Thermally Activated Delayed Fluorescence from d10‐Metal Carbene Complexes through Intermolecular Charge Transfer and Multicolor Emission with a Monomer–Dimer Equilibrium

Abstract A series of two‐coordinate AuI and CuI complexes (3 a, 3 b and 5 a, 5 b) are reported as new organometallic thermally activated delayed fluorescence (TADF) emitters, which are based on the carbene–metal–carbazole model with a pyridine‐fused 1,2,3‐triazolylidene (PyTz) ligand. PyTz features low steric hindrance and a low‐energy LUMO (LUMO=−1.47 eV) located over the π* orbitals of the whole ligand, which facilitates intermolecular charge transfer between a donor (carbazole) and an accepter (PyTz). These compounds exhibit efficient TADF with microsecond lifetimes. Temperature‐dependent photoluminescence kinetics of 3 a supports a rather small energy gap between S1 and T1 (ΔE S1-T1 =60 meV). Further experiments reveal that there are dual‐emission properties from a monomer–dimer equilibrium in solution, exhibiting single‐component multicolor emission from blue to orange, including white‐light emission.

Luminescent Group11 N-heterocyclicc arbene (NHC) complexesh ave been most extensively studied among all transition-metal-NHC complexes. [5] Recently,t here have been several reports on organometallic TADF emittersb ased on the model of carbene-d 10 metal-amides, wherein carbenes with strong paccepting abilities played ac rucial role in TADF characteris- tics. [6] The lowest-energy charge transition of this familyo f complexes was mainly concentrated on interligand charge transfer (LLCT)f rom donor (amide) to acceptor (carbene), due to the coplanarity of the ligandsa nd the contribution of metal do rbitals to an electronic bridge. Apart from intramolecular D-A charget ransfer (CT), this molecular model provides another possibility for realizing intermolecular D-A interactions (Figure 1d).
1,2,3-Triazolylidenes, ac lass of mesoionic carbenes (MICs), have attracted tremendous attention. [7] However, compared with other traditional NHCs, 1,2,3-triazolylidenes exhibit poorer p-accepting ability and extremely low-energy LUMOs, which are mainly located on the p*o rbitals of the triazole ring;t hus, this kind of carbene ligand has good potential for the construction of novel TADF metal complexes throughi ntermolecular CT.W ee nvisioned that the LUMO energy could be further loweredb yf using ap yridine ring. In this regard, we report a class of two-coordinate, neutralP yTz-M-Cz complexes 3a, 3b, 5a,a nd 5b (PyTz = pyridine-fused 1,2,3-triazolylidene;M= Au I , Cu I ;C z= carbazole;F igure 1e), in which PyTz exhibits weak paccepting ability but low-energy LUMO. With am etal bridge connecting Da nd A, these molecules are designed to have intermolecular CT between PyTz and Cz, which allowss uch complexes to emit efficient TADF with short lifetimes in the range of 0.6-2.4 msi nt he solids tate. Next, investigations into the self-assembly behavior and photophysical properties of these complexes in solution demonstrated the existence of am onomer-dimer equilibrium, in which the dimer exhibited TADF characteristics at around 600 nm. Additionally,b ased on the monomer-dimere quilibrium and TADF characteristics, this dual-emission system can be switched to exhibit single-component multicolor emission over aw ide range, from blue (450 nm) to orange (600 nm), including white-lighte mission. [8] Results and Discussion

Synthesis and structural characterization
PyTz-M-Cz complexes 3 and 5 were synthesized from the carbene precursor,p yridine-fused 1,2,3-triazolium (1;S cheme 1). Single crystals of 3a, 5a,a nd 5b have been obtained by diffusion of diethyl ether into ac oncentrateds olution of the complex in dichloromethane for structure determination by means of X-ray crystallography.X -ray crystallography of 3 and 5 indicates that all of the molecules presentn early planar structures. For 3a,t he dihedral angle between the planes upon which PyTz and Cz are located is only 4.618,w hereas, for 5a and 5b, the dihedral angles are 5.91 and 08,r espectively (TableS7i n the Supporting Information). Furthermore, these complexes could self-assemblet of orm ah ead-to-tail dimeric conformation (Scheme 1a nd Figure 2). With regardt o3a and 5a,e ach molecule is connected to another,f orming ad imer in ah eadto-tail conformation, in which the electron-rich Cz ring faces toward the electron-deficientP yTz ligand.T he distances of the p-p interactions inside the dimers are around3 .235 and 3.258 ,r espectively.T he p-p distances between dimers are about 3.548 for 3a and 3.533 for 5a.Then, for 5b,asimilar self-assembly mode of the molecule could be observed (in a head-to-tail manner). However,u nlike 3a and 5a,m olecules of 5b are linked to each other through p-p interactions, forming 1D chain structures;t he distance of the p-p interaction is around3 .389 .F urthermore, in the single-crystal structureo f 5b,C zi sn ot completely facing PyTz, but is slightly interlaced with the carbene ligand, which is probably due to steric hindrance from the tert-butyl groups ( Figure 2). It is worth noting that there is no clear metal-metal interaction among complexes (M-M distances > 3.5 ). Selected bond lengths and angles in complexes 3a and 5a-5b are collected in Ta ble S7 in the Supporting Information.
