Photoactive Metal-to-Ligand Charge Transfer Excited States in 3d⁶ Complexes with Cr⁰, Mn^I, Fe^II, and Co^III

Abstract

Many coordination complexes and organometallic compounds with the 4d⁶ and 5d⁶ valence electron configurations have outstanding photophysical and photochemical properties, which stem from metal-to-ligand charge transfer (MLCT) excited states. This substance class makes extensive use of the most precious and least abundant metal elements, and consequently there has been a long-standing interest in first-row transition metal compounds with photoactive MLCT states. Semiprecious copper(I) with its completely filled 3d subshell is a relatively straightforward and well explored case, but in 3d⁶ complexes the partially filled d-orbitals lead to energetically low-lying metal-centered (MC) states that can cause undesirably fast MLCT excited state deactivation. Herein, we discuss recent advances made with isoelectronic Cr⁰, Mn^I, Fe^II, and Co^III compounds, for which long-lived MLCT states have become accessible over the past five years. Furthermore, we discuss possible future developments in the search for new first-row transition metal complexes with partially filled 3d subshells and photoactive MLCT states for next-generation applications in photophysics and photochemistry.

Introduction

Transition metal complexes with a metal-centered HOMO and a ligand-based LUMO can have MLCT excited states with lifetimes on the order of nano- to microseconds, long enough for luminescence or diffusion-controlled photochemical processes to occur. According to a photophysical rule of thumb established by exploring many precious metal-based complexes,¹ the larger the energy gap between the lowest MLCT excited state and the electronic ground state, the longer the excited-state lifetime and the higher the luminescence quantum yield become. To obtain large energy gaps, chelating polypyridine ligands are often combined with second- or third-row transition metals having six electrons in their outermost d-subshell. These heavier d-metals embedded into an octahedral coordination environment of ligands with combined σ-donor and π-acceptor properties then adopt a low-spin d⁶ configuration, in which the HOMO corresponds to a triply degenerate set of d-orbitals (named t_2g in Figure 1a) and a LUMO with antibonding character on the ligand’s π-system (called π* in Figure 1a). Typical representatives are Ru^II,² Re^I,³ Os^II,⁴ and Ir^III,⁵ all of which belong to the rarest and most expensive stable elements; hence there is considerable interest in alternatives from the first row of transition metals, which would be more abundant and cheaper.

Figure 1.

(a) Molecular orbital (MO) scheme for low-spin d⁶ metal complexes as typically encountered for Ru^II polypyridine, cyclometalated Ir^III, and related precious metal-based compounds, simplified to octahedral symmetry. (b) Typical MO scheme for Fe^II polypyridines, simplified to O_h point group. (c) Single configurational coordinate diagram with the key electronic states in low-spin 3d⁶ complexes; Q is a nuclear coordinate, and E is relative energy.

Fe^II is the most obvious choice and by far the best investigated case among 3d⁶ compounds,⁶ in which photoactive MLCT excited states can in principle be installed, but this is very difficult.⁷ The main challenge is that the lowest MLCT excited states of Fe^II complexes very often deactivate on the femto- to picosecond time scales,⁸⁻¹⁰ making them unsuitable for many photophysical and photochemical applications. The underlying reason for this behavior is the decrease in ligand field strength when going from the second to the first row of the d-metals, and this in turn is a consequence of the more contracted nature of the 3d-orbitals compared to the 4d- or 5d-orbitals.¹¹ In Figure 1b, this manifests in a situation in which the LUMO is no longer a ligand-based π* orbital but instead becomes metal-centered, comprised of a doubly degenerate set of d-orbitals with e_g symmetry in octahedral ligand fields. In other words, the HOMO–LUMO transition is now metal-centered (MC) and the MLCT state is no longer the lowest electronically excited state, causing rapid MLCT excited state deactivation.¹² The relaxation from an initially excited MLCT into a lower lying MC state is called internal conversion (IC), and its rate and efficiency depend on the relative energies of the involved states and on the activation barrier for the IC process. In the prototypical [Fe(bpy)₃]²⁺ complex (bpy = 2,2′-bipyridine) (Figure 2a), IC is associated with an activation energy of only roughly 300 cm^–1,¹³ and the MLCT state deactivates within 50–80 fs at room temperature.^14,15 Such ultrafast relaxation is often illustrated in potential well diagrams (Figure 1c),⁷ in which the horizontal axis reflects the altering bonding situations in different electronic states caused by the redistribution of electron density between different orbitals. The lowest MC states of 3d⁶ metal complexes are strongly distorted with respect to the MLCT states and the electronic ground state,¹⁶ owing to the promotion of electrons from formally non-σ-bonding t_2g-orbitals to antibonding e_g-orbitals, thus leading to an elongation of the average metal–ligand bond distance. This property can make the activation barrier for IC from the lowest MLCT into MC states undesirably small. To maximize that activation barrier, it is helpful to establish strong ligand fields and rigid coordination environments, which shift the MC states to high energies (ideally above the lowest MLCT state) and furthermore allow only minimal displacements of the MC potential wells. Strong ligand fields can be obtained with strong σ-donors (which act on the metal e_g-orbitals and raise their energy) and strong π-acceptors (which influence the metal t_2g-orbitals and lower their energy), whereas rigidity is typically achievable with bi- or tridentate chelate ligands that offer limited degrees of conformational freedom when coordinated to the metal.¹⁷ Over the past few years, significant progress has been made along these lines, not just with 3d⁶ metal complexes, but also with other types of photoactive first-row transition metal compounds.¹⁸ As noted above, Fe^II has received most attention among 3d⁶ compounds,¹⁰ even though early studies already noted that Cr⁰ compounds are promising as 3d⁶ MLCT luminophores,^19,20 but then it seems that isoelectronic alternatives to Fe^II were largely ignored for three decades. Over the past 5 years, new Cr⁰, Mn^I and Co^III complexes with photoactive MLCT excited states were discovered, and important progress on Fe^II complexes has been made. In the following, we focus on these key developments and provide a comparative discussion of the photophysical and photochemical characteristics of these four different, yet closely related 3d⁶ species.

Figure 2.

Polypyridine complexes of Fe^II: (a) [Fe(bpy)₃]²⁺.^14,15 (b) [Fe(dcpp)₂]²⁺ complex (dcpp = 2,6-bis(2-carboxypyridyl)pyridine) with its particularly strongly π-accepting terdentate ligands.²² (c) [Fe(dcpp)(ddpd)]²⁺ complex (ddpd = N,N′-dimethyl-N,N′-dipyridine-2-yl-pyridine-2,6-diamine).²³ (d) [Fe(dqp)₂]²⁺ complex (dqp = 2,6-di(quinoline-8-yl)pyridine).²⁴ (e) [Fe(pdmmi)₂]²⁺ complex (pdmmi = 3,3′-pyridine-2,6-diyl(methylene)bis(1-methylimidazolylidene).²⁴ (f) Fe^II is coordinated by the three bpy subunits of a cage-bpy ligand to afford the [Fe(cage-bpy)]²⁺ complex; two Cu^I ions can furthermore be coordinated by the four nitrogen atoms of both imine caps of the cage to yield the [FeCu₂(cage-bpy)]²⁺ complex.⁹ (g) Trinuclear Ru^II-Fe^II-Ru^II compound.²⁶ (h) High-spin [Fe(dftpy)₂]²⁺ (dftpy = 6,6″-difluoro-2,2′:6′,2″-terpyridine), [Fe(dctpy)₂]²⁺ (dctpy = 6,6″-dichloro-2,2′:6′,2″-terpyridine) and [Fe(dbtpy)₂]²⁺ (dbtpy = 6,6″-dibromo-2,2′:6′,2″-terpyridine) complexes with ^5/7MLCT excited states.^27,28

