photorelease from Organic Photoactivatable Compounds

2.1. Nitroaryl Groups

The nitroaryl motif has proven to be a fertile scaffold for the development of photoremovable protecting groups (PPGs), leading to the emergence of several structural families, including the o-nitrobenzyl, o-nitro-2-phenethyl, and o-nitroanilide groups.¹⁰ This section focuses on efforts to bathochromically shift the absorption spectra of o-nitrobenzyl and o-nitro-2-phenethyl PPGs toward the visible part of the spectrum. The absorption spectra of some representative nitroaryl PPGs are shown in Figure 1. A comprehensive review of UV-excitable nitroaryl derivatives covering their development and photochemical properties has been published.¹⁰

Figure 1.

Absorption spectra of selected nitroaryl PPGs. Green line, a 2-nitrobenzyl derivative (LG = thymidine);¹⁴⁹ purple line, an α-methyl-(6-nitropiperonyloxymethyl) derivative (LG = thymidine);¹⁵⁰ red line, a nitrodibenzofuran derivative (LG = Fmoc-Cys-OH);¹⁵¹ blue line, an o-nitro-2-phenethyl derivative (LG = Boc-glutamate);¹⁵² black line, a π-extended 2-nitrobenzyl derivative (LG = GABA).¹⁵³

2.1.1. The o-Nitrobenzyl Group

o-Nitrobenzyl derivatives (oNB) make up a family of general-purpose PPGs that have been developed since the 1960s^4,154 and are still widely used.¹⁰ Their photorelease mechanism has been studied extensively.^155−160 Briefly, the excitation of the ground state of an oNB derivative 1 (Figure 1) is followed by intramolecular hydrogen abstraction by the nitro group to form an aci-nitro intermediate (2, Scheme 1; LG = leaving group). The decay rate constant of the aci-nitro intermediate (∼10²–10⁴ s^–1) depends on the substitution of the oNB group, the solvent, and the pH. An irreversible cyclization of the aci-nitro intermediate leads to a 1,3-dihydrobenz[c]isoxazol-1-ol (3). Subsequent ring-opening gives a hemiacetal intermediate that hydrolyzes to release the leaving group (LG) and form an o-nitrosobenzaldehyde byproduct (4). The photorelease of many functional groups including carboxylic acids,⁴ phosphates,¹⁶¹ thiols,¹⁶² alcohols,¹⁶³ and amines¹⁶⁴ has been demonstrated, although the latter two moieties are typically attached as carbonic acid derivatives.

Scheme 1. Proposed Photoreaction Mechanism of o-Nitrobenzyl PPGs¹⁵⁹.

Efforts to bathochromically shift the absorption maxima of the parental oNB 5a (λ_max^abs ≈ 260 nm) have generally met with limited success because of an inverse correlation between the bathochromic shifts of absorption bands and photochemical parameters such as the release quantum yield (Φ_r) and rate constant.^{137,163,165−167} For example, Jullien and co-workers examined a series of p-substituted nitrobenzyl derivatives 5b–5f and found that bathochromic shifts of their absorption maxima were associated with a decrease in Φ_r (Table 4).¹⁶³ This loss of efficiency could be counteracted to some extent by substitution at the benzyl position,^{4,163,166−168} leading to the development of the red-shifted α-methyl-6-nitroveratryl (6)⁴ and α-methyl-(6-nitropiperonyloxymethyl) (7) PPGs (Figure 1).¹⁶⁹ However, due to the reduction in quantum efficiency, the uncaging cross section (Φ_rε(λ_irr)) of the latter group tends to be comparable to that of the parent oNB 5a.^4,170,171 Nitrodibenzofuran 8a (NDBF; Figure 1), introduced by Ellis-Davies and co-workers, is an exceptional red-shifted oNB derivative that releases LGs efficiently.¹⁷² The photolysis of ether,¹⁷² thioether,¹⁵¹ and phosphoester^173,174 LGs caged with this group reportedly proceeded with Φ_r values of 0.5–0.7, although lower quantum yields were obtained in some cases (0.04–0.2).^175−177 The tail absorption of 8a in the visible range (398–440 nm) was sufficient to promote the photoreaction.^175,178 Introducing electron-donating groups (EDG) at the 7-position of NDBF (8b and 8c) led to a bathochromic shift in λ_max but also reduced its photouncaging quantum efficiency (Table 4).^151,174 The low quantum yield of 8c was attributed to a charge-transfer transition following photoexcitation that competes with LG release.^174,179 Ball and co-workers recently reported that derivatives of 8a and 8c undergo efficient photocleavage of C(sp²)–N bonds.¹⁸⁰ To explain this, a mechanism was proposed involving hydrogen-atom abstraction followed by selective nucleophilic attack of a solvent molecule on the resulting extended conjugated system. The absorption maximum of oNB-type PPGs can also be bathochromically shifted by extending the aromatic core,^181−183 as in the 7-methoxynaphthalene derivative 9.¹⁸³

Table 4. oNB Derivatives.

PPG	λmaxabs (nm)	εmax (M–1 cm–1)	leaving groupsa	Φr (λirr/nm)	solventb	ref
5a	262	5.2 × 103	thymidine (as carbonic acid)	0.033 (365)	CH3OH/H2O, 1:1	(167−170)
			pivalic acid	0.13 (254)	CH3CN	(149)
5b	272	6.0 × 103	4-nitrophenol	0.1 (325)	CH3CN	(163)
5c	310	9.0 × 103	4-nitrophenol	not reported	CH3CN	(163)
5d	310	8.0 × 103	4-nitrophenol	0.007 (325)	CH3CN	(163)
5e	367	1.6 × 104	4-nitrophenol	<0.001 (325)	CH3CN	(163)
5f	394	1.6 × 104	4-nitrophenol	<0.001 (325)	CH3CN	(163)
6	352	4.0 × 103	l-threo-β-benzyloxyaspartate	0.005 (355)	PBS buffer, pH 7.4	(184)
7	351	3.5 × 103 (ε365)	thymidine (as carbonic acid)	0.0075 (365)	CH3OH/H2O, 1:1	(150)
8a	325	18.4 × 103	EGTA (Ca2+), IP3	0.5–0.7 (350–400)	HEPES buffer, pH 7.2	(172,173)
8b	362	9.3 × 104	Fmoc-cysteine–OH	0.51 (350)	phosphate buffer, pH 7.4	(151)
8c	424	1.6 × 104	nucleobases	0.5–11 × 10–3 (420)	DMSO	(174)
9	339	1.1 × 104 (ε350)	hippuric acid	0.031 (420)	ethanol	(183)

^aOnly selected LGs are shown.

^bPBS = phosphate buffer saline. HEPES = 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid; DMSO = dimethyl sulfoxide; EGTA = ethylene glycol tetraacetic acid; IP₃ = inositol triphosphate.

Jullien and co-workers also found that a bathochromic shift in λ_max^abs relative to the parent PPG 5a could be achieved by substitution to form a π-extended donor–acceptor system containing an electron-donating group (EDG) such as a methoxy group (10–13, Table 5).¹⁶³ These chromophores had λ_max values of 336–371 nm but were photolyzed inefficiently to release a carboxylic acid (Φ_r = 0.001), in keeping with the previously mentioned inverse correlation between shifts in λ_max^abs and Φ_r.¹⁶³ Derivatives of biphenyl 10a exhibited a bathochromic shift in λ_max of ∼70 nm relative to 5a,^163,185,186 and an additional ∼60 nm shift was achieved by using a dialkylamine EDG (10b; Figure 1).^187,188 The release of a carboxylic acid¹⁸⁷ and the fragmentation of the selective Ca²⁺-chelator ethylene glycol tetraacetic acid²⁷ (EGTA) with subsequent Ca²⁺ release were achieved at λ_irr = 400–405 nm using PPGs of this type.¹⁸⁸ Stilbene-type derivatives 11, which bear various alkoxy EDGs, had λ_max^abs values of 369–376 nm but released carboxylic acids with low quantum yields when irradiated above 400 nm.^163,189,190 Relatively similar quantum yields were reported for release from a derivative of 11 bearing the dimethylamino group as an EDG (Φ_r = 0.8–2 × 10^–4).¹⁹¹ It was proposed that a photoinduced reversible E–Z isomerization^192−194 competes with photorelease in this case.¹⁹⁰ Accordingly, rigid derivatives 14 and 15 (Figure 1) were photolyzed more efficiently than 11 to liberate carboxylic acid LGs or to cleave an ether bond (causing EGTA bifurcation leading to Ca²⁺ release).^{153,188,195,196} The π-extended 1,2-dihydronaphthalene 15, which has a dialkylamino EDG, is the chromophore with the longest absorption wavelength in this series.¹⁵³ Visible-light uncaging from simple oNB derivatives has also been achieved through conjugation with silicon quantum dots¹⁹⁷ or upconverting nanoparticles^198−203 (see also sections 6.4.1 and 6.4.2). It should be noted that many oNB derivatives with absorption maxima in the near UV-region have proven very useful in diverse applications^{16,20,23,25,204−208} including in vivo experiments.^209−215 Several genetically encoded amino acids caged by oNB derivatives have also been reported.^216−218

Table 5. oNB Derivatives with Extended π-Systems.

PPG	λmaxabs (nm)	εmax (M–1 cm–1)	leaving groupsa	Φr (λirr/nm)	solventb	ref
10a	335–342	7.3–14.0 × 103	4-nitrophenol, chlorambucil, celecoxib	0.005–0.013 (325 or 355)	CH3CN or CH3CN/Tris pH 9.0, 1:1 or CH3CN/phosphate buffer pH 7.2, 1:1	(163, 185, 186)
10b	403	8.8 × 103	EGTA (Ca2+)	0.05 (400)	C6D6	(188)
11	369–376	1.9–2.5 × 104	coumarin, chlorambucil	3.2–15.4 × 10–4 (325 or 400)	CH3CN or CH3CN/Tris pH 9.0, 1:1	(163, 189)
12	348	1.9 × 104	4-nitrophenol, coumarin	0.001–0.005 (325)	CH3CN/Tris pH 9.0, 1:1	(163)
13	371	1.9 × 104	coumarin	0.001 (325)	CH3CN/Tris pH 9.0, 1:1	(163)
14	362–364	1.2–1.8 × 104	benzoic acid, EGTA (Ca2+)	0.09–0.3 (360)	CH3CN or DMSO	(188, 195)
15	420–443	1.8–2.9 × 104	Boc-glutamate	0.01 (355)	CH3OH	(153)

^aOnly selected LGs are shown.

^bTris = tris(hydroxymethyl)aminomethane; DMSO = dimethyl sulfoxide; EGTA = ethylene glycol tetraacetic acid.

An outstanding 1-photon (1P)-absorbing oNB derivative is compound 16, a dinitro-derivative of bisstyrylthiophene (BIST) coupled to two units of EGTA, which was recently reported by Ellis-Davies and co-workers and used for visible-light-induced (λ_irr = 473 nm) calcium uncaging (Scheme 2).²¹⁹ UV-excitable oNB derivatives are the PPGs most commonly used for photoscission of C–O or C–N bonds leading to the bifurcation of a chelator and the release of metal cations.^{27,213,220,221} The π-extended electron-poor compound 16 exhibited strong absorption maxima in the blue light region (λ_max^abs = 440 nm, ε₄₄₀ = 6.6 × 10⁴ M^–1 cm^–1) and a large two-photon (2P) absorption cross section (δ_unc of >250 GM) in the 720–830 nm range.²¹⁹ This compound is a strong Ca²⁺ chelator, but upon 1P (λ_irr = 473 nm, Φ_r = 0.23) or 2P excitation (λ_irr = 720 or 810 nm), its Ca²⁺ affinity falls markedly, leading to the release of free Ca²⁺. A BIST scaffold masked with PEG dendrons was also used to cage γ-butyric acid (GABA), although this species was found to be resilient to 1P photolysis (λ_irr = 470 nm) and released GABA only upon 2P excitation.²²² Similar effects on uncaging have been reported previously.¹⁷⁴

Scheme 2. Photouncaging of Ca²⁺ with Visible Light²¹⁹.

Simple oNB derivatives tend to have rather low 2P-uncaging cross sections (δ_unc), ranging from 0.01 to 0.035 GM.^163,165,223 Nevertheless, they have been used successfully in some biological applications.^224−226 NDBF derivative 8a is an exception, with a reported δ_unc of 0.6 GM (at 720 nm).¹⁷² The 2P-uncaging cross sections of derivatives of 6 were improved by incorporating the chromophore into dyads (δ_unc = 0.1–1.0 GM).^227,228 Jullien and co-workers observed that the δ_unc of derivatives 10–13 remained low for 2P uncaging of carboxylic acids (δ_unc = 0.02–0.05 GM, λ_irr = 730–800 nm).¹⁶³ The same authors reported that substitution at the benzyl position has similar effects on both δ_unc and Φ_r.¹⁶³ It was therefore suggested that the same excited state is involved in both 1P and 2P photolysis. Stilbene derivative 11 (OEt = EDG) exhibited 2P absorption of 20 GM and δ_unc = 0.014 GM for the release of chlorambucil,¹⁸⁹ whereas rigid stilbene derivatives of 15 and the biphenyl 10 were reported to be photolyzed more efficiently, with δ_unc = 5–21 and 7.8 GM at 740 and 800 nm, respectively.

2.1.2. The o-Nitro-2-phenethyl Group

The 1-(2-nitrophenyl)ethyloxycarbonyl (NPEOC) group¹⁷⁰17 and its α-methyl analog^170,22918 (NPPOC; the “OC” stands for the −OC(=O) group, which is typically a part of the LG) constitute a separate class of nitroaryl PPGs. Despite its close structural similarity to oNB (5a), the proposed photoreaction mechanism of o-nitro-2-phenethyl derivatives is markedly different, involving a photoinduced elimination step (Scheme 3)¹⁷⁰ reminiscent of that reported for (2-hydroxyethyl)benzophenone-type PPGs.^{118,230−235} The quantum yields obtained for o-nitro-2-phenethyl derivatives exceed those for their oNB analogs^150,170 (for example, Φ_r = 0.35 and 0.033 for 5′-O-nucleoside carbonate photorelease from 18 and 5a, respectively), leading to their use in automated light-mediated oligonucleotide synthesis (DNA-chips),^236,237 the preparation of peptide^238−240 and RNA^241,242 microarrays, the synthesis of aptamers²⁴³ and carbohydrates,²⁴⁴ and gene assembly.²⁴⁵

Scheme 3. Photorelease from o-Nitro-2-phenethyl PPGs¹⁷⁰.

The parent compounds 17 and 18 were further modified to enhance their absorption at longer wavelengths, as exemplified by the 3-(4,5-dimethoxy-2-nitrophenyl)-2-butyl group (DMNPB, 19)^246−248 and the analogous 2-(3,4-methylenedioxy-6-nitrophenyl)-propoxycarbonyl group (MNPPOC, 20).¹⁵⁰ Both these groups have a λ_max^abs at 350 nm but lack the associated decrease in Φ_r observed for oNB derivatives (Table 6). Bowman and co-workers showed that the tail absorption of 20 above 400 nm enables its use in visible-light photobase generation (see also section 5); the photorelease of tetramethylguanidine (TMG) at λ_irr = 405 and 455 nm proceeded with uncaging cross sections (Φ_rε(λ_irr)) of 38.5 and 4.6 M^–1 cm^–1, facilitating visible-light-mediated control over a thiol-Michael addition polymerization process.²⁴⁹ The thiophenyl-2-(2-nitrophenyl)propoxycarbonyl derivative 21 was shown to have spectroscopic properties comparable to those of 20 (Table 6).^250,251 Additionally, Steiner and co-workers used intra- and intermolecular energy transfer from a triplet sensitizer (section 6.1) to initiate the release of LGs from NPPOC derivative 18 at λ_irr ≥ 400 nm.^171,252,253

Table 6. Spectroscopic and Photochemical Properties of o-Nitro-2-phenethyl Derivatives.

PPG	λmaxabs (nm)	εmax (M–1 cm–1)	leaving groupsa	Φr (λirr/nm)	solvent	ref
17	∼260	0.29 × 103 (ε365)	thymidine (as carbonic acid)	0.042 (365)	CH3OH/H2O, 1:1	(170)
18	∼260	0.26 × 103 (ε365)	thymidine (as carbonic acid)	0.35 (365)	CH3OH/H2O, 1:1	(170)
19	350	3.5 × 103	GABA	0.26 (364)	phosphate buffer, pH 7.2	(246)
20	353	3.4 × 103 (ε365)	thymidine (as carbonic acid)	0.035–0.037 (365)	CH3OH/H2O, 1:1	(150)
21	∼350	1.5 × 103 (ε365)	DNA phosphoramidites	0.68 (365)	CH3OH	(250)
22	317	9.9 × 103	glutamate	0.09 (364)	phosphate buffer, pH 7.4	(254, 255)
23	296–302	6.3–7.1 × 103	glutamate	n.d.	phosphate buffer, pH 7.4	(254, 255)
24	397	7.5 × 103	GABA	0.15 (405)	phosphate buffer, pH 7.4	(152)
25	415	6.4 × 104	glutamate	0.25 (354)	phosphate buffer, pH 7.4	(267)

^aOnly selected LGs are shown. GABA = γ-aminobutyric acid.

o-Nitro-2-phenethyl derivatives such as 17 and 18 typically have higher 2P δ_unc values than simple oNB derivatives such as 5 and 6 (δ_unc = 0.1–0.9^233,246 vs 0.01–0.35^163,165,223 GM, respectively).²⁴⁶ NPPOC biphenyl systems 22 (Figure 2) have been studied to determine whether extending the π-system of o-nitro-2-phenethyl moieties could improve their 2P-absorption sensitivity. Goeldner and co-workers showed that p-methoxynitrobiphenyl platform 22 exhibits a ∼60 nm bathochromic shift in λ_max^abs relative to 18 while retaining a comparable 1P-photorelease quantum yield for glutamate (Table 6).²⁵⁴ This stands in contrast to the previously mentioned inverse correlation between bathochromic shifts of λ_max and Φ_r in oNB derivatives (see section 2.1.1). The 2P-uncaging cross sections of glutamate from 22 were 3.2 and 0.45 GM at 740 and 800 nm, respectively,^254,255 both of which are significantly higher than the corresponding values for 19 (δ_unc = 0.17 GM, 720 nm).²⁴⁶ Moving the methoxy EDG to the ortho or meta positions (23) did not affect 1P photorelease yield but reduced the 2P uncaging cross section (δ_unc = 2.2 and 1.8 GM, respectively, 740 nm).²⁵⁵ The introduction of a hydroxyl EDG was detrimental to the photouncaging of glutamate (reducing its chemical yield to <10%), presumably because it opened up photochemical pathways that compete with photorelease.²⁵⁴ The impact of varying the p-alkoxy substituent of 22 on the photorelease of various LGs at λ_irr = 300–365 nm was investigated, but no appreciable effects on photoreaction properties were observed.^{185,255−260} Specht, Goeldner, and co-workers further showed that dialkylamino substituents (24) caused an additional ∼90 nm bathochromic shift with no significant detrimental effects on the quantum yield of 1P GABA photorelease (Table 6) and also substantially increased the 2P-uncaging cross section, giving δ_unc values of up to 11 GM at 800 nm.¹⁵² The photorelease of carboxylates,^152,255,261 amines^{260,262−264} (connected as carbamates), alcohols,²⁶⁵ and phosphates²⁶⁶ from various dialkylamino derivatives of 24 proceeded with Φ_r = 0.09–0.28 at λ_irr = 390–520 nm and with δ_unc values of up to 20.5 GM at 800 nm. To improve the water-solubility of these rather hydrophobic PPGs and enable their conjugation to (intra)cellular targeting groups, hydrophilic functional groups were attached to the amino^{152,262,263,266} or alkoxy^256,260 moieties of 22 and 24.^185,258

Figure 2.

o-Nitro-2-phenethyl derivatives (LG = alkoxide, carboxylate, carbonate, carbamates, or phosphate).

The extension of the π-system of NPPOC with styrene and phenylacetylene substituents was also explored.^254,257,268 For example, Wombacher and co-workers synthesized 26 to cage the plant hormone gibberellic acid (GA₃) via an ester linkage (Scheme 4).²⁶⁸ This conjugate had a λ_max^abs of 400 nm and released GA₃ upon 1P (λ_irr = 470 nm) or 2P (λ_irr = 800 nm) excitation in cultured COS-7 cells, enabling light-mediated control over a chemically-induced dimerization system based on the gibberellin perception mechanism.^269,270 Symmetric biphenyl-substituted NPPOC structures such as 25 (Figure 2) exhibited significantly improved 1P- and 2P-absorption photorelease efficiencies (Φ_r = 0.25–0.30, δ_unc = 0.9–5.0 GM (at 840 nm),²⁶⁷ but their size and poor solubility make them more suitable for applications where they are incorporated into larger structures.²⁷¹

Scheme 4. Photouncaging of Plant Hormone GA₃ from π-Extended NPPOC Derivative²⁶⁸.

2.2. The (Coumarin-4-yl)methyl Group

Coumarin (2H-chromen-2-on) is a secondary metabolite found in many plants that was first isolated from the Tonka bean, known in French as coumarou, in 1820.^272−274 The development of coumarins as a new class of photoremovable protecting groups began with the discovery of Givens and Matuszewski that the (coumarin-4-yl)methyl group exhibits photoreactivity, enabling the release of phosphate esters (Scheme 5).²⁷⁵

Scheme 5. Release of Phosphate from 7-Methoxycoumarin 27(275).

The mechanism of the photorelease from (coumarin-4-yl)methyl derivatives has been extensively studied^276−278 and reviewed,^10,279 and it is summarized in Scheme 6.²⁷⁶ Briefly, a heterolytic C–X bond cleavage takes place from the lowest ¹π,π* singlet excited state, which competes with unproductive radiationless decay and fluorescence emission. A tight ion pair (TIP) was proposed to be the key intermediate in this process; the (coumarin-4-yl)methyl cation in this pair could react directly with adventitious nucleophiles or solvents to form a new stable (coumarin-4-yl)methyl product. Recombination of the TIP to regenerate the ground-state caged derivative would be an unproductive competing radiationless pathway in this mechanism. It should however be noted that ultrafast time-resolved visible-pump-infrared-probe spectroscopy experiments yielded no evidence of TIP formation during the photorelease of a (coumarin-4-yl)methyl azide.²⁸⁰ There are also evidences suggesting that some coumarin derivatives exhibit triplet-state reactivity.^{165,281−284}

Scheme 6. Photocleavage Mechanism of (Coumarin-4-yl)methyl-Caged Phosphates²⁷⁶.

In general, coumarin-based PPGs offer several advantages: (1) high molar absorption coefficients at wavelengths above 350 nm, (2) high photorelease efficiencies, (3) acceptable stabilities in the dark, (4) fast photolysis kinetics, and (5) practically useful 2-photon excitation cross sections. Furthermore, their spectroscopic, photochemical, and other relevant properties (e.g., solubility and conjugation) can easily be tuned by varying the substituents on the coumarin ring. Given the high diversity of known coumarins, their synthesis is outside the scope of this review; interested readers are directed to reference works for extensive surveys.^10,285 Similarly, comprehensive reviews of the biological and other applications of (coumarin-4-yl)methyl PPGs can be found elsewhere.^{16,19,21,22,26,50,285−289} The following section focuses on the evolution of coumarinyl PPGs that are excitable by light in the visible region of the spectrum. The absorption spectra of representative (coumarin-4-yl)methyl PPGs discussed in this section are shown in Figure 3.

Figure 3.

Absorption spectra of selected (coumarin-4-yl)methyl PPGs. Black line, a (coumarin-4-yl)methyl derivative (LG = cAMP);²⁹⁰ red line, a [7-(diethylamino)coumarin-4-yl]methyl derivative (LG = benzoate);²⁹⁰ magenta line, a thionated [7-(diethylamino)coumarin-4-yl]methyl derivative (LG = benzoate);²⁹¹ orange line, a 3-[3-(methylamino)-3-oxoprop-1-en-1-yl] derivative (LG = glutamate);²⁹² green line, a 7-styryl derivative (LG = 4-methoxybenzylcarbonate);²⁹³ cyan line, a bis-julolidine derivative (LG = 4-methoxybenzoate);²⁹⁴ blue line, a benzothiazolium derivative (LG = 3,5-dimethylbenzoate).²⁹⁵

The parent (coumarin-4-yl)methyl 28a has an absorption maximum at 310 nm (Table 7; Figure 3) and was shown to photorelease cyclic adenosine monophosphate (cAMP) with Φ_r = 0.085.²⁹⁰ Introducing EDGs at the C7-position led to an increased intramolecular charge-transfer (ICT) character and a greater transition dipole moment, resulting in more intense and red-shifted absorption.^{278,290,296−302} The weakly electron-donating 7-methyl substituent (28b) caused a ∼7 nm bathochromic shift in λ_max^abs,^303,304 while derivatives with stronger EDGs such as hydroxy ((7-hydroxycoumarin-4-yl)methyl, 29a) and methoxy ((7-methoxycoumarin-4-yl)methyl, 29b) exhibited more pronounced effects (Table 7). The(7-carboxymethoxycoumarin-4-yl)methyl derivative 29c was designed to provide improved water solubility,^{184,301,305−307} while esters 29d ((7-acetoxycoumarin-4-yl)methyl) and 29e ((7-propionyloxycoumarin-4-yl)methyl) were introduced to improve membrane permeability.^308−311 After penetration into live cells by diffusion, the esters of 29d and 29e are hydrolyzed by endogenous esterases to form the more polar phenolic derivative 29a, which has negligible membrane permeability and thus accumulates inside cells.^309,310 A genetically encodable lysine caged by 29a was developed to control protein functions in cell cultures and in vivo.^312−316 The photoexcitation of 29a–e and their derivatives is usually restricted to the 300–350 nm wavelength range. Photouncaging of phosphates, sulfonates, and quaternary amines from 29a–e and their derivatives typically occurs with Φ_r values of 0.05–0.39,^{276,278,281,290,301,309,317,318} whereas poorer leaving groups, such as carboxylic,^{184,276,278,319,320} carbonic,^{319,321−323} and carbamic^{305,313,319,324−326} acids are liberated less efficiently (Φ_r = 0.004–0.03). The photorelease efficiencies of amino acids connected to 29a and 29b through different linkers declined in the following order: anhydride > ester > carbamate > carbonate.³¹⁹ The carbonic or carbamic acids initially liberated by photorelease from these linkers are unstable and undergo decarboxylation to give the corresponding free alcohol or amine, respectively. These decarboxylation reactions usually have quite low rates, with k_-CO2 on the order of 10^–3 s^–1, and they are subject to both acid and base catalysis.^327−330 A single example of a C–N bond cleavage from 29b was reported.³³¹ This reaction proceeded efficiently only in the presence of an excess of a hydrogen-atom donor such as n-decanethiol or 1,4-cyclohexadiene. A radical mechanism was proposed (Scheme 7), involving electron transfer between the amine and coumarinylmethyl moieties in 36 to form the intramolecular radical ion pair 37. The subsequent cleavage of the C–N bond generates an aminyl radical and a resonance-stabilized coumarinylmethyl radical 38, both of which can be trapped by hydrogen-atom donors.

Table 7. Coumarin PPGs Substituted at the 7-Position^a.

PPG	λmaxabs (nm)	εmax (M–1 cm–1)	solventb	ref
28a	310	5.1 × 103	CH3OH/HEPES buffer pH 7.2, 1:1	(290)
28b	317	3.92 × 103	ethanol	(303, 304)
29a–e	314–328	1.0–1.6 × 104	CH3OH/HEPES buffer pH 7.2, 1:1 or MOPS buffer, pH 7.2	(165, 290, 301, 308)
30a	348	1.4 × 104	PBS buffer, pH 7.4	(324)
30b	378–398	1.5–1.8 × 104	CH3OH/HEPES buffer pH 7.2, 1:1	(290, 332, 333)
30c	387–406	1.5–2.1 × 104	CH3CN/HEPES buffer pH 7.2, 1:20 or HEPES buffer pH 7.2 or CH3OH/HEPES buffer pH 7.2, 1:4	(301, 334)
31	399–403	1.8–4.4 × 104	CH3OH/H2O, 9:1 or CH3OH/HEPES buffer pH 7.2, 4:1	(294, 335, 336)
32	371	1.6 × 104	CH3CN/PBS buffer pH 7.4, 7:3	(337)
33	450	not reported	CH3CN	(338)
34a	323	4.1 × 104	CH3OH/HEPES buffer pH 7.2, 4:1	(339)
34b	325–340	3.9-4.1 × 104	CH3OH/HEPES buffer pH 7.2, 4:1	(339)
35a	347–354	3.5–5.8 × 104	CH3OH/HEPES buffer pH 7.2, 4:1 or CH3CN/H2O, 9:1	(293, 339)
35b	366	2.8 × 104	CH3CN/H2O, 9:1	(293)
35c	407	2.9 × 104	CH3CN/H2O, 9:1	(293)

^aLG = alkoxides, carboxylates, carbonates, carbamates, phosphates, thiols, sulfonates, azide, halides.

^bHEPES = 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid; MOPS = 3-(N-morpholino)propanesulfonic acid; PBS = phosphate buffer saline.

Scheme 7. Photouncaging of Amines via Direct C–N Bond Cleavage³³¹.

The introduction of a 7-NH₂ substituent ((7-aminocoumarin-4-yl)methyl, 30a)³²⁴ caused a ∼40–45 nm bathochromic shift of λ_max^abs (Figure 3), and the liberation of carboxylic acids and amines from the corresponding esters and carbamates of 30a proceeded with Φ_r values of 0.003–0.6 (λ_irr = 350 or 419 nm).^{308,324,339,340} Alkylation of the 7-amino moiety, which increases its electron-donating ability, resulted in a more red-shifted and intense absorption band in [7-(dimethylamino)coumarin-4-yl]methyl derivative 30b(290,334,341) and [7-(diethylamino)coumarin-4-yl]methyl analog 30c(301,334) (Table 7).^278,290,301 The photorelease quantum yields for 30b and 30c exceeded those for all other compounds in this series. This was attributed to greater stabilization of the (coumarin-4-yl)methyl carbocation by the electron-donating dialkylamino substituents, leading to more efficient LG liberation from the TIP intermediate.^276,278,290 For example, the Φ_r values for cAMP release from 30b and 30c were 0.28 and 0.21, respectively, around twice that for 29b (Φ_r = 0.13).^278,290 The release of carboxylic acids from 30b and 30c occurred with Φ_r values of 0.003–0.12,^{291,321,332,333} whereas amines (as carbamic acids),^{278,342−346} alcohols (as carbonic acids),^293,344,347 and thiols (as thiocarbonic acids)^348−350 were liberated with Φ_r = 0.01–0.09. The direct release of phenols occurred with Φ_r = 0.02–0.26, but competing recombination of the primary products proceeded with similar or even higher efficiency with these LGs.^{345,351−353} The favorable spectroscopic and photochemical properties of 30c, such as its absorption above 400 nm,^332,333 have made it one of the most popular PPGs. For more examples of its applications, the reader is referred to several review articles.^{10,16,19,21,22,26,285}

Derivative 30d was shown to have similar spectroscopic and photochemical properties to 30c (Table 7)³⁵⁴ while providing an additional derivatization point for further modulation of its properties and functions.^354−360 The alkyl substituents of the (7-dialkylaminocoumaryl)methyl group can easily be replaced with other functional moieties without significantly affecting the molecule’s photophysical and photochemical properties,³⁶¹ allowing other properties to be tuned to expand the PPG’s utility. For example, long alkyl chains have been appended to the 7-amino group to increase hydrophobicity,^362−366 and highly polar or charged moieties such as bis(carboxymethyl),^{283,306,367−375} bis((dimethylamino)ethyl)carboxamide,³⁷⁶ and bis(ethylsulfonate)^377,378 groups have been used to increase water solubility and control cellular permeability. Other functionalities have been appended to the 7-amino group to enable conjugation to (sub)cellular targeting motifs,^{377,379−382} binding to surfaces and nanoparticles,^383−388 or incorporation into polymer backbones.^389,390 Analyte-dependent photoactivatable derivatives have also been reported.^383,391,392

Derivatives bearing a conformationally locked electron-donating julolidine motif^393,394 exhibited a 10–15 nm bathochromic shift of λ_max^abs relative to their corresponding open-chain analogs (Table 7) and were photolyzed with higher quantum yields.^{294,335−337} For example, the liberation of benzoic acid derivatives from coumarin 31 was 5–7-times more efficient than from 30c under the same conditions (λ_irr = 405 nm).³³⁷ The 7-azetidinyl and 7-aziridinyl substitutions significantly increased fluorescence quantum yields in coumarin fluorophores, which was related to a decrease in the population of twisted intramolecular charge transfer (TICT) states³⁹⁵ upon excitation.^313,396 Rivera-Fuentes and co-workers synthesized 7-azetidinyl coumarin 32, which released carboxylic acids with Φ_r = 1.4–1.6 × 10^–2 upon irradiation at 405 nm.³³⁷ The authors suggested that this increase in photouncaging efficiency is not due to the substituent’s effect on the population of TICT states (as was suggested for the fluorescence enhancement^313,396) but rather to suppression of an unproductive H-bond-induced non-radiative decay^397−399 (HBIND) channel.³³⁷ Photouncaging (λ_irr = 405 nm) of a fluorescein derivative from 32 in live cells was demonstrated.³³⁷ Singh and co-workers synthesized the squaric acid–coumarin conjugate 33 (LG = the anticancer drug chlorambucil, Table 7). An organic nanoparticle formulation of this compound exhibited a hypsochromically shifted and broadened absorption spectrum (λ_max ≈ 410 nm) relative to that of the free molecular species.³³⁸ Photoexcitation of 33–nanoparticle conjugates (λ_irr = 410 nm) led to the simultaneous release of chlorambucil (Φ_r = 0.083) and generation of singlet oxygen (Φ_Δ = 0.51) from the excited squaraine moiety.^400−402 This simultaneous release of a strong oxidant and an anticancer drug had synergistic effects on cell viability in cultured HeLa cells.³³⁸

Gonçalves and co-workers expanded the coumarin π-system by substituting the 7-position with phenyl (34a) or p-methoxyphenyl (34b) groups, resulting in bathochromic shifts in the absorption of 19 and 31 nm, respectively, relative to the parent coumarin 28a (Table 7). However, detectable carboxylic acid release from these derivatives occurred only upon irradiation below 350 nm.³³⁹ The introduction of a 7-styryl group^293,339 in 35a caused a more significant bathochromic shift of λ_max^abs that was further enhanced by substituting the para-position with EDGs (35b and 35c, Table 7; Figure 3).²⁹³ The liberation of alcohols (caged through a carbonate linker) from 35c proceeded with Φ_r = 8.3 × 10^–4 (λ_irr = 420 nm), which is ∼50-times lower than the corresponding value for coumarin 30c (Φ_r = 4.5 × 10^–2). Nevertheless, the uncaging cross section of 35c upon irradiation at 430 nm was around 4-times that of 30c (Φ_rε₄₃₀ = 8.28 and 2.29 M^–1 cm^–1 for 35c and 30c, respectively).²⁹³ Because of its extended D−π–A backbone,²⁶⁷35c exhibited a much stronger 2P absorption than 30c (309 vs 2.3 GM at 800 nm) and a ∼2-fold higher 2P uncaging cross section (δ_unc = 0.26 vs 0.12 GM at 800 nm).²⁹³

Coumarin derivatives bearing EDGs at the 6-position exhibited greater bathochromic shifts in absorption than their 7-EDG counterparts,^{278,290,405,406} but usually also exhibited less efficient photorelease (Table 8).^278,290 For example, the (6-methoxycoumarin-4-yl)methyl compound 39a had a 20 nm bathochromic shift of λ_max^abs relative to its 7-methoxy analog 29b and was photolyzed to release cAMP as an LG ∼4-times less efficiently.^278,290 The spectroscopic and photochemical properties of the 6,7-dialkoxy derivatives 39b–d resembled those of their 6- or 7-monosubstituted analogs. Sulfonates and phosphates such as cAMP and cGMP were released from 39b–d with Φ_r = 0.08–0.14,^{278,290,301,407−410} while poorer LGs such as carboxylic and carbamic acids were released with Φ_r = 0.6–2.0 × 10^–2.^305,321 The uncaging of cysteine residues protected with 39b in proteins was used to study their folding kinetics on a sub-microsecond time-scale.^411,412 The (6,7-dicarboxymethoxycoumarin-4-yl)methyl derivative 39c was designed to provide increased water solubility,^301,305,410 and the diethyl ester 39d was synthesized to improve membrane permeability.⁴¹³

Table 8. Coumarin PPGs Substituted at the 6-Position.

PPG	λmaxabs (nm)	εmax (M–1 cm–1)	leaving groupsa	Φr (λirr/nm)	solventb	ref
39a	337–346	4.2–4.5 × 103	cAMP	0.02-0.055 (333)	CH3OH/HEPES buffer pH 7.2, 1:1	(278, 290)
39b	341–349	1.1–1.2 × 104	cAMP	0.04 (333)	CH3OH/HEPES buffer pH 7.2, 1:1	(278, 290)
39c	346–347	1.1–1.2 × 104	cAMP	0.08–0.10 (333)	CH3CN/HEPES buffer pH 7.2, 1:20 or HEPES buffer pH 7.2	(301)
40a	370–375	1.5–1.7 × 104	acetic acid	0.37 (365)	MOPS buffer, pH	(165)
			cAMP	0.1 (350)	7.2	(403)
40b	370	1.6 × 104	acetic acid	0.01 (365)	MOPS buffer, pH 7.2	(165)
40c	320	0.6 × 104	cAMP	0.074 (350)	MOPS buffer, pH 7.2	(403)
40d	329–330	0.5–1.0 × 104	2′-deoxycytidines	0.24–0.30 (350)	MOPS buffer, pH 7.2	(404)

^aOnly selected LGs are shown.

^bHEPES = 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid; MOPS = 3-(N-morpholino)propanesulfonic acid; cAMP = cyclic adenosine monophosphate.

The effect of electron-withdrawing groups (EWGs) at the 6-position has mainly been explored in combination with an EDG at the 7-position. Introducing an EWG in the 6-position had only a minor effect on the absorption spectrum relative to the parent PPG (causing bathochromic shifts of ∼5–15 nm) and often led to reduced photouncaging quantum yields,^165,285,303 presumably due to interference with through-bond electron transfer to the C2 carbonyl in the excited state.²⁸⁵ Two exceptions to these effects were observed for the (6-bromo-7-hydroxycoumarin-4-yl)methyl compound 40a by Tsien and co-workers.¹⁶⁵ First, the 6-bromo substituent increased the acidity of the 7-OH group relative to 29a (pK_a = 6.2 vs 7.9), causing 40a to be predominantly anionic at physiological pH. Consequently, 40a has an absorption maximum at 375 nm, compared to 330 nm for its protonated form and 325 nm for 29a, and is more water-soluble.^165,414 These effects were also observed for the 6-chloro derivative 14b.^165,415 Second, acetate was liberated from 40a ∼1.5-times more efficiently than from 29a (Φ_r = 0.037 vs 0.025).¹⁶⁵ It was suggested that the heavy bromo substituent of 40a promotes ISC to the triplet excited state and that this effect outweighs its interference with through-bond electron transfer to the C2 carbonyl, resulting in increased quantum efficiency.^165,285 The introduction of an electron-withdrawing chlorine atom at the 6-position led to a lower photorelease quantum efficiency in 40b, but the heavy atom effect of two additional bromo substituents at the 3- and 8-positions increased efficiency in the case of 40a (Φ_r = 0.065), suggesting that the triplet excited state is productive in these derivatives.¹⁶⁵ Phosphates (e.g., cAMP, cGMP, deoxycytidines; Φ_r = 0.09–0.1),^403,404 carboxylic acids (Φ_r = 0.02–0.13),^165,416,417 amines (as carbamic acids; Φ_r = 0.04–0.16),^418,419 alcohols (as carbonic acids; Φ_r = 0.01–0.4),^{321,342,420,421} diols (Φ_r = 0.004–0.06),^422,423 and alkoxyamines⁴²⁴ have all been successfully released from 40a. The 2P uncaging cross section of 40a at 740 nm ranged from 0.35 to 2.0 GM depending on the caged substrate.^{165,404,417,419,423} Despite several reports of successful liberation of thiols from 40a-thioethers,^425−428 photoisomerization of the by-product 42 occurred with higher efficiency (Scheme 8). Blocking the 3-position with a methyl group as in 41 prevented the formation of 42, facilitating the clean formation of 43 and the liberation of free thiols, albeit with lower quantum efficiencies than were achieved with 40a (Φ_r = 0.01 and 0.04, respectively).^429,430

Scheme 8. Photochemistry of Thioethers 40a and 41(429,430).

Similarly, phenols could be liberated directly from 40a, but competing recombination of the primary products was observed.^345,352,431 For example, a photo-Claisen rearrangement was found to proceed ∼2.5-times more efficiently than LG photorelease from 44 (Scheme 9).³⁴⁵

Scheme 9. Photochemistry of 44(345).

The favorable spectroscopic and photochemical properties of 40a were found to be useful not only for the photorelease of bioactive small molecules^{421,427,432−438} but also in the development of photoresponsive polymers,^{426,439−446} dendrimers,⁴⁴⁷ and supramolecular materials.^448−451 A genetically encodable lysine caged by 40a was also reported.³²⁵ Compound 40c (LG = acetate) was introduced as a more cell-permeable version of 40a, which can be trapped inside cells after hydrolysis of the ester bond.⁴⁰³ The 6-bromo-7-alkoxy derivatives of 40d had spectroscopic properties comparable to those of the protonated form of 40a, and were shown to release various LGs with Φ_r = 0.01–0.3 at λ_irr = 350 nm.^{342,347,404,421,452,453}

Singh and co-workers developed coumarin 45 as a photoresponsive, dual-channel sensor for hypoxia and nitric oxide (NO; see also section 4.2) with λ_max^abs = 410 nm and very weak fluorescence (Φ_F = 0.01; Scheme 10).⁴⁵⁴ Reduction of the 6-NO₂ group to an NH₂ group (46) led to a hypsochromic-shift in the absorption maximum (λ_max = 387 nm) and intense fluorescence emission centered at 535 nm (Φ_F = 0.55). Further reaction of the diamino moiety in 46 with NO^455−457 provided triazole 47 with λ_max^abs = 355 nm and fluorescence emission at 500 nm. The liberation of chlorambucil from 47 took place with Φ_r = 0.04 (λ_irr ≥410 nm) and a chemical yield of 90%. Hypoxia-dependent detection of NO based on changes in fluorescence and subsequent light-mediated release of chlorambucil was demonstrated in cultured HeLa cells.⁴⁵⁴

Scheme 10. Photochemistry of the Hypoxia and NO Dual-Channel Sensor 45(454).

