Phosphorescent Carbene‐Gold‐Arylacetylide Materials as Emitters for Near UV‐OLEDs

A series of carbene‐gold‐acetylide complexes [(BiCAAC)AuCC]nC6H5−n (n = 1, Au1; n = 2, Au2; n = 3, Au3; BiCAAC = bicyclic(alkyl)(amino)carbene) have been synthesized in high yields. Compounds Au1–Au3 exhibit deep‐blue to blue‐green phosphorescence with good quantum yields up to 43% in all media. An increase of the (BiCAAC)Au moieties in gold complexes Au1–Au3 increases the extinction coefficients in the UV–vis spectra and stronger oscillator strength coefficients supported by theoretical calculations. The luminescence radiative rates decrease with an increase of the (BiCAAC)Au moieties. The time‐dependent density functional theory study supports a charge‐transfer nature of the phosphorescence due to the large (0.5–0.6 eV) energy gap between singlet excited (S1) and triplet excited (T1) states. Transient luminescence study reveals the presence of both nonstructured UV prompt‐fluorescence and vibronically resolved long‐lived phosphorescence 428 nm. Organic light‐emitting diodes (OLED) are fabricated by physical vapor deposition with 2,8‐bis(diphenylphosphoryl)dibenzo[b,d]furan (PPF) as a host material with complex Au1. The near‐UV electroluminescence is observed at 405 nm with device efficiency of 1% while demonstrating OLED device lifetime LT50 up to 20 min at practical brightness of 10 nits, indicating a highly promising class of materials to develop stable UV‐OLEDs.


Introduction
Organic light-emitting diodes (OLEDs) are now ubiquitous in commercial devices for display and lighting technologies, as well as others.However, despite the considerable amount of research reactions within the device.Recently, carbene-metal-amide (CMA) emitters have emerged as promising Au(I) complexes for application in OLEDs, enabling devices with up to 26% external quantum efficiency (EQE) at practical brightness of 100 nits. [12]CMA materials are two-coordinate coinage metal complexes, where the metal is coordinated with a neutral carbene and an anionic amide ligand.To prepare more stable emitters, one needs to consider the organometallic complex with a more covalent bond character between the gold atom and the ligands, thus increasing bond dissociation energy for a less polarized chemical bond.Therefore, we considered exchanging the anionic amide ligand in CMA with a carbon-based (aryl)acetylide ligand due to similar values in electronegativity (≈2.5) [13] for gold and carbon atoms.For over two decades, various cyclometallated Au(III)-(aryl)acetylide complexes have been thoroughly investigated and applied in electroluminescent devices to produce skyblue, green, and red OLEDs. [14,15]Also, Au(I)-(aryl)acetylide complexes are one of the most stable organogold compounds with well-reported photophysical behavior in the literature but are hardly applied in OLED devices.One of the plausible explanations could be a relatively long excited state lifetime (10-100 ms) and poor luminescence quantum yields.In Scheme 1, we assembled a chronological evolution of the ligated Au(I)-(aryl)acetylide complexes and their performance.For instance, the luminescence efficiency increases with an increase in the -donor property of the neutral lignad when comparing (L)Au(C≡CPh) complexes where L = PPh 3 (Ph = phenyl, poor photoluminescence quantum yield (PLQY)) [16] and PCy 3 (Cy = cyclohexyl, PLQY up to 22%). [17]Nolan [18] and Venkatesan [19] reported various (L)Au(C≡CPh) complexes (Scheme 1), where L is one of the conventional N-heterocyclic carbene ligands.These mononuclear complexes are practically nonemissive in the solid state (PLQY values are ≈1%, Scheme 1), whereas di-and multinuclear complexes may show enhanced PLQY values (up to 30%) due to the aurophilic (Au•••Au) interactions. [20]Chen et al. recently reported alkynyl complexes (L)Au(C≡C-Aryl) [21] where L is a pyrazinefused NHC-carbene having stronger -accepting properties compared with conventional NHC-carbenes (Scheme 1).This molecular design resulted in various complexes to have either blue phosphorescence (PLQY 36%) or bright green and yellow thermally activated delayed fluorescence (TADF with PLQY up to 76%) emitters while demonstrating green and yellow OLEDs with up to 9.7% EQE at practical brightness of 1000 cd m −2 . [21]ere we investigated a series of carbene-metal-acetylides (CMAc) complexes {(BiCAAC)AuCC-} n C 6 H 6−n ) as shown in Scheme 1 (n = 1 for Au1, 2 for Au2, and 3 for Au3).Unlike previous works (Scheme 1), we decided to use a bicyclic(alkyl)(amino)carbene (BiCAAC) ligand with superior donor and -acceptor properties compared to phosphine and conventional NHC-carbene ligands.Complexes Au1-Au3 are investigated to ascertain the effect of multiple gold atoms on the photophysical properties of the emitters.Unlike complexes with conventional NHC ligands, the mono-substituted complex (Au1) shows the most suitable and bright phosphorescence with PLQY up to 43%.Au1 reveals a high-energy fast, prompt fluorescence 1 (*) as well as a long-lived phosphorescence 3 (*), where the latter can be quenched by a suitable host material in the emitting layer of the OLED.By quenching the triplet emission, for instance, by using DPEPO host material for the emitting layer, we exclusively see prompt fluorescence in the OLED devices, resulting in near-UV electroluminescence of 1% and excellent device operational stability of 20 min (LT 50 ).

