Sensitization processes resulting from the photoexcitation of chromophores (sensitizers) bound to semiconductor nanoparticles are of great importance for solar energy conversion. 1 Examples are TiO 2 solar cells, where the sensitizer is usually a Ru II -polypyridyl complex substituted with COOR binding groups, such as Ru(bpy) 2 -(dme) 2+ (1). 2 To study the dynamics of long-range interfacial charge-transfer processes in sensitizer/semiconductor nanoparticle systems, 3 we prepared a series of rigid sensitizers made of a tripod-shaped base terminated with a Ru II complex and attached them to the surface of mesoporous nanocrystalline (anatase) TiO 2 thin films through three COOR groups. 4 The three-point attachment and the rigidity of these molecules allow the control of the distance and orientation of the sensitizer with respect to the nanoparticle surface. Earlier nanosecond experiments with the tripod/TiO 2 systems demonstrated that the excited-state electron injection is quantitative and occurs well within the duration of the laser pulse. 4a,b In this communication, we report the observation of ultrafast charge injection from the longest tripods prepared in these laboratories, 2 and 3 shown in Figure 1, 5 in which the Ru-to-footprint distance is 24 Å. 6 To our knowledge, this is the first example of subpicosecond photoinduced ET in sensitized TiO 2 occurring over such extended distances.Interfacial charge injection was investigated in 2 and 3 ( Figure 1) bound to TiO 2 mesoporous films, with 1 serving as the reference. The structures of 2 and 3 are identical except for the ligand connecting the Ru II center to the tripod (phenanthroline in 2 and bipyridine in 3). The thin films were cast on microscope cover glasses and treated with solutions of 1, 2, or 3, following previously described procedures. 4a The resulting 1/TiO 2 , 2/TiO 2 , and 3/TiO 2 systems were excited at 405 nm (SH) and probed with the white light continuum generated by the fundamental output of a homebuilt 1 kHz multipass Ti:sapphire amplifier. 7 The typical pulse length was 110 fs, and the beams were mildly focused to ∼0.5-1.0 mm diameter at the sample. The injection dynamics was probed over a broad range of wavelengths. The analysis focused on the 750-1100 nm spectral region, where only the MLCT excited state of the Ru II chromophore and the electron injected into the conduction band of TiO 2 , e -TiO2 , exhibit appreciable absorption 8,9 and no stimulated emission is observed.For the reference system 1/TiO 2 the electron injection occurred within the response time of our instrument, k inj > 1 × 10 13 s -1 , and only the long-lived absorption of e -TiO2 was detected ( Figure 2a, lowest trace). This is fully consistent with the reports of other groups, which place the injection rate of directly bound Ru II -polypyridyl sensitizers in the sub-100 fs range. 10 Recent stateof-the-art measurements on related sensitizers show that this rate is ∼20-30 fs, 11 i.e., much faster than the duration of our excitation and probe pulses. Upon excitation...

The inter- and intra-molecular dynamics of aqueous dimethyl sulfoxide (DMSO) solutions have been measured using optical heterodyne-detected Raman-induced Kerr effect spectroscopy. Solutions were studied over the entire range of composition at 294 K. The Kerr transients characterize both the underdamped inter- and intra-molecular vibrational motions, as well as the overdamped, diffusive orientational motions. The longer diffusive relaxation time constant τ2 is assigned to DMSO reorientation, and varies strongly with mole fraction of DMSO. The shorter time constant τ1 is assigned to water reorientation, and the value of 1.0 ps is nearly invariant across the range of solution composition. The solutions deviate substantially from hydrodynamic scaling behavior, since the ratio of DMSO reorientation time constant normalized by shear viscosity τ2/η is not a linear function of mole fraction. The peak frequencies for three of five low frequency intramolecular vibrations decrease with increasing water content. Both anisotropic and isotropic Kerr transients are recorded. The isotropic Kerr transient is dominated by the partially depolarized ν10 symmetric C–S–C stretching vibration at 670 cm−1. Electronic structure calculations for isolated DMSO, perdeuterated DMSO (d6-DMSO), and water are carried out using semi-empirical (PM3), density functional (B3LYP), and MP2 methods. Basis sets up to 6-311++G(d,p) are used and vibrational frequencies are calculated within the harmonic approximation. An unusual hemispherical shape is obtained for the electrostatic potential (ESP) about the DMSO oxygen. This hemispherical ESP was consistently observed for all levels of electronic structure theory and all basis sets that were used. Calculated polarizabilities permit us to estimate the anisotropic Kerr signal for an isolated DMSO molecule to be 79 times greater than for an isolated water molecule.

A theoretical model of ultrafast dynamics in the optical Kerr effect in molecular liquids is developed. It assumes that for short times there exist dynamic quasicrystalline structures including a central molecule and its nearest neighbors. The interaction of such structures with a femtosecond laser pulse leads to excitation of vibrational modes (local phonons) which are responsible for subpicosecond nonlinear polarizability of the liquid. The results of numerical calculations performed for benzene show a very good agreement with the experimental data. The lifetime of the dynamic quasicrystalline structures in benzene, at room temperature, is estimated to be about 200 fs.

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