EWGs or EDGs at the 8-position do not significantly affect the absorption spectra of coumarins;^406,458 thus, substitution at this position was used to tune the non-photochemical properties of coumarin-based PPGs. For example, the 7,8-dihydroxy derivative 48a and the bis(carboxymethoxy)-substituted coumarin 48b exhibited similar spectroscopic properties (Table 9) to their analogs 29a and 29c (Table 7).^367,459 The catechol motif in 48a enabled its attachment to TiO₂ nanoparticles; photorelease of chlorambucil (Cbl) from 48a (LG = Cbl) bound to such nanoparticles (λ_irr >410 nm) was accompanied by ¹O₂ generation by excited TiO₂ (Φ_Δ = 0.29).⁴⁵⁹ The bis(carboxymethoxy) moiety of 48b conferred increased water solubility (up to 2.7 mM in acetonitrile/HEPES buffer 5:95, pH 7.2).³⁶⁷ The dialkylaminomethyl C8 substituents of 49a–d (Table 9) significantly reduced the acidity of the 7-OH group in 49c (pK_a = 4.9)⁴⁶⁰ and 49d (pK_a = 3.8)⁴⁶¹ relative to the parent 40a (pK_a = 6.2), presumably because the aminomethyl group forms an intramolecular hydrogen bond with the phenolic hydroxyl group,⁴⁶² leading to greater photouncaging efficiency at the lower end of the physiological pH range. The liberation of carboxylic acids, diols, amines (as carbamic acids), and phenols (as carbonic acids) from 49a–d occurred with quantum efficiencies similar to or slightly exceeding that for 40a upon both 1P (Φ_r = 0.06–0.014, λ_irr = 360 nm) and 2P excitation (δ_unc = 0.5–1.4 GM, 755 nm).^{418,460,461,463} The 8-bis(carboxymethyl)aminomethyl moiety of 49c increased water solubility^418,460 (to >2 mM in acetonitrile/HEPES buffer 5:95, pH 7.2), and the appended alkyne of 49d enabled further conjugation of the PPG via copper-mediated click chemistry.^461,463 Singh and co-workers developed the π-extended coumarin derivatives 50 and 51 (Table 9),^464,465 which exhibited broad-range absorption extending to 400 or 550 nm, respectively. The 2-(2′-hydroxyphenyl)benzothiazole⁴⁶⁶ (HBT) moiety in 50 facilitated pH-dependent excited-state intramolecular proton transfer⁴⁶⁷ (ESIPT); at pH < 7.4, the 7-OH group enabled an ESIPT process resulting in emission at 528 nm, but at higher pH values, the hydroxy group was ionized and ESIPT was prevented, resulting in blue-shifted emission with λ_max^em = 480 nm.⁴⁶⁴ The acidochromic spiropyran moiety⁴⁶⁸ in 51 allows this PPG to undergo a reversible pH-dependent transformation between two species with distinguishable absorption spectra (Scheme 11; see also section 8).⁴⁶⁵ The closed form of the spiropyran (51_SP) has a C_spiro–O bond and has an absorption spectrum typical of 7-OR coumarin derivatives (λ_max ≈ 325 nm). Under acidic conditions (pH < 5.4), the Cspiro–O bond was cleaved to form the zwitterionic merocyanine isomer (51_MC), which has a more intense and red-shifted absorption spectrum extending up to 550 nm. The 51_MC form can thus be selectively photolyzed at λ_irr >410 nm. Chlorambucil liberation was observed upon irradiation of 50 and 51 (LG = Cbl) at 365 and 410 nm, respectively.^464,465

Table 9. Coumarin PPGs Substituted at the 8-Position.

PPG	λmaxabs (nm)	εmax (M–1 cm–1)	leaving groupsa	Φr (λirr/nm)	solventb	ref
48a	∼325	not reported	chlorambucil	0.034 (410)	ethanol	(459)
48b	324	1.1 × 104	Fmoc-cysteine	0.06 (350)	CH3CN/HEPES buffer pH 7.2, 1:20	(367)
49a–c	371–376	1.2–1.8 × 104	benzoic acid, dopamine and octopamine (as carbamic acids), capsaicin (as a carbonic acid), benzaldehyde (as a diol)	0.06–0.16 (360 or 365)	CH3CN/PBS buffer pH 7.2, 1:20 or CH3CN/HEPES buffer pH 7.2, 1:20	(418, 460)
49d	359	0.9 × 104	arachidonic acid, paclitaxels	0.06–0.14	MOPS buffer, pH 7.2	(461)
50	330	not reported	chlorambucil	0.006 (365)	ethanol	(464)
51	∼330	not reported	chlorambucil	(410)	CH3CN/H2O, 7:3	(465)
52a	356	2.1 × 104	acetic acid	0.026 (350)	MOPS buffer, pH 7.2	(416)
52b	362	2.3 × 104	acetic acid	0.059 (350)	MOPS buffer, pH 7.2	(282, 416, 469)
				0.11 (365)	PBS buffer pH 7.4	(131)
52c	365	2.3 × 104	acetic acid	0.52 (365)	MOPS buffer, pH 7.2	(282)
52d	378	2.7 × 104	acetic acid, glutamate (as ester or as carbamic acid)	0.17–0.43	PBS buffer pH 7.4	(282, 469)

^aOnly selected LGs are shown.

^bHEPES = 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid; MOPS = 3-(N-morpholino)propanesulfonic acid; PBS = phosphate buffer saline.

Scheme 11. Photochemistry of Spiropyran-Coumarin 25(465).

Tamamura and co-workers developed 8-azacoumarin derivatives 52a–c, whose absorption maxima are bathochromically shifted by ∼30 nm relative to 29a (Table 9). These compounds have rather acidic phenolic OH groups (with pK_as of 4.22–5.67) and high water solubility (5–10 mM in PBS buffer).^282,416,469 The observed trend in the efficiency of acetic acid photorelease from 52a–c (c > b > a) was attributed to the heavy atom effect of the 6-substituents on the ISC rate.^282,416,469 The bromine atom at the 3-position of 52d induced an additional bathochromic shift, approximately doubled the photouncaging efficiency, and increased the pK_a of the phenolic OH group to 5.1.^282,416

Extending the π-system at the 3-position is a well-establishedand useful way to bathochromically shift the absorption and emission maxima of coumarin fluorophores.^474−479 Jullien and co-workers synthesized 3-cyano coumarins 53a and 53b, which exhibited bathochromic shifts in λ_max^abs of 37 and 58 nm, respectively, relative to the parent coumarins 29b and 30c (Table 10).²⁹¹ The photorelease of benzoic acid from 53b was ∼5-times less efficient than from 30c. A 3-iodo derivative of 30c had a similar λ_max to 53b (441 nm) but released pyridine derivatives more efficiently than 30c (Φ_rε₄₀₅ = 202.0 vs 0.3 M^–1 cm^–1, respectively), presumably due to less efficient PeT from the pyridine to the coumarin.⁴⁸⁰ Ellis-Davies and co-workers introduced the water-soluble 3-[3-(methylamino)-3-oxoprop-1-en-1-yl] coumarin derivative 54a (DEAC450; Figure 3), which strongly absorbs blue light.²⁹² The release of carboxylic acids (e.g., glutamate, GABA), cyclic adenosine monophosphate (cAMP), and cyclic guanosine monophosphate (cGMP) from 54a proceeded quantitatively and efficiently upon either 1P (λ_irr = 473 nm, Φ_r = 0.18–0.78) or 2P (δ_unc = 0.5 GM, 900 nm) excitation, and a solvent-captured species was identified as the sole photoproduct (Scheme 12).^{140,292,481,482}

Table 10. Coumarin PPGs Substituted at the 3-Position.

PPG	λmaxabs (nm)	εmax (M–1 cm–1)	leaving groupsa	Φr (λirr/nm)	solventb	ref
53a	360	2.5 × 104	benzoic acid	not reported	CH3CN/Tris buffer (1:1), pH 7.2	(291)
53b	443	2.6 × 104	benzoic acid	0.04	CH3CN/Tris buffer (1:1), pH 7.2	(291)
54a	450	4.3 × 104	glutamate	0.39 (473)	phosphate buffer, pH 7.4	(292)
55a–f	457–472	3.3–4.7 × 104	Fmoc-Gly-OH	0.09-0.45 (455)	DMSO	(470)
55g–i	482–538	3.0–4.0 × 104	3,5-dimethyl benzoic acid	0.001–0.01 (544)	H2O	(295)
55j	470	3.5 × 104	Boc-Phe-OH	0.0044 (463)	CH3CN/HEPES buffer (2:1), pH 7.0	(471)
56a	345	2.3 × 104	benzoic acid	0.09 (360)	DMSO	(472)
56b	369	1.7 × 104	benzoic acid	0.03 (360)	DMSO	(472)
57a	407	not reported	glutamate	0.05 (410)	HEPES buffer, pH 7.4	(140)
57b	407	2.4 × 104	benzoic acid	0.16 (400)	DMSO	(473)
58a–e	430–456	3.0–4.4 × 104	4-methoxy benzoic acid	0.04–0.45 (430–456)	CH3OH/H2O (9:1)	(294)
59	467	3.5 × 104	4-methoxy benzoic acid	0.41 (467)	CH3OH/H2O (9:1)	(294)

^aOnly selected LGs are shown.

^bTris = tris(hydroxymethyl)aminomethane; DMSO = dimethyl sulfoxide; HEPES = 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid.

Scheme 12. Photochemistry of DEAC450 PPG 54a(292).

Coumarin 54a absorbs weakly in the UV region, especially in the range of 340–360 nm,^140,292 which enabled its selective and orthogonal 1P (473 and. 355 nm) and 2P (900 and 720 nm) excitation in the presence of shorter-wavelength activatable PPGs such as 4-carboxymethoxy-7-nitroindolinyl (CDNI) or dicarboxylate 2-(p-phenyl-o-nitrophenyl)propyl (dcPNPP).^140,481,483 Derivatives in which the 3-acrylamide moiety is conjugated with O-(aminoethyl)-2-azidoethyl-pentaethylene glycol (54b) or a PEG dendron (54c) were developed to increase the water solubility of caged GABA^481,483,484 and reduce antagonism towards GABA-A receptors.^481,485 Winssinger and co-workers reported that bioluminescence resonance energy transfer (BRET) from the Nanoluc-Halotag⁴⁸⁶ fusion protein (H-Luc) to coumarin 60 was sufficient to induce uncaging of the kinase inhibitor ibrutinib (Scheme 13).⁴⁸⁷ Accordingly, treatment with 60 caused furimazine-dependent covalent inhibition of the ErbB2 protein kinase in NanoLuc-HaloTag-expressing SKBR3 cells.⁴⁸⁷

Scheme 13. BRET-Induced Photouncaging of Ibrutinib in Live Cells⁴⁸⁷.

Blanchard-Desce, Kele, and co-workers independently developed a series of 3-π-extended 7-(diethylamino)coumarinylmethyl derivatives bearing electron-withdrawing end-groups (55a–i, Table 10).^295,470 Coumarins 55a–f had absorption maxima at 457–472 nm, and the charged pyridinium and benzothiazolium derivatives 55g–i exhibited even more pronounced bathochromic shifts in λ_max^abs.^295,470 The 66 nm difference in λ_max between 55c and 55i (472 and 538 nm, respectively; Figure 3) can be attributed to the effect of the N-alkyl moiety on the benzothiazolyl end-group. The release of carboxylic acids from 55a–c and 55e–f proceeded with Φ_r = 0.09–0.45, but 55d was photolyzed with Φ_r < 0.001. The authors explained this discrepancy by noting that the benzothiadiazolyl end-group of 55d is the strongest EWG in this series and suggesting that its strong electron-withdrawing effect gives rise to a strongly polarized excited state with pronounced photoinduced ICT that hinders photorelease.^470,488 Accordingly, carboxylic acid liberation was less efficient from 55g–i, which have strong cationic EWGs.²⁹⁵ The 2PE cross section for these compounds was larger in the 700–750 nm region (δ = 175–1304 GM) than in the 940–970 nm region (δ = 59–371 GM).^295,470 Coumarin 55f had the highest photoreaction efficiency for both 1PE (Φ_r = 0.45) and 2PE (δ_unc = 442 and 64 GM, at 730 and 940 nm, respectively) in this series.⁴⁷⁰ Visible-light activated liberation of carboxylic acid LGs from coumarin 55j was demonstrated to occur only after bioorthogonal transformation of the modulating tetrazine moiety.⁴⁷¹

The introduction of 3-phenyl groups bearing either a strong EWG (56a,b and 57a) or a strong EDG (57b) in the para position to create D−π–A or D−π–D systems, respectively, resulted in a ∼25–30 nm bathochromic shift of λ_max^abs relative to the parent coumarin (Table 10).^140,472,473 Photolysis of 56a,b and 57a,b led to quantitative carboxylic acid release; D−π–D derivative 57b had the highest quantum yield in this series.^140,472,473 The 2PE uncaging cross section was determined to be 3.4 GM (710 nm) for 56a and 2.1 GM (740 nm) for 56b;⁴⁷² that for 57b was estimated to be 16 GM (680 nm).⁴⁷³ Aldehyde 61 was the major photoproduct (70%) formed upon irradiation of 57b in anhydrous DMSO; the expected alcohol 62 was obtained only in the presence of water.⁴⁷³ A proposed mechanism is shown in Scheme 14.

Scheme 14. Proposed Mechanism for the Uncaging Reaction of 57b(473).

Zhu and co-workers observed similar photochemical behavior in the D−π–A and D−π–D systems 58a–e and 59, which were formed by extending the π-systems of 3-styryl coumarins at the 3-position (Table 10).²⁹⁴ Coumarin 58a exhibited a 48 nm bathochromic shift in λ_max^abs relative to 30c, and it was photolyzed to release p-methoxybenzoic acid with a similar quantum yield (Φ_r = 0.05 and 0.04, respectively) upon irradiation at the corresponding absorption maxima.²⁹⁴ The introduction of either an EDG or an EWG at the 3-styryl para position (58b–e) caused a further bathochromic shift. The uncaging quantum yields for D−π–D derivatives 58c–e were 5–10-times higher than that for D−π–A derivative 58b (Φ_r = 0.19–0.45 vs 0.04, respectively).²⁹⁴ The ∼40 nm difference between the absorption maxima of 58d and 58b is probably related to the presence of the π-bridge. Bis(julolidine) derivative 59, which bears the strongest electron donors in this series,³⁹³ also had the most bathochromically shifted λ_max (Figure 3) and was photolyzed with the highest efficiency (Φ_rε₄₆₇ = 14.3 M^–1 cm^–1).²⁹⁴ The 2PE uncaging cross sections determined for D−π–D systems 58c–e and 59 were ∼5–10-times larger than that for D−π–A system 58b (δ_unc = 17.7–39.6 vs 3.2 GM at 730 nm, respectively).

Benzocoumarin derivatives typically have similar or slightly hypsochromically shifted absorption maxima relative to 4-methylcoumarin^406,489,490 (λ_max^abs = 274–321 nm vs 310 nm), but their spectroscopic properties can be modified by introducing EDGs or EWGs to modulate their ICT states.⁴⁹⁰ Gonçalves, Costa, and co-workers reported on several benzocoumarin PPGs (63–68); their structures and photophysical and photochemical properties relating to the photorelease of various carboxylic acids are shown in Table 11.^{320,491−500}

Table 11. Spectroscopic and Photochemical Properties of Benzocoumarin-Derived PPGs.

PPG	λmaxabs (nm)	εmax (M–1 cm–1)	leaving groupsa	Φr (λirr/nm)	solventb	ref
63	345	0.8 × 104	GABA–OH	0.7 × 10–5 (350)	ethanol	(320)
64a	360	1 × 104	Phe-OH		ethanol	(491−493)
64b	344–348	0.6–1.8 × 104	various amino acids	0.1–6.2 × 10–5 (350)	ethanol or CH3OH/HEPES pH 7.2, 4:1	(320, 491−493)
65a	371−376	4.1 × 103	GABA, various amino acids, 5-aminolevulinic acid	2–24 × 10–5 (350)	CH3OH/HEPES pH 7.2, 4:1	(494, 496, 497)
				0.4–16 × 10–5 (419)		(494, 496, 497)
65b	377–418	0.7–7.7 × 103	butyric acid, 5-aminolevulinic acid	0.8–25.0 × 10–5 (350)	CH3OH/HEPES pH 7.2, 4:1	(495, 497, 498)
				0.5–31.0 × 10–5 (419)		(495, 497, 498)
66	377–398	0.7–7.7 × 103	butyric acid	0.7–13.0 × 10–5 (350)	CH3OH/HEPES pH 7.2, 4:1	(497, 498)
				0.6–7.0 × 10–5 (419)		(497, 498)
67	362	1.1 × 104	glutamate	0.006 (355)	CHCl3	(499)
68	339–361	0.3–1.1 × 104	butyric acid	8.6–12.0 × 10–5 (350)	CH3OH/HEPES pH 7.2, 4:1	(497, 500)
				0.04–1.0 × 10–5 (419)		(497, 500)

^aOnly selected LGs are shown.

^bGABA–OH = 4-(benzyloxycarbonylamino)butanoic acid; Phe-OH = N-(carbobenzyloxy)-l-phenylalanine; HEPES = 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid.

The absorption maxima of coumarins can be significantly red-shifted by increasing the electron-withdrawing capacity at the 2-position.^501−504 For example, the singlet excited states of thiocarbonyls are lower in energy than those of their carbonyl analogs, so their light absorption is bathochromically shifted.^505,506 This effect was also observed for coumarins.^507−510 Costa, Jullien, and co-workers studied thionated coumarin 69a, which had a 73 nm bathochromic shift of λ_max^abs relative to its carbonyl analog, yet it released carboxylic acids with a low quantum efficiency (Table 12).^291,304,511 The absorption maximum of 7-NEt₂ thiocoumarin 69b was bathochromically shifted by 87 nm relative to its carbonyl analog (Figure 3), and it photoreleased benzoic acid with Φ_r = 0.18, which was 2 orders of magnitude higher than the corresponding value for the carbonyl analog when both were irradiated at their absorption maxima.²⁹¹ The liberation of benzoic acid from 69b was induced by irradiating Er³⁺- or Tm³⁺-based upconverting nanoparticles (see also section 6.4.2) at 974 nm.⁵¹² However, the release quantum yield of 69b was found to be concentration-dependent, decreasing ∼30-fold when its concentration in the irradiated solution was lowered from 25 to 4 μM.²⁹¹ The photouncaging of a carbamate-linked cyclofen analog^431,513 from 75 was demonstrated (Scheme 15).⁵¹⁴ The solvent-captured derivative 76 was identified as the sole photoproduct of this reaction, and the cyclofen derivative was liberated in high chemical yield (90%, Φ_r = 5 × 10^–3) at λ_irr = 470 nm. The chromatically orthogonal photoactivation of 75 and 13-cis-retinoic acid (using blue-cyan- and UV-light sources, respectively) was used to control the development of live zebrafish embryos.⁵¹⁴ Additionally, the photouncaging of a Cas9 activator, trimethoprim, from 69b was used to control the activity of a CRISPR-Cas9 system in cell cultures.⁵¹⁵

Table 12. Spectroscopic and Photochemical Properties of Thionated Coumarin PPGs.

PPG	λmaxabs (nm)	εmax (M–1 cm–1)	leaving groupsa	Φr (λirr/nm)	solventb	ref
69a	395–398	1.4–1.7 × 104	benzoic acid, Z-Phe-OH	2–7 × 10–5 (365, 419)	ethanol or CH3CN/Tris buffer pH 7.5, 1:1	(291, 304, 511)
69b	472	3.1 × 104	benzoic acid	0.12 (365), 0.18 (487)	CH3CN/Tris buffer pH 7.5, 1:1	(291)
70a–c	400–431	0.7–2.7 × 104	butyric acid, various amino acids	1.0–13.0 × 10–5 (419)	CH3OH/HEPES pH 7.2, 4:1	(335, 516, 517)
71	427	1.8 × 104	benzoic acid		CH3CN/Tris buffer pH 7.5, 1:1	(291)
72	490	3.0 × 104	4-methoxybenzoic acid	0.4 (490)	CH3OH/H2O 9:1	(294)
73	515	2.5 × 104	4-methoxybenzoic acid	0.7 (515)	CH3OH/H2O 9:1	(294)
74	479	1.0 × 104	acetic acid	0.071 (475)	H2O	(284)

^aOnly selected LGs are shown.

^bTris = tris(hydroxymethyl)aminomethane; HEPES = 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid; Z-Phe-OH = N-(carbobenzyloxy)-l-phenylalanine.

Scheme 15. Photochemistry of Thiocoumarin 75(514).

Gonçalves and co-workers studied several thionated benzo[f]coumarins (70a–c) that exhibited bathochromic shifts of ∼65 nm relative to their carbonyl analogs and released carboxylic acids, with 3–20-times higher quantum yields upon irradiation at 419 nm (Table 12).^335,516,517 The thiocarbonyl motif proved to be compatible with other structural modifications that caused further bathochromic shifts in absorption. For example, the 3-CN derivative 71 had a λ_max^abs of 427 nm, although it was thermally unstable in a Tris buffer/acetonitrile solution (pH = 7.5).²⁹¹ Conversely, coumarin derivatives 72 and 73 bearing electron-rich p-aminostyryl moieties in the 3-position had significantly bathochromically-shifted absorption spectra (λ_max = 490 and 515 nm, respectively) but were thermally stable.²⁹⁴ Irradiation of 72 and 73 at their λ_max^abs induced photorelease of p-methoxybenzoic acid with Φ_r = 0.4 and 0.7, respectively. The cyclized derivative 78, rather than the expected 4-hydroxymethyl-coumarin, was identified as the main photoproduct of photolysis of 77. The mechanism proposed to explain this observation is shown in Scheme 16. Similar photoproducts are formed from 72 and 73; in all three cases, the loss of π-conjugation at the 3-position in the photoproducts causes a hypsochromic shift of λ_max (to 470 nm).²⁹⁴

Scheme 16. Proposed Photolysis Mechanism for 3-Styryl-Conjugated Thiocoumarins²⁹⁴.

Howorka and co-workers reported that the absorption maximum of thiocoumarin 74, which has an electron-rich p-diethylaminostyryl moiety at the 7-position, is at ∼480 nm (Table 12).²⁸⁴ Irradiation of 74 in DMSO or water liberated acetic acid with Φ_r = 0.024 and 0.071, respectively. On the basis of transient-absorption spectroscopy and steady-state kinetic studies in the presence and absence of oxygen, the authors proposed that the photorelease of the LG in DMSO proceeds through the triplet excited state, while a charge-separated state is more populated in water.²⁸⁴

Imino and hydroxyimino derivatives 79a and 79b (Figure 4) exhibited only minimal shifts in λ_max^abs relative to the carbonyl analog 29b.²⁹¹ Compound 79a was thermally unstable in a tris buffer/acetonitrile solution (pH 7.5), while 79b was photochemically inactive; it did not release acetic acid as a LG upon irradiation at 365 nm.²⁹¹ Conversely, 7-(N,N-diethylamino)-dicyano derivative 80a (λ_max = 487 nm, ε₄₈₇ = 3.3 × 10⁴ M^–1 cm^–1)²⁹¹ released carboxylic acids in high chemical yield (>90%) upon irradiation at 487 or 505 nm with Φ_r = 0.3–1.5 × 10^–3.^291,518,519 However, the liberation of an amine (as a carbamic acid) from 80a proceeded ∼3-times less efficiently than that of a comparable carboxylic acid.⁵¹⁹ Marchán and co-workers reported that the release quantum yields of carboxylic acids and amines from dicyano derivative 80b (R = Me) were up to 2.5-fold higher than those for release from 80a.^518,519 Additionally, 80b was more stable than its thiocarbonyl analog in the presence of acids and bases commonly used in Fmoc/t-Bu solid-phase peptide synthesis. It was therefore used to cage a cyclic RGD peptide-drug conjugate (82, Figure 5).⁵¹⁸ Photouncaging of the peptide-drug conjugate from 82 (λ_irr = 505 nm) proceeded with Φ_r = 7.2 × 10^–3.

Figure 4.

Structures of coumarin PPGs substituted at the 2-position. LG = alkoxide, carboxylate, or amine (as a carbamate).

Figure 5.

Structure of the 80b-caged c(RGDfK)-ruthenocene conjugate 82.

An analog of 80b (R = alkyl) was used to prepare caged morpholino oligonucleotides^{212,520−523} (cMOs) capable of perturbing targeted RNAs in vivo.³⁵⁶ Although the caged cMOs were successfully uncaged in live zebrafish embryos (λ_irr = 470 nm), their thermal stability in vivo was significantly lower than that of their carbonyl analogs.³⁵⁶ Increasing the system’s electron-withdrawing capacity at the 2-position by replacing one cyano group with a p-nitrophenyl moiety (81) caused an additional ∼15 nm bathochromic shift of the absorption maximum (λ_max^abs ≈ 502 nm) but also significantly reduced the photouncaging quantum yield (Φ_r = 0.5–2.3 × 10^–6).⁵¹⁹

A different way of utilizing the coumarin scaffold for photorelease was demonstrated by introducing a photoreactive oxime ester⁵²⁴ in the 3-position (83, Figure 6).⁵²⁵ The excitation of oxime esters typically results in homolytic scission of the N–O bond and the formation of a caged radical pair.^526−528 Photoexcitation of 83 (λ_max^abs = 436 nm, ε₄₃₆ = 3.9–4.2 × 10⁴ M^–1 cm^–1) at 450 nm led to the formation of heterocyclic radicals, and the system was used as a photoinitiator for radical polymerization of acrylate monomers.⁵²⁵

Figure 6.

Structure of coumarin-oxime-ester PPG 83.

2.3. Arylmethyl and Arylcarbonylmethyl Groups

Polyaromatic cores provide convenient platforms for developing π-extended arylmethyl and arylcarbonylmethyl PPGs. For example, the (anthracen-9-yl)methyl group was introduced as a polyaromatic benzyl-type PPG for carboxylates, alcohols, and hydroxylamines with a bathochromically shifted absorption band (λ_max^abs ≈ 385 nm)⁵²⁹ relative to those of benzyl¹ (λ_max ≈ 254 nm) and 2-naphthylmethyl³¹⁷ (λ_max^abs ≈ 280 nm) chromophores.^{323,529−531} Lam and co-workers prepared an (anthracen-9-yl)methyl that absorbs above 400 nm by extending its π-conjugation through the 10-position (84a–f, Figure 7).⁵³²

Figure 7.

π-Extended (anthracen-9-yl)methyl PPGs.

The absorption spectra of compounds 84b–d (λ_max^abs ≈ 405 nm) are bathochromically shifted relative to that of 84a (λ_max = 376 nm). Although these compounds have different substituents at the para position of the phenyl moiety, their spectra are similar. This was attributed to steric hindrance, which may cause the phenyl ring to be oriented orthogonally to the anthracenyl core.⁵³³ The presence of an acetylene bridge in 84e and 84f eliminates this steric hindrance;⁵³⁴ accordingly, the absorption maxima of these compounds are bathochromically shifted by ∼30–40 nm (λ_max^abs = 425–440 nm). The photorelease of the diphenylphosphinothioester LG from 84b–f in a THF/water mixture (3:1, λ_irr >420 nm) was demonstrated, and 84f showed the highest release efficiency within this series. The photoinduced heterolytic cleavage of the (anthracen-9-yl)methyl–phosphorus bond in 84f at λ_irr = 366 and 416 nm occurred with Φ_r = 0.08 and 0.025, respectively; these values are comparable to those for previously reported 4,5-dimethoxy-2-nitrobenzyl⁵³⁵ (DMNB) and (anthracen-9-yl)methyl⁵³⁶ caged phosphines (λ_irr = 360–400 nm). A phototriggered (λ_irr > 420 nm) traceless Staudinger ligation⁵³⁷ of caged oligopeptides (85) with azide-containing amino acids was shown to form the expected oligopeptides in chemical yields of 31–43% (Scheme 17).

Scheme 17. Visible-Light Triggered Traceless Staudinger Ligation⁵³².

The (acridin-9-yl)methyl group (86) was introduced by Zhang and co-workers as a UV-activatable (λ_max^abs ≈ 355 nm, ε₃₆₀ ≈ 1 × 10⁴ M^–1 cm^–1) PPG for alcohols⁵³⁸ and was later used with carboxylic acids.^{335,517,539,540} Its tail absorption in the visible range enabled photolysis at λ_irr = 419 nm, albeit with low quantum efficiency (Φ_r = 0.5–1.6 × 10^–4).^335,540 The photoreaction was proposed to proceed through an ion-pair intermediate.⁵³⁹ Singh and co-workers introduced the π-extended [benzo(a)acridin-12-yl]methyl derivative 87, which exhibits a bathochromic shift of ∼20 nm (λ_max ≈ 374 nm, ε_max ≈ 5 × 10⁴ M^–1 cm^–1) and extended absorption up to ∼425 nm (Scheme 18).⁵⁴¹ Photorelease (λ_irr ≥ 410 nm) of carboxylic acids from 87 proceeded in excellent chemical yields (80–92%) and with significantly higher quantum efficiencies (Φ_r = 0.08–0.13) than for 86; solvent-captured (benzo(a)acridin-12-yl)methylalcohol was identified as the sole side-photoproduct (Scheme 18).⁵⁴¹ A (benzo(a)acridin-12-yl)methyl-caged chlorambucil derivative was also shown to accumulate in the nuclei of cultured HeLa cells, presumably due to the acridine scaffold’s capacity to intercalate with DNA,^542−544 and exhibited light-dependent cytotoxicity.⁵⁴¹

Scheme 18. (Acridin-9-yl)methyl PPGs 86 and 87(541).

Singh and co-workers also introduced 1-(hydroxyacetyl)pyrene⁵⁴⁵88 as a variant of the well-established (pyren-1-yl)methyl PPG (Scheme 19).^{322,546−549} In contrast to (pyren-1-yl)methyl, which absorbs only in the UV region (with a solvent-dependent λ_max^abs = 320–340 nm), the addition of a hydroxyacetyl group in 88 caused a bathochromic shift of λ_max (∼355 nm), resulting in sufficient absorption above 400 nm (ε₄₁₀ = 2.7–3.9 × 10³ M^–1 cm^–1) to enable visible light-induced photolysis.⁵⁴⁵ The photorelease (λ_irr ≥ 410 nm) of carboxylic acids^545,550 and alcohols⁵⁵¹ (as carbonates) in a 1:1 acetonitrile/H₂O solution proceeded with near-quantitative chemical yields (>94%) and high quantum efficiencies (Φ_r = 0.30–0.41 and 0.17–0.20, respectively; Scheme 19). These results can be compared to those achieved with a (pyren-1-yl)methyl group, which released carboxylic acids and alcohols with (solvent dependent) Φ_r = 0.0029–0.139 upon excitation at 350 nm.^{322,546−548} The inherent fluorescence of 1-acetylpyrene⁵⁵² (λ_max^abs = 439 nm, Φ_F = 0.02) enabled imaging of 88 in fixed cells.⁵⁵¹ The efficiency of photorelease from 88 depended strongly on the water content of the reaction mixtures and decreased in the presence of a triplet quencher (potassium sorbate). Three photoproducts (the leaving group, 1-hydroxyacetylpyrene, and acetylpyrene) were formed upon irradiation; Scheme 20 shows a mechanism explaining these results.^545,551

Scheme 19. Photochemistry of 1-(Hydroxyacetyl)pyrene PPG⁵⁵¹.

Scheme 20. Photorelease from 1-Acetylpyrene PPG (88)⁵⁵¹.

Another arylmethyl-type PPG studied by Singh and co-workers is the (perylen-3-yl)methyl group (89), which absorbs in the visible range (λ_max^abs = 438 nm, ε_max = 2.4–3.5 × 10⁴) and displays characteristic fluorescence (λ_max = 445 nm, Φ_F = 0.9; Scheme 21).⁵⁵³ Carboxylic acids and alcohols (attached as carbonates) were successfully photoreleased from 89 (λ_irr ≥ 410 nm) in an acetonitrile/H₂O (3:1) solution with high chemical yields (>89%) and moderate quantum efficiencies (Φ_r = 7.7–9.3 × 10^–2).

Scheme 21. Photochemistry of the (Perylen-3-yl)methyl PPG⁵⁵³.

Similar to other polyaromatic arylmethyl-type PPGs,⁵²⁹ the photoreaction mechanism of 89 was proposed to proceed through the singlet excited state, followed by heterolysis of the C–O bond and solvent capture to afford the photoproducts.⁵⁵³ Heterolysis of the C–O bond was previously calculated to be energetically preferable to homolysis, especially when the carbocation is stabilized,^554−556 although homolysis dominates in simple benzyl derivatives.⁵⁵⁷ Zhao and co-workers further demonstrated that carboxylic acid leaving groups (e.g., chlorambucil) can be released from perylene 90 (Scheme 22), although quantitative chemical yields were not reported.⁵⁵⁸

Scheme 22. Photorelease of Chlorambucil from a Single (Perylen-3,4,9,10-yl)tetramethyl PPG (90)⁵⁵⁸.

The inherent hydrophobicity of polyaromatic PPGs limits their applicability in aqueous media. However, their incorporation into larger molecular structures has been demonstrated. For example, 89 was used to prepare photodegradable hydrogels^559,560 and polymer nanoparticles.^561−563 Additionally, Singh and co-workers used a reprecipitation technique⁵⁶⁴ to formulate (perylen-3-yl)methyl-caged chloambucil⁵⁶⁵ and pesticide 2,4-D⁵⁶⁶ (91 and 92, Figure 8) as globular organic nanoparticles with an average particle size of 25–30 nm, broad absorption spectra extending into the visible range (350–550 nm), and fluorescence emission at 625 nm. These nanoparticles were photolyzed (λ_irr ≥ 410 nm) to release the parent compound (perylen-3-yl)methanol and the corresponding leaving group. In the absence of light, 2–5% of the starting material hydrolyzed upon incubation in water or 10% fetal bovine serum (FBS) at 35–37 °C over 2–7 days. Photorelease (λ_irr ≥410 nm) from nanoparticles of 91 and 92 was also demonstrated in cultured HeLa cells⁵⁶⁵ and plants⁵⁶⁶ (Cicer arietinum), respectively, and light-dependent biological effects of the corresponding bioactive leaving groups were observed. Leaving group release could be monitored in real time because it caused the fluorescence emission band to shift from 625 nm (nanoparticle) to 450 nm ((perylen-3-yl)methanol). Similarly, the antimicrobial compound salicylic acid was caged with 1-(hydroxyacetyl)pyrene via an ester linkage (93, Figure 8), and the resulting conjugate was formulated into light-responsive (λ_irr ≥ 410 nm) organic nanoparticles whose photoactivation was demonstrated.⁵⁶⁷

Figure 8.

Structures of (perylen-3-yl)methyl and 1-(hydroxyacetyl)pyrene derivatives.

2.4. The (Benzothiadiazol-6/7-yl)methyl Group

VanVeller and co-workers studied compounds 94 and 95 as benzyl-type PPGs with a π-expanded heteroaromatic benzothiadiazole core (Figure 9).⁵⁶⁸ Their photoreactivity was explained based on the redistribution of electron density from the EDGs in the o- and m-positions upon excitation.^554,555,557 Although the meta effect was established for methoxy-substituted benzyl derivatives over 50 years ago,⁵⁵⁶ the corresponding NR₂ analogs were only studied recently^569−576 and achieved high photoreaction quantum yields (up to 0.45⁵⁷⁰). Derivatives 94 and 95 had broad absorption bands extending up to 500 nm with maxima at 420 nm. Acetate was photoreleased from 94a and 95 with comparable quantum yields (Φ_r = 0.067 and 0.061, respectively; λ_irr = 455 nm), and ethanol was liberated from 95 with Φ_r = 0.04.⁵⁶⁸ The presence of a bromo substituent in the 7-position (94b) roughly halved the quantum yield,⁵⁶⁸ presumably because of competing photochemical processes such as C–Br scission.⁵⁷⁷

Figure 9.

(Benzothiadiazol-6/7-yl)methyl PPGs.

2.5. The (N-Methyl-7-hydroxyquinolinium-2-yl)methyl Group

The photochemistry of the (7-hydroxyquinoline-2-yl)methyl group was initially explored and exploited by Dore and co-workers, who identified the (8-bromo-7-hydroxyquinoline-2-yl)methyl (BHQ)⁵⁷⁸ and (8-cyano-7-hydroxyquinoline-2-yl)methyl (CyHQ)⁵⁷⁹ groups (among others) as efficient PPGs for carboxylates, phosphates, diols, phenols, and amines.^{578,580−584} Derivatives of the (7-hydroxyquinoline-2-yl)methyl group can also be removed by 2P excitation, which reportedly proceeds with δ_unc values of up to 2.6 GM (740 nm) for carboxylate LGs.^579,585 The photochemical properties of (7-hydroxyquinoline-2-yl)methyl could be adjusted by varying the substituents on the quinoline core, although the λ_max^abs remained in the range of 320–385 nm.^579,585,586 Singh and co-workers showed that attaching an (8-alkoxyquinoline-2-yl)methyl derivative to carbon dots enabled their photolysis with λ_irr ≥ 410 nm to release H₂S (see also section 4.3).⁵⁸⁷ Additionally, Narumi and co-workers^588,589 extended the absorption wavelengths of quinoline-derived PPGs above 400 nm by N-alkylation of the quinoline nitrogen, a modification known to shift the absorption spectrum bathochromically.^590,591

The N-methyl-7-hydroxyquinolinium group 96 (N-Me-7HQm, λ_max^abs = 418 nm) photoreleased acetic acid as an LG upon irradiation with blue light (λ_irr = 458 nm, Φ_r = 0.045), with concomitant formation of (N-methyl-7-hydroxyquinolinium-2-yl)methanol as the sole additional photoproduct (Scheme 23). It is not yet known whether the mechanism of this photoreaction is similar to that of other (7-hydroxyquinoline-2-yl)methyl derivatives, in which heterolysis of the C–O bond proceeds from the triplet-excited state to generate an ion pair that subsequently collapses into the free leaving group and a solvent-captured side product.^{578,582,592,593} Methylation of the 7-hydroxy group caused a 60 nm hypsochromic shift of λ_max but terminated the photoreaction. 7-NMe₂ derivatives exhibited a λ_max^abs at ∼445 nm and were photolyzed with lower quantum yields (Φ_r = 1.2–2.8 × 10^–3) than the 7-hydroxy derivative, presumably due to the contribution of the NMe₂ group in the twisted intramolecular charge-transfer (TICT) excited state.^395,594 Increasing the system’s electron density by introducing an ethyl group in the 4-position more than doubled the quantum yield. As a salt, 96 was highly soluble in water (up to 20 mM, depending on the leaving group) and was successfully used for the photorelease of several amino acids and neurotransmitters (caged via their amino groups as carbamates), achieving Φ_r values of 0.025–0.068 and Φ_rε(λ_irr) values at 458 nm in the range of 96–272 M^–1 cm^–1.^588,589

Scheme 23. Photochemistry of N-Methyl-7-hydroxyquinolinium PPG⁵⁸⁸.

2.6. The Bimane Group

Singh and co-workers repurposed the fluorescent molecule bimane, often used in biochemistry for fluorescence labeling of proteins,^595−597 as a photoremovable protecting group by introducing carboxylate leaving groups on the 3-methyl or 3,5-dimethyl substituents (97 and 98, respectively, Figure 10).⁵⁹⁸ These compounds had a λ_max^abs at ∼380 nm with tail absorption above 400 nm and a fluorescence λ_max between 460 and 480 nm (Φ_F = 0.6–0.8). These values are similar to those for unsubstituted bimane.

Figure 10.

Bimane PPGs.

Photorelease (λ_irr ≥ 410 nm) of various aliphatic and aromatic carboxylic acids from bimane 97 proceeded with good chemical (75–85%) and moderate quantum yields (Φ_r = 0.06–0.07)⁵⁹⁸ that increased with the solvent’s water content.⁵⁹⁸ The authors proposed that the photoreaction proceeds through the singlet excited state and involves heterolytic C–O bond cleavage to generate an ion-pair that then undergoes solvent-mediated separation, leading to solvent capture of the bimane methyl cation (Scheme 24). In the case of 98, the captured product underwent a second photoreaction to release another equivalent of the LG with a quantum efficiency similar to that for the first LG liberation (Φ_r = 0.04).⁵⁹⁸

Scheme 24. Proposed Mechanism of Photorelease from Bimane-Derived PPGs⁵⁹⁸.

The photorelease of thiols from bimane 97 upon irradiation at 420 nm proceeded with a high chemical yield (70%) and Φ_r = 0.02.⁵⁹⁹ Because of the high nucleophilicity of thiols, the presence of strong electrophiles was required to prevent recombination of the resulting ion pair; in their absence, no photoproducts were detected. Similar behavior was observed for the release of thiols from 3-(hydroxymethyl)-2-naphthol derivatives.⁶⁰⁰ Photouncaging (λ_irr = 420 nm) of thiols from the corresponding tetra-bimane-protected dendrimeric monomer 99 in the presence of a dendritic monomer functionalized with strong electrophiles led to the formation of a photoinduced crosslinked polymeric network (Scheme 25) capable of entrapping live cells while preserving their viability.⁵⁹⁹

Scheme 25. Polymer Cross-Linking via a Light-Induced Thiol-Electrophile Reaction⁵⁹⁹.

2.7. Arylcarbonylmethyl Groups

Aromatic ketones are synthetically accessible, thermally stable, and photochemically reactive moieties that have been used extensively as PPGs.¹⁰ Both their lowest energy transition (n,π*) and the higher energy π,π* absorption bands are typically in the UV range. The liberation of leaving groups from arylcarbonylmethyl groups (Figure 11) can proceed via different reaction mechanisms depending on the substitution of the arene; for details, see an earlier review.¹⁰ Simple phenacyl PPGs (100) liberate LGs via H atom abstraction^601−603 or electron transfer (see section 6.2), o-methylacetophenones (101) react through an intramolecular H-transfer process,^{235,604−610}p-hydroxyphenacyl moieties (102) release LGs via a photo-Favorskii rearrangement,^611−613 and benzoin derivatives (103) release LGs via photocyclization to form 2-phenylbenzofuran as a side-product.^{275,614−616} Here, we discuss only arylcarbonylmethyl PPGs that absorb above 400 nm.

