title: Accumulation of multiple oxidative equivalents at a single site by cross-surface electron transfer on TiO2.
abstract: The photodriven accumulation of two oxidative equivalents at a single site was investigated on TiO2 coloaded with a ruthenium polypyridyl chromophore [Ru(bpy)2((4,4'-(OH)2PO)2bpy)](2+) (Ru(II)P(2+), bpy = 2,2'-bipyridine, ((OH)2PO)2-bpy = 2,2'-bipyridine-4,4'-diyldiphosphonic acid) and a water oxidation catalyst [Ru(Mebimpy) ((4,4'-(OH)2PO-CH2)2bpy)(OH2)](2+) (Ru(II)OH2(2+), Mebimpy = 2,6-bis(1-methylbenzimidazol-2-yl)pyridine, (4,4'-(OH)2PO-CH2)2bpy) = 4,4'-bis-methlylenephosphonato-2,2'-bipyridine). Electron injection from the metal-to-ligand charge transfer (MLCT) excited state of -Ru(II)P(2+) (-Ru(II)P(2+)*) to give -Ru(III)P(3+) and TiO2(e(-)) was followed by rapid (<20 ns) nearest-neighbor -Ru(II)OH2(2+) to -Ru(III)P(3+) electron transfer. On surfaces containing both -Ru(II)P(2+) and -Ru(III)OH2(3+) (or -Ru(III)OH(2+)), -Ru(II)OH2(2+) was formed by random migration of the injected electron inside the TiO2 nanoparticle and recombination with the preoxidized catalyst, followed by relatively slow (μs-ms) non-nearest neighbor cross-surface electron transfer from -Ru(II)OH2(2+) to -Ru(III)P(3+). Steady state illumination of coloaded TiO2 photoanodes in a dye sensitized photoelectrosynthesis cell (DSPEC) configuration resulted in the buildup of -Ru(III)P(3+), -Ru(III)OH(2+), and -Ru(IV)═O(2+), with -Ru(IV)═O(2+) formation favored at high chromophore to catalyst ratios.
authors.affiliation: Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-3290, USA.
authors.forename: Wenjing
authors.initials: W
authors.lastname: Song
authors.forename: Akitaka
authors.initials: A
authors.lastname: Ito
authors.forename: Robert A
authors.initials: RA
authors.lastname: Binstead
authors.forename: Kenneth
authors.initials: K
authors.lastname: Hanson
authors.forename: Hanlin
authors.initials: H
authors.lastname: Luo
authors.forename: M Kyle
authors.initials: MK
authors.lastname: Brennaman
authors.forename: Javier J
authors.initials: JJ
authors.lastname: Concepcion
authors.forename: Thomas J
authors.initials: TJ
authors.lastname: Meyer
mesh_terms: D015082:2,2'-Dipyridyl / Q000737:chemistry*; D004579:Electron Transport; D053758:Nanoparticles / Q000737:chemistry; D010084:Oxidation-Reduction; D055668:Photochemical Processes; D012428:Ruthenium / Q000737:chemistry*; D013499:Surface Properties; D014025:Titanium / Q000737:chemistry*
chemical_list: C009495:titanium dioxide; D015082:2,2'-Dipyridyl; D012428:Ruthenium; D014025:Titanium