Monomer-dimere quilibrium studies Figure 3a shows the absorption spectra of 3a, 3b, 5a and 5b in THF,a nd Table 1s ummarizes the photophysical properties of these complexes. Allo ft he complexes exhibit similars pectral profiles,a lthough tert-butylated complexes 3b and 5b show bathochromic shifts of about 6nmw ith respect to their non-butylated counterparts. Structured absorption bands between 280 and 310 nm are assigned to intraligand p!p*a nd n!p*t ransitions of Cz and PyTz, whereas the low-energy absorptionb ands at 358-380 nm are also attributed to the Cz ligand,w hicha re consistent with the absorption spectra of the KCz and KCz'' ( Figure S7 in the Supporting Information), suggesting that there are no MLCT or LLCT transitions for such complexes.N otably,a ll of the complexes display weak absorption tails at around 400 nm;i nv iew of the single-crystal data, these might be attributedt ot he formation of ad imer through p-p interactions.O nt he basis of the concentration-dependent UV/Vis spectra (Figure 3b and Figure S8 in the Supporting Information), the dimerizationc onstant K (THF,2 58C) was determined by applying absorbance plots at around 400 nm to a monomer-dimer equilibrium (see "Calculations of the dimerization constant K using absorption spectra"i nt he Supporting Information). [9] The dimerization constants, K,a nd the corre-spondingG ibbs free energies (DG)o fa ll complexes have been collectedi nTa ble 1.
For further proof of the existence of dimers in solution,i nvestigationso fi nduced self-assembly were performed in mixed-solvent systems.T he nominal concentration of complexes 3a, 3b, 5a,a nd 5b was kept at 1.5 10 À5 m,a nd hexane was added to trigger aggregation ( Figure 3c and Figure S9 in the Supporting Information). Upon addition of hexane (0 to 90 %) to the solution of 3a in THF,a ni sosbestic point appeared at around3 90 nm, with the growth of an absorptions houlder at around 405 nm and ad eclineo ft he absorptionb and at 375 nm. It is worth noting that increasing the proportion of the poor solvent only led to growth of the shoulder at around 400 nm and no other new absorption bands wereo bserved. These observations suggest that the series of complexesu ndergo solvent-induced aggregation, which favors the dimer upon the addition of ap oors olvent. [10] The temperature-dependent UV/Vis spectrao fc omplex 3a in THF are shown in Figure 3d.W ith increasing temperature, there was ab lueshift of the band initially centered at about 400 nm, whereas the absorption of Cza t3 65 nm was gradually enhanced, indicating that increased molecular motionsw eakened the intermolecular CT process at high temperature. [11] In addition, the 1 H-1 HC OSY spectrum of 3b shows ac ross peak between the protons of PyTz and proton of Cz ( Figure S10 in the Supporting Information), which indicates that head-to-tail p-p interactionsa re conserved in CDCl 3 .
