Light-driven generation of hydrogen: New chromophore dyads for increased activity based on Bodipy dye and Pt(diimine)(dithiolate) complexes

Significance

The light-driven generation of H₂, the reductive side of water splitting, requires a light absorber or photosensitizer (PS) for electron-hole creation and photoinduced electron transfer. To increase the effectiveness of charge transfer chromophores as PSs, this report describes the attachment of a strongly absorbing organic dye (dipyrromethene-BF₂, commonly known as Bodipy) to Pt diimine dithiolate charge transfer chromophores and examination of systems containing these dyads for the light-driven generation of H₂. The use of these dyads increases system activity under green light irradiation (530 nm) relative to systems with either chromophore alone, validating such an approach in designing artificial photosynthetic systems. One dyad system exhibits both high activity and substantial durability (40,000 turnovers relative to PSs over 12 d).

Abstract

New dyads consisting of a strongly absorbing Bodipy (dipyrromethene-BF₂) dye and a platinum diimine dithiolate (PtN₂S₂) charge transfer (CT) chromophore have been synthesized and studied in the context of the light-driven generation of H₂ from aqueous protons. In these dyads, the Bodipy dye is bonded directly to the benzenedithiolate ligand of the PtN₂S₂ CT chromophore. Each of the new dyads contains either a bipyridine (bpy) or phenanthroline (phen) diimine with an attached functional group that is used for binding directly to TiO₂ nanoparticles, allowing rapid electron photoinjection into the semiconductor. The absorption spectra and cyclic voltammograms of the dyads show that the spectroscopic and electrochemical properties of the dyads are the sum of the individual chromophores (Bodipy and the PtN₂S₂ moieties), indicating little electronic coupling between them. Connection to TiO₂ nanoparticles is carried out by sonication leading to in situ attachment to TiO₂ without prior hydrolysis of the ester linking groups to acids. For H₂ generation studies, the TiO₂ particles are platinized (Pt-TiO₂) so that the light absorber (the dyad), the electron conduit (TiO₂), and the catalyst (attached colloidal Pt) are fully integrated. It is found that upon 530 nm irradiation in a H₂O solution (pH 4) with ascorbic acid as an electron donor, the dyad linked to Pt-TiO₂ via a phosphonate or carboxylate attachment shows excellent light-driven H₂ production with substantial longevity, in which one particular dyad [4(bpyP)] exhibits the highest activity, generating ∼40,000 turnover numbers of H₂ over 12 d (with respect to dye).

Water splitting into hydrogen and oxygen is the key energy-storing reaction of artificial photosynthesis (AP) and one of the most promising long-term strategies for carbon-free energy on a potentially global scale (1). As a redox reaction, water splitting has been studied primarily in terms of its two half-reactions, the reduction of aqueous protons to H₂ and the oxidation of water to O₂ (2–13). Whereas some of these studies date back more than 30 y (14–22), recent progress on each half-reaction has been notable, particularly with regard to catalyst development and mechanistic understanding of each transformation (6, 23–29). In this paper, we focus on efforts dealing with the light-driven generation of H₂, which in its simplest form requires a light absorber or photosensitizer (PS) for electron-hole creation, a means or pathway for charge separation and electron transfer, an aqueous proton source, a catalyst for collecting electrons and protons and promoting their conversion to H₂, and an ultimate source of electrons in the form of an electron donor.

Dating from the earliest work on the light-driven generation of H₂, the photosensitizer has most often been a Ru(II) complex with 2,2′-bipyridine (bpy) and/or related heterocyclic ligands having a long-lived triplet metal-to-ligand charge transfer state (³MLCT) (5, 7, 30–33). Recent studies have included analogous Ir(III) d⁶ systems based on phenylpyridine (ppy) ligands in place of bpy (34). With charge transfer excited states, the sensitizers are poised for photoinduced electron transfer following photon absorption and intersystem crossing (ISC). Another set of charge-transfer chromophores that have been used in related systems is [Pt(terpyridyl)(arylacetylide)]⁺ complexes that also have ³MLCT states (35–37). Despite their success with photoinduced electron transfer, all of these charge transfer (CT) complexes have absorptions that are too weak for efficient photon capture with molar extinction coefficients (ε) of 7–15 × 10³ M⁻¹⋅cm⁻¹, and the energies of their singlet absorbing states (¹MLCT) are too high (typically >2.6 eV with λ < 490 nm) for effective use of the solar spectrum. More strongly absorbing organic dyes with ε of ∼10⁵ M⁻¹⋅cm⁻¹ were also examined during early studies of the light-driven generation of H₂. Whereas those with heavier halogen substituents such as Rose Bengal and Eosin Y were found to promote H₂ formation because of facile ISC to long-lived ³ππ* states, they also exhibited poor photostability, decomposing within 3–5 h (38–41).

Platinum diimine dithiolate complexes (PtN₂S₂) such as 1 constitute another class of charge transfer chromophores that are solution luminescent and undergo electron transfer quenching of both oxidative and reductive types (42–48). The excited state energies of these complexes are significantly lower than those of the Ru(bpy), Ir(ppy), and Pt(terpyridyl) complexes mentioned above and have an excited state that has been described as having both MLCT and ligand-to-ligand charge transfer (LLCT) character. In complexes such as 1, the highest occupied molecular orbital (HOMO) is of mixed character, being composed of substantial percentages of both Pt-based and dithiolate-based wavefunctions. As a consequence of the mixed character of the HOMO in PtN₂S₂, the excited states derived from HOMO to LUMO (lowest unoccupied molecular orbital) excitation have been labeled mixed-metal ligand-to-ligand′ charge transfers (MMLL′CT). Based on these charge transfer states, two of the PtN₂S₂ complexes, including 1 with R = COOH, were examined as photosensitizers for H₂ generation with TiO₂ as the electron conduit and Pt islands on the TiO₂ surface as the catalyst (49). The deprotonated carboxlyate groups of di(carboxy)bipyridine (dcbpy) served to link the PS to the platinized TiO₂ (Pt-TiO₂). The system produced hydrogen and proved to be stable for more than 70 h, using light of λ > 450 nm. However, activity of the system was low, based in part on the low absorptivity of the PtN₂S₂ chromophore and the small amount of the complex bound to TiO₂.

