Photocatalysis of a Dinuclear Ru(II)-Re(I) Complex for CO Reduction on a Solid Surface.

The development of CO-reduction photocatalysts is one of the main targets in the field of artificial photosynthesis. Recently, numerous hybrid systems in which supramolecular photocatalysts comprised of a photosensitizer and catalytic-metal-complex units are immobilized on inorganic solid materials, such as semiconductors or mesoporous organosilica, have been reported as CO-reduction photocatalysts for various functions, including water oxidation and light harvesting. In the present study, we investigated the photocatalytic properties of supramolecular photocatalysts comprised of a Ru(II)-complex photosensitizer and a Re(I)-complex catalyst fixed on the surface of insulating AlO particles: the distance among the supramolecular photocatalyst molecules should be fixed. Visible-light irradiation of the photocatalyst in the presence of an electron donor under a CO atmosphere produced CO selectively. Although CO formation was also observed for a 1:1 mixture of mononuclear Ru(II) and Re(I) complexes attached to an AlO surface, the photocatalytic activity was much lower. The activity of the AlO-supported photocatalyst was strongly dependent on the adsorption density of the supramolecular moiety, where the initial rate of photocatalytic CO formation was faster at lower density and higher photocatalyst durability was achieved at higher density. One of the main reasons for the former phenomenon is the decreased quenching fraction of the excited state of the photosensitizer unit by the reductant dissolved in the solution phase in the case of higher density. This is due to the self-quenching of the excited photosensitizer unit and steric hindrance between the condensed supramolecular photocatalyst molecules attached to the surface. The higher durability of the more condensed system is caused by intermolecular electron transfer between reduced supramolecular photocatalyst molecules, which accelerates the formation of CO in the photocatalytic CO reduction. Coadsorption of a Ru(II) mononuclear complex as a redox photosensitizer could drastically reinforce the photocatalysis of the supramolecular photocatalyst on the surface of the AlO particles: more than 10 times higher turnover number and about 3.4 times higher turnover frequency of CO formation. These investigations provide new architectures for the construction of efficient and durable hybrid photocatalytic systems for CO reduction, which are composed of metal-complex photocatalysts and solid materials.