Metal-centered (MC) excited states (ESs) of 3d transition metal complexes (TMCs) often possess rather low energies so that these represent the lowest energy ESs. Additionally, MC states are often strongly distorted, hence they efficiently decay non-radiatively to the ground state. As bimolecular photoinduced electron transfer (PET) and energy transfer (EnT) processes require contact to the substrate, the metal confinement of the ES wavefunction of MC states makes these processes challenging. Consequently, MC states are considered less useful as compared to long-lived charge transfer states of higher energy with wavefunctions extending onto the ligands. Despite these supposed drawbacks, some classes of TMCs can successfully engage in bimolecular PET and EnT processes with MC states being the photoactive states. We discuss these initial examples of MC ES reactivity covering chromium, manganese, iron, and cobalt complexes with the aim to gain a deeper understanding of these processes and to identify the decisive key parameters. Finally, we present catalytic photoredox and energy transfer processes using photosensitizers with suitable MC ESs.

The exploitation of light as an energy source to drive or assist chemical transformations is a rapidly growing research field. In homogeneous photocatalytic applications, a sensitizer, e.g., a photoactive transition metal complex (TMC), reacts either in a photoinduced electron transfer (PET, photoredox catalysis)1–7 or in a photoinduced energy transfer (EnT, energy transfer catalysis) step1,4,5,8–10 with a substrate in a bimolecular reaction as an initial chemical step of the catalytic cycle after light absorption. Excited state (ES) lifetimes of at least hundreds of picoseconds to nanoseconds are required for efficient diffusion-controlled bimolecular reactions. Otherwise, the ES decays to the ground state (GS) before any reaction can occur.

TMCs of rare and precious elements, e.g., ruthenium, iridium, or platinum, are employed in the majority of these applications. Great efforts to substitute the precious elements with the much more abundant 3d elements in photoactive TMCs are currently made with increasing success.3,11–14 Substrate activation occurs in most cases from charge transfer (CT) states of the sensitizer, mostly metal-to-ligand CT (MLCT) and in rare cases ligand-to-metal CT (LMCT) states.3,12–15 Several challenges are faced for the development of photoactive complexes with abundant 3d metal ions. Beyond increased GS substitutional lability of 3d TMCs,16 primarily based on the intrinsic weaker ligand field compared to their heavier congeners,12,17,18 which roots in the primogenic effect,19,20 fast deactivation kinetics of potential photoactive ESs occurs based on the same effect of a weak ligand field. The weak ligand fields of 3d TMCs lead to low-lying, geometrically distorted metal-centered (MC) or dd states, often below the CT states, opening pathways for the non-radiative deactivation of CT states.11,12,17,18 The tuning of the ligand field strength of 3d TMCs by enhanced ligand design is central to obtain long-lived CT states for photo-applications.12,17,18,21 Parallel to these efforts, TMCs with photoactive MC ESs appeared in the focus of recent research. MC ESs lack a charge separation and are often short-lived. Hence, the majority of the MC ES population decays non-radiatively, due to the low energy and distortion of the MC states. In addition, the limited charge-separation with the concomitant limited extension of the ES wavefunction onto the ligands (metal confinement) should mitigate bimolecular reactions, such as PET and EnT, with substrates in solution. Despite these challenges, TMCs with intriguing bimolecular MC state photoreactivities have been reported in the last few years.

The reactivity of MC ESs, in particular unimolecular dissociative pathways, has been reviewed before and has found applications in photocatalysis including examples for MC state reactivity of nickel(II) complexes.22–26 The selectivity of photosubstitution reactions often follow the empirical Adamson rules27 and have been described conceptually by the angular-overlap model of Vanquickenborne and Ceulemans.28,29 These substitution reactions arise from strongly distorted MC ESs with populated antibonding orbitals, eg* orbitals (Oh notation) for octahedral TMCs. A typical example is the photoaquation of [CrIII(NH3)6]3+, forming [CrIII(H2O)(NH3)5]3+.27 The present review focuses exclusively on recent developments of bimolecular MC state photoreactivity of mononuclear chromium, manganese, iron, and cobalt complexes in fluid solution, showing EnT to and PET from/to substrates. The discussion of the results is supported by density functional theory (DFT) calculations to illustrate the spin density distributions in the PET and EnT processes. Multinuclear photoactive complexes with M–M bonds are beyond the scope of this review and have been previously summarized.30–32 

The transition probability for radiative processes, absorption and emission of light, is determined by spin and symmetry selection rules.22,33 For TMCs with inversion symmetry, as for octahedral complexes, the Laporte selection rule comes into play.34 This leads to very low extinction coefficients for MC transitions (ε = 1–10 M−1 cm−1) in complexes of inversion symmetry, compared to CT or ligand-centered (LC) transitions (ε = 102–106 M−1 cm−1).35 The low transition probability for direct population of MC ESs may restrict the application as photosensitizers, compared to photosensitizers with strongly allowed CT transitions. Yet, this can also circumvent accumulation of reactive intermediates in photoredox catalysis (vide infra, Sec. IV A). The extinction coefficient of MC transitions is further lowered by a factor of 1/10–1/1000 for spin-forbidden electronic transitions, leading to very low transition probabilities of Laporte and spin-forbidden transitions (ε = 10−3–1 M−1 cm−1).35 Instead of direct excitation of spin-forbidden MC states, excitation of an allowed transition followed by intersystem crossing (ISC) to populate a photoactive ES of different spin multiplicities relative to the GS is advantageous. The ES deactivation of such states is then a spin-forbidden process, lowering the rate constants for radiative (kr) and non-radiative (knr) processes. This spin selection can lead to long ES lifetimes of MC states enabling bimolecular processes. ISC involving MC or MLCT/LMCT states with significant metal contribution is often ultrafast. The wavefunctions' high metal-character invokes spin–orbit coupling (SOC) by the heavy-atom effect, increasing the ISC rates.36 In addition, spin–vibronic coupling in ISC processes can be operative as breakdown of the Born–Oppenheimer approximation.36 As exemplified on chromium(III)37–40 and iron(II)41–43 complexes, ISC occurs in the femtosecond time regime.

According to the energy gap law [Figs. 1(a) and 1(b)], the MC ES lifetimes strongly depend on the ES energy.12,44 In addition, the ES distortion affects the lifetime by two limiting cases, namely, the weak coupling limit (WCL) for nested states with long lifetime and the strong coupling limit (SCL) for strongly distorted ESs with comparably short lifetime [Figs. 1(b) and 1(c)].12,44 These factors directly influence the non-radiative decay rates knr. MC states, derived from interconfigurational transitions, between the t2g and eg* orbitals in octahedral complexes, for example, are highly distorted by population of antibonding eg* orbitals (SCL). In extreme cases, the distortion provoked by the eg* population can even lead to photodissociation, e.g., in nickel(II) complexes.26 On the other hand, intraconfigurational MC states, so-called spin-flip states, are nested with small structural reorganization (WCL). The latter states can achieve lifetimes up to the millisecond range, e.g., for chromium(III) complexes.45–50 This long ES lifetime is associated with small non-radiative and radiative rate constants knr and kr, according to the WCL, the spin selection rule and in addition for kr, Laporte's rule for TMCs with inversion symmetry.46 In a fourth limiting case [Fig. 1(d)], a very large ES distortion with a horizontal displacement of the ES potential energy well outside the GS potential well leads to a weak vibrational overlap, extending the ES lifetime, as for the high-spin (hs) ES (5MC) in pseudo-octahedral low-spin (ls) iron(II) complexes.18 At low enough temperatures, this ES dynamics can lead to trapping the high-spin state 5MC for very long times (LIESST effect, Light-Induced Excited Spin State Trapping).51,52

FIG. 1.

Potential energy surface diagrams along a distortional coordinate with relevant vibrational wavefunctions (a) for nested excited states (weak vibrational wavefunction overlap and weak coupling limit), (b) a lower lying nested excited state (strong vibrational wavefunction overlap and weak coupling limit), (c) a low-lying, strongly distorted excited state (strong vibrational wavefunction overlap and strong coupling limit), and (d) a low-lying, extremely distorted excited state (weak vibrational wavefunction overlap). The ground state is in blue and the excited state is in red.

FIG. 1.

Potential energy surface diagrams along a distortional coordinate with relevant vibrational wavefunctions (a) for nested excited states (weak vibrational wavefunction overlap and weak coupling limit), (b) a lower lying nested excited state (strong vibrational wavefunction overlap and weak coupling limit), (c) a low-lying, strongly distorted excited state (strong vibrational wavefunction overlap and strong coupling limit), and (d) a low-lying, extremely distorted excited state (weak vibrational wavefunction overlap). The ground state is in blue and the excited state is in red.