Photoluminescence (PL) spectra of 3a, 3b, 5a,a nd 5b in the nondoped solid state exhibit broad emission bands centered at 539-580nm( Ta ble 2a nd Figure S11i nt he Supporting Information). However,e mission spectra of thesec omplexes in THF exhibit dual-emission bands at around 450 and 600 nm (Table 2a nd Figure4a). The high-energy, narrow,e mission bandsa ta round 450 nm are attributed to the p!p*t ransition of 1 Cz, which are in line with the emission of KCz and KCz'' that display vibronic fine structure (FigureS12 in the Supporting Information). To investigate the origin of the orange-colored emission band ( % 600 nm) thoroughly,aconcentration-dependentP Le xperiment was first carried out (Figure4ba nd Figure S13 in the Supporting Information). The intensity of the . Room-temperatureUV/Vis absorptionspectra of a) complexes 3a, 3b, 5a and 5b in THF;b )complex 3a in THF at different concentrations (1 10 À5 -2 10 À3 m); inset:Dimerization plots for the monomer-dimer equilibriumm onitored at the absorptionwavelength of the dimer;a nd c) complex 3a in THF with different volumef ractionso fh exane( 0-90 %); [3a] = 1.5 10 À5 m.d )UV/Vis absorption spectrao f3a in THF at various temperatures; [3a] = 1 10 À3 m. emission band dependso nt he concentration of the solution. According to single-crystal data and UV/Vis absorption investigation,this emission band was produced by the formation of a dimer.T he self-assembly behavior of all complexes in THF/ hexane mixtures were further investigated (Figure4ca nd Figure S14 in the Supporting Information). Allo ft he complexes were prepared at ad ilute concentration of 1.5 10 À5 m in the admixture solution, of whicht here was little emissiona t around6 00 nm at the same concentrationi np ureT HF.A st he proportion of hexane gradually increased, the emission signal at longer wavelength became more intense than that at 450 nm. Notably,t he position of the emission band of 1 Cz at around4 50 nm was not polarity dependent, but the emission band of the dimer underwent ah ypochromatic shift of about 70 nm, going from more polar (hexane 0%)t ol ess polar (hexane 90 %) solutions.

Computational studies
DFT calculations at the M062X/def2SVP level in the gas phase have been performed on the monomers and dimers of these complexes to investigate the effect of D-A interactions on the molecular geometry (Figures S16-S18 and Ta ble S8 in the Supporting Information). The optimized ground structures of the monomers were in at wisted conformation between PyTz and Cz, whereas the dimeric modelss howedaplanar conformation and were nearly consistent with the single-crystal results. This difference was due to the existence of intermolecularn oncovalent p-p interactions between Cz and PyTz, whichp ropelled the complexes into ap lanar configuration.
Notably,t he twisted conformation of the monomer plays a crucial role in the luminescence of metal complexes. [4a, 6b, 12] Time-dependent( TD) DFT calculations of 5a showed that the S 1 state originated from intramolecular CT from Cz to PyTz, but  . Room-temperaturePLspectrao fa)complexes 3a, 3b, 5a,and 5b at 1 10 À3 m in THF; l ex = 375 nm;b )complex 3a in THF at different concentrations( 1 10 À5 -2 10 À3 m); intensity maximao ft he blue emission bandsw ere normalized; l ex = 375 nm;c )complex 3a in THF mixed with hexane at different volume fractions (0-90%); intensitym aximao fthe blue emissionbandsw erenormalized;[3a] = 1.5 10 À5 m, l ex = 375 nm. d) PL spectra of complex 3a in THF recordeda td ifferent temperatures, ranging from 0t o5 08C(left) and 55 to 75 8C( right);i ntensity maximao fthe blue emission bands were normalized. e) PL intensity ratio of the dimer overt he monomer as afunction of temperature; [ 3a] = 1.0 10 À3 m; l ex = 380 nm. Chem.E ur.J.2020, 26,17222 -17229 www.chemeurj.org 2020 The Authors. Published by Wiley-VCH GmbH there was an egligible oscillator strength (f = 0.0000) because of the poor overlap between electrona nd hole distributions, which was associated with the twisted geometry of the complex ( Figure 5). The excited states S 1 to S 4 had very low oscillator strengths until S 5 gave al arger oscillator strength( f = 0.0450), which was mainly assigned to p!p*o fC z( 93 %; Ta ble S9 in the Supporting Information), showingt he consistency of the emission of 1 Cz observed in the dilute solution.
