Emission from the charge transfer state of xanthene dye-sensitized TiO2 nanoparticles:: A new approach to determining back electron transfer rate and verifying the marcus inverted regime

Electron injection and back electron transfer dynamics of xanthene dyes adsorbed on TiO2 nanoparticles have been studied by picosecond transient absorption and time-resolved fluorescence spectroscopy. When the xanthene dyes are adsorbed on the TiO2 surface, a good fraction of the dye molecules forms charge transfer (CT) complex with the TiO2 nanoparticle. On excitation of the above system, electron transfer from dye molecule to nanoparticle takes place. Electron injection has been observed by direct detection of electron in the conduction band of nanoparticle and bleach of the dye as detected by picosecond transient absorption spectroscopy. The corresponding dynamics have been determined by monitoring the recovery kinetics of the bleach of the dye in the visible region. Electron injection in the above systems can take place in two different ways: (1) through the excited state of the dye and (2) through direct injection to the conduction band on excitation of the charge transfer complex. For the charge transfer complex, when the recombination reaction takes place, charge transfer (CT) emission has been observed, Monitoring the CT emission, we have determined the back ET rate. We have also found that the back ET rate for the xanthene dye-sensitized TiO2 CT complex decreases as the relative driving force increases. Assuming a negligible change in electronic coupling, our results provide the evidence for the Marcus inverted region kinetic behavior for an interfacial ET process.