Complexes [Cr(L^MIC)2]⁺ and [Cr(L^NHC)2]⁺ were synthesized following a strategy similar to that used for the recently reported Mn^IV complex (Fig. 2).39 The pro-ligands [H3L^MIC][I]2 or [H3L^NHC][I]2 were deprotonated in situ in THF using lithium bis(trimethylsilyl)amide (LiHMDS), followed by addition of this solution to a suspension of chromium(ii) chloride (CrCl2) in THF. Subsequent oxidation of the complexes was achieved by aqueous work-up and aerobic salt metathesis using NaBF4 or KPF6 respectively. After purification by column chromatography and/or recrystallization, the target compounds were obtained in moderate yields of 24–50% (SI). Notably, the NHC complex ([Cr(L^NHC)2]⁺ was also recently reported by Kunz, Heinze and co-worker and its excited state dynamics were thoroughly investigated.44

Single orange/red crystals of X-ray diffraction quality were obtained via vapor diffusion at room temperature from DCM/hexane ([Cr(L^MIC)2]BF4) or acetone/hexane ([Cr(L^NHC)2]BF4). The single crystal quality of the [BF4]⁻ salts was found to be the best, as other counterions lead to more complicated disorder and twinning effects. Both complexes crystallize in the monoclinic system with space group P21/c with a positional disorder of the [BF4]⁻ anions over three ([Cr(L^MIC)2]BF4) or two ([Cr(L^NHC)2]BF4) positions and three disordered hexane ([Cr(L^MIC)2]BF4) or 1.7 disordered acetone ([Cr(L^NHC)2]BF4) molecules (Fig. S53 and S54). The Cr^III center is six-fold coordinated in a distorted octahedral coordination sphere by four imidazolylidene/triazolylidene units and the two amide donors. The Ccarbene–Cr–Ccarbene angles are 176.38(11)°/173.00(12)° and 175.62(12)°/172.57(12)° for C1–Cr1–C2 and C1A–Cr1–C2A, while the Namide–Cr–Namide angle N10–Cr1–N10A is found to be 179.90(13)°/178.85(11)° in [Cr(L^MIC)2]BF4/[Cr(L^NHC)2]BF4, respectively (Fig. 2, right). The values display that—despite the slightly higher steric bulk of the cyclohexyl ring in [Cr(L^MIC)2]BF4 compared to [Cr(L^NHC)2]BF4—the latter is substantially more distorted. This is also visible along the Namido–Cr–Namido axis showing the two ligands to be almost perpendicular in [Cr(L^MIC)2]BF4 (88.4(1)°), while the imidazolylidene substituents are strongly tilted in [Cr(L^NHC)2]BF4 hence resulting in a smaller angle between the carbazole planes (61.89(1)°). We propose that his distortion is caused by unfavorable C–HImidazole⋯C–Hcarbazole repulsions, causing the imidazolylidene moieties to rotate more strongly out of plane compared to the triazolylidene. In the latter, the potential formation of favorable C–Hcarbazole⋯Ntriazole interactions diminishes this rotation (Fig. 2, right). These unfavorable interactions are even more pronounced in [Cr(L^py)2]⁺ with the six-membered pyridine donors.24

In accordance with enhanced covalency, the Cr–Namido distances of 2.026(2)/2.002(2) Å (Cr1–N10) and 2.025(2)/2.004(2) Å (Cr1–N10A) are shortened compared to the Cr–Ccarbene distances of 2.136(3)–2.144(3) Å/2.127(3)–2.160(3) Å in [Cr(L^MIC)2]BF4/[Cr(L^NHC)2]BF4. This observation is in line with axial compression distortion along the Namido–Cr–Namido axis, previously reported in [Cr(L^py)2]⁺.24 Notably, the metal–donor distances in [Cr(L^MIC)2]BF4 are substantially larger, compared to the isoelectronic Mn^IV triazolylidene complex [Mn(L^MIC)2]²⁺ previously reported by some of us (M–Naverage 1.938(3); M–Caverage 2.081(3) Å).39 Additionally, the metal–carbene distances in [Cr(L^MIC)2]BF4 are longer compared to previous examples of heteroleptic and homoleptic Cr^III NHC complexes in the literature, e.g., by Gibson and Steed (2.087(6)–2.120(6) Å)45 or by Scattergood et al. (2.093(4)–2.106(4) Å).37 We rationalize this bond elongation by the steric pressure of the cyclohexyl groups in the homoleptic [Cr(L^MIC)2]BF4 complex. Further information on the structural parameters and data of complexes [Cr(L^MIC)2]BF4 and [Cr(L^NHC)2]BF4 can be found in the SI (Tables S2 and S3).