Close modal

Conceptually, for long ES lifetimes suitable for bimolecular reactions, the MC states should be nested with the GS, either by the intraconfigurational character of the MC state and/or by rigidifying the ligand sphere.11,12 The energy of the lowest MC state should be high enough to be competent for useful ET or EnT reactions and to mitigate non-radiative deactivation according to the energy gap law. This can be achieved by modulation of the ligand field strength for interconfigurational states, e.g., for FeII complexes.11,12,17,18 For intraconfigurational MC states, the nephelauxetic effect should be minimized to increase the MC state energy.22,53–55

Beyond radiative decay and monomolecular non-radiative deactivation processes, TMCs (MLn) in ESs (*MLn) can undergo PET and EnT with a substrate (Sub).44,56 Both quenching processes, PET and EnT, can compete depending on the individual rates and driving forces. The relative ES energies for EnT, the differences of ES redox potentials of the TMC and GS redox potential of the substrate for ET, respectively, as well as the respective reorganization energies are decisive for the preferred quenching path (Fig. 2).57,58 External factors, e.g., solvent polarity or pH, can switch between PET and EnT.59 

FIG. 2.

Excitation of a TMC MLn by light, followed by (a) EnT and vibrational relaxation (VR) results in an electronically excited, thermally equilibrated substrate *Sub and the TMC in its GS or by (b) PET resulting in an oxidized TMC MLn+ and reduced substrate Sub for oxidative ET or vice versa for reductive ET.

FIG. 2.

Excitation of a TMC MLn by light, followed by (a) EnT and vibrational relaxation (VR) results in an electronically excited, thermally equilibrated substrate *Sub and the TMC in its GS or by (b) PET resulting in an oxidized TMC MLn+ and reduced substrate Sub for oxidative ET or vice versa for reductive ET.

Close modal

The efficiency of bimolecular PET and EnT quenching can be determined by Stern–Volmer analyses, expressed in the quenching rate constant kq [Eq. (1)].6,44,60,61 The quenching rate constant linearly depends on the ratio of luminescence rate constants in the presence (kobs) and absence (k0) of a quencher with given substrate concentration [Sub] and on the ratio of luminescence quantum yields Φ0 and Φobs, respectively.6,44,60,61 Transient absorption (TA) spectroscopy can also be helpful for Stern–Volmer analyses, especially in the case of dark but sufficiently long-lived photoactive ESs (vide infra, Secs. IV B and IV C),

kobsk0=Φ0Φobs=k0+kqSubk0=1+kqSubk0.
(1)

Stern–Volmer plots obtained from steady-state and time-resolved emission data distinguish between dynamic (bimolecular, diffusion controlled) and static (pre-association) quenching processes.6,44 Yet, Stern–Volmer analyses cannot differentiate PET and EnT processes.6,44 Spectroscopic detection of products for oxidative quenching with the oxidized photosensitizer (MLn+) and reduced substrate (Sub) or for reductive quenching with reduced sensitizer (MLn) and oxidized substrate (Sub+) indicates a PET process, while spectroscopic detection of the electronically excited substrate *Sub supports an EnT process (Fig. 2).6,44

A molecule in an electronically ES is a stronger oxidant as well as a stronger reductant.44 The thermodynamics for a photoinduced electron transfer between a TMC and a substrate is given by the ES redox potential of the TMC and the GS redox potential difference of the substrate. The ES redox potential E*ox/red of the TMC can be estimated with the Rehm–Weller equation from the GS redox potential Eox/red and the one-electron potential E00, corresponding to the energy difference of GS and ES at zero vibrational levels (0–0 transition) given as44,56,62

Eox*=EoxE00;Ered*=Ered+E00.
(2)

The kinetics of the outer sphere ET between an electron donor (D) and acceptor (A) is described by Marcus theory shown as63–66 

kPET=4π2h(HPET(rDA))214πλkbTexpΔG0+λ24λkbT,
(3)
HPET(rDA)=HPET0expβPET2rDAr0.
(4)

The rate constant kPET for PET depends on the distant dependent electronic coupling HPET(rDA) between D and A, the reorganization energy λ, including intramolecular structural changes of D and A and of solvent molecules and the thermodynamic driving force ΔG0 of the ET.64 The electronic coupling HPET(rDA) decreases exponentially with distance rDA from its maximum HPET(0) with a damping factor β, specific for the intervening medium [Eq. (4)].63–66 For the overall reaction of PET, the lifetime of the encounter complex and ES lifetime τ have to be taken into account. The typically small radial expansion of MC ES wavefunctions (nephelauxetic effect) leads to small overlap with the wavefunction of the substrate, resulting in a weak electronic coupling HPET. To compensate the small electronic coupling HPET and often short MC ES lifetimes, PET to substrates with large driving force ΔG0 are required. Therefore, PET from/to MC states is very rare as compared to PET involving CT states. The influence of encounter complex stability and ES lifetime in PET with respect to electrostatic interactions between sensitizer and substrate has been studied on PET from MLCT states with positively and negatively charged copper(I) as the sensitizer with neutral and cationic methyl viologen as the substrate,67 yet analogous studies including MC ESs have not yet been reported.

From these basic theoretical considerations, potential screws to enhance the PET efficiency involving MC ES of TMC sensitizers are (1) increasing the MC ES lifetime, (2) increasing the electronic coupling HPET by extension of the MC ES wavefunction to the ligand (nephelauxetic effect), (3) control of the encounter complex lifetime and structure by electrostatic and other interactions, e.g., hydrogen bonding or ion pair formation between TMC and substrate, and (4) exploiting ligand-centered instead of metal-centered redox processes for better wavefunction overlap with substrates.68,69

The Gibbs free energy of EnT as a bimolecular process between an electronically excited donor (D), denoted *MLn in Fig. 2, and an acceptor (A), Sub, is given by ΔG0 = E00AE00D.44,56,70E00A and E00D are the transition energies at 0–0 vibronic levels of acceptor and donor, respectively, with the assumption of negligibly small entropic effects.70 EnT in an encounter complex can be described with a similar “Golden Rule” expression in the Marcus-type description for ET with the two-electron matrix element HEnT for the electronic coupling and the Franck–Condon (FC) weighted density of states FCEnT [Eq. (5)].44,56HEnT includes the electronic interaction of the HOMOs and LUMOs of D and A. HEnT consists of an exchange and a Coulomb part. Two limiting cases arise, namely, the Dexter-EnT71 following an exchange mechanism, which is a simultaneous two-electron exchange and the Förster72–74 resonance EnT (FRET) with a dipolar Coulomb mechanism (Fig. 3).44,56 An effective orbital overlap between D and A is required for the exchange EnT, while FRET typically relies on the local spin conservation of D and A.44,56

Since Dexter-EnT is a double-electron transfer, the description of the rate constant kEnTD closely relates to that of single-electron transfer [Eqs. (3) and (6)]. kEnTD depends on the Dexter overlap integral JD and on the electronic coupling HEnT between D and A [Eq. (6)].44,56 Electronic coupling HEnT exponentially depends on the distance between D and A, including the attenuation parameter βEnT [Eq. (7)],44,56

kEnT=4π2hHEnT2FCEnT,
(5)
kEnTD=4π2hHEnT2JD,
(6)
HEnT(rDA)=HEnT0expβEnT2rDAr0.
(7)

Dexter-EnT only requires conservation of the total spin of the D/A pair, allowing EnT when GS and ES of D and A, respectively, are of different spin multiplicities. This is the typical situation of a triplet–triplet EnT, e.g., for *[Ru(bpy)3]2+ (*D(T1)) and organic acceptors A(S0) [Eq. (8) and Fig. 3(a), bpy = 2,2′-bipyridine]. Another prominent example for Dexter-type EnT in TMCs is quenching of excited triplet states by molecular oxygen (3O2, Σg3), forming singlet oxygen (1O2, 1Δg),75 

*D(T1)+A(S0)D(S0)+*A(T1).
(8)

FRET normally requires the additional conservation of the local spins of D and A. Hence, Förster-type EnT is operative in singlet–singlet EnT [Eq. (9) and Fig. 3(b)].44,56 A further generalization for dipolar EnT is that the coupled spins of *D/A and D/*A are of same total spin angular momentum76 or that the locally spin forbidden transitions are enabled by strong spin–orbit coupling.77–79 For FRET, a direct or indirect orbital overlap between D and A is not required.44,56 The distance between D and A for effective FRET ranges between 10 and 100 Å,80 

*D(S1)+A(S0)D(S0)+*A(S1).
(9)
FIG. 3.

