Phosphorescent properties of metal-free diphosphine ligands and effects of copper binding

We studied the luminescence properties of copper(I) complexes containing bis[2-(diphenylphosphino)phenyl]ether (DPEphos), [Cu(DPEphos)(CH3CN)]PF6 (1), [Cu(DPEphos)2]PF6 (2), and copper(I) complexes with 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene (xantphos), [Cu(xantphos)(CH3CN)2]PF6 (3), [Cu(xantphos)2]PF6 (4) in the solid state. The metal-free xantphos ligand shows weak phosphorescent emission at around 455 nm with an emission lifetime in the sub-microsecond range in the solid state at room temperature. When xantphos forms complex 4, it results in a nearly 4-fold increase in the emission quantum yield and emission lifetime, with a small shift in the emission maximum. In contrast, no such enhancement in the luminescence or increment in the emission lifetime was observed in complexes 1-3. The X-ray structural analysis of complexes 1-4 reveals a large vacant space in complexes 1-3 and in contrast, close packing of the ligands in complex 4 around the metal center. This indicates that the decrease in the free-space around the metal center results in a decrease in the geometric relaxation, suppressing the excited-state deactivation pathway.