Phosphorescent transition metal complexes have attracted considerable attention for their application in organic light emitting devices(OLEDs), as they can make use of both singlet and triplet excitons resulting in high electroluminescence (EL) efficiency ^[1-3]. However the central ions of most phosphorescent complexes are noble metals, such as Ir(Ⅲ) and Pt(Ⅱ), which are expensive and have highly limited resources. In 1999, Ma and coworkers ^[4] reported the EL device with a phosphorescent copper(Ⅰ) complex as emitting layer for the first time, which offered a new type of phosphorescent emitter for OLEDs. Over the past decade, luminescent copper(Ⅰ) complexes have gained great progress in molecular structure designing, luminescence mechanism studying and EL performance, and so forth ^[5-14]. Due to the low cost and abundant resource of copper in comparison to other transition metals, luminescent copper(Ⅰ) complexes are seen as the promising alternative emitters for OLEDs.

In recent years, most studies focused on the homoleptic and heteroleptic cationic copper(Ⅰ) complexes ^{[5-8, 15-23]}. Nevertheless, it has been proven that neutral copper(Ⅰ) complexes have superior emission characteristics in contrast to their charged counterparts ^[24]. In addition, neutral copper(Ⅰ) complexes facilitate vacuum deposition and the balance of charges injection for OLEDs due to the lack of small mobile counterions.As we know, most neutral copper(Ⅰ) complexes showed excellent luminescence properties and some of them achieved high EL efficiencies ^{[9-14, 25-30]}. In the past, the diimine ligands with imidazole moiety are usually employed to synthesize cationic copper(Ⅰ) complexes ^[31-34], in fact, these ligands are suitable for neutral copper(Ⅰ) complexes, because the active H atom in imidazole unit is easy to be removed by strong bases resulting in anionic ligands ^{[35, 36]}. Herein, we used 2-(2-pyridyl)-1H-imidazole (Hpim) as the precursor of anionic ligand to synthesize a new neutral heteroleptic copper(Ⅰ) complex (pim) Cu(POP) (Scheme 1), which shows high stability and intense phosphorescence in solid state, where POP denotes bis[2-(diphenylphosphino)phenyl]ether.

1 Experiment 1.1 Materials and Equipments

All reagents and solvents were purchased from commercial suppliers and used as received unless otherwise stated. ¹HNMR, ¹³CNMR and ³¹PNMR spectra were recorded on a Bruker Avance500 spectrometer. Mass spectra were performed on a LCQ DecaXp Max instrument. Thermal gravimetric analyses were performed on a SII instrument TGA/DTA 6300 with a heating rate of 10 ℃/min under N₂ atmosphere. Absorption spectra were recorded on a Hitachi U-3010 UV-Vis spectrophotometer. Steady-state emission spectra, emission lifetimes, and absolute emission quantum yields were recorded under N₂ atmosphere on an Edinburgh FLS980 spectrometer equipped with a barium sulfate coated integrating sphere.

1.2 Synthesis of Hpim

An ice-cold 2-pyridine carboxaldehyde (10 mmol, 1.1 g) in ethanol (10 mL) was added to 40% aqueous glyoxal (1.5 mL) in ethanol (5 mL) at 0 ℃, and then ice-cold concentrated aqueous NH₃ (3.5 mL) was added without delay. The mixture was held at 0 ℃ for 1 h, and then was allowed to stir for 5 h at 25 ℃. After the ethanol was removed under reduced pressure, the resulting solution was extracted with ethyl ether for three times. The combined organic solution was dried over anhydrous MgSO₄ and evaporated under reduced pressure. The crude product was purified by column chromatography (white powder, yield: 0.55 g, 38 %). ¹HNMR (500 MHz, CDCl₃): 11.46 (1H, s), 8.53 (1H, d, J=5.0), 8.23 (1H, d, J=8.0), 7.81(1H, t, J= 9.0), 7.27-7.17 (3H, m). ESI-MS m/z (M⁺-1): 144.1.