To improve absorptivity of the PS in such H₂ generating systems, an approach was initiated in which a strongly absorbing organic dye was linked directly to the PtN₂S₂ moiety for more efficient photon absorption and subsequent energy transfer to the CT chromophore. Such a strategy had been adopted by Ziessel through the synthesis of a bipyridine linked to a dipyrromethene-BF₂ (Bodipy) dye similar to 2 and was examined for Ru complexes containing this ligand, resulting in sensitization of the lower-energy ³ππ* of the Bodipy part of the molecule (50). In a study by Lazarides et al. (51), the dye-CT dyad 3 that contained 2 and the dithiolate-linked Bodipy dyad 4 were synthesized and examined by absorption spectroscopy and transient absorption measurements to determine the potential success of photoinduced charge separation. In that study, it was determined that the absorption spectra of the two dyads are essentially the sum of each constituent chromophore and that the redox potentials of each component are virtually unaffected by bringing the two units together in a dyad. The measurements also showed that energy transfer from the ¹ππ* state of the Bodipy moiety to the ¹MMLL′CT state was energetically favorable and that for both 3 and 4, the dynamics progressed as shown in Scheme 1. Although the rates of both singlet energy transfer (SenT) and ISC for the two dyads were similar and proceeded in less than 1 ps, a significant difference was seen for the triplet energy transfer (TEnT) step with the time constant for 3 being 8 ps whereas that for 4 was 160 ps, indicating that for productive electron transfer to TiO₂, the arrangement in 4 with Bodipy attached to the dithiolate was preferred.

Scheme 1.

Jablonski diagram of the Bodipy-PtN₂S₂ dyads showing the transitions between the different states (SEnT, singlet energy transfer; ISC, intersystem crossing; TEnT, triplet energy transfer).

In this paper, the synthesis and characterization of dyads 4 with R = COOMe, P(O)(OEt)₂, CH₂(P(O)(OEt)₂) and an analogous phenanthroline (phenP) derivative 5 are described, along with studies of their ability to promote light-driven production of H₂ from aqueous protons in conjunction with Pt-TiO₂ as both electron conduit and catalyst. The benefit of attaching the strongly absorbing bodipy chromophore to the charge transfer PtN₂S₂ complex is discussed.

Results and Discussion

Syntheses and Characterization.

PtCl₂(diimine) complexes in which the diimine is a derivatized bpy or phen were prepared by the reaction of cis-Pt(DMSO)₂Cl₂ with the corresponding diimine and isolated as a precipitate from the reaction mixture. The particular diimines used were 4,4′-bis(methoxycarbonyl)-2,2′-bipyridine, 4,4′-bis(diethylphosphonyl)-2,2′-bipyridine, 4,4′-bis(diethylphosphonomethyl)-2,2′-bipyridine, and 5-diethylphosphonyl-1,10-phenanthroline. The subsequent reaction of the particular PtCl₂(diimine) complex with 1,2-benzenedithiol (bdt) led to the corresponding derivative of 1. For simplicity, the diimines are denoted, respectively, as bpyC, bpyP, bpyCH₂P, and phenP in complexes 1 and dyads 4.

The dithiolene ligand precursor that led to formation of dyad 4 with R=H [that is, 2-oxobenzo[d][1,3]dithiole-5-[1,3,5,7-tetramethyl-2,6-diethyl(4,4-difluoro-4-bora-3a,4a-diaza-s-indacene)] (Bodipy-bdtCO)] was prepared following the previously reported procedure involving the compound denoted as (OHC)-bdtCO (51). Dyads 4 with R = COOMe, P(O)(OEt)₂, CH₂(P(O)(OEt)₂) were synthesized according to Scheme 2. The resultant dyads were characterized by elemental analyses, mass spectrometry, and ¹H NMR spectroscopy (see Experimental section and SI Appendix for NMR spectra). In addition to the complexes containing the PtN₂S₂ charge transfer chromophore, compound 6 was made by carrying out the Bodipy synthesis using 3-ethyl-2,4-dimethyl-1H-pyrrole and diethyl(4-formylphenyl)phosphonate.

Scheme 2.

Synthesis of the derivatives of dyads 4 and 5.

UV-Vis Absorption Studies.

The absorption spectra for Pt complexes 1 with R = COOMe, P(O)(OEt)₂, CH₂(P(O)(OEt)₂) and the corresponding dyads 4 in CH₃CN at room temperature are presented in Figs. 1 and 2, and relevant absorption maxima are summarized in Table 1. Solutions of dyads 4 and the charge transfer chromophores 1 exhibit strong absorptions around 300 nm due to ππ* transitions involving the polypyridine ligands (bpy or phen) (see Figs. 1 and 2, respectively). The spectra of the dyads exhibit the strong absorption band of the Bodipy chromophore at 530 nm that shows little change as R is varied and agrees with that of 6, consistent with electronic separation of the two chromophores in the dyads. Another prominent feature in the visible spectra for the derivatives of 1 and 4 corresponds to the band for the ¹MMLL′CT absorption in the 500- to 700-nm region. The absorption bands are broad and shift significantly with the bpy substituent, moving from 525 nm with bpy R = H (51) to 613 nm and 594 nm for R = COOMe, P(O)(OEt)₂, respectively. The change is consistent with the LUMO for the ¹MMLL′CT absorption being bpy based and the COOMe and P(O)(OEt)₂ substituents being strongly electron withdrawing. For 1 with R = CH₂(P(O)(OEt)₂ and for the phenanthroline dyad 5, there is little observable effect of the electron withdrawing ability of the phosphonate ester group (52).

Fig. 1.

UV-vis absorption spectra of dyads 4(bpyC) (blue), 4(bpyP) (red), 4(bpyCH₂P) (black), and 5(phenP) (green), as well as the Bodipyphosphonate ester 6 (pink) in acetonitrile. Spectra have been normalized to give a maximum Bodipy absorbance of 1.

Fig. 2.

UV-vis absorption spectra of PtN₂S₂ complexes 1(bpyC) (blue), 1(bpyP) (red), 1(bpyCH₂P) (black), and 1(phenP) (green) in acetonitrile. Spectra have been scaled for the same maximum absorbance of the MMLL′CT absorption bands at ∼600 nm.

Table 1.

Absorption maxima for PtN₂S₂ complexes and associated Bodipy dyads 4 and 5 in CH₃CN at RT

Sensitizer	Absorption λmax, nm (molar absorptivity ε, M−1⋅cm−1)
1(bpy)	307 (27,200), 542 (6,890)
1(bpyC)	315 (12,500), 326 (14,500), 620 (7,100)
1(bpyP)	317 (28,400), 610 (9,700)
1(bpyCH2P)	303 (32,100), 546 (8,000)
1(phenP)	318 (6,900), 439 (3,900), 552 (8,600)
4(bpyC)	326 (26,500), 383 (13,600), 523(68,400), 613 (8,900)
4(bpyP)	306 (29,200), 317 (33,000), 370 (11,900), 522 (71,600), 594 (10,700)
4(bpyCH2P)	303 (32,800), 373 (9,200), 522 (63,900), 537 (8,400)
5(phenP)	321 (12,000), 379 (9,600), 397 (9,900), 522 (75,500), 541 (9,600)
6	379 (7,200), 397 (6,300), 523 (66,400)

Crystal Structure Characterizations.

Single crystals of 1(bpyC) and 4(bpyP) were obtained by diffusion of diethyl ether into dichloromethane and acetonitrile solutions of the respective compounds. For the Bodipy dye with a phosphonate ester 6, crystals were obtained by layering hexane onto a dichloromethane solution of the dye. Each of these compounds was structurally characterized by X-ray crystallography (Fig. 3). Complete parameters for each structure determination are available in SI Appendix, a summary of crystallographic data is given in SI Appendix, Table S1, and key bond lengths and angles for 1(bpyC) and 4(bpyP) are presented in SI Appendix, Table S2.