One-electron description of (a) triplet–triplet Dexter-type EnT and (b) singlet–singlet Förster-type EnT.

FIG. 3.

One-electron description of (a) triplet–triplet Dexter-type EnT and (b) singlet–singlet Förster-type EnT.

Close modal

The Förster-EnT rate constant kEnTF strongly depends on the distance rDA of D and A with the inverse sixth power, on the Förster overlap integral JF, on the refractive index of the medium n, on the ES lifetime τD, on the quantum yield ΦD of the donor, and on an orientation factor K with K2 = 3/2 for random orientation given as,44,56

kEnTF=8.8×1025K2ϕDn4rDA6τDJF.
(10)

ISC is often efficient in TMCs leading to the population of the lowest spin-forbidden ES.43,81–83 Therefore, an exchange mechanism (Dexter) predominantly operates for EnT from excited TMCs.44,56,70

Studies on EnT from and to MC states of chromium(III) complexes clearly illustrate that the expansion of purely metal-centered orbitals on the ligand by the nephelauxetic effect increases kEnTD, due to more effective donor acceptor orbital overlap.70,84–86 For increasing the Dexter EnT efficiency from MC states, analogous arguments apply as for ET processes involving MC states (vide supra).

Pseudo-octahedral chromium(III) complexes with a strong ligand field (Δo ≫ 20 B) beyond the 2Eg, 2T1g/4T2g crossing points can act as spin–flip emitters, showing phosphorescence in the red to near-infrared (NIR) from the lowest doublet states 2Eg and 2T1g after 4T2g/4T1g (Oh notation) or CT excitation and ISC (Fig. 4).22,53–55,87–90 These weakly distorted intraconfigurational ESs possess lifetimes up to the millisecond range,45–50 sufficiently long to undergo bimolecular processes in the ESs (Fig. 5).12,88,89 Apart from 2Eg/2T1g → 4T2g back-ISC (bISC) for weak-field ligands repopulating the strongly distorted, often dissociative24,25,28,914T2g state (Fig. 5),22,88,90 typical non-radiative relaxation pathways are a trigonal distortion in tris(bidentate) complexes88,90,92,93 or multiphonon relaxation via high-energy oscillators (C–H, N–H, and O–H, Fig. 6).48,88,94,95

FIG. 4.

Tanabe–Sugano diagram for octahedral TMCs with d3 electron configuration; C/B =4.50.96,97 The 4T2g/2Eg(2T1g) ES crossing point is indicated by a circle. The relevant excited quartet states shown in blue and the relevant doublet states in red.

FIG. 4.

Tanabe–Sugano diagram for octahedral TMCs with d3 electron configuration; C/B =4.50.96,97 The 4T2g/2Eg(2T1g) ES crossing point is indicated by a circle. The relevant excited quartet states shown in blue and the relevant doublet states in red.

Close modal
FIG. 5.

Schematic potential energy surface diagram of a d3-TMC in an octahedral ligand field (Oh notation) with Δo ≫ 20 B with respective microstates and energies of the Franck–Condon (FC) states depicted on the left (adapted from Ref. 53). Rate constants for various processes indicated; dissociation, kdiss; back-ISC, kbISC; fluorescence, kf; non-radiative relaxation, knr; phosphorescence, kp.

FIG. 5.

Schematic potential energy surface diagram of a d3-TMC in an octahedral ligand field (Oh notation) with Δo ≫ 20 B with respective microstates and energies of the Franck–Condon (FC) states depicted on the left (adapted from Ref. 53). Rate constants for various processes indicated; dissociation, kdiss; back-ISC, kbISC; fluorescence, kf; non-radiative relaxation, knr; phosphorescence, kp.

Close modal
FIG. 6.

Simplified Jablonski diagram with FC state energies illustrating relaxation pathways in a d3-TMC of Oh symmetry after photoexcitation with quartet and doublet excited states shown in blue and red, respectively (adapted from Ref. 12). Rate constants for various processes indicated; ISC, kISC; back-ISC, kbISC; internal conversion, kIC; back-IC, kbIC; fluorescence, kf; non-radiative relaxation, knr; phosphorescence, kp; vibrational relaxation, kVR; multiphonon energy transfer, kEnT.

FIG. 6.

Simplified Jablonski diagram with FC state energies illustrating relaxation pathways in a d3-TMC of Oh symmetry after photoexcitation with quartet and doublet excited states shown in blue and red, respectively (adapted from Ref. 12). Rate constants for various processes indicated; ISC, kISC; back-ISC, kbISC; internal conversion, kIC; back-IC, kbIC; fluorescence, kf; non-radiative relaxation, knr; phosphorescence, kp; vibrational relaxation, kVR; multiphonon energy transfer, kEnT.

Close modal

Since the spin–flip emission energies are very similar in chromium(III) complexes (Table I), the GS and ES redox potentials and the location of reduction—metal-centered vs ligand-centered—are decisive for potential applications as PET or EnT photosensitizer, respectively (Table I). The one-electron reductions of chromium(III) complexes with electron-poor pyridine ligands [Cr(tbpy)3]3+ (tbpy= 4,4′-di-tert-butyl-2,2′-bipyidine) and similarly for [Cr(bpy)3]3+13+,98 [Cr(phen)3]3+23+ (phen = 1,10-phenanthroline)99 and [Cr(tpy)2]3+ (tpy = 2,2′:6′,2″-terpyridine)100 are ligand-centered (Scheme 1 and Table I), corresponding to a description of 12+ and 22+ as [CrIII(L•–)L2]2+.

TABLE I.

Photo and redox properties of selected mononuclear chromium(III) complexes with phosphorescence emission wavelengths λem, luminescence lifetimes τ, as well as room temperature quantum yields Φ under deaerated conditions at room temperature, electrochemical data for the first reduction with respective excited state redox potentials and rate constants kq with Stern–Volmer constants KSV for quenching with molecular oxygen.

λem/nmτRT,deox/μsΦdeox/%E½ [Cr]3+/2+/VaE* [Cr]3+/2+/V akq O2/× 107 M−1 s−1KSV/M−1
[Cr(bpy)3]3+13+101,104 729b 69b 0.25b −0.63c 1.08 1.7b 1200b,d 
[Cr(phen)3]3+23+101,104 730b 304b 1.2b −0.65c 1.05 2.7b 8200b,d 
[Cr(Ph2phen)3]3+33+101,104 744b 425b 3.0b −0.67c 1.00 31b 130 000b,d 
[Cr(dmcbpy)3]3+43+101,105 733b,e 7.7b/25c 0.014b/0.14c −0.26c 1.44 0.82c,f 200c,f 
[Cr(dqp)2]3+53+47,102 724/747c,e 1200e/2140c 5.2e −0.80c 0.86 1.3c 28 000c,d 
[Cr(ddpd)2]3+63+50,105 738/775e/776c 898e/1136c 11.0e/12.1c −1.11c 0.49 1.77e/1.3c,f 15 900e/15 000c,f 
[Cr(tpe)2]3+73+46,105 748c,g 1965c/2800h 4.0c/5.4h −0.88c 0.87 0.19c,f 3800c,f 
[Cr(bpmp)2]3+83+45  709h 1550h 15.8h −0.81c 0.94 0.29h 4500h 
λem/nmτRT,deox/μsΦdeox/%E½ [Cr]3+/2+/VaE* [Cr]3+/2+/V akq O2/× 107 M−1 s−1KSV/M−1
[Cr(bpy)3]3+13+101,104 729b 69b 0.25b −0.63c 1.08 1.7b 1200b,d 
[Cr(phen)3]3+23+101,104 730b 304b 1.2b −0.65c 1.05 2.7b 8200b,d 
[Cr(Ph2phen)3]3+33+101,104 744b 425b 3.0b −0.67c 1.00 31b 130 000b,d 
[Cr(dmcbpy)3]3+43+101,105 733b,e 7.7b/25c 0.014b/0.14c −0.26c 1.44 0.82c,f 200c,f 
[Cr(dqp)2]3+53+47,102 724/747c,e 1200e/2140c 5.2e −0.80c 0.86 1.3c 28 000c,d 
[Cr(ddpd)2]3+63+50,105 738/775e/776c 898e/1136c 11.0e/12.1c −1.11c 0.49 1.77e/1.3c,f 15 900e/15 000c,f 
[Cr(tpe)2]3+73+46,105 748c,g 1965c/2800h 4.0c/5.4h −0.88c 0.87 0.19c,f 3800c,f 
[Cr(bpmp)2]3+83+45  709h 1550h 15.8h −0.81c 0.94 0.29h 4500h 

aVs ferrocene.107 

b1 M HCl.

cAcetonitrile.

dCalculated as τRT,deoxkq.

eH2O.

fCalculated with c(O2) = 1.9 mM and pO2 = 213 hPa for air-saturated acetonitrile.106 

gD2O/DClO4.

hH2O/HClO4.