Evans NMR spectroscopy of the complexes revealed strong paramagnetism (Fig. S1–S8 displaying a magnetic moment of 3.91 µB for [Cr(L^MIC)2]BF4 (Fig. S3) and 3.85 µB for [Cr(L^NHC)2]BF4 (Fig. S7)), consistent with the presence of three unpaired electrons (expected spin-only value = 3.87 µB) and a high-spin d³ configured Cr^III center. Additionally, [Cr(L^MIC)2]BF4 and [Cr(L^NHC)2]X (X = [PF6]⁻ or [BF4]⁻) were characterized by IR spectroscopy, high-resolution mass-spectrometry and elemental analysis (see SI for further information).

Cyclic voltammetry studies in acetonitrile at room temperature revealed the presence of three reversible redox events for both complexes (Fig. 3 and Table 1). Two reversible oxidations are observed at 0.11 V and 0.48 V vs. Fc/[Fc]⁺ for [Cr(L^MIC)2]⁺, and at 0.08 V and 0.50 V 44 vs. Fc/[Fc]⁺ for [Cr(L^NHC)2]⁺. Given the proximity of the two redox processes and our previous investigations on the manganese analogue of [Mn(L^MIC)2]²⁺,39 we propose that both oxidations are ligand-centered. Compared to [Cr(L^py)2]⁺ (0.46 V and 0.78 V vs. Fc/[Fc]⁺, Fig. 3) the oxidations in [Cr(L^MIC)2]⁺ and [Cr(L^NHC)2]⁺ are anodically shifted, which is in agreement with the stronger σ-donor and weaker π-acceptor properties of NHC and MIC moieties relative to neutral N-donor ligands such as pyridines. Additionally, reversible reduction processes were recorded at −2.15 V and −1.91 V 44 vs. Fc/[Fc]⁺ for [Cr(L^MIC)2]⁺ and [Cr(L^NHC)2]⁺, respectively, while [Cr(L^py)2]⁺ shows a reduction event at −1.54 V vs. Fc/[Fc]⁺. The strong anodic shift of these values aligns with the σ-donor capacity of the ligand series and tentatively suggests—in combination with the pronounced shift compared to the previously reported Mn^IV complex [Mn(L^MIC)2]²⁺—a metal-centered reduction process.39 However, spectroelectro-EPR spectroscopic measurements (Fig. S55 and S56) and DFT calculations (vide infra) rather indicate a ligand-centered process instead of the expected metal-centered reduction. Notably, all attempts to isolate any reduced or oxidized materials failed and EPR measurements after reduction further indicate instability of resulting complex (Fig. S55 and S56). For the oxidation, a fast colour change of the solutions is observed after addition of the oxidants, however the solutions quickly convert back to their original colour, indicating photo-instability of oxidizes species. This observation is in line with the ligand centered redox-process in the corresponding Mn^IV complex [Mn(L^MIC)2]²⁺ which also rapidly decomposed at room temperature.39

To gain further information on the electronic structure of the native, mono-oxidized and mono-reduced complexes [Cr(L^MIC)2]⁺, [Cr(L^MIC)2]²⁺ and [Cr(L^MIC)2]⁰ as well as its NHC and pyridine congeners [Cr(L^NHC)2]⁺ and [Cr(L^Py)2]⁺, computational investigations were performed at the density functional theory (DFT) level of theory. The DFT calculations support ligand-based oxidation as well as ligand reduction of [Cr(L^MIC)2]⁺ (Tables S6–S7 and Fig. S57–S58).