Iron(II) Compounds

With classical polypyridines, the achievable ligand field strengths for octahedral Fe^II complexes are typically such that MC states depopulate MLCT excited states in ultrafast manner, and this is usually monitored by following excited state absorption (ESA) bands that are characteristic for the one-electron reduced polypyridine ligands.²¹ A particularly strongly π-accepting variant of a tridentate polypyridine came close to a point (Figure 2b) at which the order of the two lowest ³MC and ⁵MC excited states was reversed when compared to the majority of previously investigated compounds, yet MLCT deactivation remained ultrafast.²² When combining that strongly π-accepting chelate with a strongly electron-donating tridentate ligand, a heteroleptic push–pull type Fe^II complex (Figure 2c) with a similarly strong ligand field was obtained, but its MLCT lifetime could not be measured.²³ The use of tridentate polypyridine ligands providing close to perfect octahedral coordination geometries gave access to Fe^II complexes (Figure 2d/e) with MLCT lifetimes approaching the ps time regime (Table 1).²⁴ A particularly ambitious study reported on a macrocyclic molecular cage (Figure 2f), in which one Fe^II cation was coordinated by the three central bpy units.^9,25 The MLCT decay of the resulting [Fe(cage-bpy)]²⁺ complex exhibits vibronic coherence, from which information concerning the structural distortions responsible for the internal conversion of the MLCT into the MC state can be gained. The coordination of Cu^I ions by four nitrogen atoms at the two caps of the cage rigidifies the overall molecular structure in [FeCu₂(cage-bpy)]²⁺ and decelerates the MLCT decay from 110 fs to 2.6 ps (Table 1, entry 7). The authors noted that this represented the longest reported Fe^II 3MLCT lifetime in a pure polypyridine coordination environment at the time of publication.⁹ For a trinuclear Ru^II-Fe^II-Ru^II compound integrating a central [Fe(tpy)₂]²⁺-like unit (Figure 2g), a decay with a time constant of 23 ps (Table 1, entry 8) was tentatively attributed to the relaxation of the lowest ³MLCT excited state into MC states.²⁶

Table 1. MLCT Lifetimes (τ_MLCT) of Fe^II Complexes with Polypyridine Ligands in Solution at Room Temperature.

entry	compound	molecular structure	τMLCT
1	[Fe(bpy)3]2+	Figure 2a	50–80 fs14,15
2	[Fe(dcpp)2]2+	Figure 2b	N/A22
3	[Fe(dcpp)(ddpd)]2+	Figure 2c	N/A23
4	[Fe(dqp)2]2+	Figure 2d	450 fs24
5	[Fe(pdmmi)2]2+	Figure 2e	0.8–1.5 ps24
6	[Fe(cage-bpy)]2+	Figure 2f	110 fs9
7	[FeCu2(cage-bpy)]2+	Figure 2f	2.6 ps9
8	RuII–FeII-RuII	Figure 2g	23 ps26
9	[Fe(dftpy)2]2+a	Figure 2h	14.0 ps28
10	[Fe(dctpy)2]2+a	Figure 2h	16.0 ps27,28
11	[Fe(dbtpy)2]2+a	Figure 2h	17.4 ps28
12	[Fe(tpy)(CN)3]−	Figure 3a	ca. 10 ps29
13	[Fe(bpy)(CN4)]2–	Figure 3b	18 ps30,31

^aThe indicated lifetime is for a ^5/7MLCT excited state. The respective compounds have a high-spin d⁶ valence electron configuration in the ground state (⁵T₂).

In a radically different approach, the MLCT lifetimes of [Fe(tpy)₂]²⁺ derivatives were tuned by ligand halogenation (Figure 2h) and steric strain.^27,28 With increasing halogen size, steric interactions between the two coordinated tpy ligands increasingly restrict the conformational degrees of freedom, which likely contributes to the MLCT lifetime elongation from 14.0 to 16.0 and 17.4 ps (Table 1, entries 9–11) when going from F to Cl and Br substituents. However, the repulsive interactions between the halogen atoms from one tpy ligand and the other coordinated tpy furthermore cause a weak ligand field; hence these compounds have a high-spin ⁵T₂ ground state (instead of ¹A₁ as all other complexes in Figure 2). The photoactive MLCT excited states in this situation are either quintets or septets, different from the triplets commonly observed for low-spin Fe^II complexes. Thus, suitably designed coordination spheres around Fe^II can provide access to photochemistry via quintet MLCT excited states.

In combination with strong-field cyanido ligands, a tpy ligand (tpy = 2,2′:6′,2″-terpyridine) on Fe^II (Figure 3a) recently afforded a ³MLCT lifetime of ca. 10 ps (Table 1, entry 12).²⁹ Earlier studies on the related [Fe(bpy(CN)₄]^2– complex (Figure 3b) revealed an MLCT lifetime of 18 ps (Table 1, entry 13), suggesting that cyanido ligands are helpful to decelerate relaxation to MC states,^30,31 but they also tend to make the photophysics of the resulting complexes very susceptible to interactions with the solvent.³² Borylation of the peripheral nitrogen atoms of the cyanido ligands presumably further strengthens the ligand field,³³ yet the detectable excited-state lifetime of 28 ps in [Fe(bpy)(BCF)₄]^2– (BCF = tris(pentafluorophenyl)) (Figure 3c) was attributed to an MC rather than an MLCT state.³⁴ Further MLCT excited-state tuning in iron(II) cyanido complexes is possible through variation of the α-diimine ligands.³⁵

Figure 3.

Heteroleptic Fe^II complexes with coordination environments containing a combination of polypyridine and cyanido ligands: (a) [Fe(tpy)(CN)₃]⁻.²⁹ (b) [Fe(bpy)(CN)₄]^2–.^30,31 (c) [Fe(bpy)(BCF)₄]^2–.³⁴

N-Heterocyclic carbene (NHC) ligands emerged as better alternatives to polypyridines as far as long ³MLCT lifetimes are concerned, largely owing due their stronger σ-donor properties.^6,7,36,37 Following an initial study published in 2013,³⁸ a series of conceptually related Fe^II compounds (Figure 4a-f) with chelating NHC ligands featuring MLCT lifetimes on the order of a few picoseconds were discovered (Table 2),³⁹⁻⁴⁵ which represented record lifetimes at that point.⁷ The only outlier in this series is [Fe(pbbi)₂]²⁺ (Figure 4c) with its lifetime of merely 0.3 ps,³⁸ which is likely the result of a weakened ligand field caused by the steric strain imposed by the tert-butyl substituents. The attachment of peripheral anthracenyl- or pyrenyl-substituents has no important effect on the ³MLCT lifetime (Figure 4g/h),⁴⁶ because the triplet energies of the respective molecular entities are too high relative to the lowest ³MLCT state; hence triplet reservoir effects cannot be expected.⁴⁷ In the very close to perfectly octahedral coordination environment of [Fe(dpmi)₂]²⁺ (Figure 4i), a ³MLCT lifetime of 9.2 ps was obtained (Table 2, entry 9).⁴⁸ The ³MLCT excited-state decay behavior has furthermore been investigated in what could be considered an Fe^II NHC–Zn^II porphyrin dyad (Figure 4j), and a lifetime of 160 ps was reported (Table 2, entry 10).⁴⁹ This dyad was furthermore reported to be emissive from an Fe^II NHC-based ³MLCT state.

