TADF-emitting copper(i) and silver(i) complexes featuring intra-ligand charge transfer based on a donor–acceptor–donor ligand†

Copper(I) and silver(I) complexes have garnered significant interest as photoluminescent and electroluminescent emitters. Developing efficiently luminescent copper(I) and silver(I) complexes relies on the rational design of ligands, based on a deep understanding of the relationships between structure, excited states, and photophysical properties. Herein, a copper(I) complex and a silver(I) complex, namely Cu(Ac-phen)XP·BF₄ and Ag(Ac-phen)XP·BF₄, were synthesized based on a donor–acceptor–donor (D–A–D) type diimine ligand and a bisphosphine ligand. Cu(Ac-phen)XP·BF₄ and Ag(Ac-phen)XP·BF₄ in the solid state exhibit red and orange-yellow emission with photoluminescent quantum yields of 18% and 40%, and emission lifetimes of 1.2 μs and 7.8 μs, respectively. Theoretical and experimental investigations demonstrate that both complexes emit efficient thermally activated delayed fluorescence (TADF) from excited states dominated by intra-ligand charge transfer (ILCT) transitions. The ILCT emissive states are attributed to the stronger electron-donating ability of the A–D–A type ligand compared to the d¹⁰ copper and silver centers. The distinct metal coordination effects of the copper and silver centers lead to different levels of LUMO stabilization in the diimine ligand, resulting in different emissions for the copper(I) and silver(I) complexes.