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

The excited-state dynamics of Re(CO)₃Cl(dcbpy) (dcbpy = 4,4'-dicarboxy-2,2'-bipyridine) in dimethyl formamide (DMF) solution and on the surface of ZrO₂ and TiO₂ nanocrystalline thin films were studied using femtosecond IR spectroscopy. For Re(CO)₃Cl(dcbpy) in DMF solution, a long-lived metal-to-ligand charge-transfer (MLCT) state with a lifetime of > 1 nsec was observed after 400-nm excitation of the MLCT band. The frequencies for the CO stretching bonds were blue-shifted compared with those in the ground state, which was consistent with the MLCT nature of the excited state. Rapid spectral evolution of the excited-state CO stretching bands was observed within the first 12 psec. For Re(CO)₃Cl(dcbpy) on ZrO₂ thin films, a similar MLCT state was observed. However, the spectral blue shift was much less pronounced and occurred on a faster time scale. Vibrational relaxation was suggested as the primary contribution to the spectral evolution of Re(CO)₃Cl(dcbpy) on the ZrO₂ films, whereas both vibrational relaxation and solvation of the MLCT state contributed 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. Broad mid-IR absorption of injected electrons was observed for Re(CO)₃Cl(dcbpy)-sensitized TiO₂ thin films. The rise time of the electron absorption signal in TiO₂ was < 100 fsec. The adsorbate CO stretching bands and the detailed information about the electron-injection process that can be obtained from the adsorbate vibrational spectra were also discussed.