Figure 4.

Fe^II complexes with N-heterocyclic carbene ligands: (a) [Fe(bpmi)₂]²⁺ (pbmi = pyridine-2,6-diyl)bis(1-methyl-imidazol-2-ylidene).³⁸ (b) [Fe(pbmbi)₂]²⁺ (pbmbi = (pyridine-2,6-diyl)bis(1-methyl-benzimidazol-2-ylidene).⁴¹ (c) [Fe(pbbi)₂]²⁺ (pbbi = pyridine-2,6-diyl)bis(1-tert-butyl)-imidazol-2-ylidene).³⁸ (d) [Fe(cpbmi)₂]²⁺ (cpbmi = (carboxypyridine-2,6-diyl)bis(1-methyl-imidazol-2-ylidene).^40,42 (e) [Fe(cpbmbi)₂]²⁺ (cpbmbi = (carboxypyridine-2,6-diyl)bis(1-methyl-benzimidazol-2-ylidene).^7,41 (f) [Fe(pzbmi)₂]²⁺ (pzbmi = (pyrazine-2,6-diyl)bis(1-methyl-imidazol-2-ylidene).⁴⁴ (g) [Fe(bim-ant)₂]²⁺ (bim-ant = (4-anthracene-pyridine-2,6-diyl)bis(1-methylimidazol-2-ylidene)).⁴⁶ (h) [Fe(bim-pyr)₂]²⁺ (bim-pyr = (4-pyrene-pyridine-2,6-diyl)bis(1-methylimidazol-2-ylidene)).⁴⁶ (i) [Fe(dpmi)₂]²⁺ (dpmi = di(pyridine-2-yl)(3-methylimidazol-2-yl)methane).⁴⁸ (j) Dinuclear [FeNHC-ZnP]²⁺ compound.⁴⁹

Table 2. MLCT Lifetimes (τ_MLCT) of Fe^II Complexes with NHC or Mesoionic Carbene Ligands in Solution at Room Temperature.

entry	compound	molecular structure	τMLCT
1	[Fe(pbmi)2]2+	Figure 4a	9 ps38
2	[Fe(pbmbi)2]2+	Figure 4b	16 ps41
3	[Fe(pbbi)2]2+	Figure 4c	0.3 ps38
4	[Fe(cpbmi)2]2+	Figure 4d	16.5–18 ps40,42
5	[Fe(cpbmbi)2]2+	Figure 4e	26 ps7,41
6	[Fe(pzbmi)2]2+	Figure 4f	21–25 ps44
7	[Fe(bim-ant)2]2+	Figure 4g	13.4 ps46
8	[Fe(bim-pyr)2]2+	Figure 4h	12.8 ps46
9	[Fe(dpmi)2]2+	Figure 4i	9.2 ps48
10	[FeNHC-ZnP]2+	Figure 4j	160 ps49
11	[Fe(btz)3]2+	Figure 5a	528 ps50
12	[Fe(btz)2(bpy)]2+	Figure 5b	7.6/13 ps30,57
13	[Fe(btp)2]2+	Figure 5c	8.7 ps58

Mesoionic carbenes allowed an MLCT lifetime elongation up to 0.5 ns (Figure 5a),⁵⁰ owing to a further increase in ligand field strength caused by their strong π-acceptor properties, while remaining similarly strong σ-donors as NHCs (Table 2, entry 11).⁵¹ The electron donating properties of these mesoionic carbenes are in fact sufficiently pronounced to make Fe^III the most stable oxidation state in a homoleptic tris(bidentate) complex,⁵² and this led to the (presumably accidental) discovery of Fe^III complexes with luminescent ligand-to-metal charge transfer (LMCT) excited states with lifetimes up to the nanosecond regime.^53,54 The accessibility of both the Fe^II and Fe^III oxidation states in the compound of Figure 5a furthermore gave access to an unusual operation mode for photoredox catalysis, in which the two different oxidation states were excited consecutively.⁵⁵ Moreover, an uncommon symmetry-breaking charge separation reaction was observable after excitation of [Fe(btz)₃]³⁺, leading to Fe^II and Fe^IV photoproducts, thus corresponding essentially to a photoinduced disproportionation reaction.⁵⁶ Turning back to Fe^II 3MLCT excited states and their lifetimes, we note that a heteroleptic Fe^II complex with the same btz ligand (Figure 5b)^30,57 and a tridendate ligand combining mesoionic carbene and pyridine ligand motifs (Figure 5c)⁵⁸ give less spectacular results in the sense that the MLCT lifetime is much shorter (Table 2, entries 12–13).

Figure 5.

Fe^II complexes with mesoionic carbene ligands: (a) [Fe(btz)₃]²⁺ (btz = 3,3′-dimethyl-1,1′-bis(p-tolyl)-4,4′-bis(1,2,3-triazol-5-ylidene)).⁵⁰ (b) [Fe(btz)₂(bpy)]²⁺.^30,57 (c) [Fe(btp)₂]²⁺ (btp = 4,4′-(pyridine-2,6-diyl)bis(1-ethyl-3-methyl-1,2,3-triazol-5-ylidene)).⁵⁸

In a conceptually much different approach, tridentate ligands incorporating π-donating amido groups enabled strong mixing between the metal t_2g orbitals and the filled ligand orbitals (Figure 6a), such that the resulting HOMO obtained substantial ligand character,⁵⁹ an effect that had been predicted computationally.^60,61 In combination with π-accepting phenanthridine and quinoline moieties in the same (tridentate) chelate (Figure 6b), this “HOMO inversion” effect led to absorption over large parts of the UV–vis spectrum.⁶² The lowest electronically excited states in this compound class (leading to absorption bands between 650 to 900 nm) involve the relocation of electron density from the π* (p+d) N_amido-Fe antibonding orbitals (HOMO in Figure 6a) to phenanthridine-based π* orbitals, and were termed PALCT for “π_antibonding-to-ligand charge transfer”.⁵⁹ Transitions with cleaner MLCT character seem to occur at higher energy in those compounds. The ³PALCT excited states have lifetimes on the order of 2–3 ns in solution at room temperature (Table 3, entries 1–3). X-ray techniques confirmed the high degree of metal–ligand bond covalence in this compound class,⁶³ and furthermore led to the insight that the ensuing low Racah B parameter overcompensates for the comparatively weak ligand field splitting (10 Dq) imparted by the π-donating amido units. Consequently, the 10 Dq/B ratio is larger for [Fe(pqa)₂^(Cl)] than for [Fe(tpy)₂]²⁺. The combined effects of a stabilized PALCT state, minimal stabilization of the MC states, and a substantial inner-sphere reorganization energy cause a greater barrier for deactivation of the lowest charge-transfer excited state by MC states.⁶³

Figure 6.

