The synthesis of M^BZI complexes is depicted in Figure 2. Cyclization of the dianiline derivative (a) to the benzoimidazolium salt is reported to be extremely challenging, due to steric bulk imposed by the isopropyl groups, with a reaction yield of only 16% (Grieco et al., 2015). We modified the literature procedure, using excess triethyl orthoformate [HC(OEt)₃] and distilling the excess off during the reaction, which increases the yield of the reaction to 76%. Similar to M^CAAC and M^MAC complexes, the synthesis of M^BZI complexes starts with addition of Ag₂O to the benzoimidazolium salt or CuCl to BZI carbene generated in situ with base to form the respective benzoimidazole silver(I) or copper(I) chloride complexes. The isolated BZIAgCl is transmetallated with (Me₂S)AuCl to form the BZIAuCl complex. Reaction of the chloride complexes with carbazole in the presence of NaOtBu forms the M^BZI complexes in good yields (70–85%).

The structures were determined for Cu^BZI, Ag^BZI, and Au^BZI by single crystal X-ray diffraction (details can be found in the Supplementary Information and the datasets generated for this study can be found in the Cambridge Crystallographic Data Center, https://www.ccdc.cam.ac.uk/structures/, under the identifiers Cu^BZI: 1984269, Ag^BZI: 1984268 and Au^BZI: 1984267). Cu^BZI and Au^BZI show only a single conformer, with bond distances and interligand torsion angles similar to our previously reported linear coinage metal complexes (summarized in the Supporting Information) (Di et al., 2017; Romanov et al., 2018, 2019; Hamze et al., 2019a,b; Shi et al., 2019). In contrast, the unit cell for the Ag^BZI complex contains two conformers (Figure 3). The first, like its copper and gold analogs, has a coplanar conformation of its carbene and amide ligands (dihedral angle = 0°), whereas the second displays an orthogonal conformation (dihedral angle = 95°). The C_carbene···N_Cz distances of M^BZI fall in the order Cu (~3.73 Å) < Au (~4.00 Å) < Ag (~4.11 Å). The C–M–N bond angles are all close to 180° (range = 174–180°).

The electrochemical properties of the complexes were determined using cyclic voltammetry (CV) and differential pulse voltammetry (DPV). The copper and the silver complexes show irreversible reduction, whereas the gold analog shows a quasi-reversible reduction (see section Supporting Information). The reduction potentials for the M^BZI series are identical (E_red = −2.84 ± 0.02 V) and greater (more negative) than those of M^CAAC (E_red = −2.78 ± 0.06 V) and M^MAC (E_red = −2.45 ± 0.06 V) complexes. The reduction potentials of M^BZI relative to their M^CAAC and M^MAC analogs indicates that the electrophilicity of the coordinated BZI carbene is lower than the CAAC and MAC ligands in similar complexes. All the complexes undergo irreversible oxidation (Table 1) and, unlike the M^CAAC and M^MAC complexes where the oxidation potential is the same across the series, the potential of the M^BZI complexes increases from Cu (E_ox = 0.11 ± 0.06 V) to Au (E_ox = 0.32 ± 0.06 V), suggesting participation of the metal in the oxidation process. Thus, values for the redox gap (ΔE_redox) are greater for the silver and gold complexes than the copper analog.

The structure calculated using density function theory (DFT) for the ground state of Au^BZI is shown in Figure 4. The HOMO density is localized largely on the carbazolide (Cz) ligand, whereas the LUMO is primarily confined to the carbene ligand, with smaller contributions from the metal d-orbitals to both MOs. Time dependent DFT (TD-DFT) calculations find that ³Cz is the lowest-energy state and lies <0.09 eV below the manifold of the ^1/3ICT states (Table S8). Additionally, the oscillator strength calculated for the silver complex is weaker than that of its Cu and Au analogs (Table S8), consistent with the lower molar absorptivity observed for the ICT transition in the absorption spectra of the silver complex (see below). Large molecular dipole moments calculated for the ground state are directed along the metal-ligand bond axis toward the carbazolide ligand (μ_calc ≈ −12.5 debye), whereas the moments for the excited ¹ICT state is comparable in magnitude but directed toward the BZI ligand (μ_calc ≈ 13.5 debye).

