We found a very suitable sacrificial electron donor, 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzoldlimidazole (BIH), to demonstrate the proper ability of a Ru(II)-Re(I) supramolecular photocatalyst for CO2 reduction. The efficiency, durability, and rate of the photocatalytic reaction were drastically improved (Phi(CO) = 0.45, TONCO = 3029, TOECO = 35.7 min(-1)) using BIH as an electron donor compared to that using 1-benzyl-1,4-dihydronicotinamide as an electron donor (Phi(CO) = 0.15, TONCO = 207, TOFCO = 4.7 min(-1)). The reasons for the improvement were clarified based on the detailed mechanistic investigations. (C) 2013 Elsevier Inc. All rights reserved.