(a) Simplified molecular orbital diagram illustrating the situation with phenanthridine-based π* orbitals, metal-based t_2g orbitals, and amido-centered N(2p) orbitals, leading to a mixed and destabilized HOMO.⁵⁹ Molecular structures of: (b) [Fe(pqa)₂^(R)] (pqa = (phenanthridin-4-yl)(quinoline-8-yl)amido, R = ^tBu, CF₃, Cl).^59,62,63 (c) [Fe(pbpy)(tpy)]²⁺ (pbpy = 6-phenyl-2,2′-bipyridine).⁷² (d) [Fe(C^N_py^N_py)(6-NHC-bpy)]⁺ (HC^N_py^N_py = 6-(phenyl)-2,2′-bipyridine; 6-NHC-bpy = 6-(1-methyl-imidazol-2-ylidene)-2,2′-bipyridine.⁷³ (e) [Fe(pphen)₂] (pphen = 9-phenylphenanthroline).⁷⁴

Table 3. Charge-Transfer Excited-State Lifetimes (τ_CT) of Fe^II Complexes with Amido or Cyclometalating Ligands in Solution at Room Temperature.

entry	compound	molecular structure	τCT
1	[Fe(pqa)2(tBu)]a	Figure 6b	2.0–2.6 ns59
2	[Fe(pqa)2(CF3)]a	Figure 6b	2.5–2.7 ns59
3	[Fe(pqa)2(Cl)]a	Figure 6b	∼3 ns63
4	[Fe(pbpy)(tpy)]2+	Figure 6c	0.8 ps72
5	[Fe(C^Npy^Npy)(6-NHC-bpy)]+	Figure 6d	21.4 ps73
6	[Fe(pphen)2]	Figure 6e	1 ns74

^aThe lowest excited state is a PALCT (π_antibonding-to-ligand charge transfer) state in this case.^59,63

Cyclometalated Fe^II complexes have long received attention from a computational chemistry perspective,⁶⁴⁻⁷⁰ but air- and water-stable cyclometalated Fe^II compounds amenable to experimental photochemistry have been elusive.⁷¹ Recently, a robust heteroleptic Fe^II complex with one N^N^C coordinating ligand and one tpy chelate (Figure 6c), was found to have an MLCT lifetime of 0.8 ps (Table 3, entry 4),⁷² a factor of 5 longer than in the structurally related [Fe(tpy)₂]²⁺. This finding is in line with earlier theoretical studies that considered N^N^C_aryl ligands promising for elongating the MLCT lifetimes of Fe^II complexes,^61,67,68 though a lifetime of 0.8 ps for [Fe(pbpy)(tpy)]²⁺ is short in comparison to 528 ps for [Fe(btz)₃]²⁺ (Table 2, entry 11). The combination of a cyclometalating ligand with a bpy ligand bearing an additional NHC coordination site in the [Fe(C^Npy^Npy)(6-NHC-bpy)]⁺ complex (Figure 6d) led to a ³MLCT lifetime of 21.4 ps (Table 3, entry 5).⁷³

9-Phenylphenanthroline was later used as a tridentate chelate ligand for a homoleptic Fe^II complex (Figure 6e) with a rigid N^N^C coordination environment that led to an MLCT excited state featuring a 1 ns lifetime in solution at room temperature (Table 3, entry 6).⁷⁴ Photoluminescence in the NIR spectral range between 1170 and 1230 nm was reported for this compound, which seems very unusual given that in isoelectronic Ru^II and Os^II compounds, the energy gap law typically makes MLCT emission in the NIR improbable,¹ unless specifically tailored ligands (with particularly π-extended frameworks) are present.⁷⁵ To date, this is the only mononuclear Fe^II complex for which an emissive MLCT excited state has been claimed, whereas selected earlier studies merely reported on ligand-based fluorescence from Fe^II compounds.⁷⁶⁻⁷⁸ As long as the respective ligand-centered (singlet) excited states are sufficiently decoupled from the metal-centered excited states, the occurrence of such fluorescence seems very plausible.

Chromium(0) Carbonyls and Isocyanides

To stabilize the low oxidation state of Cr⁰ and to obtain a low-spin 3d⁶ valence electron configuration in octahedral complexes, strongly π-accepting ligands such as carbonyls and isocyanides are well-suited. Studies performed in the 1970s already noted the similarity between the electronic structures of zerovalent group 6 metal isocyanide complexes (Figure 7a) and Ru^II polypyridines.^19,79 For hexakis(arylisocyanide) complexes with the heavier homologues Mo⁰ and W⁰, emission in solution at room temperature was readily observable.¹⁹ Related studies reported on luminescence from [M(α-diimine)(CO)₄] complexes (Figure 7b; M = Cr⁰, Mo⁰, W⁰), with some of the Cr⁰ compounds apparently featuring dual emission from two MLCT excited states in benzene at room temperature.^20,80 Aside from this uncommon behavior in conflict with Kasha’s rule,⁸¹ later studies demonstrated that dissociation of CO ligands can be an important excited-state deactivation pathway in [Cr(α-diimine)(CO)₄] complexes.⁸²

Figure 7.

(a) Generic molecular structure of hexakis(arylisocyanide) complexes with low-valent metals (Cr⁰, Mn^I, Mo⁰, W⁰).^19,79 (b) Generic molecular structure of [M(α-diimine)(CO)₄] compounds (M = Cr⁰, Mo⁰, W⁰), here with α-diimine = 1,10-phenanthroline.^20,80 (c) Molecular structures of Cr⁰ complexes with bidentate arylisocyanide ligands (R = H: L^H, R = pyrenyl: L^Py), emitting from ³MLCT excited states.^83,85 (d) Competing MLCT decay pathways in [Cr(L^Py)₃] depending on solvent polarity.⁸⁵ (e) Molecular structures of [Mn(L^bi)₃]⁺ (L^bi = 2,5-bis(3,5-di-tert-butyl-2-isocyanophenyl)thiophene).⁹⁰ (f) [Mn(L^tri)₂]⁺ (L^tri = 5,5′-(2-isocyano-5-methyl-1,3-phenylene)bis(2-(3,5-di-tert-butyl-2-isocyanophenyl)thiophene)).⁹⁰ (g) [Mn(CNdippPh^OMe2)₆]⁺ (CNdippPh^OMe2 = 4-(3,5-dimethoxyphenyl)-2,6-diisopropylphenylisocyanide)),⁹⁴ and (h) [Co(bimca)₂]⁺ (bimca = 1,8-bis(imidazoline-2-yliden-1-yl)carbazolide).¹⁰⁸

A chelating arylisocyanide ligand provided access to clear-cut ³MLCT emission in a homoleptic Cr⁰ complex with a tris(bidentate) coordination environment (Figure 7c, R = H).⁸³ In dry and deaerated THF at room temperature, the ³MLCT lifetime was 2.2 ns (Table 4, entry 1), but the photoluminescence quantum yield (ϕ) was only 10^–5, barely above the detection limit. A Mo⁰ complex with the exact same coordination environment showed ³MLCT photoluminescence with a quantum yield of 5.8 × 10^–2 (5800 times higher) and an average lifetime of 880 ns (400 times longer) under identical conditions,⁸⁴ hence it seems plausible that the major nonradiative deactivation pathway in the Cr⁰ complex involves the same types of internal conversion processes into lower-lying ³MC (and possibly ⁵MC) states as in typical octahedral Fe^II compounds (Figure 1c).⁸³

Table 4. MLCT Lifetimes (τ_MLCT) and Emission Properties of Cr⁰, Mo⁰, Mn^I, and Co^III Complexes in Solution at Room Temperature^a.

entry	compound	molecular structure	τMLCT	emission properties
1	[Cr(LH)3]b	Figure 7c (R = H)	2.2 ns	ϕ = 10–5, λmax = 630 nm83
2	[Cr(LPy)3]c	Figure 7c (R = pyrenyl)	6.1 nsd	ϕ = 9 × 10–4, λmax = 682 nm85
3	[Mn(Lbi)3]+e	Figure 7e	0.74 nsf	ϕ = 5 × 10–4, λmax = 478 nm90
4	[Mn(Ltri)2]+e	Figure 7f	1.73 nsf	ϕ = 3 × 10–4, λmax = 525 nm90
5	[Mn(CNdippPhOMe2)6]+	Figure 7g	N/A	nonemissive94
6	[Co(bimca)2]+g	Figure 7h	1.2 nsh	nonemissive108

^aϕ is the photoluminescence quantum yield, λ_max is the emission band maximum.

