Excited State Electron Transfer from Ru(II) Polypyridyl Complexes Anchored to Nanocrystalline TiO2 through Rigid-Rod Linkers

Abstract

Rigid-rod linkers varying in length were used to bind Ru(II) polypyridyl complexes to the surface of TiO2 (anatase) and of ZrO2 nanoparticle thin films. The linkers were made of p-phenyleneethynylene (Ph-E)n bridges carrying two COOR anchoring groups at the end and were capped with Ru(II) polypyridyl complexes as the sensitizing chromophores. Two series of rigid-rod sensitizers were prepared: Ru complexes having bpy or 4,4‘-(Cl)2-bpy as the ancillary ligands. In the first series, the excited state was localized on the rigid-rod linker, in the second series, the excited states were localized on the 4,4‘-(Cl)2-bpy ligands. The rigid-rod sensitizers with Ru(bpy)2 complexes did bind strongly (Kad ∼ 105 M-1) with high surface coverages (∼10-8 mol/cm2) on the nanostructured metal oxide films, ZrO2 and anatase TiO2. The length of the fully conjugated rigid-rod linker influences the photophysical properties of the sensitizer, and nanosecond transient absorption measurements indicated long-lived metal-to-ligand charge-transfer (MLCT) excited states (∼2 μs) with evidence for delocalization onto the rigid-rod linker. The interfacial electron transfer behavior on TiO2 was found to be dependent on the Brønsted acidity or basicity of the surface. On base pretreated TiO2, the excited state electron injection yields were low and could be increased by addition of LiClO4 to an external CH3CN solution. Under these conditions, a fraction of the injection process could be time resolved on a 10 ns time scale. On acidic TiO2, ultrafast excited state electron injection was observed for both series. Recombination was found to be second order with average rate constants independent of which rigid-rod sensitizer was excited. For rigid-rod sensitizers with Ru(4,4‘-(Cl)2-bpy)2, there was evidence for a direct interaction between the 4,4‘-(Cl)2-bpy ligands and the TiO2 surface. Photophysical and interfacial electron transfer properties of these Cl-substituted complexes were nearly independent of the rigid-rod length.