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

Investigations on the ultrafast electron injection mechanism from the dye a lizarin to wide band gap semiconductor colloids in aqueous medium are prese nted, combined with detailed studies on population, depopulation, and relax ation phenomena in trap states and their influence on the injection process . Because of the very strong electronic coupling between dye and semiconduc tor in an alizarin/TiO2 system, a very fast electron injection from the exc ited 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 co nduction band of the semiconductor colloid and the dye cation to the ground state. We find highly multiphasic recombination dynamics with time constan ts from 400 fs to the nanosecond time scale. The nonexponential character o f the recombination reaction is attributed to fast relaxation processes. Th e crucial contribution of surface trap states and their influence on the ob served dynamics was investigated with alizarin adsorbed on the insulating s ubstrate ZrO2. Since the conduction band edge lies far above (approximate t o 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 occu rs. This observation, is explained by fast electron injection into surface trap states near the docking site on the colloid. For the alizarin/ZrO2 sys tem the time scale for the injection into these traps is determined to be f aster than 100 fs. The relaxation processes in the traps and the repopulati on of the S-1 state occur within 450 fs, the subsequent ground-state relaxa tion takes 160 ps. The ultrafast injection dynamics into the traps, recorde d for alizarin/ZrO2, underlines the importance of surface states for the in itial charge separation also for systems with a lower band edge such as TiO 2. We show that in the dye/ZrO2 system the process of electron injection is not suppressed but "stopped" after the ultrafast transition into trap stat es. It is therefore a valuable system for probing the electron dynamics in surface states.