Distance Dependent Electron Transfer at TiO₂ Interfaces Sensitized with Phenylene Ethynylene Bridged Ru^II–Isothiocyanate Compounds

Excess electrons present in semiconductor nanocrystallites generate a significant electric field, yet the role this field plays in molecular charge transfer processes remains poorly understood. Three ruthenium bipyridyl cis-Ru­(bpy)­(LL)­(NCS)₂ compounds, where LL is a 4-substituted bpy, with zero, one, or two phenylene ethynylene bridge units, were anchored to mesoporous nanocrystalline TiO₂ thin films to specifically quantify interfacial charge transfer with chromophores designed to be set at variable distances from the surface. Injection of electrons into TiO₂ resulted in a blue shift of the metal-to-ligand charge transfer absorption consistent with an underlying Stark effect. The electroabsorption data were used to quantify the electric field experienced by the compounds that decreased from 0.85 to 0.22 MV/cm as the number of OPE spacers increased from 0 to 2. Charge recombination on the 10^–8–10^–5 s time scale correlated with the magnitude of the electric field with an apparent attenuation factor β = 0.12 Å^–1. Slow components to charge recombination observed on the 10^–4–10^–1 s time scale that were unaffected by temperature, irradiance, or the bridge units present on the molecular sensitizer were attributed to electron tunneling between TiO₂ acceptor states. The photocurrent efficiencies of solar cells based on these compounds decreased markedly when the bridge units were present on the sensitizer. Iodine was found to form adducts with all three compounds, K = 1.8 ± 0.2 × 10⁴ M^–1, but only significantly lowered the excited state injection yield for those that possessed the bridge units.