Comparative ab initio NEVPT2/CASSCF calculations were performed using the solid-state structural parameters of complexes [Cr(L^MIC)2]⁺ (Fig. 4 and S62–S64), [Cr(L^NHC)2]⁺ (Fig. S66), and [Cr(L^py)2]⁺ (Fig. S67). Active spaces of saCASSCF(7,11) were chosen for [Cr(L^MIC)2]⁺ (Fig. S63) and [Cr(L^py)2]⁺ (Fig. S67), comprising the five 3d orbitals in combination with six ligand-based orbitals, thereof two formally occupied (carbazolide π-donor functionality) as well as four unoccupied π*-orbitals delocalized mainly across the carbazolides. For [Cr(L^NHC)2]⁺, saCASSCF(7,10) with one π*-orbital less in the active space was required, as the considerably π-acidity of the NHCs led to otherwise difficult-to-converge wavefunctions (Fig. S66). Despite that the carbene complexes show near-ideal octahedral coordination geometries with orthogonal/coplanar ligand-π-systems (vide supra) and that the pyridine-congener is significantly distorted (e.g., dihedral angle between the two pyridine ligands ∠C–N^py–N^py–C = 34°), the electronic structures of all complexes are similar. Fig. 4 depicts the computed molecular orbital diagram for [Cr(L^MIC)2]⁺. The computations confirm that an idealized octahedral ligand field with a 3 + 2d-orbital splitting pattern is appropriate to understand the electronic structure, and that the covalency in the bonds with the donor ligands is moderate.

Notably, the two energetically lowest π*-orbitals of the carbazolido ligands are lower in energy than the d(x²–y²) and d(z²) orbitals. Hence, four low-intensity (starting at 525 nm, 2.36 eV, Q1–Q4; Table S10) and four high-intensity (starting at 460 nm, 2.71 eV, Q5–Q8) quartet mixed intra-ligand charge transfer (⁴ILCT) and ligand-to-ligand charge transfer (⁴LLCT) bands with predominant ⁴ILCT character are predicted. The weak d–d (MC) transitions (Q9–Q15) are predicted to occur in the energy range of 405–295 nm (⁴T2: 3.03 eV) and hence cannot be experimentally observed due to superposition by the intense charge transfer bands in the same spectral region. Corresponding vertical, that is still referring to the structural parameters of the quartet ground state, metal-centered doublet excited states are found at an energy range of 730 nm to 480 nm. The two energetically lowest D1 (1.70 eV, 730 nm) and D2 (1.75 eV, 710 nm) excited states represent the ²E and ²T1 spin-flip states (Fig. 5). Indeed, in the experimental UV-vis absorption spectrum of [Cr(L^MIC)2]⁺ in acetonitrile, the most prominent band at 432 nm with a molar absorptivity (ε) of 19 300 M⁻¹ cm⁻¹ is assigned to ⁴LLCT/ILCT (vide supra). Two weaker intensity bands at 500 and 543 nm with ε of 2500 M⁻¹ cm⁻¹ and 1800 M⁻¹ cm⁻¹ similarly exhibit substantial charge transfer character.

Upon moving to [Cr(L^NHC)2]⁺, the energies of the ligand as well as the metal-centered states decrease in energy (Fig. 6). The lowest quartet state Q1 excited state appears at 690 nm (1.80 eV), and is anticipated to be of CT (charge-transfer) character akin to [Cr(L^MIC)2]⁺. The ⁴T2 quartet state is predicted at 420 nm (Q3, 2.94 eV), and the ²T1 and ²E doublet states at 860 nm (D1, 1.44 eV) and 685 nm (D2, 1.80 eV), respectively.