Results and Discussion
Gold complexes Au1-3 (Chart 1) were prepared in high yields by reaction of (BiCAAC)AuCl [22] with KO t Bu and ethynylbenzene (Au1), 1,4-diethynylbenzene (Au2), or 1,3,6-triethynylbenzene (Au3), respectively.All gold complexes are off-white solids soluble in toluene and polar aprotic solvents (dichloromethane and tetrahydrofuran) and stable for several months in the solid state in air.The carbene-carbon resonances exhibit minor deviation from 261.6 ppm (±0.1) in 13 C NMR spectra for Au1-3, indicating similar chemical and electronic environments for the (Bi-CAAC)Au moiety.][19][20]23] Thermogravimetric analysis has been used to determine the decomposition temperature (T d, 5% mass loss as shown in Figure S1, Supporting Information), indicating lower thermal stability for the monogold complex Au1 (297 °C) compared with Au2 (342 °C) and Au3 (331 °C).Regardless of lower thermal stability for Au1, it was successfully sublimed on a gram scale at 210 °C and 5 × 10 −6 mbar pressure to obtain high purity samples for the fabrication of the OLED devices, vide infra.
Crystals of Au1 and Au2 suitable for X-ray diffraction were obtained by layering CH 2 Cl 2 solutions with hexane.The molecular structures and key geometric parameters are collected in Figure 1, together with key intermolecular contacts.All compounds are monomeric with no close aurophilic interactions between the gold atoms.Both Au1 and Au2 exhibit linear geometry around the gold atom with an identical C1-Au-C25 angle of 176.7(4)°.Both the gold-carbene(C1) and gold-acetylide(C25) bond lengths are shorter by 0.02-0.04Å for the digold complex Au2 compared with mononuclear Au1.Such deviations in bond lengths result in 0.06(1) Å longer carbene(C1)•••acetylide(C25) distance for the compound Au1.The acetylide C25≡C26 triple bond length is slightly longer for the Au2 compared with Au1, however, the deviation is within the experimental error of 0.01 Å.
Intermolecular contacts have been analyzed for compounds Au1 and Au2 to gain more insight into measured gold─carbon bond length deviations.The neighboring molecules are arranged into 3D networks by weak hydrogen bonds between soft C─H acids and soft -bases (double, triple, and aromatic moieties), as shown in Figure 1.Complex Au2 exhibits a bifurcated hydrogen bond where C3─H3(carbene) is pointed to the center of the C26 (C≡C) ─C27 (phenyl) bond (Figure 1).This results in 0.04 Å elongation of the C26─C27 bond for Au2 compared with Au1, where a similar hydrogen bond C6─H6(carbene) is formed with three sp 2 -hybridized carbons (C26, C27, and C32), resulting in much smaller bond elongation (0.02 Å for C26─C27 and C27─C32) due to better delocalization for Au1.Unlike Au2, complex Au1 exhibits a short T-shape hydrogen bond C16─H16 pointed toward the acetylide terminal carbon atom C25 which is attached to a gold atom Au1.This fact corroborates the 0.06 Å longer gold─acetylide bond length for Au1 compared with Au2 due to the weakening of the bonds attached to the somewhat more electron-deficient C25 carbon atom participating in a weak hydrogen bonding.
Cyclic voltammetry was performed in THF solutions, with data summarized in Table 1.Combined cyclic voltammograms are Fc/Fc+ + 5.39) eV; E LUMO = -(E onset red Fc/Fc+ + 5.39) eV. [ 24]own in Figure 2. Complexes Au1-3 show a quasireversible reduction process at −2.90 ± 0.03 V with little variation in E 1/2 .This value is closely similar to the previously reported CMA1 ( Ad CAACAuCz) complex. [12]The quasireversibility of the reduction peak can be demonstrated by the increase in the peak-to-peak separation values ΔE p from 162 to 323 mV (at 100 mV s −1 ) for Au1-Au3, which deviates from the ideal value of 59 mV for a oneelectron reversible couple (Table 1).The reduction wave for complexes Au1-Au3 exhibits the ratio between cathodic and anodic currents i pc /i pa in the range of 0.73-0.85,somewhat deviating from the ideal unity value for the fully reversible couple.Both cathodic and anodic currents experience concurrent increase with the number of the (BiCAAC)Au moieties in complexes Au1-Au3, implying that the LUMO (−2.61 ± 0.01 eV) is largely localized on the (BiCAAC)Au moiety for all complexes.Oxidation of the compounds Au1-Au3 results in an irreversible wave with no reduction back-peak.Significant variation in E p values was measured, 0.59 ± 0.12 V, thus indicating the HOMO is largely localized on the arylacetylide of the compounds.The change in E p demon- strates that the energy of the HOMO varies with the number and position of the (BiCAAC)Au-C≡C moieties on the central benzene ring (Figure 2 and Table 1).This affects the energy gap values, which increase in the order of Au2 (3.24 eV) < Au3 (3.38 eV) < Au1 (3.49 eV).