SCHEME 1.

Molecular structures of chromium(III) complexes.

SCHEME 1.

Molecular structures of chromium(III) complexes.

Close modal

The S =1 ground state of 12+ and 22+ results from an antiferromagnetic coupling between a chromium(III) ion (d3, S =3/2) and the ligand radical anion L•− (S = ½).98,99 A similar situation holds for [Cr(Ph2phen)3]3+33+ (Ph2phen = 4,7-diphenyl-1,10-phenanthroline) and [Cr(dmcbpy)3]3+43+ (dmcbpy = dimethyl 2,2′-bipyridine-4,4′-dicarboxylate), respectively, which is supported by the very similar and comparably high [Cr]3+/2+ reduction potentials (Table I and Scheme 1).101 A ligand-centered reduction was also stated for [Cr(dqp)2]3+53+ (dqp = 2,6-bis(8′-quinolinyl)pyridine)47 with a significantly lower reduction potential of E½ = −0.80 V.102 The reduction in [Cr(ddpd)2]3+63+ with the electron-rich ddpd ligand (ddpd = N,N′-dimethyl-N,N′-dipyridin-2-yl-pyridine-2,6-diamine) occurs at significantly lower potential and is clearly metal-centered, giving the labile high-spin chromium(II) complex (d4, S =2).50,103 The reduction potentials of [Cr(tpe)2]3+73+46 and [Cr(bpmp)2]3+83+45 (tpe = 1,1,1-tris(pyrid-2-yl)ethane, bpmp= 2,6-bis(2-pyridyl-methyl)pyridine) are between those of 13+43+ and 63+ leading to a more complex electronic situation in [CrL2]2+ (Table I and Scheme 1). According to density functional theory (DFT) calculations on 72+ and 82+ with S =1, the electronic structures are best described as an admixture of a low-spin chromium(II) ion and a chromium(III) ion antiferromagnetically coupled to a ligand radical anion.45,46 The differences in the ES redox potentials of pseudo-octahedral low-spin d6 TMCs in their MLCT states, e.g., [Ru(bpy)3]2+, can be rationalized by an electron/hole formalism (vide infra, Fig. 10). In contrast, for 2Eg spin–flip ESs with their intraconfigurational character, the ES potential is merely depending on the decrease in electron exchange interaction from the 4A2g GS to the 2Eg ES.102 

The high ES reduction potentials of 13+53+ and 73+ recommend them as photocatalysts PC3+ in PET to organic substrates, e.g., in photoredox catalyzed Diels–Alder cycloadditions of dienes such as 2,3-dimethyl-1,3-butadiene (DMB) with styrenes such as trans-anethole (tAn) to yield the Diels–Alder product (DAP) [Schemes 2(a) and 2(b)].105,108,109 The ligand-centered reductions of 13+53+ and 73+ lead to an efficient PET with large kPET, based on a strong electronic coupling HPET between donor (organic substrate) and acceptor (electronically excited 13+53+ and 73+, 2Eg state) [Eq. (3)]. As required for a sustainable photoredox catalysis, the ligand-centered reduction circumvents the formation of labile high-spin chromium(II) complexes.110 The initially proposed mechanism by Shores, Ferreira, and co-workers108,109 for the photoredox catalyzed Diels-Alder reactions has been recently updated.105 The proposed catalytic cycle starts with a reductive quenching of the excited complex in its doublet state *2[PC]3+ by the styrene tAn, forming the reduced photocatalyst 3[PC]2+ and the trans-anethole radical cation tAn•+. The latter reacts with the diene DMP in a [4 + 2] cycloaddition as the rate-determining step to a cyclohexene radical cation DAP•+ as Diels–Alder product intermediate. DAP•+ is reduced to the product DAP by superoxide O2•–, which stems from the regeneration of the photocatalyst by reoxidation of 3[PC]2+ with triplet oxygen 3O2, closing the photocycle and mediator cycle [Scheme 2(a)].105 The latter step contrasts to the previously proposed mechanism including a second EnT photocycle. In this second photocycle, the excited catalyst *2[PC]3+ forms 1O2 in an EnT step, which acts as competent oxidant for 3[PC]2+ [Scheme 2(b)].1083[PC]2+ itself is thermodynamically competent to oxidize DAP•+, but this path seems to be negligible, due to electrostatic repulsion of both the cationic reactants 3[PC]2+ and DAP•+, while O2•– is negatively charged [Scheme 2(a)].108 One key aspect might be the presence of water in the acetonitrile solvent, which significantly raises the 3O2/O2•– potential for a fast oxidative regeneration of 4[PC]3+.105 This fast regeneration significantly prolongs the lifetime of tAn•+ by diminishing non-geminate back-ET between tAn•+ and 3[PC]2+. The formed superoxide prevents the accumulation of DAP•+ by an efficient thermal reduction to DAP.105 Compared to the established noble metal photoredox catalyst [Ru(bpz)3]2+ (bpz = 2,2′-bipyrazine, E*[Ru]2+/+ = 1.07 V vs ferrocene107)111–113 and the bipyridine based chromium(III) complex 43+, the CrIII complex 73+ showed a much higher photostability and possesses an ultralong ES lifetime, achieving quantitative conversion and reusability.105 

SCHEME 2.

(a) Proposed mechanism for the photo(redox) catalized Diels–Alder cycloaddition of electron-rich olefins, (b) including the previously proposed but unnecessary mediator cycle in a second photocycle, and (c) formation of reversed Diels–Alder adduct with electron-poor olefins.

SCHEME 2.

(a) Proposed mechanism for the photo(redox) catalized Diels–Alder cycloaddition of electron-rich olefins, (b) including the previously proposed but unnecessary mediator cycle in a second photocycle, and (c) formation of reversed Diels–Alder adduct with electron-poor olefins.

Close modal

With electron poor olefins, e.g., 4-methoxychalcone, the doublet state of 33+ is incompetent for direct reductive quenching. Hence, other PET and EnT processes come into play and lead to Diels–Alder adducts of different regioselectivities [Scheme 2(c)].114,115 Intermediate formation of [2 + 2] photo cycloaddition products of 4-methoxychalcone with the diene (path A) and with itself (path B; not preferred) are discussed. The vinyl cyclobutane intermediate from path A possesses a low oxidation potential, suitable for PET from excited 33+, followed by cycloreversion or cyclorearrangement, respectively. Alternatively, a [4 + 2] cycloaddition operates via a triplet state pathway after initial population of the 4-methoxychalcone triplet state (path C), which is relatively long-lived (29 ns in methanol).116 The triplet state is obtained by EnT from excited 33+ after forming an associate with 4-methoxychalcone.114 This triplet formation facilitates geometric distortion of the electron-poor olefin leading to the formation of the reverse-Diels–Alder product with the diene. Stern–Volmer analyses suggest that dynamic quenching is operative.114 

73+47,102 shows very similar photophysical and electrochemical properties to 53+46,105 (Table I). Hence, 73+ was utilized successfully as photoredox catalyst in the aerobic bromination of methoxyaryls, the oxygenation of 1,1,2,2-tetraphenylethylene, the aerobic hydroxylation of arylboronic acids, and the vinylation of N-phenyl pyrrolidine (Scheme 3).102 Analogously to the above described Diels–Alder cycloaddition, the ES of the photocatalyst 53+ is reductively quenched [Scheme 2(a)]. Apart from the vinylation reaction, the reduced catalyst 52+ is regenerated by O2 with the formation of superoxide O2•−.102 Interestingly, the bromination of 1,3,5-trimethoxybenzene (1,3,5-TMB) occurs selectively and stepwise with the initial formation of the monobrominated product in 78% maximum yield after 30 min irradiation time. A 91% yield of the dibrominated product is obtained after 210 min of irradiation [Scheme 3(a)].

SCHEME 3.

(a) Bromination of methoxyaryls, (b) oxygenation of 1,1,2,2-tetraphenylethylene, (c) hydroxylation of arylboronic acids and pinacol esters, and (d) vinylation of N-phenyl pyrrolidine.

SCHEME 3.

(a) Bromination of methoxyaryls, (b) oxygenation of 1,1,2,2-tetraphenylethylene, (c) hydroxylation of arylboronic acids and pinacol esters, and (d) vinylation of N-phenyl pyrrolidine.