In addition, the electronic effect of the carbenel igand is also ak ey factor affecting the photophysical properties. Molecular orbitala nalysis demonstrates that, unlike other carbenes, in which LUMOs are mainly located on the unfilledporbital of the carbene carbon,t he LUMOs of PyTz and MIC are primarily distributed over the p*o rbitals of the whole ligand and LUMO + 2, which are mainly situatedo nt he carbene carbon, truly reflect the p-accepting ability of the carbene (Figures S19-S25 in the Supporting Information). The energyo f LUMO + 2o fP yTz (0.93 eV) is relativelyh igher than that of other carbenes. In fact, MLCT or intramolecular LLCT cannot be observedf or this series of 1,2,3-triazolylidene complexes. [13] However,i ti sn oteworthy that the low-energy LUMO and lesser steric hindrance enable PyTzt oh ave the potentialt of acilitate intermolecular CT.

TADF studies
To probe the nature of dual emission,atemperature-dependent PL experiment has been carried out (Figure 4d and Figure S26 in the Supporting Information). Interestingly,f or complex 3a,w ith increasing temperature from 273 to 323 K, the emission intensity of the dimer becamei ncreasingly stronger, which reminded us of the characteristics of TADF.H owever,a s the temperature continued to rise, this emission band gradually weakened, in comparison with the fluorescenceo f 1 Cz. Combined with the temperature-dependent UV/Vis spectra ( Figure 3d), the reason shouldb ea ttributedt o, with an increase of temperature, the thermal motionso ft he molecules strengthening, leading to the destructiono fd imer molecules.
The transientP Ld ecay for the emission wasi nvestigated in both solution and nondoped solid state (Table 2a nd Figure S27 in the Supporting Information). All of the complexes in THF under an inert atmosphere show short lifetimesa ta round 450 nm for about2 .8-4.0 ns, whichc orresponds to the emission originating from 1 Cz. Meanwhile, it is interesting to note that the emission at longerw avelength exhibits ab iexponential decay with as horterc omponent of 4.1-6.7 ns and al onger component of 28. 8-182.5 ns. Combined with temperature-de-pendentP Le xperiments,t he emission band at around 600 nm might be attributed to the prompt fluorescencea nd TADF of the dimer,w hichp robably originates from the separated HOMO and LUMO distributions in the dimer molecule. To investigate the effect of the triplet on the delayedc omponent, excited-state lifetimes of 3a and 5a were measured in the presence of triplet-quenching oxygen (Figure 6a and b). [3e, 12] If oxygen was bubbled through the solution for 3min, for 3a, the delayedf luorescencel ifetime was decreased from 182.5 to 35.4 ns, and, for 5a,t he delayedc omponent dropped to 18.4 ns, which indicated that the triplet had ac rucial role in the delayed-component emission because of the efficient RISC from T 1 to S 1 .
Longer delayed-component lifetimesw ere observed in the nondoped solid state in the microsecond range (0.6-2.4 ms; Figure 6c). To examinet he small DE S 1 ÀT 1 of TADF,t emperature-dependentP Lk inetics of 3a were recorded from 300 to 160 K (Figure 6d), which presented at emperature dependence of increasingd elayed-component lifetimes with decreasing temperature. Because the PLQYs of these complexes are limited at around5 0%,w ea ttempted to use at wo-level model to evaluate DE S 1 ÀT 1 through Equation (1), [6c] in which t TADF is the lifetime of TADF; DE S 1 ÀT 1 is the singlet-triplet splitting energy; T is temperature; k B is the Boltzmann constant;a nd k S 1 and k T 1 are the prompt fluorescencea nd phosphorescent rate constants, respectively.