Photophysical Properties and Theoretical Considerations
The UV-vis spectra for all complexes are shown in Figure 3 (Figure S3a,b, Supporting Information), while the photophysical parameters are collected in Table 2 in various media.The UV-vis spectra show a broad and weakly-resolved absorption band in the 300-375 nm region, see Figure 3a.This band experiences a 40 nm blueshift with an increase in the solvent polarity (Figure 3b), thus demonstrating a negative solvatochromism similar to carbenemetal-amide materials. [12]The energy gap values between the highest occupied and lowest unoccupied electronic levels were estimated from the red onset of the lowest-in-energy absorption band in the UV-vis spectra in THF solution for all gold complexes (3.57eV for Au1, 3.28 eV for Au2, and 3.42 eV for Au3).
The values correlate with the energy gap calculated from the electrochemistry experiment in THF, indicating luminescence for monogold complex Au1 to be more blueshifted, whereas being redshifted for complex Au2.
We ascribe the lowest-energy bands to a metal-mediated charge-transfer (L(M)LCT) from arylacetylide (HOMO) to carbene (LUMO) in line with our theoretical calculations, see Figure 4.The lowest-energy bands of monogold compound Au1 possess a molar extinction coefficient of ≈1×10 4 m −1 cm −1 , while di-and trigold complexes Au2-3 show molar extinction coefficients of ≈4 × 10 4 m −1 cm −1 , thus indicating a higher allowance of the excitation when increasing the number of (BiCAAC)Au(acetylide) moieties.Theoretical calculations support this experimental fact with higher oscillator strength coefficients for the S 0 →S 1 transition in the case of the digold complex Au2 (f = 1.3447; f = 0.7947 for Au3) compared with monogold complex Au1 (f = 0.4426).Such increases in the UV-vis extinction coefficients have been observed before for the organometallic and organic TADF materials. [25,26]For instance, Wright et al. demonstrated that the increase of the spatial overlap of the molecular orbitals or homoconjugation enhances the photophysical properties of the materials by facilitating an intramolecular charge-transfer process when compared to the classic donor-acceptor systems. [25]Indeed, theoretical calculations show an increase in the degree of the homoconjugation between HOMO and LUMO orbitals reflected by the increase of the HOMO-LUMO overlap integrals for the gold complexes: 0.42 for Au1 and 0.47 for Au2.This clearly illustrates that greater homoconjugation in Au2 leads to an enhancement in the extinction coefficient of the CT band. [25]omplex Au3 shows no significant trend since the S 0 →S 1 transition corresponds to largely a combination of the HOMO-LUMO (20%), HOMO-1-LUMO+1 (20%), and HOMO-1-LUMO+2 (36%) transitions (Table S2, Supporting Information).The isosurfaces and overlap integrals for the pertinent orbitals of the trigold complex (Au3) indicate a somewhat smaller homoconjugation compared with the digold complex (Au2), explaining a slightly lower enhancement of the extinction coefficients regardless of the larger number of the (BiCAAC)Au(acetylide) moieties.The photoluminescence (PL) spectra for all complexes are shown in Figure 5, while the photophysical parameters are collected in various media and presented in Table 2.The steady-state emission of complexes Au1-Au3 in all media shows an acetylidecentered and vibronically resolved 3 (*) phosphorescence profile with peak PL at 429 nm (Au1), 505 nm (Au2), and 455 nm (Au3).Complexes Au2 and Au3 demonstrate 26-76 nm redshift of the PL profiles compared with Au1, which results from the perturbation produced by increasing the number of the gold atoms coordinated with the acetylide core ligand. [27]Gold complexes Au1-Au3 demonstrate only minor solvatochromism (up to 5 nm) in various media and solvents of different polarities (see Figure 5; and Figure S4, Supporting Information).The trend of  em values of Au1-3 follow the bandgap energies measured by cyclic voltammetry and UV-vis spectroscopy, see above, as well as TD-DFT calculations (Table S5, Supporting Information), where Au1 shows a deep-blue emission, Au2 shows a green emission, and Au3 shows a blue emission in all media.Moreover, a highly-resolved PL profile remains intact upon freezing the solution and films down to 77 K (Figure 5), indicating an acetylide-centered 3 (*) PL with a similar nature at room and 77 K temperature.
