A Tetranuclear Cuprous Halide Cluster Exhibiting Efficient Blue Thermally Activated Delayed Fluorescence①

WANG Pei HUANG Chun-Hua CHEN Xu-Lin LU Can-Zhong②

a (CAS Key Laboratory of Design and Assembly of Functional Nanostructures, and Fujian Provincial Key Laboratory of Nanomaterials, Fujian Institute of Research on the Structure of Matter,Chinese Academy of Sciences, Fuzhou 350002, China)

b (Xiamen Institute of Rare-earth Materials, Haixi Institutes,Chinese Academy of Sciences, Xiamen 361021, China)

c (Fujian Science & Technology Innovation Laboratory for Optoelectronic Information of China, Fuzhou 350108, China)

d (Huaqiao University, Xiamen 361021, China)

e (University of Chinese Academy of Sciences, Beijing 100049, China)

ABSTRACT A new tetranuclear cuprous halide complex, (CuBr)4(PN)2 (PN = 2-(diphenylphosphaneyl)-6-methoxypyridine), was synthesized and fully characterized. In solid state, this complex exhibits efficient blue emission with a photoluminescence quantum yield of 43.3% and a decay time of 19 μs at room temperature. The theoretical calculations, combined with the temperature dependence of spectroscopic properties and emission decay behaviors, indicate that the emission in the solid state originates from the 1,3(MLCT + XLCT) excited states, which are in thermal equilibrium with a small energy gap of about 0.1 eV.

Keywords: cuprous complex, thermally activated delayed fluorescence, blue emission, photoluminescence;

1 INTRODUCTION

Cuprous complexes have recently emerged as promising inexpensive alternatives for iridium complexes in electroluminescent applications because of their rich structural and photophysical properties[1], as well as their thermally activated delayed fluorescence (TADF) mechanism[2-4], which can harness both triplet and singlet excitons via efficient reverse intersystem crossing. Due to the low oxidation potential of the Cu+ion, the emissive states of cuprous complexes usually exhibit significant metal-to-ligand charge transfer (MLCT) characteristic, endowing a narrow energy gap between the emissive states (1MLCT and3MLCT) and in turn ensuring efficient TADF emission at room temperature[5].Whereas, at the same time, the low oxidation potential of the Cu+ion would cause these emissive MLCT states too low in energy to realized blue emission[6].

The diverse coordination modes of Cu+ion stimulate the search for emissive cuprous complexes with various structural features, amongst which cuprous halide clusters are a promising type of photoluminescent and electroluminescent materials due to their distinctive advantages[7]. First, most of cuprous halide clusters are neutral and sublimable, and hence,are conductive to construct multilayer optoelectronic device[8,9]. Second, due to the presence of heavy halide atoms(Br or I), these complexes generally show efficient spin-orbit coupling (SOC)[7,10,11], which play a crucial role in accelerating the intersystem crossing process and enhancing luminous efficiency. Herein, we report a novel tetranuclear cuprous complex, (CuBr)4(PN)2(PN = 2-(diphenylphosphaneyl)-6-methoxypyridine), which emits blue TADF at ambient temperature with a photoluminescence quantum yield of 43.3% and a decay time of 19μs. The crystal structures, photophysical properties, and theoretical calculations have been systematically studied.

2 EXPERIMENTAL

2. 1 Materials and methods

The ligand and other chemicals were purchased from commercial sources and used without further purification.1H NMR spectra were recorded on a Bruker Avance III 500MHz NMR spectrometer. Elemental analyses (C, H, N) were carried out with an Elemental Vario EL III elemental analyzer.The PL quantum yields, which were defined as the number of photons emitted per photon absorbed by the system, were measured by FluoroMax-4 equipped with an integrating sphere. Photoluminescence spectra and decay time were recorded on an Edinburgh Instruments FLS980 spectrometer using Xenon Lamp or OPO laser as the excitation light source. The variable-temperature measurements (77～313 K)of emission decay time and spectra were performed using a LINKAM THMS600 system with a range of 77～873 K.X-ray structure determination was carried out on the Bruker D8 Venture at 200 K.

