Real-Time Observation of Photoinduced Adiabatic Electron Transfer in Strongly Coupled Dye/Semiconductor Colloidal Systems with a 6 fs Time Constant

Electron transfer from organic dye molecules to semiconductor−colloidal systems is among the fastest reported charge-separation reactions. We present investigations on alizarin complexing the surface of TiO2 semiconductor colloids in solution. Because of the very strong electronic coupling between the sensitizer and the semiconductor in the alizarin/TiO2 system, very fast electron injection from the photoexcited dye to the conduction band of TiO2 occurs. The real-time observation of the injection process is achieved by transient absorption spectroscopy using a 19-fs excitation pulse provided by a pump pulse from a noncollinear optical parametric amplifier and a probe pulse from a quasi-chirp-free supercontinuum. An injection time τinj of 6 fs can be unambiguously derived in three different ways from the experimental data:  (i) analysis of individual transients at spectral positions without contributions from subsequent reactions (relaxation, recombination); (ii) global fitting procedure for 31 wavelengths over a wide spectral range; and (iii) calculation of the S* state and comparison to the “nonreactive” system alizarin/ZrO2. The spectral signature of the 6-fs kinetic component can be assigned to electron transfer from the excited dye molecule to the TiO2 colloid. Even for this strongly coupled system, we propose a localized excitation with a subsequent adiabatic electron transfer reaction that is, to our knowledge, the fastest electron-transfer reaction that has been directly measured by transient spectroscopy.