Absorption spectra of the M^BZI complexes in polar (2-methyltetrahydrofuran, 2-MeTHF) and nonpolar (methylcyclohexane, MeCy) solvents are shown in Figure 5. Absorption bands between 300 and 375 nm of M^BZI are assigned to π-π^* transitions localized on the carbazolyl ligand (Hamze et al., 2019a,b; Shi et al., 2019). The band at lower energy (>375 nm) is assigned to an intramolecular ligand-to-ligand charge transfer (ICT/LLCT) transition from Cz to BZI. The energy of the ICT band of M^BZI is higher than in the M^CAAC and M^MAC complexes, consistent with the order of reduction potentials in these complexes. The ICT band extends to 410 nm in MeCy and has two features separated by 1100 cm⁻¹ indicative of vibronic coupling. Similar to M^CAAC and M^MAC complexes, the ICT band of the M^BZI complexes displays negative solvatochromism and merges into the higher lying ligand π-π^* transitions in polar solvents. These shifts with solvent polarity are due to the large change in the molecular dipole moments between the ground and excited ICT states. The molar absorptivities of the M^BZI complexes decrease in the order Au > Cu > Ag in all media. The same trend was observed in the M^CAAC and M^MAC analogs and attributed to a decrease in the overlap integrals between orbitals on the donor Cz and acceptor ligands mediated by the metal center (Hamze et al., 2019b).

Emission spectra of the M^BZI compounds in MeCy, 2-MeTHF solution and polystyrene (PS) films at room temperature and 77 K are shown in Figure 5 and tabulated in Table 2. The spectra from the M^BZI complexes are blue-shifted relative to their M^CAAC and M^MAC counterparts, yet display similar solvatochromic behavior, undergoing red shifts in polar solvents. Spectra recorded in MeCy are relatively narrow (FWHM = 44 nm, 2,300 cm⁻¹) and show underlying vibronic features. The photoluminescence quantum yields (Φ_PL) of M^BZI complexes are close to unity in MeCy and PS films but decrease with increasing solvent polarity (Table 2). Furthermore, increasing solvent polarity is correlated with increased spectral width and loss of the vibronic features, suggesting structural distortion in the excited states. To explain the blue-shift in absorption spectra and red-shift in emission spectra observed in these complexes with increasing solvent polarity, a diagram representing the potential energy surfaces for the ground state (S₀) and excited states (³Cz and ^1,3ICT) as a function of nuclear coordinates in MeCy and CH₂Cl₂ is proposed (Figure 6). The vibronically structured absorption and emission spectra in nonpolar solvents (MeCy) indicate that the potential energy surfaces are well-nested, such that ICT transitions are induced with small reorganization energies. In contrast, the blue-shifted absorption and broad, featureless red-shifted emission observed in polar solvents (CH₂Cl₂) indicate that significant reorganization occurs within the metal complex and its surrounding media (to a larger extent) as a result of the large change in dipole moment upon excitation (Δμ_calc > 24 debye). Unlike M^CAAC and M^MAC complexes, where the radiative rate constant (k_r) is fastest for the silver analog (Romanov et al., 2018; Hamze et al., 2019b), Au^BZI has the fastest k_r in accord with gold having the largest SOC constant.

The biexponential character of the emission decay in PS thin films could be due to the presence of M^BZI complexes in different conformations, one where the carbene and carbazole are in a coplanar orientation and another where the two ligands are twisted relative to each other. In our previous studies of (CAAC)Cu(carbazole) complexes we found that while the two forms display similar emission spectra the twisted form has a markedly longer excited state lifetime and lower oscillator strength than the coplanar form (Hamze et al., 2019a). In solution the excited M^BZI can effectively rotate to the coplanar form prior to relaxing to the ground state. However, the rigid PS matrix will prevent the conformers from equilibrating in the excited state and thus they are expected to emit independently with different individual emission lifetimes.

The emission spectra display a pronounced rigidochromic shift upon cooling to 77 K and become extremely narrow and vibronically structured, with luminescence lifetimes in the millisecond regime. Thus, emission at low temperatures is consistent with a triplet transition localized on the carbazolide ligand (³Cz). This change in emission properties with temperature is attributed to the close energy separation between the ³Cz and ^1/3ICT manifolds, making the ICT manifold thermally accessible at room temperature, but inaccessible in frozen MeCy and 2-MeTHF at 77 K. The fact that the M^BZI complexes display ³Cz emission in PS (as well as MeCy and 2-MeTHF) at 77 K is different from the behavior observed in M^CAAC and M^MAC complexes. Emission from the latter complexes remains broad and featureless in a polystyrene matrix at all temperatures, even down to 4 K (Hamze et al., 2019b). Thus, in the case of the M^CAAC and M^MAC complexes, the ³ICT state lies below the energy of the ³Cz state in PS at all temperatures (Hamze et al., 2019b). However, for the M^BZI complexes in PS films, it is evident that the lowest excited triplet state is indeed ³Cz at all temperatures. This difference suggests that the ³Cz and ³ICT states in the M^BZI complexes are near degenerate in energy, and TADF emission occurs via thermal activation from the ³Cz to ¹ICT states, not just within the ICT manifold as in the case of the M^CAAC and M^MAC complexes.

