Activation Energies for Electron Transfer from TiO2 to Oxidized Dyes: A Surface Coverage Dependence Correlated with Lateral Hole Hopping

Three ruthenium(II) sensitizers, [Ru(L)(2)(dcb)](2+), were anchored to mesoporous TiO2 thin films where the ligand L = 4,4'-(CH3)(2)-bpy (dmb), 4,4'-(C(CH3)(3))(2)-bpy (dtb), and 4,4'-(CF3)(2)-bpy (bpyCF(3)) controls the thermodynamics and electronic coupling for self-exchange intermolecular Ru-III/II "hole hopping". Apparent electron difussion coefficients, D-app, were reported to increase in the order bpyCF(3) << dtb < dmb. Nanosecond transient absorption measurements made over an 80 degrees temperature range were conducted to abstract average charge recombination rate constants, k(cr), under conditions of sub-percolation and saturated sensitizer surface coverages. For sensitizers [Ru(dmb)(2)(dcb)](2+) and [Ru(dtb)(2)(dcb)](2+), the kcr values at saturation coverages were significantly larger than those at low coverages, by a degree that followed the trend in D-app. The inability of [Ru(bpyCF(3))(2)(dcb)](2+) to introduce hole transport was afirmed by recombination kinetic data that were insensitive to the sensitizer surface converage. An Arrhenius analysis indicated that lateral Ru-III/II hole hopping decreased the barrier for electron transfer that ultimately led to faster recombination rates.