Femtosecond dynamics of interfacial and intermolecular electron transfer at eosin-sensitized metal oxide nanoparticles

Photoinduced electron injection from eosin Y into the conduction band of titanium dioxide was further scrutinized, as previous studies on the charge injection from xanthene dyes have led to diverging conclusions. Eosin-sensitized TiO2 constitutes in many aspects a model system for studying the dynamics of charge injection: Adsorption of the sensitizer onto the oxide surface through electrostatic interaction and hydrogen bonding make this system exemplary of the weak electronic coupling case. The formation of dimeric eosin on the surface of metal oxide nanoparticles in an aqueous suspension was inferred from the study of the deactivation of the dye's singlet excited state on insulating particles, such as ZrO2 and Al2O3, and of the formation of the resulting radical ion pair, using femtosecond stimulated emission and transient absorption. It was found that dimers undergo ultrafast dismutation with a time constant of 500 fs. The same process occurs also on TiO2 particles and results in a competition between interfacial and intermolecular electron transfer. Two time constants of 160 fs (58%) and 1 ps were obtained for the charge injection from the singlet excited state of eosin into this semiconductor. The presence of poly(vinyl alcohol), often used as a stabilizer for aqueous colloidal suspensions, was observed to prevent the dimer formation on the surface but also to slow the rate of electron injection by orders of magnitude. This observation is rationalized in terms of an increased distance between the semiconductor surface and the chromophore because of the adsorption of polymeric chains onto the particles and a decrease of the electronic coupling matrix element associated to the charge transfer process. These findings remove the apparent discrepancies perceived between early measurements and more recent reports and highlight the role played by the mode of adsorption of the sensitizer in controlling electron injection kinetics.