In agreement with the computational data, the ground state UV-vis electronic absorption spectrum of [Cr(L^NHC)2]⁺ in acetonitrile (Fig. 7b) reveals an intense band at 402 nm with a molar absorptivity of 23 000 M⁻¹ cm⁻¹, which can be attributed to ⁴ILCT/LLCT transitions, while the bands at 450 and 477 nm (ε ∼2000 M⁻¹ cm⁻¹) have mainly ⁴LMCT character. In case of the literature-known [Cr(L^py)2]⁺, the lowest-energy excited quartet state is not any more CT, but an MC state (Q1, ⁴T2, 2.17 eV). The Q3 state at 2.25 eV is associated with the first ILCT/LLCT transition, and the ²T1 and ²E states are found at 685 nm (1.80 eV) and 680 nm (1.82 eV), respectively. As the ⁴MC states are predicted at 24 400 cm⁻¹ (409 nm, 3.03 eV) in [Cr(L^MIC)2]⁺ and at 23 700 cm⁻¹ (421 nm, 2.94 eV) in [Cr(L^NHC)2]⁺, and according to the Tanabe–Sugano formalism (Fig. 1c), these energies correspond to the ligand field strength (10 Dq). Comparing these to the pyridine analogue [Cr(L^py)2]⁺, which has a 10 Dq value of 17 500 cm⁻¹ (2.17 eV), a consistent trend emerges: the ligand field splitting increases progressively going from pyridine to NHC to MIC complex, correlating with the enhanced σ-donating properties of the equatorial ligands. A larger magnitude of σ-donation leads to destabilization of the antibonding eg orbitals and subsequent increase in the 10 Dq. In the orbital picture (cf. Fig. 6 and S65), complemented by ab initio ligand field theory (AILFT; Table S14),46 we find that the energy level of the vacant eg orbital set is most elevated for the MIC-ligand, followed by the NHC and then pyridine. This observation is in line with the anticipated σ-donor strengths of these ligands MIC > NHC > py,33,47,48 namely in respect to their stereoelectronic properties, yet also their behavior in Co^III and Pd^II/IV complexes.6,49

A similar trend, yet weaker, is found for the populated t2g orbital set, with the energies decreasing in the order MIC > NHC > py. Indeed, NHCs are better π-acceptors than MICs,33,47,48,50 and hence are expected to also comparatively lower the energies of the t2g orbitals. We believe that the position of pyridine, albeit commonly considered to be less π-acidic than NHCs, is due to the distortion of the ligand framework MIC < NHC < py (vide supra), that is steric reasons. Indeed, the computations also suggest increasing D4h-character in the order MIC < NHC < py (Fig. S65 and Table S14).

Hand in hand with the computational predictions, we investigated the excited-state dynamics of the newly synthesized complexes using UV-vis transient absorption (TA) spectroscopy in acetonitrile. Following the excitation of [Cr(L^MIC)2]⁺ at 532 nm with nanosecond pulses, the TA spectrum revealed a ground state bleach (GSB) at 434 nm, matching with the ⁴ILCT/LLCT band observed in the ground state absorption spectrum (Fig. 7a). Additionally, three intense excited state absorption (ESA) bands at 395, 490 and 670 nm are observed. These bands are associated with the electronic transitions originating from the ²MC state, which, based on NEVPT2/CASSCF calculations (SI, Table S10 and Fig. 4), represent the energetically lowest excited state. Given their intensity, those transitions are spin-allowed and occur within the doublet excited state manifold, leading to the population of the higher-lying ²MC or ²CT excited states.

Kinetic mono-exponential traces of ESA and GSB signals yield a ²MC excited state lifetime of 59 ns. Compared to the 4.4 ns lifetime observed in the parent complex [Cr(L^py)2]⁺ (Fig. S42), this represents an increase of more than tenfold. This experimental observation aligns well with our computational results, which predict destabilization of the ⁴MC states and an increase in 10 Dq in the complex [Cr(L^MIC)2]⁺ due to the strong σ-donation from the MICs (cf. Fig. 6). In addition, we anticipate minor changes in ²MC energies, as electron repulsion parameters are expected to be influenced to a relatively minor extent by the introduced ligand structural modification from pyridine to MIC. However, since the anionic carbazolide unit, primarily contributing to the Racah parameter B, remains unchanged, the destabilization of ⁴MC is projected to be significantly more pronounced than any variation in ²MC energy (see Table 2 and further discussion). This further leads to a larger energy gap between the lowest ⁴MC state and the key photoactive ²MC state, reducing the efficiency of back-intersystem crossing and ultimately contributing to a slower deactivation rate.

