Cyclometalated Iridium(III) Complexes Containing 2-Phenylbenzo[d]oxazole Ligand: Synthesis, X-ray Crystal Structures, Properties and DFT Calculations

Two new iridium(III) complexes were synthesized and fully characterized, [(bo) 2 Ir(pzpy)] ( 2a ) and [(bo) 2 Ir(pzpyz)] ( 2b ) (where bo = 2-phenylbenzo[d]oxazole, pzpy = 2-(1H-pyrazol-3-yl)pyridine, pzpyz = 2-(1H-pyrazol-3-yl)pyrazine). The single crystal structures of 2a-2b have been determined. Considering the relationship between their structures and photophysical properties, DFT calculations have been used to further support this inference. These Ir(III) complexes emit from the excited state of 3 MLCT/ 3 LLCT in the green and yellow region, and the quantum yields in the degassed CH 2 Cl 2 solution at room temperature are 35.2% and 46.1%. Theoretical and experimental results show that iridium(III) complexes 2a-2b are promising phosphorescent material.


Introduction
Neutral mononuclear cyclometalated iridium complexes have been found to be suitable for use in organic light emitting diodes (OLEDs). [1][2][3] The privileged use is due to their interesting luminescence properties, [4][5] such as high quantum yields, long excited-state lifetimes, and tunable emission color over the entire visible spectrum. [6][7] Most often, the variation of their emission color was largely governed by the cyclometalated and/or ancillary ligands structures.
2-phenylbenzo[d]oxazole (bo) is one typical ligand framework for constructing Ir(III) complexes, and can be used to fine-tune the emission color of complexes by judicious modification. [8][9][10] For example, in 2015, we reported Ir(bo) 2 (acac) derivatives with substituents on the benzoxazole ring and their emissions covered a narrow range from 560 to 566 nm. 11 The color adjusting by the change of cyclometalated ligands structures were not very satisfactory, although the quantum yield was up to 53.5%. Afterward, we designed a series of bo-based iridium(III) com-plexes with different N^O ancillary ligands. They exhibited a wide range of emission wavelengths (λ max = 531-598 nm) with high quantum yields (19%-94%). 12 The research findings showed that the structures of ancillary ligands have obvious effect on tuning the emission color of bobased iridium(III) complexes. Therefore, we wanted to further investigate other types of ancillary ligands. In this paper, we design two N^N ancillary ligands (a and b) and synthesize two bo-based iridium(III) complexes (2a and 2b) (Scheme 1). The photophysical and electrochemical properties of these complexes were investigated, and the lowest energy electronic transitions were analyzed based on density functional theory (DFT) and time-dependent DFT (TDDFT).

4. Crystallographic Studies
X-ray diffraction data were collected with an Agilent Technologies Gemini A Ultra diffractometer equipped with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) at room temperature. Data collection and reduction were processed with CrysAlisPro software. 16 The structure was solved and refined using Full-matrix leastsquares based on F 2 with program SHELXS-97 and SHELXL-97 17 within Olex2. 18 All non-hydrogen atoms were found in alternating difference Fourier syntheses and least-squares refinement cycles and, during the final cycles, refined anisotropically. Hydrogen atoms were placed in calculated positions and refined as riding atoms with a uniform value of Uiso.

5. Computational Method
The geometry of complexes 2a-2b was optimized starting from the X-ray data by the DFT (density functional theory) method with B3LYP (Becke three-parameter Lee-Yang-Parr) hybrid density functional theory and the 6-31G* basis set. All calculations were carried out with Gaussian 09 software package. 19

1. Description of Crystal Structure
The single crystal structures of 2a-2b were obtained by X-ray diffraction studies, and ORTEP diagrams are shown in Fig. 1. The crystallographic data and structural details are given in Table 1. The selected bond lengths and  bond angles are collected in Table S1.
For the structures of these complexes, the Ir(III) centre adopts a twisted octahedral geometry, and the C^N ligands are in cis-C,C' and trans-N,N' configurations. The Ir-N average bond lengths of 2a (2.077Å) and 2b (2.100Å) are longer than the Ir-C average bond lengths of 2a (2.019Å) and 2b (2.041Å), which are reported in other iridium complexes. 20 Furthermore, the Ir-N bonds between the iridium and N^N ligands (2.081-2.152 Å) are longer than those between the iridium and C^N ligands (2.037-2.073Å), consistent with strong trans influence of the C^N ligands. The octahedral para-orbital angles range from 171.0(5)° to 174.6(5)° for 2a and from 171.9(4)° to 174.9(4)° for 2b, which is close to a straight line. The metric parameters of the two iridium complexes are similar owing to the same cyclometalated ligands and analogous ancillary ligands.

