Highly Efficient Deep Blue Luminescence of 2-Coordinate Coinage Metal Complexes Bearing Bulky NHC Benzimidazolyl Carbene

The structural, photophysical and electrochemical properties of three luminescent 2-coordinate coinage metal (i.e., M = Cu, Ag, Au) complexes bearing a sterically bulky benzimidazolyl carbene, 1,3-bis(2,6-diisopropylphenyl)-1-H-benzo[d]imidazol-2-ylidene (BZI), and carbazolide (Cz) as the anionic ligand were investigated. All the complexes emit in the deep blue region (~430 nm) with relatively narrow spectra (full width at half maximum = 44 nm, 2,300 cm−1) characterized by vibronic fine structure in nonpolar media (methylcyclohexane at room temperature), and with high photoluminescence quantum yields (ΦPL > 80%) and radiative rate constants (kr ~ 7.8 × 105 s−1). The luminescence is solvatochromic, undergoing a red-shift in a polar solvent (CH2Cl2) at room temperature that are accompanied by a decrease in quantum yields (ΦPL < 23%) and radiative rate constants (kr < 4.0 × 104 s−1), whereas the non-radiative rate constants remain nearly constant (knr ~ 1.0 × 105 s−1). The radiative rate is controlled via thermally assisted delayed fluorescence (TADF) and temperature-dependent luminescence studies of the gold complex (AuBZI) in methylcyclohexane solution reveal an energy difference between the lowest singlet and triplet excited states of 920 cm−1. An organic light-emitting diode (OLED) fabricated using AuBZI as a luminescent dopant has an external quantum efficiency of 12% and narrow, deep-blue emission (CIE = 0.16, 0.06).