To gain further insight into excited-state dynamics on faster timescales, we analyzed the TA spectra of [Cr(L^MIC)2]⁺ following the femtosecond-pulse excitation at 430 nm, with delay times up to 300 ps, using a global fit with a sequential excited-state population model (Fig. 8 and S24–S25). Within 1 ps after the excitation, a characteristic intense ESA double band at 550–560 nm appears, along with ESA bands at 650 and 720 nm formed. Based on spectroelectro chemical data, we attribute these spectral features to the population of ⁴LLCT/ILCT states. Specifically, bands at 550 and 645 nm appear in the UV-Vis differential absorption spectrum of [Cr(L^MIC)2]⁺ upon ligand oxidation (Fig. S26c), while bands at 580 and 720 nm emerge upon ligand reduction (Fig. S26b), supporting our assignment. Subsequently, with a lifetime of 72 ps (obtained from the global fit, see Fig. 7 and S24–S25) the 550 and 720 nm bands decay, giving rise to new ESA bands at 500 and 670 nm. As previously discussed (Fig. 8 and Table 2), these new bands are attributed to the long-lived ²MC state (59 ns), which does not decay within the experiment time window considered here. Moreover, the absence of spectral features associated with ligand oxidation or/and reduction further confirms our previous ²MC state assignment. In well-investigated Cr(acac)3 (acac = acetylacetonate) and [Cr(btmp)2]³⁺ (btmp = 2,6-bis(4-phenyl-1,2,3-triazol-1-yl-methyl)pyridine) complexes, intersystem crossing to the doublet states is known to occur on the sub-picosecond timescale and it proceeds more rapidly than internal conversion within the quartet manifold.27,52 In our measurements, the spectral signatures of the ⁴CT state persist at delay times beyond 70 ps. We therefore speculate that the 70 ps time constant obtained from the global fit likely reflects a combination of intersystem crossing, internal conversion and vibrational cooling. Consistent with this interpretation, internal conversion and vibrational cooling have previously been reported to occur on the timescales up to 300 ps in polypyridine Cr^III complexes.12

We anticipated that [Cr(L^NHC)2]⁺ would exhibit photophysical properties intermediate between the parent [Cr(L^py)2]⁺ and the novel [Cr(L^MIC)2]⁺, as the σ-donation from NHC units is known to be weaker than that of MIC, bringing the 10 Dq value between the two. In the TA spectrum following excitation of [Cr(L^NHC)2]⁺ at 355 nm with femtosecond pulses (pulse duration of ∼250 fs) at a delay time of 750 ps, a GSB at 405 nm and ESA bands at 425, 480 nm are observed (Fig. 7b). Similar to [Cr(L^MIC)2]⁺, the GSB corresponds to the ⁴ILCT/LLCT band in the ground state absorption, while ESA features are attributed to the electronic transitions originating from ²MC state, predicated by DFT. The ²MC state in [Cr(L^NHC)2]⁺ undergoes deactivation to the ground state with a lifetime of 1.1 ns. Unexpectedly, its faster excited-state relaxation compared to the parent complex [Cr(L^py)2]⁺ suggests additional factors needed to be considered when rationalizing excited state dynamics. Aiming to do that, we examined the doublet and quartet excited state energies accessible upon the excitation at 355 nm (3.5 eV) (Table S13). Computational simulations predict low-lying ⁴LMCT states (1.8 and 1.9 eV) in close proximity to the ²MC states (1.8–1.85 eV), potentially contributing to a rapid deactivation of the latter (Fig. S65–S66 and Table S11). An analogical scenario was proposed for the homoleptic NHC complex [Cr(ImPyIm)2]³⁺, where excited-state decay occurred within the picosecond regime due to the low-lying charge-transfer states, populated through back-intersystem crossing.37

When discussing the excited-state properties of the parent complex [Cr(L^py)2]⁺ and the novel complexes [Cr(L^MIC)2]⁺ and [Cr(L^NHC)2]⁺, we must address the key limitation regarding carbazolide ligands, namely unusually short excited state lifetimes relative to the other known NIR photoactive Cr^III species (²MC lifetimes typically in the microsecond range) and the absence of emission in solution at room temperature. There are several plausible explanations for that excited state behavior, which are applicable for all complexes discussed herein. The most significant factor is the electronic nature of the carbazolide unit. Low-energy charge-transfer transitions from π-orbitals localized on the carbazolide moiety to the metal's d-orbitals or ligand π* orbitals (mainly localized on pyridine or carbene units) become feasible, resulting in a high density of quartet and doublet charge-transfer and metal-centered states. This complicates predictions regarding the effects of structural modifications on excited-state dynamics, as is also reflected in our experimental and computational analysis (Table S1 and Fig. S43–S44). Additionally, the greater covalency of the axial Cr–Namido bonds shortens their length compared to meridional Cr–Ccarbene or Cr–Npyridine, inducing axial compression, which is observed already in the ground state (see XRD data, Fig. S53–S54 and Table S3). This, in turn, can potentially enhance Jahn–Teller distortion in the ⁴MC and ⁴CT excited states and facilitate faster non-radiative deactivation.53

