Investigation on the photophysical properties of Cu(I) complexes supported by N‐heterocyclic carbene ligands with electron‐donating/withdrawing groups on imidazolylidene unit

Abstract

Two new four-coordinate Cu(I) complexes consisted of bidentate NHC ligand and diphosphine ligand were successfully synthesized and characterized. The electron-donating methyl and electron-withdrawing chlorine groups were introduced into the imidazolylidene unit of N-heterocyclic carbene (NHC) ligands to investigate their influences on the photophysical properties of complexes. The red-shifted low-energy absorption bands indicate that both methyl and chlorine decreased the band gaps between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). Theoretical calculation results show the methyl group raised the energy level of HOMO more than that of LUMO, while the chlorine substituent lowered the energy level of LUMO more than that of HOMO. Both these Cu(I) complexes show high emission efficiencies and microsecond-scale emission lifetimes. The photophysical data at 77 K show that emissions of these complexes are the mixture of phosphorescence and thermally activated delayed fluorescence (TADF).