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)(3)Cl(dcbpy) (ReCOA) and related derivatives on the surfa ce of TiO2 nanocrystalline thin films. We prepared two derivatives of ReCOA by inserting 1 and 3 CH2 spacers (ReC1A and ReC3A respectively) between th e bipyridine rings and the carboxylate anchoring groups. We directly observ ed electron injection from the excited states of Re dyes to TiO2 by simulta neously measuring the broad absorption of injected electrons and the vibrat ional spectrum of the complex in its ground, excited, and oxidized states. Electron injection from ReCOA to TiO2 was found to occur on the <100-fs tim e scale, whereas the excited state vibrational spectrum evolved with a time constant of a few picoseconds, indicating that injection occurred before c omplete vibrational relaxation in this system. We found that the electron i njection rate decreased by a factor of >200 from ReCOA to ReC1A. This decre ase is much larger than the predicted change for nonadiabatic electron tran sfer (ET) processes, in which ET rate depends exponentially on the length o f CH2 spacers. This result as well as ultrafast injection rate indicate an adiabatic electron injection process in ReCOA-sensitized TiO2. In contrast, the electron injection rate decreased by a factor of 13.7 from ReC1A to Re C3A, which agrees qualitatively with the trend predicted for nonadiabatic E T processes.