Synthesis, Crystal Structure and Photoluminescence of a TADF Cuprous Complex①

GAN Xue-Min WU Xio-Yun YU Rong-Min LU Cn-Zhong

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Synthesis, Crystal Structure and Photoluminescence of a TADF Cuprous Complex①

GAN Xue-Mina, bWU Xiao-YuanaYU Rong-Mina②LU Can-Zhonga②

a(350002)b(100049)

A four-coordinate mononuclear cuprous complex oCBP-Cu-Pym (1, oCBP = 1,2-bis(diphenylphosphine)-nido-carborane, Pym = 2-methyl-6-(1H-pyrazol-1-yl)pyridine) was synthe- sized and characterized by elemental analysis,NMR, UV-Vis and X-ray single-crystal structure analysis. It crystallizes in monoclinic space group2/with= 28.4182(8),= 16.2994(4),= 22.2708(5) ?,= 127.219(2)°,= 8214.8(3) ?3,= 8,M= 766.92,ρ= 1.24 g/cm3,(000) = 3160,= 2.30 mm–1,= 1.063, the final= 0.0700 and= 0.1903 for 7158 observed reflections with> 2(). The Cu(I) ion adopts a highly distorted tetrahedral geometry defined by two nitrogen and two phosphorous atoms. Under UV 365 nm at room temperature, this complex exhibits green emission with maximum emission peak at 516 nm, lifetime 32.4 μs and quantum yield (= 0.461) in the solid state. Photophysical investigation suggests that the emission of complex 1at room temperature was attributed to TADF, which is strongly supported by theoretic calculation.

Cu(I) complex, crystal structure, TADF, DFT calculation;

1 INTRODUCTION

Researches on transition luminescent materials have attracted enormous interest due to their attractive photophysical properties, which make them potentially amenable to applications in light-emitting technologies, dye-sensitized photovoltaics, biological imaging microscopy and light-emitting electro- chemical cells (LECs)[1-3]. Copper-based lumino- phores usually exhibit metal-to-ligand charge transfer (MLCT) state and small singlet-triplet energy gap, which allow fast reverse intersystem crossing (RISC) from the singlet state to triplet state, leading to highly efficient thermally activated delayed fluorescence (TADF) emission[4-6]. Theoretically, we can obtain luminescent materials with 100% internal quantum efficiency compared with those of the noble-metal phosphorescent materials[7]. Therefore, the Cu(I) emissive materials are promising candidates for highly efficient OLEDs[8-11]. In this work, a novel mono-nuclear neutral cuprous complex, oCBP-Cu- Pym, was designed and synthesized from the reactionof [Cu(CNCH3)4BF4], diimine ligand 2-methyl-6-(1H-pyrazol-1-yl)pyridine (Pym) and a new phosphine ligand 1,2-bis(diphenylphosphine)- nido-carborane (oCBP) in methanol. Herein, we report the synthesis, structure, spectroscopic charac- terization and theoretical calculation of the title compound.

2 EXPERIMENTAL

2.1 Materials and instruments

All the chemicals were used as commercially obtained without purification. NMR spectra were recorded on a Bruker Avance III 400MHz NMR spectrometer. Elemental analyses (C, H, N) were carried out with an Elemental Vario EL III elemental analyzer. Photo-luminescence spectra were recorded on a HORIBA Jobin-Yvon FluoroMax-4 spectro- photometer. The UV-vis absorption spectra were recorded using a Perkin-Elmer Lambda-365 UV/vis spectrophotometer. The lifetimes of powder samples at different temperature (77～298 K) were carried out by a HORIBA Jobin-Yvon FluoroMax-4 in- strument with a Multi-channel scaling (MCS) peripheral equipment and a spectra LED (373 nm). 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.

2. 2. 1 Synthesis of 2-methyl-6-(1H-pyrazol-1-yl)pyridine)[12](Pym)

To a Schlenk tube with a magnetic bar was added 2-bromo-6-methylpyridine (1.72 g, 10 mmol), potassium tert-butoxide (1.35 g, 12 mmol) and 1H- pyrazole (0.82 g, 12 mmol) in 1,4-dioxane (9.2 mL, 100 mmol, 10 equiv). The reaction mixture was stirred and heated under reflux in nitrogen for 48 h with an oil bath. Then the mixture was cooled to room temperature and poured into water (90 mL). The solution was neutralized by ammonia aqueous solution, and then extracted with dichloromethane. The organic phase was washed with brine, dried over sodium sulfate, and evaporated to dryness under vacuum. Finally, the crude product was purified by column chromatography on silica gel to a?ord a white solid (1.40 g, 88%).1H NMR (400 MHz, CDCl3)8.65～8.67 (m, 1H), 7.82 (d,= 8.2 Hz, 1H), 7.74～7.65 (m, 2H), 7.11 (d,= 7.4 Hz, 1H), 6.43～6.48 (m, 1H), 2.61 (s, 3H).

