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)₃Cl(dcbpy) in DMF solution and on the surface of ZrO₂ and TiO₂ nanocrystalline thin films. For Re(CO)₃Cl(dcbpy) in DMF solution, we observed a long-lived ³MLCT 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)₃Cl(dcbpy) on ZrO₂ thin films, a similar ³MLCT 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)₃Cl-(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)₃Cl(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)₃Cl(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.