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

The redox photosensitizing ability of heteroleptic Cu(I) com-plexes containing a 2,2 & curren;-bipyridine (bpy) derivative as an & alpha;- diimine ligand was investigated in a photocatalytic CO2 reduc-tion. 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 con-ventional Cu(I)-phenanthroline analog by visible light irradi-ation in the presence of a reductant. Incorporating phenyl (ph) groups at the 4,4 & curren; -positions of the bpy ligand significantly affected the redox-photosensitization of the Cu(I)-bpy com-plexes 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 dem-onstrated 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.