Close modal

All proposed reaction mechanisms are described with one photocycle, apart from the photocatalytic oxygenation of 1,1,2,2,-tetraphenylethylene with two photocycles and one mediator cycle utilizing O2 (Scheme 4).102 The studies102,105,108,109 presented above indicate that a small thermodynamic driving force of the PET with a slower PET step (kPET = kq), according to Marcus theory,63–66 can be efficiently compensated with the often very long lifetimes (τ0) of spin–flip states in chromium(III) complexes (Table I), expressed with the Stern–Volmer constant KSV = τ0 · kPET.6 

SCHEME 4.

Proposed mechanism for the photo(redox) catalyzed oxygenation of 1,1,2,2-tetraphenylethylene with [Cr(dqp)2]3+53+.

SCHEME 4.

Proposed mechanism for the photo(redox) catalyzed oxygenation of 1,1,2,2-tetraphenylethylene with [Cr(dqp)2]3+53+.

Close modal

For a strategy using spin–flip states of chromium(III) complexes for sensitizing substrates selectively by EnT, the PET pathway as a conceivable side reaction must be closed. [Cr(ddpd)2]3+63+ with its comparably low ES reduction potential is suitable for selective EnT (Table I). 63+ forms 1O2 in a Dexter-type EnT in DMF or CH3CN with a quantum efficiency of Φ(1O2) = 0.61.50,117 The thus-formed 1O2 was exemplarily used as reagent in α-cyanation reactions of aliphatic amines with Me3SiCN (Scheme 5).117 Reductive quenching of *63+ by amine oxidation is less preferred for 63+. Yet, for chromium(III) complexes with ligand centered reduction, the reductive quenching of *[Cr]3+ giving the amine radical cation [Sub]•+/0 is thermodynamically allowed with driving forces of about ΔG = (–0.32)–(–1.06) eV (Table I and Scheme 5). This prohibits utilizing 13+53+, 73+, and 83+ as selective O2 sensitizers in these reactions but would enable amine radical cation initiated reactions.

SCHEME 5.

Proposed mechanism for the photocyanation of amines with Me3SiCN via 1O2 with [Cr(ddpd)2]3+63+ as sensitizer, substrates with redox potentials [Sub]•+/0 (vs ferrocene), and cyanation products below.

SCHEME 5.

Proposed mechanism for the photocyanation of amines with Me3SiCN via 1O2 with [Cr(ddpd)2]3+63+ as sensitizer, substrates with redox potentials [Sub]•+/0 (vs ferrocene), and cyanation products below.

Close modal

Chromium(III) ions can act as energy acceptors or donors with lanthanide ions in hetero-oligometallic sensitizer–activator assemblies or complex salts leading to upconversion (UC) luminescence in the solid state, which is beyond the scope of this review.118–124 In contrast, 83+ has been successfully employed in solution in sensitized triplet–triplet annihilation-UC (sTTA-UC)125,126 of 9,10-diphenyl anthracene [DPA, Scheme 1, Fig. 7(a)].127 sTTA-UC involves two bimolecular EnT processes. The first process is a Dexter-type EnT from the excited photosensitizer to an acceptor, such as DPA, to populate the long-lived T1 state (*3DPA) of the latter. For [Ru(bpy)3]2+ type sensitizers,17,18,128–131 the long-lived 3MLCT ES (vide infra) acts in a Dexter-type triplet–triplet energy transfer (TTET).125,126 The second is the TTA process of two excited acceptors in their T1 states, giving an acceptor in S0 and one in the S1 luminescent state.125,126 Overall, two photons are converted to one of higher energy.

FIG. 7.

(a) Simplified Jablonski diagram illustrating the sTTA-UC process between [Cr(bpmp)2]3+83+ and DPA and (b) schematic representation of microstates involved in the DTET/TDET processes with spin densities of geometry optimized structures of the 83+/anthracene pair in the 2Eg/S0 and 4A2g/T1 states (isosurface value: 0.02; hydrogen atoms are omitted; and see the supplementary material).

FIG. 7.

(a) Simplified Jablonski diagram illustrating the sTTA-UC process between [Cr(bpmp)2]3+83+ and DPA and (b) schematic representation of microstates involved in the DTET/TDET processes with spin densities of geometry optimized structures of the 83+/anthracene pair in the 2Eg/S0 and 4A2g/T1 states (isosurface value: 0.02; hydrogen atoms are omitted; and see the supplementary material).

Close modal

83+ fulfills the requirements for sTTA-UC with its low excitation energy (2.33 eV, 532 nm),45,127 compared to other CrIII complexes, outside the DPA S0/S1 absorption/emission132,133 window [Fig. 7(a)]. The unprecedented high 2Eg/2T1g emission energy (1.75 eV, 709 nm) of 83+ is in the range of the DPA T1 state (1.6–1.8 eV, experimental134,135 and 1.72–1.76 eV, calculated132,133) suitable for doublet–triplet EnT (DTET).127 Together with the exceptional long lifetime of the doublet states of τ0 = 890 μs in DMF/HClO4, the phosphorescence is efficiently dynamically quenched by DPA (KSV = 5.0 × 104 M−1) with a DTET rate of kDTET = KSV/τ0 = 5.6 × 107 M−1 s−1 via Dexter-EnT. DFT calculations on the 83+/anthracene pair located the 2Eg/S0 state pair as lowest geometry optimized excited MC state. This contrasts higher-level complete active space self-consistent field (CASSCF) calculations with one 2T1g microstate as lowest doublet state of 83+;45 yet, the very small energy gap between these two lowest microstates enables thermal equilibration. Dexter-EnT with DPA produces 83+ in its 4A2g GS and the anthracene in its T1 state with the local unpaired electrons antiferromagnetically coupled so that the total spin is conserved [Fig. 7(b)]. The experimental DTET efficiency ΦDTET = 1 – τ/τ0 = 0.84 of 83+ exceeds that of the TTET of [Ru(bpy)3]2+ with only 0.56 with 1 mM DPA. Dexter-EnT can obviously be efficient for MC states with long ES lifetimes, even with the weak MC ES wavefunction overlap with the acceptor's GS wavefunction, compared to e.g., MLCT states. The small 2Eg/2T1g to T1 energy gap between 83+ and DPA together with their long ES lifetimes allows for back-EnT (TDET) in a DTET/TDET thermal equilibrium, which was observed by the decreased *3DPA lifetime and delayed phosphorescence of 83+.127 The TDET does not lead to energy loss in the sTTA-UC process, due to the extreme long ES lifetime of 83+, giving an excellent UC quantum yield of ΦUC = 0.12 under saturation conditions after excitation at 520 nm.127 With sterically less shielded anthracenes, the excited singlet state S1 of the anthracenes prepared by DTET with 83+ and TTA yields the corresponding [4 + 4] cycloaddition product.127 

Further examples for potential applications of chromium(III) complexes with long-lived excited doublet states are the photooxidation of DNA136–138 or the oxygen photosensitization in photodynamic therapy.139,140 All these first promising applications of the MC state photoreactivity of pseudo-octahedral chromium(III) complexes in organic transformations, acting as either photoredox catalysts or sensitizers for singlet oxygen or arenes via EnT, give an idea of the huge potential in substituting noble metal photoactive complexes by abundant 3d metal ions. The very long ES lifetimes in the milliseconds range compensate the metal confinement of the ES wavefunction with small electronic coupling HPET(rDA)/HEnT(rDA) with substrates in bimolecular PET and EnT reactions, respectively. The narrow range of spin–flip state energies in chromium(III) complexes allows for switching between reductive PET and EnT by the location of reduction, namely, ligand vs metal-centered. Ligand-centered reductions seem to lead to higher reduction potentials concomitant with large driving forces ΔG0 for reductive PET. Ligand-centered reductions should also favor a stronger electronic coupling HPET(rDA), thanks to a shorter electron donor acceptor distance and a better overlap of the involved orbitals in PET [Eqs. (3) and (4)]. This could outcompete Dexter-type EnT from pure MC states with a presumably larger chromium acceptor distance rDA and resulting smaller HEnT(rDA). Hence, ET can be kinetically and thermodynamically favored over EnT in optimized photoactive chromium(III) complexes.

