Mechanism of Photocatalytic CO₂Reduction by Iron Spin-Crossover Complex with Copper Photosensitizer

The mechanism of CO2 reduction and hydrogen production by a homogeneous photocatalytic system consisting of a copper (Cu) complex as a photosensitizer and an iron (Fe) spin-crossover complex as a catalyst was investigated using density functional theory. The photocatalytic reaction is initiated by electronic excitation upon light irradiation, characterized by a charge transfer from the metal d-orbital to the ligand orbital of the Cu photosensitizer (Cu PS). The long-lived lowest energy triplet state exhibits a larger oxidation power than the ground state, and electron transfer from the sacrificial electron donors results in the formation of a reduced species of Cu PS. The electron transfer from the one-electron reduced species of Cu PS to the Fe complex leads to CO2 reduction. The calculations suggest that the two-electron reduction of Fe complexes is necessary for CO2 reduction and that the complexes with CO2 bound to the Fe center are the target species for the second electron reduction, but the one-electron reducing species of Cu complexes has a weak reducing power and the Fe complexes are not reduced efficiently. For the subsequent catalytic reaction, the binding of CO2 to the Fe center was substantially endergonic, and the binding stability was low. Therefore, in order to increase the catalytic efficiency, a ligand design that improves the reducing power for the Cu complexes and the CO2 binding stability to the Fe center for the Fe complexes is essential.