Photocatalytic function of the B₁₂ complex with the cyclometalated iridium(III) complex as a photosensitizer under visible light irradiation

A visible light induced three-component catalytic system with the cobalamin derivative (B₁₂) as a catalyst, the cyclometalated iridium(iii) complex (Irdfppy, Irppy, Irpbt and [Ir{dF(CF₃)ppy}₂(dtbpy)]PF₆) as a photosensitizer and triethanolamine as an electron source under N₂ was developed. This catalytic system showed a much higher catalytic efficiency than the previous catalytic system using [Ru(ii)(bpy)₃]Cl₂ as the photosensitizer for the dechlorination reaction of 1,1-bis(4-chlorophenyl)-2,2,2-trichloroethane (DDT). Noteworthy is the fact that the remarkable high turnover number (over ten thousand) based on B₁₂, which ranks at the top among the reported studies, was obtained when Irdfppy was used as a photosensitizer. This photocatalytic system was also successfully applied to the B₁₂ enzyme-mimic reaction, i.e., the 1,2-migration of the phenyl group of 2-bromomethyl-2-phenylmalonate. The plausible reaction mechanism was proposed, which involved two quenching pathways, an oxidative quenching pathway and a reductive quenching pathway, to be responsible for the initial electron transfer of the excited-state photosensitizers during the DDT dechlorination reaction. Transient photoluminescence experiments revealed that the oxidative quenching of the photosensitizer dominated over the reductive quenching pathway.