Orientational and interaction induced dynamics in the isotropic phase of a liquid crystal: Polarization resolved ultrafast optical Kerr effect spectroscopy

Abstract

The ultrafast dynamics of the isotropic phase of a liquid crystal 4'-pentyl-4-p-biphenylcarbonitrile (5CB) have been investigated using polarization resolved optical Kerr effect spectroscopy. Measurements were made as a function of both temperature and dilution in nonpolar solvents. To separate single molecule and interaction induced components to the relaxation of the induced birefringence, measurements of both the anisotropic and isotropic response were made. The isotropic response was found to be dominated by a damped low-frequency mode of intramolecular origin. There is a minor additional component assigned to an interaction induced contribution. There is at most an extremely weak isotropic signal beyond 1 ps, showing that the picosecond time scale dynamics of 5CB are dominated by orientational relaxation. The isotropic response is independent of temperature in the range studied (0.2-50 K above the nematic to isotropic phase-transition temperature). The anisotropic response exhibits relaxation dynamics on time scales spanning subpicosecond to several hundred picoseconds and beyond. The fastest components are dominated by a librational response, but there are smaller contributions from three low-frequency intramolecular modes, and a contribution from interaction induced effects. The low-frequency spectral density extracted from these data are independent of temperature in the range studied, 0.2-30 K above the phase-transition temperature, but shift to lower frequency on dilution in alkane solvents. In neat 5CB the picosecond time scale orientational dynamics are dominated by temperature-independent reorientation within the pseudonematic domains, while in solution these are disrupted, and the orientational response becomes faster and temperature dependent.

Ultrafast dynamics of pyrrolidinium cation ionic liquids

“…Several line shape models have been used to analyze the low-frequency intermolecular part of OHD-RIKES spectrum for molecular liquids. 33,34,38,50,[80][81][82][83][84][85][86][87][88][89][90][91] The low-frequency intermolecular spectrum of liquids contains the interactioninduced ͑translational͒ and librational ͑reorientational͒ motions. [92][93][94][95][96] The recent study by Ryu and Stratt on the intermolecular dynamics and calculated Kerr spectra of benzene and CS 2 liquids provides insight for how we may consider ionic liquids in a way that goes beyond line shape analyses.…”

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

“…6 Experimental and phase matching geometry (inset) for the diffractive optic based transient grating OKE experiment described by Goodno et al 111,112 The figure was adapted from that described by Brixner et al 114 T NI . [18][19][20][21] The isotropic phase is the one most readily investigated by OHD-OKE because it best fulfils the requirement for an isotropic non-scattering sample. However, a few measurements have been reported in the smectic phase.…”

“…7 Such fluids are of considerable significance in materials chemistry and life sciences, and an understanding of the microscopic dynamics which underlie their complex macroscopic behaviour will be of great value. Examples where the OHD-OKE has uncovered such microscopic details include ionic liquids, [8][9][10][11][12][13][14][15][16] liquid crystals, [17][18][19][20][21][22][23][24][25][26][27][28] microemulsions, [29][30][31][32] polymer solutions 29,[33][34][35][36][37] and nanoconfined liquids. [38][39][40][41][42][43][44][45][46][47][48] The measurement of molecular dynamics in such media poses a variety of experimental difficulties, which, we aim to show, have at least in part been addressed by OHD-OKE based methods.…”

A new ultrafast technique for measuring the terahertz dynamics of chiral molecules: The theory of optical heterodyne-detected Raman-induced Kerr optical activity

“…It is impossible to predict at this point whether the various intermediate-frequency modes that are typically identified in a fit to an OHD-RIKES spectrum will have very different or similar tensor invariants. From previous SMOKE measurements 8,22 it is known that the isotropic invariant of the ͑electric-dipole͒ polarizability tensor is very small compared to the anisotropic invariant for the intermediate frequency modes in liquids. This suggests that the OHD-RIKOA signal will mostly depend on the magnitude and sign of ␤͑␥͒ and ␤͑͒.…”

“…Using this technique, one can separately measure the anisotropic and isotropic invariants of the molecular polarizability tensor. Unfortunately, for the intermediate frequency collective modes in liquids, the anisotropic invariant is significantly larger than the isotropic invariant 8,22 and the isotropic spectrum is very similar in shape to the anisotropic spectrum. Significant differences between the isotropic and anisotropic spectra can only be seen at high frequencies in hydrogen-bonding liquids and proteins.…”