A series of highly emissive three-coordinate copper(I) complexes, (dtpb)Cu(I)X [X = Cl (1), Br (2), I (3); dtpb =1,2-bis(o-ditolylphosphino)benzene], were synthesized and investigated in prototype organic light-emitting diodes (OLEDs). 1-3 showed excellent photoluminescent performance in both degassed dichloromethane solutions [quantum yield (Φ) = 0.43-0.60; lifetime (τ) = 4.9-6.5 μs] and amorphous films (Φ = 0.57-0.71; τ = 3.2-6.1 μs). Conventional OLEDs containing 2 in the emitting layer exhibited bright green luminescence with a current efficiency of 65.3 cd/A and a maximum external quantum efficiency of 21.3%.

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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 ¼ 1,2-bis(diphenylphosphino)benzene and PS À ¼ 2-diphenylphosphinobenzenethiolate] were synthesized.X-ray crystallography demonstrated that 1-3 possessed tetrahedral structures containing two types of bidentate ligands, PP and PS À . 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 À ) 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.

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