Electron injection, charge recombination, and energy migration in surface-modified TiO2 nanocrystallite layers. A laser photolysis study

Ru(bpy)(3)(2+) (bpy = bipyridine) and Ru(o-phen)(3)(2+) (o-phen = o-phenanthroline) have been adsorbed to a membrane layer made of TiO2 nanocrystallites from aqueous solutions at pH 2.5 by means of pretreatment of the surface with Nafion, sodium dodecyl sulfate (SDS) or sodium dodecyl benzyl sulfate (SDBS), Pulsed laser-induced emission and absorbance changes have been studied. The time profiles provide in formation concerning environmental effects (charge and hydrophobic interactions) on the rates and yields of electron injection from the excited dyes to the TiO2 membrane and subsequent electron recapture by Ru(III). The differences between the rates of electron injection and recapture, the multiexponential nature of these reactions, and the differences between specific photosensitizers and binders are discussed in light of the semiconducting properties of the TiO2 nanocrystallites and the hydrophobic and ionic interactions between the photosensitizers and the binders. Oxidation of iodide ions by Ru(III) was also studied. iodide ions react efficiently with Ru(III) despite the negative charge of the binders, indicating that most of the charge is neutralized by the surface charge of TiO2 and by the Ru(III) ions. Quantum yields for net electron injection were determined from the initial (extrapolated) bleaching. In most systems observed bleaching corresponds to 30-80% of the absorbed photons. At relatively high laser pulse intensities, emission measurements show that bimolecular (and apparent higher order) processes take place, involving fast tripler annihilation, A detailed mechanism provides quantitative kinetic treatment of the data. Comparison of results in dry and wet layers indicates that energy migration is responsible for the enhanced triplet annihilation.