INTRODUCTION
Highly luminescent, neutral two-coordinate, linear d 10 metal complexes of coinage metals i.e., Cu(I), Ag(I), Au(I) have recently been reported (Di et al., 2017;Romanov et al., 2018Romanov et al., , 2019Hamze et al., 2019a,b;Shi et al., 2019). These complexes have redox active carbene (acceptor) and amide (donor) ligands connected by a metal in a linear fashion i.e., (carbene)M (I) (amide). Their photoluminescence efficiencies are close to 100% in solution and in thin films, with phosphorescent lifetimes in the range of 1-3 µs. These (carbene)M (I) (amide) complexes emit from an amide-N (donor) to carbene-C (acceptor) intramolecular charge transfer (ICT) state, a transition also referred to as ligand-to-ligand charge transfer (LLCT). Because of their excellent photophysical properties, these complexes are potential candidates for application in photocatalysis (Kalyanasundaram, 1982), chemo-and biosensing (Keefe et al., 2000;Lo et al., 2010), dye-sensitized solar cells (Grätzel, 2005) and organic electronics (Lamansky et al., 2001). In particular, their fast radiative lifetimes make them promising candidates as dopants in organic light emitting diodes (OLEDs) (Di et al., 2017;Romanov et al., 2018Romanov et al., , 2019Hamze et al., 2019a,b;Shi et al., 2019). Unlike phosphors like iridium and platinum complexes, which rely on strong spin-orbit coupling (SOC) to induce what is principally triplet metal ligand charge transfer ( 3 MLCT) emission (Yersin et al., 2011), the two-coordinate coinage metal complexes emit via E-type fluorescence or thermally activated delayed fluorescence (TADF). Their fast radiative lifetimes are due to two factors; the small energy separation between their lowest singlet and triplet excited states ( E ST ) and spin orbit coupling via the metal ion. Together these two parameters lead to rapid endothermic intersystem crossing from the longlived triplet to the faster radiating singlet state. Recent work has shown that E ST in these complexes is dependent on the identity of the metal atom. The copper and gold complexes have similar values for E ST , whereas the E ST for the silver complexes is smaller (Romanov et al., 2018;Hamze et al., 2019b). The small E ST in the silver analogs leads to extremely fast radiative rate constants (k r > 10 6 s −1 ), faster than the Ir and Pt phosphorescent emitters. The emission energy of complexes, FIGURE 1 | The compounds discussed in this paper have the general formula (carbene)M(N-carbazolyl), with three carbenes, i.e., CAAC, MAC, and BZI. The BZI compounds are new and the CAAC and MAC derivatives have been reported previously (Hamze et al., 2019a,b;Shi et al., 2019). Compounds are abbreviated using the symbol for the metal ion and superscripted carbene, e.g., Cu CAAC or Au BZI . although nearly independent of the metal, can be altered using different carbenes with varying electrophilicity or amides with different nucleophilicity, allowing the luminescence color to be varied from deep blue to deep red (Hamze et al., 2019a;Shi et al., 2019). Developing alternative blue dopants is crucial in tackling the long-standing problem of stability in blue OLEDs. Complexes based on the CAAC ligand (M CAAC , Figure 1) are reported to have efficient blue photoluminescence and give good efficiencies as dopants in OLEDs (Hamze et al., 2019a). Unfortunately, the emission spectra of these complexes are broad, which is not ideal for display applications. Additionally, the ability to vary the physical and electronic properties of CAAC ligands is limited and therefore inconvenient for modifying the characteristics needed for OLEDs. Complexes based on the MAC ligand (M MAC ) can be used to fabricate high efficiency OLEDs; however, emission in these derivatives is bathochromically shifted to green owing to the electrophilic MAC ligand (Hamze et al., 2019b). Substitution of the carbazole ligand with cyano groups was therefore used to stabilize the HOMO and blueshift the emission (Shi et al., 2019). Alternatively, carbenes based on benzoimidazoles (BZI), originally used in luminescent twocoordinate Au complexes (Wang et al., 1999), lead to metal complexes with LUMO energies similar to CAAC (Krylova et al., 2014;Hamze et al., 2017Hamze et al., , 2020, suggesting that replacing CAAC with benzoimidazolyl-carbene ligands should give similar photophysical and electrochemical properties as M CAAC . Herein, we examine monovalent, linear, 2-coordinate coinage metal (i.e., M = Cu, Ag, Au) complexes (M BZI ) bearing a sterically bulky benzimidazolyl carbene, 1,3-bis(2,6-diisopropylphenyl)-1-H-benzo[d]imidazol-2-ylidene (BZI), and carbazolide (Cz) as the anionic ligand. We have investigated the structural and photophysical properties of the M BZI derivatives to elucidate the role of the carbene and the metal ion in the excited-state properties. The M BZI complexes have structures, redox potentials and photoluminescent efficiencies ( PL = 0.8-1.0) similar to the M CAAC analogs, but different excited-state dynamics. Analysis of the luminescence at low temperature reveals that the triplet carbazole ( 3 Cz) state and the singlet/triplet intramolecular charge transfer ( 1/3 ICT) manifold of the M BZI complexes are near degenerate, resulting in photophysical properties that are distinct from the M CAAC complexes. The Au BZI complex has also been successfully employed a dopant to fabricate efficient blue OLEDs.

Synthesis
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 Ag 2 O 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 (Me 2 S)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%).

X-Ray Analysis
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., 2018Romanov et al., , 2019Hamze 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 • ).

Electrochemistry
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 ( E redox ) are greater for the silver and gold complexes than the copper analog.

Computational Analysis
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 3 Cz is the lowest-energy state and lies <0.09 eV below the manifold of the 1/3 ICT 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 1 ICT state is comparable in magnitude but directed toward the BZI ligand (µ calc ≈ 13.5 debye).