Fig. 3.

(A–C) Molecular structures of 1(bpyC) (A), 4(bpyP) (B), and 6 (C). Thermal ellipsoids at the 50% probability level are shown. The hydrogen atoms are omitted for clarity. In 4(bpyP), the severely disordered ethyl groups of the PO(OEt)₂ and a diethyl ether molecule of crystallization are omitted for clarity.

In the crystal structure of 1(bpyC), the geometry about the Pt(II) ion is essentially square planar with minor distortions caused by bite angle limitations of the chelating ligands and a very minor torsional deformation of 2.0° (the dihedral angle between the S1-Pt-S2 and N1-Pt-N2 planes). The observed Pt-S and Pt-N bond lengths are similar to those observed for other Pt(II) diimine dithiolate complexes (42–46, 49, 51). In the crystal structure of dyad 4(bpyP), the two essentially planar moieties, Bodipy and Pt(bdt)(bpy), which are covalently linked via the bdt ligand, have a dihedral angle of 74.6°, as measured between the dipyrromethene and bdt planes. As a consequence of the methyl substituents on the pyrrole rings, rotation about the C-C bond connecting the two chromophores is restricted. Severe disorder of ethyl groups of the two phosphonate ester substituents on bpy was observed, precluding a fully satisfactory refinement. The structure of Bodipy dye 6 is very similar to related Bodipy structures (53). The p-phenylene ring attached to the P(O)(OEt)₂ ester makes a dihedral angle of 73° with the dipyrromethene-boron plane, making possible only very modest conjugation between the two moieties. The observed dihedral angle is very similar to that found in the structure of 4(bpyP) between the planar Bodipy and bdt rings.

Electrochemical Characterization of the Dyads.

All of the Bodipy-Pt(diimine)(dithiolate) dyads, as well as the corresponding component compounds, were studied by cyclic voltammetry and their electrochemical data are collected in Table 2. The two Bodipy compounds, Bodipy-bdt(CO) and 6, exhibit similar cyclic voltammetries (CVs) with a single reversible reduction at −1.58 V and −1.56 V and a single reversible oxidation at 0.66 V and 0.65 V, respectively (Fig. 4 A and B), that are attributable to the dye. A silver wire was used as the quasi-reference electrode and calibrated against ferrocene. All redox potentials are reported vs. the Fc/Fc⁺ couple that has a value of 0.40 V vs. SCE with SCE = 0.24 relative to NHE (50, 53–57). The cyclic voltammograms of the Pt(diimine)(bdt) complexes 1(bpyC) and 1(bpyP) each exhibit two reversible reductions in the ranges between −1.39 V to −1.41 V and −1.9 V to −2.03 V, as well as two irreversible oxidations in the range 0.04–0.54 V (Fig. 4 C and D), whereas for 1(bpyCH₂P) and for 1(phenP), only one reversible reduction at more cathodic values (between −1.62 V and −1.74 V), as well as two irreversible oxidations, are seen in SI Appendix, Fig. S1. The less negative first reduction potential and observation of a second reduction for 1(bpyC) and 1(bpyP) are a consequence of the ester electron withdrawing abilities on the energy of the π* LUMO that serves as the acceptor orbital upon reduction. Both –COOMe and –P(O)(OEt)₂ are strongly electron withdrawing groups, whereas in complexes with bpyCH₂P and phenP, the effect is essentially muted. These observations are consistent with the spectroscopic results discussed above regarding the position of the ¹MMLL′CT absorption.

Table 2.

Summary of oxidation and reduction potentials (V vs. Fc/Fc⁺)

	E1/2*/V (ΔEp/mV)†: Epc/V (irrev)	E1/2*/V (ΔEp /mV)†: Epa/V (irrev)
1(bpyC)	−1.90 (73); −1.39 (65)	+0.17 (irrev); +0.54 (irrev)
1(bpyP)	−2.03 (71); −1.41 (80)	+0.13 (irrev); +0.54 (irrev)
1(bpyCH2P)	−1.74 (76)	+0.04 (irrev); +0.31 (irrev)
1(phenP)	−1.62 (irrev)	+0.06 (irrev); +0.32 (irrev)
Bodipy-bdtCO	−1.58 (94)	+0.66 (81)
6	−1.56 (74)	+0.65 (76)
4(bpyC)	−1.90 (81); −1.66 (72); −1.31 (62)	+0.36 (irrev); 0.69 (irrev)
4(bpyP)	−1.98 (106); −1.67 (75); −1.32 (81)	+0.42 (irrev); +0.72 (irrev)
4(bpyCH2P)‡	−1.85 (irrev)	+0.36 (irrev); 0.72 (irrev)
5(phenP)‡	−1.74 (irrev); −1.85 (irrev)	+0.35 (irrev); 0.73 (irrev)

All measurements were at room temperature in dry CH₃CN at ∼1 × 10⁻³ M concentration in solutions containing 0.1 M (TBA)PF₆ as supporting electrolyte. A silver wire quasi-reference electrode was used and calibrated vs. Fc/Fc⁺ couple [0.40 V vs. SCE (58) with SCE vs. NHE = 0.24 V]. irrev, irreversible.

^*E_1/2 = (E_pa + E_pc)/2; E_pa and E_pc are anodic and cathodic peak potentials, respectively.

^†ΔE_p= (E_pa – E_pc). ΔE_p = 80 mV for Fc/Fc⁺ couple.

^‡4(bpyCH₂P) and 5(phenP) were measured in dry CH₂Cl₂.

Fig. 4.

(A–F) Cyclic voltammograms of (A) 6, (B) Bodipy-bdtCO, (C) 1(bpyC), (D) 1(bpyP), (E) 4(bpyC), and (F) 4(bpyP) were measured in 0.1 M TBAPF₆/dry CH₃CN solution. Potentials are referenced to Fc/Fc⁺.

The cyclic voltammograms of all of dyads 4, shown in Fig. 4, correspond to a sum of the CVs of individual Pt(bdt)(bpy) and Bodipy components. For 4(bpyC) and 4(bpyP), three reversible reduction couples between −1.31 V and −1.98 V are observed, as shown in Fig. 4 E and F, corresponding to the one Bodipy and two PtN₂S₂ reductions. For the dyad 4(bpyCH₂P), only one quasi-reversible reduction couple around −1.85 V is found (SI Appendix, Fig. S1C) and for 5(phenP), evidence for two reduction waves that are quasi-reversible at best is seen. For all of the dyads, one reversible anodic wave and one irreversible anodic wave are seen, corresponding to Bodipy and PtN₂S₂ oxidations, respectively.

Attachment of Dyads to TiO₂.

