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 TiO 2 semiconductor colloids in solution. Because of the very strong electronic coupling between the sensitizer and the semiconductor in the alizarin/TiO 2 system, very fast electron injection from the photoexcited dye to the conduction band of TiO 2 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 τ inj 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/ZrO 2 . The spectral signature of the 6-fs kinetic component can be assigned to electron transfer from the excited dye molecule to the TiO 2 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.