Theoretical studies on structures and spectroscopic properties of cyclometalated gold(III) complexes

The electronic structures and spectroscopic properties of the four tridentate cyclometalated Au(III) complexes [Au(CNC)C[triple bond]CPh] (1), [Au(NCC)C[triple bond]CPh] (2), [Au(NNC)C[triple bond]CPh]+ (3), and [Au(NCN)C[triple bond]CPh]+ (4) [HCNCH = 2,6 -diphenylpyridine, NCHCH = 3-(2-pyridyl)biphenyl, NNCH = 6-phenyl-2,2'- bipyridine, NCHN = 1,3-di(2-pyridyl)benzene] were calculated to explore their spectroscopic nature. The geometry structures of 1-4 in the ground and excited states were optimized under the density functional theory (DFT) and the single-excitation configuration interaction (CIS) level, respectively. The absorption and emission spectra in CH(2)Cl(2) solution were calculated by the time-dependent density functional theory (TD-DFT) with the PCM solvent model. As revealed from the calculations, with the variation of pyridyl in position and the number, the electron-accepting ability of pyridyl in 1-4 are different. With increasing the electron-accepting ability of pyridyl, the HOMO-LUMO energy gaps of 1-4 decrease, and the lowest-energy absorption bands and emission bands are red-shifted in the order 1 < 2 < 4 < 3. All of the lowest-energy absorptions are assigned as the LLCT character, and the solvent polarity has little impact on the absorption spectra. The 477 nm emission of 1 arises from the 3ILCT transition, whereas the 517 and 634 nm emission of 2 and 3, respectively, come from the 3LLCT and 3ILCT. In addition, the 577 nm emission of 4 is assigned as 3LLCT/3LMCT character.