2. 2 Synthesis of (CuBr)4(PN)2

The complex was prepared according to the following procedure: A certain amount of CuBr (143 mg, 1 mmol) was dissolved into CH3CN (2 mL), and then the mixture was poured into a solution of the PN ligand (293 mg, 1 mmol) in CH2Cl2(2 mL). After stirring at room temperature for 1 hour,the reaction mixture was filtered. Single crystals of the cuprous complex suitable for X-ray diffraction measurement were obtained by slow diffusion of ether into the resulting filtrate.1H NMR (500 MHz, CD2Cl2)δ8.53 (s, 4H),8.10～6.91 (m, 22H), 3.76 (s, 6H). Elemental Analysis (%)Calcd. for C36H32Br4Cu4N2O2P2: C, 37.26; H, 2.78; N, 2.41.Found: C, 37.32; H, 2.82; N, 2.45.

2. 3 X-ray structure determination

A pale green crystal of the complex with dimensions of 0.28mm × 0.19mm × 0.15mm was used for X-ray diffraction analysis. The diffraction data were collected on a MoKαradiation (λ= 0.71073 ?). A total of 49199 reflections were collected at 200 K in the range of 4.85≤2θ≤59.21o by using anω-scan mode, of which 5413 were unique withRint=0.0279 and 3735 were observed withI> 2σ(I). The structures were solved by direct methods and refined by full-matrix least squares methods using the SHELXL-13 program package. All non-hydrogen atoms were refined with anisotropic thermal parameters. Selected bond lengths and bond angles of (CuBr)4(PN)2are listed in Table 1.

Table 1. Selected Bond Lengths (?) and Bond Angles (o) of (CuBr)4(PN)2

2. 4 Computational methodology

The density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations were performed based on X-ray single-crystal structure at the hybrid Becke three-parameter Lee-Yang-Parr (B3LYP) functional level[3,12]. The input data came from X-ray crystal structure.In this calculation, a “double-ζ” quality basis set composed of Hay and Wadt’s effective core potentials (LANL2DZ) was employed for the Cu and Br atoms, and all-electron basis set of 6-31G* was used for P, O, N, C, and H atoms. All calculations were carried out using Gaussian 09[13]. The optimized structure and the visualization of frontier molecular orbitals were carried out through GaussView 5[13]. The partition orbital composition was analyzed by using the Multiwfn 3.7 program[14].

3 RESULTS AND DISCUSSION

The crystal structure of (CuBr)4(PN)2was determined by single-crystal X-ray diffraction analysis. The molecular structure drawing and the ORTEP diagram is shown in Fig. 1.The four copper atoms are arrayed in a parallelogram with two obtuse angles of 98.30oand two acute angles of 81.70o.The adjacent sides of the parallelogram exhibit unequal lengths of 2.686 and 2.691 ?, respectively, which are both shorter than twice the van der Waals radius of copper atom(2.8 ?), indicating significant metallophilic interactions between each two copper atoms. Each Cu3array which makes the acute angles in the parallelogram isμ3-capped with a bromine atom. The other two bromine atoms formμ2-bonds on each long side of the Cu4parallelogram. The two copper atoms on each short side of the parallelogram are chelated by a PN ligand in aC2-symmetric arrangement. The steric hindrance of methoxy group may play a key role in forming the Cu4Br4cluster. Yersin et al have synthesized cuprous complexes with Cu2X2(X = Cl, Br or I) core by using a similar PN ligand without any substituents on the pyridine ring or benzene ring[11](see Supporting Information for detail).

UV-vis absorption spectra of (CuBr)4(PN)2and the PN ligand in CH2Cl2are shown in Fig. 2. The strong absorption bands below 300 nm are assigned toπ-π* transition[6,15]. And the weak absorption bands above 300 nm are assigned to n-π* transition. In addition, the absorption bands of(CuBr)4(PN)2around 350 nm can be assigned to halide-to-ligand charge transfer (XLCT) and metal-to-ligand charge transfer (MLCT) according to the results of DFT and TD-DFT calculations.

