Varying the Electronic Structure of Surface-bound Ruthenium(II) Polypyridyl Complexes

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.