2. 2. 2 Synthesis of ligand 1,2-bis(diphenyl-phosphine)-nido-carborane[13](oCBP)

2.5 M-BuLi solution (22.5 mL, 9 mmol) was added slowly to a solution of 1,2-dicarbadode- caborane (0.43 g, 3 mmol) in distilled THF (15 mL) at 0 ℃. The resultant mixture was stirred at 0 ℃ for 1 h under nitrogen atmosphere. A solution of PPh2Cl (1.46 g, 6.6 mmol) in distilled THF (5 mL) was added slowly to the resultant mixture at 0 ℃. The resultant mixture was stirred at room tem- perature for 1 h and then refluxed for 1 h under nitrogen atmosphere. After cooling to room temperature, H2O (20 mL) was added to the reaction mixture. After stirring at room temperature for 1 h, the precipitate was filtered and washed with H2O (3 × 50 mL) to a?ord a white solid. Yield: 0.74 g (49%).1H NMR (400 MHz, CDCl3):7.92～7.85 (m, 8H), 7.52～7.39 (m, 12H), 2.38 (br, 10H).31P NMR (162 MHz, CDCl3):7.38.

2. 2. 3 Synthesis of complex oCBP-Cu-Pym (1)

A mixture of [Cu(CH3CN)4]BF4(31 mg, 0.1 mmol) and ligand oCBP (50 mg, 0.1 mmol) in methanol (5 mL) was stirred at room temperature for 0.5 h. Pym (42 mg, 1 mmol) was added and the mixture was stirred at 80 ℃ for 1 h. After cooling to room temperature, the solution was filtrated, and an air- stable product was recrystallized by slow solvent evaporation of the product in a mixture of CH2Cl2/ hexane.1H NMR (400 MHz, DMSO-d6)9.18(d,= 7.9 Hz, 1H), 8.72 (d,= 8.1 Hz, 1H), 8.02 (m, 3H), 7.74～6.92 (m, 22H), 1.60 (s,3H), 0.60 (s,= 7.0 Hz, 3H), –1.98 (B-H). Anal. Calcd. for C35.5H40B9Cl- CuN3P2: C, 55.5; H, 5.22; N, 4.47%. Found: C, 54.65; H, 5.24; N, 5.23%.

2.3 Structure determination

A yellow crystal of complex 1 with dimensions of 0.2mm × 0.15mm × 0.12mm was used for X-ray diffraction analysis. Diffraction data of the complex were collected on a SuperNova, Dual, Cu at zero, Atlas diffractometer equipped with graphite-mono- chromated Curadiation (= 1.54184 ?). A total of 16198 reflections were collected at 100.01(16) K in the range of 6.780≤2≤148.852o by using anscan mode, of which 8146 were unique withint= 0.0350 and 7158 were observed with> 2(). The structure was solved by direct methods with SHELXS-97 and refined by full-matrix least-squares methods with SHELXL-97 program package. All of the non-hydrogen atoms were located with succes- sive difference Fourier synthesis. Hydrogen atoms were added in the idealized positions. The non- hydrogen atoms were refined anisotropically. The final= 0.0786,= 0.1969 (= 1/[2(F2) + (0.0943)2+ 55.1329], where= (F2+ 2F2)/3),= 1.060, (Δ/)max= 0.001, (Δ)max= 1.940 and (Δ)min= – 1.690 e/?3. Selected bond lengths and bond angles from X-ray structure analysis are listed in Table 1.

Table 1. Selected Bond Lengths (?) and Bond Angles (°)

2.4 Computational methodology

Calculations on the electronic structures of 1 were performed by using density functional theory (DFT) with the hybrid Becke three-parameter Lee-Yang- Parr (B3LYP) functional level[14]. The input data came from X-ray crystal structure. In this calculation, a “double-ζ” quality basis set consisting of Hay and Wadt’s effective core potentials (LANL2DZ)[15]was employed for the Cu atom, and all-electron basis set of 6-31G* was used for P, B, N, C, and H atoms. All calculations were carried out using Gaussian 09[16-18]. Visualization of the optimized structures and Frontier molecular orbitals were performed by GaussView. The partition orbital composition was analyzed by using the Multiwfn 2.4 program[19].

3 RESULTS AND DISCUSSION

Complex 1 is a neutral copper(I) complex of the type Cu(PP)(NN). Fig. 1 shows its molecular struc- ture and ORTEP diagram. The X-ray crystallo- graphic study reveals that the metal ion in this complex exhibits highly distorted tetragonal coordination, with N–Cu–N and P–Cu–P angles of 79.50and 91.09?, respectively (Table 1). All Cu–P bond lengths ranging from 2.260 to 2.246 ? are typically within the normal range for a copper(I) center chelated by phosphine heterocycles[20]. The steric hindrance of these two ligands is expected to improve the rigidity of the cuprous complex and minimize the structural rearrangement distortion in its excited states, which can probably reduce the deactivation of excited states and increase the light emission efficiency[21].

