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

Abstract

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 (³MLCT) 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 IR 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.