Remote and Adjacent Excited-State Electron Transfer at TiO₂ Interfaces Sensitized to Visible Light with Ru(II) Compounds

The ruthenium polypyridyl compounds, Ru(dpp)2(deeb)(PF6)2 (Ru-deeb) and cis-Ru(dpp)2(eina)2(PF6)2 (Ru-eina), where dpp is 4,7-diphenyl-1,10-phenanthroline, deeb is 4,4'-diethyl ester-2,2'-bipyridine, and eina is 4-ethyl ester pyridine, have been prepared and characterized to sensitize nanocrystalline TiO2 (anatase) thin films. In neat acetonitrile at room temperature, Ru-deeb was emissive with lambdaem=675 nm, tau=780 ns, and emission quantum yield phiem=0.067, whereas Ru-eina was nonemissive with tau<10 ns. The short lifetime and observed photochemistry for Ru-eina are consistent with the presence of low-lying ligand-field (LF) excited states. The metal-to-ligand charge transfer (MLCT) excited states of Ru-deeb were found to be localized on the surface-bound deeb ligand, and on the remote dpp ligand for Ru-eina. Interfacial proton concentration was employed to tune the relative sensitizer-semiconductor energetics. Injection quantum yields, phiinj, varied from approximately 0.2 at pH=5 to approximately 1 at pH=1, with a slope of approximately 0.15/pH for both compounds. At pH=12, long-lived excited states were observed with phiinj<0.05. At pH<or=2, phiinj became temperature-dependent for Ru-eina, but not for Ru-deeb. A mechanism is proposed wherein population of LF states at elevated temperatures lowers phiinj.