Figure 11.

Arylcarbonylmethyl PPGs.

A phenacyl-based polyaromatic scaffold containing tetraphenylethylene⁶¹⁷ was used by Singh and co-workers to create a PPG with an aggregation-induced emission chromophore^618,619 by installing a pendant acetyl group on each aryl ring (104).⁶²⁰ Sequential photorelease (λ_irr ≥ 410 nm) of four carboxylic acid moieties (chlorambucil, Cbl) from an organic nanoparticle formulation of 104 was demonstrated (Scheme 26).⁶²⁰ The chemical yield and quantum efficiency of release depended strongly on the water fraction of the solution (f_w): there was no appreciable release below f_w = 85% but photorelease proceeded in chemical yields of up to 96% for f_w = 99% (Φ_r = 0.52). It was suggested that the photoreaction requires a restriction of intramolecular rotation (RIR)⁶²¹ that is imposed in the aggregated state. Singlet oxygen generation by both 104 as a nanoparticle and the photoproduct 105 (Φ_Δ = 0.31 and 0.27, respectively) provided a complementary cell-killing mechanism whose effects were additive with those of the photoreleased drug in HeLa cells.⁶²⁰

Scheme 26. Photorelease of Chlorambucil from a Phenacyl-type Tetraphenylethylene PPG (104) via a Photoreaction That Occurs Only in an Aggregated State⁶²⁰.

The p-hydroxyphenacyl (pHP) group (102) is an established⁶¹² UV-excitable (λ_max^abs ∼ 275 nm), arylcarbonylmethyl-type⁶²² PPG with several favorable properties including high rate constants (10⁷–10⁸ s^–1), quantum yields (0.2–1.0), and chemical yields (typically 60–90%) for LG release, and the formation of a major biocompatible photoproduct (p-hydroxyphenylacetic acid) with a hypsochromically-shifted absorption spectrum. It has therefore found many applications in chemistry and biology.^22,622 The mechanism of the photoreaction has been studied extensively and previously reviewed.^22,622 Briefly, photoexcitation and subsequent ISC are thought to generate an intermediate^622−628 reminiscent of a Favorskii rearrangement^624−626 cyclopropanone intermediate (106) that either hydrolyzes to form p-hydroxyphenylacetic acid or undergoes decarbonylation to form p-hydroxybenzylalcohol (Scheme 27).^629,630

Scheme 27. Photochemistry of 102 (pHP) as a PPG⁶¹³.

A bathochromic shift in the absorption spectrum of pHP was induced by extending its π-system at the 3-position (107 and 108, Scheme 12; λ_max^abs ≈ 380 nm in polar protic solvents). The nitrogen-containing substituents in 107 and 108 are positioned in a way that was expected to facilitate an excited-state intramolecular proton transfer (ESIPT) that would assist in the deprotonation of the p-hydroxyl group. Specifically, it was proposed that upon excitation to the singlet excited state, an ESIPT would occur,⁶³¹ followed by a transition to a triplet excited state, allowing the reaction to proceed as shown in Scheme 27.^631−633 The hydrophobicity of the tetraphenylethylene⁶¹⁷ moiety in 108 enabled the formation of organic nanoparticles⁶³³ using a reprecipitation technique.⁵⁶⁴ Visible light-mediated photorelease (λ_irr ≥ 410 nm) of a carboxylic acid^631,633 (chlorambucil) and a hydroxylamine⁶³² (an NO-donating NONOate;⁶³⁴ see section 4.2) from 107 and 108 proceeded in high chemical yields, with the corresponding p-hydroxyphenylacetic acid derivative being the sole additional photoproduct^631,633 (Scheme 28). Uncaging was accompanied by a 70–100 nm blue-shift of the fluorescence emission spectrum, enabling real-time quantitative monitoring of the reaction’s progress in live cultured cells.^631−633

Scheme 28. Photouncaging of Chlorambucil and DEA NONOate from 107(631,632,635).

A two-step activation system was developed by capping the p-hydroxyl group in 110 with a 4-benzylboronic acid pinacol ester.⁶³⁶ Phenylboronic acid and its esters react with hydrogen peroxide (H₂O₂) to form the corresponding phenol,^637−639 which in turn can lead to the release of a leaving group from the p-benzyl position via 1,6-elimination.^640−642 The photorelease of chlorambucil from 110 thus occurred only in the presence of H₂O₂ (Scheme 29).⁶³⁶

Scheme 29. Two-Step Sequential Uncaging of Chlorambucil (Cbl) from 110(636).

Goeldner and co-workers introduced donor–acceptor biphenyl pHP derivatives 109a and 109b (λ_max^abs = 313 and 369 nm, respectively; Scheme 27).²⁵⁵ The photorelease of glutamate was more efficient from 109a than from 109b (Φ_r = 0.21 and 0.015, respectively). The 2P-uncaging cross section of glutamate liberation from 109a was 0.21 GM at 740 nm,²⁵⁵ which is comparable to the values previously observed for 2P release of diethyl phosphate and ATP from the parent pHP (0.5–1.1 GM at 550 nm).⁶⁴³

2.8. The 4-(p-Hydroxybenzylidene)-5-imidazolinone Group

Campbell and co-workers developed a photocleavable variant (PhoCl) of the photoconvertible fluorescent protein mMaple⁶⁴⁴ that exploits the phototransformation of the protein’s chromophore, a 4-(p-hydroxybenzylidene)-5-imidazolinone group formed autocatalytically from the tripeptide serine-tyrosine-glycine in the presence of oxygen.⁶⁴⁵ This moiety is non-fluorescent in its neutral form (111; Scheme 30). Upon excitation with UV or violet (∼400 nm) light, it is deprotonated to form an excited intermediate, 112*,^646,647 which undergoes irreversible β-elimination to liberate a carboxamide-containing peptide.⁶⁴⁸ The photocleavage of PhoCl has been used in several applications, for example, to control protein function^{645,649−652} or modulate the mechanical properties of hydrogels.⁶⁵³

Scheme 30. Photocleavage of a Peptide in PhoCl⁶⁴⁸.

2.9. The Stilbene Group

Singh and co-workers harnessed the photocyclization of stilbenes to induce elimination of alcohol and carboxylic acid leaving groups from the 2-position of E-4-(N,N-dimethylamino)-4′-nitrostilbene⁶⁵⁴ (DANS, 113, Scheme 31). The light-mediated E–Z isomerization of stilbenes followed by a photochemically allowed conrotatory 6π-electrocyclization to form an E-dihydrophenanthrene is a well-studied process.^655−657 This intermediate tends to spontaneously undergo a subsequent oxidative dehydrogenation to yield a phenanthrene derivative.^656−658 However, in the absence of an oxidant, the dihydrophenanthrene intermediate may reversibly open to give the corresponding Z-stilbene; alternatively, if suitable substituents are present, hydrogen-shift processes may occur.^659,660 However, the presence of substituents such as methoxy^661−663 or halide^663,664 groups in the 2-position resulted in thermal non-oxidative elimination to form a phenanthrene derivative 114 (Scheme 31).

Scheme 31. Photorelease from E-4-(N,N-Diethylamino)-4′-nitrostilbene (DANS) PPG⁶⁵⁴.

The spectroscopic properties of 113 were solvent-dependent: on going from non-polar to polar solvents, λ^abs shifted from 410 to 430 nm, λ_max^em shifted from 502 to 720 nm, and there was a marked decrease in the fluorescence quantum yield (from 0.55 to <0.002), presumably because of the formation of a TICT state.⁶⁶⁵ The molar absorption coefficient remained relatively constant (ε_max = 2.5–2.8 × 10⁴ M^–1 cm^–1). Photorelease of alcohols and carboxylic acids from 113 was demonstrated in hexane, acetonitrile, and water. In acetonitrile, the Φ_r was in the range of 0.10–0.14 for all tested leaving groups, and the chemical yields of photorelease were between 88 and 93%. On the basis of previous studies,^{661,666−668} a photoreaction mechanism was proposed (Scheme 32) in which photoexcitation of 113 leads to a singlet excited state, allowing the E-stilbene to isomerize into its Z-isomer. The subsequent photoexcitation of the Z-isomer to its singlet state leads to a conrotatory 6π-electrocyclization to form E-dihydrophenanthrene. Orbital symmetry and energy considerations⁶⁶⁹ suggest that the relative configuration of the dihydrophenanthrene product is E. Spontaneous re-aromatization of the E-dihydrophenanthrene by elimination then yields phenanthrene 114 and releases the leaving group. The fluorescence of the phenanthrene photoproduct (λ^em = 450 nm) was monitored to follow the reaction in real time. The photorelease of chlorambucil from 113 (λ_irr ≥ 410 nm), giving rise to light-dependent cytotoxicity, was observed in MCF-7 breast cancer cell lines.⁶⁵⁴

Scheme 32. Proposed Release Mechanism from Stilbene Derivatives 113(654).

2.10. Quinones

The photoreduction of quinones has been studied extensively.^670−675 The reaction can proceed intermolecularly in the presence of a hydrogen donor^676−681 (e.g. an alcohol or amine) or intramolecularly via H atom transfer^682−689 from a favorably positioned C–H bond on a side-chain. Examples of hydrogen abstraction from the C–H bonds of amino^690−693 and sulfido^694,695 substituents are also known.

Iwamura and co-workers were the first to exploit the solvent-assisted intermolecular photoreduction of quinones to release leaving groups.³¹⁷ The (anthraquinon-2-yl)methyl derivative 115 (λ_max^abs = 325 nm), originally developed as a protecting group for carboxylic acids that could be removed with reducing agents,⁶⁹⁶ efficiently released cyclic adenosine monophosphate (cAMP, Φ_r = 0.2) upon irradiation at 350 nm (Scheme 33). This photoreaction was proposed to involve two photochemical steps.^697−699 The first is quinone photoreduction via H atom transfer from the solvent in the triplet excited state to form a ketyl radical that gives rise to the final 1,4-dihydroxyanthraquinone derivative. The second is the photorelease of the leaving group from the resulting dihydroxyanthraquinone via a mechanism similar to that of photorelease from o-hydroxybenzyl PPGs.^{570,700−702} Photochemical liberation (λ_irr = 350 nm) of alcohols,^322,697 carboxylates,^698,699 and ketones/aldehydes⁷⁰³ from 115 (and derivatives thereof) proceeded with Φ_r = 0.03–0.12. However, the solvent-dependence of the intermolecular photoreduction of 115 may limit its usefulness.^322,697,698

Scheme 33. Photorelease of cAMP from an (Anthraquinon-2-yl)methyl PPG³¹⁷.

The presence of electron donors on the quinone core can give rise to broad charge-transfer absorption bands in the visible range.⁷⁰⁴ Although 1,4-naphthoquinone and 1,4-anthraquinone appear to have more extended π-systems than 1,4-benzoquinone, a hypsochromic shift in λ_max^abs of ∼50 nm was observed for each fused benzene ring.⁷⁰⁵ The photochemistry of naphthoquinone and anthraquinone derivatives resembles that of benzoquinones bearing aryl substituents. However, the presence of electron donors and acceptors on the distal rings causes substantial absorption spectrum shifts.⁷⁰⁶ For example, the absorption of 1,4-benzoquinone (λ_max = 281 nm) was significantly extended into the visible range (400–600 nm) upon introducing electron-donating substituents.^707−710 The magnitude of this shift correlated qualitatively with the magnitude of the HOMO and LUMO coefficients in the substituted quinone.⁷¹¹

Chen and Steinmetz used the photocyclization of 2-dialkylamino-1,4-benzoquinones^{690,691,712,713} into benzoxazoline photoproducts to drive 1,4-elimination of carboxylates and phenolates bound through the 5-methylene group (116, Scheme 34).^714,715

Scheme 34. (1,4-Benzoquinon-5-yl)methyl Derivatives 116 as PPGs⁷¹⁵.

2-Pyrrolidino-1,4-benzoquinone 116a exhibited a strong absorption band in the UV region and a weaker charge-transfer absorption band extending into the visible range (450–650 nm, ε = 1.9–2.6 × 10³ M^–1 cm^–1),^{697,714−716} which is typical for amino-substituted quinones.^717,718 Carboxylates were released from 116a (λ_irr = 458 or 532 nm) in chemical yields of 50–60% at full conversion. An additional photoproduct resulting from cycloaddition of an o-quinone methide photoproduct with 116a was formed with a chemical yield of 36%. The photorelease yield increased to 89–92% upon irradiation in the presence of 3-(dimethylamino)cyclohexen-1-one (0.1 M) as an o-quinone methide trapping agent.⁷¹⁵ The quantum yield for the disappearance of 116a was similar to that for carboxylate formation and was sensitive to solvent polarity (dropping from 0.10 in dichloromethane to ∼0.005 in water) but not to the presence of oxygen. Kalow and co-workers improved the quantum efficiency of release in water by changing the substituents around the side-chain γ-hydrogen:^697,716 replacing the pyrrolidine in 116a with an isoindoline (116c) or tetrahydroisoquinoline (116d), in which the C–H bond is weaker, led to an 8- to 10-fold increase in Φ_r in water (5.8 × 10^–3 vs 4.7 × 10^–2 and 5.7 × 10^–2, respectively). The release efficiency correlated strongly with the leaving group pK_a,^714,715 suggesting that the thermal 1,4-elimination step is rate determining. Rate constants for the release of phenols in this step were in the range of k = 5.1–20.1 × 10^–4 s^–1,⁷¹⁵ in keeping with previous measurements.^719,720 The proposed photoreaction mechanism is shown in Scheme 35.

Scheme 35. Proposed Release Mechanism from a 2-Pyrrolidino-1,4-Benzoquinone PPG⁷¹⁵.

The sensitivity of the reaction’s quantum yields to solvent polarity suggested the involvement of an intramolecular charge-transfer (ICT) excited state^679,721,722 (117) formed prior to excited-state hydrogen transfer from the pyrrolidino group to benzoquinone (118). The initial ICT excited state undergoes rapid back electron transfer (eT) to regenerate the ground state 116a, competing with the hydrogen-transfer step. A possible immediate precursor to benzoxazoline 121 in the photocyclization is the ground-state zwitterionic species 119.^684,685,723 In keeping with this hypothesis, photolysis of α-keto amides bearing leaving groups at the α-position yielded intermediates analogous to 119 that could cyclize with concomitant expulsion of a phenolate or carboxylate leaving group.⁷²⁴ Alternatively, leaving groups can be liberated from 121 via deprotonation and subsequent 1,4-elimination.⁷²⁵ Evidence for this pathway was obtained by isolating intermediate 121 (with LG = OPh) and showing that it could undergo elimination under sufficiently basic conditions.⁷¹⁵ Additionally, nanosecond laser flash photolysis experiments provided no evidence supporting direct elimination of carboxylates from 119.⁷¹⁵ Chromatically orthogonal photorelease from 116d (λ_irr = 626 nm) and a 2-nitrobenzyl derivative (λ_irr = 365 nm) has been demonstrated.⁷¹⁶

Almutairi and co-workers overcame the inefficiency of photorelease from 116a in water by formulating 116a-caged drug systems (with drugs including paclitaxel, dexamethasone, and chlorambucil) by applying a microemulsion probe-sonication method to a mixture of poloxamer 407 (1% w/v) in water to form monodisperse water-dispersible nanoparticles with hydrophobic interiors. The particles had diameters of 108 ± 20 to 305 ± 101 nm, and their caged drug content ranged from 69 to 94 mol %.⁷²⁶ Photolysis of these nanoparticles (λ_irr > 590 nm) using various mouse tissue filters released the caged drugs, and the progress of the photorelease process was monitored by co-loading fluorescent dyes (DiD and IR780) into the nanoparticles.⁷²⁶

Dougherty and co-workers^723,727 studied analogs of 123, an o-hydroxydihydrocinnamic acid derivative with a conformationally restrictive “trimethyl lock” that enables thermal uncaging and photoreduction to release alcohols and amines via a 1,6-rearrangement mechanism^728−730 (Scheme 36). The UV-light-induced release of 2-nitrobenzyl-protected phenols to activate the trimethyl lock mechanism has been demonstrated previously.^{223,731−733}

Scheme 36. Photochemistry of 1,4-Benzoquinone Trimethyl Lock PPGs⁷²³.

The broad visible absorption bands of 124, 125 (400–600 nm), and 126 (350–500 nm; Figure 12) are indicative of transitions to charge-transfer states.⁷⁰⁷ The absorption maxima of 124 and 125 are bathochromically shifted relative to that of 116,^715,716 which was attributed to the steric effect of the trimethyl lock moiety. Photolysis of 126 (λ_irr = 455 nm), 124 (455 nm), and 125 (565 nm) in polar protic solvents liberated caged alcohols in quantitative chemical yield.⁷²³ Additionally, the orthogonal release of two different alcohols from 126 (R = Me) and 124a was demonstrated (λ_irr = 455 and 565 nm, respectively),⁷²⁷ and a mechanism for their photoreaction was proposed on the basis of studies on 126(723) (Scheme 37).

Figure 12.

Structures of 2-amino- and 2-sulfido-substituted 1,4-benzoquinone trimethyl lock PPGs.

Scheme 37. Proposed Uncaging Mechanism of Photoinduced Quinone Trimethyl Lock PPGs⁷²³.

Photochemical transformations are shown for R¹ = R² = H or Ph.

Zwitterion 127 was proposed to be the penultimate intermediate in this photoreaction; it can undergo nucleophilic capture at the carbocationic center by the solvent or an intramolecular nucleophile to give 128 or 129, respectively. Either of these species can then undergo trimethyl lock-facilitated ring closure to provide the final products in high chemical yield (>95%).⁷²³ The singlet excited state, ¹126*, has a charge-transfer character. In contrast to the photoreactions of typical benzoquinones, which undergo ISC to their triplet excited state with a near-unity quantum yield,⁷³⁴¹126* predominantly undergoes nonemissive return to the ground state (with >90% efficiency). The triplet excited state ³126* is formed with low efficiency (Φ_ISC = 1–5%) and was suggested to have a charge-transfer character based on a spin density calculation (DFT M06/6-311++G**),⁷²³ in contrast to the n,π* triplet-excited state of simple benzoquinones.⁷³⁵ It is still unclear whether the nonproductive triplet decay of ³126* to 127 occurs directly or via ³127*. Formation of 127 can thus occur through both the singlet and triplet manifolds. For simple S-alkyl derivatives, product formation via the singlet pathway dominated, but the triplet pathway contributed more substantially in the case of S-benzyl analogs.⁷²³ Although the triplet excited state is formed with low efficiency, it forms the product more efficiently than the singlet excited state.

The fluorophore 4-methylumbelliferone and the neurotransmitter GABA were both successfully photoreleased from derivatives of 126 under physiological conditions.⁷²³ Additionally, Forsythe and co-workers⁷³⁶ showed that S-substituted analogs of 126 can be formed in water under slightly basic conditions via a thio-bromo coupling.^737,738 Dendrimeric monomers with bromo-substituted 1,4-benzoquinone trimethyl lock moieties (132) were reported to react with dendrimeric monomers bearing sulfhydryl end groups (133) to form a polymeric network with sulfido-substituted 1,4-benzoquinone trimethyl lock PPGs incorporated into its backbone (Scheme 38).⁷³⁶ Encapsulation of cells (mouse fibroblast L929, human foreskin fibroblast, and human mesenchymal stem cells) in this polymeric network and their subsequent release by photodegradation of the polymer (λ_irr > 420 nm) were demonstrated, with the cells retaining high viability throughout the process.⁷³⁶

Scheme 38. Photocleavable Polymer Based on a Sulfido-Group-Substituted 1,4-Benzoquinone Trimethyl Lock PPG⁷³⁶.

The 2-hydroxycinnamyl moiety (134, Scheme 39) was first utilized as a PPG by Porter and co-workers to achieve photochemical activation of thrombin.^739,740 This system undergoes an initial photoinduced isomerization followed by cyclization to form a coumarin derivative, causing the release of caged substrates such as alcohols.^741−750 The cyclization rates of 2-hydroxycinnamyl esters or amides approached those of the trimethyl lock system (k = 0.03–50 × 10⁵ s^–1).^{723,728,729,751−754} The absorption maxima of 134 could be further bathochromically shifted (up to λ_max^abs = 394 nm) by introducing electron-donating substituents on the phenyl ring^743,755,756 or extending the system’s π-conjugation.^750,757 A small set of 2-hydroxycinnamic derivatives was synthesized to study and improve the 2P-absorption-mediated release process.^755,756,758

Scheme 39. Photochemistry of o-Hydroxycinnamic Derivatives⁷⁴⁰.

Dougherty and co-workers incorporated conformationally locked Z-cinnamyl esters (with a cis-alkenyl lock⁷⁵⁹) into amino-substituted 1,4-benzoquinones 135 and 136, and 1,4-naphthoquinone 137 (Figure 13), enabling visible-light mediated quinone photoreduction to serve as an intramolecular cyclization initiator and leaving-group release trigger (Scheme 40).⁷⁶⁰

Figure 13.

Structures of amino-substituted 1,4-benzo- and naphthoquinone cis-alkenyl lock PPGs.

Scheme 40. Photochemistry of 1,4-Benzoquinone cis-Alkenyl Lock PPG 18(760).

Amino-substituted quinones 135–137 have broad charge-transfer absorption bands in the range of 495–535 nm (ε_max = 2.9–3.6 × 10³ M^–1 cm^–1). Upon irradiation at 565 nm, methanol or ethanol was liberated with Φ_r = 0.03–0.04 in quantitative chemical yield.⁷⁶⁰ Solvent effects on the photoreduction⁷¹⁵ and cyclization⁷⁵⁹ processes were observed as expected; in polar protic solvents, photoreduction was less efficient and cyclization was more efficient, whereas the opposite was true in non-polar aprotic solvents.⁷⁶⁰

A different mechanism involving photoenolization^609,761 followed by heterolytic elimination^{230,231,233,762} was demonstrated by Kamdzhilov and Wirz²³² for substituted 5-(ethylene-2-yl)-1,4-naphthoquinone 138 (Scheme 41), which absorbs below 405 nm. This compound released HBr and diethyl phosphate upon irradiation at 365 nm (Φ_r = 0.35 and 0.70, respectively) in a neutral aqueous solution, but a poorer leaving group (acetic acid) was released less efficiently (Φ_r < 0.01). Unfortunately, the sensitivity of this photoreaction to acids and the instability of naphthoquinones toward bases limit the use of this photochemical protecting group.

Scheme 41. Photochemistry of 5-(Ethylene-2-yl)-1,4-naphthoquinone PPG²³².

2.11. Xanthene and Pyronin Groups

Wirz, Klán, and co-workers showed that acetate and diethyl phosphate can be released from a 1:1 complex of 139 with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) upon irradiation at >500 nm with Φ_r = 0.3–2.4 × 10^–2 (Scheme 42).⁷⁶³ This relatively low photorelease quantum yield was compensated for by a large molar absorption coefficient at the irradiation wavelength (ε ≈ 4 × 10⁴ M^–1 cm^–1), resulting in an uncaging cross section Φ_rε(λ_irr) of ∼100 M^–1 cm^–1.⁷⁶³ Efforts to synthesize 139 by alternative methods or to separate this DDQ complex were unsuccessful.^11,763 It was assumed that the primary photoproduct in aqueous solutions, a meso-methylhydroxy derivative formed from the corresponding cationic intermediate, is further oxidized by DDQ to give 6-hydroxy-3-oxo-3H-xanthene-9-carboxylic acid (140) as the major photoproduct. HMO calculations of the xanthene frontier molecular orbitals revealed that the HOMO and LUMO are disjoint at the meso-position, resulting in an increase in its negative charge upon HOMO–LUMO excitation. This behavior was linked to specific photoreactivity such as the extrusion of leaving groups (Scheme 42)¹¹ (see also section 2.12).

Scheme 42. Photochemistry of Xanthene PPG 139(763) and Schematic Representation of the HOMO and LUMO of Its π-System¹¹.

A different approach involved placing a 1,3-dithian-2-yl substituent at the meso-position of pyronin 141 (Scheme 43).⁷⁶⁴ The resulting compound exhibited strong absorption (λ_max^abs = 584 nm) and emission (λ_max = 607 nm) bands in aqueous solution, and irradiation in its major absorption band (Φ_r = 3 × 10^–4) gave a stable photoproduct that absorbed at 382 nm and emitted at 448 nm. It was proposed that direct thiolate release could explain the observed meso C–C bond cleavage in the pyronin chromophore.⁷⁶⁴ Recently, Klán, Roithová, and co-workers subsequently showed that transformation of 141 is a multi-photon multi-step process. Many intermediates in this process were identified and their interrelationships were clarified using steady-state and time-resolved optical spectroscopy, mass spectrometry, and NMR spectroscopy.⁷⁶⁵Scheme 43 presents a simplified mechanism for this complex reaction, which involves at least three light-initiated steps. A different mechanism involving photooxidative cleavage by singlet oxygen was observed for the photolysis of exo-alkylidene xanthenes.⁷⁶⁶

Scheme 43. Photochemistry of Pyronin 141(764,765).

Jo and co-workers recently demonstrated the release of 10-N-carbamoyl substituents from a 3,7-bis(dimethylamino)-10H-phenothiazine chromophore upon excitation with red light in aqueous solution (142, Scheme 44).⁷⁶⁷ The side product of this transformation is the fluorescent indicator methylene blue. Because 3,7-bis(dimethylamino)-10H-phenothiazine is a UV-light absorbing chromophore,⁷⁶⁸ its excitation at 660 nm was unexpected. Although no mechanistic details were provided in the report, the authors suggested that the reaction starts with the heterolytic cleavage of the N–C(O) bond upon irradiation.

Scheme 44. Photorelease from 10H-Phenothiazine Derivatives⁷⁶⁷.

2.12. BODIPY Groups

4,4-Difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) and its derivatives are highly fluorescent dyes that are widely used in chemistry^769−773 and biology.^774−778 Their favorable spectroscopic properties are mainly due to their relatively compact, rigid, and planar structures, which make the potential energy surfaces of their S₀ and S₁ states very similar. Consequently, narrow Gaussian-shaped absorption and emission bands are typically observed for their lowest energy transition.^779,780 These compounds have high Φ_HF values (typically >0.5) because of inefficient nonradiative decay and ISC.^781,782 Simple BODIPY chromophores typically have two narrow absorption bands in the visible range (λ_max ≈ 490–540 nm, ε ≈ 3–9 × 10⁴ M^–1 cm^–1): an intense 0–0 band due to the S₀–S₁ transition, and a shoulder attributed to the 0–1 vibrational transition.^783−787 The BODIPY scaffold tends to resist photobleaching^781,788,789 and degradation by acids/bases,^790,791 but it is highly amenable to synthetic transformations that modulate its spectral properties.^{779,780,792,793} This section focuses on the different strategies reported to date utilizing the BODIPY chromophore as a photoreleasing system (Figure 14).

Figure 14.

Structures of BODIPY PPGs with LGs at different positions.

The groups of Winter⁷⁹⁵ and Weinstain⁷⁹⁴ independently studied the release of leaving groups from meso-methyl BODIPY derivatives upon photoexcitation with green light (a general structure 143 is shown in Scheme 45). The basis for this photoreactivity can be traced to the properties of the chromophore’s meso position. Heterolytic photodissociation to give a meso-methyl carbocation is favored when electronic excitation to the lowest singlet-excited state involves a substantial transfer of electron density to the sp² carbon atom of a chromophore bearing a LG in the α-position such that the antibonding σ* orbital is mixed with the chromophore’s LUMO.¹¹ In such cases, the promotion of an electron from the HOMO to the LUMO weakens the C–LG bond, facilitating heterolytic dissociation of the leaving group. This mechanism is responsible for, among other things, Zimmerman’s meta-effect in the photosolvolysis of methoxybenzyl acetates⁵⁵⁶ and the photochemical reactivity of coumarinyl-4-methyl PPGs (section 2.2).^22,278 HMO calculations suggested that BODIPY also reacts in this way because its HOMO and LUMO are disjoint at the meso position (Scheme 45), and the negative charge at this position increases upon excitation,^11,780 as also predicted for xanthene chromophores (Scheme 42). meso-Methyl carbocations were reported to have a low-lying excited state with a vertical S₀–S₁ energy gap of 8–13 kcal mol^–1 (TD-DFT), suggesting a near-degenerate diradical configuration.⁷⁹⁵ Such low-energy diradicals can have a close-lying conical intersection between the closed-shell singlet and singlet diradical forms.⁷⁹⁶ The photoheterolysis of C–LG bonds to generate ion pairs was shown to be favored when the ion pair has access to a nearby productive conical intersection that provides an efficient channel for the excited state of the precursor to decay to the ground-state ion pair.^796,797

Scheme 45. Photorelease of LG from meso-Methyl BODIPY Derivatives,⁷⁹⁴ and Schematic Representation of the HOMO and LUMO of the BODIPY π-System (BF₂ Is Omitted)¹¹.

The synthesis of meso-methyl functionalized BODIPYs generally follows known routes,^{481,779,780,792,793} and the attachment of a leaving group is achieved by one of (a) incorporation during BODIPY core synthesis,^{795,798−801} (b) displacement of a formerly installed good leaving group,^{295,798,801−803} (c) utilizing a meso-methylhydroxy derivative as a nucleophile to attack a pre-activated form of the leaving group,^{795,804−808} or (d) modifying a previously installed carbonic acid derivative^{794,805,809−811} (Scheme 46). The latter process was shown to compete with an undesired direct attack at the meso-methyl position.⁸⁰⁹

Scheme 46. Synthetic Pathways to meso-Methyl BODIPY PPGs.

meso-Methyl BODIPY PPGs such as 144, 145, and 146 (Figure 15) typically have absorption bands with λ_max^abs at 490–545 nm (Figure 16), although a family of π-extended meso-methyl BODIPYs with λ_max at 586–693 nm was also reported (147; Figure 16).^807,812 The absorption spectra of representative meso-methyl BODIPY PPGs discussed in this section are shown in Figure 16.

Figure 15.

Structures of meso-methyl BODIPY PPGs.

Figure 16.

Absorption spectra of selected meso-methyl BODIPY PPGs. Black line, 1,3,5,7-tetramethyl derivative (LG = acetate);⁷⁹⁵ blue line, a 2,6-diethyl-1,3,5,7-tetramethyl derivative (LG = (4-nitrophenyl)carbamate);⁷⁹⁴ red line, a 2,6-diiodo-B-dimethyl derivative (LG = acetate);⁷⁹⁸ green line, a bis(p-methoxystyryl) derivative (LG = benzoate);⁸⁰⁷ cyan line, a bis(p-dimethylaminostyryl) derivative (LG = benzoate).⁸⁰⁷

The photorelease of diverse functional groups, including primary and secondary amines^{794,805,809,810,812} (as carbamates), carboxylic acids,^{295,794,795,798,802,804,806−808} alcohols^794,798,805 (as carbonic acid esters⁷⁹⁴ or ethers^798,801,805), halogens,⁷⁹⁸ hydroxylamines,⁸⁰³ thiocarbamic acid,⁷⁹⁹ thioacetic acid,⁷⁹⁸ and thiols⁸⁰⁰ (Figure 15) from BODIPY-based PPGs, has been reported. The liberation efficiency of LGs correlated well with the pK_a of their conjugate acids,^794,798,809 and the photoreaction quantum efficiency of unsubstituted BODIPYs (other than 1,3,5,7-tetramethyl derivatives) was moderate to low. The photoreaction quantum yields determined in aerated protic solvents were around half those measured in degassed solutions.⁷⁹⁸ The typically small values of Φ_r for these systems (Φ_r^aer = 1–800 × 10^–4) are compensated by high molar absorption coefficients (typically, ε_max = 3–7 × 10⁴ M^–1 cm^–1), giving rise to uncaging cross sections, Φ_rε(λ_irr), of 1–100 M^–1 cm^–1, that are similar in magnitude to those of traditional UV-absorbing PPGs.¹⁰ The major side-photoproducts were identified as solvent-captured meso-methyl BODIPY derivatives (Scheme 45).^{719,794,795,798,803,808} These compounds (e.g., meso-methyl ethers formed when using methanol as the solvent) absorb at very similar wavelengths to the original PPG and may thus act as internal optical filters, but they are eventually degraded upon extensive irradiation.⁷⁹⁸

Klán, Weinstain, Winter, and co-workers showed that in addition to the nature of the LG, the efficiency of photorelease from BODIPY chromophores depended on their substitution: halogen substituents at the 2,6-positions increased both the reaction efficiency and Φ_ISC,^795,798 with a substituent heavy-atom effect that decreased in the order I > Br > Cl > H.^813,814 However, electronegative halogen substituents increased the electrophilicity of the meso-methyl position, making it prone to nucleophilic attacks that caused LG liberation even in the dark.⁸⁰⁹ Stronger EWGs such as aldehydes or sulfonates in the 2,6-positions substantially raised the barrier to C–O bond heterolysis on the triplet surface of 144 and thus impeded photorelease.⁸¹¹ For example, the calculated C–O bond (heterolytic) dissociation energies for derivatives of 144 bearing 2,6-disulfonate, -dihydrogen, and -diethyl substituents were 18.9, 15.7, and 13.5 kcal mol^–1, respectively, with photouncaging quantum yields of 0, 0.6 × 10^–4 and 3.9 × 10^–4, respectively.⁸¹¹ The absence of the 1,7-methyl groups reduced Φ_r by a factor of ∼1.5, probably due to a reduced electron density in the BODIPY core and thus a reduced capacity to stabilize the putative cationic diradical intermediate.^795,798 In addition, dialkylborano analogs had significantly higher Φ_r values than their BF₂ counterparts (up to 30-fold).^{798,801,807,812} These substituent effects were additive: the Φ_r of 149 (Figure 16) was 100-times that of 148 (Figure 17).⁷⁹⁸

Figure 17.

Structures and quantum efficiencies of BODIPY PPGs 148 and 149, the latter of which combines the 2,6-diiodo and BMe₂ modifications.

Winter and co-workers recently studied conformationally-restrained π-extended boron-methylated meso-methyl BODIPYs that absorb light at around 700 nm to liberate acetic acid with Φ_r = 0.018–0.037. These efficiencies are ∼50-times those reported for the comparable non-restrained 147 derivatives.⁸¹⁵ It was suggested that conformational restriction inhibited competing excited state decay pathways such as internal conversion, leading to higher photorelease quantum yields. The Jabłoński diagram describing the photochemistry of 150 shown in Figure 18 indicates that photorelease proceeds from both the singlet and triplet excited states.⁷⁹⁸ The increase in quantum efficiency correlated with enhanced ISC,^795,798 which can reduce competition from radiative (Φ_F) and nonradiative (Φ_NR) processes.⁷⁹⁸ It should be noted that the reaction efficiencies are relatively low for mediocre LGs such as carboxylates, presumably because of ion-pair recombination and nonradiative decay from the triplet excited state, which do not occur during photolysis of good LGs such as Cl^–.⁷⁹⁸

Figure 18.

Jabłoński diagram of the photochemistry of 150.⁷⁹⁸

Photorelease from meso-methyl BODIPYs has been used in various biological and synthetic applications. For example, the uncaging of signaling molecules including the gasotransmitter H₂S (see also section 4.3),⁷⁹⁹ histamine, and the neurotransmitter dopamine,^794,811,812 has been demonstrated in cellular environments. Sortino and co-workers reported that light-dependent NO-induced vasodilatation of rat aorta can be achieved by uncaging the NO donor (see also section 4.2) N-nitroso-N-phenylhydroxylamine.⁸⁰³ Weinstain and co-workers introduced a protecting-group-free, late-stage functionalization of meso-methyl BODIPYs that enabled their targeting to specific cellular organelles⁸⁰⁵ and the development of water-soluble derivatives.⁸¹¹ Other notable applications include the selective photorelease of the protonophore 2,4-dinitrophenol in mitochondria and the protein synthesis inhibitor puromycin in the endoplasmic reticulum,⁸⁰⁵ as well as the light-dependent delivery of cytotoxic molecules including chlorambucil and a cathepsin B inhibitor (CA-074⁸¹⁶) to cells.^804,806 In both of the latter cases, the observed cytotoxicity was partly due to photosensitized ¹O₂ generation by the BODIPY PPG.^804,806 Sebastián and co-workers used a meso-methyl BODIPY-caged diethylamine as an organic light-responsive nucleophilic cyanoacrylate initiator capable of fast on-demand photocuring of commercial formulations of various cyanoacrylates including biologically-relevant long alkyl chain monomers.⁸¹⁰ Klán and co-workers reported that controlled photorelease of alkynoic acids was followed by efficient decarboxylation, giving terminal alkynes that could subsequently undergo Cu^I-catalyzed azide/alkyne cycloaddition reactions.⁸⁰² Similarly, Truong and co-workers developed a phototriggered thiol-propiolate addition that is initiated by uncaging methyl-3-mercaptopropionate.⁷¹⁹ The utility of this photochemical ligation strategy was demonstrated by fabricating hydrogels with specific architectures, photo-immobilization of biomacromolecules, and live-cell encapsulation within a hydrogel scaffold.⁸⁰⁰

The BODIPY boron center has also been identified as a photoreactive center. Urano, Nagano, and co-workers showed that the B–O bond in 4-aryloxy BODIPYs can be photolyzed (λ_irr = 475–490 nm) to release phenols (Scheme 47).^817,818 The absorption/emission wavelengths (λ^abs = 495–525; λ^em = 506–541 nm) and molar absorption coefficients (ε_max = 6.2–9.8 × 10⁴ M^–1 cm^–1) of derivatives 151 were only slightly affected by modulation of the HOMO/LUMO energy gap. However, their fluorescence quantum yields, Φ_F, decreased dramatically upon raising the HOMO energy of the aryl group or reducing the LUMO energy of the BODIPY core.^817,818 This effect on Φ_F was attributed to electron transfer (eT) from the adjacent aryl group to the BODIPY core.⁸¹⁹ The release quantum efficiencies were generally inversely correlated with Φ_F, suggesting that the photoreaction proceeds via a competing PeT process in which B–O bond solvolysis is preceded by the formation of a charge-separated intermediate with a cationic aryl group radical and an anionic radical BODIPY core. Derivatives with a high calculated PeT driving force⁸²⁰ had low uncaging efficiencies (Φ_r = 0.3–2 × 10^–3). The same was true when the calculated PeT driving force was low (Φ_r = 0.7–2.6 × 10^–3), presumably because fast reverse-electron transfer from a charge-separated intermediate⁸²¹ competes with uncaging. Derivatives with moderate predicted PeT driving forces thus had the highest uncaging efficiencies (Φ_r = 1.4–5.4 × 10^–3).^817,818 Photouncaging of phenols from π-extended BODIPY analogs at λ_irr = ∼620 nm was demonstrated, albeit with very low quantum efficiencies (Φ_r = 1.5 × 10^–7–7.4 × 10^–5).⁸¹⁸ This capability was used to achieve intracellular photorelease of the transient receptor potential cation channel V1 (TRPV1) agonist capsaicin in cultured HEK293 cells transiently transfected with TRPV1, leading to the induction of light- and ligand-dependent Ca²⁺ uptake.⁸¹⁷

Scheme 47. Photorelease of Phenols by B–O Photolysis⁸¹⁸.

The range of functionalities releasable via B–O bond photocleavage was expanded^{818,822−824} by introducing a benzyloxycarbonyl linker that undergoes a spontaneous 1,6-elimination to release a leaving group from its benzyl position after the phenol moiety is liberated (Scheme 48).⁸²⁵ For example, a carboxylic acid derivative of the fluoroquinolone antibiotic levofloxacin was directly photoreleased (λ_irr = 470 nm) from ester 152 in 31% chemical yield.⁸²² As a result, this ester exhibited light-dependent bactericidal effects in E. coli and the Gram-positive S. aureus. The terminal primary amine of biogenic histamine was similarly caged through a carbamate bond (153a and 153b), and its subsequent photorelease (λ_irr = ∼480 nm) was achieved in 40% chemical yield with Φ_r = 3–3.9 × 10^–4.⁸¹⁸ Photoexcitation (λ_irr = 488 nm using an argon laser) of the cell-impermeable 153b in HeLa cells induced light- and H₁-receptor-dependent Ca²⁺ oscillations. The photolysis of carbamothioate 154 (λ_irr = 470 nm) also resulted in the release of a free amine;⁸²³ the carbamothioic acid subsequently underwent thermal B–O bond cleavage (k = 0.02 min^–1, pH 7.4) to release carbonyl sulfide (COS) and a free amine.^823,826 COS is hydrolyzed into CO₂ and H₂S (spontaneously or under carbonic anhydrase catalysis⁸²⁷), making it a useful H₂S generator for research with potential therapeutic applications (see also section 4.3).^{799,828−830}

Scheme 48. Expansion of Chemical Functionalities Amenable to Uncaging from BODIPYs via B–O Photolysis Using a Self-Immolative Linker^{818,822−824}.

Smith, Winter, and co-workers developed 155 as a photoswitchable probe⁸³¹ for single-molecule localization spectroscopy,⁸³² demonstrating that the B–C bond is also amenable to photocleavage. Photoexcitation of 155 (λ_irr = 488 or 532 nm) caused the cleavage of its two B–alkyl groups and their subsequent replacement by solvent adducts (156), which was accompanied by a ∼6-fold increase in Φ_F (from 0.15 to 0.96; Scheme 49). Conjugation of 155 with paclitaxel via its styryl group enabled super-resolution imaging of microtubules in live HeLa cells with an average full-width-at-half maximum (FWHM) diameter of 94 ± 10 nm.⁸³¹ The photoactivation of 155 required only a single laser wavelength and relatively low laser power.^833−835

Scheme 49. Photoswitchable BODIPY Probe Based on B–C Bond Photocleavage⁸³¹.

A different approach to using BODIPY scaffolds for photorelease is to introduce photoreactive functional groups in the 2,6-positions (157).^836,837 This approach is exemplified by the photorelease of amines from sulfonamide groups in the BODIPY 2,6-positions (λ_irr = 470 ± 20 nm, chemical yields = 45–50%, Φ_r = 0.022–0.11).⁸³⁶ These photoreactions were insensitive to the presence of oxygen, required a carboxylic functional group in the α-position of the amide chain, and exhibited pH-dependent rates, proceeding less efficiently under acidic conditions. These observations are consistent with the known photochemistry of arylsulfonamides,⁸³⁸ which have been studied extensively in the context of their use as potential PPGs^839−842 and due to their prevalence in pharmaceuticals.^843−845 The proposed photoreaction mechanism involves a sequence of electron-transfer, decarboxylation, hydrogen-transfer, and fragmentation steps that are shown in Scheme 50.

