Journal of Physical Chemistry B, Vol.105, No.5, 967-974, 2001
Electron injection and recombination in fluorescein 27-sensitized TiO2 thin films
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 dynamics of photoproducts, which are conduction-band electrons in TiO2 and oxidized Fluorescein 27 dye molecules. The kinetics of SE decay and the rise times of induced absorption of photoproducts ale ultrafast and nonexponential, requiring at least three time constants ranging from < 100 fs to 8 ps. The depopulation of the excited state monitored by SE decay corresponds very well to the formation of the photoproducts. Recombination is also nonexponential, part of it happens in tens of picoseconds, but the major part does not occur on the investigated time scale (until 500 ps). The possibility of resolving the dynamics of both precursor and product species in Fluorescein 27-TiO2 nanoparticle thin films makes it suitable for spectroscopicaly studying interfacial electron transfer as a function of system parameters.