As shown in Figure 6e,t hese plots weref itted to afford DE S 1 ÀT 1 = 60 meV,w hich indicated that the triplet energy level was quite close to the singlet energy level.M eanwhile,t he TADF process could also be confirmed by Figure4da nd e. It is necessary to compare the excited-state energies of the dimer (Dm) and localized Cz. To improvet he reverse ISC efficiency,t he 1, 3 Dm energiesm ust be kept lower than that of 3 Cz. The emission spectrao f3a, 3b, 5a,a nd 5b in 2-MeTHFw ere measured at 300 and 80 K( FigureS28 in the Supporting Information). The emission bandso fd imers were located at 600 nm at room temperature. However,the spectralprofiles underwent ah uge change in frozen solvent if the temperature dropped to 80 K. At this point, there was as tronge mission band at around5 00 nm, which displayed vibronic fine structure. The Figure 5. a) The optimized ground-state structure of monomer 5a.The electron and hole distributions of b) the lowest-energy excited state (S 1 )a nd c) the fifth-lowest-energy excitedstate(S 5 ). The geometryof5a is optimized at S 1 by using TDDFTcalculations. Chem. Eur.J.2020, 26,17222 -17229 www.chemeurj.org 2020 The Authors. Published by Wiley-VCH GmbH spectrals hifts were the result of destabilization of intermolecular CT due to the rigid environmenti nt he freezing solution. [6c-e] Therefore, the proportion of 3 Cz/Dm gradually increasedw ith decreasing temperature:o nce the temperature reachedt o8 0K, 3 Cz emission was observed because the energies of the 1, 3 Dm states were above the 3 Cz state. The vibronic fine structure at 500 nm, along with the long lifetimea t8 0K (= 0.7 ms), was in agreement with the locally excited 3 Cz state of KCz ( Figures S29 and S30 in the SupportingI nformation). These results demonstrate that the 1, 3 Dm levels are below that of 3 Cz at room temperature.B ased on the above experimental results, the singlet and triplet distributions of the monomer and dimer are illustrated in Figure 6f.

Single-componentm ulticolor emission studies
After confirmingt he origin of the dual emissions, it was found that the contributions from the monomer and dimer emissions could result in single-componentm ulticolor emission. The position and intensity of dual-emission bands of these complexes could be modulated by the concentration,d egree of aggregation, and temperature based on the above experiments S14,and S26 in the Supporting Information). The fluorescencep hotographs of 3a at different concentrations and with different volumef ractions of hexane are shown in Figure7a and b, respectively.O wing to the TADF nature and switchingb etween monomer and dimer,t he color change is sensitive to temperature. As shown in Figure 7c,a st he temperature rose, the color of 3a in the 1 10 À3 m solution in THFu nder 380 nm excitation changed from white to yellow to white to blue, which is promising for at emperature sensor.
In addition, modulation of the dual emissions is also investigated by changing the excitation wavelengths (Figure 7d and FigureS31 in the Supporting Information). Ta king 3b as an example, as the excitation wavelengths varied from 360 to 420 nm, the dimer emission bandsg radually enhanced and the emissiono f 1 Cz weakened. So, the overall emission color rangedf rom blue to white to yellow,a nd the corresponding CIE [14] coordinates are displayedi nF igure 7d.

Conclusion
We have designed and synthesized as eries of twocoordinate Au I or Cu I complexes (3a, 3b, 5a,a nd 5b)b ased on as imple molecular model of PyTzmetal-Cz. The complexes could self-assemble into a head-to-tail configurationt hrough intermolecular pp interactions that facilitated intermolecularC Tb etween Cz and PyTz, taking advantage of the lowenergy LUMO of the carbene ligand.P Lk inetics demonstrated that these molecules exhibited ad elayed component in both solution and the solid state in the absence of triplet-quenching oxygen, which was ascribed to TADF emitters. In addition, thesec omplexese xhibited longer delayed-component lifetime( 0.6-2.4 ms) and enhanced QYs in the solid state. Te mperature-dependent studies of 3a revealed ar ather small energy separation betweenS 1 and T 1 (DE S 1 ÀT 1 = 60 meV).
Efficient absorption and emission spectra demonstrated the existence of the monomer-dimer equilibrium in solution, presenting dual-emissionb ands at around4 50 nm and 600 nm, respectively.The emission band in the low-energy region is solvatochromic as the polarity of the solutionc hanges. Furthermore,t here exists al ocally excited triplet state, 3 Cz, that dominates emissioni nf rozen solvent because of the destabilization of the intermolecularC Ts tates. Wea lso demonstrate that, through combining the dual emissions generated by the monomer and dimer,t he series of complexes displayeds inglecomponent multicolor emission. The color tunability couldb e realized by changing the excitation wavelength, temperature, degree of aggregation, and concentration of the complex. Further studies to explore TADF materials and the corresponding application of this dimericmode are in progress.

Experimental Section
Experimental details, including materials, experimental procedures, chemical preparation of the compounds, photophysical characterization, X-ray crystallographic studies, calculations of the dimerization constant K,c omputational details, and NMR spectroscopy data for the compounds, are provided in the Supporting Information.