To confirm the nature of the phosphorescence nature of the PL, we measured the excited state lifetimes for all gold complexes at 295 and 77 K (see Table 2).At room temperature, all complexes demonstrate long excited state lifetimes in the range of 24-80 μs (with biexponential PL decays), and the values increase with the number of the (BiCAAC)Au moieties from Au1 to Au3.We measured a nearly twofold increase of the excited state lifetime upon cooling to 77 K, thus enabling us to assign the nature of the PL to phosphorescence from the acetylide triplet intraligand excited state ( 3 IL(C≡C)) and facilitated by the large spin-orbit coupling from the Au(I) atom.Complex Au1 in a 2% PS host shows only a marginal increase for the excited state lifetime on cooling from 296 to 16 K (Figure S4, Supporting Information).The phosphorescence profile attains a more vibronically resolved structure at 16 K (varied temperature experiment as shown in Figure S4, Sup-porting Information), thus further supporting the assignment of the phosphorescence and ruling out TADF.
Our theoretical calculations (Table S6, Supporting Information) reveal a different conformation between the planes of the BiCAAC carbene and phenylacetylide ligands for S 1 (twisted) and T 1 (nearly coplanar like S 0 ) excited state geometry for Au1-2.We calculated the large energy difference between S 1 and T 1 excited state of ≈0.6 eV for Au1-3 (see Figure 5; and Table S5, Supporting Information).This result correlates well with the theoretically calculated larger overlap integral between HOMO and LUMO orbitals (O H/L ) for these complexes; for instance, O H/L = 0.47 for Au2 is larger than 0.36 for the archetype CMA1 complex. [12]The larger the O H/L , the greater the stabilization of the T 1 state for the phosphorescent materials.Since the S 0 -S 1 transition character is predominantly HOMO-LUMO (85% and 91% for Au1 and Au2, respectively), we expect the initial photoexcitation to populate the 1 CT state, followed by rapid intersystem crossing ISC to a 3 CT state and then nonradiative relaxation to the lowest in energy intraligand 3 IL(*) state demonstrating a vibronically resolved phosphorescence.
Lifetimes in films are fitted biexponentially and in solution monoexponentially.A marked increase in lifetimes in both solution and films on cooling from 295 to 77 K indicates quenching of phosphorescence at 295 K by a nonradiative process.Despite this, complexes Au1-3 show high phosphorescent quantum yields of 43%, 37%, and 22%, respectively, in 20% DPEPO films at 295 K.In polystyrene (PS) and DPEPO films, the lifetimes of the complexes increase with the number of gold units (103, 135, and 250 μs for Au1, Au2, and Au3, respectively, in 20% DPEPO films at 295 K).The highest PLQYs are measured in the DPEPO host and decrease from Au1 (up to 43%) to Au2 (up to 37%) and Au3 (up to 22%, Table 2).The PLQY for the Au1 phosphorescence is one of the highest reported to date for mononuclear (carbene)Au(I)(phenylacetylides) that are not enhanced by aurophilic interactions.Therefore, the radiative rates decrease with an increase in the number of (BiCAAC)Au units and indicate that distortion of the linear geometry in the excited state is more likely for the larger complexes, i.e., Au3, thus outcompeting any benefits gained by increasing the heavyatom effect with multiple (BiCAAC)Au-C≡C moieties.In fact, the toluene solution for Au3 shows shorter lifetimes than Au2 (47.6 and 77.3 μs for Au3 and Au2, respectively).This is likely a result of increased nonradiative decay in solution due to a larger number of (BiCAAC)Au moieties and, therefore, a greater number of possible nonradiative pathways present in a fluid medium than compared to a rigid medium such as film or frozen solution.
To reveal the distortions responsible for the nonradiative processes, we overlayed the molecular coordinates from the X-ray single crystal diffraction experiment and calculated coordinates for the triplet excited state (T 1 ) for phosphorescent complexes Au1 and Au2 (Figure 4).Complex Au1 has only minor 10°rotational distortion whereas complex Au2 experiences a severe out of plane bending and rotation of the phenyl(alkynyl) moiety which occurs upon photoexcitation as shown in Figure 4.Such bending and rotational distortions are likely responsible for the gradual decrease in PLQY with increase of the (BiCAAC)Au moieties for Au1 to Au3 (see Table 2).Given such severe distortions for Au2, this problem will be even more pronounced for complex Au3, which has the highest number of the (BiCAAC)Au moieties and the lowest PLQY values in the series.Such Renner-Teller type distortions are associated with the bending in the linear fragment and were discussed previously for another class of carbon-based emitters-Carbene-Au(I)-Aryls. [28]

