Electron Injection and Recombination in Fluorescein 27-Sensitized TiO2 Thin Films

Abstract

Electron injection and recombination dynamics of the dye Fluorescein 27 adsorbed to a nanocrystalline titanium dioxide (TiO2) thin film in acetonitrile (CH3CN) were studied by femtosecond pump−probe spectroscopy. After excitation of the dye at the absorption maximum, transient absorption spectra and kinetics were recorded in the spectral region between 400 and 2000 nm. It was found that most of the transient spectrum is dominated at early times by induced excited-state absorption (ESA). Even in the near-IR spectral region, where the ESA of the dye in aqueous solution is less than the noise level of our measurements, a pronounced ESA of the dye/TiO2 system has been observed. Photoproduct formation following electron injection from the dye molecule into the conduction band of the semiconductor can be resolved in spectral regions where ESA is either canceled by stimulated emission (SE) or is compensated by ground-state absorption bleach at early times. The remaining signals at these wavelengths reflect the d...