Manganese complexes with long-lived photoactive MC ESs are rare. The oxido manganese(IV) complex with a pentadentate N-donor ligand [MnIV(Bn-TPEN)(O)]2+92+ (Bn-TPEN = N-benzyl-N,N′,N′-tris(2-pyridylmethyl)-1,2-diamino-ethane) is photoinactive as well [Fig. 8(a)]. Yet, coordination of two equivalents of the Lewis acid Sc(CF3SO3)3 or one equivalent of Sc(NO3)3 to the oxido ligand increases the lifetime of the lowest non-emissive excited doublet state to 6.4 and 7.1 μs, respectively, as determined by TA spectroscopy.141–143 The adducts 92+–[Sc(OTf)3]2 and 92+–Sc(NO3)3 are reductively quenched by a series of benzene derivatives.141–143 The electronic situation in 92+ is closely related to that of [Mn(N4Py)(O)]2+ (N4Py = N,N-bis(2-pyridylmethyl)-N-b(2-pyridylmethyl)methylamine) and the model [Mn(NH3)5(O)]2+ [Fig. 8(b)].142,144 The dxz, dyz, and dz2 ligand field orbitals in idealized C4v symmetry possess significant Mn=O π* and σ* character, respectively, which causes a destabilization relative to the dxy and dx2–y2 orbitals, respectively [Fig. 8(b)].144 The respective molecular orbitals possess significant oxido ligand contributions.144 From this simple molecular orbital inspection, the 4E states [(dxz, dyz) → dz2] and [(dxz, dyz) → dx2–y2] may possess some LMCT contribution aside from the MC character 4[MC+LMCT]. The same argument holds for the 2E state 2[MC+LMCT]. CASSCF calculations on [Mn(N4Py)(O)]2+ give the energies of the ESs as 4E [(dxz, dyz)→dx2–y2; 7176, 7552 cm−1], 4E [(dxz, dyz)→dz2; 17 085, 17 252 cm−1], and 2E (4300 cm−1).144,145 The calculations reveal a multiconfigurational character of the 4E states with 15% and 21% admixture of MnIII–O manganese(III) oxyl character.144,145 According to CASSCF calculations, thermal concerted pro-ton- transfer (CPET) in C–H bond activation reactions of alkanes, alkenes, and arenes as substrates146–148 with oxido manganese(IV) complexes follows a multi-state reactivity profile.144,145,149,150 Lewis acid [Sc(OTf)3]2 coordination to the oxido ligand weakens the Mn=O bond with a bond elongation from 1.69 Å for 92+ to 1.74 Å 92+–[Sc(OTf)3]2, determined by Mn K-edge EXAFS (extended x-ray absorption fine structure).151 The Lewis acid coordination destabilizes the dz2 orbital with Mn=O σ* character more than the respective dxz and dyz orbitals of π* character.142 The authors ascribe the excitation to the population of the 4E [(dxz, dyz)→dz2] state, which is destabilized from 9804 cm−1 (1020 nm) to 14500 cm−1 (690 nm) upon coordination of the Lewis acid. This destabilization enhances the lifetime of these ESs and enables efficient ISC to the very low energy photoactive 2E state at 5600 cm−1 (0.7 eV), estimated from PET rates.142,151 Additionally, the 2E state of 92+–[Sc(OTf)3]2 is with a lifetime of 6.4 μs relatively long-lived due to spin forbidden relaxation to the 4B1 GS.142 The impact of Lewis acid coordination to 92+ on the lower lying 4E [(dxz, dyz)→dx2–y2] state was not discussed.142 The multi-state reactivity under thermal reaction conditions of CPET reactions 144,145 implies that the perturbation of the electronic structure and concomitantly the photophysics and reactivity by Lewis acid coordination could be more intricate. DFT and CASSCF calculations on how coordination of [Sc(OTf)3]2 or Sc(NO3)3 affects the electronic structure of 92+ may shed more light onto the photophysics of 92+–[Sc(OTf)3]2 and 92+–Sc(NO3)3 and may give valuable information for a conceptual utilization of Lewis acids in tuning photophysical properties of photoactive complexes. The influence of Lewis/Brønsted acids on thermal ET, oxygen atom transfer, and hydrogen atom transfer (HAT) reactions on related manganese(IV) oxido and manganese(III) hydroxido complexes had been investigated.152–154 Employing different Lewis/Brønsted acids should also affect the decisive photophysical properties such as ES reduction potentials and ES lifetimes for reductive PET.

FIG. 8.

(a) Structure of [Mn(Bn-TPEN)(O)]2+–[Sc(CF3SO3)3]292+–[Sc(CF3SO3)3]2 and (b) schematic representation of microstates with molecular orbitals in local C4v symmetry (orbital illustrations derived from DFT calculations of the model complex [Mn(NH3)5(O)]2+; hydrogen atoms are omitted; and see the supplementary material).

FIG. 8.

(a) Structure of [Mn(Bn-TPEN)(O)]2+–[Sc(CF3SO3)3]292+–[Sc(CF3SO3)3]2 and (b) schematic representation of microstates with molecular orbitals in local C4v symmetry (orbital illustrations derived from DFT calculations of the model complex [Mn(NH3)5(O)]2+; hydrogen atoms are omitted; and see the supplementary material).

Close modal

The GS reduction potential positively shifts by 0.58 V upon Lewis acid coordination from E([MnO]2+/+) = 0.38 V for 92+ to E([MnO-(Sc)2]2+/+) = 0.96 V for 92+–[Sc(OTf)3]2 (vs ferrocene)107 with reorganization energies for outer-sphere ET of λ = 2.24 and 2.12 eV, respectively.151,155 The ES reduction potential of the non-emissive state is significantly larger (E*([Mn-(Sc)n]2+/+) = 1.7 V vs ferrocene107) and was obtained from PET rate constants with various substrates, such as benzene. The reductive quenching occurs with much smaller molecular reorganization for 92+–[Sc(OTf)3]2 (λ = 0.64 eV) and 92+–Sc(NO3)3 (λ = 0.53 eV), respectively, which was ascribed by the authors to a ligand centered PET to a LMCT state.141,142 Both characteristics favor fast PET, according to Marcus theory [Eq. (3)].63–6692+–[Sc(OTf)3]2 undergoes reductive PET to produce the benzene radical cation, which subsequently reacts with OH or Br to give phenol and bromobenzene, respectively (Scheme 6).142,156 In the presence of water, phenol is obtained in a formal HAT from the formed OH benzene adduct radical with reduction of the initial photoproduct 9+–[Sc(OTf)3]2 to the manganese(II) complex [MnII(Bn-TPEN)]2+ and loss of Sc(OTf)3.142 [MnII(Bn-TPEN)]2+ comproportionates with 92+–[Sc(OTf)3]2 to [{MnIII(Bn-TPEN)}2(O)]4+.142 A photocatalytic cycle is not achieved. However, for the oxybromination of electron-rich methoxy substituted arenes under similar thermal ET reactions with 92+, a catalytic cycle was established with iodosyl benzene as terminal oxidant and Sc(OTf)3 as activator.156 The catalyst was regenerated from intermediate [MnII(Bn-TPEN)(OH)]+–[Sc(OTf)3]2 by iodosyl benzene and Sc(OTf)3.156 Based on the presented accumulated data, the combination of long lived excited doublet states of oxido manganese(IV) complexes by Lewis acid coordination with suitable sacrificial oxidants might enable challenging photocatalytic transformations. The exact nature of the reactive doublet state, 2MC/2LMCT, might vary between different complexes and should be identified in each case to extract clear structure–photoreactivity relationships.

SCHEME 6.

Proposed schematic mechanism for the non-catalytic photochemical benzene hydroxylation with 92+–[Sc(OTf)3]2 as a photooxidant, abbreviated as [Mn(O)]2+–[Sc]2.

SCHEME 6.

Proposed schematic mechanism for the non-catalytic photochemical benzene hydroxylation with 92+–[Sc(OTf)3]2 as a photooxidant, abbreviated as [Mn(O)]2+–[Sc]2.

