Insight into the Origin of Competitive Emission of Copper(I) Complexes Bearing Diimine and Diphosphine Ligands

Abstract

Abstract The photophysical properties of three luminescent Cu(I) complexes, [Cu(dmp)(xantphos)]+ (Cu-1; dmp = 2,9-dimethyl-1,10-phenanthroline, xantphos = 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene), [Cu2(dmp)2(µ-dppa)2]2+ (Cu-2; dppa = bis(diphenylphosphino)acetylene), and [Cu2(Ph2dmp)2(µ-dppa)2]2+ (Cu-2ph; Ph2dmp = 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline), were investigated. The luminescence of Cu-1 at room temperature was assigned to the thermally activated delayed fluorescence (TADF) from the singlet metal-to-ligand charge transfer (1MLCT) excited state. However, the emission of Cu-2 and Cu-2ph exhibited lower radiative rate constants than Cu-1 at room temperature, which can be assigned to phosphorescence from the triplet metal-to-ligand charge transfer (3MLCT) excited state at room temperature. In addition, emission of Cu-2 and Cu-2ph at 77 K indicated significant contributions of the 3ππ* excited state. Theoretical calculations suggested that the energy difference between the optimized S1 and T1 states of Cu-1 is relatively smaller than that of Cu-2. As a result, Cu-1 shows TADF and Cu-2 shows phosphorescence.