Another difference in the properties of the M^BZI complexes compared to the M^CAAC and M^MAC analogs is the pronounced decrease in luminescence efficiency with increasing solvent polarity. For example, the quantum yield of Cu^BZI is severely diminished in CH₂Cl₂ relative to that recorded in MeCy (Φ_PL = 0.03 in the former and 0.80 in the latter), whereas this decrease in efficiency is less pronounced for Cu^CAAC (Φ_PL = 0.4 in CH₂Cl₂ and 0.92 in MeCy) and Cu^MAC (Φ_PL = 0.5 in CH₂Cl₂ and 0.90 in MeCy). To better understand the origin of this decrease in Φ_PL with solvent polarity, photophysical properties of Au^BZI were characterized in mixtures of MeCy and CH₂Cl₂ at various ratios. The ICT band in the absorption spectra gradually blue shifts with increasing CH₂Cl₂ concentration and the vibronic fine structure observed in MeCy disappears in mixtures with ≥ 5 vol% CH₂Cl₂ (Figure 7A and Figure S6). Figure 7B shows that the radiative rate constant of Au^BZI decreases with increasing solvent polarity, whereas the non-radiative rate constant (k_nr) remains near constant, consequently decreasing the Φ_PL. The fact that the non-radiative rate constant of Au^BZI is largely independent of CH₂Cl₂ concentration, despite the similar reduction potentials for the Au^BZI excited state (E^0/+* = −2.67 V) and CH₂Cl₂ (E^0/− = −2.73 V), indicates that there is no oxidative quenching of excited Au^BZI by CH₂Cl₂. Therefore, the lower Φ_PL of Au^BZI in CH₂Cl₂ comes about from a decrease in the radiative rate constant. This change is likely due to a decrease in the Franck-Condon factors for the ICT transition caused by the shift of the excited state surface in the polar solvent, as illustrated in Figure 6.

Emission studies of Au^BZI at variable temperature were conducted in MeCy and CH₂Cl₂ to investigate the parameters controlling TADF (Figure 8). Emission in MeCy slightly increases in intensity and displays limited changes in line shape with increasing temperature. In contrast, spectra recorded in CH₂Cl₂ reveal a drop in intensity with increasing temperature. Vibronic features resolved at low temperatures (190–230 K) are found to broaden abruptly at 240 K. The k_rvalues calculated from the quantum yields measured at various temperatures (see Supplementary Information) were fit to a two level model using Equation 1 (Figure 9A) (Hamze et al., 2019b).

Where, k_B is the Boltzmann constant, k_TADF and k_fl are the radiative rate constants of the TADF and fluorescence, respectively and kISCS1→T1 is the intersystem crossing rate (see Figure 9A). The best fit to the MeCy data (Figure 9B) give an energy difference between the triplet and emitting singlet state of Au^BZI of 920 cm⁻¹. However, the Arrhenius plot of the radiative rate constant of Au^BZI recorded in CH₂Cl₂ at variable temperature is decidedly non-linear (Figure S10). Variable temperature NMR spectra indicate that an aquo species is formed with residual water below 240 K (Figure S11). The formation of this aquo complex likely leads to the anomalous behavior observed in CH₂Cl₂ at lower temperatures, making the analysis using the simple two-level model problematic.

The Au^BZI complex is stable to sublimation and was thus used as dopant to fabricate OLEDs by thermal evaporation. Device optimization and details are shown in the Supplementary Information. Considering the high triplet energy of Au^BZI (E_T = 3.1 eV), 1,3-bis(triphenylsilyl)benzene (UGH3, E_T = 3.5 eV) was employed as the host matrix. Devices were fabricated using different doping levels (5, 10 and 15 wt%) and the best performance was obtained with 5 wt% Au^BZI (see Supplementary Information). Optimized devices achieved reasonably high efficiencies (maximum EQE = 12%, Figure 10) and electroluminescence (EL) spectra (λ_max = 430 nm, FWHM = 45 nm) identical to the PL spectrum in PS, demonstrating efficient exciton confinement on the complex. The color coordinates of the EL spectrum (CIE = 0.16, 0.06) make Au^BZI an efficient deep blue dopant for phosphorescent OLEDs.