The main objective of the present study was to determine whether photofunctional dyads such as 4 were efficient when attached to TiO₂ for electron photoinjection into the semiconductor. The most common method for attachment of light absorbers to TiO₂ nanoparticles is through various anchoring groups such as carboxylate, phosphonate, sulfonate, salicylate, acetylacetonate, catecholate, and hydroxamate (59–64). In the present study, with phosphonate esters and carboxylate esters as diimine substituents on PtN₂S₂ chromphores and dyads, attempts to hydrolyze the esters prior to TiO₂ attachment proved problematic because of poor stability of the PtN₂S₂ moiety under the hydrolysis conditions. However, in light of an earlier observation of partial hydrolysis of a phosphonate ester of a rhodamine derivative when examined with TiO₂ (65), the idea of using in situ hydrolysis for attachment was attempted with complexes 1 and dyads 4, 5, and 6 in the presence of TiO₂.

An effective procedure was found that involved sonication of a slurry of platinized TiO₂ (Pt-TiO₂) in an acetonitrile solution of the PtN₂S₂ complex or dyad containing the phosphonate ester, followed by isolation of the solid, which was then washed with CH₃CN twice and dried under vacuum. Higher attachment was obtained with 20 min sonication. A comparison of the UV-vis spectra of dyes in CH₃CN before and after sonication and removal of Pt-TiO₂ showed a substantial decrease in the intensities of the MMLL′CT and/or Bodipy absorptions, which gave an approximate percentage of PtN₂S₂ complex or dyad attached to the surface of TiO₂. With no anchoring group present as in compounds 1(bpy) and 4(bpy), UV-vis spectra revealed a decrease of 11–16% of the characteristic bands between the initial and final solutions, indicating little if any binding to Pt-TiO₂ (Table 3 and SI Appendix, Figs. S14 and S19). For complexes having a single anchoring group such as 1(phenP) and 5(phenP), the decreases were much more notable, corresponding to binding of ∼70%. For the systems having two phosphonate ester substituents as possible anchoring groups such as in PtN₂S₂ complexes 1(bpyP), 1(bpyCH₂P) and dyads 4(bpyP), 4(bpyCH₂P), the degree of attachment increased significantly, corresponding to 82–99% attachment. The results are clearly consistent with binding of the PtN₂S₂ complexes and dyads occurring via the anchoring groups, rather than simply through physisorption. Due to poor solubility in acetonitrile, the sonication procedure did not work initially with the carboxylate PtN₂S₂ complex 1(bpyC) and the corresponding dyad 4(bpyC). However, through the use of methylene chloride solutions (rather than acetonitrile), these systems also showed excellent attachment to Pt-TiO₂ (87–89%).

Table 3.

Light-driven hydrogen production with different photosensitizers

Complexes	Irradiation time, h	H2, mL	Attachment, %	H2 TON*	H2 TOF†
1(bpy)	60	1.0	16	2,100	114
1(bpyP)	60	16.9	82	7,000	210
1(bpyCH2P)	60	4.9	99	1,700	84
1(phenP)	60	3.9	72	1,800	54
1(bpyC)	60	4.1	89	1,600	84
4(bpy)	60	2.3	11	7,100	361
4(bpyP)	60	44.1	97	15,400	547
4(bpyCH2P)	60	20.6	99	7,100	318
4(bpyC)	60	11.7	87	4,600	185
5(phenP)	60	7.3	70	3,500	171
6	60	1.6	36	1,500	57
4(bpyP)	288	115.8	97	40,300	—

General conditions: for attachment, 20 mg of Pt-TiO₂ and 2.5 mL of 50 µM solution of photosensitizer in CH₃CN or CH₂Cl₂ were sonicated for 20 min; for H₂ generation, 20 mg of dried Pt-TiO₂ containing the attached photosensitizer was added to 5 mL of 0.5 M ascorbic acid in H₂O at pH 4.0 and irradiated with 530 nm light.

^*Turnovers of H₂ are given with respect to photosensitizer attached to Pt-TiO₂.

^†The initial turnover frequency (TOF) of H₂ is given per mole of photosensitizer per hour.

All of the attachment results are given in Table 3 and SI Appendix. Fig. 5 shows the corresponding spectra for dyad 4(bpyP) in CH₃CN leading to attachment estimated at 97%. Fig. 6 provides results from diffuse reflectance spectroscopy of the dye-attached Pt-TiO₂ powders showing the MMLL′CT absorption for the PtN₂S₂ complexes 1 and both the Bodipy and MMLL′CT absorptions for dyads 4 and 5. When linked to TiO₂ in this manner, neither the complexes nor the dyads exhibited any evidence of dissociation from the nanoparticles when placed in water or aqueous/acetonitrile solvents.

Fig. 5.

UV-vis spectra of the solution of 4(bpyP) in CH₃CN (2.5 mL, 50 μM) before (black line) and after (red line) sonication with Pt-TiO₂ (20 mg), followed by Pt-TiO₂ removal. Sample solutions were in 2-mm cuvettes.

Fig. 6.

(A) Diffuse reflectance of series 1 attached to Pt-TiO₂. (B) Diffuse reflectance of series 4, as well as 5(phenP) and 6 attached to Pt-TiO₂. Each spectrum has been baseline corrected with a blank of Pt-TiO₂.

Light-Driven H₂ Production.

In this study, all of the dyads and PtN₂S₂ complexes, as well as Bodipy dye 6, were attached to Pt-TiO₂ in the manner described above and then examined for light-driven H₂ generation in pH 4 water with ascorbic acid (AA) as the sacrificial electron donor under 530 nm irradiation. For each sample, system pressure was monitored in real time, and at the end of each irradiation period, gas from the headspace of each sample was examined for H₂ content quantitatively, as previously described (26, 27, 66, 67). In Fig. 7, H₂ production using various chromophores is shown, whereas in Table 3 the results in terms of turnover numbers (TONs) and initial turnover frequency of H₂ with respect to photosensitizer attached to Pt-TiO₂ are given.

Fig. 7.

(A) Hydrogen production using different chromophores. (B) H₂ production using 1(bpyP) (black line), 4(bpyP) (cyan line), and 6 (orange line). Each sample contained 5 mL of 0.5 M ascorbic acid in H₂O at pH 4.0 to which was added 20 mg of Pt-TiO₂ with the photosensitizer attached as described in the text. Each sample was irradiated with 530 nm light.

Of the different light absorbers examined after 60 h, dyad 4(bpyP) exhibits the highest activity with a TON of 15,400, whereas systems with dyads 4(bpyCH₂P), 4(bpyC), and 5(phenP) are less active (7,100, 4,600, and 3,500 TONs, respectively). In each case, the activity of the dyad was found to be two to four times greater than that of the corresponding PtN₂S₂ complex (SI Appendix, Figs. S25–S29), suggesting that the direct binding of Bodipy to the PtN₂S₂ chromophore increases the potential light harvesting of the latter’s CT excited state. The value of having the CT chromophore also present for photoinduced electron transfer into TiO₂ and H₂ generation was demonstrated when the Bodipy dye 6 was used as photosensitizer without the PtN₂S₂ chromophore present, leading to a greatly diminished TON compared with both dyad 4(bpyP) and complex 1(bpyP) (Fig. 7B). In an analysis of the activity between 1(bpyP) and 1(bpyCH₂P) or between 4(bpyP) and 4(bpyCH₂P), the methylene spacer negatively impacts system activity, presumably by decreasing the effectiveness of electron transfer through the additional −CH₂− moiety for the latter. Finally, attachment of the photosensitizer (PtN₂S₂ complex or dyad) to TiO₂ through a linker is revealed by the fact that when either complex 1(bpy) or dyad 4(bpy) with no anchoring group is used, significantly smaller amounts of H₂ are obtained (Table 3).

