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 TiO 2 thin films where the ligand L = 4,4′-(CH 3 ) 2 -bpy (dmb), 4,4′-(C(CH 3 ) 3 ) 2 -bpy (dtb), and 4,4′-(CF 3 ) 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° 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 k cr 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.