Photochemical reduction of CO2 with ascorbate in aqueous solution using vesicles acting as photocatalysts.

We report a novel system of visible-light-driven CO2 reduction to CO in an aqueous solution, in which DPPC vesicles dispersed in the solution act as a photocatalyst using ascorbate (HAsc(-)) as an electron source. In the vesicles metal complexes Ru(dtb)(bpy)2 and Re(dtb)(CO)3Cl (dtb = 4,4'-ditridecyl-2,2'-bipyridyl) are incorporated, which act as a photosensitizer and a catalyst for CO2 reduction, respectively. The reaction is initiated with the reductive quenching of the (3)MLCT excited state of the Ru complex with HAsc(-), followed by an electron transfer from the reduced Ru complex to the Re complex to give a one-electron reduced Re species having catalytic ability for CO2 reduction. In order to search for optimum conditions for the CO production, the dependence of the initial rate of CO formation, vi, on the concentration of the metal complexes and HAsc(-) in the vesicle solution was examined. Consequently, we obtained ∼3.5 μmol h(-1) and 190 for vi and the turnover number for CO formation with respect to the Re catalyst, respectively. On the basis of the dependence of vi on the incident light intensity, we have concluded that the photocatalytic reduction of CO2 to CO with HAsc(-) in this system requires only one photon, and propose that HAsc(-) donates an electron not only to the excited state of the Ru complex, but also to the Re-CO2 adduct involved in the catalytic cycle for CO2 reduction.