To date, a variety of organic and inorganic chromophores have been used in conjunction with Pt-TiO₂ as the catalyst. Of several Pt(II) complexes that have been used with triethanolamine (TEOA) as the sacrificial donor (SD), Pt(II) terpyridyl acetylide complexes were found to generate 115 TONs in 12 h (λ > 410 nm) (37). Systems containing Ru bipyridyl complexes with EDTA as SD were reported to generate over 500 TONs in 3 h (λ > 420 nm) (68). In another study using TEOA as SD, several iodinated boron-dipyrromethene (Bodipy) dyes were shown to generate H₂, with one derivative promoting formation of 70 TONs of H₂ in 2 h (λ > 420 nm) (69) and another yielding ∼100 TONs in 25 h (λ > 420 nm) (70). Illumination of a selenorhodamine dye with white-light light-emitting diodes (LEDs) using ascorbic acid as SD resulted in ∼300 TONs of H₂ in 67 h (65). Two other systems were reported to have much higher activity in H₂ production. The first (71) used Eosin Y (λ > 460 nm) with TEOA as SD and produced ∼10,000 TONs of H₂ in 5 h, whereas the second (72) used Alizarin (λ > 420 nm) with TEOA as SD and yielded ∼10,000 TONs in 80 h. Earlier experiments performed here and elsewhere have found Eosin Y to be unstable upon reductive quenching, suggesting that if correct, these very active systems involve photoinduced electron transfer from the dye into the platinized TiO₂. Direct comparison of the present results with these earlier reports is not possible because of differences in the conditions used such as relative concentrations of system components, pH, the nature of the sacrificial donor, the light source used and its intensity, and the solvent.

Fig. 8A shows photographs for essentially complete attachment of 4(bpyP) to the semiconductor surface. The powder of Pt-TiO₂ changed from gray to pinkish whereas the purple stock solution of 4(bpyP) became colorless after 20 min of sonication. After 60 h of irradiation, the H₂-generating solution is also shown in Fig. 8B. The yellow color of the solution is indicative of dehydroascorbic acid (the oxidation product), whereas the solid of Pt-TiO₂ attached with 4(bpyP) retains the characteristic color shown in Fig. 8A. The durability of the 4(bpyP) system was further probed by the addition of more ascorbic acid as system activity appeared to decline after 4 d. The AA addition led to a resumption of activity that continued for another ∼5 d, and it was again repeated, generating ∼40,000 TONs over 12 d. The results are shown in Fig. 8C. The Bodipy-PtN₂S₂ photosensitizer thus appears to be stable under the irradiation conditions used.

Fig. 8.

(A) Photographs showing the color change in Pt-TiO₂ before and after sonication, with the initial purple color of 4(bpyP) in CH₃CN becoming almost colorless. (B) solid and solution color after 60 h of 530 nm irradiation with ascorbic acid present (the yellow color of the solution corresponds to dehydroascorbic acid). (C) H₂ production over 12 d for 4(bpyP) (2.5 mL, 50 μM), AA (5 mL, 0.5 M, pH 4.0), 20 mg of Pt-TiO₂, with λ_irrad 530 nm at 15 °C.

Conclusions

The new dyads reported here consisting of a Bodipy dye and a PtN₂S₂ CT chromophore have been prepared and studied in the context of light-driven generation of H₂ from aqueous protons with ascorbic acid as the sacrificial electron donor. The dyads exhibit both spectroscopic and electrochemical properties characteristic of the individual components with little electronic coupling between them. Each of the dyads has a diimine-attached substituent for binding to TiO₂. Connection of the dyads and their respective PtN₂S₂ complexes to platinized TiO₂ is achieved by a sonication procedure, and the resultant assemblies have been examined for the light-driven reduction of protons to H₂. Under 530 nm irradiation in pure aqueous solution, dyad 4(bpyP) is found to be most active with ca. 40,000 TONs based on the dyad over a period of 12 d. For all of the dyads, the activity was found to be two to four times greater than that of the corresponding PtN₂S₂ complex. When the Bodipy dye is used alone as the light absorber and linked to Pt-TiO₂, only minimal yields of H₂ are obtained upon 530 nm irradiation, indicating that photoinduced electron transfer does not readily occur from the dye alone. The value of attachment of the light absorber to Pt-TiO₂ is also demonstrated by substantially reduced H₂ generation when a Bodipy-PtN₂S₂ dyad without a linkage to the TiO₂ surface is used. From these results, we conclude that light absorption by the organic dye enhances formation of the ³MMLL′CT state, from which electron transfer occurs, and that attachment of this dyad assembly to Pt-TiO₂ increases the overall H₂-generating ability of the system. The results thus indicate the viability of a two-component light absorber (one strongly absorbing for photon capture and energy transfer, and the other set for excited state charge transfer) for generating hydrogen in the reductive side of water splitting.

Experimental

Chemicals.

All reactions were conducted under N₂ atmosphere. The compounds 2,2′-bipyridine-4,4′-dicarboxylic acid, potassium tetrachloroplatinate (K₂PtCl₄), and tetrakis(triphenylphosphine)palladium(0) purchased from Aldrich and 4,4′-dibromo-2,2′-bipyridine purchased from TCI were used without further purification. All solvents were dried before use. Syntheses of the following were done using published procedures: 2-oxobenzo[d][1,3]dithiole-5-[1,3,5,7-tetramethyl-2,6-diethyl(4,4-difluoro-4-bora-3a,4a-diaza-s-indacene)] (bodipy-bdtCO) (51), 4,4′-bis(methoxycarbonyl)-2,2′-bipyridine (4,4′-dcbpy) (73), 4,4′-bis(diethylphosphonate)-2,2′-bipyridine (4,4’-(P(O)(OEt)₂)bpy) (74), 4,4′-bis(diethylphosphonomethyl)-2,2′-bipyridine (4,4′-(CH₂P(O)(OEt)₂)bpy) (74), diethyl 1,10-phenanthrolin-5-ylphosphonate (5-(P(O)(OEt)₂)Phen) (75), cis-Pt(DMSO)₂Cl₂ (76), PtCl₂(4,4′-dcbpy) (77, 78), PtCl₂(4,4′-(P(O)(OEt)₂)bpy) (77, 78), Pt(4,4′-(P(O)(OMe)₂)bpy)Cl₂ (77, 78), PtCl₂(5-(P(O)(OEt)₂)Phen) (77, 78), PtCl₂(4,4′-(CH₂P(O)(OEt)₂)bpy) (77, 78), [Pt(4,4′-dcbpy)(CH₃CN)₂](CF₃SO₃)₂ (79), and diethyl (4-formylphenyl)phosphonate (80). All other chemical precursors and solvents were obtained from commercial sources and were used as received.

