Rhenium(i) trinuclear rings as highly efficient redox photosensitizers for photocatalytic CO2 reduction† †Electronic supplementary information (ESI) available: Franck–Condon analysis; photochemical one-electron-reduced species formation and characterisation; photophysical, electrochemical and quenching properties of R(4·5) in DMA; photophysical and electrochemical properties of the catalysts; photocatalytic CO2 reduction experiments and additional data. See DOI: 10.1039/c6sc01913g Click here for additional data file.

We developed new cyclic Re(I)-based trinuclear redox photosensitizers with both high oxidation power in the excited state and strong reduction power in the reduced form. These excellent properties were achieved by introducing electron-donating groups on the diimine ligand of the Re(I) metal centre and by connecting each Re(I) unit with polyphenyl–bisphosphine bridging ligands. These Re-rings were applied to homogenous visible light-driven photocatalytic CO2 reduction in conjunction with various mononuclear catalysts, such as Re(I), Ru(II) and Mn(I) metal complexes, employing a relatively weak sacrificial electron donor, triethanolamine. Each system showed good product selectivity (CO or HCOOH) and an excellent quantum yield of product formation FCO 1⁄4 0.60 to 0.74 using fac[Re(bpy)(CO)3(CH3CN)] , FHCOOH 1⁄4 0.58 using trans(Cl)–Ru(dtbb)(CO)2Cl2 and FHCOOH 1⁄4 0.48 using a fac-[Mn(dtbb)(CO)3(CH3CN)] + catalyst. The high photocatalytic efficiencies for CO2 reduction are attributed to efficient reductive quenching of the Re-ring by triethanolamine and fast electron transfer from the generated one-electron-reduced species of the ring to the catalyst.