Scheme 50. Proposed Mechanism for Photolysis of 157(836).

The key steps in this reaction are electron transfer from the donor (carboxylate) to the acceptor (sulfonamide)^839,840,846 and the subsequent formation of a charge-separated state.^845,846 In the case of 157, the ability of carboxylates to transfer a single electron to the excited BODIPY is yet to be established, but a similar process was proposed for the closely related compounds 225a,b (section 4.1.1).⁸⁴⁷ The formation of a charge-separated state was supported by the need for the carboxylate and sulfonamide groups to be in close proximity.^836,848 Decarboxylation occurs from this state^842,849 and is followed by a radical reaction that can proceed through competing pathways, resulting in either the cleavage of the S–N bond with the formation of a reduced BODIPY derivative (pathway a) or deboronation (pathway b).⁸⁵⁰ In both cases, back electron transfer terminates the reaction. The formation of equal amounts of amine and aldehyde in the pathway suggests the formation of an imine intermediate that is subsequently hydrolyzed.⁸³⁶ The observed chemical yields of pathways a and b were consistent with previous observations of competition between different reaction pathways in arylsulfonamides.^849,850 When the carboxylate group is not ionized, the yields of the amine are reduced; this was attributed to a need for H atom transfer^847,851 and makes other reaction pathways more competitive.⁸⁵⁰ Photolytic cleavage of GABA from 158 (λ_irr = 488 nm; Scheme 51) enabled the controlled induction of light-triggered membrane-potential responses in patch-clamped mouse basolateral amygdala brain slices.⁸³⁶

Scheme 51. Release of GABA upon Photolysis of 158(836).

Zhang and co-workers introduced a PPG based on a BODIPY derivative bearing photoreactive oxime esters⁵²⁴ at the 2-position (Scheme 52).⁸³⁷ Compounds 159a and 159b have strong absorption bands centered at 507 and 516 nm (ε_max = 5.2–5.7 × 10⁴ M^–1 cm^–1) with low Φ_F values (0.09–0.12). This was attributed to the proximity of the ³n,π* state to the lowest ¹π,π* excited state, suggesting that ISC competes with fluorescence. The quantum yields of photorelease for carboxylic acid LGs at λ_irr = 503 nm were comparable to those for other BODIPY PPGs (Φ_r = 5.2–7.2 × 10^–4). The proposed photoreaction mechanism for this process is shown in Scheme 53. The excitation of oxime esters was shown to result in homolytic scission of the N–O bond and formation of a caged radical-pair.^526−528 This process is considered to occur mainly from the triplet-excited state,^852−856 although dissociation proceeded efficiently from the singlet-excited state in some oxime esters.⁸⁵⁴ The caged radical pair can undergo recombination or in-cage reactions,⁸⁵⁴ but escape of the carboxyl radical from the cage and subsequent hydrogen atom abstraction from a polar protic solvent creates a carboxylic acid leaving group.^{412,528,837,856} The corresponding iminyl radical can undergo several transformations^796,857,858 or fast back-electron transfer to form an iminyl cation that is subsequently converted into a nitrile by deprotonation.⁸⁵⁹ In the case of 159, back electron transfer was found to be favorable; the 2-cyano BODIPY photoproduct was obtained in 60% chemical yield.⁸³⁷ The histone deacetylase inhibitor valproic acid (VPA) was caged as a BODIPY oxime ester in this way; in HeLa cells, this compound caused light- and dose-dependent toxicity with an IC₅₀ 100-times lower than that of free VPA.⁸³⁷

Scheme 52. Photorelease of Carboxylic Acids from BODIPY Oxime Esters⁸³⁷.

Scheme 53. Proposed Mechanism for Photolysis of 159(837).

3. Photorelease from Coordination Compounds

Coordination compounds, which consist of a central (usually metal) atom or ion surrounded by ligands, have rich and varied photochemistry.^860,861 They often have unique ground- and excited-state properties that can be tuned by varying the central atom or the coordinating ligands to allow light-triggered release of chemically or biologically active species. Many organometallic complexes also exhibit photonuclease, photo-crosslinking, or photocytotoxic activities, accompanied by diverse types of photofragmentation reactions or photodynamic effects.^862−869 This section surveys coordination compounds that have been used as photoreleasable systems activated by visible/NIR light in the past decade. However, comprehensive coverage of all their applications would be beyond the scope of this review. A more extensive discussion of these applications can be found in several review articles and perspectives that have been published in recent years.^{8,40,58,65,66,68,69,76,98,107,113,869−876} Representative spectra of selected PPGs discussed in this section are shown in Figure 19.

Figure 19.

Representative spectra of transition metal-containing PPGs: black line, a vitamin B₁₂ derivative⁸⁷⁷ (LG = alkyl; section 3.1); red line, a phthalocyanine derivative (LG = aryloxy; section 3.2);⁸⁷⁸ green line, a ruthenium(II) bipyridyl derivative (L = tyramine; section 3.3);⁸⁷⁹ blue line, a dirhodium(II,II) derivative (L = acetonitrile; section 3.4; the ε values are 50-fold smaller than shown);⁸⁸⁰ and magenta line, a cobalt(III) derivative (section 3.5).⁸⁸¹

Transition-metal complexes are usually colored compounds and are therefore readily excited using visible light. The accessibility of multiple excited states with different spin multiplicities⁸⁸² and competing photophysical and chemical processes can result in complex and sometimes unpredictable photochemistry. Many different primary photophysical processes, including metal-to-ligand charge transfer (MLCT), ligand-to-metal charge transfer (LMCT), and ligand-to-ligand charge transfer (LLCT), may precede the release of species.^883,884 However, ligand exchange is usually the key process in ligand (species) liberation.

3.1. Photochemistry of Vitamin B₁₂ Derivatives

Vitamin B₁₂ is a water-soluble metal complex bearing a cobalt ion in the center of a conjugated corrin ring; its photophysical and photochemical properties are relatively well understood.^{60−62,64,67,885,886} The corrin Co^III complex absorbs light below 580 nm,⁸⁷⁷ and its derivatives such as 160 (Scheme 54, Figure 19; LG = alkyl, CN, OH, oradenosyl) can undergo homolysis⁸⁸⁷ of the Co–C bond (BDE = 30–44 kcal mol^–1)⁸⁸⁶ in the singlet excited state⁸⁸⁸ to give a charge-transfer Co^III intermediate within tens of picoseconds.^{877,889−896} The intermediate then dissociates into a close radical pair of LG^• and Co^II side-product radicals that either recombine within nanoseconds or escape the solvent cage (Scheme 54).^{61,67,885,886,892,897−900} The solvent’s properties affect the efficiency of recombination.⁹⁰¹ Depending on the ligands, the relaxed singlet excited states have been characterized as either MLCT or LMCT states using DFT and TD-DFT methods.^{62,886,902−912} In addition, magnetic field effects on the photolysis of 5′-deoxyadenosylcobalamin have been reported.^913,914 The rate of radical pair recombination was found to be sensitive to external magnetic fields on the order of tens to hundreds of mT in viscous solutions. Although the involvement of a triplet state in the dissociation of ligands from cobalamin complexes has not been precluded by calculations,^886,909 the formation of a triplet radical pair seems inconsistent with the observed magnetic field effects.^895,913 The photobiological role of vitamin B₁₂ in the photoreception of photosynthetic and non-photosynthetic bacteria was studied by Kutta, Jones, and co-workers.⁹¹⁵ In contrast to the mechanism described above, the photochemistry of the coenzyme B₁₂-dependent photoreceptor protein, a bacterial transcriptional regulator that controls carotenoid biosynthesis, does not proceed via radical pair intermediates but through Co–C bond heterolysis.

Scheme 54. Vitamin B₁₂ PPGs⁸⁷⁷.

It was demonstrated that visible-light-induced hydrogel formation can be facilitated using alkyl-cobalamin-based photoinitiators whose photochemistry induces radical photopolymerization.⁹¹⁶ The production of free radicals from thiolato-Cob(III)alamins was also supported by electron paramagnetic resonance.⁹¹⁷ Irradiation of alkylcobalamins using >500 nm light was shown to form carbon-centered radicals that cause DNA damage via strand scission of polynucleotides.⁹¹⁸ Additionally, in the presence of reductants such as TiO₂ or Zn/NH₄Cl, cobalamin derivatives undergo photochemical reduction to strongly nucleophilic Co^I complexes that can react with electrophiles via an S_N2 mechanism.⁹¹⁹

The quantum yields of LG release from cobalamin derivatives are often irradiation-wavelength dependent.^892,894,920 For example, the Φ_r for ^•CH₃ liberation from methylcobalamin in aerated aqueous solutions varies from 0.35 at λ_irr = 490 nm to 0.24 at Φ_r = 550 nm.^921,922 Similarly, 5′-deoxyadenosylcobalamin undergoes Co–C bond cleavage with Φ_r = 0.20 under anaerobic conditions,⁹²³ although a near-unity quantum yield for bond homolysis in aqueous solution has also been reported for this compound.⁹²⁴ The Co^II side-product can be trapped by oxygen.^923,925,926 The photolysis mechanism and release quantum yields depend not only on the type of LG but also on experimental parameters including the solvent, the pH, the presence of specific enzymes,^{888,894,901,927−929} and the nature of the lower axial base.^895,923 Cobalamin release mechanisms are discussed in more detail in a recent review by Jones.⁶¹

Photoactivatable vitamin B₁₂ systems have been used for visible-light-initiated drug release. The absorption limit of the corrin ring (>580 nm) can be extended by appending a sensitizer absorbing in the NIR region, by exploiting 2P excitation, or via upconversion (see section 6.4.2).^885,930 Building on the earlier studies on the photochemical decomposition of adenosylcobalamin and other vitamin B₁₂ analogs discussed above, Lawrence and co-workers showed that the photochemical cleavage of the Co–C bond in cobalamins can be used in the design of caged compounds.⁸⁸⁵ Cobalamin 160, which bears a rhodamine fluorophore as an LG connected through an alkyl linker, was shown to undergo selective photochemical homolysis upon irradiation at 560 nm in high chemical yield (97%), even when mixed with two different caged compounds absorbing only UV light.⁹³¹ The fluorescence of the appended rhodamine in 160 is quenched by the cobalamin, allowing its release to be monitored by observing its fluorescence under a confocal microscope in microwells and living cells.

The portfolio of leaving groups used with these PPGs was subsequently extended beyond fluorophore indicators by caging biologically active species including the anti-inflammatory agents methotrexate (161a), colchicine (161b), and dexamethasone (161c) with the cobalamin lipid conjugate 161 (Figure 20).⁹³² These caged derivatives were loaded onto human erythrocytes and the agents (which were connected via an auxiliary linker that was subsequently removed by esterase hydrolysis) were released in quantitative yield upon irradiation at 525 nm. To shift the absorption into the phototherapeutic window, the C₁₈ derivatives of pentamethine cyanine (Cy5; λ_irr = 646 nm), AlexaFluor700 (λ_irr = 700 nm), heptamethine cyanine (Cy7; λ_irr = 747 nm), and DyLight 800 (λ_irr = 784 nm) were used as sensitizers. Irradiation at the dyes’ maxima led to drug release and the induction of the expected biological responses.⁹³² A similar strategy was used to release cAMP from conjugate 162a to control the activity of a cAMP-dependent protein kinase and to release the anticancer agent doxorubicin from 162b (Figure 21).⁹³⁰ In these studies, several commercially available sensitizers including 5-carboxytetramethylrhodamine, SulfoCy5, Atto725, DyLight800, Alexa700, and BODIPY650 were used to facilitate excitation of cobalamin conjugates with visible-to-NIR light.

Figure 20.

Cobalamin PPGs designed to release methotrexate (161a), colchicine (161b), and dexamethasone (161c).⁹³²

Figure 21.

Cobalamin PPGs designed to release cyclic adenosine monophosphate (162a) and doxorubicin (162b).⁹³⁰

A photorelease strategy for liberating membrane-permeable bioagents such as colchicine, paclitaxel, and methotrexate from cobalamin–bioagent conjugates confined within lipid-enclosed compartments in the interior of erythrocytes was reported by Lawrence and co-workers.⁹³³ Upon photolysis of the conjugates by visible-to-NIR light, enabled by a Cy5 sensitizer attached via a dimethylbenzimidazole ligand, the drugs were liberated inside red blood cells. Janovjak and co-workers recently used the 5′-deoxyadenosylcobalamin binding domains of bacterial CarH transcription factors to induce growth factor receptor 1 dissociation.⁹³⁴ Several other relevant biological applications of cobalamin photochemistry have also been reported.^935−937

3.2. Photochemistry of Phthalocyanine and Porphyrin Derivatives

Si-phthalocyanine macrocycles (Figure 22) are photostable, hydrophobic, and non-toxic.^938−940 Their usefulness in aqueous media is limited by their low aqueous solubility, although this can be overcome through structural modification.^941−943 Their absorption spectra feature an intense Q-band at approximately 670 nm and a Soret band in the region of 300–400 nm,^939,944 and the quantum yield of ISC is reduced by dye aggregation.⁹⁴⁴ Interestingly, however, the efficiency of singlet oxygen production by the triplet state is very similar to that for the singlet states.⁹⁴⁵ These complexes can thus serve as efficient oxygen photosensitizers in photodynamic therapy (see also section 6.3),^{938,940,946−948} although photobleaching by self-sensitized photooxidation can limit their usefulness.⁹⁴⁵ Interestingly, the properties of the axial ligands (Figure 22, R and R′) and the pH of the solution were found to profoundly affect their photophysics.⁹³⁹

Figure 22.

Si-phthalocyanine macrocycles as PPGs.

Axial alkyl ligands in various Si-porphyrin derivatives (Figure 22) undergo homolytic cleavage upon irradiation with visible light;^949−951 the dissociation energy of the axial Si–C bond is relatively low⁹⁵⁰ (around 40 kcal mol^–1).⁹⁵² Accordingly, Ziady, Burda, and co-workers found that an axial alkyl tether used to link Si-phthalocyanines to Au nanoparticles (163) underwent efficient photochemical homolytic cleavage upon irradiation with 660 nm light (Scheme 55).⁹⁵³ This Au-drug delivery system is initially PDT-inactive because the excited state of the Si-phthalocyanine is quenched by the Au nanoparticle (see also section 6.4). Upon irradiation, the chromophore is liberated and undergoes ligand exchange⁹⁵⁰ with water to give a PDT-active species that produces singlet oxygen with a quantum yield of 49%. The homolytic photocleavage of axial alkyl groups was also investigated in methanol,⁹⁵⁴ and a mechanism was proposed involving the initial formation of a radical centered on the Si atom of the Si-phthalocyanine and an alkyl radical that subsequently abstracts hydrogen from methanol. Several Si-phthalocyanine derivatives bearing amino acrylate axial linkers cleavable by singlet oxygen produced by in situ phthalocyanine sensitization have been reported.^955−957

Scheme 55. Si-Phthalocyanine Attached to Au Nanoparticle as a PPG⁹⁵³.

The dissociation energy of axial Si–O bonds in Si-phthalocyanines is much higher (≥80 kcal mol^–1) than that of comparable Si–C bonds.⁹⁵² Irradiation of Si-phthalocyanines bearing both axial alkyl and alkylsiloxy ligands (Figure 22, R = alkyl, R′ = alkylsiloxy groups) leads to the exclusive homolytic liberation of the alkyl group.⁹⁵²

The photochemistry of a series of Si-octaphenoxyphthalocyanines bearing aryloxy, siloxy, aminoalkoxy, carboxyl, and sulfonyloxy groups as axial ligands was studied by Nyokong and co-workers.⁹⁴⁵ Axial ligand exchange to give the corresponding hydroxy derivatives in DMSO solutions was suggested to proceed via intermolecular electron transfer between the phthalocyanine π,π* excited state and an electron acceptor. Schnermann and co-workers showed that the Si–OAr bond in Si-phthalocyanines with axial aryloxy ligands can be cleaved in aqueous solutions using NIR light.⁸⁷⁸Scheme 56 shows the synthesized aryloxy derivatives 164 (Figure 19), which liberated substituted coumarin and stilbene moieties in degassed (hypoxic) aqueous solutions upon irradiation at 690 nm. Complex 164a released the fluorescent reporter 4-methylumbelliferone, and the photorelease of combretastatin-A4 and its E-isomer from 164b and 164c, respectively, was used to study the effects of tubulin polymerization inhibition under low-O₂ conditions typical of tumor microenvironments. Under normoxic (normal oxygen concentration) conditions, complexes 164 exhibited reactive oxygen species-mediated phototoxicity. Both spectroscopic and computational studies provided evidence of photoinduced electron transfer to the Si-phthalocyanine triplet to form a radical anion intermediate that undergoes ligand exchange with water⁹⁵⁸ (Scheme 57). DFT calculations indicated that the attack of water as a nucleophile on the radical-anion center is more feasible than an attack on the neutral complex.⁹⁵⁸

Scheme 56. Si-Phthalocyanine PPGs to Release Aryloxy Groups⁸⁷⁸.

Scheme 57. Ligand Exchange in a Si-Phthalocyanine PPG⁹⁵⁸.

A different way of exploiting photoinduced ligand release from Si phthalocyanines is embodied by the phthalocyanine derivative IR700 (165, Scheme 58), which releases a ligand upon irradiation at 676.5 nm in the presence of l-ascorbate as an electron donor.^959,960 It was proposed that this reaction changes the dye’s hydrophilicity and propensity to aggregate in aqueous solutions, which contributes significantly to the induction of cell death.

Scheme 58. Ligand Exchange in the Phthalocyanine IR700⁹⁵⁹.

Herges and co-workers designed Ni^II-porphyrin systems 166, which undergo photochemical axial coordination/de-coordination (Scheme 59)^961,962 in a manner that enables controlled coordination-induced spin-state switching.⁹⁶³ Photochemical isomerization of the E-azopyridine ligand to the Z form weakens its binding due to steric clashing between the substituents at the 2-positions of the pyridyl rings, resulting in ligand release. When the axial ligand is bound, the Ni complex is pentacoordinate, high spin, and paramagnetic; dissociation of the axial ligand causes switching to the diamagnetic tetracoordinate low spin state. Similarly, the spin state of Fe^III porphyrins bridged with 1,2,3-triazole ligands can be changed by adding phenylazopyridine as a photodissociable ligand,⁹⁶⁴ or the pyridine-bearing dithienylethene (DTE) photoswitch can be used to induce metal–ligand interaction between two Zn^II-porphyrin moieties connected through a diethyne linker.⁹⁶⁵

Scheme 59. Ni^II–Porphyrin PPGs.

3.3. Photochemistry of Ruthenium(II) Polypyridyl Complexes

The photochemical activity of metal polypyridyl complexes has been known for decades, and the archetypical chromophore of this type, the [Ru(bpy)₃]²⁺ cation (bpy = 2,2′-bipyridyl; Figure 23), has received considerable attention because of its unique optical and physicochemical properties.^{56,70,860,966,967} It absorbs in the visible region (λ_max^abs ≈ 450 nm) and the metal-to-ligand charge-transfer (MLCT) d → π* transition populates the singlet state, formally represented as a Ru^III–bpy^– state, which is converted into the triplet ³MLCT state in about 10 fs⁹⁶⁸ with an ISC quantum yield of almost unity.⁹⁶⁶ The long-lived triplet state ³MLCT can be deactivated by radiative, non-radiative, and electron transfer pathways or be thermally activated to give a low-lying triplet ligand-field state (³LF) with an Ru–ligand antibonding character that can lead to ligand release.^{70,72,969−971} A relationship between the π-accepting ability of the ligands and the photosubstitution efficiency has been demonstrated.^972−975 In the triplet state, this complex is an efficient oxygen sensitizer.⁹⁷⁶ The photochemical applications of [Ru(bpy)₃]²⁺ range from solar energy conversion, photocatalysis, and sensing to photochemotherapy and bioimaging.^57,70,71,966

Figure 23.

Ru^II polypyridyl complexes.

The photoreactions of analogous [Ru(bpy)₂L¹L²]²⁺ complexes (where L¹ and L² may belong to the single or separate ligands; see Figure 23) have attracted great interest in the context of photocaging and are discussed at length in recent reviews and perspectives.^{70,72,977−979} The major advantage of these and many other transition metal-containing photoactivatable systems is that they can release neutral bioactive small molecules (ligands) including nitriles, amines, and aromatic heterocycles.^980,981 The absorption spectra of [Ru(bpy)₂L¹L²]²⁺ feature strong bands in the visible region (λ_max^abs ≈ 420 nm), but larger conjugated terpyridine or bisquinoline ligands shift the absorption maximum up to as much as 600 nm, with tail absorption extending into the phototherapeutic window in the NIR and IR regions.⁹⁸² In an early study, Pinnick and Durham found that the quantum yields of photosubstitution (ligand exchange) in [Ru(bpy)₂L¹L²]²⁺ derivatives correlated with the energy of the lowest energy charge-transfer transition.⁹⁸³ It has been suggested that the direct population of the reactive ³LF state from ¹MLCT along with the population of the emissive ³MLCT state are the first photophysical events to occur in these complexes.^969,977,984 However, Dunbar and Turro showed that the population of ³MLCT competes with ligand liberation on time scales of fs to ps.^985,986 In their work, irradiation of [Ru(bpy)₂(CH₃CN)₂]²⁺ in water resulted in stepwise CH₃CN release to give [Ru(bpy)₂(CH₃CN)(H₂O)]²⁺ and [Ru(bpy)₂(H₂O)₂]²⁺ as the first and second intermediates, with the former complex being detected after only 77 ps.

Etchenique and co-workers created a [Ru(bpy)₂L¹L²]²⁺ PPG by coordinating two K⁺ channel-blocking 4-aminopyridine (4AP) ligands to obtain complex 167 (Scheme 60). These ligands were released sequentially upon irradiation at 480⁹⁸⁷ or 800 nm (2P absorption).⁹⁸⁸ A similar strategy was used to cage nicotine,⁹⁸⁹ γ-aminobutyric acid (GABA, for which the release quantum yield was 0.036⁹⁹⁰) and other amines.^{879,991−993} When one of the monodentate ligands is triphenylphosphine, which is a weaker σ-donor than an amine but a stronger π-acceptor (168, Scheme 60, Figure 19), the Ru^II center becomes electronically depleted, resulting in more efficient GABA liberation (Φ_r > 0.21).⁹⁹⁰ A kinetic flash photolysis study showed that [Ru(bpy)₂(PMe₃)(glutamine)] photoreleases glutamine within 50 ns.⁹⁹⁴ The analogous [Ru(bpy)(dcbpy)py₂]²⁺ and [Ru(dcbpy)₂ py₂]²⁺ complexes (bpy = 2,2′-bipyridine, dcbpy = 4,4′-dicarboxy-2,2′-bipyridine, and py = pyridine) released their pyridine ligands upon irradiation at 450 nm at physiological pH.⁹⁹⁵ Similarly, [Ru(ane)(chel)(py)]²⁺ (ane = 1,4,7-trithiacyclononane, chel = chelating diimine) photoreleased pyridine at 470 nm.⁹⁹⁶ In another application, a weakly fluorescent rhodamine-substituted Ru^II complex was shown to photorelease a rhodamine dye, increasing its fluorescence intensity almost six-fold.⁹⁹⁷

Scheme 60. [Ru(bpy)₂L¹L²]²⁺ Complexes 167(988) (4AP = 4-Aminopyridine) and 168(990) as PPGs.

Sterically bulky ligands were introduced to distort the pseudo-octahedral geometry of the Ru^II complexes, which reduces the energy of the locally-excited ³LE state, and increases the efficiency of ligand exchange.⁷⁰ For example, 2,2′-biquinoline (biq) is photoreleased from [Ru(biq)(phen)₂]²⁺ (phen = 1,10-phenathroline), whereas [Ru(phen)₃]²⁺ is photochemically inactive.⁹⁹⁸ This phenomenon was demonstrated by Turro and co-workers in a series of Ru complexes bearing tridentate ligands, such as [Ru(tpy)(Me₂ bpy)(py)]²⁺ (Me₂ bpy = 6,6′-dimethyl-2,2′-bipyridine, tpy = terpyridine, py = pyridine).⁹⁹⁹ As shown in Scheme 61, this complex (169) undergoes ligand exchange with a quantum yield of 0.16 upon irradiation at 500 nm. This value is approximately 3 orders of magnitude higher than that for [Ru(tpy)(bpy)(py)]²⁺, which features the sterically undemanding bpy ligand rather than the sterically bulky Me₂ bpy. The ³LE state was found to form within 3–7 ps, and it can be deactivated by ligand dissociation or non-radiative decay. Building on preceding theoretical studies,^1000,1001 Alary and co-workers performed DFT calculations indicating that these results can be attributed to the formation of a quasi-degenerate triplet metal-centered state and triplet excited-state potential energy surfaces with differing topologies.^1002,1003 An analogous photoactivatable ruthenium complex [Ru(tpy)(bpy)(L)]²⁺, where L is a rigidin derivative caged through its thioether group, released the caged ligand upon irradiation at 530 nm.¹⁰⁰⁴ The rigidins are cytotoxic marine alkaloids known to kill cancer cells. A series of related Ru terpyridine complexes bearing acetylacetonate-based ligands (Scheme 61, 170, X = H or halogen) was synthesized to bathochromically shift the absorption of these systems (λ_max^abs < 517 nm).¹⁰⁰⁵ These complexes exhibited quantum yields of ligand release five- to seven-times higher than that of [Ru(tpy)(bpy)(CH₃CN)]²⁺.

Scheme 61. Ru^II Complexes 169(999) and 170(1005) as PPGs.

Another class of Ru^II complexes features tetradentate ligands such as tris(2-pyridylmethyl)amine 171 (Figure 24).^{72,977,1006−1008} These stable complexes can cage a wide range of different ligands L, including the cathepsin K inhibitor Cbz-Leu-NHCH₂CN and nicotinamide, which are released upon irradiation at >400 nm. The selective release quantum yields of cis-nitrile ligands (∼0.01) were higher than those for cis-heterocyclic ligands in water, which was attributed to aromatic heterocycles being stronger σ-donors than nitriles.⁹⁷⁷ Rigid complexes 172 and 173 bearing tetradentate piperidine ligands (Figure 24) also underwent photochemical ligand exchange with quantum yields of 0.001–0.03.¹⁰⁰⁹ DFT studies on the two isomers of the tris(2-quinolinylmethyl)amine (TQA) complexes [Ru(TQA)(MeCN)₂]²⁺172⁹⁷⁰ and 173(1009) showed that orbital mixing is crucial for effective ligand photodissociation.

Figure 24.

Ru^II complexes with the tetradentatetris(2-pyridylmethyl)amine ligand and its analogs as PPGs.

Bonnet and co-workers demonstrated the photorelease of 2-(methylthio)ethanol from Ru^II complexes such as 174 (Scheme 62).¹⁰¹⁰ This ligand was released in water upon irradiation at 465 nm with a quantum yield of 0.13. An analogous complex bearing 6,6′-dichloro-2,2′-bipyridine as a ligand was used to control light-responsive supramolecular interactions.¹⁰¹¹ The photorelease of a microtubule-targeted rigidin analog from [Ru(tpy)(bpy)L]²⁺ derivative 175 (Figure 25) in hypoxic cancer cells is another notable practical application of Ru^II-based PPGs.¹⁰⁰⁴ A series of Ru^II polypyridyl complexes bearing 6-mercaptopurine as a photocleavable ligand was prepared by Renfrew and co-workers.¹⁰¹² The highest release quantum yield (0.6) in this series was achieved with complex 176 (Scheme 62), which liberates 6-mercaptopurine upon irradiation at 465 nm in acetonitrile. The 1,4,7-trithiacyclononane Ru^II complexes 177–179 (Figure 25), bearing photocleavable pyridine, DMSO, 3-acetylpyridine, and imidazole ligands, were designed and studied by Alessio, Sadler, and co-workers.^1013−1015 These complexes release their ligands upon irradiation with blue light (400–490 nm).

Scheme 62. Ru^II Photoreleasable Complex 174(1010).

Figure 25.

Ru^II complexes as PPGs.

Many other applications of Ru^II complexes as photoactivatable groups have been reported. The liberation of neurotransmitters to enable control over receptor activity in neuronal cells was mentioned above.^56,879 These complexes have also been used for controlled release of small molecule drugs and enzyme inhibitors. Enzymes successfully targeted in this way include proteases,¹⁰¹⁶ cathepsin B (inhibited with a novel dipeptidyl nitrile¹⁰¹⁷),^980,1018 nicotinamide phosphoribosyl transferase,¹⁰¹⁹ cytochromes P450,¹⁰²⁰ and CYP17A1.¹⁰²¹ Drugs successfully photoreleased from Ru^II complexes include the anticancer agent CHS-828,¹⁰²² the imidazole-based cytotoxic drug econazole,¹⁰²³ the anti-tuberculosis drug isoniazid,¹⁰²⁴ and 5-cyanouracil.^1025,1026 Additionally, a photoactivatable histidine building block for Fmoc/t-Bu solid-phase peptide synthesis based on a Ru^II complex with an imidazole ligand was used to prepare caged histidine peptides.¹⁰²⁷ A library of tetra- and pentadentate ligands was attached to a polystyrene resin to prepare the corresponding photolabile Ru^II complexes for a solid-phase synthesis application.¹⁰²⁸ [Ru(bpy)₂(4AMP)₂] (4AMP = 4-(aminomethyl)pyridine) was incorporated into polyurea organo- and hydrogels and used as a photoremovable moiety to induce de-gelation upon 1P or 2P excitation.¹⁰²⁹ Similarly, supramolecular crosslinked gels with a photosensitive ruthenium bipyridine complex functioning as a crosslinker and poly(4-vinylpyridine) as a macromolecular ligand were developed by Teasdale and Monkowius.¹⁰³⁰ Photolysis of these organogels with visible (>395 nm) and NIR light (1028 nm; a multiphoton process) resulted in the liberation of the pyridine moieties and degelation.

Photoinduced ligand dissociation from ruthenium complexes can also be accompanied by singlet oxygen production.⁷⁰ Turro and co-workers showed that the triplet excited state of the [Ru(bpy)(dppn)(CH₃CN)₂]²⁺ (dppn = benzo[i]dipyridophenazine) complex efficiently sensitizes oxygen to give ¹O₂ in aqueous solution (Φ_Δ = 0.72) and also releases acetonitrile in a less efficient competing process (Φ_r <0.01).¹⁰³¹ Similar dual reactivity was demonstrated for [Ru(tpy)(Me₂ dppn)(py)]²⁺ (dppn = dimethylbenzo-[i]dipyridophenazine)¹⁰³² and [Ru(pydppn)(biq)(py)]²⁺ (pydppn = (pyrid-2-yl)benzo[i]dipyridophenazine)¹⁰³³ complexes. Additionally, a structurally distinct nitrosyl phthalocyanine ruthenium complex was shown to produce singlet oxygen and release nitric oxide (see section 4.2).¹⁰³⁴

Rh^III complexes with polypyridyl and phenanthrene quinone diamine ligands are also photoactive and have been used to achieve photoinduced DNA cleavage. However, ligand exchange reactions are not the primary processes responsible for their photochemical activity.¹¹³

3.4. Photochemistry of Dirhodium(II,II) Complexes

Dirhodium (Rh^II–Rh^II) complexes have also attracted attention as photoactivatable species,^71,113 although there have been few studies on this aspect of their behavior. The complex 180, reported by Turro and co-workers, has an absorption maximum at 525 nm (ε_max = 218 M^–1 cm^–1; Scheme 63; Figure 19) in acetonitrile, which was attributed to a Rh₂(π*) → Rh₂(σ*) transition on the basis of TD-DFT calculations.⁸⁸⁰ This compound selectively exchanges its axial CH₃CN ligands with H₂O in aqueous solutions in the dark. Upon irradiation of the product with visible light, two equatorial CH₃CN ligands dissociate and are replaced with water to give three different isomers of cis-[Rh₂(μ-O₂CCH₃)₂(CH₃CN)₂(H₂O)₄]²⁺, causing a slight bathochromic shift of the absorption maximum. The liberation quantum yields depended on the irradiation wavelength: Φ_355 nm = 0.37 and Φ_509 nm = 0.09. Irradiation of 180 in water in the presence of 2,2′-bipyridine or 9-ethylguanine led to the coordination of these ligands to the dirhodium core. Similar results were obtained with cis-[Rh₂(HN(O)CCH₃)₂(CH₃CN)₆]²⁺, which releases two molecules of acetonitrile upon irradiation at >495 nm to form bis-aqua products,¹⁰³⁵ and with [Rh₂(O₂CCH₃)₂(CH₃CN)₆]²⁺, which releases its axial CH₃CN ligands upon irradiation at >455 nm.¹⁰³⁶

Scheme 63. Dirhodium (Rh^II–Rh^II) Complex as a PPG⁸⁸⁰.

The 1,10-phenanthroline complex 181 was shown by Turro and Dunbar to release two equatorial CH₃CN ligands in water upon irradiation with visible light (λ_irr >590 nm), whereas mononuclear radical Rh^II fragments were formed upon homolytic photocleavage of the metal–metal bond (Figure 26).¹⁰³⁷ Remarkably, the release quantum yield measured upon irradiation at 550 nm exceeded unity (Φ_r = 1.38), suggesting that a dark release follows the initial photoreaction. Another photoactivatable dirhodium complex, 182, bearing a benzo[i]dipyridoquinoxaline ligand (Figure 26) was designed to serve as a DNA-intercalating singlet oxygen generator (Φ_Δ = 0.22 at 477 nm) thanks to its low-lying dppn-centered ³π,π* state.¹⁰³⁸ Upon irradiation in water, acetonitrile is released from this compound and replaced by H₂O as a ligand (Φ_r = 0.0033 at 450 nm).

Figure 26.

Dirhodium (Rh^II–Rh^II) complexes 181(1037) and 182.¹⁰³⁸

3.5. Photochemistry of Pt-, Co-, and Fe-Containing Organometallic Complexes

Usually unreactive Pt^IV prodrugs are important anticancer compounds¹⁰³⁹ that are designed to be converted into toxic Pt^II species in vivo by reducing agents such as ascorbic acid.¹⁰⁴⁰ Well-known Pt^II drugs such as cisplatin and carboplatin have very narrow therapeutic indexes, so there is great interest in their controlled photochemical production in target tissues. Several visible-light absorbing photoactivatable Pt^IV complexes with the general structure trans,trans,trans-[Pt(N₃)₂(OH)₂(N¹)(N²)] (183, N¹, N² = pyridine or amines; Figure 27) were studied by Sadler and co-workers and shown to be cytotoxic to cancer cells upon irradiation with blue light.^1041−1044 Compounds 183 do not liberate pyridine or amines upon excitation but do exhibit Pt–N₃ bond elongation, eventually leading to the release of azidyl radicals.¹⁰⁴¹ This concept was also used in the design of a photoactivatable dopamine-conjugated Pt^IV anticancer complex that was incorporated into borate hydrogels,¹⁰⁴⁵ as well as Pt^IV triazolato azido complexes that photorelease Pt^IV and Pt^II 5′-guanosine monophosphate species.¹⁰⁴⁶ Additionally, the oxaliplatin-based photocaged Pt^IV prodrug coumaplatin (184), was shown to release an axial ligand upon irradiation at 450 nm, forming a cationic Pt^IV intermediate that oxidizes water and generates oxygen under biological conditions.¹⁰⁴⁷

Figure 27.

Pt^IV complexes as PPGs.

Chakravarty and co-workers showed that curcumin (185, Figure 28), a compound with significant antioxidant, anti-inflammatory, antiseptic, and anticancer activities,⁵⁹ can form photoactivatable Pt^II complexes.^1048−1050 For example, [Pt(NH₃)₂(cur)](NO₃) (186, cur = curcumin) exhibits a strong absorption band with a λ_max^abs of ∼430 nm and releases two anticancer agents, curcumin and a cisplatin analog (which crosslinks DNA), upon irradiation with visible light.¹⁰⁴⁸ The analogous [Pt(en)(cur)](NO₃) and [Pt(dach)(cur)](NO₃) (187, en = ethylenediamine, dach = 1R,2R-(−)-1,2-diaminocyclohexane) complexes also liberated curcumin under similar conditions.¹⁰⁴⁹ The use of a photosensitizer as a ligand (see also section 6) can lead to dual photochemotherapeutic effects. This was demonstrated using [Pt(L)(R-BODIPY)]Cl complexes, where R-BODIPY is a distyryl-BODIPY derivative (sensitizer) and L are different terpyridine ligands. Irradiation of these species with red light (600–720 nm) caused both singlet oxygen production and the release of photoactive BODIPY ligands, resulting in appreciable photocytotoxicity.¹⁰⁵¹ Similarly, platinum(II) ferrocenylterpyridine (Fc-tpy) complexes [Pt(Fc-tpy)(L)]Cl (L = a biotin-containing ligand) released their biotinylated ligands upon irradiation with red light (647 nm) because of the photosensitizing behavior of the Fc-tpy ligand.¹⁰⁵² Finally, the very interesting heptamethine cyanine-based Pt^II complex 188 (Figure 28) was reported to undergo Pt–O bond scission and to generate singlet oxygen upon irradiation with near-IR light.¹⁰⁵³

Figure 28.

Photoactivatable Pt^II complexes.

Unlike Ru^II and Pt^IV complexes, Co^III complexes (see also section 3.1) usually have very weak absorption bands in the visible region.⁸⁷⁵ Therefore, strongly absorbing ligands that can photoreduce the Co^III ion to induce ligand release have been developed. The first reported complex of this type was the Ru^II–Co^III heterodinuclear species 189 (Figure 29), which has an absorption maximum close to 400 nm.¹⁰⁵⁴ Upon irradiation with visible light, the Ru^II moiety probably transfers an electron to the Co^III complex to produce a Co^II species with concomitant release of the ethylenediamine ligands. Renfrew and co-workers reported the release of curcumin from Co^III curcumin complexes such as 190 (Figure 29, Figure 19).⁸⁸¹ This complex absorbs at λ_max^abs = 451 nm, and the authors hypothesized that irradiation at 520 nm causes electron transfer from curcumin to the cobalt ion. The photodegradation quantum yield for this compound was found to be 0.01. A similar strategy was demonstrated using ternary Co^III complexes of mitocurcumin (a water-soluble curcumin derivative) bearing a tetradentate phenolate-based ligand.¹⁰⁵⁵ Mitocurcumin was released upon irradiation with visible light and was shown to act as a phototoxin that generated reactive oxygen species in cells.

Figure 29.

Ru^II–Co^III, Co^III, and Fe^III complexes.

The analogous charge-neutral Fe^III complex 191 (Figure 29) and two other high-spin iron complexes reportedly released curcumin upon irradiation with visible light, and thus exhibited cytotoxicity in multiple cell lines.¹⁰⁵⁶ In addition, Fe^III–polysaccharide hydrogels were found to be visible-light (405 nm) responsive because of the photoreduction of the Fe^III ions to Fe^II, which rendered the Fe complexes incapable of functioning as cross-linkers for the polymer.¹⁰⁵⁷

4. Photorelease of Gasotransmitters

Gasotransmitters are small gaseous endogenously-produced signaling molecules that are involved in the control of a vast array of physiological processes in the cardiovascular, nervous, gastrointestinal, excretory, and immune systems as well as cellular functions including apoptosis, proliferation, inflammation, metabolism, oxygen sensing, and gene transcription.¹⁰⁵⁸ The most important gasotransmitters identified to date are nitric oxide (NO), carbon monoxide (CO), and hydrogen sulfide (H₂S). These small molecules are freely permeable through membranes and are perceived without cognate receptors.¹⁰⁵⁹ Their molecular targets can be divided into two groups. The first are metal-containing prosthetic groups of proteins that form coordination complexes with CO and NO; examples include heme-imidazoles (as in hemoglobin and cytochrome c oxidase), thiolated hemes (as in cytochromes P450), and non-heme iron complexes (as in prolyl hydroxylase and superoxide dismutase). The second group consists of organic thiols, which can be nitrosylated by NO and sulfhydrated by H₂S.¹⁰⁵⁸ Gasotransmitter perception can induce diverse macroscopic biological responses, many of which are therapeutically relevant such as vasodilatation,^1060,1061 protection of tissues against hypoxia,¹⁰⁶² anti-inflammatory processes,¹⁰⁶³ wound healing,^1064,1065 platelet aggregation inhibition,¹⁰⁶⁶ postsynaptic plasticity augmentation, and hormone secretion.¹⁰⁶⁷ The simplest method of administering gasotransmitters is by direct inhalation of small quantities of the gaseous species. While this approach induces therapeutic effects in some contexts,^1068,1069 its usefulness is limited by narrow pharmaceutical windows and it requires precise control of the gasotransmitter’s concentration, which is very difficult to achieve.¹⁰⁷⁰ Consequently, there is considerable interest in the development of gasotransmitter-releasing molecules.^80,82,94

Most known gasotransmitter-releasing molecules are based on metal complexes that release a weakly bound gasotransmitter ligand via simple hydrolytic ligand exchange upon dissolution in aqueous media. Such complexes are prone to rapid initial releases of the bound gasotransmitter prior to administration to the target organism but do not allow for precise control over the release. Enzymatically triggered reactions that offer more controlled release profiles have also been demonstrated.¹⁰⁷¹ An alternative activation strategy that enables precise spatial and temporal control over gasotransmitter release is to use photochemically activatable gasotransmitter-releasing molecules such as photoactivatable CO-releasing moieties (photoCORMs) or photoactivatable nitric oxide-releasing moieties (photoNORMs).⁷⁷⁻¹¹⁴ In therapeutic applications, organic (transition-metal-free) photoCORMs can have important advantages over metal carbonyl complexes such as more favorable biodistribution and lower toxicity.⁸⁰⁻⁸² Because of their distinct physicochemical properties (which typically include small size or neutral charge), the photorelease of gasotransmitters requires unique and highly specific strategies that may differ appreciably from those used for photorelease of larger ligands. Therefore, we present this research area in its own section. Research on photochemically activatable gasotransmitter-releasing molecules has advanced rapidly in the past decade, and many reviews are available.^{49,80−102,1072−1074} This section focuses solely on visible-light-absorbing photorelease systems. Figure 30 shows the absorption spectra of selected transition-metal-free molecules that release gasotransmitters upon excitation with visible or NIR light, and Figure 31 shows the absorption spectra of some transition metal complexes with such activity.