Characterization.

The ¹H NMR spectra were recorded on a Bruker Avance 400-MHz spectrometer and referenced to either a residual proton resonance of the deuterated solvent or tetramethylsilane as an internal standard. Elemental analyses were performed using a PerkinElmer 2400 Series II Analyzer. Mass spectrometry was performed on all compounds, using a Shimadzu LCMS and an LTQ VELOS Thermo LCMS (positive mode), respectively. CV measurements were performed with a CHI 680D potentiostat at room temperature under Ar, using a one-compartment cell with a glassy-carbon working electrode (3 mm diameter), a glassy-carbon auxiliary electrode (3 mm diameter), and an Ag wire quasi-reference electrode (QRE). For all measurements, tetrabutylammonium hexafluorophosphate was used as the supporting electrolyte with ferrocene as an internal reference. UV-vis spectra were recorded on a Cary 60 UV-vis spectrophotometer, using a 1-cm or 2-mm path length quartz cuvette in CH₃CN. Diffuse reflectance spectroscopy was performed using a Dip Probe Coupler (Agilent) and a VideoBarrelino (Harrick).

Syntheses.

1(bpy).

bdt (16 mg, 0.11 mmol) was added to a solution of potassium tert-butoxide (25 mg, 0.22 mmol) in 5 mL of CH₃CN and the solution was stirred for 15 min. Under N₂, the solution was added to Pt(4,4′-bpy)Cl₂ (46 mg, 0.11 mmol) in 2 mL of CH₂Cl₂. The resultant solution was stirred at room temperature for 11 h, after which it was reduced to dryness. The solid product was purified by column chromatography over silica gel using CH₃CN as eluant. The main blue band was collected, and the solvent was removed, affording the desired product as a purple crystalline solid. Yield: 30 mg (55%). ¹H NMR (d₆-DMSO, 400 MHz): 9.11 (d, 2H, bpy aromatic), 8.66 (d, 2H, bpy aromatic), 8.40 (t, 2H, bpy aromatic), 7.79 (t, 2H, bpy aromatic), 7.22 (t, 2H, dithiolate aromatic), 6.71 (t, 2H, dithiolate aromatic). Anal. Calcd for PtC₁₆H₁₂N₂S₂: C, 39.10; H, 2.46; N, 5.70. Found: C, 39.05; H, 2.38; N, 5.63.

1(bpyC).

Method A.

Pt(4,4′-dcbpy)Cl₂ (59 mg, 0.11 mmol) was suspended in 10 mL of deaerated methanol and bdt (16 mg, 0.11 mmol) was added, followed by potassium tert-butoxide (25 mg, 0.22 mmol). The suspension gradually darkened, and a purple precipitate appeared. The suspension was further stirred at room temperature under an atmosphere of nitrogen for 48 h, and the solid was collected by filtration. The crude product was purified by column chromatography over silica gel using CH₂Cl₂/EtOAc (5:1) as eluant. The main purple band was collected, and the solvent was removed, affording the desired product as a purple crystalline solid. Yield: 17 mg (25%). ¹H NMR (CD₂Cl₂, 400 MHz): 9.34 (d, 2H, bpy aromatic), 8.55 (s, 2H, bpy aromatic), 7.95 (d, 2H, bpy aromatic), 7.23 (s, 2H, dithiolate aromatic), 6.69 (s, 2H, dithiolate aromatic), 4.03 (s, 6H, CH₃). EI-MS(m/z): 608 [M+H]⁺. Anal. Calcd for PtC₂₀H₁₆N₂O₄S₂: C, 39.54; H, 2.65; N, 4.61. Found: C, 39.02; H, 2.52; N, 4.49.

Method B.

A suspension of Pt(4,4′-dcbpy)Cl₂ (54 mg, 0.10 mmol) and AgOTf (100 mg, 0.39 mmol) in 6 mL CH₃CN was refluxed for 36 h. After cooling to room temperature (RT), the reaction mixture was filtered and the filtrate was reduced to dryness to give a red solid. The isolated material, [Pt(4,4′-dcbpy)(CH₃CN)₂](CF₃SO₃)₂ was washed with acetone and diethyl ether and then dried in vacuo. Yield: 69.5 mg (82%). Potassium tert-butoxide (18 mg, 0.16 mmol) was added to a solution of bdt (11 mg, 0.08 mmol) in 2 mL of acetonitrile and stirred for 20 min under N₂, after which the solution was added to a solution of [Pt(4,4′-dcbpy)(CH₃CN)₂](CF₃SO₃)₂ (69.5 mg, 0.082 mmol) in 2 mL of acetonitrile, which became dark blue immediately. The mixture was stirred at room temperature overnight and the solid was isolated by filtration. The crude product was purified by column chromatography over silica gel using pure CH₂Cl₂ as eluant. The main blue band was collected, and the solvent was removed, affording the desired product as a blue crystalline solid. Yield: 21 mg (43%). Characterization data match method A values.

1(bpyP).

Potassium tert-butoxide (25 mg, 0.22 mmol) was added to a solution of bdt (16 mg, 0.11 mmol) in 5 mL of acetonitrile. After stirring for 15 min under N₂, the solution was added to a suspension of Pt(4,4′-(P(O)(OEt)₂)bpy)Cl₂ (76 mg, 0.11 mmol) in 1 mL of acetonitrile, which became dark blue immediately. The mixture was stirred at room temperature for 11 h and the solid was isolated by filtration. The crude product was purified by column chromatography over silica gel using a gradient of CH₃CN/CH₂Cl₂ (1:3–1:1) to afford the product as a blue solid. Yield: 72 mg (86%). ¹H NMR (CD₂Cl₂, 400 MHz): 9.42 (t, 2H, bpy aromatic), 8.41 (d, 2H, bpy aromatic), 7.76 (dd, 2H, bpy aromatic), 7.27 (d, 2H, dithiolate aromatic), 6.70 (d, 2H, dithiolate aromatic), 4.19 (m, 8H, CH₂ of ethyl groups), 1.31 (t, 12 H, CH₃ of ethyl groups). EI-MS(m/z): 764 [M+H]⁺. Anal. Calcd for C₂₄H₃₀N₂O₆P₂PtS₂: C, 37.75; H, 3.96; N, 3.67. Found: C, 37.23; H, 3.77; N, 3.61.

1(bpyCH₂P).