Fig. 1. Molecular structure (left) and ORTEP diagram (right) of complex 1. Thermal ellipsoids are drawn at 50% probability. Hydrogen atoms are not displayed for clarity

Fig. 2. Absorption spectra (left) of complex 1 as well as free ligands and the emission spectra (right) of the cuprous complexes in degassed CH2Cl2(c ≈ 2 × 10-5M) at room temperature

Fig. 2 shows the UV-vis absorption spectra of complex 1, oCBP ligand and Pym ligand in CH2Cl2at room temperature. Complex 1 exhibits multiple intense absorption peaks in the region below 305 nm (> 104M-1·cm-1), which can be assigned to spin-allowed-* transition of both Pym and oCBP ligands. And the broad absorption band at 308～439 nm of complex 1, which is weakly observed in the spectrum of ligand, is assigned to(Cu)-*(NN) metal-to-ligand charge-transfer(MLCT) transitions and(PP)-*(NN) ligand-ligand charge transfer (LLCT). It is supported by DFT calculations (Fig. 3). More specifically, the compositions of the involved orbitals are provided in Table 2. Orbital component analysis of Frontier orbitals reveals that the HOMO is composed of the contributions from Pym moiety (96.39%). In contrast, the LUMO localizes on Pym (91.13%), oCBP (5.50%) and Cu (3.37%). The computational results indicate that the lowest lying transitions of 1 mainly consist of LLCT and MLCT characters. Natural transition orbital (NTO) analyses were performed to investigate the origin of luminescence in 1 (Fig. 4). The maps of S1and T1states are similar to each other; the transition from HOMO → LUMO is 100% for S1and 83% for T1. The corrected emission spectrum of complex 1 in degassed CH2Cl2showed a single band, maximized with a peak wavelength at 550 nm.

Fig. 3. Frontier molecular orbitals (HOMO, HOMO-1, LUMO, and LUMO+1) for complex 1 from DFT calculations

Table 2. Partition Orbital Composition Analyses for the Frontier Molecular Orbitals of Complex 1

Fig. 4. Redistribution of electron densities of the lowest singlet excited state and the lowest triplet excited state from TD-DFT calculations

Fig. 5. Emission spectra of complex 1in the solid state at 77 and 298 K

Fig. 5 shows the emission spectra of 1 in solid state at 298 and 77 K. Complex 1 exhibits green emission with photo-luminescence quantum yield of 46.1% at 298 K. With the decrease of temperature, the complex displays red-shifted emission with the peak maxima changing from 516 to 530 nm, and lifetime from 32.4 to 972.6 μs. This indicates that the emission of 1 originates from two different excited states (S1andT1) that are convertible and in thermal equilibrium. Fig. 6a displays emission decay curves measured at different temperature. For getting insight into the nature of emission, the lifetimes at varied temperature in the range of 77～298 K were measured and summarized in Fig. 6b. The red curve is fitted according to the following equation[6, 22, 23]: Herein, kB, τ(S1) and τ(T1) are the Boltzmann constant, the decay time of S1state and the decay time of T1state, and ESTdenotes the energy gap between the S1and T1states. The fitted results are present in Fig. 6b,, values of τ(S1) = 304 ns, τ(T1) = 943 μs, and EST= 0.105 eV. Owing to the small EST, the triplet excitons in T1states can convert thermally to the singlet excitons in the S1state. At room temperature, the conversion from T1states to the S1states occurs easily, and complex 1 emits thermally activated delayed fluorescence (TADF). TADF is often found in Cu(I) complexes and the donor- acceptor charge transfer organic compounds with small EST[24, 25].

Fig. 6. (a) Transient decay curves of complex 1 at different temperature;(b) Temperature dependence of the decay time for complex 1 in the solid state(the solid line represents a fit curve according to Eq. 1)

In summary, we report a new neutral cuprous complex which exhibits high luminescence quantum yield reaching 46.1%. The luminescent properties of the complex have been studied experimentally and theoretically, indicating that the complex displays TADF at room temperature.

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14 November 2017;

21 May 2018 (CCDC 1822329)

① This project was supported by the National Natural Science Foundation of China (21373221, 21521061, 51672271, 21671190, 21403236) and the Natural Science Foundation of Fujian Province (2006L2005)

Lu Can-Zhong, professor in chemistry. E-mail: czlu@fjirsm.ac.cn;Yu Rong-Min, professor in chemistry. E-mail: rongminyu@fjirsm.ac.cn

10.14102/j.cnki.0254-5861.2011-1989