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 IrO(2) nanoparticles, such species are encapsulated within the nanofibers, thus generating the IrO(2)⊂NS‐RuCh system, in which both Ru(II) photosensitizers and IrO(2) water oxidation catalysts are within the nanofiber structures. NS‐RuCh and IrO(2)⊂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 IrO(2) species. Photochemical water oxidation has been investigated by using IrO(2)⊂NS‐RuCh as the chromophore/catalyst assembly and persulfate anions as the sacrificial species: photochemical water oxidation yields O(2) with a quantum yield (Φ) of 0.21, definitely higher than the Φ obtained with a similar solution containing separated Ru(bpy)(3) (2+) and IrO(2) nanoparticles (0.05) or with respect to that obtained when using NS‐RuCh and “free” IrO(2) nanoparticles (0.10). A fast hole‐scavenging process (rate constant, 7×10(4) s(−1)) involving the oxidized photosensitizer and the IrO(2) catalyst within the IrO(2)⊂NS‐RuCh system is behind the improved photochemical quantum yield of IrO(2)⊂NS‐RuCh.