The role of surface states in the ultrafast photoinduced electron transfer from sensitizing dye molecules to semiconductor colloids

Investigations on the ultrafast electron injection mechanism from the dye alizarin to wide band gap semiconductor colloids in aqueous medium are presented, combined with detailed studies on population, depopulation, and relaxation phenomena in trap states and their influence on the injection process. Because of the very strong electronic coupling between dye and semiconductor in an alizarin/TiO2 system, a very fast electron injection from the excited dye to the conduction band of TiO2 is expected. Our measurements show an injection time tau(inj) < 100 fs, suggesting that the electron transfer follows an adiabatic mechanism. Furthermore, we present experiments over a wide spectral range on the recombination reaction of the electron in the conduction band of the semiconductor colloid and the dye cation to the ground state. We find highly multiphasic recombination dynamics with time constants from 400 fs to the nanosecond time scale. The nonexponential character of the recombination reaction is attributed to fast relaxation processes. The crucial contribution of surface trap states and their influence on the observed dynamics was investigated with alizarin adsorbed on the insulating substrate ZrO2. Since the conduction band edge lies far above (approximate to 1 eV) the S-1 state of alizarin, the electron injection into this band is completely suppressed. Despite this fact our spectroscopic investigations show that on ultrafast time scales the formation of an alizarin cation occurs. This observation, is explained by fast electron injection into surface trap states near the docking site on the colloid. For the alizarin/ZrO2 system the time scale for the injection into these traps is determined to be faster than 100 fs. The relaxation processes in the traps and the repopulation of the S-1 state occur within 450 fs, the subsequent ground-state relaxation takes 160 ps. The ultrafast injection dynamics into the traps, recorded for alizarin/ZrO2, underlines the importance of surface states for the initial charge separation also for systems with a lower band edge such as TiO2. We show that in the dye/ZrO2 system the process of electron injection is not suppressed but "stopped" after the ultrafast transition into trap states. It is therefore a valuable system for probing the electron dynamics in surface states.