Figure 30.

Representative absorption spectra of transition-metal-free molecules that photorelease CO, NO, and H₂S. Black,¹⁰⁷⁵ red,¹⁰⁷⁵ cyan¹⁰⁷⁶ lines, flavonol-based photoCORMs (section 4.1.1); blue line, a xanthene-based photoCORM (section 4.1.1);⁷⁶³ magenta line, a BODIPY-based photoCORM (R = a styryl group; section 4.1.1);⁸⁴⁷ yellow line, a BODIPY-based photoNORM (section 4.2.1);¹⁰⁷⁷ orange line, a naphthalimide-based photoNORM (section 4.2.1);¹⁰⁷⁸ violet line, a BODIPY-based H₂S releasing molecule (section 4.3).⁷⁹⁹

Figure 31.

Representative spectra of transition-metal complexes photoreleasing CO, NO, and H₂S. Red,¹⁰⁷⁹ blue,¹⁰⁷⁹ green,¹⁰⁸⁰ cyan¹⁰⁸⁰ lines, Mn^II tricarbonyl photoCORMs (section 4.1.2); magenta¹⁰⁸¹ and orange¹⁰⁸² lines, Mn^II photoNORMs (section 4.2.2); black line, a Ru^II-based H₂S releasing complex (section 4.3).¹⁰⁸³

4.1. Release of Carbon Monoxide

4.1.1. Transition-Metal-Free PhotoCORMs

The lowest excited state of simple ketones and aldehydes corresponds to the excitation of an electron from the n lone pair to the π* molecular orbital.¹³⁶ n,π*-Transitions are generally weak and often hidden by the red tail of a stronger π,π*-absorption. Homolytic cleavage of the α-bond in ketones (α-cleavage; Norrish type I reaction) often results in decarbonylation, that is, carbon monoxide (CO) release. Therefore, carbonyl compounds have been widely used as photoCORMs.^80,82

The release of CO from prototypical aliphatic acyclic and especially small cyclic ketones by radical decarbonylation occurs only at the edge of the vacuum UV range (e.g., λ_irr = 193 nm for 3-cyclopentenone).¹⁰⁸⁴ However, an extension of the ketone π-system results in bathochromic shifts of their absorption maxima.¹³⁶ 1,2-Dicarbonyl compounds (which typically absorb above 300 nm) can also be photolyzed to produce CO.¹⁰⁸⁵ Irradiation of bicyclo[2.2.2]octane-2,3-dione 192a in toluene at the edge of the visible region (395 ± 25 nm) resulted in the formation of aromatic side-products 193a–f and the release of two equivalents of CO (Φ_r = 0.02, εΦ_CO = 6 at 395 nm) (Scheme 64).¹⁰⁸⁵ 1,2-Diketone 192a was used as an additive in poly(ε-caprolactone) electrospun scaffolds designed for vascular tissue engineering¹⁰⁸⁶ and was shown to release CO upon irradiation at λ_irr = 470 nm in this environment.^1086,1087 To increase the hydrophilicity of the bicyclo[2.2.2]octane-2,3-dione scaffold, a derivative substituted with oligo-(ethylene)glycol side chains (192b) was prepared.¹⁰⁸⁷ However, 192b did not release CO when dissolved in a water/DMSO (99:1) mixture due to hydration of the carbonyl groups. This was circumvented by encapsulating 192b in micelles of Pluronic F127, a biocompatible block copolymer of poly(ethylene oxide) and polypropylene oxide. The encapsulated compound efficiently released CO upon irradiation at 470 nm, and the system was successfully used in vitro.¹⁰⁸⁷

Scheme 64. 1,2-Diketones as PhotoCORMs^1085−1087.

Liao and co-workers recently overcame the hydration-induced deactivation of photoactivity in bicyclo[2.2.2]octane-2,3-dione by preparing derivative 192c, which carries t-butyl substituents (Scheme 64) that sterically hinder hydrate formation.¹⁰⁸⁸ This compound was incorporated into poly(butyl cyanoacrylate) nanoparticles and used as a tissue adhesive with possible applications in CO delivery to the brain. Additionally, Raymo and co-workers have designed an autocatalytic reaction based on the photoinduced decarbonylation (λ_irr = 420 nm) of 192a and 192d, which is sensitized by its own photoproducts, the anthracene derivatives 193. The quantum yields of decarbonylation for 192a and 192d were 0.20 and 0.50, respectively.¹⁰⁸⁹

The group of Yamada substituted the bicyclo[2.2.2]octane-2,3-dione scaffold with BODIPY antennas to obtain 192e and 192f (Figure 32).¹⁰⁹⁰ These compounds have a major absorption maximum in the green-to-yellow region (λ_max^abs = 534 nm for 192e and λ_max = 605 nm for 192f) and release CO upon irradiation at 450 nm via initial photoinduced electron transfer from the BODIPY moiety to the 1,2-diketone functionality. Moreover, 192e releases 2 equiv of ethylene via a thermal process that occurs upon heating to 220 °C. The release of CO and ethylene are thus orthogonal and can be performed sequentially. Unfortunately, 192e and 192f are insoluble in polar media, which limits their biological applications. Diketones 194a and 194b (Figure 32) decarbonylate upon irradiation at λ_irr = 395 ± 25 nm, but their aromatic photoproducts (hexacene and heptacene, respectively) do not accumulate in the reaction system due to their fast oxidation and dimerization.^{1085,1091,1092}

Figure 32.

1,2-Dicarbonyl PhotoCORMs.

2-Ketocarboxylic acids can also be used as photoCORMs. Visible light irradiation (>390 nm) of the tetra-(2-N-methylpyridyl)porphyrin-Fe^III complex with 2-ketocarboxylic acid 195 led to photoinduced electron transfer from the carboxylate anion to the central metal ion, yielding an Fe^II complex and a carboxyl radical (Scheme 65)¹⁰⁹³ that underwent simultaneous decarboxylation and decarbonylation. The released CO was then efficiently trapped by the Fe^II-porphyrin complex in the solvent cage.

Scheme 65. Photolysis of 2-Ketocarboxylic Acids (R = iPr, Ph)¹⁰⁹³.

Flavonol or 3-hydroxyflavone (3-hydroxy-2-phenylchromen-4-one; 196, Scheme 66) belongs to the family of flavonoids, well-known natural antioxidants^1094,1095 that have been recognized as CO-releasing molecules. Unsubstituted flavonols absorb only in the UV region and, thanks to their biological relevance, the photodecomposition mechanism responsible for the resulting CO release has been studied since the 1960s. The photosensitized oxygenation of 196 by singlet oxygen generated photochemically in situ was reported in the seminal work of Matsuura and co-workers,¹⁰⁹⁶ who showed that it results in the formation of CO together with o-benzoyl salicylic acid 197 as a side-photoproduct. The reaction was suggested to proceed via an endoperoxide intermediate. In the absence of oxygen, 3-hydroxyflavone rearranges into the 3-arylphthalide derivative 198 with concomitant CO liberation. The 3-hydroxy group was found to be essential for this reaction because the analogous 3-methoxyflavone derivative is photostable. These mechanistic pathways were later studied in detail.^1097−1100 Kubinyi and co-workers introduced the push-pull substituted 4′-diethylamino-3-hydroxyflavone and its Mg^II complex.¹¹⁰¹ However, despite the ESIPT character of this compound and its absorption in the visible part of the spectrum, only UV-light-initiated CO release was studied. The photochemistry of flavonol-based CORMs was recently reviewed.⁹²

Scheme 66. Photodecarbonylation of 3-Hydroxyflavone¹⁰⁹⁶.

Flavonols are excellent ligands for d-block elements; complexation of metal cations with flavonolate anions causes bathochromic shifts of their lowest energy absorption bands into the visible part of the spectrum and also increases their molar absorption coefficients in some cases.¹⁰⁹⁹ Because of their biological relevance in plant metabolism and occurrence in soil,¹¹⁰² the photochemistry of these metal complexes has been studied in detail. The photoreactivity of Pb^II and Al^III flavonolato complexes is suppressed, whereas Zn^II complexes exhibit similar reactivity to free 3-hydroxyflavone.¹⁰⁹⁹ The flavonolate complex [(6-R₂TPA)Zn(3-Hfl)]ClO₄ (TPA = tris(2-pyridylmethyl)amine) 199 (Figure 33),¹¹⁰³ Zn^II complexes bearing tetradentate tripodal nitrogen donor ligands and flavonol derivatives 200 or 201,¹¹⁰⁴ and the bipyridine-ligated Zn^II complex 202 with a bridging flavonolate ligand¹¹⁰⁵ all released CO upon irradiation at λ_irr > 400 nm. In addition, the Ru^II cymene complex 203 released CO upon irradiation at either 300 or 419 nm.¹¹⁰⁶ Additionally, Farmer and co-workers synthesized and characterized a series of Ru^II bipyridine-substituted flavonolato complexes 204,^1107,1108 and studied the mechanism of their photooxygenation by ¹O₂ at low temperatures. Their results suggest that this process occurs via 1,2- or 1,3-addition to the flavonol core.

Figure 33.

Metal flavonolato complexes and their ligands.

The π-extended 3-hydroxyflavone 205 (Figure 34) and its derivatives absorb in the visible region.^1076,1109 Displaying photochemistry similar to that of 196, Berreau and co-workers have demonstrated that 1 equiv of CO is photoreleased from 205. The reported quantum yield of CO release for 205 (Φ_r = 0.007, λ_irr = 419 nm) in DMSO/aqueous buffer (1:1, v/v, pH = 7.4) increased by a factor of 2 upon complexation with Zn^II but was reduced by an order of magnitude upon binding to bovine serum albumin.¹¹¹⁰ The 4′-diethylamino-substituted 206 exhibits bathochromically shifted absorption (λ_max^abs = 442 nm) but with an unchanged photochemical efficiency of CO release.¹⁰⁷⁶ The structure of 205 was further modified to obtain 4-flavonothione analog 207 and the 4′-diethylamino derivative 208, which have bathochromically shifted absorption bands (Figure 30).¹⁰⁷⁶ Compound 207 also had a higher quantum yield of CO release than 205 (Φ_r = 0.4 at 419 nm). Because a free 3-hydroxy group was found to be essential for the photoreactivity of 205, its substitution with a protecting group sensitive to an external trigger allowed Berreau and co-workers to construct a series of RS^– co-triggered “AND logic gates” that release CO only in the simultaneous presence of oxygen, light, and a thiol. An acryloyl-protected derivative 209(1111,1112) was activated by thiols including cysteine, while the cyanate-substituted compound 210 proved suitable for intracellular H₂S sensing.¹¹¹¹ When combined with PdCl₂, the allyl-protected flavonol derivative 211 was shown to act as an OFF-ON fluorescent CO sensor that replenishes the CO consumed during detection.¹¹¹³ A similar approach was used by Tang and co-workers, who designed the hydrogen peroxide-sensitive compound 212.¹¹¹⁴ Oxidation of this compound’s pinacol boronate ester by hydrogen peroxide liberates free 205, which can subsequently photorelease CO. The 2-nitrobenzyl-protected flavonol 213 was used by Hu and co-workers to prepare CO-releasing micelles that undergo a tandem photochemical reaction in which 2-nitrobenzyl deprotection is followed by CO release, accompanied by a dual fluorescence response.^1115,1116 Flavonol derivatives substituted with polar groups such as triphenylphosphonium (214) were found to target mitochondria and affect cellular bioenergetics.¹¹¹⁷ However, the sulfonated analog 215 did not penetrate through the cell membrane and thus enabled extracellular CO release.¹¹¹⁸ Both derivatives had photochemical properties similar to 205, allowing Berreau and co-workers to compare the effects of extracellular (215), cytosolic (205), and mitochondrial-localized (214) photoinduced CO release.^1117,1118 The 3-hydroxybenzo[g]quinolone derivative 216a releases one equivalent of CO upon illumination at 465 nm under physiological conditions.¹¹¹⁹216b, an oxidized form of 216a, is photochemically stable and can act as a prodrug that is activated by thiol-mediated reduction in vivo.¹¹¹⁹ The group of Feng recently introduced a coumarin-flavone hybrid 217 that combines the excellent absorption properties of the 7-(diethylamino)coumarinyl moiety with the CO-releasing ability of 3-hydroxyflavone.¹¹²⁰ Upon irradiation at 460 nm, the excited coumarinyl moiety transfers energy to the fluorescent flavone CO-releasing group. Following CO release, the molecule is transformed into a coumarinyl-substituted salicylic acid derivative with fluorescence similar to that of free 7-diethylaminocoumarin.

Figure 34.

π-Extended flavonol derivatives.

The mechanism of the aerobic photodecarbonylation of 205 was the subject of several investigations.^1109,1110 Like its parent flavonol 196,^{1110,1121,1122} the visible-light absorbing 205 exists in both acid (205a) and base (205b) forms (Schemes 67 and 68; Figure 30).¹¹²³ Klán and co-workers showed that the singlet excited state of 205a undergoes rapid excited-state intramolecular proton transfer (ESIPT) in methanol to give ¹205z* (z = zwitterion), which intersystem crosses to the triplet ³205z*. The triplet then reacts with ground-state oxygen, possibly via an endoperoxide intermediate, to release CO (Φ_r = 0.031; Scheme 67).¹⁰⁷⁵ The conjugate base 205b releases CO via an oxygenation reaction with singlet oxygen formed by the sensitizing action of the triplet ³205b* (Φ_Δ = 0.07; Φ_r = 0.018), and partially via oxidation with ³O₂ (Φ_r = 0.003; Scheme 68). There are thus three major orthogonal pathways of CO release. In addition, both forms undergo a very inefficient photorearrangement to release CO in the absence of oxygen. An isotopic labeling study with ¹⁸O₂ revealed that the photoproduct 218 exclusively incorporates ¹⁸O atoms.¹⁰⁷⁶

Scheme 67. Photochemistry of Conjugate Acid 205a(1075).

Scheme 68. Photochemistry of the Conjugate Base 205b(1075).

Štacko, Klán, and co-workers developed a new class of transition-metal-free photoCORMs by fusing two CO-releasing flavonol moieties with a heptamethine cyanine chromophore (219a,b, Figure 35).¹¹²⁴ The resulting hybrids released CO in high chemical yields of ∼130% (in principle, 2 equiv of CO can be liberated) upon activation with NIR light of up to 820 nm, with excellent uncaging cross sections (Φ_rε(λ_793 nm) = 75 for 219b). The biocompatibility and applicability of these systems in vitro and in vivo were also demonstrated.

Figure 35.

Cyanine-flavonol hybrid 219(1124) and Zn^II complexes 220 and 221.

Complexation of 205, 207, and 208 with [Zn^II(Ph₂TPA)]²⁺ (TPA = tris(2-pyridylmethyl)amine) in 220 (Figure 35) bathochromically shifts their absorption bands by 60–80 nm (e. g., λ_max^abs = 600 nm for 220, X = S, R = NEt₂) and increases the CO release quantum yield to almost unity (Φ_r = 0.95 for both 220, X = S, R = H, and 220, X = S, R = NEt₂).^1125,1126 One equivalent of CO is always released upon irradiation, and the complexes are active in both the solution and solid phases. In the absence of the TPA ligand, bis-flavonolato-Zn^II complexes 221 are formed. These compounds have even more bathochromically shifted absorption spectra (by ∼10 nm) and can release, in principle, 2 equiv of CO originating from the two flavonolato ligands upon irradiation at >545 nm.¹¹²⁵

Cyclopropenones are strained systems that liberate CO upon irradiation by UV light.¹¹²⁷ Most 2,3-alkylcyclopropenones absorb only in the deep UV-region, but their absorption band can be bathochromically shifted by substitution, for example, 2,3-bis(4-methoxyphenyl)-cyclopropenone 222 has a λ_max^abs of 340 nm (Figure 36).¹¹²⁷ 2,3-Bis-naphthyl-cyclopropenone derivatives 223 have an absorption tail in the range of 400–440 nm, but 1P absorption leading to decarbonylation occurs only under illumination with UV light (λ_irr = 350–380 nm).¹¹²⁸ However, they also efficiently decarbonylate upon non-resonant two-photon absorption at 800 nm. Unfortunately, their strong π-stacking makes them poorly soluble in polar protic solvents, which often limits their usefulness as photoCORMs.

Figure 36.

Cyclopropenone photoCORMs.

The doubly 9-anthryl substituted cyclopropenone 224 absorbs in the visible region (λ_max^abs = 465 nm), although excitation at these wavelengths does not induce decarbonylation.^1129,1130 The lowest absorption band of this compound corresponds to an intramolecular excimer of the 9-anthryl substituents and does not weaken the C–C bonds in the cyclopropenone moiety. Derivative 224 thus releases CO only upon excitation with shorter wavelength light (λ_irr = 366 nm) due to a very fast adiabatic reaction from an upper excited state that is largely localized in the cyclopropenone chromophore.¹¹³¹

Klán and co-workers discovered that xanthene-based carboxylic acid 140, isolated as a product from the photoreaction of 139 (Scheme 42, section 2.11), can release carbon monoxide via the triplet-excited state with Φ_r = 6.8 × 10^–4 in aqueous solutions of pH 7.4 upon irradiation at 500 nm (Scheme 69).⁷⁶³ A 6-fold increase in the quantum yield was obtained at pH 5.7; under these conditions, 140 and its conjugate base exhibit equal absorbance at the irradiation wavelength (Figure 30). The photoreaction cross section Φ_rε(λ_irr) for 140 was on the order of 10 M^–1 cm^–1 at λ_irr ∼500 nm and pH 7.0 (Table 13). Irradiation of 140 at 503 nm in the presence of non-complexed methemoglobin (MetHb, Fe^II) in aqueous solution led to the formation of carbonylhemoglobin (COHb). Studies on isotopically labeled 140 (C¹⁸O₂H) and DFT calculations suggested that an α-lactone intermediate is formed upon irradiation (via a mechanism analogous to that shown for BODIPY-based photoCORMs, vide infra), which subsequently thermally decarbonylates to release CO^{847,1132,1133} and form 3,6-dihydroxy-9H-xanthen-9-one as a photoproduct.⁷⁶³

Scheme 69. Photochemistry of photoCORM 140(763).

Table 13. Photophysical Properties of Some Organic PhotoCORMs.

CORM	λmaxabs (nm)	εmax (M–1 cm–1)	nCO	Φr (λirr/nm)	solvent	ref
140	488	18.6 × 103	(1)a	6.8 × 10–4 (500)	PBS	(763)
191a	461	0.3 × 103	(2)a	0.02 (395)	toluene	(1085)
191e	534	53.8 × 103	(2)a	n.d. (450)	DCM	(1090)
191f	605	135 × 103	(2)a	n.d. (450)	DCM	(1090)
205	409	16.2 × 103	0.96	0.0073 (419)	CH3CN	(1075, 1076, 1109)
208	543	79.4 × 103	1.00	n.d.	CH3CN	(1076)
224	465 (inactive)	17.8 × 103
	374 (active)	8.9 × 103	(1)a	0.14 (366)	cyclohexane	(1131)
225a	502	49.0 × 103	0.87	2.7 × 10–4 (500)	PBS	(847)
225b	652	52.5 × 103	0.91	1.2 × 10–5 (365)	PBS	(847)

^aTheoretical yield of CO equivalents. PBS = phosphate-buffered saline; DCM = dichloromethane.

Klán and co-workers also introduced two organic photoCORMs^10,80 based on the BODIPY chromophore, 225a and 225b(847) (Scheme 70). The release of CO from 225a was achieved in 45% chemical yield with Φ_r = 1.1 × 10^–4 in an aerated PBS solution (pH = 7.4) to give 2-methylpyrrole and 2H-pyrrole-4-carbaldehyde as the major additional photoproducts.⁸⁴⁷ These compounds are typical products of photochemical degradation of BODIPYs.¹¹³⁴ The release of CO from the π-extended BODIPY 225b(847) (λ_irr = 652 and 732 nm; Figure 30) occurred with a lower quantum efficiency (Φ_r = 1.4 × 10^–5), presumably due to an enhancement of radiationless decay related to the presence of the two flexible styryl groups.^1135,1136 Nevertheless, the release was efficient enough for use in vivo: white light-induced photoactivation of 225b in mice noticeably increased CO levels in the blood and some tissues.⁸⁴⁷ The involvement of a triplet excited state was established by transient spectroscopy, oxygen quenching experiments, and experiments using CsCl as a heavy-atom-effect mediator.¹¹³⁷ The benzyl ester derivative of 225a was photostable, and the photolysis of 225a at pH 2.5 proceeded with a ∼4-fold lower quantum efficiency than at pH = 7.0. This was in agreement with the calculated ΔG_eT, which predicted a more efficient intramolecular electron transfer from the carboxylate to the triplet-excited BODIPY core than for the protonated form.⁸⁴⁷ The proposed mechanism of the photoreaction is shown in Scheme 71. Upon excitation, a strongly fluorescent singlet excited state of 225 undergoes relatively inefficient ISC to the triplet state followed by an exergonic electron transfer (eT) from the carboxylate to the BODIPY core to form an oxyallyl-type triplet diradical.¹¹³⁸ The diradical then intersystem crosses to an open-shell singlet state, followed by the formation of α-lactone on the singlet ground-state potential energy surface.⁸⁴⁷ Finally, thermal fragmentation of the lactone releases CO.^1132,1133

Scheme 70. Photorelease of CO from BODIPY PhotoCORMs⁸⁴⁷.

Scheme 71. Proposed Mechanism of CO Photorelease from BODIPY PhotoCORMs 225a,b⁸⁴⁷.

4.1.2. Release of Carbon Monoxide from Transition-Metal-Containing PhotoCORMs

Most of the known photochemically activatable CO-releasing molecules (photoCORMs) are based on metal carbonyl complexes that undergo photoinduced cleavage of the carbonyl moiety followed by the addition of a solvent molecule to the vacant position in the metal’s coordination sphere.⁸³ The mechanisms of photochemical CO release from the coordination sphere of a transition metal have been studied in detail.^1139−1141 The carbonyl–metal bond is relatively strong (∼20–40 kcal mol^–1)¹¹⁴² because of its π-backbonding character. Delocalization of the LUMO on the carbonyl moiety is a general requirement for the photochemical liberation of CO.¹¹⁴⁰ The photocleavage is a reversible process; the recombination of the liberated CO molecule with the vacancy on the metal’s coordination sphere regenerates the photoCORM and thus reduces the quantum yield of CO release. This can be avoided by using ancillary ligands that shift electron density away from the metal center and reduce the amount of metal–CO backbonding.¹¹³⁹ It has been shown that not all carbonyl ligands are cleavable from complexes containing multiple carbonyl moieties and not all CO molecules are released in the primary photochemical step. CO can also be liberated by subsequent solvolysis or oxidative steps.^82,1140,1143

The first report on a photoCORM was published by Motterlini and co-workers, who described the photodissociation of Fe(CO)₅ and complex 226 (Figure 37).¹¹⁴⁴ The term photoCORM was introduced by Rimmer and co-workers in reference to W⁰ complex 227 (Figure 37), which releases one equivalent of CO upon irradiation.¹¹⁴³ The first transition-metal-containing photoCORMs required excitation at wavelengths in the range of 310–360 nm,^1139,1145 but strategies for bathochromically shifting their spectra were introduced by Mascharak and co-workers.¹¹⁴⁶ The use of nitrogen-based ligands with extended π-conjugation can lower the energy of the LUMO, while strongly donating ancillary σ-and π-donors raise the HOMO energy. The combination of such ligands with highly thermostable carbonyl complexes of electron-rich d⁶ metal ions such as Mn^I, Re^I, Fe^II, or Ru^II gives rise to bathochromically shifted MLCT absorption bands.

Figure 37.

Structures of some UV-absorbing photoCORMs.

Many visible-light-absorbing photoCORMs are based on Mn^I complexes. For example, Mn^I complexes with bidentate heteroaryl-imine ligands (Table 14) exhibit absorption maxima in the range of 390–700 nm. The absence of the σ-donating ligand Br^–(as in 228b, 229b, and 231b) caused a hypsochromic shift of 60–110 nm (Figure 31) and reduced the CO release efficiency by a factor of ∼1.1–3.5 relative to the reference analogs (228a, 229a, and 231a).^{1079,1146,1147} Extending the conjugation of the aromatic ligand, for example, by replacing pyridine with quinoline (as in complexes 229a, 229b, and 234), bathochromically shifted the MLCT band absorption maximum by ∼35–45 nm and also increased the quantum yield of CO release. Complexes 232, 233, and 234 (Table 14) containing α-diimine ligands were designed to be more soluble in water.¹¹⁴⁸ Irradiation of 233 released 3 equiv of CO, but the quantum yield of this process declined by factors of ∼2 and 3 in dichloromethane and aqueous solutions, respectively. The CO release efficiency of these complexes increased in the order 232 < 233 < 234, paralleling the increase in the electron-donating abilities of their ligands. Studies on analogous Mn^I and Re^I complexes containing 4-aminophenyl instead of adamantyl ligands revealed that only manganese complex 235 photoreleased CO upon visible light irradiation.¹¹⁴⁹ Zobi and co-workers studied a series of Mn^I-tricarbonyl complexes bearing azobipyridine ligands (236a–e, Table 14).¹¹⁵⁰ CO was liberated upon their illumination with red light (≥625 nm), and DFT calculations indicated that electron-withdrawing substituents lowered the LUMO energy more than that of the HOMO, resulting in a bathochromic shift of the MLCT band maximum. Complex 236e, which bore the strongest electron-withdrawing groups (CF₃ and Cl) was even activatable at the tail of the absorption range (810 nm). In the dark, complexes 236a and 236b were stable but the electron-poor complexes 236c–e slowly released CO. A series of 8 benzimidazole-based photoCORMs 230 was recently studied by Schatzschneider.¹¹⁵¹ These compounds rapidly released CO upon illumination, and their photochemistry was sensitive to their substitution. The 4-NO₂ substituted derivative 230 exhibited the most bathochromically shifted absorption but released CO with the lowest observed chemical yield because of a competing photodecomposition process.

Table 14. Mn^I-Based photoCORMs Containing Bidentate Heteroaryl-Imine Ligands^a.

CORM	λmaxabs (nm)	εmax (M–1 cm–1)	nCO	Φr (λirr/nm)	solvent	ref
228a	500	2.5 × 103	<3	0.34 (509)	THF	(1079)
228b	390	3.6 × 103	<3	0.12 (509)	CH3CN	(1079)
229a	535	2.2 × 103	<3	0.37 (509)	THF	(1079)
229b	435	3.7 × 103	<3	0.34 (509)	CH3CN	(1079)
230	386–495	1.3–2.2 × 103	0.8–1.8	n.d. (412 or 468)	DMSO	(1151)
231a	586	3.9 × 103	n.d.	0.48 (550)	CH3CN, DCM	(1146, 1147)
231b	520	4.1 × 103	n.d.	∼0.33b (420)	DCM	(1146, 1147)
232	445	1.8 × 103	n.d.	∼0.18b(≥450)e	DCM	(1148)
233	455	2.1 × 103	3	0.35 (≥450)e	DCM	(1148)
				∼0.16b	PBS
				∼0.10b	H2O
234	490	1.9 × 103	n.d.	∼0.78b(≥450)e	DCM	(1148)
235	437	n.d.	3	n.d. (525 or 468)	DMSO	(1149)
236a	625	4.35 × 103	n.d.	n.d.	DCM	(1150)
				(τ1/2 = 3.52 h)c	(H2O)d
236b	630	4.43 × 103	n.d.	n.d.	DCM	(1150)
				(τ1/2 = 3.60 h)c	(H2O)d
236c	661	3.46 × 103	n.d.	n.d.	DCM	(1150)
				(τ1/2 = 1.21 h)c	(H2O)d
236d	678	3.76 × 103	n.d.	n.d.	DCM	(1150)
				(τ1/2 = 0.48 h)c	(H2O)d
236e	693	4.85 × 103	n.d.	n.d.	DCM	(1150)
				(τ1/2 = 0.41 h)c	(H2O)d

^an.d.: not determined, PBS = phosphate-buffered saline, THF = tetrahydrofuran, DMSO = dimethyl sulfoxide, DCM = dichloromethane.

^bValues estimated from the relative apparent CO release rates (k_CO).

^cRelative half-lives in the series 236a–e; samples were irradiated at λ_max^abs.

^dAqueous solutions were used in the Mb assay¹¹⁵² to determine CO release.

^eBroadband visible light with a cut-off filter was used.

α,α′-Diimines and related ligands can also be used to tune the properties of Mn^I-based photoCORMs. For example, the Mn^Ifac-complex 237 (Table 15, L = Br⁻, Ar = 2,6-iPr₂Ph) releases CO upon irradiation with green light.¹¹⁵³ Its photoactivity can be attributed to an MLCT transition from the Mn^I–CO π and Br-centered orbitals to the π* orbitals of the diamine ligand, which weakens Mn–CO π-backbonding and thus facilitates CO release. The substitution of the bromide ligand with a neutral molecule (237, L = CO, THF, CH₃CN, tBuCN) afforded complexes absorbing at 420 nm. A tetracarbonyl complex (237, L = CO) was reactive in the dark and rapidly released CO upon dissolution in acetonitrile or THF. UV-photolysis of 237 (L = Br⁻) in THF released one equivalent of CO along with the Br^– ligand isomerization, and the resulting dicarbonyl complex coordinated CO to form meridional isomer of 237. The complexation of highly conjugated ligands derived from α,α′-diimines with the [Mn(CO)₃Br] moiety, as in 238a (Figure 31) and 238b (Table 15), led to exceptionally efficient CO release.^1080,1154 Complex 238b reportedly released CO in the IR region upon irradiation above 780 nm, where the compound does not absorb noticeably.¹¹⁵⁴ The authors attributed this to a weak but not completely forbidden optical population of the lowest triplet excited state of the complex. Similar S₀–T₁ absorption was later observed for carbazole derivatives.¹¹⁵⁵ The complex 238b exhibited strong solvatochromism, which was rationalized by suggesting that its lowest-lying singlet excited state has a charge-transfer character. Complex 239 (Table 15), bearing an iminoketone ligand, has a significantly bathochromically-shifted absorption maximum at 630 nm (Figure 31) and retains CO releasing ability.¹⁰⁸⁰ A series of thiourea- and thiazolyl-benzotriazolyl-carbothioamide-based Mn^I photoCORMs 240a, 240b, 241, and 242 (Table 15) were also reported to release 1-2 equiv of CO.^1156,1157

Table 15. Mn^I-Based PhotoCORMs Containing α,α′-Diimino, Iminoketone, or Carbothioamide Ligands^a.

CORM	λmaxabs (nm)	εmax (M–1 cm–1)	nCO	Φr (λirr/nm)	solvent	ref
237 (L = Br–)	582	n.d.	<1	n.d. (560)	THF	(1153)
238a	570	4.6 × 103	3	0.70 (545)	CH3CN	(1080)
238b	513 (H2O)	2.1 × 103	1	0.54 (545)	H2O	(1154)
	568 (MeOH)			0.30 (623)
				0.38 (623, deg.)c
239	630	3.7 × 103	n.d.	∼0.46b (>520)	CH3CN	(1080)
240a	398	n.d.	2	n.d. (468)	DMSO	(1156)
240b	407	n.d.	2	n.d. (468)	DMSO	(1156)
241	387	n.d.	1	n.d. (468)	DMSO	(1156)
242	437	6.1 × 103	1.5	n.d. (468 or 525)	DMSO	(1157)

^aTHF = tetrahydrofuran, DMSO = dimethyl sulfoxide.

^bValues estimated from relative apparent CO release rates (k_CO).

^cdeg.: degassed.

A Mn^I tricarbonyl structural motif was used to develop several bipyridine-based visible-light absorbing photoCORMs. Polypyridyl-containing metallodendrimers 243 (Table 16, n = 4, 8; R = 1,4-diaminobutane-poly(propyleneimine); DAB-PPI) released ∼65% of their total content of CO ligands upon irradiation with 410 nm light.¹¹⁵⁸ Complex 244 (Table 16) releases CO upon irradiation with blue light, while photoCORMs combining 244 and lanthanide ion-doped upconversion nanoparticles (see section 6.4.2) based on Yb^III- and Tm^III-doped Gd^III salts coated with a polymer matrix consisting of phospholipid-functionalized poly(ethylene glycol) released CO upon irradiation at 980 nm.¹¹⁵⁹ Blakemore, Elles, and co-workers recently studied 4,4′-disubstituted 2,2′-bipyridyl complexes 245 (R₁ = R₂ = NO₂, CF₃, H, tBu; Table 16) and the influence of the ligand’s electronic properties on the CO release rate,¹¹⁶⁰ showing that irradiation into the MLCT band caused rapid CO release (τ_CO = 0.46–0.68 ps) followed by solvent coordination (τ_solv = 18–39 ps). A recent mechanistic study by Pordel and White examined a series of tricarbonylmanganese(I) complexes with 4,4′-substituted 2,2′-bipyridine ligands (bpy′) fac-[Mn(bpy′)(CO)₃L; L = Br^– or py] 245 (Table 16).¹¹⁶¹ In accordance with the findings of Mascharak and co-workers,⁸⁶ substituting the electron-donating Br^– ligand with a π-accepting pyridine stabilized the Mn^I-based HOMO, causing a hypsochromic shift of the absorption maxima (by 100–150 nm), and reduced the quantum yield of CO release. The regioselectivity of CO release could also be tuned: the CO ligand cis to L was liberated first when L = Br^–, but the trans-CO was liberated first when L = py. Increasing the π-acidity of the bipyridine ligands also increased the efficiency of the CO release, although this effect was comparatively weak. The absorption spectra and energies of the MLCT states of 50 different fac-[M(CO)₃]⁺ complexes (M = Mn^I, Re^I) evaluated as potential photoCORMs were recently analyzed and mathematically correlated by the group of Zobi.¹¹⁶²

Table 16. Mn^I-Based PhotoCORMs with Heteroaryl Bidentate Ligands and Their Analogs^a.

CORM	λmaxabs (nm)	εmax (M–1 cm–1)	nCO	Φr (λirr/nm)	solvent	ref
243, n = 4	410	10.3 × 103	7.56	2.66 × 10–3 (410)	DMSO:H2O	(1158)
n = 8	420	18.8 × 103	15.24	2.71 × 10–3 (410)	1:9, v/v
244	400	5.4 × 103	1.85	0.26 (470)	DCM	(1159)
245
L = Br–, R1 = R2 = CO2Me	460	3.2 × 103	∼3	0.32 (405)	CH3CN	(1161)
L = Br–, R1 = R2 = H	416	2.9 × 103	∼3	0.22 (405)	CH3CN
L = Br–, R1 = R2 = Me	411	2.9 × 103	∼3	0.20 (405)	CH3CN
L = py, R1 = R2 = CO2Me	420	4.0 × 103	∼3	0.19 (405)	CH3CN
L = py, R1 = R2 = H	383	3.4 × 103	∼3	0.17 (405)	CH3CN
L = py, R1 = R2 = Me	378	3.4 × 103	∼3	0.15 (405)	CH3CN
L = Br–, R1 = R2 = tBu	412	2.4 × 103	n.d.	n.d. (415)	CH3CN	(1160)
L = Br–, R1 = R2 = H	415	2.3 × 103	n.d.	n.d. (415)	CH3CN
L = Br–, R1 = R2 = CF3	457	1.5 × 103	n.d.	n.d. (415)	CH3CN
L = Br–, R1 = R2 = NO2	510	0.2 × 103	n.d.	n.d. (415)	CH3CN
246	410	n.d.	3	n.d. (460)	DMSO	(1164)
247	385	n.d.	2.84	n.d. (456)	PBS, pH = 7	(1165)
248a, R = OH	422	n.d.	n.d.	(kCO = 0.07 min–1)b (410)	CH3CN	(1166)
R = O–	490	n.d.		(kCO = 0.81 min–1)b (410)	CH3CN
248b, L = Br–	428	35.3 × 103	3	0.19 (451)c	ethanol:PBS	(1167)
L = CF3SO3–	428	28.6 × 103	3	0.04 (451)e	2:1, v/v
				0.22 (451)c
				0.06 (451)e
249a	396	n.d.	∼2.5	(kCO = 16 × 10–4s–1)d(468)	DMSO	(1168)
249b	401	n.d.	∼2.5	(kCO = 37 × 10–4s–1)d (468)	DMSO	(1168)
250	450	3.2 × 103	n.d.	n.d. (>440)	DCM	(1169)
251	465	3.2 × 103	2.85	n.d. (470)	DMSO	(1170)
252	440–640	0.15–3.0 × 103	n.d.	n.d. (400–700)	CH3CN	(1171)
253	393	n.d.	0.09f	n.d. (480)	CH3OH	(1172)
254a	375	1.5 × 103	3	0.18 (405)	PBS	(1173)
				0.31 (470)
254b	379	1.4 × 103	3	0.17 (405)	PBS	(1173)
				0.43 (470)
255	340	3.6 × 103	2.5	n.d. (400)	DMSO	(1174)
256a	370	5 × 103	3	0.35 (380)	CH3CN	(1175)
256b	350	6.2 × 103	3	0.39 (>410)	CH3CN	(1176)

^aDMSO = dimethyl sulfoxide, DCM = dichloromethane, PBS = phosphate buffer saline.

^bRelative rate of the CO release of 248a.

^cQuantum yield for the release of the first equivalent of CO.

^dQuantum yield for the release of the second equivalent of CO.

^eRelative rates of CO release from 249a and 249b.

^fMeasured by irradiation of solid crystalline phase. dend. = 1,4-diaminobutane dendrimer. 247: the sphere represents the membrane-puncturing needle domain of bacteriophage T4.

Zobi and co-workers also developed hybrid systems referred to as quantum-CORMs that combine Mn^I-tricarbonyl complexes 245 (Table 16) with bipyridyl ligands containing anchoring groups (R₁ = H, COOH; R₂ = COOH, NH₂, (4-carboxyphenyl)ethynyl, and (4-aminophenyl)ethynyl) that were used to bind the Mn complexes to the surfaces of CdSe/ZnS core/shell semiconductor quantum dots (see also section 6.4.1).¹¹⁶³ These quantum dots have a band-gap wavelength of 504 nm and bright emission at 512 nm. Upon irradiation at 510 nm, they sensitize the release of CO from the photoCORM, increasing its efficiency 2- to 6-fold compared to non-sensitized excitation.

Furukawa and co-workers developed light-responsive metal-organic frameworks as controllable CO-releasing cell-culture substrates.¹¹⁶⁴ These materials combine a Mn^I tricarbonyl bipyridyl complex 246 (Table 16) with a highly robust Zr^IV-based MOF. The group of Ueno developed a construct containing an artificial protein needle by conjugating the membrane puncturing needle domain of bacteriophage T4 to the Mn^I carbonyl photoCORM complex 247 (Table 16) via a maleimide thiol linkage.¹¹⁶⁵ This system was used as an in vivo magnetic-resonance-imaging contrast reagent. Allosteric regulation of CO release was demonstrated in complex 248a (Table 16),¹¹⁶⁶ in which the phenolic substituent of the terpyridyl ligand responds to fluoride ions by undergoing deprotonation, leading to allosteric activation of CO release; the deprotonated complex released CO approximately 1 order of magnitude more efficiently than its neutral form. Ford and co-workers synthesized another terpyridine-based manganese tricarbonyl complex 248b (Table 16), which can release CO both by 1-photon excitation in the visible region and also by 2-photon excitation at 750 and 800 nm because the terpyridine ligand acts as an efficient 2-photon antenna.¹¹⁶⁷

Potentially bioactive Mn^I tricarbonyl complexes with 2-(2′-pyridyl)benzimidazole ligands bearing morpholino (249a) or phthalimido (249b, Table 16) substituents were studied spectroscopically and computationally by Mansour and Ragab.¹¹⁶⁸

Another Mn^I tricarbonyl photoCORM, 250 (Table 16), contains a benzothiazole ligand that functions as a turn-on fluorescent signal.¹¹⁶⁹ Upon photoexcitation, this complex releases both CO and the 2-(2-pyridyl)-benzothiazole ligand, whose fluorescence was successfully used to monitor the CO-induced death of human breast cancer cells treated with 250. The similar Mn^I complex 251 (Table 16), which has a ligand derived from the anti-anxiety drug bromazepam, was reported by Mansour.¹¹⁷⁰

A series of four photoCORMs 252 (Table 16) bearing 2-(benzo[d]thiazol-2-yl)phenol ligands, was developed by Roy and co-workers,¹¹⁷¹ and tricarbonyl Mn^I complexes with 3-(2-pyridyl)pyrazole ligands 253 (Table 16) were shown to release CO independently of the choice of R substituents.¹¹⁷²

Mn^I tricarbonyl photoCORMs 254a and 254b (Table 16), containing substituted bispyrazolylmethane ligands, were prepared by the group of Westerhausen.¹¹⁷³ These complexes are initially neutral but their ligands have terminal acetyl groups that are hydrolyzed into carboxylates upon cellular uptake. As a result, the photoCORMs become anionic and are trapped inside the cells. Fairlamb, Lynam, and co-workers synthesized a series of biotin-conjugated Mn^I-based photoCORMs 255 (R = biotin; Table 16) that release CO upon irradiation at 400 nm and bind efficiently to avidin.¹¹⁷⁴

Fluorescent dansylimidazole-substituted complex 256a (Table 16) released CO upon irradiation with visible light.¹¹⁷⁵ Its analog 256b exhibited strong green luminescence that could be visualized in vitro.¹¹⁷⁶ This class of luminescent Mn^I-based photoCORMs was extended by preparing dansylimidazole complexes with diazabutadiene ligands bearing sterically similar adamantyl (lipophilic) or 1,3,5-triazaadamantyl (hydrophilic) substituents.¹¹⁷⁷ Changing the ligand’s lipophilicity altered the localization of the photoCORMs in cellular organelles.