A solution of bdt (27 mg, 0.19 mmol) and potassium tert-butoxide (45 mg, 0.40 mmol) in 15 mL methanol was added dropwise to a solution of Pt(4,4′-CH₂P(O)(OEt)₂bpy)Cl₂ (137 mg, 0.19 mmol) in 15 mL of CH₂Cl₂. The mixture was stirred for 12 h and the solution was evaporated to give a purple solid. The crude product was purified by column chromatography over silica gel using CH₃CN as eluant. The main purple band was collected, and the solvent was removed affording the product, which was recrystallized from CH₂Cl₂/hexane to give needle-like crystals. Yield: (98 mg, 65%). ¹H NMR (CD₂Cl₂, 400 MHz): 9.11 (d, 2H, bpy aromatic), 8.05 (s, 2H, bpy aromatic), 7.43 (d, 2H, bpy aromatic), 7.32 (s, 2H, dithiolate aromatic), 6.74 (s, 2H, dithiolate aromatic), 4.12 (m, 8H, CH₂ of ethyl groups), 3.23 (s, 2H, CH₂), 3.17 (s, 2H, CH₂), 1.31 (t, 12 H, CH₃ of ethyl groups). EI-MS(m/z): 792 [M+H]⁺. Anal. Calcd for C₂₆H₃₄N₂O₆P₂PtS₂•H₂O: C, 38.57; H, 4.48; N, 3.46. Found: C, 38.56; H, 4.28; N, 3.40.

1(phenP).

A solution of bdt (27 mg, 0.19 mmol) and potassium tert-butoxide (45 mg, 0.40 mmol) in 15 mL methanol was added dropwise to a solution of Pt(5-phen-P(O)(OEt)₂)Cl₂ (110 mg, 0.19 mmol) in 15 mL of CH₂Cl₂.The mixture was stirred for 8h and the solution was evaporated to give a purple solid. The crude product was recrystallized from CH₂Cl₂/hexane. The purple solid obtained was purified by column chromatography on silica gel using CH₂Cl₂/hexane (2:1) as the eluant. The main purple band was collected, and the solvent was removed affording the product, which was recrystallized from CH₂Cl₂/pentane to give a purple powder. Yield: (64 mg, 52%). ¹H NMR (CD₂Cl₂, 400 MHz): 9.52 (d, 1H, Phen aromatic), 9.28 (d, 1H, Phen aromatic), 9.17 (d, 1H, Phen aromatic), 8.64 (d, 1H, Phen aromatic), 8.47 (d, 1H, Phen aromatic), 7.79 (t, 2H, Phen aromatic), 7.21 (s, 2H, dithiolate aromatic), 6.69 (s, 2H, dithiolate aromatic), 4.35 (m, 4H, CH₂ of ethyl groups), 1.38 (t, 6 H, CH₃ of ethyl groups). EI-MS(m/z): 652 [M+H]⁺. Anal. Calcd for C₂₂H₂₁N₂O₃PPtS₂: C, 40.55; H, 3.25; N, 4.30. Found: C, 41.74; H, 3.48; N, 4.08.

4(bpyC).

Pt(4,4′-dcbpy)Cl₂ (30 mg, 0.055 mmol) was suspended in 5 mL of deaerated methanol and bodipy-bdtCO (26 mg, 0.055 mmol) was added, followed by potassium tert-butoxide (12 mg, 0.11 mmol). The suspension was stirred at room temperature under an atmosphere of nitrogen for 48 h. The crude product was purified by column chromatography on silica gel using CH₂Cl₂/EtOAc (4:1) as eluant. The main purple band was collected, and the solvent was removed. The residue was recrystallized from CH₂Cl₂/ether to give the product as 18 mg (36%) of black needle-like crystals. ¹H NMR (CD₂Cl₂, 400 MHz): 9.40 (dd, 2H, bpy aromatic), 8.61 (s, 2H, bpy aromatic), 8.06 (dd, 2H, bpy aromatic), 7.45 (d, 1H, dithiolate aromatic), 7.25 (s, 1H, dithiolate aromatic), 6.74 (d, 1H, dithiolate aromatic), 4.02 (s, 6H), 2.49 (s, 6H, CH₃ on bodipy), 2.37 (q, 4H, CH₂ of ethyl groups), 1.48 (s, 6H, CH₃ on bodipy), 1.02 (t, 6H, CH₃ of ethyl groups). EI-MS(m/z): 910 [M+H]⁺, 890 [M-F]⁺. Anal. Calcd for C₃₇H₃₇BF₂N₄O₄PtS₂•(C₂H₅)₂O: C, 50.05; H, 4.82; N, 5.69. Found: C, 50.31; H, 4.55; N, 5.56.

4(bpyP).

The preparation of 4(bpy-P(O)(OEt)₂) is similar to that of 4 (bpy-COOMe) except that Pt(4,4′-dcbpy)Cl₂ is replaced by Pt(4,4′-(P(O)(OEt)₂)bpy)Cl₂. The crude product was purified by column chromatography on silica gel using CH₂Cl₂/acetone (3:2) as eluant. The main purple band was collected, and the solvent was removed. The residue was recrystallized from CH₂Cl₂/ether to give the product as 14 mg (39%) of black needle-like crystals. ¹H NMR (CD₂Cl₂, 400 MHz): 9.52 (d, 2H, bpy aromatic), 8.56 (d, 2H, bpy aromatic), 7.88 (m, 2H, bpy aromatic), 7.51 (d, 1H, dithiolate aromatic), 7.27 (s, 1H, dithiolate aromatic), 6.72 (d, dithiolate aromatic), 4.29 (m, 8H, CH₂ of ethyl groups), 2.48 (s, 6H, CH₃ on bodipy), 2.34 (q, 4H, CH₂ of ethyl groups on bodipy), 1.42 (s, 6H, CH₃ on bodipy), 1.40 (m, 12H, CH₃ of ethyl groups), 1.00 (t, 6H, CH₃ of ethyl groups on bodipy). EI-MS(m/z): 1,066 [M+H]⁺, 1,046 [M-F]⁺. Anal. Calcd for C₄₁H₅₁BF₂N₄O₆PtS₂•0.5CH₂Cl₂: C, 44.97; H, 4.73; N, 5.06. Found: C, 44.52; H, 4.53; N, 5.17.

4(bpyCH₂P).

