Highly Efficient Supramolecular Photocatalysts for CO2 Reduction with Eight Carbon-Carbon Bonds between a Ru(II) Photosensitizer and a Re(I) Catalyst Unit

Abstract

Supramolecular photocatalysts, wherein redox photosensitizer (PS) and catalyst (CAT) molecules are connected to each other, have been extensively studied because of their high photocatalytic activity in both homogeneous and heterogeneous environments compared with the corresponding mixed systems of separated PS and CAT. A supramolecular photocatalyst RuC2PhC2Re, wherein [Ru(diimine)3]2+ redox PS and fac-[Re(diimine)(CO)3{OC(O)OCH2CH2N(CH2CH2OH)2}] CAT units were spatially separated by a bridging ligand p(-C2H4)2Ph consisting of 8 C-C bonds including a p-phenylene ring, was developed. Although the rate of intramolecular electron transfer of RuC2PhC2Re from one-electron-reduced Ru unit to the Re unit, which is a critical process of photocatalysis proceeding through the bond mechanism, was slower than that of RuC2Re having shorter bridging ligand with an ethylene chain, it could reduce CO2 to CO with higher durability (TON = 3880) than RuC2Re (TON = 2800). These results clearly suggest that the PS and CAT units can be separated further without lowering photocatalysis of supramolecular photocatalysts because the rate of intramolecular electron transfer is much faster, even in RuC2PhC2Re, than that of the subsequent processes in photocatalytic CO2 reduction.