^bIn deaerated THF at 20 °C.⁸³

^cIn cyclooctane at 20 °C.

^dWeighted average lifetime of a biexponential decay, attributed to different conformers.

^eIn deaerated CH₃CN at 20 °C.

^fWeighted average lifetime of a triexponential decay, attributed to different conformers.

^gIn deaerated CH₃CN at 22 °C.¹⁰⁸

^hLikely not a pure MLCT state, but instead containing substantial intraligand character.

Nonetheless, the distortion of the emissive ³MLCT state plays an non-negligible role according to a follow-up study, in which pyrenyl substituents were attached at the periphery of the diisocyanide chelate (Figure 7c, R = pyrenyl).⁸⁵ Though these substituents are oriented more or less orthogonally to the m-terphenyl backbone in the electronic ground state, transient absorption spectroscopy provides evidence that the emissive MLCT state has admixed pyrene character, compatible with the view that the MLCT-excited electron is more delocalized in [Cr(L^Py)₃] than in [Cr(L^H)₃]. Thus it seems plausible that the luminescent MLCT excited state of [Cr(L^Py)₃] is somewhat less distorted than in [Cr(L^H)₃], analogously to what is known from Ru^II and Os^II polypyridines with enlarged π-conjugation networks.^75,86−88 This picture of weaker excited-state distortion seems useful to help understand the 90 times higher luminescence quantum yield of [Cr(L^Py)₃] in comparison to [Cr(L^H)₃] (Table 4, entry 2), and it is supported by the observation of a roughly 20% narrower MLCT luminescence bandwidth for [Cr(L^Py)₃] compared to [Cr(L^H)₃]. An uncommon bell-shaped dependence of the MLCT lifetime on solvent polarity is obtained for [Cr(L^Py)₃], because the highest polarity solvents lower the MLCT energy and thereby facilitate nonradiative relaxation directly to the ground state, whereas too strongly apolar solvents raise the MLCT energy to the extent that internal conversion to MC states becomes increasingly favorable (Figure 7d).^85,89 The longest lifetime was measured in cyclooctane (6.1 ns), representing the record ³MLCT lifetime reported for a 3d⁶ complex in solution at room temperature so far.

Manganese(I) Arylisocyanides

[Mn(L^bi)₃]⁺ and [Mn(L^tri)₂]⁺ (Figure 7e/f) are analogues of [Ru(bpy)₃]²⁺ and [Ru(tpy)₂]²⁺, and furthermore represent the first examples of Mn^I compounds emitting from an MLCT excited state in solution at room temperature.^89,90 Owing to the higher oxidation state of Mn^I with respect to Cr⁰ (implying a more stabilized HOMO), the MLCT absorption and emission bands are considerably blue-shifted when using similar types of ligands. Thiophene instead of benzene units were used in the ligand backbones to connect individual arylisocyanide subunits, in an attempt to optimize the bite angles of the chelate ligands and the LUMO energies. Studies of Mo⁰ complexes indicated that the change from six-membered benzene rings to five-membered thiophene linkers on isocyanide ligand backbones can decrease the energy of the lowest MLCT excited state by roughly 3000 cm^–1,^91,92 and this seemed desirable to ensure that an MLCT state indeed becomes the energetically lowest excited state, even for the less readily oxidizable Mn^I. Despite this design principle, the lowest intraligand π–π* state is energetically close, and a triplet state of this type seems to play a non-negligible role in the photophysics and photochemistry of [Mn(L^bi)₃]⁺ and [Mn(L^tri)₂]⁺.⁹⁰ The MLCT luminescence quantum yields in deaerated acetonitrile at 20 °C are below 0.1% and the excited-state lifetimes are shorter than 2 ns (Table 4, entries 3–4), yet both photoinduced electron transfer and triplet–triplet energy transfer to various substrates proceeded readily. L^bi and L^tri require multistep syntheses, and therefore more readily accessible monodentate arylisocyanide ligands would be attractive alternatives for luminescent Mn^I complexes. However, with a monodentate ligand that gave a brightly emissive W⁰ complex in earlier studies,⁹³ the resulting hexakis(arylisocyanide)manganese(I) complex (Figure 7g) was nonluminescent and instead featured light-induced ligand dissociation,⁹⁴ similar to what is known from related carbonyl compounds.⁹⁵

Cobalt(III) Complex in a Coordination Environment of Amido and Carbene Ligand Motifs

The high oxidation state of Co^III relative to the other 3d⁶ species considered herein leads to comparatively strong ligand fields and furthermore increases the energy of the lowest MLCT state.⁹⁶ When combined with strong-field π-acceptor ligands such as cyanides or strongly σ-donating NHC ligands, the energetically lowest-lying MC state is sufficiently high above the ground state to become emissive, despite its substantial distortion.⁹⁷⁻¹⁰⁰ The same electronic state that causes ultrafast nonradiative MLCT deactivation in Fe^II polypyridines then features microsecond lifetimes in solution at room temperature.¹⁰⁰ In combination with electron-rich imine ligands, an LMCT state can become the energetically lowest (and luminescent) excited state,¹⁰¹ representing a reversal of the preferred charge transfer direction relative to Cr⁰, Mn^I, and Fe^II. This is unsurprising, because higher oxidation states are of course more easily reduced, and this design principle has found much recent attention for the development of LMCT luminophores,¹⁰² for example, based on Zr^IV,^103,104 Mn^IV,¹⁰⁵ or Fe^III.^52,53,106 Installing an energetically lowest-lying MLCT state in a Co^III complex therefore requires particularly careful ligand design.

