We found a very suitable sacrificial electron donor, 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH), to demonstrate the proper ability of a Ru(II)–Re(I) supramolecular photocatalyst for CO 2 reduction. The efficiency, durability, and rate of the photocatalytic reaction were drastically improved (Φ CO =0.45, TON CO =3029, TOF CO =35.7min −1 ) using BIH as an electron donor compared to that using 1-benzyl-1,4-dihydronicotinamide as an electron donor (Φ CO =0.15, TON CO =207, TOF CO =4.7min −1 ). The reasons for the improvement were clarified based on the detailed mechanistic investigations.