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 RuIII/II redox chemistry was ...