The absence of emission at room temperature in solution and technical limitations in detecting luminescence at 77 K in a glass matrix hindered the experimental determination of the ²MC excited state energies for both [Cr(L^MIC)2]⁺ and [Cr(L^NHC)2]⁺ complexes. Consequently, two alternative approaches were considered for estimating the doublet excited state energy. The first involves detecting spin-forbidden transitions via UV-vis ground state absorption spectroscopy, which however was not feasible in our case due to technical limitations. The second approach relies on studying a series of photoinduced electron transfer (PET) reactions, enabling estimation of the excited-state redox potential (E⁰(D⁺*/D²⁺)) and zero-point energy (E00) using eqn (1).54 This methodology has previously been applied to estimate the excited-state reduction potential of the photoactive ³MC state in Co(iii) polypyridine complex.55 The excited state lifetime of 59 ns for [Cr(L^MIC)2]⁺ in solution allows for diffusion-based excited-state redox reactivity, and consequently renders it suitable for this type of analysis.1E0(D⁺*/D²⁺) = E0(D⁺/D²⁺) – E00/e2ΔGET = [E⁰(D⁺*/D²⁺) – E⁰(A/A^˙−)] × e34ΔG‡ET = [(ΔGET/2)² + ΔG‡ET(0)²]^1/2 + ΔGET/2

Based on the NEVPT2/CASSCF calculations, the doublet excited state energy is estimated at 1.70 eV, and the ground state oxidation potential (E⁰(D⁺/D²⁺)) of the complex was determined (via cyclic voltammetry) to be +0.05 V (vs. Fc/[Fc]⁺). Applying eqn (1), the excited state oxidation potential can be estimated to be higher than −1.65 V vs. Fc/[Fc]⁺. With this threshold in mind, we screened a series of electron acceptors for the oxidative quenching, including nitrobenzene and benzoquinone derivatives, with reduction potentials (E⁰(A/A˙⁻)) ranging from −1.48 V to −0.42 V vs. Fc/[Fc]⁺. We then performed PET experiments between the complex and selected quenchers (Table 3) in acetonitrile, using TA spectroscopy to monitor the reaction kinetics. The excited state lifetime of the ²MC state was analyzed as a function of quencher concentration using Stern–Volmer plots (Fig. S27–S33). The obtained bimolecular quenching rate constants (kq) were then evaluated as a function of the reaction driving force (ΔGET, eqn (2)) within the framework of the Rehm–Weller formalism (Fig. 9). Full Rehm–Weller plot analysis using eqn (3) and (4) with varied E⁰(D⁺*/D²⁺) was performed, allowing the best fit for E⁰(D⁺*/D²⁺) = −1.24 V. Further, a diffusion rate constant of kd = (1.49 ± 0.02)·10¹⁰ M⁻¹ s⁻¹ and the self-exchange activation free energy ΔGET^‡(0) = 0.131 ± 0.002 eV were obtained from the fit. Those values are in line with the observations for quenching studies between [Cr(dqp)2]³⁺ and a series of electron donors, with kd = 1.7 × 10¹⁰ M⁻¹ s⁻¹ and ΔGET^‡(0) = 0.14 eV, as well as with other related studies with Cr^III complexes.15,56 Finally, using eqn (1), an E00 value of 1.35 eV was calculated, allowing us to construct a complete Latimer diagram for [Cr(L^MIC)2]⁺ (Fig. 10).

Turning attention back to the NHC complex [Cr(L^NHC)2]⁺, the excited state lifetime of 1.1 ns is too short for efficient quenching studies and the application of the same E00 estimation method used for [Cr(L^MIC)2]⁺. According to the computational simulation, the doublet excited state energy is calculated at 1.44 eV, combining it with E⁰(D⁺/D²⁺) = 0.08 V vs. Fc/[Fc]⁺ in eqn (1), we estimate E⁰(D⁺*/D²⁺) to be above −1.36 V vs. Fc/[Fc]⁺. To probe the feasibility of the PET reaction, we tested methyl viologen (E = −1.00 V vs. Fc/[Fc]⁺) as an electron acceptor (Fig. S39). Less than 5% reduction in the excited state lifetime was observed for 100 mM concentration of the quencher, allowing the bimolecular quenching rate constant for this reaction to be estimated around 6.5 × 10⁸ M⁻¹ s⁻¹.