Photophysical Characterization
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).    Sworakowski et al. (2016).
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 ( 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 (S 0 ) and excited states ( 3 Cz and 1,3 ICT) as a function of nuclear coordinates in MeCy and CH 2 Cl 2 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 Frontiers in Chemistry | www.frontiersin.org contrast, the blue-shifted absorption and broad, featureless redshifted emission observed in polar solvents (CH 2 Cl 2 ) 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 ( µ 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 FIGURE 6 | Qualitative energy diagram representing the ground state (S 0 ) and both excited state potential energy surfaces ( 3 Cz and 1,3 ICT) as a function of nuclear coordinate in MeCy (blue) and CH 2 Cl 2 (red) solution along with absorption (solid) and emission (dashed) transitions.
ligand ( 3 Cz). This change in emission properties with temperature is attributed to the close energy separation between the 3 Cz and 1/3 ICT 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 3 Cz 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 3 ICT state lies below the energy of the 3 Cz 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 3 Cz at all temperatures. This difference suggests that the 3 Cz and 3 ICT states in the M BZI complexes are near degenerate in energy, and TADF emission occurs via thermal activation from the 3 Cz to 1 ICT 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 CH 2 Cl 2 relative to that recorded in MeCy ( PL = 0.03 in the former and 0.80 in the latter), whereas this decrease in efficiency is less pronounced for Cu CAAC ( PL = 0.4 in CH 2 Cl 2 and 0.92 in MeCy) and Cu MAC ( PL = 0.5 in CH 2 Cl 2 and 0.90 in MeCy). To better understand the origin of this decrease in PL with solvent polarity, photophysical properties of Au BZI were characterized in mixtures of MeCy and CH 2 Cl 2 at various ratios. The ICT band in the absorption spectra gradually blue shifts with increasing CH 2 Cl 2 concentration and the vibronic fine structure observed in MeCy disappears in mixtures with ≥ 5 vol% CH 2 Cl 2 ( 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 PL . The fact that the non-radiative rate constant of Au BZI is largely independent of CH 2 Cl 2 concentration, despite the similar reduction potentials for the Au BZI excited state (E 0/+ * = −2.67 V) and CH 2 Cl 2 (E 0/− = −2.73 V), indicates that there is no oxidative quenching of excited Au BZI by CH 2 Cl 2 . Therefore, the lower PL of Au BZI in CH 2 Cl 2 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 CH 2 Cl 2 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 CH 2 Cl 2 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 r values 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 k S 1 →T 1 ISC 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 CH 2 Cl 2 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 CH 2 Cl 2 at lower temperatures, making the analysis using the simple two-level model problematic.

OLED Characterization
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.

CONCLUSION
A series of 2-coordinate coinage metal (i.e., M = Cu, Ag, Au) complexes bearing a sterically bulky benzimidazolylcarbene, 1,3-bis(2,6-diisopropylphenyl)-1-H-benzo[d]imidazol-2-ylidene (BZI), and carbazolide (Cz) as the anionic ligand were investigated. X-ray analysis reveals a linear geometry at the metal center with the ligands in a coplanar conformation, as well as orthogonal for Ag BZI . The redox gap of all the complexes is large ( E redox > 3 V), in agreement with the highenergy absorption band (> 400 nm in CH 2 Cl 2 ) corresponding to the carbazolide-to-carbene ICT transition. The complexes have high luminescence efficiencies ( PL >80%) and display deep blue narrow emission in MeCy and PS films. Their absorption spectra display negative solvatochromism, whereas their emission spectra undergo bathochromic shifts in polar solvents that is accompanied by decrease in quantum yields ( PL < 23%) and radiative rate constants (k r < 4.0 × 10 4 s −1 ). The non-radiative rate constants, however, are unaffected by the medium, remaining nearly the same in polar and nonplar media (k nr ∼ 1 × 10 5 s −1 ). Temperature-dependent studies reveal that the energy difference between the singlet and triplet excited states in methylcyclohexane is 920 cm −1 . Vapor-deposited OLEDs fabricated using Au BZI as an emissive dopant have high efficiency (EQE = 12%) and a narrow and deep blue emission (CIE = 0.16, 0.06). These two-coordinate complexes present new opportunities for use as dopants in blue OLEDs. Lifetime studies on devices will need to be carried out to determine if these coinage metal-based emitters could serve as alternatives to state-of-the-art Ir(III) complexes commonly used in OLEDs.

SUPPORTING INFORMATION
Synthesis of precursors and complexes, differential pulse voltammetry, cyclic voltammetry curves, photophysical data of the final complexes in various solvents, computational data and x-ray crystallographic data of the final complexes, OLED device fabrication procedure and device characteristics, 1 H and 13 C NMR of precursors and final complexes.

DATA AVAILABILITY STATEMENT
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 CuBZI: 1984269, AgBZI: 1984268, and AuBZI: 1984267.

AUTHOR CONTRIBUTIONS
RHam synthesized and measured the photophysical properties of the copper, silver, and gold compounds. MI measured and analyzed the temperature dependent photophysical properties of the gold complex. DM carried out the theoretical modeling. MJ prepared and characterized the OLEDs. RHai determined all of the crystal structures. RHam, PD, and MT conceived of the project. MI, PD, and MT wrote the manuscript.

FUNDING
This work was supported solely by the Universal Display Corporation. No funds have been received from Universal Displays, our institution or any other source to cover publication fees or open access fees.