Ultrafast excited-state dynamics of Re(CO)3Cl(dcbpy) in solution and on nanocrystalline TiO2 and ZrO2 thin films

Femtosecond infrared spectroscopy was used to study the excited-state dynamics of Re(CO)(3)Cl(dcbpy) in DMF solution and on the surface of ZrO2 and TiO2 nanocrystalline thin films. For Re(CO)(3)Cl(dcbpy) in DMF solution, we observed a long-lived (MLCT)-M-3 state with a lifetime of >1 ns. The frequencies for the CO stretching bands were blue-shifted compared to those in the ground state, consistent with the metal-to-ligand charge-transfer nature of the excited state. Rapid spectral evolution of the excited-state CO stretching bands was observed within the first 12 ps. For Re(CO)(3)Cl(dcbpy) on ZrO2 thin films, a similar (MLCT)-M-3 state was observed. However, the spectral blue shift was much less pronounced and occurred on a faster time scale. We suggest that vibrational relaxation is the primary contribution to the spectral evolution of Re(CO)(3)Cl(dcbpy) on the ZrO2 film, whereas both vibrational relaxation and solvation of the MLCT state contribute to the spectral evolution in DMF solution. The excited-state decay rate of Re(CO)(3)Cl(dcbpy) on ZrO2 films was faster than the rate in DMF and increased with higher excitation power. The faster excited-state decay is attributed to the occurrence of an excited-state quenching process between neighboring excited molecules on the film. For Re(CO)(3)Cl(dcbpy)-sensitized TiO2 thin films, broad mid-IR absorption of injected electrons was observed. The rise time of the electron absorption signal in TiO2 was found to be less than 100 fs. In addition, the adsorbate CO stretching bands were also observed, We discuss the detailed information about the electron-injection process that can be obtained from the adsorbate vibrational spectra.