When comparing the experimentally determined doublet excited state energy (E00) of 1.35 eV for [Cr(L^MIC)2]⁺ complex and 1.16 eV for the parent [Cr(L^py)2]⁺ complex (Table 2),24 we observe a minor destabilization of the ²MC excited state by 0.19 eV. This can be rationalized by the decreased covalency of the Cr–Ccarbene bond compared to the Cr–Npyridine bond. Indeed, we calculated the electron repulsion parameter B, which inversely correlates with metal-bond covalence, using the Tanabe–Sugano formalism (see equation in Fig. 1c), and the value for [Cr(L^MIC)2]⁺ was determined to be 600 cm⁻¹ compared to 550 cm⁻¹ for [Cr(L^py)2]⁺.24

From these observations and previous results, we can now conclude how structural modifications in our new complex affect their excited state properties. Confirming our initial hypothesis, replacing the equatorial ligand moieties from pyridines to strong σ-donating MICs leads to a significant increase in the ligand field strength while maintaining minor changes in the Racah B parameter, keeping it sufficiently low (see Table 2). Furthermore, the increased ⁴MC–²MC energy gap, from 1.01 eV in [Cr(L^py)2]⁺ to 1.68 eV in [Cr(L^MIC)2]⁺ complex, supports the observed reduction in the non-radiative deactivation rate of the ²MC excited state in the new complex.

Based on these insights, we investigated the [Cr(L^MIC)2]⁺ complex, which exhibits the longest excited state lifetime (59 ns) in the carbazolide series and an excited state oxidation potential of −1.24 V (vs. Fc/[Fc]⁺), for photoinduced electron transfer catalysis. Aryl diazonium salts are known to form diazo-radicals upon single electron reduction, which can further undergo C–N bond dissociation, resulting in the generation of aryl radicals and the evolution of nitrogen. The produced aryl radical can be used for a vast scope of transformations, including C–H arylations, borylations, and phosphorylations.57–59 In our study, we focused on a para-methoxy-substituted phenyldiazonium salt with a reduction potential of −1.07 V (vs. Fc/[Fc]⁺). Using our novel Cr^III complex as a photocatalyst, we successfully demonstrated model C–H arylation and borylation reactions (Fig. 11) involving the mentioned substrate. Both photocatalytic transformations are anticipated to proceed via well-known, literature-reported mechanisms (Fig. S48 and S52).36,57–59

The photocatalytic performance of [Cr(L^MIC)2]⁺ was first evaluated for the C–H arylation of furan with a p-methoxyphenyl diazonium salt (Fig. S45–S48). Under an argon atmosphere, a catalyst loading of 1 mol%, 10 eq. of furan, and 0.11 mM of the diazonium substrate were irradiated using 520 nm LED for 3 hours. A substrate conversion of >95% and yield of 66% (NMR, relative to an internal standard, Fig. S45) were obtained. The photocatalytical borylation of p-methoxyphenyl diazonium salt with bis(pinacolato)diboron was tested as a second model transformation. Substrate conversion of >99% and 74% product yield were achieved after 16 hours of irradiation with a 520 nm LED of the reaction mixture (see Fig. S49–S51, catalyst loading of 1 mol%, 3.0 eq. of bis(pinacolato)diboron and 0.11 mM of substrate).

As indicated by the NMR yields, selected model transformations proceed efficiently under our photocatalytic conditions. For comparison, this class of reactions can be also performed using Fe^III, Cu^I, Ru^II, Os^II and Ir^III transition metal complexes as photocatalysts, as well as metal-free systems such as eosin Y, under visible-light irradiation, typically affording moderate to high yields.36,57–59 It should be noted that the literature-reported systems display appreciable thermal reactivity, which accounts for the yields of up to 20% observed in the control experiments, both in related studies and this work (Fig. S46, S47, S50 and S51).36,57–59