Photocatalytic CO₂ reduction using a diazabenzacenaphthenium photosensitizer and a Mn catalyst

Redox photosensitizers are key components in photoredox catalysis, mediating photoinduced electron transfer from an electron donor to a catalyst or substrate. In systems proceeding through reductive quenching, the efficiency of forming the one-electron-reduced photosensitizer critically determines the overall quantum yield. Here, we report a comprehensive photophysical investigation of 4,5,9,10-tetraethyl-1,2-dihydrobenz[de]imidazo[1,2,3-ij]-1,8-naphthridinium cation (N-BAP⁺), originally developed as a redox photosensitizer for oxidative quenching in photocatalytic organic synthesis. The photochemical reduction of N-BAP⁺ by 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH) was also analyzed, focusing on the contributions of the singlet and triplet excited states of N-BAP⁺. Spectroscopic studies demonstrate that N-BAP⁺ undergoes efficient reductive quenching with BIH, generating the one-electron-reduced species N-BAP˙. Based on these findings, we applied N-BAP⁺ to photocatalytic CO₂ reduction using a Mn-complex catalyst and BIH as a reductant, resulting in an efficient and durable photocatalytic CO₂ reduction system. Kinetic analysis revealed that the reductive quenching of the triplet excited state was the essential pathway governing the activity of this photocatalytic CO₂ reduction. These findings suggest that numerous candidates, with or without TADF properties, could serve as organic redox photosensitizers free of heavy metal ions. For optimal performance, such molecules should achieve as high a triplet excited-state formation yield as possible while keeping the singlet excited-state lifetime as short as possible.