The bimca ligand (Figure 7h) known from previous studies of four-coordinate square-planar complexes in other contexts¹⁰⁷ seemed promising to increase the electron density at the cobalt center, because bimca’s central amido subunit is strongly π-donating, which could help establish an energetically low-lying MLCT state. At the same time, the tightly flanking NHC units of bimca provide an overall rigid strong-field coordination environment that looked suitable to shift MC states to high energies. Compared to related cobalt complexes, the potential of the Co^IV/III redox couple of [Co(bimca)₂]⁺ is shifted cathodically by roughly 0.5 V,¹⁰⁸ confirming the anticipated strong influence of the amido subunit regarding the ease of metal oxidation. Transient UV–vis absorption spectroscopy combined with cyclic voltammetry and spectro-electrochemical studies support a picture, in which an intraligand charge transfer (ILCT) and an MLCT excited state are energetically close, and in which a photoactive excited state at ∼2.6 eV featuring a lifetime of 1.2 ns in acetonitrile at 22 °C (Table 4, entry 6) has substantial MLCT character, thus resembling in its composition the luminescent excited states of many cyclometalated Ir^III compounds.¹⁰⁹ In [Co(bimca)₂]⁺ the respective excited state is nonemissive, but readily undergoes photoinduced electron transfer to methyl viologen, albeit only with a low cage-escape quantum yield of 2% in acetonitrile. Similar cage escape yields, in the range 1–7% were reported for electron transfers between an Fe^III photosensitizer and organic electron donors (dimethyltoluidine, dimethylaniline, and tritolylamine) in polar solvents (acetonitrile and dimethylformamide).^53,110 Sub-picosecond transient absorption spectroscopy signaled the presence of a higher-lying excited state with a lifetime of roughly 100 ps and a spectral signature compatible with the above-mentioned ILCT state, suggesting that internal conversion from the ILCT to the MLCT state is unusually slow, perhaps because the respective two states are largely decoupled from one another. The roles and the energies of MC excited states in [Co(bimca)₂]⁺ have remained somewhat unclear, and in principle the above-mentioned photoinduced electron transfer reactivity could also emerge from an MC state, similar to what is known for Fe^II polypyridines.¹¹¹ Pump–probe X-ray experiments and detailed (time-dependent) density functional calculations would seem desirable to further elucidate the roles and energies of ILCT, MLCT, and MC states in [Co(bimca)₂]⁺.

Lessons Learned and Future Challenges

Over the past ten years, carbene ligands came into focus for Fe^II complexes, because they allowed an elongation of the ³MLCT lifetimes from the subpicosecond range^14,112,113 to tens of picoseconds for N-heterocyclic carbenes^7,10,36,114 and up to the subnanosecond time scale for a mesoionic carbene.⁵⁰ This ³MLCT lifetime elongation is largely attributable to slower relaxation into MC states, principally ³T₁ and ⁵T₂ in octahedral symmetry (Figure 1c), because these MC states are shifted to higher energies when using carbene instead of polypyridine ligands.¹¹⁵ The strongly σ-donating character of NHCs raises the ³T₁ and ⁵T₂ state energies due to the destabilization of the metal e_g (d_z2 and d_x2-y2) orbitals, and the additional π-acceptor character of mesoionic carbenes stabilizes the t_2g (d_xy, d_xz, d_yz) orbitals. Thus, the very classical design principle of maximizing the ligand field strength (Figure 8a) is pursed with the carbene ligand approach, and the same holds true for heteroleptic Fe^II complexes incorporating cyanido or cyanoborylated ligands in addition to polypyridines, resulting in MLCT lifetimes in the tens of picoseconds (Table 5, entry 1).^29−32,34 Recent studies of Co^III complexes suggest that the factors governing the energies of MC and MLCT states can indeed be controlled independently,¹¹⁶ which seems plausible given that MC states usually depend more strongly on ligand field parameters whereas MLCT states are more influenced by the metal and ligand redox properties.

Figure 8.

Illustration of effects having an important influence on MLCT lifetimes in 3d⁶ complexes. (a) σ-donating and π-accepting ligands to maximize the ligand field strength. (b) π-donor ligands can increase the metal–ligand bond covalence and lower the Racah B parameter. (c) Optimized bite angles are important to maximize the overlaps between metal and ligand orbitals. (d) Rigidity of the ligands and the overall complexes can help minimize unwanted excited-state distortions and decelerate nonradiative relaxation. (e) Delocalization of MLCT-excited electron density can help reduce excited state distortion. (f) Deuteration can slow down nonradiative deactivation. (g) Faster radiative relaxation (k_r) can make photoluminescence more competitive with nonradiative processes.

Table 5. Possible Strategies to Enhance Photophysical and Photochemical Properties of d⁶ (and Other Types of) Metal Complexes^a.

entry	strategy	approaches used	examplesb
1	increase ligand field strength	strong σ donor ligands, π acceptor ligands	Cr0,83,85 MnI,90 FeII6,7,10,12,24,29,30,34,36,38,39,40−46,50 CoIII,100 Mo084,91,133
2	increase metal–ligand bond covalence	π donor ligands	FeII,59,63 CoIII,108 CrIII117,118,134,135
3	optimize metal–ligand orbital overlap	chelates with optimized bite angles	MnI,90 FeII,22,23,48 CoIII,108 CrIII124,125,128,136
4	minimize excited state distortion	structural rigidity, enlarged π conjugation networks	Cr0,83,85 FeII,9 CoIII,108 NiII,137 CuI,130,131,138 ZrIV104
5	decrease vibrational energies	ligand deuteration	VIII,139,140 CrIII124,141
6	increase radiative relaxation rates	enhanced transition probability	FeII,142 CuI138

^aNot restricted to MLCT transitions.

^bParticular focus here is on 3d⁶ compounds and on other types of coordination complexes based on Earth-abundant metals; not a comprehensive list.

The energies of many MC states are however not merely a function of the ligand field parameter 10 Dq, but rather depend on the ratio of 10 Dq and B, where B is the Racah parameter characterizing the metal–ligand bond covalence. This aspect is now receiving increased attention in the design of photoactive first-row transition metal complexes,¹¹⁷ even though the nature of bonding in this compound class is usually considered more ionic in comparison to the second or third row.^11,96 Amido π-donor ligands (Figure 6b/Table 5, entry 2) help reduce the repulsion between d-electrons owing to the nephelauxetic effect,^63,108 and the Racah B parameter can be lowered markedly (Figure 8b).¹¹⁸ With Fe^II complexes, this has been useful to approach the HOMO inversion scenario predicted computationally,⁶⁰ leading to so-called PALCT excited states,^59,63 which can be viewed as a form of charge transfer with a particularly strong contribution of electron density from the covalent Fe–N_amido interaction. Whereas the π-donor ability of amido ligands is evident, the investigation of a spectrochemical series of Co^III complexes revealed that polypyridines can act as net π-donors, thus contrasting the behavior of these ligands toward second- and third-row transition metal complexes, in which they are net π-acceptors.⁹⁶ This illustrates that well-established principles for precious metal compounds are not necessarily applicable to the first-row of transition metals, here in this case mainly because the respective metal orbital energies differ strongly.⁹⁶ Computational studies forecasted this effect,¹¹⁹ and furthermore predicted that cyclometalating ligands could lead to long-lived MLCT states.^61,64−68 Synthetic advances now provide access to such cyclometalated Fe^II complexes,^6,72,120 and in one case led to the claim of an emissive MLCT state in solution at room temperature (Figure 6d).⁷⁴ The energy gap between this MLCT state and the electronic ground state is only 1.1 eV, whereas Ru^II and Os^II polypyridines with comparable energy gaps are usually nonemissive.^87,121 Thus, the results reported for this Fe^II compound seem surprising, particularly for an excited state that is substantially distorted, as the apparent Stokes shift of ∼5000 cm^–1 and the emission bandwidth of ∼1300 cm^–1 imply.⁷⁴

