Bridge Length-Dependent Ultrafast Electron Transfer from Re Polypyridyl Complexes to Nanocrystalline TiO₂ Thin Films Studied by Femtosecond Infrared Spectroscopy

Femtosecond infrared spectroscopy was used to study the electron injection dynamics of Re(CO)₃CI(dcbpy) (ReC0A) and related derivatives on the surface of TiO₂ nanocrystalline thin films. We prepared two derivatives of ReC0A by inserting 1 and 3 CH₂ spacers (ReClA and ReC3A respectively) between the bipyridine rings and the carboxylate anchoring groups. We directly observed electron injection from the excited states of Re dyes to TiO₂ by simultaneously measuring the broad absorption of injected electrons and the vibrational spectrum of the complex in its ground, excited, and oxidized states. Electron injection from ReC0A to TiO₂ was found to occur on the <100-fs time scale, whereas the excited state vibrational spectrum evolved with a time constant of a few picoseconds, indicating that injection occurred before complete vibrational relaxation in this system. We found that the electron injection rate decreased by a factor of >200 from ReCOA to ReClA. This decrease is much larger than the predicted change for nonadiabatic electron transfer (ET) processes, in which ET rate depends exponentially on the length of CH₂ spacers. This result as well as ultrafast injection rate indicate an adiabatic electron injection process in ReC0A-sensitized TiO₂. In contrast, the electron injection rate decreased by a factor of 13.7 from ReClA to ReCSA, which agrees qualitatively with the trend predicted for nonadiabatic ET processes.