Transient Emission and Absorption Spectroscopy
Next we measured transient luminescence and absorption kinetics for the CMAc complexes Au1-Au3, where the most bright acetylide complex, Au1, was considered in more detail to gain more insights about its luminescence nature.The transient PL kinetic profile for polystyrene (PS) film of Au1 (Figure 6a) has been probed at several time domains.Early time regime (up to 5 ns) for Au1 shows a high energy fluorescence with unstructured profile peaking at 410 nm (Figure 6b, brown profile).We ascribe this luminescence to the singlet charge transfer state 1 CT (HOMO→LUMO, Scheme 2).The PS films and toluene solutions of complexes Au2 (Figure S6, Supporting Information) and Au3 (Figure S7, Supporting Information) possess similar featureless and broad CT prompt fluorescence with up to 30 nm redshift compared with Au1.Next we analyzed profiles (Figure 6b) from a submicrosecond regime (red profile) and after a delay of 10 μs (orange) and 100 μs (yellow).The red PL profile in the submicrosecond regime (10-100 ns) demonstrates a well-resolved vibronic structure (Figure 6b) which is very similar to the highlyresolved PL profile measured for Au1 at 16 K (Figure 6d, blue).To explain such vibronic structure, we correlate the three major vibrational spacings ( 0 −  1 = 1087 cm −1 ;  0 −  2 = 1559 cm −1 ; and  0 −  3 = 2121 cm −1 ) with the experimental IR-spectrum for in-plane C─H bending (exp.IR 1074 cm −1 ), symmetric phenyl ring C─C bond stretch (exp.IR 1606 cm −1 ), and acetylenic C≡C stretching frequencies (exp.IR 2117 cm −1 ).Such a good fit with the IR data points out on the intraligand IL(*, C≡CPh) excited state nature.Previous reports of the detailed vibronic analysis for analogous (phosphine)gold(C≡CPh) complexes support our explanations, [16,17] thus enabling us to ascribe the long-lived vibronic PL to triplet intraligand 3 IL(*) phosphorescence at 2.96 eV.To correlate the associated fluorescence with the 1 IL(*, C≡CPh) excited state, we measured PL in air to quench the dominating phosphorescence and reveal the vibronically-resolved fluorescence at 3.44 eV (excited state lifetime 5 ns, as shown in Figure 6c and Scheme 2).][19]21] All these factors further support the assignment of the long-lived phosphorescent component (Figures 5c and 6b orange and yellow) to the phosphorescence having an intraligand origin 3 IL(* for C≡CPh).
The long-lived and bright 3 IL-phosphorescence (PLQY 43% for Au1) is enabled by the strong spin-orbit coupling coefficients (H SOC ) from the gold(I) atom and facilitated by the strong -donor and -acceptor of BiCAAC-carbene ligand.We observe similar PL behavior for complex Au2 (Figure S6, Supporting Information) and Au3 (Figure S7, Supporting Information).The energy state diagram and various emission pathways are summarized for complex Au1 in Scheme 2.
Transient absorption kinetic and spectra (Figure 6c,d) were measured for complex Au1 in PS matrix to estimate the effect of the gold perturbation in promoting the intersystem crossing (ISC) rate and compare with the archetype CMA1.Complex Au1 shows a broad excited state absorption band with little change in the spectral shape over any timescales (Figure 6d).The early-time species fully convert into a late time species on a timescale up to 9 ps for Au1 (Figure 6c), which is somewhat shorter compared with 13 ps measured for CMA1.Faster ISC rate for complex Au1 suggests greater contribution of the gold atom into the spin flip process compared to complex CMA1.

