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

Femtosecond infrared spectroscopy was used to study the excited-state dynam ics of Re(CO)(3)Cl(dcbpy) in DMF solution and on the surface of ZrO2 and Ti O2 nanocrystalline thin films. For Re(CO)(3)Cl(dcbpy) in DMF solution, we o bserved a long-lived (MLCT)-M-3 state with a lifetime of >1 ns. The frequen cies for the CO stretching bands were blue-shifted compared to those in the ground state, consistent with the metal-to-ligand charge-transfer nature o f the excited state. Rapid spectral evolution of the excited-state CO stret ching 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 spec tral blue shift was much less pronounced and occurred on a faster time scal e. We suggest that vibrational relaxation is the primary contribution to th e spectral evolution of Re(CO)(3)Cl(dcbpy) on the ZrO2 film, whereas both v ibrational relaxation and solvation of the MLCT state contribute to the spe ctral evolution in DMF solution. The excited-state decay rate of Re(CO)(3)C l(dcbpy) on ZrO2 films was faster than the rate in DMF and increased with h igher excitation power. The faster excited-state decay is attributed to the occurrence of an excited-state quenching process between neighboring excit ed 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 dis cuss the detailed information about the electron-injection process that can be obtained from the adsorbate vibrational spectra.