Mechanistic study of photocatalytic CO₂reduction using a Ru(ii)-Re(i) supramolecular photocatalyst

Supramolecular photocatalysts comprising [Ru(diimine)₃]²⁺photosensitiser andfac-[Re(diimine)(CO)₃{OC(O)OC₂H₄NR₂}] catalyst units can be used to reduce CO₂to CO with high selectivity, durability and efficiency. In the presence of triethanolamine, the Re catalyst unit efficiently takes up CO₂to form a carbonate ester complex, and then direct photocatalytic reduction of a low concentration of CO₂,e.g., 10% CO₂, can be achieved using this type of supramolecular photocatalyst. In this work, the mechanism of the photocatalytic reduction of CO₂was investigated applying such a supramolecular photocatalyst,RuC2Rewith a carbonate ester ligand, using time-resolved visible and infrared spectroscopies and electrochemical methods. Using time-resolved spectroscopic measurements, the kinetics of the photochemical formation processes of the one-electron-reduced speciesRuC2(Re)⁻, which is an essential intermediate in the photocatalytic reaction, were clarified in detail and its electronic structure was elucidated. These studies also showed thatRuC2(Re)⁻is stable for 10 ms in the reaction solution. Cyclic voltammograms measured at various scan rates besides temperature and kinetic analyses ofRuC2(Re)⁻produced by steady-state irradiation indicated that the subsequent reaction ofRuC2(Re)⁻proceeds with an observed first-order rate constant of approximately 1.8