Near-UV OLED Fabrication
Complex Au1 demonstrated the highest energy PL with the highest PLQY up to 43% in the DPEPO host (bis[2-(diphenylphosphino)phenyl]ether oxide, Figure 7), indicating its suitability to test in an OLED device.To achieve a stable near UV-OLED from complex Au1, the electroluminescence (EL) should predominantly show the prompt-fluorescence component while minimizing the deep-blue phosphorescence component of Au1 (Figure 6b).We proposed that it is possible to minimize the phosphorescence component via triplet energy back transfer to a host, for instance, DPEPO or PPF (Figure 8a).The T 1 energy level of DPEPO is reported to be 2.995 eV, [29] which is close to the T 1 energy (2.99 eV) of Au1.The S 1 energy for DPEPO is 3.94 eV [29] and is significantly higher than that of Au1 (S 1 = 3.32 eV), thus suggesting that efficient energy transfer of the triplet excitons whereas singlet excitons could originate electroluminescence from the prompt-fluorescence (Figure 7b; and Figure S8a, Supporting Information).Indeed, when DPEPO was used as the host in the emissive layer, near-UV EL was successfully measured with high practical EQE values of 1% at 10 cd m −2 (Figure 7d; and Figure S8d, Supporting Information).The EQE efficiency of the first CMAc near-UV OLED is similar to those reported for organic fluorescent and TADF emitters reported for the UV-OLED (rarely exceeding 1% at practical brightness). [30]imilarity between PL and EL-transient kinetics indicates that molecular design concepts for new CMAc emitters can be successfully translated into near-UV OLED devices.Time-resolved EL kinetics of the near-UV OLED devices from complex Au1 (Figure 8b,c) were measured to further support the dominating fluorescence nature of the EL.The intensity of the EL reduced by 15 times within several ns after turning off the bias (Figure 8c), while the residual phosphorescence still remains in a microsecond regime.In fact, the transient EL-profiles are dominated by prompt near-UV fluorescence at 3.32 eV (orange profile, Figure 8b; and Figure S9, Supporting Information) whereas the long microsecond decay profile demonstrates a weak phosphorescence at 2.99 eV (cyan and green profiles, Figure 8b; and Figure S9, Supporting Information).Such a weak phosphorescence profile clearly indicates that the triplet state energy can be effectively transferred to the DPEPO or PPF host.The later result indicates that such triplet exciton management can be used as a strategy to extend the OLED device operating stabilityone of the major problems of the near-UV OLED on par with relatively low efficiency EQEs (rarely exceeds ≈1%).More stable near-UV OLEDs can be achieved by reducing the density of the long-lived triplet state for CMAc materials, thus reducing bimolecular quenching process at high current densities (Figure 7d) that serves as one of the major reasons for the OLED degradation. [31,32,33]A near-UV OLED device lifetime LT 50 of ≈1 min (Figure 8d; and Table S3) was measured for Au1 doped in DPEPO host.A more stable analog of DPEPO, such as PPF (Figure 7a) was used as a host material to further improve the operational stability in the emitting layer.Thanks to the rigid and conjugated dibenzofuran core, PPF has a slightly higher triplet energy level of 3.1 eV that was key to fabricating high efficiency PhOLEDs. [34]In fact, the device lifetime LT 50 increased up to ≈20 min (Table S4), which is a marked improvement and the longest operating stability reported for the near-UV OLED.These exciting results demonstrate a high promise of the CMAc materials for further improvement and application in near-UV OLED devices.
The near-UV OLED generated using a neat Au1 film as the emitting layer shows a similar emission as the PPF-host OLED (Table 3) and radiates with a violet-blue EL at 427 nm, due to a combination of the 1 IL-fluorescence and 3 IL-phosphorescence (Figure S10, Supporting Information).The Au1 neat OLED device can achieve a respectable EQE of 0.7% which is likely a result of the concentration quenching that is common for phosphorescent emitters. [35]However, the very poor device stability for the neat OLED (a few seconds) clearly indicates that our triplet management approach and the use of stable PPF hosts is the way forward to develop future stable and efficient near-UV OLEDs.

