title: Varying the electronic structure of surface-bound ruthenium(II) polypyridyl complexes.
abstract: In the design of light-harvesting chromophores for use in dye-sensitized photoelectrosynthesis cells (DSPECs), surface binding to metal oxides in aqueous solutions is often inhibited by synthetic difficulties. We report here a systematic synthesis approach for preparing a family of Ru(II) polypyridyl complexes of the type [Ru(4,4'-R2-bpy)2(4,4'-(PO3H2)2-bpy)](2+) (4,4'(PO3H2)2-bpy = [2,2'-bipyridine]-4,4'-diylbis(phosphonic acid); 4,4'-R2-bpy = 4,4'-R2-2,2'-bipyridine; and R = OCH3, CH3, H, or Br). In this series, the nature of the 4,4'-R2-bpy ligand is modified through the incorporation of electron-donating (R = OCH3 or CH3) or electron-withdrawing (R = Br) functionalities to tune redox potentials and excited-state energies. Electrochemical measurements show that the ground-state potentials, E(o')(Ru(3+/2+)), vary from 1.08 to 1.45 V (vs NHE) when the complexes are immobilized on TiO2 electrodes in aqueous HClO4 (0.1 M) as a result of increased Ru dπ-π* back-bonding caused by the lowering of the π* orbitals on the 4,4'-R2-bpy ligand. The same ligand variations cause a negligible shift in the metal-to-ligand charge-transfer absorption energies. Emission energies decrease from λmax = 644 to 708 nm across the series. Excited-state redox potentials are derived from single-mode Franck-Condon analyses of room-temperature emission spectra and are discussed in the context of DSPEC applications.
authors.affiliation: Department of Chemistry, University of North Carolina at Chapel Hill , CB 3290, Chapel Hill, North Carolina 27599, United States.
authors.forename: Dennis L
authors.initials: DL
authors.lastname: Ashford
authors.forename: M Kyle
authors.initials: MK
authors.lastname: Brennaman
authors.forename: Robert J
authors.initials: RJ
authors.lastname: Brown
authors.forename: Shahar
authors.initials: S
authors.lastname: Keinan
authors.forename: Javier J
authors.initials: JJ
authors.lastname: Concepcion
authors.forename: John M
authors.initials: JM
authors.lastname: Papanikolas
authors.forename: Joseph L
authors.initials: JL
authors.lastname: Templeton
authors.forename: Thomas J
authors.initials: TJ
authors.lastname: Meyer