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A series of heteroleptic coinage metal(I) complexes Cu(PP)(PS) 1, Ag(PP)(PS) 2, and Au(PP)(PS) 3 PP = 1,2-bis(diphenylphosphino)benzene and PS− = 2-diphenylphosphinobenzenethiolate were synthesized. X-ray crystallography demonstrated that 1–3 possessed tetrahedral structures containing two types of bidentate ligands, PP and PS−. Photophysical studies and time-dependent density functional theory calculations indicated that the emission from 1–3 in the solid state at room temperature originated from thermally activated delayed fluorescence (TADF). The thiolate ligand with strong electron-donating character (PS−) reduced the contribution from metal orbitals to the highest occupied molecular orbitals of the complexes, decreasing the metal-to-ligand charge-transfer character of the excited states of 1–3. The origin of TADF in 1–3 was attributed to ligand-to-ligand charge-transfer on the basis of molecular orbital calculations. Au(I) complex 3 was unstable in solution because of a rapid ligand exchange reaction, and Ag(I) complex 2 showed limited solubility in organic solvents. Cu(I) complex 1, which exhibited efficient green TADF with a maximum emission wavelength of 521 nm and a quantum yield of 0.52 in the solid state, was used to fabricate TADF-type organic light-emitting diodes via a wet process.
Osawa et al. (Tue,) studied this question.