Close modal

Optimization of the photophysical properties of pseudo-octahedral low-spin iron(II) complexes is subject of intense research.18,157,158 Commonly, d6 low-spin iron(II) complexes with (poly)pyridine ligands follow ultrafast ES dynamics from initially populated 1MLCT states often to the lowest MC ES.18,157–159 The intrinsically weaker ligand field of Fe2+ in comparison to the heavier homologues Ru2+ or Os2+ leads to low-lying distorted MC states. These MC states provide an efficient pathway for the non-radiative deactivation of MLCT states. The prototypical [Ru(bpy)3]2+ complex with energetically higher-lying MC states shows MLCT phosphorescence (λem ≈ 620 nm) with a long ES lifetime in the nanosecond regime.17,18,128–131 For the lighter congener [Fe(bpy)3]2+102+, the non-luminescent high-spin 5T2g state is populated with unity quantum efficiency in less than 50 fs after 1MLCT excitation, for example (Scheme 7, Figs. 9 and 10).42,160 The 5T2g MC state of 102+ possesses a lifetime in the 650 to ca. 1000 ps range, depending on the solvent.161–163 This is close to the diffusion limit for bimolecular reactions of ≈1 ns.6 The comparably high 5MC lifetimes of low-spin iron(II) complexes arise from the required double spin change and the comparably large distortion relative to the ground state with a horizontal displacement of the ES minimum outside the GS potential well [Fig. 1(d)]. Different decay channels in the ES dynamics of 102+ after population of the 1MLCT FC state are discussed, underlining the intricate energy landscape with a high density of ESs of various multiplicities in the FC region and beyond (Fig. 9).41–43,81,161,162,164–169 The relaxation into the low-lying 5T2g state (0.69 eV) significantly lowers the ES electrochemical potential by 1.81 V compared to the 3MLCT state (2.50 eV), according to adiabatic energies derived from complete active space second-order perturbation theory (CASPT2) calculations.164 Despite that limitation for PET to substrates, evidence for MC state reactivity in photo(redox) reactions with iron(II) polypyridine complexes, such as 102+, has been reported.170–173 However, the detailed photophysical mechanisms for these photoreactions have not yet been disclosed.

SCHEME 7.

Molecular structures of [Fe(bpy)3]2+102+, [Fe(tren(py)3)]2+112+, and [Fe(phen ⁁ CAr)2] 12.

SCHEME 7.

Molecular structures of [Fe(bpy)3]2+102+, [Fe(tren(py)3)]2+112+, and [Fe(phen ⁁ CAr)2] 12.

Close modal
FIG. 9.

Tanabe–Sugano diagram for octahedral TMCs with d6 electron configuration; C/B =4.42.96,97 The decisive 1A1g/5T2g crossing point is indicated by a circle and the 5T2g/3T1g excited state crossing point is indicated by a square. Relevant excited singlet states are shown in blue, relevant triplet states are in red, and the 5T2g state is in black.

FIG. 9.

Tanabe–Sugano diagram for octahedral TMCs with d6 electron configuration; C/B =4.42.96,97 The decisive 1A1g/5T2g crossing point is indicated by a circle and the 5T2g/3T1g excited state crossing point is indicated by a square. Relevant excited singlet states are shown in blue, relevant triplet states are in red, and the 5T2g state is in black.

Close modal
FIG. 10.

Simplified Jablonski diagram, including microstates of 102+ with relevant photophysical processes, absorption, ISC, IC, and VR are indicated. 3T comprises 3T1g and 3T2g states. Electronic state ordering according to Ref. 164.

FIG. 10.

Simplified Jablonski diagram, including microstates of 102+ with relevant photophysical processes, absorption, ISC, IC, and VR are indicated. 3T comprises 3T1g and 3T2g states. Electronic state ordering according to Ref. 164.

Close modal
FIG. 11.

Schematic representation of microstates of conceivable PET steps between [Fe(tren(py)3)]2+112+ and benzoquinones with spin densities of geometry optimized structures of the 112+/p-TCBQ pair (isosurface value: 0.02; hydrogen atoms are omitted; p-TCBQ = 2,3,5,6-tetrachloro-1,4-benzoquinone; and see the supplementary material) and subsequent SCO and recombination/cage escape reactions.

FIG. 11.

Schematic representation of microstates of conceivable PET steps between [Fe(tren(py)3)]2+112+ and benzoquinones with spin densities of geometry optimized structures of the 112+/p-TCBQ pair (isosurface value: 0.02; hydrogen atoms are omitted; p-TCBQ = 2,3,5,6-tetrachloro-1,4-benzoquinone; and see the supplementary material) and subsequent SCO and recombination/cage escape reactions.

Close modal

The bimolecular reactivity of [Fe(tren(py)3)]2+112+ (tren(py)3 = tris(2-pyridyl-methylimino-ethyl)amine; Schemes 7 and 8) in its 5T2g state has been investigated in detail using a series of benzoquinoid (BQ) electron acceptors as oxidative quenchers.174 The 1MLCT state of 112+ (580 nm) evolves into the dark 5T2g state within ≈200 fs. This high-spin state possesses a lifetime of 55 ns at room temperature.174–178 The ES redox potential of the non-emissive 5T2g state was estimated as E*([FeIII/II]3+/2+) ≈ –0.35 V vs ferrocene in acetone from TA spectroscopic monitoring of the oxidative quenching reactions, using a series of BQ substrates with known redox potentials from –0.42 to 0.07 V vs ferrocene. Together with the FeIII/II GS redox potential E(113+/2+) = +0.51 V, the 5T2g state energy was determined to 0.86 eV [Eq. (2)].174 

SCHEME 8.

Oxidative quenching of the 5T2g excited MC state of [Fe(tren(py)3)]2+112+ by substituted benzoquinones BQ.

SCHEME 8.

Oxidative quenching of the 5T2g excited MC state of [Fe(tren(py)3)]2+112+ by substituted benzoquinones BQ.

Close modal

After PET, the final relaxed iron(III) complex is assumed to possess a low-spin configuration,174 similar to other polypyridine iron(III) complexes.179 Yet, this cannot be the initially formed spin multiplicity based on spin conservation during the ET from 112+ in its quintet state and the BQ in its singlet state (S0). Despite the large overall reorganization energy for the PET from FeII112+ ((t2g)4(eg*)2, 5T2g) to low-spin FeIII113+ ((t2g)5(eg*)0, 2T2g, Oh notation), the initial PET step might be fast. Exemplary DFT calculations on the 112+/p-TCBQ pair (p-TCBQ = 2,3,5,6-tetrachloro-1,4-benzoquinone) identified two possible PET pathways (Fig. 11). PET from the 5T2g state can occur either from the doubly occupied t2g orbital (π path) yielding 113+ in the high-spin state ((t2g)3(eg*)2, 6A1g, Oh notation) and BQ•– antiferromagnetically coupled to the Fe spins or from an eg* orbital (σ path) forming the intermediate-spin state 4T1g ((t2g)4(eg*)1) and BQ•– ferromagnetically coupled to the Fe spins (Fig. 11). Since the accuracy of predicting the energy differences of spin states in iron(II) spin-crossover (SCO) complexes strongly depends on the DFT functional, the relative energies of the PET π and σ pathways are not further discussed here. Yet, geometries are often well reproduced.180,181 The 5T2g6A1g PET π path requires a smaller geometric reorganization with Fe–N distances decreasing from 2.140–2.295 to 2.080–2.281 Å, while the Fe–N bonds in the σ path contract significantly stronger (Fe–N distances of 1.994–2.266 Å in the 4T1g state). This is expected considering the removal of an electron from an antibonding eg* orbital. The final low-spin iron(III) electron configuration is reached after 6A1g2T2g (ΔS = 2) and 4T1g2T2g (ΔS = 1) SCO, respectively and cage escape of BQ•–. The overall rate for the electron transfer reaction via the 6A1g and the 4T1g states depends on the individual rates of the PET steps (π/σ path) and the subsequent SCO rates with ΔS = 2 and 1, respectively (Fig. 11). The former path might enable a fast nonproductive recombination to ls-FeII and BQ before cage escape, while the latter might promote cage escape before recombination due to the spin conservation constraint. These potential pathways, including SCO processes, cage escape, and recombination reactions, deserve further detailed experimental and computational investigations.

Very recently, the iron(II) complex [Fe(phen ⁁ CAr)2] 12 with cyclometalated 2–(3-tert-butyl-phenyl)-1,10-phenanthroline ligands was reported. 12 shows room temperature luminescence and photoreactivity as catalyst in cross coupling reactions (Scheme 7).182 The authors ascribe the luminescence at λem = 1220 nm (1.02 eV), corresponding to an ES redox potential of E*([FeIII/II]+/0) = –2.0 V (vs ferrocene), with a lifetime of τRT,deox ≈ 1 ns at room temperature in benzene solution to 3MLCT state phosphorescence.18212 was used in a stoichiometric amount to the substrate 4-chlorobromobenzene. Despite the presence of intense, broad CT absorptions around 750 nm in 12, the cross coupling reaction was performed under irradiation with light of high energy (405 nm).182 Comparing the iron(II) complexes 112+ and 12 with photoactive 5MC and 3MLCT states, respectively, is highly instructive. Since the ES energies are quite similar with 1.02 (3MLCT) and 0.86 eV (5T2g), the GS redox potentials determine differences of ES redox potentials.174,182 The 5MC lifetime of 112+ exceeds the 3MLCT lifetime of 12 by a factor of 55, probably balancing the reduced charge separation in the 5MC state relative to the 3MLCT state and the metal-confined nature of the 112+113+ redox process, as already discussed for chromium(III) complexes. Other low-spin FeII 1MC→5MC SCO complexes with high enough 5MC state lifetimes at room temperature and high enough ES oxidation potentials could give valuable access for bimolecular photoreactivity of iron(II) complexes in excited MC states in the future.

