Electric Fields and Charge Screening in Dye Sensitized Mesoporous Nanocrystalline TiO₂ Thin Films

The photophysical and electron transfer properties of mesoporous nanocrystalline (anatase) TiO₂ thin films sensitized to visible light with [Ru­(dtb)₂(dcb)]­(PF₆)₂, where dtb is 4,4′-(tert-butyl)₂-2,2′-bipyridine and dcb is 4,4′-(CO₂H)₂-2,2′-bipyridine, were quantified in acetonitrile solutions that contained 100 mM concentrations of Li⁺, Na⁺, Mg²⁺, or Ca²⁺ perchlorate salts. The presence of these salts resulted in a dramatic and cation dependent bathochromic (red) shift of the metal-to-ligand charge transfer (MLCT) absorption and photoluminescence (PL) spectra of Ru­(dtb)₂(dcb)/TiO₂ relative to the value measured in neat or 100 mM TBAClO₄, where TBA is tetrabutyl ammonium cation, acetonitrile solutions. The magnitude of the shifts followed the trend: Na⁺ < Li⁺ < Ca²⁺ < Mg²⁺. The PL intensity was also found to decrease in this same order and comparative actinometry studies showed that this was due to MLCT excited state electron transfer quenching by the TiO₂ acceptor states. The Ru^III/II redox chemistry was found to be non-Nernstian; the ideality factors were cation-dependent, suggestive of an underlying electric field effect. Electrochemical reduction of the TiO₂ resulted in a black coloration and a blue shift of the fundamental (VB → CB) absorption, the normalized spectra were cation independent. Reduction of sensitized TiO₂ also resulted in a blue shift of the MLCT absorption, the magnitude of which was used to determine the surface electric fields. Under conditions where about 20 electrons were present in each anatase nanocrystallite, the electric field strength reported by the Ru compound followed the trend Na⁺ < Li⁺ < Mg²⁺ < Ca²⁺, with Na⁺ being 1.1 MV/cm and Ca²⁺ 2.3 MV/cm. In pulsed laser experiments, the first-derivative absorption signature was observed transiently after excited state injection and iodide oxidation. These absorption amplitudes were time-dependent and decayed over time periods where the number of injected electrons was constant, with behavior attributed to screening of the surface electric field by cations present in the electrolyte. The monovalent cations screened charge much more rapidly than did the dications, k_Li⁺,Na⁺ = 5.0 × 10⁴ s^–1 and k_{Mg²⁺,Ca²⁺} = 5.0 × 10² s^–1, presumably because the small number of injected electrons resulted in spatially isolated singly reduced Ti­(III) sites that were more easily screened by the monocations.