The optimization of ligand field strength (10 Dq) and covalence (B) discussed above is intimately tied to the bite angles of chelating ligands (Figure 8c/Table 5, entry 3), which crucially affect the overlap of metal and ligand orbitals.^22,96 This aspect is illustrated by the remarkable ³MLCT lifetime difference between [Ru(bpy)₃]²⁺ and [Ru(tpy)₂]²⁺,^122,123 and is furthermore seen in first-row transition metal complexes.^124−126 Various types of bite-angle optimized chelate ligands, some of them known from earlier studies of precious metal complexes,^107,127 have now been used successfully to enhance the photophysical properties of 3d compounds.^{22,48,108,118,125,126,128}

The issue of bite angles is connected to structural rigidity (Figure 8d/Table 5, entry 4). While the ligand rigidity in itself can be helpful, as illustrated for example by the difference between 1,10-phenanthroline (phen) and bpy ligands in Cu^I complexes,¹²⁹ an equally important question is to what extent cooperative steric effects between individual ligands can increase the overall rigidity of the entire coordination unit. The significance of this aspect is illustrated by Cu^I complexes featuring sec-butyl or cyclohexyl substituents on their phen ligands,^130,131 and by Zr^IV complexes with mesityl substituents on the ligand periphery.^103,104 As far as photoactive 3d⁶ complexes are concerned, the strategy of cooperative rigidity seems underexplored yet, though NMR experiments on a Cr⁰ complex with tert-butyl substituents in α-position to the ligating isocyanide units (Figure 7c) provide direct evidence for steric interactions between substituents from different chelate ligands.⁸⁵ Bulky tert-butyl substituents seemed indispensable for obtaining luminescent Cr⁰ and Mn^I complexes,⁸⁹ because in earlier studies with Mo⁰ (4d⁶) complexes the replacement of methyl by tert-butyl groups improved the MLCT lifetimes and luminescence quantum yield by an order of magnitude.⁸⁴ Overall rigidity with mutually interlocked ligands can be useful to minimize excited state distortion and to restrict the vibrational degrees of freedom, both of which can help decelerate nonradiative relaxation processes. Not all vibrational modes deactivate a given excited state equally well, and a recent study of a tailor-made Fe^II compound to monitor coherence phenomena provided direct insight into what kinds of complex distortions are particularly relevant.^9,25 Such investigations can inform future ligand designs,¹³² and this specific case study furthermore illustrated how rigidity slows MLCT excited state decay.

Another approach to minimize excited state distortion involves the use of extended π-conjugation networks on the ligands, to delocalize the MLCT-excited electron density (Figure 8e/Table 5, entry 4). This strategy is well-known from Ru^II polypyridines,^75,87 and recently has been applied successfully to a Cr⁰ compound (Figure 7c), resulting in the longest-lived ³MLCT excited state of a 3d⁶ compound in solution at room temperature known to date.⁸⁵ Alternatively, polyaromatic hydrocarbon (PAH) groups (for example anthracenyl or pyrenyl substituents) at the ligand periphery could lead to the so-called triplet reservoir effect,⁴⁷ in which the long-lived T₁ state of the PAH is in equilibrium with the photoactive ³MLCT state. However, for this to occur, energy matching between the respective two states is indispensable, otherwise the PAH groups have no significant effect in terms of MLCT lifetime elongation (Figure 4g/h).⁴⁶

Higher energy vibrations mediate nonradiative relaxation better than lower energy modes, and hence ligand deuteration can have a beneficial influence (Figure 8f). This strategy has long been applied to lanthanide complexes as well as 4d⁶ and 5d⁶ polypyridines.^143,144 Recent work demonstrated that it can also be very relevant for first-row transition metal compounds,^{128,139−141} though 3d⁶ complexes played no important role until now. Site-selective deuteration experiments showed that the C–H/C–D oscillators in immediate proximity to the metal core have the biggest influence, and furthermore the energetic match between vibrational overtones and the photoactive excited state is a key factor (Table 5, entry 5).^114,120 The isocyanide ligands needed to obtain emissive Cr⁰ and Mn^I complexes feature high-energy CN stretching vibrations (∼1950 cm^–1) in immediate proximity to the metal core,^{83,85,89,90,92,94,145} which could represent an inherent disadvantage of this class of compounds.

In addition to slowing down nonradiative MLCT excited state relaxation, accelerating radiative emission can help enhance the photoluminescence quantum yields (Figure 8g/Table 5, entry 6). While the energy gap law correctly predicts the nonradiative rate constants for a family of related complexes, the current understanding of how to control radiative MLCT excited state decay seems less well developed. Radiative relaxation from ³MLCT states in d⁶ metal complexes is associated with comparatively low rate constants due to the spin-forbidden nature of the transition to the electronic (singlet) ground state. Mixing of the relevant ³MLCT state with singlet excited states could in principle be expected to lead to higher radiative decay rates, but this aspect seems underexplored for complexes of Cr⁰, Mn^I, Fe^II, and Co^III. Stark spectroscopy has provided evidence for spin disallowed absorption transitions in [Fe(bpy)₃]²⁺,¹⁴² which seems encouraging for the idea that greater radiative excited-state decay rates could be obtained by singlet–triplet mixing in low-spin d⁶ compounds.

No matter what exact design principles are applied, the photoactive MLCT excited states of d⁶ complexes will likely remain considerably more strongly distorted than the spin-flip excited states in d³ metal compounds, as becomes evident when comparing the Huang–Rhys factors of ³MLCT states in [Ru(bpy)₃]²⁺ and its congeners with those of the lowest spin-flip excited states of Cr^III.^146,147 Fighting nonradiative relaxation is therefore inherently more difficult when dealing with 3d⁶ MLCT excited states, and this should be kept in mind when comparing photophysical figures of merit between d⁶ MLCT luminophores and d³ spin-flip (MC) emitters.^128,148

Photocatalysis

Whereas 4d⁶ and 5d⁶ complexes and their MLCT excited states are extensively used in organic photoredox and energy transfer catalysis,^2,149,150 this is not yet the case for 3d⁶ analogues. There is an increasing body of literature reporting on photocatalysis with Fe^II complexes (mostly with polypyridine ligands),^151−154 though this usually implies reactivity from the lowest MC excited state.¹¹¹ A recent study claimed that the MLCT excited state of a cyclometalated Fe^II complex (Figure 6e) undergoes direct photoreaction, though this was a stoichiometric, not catalytic process.⁷⁴ A clear-cut case of catalytic MLCT photoreaction with 3d⁶ metal complexes has been observed with Mn^I isocyanides (Figure 7e/f), but only for stilbene photoisomerization, as a proof of principle for triplet energy transfer catalysis.^89,90 Clearly, the photoreactivity of ³MLCT excited states of 3d⁶ complexes is underexplored yet, mainly due to the fact that most of the compounds known to date have too short MLCT lifetimes. Work on Mo⁰ and W⁰ isocyanide complexes suggests that Cr⁰ isocyanides could be very potent photoreductants.^{84,93,133,145,155−157} Photochemical reduction reactions seem therefore most attractive with Cr⁰ and Mn^I complexes, going through an oxidative ³MLCT excited-state quenching pathway, followed by subsequent reduction of the resulting 3d⁵ species to regenerate the initial 3d⁶ complex. Reductive excited-state quenching does not currently seem to be a viable option for these electron-rich isocyanide complexes, simply because they are very difficult to reduce. Conversely, Co^III complexes are potentially attractive photo-oxidants, and, in the form of LMCT chromophores, have already been used in this capacity.¹⁰¹ The observation of photoinduced electron transfer from [Co(bimca)₂]⁺ to methyl viologen indicates however that photoreductions are also realistic, despite the high (formal) metal oxidation state.¹⁰⁸ Fe^II complexes would seem well-suited to tackle both photo-oxidations and photoreduction reactions, analogously to Ru^II compounds. In a recent study, the ²LMCT excited state of [Fe(btz)₃]³⁺ was quenched reductively, and the formed [Fe(btz)₃]²⁺ photoproduct was then excited again to its ³MLCT excited state, which initiated an atom transfer radical addition (ATRA) reaction.⁵⁵ Such excitation strategies resembling the Z-scheme of natural photosynthesis have recently become of interest also with other first-row transition metal complexes.^158,159