Pseudo-octahedral iron(II) complexes already find application in photochemical organic transformations. Yet, a profound knowledge of the underlying photophysical processes and even the type of ESs involved are lacking.170–173 Now, first steps are made toward the understanding that 5MC states of low-spin iron(II) complexes can be long-lived and oxidizing enough for bimolecular PET reactions. Concepts to increase the MLCT lifetimes by controlling the 3/5MC/1/3MLCT ES ordering and distortion of pseudo-octahedral iron(II) complexes18,183 may help to optimize the 3/5MC states lifetime and ES electrochemical potentials. In complexes with (local) inversion symmetry and hence weak ligand field absorptions, 1MLCT states may be exploited as antenna states to circumvent Laporte's rule.22,34 Irradiating into strongly allowed 1MLCT transitions efficiently populates 1MLCT states, which evolve with high quantum efficiency to lower-lying 5MC states, as observed for the prototypical iron(II) complex 102+.160 

Pseudo-octahedral low-spin cobalt(III) complexes with d6 electron configuration are isoelectronic to respective iron(II) and ruthenium(II) complexes (Fig. 9). Compared to the Fe2+ central ion, the higher charged Co3+ ion with its stronger intrinsic ligand field mitigates the problem of low-lying MC states as non-radiative deactivation path (vide supra Sec. IV C). Cobalt(III) complexes with electron-rich bis(tridentate) ligands with guanidinyl moieties, forming 6-membered chelate rings, with long-lived phosphorescent 3LMCT states were successfully employed in photoredox catalysis.184 In the cobalt(III) complex [Co(PhB(MeIm)3)2]+13+ with the strong σ-donating tris(carbene) ligand tris(3-methylimidazolin-2-ylidene)(phenyl)borate, the lowest ES is of 3MC (3T1g in Oh notation) character instead of 3LMCT nature or 5MC character as in 112+ [Fig. 12(a)]. The strong ligand field imposed by the high oxidation state and the strong donating ligands in a close-to-octahedral geometry shift the 3MC state to quite high energy, even to the right of the 3T1g/5T2g crossing point in the Tanabe-Sugano diagram (Fig. 9, crossing point indicated by a square). Phosphorescence is observed at 690 nm (1.80 eV) with a luminescence quantum yield of  >0.0001 after excitation at 266 nm. The ES lifetimes of 13+ are high with 0.82 and 1.25 μs in MeCN and MeOH, respectively [Fig. 12(a)].185,186 The luminescent 3MC state, as lowest ES, is populated after initial 1MC (310 nm) or 1CT (250 nm) state excitation. Time-dependent DFT (TD-DFT) calculations describe the 1CT state as a combination of MLCT and ILCT character.185 The 3MC ES lifetime of 13+ drastically exceeds that of [Co(CN)6]3– (<5 ns) with similar electronic properties at first sight.187–189 The long ES lifetime of 13+ was ascribed to the strong ligand field (Δo = 38600 cm−1) decreasing the rate constant for 3MC → 1GS non-radiative decay and the rigidity of the tripodal [PhB(MeIm)3] ligands decreasing the ES distortion, further decreasing the rate constants for non-radiative deactivation [Fig. 1(a)].12,185 The orange emission of 13+ is quenched by oxygen in an EnT process, indicated by 30% loss of emission intensity in relation to deaerated samples and by detection of the characteristic emission of 1O2 as photoproduct [1275 nm, Fig. 12(b)].185 The application of 13+ as sensitizer for singlet oxygen in organic transformations has not been reported yet, likely because high energy UV-B light is required for efficient excitation. 13+ is not reduced at potentials above –2 V vs ferrocene in acetonitrile,185 indicating that reductive PET might be hampered. The ES redox potential for the 132+/+ oxidation can be estimated from the calculated 3MC/1GS energy difference with 2.1 eV (E00) and the GS redox potential with 0.96 V (Eox) vs ferrocene as E*([Co]2+/+) = –1.14 V.185 The ES redox potential of 13+ significantly exceeds that of 112+. However, oxidative PET has not been described for 13+, again likely because high energy light is required. A challenge for implementing cobalt(III) photosensitizers in photocatalytic settings utilizing their 3MC (3T1g) states is certainly the poor absorption cross section in the visible spectral region in particular, for centrosymmetric complexes, such as 13+. Similar to the iron(II) scenario (vide supra Sec. IV C), suitable antenna states with CT character might be introduced to address the poor absorptivity of cobalt(III) complexes in the visible spectral region.

FIG. 12.

(a) Molecular structure of [Co(PhB(MeIm)3)2]+13+ and (b) schematic representation of microstates involved in the EnT process with spin densities of geometry optimized structures of the 13+/oxygen pair in the 3T1g/Σg3 and 1A1g/1Δg states (isosurface value: 0.02; hydrogen atoms are omitted; and see the supplementary material).

FIG. 12.

(a) Molecular structure of [Co(PhB(MeIm)3)2]+13+ and (b) schematic representation of microstates involved in the EnT process with spin densities of geometry optimized structures of the 13+/oxygen pair in the 3T1g/Σg3 and 1A1g/1Δg states (isosurface value: 0.02; hydrogen atoms are omitted; and see the supplementary material).

Close modal

Intriguing examples of bimolecular metal-centered state reactivity of 3d transition metal complexes indicate a clear paradigm change that metal-centered excited states are not long-lived enough and lack sufficient charge separation to undergo bimolecular electron or energy transfer processes with substrates as typically provided by charge transfer states. First indications of oxidative photoinduced electron transfer with quintet metal-centered states of iron(II) complexes, reductive photoinduced electron transfer with doublet excited states of oxido manganese(IV) complexes, or energy transfer to oxygen from a triplet metal-centered state of a hexacarbene cobalt(III) complex and valuable catalytic and photonic applications in bimolecular organic transformations are already reported for the doublet metal-centered excited states of chromium(III) sensitizers. The metal confinement of metal-centered states concomitant with weak electronic coupling of electron donor and acceptor can be overcome in photoinduced electron transfer reactions with ligand-centered redox chemistry as shown for the reductive quenching of chromium(III) complexes with electron-poor polypyridine ligands. Ligand-centered redox chemistry furthermore by-passes substitutionally labile metal redox states with populated eg* orbitals. Tuning of the ground state electrochemical potential can be used to open or close the photoinduced electron transfer pathway to favor electron or energy transfer processes for chromium(III) complexes. Lewis acid adduct formation in oxido manganese(IV) complexes modifies the ground and excited state electrochemical potentials, as well as the excited state lifetime to enable bimolecular photoinduced electron transfer. The required high excited state lifetimes have been achieved so far by exploiting nested intraconfigurational states [chromium(III) and manganese(IV)], by requiring a double spin state change [iron(II)] or by shifting the metal-centered states to sufficiently high energy [cobalt(III) with a very strong ligand field and high octahedricity]. Charge transfer states can be used purposefully as antenna states to populate photoactive but weakly absorbing metal-centered states. With these spectacular initial results, the bimolecular photochemistry of metal-centered states can contribute novel reactivities and unprecedented mechanisms, as well as large-scale applications of complexes of Earth-abundant 3d metal ions in photocatalysis.

See the supplementary material for details of the computational methods and coordinates of the optimized geometries.

Parts of this article were conducted using the supercomputer Elwetritsch and advisory services offered by the TU Kaiserslautern (https://elwe.rhrk.uni-kl.de), which is a member of the AHRP and the Gauss Alliance e.V. The financial support from the Deutsche Forschungsgemeinschaft [DFG, Priority Program SPP 2102 “Light-controlled reactivity of metal complexes,” HE 2778/18‐1] is gratefully acknowledged.

The authors have no conflicts to disclose.

Christoph Förster: Conceptualization (equal); Data curation (lead); Formal analysis (lead); Methodology (lead); Supervision (equal); Validation (equal); Visualization (equal); Writing – original draft (equal); Writing – review & editing (equal). Katja Heinze: Conceptualization (equal); Funding acquisition (lead); Supervision (equal); Validation (equal); Visualization (equal); Writing – original draft (equal); Writing – review & editing (equal).

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