1.3 Synthesis of (pim)Cu(POP)

At room temperature, Hpim (2 mmol, 0.29 g) in dichloromethane (10 mL) was slowly added to the dichloromethane solution(10 mL) of [Cu(CH₃CN)₄](BF₄) (2 mmol, 0.63 g), and the mixture was stirred for 1 h. Then, POP (2 mmol, 1.08 g) in dichloromethane (10 mL) was slowly added and stirred for another 1 h. After the solution was filtered and evaporated to dryness, the residue was dissolved in methanol(25 mL). Solid state KOH (3 mmol, 0.17 g) was added to the solution, and the mixture was stirred for 6 h at room temperature. The precipitate was collected and washed with water for three times. After the precipitate was dissolved in 30 mL ethanol/dichloromethane (3/1), pure complex was obtained as pale green crystal by slowly evaporating the solvent (yield: 0.91 g, 61%). ¹HNMR (500 MHz, CDCl₃): 7.79 (1H, d, J= 5.0), 7.59 (1H, s), 7.24~7.22 (9H, m), 7.19~7.05(14H, m), 6.99 (3H, d, J=8.0), 6.91 (3H, t, J=7.5), 6.73~6.68(3H, m). ¹³CNMR (125 MHz, CDCl₃): 158.55, 158.50, 158.45, 153.02, 148.12, 136.50, 134.34, 133.62, 132.74, 132.62, 132.50, 130.85, 129.32, 128.27, 128.23, 128.20, 125.88, 125.78, 125.69, 124.41, 120.23, 119.53. ³¹PNMR (200 MHz, CDCl₃): -13.94.

1.4 X-ray Crystallography

Crystal data were collected on a Bruker SMART APEX-Ⅱ CCD diffractometer with Mo Kα radiation. The data were corrected for Lorentz-polarization factors as well as for absorption. Structures were solved by direct methods and refined by full-matrix least-squares methods on F² with the SHELXL-97 program. All nonhydrogen atoms were refined anisotropically, while hydrogen atoms were placed in geometrically calculated positions. CCDC reference number of (pim)Cu(POP) is 1511744.

1.5 Density Functional Calculations

Density functional theory (DFT) calculations were performed with Gaussian 09 software package, which employing the B3LYP functional with the LanL2DZ basis set for Cu and 6-31G^* basis set for C, N, H, O, and P. Geometric parameters obtained from X-ray analyses were used as a starting point for geometry optimization in the ground state. Geometry optimization was carried out in the gas phase, and frequency calculation was performed to confirm the optimized structure to be true minimum value on the potential energy surface. The optimized geometry was used for time-dependent density functional (TD-DFT) calculations.

2 Results and Dicussiion 2.1 Synthesis and Structure

The diimine compound Hpim was prepared by the reported method ^[37]. The neutral copper(Ⅰ) complex (pim)Cu(POP) was synthesized via a two-step reaction as shown in Scheme 1. Firstly, [Cu(CH₃CN)₄](BF₄) reacted with POP and Hpim in sequence at room temperature. The product of this step is the charged counterpart of (pim)Cu(POP), which was not purified as the intermediate. Then, the intermediate product was deprotonated with KOH in methanol to give the target complex. Pure complex was obtained as crystal by slowly evaporating the dichloromethane/ethanol solution of crude product. The structure of (pim)Cu(POP) was characterized by ¹HNMR, ¹³CNMR and ³¹PNMR, and confirmed by X-ray crystallography. In solid state, (pim)Cu(POP) is very stable to air and has high thermal stability with a decomposition temperature (T_d) of 303 ℃(Figure 1).

The crystal structure of (pim)Cu(POP) was determined by single-crystal X-ray diffraction as shown in Figure 2. The complex crystallizes in monoclinic crystal system P2₁/c space group, and adopts a typical distorted tetrahedral geometry. The bond distances of Cu1-N1, Cu1-N3, Cu1-P1 and Cu1-P2 are 0.2006(4) nm, 0.2112(4) nm, 0.22511(13) nm and 0.22364(13) nm, respectively. The bond angles are N1-Cu1-N3=81.58(15)°, P1-Cu1-P2=111.84(15)°, N1-Cu1-P1=119.40(11)°, N3-Cu1-P1=106.12(10)°, N1-Cu1-P2=118.96(11)° and N3-Cu1-P2=114.20(11)°. These important bond distances and bond angles are similar to those of reported four-coordinate copper(Ⅰ) complexes ^{[8-11, 22-25]}. Analysis of the structural data shows that there is no π-π interaction or hydrogen bond in this complex.