Tridentate heteroaryl ligands form stable complexes with Mn^I and were used for the successful design of photoCORMs. Very recently, the group of Schiller introduced a novel class of 2-photon absorbing naphthalimide-containing photoCORMs 257 (Figure 38).¹¹⁷⁸ These compounds release CO by both 1- (405 nm) and 2-photon (800 nm) excitation. CO liberation is accompanied by the release of the naphthalimide ligands, which are fluorescent in solution, in non-woven fabrics, and in HeLa cells. Similar naphthalimide-substituted photoCORMs 257 (X = NR) were used to synthesize green light-responsive (550 nm) CO-releasing polymeric materials by ring-opening metathesis polymerization.¹¹⁷⁹

Figure 38.

Naphthalimide substituted photoCORM.

Schiller and co-workers developed the dabsyl-substituted Mn^I tricarbonyl complex 258 (Figure 39), which releases CO upon irradiation at 405 nm.¹¹⁸⁰ The complex was loaded onto paper strips to form a material whose light-triggered CO release could be monitored with the naked eye by observing the change in its color. An analogous approach was used in the design of the colorimetric and fluorometric dual response photoCORM 259 (Figure 39), which is based on the nitrobenzoxadiazole fluorophore and releases CO upon irradiation at 490 nm.¹¹⁸¹

Figure 39.

Dabsyl- and nitrobenzoxadiazole-substituted photoCORMs.

Mn^I-based photoCORMs with other structures have also been reported (Table 17). For example, Ueno and co-workers developed a series of engineered protein crystals using the photoCORM Mn(CO)₅Br.¹¹⁸² Polyhedral crystals containing histidine as a ligand were used to immobilize the Mn^I carbonyl complex 260 (Table 17). Two proteins were prepared, a wild-type (WT) with 3 histidine units and mutants with hexahistidine tags containing 3 or 6 equiv of the photoCORM. The CORM loading and the corresponding quantum yields of CO release correlated with the number of histidine residues in the protein.

Table 17. Other Mn-Based PhotoCORMs^a.

CORM	λmaxabs (nm)	εmax (M–1 cm–1)	nCO	Φr (λirr/nm)	solvent	ref
260	n.d.	n.d.	1.9b	0.013b (456)	PBS	(1182)
			2.9c	0.047c (456)
261	435	n.d.	∼1	n.d. (468)	DMSO	(1183)
262	395	n.d.	∼0.5	n.d. (410)	DMSO	(1183)
263	465–486	n.d.	1.7–1.9	n.d. (520–560)	DCM	(1184)
264	360	1.4 × 103	2	n.d. (400)	CH3CN	(1185)
			1.4	n.d. (465)
265	388	n.d.	3	n.d. (470)	H2O	(1188)
266	354	n.d.	2.86 (405)	0.1 (365)	PBS	(1189)
267	350	n.d.	2.61 (405)	0.06 (365)	PBS	(1189)
268	344	1.1 × 103	3	0.054 (385)	H2O	(1190)
				0.030 (410)
269					H2O	(1190)
R = H	349	1.3 × 103	3	0.058 (385)
				0.044 (410)
R = CH3	354	1.4 × 103	3	0.081 (385)
				0.033 (410)
270	384	n.d.	6	0.11 (365)	PBS	(1191)
				0.06 (470)
271						(1193)
R = nPr	385	1.8 × 103	n.d.	n.d. (405)	CHCl3
R = nBu	n.d.	n.d.	n.d.	n.d. (405)	CHCl3
272						(1194)
N∩N = phe	550	5.8 × 103	2	0.41 (659)	CH3CN
N∩N = bpy	550	4.9 × 103	n.d.	0.39 (659)	CH3CN
N∩N = biq	719	1.3 × 103	n.d.	0.24 (659)	CH3CN
N∩N = phen–CHO	652	7.6 × 103	n.d.	0.02 (659)	CH3CN
273	340	n.d.	5.3	0.24 (405)d	DMA	(1195)
				0.006 (635)e

^aPBS = phosphate buffer saline, DMSO = dimethyl sulfoxide, DCM = dichloromethane, Phe = 1,10-phenanthroline, Bpy = 2,2′-bipyridyl, Biq = 2,2′-biquinoline, Phen-CHO = phenanthrolinecarboxaldehyde, DMA = dimethylacetamide.

^bWild-type protein (3 histidine units).

^cMutant protein with hexahistidine tag.

^dDirect irradiation.

^eSensitized release.

Mn^I tricarbonyl complexes with tazarotene (261) and metamizole (262) as bidentate and tridentate ligands, respectively, were developed by Mansour and Shebab (Table 17).¹¹⁸³ Both ligands dissociate from the metal center upon the CO photorelease. In addition, Manimaran and co-workers developed a series of 10 Mn^I-based aminoquinonato-bridged dinuclear complexes 263 (Table 17) that release CO upon irradiation with green light.¹¹⁸⁴ The mechanism of CO release from the Mn^I tryptophanate complex 264 (Table 17)¹¹⁸⁵ was studied by time-resolved ultrafast infrared spectroscopy (TRIR) and TD-DFT,¹¹⁸⁶ which revealed that excitation leads to an LMCT from the indole moiety of the tryptophan ligand to the metal d-orbitals. The loss of CO then occurs within 3 ps, resulting in the formation of the triplet state of the dicarbonyl product ³[Mn^I(tryp)(CO)₂(MeCN)], which is solvated within 20 ps. The mechanism of CO release from 264 was further studied by laser-interfaced mass spectrometry (LIMS) across a wide wavelength range (λ_irr = 234–580 nm).¹¹⁸⁷

Zobi and co-workers studied a fac-Mn^I tricarbonyl complex with a tetraazacyclotetradecane ligand attached to the 5′–OH ribose group of vitamin B₁₂ (265, Table 17).¹¹⁸⁸ Vitamin B₁₂ acts as a biocompatible water-soluble scaffold that allows the photoCORM to be actively transported into cells. Because of its remote attachment, it does not affect the photochemistry of the Mn^I complex. The compound was successfully delivered into fibroblasts, where the photoinduced release of CO protected them against death under conditions of hypoxia and metabolic depletion.

The group of Westerhausen developed a series of water-soluble manganese tricarbonyl complexes 266 and 267 (Table 17) based on tridentate aminoalkylsulfide ligands.¹¹⁸⁹ Accumulation of these complexes inside cells was observed by FT-IR imaging. Peralta and co-workers recently synthesized three analogous water-soluble complexes 268 and 269 (R = H, CH₃) that sequentially release three equivalents of CO upon irradiation with blue light.¹¹⁹⁰ The thiolato-bridged Mn^I-dimer 270 (Table 17) prepared from cysteamine by Westerhausen and co-workers was shown to photochemically release all 6 of its bound CO ligands.¹¹⁹¹

Schiller and co-workers demonstrated the photochemically controlled release of CO from non-woven polylactide fibers containing the Mn⁰ decacarbonyl complex (Mn₂(CO)₁₀, CORM-1).¹¹⁹² They later constructed a device for remote-controlled delivery of CO using tetranuclear Mn^I-based complexes 271 (Table 17, R = nPr, nBu) embedded in non-woven polylactide or polymethacrylate fabrics¹¹⁹³ that were shown to be nontoxic towards 3T3 mouse fibroblast cells.

A new strategy for triggering the photochemical release of caged CO using long-wavelength and NIR light was described by Ford and co-workers using dinuclear Rh⁰–Mn⁰ carbonyl complexes 272 (Table 17).¹¹⁹⁴ These complexes have strong metal–metal-bond-to-ligand charge-transfer (MMLCT) absorption bands from ∼550 to ∼720 nm. Their photoexcitation leads to homolytic cleavage of the Re–Mn bond, yielding mononuclear metal radicals that tend to recombine in the absence of trapping agents but react with dioxygen to form active species that efficiently release CO via secondary thermal or photochemical processes in aerated solutions. Schiller and co-workers combined the classical UV-absorbing photoCORM Mn₂(CO)₁₀273 (Table 17) with the Pd^II tetraphenyltetrabenzoporphyrin complex 274, which is a triplet photosensitizer (see also section 6.1) excitable by red or NIR light.¹¹⁹⁵ The triplet-excited photosensitizer transfers its energy to the photoCORM, which then liberates CO. The authors combined these components in the solid state to prepare a CO-releasing material supported on electrospun non-woven fabrics.

Ru^II complexes have d⁶ configurations similar to those of Mn^I complexes and have also been successfully used as visible-light activatable photoCORMs. Ru^II analogs of 229a (Table 14), 275, and 276 (Figure 40) are carbonyl complexes where the 2-quinoline-N-(2′-methylthiophenyl)-methylenimine (qmtpm) moiety acts as a tridentate ligand due to the high affinity of Ru^II for sulfur. Complex 275 has a MLCT band at λ_max^abs = 405 nm.¹¹⁹⁶ The replacement of a strongly π-accepting CO ligand with PPh₃ resulted in a bathochromic shift of the MLCT band to 465 nm. Complex 275 releases CO only in acetonitrile upon UV-light (310 nm) irradiation, whereas phosphine-substituted complex 276 is active in the visible region and releases CO under visible light (≥440 nm). Mansour studied the Ru^II dicarbonyl complex 277 (Figure 40), which incorporates a ligand derived from the anti-anxiety drug bromazepam and liberated 2 equiv of CO upon excitation with 470 nm light.¹¹⁷⁰ The complexation of this photoCORM to bromozepam increased its antibacterial toxicity relative to the non-complexed drug. Complex 278, which has a bisquinoline ligand, sequentially releases two equivalents of CO upon irradiation at 350 or 420 nm, with the first equivalent being liberated more efficiently.¹¹⁹⁷ Finally, Oyama and co-workers recently introduced a series of Ru^II dicarbonyl photoCORM complexes with asymmetric bipyridine ligands.¹¹⁹⁸

Figure 40.

Ru^II-based photoCORMs.

Some Fe^II carbonyl complexes were reported as visible-light excitable photoCORMs. For example, irradiation of complex 279 (Figure 41) at 470 nm resulted in decarbonylation, which was monitored using a myoglobin assay and ion channels sensitive to CO.^1199,1200 The diiron hexacarbonyl complex 280 (Figure 41)¹²⁰¹ is a water-soluble analog of an iron–iron hydrogenase model complex¹²⁰² and can liberate 6 equiv of CO upon irradiation at 390 nm. It is not clear whether all 6 CO ligands are liberated from the excited state. Nakajima and co-workers reported a series of N,C,S-pincer iron(III) carbonyl complexes 281 (Figure 41) with two phosphorous ligands (281a: R₁ = Me, R₂ = Ph; 281b: R₁ = R₂ = Me; 281c: R₁ = R₂ = OEt) in the trans-positions.¹²⁰³ A detailed study of their wavelength dependence showed that all of these complexes released CO upon irradiation at <500 nm and that complex 281c was photoactive even at 800 nm. The release quantum yields were in the range of 0.03–0.01 for all complexes. A core-shell-based material that releases CO via an upconversion process (see section 6.4.2) was developed by Liu and co-workers.¹²⁰⁴ The core, in this case, consists of upconverting nanoparticles of β-NaYF₄:Yb^III/Er^III, while the shell consists of [Fe^II(η⁵-Cp)(CO)₂] complexes with the structure 282 (Figure 41) that are anchored to the surfaces of the nanoparticles via thiol groups. Irradiation of this material with a 980 nm laser led to the sensitization and subsequent decarbonylation of the CORM shell. Wright and co-workers introduced 2-aminopyridine and 1-aminoisoquinoline-based iron(II) complexes 283 bearing two CO molecules¹²⁰⁵ that can be substituted by thioglucose to obtain the ferracyclic dimeric complexes 284, which exhibit enhanced water solubility and Φ_r values of 0.9–1.7 × 10^–4.

Figure 41.

Iron-based photoCORMs.

Re^I complexes 285 (L = Br^–, PPh₃, Figure 42), which are analogous to Mn^I-based complexes 231 (Table 14), were found to release CO only under UV illumination.¹¹⁴⁶ This was rationalized by TD-DFT calculations indicating that ISC to the triplet state promoted by strong spin-orbit coupling competed with CO release. Conversely, the water-soluble Re^I-based photoCORM 286 (Figure 42) released 1 equiv of CO upon irradiation at 405 nm with Φ_r = 0.11¹²⁰⁶ (Φ_r (365 nm) = 0.024).¹²⁰⁷ This complex and its photoproduct are both fluorescent, with distinguishable maxima at 515 (Φ_F (365 nm) = 0.08) and 585 nm, respectively, enabling qualitative monitoring of CO release in cells using confocal fluorescence microscopy.

Figure 42.

Re^I-based photoCORMs.

4.2. Release of Nitric Oxide

Nitric oxide (NO) is a gasotransmitter with many important biological roles¹²⁰⁸ in processes including vasodilatation,^{1061,1209−1212} platelet aggregation inhibition,¹⁰⁶⁶ wound healing,¹⁰⁶⁵ postsynaptic plasticity augmentation, and hormone secretion.¹⁰⁶⁷ The development of NO-releasing molecules (NORMs) is therefore of interest for both therapeutic applications¹²¹³ and mechanistic chemico-biological studies. Advances in the photochemical release of NO from photoNORMs have been reviewed previously.^103−109

4.2.1. Transition-Metal-Free PhotoNORMs

There are three types of transition-metal-free photoactivatable nitric oxide (NO) donors (often termed photoactivatable nitric-oxide-releasing moieties, or photoNORMs): (i) N-nitroso amines, (ii) diazeniumdiolates (NONOates, R₂N–(NO^–)–N=O) and related structures, and (iii) o-substituted nitroarenes and their derivatives. NO release from these species can be induced by direct excitation or energy/electron transfer from an excited sensitizer.

N-Nitroso amines (Scheme 72) are popular NO donors because they are easily prepared by nitrosylation of amines.¹²¹⁴ The N–NO bond is typically very weak (∼39 kcal mol^–1); its energy is comparable to that of a photon with a wavelength of ∼730 nm.¹⁰³ However, the N–NO group exhibits very limited absorption in the visible/NIR part of the spectrum and is therefore always combined with a suitable sensitizer. Upon NO photorelease, N-nitroso amines form aminyl radicals that may abstract hydrogen atoms from other molecules or undergo reduction or oxidation.^1215,1216 They can also transnitrosate with nucleophiles such as thiols.¹²¹⁷

Scheme 72. Photoreactions of N-Nitroso Amines^{1214,1215,1217}.

The thermally stable and non-cytotoxic aza-BODIPY-based N-nitrosamine 287 (Figure 43) was shown to release NO upon irradiation at λ_irr = 700 nm in vitro and in vivo.¹²¹⁸ This molecule also acts as an excellent photoacoustic sensor, allowing the local, irradiation-dependent release of NO to be monitored in vivo by photoacoustic tomography. The BODIPY scaffold was also used to prepare 288,¹⁰⁷⁷ which releases NO upon irradiation at wavelengths in the range λ_irr = 470–500 nm with a Φ_r of 0.0019 at 488 nm (Figure 30).

Figure 43.

Structures of BODIPY N-nitroso amine-based photoNORMs.

Like 288, the rosamine photoNORM 289 releases NO upon irradiation at λ_irr = 530–590 nm (Scheme 73).¹²¹⁹ The suggested mechanism involves photoinduced electron transfer from the N-nitrosoaminophenol group to the excited rosamine moiety to form unstable phenoxyl radical 290, which decomposes to release NO and form the stable quinoneimine 291.

Scheme 73. Mechanism of Photochemical Release of NO from 289(1219).

This approach was further developed by the synthesis of photoNORMs 292 and 293 (Figure 44), and it was found that the distance between the NO-releasing N-nitrosoaminophenol group and the rosamine sensitizer profoundly affects the efficiency of NO release.¹²²⁰ Compound 293 exhibited the most efficient NO release (Φ_r = 1.01 × 10^–3); the other derivatives were strongly fluorescent. This proximity effect was attributed to π–π stacking and the formation of conformational isomers that enable efficient electron transfer. Conjugation of d-galactose with the phenolic hydroxy group completely blocked NO release, which was restored by selective enzymatic hydrolysis with β-galactosidase, enabling NO release using two independent triggers that can be activated simultaneously. The formation of phenoxyl radical 290 (Scheme 73) was shown to be the key step in NO release. The release of NO from compound 293 upon irradiation at 530–590 nm was tested in HET293T cells and used to control the response of rat aortas to NO in an ex vivo system.¹²²¹ Compounds 288, 289, and 293 were subsequently evaluated as potential NO donors for treatment of erectile dysfunction.¹⁰⁵

Figure 44.

Structures of rosamine-based photoNORMs.

Yang and co-workers recently developed a visible-light absorbing photoNORM based on N-nitroso rhodamine derivative 294 (Figure 45).¹²²² Upon irradiation with λ_irr = 532 nm, the weakly fluorescent molecule 294 was converted into NO and a fluorescent rhodamine-based photoproduct (Φ_F = 0.43 at 560 nm in phosphate buffer, pH = 7.4), which was used to monitor the localization, flux, and dose of the released NO. NO release was found to occur within 7 ps after excitation. A sulfonate group was attached to the N-nitrosamino group in photoNORM 295 to increase its water solubility,¹²²³ while the morpholine-substituted derivative 296 was designed to target the lysosomes and release NO in living cells and zebrafish.¹²²⁴ Interestingly, a chromenylium analog of 296, 297, released NO in 91% yield only upon excitation with 365 nm light despite absorbing in the visible region (λ_max^abs = 537 nm).¹²²⁵ Additionally, the dihedral angle between the nitroso moiety and the rhodamine core was found to influence the efficiency of NO photorelease. The nitrosamine group is almost orthogonal to the plane of the chromophore in 294–296, whereas in the ring-restricted compound 298, the nitrosamine moiety is locked in a coplanar geometry.¹²¹⁶ The direct conjugation of the chromophore and nitrosamine systems enables more efficient photoinduced intramolecular charge transfer, leading to NO release at λ_irr = 532 nm, ∼20-times more efficiently than from 294. A doubly N-nitrosylated analog of 294, 299, released NO only upon UV light irradiation (λ_irr = 365 nm; Figure 45).¹²²⁶ Unlike 294, which exists as an equilibrating mixture of a fluorescent visible-light-absorbing open form and a non-fluorescent UV-light-absorbing lactone form, photoNORM 299 exists exclusively as a lactone. This molecule was used to study changes in mitochondrial dynamics following NO release induced by irradiation at 375 nm.¹²²⁷

Figure 45.

N-Nitroso amine-based photoNORMs.

The naphthalimide derivative 300 is another notable N-nitrosoamino photoNORM (Figures 46 and 30).¹⁰⁷⁸ It releases NO only upon irradiation with UV light (λ_irr = 365 nm) or 2P excitation at λ_irr = 740 nm. Its coumarinyl-substituted analog 301 also releases NO upon UV irradiation or 2P excitation at λ_irr = 800 nm, with a chemical yield of 79%.¹²²⁸

Figure 46.

Napththalimide-based photoNORMs.

Diazeniumdiolates (NONOates) release two equivalents of NO via thermal processes and have been investigated by Keefer and co-workers as potential liver-selective NO donating prodrugs.¹²²⁹ NONOates release NO in neutral aqueous solutions at a rate that depends on their structure. Because of their simple preparation¹²³⁰ and generation of predictable amounts of NO, they have attracted considerable attention as NORMs.^1231,1232 NO release from NONOates is insensitive to biological factors, so they do not induce resistance.¹²³³ The first attempt to control NO release from NONOates with light was reported by Tsien and co-workers, who used the o-nitrobenzyl protecting group (section 2.1.1) as a PPG in 302 (Scheme 74).¹²³⁴ This group is activated only by UV-irradiation (λ_irr = 365 nm); upon deprotection of the NO group, it thermally releases two equivalents of NO and a secondary amine. Another approach was used by Sortino and co-workers, who used the thermal NO releaser cupferron (which exists in two resonance forms: phenyldiazenium diolate 303 and N-nitroso-N-phenylhydroxylaminate 304)⁶³⁴ in the BODIPY-based (see section 2.11) photoNORM 305 (Figure 47).⁸⁰³ Its irradiation at 530–550 nm resulted in the heterolytic cleavage of the BODIPY protecting group, unmasking cupferron, which thermally releases 1 equiv of NO (Φ_r = 0.008 ± 0.001 at 532 nm; Φ_rε(λ_irr) = 550 M^–1 cm^–1). Iodo analog 306 was introduced as a photosensitizer for photodynamic therapy that simultaneously photoreleases NO (Figure 47).⁷⁹⁸ The iodine atoms enhance the quantum yield of ISC, which increases singlet oxygen production in aerated solutions because of the sensitizing effect of the triplet-excited BODIPY group.

Scheme 74. Photochemical Deprotection of NONOates¹²³⁴.

Figure 47.

Structures of cupferron and cupferron-based photoNORMs.

o-Substituted nitroarenes have also been identified as potential photoNORMs. The o-trifluoromethyl nitrobenzene derivatives 307 (R = H, Scheme 75) isomerize upon irradiation with UV light (λ_max^abs ≈ 300 nm) to give arylnitrite derivatives 308,¹²³⁵ which thermally release NO to form phenoxy radicals 309. These radicals subsequently abstract hydrogen atoms to give phenol derivatives 310. The substituent in the ortho-position plays a key role in this process because it forces the nitro group to adopt a twisted geometry. Introducing an amino substituent para to the nitro group yields a chromophore with push-pull character, bathochromically shifting the absorption bands by ∼80 nm (307, R = NHR′, R′ = alkyl, acyl).¹²³⁵

Scheme 75. Photochemical Release of NO from o-Substituted Nitroarenes¹²³⁵.

Hexadecylamine-substituted 4-nitro-3-(trifluoromethyl)aniline 311 (Figure 48) was used by Jose and co-workers to prepare nanoscale lipid vesicles for photoinduced NO delivery (λ_irr = 410 nm).¹²³⁶ However, even push-pull-substituted nitroarenes are poor chromophores (ε_max ≈ 1000 M^–1 cm^–1) and must be sensitized to enable visible-light activation. Sortino and co-workers introduced the anthracene-based photoNORM 312, which releases NO upon irradiation with 420 nm light.¹²³⁷312 was used as a photoNORM in the construction of a fluorescein-labeled β-cyclodextrin-based supramolecular nanoassembly with a red-emitting singlet oxygen photosensitizer (zinc phthalocyanine).¹²³⁸ This system exhibits “five-in-one” photochemical features: visible-light or 2P (740 nm) excitation, facile visualization due to distinct fluorescence, production of cytotoxic singlet oxygen, and NO release. The coumarin-based compound 313 was incorporated into photo-antimicrobial polymeric films to release NO upon irradiation at λ_irr > 400 nm.¹²³⁹o-Trifluoromethyl-substituted nitroarenes were similarly used to prepare diketopyrrolopyrrole-based nanoplatforms for pH-responsive photodynamic/photothermal synergistic cancer therapy.¹²⁴⁰

Figure 48.

Structures of o-trifluoromethyl substituted nitroarene-based photoNORMs.

Miyata and co-workers developed a series of 2,6-dimethylnitrobenzene-based photoNORMs with properties similar to their trifluoromethyl analogs.^110,1241 However, π-extension of the aromatic system did not lead to visible-light activated NO release. The most active derivative 314 (Table 18) generated 0.55 equiv of NO upon irradiation with UV light. The attachment of fluorescein as a sensitizer generated the visible-light-absorbing molecule 315, which releases NO only upon UV-irradiation or 2P excitation (σ_TPA = 0.12 GM).^1242,1243 The conjugated analog 316 releases NO both by 1P and 2P excitation with a 2P absorption cross section 8-times higher than that of 315 (σ_TPA = 0.98 GM).¹²⁴⁴ A similar approach is embodied in rhodamine-based derivatives 317, which liberate NO upon irradiation with yellow light,¹²⁴⁵ and in compound 318, which incorporates 6-bromo-7-hydroxycoumarin as a sensitizer.¹²⁴⁶

Table 18. 2,6-Dimethylnitrobenzene-Based PhotoNORMs.

NORM	λmaxabs (nm)	εmax (M–1 cm–1)	λirr (nm)	Φr (λirr/nm)	solvent	ref
314	335	n.d.	UVa	n.d.	H2O:DMSO	(110, 1241)
					3:1 (v/v)
315	452	n.d.	330–380	n.d.	H2O:DMSO	(1242)
			450–480		3:1 (v/v)
			720–735c
316	450	n.d.	450–480	n.d.	H2O:DMSO	(1244)
			720c		3:1 (v/v)
317, X = pyrrolidine	563	26 300	530–590	0.0023 (550)	phosphate bufferb	(1245)
317, X = morpholine	553	15 500	530–590	n.d.	phosphate bufferb	(1245)
318	360	11 200	400–430	0.053 (358)	H2O:DMSO	(1246)
					1:1 (v/v)

^aUV irradiation with a Pyrex filter.

^bSodium phosphate buffer, c = 100 mM, 1% DMSO (v/v).

^c2-photon excitation.

Sortino and co-workers recently synthesized two hybrid fluorescent photoNORMs 319a and 320 (Figure 49) that combine an N-nitrosamine moiety with an o-trifluoromethyl nitroarene and a BODIPY or rhodamine sensitizer.¹²⁴⁷ Both compounds released NO upon irradiation with green light (λ_irr = 510 nm for 319a and 532 nm for 320), but release was more efficient from the BODIPY-substituted derivative 319a (Φ_r = 0.031) than from the rhodamine-substituted compound 320 (Φ_r = 0.001). The reaction was suggested to be initiated by electron transfer from the N–NO group to the excited dye moiety. While these compounds should release 2 equiv of NO (one from the N-nitrosamine and one from the aryl nitro group), only the cleavage of the N-nitrosamine N–NO bond was observed. The iodo analog of 319a, 319b, releases NO upon irradiation with 532 nm light, which is accompanied by singlet oxygen production.¹²⁴⁸

Figure 49.

Hybrid photoNORMs combining N-nitrosamines and o-trifluoromethyl nitroarenes.

4.2.2. Transition-Metal-Containing PhotoNORMs

The most accessible sources of NO are nitrosyl and nitrito transition metal complexes with weakly bound NO ligands that can be released by external triggers. The oldest and best known NO donors are pentacyanonitrosylmetallates such as disodium pentacyanonitrosylferrate (sodium nitroprusside), which has been known since the mid-19th century.^1249−1251 The M–NO bond is readily cleaved by excitation into the MLCT (d_M → π*_NO) band of an organometallic complex. These bands exist in the visible region (above 400 nm); excitation weakens π-back-bonding to the NO ligand and facilitates electron transfer from the metal center to NO⁺, which is then liberated as a neutral NO molecule.¹²⁵² Transition metal complexes releasing NO have been reviewed by Ford^{77−79,98,107} Mascharak,⁶³ and Liu¹²⁵³ and their co-workers.

The nitrito complex trans-[Cr^III(cyclam)(ONO)₂]⁺321 (Scheme 76) was shown to release one equivalent of NO upon irradiation at 436 nm with Φ_r = 0.0092 in a degassed aqueous solution.¹²⁵⁴ Upon irradiation in aerated aqueous solutions, the complex releases NO with Φ_r = 0.25. This dichotomy was attributed to oxidation of the Cr^IV complex formed upon NO release by O₂ to give Cr^V species 322. The presence of glutathione increased the quantum yield of NO release to Φ_NO = 0.25 by reducing an intermediate Cr^IV complex to a Cr^III–OH complex.^1255,1256 Complex 321 was combined with anthracene or pyrene “antenna” ligands (323a and 323b, Figure 50) that harvest light and act as fluorescent reporters.^1255,1256

Scheme 76. Cr^III-Nitrito Complexes and Their Photochemistry¹²⁵⁴.

Figure 50.

Cr^III-Nitrito complexes substituted with antenna ligands.

Ford and co-workers recently developed a Cr^III nitrito complex 324 (Figure 51), which photoreleased NO upon irradiation at 451 nm¹²⁵⁷ and also upon irradiation at 800 nm when loaded onto polymer disks containing Nd-sensitized upconverting nanoparticles (see also section 6.4.2).

Figure 51.

Cr^III nitrito complex 324.

Fe^III and Fe^II nitrosyl complexes have long been known as photoNORMs. The most established Fe^II nitrosyl complex, sodium nitroprusside 325 (Scheme 77), was shown to photochemically release both CN^– and NO upon irradiation with 314–456 nm light.¹²⁵⁰ When irradiated in aqueous solution at >480 nm, NO was the sole photoproduct together with the oxidized Fe^III aqua complex as a side-product. NO release from nitrosyl complexes of electron-rich d-elements generally proceeds via electron transfer from the metal center to the NO⁺ ligand and subsequent release of the NO radical.

Scheme 77. Photochemistry of Sodium Nitroprusside¹²⁵⁰.

Roussin’s black salt 326 (Figure 52) and Roussin’s red salt 327 were also found to release NO (5.9 equiv for 326 and 4 equiv for 327) upon photolysis at wavelengths of 313–546 nm.¹¹⁴¹ The NO release quantum yield of 326 was Φ_r = 0.007, while that of 327 was one order of magnitude higher. Like Cr^III complexes 321, the NO release efficiency was increased in the presence of oxygen. Encapsulation of 326 in NIR-absorbing nanocarriers resulted in efficient NO photorelease upon 980 nm excitation.^1258,1259 Derivatives of 327 and 328 were used as photoNORMs to efficiently release NO in aerated solutions.^1260−1263 Protoporphyrin IX was used to sensitize NO release from compound 328 (R = CH₂CH₂OH) at 436 and 546 nm with quantum yields of Φ_r = 5.2 × 10^–4 and 2.5 × 10^–4, respectively.¹²⁶² Ford and co-workers showed that 328 (R = CH₂CH₂OH) could also be activated by attaching either sensitizing fluorescein derivatives absorbing at λ_irr = 400 (1PE) and 800 nm (2PE)¹²⁶⁰ or a benzothiazolyl-substituted fluorenyl two-photon antenna with a large 2P absorption cross section (σ_TPA = 246 GM).¹²⁶⁴ Patra, Mascharak, and co-workers prepared Fe^III complex 329 (Figure 52), which incorporates pentadentate carboxamide-containing ligands.^63,1265,1266 This complex releases NO upon irradiation with visible light at 500 nm with Φ_r = 0.19. Lee, Chiang, Tsai, and co-workers recently introduced novel Fe^II-based photoCORMs with pendant thiols or thioethers (330, R = H, CH₃).¹²⁶⁷ The S-methylated complex releases NO upon irradiation with visible light (λ_irr > 400 nm), but the free thiol in 330 (R = H) interacts with the departing NO, generating HNO as the main photoproduct.

Figure 52.

PhotoNORMs based on iron–sulfur clusters.

Complex 331, a Mn^II analog of complex 329 (Table 19; Figure 31), irreversibly released NO upon visible-light activation^1081,1268 and was used to construct NO-releasing polyurethane-coated sol-gel hybrid materials.¹²⁶⁹ Replacing one pyridyl ligand of 331 with a quinoline unit yielded complex 332, in which the absorption band is bathochromically shifted but photoactivity upon irradiation is retained at up to 810 nm.¹⁰⁸¹ Additionally, the absorption maxima of the related complexes 333 and 334 (Figure 31), which contain imine nitrogens trans to the NO ligand, are bathochromically shifted by ∼100 nm relative to their carboxamide counterparts 331 and 332.¹⁰⁸²

Table 19. Manganese(II) Multidentate Complexes Activatable in the Visible and NIR Region.

NORM	λmaxabs (nm)	εmax (M–1 cm–1)	λirr (nm)	Φr (λirr/nm)	solvent	ref
331	635	220	500, 550	0.326 (500)	CH3CN	(1081, 1268)
				0.309 (550)	CH3CN
				0.400 (500)	H2O
				0.385 (550)	H2O
332	650	450	500, 550	0.623 (500)	CH3CN	(1081)
				0.579 (550)	CH3CN
	670			0.742 (500)	H2O
				0.694 (550)	H2O
333	720	750	500, 550	0.41 (500)	CH3CN	(1082)
				0.58 (550)
334	785	1200	500, 550	0.39 (500)	CH3CN	(1082)
				0.43 (550)
335			460, 530, 650		MESa	(1270)
R = OCH3	457	4740		0.58 (460)
				0.47 (530)
				0.49 (650)
R = H	461	3120		0.61 (460)
				0.51 (530)
				0.47 (650)
R = Cl	475	6940		0.66 (460)
				0.66 (530)
				0.73 (650)
R = NO2	523	13.6 × 103		0.61 (460)
				0.63 (530)
				0.78 (650)

^aMES = a 2-(N-morpholino)ethanesulfonic acid-based buffer.

Hitomi and co-workers studied the effects of varying the electronic properties of the ligands and the irradiation wavelength on the NO photorelease quantum yields of substituted complexes 335.¹²⁷⁰ Electron-neutral and electron-donating groups gave the highest NO liberation efficiency at 460 nm, whereas electron-withdrawing groups provided the most efficient release at 650 nm.

Thanks to their robustness, thermal stability, and photoreactivity, Ru^II nitrosyl complexes have become established as useful photoNORMs.^{113,1271−1286} The applications of these complexes are quite broad and beyond the scope of this review. Several representative complexes of this type, namely the nitrosyl-substituted Ru^II trichloride complex 336,¹²⁷⁷trans-tetraamine Ru^II nitrosyl complex 337 substituted with N- or P-based ligands,¹²⁷⁶ and theporphyrin-based Ru^II nitrosyl complexes 338,¹²⁷⁸ are shown in Figure 53.

Figure 53.

Structures of some Ru^II-based photoNORMs, Sol = DMSO, CH₃CN.

Cyclam Ru^II complex 339, prepared by Tfouni and co-workers, releases NO only upon irradiation with near-UV light (Φ_r = 0.14 at 355 nm).¹²⁸⁷ Salen complexes 340 bearing π-extended ligands were extensively studied as potential photoNORMs because of their visible-light absorption.^{1275,1283,1288} Upon irradiation at λ_irr = 546 nm, 340 (in which X = Cl^–) released NO more efficiently (Φ_r = 0.07) than complexes with other X ligands (ONO^– or H₂O).^1275,1283

Mascharak and co-workers developed a series of Ru^II complexes 341 bearing tetradentate ligands.^{1272,1289,1290} A systematic study of complexes in this series with π-extended ligands revealed factors important for release in the visible region.¹²⁷² For example, 341 (R = OMe, X = Cl^–) releases NO with Φ_r = 0.01 at 500 nm (λ_max^abs = 420 nm), while its π-extended quinoline analogue (341, R = OMe, X = Cl^–, quinoline ligand) is photolyzed more efficiently (Φ_r = 0.025 at 500 nm, λ_max = 490 nm).¹²⁸⁹ Ru^II nitrosyl complex 342 containing a tridentate N-(pyridin-2-ylmethylene)quinolin-8-amine ligand released NO upon irradiation with both 365 nm UV light and visible light with Φ_r = 0.004 (at 365 nm) in acetonitrile.¹²⁹¹ Similarly, Ru^II complexes 343 and 344 (Figure 54) released NO upon irradiation at 355 nm in water (Φ_r = 0.12 and 0.20, respectively) and at 410 nm in acetonitrile (Φ_r = 0.05 and 0.17, respectively).¹²⁹²

Figure 54.

Structures of Ru^II NO complexes with pentadentate ligands.

Malfant and co-workers investigated the mechanism of NO photorelease in Ru^II nitrosyl complexes with terpyridyl ligands bearing substituents having different electron-donating abilities (345a–345c; Table 20),^1293,1294 showing that low-lying electronic transitions that drive NO release exhibit strong charge-transfer interactions with the nitrosyl moiety. Upon excitation, the nitrosyl MLCT state is reduced to form a free NO radical and an oxidized Ru^III metal complex. Additionally, the 9-dibutyl-9H-fluoren-2-yl substituted Ru^II terpyridine complexes 346 (Table 20) released NO upon irradiation with blue light.¹²⁹⁵ The fluorenyl substituents of these complexes make them excellent chromophores, with a 2-photon absorption cross section of σ_TPA = (156 ± 23) GM. Substitution of the 9-dihexyl-9H-fluoren-2-yl groups in 347a with N-ethylcarbazol-3-yl ligands (as in 347b) strengthened the bathochromic shift of the charge-transfer transitions toward the electron-withdrawing Ru-NO fragment, resulting in excellent 2-photon absorption (σ_TPA = (159 ± 22) GM) but reducing the rate of NO release.¹²⁹⁶ A similar group of complexes 348 (Table 20) bearing zero, one, two, or three 4′-(4-methoxyphenyl) electron-donating substituents was also investigated.¹²⁹⁷ The degree of intramolecular charge-transfer toward the strongly electron-withdrawing nitrosyl ligand increased with the number of methoxyphenyl substituents. However, irradiation of these complexes in the charge-transfer absorption band revealed only minor differences in the quantum yield of NO release, indicating that the CT band is not the sole determinant of NO release efficiency and that other factors must be involved. Malfant and co-workers further extended the study of terpyridine Ru^II complexes by examining derivatives 349 (Table 20),¹²⁹⁸ which released NO upon green-light irradiation. Similar complexes were used by Liu and co-workers to create NO-releasing Ru^II nitrosyl-containing nanoplatforms bearing BODIPY (350)¹²⁹⁹ or naphthalimide (351) ligands.¹³⁰⁰ Maji and co-workers recently synthesized two nitrosyl complexes 352, which can be classified as {RuNO}⁶ and {RuNO}⁷ complexes using Enemark-Feltham notation.¹³⁰¹ Irradiation with visible light caused NO release from both complexes, but the {RuNO}⁷ complex (352²⁺) was more active. This was attributed to the more efficient formation of the MLCT state in the {RuNO}⁷ complex, which contains a Ru^II–NO• fragment, than in the {RuNO}⁶ complex containing a Ru^II–NO⁺ fragment.

Table 20. Structures and Properties of Ru^II PhotoNORMs with Terpyridine Ligands^a.

NORM	λmaxabs (nm)	εmax (M–1 cm–1)	λirr (nm)	Φr (λirr/nm)	solvent	ref
345a – trans			365		CH3CN	(1294)
R = NO2	357	9900		0.05
R = H	350	18 000		0.12
R = Br	354	22 900		0.11
R = OCH3	387	18 500		0.07
345b – cis			365		CH3CN	(1294)
R = NO2	352	6700		0.24
R = H	330	17 400		0.39
R = Br	340	23 600		0.32
R = OCH3	366	15 600		0.28
345c	420	12.4 × 103	365, 436	0.08 (365)	CH3CN	(1293)
				0.03 (436)
346			400, 405		CH3CN	(1295)
R1 = Fl, R2 = H	455	16 700		0.06 (400)
R1 = H, R2 = Fl	362	39 400		0.033 (400)
347a	453	16 700	405, 436	0.06 (405)	CH3CN	(1296)
				0.03 (436)
347b	517	14 600	436	0.01	CH3CN	(1296)
348			365, 436		CH3CN	(1297)
R1 = H, R2 = H	352	n.d.		0.086 (365)
R1 = Ar, R2 = H	425	n.d.		0.011 (436)
R1 = H, R2 = Ar	360	33 000		0.024 (365)
R1 = Ar, R2 = Ar	365	39 000		0.002 (436)
	421	n.d.b
349 – trans			365, 546		CH3CN	(1298)
R = NEt2	550	20 200		0.09 (365)
				0.01 (546)
R = (NO)NEt2	497	3200		0.13 (365)
349 – cis			365, 546		CH3CN	(1298)
R = NEt2	516	17 200		0.12 (365)
				0.045 (546)
350	548	n.d.	>400, 470, 530, 672	0.034 (470)	H2O	(1299)
				0.083 (530)
				0.017 (627)
351	519	n.d.	808	0.017	salinec	(1300)
352			–d	n.d.	CH3CN	(1301)
{RuNO}6	298	34 500		(74 min)e
{RuNO}7	479	15 400		(17 min)e

^aFl = 9,9′-dibutyl-9H-fluoren-2-yl, Ar = 4′-(4-methoxyphenyl).

^bA shoulder in the absorption spectrum.

^cNaCl aqueous solution (c = 150 mM).

^dUnspecified wavelength, Xe light source.

^eHalf-life of the NO release for 352.

Slep and co-workers studied complexes 353–355 (Figure 55), which release NO upon visible-light irradiation (λ_irr = 455 nm).¹³⁰² Their quantum yields of NO release span 3 orders of magnitude, ranging from Φ_r = 0.06 × 10^–3 for 353, to Φ_r = 1.63 × 10^–3 for 355, and Φ_r = 0.04 for 354. DFT analysis revealed that the presence of a second Ru^II center increases the molar absorption coefficient but does not necessarily influence the electronic distribution of the excited state responsible for NO release. Nikolaou and co-workers developed a ruthenium-based trinuclear complex [Ru₃O(CH₃COO)₆(4-pic)₂(NO)]PF₆ ((4-pic) = 4-methylpyridine) that releases NO upon irradiation at λ_irr = 532 nm.¹³⁰³

Figure 55.

Ru^II-based dinuclear photoNORMs.

Cho and co-workers recently synthesized a Co^III-nitrosyl complex 356 (Figure 56) that efficiently released NO upon white-light irradiation (λ_irr = 385–740 nm; Φ_r = 0.78)¹³⁰⁴ and was used in a real-time simulation of cell signaling to study extracellular signal-regulated kinases.

Figure 56.

Co^III-nitrosyl photoNORM.