A solution of bodipy-bdtCO (71 mg, 0.15 mmol) and potassium tert-butoxide (34 mg, 0.30 mmol) in 10 mL methanol was added dropwise to a solution of Pt(4,4′-(CH₂P(O)(OEt)₂)bpy)Cl₂ (108 mg, 0.15 mmol) in 25 mL of CH₂Cl₂. The mixture was stirred for 35 h and the solution was evaporated to give a solid. The crude product was purified by column chromatography on silica gel using CH₂Cl₂/hexane (2.5:1) as eluant. The main purple band was collected, and the solvent was removed, affording the product as a purple powder. Yield: (134 mg, 82%). ¹H NMR (CD₂Cl₂, 400 MHz): 9.20 (dd, 2H, bpy aromatic), 8.11 (s, 2H, bpy aromatic), 7.47 (m, 3H, 2H of bpy aromatic and 1H of dithiolate aromatic), 7.22 (s, 1H, dithiolate aromatic), 6.68 (d, 2H, dithiolate aromatic), 4.12 (m, 8H, CH₂ of ethyl groups), 3.29 (dd, 4H, CH₂ on bpy), 2.48 (s, 6H, CH₃ on bodipy), 2.33 (q, 4H, CH₂ of ethyl groups on bodipy), 1.43 (s, 6H, CH₃ on bodipy), 1.31 (m, 12H, CH₃ of ethyl groups), 1.02 (t, 12 H, CH₃ of ethyl groups on bodipy). EI-MS(m/z): 1,094 [M+H]⁺, 1,074 [M-F]⁺. Anal. Calcd for C₄₃H₅₅N₄O₆P₂PtS₂: C, 47.21; H, 5.07; N, 5.12. Found: C, 47.07; H, 5.11; N, 5.03.

5(phenP).

A solution of bodipy-bdtCO (71 mg, 0.15 mmol) and potassium tert-butoxide (34 mg, 0.30 mmol) in 10 mL methanol was added dropwise to a solution of PtCl₂(5-(P(O)(OEt)₂)Phen) (87 mg, 0.15 mmol) in 25 mL of CH₂Cl₂.The mixture was stirred for 30 h and the solution was evaporated to give a solid. The crude product was recrystallized by CH₂Cl₂/pentane and the purple solid obtained was purified by column chromatography on silica gel, using CH₂Cl₂/hexane (2.5:1) as eluant. The main purple band was collected, and the solvent was removed, affording the product. Yield: (102 mg, 71%). ¹H NMR (CD₂Cl₂, 400 MHz): 9.65 (m, 2H, Phen aromatic), 9.23 (t, 1H, Phen aromatic), 8.77 (m, 2H, Phen aromatic), 7.97 (m, 2H, Phen aromatic), 7.52 (d, 1H, dithiolate aromatic), 7.27 (s, 1H, dithiolate aromatic), 6.71 (d, 1H, dithiolate aromatic), 4.32 (m, 4H, CH₂ of ethyl groups), 2.48 (s, 6H, CH₃ on bodipy), 2.35 (q, 4H, CH₂ of ethyl groups on bodipy), 1.45 (s, 6H, CH₃ on bodipy), 1.36 (t, 6H, CH₃ of ethyl groups), 0.99 (m, 6H, CH₃ of ethyl groups). EI-MS(m/z): 954 [M+H]⁺, 934 [M-F]⁺. Anal. Calcd for C₃₉H₄₂BF₂N₄O₃PPtS₂: C, 49.11; H, 4.44; N, 5.87. Found: C, 49.54; H, 4.54; N, 5.57.

Bodipy-C₆H₄P(O)(OEt)₂, 6.

The compounds 3-ethyl-2,4-dimethyl-1H-pyrrole (356 mg, 2.89 mmol) and diethyl (4-formylphenyl)phosphonate (349 mg, 1.44 mmol) were dissolved in dry CH₂Cl₂ (110 mL). Trifluoroacetic acid (two drops) was added to the solution, which was stirred at room temperature overnight. DDQ (327 mg, 1.44 mmol) was added in a single portion, and the mixture was stirred at room temperature for 10 h. N,N-diisopropylethylamine (2.8 mL, 16 mmol) and BF₃·Et₂O (2.8 mL, 23 mmol) were added, and the mixture was stirred at room temperature for 10 h. The reaction mixture was washed with water (3 × 20 mL) and brine (3 × 20 mL). The separated organic fractions were dried with MgSO₄ before the solvent was removed. The residue was purified by column chromatography on silica gel, using toluene/CH₃CN (4:1) as eluant (297 mg, 40% yield). ¹H NMR (CDCl₃, 400 MHz): δ = 7.97 (m, 2 H, benzene aromatic), 7.44 (m, 2 H, benzene aromatic), 4,20 (m, 4H, CH₂ of ethyl groups), 2.53 (s, 6H, CH₃ on bodipy), 2.32 (q, 4H, CH₂ of ethyl groups on bodipy), 1.36 (t, 6H, CH₃ of ethyl groups), 1.25 (s, 6H, CH₃ on bodipy), 0.99 (t, 6H, CH₃ of ethyl groups). EI-MS(m/z): 517 [M+H]⁺, 497 [M-F]⁺. Anal. Calcd for C₂₇H₃₆BF₂N₂O₃P: C, 62.80; H, 7.03; N, 5.43. Found: C, 62.68; H, 7.23; N, 5.45.

Attachment of Dyads, Complexes, and Bodipy Dye 6 to TiO₂.

A total of 2.5 mL of a stock solution of the photosensitizer in CH₃CN (50 μM) [or in CH₂Cl₂ for 1(bpyC) and 4(bpyC)] was added to a 20-mL vial containing 20 mg of platinized TiO₂. The sample vial was sonicated for 20 min in the dark and the platinized TiO₂ particles were immediately separated by centrifugation and dried under vacuum. The photosensitizer-attached Pt-TiO₂ sample was then transferred to a 40-mL vial to which 5 mL of aqueous ascorbic acid solution (0.5 M, pH 4.0) was added. For samples prepared by this method, hydrogen evolution studies were conducted in aqueous media, rather than 1:1 vol/vol CH₃CN/H₂O.

Hydrogen Evolution Studies.

The 40-mL sample vials were placed into a temperature-controlled block at 15 °C and each vial was sealed with an air-tight cap fitted with a pressure transducer and a rubber septum. The samples were then purged with a mixture of gas containing N₂/CH₄ (79:21 mol %). The methane present in the gas mixture serves as an internal reference for GC analysis at the end of the experiment. The samples were irradiated in a locally built 16-well photolysis apparatus with high-power green (530 nm) LUXEON Rebel LEDs, mounted on a 20-mm Star CoolBase-161 lm at 700 mA atop an orbital shaker. The light power of each LED was set to 130 mW and measured with an L30 A Thermal sensor and a Nova II power meter (Ophir- Spiricon). The pressure changes in the vials were recorded using a Labview program from a Freescale semiconductor sensor (MPX4259A series). At the end of the experiment, the headspaces of the vials were characterized by gas chromatography to ensure that the measured pressure change was a consequence of hydrogen generation and to confirm the amount of hydrogen generated. The amounts of hydrogen evolved were determined using a Shimadzu GC-17A gas chromatograph with a 5-Å molecular sieve column (30 m, 0.53 mm) and a thermal conductivity detector and were quantified by a calibration plot to the internal CH₄ standard.

SI Appendix contains the following: ¹H NMR spectra of the Pt chromophores; cyclic voltammograms of 1(bpyCH₂P), 1(phenP), 4(bpyCH₂P), 5(phenP); preparation of Pt-TiO₂; comparison of the UV-vis spectra of chromophores in CH₃CN (or CH₂Cl₂) before and after sonication; and H₂ production using various chromophores. Complete crystallographic information is available in CIF format.