Tris(bipyridine)ruthenium(II) photosensitized reduction of a macrocyclic cobalt(III) complex using phenol as a donor in aqueous acidic solutions

In an aqueous acid solution, the Ru(bpy)(3)(2+) photosensitized reduction of a macrocyclic Co-III complex, Co(N-4)(OH2)(2)(3+) (N-4 = 5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradeca-4,11-ditraazacyclotetradeca-4,11-diene), has been found to proceed in the presence of phenol. In Ru(bpy)(3)(2+)-Co(N-4)(OH2)(2)(3+)-phenol, the photosensitized reaction proceeds by electron transfer from the excited state Ru(bpy)(3)(2+) to the Co-III complex. Net charge separation is achieved owing to rapid reduction of the oxidized species of Ru(bpy)(3)(2+) by phenol molecules. The pH dependence of the charge-separation efficiency in the presence of phenol is quite different from that in the corresponding photosensitized system using teoa as a sacrificial donor. A partially reversible redox reaction such as slow reduction of the oxidized species of phenol and the oxidation of the Co-II generated takes place simultaneously with repetitive irradiations. The present results reveal that phenol is an excellent sacrificial donor, preferential to the usual ones such as teoa and EDTA for the photosensitized reaction in an aqueous acidic solution.