Conclusion
We have successfully demonstrated a donor-acceptor molecular design strategy toward bright phosphorescent carbene─gold(I)acetylide (CMAc) containing only carbon─gold chemical bonds.The bulky BiCAAC carbene prevented aggregation of the CMAc molecules and enabled bright phosphorescence from 425 to 500 nm in all media with PLQY values up to 43%, which is unprecedentedly high for gold(I)acetylide complexes without aurophilic interactions.We explored the effect of an increase in substitution of the (BiCAAC)Au moieties from one to three that evoked hyperconjugation enhancing the extinction coefficients and calculated oscillator strength coefficients for complexes with multiple gold atoms Au2 and Au3.The monogold complex Au1 appeared the most efficient in the family due to

Figure 1 .
Figure 1.Crystal structures (left) and intermolecular contacts (right) for gold complexes Au1 (top) and Au2 (bottom) with key geometrical parameters.Ellipsoids are shown at the 50% level.

Figure 3 .
Figure 3. a) UV-vis spectra of complexes Au1-3 in THF at 295 K. b) UV-vis spectra of complex Au1 in various solvents at 295 K.

Figure 4 .
Figure 4. Top: HOMO and LUMO plots of Au1 (left) and Au2 (right).Bottom: superposition of the crystal structure (green) and excited triplet T 1 (blue) geometries for complexes Au1 and Au2 (overlay via C1, C2, and N1 atoms) determined by theoretical calculations and showing rotational and bending distortions.

Figure 6 .
Figure 6.Complex Au1 in polystyrene (PS) host film (2 wt%).a) Time-resolved emission kinetics (integrated over the emission range).b) Transient photoluminescence spectra (TRPL).c) Phosphorescence quenching experiment to reveal 1 LE and 3 LE states.d) Steady state phosphorescence PL at 16 K and in the range from 10 to 100 ns at 295 K with vibrational analysis.e) Transient absorption (TA) kinetic measurements with fitting parameters.f) TA spectra.Pump wavelength 320 nm.

Figure 7 .
Figure 7. a) Vapor-deposited OLED device architecture based on complex Au1 dopped in different hosts at 10% by weight and electron transport layers.b) Electroluminescence spectra of near-UV OLEDs.c) Luminance-voltage plot.d) EQE versus luminance of near UV-OLEDs in DPEPO and PPF hosts at 10% doping concentration.

Figure 8 .
Figure 8. a) Exciton management diagram for complex Au1 showing a triplet exciton transfer to the DPEPO host.b) Transient electroluminescence profiles for Au1 showing strong prompt fluorescence and weak phosphorescence profile after 1 and 5 μs delays.c) Time-resolved electroluminescence (EL) decays for complex Au1 in various near-UV OLED architectures.d) Near-UV OLED device operating lifetime curves (initial luminance 10 cd m −2 ) at constant current under vacuum.

Table 2 .
Photophysical properties of all complexes.

Table 3 .
Performance data of vapor-deposited near UV-OLEDs at 10% doping concentration of complex Au1.