Molecular dynamics of thiophene homologues investigated by femtosecond optical Kerr effect and low frequency Raman scattering spectroscopies

Abstract

We present the results of femtosecond optical heterodyn detected optical Kerr effect (OHD-OKE) spectroscopy and low frequency Raman scattering spectroscopy for three thiophene homologues (C4H4X, X=O, S, and Se). The observed OHD-OKE transients were analyzed both in time and frequency domains. Fourier transformed OHD-OKE data were directly compared with the corresponding spontaneous Raman scattering spectra. The reorientation lifetimes for furan (O), thiophene (S) and selenophene (Se) were determined to be 0.9, 2.0 and 3.2 ps, respectively, and were fairly explained in terms of the Stokes-Einstein-Debye relation. Their subpicosecond dynamics was analyzed with an antisymmetric Gaussian function and a skewed function developed by Bucaro and Litoviz, which gave good curve fits to observed data. Librational and translational dynamics of the molecules was discussed in terms of microscopic environments formed by neighboring molecules and the results of intermolecular interaction energy calculations by an ab initio molecular orbital method.

Optically-heterodyne-detected optical Kerr effect (OHD-OKE): Applications in condensed phase dynamics

“…However, it should be noted that there are data which do not ®t this simple picture. The selenophene data in the study of Kamada et al reverse the trend predicted by the harmonic model [55]. The series of three halobenzenes studied by Friedman and She also reveal a trend which is the opposite to that expected (if it is assumed that k is similar in the three liquids) [30].…”

“…Scatter plot of fast and slow exponential relaxation times for aromatic liquids (following [54]). Data taken from Smith et al [45,47], Kamada et al [55] and Neelakandan et al [43,44]. Error bars are an estimated §0.5 ps for the fast component.…”

“…BL , the exponent ¬ and the relative amplitude g 1 =b. Although this ®tting procedure is partly empirical (see below), this pair of functions has been remarkably successful at describing the ultrafast spectral densities of a large number of liquids [33,38,39,41,44,45,47,55].…”

“…Kamada et al measured the librational frequencies for the series furan, thiophene and selenophene [55]. They calculated a value for k based on the free volume of the liquids.…”

“…Clearly some means of deconvoluting these contributions is desirable, and this inevitably involves a degree of curve ®tting. Chang and Castner [38], in a procedure subsequently adopted by a number of groups [33,41,44,45,47,55], showed that Im‰D 0 …! †Š can often be well represented by the sum of an antisymmetrized Gaussian…”