Fig. 1. Molecular structure (a) and ORTEP diagram (b) of the complex (CuBr)4(PN)2

DFT and TDDFT calculations were carried out to investigate the molecular orbitals and electronic states of the presented cuprous complex. The major contributions of the lowest excited singlet state (S1) and the lowest excited triplet state (T1) involve charge transfer transitions from the highest occupied molecular orbital (HOMO) and HOMO-2 to the lowest unoccupied molecular orbital (LUMO) and LUMO+1.As shown in Fig. 3, the HOMO and HOMO-2 of(CuBr)4(PN)2are mainly localized on the Cu4Br4cluster,while the LUMO and LUMO+1 are almost entirely distributed over the pyridine rings on the NP ligands.Therefore, as summarized in Table 2, the low-lying transitions are characterized as metal-to-ligand charge transfer(MLCT) transitions mixed with halogen-to-ligand charge transfer (XLCT) transitions.

Fig. 2. UV-vis absorption spectra of (CuBr)4(PN)2 and the PN ligand

Fig. 3. HOMO, HOMO-2, LUMO, and LUMO+1 of (CuBr)4(PN)2 in the optimized ground state structure

Table 2. Transition Contribution of the Lowest Excited States

.

The emission spectra and transient photoluminescence (PL)decay spectra of the investigated cuprous complex in solid state at 298 and 77 K are shown in Figs. 4 and 5. This complex exhibits blue emission with a spectral maximum of 488 nm with a photoluminescence quantum yield of 43.3%when excited at room temperature. As compared to the reported binuclear cuprous complexes based on the similar NP ligand and bromine ion, the emission spectrum peak of(CuBr)4(PN)2blue shift by 13 nm (see Supporting Information for detail comparison). Considering the dominant(M+X)LCT transition characteristics of the lowest excited states as revealed by the calculation results, such emission blue-shift should be attributed to the weaken electron-withdrawing ability of the PN ligand by methoxy substituent. This result indicates that it should be a promising strategy to obtain blue-emitting cuprous complexes by introducing electron-donating group into the ligand to destabilize the LUMO level. As the temperature decreases from 300 to 77 K, the emission spectrum peak red shifts from 488 to 503 nm accompanied with the decay time increase from 19to 37.6μs. The temperature dependence of spectroscopic properties and emission decay behaviors reveal the presence of two thermally equilibrated emissive states,namely S1and T1. To further investigate the behaviors of the two excited states, decay time at different temperature ranging from 77 to 298 K was measured and summarized in Fig. 6. The observed overall decay timeτcan be expressed by the following Boltzmann-type equation:

in which kBand T represent the Boltzmann constant and the absolute temperature, respectively. The red curve is fitted according to Eq. 1.τ(S1) andτ(T1) are the decay time of the lowest singlet and triplet excited states, respectively, and ΔESTis the energy gap between these two states. The fitting curve functionalized by the equation is consistent with the obtained experimental data of decay time. The parameters obtained are as follows:τ(S1) = 319 ns,τ(T1) = 37.12μs, and ΔEST= 0.097 eV. Owing to the small ΔEST, the triplet excitons in T1state can be thermally activated and convert to the singlet excitons in the S1state at room temperature, and in turn return to the ground state via radiative transition with TADF emission. This typical TADF mechanism has been observed from Cu(I) complexes and pure organic compounds consisting of donor-acceptor with small S1-T1energy gaps[16-18].

Fig. 4. Emission spectra of (CuBr)4(PN)2 at 77 and 298 K

Fig. 5. Transient PL decay curves of (CuBr)4(PN)2 at 77 and 298 K

Fig. 6. Temperature dependence of the decay time for complex in the solid state.The black cubes represent experiment data and the red solid line represents a fit curve according to Eq. 1

4 CONCLUSION

In summary, a new tetranuclear cuprous halide cluster chelated by two same PN ligands was designed and synthesized. This complex exhibits intense blue photoluminescence with a PLQY of 43.3% and a lifetime of 19μs at room temperature. Theoretical and experimental studies revealed the TADF nature of the blue emission of the investigated complex. Introducing electron-rich substituents onto the ligand to destabilize the LUMO level has been demonstrated as an effective strategy to tune the emission color to blue region by theoretical predictions and comparative studies. These results may provide a reference for designing efficient cuprous halide materials with blue emission which are required in electroluminescence applications. The application of the investigated complex in electroluminescence devices is under way.