Photoinduced Water Oxidation in Chitosan Nanostructures Containing Covalently Linked Ru-II Chromophores and Encapsulated Iridium Oxide Nanoparticles

The luminophore Ru(bpy)(2)(dcbpy)(2+) (bpy=2,2'-bipyridine; dcbpy=4,4'-dicarboxy-2,2'-bipyridine) is covalently linked to a chitosan polymer; crosslinking by tripolyphosphate produced Ru-decorated chitosan fibers (NS-RuCh), with a 20 : 1 ratio between chitosan repeating units and Ru-II chromophores. The properties of the Ru-II compound are unperturbed by the chitosan structure, with NS-RuCh exhibiting the typical metal-to-ligand charge-transfer (MLCT) absorption and emission bands of Ru-II complexes. When crosslinks are made in the presence of IrO2 nanoparticles, such species are encapsulated within the nanofibers, thus generating the IrO2 subset of NS-RuCh system, in which both Ru-II photosensitizers and IrO2 water oxidation catalysts are within the nanofiber structures. NS-RuCh and IrO2 subset of NS-RuCh have been characterized by dynamic light scattering, scanning electronic microscopy, and energy-dispersive X-ray analysis, which indicated a 2 : 1 ratio between Ru-II chromophores and IrO2 species. Photochemical water oxidation has been investigated by using IrO2 subset of NS-RuCh as the chromophore/catalyst assembly and persulfate anions as the sacrificial species: photochemical water oxidation yields O-2 with a quantum yield (phi) of 0.21, definitely higher than the phi obtained with a similar solution containing separated Ru(bpy)(3)(2+) and IrO2 nanoparticles (0.05) or with respect to that obtained when using NS-RuCh and "free" IrO2 nanoparticles (0.10). A fast hole-scavenging process (rate constant, 7x10(4) s(-1)) involving the oxidized photosensitizer and the IrO2 catalyst within the IrO2 subset of NS-RuCh system is behind the improved photochemical quantum yield of IrO2 subset of NS-RuCh.