title: Photophysical characterization of a chromophore/water oxidation catalyst containing a layer-by-layer assembly on nanocrystalline TiO2 using ultrafast spectroscopy.
abstract: Femtosecond transient absorption spectroscopy is used to characterize the first photoactivation step in a chromophore/water oxidation catalyst assembly formed through a "layer-by-layer" approach. Assemblies incorporating both chromophores and catalysts are central to the function of dye-sensitized photoelectrosynthesis cells (DSPECs) for generating solar fuels. The chromophore, [Rua(II)](2+) = [Ru(pbpy)2(bpy)](2+), and water oxidation catalyst, [Rub(II)-OH2](2+) = [Ru(4,4'-(CH2PO3H2)2bpy)(Mebimpy)(H2O)](2+), where bpy = 2,2'-bipyridine, pbpy = 4,4'-(PO3H2)2bpy, and Mebimpy = 2,6-bis(1-methylbenzimidazol-2-yl)pyridine), are arranged on nanocrystalline TiO2 via phosphonate-Zr(IV) coordination linkages. Analysis of the transient spectra of the assembly (denoted TiO2-[Rua(II)-Zr-Rub(II)-OH2](4+)) reveal that photoexcitation initiates electron injection, which is then followed by the transfer of the oxidative equivalent from the chromophore to the catalyst with a rate of kET = 5.9 × 10(9) s(-1) (τ = 170 ps). While the assembly, TiO2-[Rua(II)-Zr-Rub(II)-OH2](4+), has a near-unit efficiency for transfer of the oxidative equivalent to the catalyst, the overall efficiency of the system is only 43% due to nonproductive photoexcitation of the catalyst and nonunit efficiency for electron injection. The modular nature of the layer-by-layer system allows for variation of the light-harvesting chromophore and water oxidation catalyst for future studies to increase the overall efficiency.
authors.affiliation: Department of Chemistry, University of North Carolina , CB 3290, Chapel Hill, North Carolina 27599, United States.
authors.forename: Stephanie E
authors.initials: SE
authors.lastname: Bettis
authors.forename: Kenneth
authors.initials: K
authors.lastname: Hanson
authors.forename: Li
authors.initials: L
authors.lastname: Wang
authors.forename: Melissa K
authors.initials: MK
authors.lastname: Gish
authors.forename: Javier J
authors.initials: JJ
authors.lastname: Concepcion
authors.forename: Zhen
authors.initials: Z
authors.lastname: Fang
authors.forename: Thomas J
authors.initials: TJ
authors.lastname: Meyer
authors.forename: John M
authors.initials: JM
authors.lastname: Papanikolas