Heteroleptic Cu(I) Bipyridine Complexes as Redox Photosensitizers for Photocatalytic CO₂ Reduction

The redox photosensitizing ability of heteroleptic Cu(I) complexes containing a 2,2′–bipyridine (bpy) derivative as an α–diimine ligand was investigated in a photocatalytic CO2 reduction. When combined with fac–Mn(bpy)(CO)3Br as a multi-electron catalyst, the reaction systems generated a mixture of CO and HCOOH to a similar extent to the case using a conventional Cu(I)–phenanthroline analog by visible light irradiation in the presence of a reductant. Incorporating phenyl (ph) groups at the 4,4′–positions of the bpy ligand significantly affected the redox-photosensitization of the Cu(I)–bpy complexes due to improved visible light absorption. Owing to coplanarity between the ph groups and bpy scaffold, the former effectively increased and shifted the lowest-energy metal-to-ligand charge transfer absorption into the visible region more than the case of Cu(I) phenanthroline analogs. Furthermore, photophysical measurements and quenching experiments demonstrated that the heteroleptic Cu(I)–bpy complexes underwent efficient photoinduced-electron transfer from the reductant, generating the corresponding one-electron reduced species. Electrochemical measurements also demonstrated that Cu(I)–bpy complexes had sufficient reducing power and stability to transfer the extra electron to the Mn-complex catalyst in the one-electron reduced state, thereby reproducing the original photosensitizer. The redox photosensitizing ability of heteroleptic Cu(I) bipyridine complexes was investigated in photocatalytic CO2 reduction. The introduction of phenyl groups in the bipyridine ligand effectively expanded the lowest-energy absorption band corresponding to metal-to-ligand charge transfer in the visible region. The CO2 reduction proceeded photocatalytically with a Mn-complex catalyst and a reductant (D) by visible light irradiation similar to Cu(I) phenanthroline analogs.