Optical Kerr-effect investigation of the reorientational dynamics of CS2 in CCl4 solutions

Femtosecond optical Kerr effect setup with signal "live view" for measurements in the solid, liquid, and gas phases

We present the experimental setup constructed in our laboratory for measurement of the femtosecond optical Kerr effect. The setup allows measurements with high temporal resolution and acquisition speed. The high signal to noise ratio is obtained with use of a homemade balanced detector. Due to the high acquisition speed and good signal to noise ratio, it is possible to have a "live view" of the signal and to easily tune the sample position and orientation before the measurement. We show the example results obtained in the solid, liquid, and the gas phases and we use them in order to check on the precision of our setup. As the samples we have used a YAG crystal, liquid acetone, and atmospheric air. In the latter two cases, a good agreement with the literature data has been found. The measurements in the gas phase confirm that our setup, although utilizing low energy pulses from the sapphire oscillator, is able to acquire high quality rotational signal in a low density sample.

Stokes-Einstein-Debye failure in molecular orientational diffusion: exception or rule?

The Stokes-Einstein-Debye (SED) expression is used routinely to relate orientational molecular diffusivity quantitatively to viscosity. However, it is well-known that Einstein's equations are derived from hydrodynamic theory for the diffusion of a Brownian particle in a homogeneous fluid and examples of SED breakdown and failure for molecular diffusion are not unusual. Here, using optical Kerr-effect spectroscopy to measure orientational diffusion for solutions of guanidine hydrochloride in water and mixtures of carbon disulfide with hexadecane, we show that these two contrasting systems each show pronounced exception to the SED relation and ask if it is reasonable to expect molecular diffusion to be a simple function of viscosity.

Efficient photochromic transformation of a new fluorenyl diarylethene: one- and two-photon absorption spectroscopy

Efficient reversible phototransformation of a new diarylethene-fluorene derivative, 1,2-bis(5-(9,9-didecyl-7-nitro-9H-fluoren-2-yl)-2-methylthiophen-3-yl)cyclopent-1-ene (1), was demonstrated in organic media under low-intensity laser excitation. Linear photophysical characterization of 1 was performed at room temperature in solvents of different polarity and viscosity. Significantly, close to unity quantum yield for the cyclization reaction of 1 was shown in nonpolar solutions. The lifetimes of the excited states of the open (OF) and closed (CF) forms of 1 were measured by a femtosecond transient absorption technique, and corresponding values of ∼0.7 and ∼0.9 ps were shown in dichloromethane (DCM), respectively. Degenerate two-photon absorption (2PA) spectra of the OF and CF of 1 were obtained over a broad spectral range by the open aperture Z-scan method under 1 kHz femtosecond excitation. The values of 2PA cross sections of the OF in DCM (∼50-70 GM) were found to increase up to 1 order of magnitude (∼600 GM) after cyclization to the CF. The nature of cyclization and cylcoreversion processes were investigated by quantum chemistry with employment of DFT-based methods implemented in the Gaussian'09 program. The potential of 1 for application in optical data storage was shown using poly(methyl methacrylate)-doped films and two-photon fluorescence microscopy readout.

Ultrafast dynamics in complex fluids observed through the ultrafast optically-heterodyne-detected optical-Kerr-effect (OHD-OKE)

The ultrafast molecular dynamics of complex fluids have been recorded using the optically-heterodyne-detected optical-Kerr-effect (OHD-OKE). The OHD-OKE method is reviewed and some recent refinements to the method are described. Applications to a range of complex fluids, including microemulsions, polymer melts and solutions, liquid crystal and ionic liquids are surveyed. The level of detail attainable with the OHD-OKE method in these complex fluids is discussed. The prospects for future experiments are discussed.

Molecular dynamics investigation with the time resolved optical Kerr effect on the CS2–C6H6 mixtures

An investigation of the molecular dynamics in pure liquids and in mixtures through the technique of time resolved optical Kerr effect is performed. The samples studied were the mixtures of carbon disulfide (CS(2)) with benzene (C(6)H(6)). The molecular dynamics of the pure liquids is briefly discussed while the main results are obtained for the mixtures. A slow dynamics component is observed for the optical heterodyne detected optical Kerr effect transient decaying exponentially with time constants on picosecond time scale. The fast subpicosend time relaxations are analyzed in terms of the nondiffusive component of the spectral response that is associated with the molecular dynamics. The modifications of the spectrum are quantified, and the explanation of the observed changes is given in terms of the structural interaction configurations that produced changes in the intermolecular potential within which the molecules execute librational motions.

Temperature- and solvation-dependent dynamics of liquid sulfur dioxide studied through the ultrafast optical Kerr effect

The ultrafast dynamics of liquid sulphur dioxide have been studied over a wide temperature range and in solution. The optically heterodyne-detected and spatially masked optical Kerr effect (OKE) has been used to record the anisotropic and isotropic third-order responses, respectively. Analysis of the anisotropic response reveals two components, an ultrafast nonexponential relaxation and a slower exponential relaxation. The slower component is well described by the Stokes-Einstein-Debye equation for diffusive orientational relaxation. The simple form of the temperature dependence and the agreement between collective (OKE) and single molecule (e.g., NMR) measurements of the orientational relaxation time suggests that orientational pair correlation is not significant in this liquid. The relative contributions of intermolecular interaction-induced and single-molecule orientational dynamics to the ultrafast part of the spectral density are discussed. Single-molecule librational-orientational dynamics appear to dominate the ultrafast OKE response of liquid SO2. The temperature-dependent OKE data are transformed to the frequency domain to yield the Raman spectral density for the low-frequency intermolecular modes. These are bimodal with the lowest-frequency component arising from diffusive orientational relaxation and a higher-frequency component connected with the ultrafast time-domain response. This component is characterized by a shift to higher frequency at lower temperature. This result is analyzed in terms of a harmonic librational oscillator model, which describes the data accurately. The observed spectral shifts with temperature are ascribed to increasing intermolecular interactions with increasing liquid density. Overall, the dynamics of liquid SO2 are found to be well described in terms of molecular orientational relaxation which is controlled over every relevant time range by intermolecular interactions.

Molecular structure and dynamics of liquids: aqueous urea solutions

In this review a multi-technical approach to the analysis of the structure and dynamics of the urea/water system is described. The reorientational movement of the solute molecule is investigated by the analysis of spectral band-shapes, as well as with the use of the optical Kerr effect (OKE) and molecular dynamics simulation (MDS). The effect of solute concentration on the structure and dynamics of the aqueous solutions (aggregation, orientational distribution, solvation...) is studied by molecular dynamics simulation and neutron scattering. The results obtained by other techniques are included to provide a critical analysis. Finally, the low-frequency Raman spectra of the system are interpreted on the basis of the semi-quantitative information obtained by molecular dynamics simulation.