Ru(II)-Bpy complexes bound to nanocrystalline TiO2 films through phenyleneethynylene (OPE) linkers:: Effect of the linkers length on electron injection rates

Three rigid-rod Ru-bpy sensitizers with linkers made of an oligo(phenyleneethynylene) (OPE) bridge varying in length (9.6-23.3 angstrom) and ending with an isophthalic acid anchroing unit were bound to TiO2, (anatase) nanoparticle films, and the photoinduced electron injection rates for all three compounds were studied by femtosecond laser spectroscopy. The sensitized films were excited at 530 nm, in the MLCT region, and the injection dynamics was probed in the 600-1000 nm range. The excited state for all three compounds (tau-2 mu s in acetonitrile solution) was readily quenched upon binding to TiO2. All injection traces could be fitted with bi-exponential decays on a measured timescale of 130 ps. Plots of the fast or slow component of the electron injection time constants probed at 670 nm vs the spacer length yielded a very low value for the attenuation factor (beta similar to 0.04 angstrom(-1)). Although the dependence was weak, the length of the linker clearly resulted in slower injection rates. The weak and monotonic linker length dependence had not been observed with other rigid-rod models, demonstrating the importance of anchoring groups in the design of model compounds for electron transfer studies at the semiconductor interfaces.