Photocatalytic CO2 Reduction Mediated by Electron Transfer via the Excited Triplet State of Zn(II) Porphyrin.

A porphyrin/rhenium complex dyad, ZnP-phen=Re, in which the 2-position of the 1,10-phenanthroline (phen) of fac-Re(phen)(CO)3Br is directly connected with the meso-position of zinc(II) porphyrin, showed phosphorescence from the zinc porphyrin part under Ar, even at room temperature. The phosphorescence was efficiently quenched by 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH) used as an electron donor, under Ar, with the Stern-Volmer constant KSV = 180 000 M-1, indicating that the quantitative photoinduced electron transfer occurred from BIH to the excited triplet state (T1) of ZnP-phen=Re. The system affords CO with a high turnover number (>1300) and >99.9% CO selectivity in the photocatalytic CO2 reduction.