Kinetic Resolution of Charge Recombination and Electric Fields at the Sensitized TiO2 Interface

The sensitizer [Ru(dtb)₂(dcb)]²⁺, where dtb is 4,4′-di-tert-butyl-2,2′-bipyridine and dcb is 4,4′-dicarboxylic acid-2,2′-bipyridine, was anchored to mesoporous TiO₂ thin films and characterized by visible spectroscopy in 0.1 M Na⁺, Li⁺, Mg²⁺, and Ca²⁺ perchlorate acetonitrile solutions on nanosecond and longer time scales. Relative to neat acetonitrile, the presence of these electrolyte cations induced a red shift in the metal-to-ligand charge transfer (MLCT) absorption of Ru(dtb)₂(dcb)/TiO₂. The magnitude of the shift increased with increasing valence of the metal cation. Pulsed 532 nm light excitation of Ru(dtb)₂(dcb)/TiO₂ resulted in the appearance of a long-lived bleach that returned to pre-excitation values on an approximately millisecond time scale under all conditions studied. Global analysis, spectral modeling, and single wavelength kinetic analysis revealed that two dynamic processes were operative: (1) charge recombination, Ruᴵᴵᴵ(dtb)₂(dcb)/TiO₂(e–) → Ruᴵᴵ(dtb)₂(dcb)/TiO₂, and (2) an electric field created by the injected electron. These two distinct nonexponential processes were observed in the same spectral region and on similar time scales. The ability of global analysis, specifically the decay-associated spectra, to kinetically and spectrally resolve these two processes was assessed. Single wavelength kinetic measurements and spectral modeling provided quantitatively different rate constants, but both led to the surprising conclusion that there was no evidence for charge screening of the electric field by cations present in the electrolyte. The decay of the electric field was cation independent, behavior very different from that previously reported in the presence of redox mediators. The charge recombination kinetics revealed a small yet measurable dependence on the nature of the cation present in the electrolyte with the divalent cations inducing the fastest recombination.