4.2.3. Sensitized Release of NO from Metal Nitrosyl Complexes

Transition metal nitrosyl complexes are often efficient photoNORMs that can be activated by visible light. However, their absorption bands have often lower molar absorption coefficients than common organic dyes (ε ≈ 10³ M^–1 cm^–1)¹⁰⁸² and absorption maxima in the 400–500 nm region, which is unsuitable for deep tissue irradiation. Unfortunately, modifications that bathochromically shift their absorption into the 600–800 nm region often render these complexes unstable to hydrolysis (i.e., NO and ligand solvolysis).⁹⁸ Many alternative strategies have therefore been developed to shift their absorption maxima into the red and infrared regions while maintaining good dark stability and enhancing their molar absorption coefficients. These include (i) conjugation of photoNORMs with antenna moieties,^{1081,1255,1261,1270,1272,1305,1306} (ii) multiphoton excitation of photoNORMs,^{78,79,1243,1260,1305} (iii) the use of semiconductor quantum dots (see also section 6.4.1),^79,1305,1307 and (iv) combining photoNORMs with upconverting nanoparticles (see also section 6.4.2).^{1258,1259,1305}

Chromium(III) nitrito complexes 323a and 323b (Figure 50) that release NO upon intramolecular sensitization by pyrene or anthracene antennae are representative implementations of the first strategy.¹²⁵⁵ These complexes typically become fluorescent after releasing NO, enabling the reaction to be monitored. Roussin’s red salt derivatives 328 (R = CH₂CH₂OH, Figure 52) bearing protoporphyrin IX as a sensitizer are another notable implementation.¹²⁶² Ru^II complexes 357 (Figure 57), which have tetradentate quinoline-based ligands containing various antennae (X = O, resorufin; X = S, thionol; X = Se, selenophore) were developed by Mascharak and co-workers.^{1289,1308,1309} The attachment of the dye antenna introduces an absorption band (ε ≈ 28 000 M^–1 cm^–1) in the visible region (500–550 nm), and sensitized NO release occurs with Φ_r = 0.1–0.2. The derivative 357 (X = Se) was shown to be photoactive even at λ_irr = 600 nm with Φ_r = 0.04.¹³⁰⁹ Fluorescein- and dansyl-substituted analogs 358(1310) and 359(1311) also efficiently released NO (358: Φ_r = 0.306 ± 0.01 at 500 nm; 359: Φ_r = 0.08 at 400 nm) upon irradiation with visible light. Complex 358 has an internal fluorescence turn-on indicator of NO release because release is accompanied by the photochemical formation of a highly emissive fluorescein methyl ester, while complex 359 acts as a fluorescence turn-off indicator of NO release because it is converted into a non-emissive paramagnetic Ru^III-dansyl aqua complex upon irradiation.^1310,1311 Schiller and co-workers reported a combined spectroscopic-theoretical investigation of Ru^II complex 360,¹³¹² which has a tetradentate ligand and releases NO upon irradiation at 475 nm.

Figure 57.

Ru^II photoNORMs with antennae.

Multiphoton excitation is another approach for NIR activation of NO release.^{1258,1260,1263,1264} The fluorescein-conjugated iron–sulfur cluster 361 (Figure 58) releases 4 equiv of NO upon both 2P- (λ_irr = 800 nm) and 1P- (λ_irr = 436 nm; Φ_NO = 0.014, calculated per NO molecule) excitation.¹³¹³ Similar complexes were used to deliver NO to cells and tissues.^1258,1263

Figure 58.

PhotoNORM based on an iron–sulfur cluster sensitized with fluorescein.

Semiconductor quantum dots (see also section 6.4.1) and related nanoparticles can also be used to induce NO release upon irradiation with red or NIR light.^{1258,1259,1314−1319} This approach is exemplified by the photosensitized release of NO from Cr^III nitrito complex 321 (Scheme 76) with CdSe(ZnS) core/shell quantum dots upon irradiation with 450 nm light.^1307,1320 Tan and co-workers used a similar approach to design Mn^II-doped ZnS quantum dots that were encapsulated in the polysaccharide chitosan and conjugated to Roussin’s black salt 326 (Figure 52).^1321,1322 NIR excitation (λ_irr = 1160 nm) of this system caused 2P-induced photoluminescence at λ_max^em = 589 nm. The emitted photons were then absorbed by 326, inducing NO release.

A fourth way of inducing NO release with long-wavelength light is to use upconverting nanoparticles (see also section 6.4.2)^{1258,1323−1325} that are excited via sequential absorption of 2 or more NIR photons and then emit upconverted blue-shifted light that is absorbed by attached photoNORMs. Nanoparticles of this type have been used in combination with Roussin’s black salt 326(1258,1259) and chromium(III) nitrito complex 321.¹³²⁶

4.2.4. NO-Photoreleasing Materials

NO-releasing materials (see also section 6.4) have attracted considerable research interest because of their potential to offer lower toxicity and better solubility and photoreactivity than molecular photoNORMs. This field has recently been thoroughly reviewed,^{98,104,106,107,803} and a comprehensive discussion would be beyond the scope of this review. In general, NO-releasing materials contain NO donors attached to an inert carrier, which increases water solubility and may influence many (bio)physical properties including in vivo stability, biodistribution, and pharmacokinetics. NO donors are often coupled with a visible-light absorbing sensitizer or a NIR-absorbing upconverting species to improve photorelease.¹⁰⁴ The carriers are often biocompatible polymers,¹²⁵⁷ and the NO donors may either be present in a mixture or covalently attached.¹³²⁷ Polymeric gels are another common tool for delivering NO into organisms.^{1081,1288,1309,1328−1330} For example, a Mn^II nitrosyl complex-based sol-gel was demonstrated to release NO upon irradiation at 780 nm.¹⁰⁸¹ Additionally, self-assembled NO-releasing amphiphiles based on N-nitrosamine moieties have been used to achieve NO delivery with polymersomes.¹³³¹ Another common strategy is to combine polymers with NIR-active nanoparticles^1322,1332 and upconverting nanoparticles.^{1258,1323,1325,1333} Finally, Furukawa and co-workers introduced an alternative approach for NO release using a zeolitic imidazole framework with nitroimidazole ligands.¹³³⁴

4.3. Release of Hydrogen Sulfide and Sulfur-Based Small Molecules

Hydrogen sulfide is a gasotransmitter produced endogenously from cysteine and homocysteine.¹³³⁵ It acts as a signaling agent involved in antioxidative, antiinflammatory, vasorelaxant, and cytoprotective processes,^1336−1338 and there have been several efforts to develop methods for its controlled release. Various H₂S-liberating systems activatable by pH, the presence of thiols, redox processes, and light, have been designed.^{826,828,1339,1340} A complementary approach for H₂S release is to exploit photothermal effect using NIR light.^1341,1342 Developments in this field have been summarized in several reviews.^110−112

The first H₂S-releasing systems activatable by UV light were reported only recently.¹³⁴³ The o-nitrobenzyl caged geminal dithiol 362 (Figure 59) was prepared by the TiCl₄-catalyzed condensation of the corresponding thiol with acetone. Upon irradiation of this compound at 365 nm in the presence of water, the free gem-dithiol is released and then hydrolyzed to liberate H₂S.¹³⁴⁴ The ketoprofenate-based donor 363 also releases H₂S with simultaneous decarboxylation upon irradiation at 300–350 nm,¹³⁴⁵ while its xanthone analog 364 liberates H₂S under UVA irradiation (325–385 nm).¹³⁴⁶ Other H₂S-releasing systems are based on the Norrish type II hydrogen abstraction-induced photoproduction (λ_irr = 365 nm) of thiobenzaldehydes¹³⁴⁷ from compounds such as 365. The thiobenzaldehydes formed in this way release H₂S in the presence of amines (Scheme 78).¹³⁴⁸ Another successful system was prepared by encapsulating the hydrosulfide-containing leuco-form of malachite green 366 into vesicles that released H₂S upon irradiation with UV light (Φ_r = 0.01 at 365 nm; Φ_r = 0.22 at 254 nm).¹³⁴⁹ Interesting results were also achieved with the meta-effect-based H₂S photodonor 367, which bears water-solubilizing substituents (Φ_r = 0.14 at 365 nm).⁵⁷³ Compound 368 is an analog of 362 that also releases H₂S upon irradiation with UV light. It was also used in combination with upconverting nanoparticles based on LiYF₄:Yb/Tm coated with polyethylene glycol-octadecylamine, which convert NIR excitation (λ_irr = 980 nm) into UV photoemission (λ^em = 365 nm) to trigger H₂S liberation.¹³⁵⁰ Finally, compounds 369 release biologically active persulfides upon irradiation at 365 nm with Φ_r = 0.07 for 369 (R = H) and Φ_r = 0.36 for 369 (R = CH₃).¹³⁵¹

Figure 59.

H₂S-releasing systems.

Scheme 78. UV-Absorbing H₂S Photodonors.

Chakrapani and co-workers were the first to develop an H₂S photodonor activatable by direct excitation with visible light.⁸²³ Their BODIPY-based molecule 370 undergoes photoinduced B–O bond cleavage (section 2.12) to release a thiocarbamate-substituted phenolate upon irradiation with 470 nm light. Subsequent thermal self-immolation of this phenolate (k_immol = 0.02 min^–1) then liberates carbonyl sulfide (COS), which is transformed to H₂S (k_hydrol = 1.82 s^–1) in the presence of carbonic anhydrase, an omnipresent enzyme that catalyzes the hydration of carbon dioxide and the dehydration of bicarbonate.¹³⁵² The H₂S yield was 30–40% and its formation was tracked in vitro by monitoring the fluorescence enhancement due to the highly emissive photoproduct.

Štacko, Klán, and co-workers developed H₂S-releasing molecules 371–373 (Figure 60; 371: Figure 30) based on a BODIPY PPG (section 2.12).⁷⁹⁹ Upon photochemical excitation of the BODIPY core, the thiocarbamate leaving group installed at its meso-methylene position dissociates, leading to the release of COS, which is then converted into H₂S using carbonic anhydrase. Unlike 371, the polyethylene glycol-substituted analog 372 is water-soluble and efficiently releases COS together with Ph₂NH (λ_max^abs = 513 nm in degassed aq. PBS, Φ_r = 15.1 × 10^–2 at 365 nm; yield ≈ 86%). The π-extended derivative 373 has a bathochromically shifted absorption band and photoreleases H₂S upon irradiation at 700 nm (λ_max = 688 nm in degassed aq. PBS, Φ_r = 9.7 × 10^–2 at 365 nm, yield 69%). Oxygen quenches the productive triplet state, but H₂S release can proceed through both the singlet and triplet states.⁷⁹⁸ The thiocarbamates are synthesized by the reaction of thiols with a suitable carbamoyl donor (4-nitrophenyl carbamate⁸²³ or carbamoyl chloride,⁷⁹⁹Figure 60). The strategy of thiocarbamate caging and COS release was originally conceived by Pluth and co-workers and implemented in the form of compound 374, which bears an o-nitrobenzyl PPG (section 2.1.1) and absorbs below 400 nm.¹³⁵³

Figure 60.

H₂S photoreleasing molecules.

Singh and co-workers developed tetraphenylethylene-conjugated p-hydroxyphenacyl H₂S donors 375 (Scheme 79), which aggregate in aqueous media to form visible-light activatable (λ_irr >410 nm) nanoparticles that exhibit both aggregation-induced emission (AIE) and excited-state intramolecular proton transfer (ESIPT).¹³⁵⁴ These nanoparticles material offer efficient H₂S release (Φ_r = 0.18) that can be monitored in real time due to a fluorescence color change (λ_em = 549 nm for the starting material and 486 nm for the photoproduct).

Scheme 79. Tetraphenylethylene-Conjugated p-Hydroxyphenacyl-Based H₂S Donor.

Singh and co-workers also reported H₂S photorelease from the benzo[d]thiazol-2-yl-substituted p-hydroxyphenacyl compound 376 (Figure 61) upon irradiation with visible light (λ_irr > 410 nm).¹³⁵⁵ The closely related derivative 377 liberated hydrogen persulfide (H₂S₂) under similar conditions,¹³⁵⁶ while sulfide dimers analogous to 376 photoreleased H₂S when formulated as organic nanoparticles.⁶³³

Figure 61.

p-Hydroxyphenacyl-based H₂S photodonors.

Another H₂S releasing system developed by Singh and co-workers is the visible light-responsive (λ_irr > 410 nm) nanocarrier system 378, which is based on a quinoline derivative (Figure 62) attached to a fluorescent carbon dot (see also section 6.4).⁵⁸⁷ The system fluoresces in the visible region (λ^em = 425 nm, Φ_F = 0.078) and releases H₂S with a quantum yield of Φ_r = 0.09.

Figure 62.

Fluorescent carbon dot-based H₂S photodonor (CD = carbon dot).

The group of You used a hybrid approach to develop Pluronic F-127-based vesicles containing a photosensitizer that generates singlet oxygen upon irradiation with visible light.¹³⁵⁷ Two such photosensitizers were tested, as shown in Scheme 80: Pt^II octaethylporphine 379 (Φ_r = 0.30) and [Ir^III bis(2-(3-methoxyphenyl)pyridinate)(1,10-phenanthroline)]PF₆380 (Φ_Δ = 0.41). The singlet oxygen generated by these complexes upon irradiation (λ_irr = 500–550 nm for 379 and 380–500 nm for 380) reacts with 1,3-diphenylisobenzothiophene 381 to form an endoperoxide, which then undergoes thermal decomposition to release H₂S (Φ_r = ∼2× 10^–3).

Scheme 80. Sensitizer-Based System for Photorelease of H₂S by Visible Light.

Visible light-induced H₂S release can also be achieved using organometallic complexes such as 382 and 383 (Figures 63 and 31), as demonstrated by Wilson and co-workers. These Ru^II terpyridyl complexes have low energy metal-to-ligand charge transfer (MLCT) absorption bands in the red region (λ_max^abs = 581 nm for 382 and 570 nm for 383).¹⁰⁸³ Efficient ISC from ¹MLCT* leads to a dissociative triplet ligand-field excited state (³LF) that liberates the monodentate ligand phosphinodithioate with near-quantitative quantum yields (Φ_r = 0.85 for 382, Φ_r = 1.02 for 383 at 626 nm). The released phosphinodithioate acts as a thermal H₂S donor, undergoing hydrolytic decomposition to give two equivalents of H₂S.^1358,1359 Complexes 382 and 383 were used successfully in living cells both to protect H9c2 cardiomyoblasts and in an in vitro model of ischemia-reperfusion injury.

Figure 63.

Ru^II-based H₂S donors.

Carbon disulfide is another small gaseous molecule that was recently identified as an important bioregulatory and therapeutic agent¹³⁶⁰ and has thus become an interesting target for uncaging and triggered delivery. Ford and co-workers developed a photocatalytic method for CS₂ production from potassium 1,1-dithiooxalate 384 (Figure 64) by oxidative cleavage photosensitized by CdSe quantum dots (see also section 6.4.1).¹³⁶¹ This system releases CS₂ upon irradiation between 365 and 530 nm with Φ_r = 0.029–0.045. The mechanism of CS₂ release involves photoinduced two-electron oxidation of 384 to give CS₂ and CO₂.

Figure 64.

Carbon disulfide donor.

5. Photoacid and Photobase Generators

Photopolymerization processes use light to initiate polymerization, usually via radical reactions. Alternatively, polymerization may be triggered by an acid or a base formed by irradiation of a photoacid or photobase generator. This field has been covered by several recent reviews,^51,1362 so we discuss only a few particularly notable visible-light absorbing generators. Thiophene-containing oxime sulfonates 385 release sulfonic acids upon irradiation at 365–475 nm (Figure 65).¹³⁶³ The first step in the release mechanism was proposed to be the liberation of the corresponding sulfonyl radical via homolytic cleavage of the N–O bond. Also notable are the BODIPY-based donor–acceptor triarylsulfonium salt-based photoacid generator systems 386 and 387, which are photoactivated by green and red LED light, respectively, and were used to trigger cationic polymerization (Figure 65).¹³⁶⁴

Figure 65.

Photoacids 385–387.

Visible-light initiated polymerization in the presence of merocyanine-based photoacid 388 was demonstrated by Boyer and co-workers (Scheme 81).¹³⁶⁵ The proton dissociation was reversible, enabling temporal control of the process.

Scheme 81. Photoconversion of Merocyanine-Based Photoacid 388(1365).

Scheme 82 shows a rare example of a visible-light absorbing photobase generator. In the first case, benzothiophene imino derivative 389 releases an amine base in a two-stage photoprocess.¹³⁶⁶ The oxamic acid ester 390 then undergoes homolytic N–O bond cleavage, followed by decarboxylation and radical addition into the adjacent aryl ring. In another example, tetramethyl guanidine (a basic polymerization initiator) was liberated from a coumarinyl-4-methyl PPG (see also section 2.2) upon irradiation at 400–500 nm.³⁴⁶

Scheme 82. Visible-Light Absorbing Photobase Generator¹³⁶⁶.

6. Photosensitized Release: From Small Molecules to Nanoparticles and Nanomaterials

Photochemically induced uncaging using visible/NIR light can be achieved by various approaches. Direct release following one-photon (1P) absorption is the most desirable but is also rather challenging to achieve. The low energy of red and near-infrared photons is usually insufficient to initiate chemical processes; thus, a major goal when developing photoactivatable moieties is to identify feasible photochemical transformations. Many additional criteria may also need to be addressed; in particular, useful compounds must have suitable photochemical (good quantum yields and release rate constants, non-absorbing side-products), chemical (non-reactive side-products), and biological (non-toxicity of all species in the photoreaction pathway, and potentially water solubility) properties.¹⁰ The release of a leaving group, usually an anion or neutral species, can generally proceed directly from an excited state of different multiplicity (Scheme 83a) or a reactive ground-state intermediate formed from the excited chromophore (Scheme 83b). Near-infrared absorption is often related to molecular overtone and combination vibrations that are forbidden by the quantum-physical selection rules, so the corresponding molar absorptivities are usually small.¹³⁶⁷ Only dyes with extensive conjugated systems such as cyanines or squaraines¹³⁶⁸ exhibit intense electronic transitions in the NIR region. A potentially expensive solution is to induce absorption of two (2P) or more photons (multi-photon absorption) by a single molecule using a high-power femtosecond laser, which enables access to excited states with energies equal to the sum of the absorbed photon energies.^1369,1370 This method can thus be used to excite chromophores absorbing in the UV region with NIR or visible light, as discussed extensively in our previous review.¹⁰

Scheme 83. Direct Photorelease versus Photosensitized Release.

Another strategy for activating release with visible/NIR-light is to use two separate molecular components or bi-/multi-chromophoric systems, with one being a light-harvesting molecular or nanoscale sensitizer (see also section 6.1) that can transfer energy to^{10,1371−1373} (Scheme 83c) or exchange an electron with^10,1374,1375 (Scheme 83d; the excited sensitizer is either an electron donor or acceptor) a separate molecule or complex bearing the leaving group. The excited chromophore can also be the photoremovable moiety, as in the case of Schemes 83a and 83b; in such cases, electron transfer to or from an auxiliary ground-state electron acceptor or donor, respectively, is responsible for leaving group release and the advantage of the auxiliary light-harvesting system is lost (Scheme 83e).

Scheme 83f depicts an alternative strategy that relies on a photosensitizer acting via the photodynamic effect: it generates singlet oxygen or another reactive oxygen species (ROS) upon irradiation,^1376−1379 which then reacts with an oxidizable moiety bearing the leaving group.

6.1. Molecular Photosensitizers: Energy Transfer

Photoinduced energy transfer is a practical way to generate (usually) a triplet excited state, particularly when the desired state is not accessible by direct excitation or the molecule does not absorb sufficiently at the desired wavelength.^136,1371 An efficient triplet–triplet energy transfer should be exergonic to avoid reverse transfer, and the sensitizer should have a high molar absorption coefficient, undergo efficient ISC, and have a sufficiently long triplet lifetime. Intramolecular energy transfer via either through-space or through-bond mechanisms might be preferred because it avoids the bimolecular entropic restrictions associated with diffusion and its efficiency can be finely tuned by adjusting the inter-chromophore distance.^{10,1372,1373,1380,1381} Intramolecular energy transfer necessarily involves the use of an “equimolar” quantity of the sensitizer.

Several bichromophoric photoremovable protecting groups containing various UV-absorbing light-harvesting chromophores have been designed and studied by Corrie and co-workers over the past decade.^{842,1382−1385} Benzophenone, which has substantially higher molar absorption coefficients above 300 nm than the photoactivatable nitroindoline group, was found to act as a triplet sensitizer to promote the nitroindoline moiety in compound 391 into its triplet state and trigger the subsequent release of a carboxylic acid (Scheme 84).¹³⁸⁵

Scheme 84. Energy-Transfer-Mediated Release Involving the Nitroindoline Group¹³⁸⁵.

Steiner and co-workers used 9H-thioxanthen-9-one, which absorbs at slightly above 400 nm, to improve the light sensitivity of the weakly absorbing o-nitro-2-phenethyl PPG (see also section 2.1.2).^{171,252,253,1386} For example, compounds 392 consisting of two chromophores connected via flexible tethers of different lengths were tested in the photolithographic synthesis of high-density DNA chips (Scheme 85).²⁵³ It was found that in addition to triplet–triplet energy transfer, the singlet excited state of the sensitizer was important, especially in systems with short tethers. Similarly, the photocleavage of the 2-(2-nitrophenyl)propyl group was sensitized intramolecularly by 9H-thioxanthen-9-one in the triplet excited state to release a fluorescent rhodamine dye.¹³⁸⁷

Scheme 85. Energy-Transfer-Mediated Release Involving the o-Nitrobenzyl Group²⁵³.

The triplet excited state of 9H-thioxanthen-9-one was also shown to sensitize a linked benzothiophene-2-carboxanilide ring system (393, Scheme 86) via electrocyclic ring closure of the anilide moiety to liberate leaving groups including halides, thiolates, carboxylates, and phosphates (Φ_r = 0.14–0.41 at 395 nm).¹³⁸⁸

Scheme 86. Energy-Transfer-Mediated Release Involving the Benzothiophene-2-carboxanilide Group¹³⁸⁸.

Wang and co-workers recently demonstrated that intermolecular triplet–triplet energy transfer between a Pt^II tetraphenyltetrabenzoporphyrin sensitizer excited at 625 nm and a photoactivatable meso-methyl-substituted BODIPY derivative (394) (see also section 2.12) leads to the release of a carboxylate moiety (Scheme 87).^808,1389 The use of photosensitizers with a higher T₁ energy and a lower S₁ energy than that of the photocleavable group was recommended to enable exergonic energy transfer from a sensitizer excited at longer wavelengths.

Scheme 87. Energy-Transfer-Mediated Release Involving the BODIPY PPG⁸⁰⁸.

6.2. Molecular Sensitizers and Photocatalysts: Electron Transfer

The liberation of a leaving group can also be facilitated by (inter-/intramolecular) photoinduced electron transfer (PET), where the excited species is either the sensitizer or the substrate itself (Scheme 83d,e).^10,1374 For uncaging purposes, the sensitizer should have a high molar absorption coefficient and satisfy the other criteria mentioned in the previous section. If both reactants are neutral prior to the reaction, the resulting radical ion pair will undergo chemical transformations that eventually lead to leaving group release or recombination to restore the starting material. The Gibbs free energy of PET can be calculated from the corresponding redox potentials of both reactants and the excitation energy of the excited molecule.^{10,136,1390−1392}

UV-light-initiated PET-assisted uncaging was reviewed several years ago.^10,1374 Hamada’s pioneering photofragmentation of tosylamides in the presence of a reducing agent to give amines,⁸⁴⁰ and especially the work of Falvey and co-workers on photosensitized uncaging of phenacyl esters,^1393,1394 picolinium esters,^{1393,1395−1397} or 9-phenyl-9-tritylone¹³⁹⁸ were key studies in this area.

Falvey and co-workers also demonstrated that the sensitized release of carboxylic acids from phenacyl esters using a visible-light-absorbing electron donor (anthracen-2-amine; λ_irr > 400 nm) proceeds in near-quantitative chemical yield (Scheme 88).¹³⁹⁹ The sacrificial sensitizer can be regenerated in the presence of ascorbic acid by donating a hydrogen atom (or electron) to the aryloxy radical. Their experiments indicated that the phenacyl moiety interacts with the singlet excited state of the sensitizer.¹³⁹³ More recently, Speckmeier and Zeitler reported the catalytic deprotection of analogous phenacyl anddesyl (395) protecting groups using substoichiometric quantities of [Ru(bpy)₃](PF₆)₂ (1 mol %) as a photocatalyst excited at 455 nm and ascorbic acid (Asc–H) as a sacrificial electron donor (Scheme 89).¹⁴⁰⁰

Scheme 88. Sensitized Release of Carboxylic Acids from Phenacyl Esters¹³⁹⁹.

Scheme 89. Photocatalyzed Release of Carboxylic Acids¹⁴⁰⁰.

A similar strategy was used by Falvey’s group to release carboxylic acids, amino acids, and phosphates from N-alkylpicolinium, which has a favorable reduction potential of E_red = −1.1 V. Scheme 90 shows a bimolecular photodeprotection of a carboxylic acid using BODIPY and coumarin derivatives as photosensitizers absorbing at λ_max^abs ≈ 500 and 467 nm, respectively.¹⁴⁰¹ The PET-induced uncaging of carboxylic acids from an N-alkylpicolinium derivative by visible light was also demonstrated in the presence of substoichiometric amounts of tris(bipyridyl)ruthenium(II) (λ_max ≈ 450 nm) acting as both a sensitizer and a mediator of electron transfer between a good donor and the protecting group.¹⁴⁰² Ascorbic acid, N,N-dimethylaniline, or 1,4-diazabicyclo[2.2.2]octane served as sacrificial electron donors in this case. Fluorescence quenching and transient spectroscopy experiments showed that the reaction rate constants were near the diffusion limit. Analogous visible-light promoted reactions were performed with ketocoumarin derivatives (λ_max^abs ≈ 450 nm) as sensitizers/mediators,¹⁴⁰³N-methylpyridinium iodide esters that undergo charge-transfer excitation,¹⁴⁰⁴ and an anthraquinone-based chromophore covalently attached to an N-alkylpicolinium ester.¹⁴⁰⁵ Similarly, Boncella and co-workers used tris(bipyridyl)ruthenium(II) to mediate PET to N-methylpicolinium carbamates to release amines in very high chemical yields,¹⁴⁰⁶ Cui reported the release of N-alkyl substituted 4-picolinium ions conjugated with self-assembled monolayers via an ester group using [Ru(bpy)₃]²⁺ as a photocatalyst under irradiation at 452 nm,¹⁴⁰⁷ and Anderson, Flamigni, and co-workers showed that electron-accepting N-methylpyridinium, phenacyl, or p-nitrobenzoate moieties can be activated via intramolecular PET via two-photon absorption if covalently attached to electron-donating fluorene derivatives (λ_abs < 450 nm).¹⁴⁰⁸

Scheme 90. Photosensitized Release from N-Alkylpicolinium Ions¹⁴⁰¹.

A photoactivatable system based on a 9-phenyltritylone protecting group that releases alcohols upon irradiation at 447 nm in the presence of fac-(tris(2,2′-phenylpyridine))iridium(III) (fac-Ir(ppy)₃) or tris(bipyridine)ruthenium(II) chloride ([Ru(bpy)₃]²⁺) as photosensitizers and triethylamine as a sacrificial electron donor was reported by Falvey and co-workers.¹⁴⁰⁹ The authors proposed photodeprotection mechanisms involving both oxidative and reductive quenching scenarios (Scheme 91) corresponding to the general mechanism shown in Scheme 83d. In another case, these authors demonstrated the efficient release of calcium ions (Ca²⁺) from an EDTA complex facilitated by photolysis of riboflavin photocatalysts at λ_irr > 440 nm.¹⁴¹⁰

Scheme 91. Photocatalyzed Release of Alcohols¹⁴⁰⁹.

An interesting visible-light uncaging reaction using photocatalytic deboronative hydroxylation was recently reported by Chen and co-workers (one example is presented in Scheme 92.¹⁴¹¹ Phenol, alcohol, and amine derivatives were liberated from the corresponding boronates in high chemical yields in bacteria and mammalian cells by reaction with transient hydrogen peroxides generated in the presence of molecular oxygen using fluorescein or rhodamine derivatives as photocatalysts and ascorbate as a reductant. In a different approach, Winssinger and co-workers used an azide-reduction-triggered immolative linker¹⁴¹² to release rhodamine using a [Ru(bpy)₂ phen]²⁺ conjugate as a photocatalyst in the presence of ascorbate.¹⁴¹³

Scheme 92. Photocatalytic Deboronative Hydroxylation¹⁴¹¹.

6.3. Release via the Photodynamic Effect

A different way to release molecules of interest is to exploit the photodynamic effect, in which an excited photosensitizer and ground-state (triplet) oxygen (³O₂) react to produce reactive oxygen species (ROS) or radicals that then react with molecules in the vicinity. The most common ROS is singlet oxygen (¹O₂) produced from ³O₂ by the triplet–triplet annihilation mechanism using triplet-excited organic dyes such as porphyrins, phthalocyanines, cyanines, pyropheophorbide, rhodamine, methylene blue, or eosin, which typically absorb in the red or NIR regions.¹³⁶ The use of this phenomenon to induce cell death in medical applications is known as photodynamic therapy. Diverse chemical functionalities and entities can be cleaved by reaction with singlet oxygen (Type II photooxygenation¹³⁶) including olefins, vinyl ethers, vinyl disulfides, thioketals, and lipids.^1414,1415 Such singlet oxygen-sensitive groups can be inserted into tethers connecting drug molecules to structures such as membranes, nanomaterials, surfaces, or supramolecular carriers. The drug can then be liberated in the presence of a sensitizer, oxygen, and light.^1414−1417 The triplet-excited photosensitizer may also participate in electron exchange, that is, in Type I photooxygenation, as discussed in section 6.2.

In an early work, Anderson and Thompson demonstrated that singlet oxygen oxidation of liposome membranes by irradiating a membrane-incorporated sensitizing zinc phthalocyanine at 640 nm resulted in the release of encapsulated glucose.¹⁴¹⁸ The destruction of the membranes was attributed to a [2+2] cycloaddition reaction between ¹O₂ and membrane alkenyl groups to form dioxetanes that subsequently decompose into two aldehydes.¹⁴¹⁹ This mechanism was demonstrated to be responsible for the release of chlorin, which was used as a sensitizer (sens) that was covalently attached to silica via a tether containing a 1,2-diphenoxyethene unit (Scheme 93).¹⁴¹⁷ Many other studies have exploited the reaction between ¹O₂ and ethene derivatives such as vinyl ethers, bis(alkylthio)alkenes, or aminoacrylates for substrate liberation.^{955,956,1420−1437} The uncaging photooxidation of lipids or liposomes¹⁴³⁸ in the presence of organic sensitizers may also proceed via mechanisms involving singlet oxygen.^1439−1446

Scheme 93. Cleavage via the Photodynamic Effect (sens = Chlorin)¹⁴¹⁷.

The second example of uncaging via the photodynamic effect is the release of siRNA bearing a 9-anthracenyl group and a photosensitizer (pyropheophorbide or eosin Y derivatives) attached to the 3′-terminus of the lagging strand (Scheme 94).¹⁴⁴⁷ Upon irradiation at 650 nm, singlet oxygen is formed by sensitization and attacks the 9-anthracene moiety to form an endoperoxide intermediate, which is then detached as anthracene-9,10-dione to liberate the siRNA strand. Similarly, methylene blue and alkoxyanthracene were used as a photosensitizer and a cleavable group, respectively, to disrupt micelles loaded with a chemotherapeutic agent.¹⁴⁴⁸ Other singlet-oxygen sensitive groups including thioketals,^1449−1462 thioethers,¹⁴⁶³ imidazoles,¹⁴⁶⁴ indolizines,¹⁴⁶⁵ and hydrazones,¹⁴⁶⁶ as well as selenium-^1467−1477 and tellurium-containing^1478−1480 moieties have also been used for uncaging. A qualitatively different photosensitizer, TiO₂ nanotube-doped PbS quantum dots combined with S-nitrosocysteine, was found to generate singlet oxygen upon irradiation at <600 nm, leading to the release of nitric oxide (Scheme 94).¹⁴⁸¹

Scheme 94. Photodynamic Cleavage of a 9-Anthracenyl Group from siRNA¹⁴⁴⁷.

An interesting approach to fluorophore photoactivation was recently reported by Wensel, Xiao, and co-workers,¹⁴⁸² who replaced the carbonyl groups in common fluorophores with thiocarbonyl groups. This significantly reduced their fluorescence because of a photoinduced electron transfer-quenching mechanism. Upon irradiation, these compounds generate singlet oxygen, with which they then react to form their oxo derivatives, thereby restoring their original strong fluorescence (only one example is shown in Scheme 95).

Scheme 95. Fluorescence Switch via Self-Oxygenation¹⁴⁸².

Two additional very interesting uncaging strategies involving photosensitized singlet-oxygen-mediated self-destruction of the photosensitizer leading to the release of a desired species are the liberation of carbon monoxide from flavonols (see section 4.1.1) and the use of cyanine dyes to release various leaving groups upon irradiation with red and NIR light (see below).

Cyanine dyes are invaluable fluorophores in chemistry and biology.¹⁴⁸³ They feature odd-numbered methine bridges connecting two nitrogen-containing heterocycles,¹⁴⁸⁴ which are responsible for their unique photophysical properties.¹⁴⁸⁵ In particular, heptamethine dyes with seven-carbon bridges are widely used as fluorescent tags in biological studies^1486−1488 and as markers in medical diagnostic tests.^1489,1490 Heptamethine cyanines (Cy7) have narrow absorption bands with high molar absorption coefficients (ε_max = 0.5–2.5 × 10⁵ M^–1 cm^–1) in the red to near-infrared (NIR) parts of the spectrum (650–800 nm). The peak absorption of heptamethine cyanines lies within the “first optical window” of mammalian tissue,^125,1486 where light attenuation due to absorption and scattering is minimal, making them particularly suitable for in vivo applications. This section focuses on the use of cyanine chromophores, particularly Cy7, as photoreleasing systems. Several aspects of this topic have been addressed by various recent review articles and perspectives.^{20,21,50,878,890,1491−1494}

The photodegradation of heptamethine and other cyanines is a known phenomenon¹⁴⁹⁵ and has been shown to proceed via photooxidative cleavage of one or more heptamethine C=C bonds to form the corresponding carbonyl photoproducts.^1496−1506 The most common mechanism of Cy7 photooxygenation involves photosensitization of ambient (ground-state) molecular oxygen by the triplet excited state of Cy7 (³396*) to form singlet oxygen (¹O₂), followed by a [2+2] cycloaddition to form dioxetanes 397 that undergo thermal decomposition to form the carbonyl photoproducts (Scheme 96). Supportive evidence for this pathway include the findings that (1) triplet–triplet annihilation is exergonic^1507,1508 even though the quantum yields of ¹O₂ production (Φ_Δ) tend to be low (∼0.01–0.001);^1509,1510 (2) the extent of photooxygenation depends on the oxygen concentration;¹⁵⁰¹ (3) the reaction is suppressed in the presence of ¹O₂ quenchers^1499,1501 or traps;^{1497,1504,1511} (4) reaction rates are higher in deuterated solvents, which extend the lifetime of ¹O₂;¹⁵⁰⁴ and (5) dioxetane intermediates can be detected by mass spectrometry.^{1499,1512,1513} The generated singlet oxygen attacks the C2=C1′ or C2′=C3′ bonds to form the corresponding dioxetanes, which undergo thermal decomposition to give two carbonyl compounds.^{1499,1512−1514} For example, the paired carbonyl products 398 + 399 and 400 + 401 were formed in a ∼4:1 concentration ratio during the photooxidative cleavage of 396, accounting for 70% of the photodegradation chemical yield (Scheme 96).¹⁵¹³ Computational analysis (B3LYP/cc-pVTZ) of 396 suggested that the observed regioselectivity is determined by the energies of the dioxetane intermediates; it was found that only dioxetanes at the C2/C1′ and C2′/C3′ positions, which give rise to carbonyl compounds 398–401, are formed exergonically (ΔG = −2.8 and −0.7 kcal mol^–1, relative to 396),¹⁵¹³ in agreement with the experimental findings.^{1494,1499,1512,1513} Cyanine photobleaching may also involve other pathways,¹⁵⁰¹ such as photoinduced electron transfer from the Cy7 triplet state to oxygen to form O₂^–, which may subsequently generate hydroxyl radicals or other reactive oxygen species (ROS), together with a potentially reactive cyanine-radical cation.^{1497,1498,1505,1515} The photodegradation of cyanines has mainly been studied to identify factors affecting their stability^{1495,1502,1505,1516−1520} to improve their performance as fluorescent imaging agents,^1495,1521 although uses of their photodegradation, for example in sensing,^1522,1523 have also been reported.

Scheme 96. Proposed Mechanism of Cy7 Photodegradation¹⁵¹³.

Schnermann and co-workers pioneered the repurposing of heptamethine cyanines as photocages by developing two Cy7 scaffolds (402 and 403, Figure 66) that harness the regioselectivity of the photooxidative degradation process to drive either C–N bond cleavage^{1491,1512,1524,1525} (402) or a β-elimination reaction¹⁵¹⁴ (403), both ultimately leading to leaving group release. Both scaffolds contain a cyclohexenyl moiety attached at the C3′/C5′ positions of the heptamethine bridge, which was originally used to increase the rigidity (and hence the fluorescence quantum efficiency) of Cy7.^1526,1527

Figure 66.

General structures of Cy7 photocages undergoing photooxidative-cleavage and C4′–N bond hydrolysis (402) or β-elimination (403).

Cy7 derivatives 402 were synthesized from the corresponding cyanine dye featuring a chlorocyclohexenyl group.^{1491,1512,1524,1525} The chlorine atom of this group is conveniently displaced via an S_NR1 reaction under mild conditions,¹⁵²⁸ in this case using ethylenediamine as the nucleophile. The leaving groups, such as chloroformate or p-nitrophenylcarbonate, were introduced in the last step. Photoexcitation (λ_irr = 690 nm) of 404 (λ_max^abs = 676 nm, ε = 5.15 × 10⁴ M^–1 cm^–1) resulted in the formation of products 405 and 406 in a ∼4:1 ratio (Scheme 97), as also observed for Cy7 396. The Cy7 derivative 404 is less prone to the hydrolytic release of the tertiary amine than the photooxidative cleavage products 405 and 406, which was attributed to its more extensive π-conjugation, which reduces the electrophilicity of the key C4′–N bond (i.e., weakens its iminium character). Electrophilic reactivity at the C4′ position of heptamethine cyanines has previously been documented.^1529−1531 The inertness of the resulting aldehydes to both light and reactive oxygen species further supports the assumption that hydrolysis is the main pathway of amine release.¹⁵¹² The direct hydrolytic release of an aniline derivative (7-aminocoumarin) bound to the C4′ position was inefficient (with a chemical yield of <8%) despite rapid photooxidation of the corresponding heptamethine cyanine.¹⁵¹⁴ It was therefore proposed that hydrolysis requires the prior protonation of the amine, and that the difference in the efficiency of hydrolysis between the tertiary amine and the aniline is due to their different basicities.¹⁵¹⁴ Hydrolysis of the C4′–N bond is followed by intramolecular cyclization of the ethylenediamine linker,^1532,1533 resulting in the release of an alcohol as a leaving group.^{1512,1524,1525,1534} Although the cyclization step is pH-dependent¹⁵³³ (proceeding more slowly at low pH), leaving group release efficiency was only reduced by a factor of 1.5 upon lowering the pH from 7.4 to 5.0.¹⁵²⁵ The overall chemical yield of uncaging (66–70%) correlated with the quantities of 405 and 406 formed during the reaction, suggesting that the light-independent steps are efficient.¹⁵¹² The kinetics of leaving group release appear to depend mainly on the rate of C4′–N bond hydrolysis.¹⁵¹²

Scheme 97. Mechanism of Uncaging from Cy7 Photocages 404(1512).

The effects of structural variation of 404 on its spectroscopic and photochemical properties were also explored (Figure 67).¹⁵²⁴ Replacing the butyl sulfonic acid substituents on the indolenine nitrogens with n-propyl (407) or n-butyl pentanoates (408) did not affect the compound’s spectroscopic and photochemical properties but significantly improved cellular penetration.^1491,1524 Replacing the N,N′-dimethylethylenediamine linker with N,N′-diethylethylenediamine (409) caused a 40 nm bathochromic shift of the absorption band, reduced the background (dark) hydrolysis rate (k_rel = 0.73), and increased the photooxidation rate (k_rel = 2.8) under the experimental conditions, although overall uncaging efficiency was reduced (k_rel = 0.81).¹⁵²⁴ Efforts to introduce more sterically demanding amines were hampered by the substantially lower reactivity of such amines in the chlorine substitution reaction.¹⁵²⁴ The lower background hydrolysis rate of 409 was attributed to increased steric hindrance either around the amine-heptamethine bond or the carbamate group.¹⁵³² The introduction of sulfonates on the indolenine rings (410), which is often done to prevent aggregation,¹⁵³⁵ reduced photooxidation efficiency (k_rel = 0.43) and increased the rate of background hydrolysis (k_rel = 1.3) but also improved the kinetics of release (k_rel = 4.2).¹⁵²⁴ An alkyne group allowing the photocage to be connected to targeting molecules using click chemistry was introduced by using a branched carbamate linker (411).¹⁵²⁵ Replacing one (412a) or both (412b) flanking heterocycles with benzothiazole rings significantly improved oxidation efficiencies (k_rel = 3.7 and 6.7, respectively), in accordance with earlier studies on Cy7 fluorophores.¹⁵⁰⁴ However, this also dramatically increased the background hydrolysis rate (k_rel = 4.7 and 8.5, respectively).¹⁵²⁴ Replacing the central cyclohexenyl ring with a cyclopentenyl moiety (413) also increased the photooxidation rate (k_rel = 3.5) but significantly reduced the uncaging rate (k_rel = 0.013) and the overall uncaging chemical yield (<20%). The reason for the decreased uncaging efficiency was determined to be inefficient hydrolysis of the carbonyl intermediates.¹⁵²⁴ On the other hand, replacing the sulfonated indolenine rings with sulfonated benzindolenines and installing an alkoxy substituent on the cyclohexyl group (414a and 414b) yielded PPGs with properties comparable to those of the original 404 but with a significantly red-shifted absorption spectrum (λ_max^abs = 690 and 732 nm, respectively).¹⁵²⁴ The individual contributions of each of these modifications have not yet been determined. In principle, the flexibility of the synthesis^1536−1538 and post-synthetic functionalization^{1494,1531,1539,1540,1510} of cyanines enables useful structural modifications of the bridge or terminal heterocycles, providing considerable scope for modulation of their spectroscopic and photoreaction properties.

Figure 67.

Structures of Cy7 photocage derivatives.

An analogous photooxidative cleavage mechanism was used by Schnermann and co-workers in the case of 403 (Figure 66) to drive the release of a leaving group through a β-elimination reaction.¹⁵¹⁴ Several examples utilizing similar photooxygenation/β-elimination sequences to drive leaving group release have been reported.^{187,1541−1545} Cy 7 photocages similar to 402 have mainly been used to release of phenols, while β-elimination of a carbamic acid functionality in 403 followed by spontaneous decarboxylation¹⁵⁴⁶ was used to release a free amine. Photoexcitation (λ_irr = 780 nm) of 415 (λ_max^abs = 781 nm, ε_max = 3 × 10⁵ M^–1 cm^–1, LG = coumarin 151) proceeded rapidly in PBS buffer to form two carbonyl intermediates, 416 and 417 (Scheme 98).¹⁵¹⁴ Only 417 underwent efficient β-elimination, however. The formation ratio of 416 and 417 (∼4:1; 70% chemical yield) explains the relatively low overall uncaging yield of this process (∼14%).¹⁵¹⁴

Scheme 98. Mechanism of Uncaging from Cy7 PPGs via a Photooxidative Cleavage/β-Elimination Sequence¹⁵¹⁴.