Parallels and Differences between 3d⁶ and 3d¹⁰/3d⁸ Complexes with MLCT Excited States

Energetically low-lying MLCT excited states are typically obtained for late, electron-rich transition metals, and aside from d⁶, the d⁸ and d¹⁰ electron configurations are particularly relevant. Semiprecious Cu^I with its completely filled d-subshell has received vast attention.^{129,160−163} In the absence of low-lying MC states, the installment of long-lived MLCT states becomes relatively straightforward, particularly when compared to the 3d⁶ configuration. The focus has long been on four-coordinate, tetrahedral Cu^I complexes, but recently there is a trend toward two-coordinate linear Cu^I complexes with carbene ligands, often featuring a push–pull design between different ligands, such that ligand-to-ligand charge transfer (LLCT) states become the lowest excited states.^18,138,164 Since those LLCT states cause less molecular distortion than the Jahn–Teller susceptible MLCT excited states of d¹⁰ complexes,¹⁶⁵ the emission properties of this new compound class are particularly favorable and will likely enable future applications in lighting and sensing.¹⁶⁶

From a more fundamental curiosity-driven perspective, isoelectronic analogues based on Ni⁰ and Zn^II seem attractive. Photoactive tetrahedral Ni⁰ complexes are underexplored in comparison to Cu^I,¹⁶⁷ likely due to stability issues that seem however not insurmountable.¹⁶⁸ Zn^II complexes are better explored,¹⁶⁹ though they typically feature ligand-based fluorescent excited states rather than photoactive MLCT states.¹⁷⁰ Recently, there has been increasing focus on harnessing triplet excited states of Zn^II complexes and on introducing charge transfer character into the photoactive excited states,^171−173 including the development of emitters displaying thermally activated delayed fluorescence (TADF).¹⁷⁴ Lacking ligand field stabilization energy, Zn^II coordination compounds are often substitution-labile and tend to form pentacoordinate or even polynuclear compounds,¹⁷⁵ and multiple species can exist in dynamic equilibrium with each other.¹⁷⁶ Possible future focal points with Zn^II could be the design of compounds with photoactive LLCT or ILCT excited states, whereas Ni⁰ compounds seem attractive as classical MLCT luminophores and photosensitizers.

Even bigger challenges await with photoactive MLCT states in 3d⁸ complexes. Many square-planar Pt^II and Au^III compounds luminesce from charge-transfer excited states,^177−179 and with the recent surge of interest in combined photoredox and nickel cross-coupling catalysis, the possible involvement of MLCT states in square-planar Ni^II complexes has received increasing attention.¹⁸⁰ Complexes of the type in Figure 9a are thermally stable variants of catalytically relevant reaction intermediates in cross-coupling reactions,^181−183 and time-resolved spectroscopic studies revealed that their MLCT excited states relax to an MC state within a few picoseconds, thereby undergoing a structural distortion from square-planar to tetrahedral geometry.¹⁸⁴ Recent work demonstrated that when this structural distortion is made more difficult by suitable molecular design (Figure 9b), then longer MLCT lifetimes are achievable.¹³⁷ Many different research avenues are currently pursued with photoactive Ni^II complexes,^{183,185−187} and much remains to be discovered yet concerning the basic photophysics and photochemistry of square-planar 3d⁸ complexes. Luminescent and photochemically exploitable MLCT excited states could be one of the future targets, and this challenge seems even greater than in the case of 3d⁶ complexes.¹³⁷

Figure 9.

Next big fundamental challenge concerning long-lived MLCT excited states in first-row transition metal complexes with a partially filled d-subshell:¹⁸⁰ (a) Square-planar Ni^II complexes undergoing facile distortion to tetrahedral geometries; R¹ = OMe, ^tBu, H, Ph, CO₂Et with R² = H and X = Cl; R¹ = ^tBu with R² = OMe, H, CF₃ and X = Br.¹⁸⁴ (b) Longer MLCT excited state lifetimes are obtained by making the structural distortion from square-planar to tetrahedral more difficult.¹³⁷ (c) Square-planar Ni^II complex with an MLCT excited state lifetime of 48 ps in acetonitrile at room temperature.¹³⁷

Conclusions

In modern research on first-row transition metals and their coordination complexes, the relatively high abundance as well as the lower costs of these elements in comparison to platinum group metals are often emphasized.^188,189 In the big picture, societal, environmental, and geopolitical aspects are furthermore very relevant. The mining of some first-row transition metals can be problematic.^190,191 The overall complexity of the situation when trying to identify advantages of using first-row transition metals instead of precious metals therefore necessitates a very nuanced approach.

In the specific field of photoactive transition metal complexes, the cost argument is currently particularly tricky, because many of the ligands required to install the desirable photophysical and photochemical properties rely on multistep syntheses. It has therefore been questioned whether these expensive ligands outweigh the cost savings made with the metal.¹⁹² Most of the contemporary research in this field seems however driven by curiosity and the desire to understand fundamentally new aspects.^17,95,193 Even after decades of research dominated by work on the platinum group metals, many key aspects of excited-state relaxation, which are particularly relevant to the first row of the transition series, have remained poorly understood. The combined advances in synthesis, spectroscopy, and computational chemistry now permit much progress in understanding the fundamental photophysics and photochemistry of 3d compounds.

Keeping an eye on possible future applications in solar energy conversion, lighting, sensing, or photocatalysis, it seems desirable that the full diversity of electronically excited states occurring in metal complexes is accessible, including LMCT,^102,106 LLCT,¹⁶⁶ ILCT,¹⁷³ and MC states¹²⁴ in addition to MLCT excited states.¹⁹⁴ For complexes with a partially filled 3d subshell, rapid nonradiative MLCT excited state deactivation is particularly prevalent, and to date luminescent MLCT excited states with nanosecond lifetimes have remained very rare among 3d⁶ compounds.^{18,83,85,89,90} Consequently, MLCT-based photocatalysis has remained extremely scarce in this compound class.⁹⁰ Most attention still goes to Fe^II,⁶ but Co^III seems to attract growing interest,^96,100,108 particularly in combination with carbene ligands. The best performers in terms of luminescence and MLCT lifetimes are currently based on Cr⁰ and Mn^I, for which chelating arylisocyanides are important.⁸⁹ Ni^IV does not seem to have been considered yet from a photophysical perspective.^195,196 Square-planar Ni^II (3d⁸) compounds represent a great challenge that has now been taken on by several research groups.

The authors declare no competing financial interest.