2.2 Density Functional Calculations

To gain insight into the electronic structure of (pim)Cu(POP), DFT calculations were performed using the geometric parameters obtained from X-ray analysis as the starting structure. Unlike most other copper(Ⅰ) complexes, the molecular orbitals (MOs) of (pim)Cu(POP) have very little metal character as shown in Figure 3. The highest occupied molecular orbital (HOMO) is almost concentrated on the anionic ligand pim, while the lowest unoccupied molecular orbital (LUMO) is dominated by the neutral ligand POP. TD-DFT calculations were performed to determine the lowest energy electronic transition in this complex, and the calculation results are summarized in Table 1. The lowest singlet state (S₁) excitation is dominated by HOMO→LUMO transition, while the lowest triplet state(T₁) excitation is mainly HOMO→LUMO + 4 transition. According to the DFT and TD-DFT calculation results, the lowest energy electronic transition of (pim)Cu(POP) is principally ligand-to-ligand charge transfer (LLCT) transition which mixed with minimal ligand-centered (LC) transition and metal-to-ligand charge transfer (MLCT) transition.

2.3 Photophysical Properties

The UV-visible absorption spectrum of (pim)Cu(POP) was measured in ethanol solution as shown in Figure 4. Three intense high-energy absorption bands at around 225 nm (ε=3.16×10⁴ mol^-1·L·cm^-1), 260 nm (ε=1.60×10⁴ mol^-1·L·cm^-1) and 300 nm (ε=1.44×10⁴ mol^-1·L·cm^-1) can be assigned to the π-π^* transitions of ligands. The weak low-energy absorption band ranging from 340 nm to 415 nm (ε < 0.42×10⁴ mol^-1·L·cm^-1) cannot be found in the absorption spectra of POP and Hpim(Figure 4 insert). According to the theory calculation results, the weak absorption band mainly originates from LLCT transition.

The complex (pim)Cu(POP) does not show detectable emission in solution at room temperature, which should be caused by Jahn-Teller distortion and solvent-induced exciplex, this luminescence quenching mechanism has been confirmed by several groups ^{[38, 39]}. In contrast, this complex shows intense emission in solid state at room temperature, the photoluminescence (PL) spectrum and photophysical data are given in Figure 5 and Table 2. The maximum emission peak locates at 495 nm, and two shoulder peaks appear at around 468 nm and 520 nm, respectively. The emission of (pim)Cu(POP) exhibits a moderate absolute quantum yield of 0.23 and a long lifetime of 855.8 μs.

It is well known, copper(Ⅰ) complexes probably give a thermally activated delayed fluorescence (TADF) at room temperature due to their small band gaps between S₁ and T₁. To understand the emissive states, low-temperature measurements were performed for (pim)Cu(POP) in solid state. As shown in Figure 5, the PL spectrum recorded at 77 K shows obvious fine structure, but do not red-shift compared to that recorded at 298 K. The emission lifetime was measured to be 2554.9 μs at 77 K. It can be found that the emission lifetime recorded at 77 K is only three times longer than that recorded at 298 K. The 77 K/298 K lifetime ratio is much smaller than those observed for most copper(Ⅰ) complexes with TADF ^{[18, 29-32]}. These experimental results indicate that the emission of (pim)Cu(POP) in solid state at room temperature is phosphorescence stemming from triplet state.

3 Conclusion

In summary, a new neutral copper(Ⅰ) complex with high stability were successfully prepared and characterized. The coordinate geometry of the complex is typical distorted tetrahedral configuration, and the bond lengths and bond angles are in the normal range. Unlike most other copper(Ⅰ) complexes, the lowest energy electronic transition of this complex is mainly LLCT transition in stead of MLCT transition. The complex shows intense emission with relative high efficiency and long lifetime in solid state. The emission can be identified as phosphorescence by comparing the photophysical data recorded at 298 K and 77 K.