2. Electronic Absorption Spectra
The UV-Vis absorption spectra of 2a-2b were recorded at room temperature in CH 2 Cl 2 solutions, as shown in Fig. 2, and the data are summarized in Table 2. All of the complexes exhibit intense absorption bands in the ultraviolet region at wavelengths below 310 nm, which are assigned to the spin-allowed π-π* transitions on the C Ù N main ligands and the N^N ancillary ligands. The weaker absorption bands in the range 350-450 nm are likely attributed to metal-to-ligand charge-transfer transitions ( 1 MLCT and 3 MLCT). [21][22] Compared with complex 2a, complex 2b has a red-shifted, which may be caused by the ancillary ligand. This speculation will be confirmed by electrochemical analysis and DTF calculations. be 35.2% and 46.1% with reference to fac-Ir(ppy) 3 (Φ = 0.40). 25

4. Theoretical Calculations
Density functional theory (DFT) and time-dependent DFT (TD DFT) calculations have been performed on the complexes 2a-2b to obtain an insight into the lowest energy electron transition. The most representative molecular front orbital diagram of these complexes is shown in Fig. 4. The calculated spin-allowed electron transitions are provided in Table 3 and compared with the experimental absorption spectra data. The electron density distribution data are summarized in Table S2.
As shown in Fig. 4, the HOMOs of these complexes are mainly located on the metal center and C^N ligands. Meanwhile, the LUMO of 2a is mostly dominated on C^N ligands, while LUMO of 2b is mainly located on the whole ancillary ligands. In addition, the LUMO+1s of these complexes are primarily centered on the C^N main ligands, while HOMO-1s are delocalized over the metal center, C^N ligands and ancillary ligands. The theoretical calculation of DFT shows that the lowest energy spin-allowed transitions of 2a-2b come from HOMO→LUMO/ LUMO+1 and HOMO→LUMO+1/HOMO-1→LUMO transitions (Table 3), and therefore attributed to metal-to-ligand charge transfer transition and ligand-to-ligand π-π * transition. These calculations support the photophysical properties discussed above.

Emission Properties
The photoluminescence emission spectra of iridium complexes 2a-2b in degassed CH 2 Cl 2 solution at room temperature and corresponding data are described in Fig.  3 and Table 2, respectively. Complex 2a exhibits green phosphorescence with the broad emission maxima peak at 518 nm and a shoulder peak at 547 nm, whereas 2b is yellow emissive with the broad emission maxima peak at 529-552 nm. For their emission, the excited state of 2a is attributed to the mixing of 3 MLCT and 3 LC, 23 while that of 2b is mainly attributed to 3 MLCT. 24 As expected, the emission band of 2b has red-shifted with respect to 2a due to different ancillary ligands, which is consistent with absorption analysis. In addition, the quantum yields of 2a and 2b in solution at room temperature were measured to

5. Electrochemical Properties
The electrochemical properties of 2a-2b were studied by cyclic voltammetry and shown in Fig. 5. The corre-sponding electrochemical data and estimated HOMO energy levels are summarized in Table 2. The complexes 2a-2b exhibit quasi-reversible oxidation peaks at 1.57 V and 1.62 V, respectively. From DFT calculations (Table S2), HOMO is mainly located on Ir ions (47.77% for 2a and 46.90% for 2b) and C^N ligands (40.97% for 2a and 44.91% for 2b). Thus, their oxidation processes are assigned to Ir (III) to Ir (IV) and some contributions of the C^N ligands. 26 Based on the oxidation potential, the HOMO energy is derived from the equation E HOMO = -(E ox + 4.8 eV), and the trend is consistent with the theoretical calculations ( Table 2). From these results, it can be seen that because of the different number of nitrogen atoms in the ancillary ligands, the HOMO level of 2b is more stable than that of the analogue 2a, and the oxidation process of 2b is more difficult than that of 2a.

Conclusions
In summary, the syntheses, characterization, as well as electrochemical, spectroscopic and photophysical prop-  erties of two new bo-based iridium(III) complexes are reported. The room-temperature phosphorescence of these complexes is tunable from green to yellow depending on the different ancillary ligands. It was also found that as the number of nitrogen atoms increased on the ancillary ligands, the quantum yields became larger and emission became brighter. The DFT calculated results are in good agreement with the actual absorption spectra, indicating that the lowest absorption is assigned to the MLCT/LLCT transition. These results will facilitate the design of new bo-based iridium(III) complexes for highly efficient OLEDs.

Acknowledgments
This work was supported by the Natural Science Foundation of Hainan Province (218QN236) and Program for Innovative Research Team in University (IRT-16R19).

Supplementary Material
The selected bonds and angles of complexes 2a-2b, the frontier orbital energy and electron density distributions of complexes 2a-2b, as well as the FTIR spectra of complexes 2a-2b. Crystallographic data for the structural analyses have been deposited in the Cambridge Crystallographic Data Centre, CCDC reference number 1881007 (2a) and 1881008 (2b). Copies of this information may be obtained free of charge from The Director, CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: +44 1223 336033; e-mail: deposit@ccdc.cam. ac.uk).