Near-white light emission from single crystals of cationic dinuclear gold(I) complexes with bridged diphosphine ligands

Three cationic dinuclear Au(I) complexes containing acetonitrile (AN) as an ancillary ligand were synthesized: [μ-LMe(AuAN)2]·2BF4 (1), [μ-LEt(AuAN)2]·2BF4 (2), and [μ-LiPr(AuAN)2]·2BF4 (3) (LMe = {1,2-bis[bis(2-methylphenyl)phosphino]benzene}, LEt = {1,2-bis[bis(2-ethylphenyl)phosphino]benzene}, and LiPr = {1,2-bis[bis(2-isopropylphenyl)phosphino]benzene}). The unique structures of complexes 1–3 with two P–Au(I)–AN rods bridged by rigid diphosphine ligands were determined through X-ray analysis. The Au(I)–Au(I) distances observed for complexes 1–3 were as short as 2.9804–3.0457 Å, indicating an aurophilic interaction between two Au(I) atoms. Unlike complexes 2 and 3, complex 1 incorporated CH2Cl2 into the crystals as crystalline solvent molecules. Luminescence studies in the crystalline state revealed that complexes 1 and 2 mainly exhibited bluish-purple phosphorescence (PH) at 293 K: the former had a PH peak wavelength at 415 nm with the photoluminescence quantum yield ΦPL = 0.12, and the latter at 430 nm with ΦPL = 0.13. Meanwhile, complex 3 displayed near-white PH, that is dual PH with two PH bands centered at 425 and 580 nm with ΦPL = 0.44. The PH spectra and lifetimes of complexes 2 and 3 were measured in the temperature range of 77–293 K. The two PH bands observed for complex 3 were suggested to originate from the two emissive excited triplet states, which were in thermal equilibrium. From theoretical calculations, the dual PH observed for complex 3 is explained to occur from the two excited triplet states, T1H and T1L: the former exhibits a high-energy PH band (bluish-purple) and the latter exhibits a low-energy PH band (orange). The T1H state is considered 3ILCT with a structure similar to that of the S0-optimized structure. Conversely, the T1L state is assumed to be a 3MLCT with a T1-optimized structure, which has a short Au(I)–Au(I) bond and two bent rods (Au–AN). The thermal equilibrium between the two excited states is discussed based on computational calculations and photophysical data in the temperature range of 77–293 K. With regard to the crystal of complex 1, we were unable to precisely measure the temperature-dependent emission spectra and lifetimes, particularly at low temperatures, because the cooled crystals became irreversibly turbid over time.