Femtosecond IR study of excited-state relaxation and electron-injection dynamics of Ru(dcbpy)2(NCS)2 in solution and on nanocrystalline TiO2 and Al2O3 thin films

The photophysics and electron injection dynamics of Ru(dcbpy)(2)(NCS)(2) [dcbpy = (4,4'-dicarboxy-2,2'-bipyridine)] (or Ru N3) in solution and adsorbed on nanocrystalline Al2O3 and TiO2 thin films were studied: by femtosecond mid-IR spectroscopy. For Ru N3 in ethanol after 400 nm excitation, the long-lived metal-to-ligand charge transfer ((MLCT)-M-3) excited state with CN stretching bands at 2040 cm(-1) was formed in less than 100 fs. No further decay of the excited-state absorption was observed within 1 ns consistent with the previously known 59 ns lifetime. For Ru N3 absorbed on Al2O3, an insulating substrate, the 3MLCT state was also formed in less than 100 fs. In contrast to Ru N3 in ethanol, this:excited state decayed by 50% within 1 ns via multiple exponential decay while no ground-state recovery was observed. This decay is attributed to electron transfer to surface states in the band gap of Al2O3 nanoparticles. For Ru N3 adsorbed onto the surface of TiO2, the transient mid-IR signal was dominated by the TR absorption of injected electrons in TiO2 in the 1700-2400 cm(-1) region. The rise time of the IR signal can be fitted by biexponential rise functions: 50 +/- 25 fs(>84%) and 1.7 +/- 0.5 ps (<16%) after deconvolution of instrument response function determined in a thin silicon wafer. Because of the scattering of the pump photon in the porous TiO2 thin film, the instrument response may be slightly lengthened, which may require a faster rise time for the first component to fit the data. The first component is assigned to the electron injection from the Ru N3 excited state to TiO2. The amplitude of the slower component appears to vary with samples ranging from ca. 16% in new samples to <5% in aged samples. The subsequent dynamics of the injected electrons have also been monitored by the decay of the IR signal. The observed 20% decay in amplitude within 1 ns was attributed to electron trapping dynamics in the thin films.