The photooxidative cleavage/β-elimination sequence was also applied to merocyanines such as 418 and 419 (λ_max = 664 and 713 nm, respectively).¹⁵¹⁴ It was previously shown that oxidative cleavage takes place preferentially in the position adjacent to the more electron-rich heterocycle in unsymmetrical merocyanines.^1505,1511 Accordingly, irradiation of 418 and 419 (λ_irr ≈ 660 and 690 nm, respectively, in PBS buffer, pH 7.4) resulted in the release of coumarin 151 with chemical yields of 33% and 22%, respectively¹⁵¹⁴ (Scheme 99). The difference in yield was attributed to differences in the extent of dye aggregation in solution.¹⁵¹⁴ For both compounds, additional release (4–5%) was observed after irradiation was stopped, suggesting that β-elimination is the rate-limiting step.¹⁵¹⁴

Scheme 99. Photochemistry of Merocyanine Photocages 418 and 419 Involving a Photooxidative Cleavage/β-Elimination Sequence¹⁵¹⁴.

The estrogen receptor antagonist/agonist 4-hydroxycyclofen was caged with a Cy7 derivative (420) and its light-induced release was used to regulate gene expression in cell cultures (λ_irr = 690 nm; Scheme 100).^1491,1512 The reactive oxygen species generated during the photodecomposition process and the potentially reactive carbonyl photooxidation products were both well-tolerated in the studied cell cultures. Compound 420 also enabled light-mediated regulation of gene expression under similar irradiation conditions (λ_irr = 690 nm) in a CreER/LoxP system in transgenic mouse embryonic fibroblasts (MEFs).¹⁵¹² Exchanging the butyl sulfonic acids on the indolenine nitrogens with n-butyl pentanoates (421) significantly improved cellular penetration and increased spatial control over photoactivation by causing intracellular entrapment of the photocage.¹⁴⁹¹ A prolonged irradiation time was required in experiments using this compound,^1491,1512 which was a limitation in applications requiring efficient substrate release. Fluorescence imaging of 420 in MEF and HeLa cells revealed that its intracellular distribution displayed a distinct punctate pattern and that it co-localized with LysoTracker staining,¹⁵¹² whereas 421 co-localized with MitoTracker.¹⁴⁹¹ This difference in subcellular localization was attributed to the different charges of the two compounds. The cellular uptake mechanism was not determined, but other non-sulfonated heptamethine cyanines were shown to be captured by cells via endosomal uptake.^1547−1549

Scheme 100. Photouncaging of Cyclofen Derivatives from Cyanine Photocages¹⁵¹².

Cy7 PPGs have also been used for antibody-targeted drug-release.^{1534,1550,1551} The two strategies discussed above were used to non-specifically conjugate an NHS ester, the caged combretastatin A4¹⁵²⁵ (CA4, a microtubule polymerization inhibitor) derivative 422, and the caged duocarmycin^1524,1552 (a DNA alkylating agent) derivative 423 to panitumumab (Pan), a clinically used anti-human epidermal growth factor receptor (EGFR) monoclonal antibody (Figure 68). The latter conjugate was injected in vivo into mice bearing MDA-MB-468 EGFR+ tumor xenografts, and the tumor area was irradiated at 690 nm 4 days after its administration. This single-dose treatment was sufficient to significantly reduce tumor size and improve overall survival compared to control groups.¹⁵²⁴ A combination therapy using 423 and Pan-IR700⁹⁴⁷ (a near-IR photodynamic therapy agent) exhibited greater treatment efficacy than either therapeutic agent alone.¹⁵⁵² The location of this antibody-drug conjugate was verified prior to its photoactivation by exploiting the fluorescence of the heptamethine cyanine photocage.^{1524,1525,1552}

Figure 68.

Structures of CA4 and duocarmycin caged with Pan-targeted heptamethine cyanine photocages.

6.4. Photosensitization by Nanoparticles and Nanomaterials

Nanotechnology has found a remarkable array of applications in science and technology including biotechnology and biomedicine.¹⁵⁵³ Nanoparticles (NPs) and nanocarriers are frequently used for diagnostics, biosensing, photodynamic therapy, photothermal therapy, and targeted and controlled drug delivery/release. Developments in this field have been reviewed extensively in the past decade.^{8,14,15,114,129,130,1415,1438,1554−1585} Various materials can be used in the design of nanocarriers including metal NPs, semiconductor NPs, nanocarbons, virus- and bacteriophage-based NPs, microcapsules, and hydrogel-based systems.^{1554,1556,1586} NPs can be both carriers (transporters) or active participants in drug (species) delivery. Photoactivatable NP systems may feature direct covalent bonds to drug molecules, or drugs may be encapsulated via non-covalent interactions. These systems must be stable in the relevant environment until an external trigger is applied to induce release. Light-activated release mechanisms include photochemical bond cleavage, photoreduction, photooxidation, photochemically-induced hydrophobicity switching, photo-cross-linking, photoisomerization, and photothermal processes.¹⁵⁵⁴ The following paragraphs briefly review the fundamental principles of species release from NPs upon irradiation with visible/NIR light and present some notable examples of related NP systems. This review is not fully comprehensive because many more specific reviews already exist, as noted above.

6.4.1. Photosensitization by Quantum Dots

Quantum dots (QDs), nanoscale semiconductor particles with interesting optical and electronic properties, typically consist of binary compounds such as PbS, PbSe, or CdS,¹⁵⁸⁷ although carbon¹⁵⁸⁸ and silicon quantum dots¹⁵⁸⁹ have also been used for uncaging purposes. They usually exhibit broad absorption spectra and narrow emission peaks and have large two-photon absorption cross sections. Their irradiation generally causes electron excitation from the valence band to the conduction band, and the resulting electron and hole can interact to produce an exciton. Their photophysics can be controlled using appropriate ligands.^1590,1591 Photoinduced energy transfer (Förster resonance energy transfer, FRET) or electron exchange between an excited QD and a ligand that undergoes subsequent chemical change is another way of using QDs for photosensitization.^1590,1592 QDs can interact with both electron acceptors and donors upon excitation. It should be noted that QDs are considerably larger than molecular species.

The QDs can serve as antennas that sensitize the photoreaction. Ford and co-workers reported that NO (section 4.2) is generated from electrostatic assemblies of water-soluble CdSe/ZnS and CdSeS/ZnS QDs loaded with negatively charged dihydrolipoic acid surface ligands and the cationic complex trans-Cr^III(cyclam)(ONO)²⁺ (cyclam = 1,4,8,11-tetraazacyclotetradecane) upon irradiation with visible light (Scheme 101).¹⁵⁹³

Scheme 101. QD-Mediated Photorelease¹⁵⁹³.

Carbon quantum dots (CQDs) have high photostability and large two-photon cross sections.¹⁵⁸⁸ CQDs covalently linked to a nitroaniline derivative as a NO photodonor were shown to release NO (section 4.2) upon one-photon (<450 nm) or two-photon (800 nm) absorption via anenergy-transfer mechanism.¹⁵⁹⁴ Another example of drug delivery from a CQD using a quinoline-based phototrigger was reported by Ghosh, Singh, and co-workers.¹⁵⁹⁵

Photoinduced release via electron transfer from QDs to a ligand requires rather small nanoparticles to enable close contact between the two species.¹⁵⁹² Bao, Zhu, and co-workers prepared water-soluble nanocrystalline CdSe/ZnS particles functionalized with an N-alkyl-4-picolinium ester linked to the anticancer drug 5-fluorouracil and l-cysteine (Scheme 102).¹⁵⁹⁶ Upon irradiation at λ_irr > 400 nm, 5-fluorouracil was liberated via electron transfer from the QD to the picolinium moiety, with l-cysteine acting as an electron donor.

Scheme 102. QD-Mediated Photorelease¹⁵⁹².

Water-soluble CdTe QDs capped with mercaptopropionic acid and a ruthenium nitrosyl complex cis-[Ru^II(NO)(4AP)(bpy)₂]³⁺ (bpy = 2,2′-bipyridine, and 4AP = 4-aminopyridine) were shown by Ford, da Silva, and co-workers to release NO (section 4.2) upon irradiation at 530 nm via a charge-transfer mechanism.¹⁵⁹⁷ Visible light excitation of CdSe QDs was also demonstrated to trigger the release of coumarin from cinnamate surface ligands.¹⁵⁹⁸ In this system, electron transfer from the excited nanocrystal to the surface-bound cinnamate triggers E–Z isomerization and subsequent lactonization. o-Nitrobenzyl (oNB) groups can also be liberated from CdTe/CdS core/shell QDs under UV illumination to control QD emission.¹⁵⁹⁹ The possibility of exciton energy transfer was ruled out in this case, and because there was no overlap between QD emission and oNB absorption, it was suggested that an electron or hole transfer from the QD to the oNB occurred. Other examples of UV- or near-visible light-activated release via oNB group cleavage have also been reported.^{197,1600,1601}

6.4.2. Photorelease Mediated by Upconversion and Second-Harmonic Nanoparticles

Several photophysical phenomena combine the energies of two or more photons to produce that of one higher-energy photon. Photon upconversion in organic molecules converts two or more low-energy photons into one higher-energy photon via two fundamental mechanisms: two (multi)-photon absorption (Figure 69b) or sensitized triplet–triplet annihilation (TTA).^136,1602 The former mechanism leads to an excited state of higher energy (which would also be accessible by one-photon absorption; see Figure 69a) via a virtual state, whereas the latter intermolecular process typically involves two molecules in their triplet states that interact to leave one molecule in the ground state with the second molecule being excited to a higher electronic state. Some nonlinear crystal materials and non-centrosymmetric compounds and structures can exhibit second-harmonic generation (SHG), in which two photons with the same frequency interact with matter and coalesce to a virtual state (Figure 69c).¹⁶⁰³ The resulting second-harmonic photon is generated practically instantaneously (within a few fs), so the signal is coherent (frequency doubling). Another interesting phenomenon is observed in so-called upconversion nanoparticles (UCNPs), in which two or more sequentially absorbed photons are converted into one emitted photon with higher energy via real metastable excited states (Figure 69d).^{1324,1604−1609} UCNPs typically absorb in the IR region and emit in the visible or ultraviolet regions. Most UCNPs consist of rare-earth-based lanthanide- or actinide-doped transition metals. The theoretical quantum yield of upconversion cannot exceed 0.5 because at least two photons are required to produce one upconverted photon.

Figure 69.

(a) Single-photon absorption; (b) two-photon absorption; (c) second-harmonic generation; (d) upconversion. ψ_g = ground state; ψ_f = final excited state; ψ_v = virtual state; ψ_i = intermediate state.

Because of their unique optical and chemical properties, UCNPs can be used for drug release/delivery.^1610−1620 They are convenient and biologically favored “UV-vis lightbulbs”^49,1621 because of their ability to convert NIR light into UV and visible photons. Many conventional UV-absorbing photoactivatable groups that undergo photocleavage or photoswitching processes can thus be activated through the tissue-transparent window. Because of the many recent excellent reviews cited above, we discuss only a few illustrative examples.

An application using o-nitrobenzyl derivatives was presented by Liu, Xing, and co-workers (Scheme 103).²⁰² Monodispersed core-shell UCNPs consisting of NaYF₄ nanocrystals doped with Yb³⁺ and Tm³⁺ were functionalized with cationic photoreleasable linkers via covalent bonding, enabling the adsorption of anionic siRNA molecules via electrostatic interactions. Upon NIR light irradiation (980 nm), the photolabile linker was cleaved by upconverted UV light, initiating the intracellular release of the siRNA.

Scheme 103. Release of siRNA from UCNPs²⁰².

The literature provides many examples of applications in which different types of UCNPs serve as mediators in species release. Most works of this type published in recent years have used o-nitrobenzyl derivatives as photocleavable moieties.^{203,1622−1641} However, other systems have also been studied, including coumarin-4-ylmethyl,³⁶⁵ pyrenemethyl,¹⁶⁴² or o-hydroxycinnamic¹⁶⁴³ PPGs as well as photoactivatable ruthenium,^1333,1644 platinum,^1645,1646 and manganese¹¹⁵⁹ complexes, and Roussin’s black salt.¹⁶⁴⁷ Photochromic moieties have also been used for species liberation involving UCNPs, for example, by incorporating azobenzene^1648−1650 or spiropyran/merocyanine¹⁶⁵¹ photoswitches.

Second-harmonic emission has recently been used for uncaging. Bismuth ferrite harmonic nanoparticles (HNPs) were used to release l-tryptophan linked to a coumarin-4-ylmethyl photoactivatable group via a carbamate functionality (Scheme 104).³⁸⁷ Light (790 nm) from a femtosecond pulsed laser was converted into emission at 395 nm, which was responsible for the excitation of the PPG.

Scheme 104. Release of l-Tryptophan from UCNPs³⁸⁷.

6.4.3. Photothermally Controlled Release

Visible or NIR irradiation of some nanoparticles consisting of noble metals (gold or silver), carbon (graphene derivatives or carbon nanotubes), metallic composites (CuS, MoS₂), or polymers (polyanilines and liposomes) can result in the production of thermal energy (heat), which is dissipated into the surroundings of the nanostructure. This process is referred to as the photothermal effect,^1652,1653 and it can be regarded as a distinct type of photosensitization. Photothermal effects have been used to achieve spatially and temporally controlled release of species such as drugs and metal ions.^1653−1656

Upon excitation, noble metal nanoparticles (particularly those made of gold, AuNPs) exhibit localized surface plasmon resonance, that is, resonant oscillations of the conduction electrons, which are transformed into phonons, followed by rapid relaxation and heating.¹⁶⁵⁴ The wavelengths of the absorption maxima of AuNPs are related to the size of the particles: 10–40 nm AuNPs absorb in the green region, while larger particles have bathochromically shifted maxima. AuNPs have been used to uncage, among other things, (a) drugs embedded in a polymeric matrix surrounding AuNPs, (b) drugs embedded in liposomes together with AuNPs, and (c) drugs covalently or non-covalently attached to AuNPs via a tether.¹⁶⁵⁴ In all cases, photothermal heating disrupts the interactions confining the drug, leading to its release.

Scheme 105 shows a photothermally releasable system based on Au nanocages covered with poly(N-isopropylacrylamide) chains that undergo conformational changes when heated.¹⁶⁵⁷ Upon irradiation with a NIR laser, the light is absorbed by the AuNPs and is converted into heat via the photothermal effect. When the polymer chains collapse, a pre-loaded drug such as doxorubicin is released through the resulting pores. The polymer chains return to their original conformation in the dark and the pores close. Many other photothermal release systems using AuNPs have been reported.^1658−1672 Releasable systems consisting of cobalt nanowire-based particles,¹⁶⁷³ NaYbF₄:Er³⁺ UCNP nanocomposites,¹⁶⁷⁴ carbon nanotube thermosensitive hydrogel,¹⁶⁷⁵ biochar,¹⁶⁷⁶ and photothermal heating of water droplets confined in polymeric particles¹⁶⁷⁷ have also been reported.

Scheme 105. Au Nanocage Opening via the Photothermal Effect¹⁶⁵⁷.

7. Photoactivatable Polymers, Micelles, and Vesicles

Many photoactivatable polymeric materials, micelles, and vesicles have been developed for drug/species delivery in recent decades.^{109,1573,1678−1689} Photocleavable polymer nanostructures are particularly interesting platforms for targeted drug delivery.¹⁴ Several release mechanisms are available: these systems can serve as photoresponsive/degradable nanocarriers for drug delivery, polymeric films can facilitate photochemical species detachment or patterning,¹⁶⁸³ and hydrogels can alter the properties of biomaterials and affect the microenvironment.^1575,1690

Photocleavage of covalent bonds in UV-absorbing chromophores such as o-nitrobenzyl or coumarin-4-ylmethyl groups^10,1683 connected to polymers and vesicles is an appealing strategy because the photochemistry and applications of these chromophores are well known.¹⁶⁹¹ Their activation with red or NIR light is usually enabled by two-photon absorption or the use of upconverting nanoparticles (section 6.4.2). For example, two-photon or blue-light activation was used to remove o-nitrobenzyl-derived PPGs to release payloads from micelles^1692−1694 or polymers.^{271,1695−1697} Similarly, coumarin-4-ylmethyl groups have been used to release drugs from micelles,⁴⁴⁵ polymers,^1696,1698 hyaluronic acid nanogels,¹⁶⁹⁹ nanoparticles,^383,415 and nanocomposites.³⁶⁴ Applications of photodegradable micelles consisting of amphiphiles containing a diazonaphthoquinone group have also been reported.^1700−1705

One-photon visible-light photoactivation (at 420 nm) of cascade depolymerization of self-immolative polymersomes with photoremovable perylen-3-yl protecting groups⁵⁵³ was shown to release encapsulated bioactive agents (section 2.3) via photosolvolysis (Scheme 106).⁵⁶³

Scheme 106. Photochemical Depolymerization of Self-Immolative Polymersomes⁵⁶³.

Wu and co-workers designed red-light-responsive Ru^II-containing block copolymers for anticancer phototherapy. These copolymers can be assembled into micelles, vesicles, or large compound micelles depending on their molecular weights (Scheme 107).¹⁴⁸³ Upon excitation at 656 nm, they release the ¹O₂ generating anticancer agent [Ru(tpy)(biq)(H₂O)]²⁺ via ligand exchange (see section 3). Similar strategies were used with block copolymers bearing [Ru(Biq)₂(Hob)₂](PF₆)₂ (Biq = 2,2′-biquinoline, Hob = 4-((6-hydroxyhexyl)oxy)benzonitrile)¹⁷⁰⁶ or surface-grafted ruthenium complexes to release cytotoxic molecules into cancer cells from mesoporous silica nanoparticles.¹²⁷⁴

Scheme 107. Release from Ru^II-Containing Block Copolymers¹⁴⁸³.

The amino-1,4-benzoquinone (424; see also section 2.10) photoactivatable moiety was used to prepare a nanoparticle-bound photocage–drug conjugate. Upon irradiation with red light, the nanoparticles dissolved in aqueous media, releasing the drug (Scheme 108; drug = paclitaxel, dexamethasone, or chlorambucil).⁷²⁶ It was also shown that an encapsulated cyanine NIR-fluorescent dye such as DiD or IR780 could facilitate the location of the nanoparticles and monitoring of the photorelease process.

Scheme 108. Amino-1,4-benzoquinone Derivatives as PPGs for Drug Uncaging from Nanoparticles⁷²⁶.

Photorelease from light-responsive polymeric micelles made from an amphiphilic block polymer incorporating a BODIPY derivative (see section 2.12) is shown in Scheme 109.⁸²⁴ Upon irradiation, the micellar assembly of this polymer is disrupted due to the release of phenolate from the polymers to release a payload (Nile red). Another example of photoactivatable drug delivery is the photochemical release of dexamethasone from subcutaneously implanted polymeric particles, in which a π-extended o-nitrobenzyl derivative (section 2.1.1) absorbing below 500 nm serves as the photocleavable moiety.²⁶¹

Scheme 109. Release from BODIPY-Containing Amphiphilic Block Polymer⁸²⁴.

Mesoporous silica nanoparticles (MSNs) are important drug delivery nanocarriers with high surface areas and large pore volumes for drug loading, and they are readily functionalized with light-responsive groups or photoswitches.^1707−1710 Most known systems of this type rely on doped upconversion nanoparticles that convert NIR radiation into UV/vis radiation (section 6.4.2) or Au-based, CuS, or graphene oxide nanoparticles that absorb and convert NIR light into thermal energy via the photothermal effect (see section 6.4.3).

For example, 1P (420 nm) or 2P (800 nm) irradiation was used to release the anticancer drug chlorambucil, which was connected to a 7-amino-coumarin derivative and grafted onto the surface of aminopropyl-functionalized MSNs (Scheme 110).³⁸⁴ The drug is liberated by a photosolvolysis reaction. Multi-photon-absorption (808 nm) leading to dissociation of o-nitrobenzyl-containing poly(ethylene glycol) on the surface of gold nanostars coated with a mesoporous silica shell was also shown to release doxorubicin.¹⁷¹¹

Scheme 110. Release from Aminopropyl-Functionalized MSNs³⁸⁴.

8. Release Mediated by Photoswitching

Host–guest interactions are affected by several factors, such as the nature of the host and guest molecules and the properties of the solvent.¹⁷¹² The host entity can consist of a single molecule, usually a photoswitchable (photochromic) system whose photoreaction leads to the release of a guest molecule due to a change in binding affinity. The two isomers of a photoswitchable molecule have distinct chemical, physical and optical properties, which can be used to tune the properties of the host material. The most commonly used photoswitches¹⁷¹³ for this purpose are azobenzene,¹⁷¹⁴ spiropyran, and diarylethene derivatives (whose photoswitching involves ring-opening/closing) (Scheme 111), but stilbene and fumaric acid derivatives (which undergo light-induced E–Z isomerization) or anthracene and coumarin chromophores (which undergo reversible photodimerization) have also been used.¹⁷¹² An alternative approach uses multi-component supramolecular cages or capsules that incorporate a photoactivatable moiety to control guest release. To achieve visible or NIR light absorption, UV-absorbing chromophores can be modified by π-extension to bathochromically shift their absorption bands. Alternatively, they can be excited via a 2-photon absorption, upconversion emission, or sensitization.^1411,1715 Research in this area has been reviewed on several occasions in the past decade,^{1712,1714−1722} so here we present only some particularly notable systems bearing chromophores absorbing over 400 nm.

Scheme 111. Examples of Photochromic Systems.

Since Ueno and co-workers showed in 1978 that azobenzene-capped β-cyclodextrin can regulate the binding of various substrates (including toluene, cyclohexanol, and geraniol) upon irradiation at wavelengths of 320–390 nm,¹⁷²³ the azobenzene unit has become one of the most widely used photoswitches. Azobenzene-bridged cryptand 425 is a typical example; its irradiation with visible light selectively triggers Z → E isomerization, while E → Z isomerization is triggered by UV light, enabling control over its binding affinity towards the guest 2,7-diazapyrenium ion (Scheme 112).¹⁷²⁴

Scheme 112. Azobenzene Isomerization to Control Binding Affinity of a Guest¹⁷²⁴.

Tian and co-workers observed room-temperature phosphorescence emission as a result of photochemically controlled complexation of 2-hydroxy-5-((4-nitrophenyl)-diazenyl)benzoate (426) in β-cyclodextrin (Scheme 113), displacing the fluorescent heavy-atom containing α-bromonaphthalene (427). The fluorescence of the bromonaphthalene was suppressed when complexed with the cyclodextrin but not when it was displaced by the E-isomer of the azadiene.¹⁷²⁵ Similarly, Wang and Wu constructed supramolecular valves from tetra-o-methoxy-substituted azobenzene and β-cyclodextrin to control the release of doxorubicin from nanopores of mesoporous silica nanoparticles (see section 7) using red light.¹⁷²⁶

Scheme 113. Photochemically Controlled Complexation in β-Cyclodextrin¹⁷²⁵.

An application of a self-assembled coordination cage consisting of two square-planar-coordinated Pd^II ions and four photochromic dithienylethene-containing ligands was reported by Clever and co-workers.¹⁷²⁷ The photorelease and encapsulation of the guest, [B₁₂F₁₂]^2–, was accomplished using UV and white light, respectively. Photoactivatable metal-containing complexes^40,870 can also serve as excellent platforms for metal ion photorelease. Metal ions can induce profound biological responses, so it is desirable to control their concentrations with a high spatiotemporal resolution using photoactivatable systems. To this end, Yu and collaborators introduced the terthiazole-based molecular switch 428, which enables photoswitchable release and uptake of Zn²⁺ ions based on a 6π-electrocyclization/cycloreversion reaction of the chromophore and excited-state intramolecular proton transfer (ESIPT; Scheme 114).¹⁷²⁸ Additionally, visible-light triggered switching of the G-quadruplex ligand binding mode and G-tetrad structure formation using a pyridinium-substituted dithienylethene has been demonstrated under physiologically relevant conditions.¹⁷²⁹

Scheme 114. Terthiazole-Based Molecular Photoswitch¹⁷²⁸.

Another notable photoswitchable system for controlled metal ion release is a bisstyrylthiophene derivative that incorporates both a π-extended photoactivatable nitrobenzyl group and a conjugated Ca²⁺ chelator (see also section 2.1).²¹⁹ This species has a large two-photon cross section (350 GM) at 775 nm. Ca²⁺ photorelease (/uptake) has also been accomplished using a photoswitchable diarylethene-containing chelator,¹⁷³⁰ visible-light irradiation of flavin photosensitizers in the presence of Ca²⁺-EDTA,¹⁴¹⁰ and two-photon excitation of a 5-bromo-2-nitrobenzyl-substituted ethylene glycol tetraacetic acid chelator.¹⁸⁸ The last example discussed here is a system that releases metal ions such as Ca²⁺¹⁷³¹ or Zn²⁺ from polymersomes, that is, bilayer vesicles that self-assemble from amphiphilic diblock copolymers.¹⁷³² A photoresponsive polymersome system containing an ethyne-bridged bis[(porphinato)zinc] fluorophore as a hydrophilic membrane solute and dextran in the aqueous core undergoes deformation upon irradiation at 488 nm, liberating the metal ion.¹⁷³²

The incorporation of photoswitchable moieties into the backbones of polymer nanoparticles, micelles, polymersomes, vesicles, microgels, liposomes, mesoporous silica nanoparticles, and so on offers another way of controlling the photochemical delivery/release of cargoes encapsulated within the assembled nanocarriers.^{130,1581,1707−1709,1714,1715,1733−1738} As in the supramolecular systems discussed above, photoisomerization (e.g., azobenzene) and 6π-electrocyclization of triene systems (e.g., spiropyrans and diarylethenes; Scheme 111) are commonly used mechanisms for species release from nanocarriers. This approach was exemplified by photochromic polymersomes composed of self-assembled poly(ethylene oxide) diblock copolymers containing a spiropyran-based monomer, which exhibited reversible bilayer permeability upon photoisomerization of the hydrophobic spiropyran to give the zwitterionic merocyanine upon irradiation at >450 nm; this process could be reversed by irradiation at <420 nm (Scheme 115).¹⁷³⁹ The authors assumed that the structure of the polymersomes is determined by multiple cooperative noncovalent interactions including hydrophobic, hydrogen bonding, π–π stacking, and electrostatic interactions, and that the isomerization changes the pattern of these interactions. This system was successfully used to release a nuclei-staining dye, 4′,6-diamidino-2-phenylindole, in living HeLa cells.

Scheme 115. Photoswitching of a Spiropyran/Merocyanine Pair in Polymersome¹⁷³⁹.

Similarly, photoinduced isomerization of two different donor–acceptor Stenhouse adducts upon irradiation with visible light (Scheme 116) was used to switch the permeability of polymersome by inducing the isomerization of a nonpolar triene-enol into a polar cyclopentenone within amphiphilic block copolymers containing poly(pentafluorophenyl methacrylate).¹⁷⁴⁰ The hydrophilic anticancer drug 2′-deoxy-5-fluorouridine and the DNA-intercalating dye 4′,6-diamidino-2-phenylindole were used as payloads in this system. Other photoswitchable systems have also been used for controlled delivery upon irradiation with visible or NIR light including a spiropyran–merocyanine-containing polymer,¹⁷⁴¹ a nanocarrier-based on hollow mesoporous silica (HMS) nanoparticles,¹⁷⁴² polymer nanoparticles incorporating a donor–acceptor Stenhouse adduct,¹⁷⁴³ sulfonatocalix[4]arene with bound flavylium ions,¹⁷⁴⁴ azobenzene-containing micelles,¹⁷⁴⁵ β-cyclodextrin-grafted hyperbranched conjugated polymers,¹⁷⁴⁶ and catanionic vesicles.¹⁷⁴⁷ Additionally, the azobenzene chromophore has been used for the photochemical control of drug release from supramolecular¹⁷⁴⁸ and poly(ethylene glycol)¹⁷⁴⁹ hydrogels.

Scheme 116. Switching of a Stenhouse Adduct within Amphiphilic Block Copolymers¹⁷⁴⁰.

Photoisomerization-induced release of luminescent dyes and anticancer drugs from functionalized azobenzene molecules attached to the interiors of MSN pores upon irradiation at 413 nm was reported by Tamanoi, Zink, and co-workers (Figure 70).¹⁷⁵⁰ Another photoactivatable system was reported by Kneževič and co-workers,¹⁷⁵¹ who entrapped the model dye sulforhodamine 101 inside the mesopores of mercaptopropyl-functionalized MSNs in the presence of a Ru(bpy)₂(PPh₃) moiety coordinated to mercaptopropyl functional group. The dye was liberated upon irradiation at 455 nm via a ligand exchange reaction.

Figure 70.

Azobenzene-controlled release from the pores of mesoporous silica nanoparticles.¹⁷⁵⁰

9. Photoactivation and Photodeactivation of Drugs: Photopharmacology

Most of the photochemical release systems discussed in this review rely on irreversible release (uncaging) from auxiliary photochemically active/reactive chromophores that undergo various photochemical side-reactions, leading to their destruction and thus potentially to the formation of unwanted materials (Scheme 117a). These processes must be carefully considered during the design and development of new photoactivatable systems, especially in terms of their compatibility with biological systems where relevant. Despite this limitation, irreversible uncaging remains the dominant approach to photorelease in both fundamental and practical research. However, as discussed in the preceding sections, reversible release systems using (photoswitchable) photochromic moieties also exist and can be incorporated into more complex systems, where their reversible (photo)reactions such as E–Z-photoisomerization can induce changes in the binding affinity of a releasable guest upon irradiation (Scheme 117b, section 7). Active species can also be incorporated into a tether whose conformation/length changes upon irradiation to bring an attached active molecule into close proximity with a site of interest (Scheme 117c).¹⁷⁵²Scheme 117d shows a different approach involving a so-called photochromic ligand that can exist in two or more different isomeric forms that are interconvertible upon irradiation, of which only one is active in a specific application. The isomerization, in this case, may also be triggered by heat (Δ) in one direction, and may thus proceed spontaneously under certain conditions. The term “photopharmacology” is used in reference to bioactive molecular systems that undergo reversible photochemical transformations that alter their pharmacokinetic or pharmacodynamic properties. This is a relatively new concept but one that has attracted considerable attention and has therefore been reviewed several times in recent years.^{145,874,1752−1764} Consequently, we present only a few representative examples here.

Scheme 117. Irreversible versus Reversible Photoactivation.

Azobenzene and diarylethene photoswitches and their analogs (section 8) are the most frequently used photochromic systems in photopharmacology.^{1752,1753,1758,1763,1765} Most of them are activatable by UV light in one direction and by visible light in the reverse process. Substituents attached to a photochromic group can bathochromically shift its absorption maxima such that both forward and backward photoactivation occurs in the visible part of the spectrum.

Both E and Z isomers of azobenzenes can be pharmacologically active. Trauner and co-workers introduced azobenzene derivative 429, which is a freely diffusible photoswitchable antagonist of the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) glutamate receptor (Schemes 117 and 118).¹⁷⁶⁶ This compound is active in its stable E-form but considerably less active as the Z-isomer. Because of a significant bathochromic shift of its absorption maxima, the two isomers can be interconverted by irradiation at 460 and 600 nm, respectively.

Scheme 118. Photoswitchable Quinoxaline-2,3-dione Antagonist.

Another azobenzene-based protein ligand, 430, was developed by Yuste, Gorostiza, and co-workers to activate the light-gated glutamate receptor LiGluR in living cells (Scheme 119; Glu = glutamate).¹⁷⁶⁷ The E-form of the tether is inactive, but E–Z isomerization induced by one- (>400 nm) or two-photon irradiation brings the glutamate residue at the end of the ligand chain into the vicinity of the receptor’s glutamate-binding site. In the dark, the receptor is inactivated by the rapid and spontaneous reverse isomerization of the ligand.

Scheme 119. Azobenzene-Based Protein Switch.

Szymanski, Feringa, and co-workers developed switchable antibacterial agents¹⁷⁶⁸ based on the azobenzene chromophore whose activity is controlled by visible-light irradiation.¹⁷⁶⁹ The Z-azobenzene derivative 431, formed upon photoisomerization of the corresponding E-isomer at 652 nm, exhibited at least an 8-fold increase in activity (Scheme 120).

Scheme 120. Photoactivation of Antibacterial Azobenzene Derivative.

A different photoswitch based on a photochromic dithienylethene (432) was studied by König and co-workers who showed that the open isomer of this compound activated the dopamine D_2S receptor considerably more efficiently than the closed isomer (Scheme 121).¹⁷⁷⁰ Notably, the photophysical properties of these dithienylethene dopamine ligands exhibited high fatigue resistance.

Scheme 121. Photochromic Dithienylethene Derivative.

Bifunctional molecules targeting proteins for ubiquitylation by an E3 ligase complex and subsequent degradation by the proteasome (PROTACs; proteolysis targeting chimeras) are powerful tools for regulating the levels of certain cellular proteins.¹⁷⁷¹ Photoswitchable PROTACs, or PHOTACs (photochemically targeting chimeras), that enable optical (photopharmacological) control of protein levels using azobenzene-containing photoswitches, were recently developed by Pagano and Trauner.¹⁷⁷² Conceptually similar photoswitchable azobenzene-proteolysis targeting chimeras (Azo-PROTACs) were introduced by You and Jiang.¹⁷⁷³

Glossary

Abbreviations

λ_max^abs

absorption maximum

λ_max^em

emission maximum

λ_irr

irradiation wavelength

Φ_r

reaction quantum yield

Φ_Δ

quantum yield of singlet oxygen production

Φ_rε(λ_irr)

uncaging cross section

σ_TPA

2-photon absorption cross section

δ_unc

2-photon uncaging cross section

1P

one-photon

1PE

1-photon excitation

2P

two-photon

2PE

2-photon excitation

4AMP

4-(aminomethyl)pyridine)

4AP

4-aminopyridine

4-pic

4-methylpyridine

AIE

aggregation-induced emission

AMPA

α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid

ATP

adenosine triphosphate

AuNP

Au nanoparticle

BDE

bond dissociation energy

BHQ

(8-bromo-7-hydroxyquinoline-2-yl)methyl

BIST

bisstyrylthiophene

biq

2,2′-biquinoline

BODIPY

4,4-difluoro-4-bora-3a,4a-diaza-s-indacene

bpy

2,2′-bipyridyl

BRET

bioluminescence resonance energy transfer

cAMP

cyclic adenosine monophosphate

Cbl

chlorambucil

CD

carbon dot

CDNI

4-carboxymethoxy-7-nitroindolinyl

cGMP

cyclic guanosine monophosphate

cMO

caged morpholino oligonucleotide

COHb

carbonylhemoglobin

COS

carbonyl sulfide

CQD

carbon quantum dot

CRISPR

clustered regularly interspaced short palindromic repeats

CT

charge transfer

cur

curcumin

Cy5

pentamethine cyanine

Cy7

heptamethine cyanine

cyclam

1,4,8,11-tetraazacyclotetradecane

CyHQ

(8-cyano-7-hydroxyquinoline-2-yl)methyl

DANS

E-4-(N,N-dimethylamino)-4′-nitrostilbene

dach

1R,2R-(−)-1,2-diaminocyclohexane

DDQ

2,3-dichloro-5,6-dicyano-1,4-benzoquinone

DEA

diethylamine

DEAC

7-diethylaminocoumarin

dend.

1,4-diaminobutane dendrimer

DFT

density functional theory

DMA

dimethylacetamide

DMNB

4,5-dimethoxy-2-nitrobenzyl

DMNPB

3-(4,5-dimethoxy-2-nitrophenyl)-2-butyl

DMSO

dimethyl sulfoxide

DNA

deoxyribonucleic acid

dppn

benzo[i]dipyridophenazine

DTE

dithienylethene

EDG

electron-donating group

EDTA

ethylenediaminetetraacetic acid

EGFR

epidermal growth factor receptor

EGTA

ethylene glycol tetraacetic acid

en

ethylenediamine

ESIPT

excited-state intramolecular proton transfer

Et

ethyl

EWG

electron-withdrawing group

FBS

fetal bovine serum

Fl

9,9′-dibutyl-9H-fluoren-2-yl

FRET

Förster resonance energy transfer

FT-IR

Fourier transform infrared

FWHM

full-width-at-half maximum

GA₃

gibberellin A₃

GABA

γ-butyric acid

glu

glutamate

GM

Goeppert-Mayer

HBIND

H-bond-induced non-radiative decay

HBT

2-(2′-hydroxyphenyl)benzothiazole

HEPES

4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid

HMO

Hückel molecular orbital

HMS

hollow mesoporous silica

HNP

harmonic nanoparticle

Hob

4-((6-hydroxyhexyl)oxy)benzonitrile)

HOMO

highest occupied molecular orbital

hydrol

hydrolysis

ICT

intramolecular charge transfer

immol

self-immolation

IP₃

inositol triphosphate

IR

infrared

ISC

intersystem crossing

L

ligand

LE

locally excited

LED

light-emitting diode

LF

ligand field

LG

leaving group

LLCT

ligand-to-ligand charge transfer

LMCT

ligand-to-metal charge transfer

LUMO

lowest unoccupied molecular orbital

Me

methyl

Me₂bpy

6,6′-dimethyl-2,2′-bipyridine

MetHb

methemoglobin

MES

2-(N-morpholino)ethanesulfonic acid–based buffer

MLCT

metal-to-ligand charge transfer

MMLCT

metal–metal-bond-to-ligand charge-transfer

MNPPOC

2-(3,4-methylenedioxy-6-nitrophenyl)-propoxycarbonyl

MOF

metal–organic framework

MOPS

3-(N-morpholino)propanesulfonic acid

MSN

mesoporous silica nanoparticle

NDBF

nitrodibenzofuran

NHS

N-hydroxysuccinimide

NIR

near-infrared

NO

nitric oxide

NONOate

diazeniumdiolate

NP

nanoparticle

NPEOC

1-(2-nitrophenyl)ethyloxycarbonyl

NPPOC

1-(2-nitrophenyl)propyloxycarbonyl

oNB

o-nitrobenzyl

PBS

phosphate buffer saline

PDT

photodynamic therapy

PEG

polyethylene glycol

PET

photoinduced electron transfer

Ph

phenyl

phen

1,10-phenanthroline

phen-CHO

phenanthrolinecarboxaldehyde

photoCORM

photoactivatable CO-releasing moiety

photoNORM

photoactivatable NO-releasing moiety

pHP

p-hydroxyphenacyl

PPG

photoprotecting group

py

pyridine

Py

pyrene

pydppn

(pyrid-2-yl)benzo[i]dipyridophenazine

QD

quantum dot

qmtpm

2-quinoline-N-(2′-methylthiophenyl)-methylenimine

RNA

ribonucleic acid

sens

sensitizer

SHG

second-harmonic generation

sol

solvent

TD-DFT

time-dependent density functional theory

TICT

twisted intramolecular charge transfer

TIP

tight ion-pair

TMG

tetramethylguanidine

TPA

two photon absorption

TPA

tris(2-pyridylmethyl)amine

TPE

tetraphenylethylene

tpy

terpyridine

TQA

tris(2-quinolinylmethyl)amine

TRIR

time-resolved ultrafast infrared spectroscopy

Tris

tris(hydroxymethyl)aminomethane

TRPV1

transient receptor potential cation channel V1

TTA

triplet–triplet annihilation

UCNP

upconversion nanoparticle

UV

ultraviolet

VPA

valproic acid

WT

wild type

z

zwitterion

Biographies

Roy Weinstain received his B.Sc. degree in chemistry and biology in 2005 from Tel Aviv University, Israel. He obtained his Ph.D. from the same institution in 2010 for the development of self-immolative molecular systems under the supervision of Prof. Doron Shabat. He then joined Prof. Roger Y. Tsien’s group at the University of California San Diego as a post-doctoral fellow, working on the synthesis and application of fluorescent probes to study dynamic processes in vivo. Since 2014, he is a Senior Lecturer in the School of Plant Science and Food Security at Tel Aviv University, Israel. His research focuses on the development and implementation of chemical-biology methods to study the functions and regulation mechanisms of plant signaling molecules.

Tomáš Slanina received his M.S. degree from Masaryk University, Brno, Czech Republic in 2012. He received his Ph.D. in organic chemistry in 2015 in a joint programme between Masaryk University and the University of Regensburg, Germany, under the supervision of Prof. Petr Klán and Prof. Burkhard König. He later worked as a postdoctoral researcher in Prof. Alexander Heckel’s group at Goethe University, Frankfurt am Main, Germany, and in the research group of Prof. Henrik Ottosson at Uppsala University, Sweden. He is currently a leader of the junior research group or redox photochemistry at the Institute of Organic Chemistry and Biochemistry in Prague, Czech Republic. His research interests include organic chemistry, photochemistry, physical organic chemistry, electrochemistry, time-resolved and steady-state spectroscopy, investigation of reaction mechanisms, and chemical biology.

Dnyaneshwar Kand received his B.Sc. and M.Sc. degrees in chemistry from Pune University, India. He then joined the Indian Institute of Science, Education and Research (IISER), Pune, India with Dr. Pinaki Talukdar, receiving his Ph.D. degree in organic chemistry in 2015 for the development of colorimetric and fluorescent selective thiols sensors. In 2015, he joined the group of Dr. Roy Weinstain at Tel Aviv University, Israel, as a post-doctoral fellow (and a PBC fellow), where he worked on the development of meso-methyl BODIPY photocages. When not doing chemistry, he enjoys playing cricket.

Petr Klán received an M.Sc. degree in organic chemistry from Masaryk University, Brno, Czech Republic in 1986. After working in the industry for five years, he stayed at Michigan State University to pursue a Ph.D. in chemistry under the tutelage of Prof. Peter J. Wagner. After receiving his Ph.D. in chemistry in 1998, he joined the faculty at Masaryk University where he is now a full professor. His current research focuses on photochemistry, mechanisms of organic reactions, kinetic flash photolysis, spectroscopy, photoremovable protecting groups, and environmental photochemistry. He co-authored the book “Photochemistry of Organic Compounds” (Wiley, 2009) with Prof. Jakob Wirz.

The authors declare no competing financial interest.