Dynamics of electric-field-induced molecular reorientation and segmental mobility in the smectic-C^{*} phase of a ferroelectric liquid crystal: Faster reorientation of cores than the alkyl chains revealed by time-resolved infrared spectroscopy

Abstract

We report a study of the dynamics of molecular reorientation and segmental mobility of a ferroelectric liquid crystal in the smectic-${C}^{*}$ (Sm-${C}^{*}$) phase having 2-hydroxy-benzilidenianiline as the mesogen core and $(S)\ensuremath{-}$ 2-chloro-3-methylbutanoyl as the chiral terminal group, at different temperatures during electric-field-induced switching between the two surface-stabilized states by use of a time-resolved Fourier-transform infrared technique. From polarized infrared spectra under static dc electric fields, details of mutual arrangement of different molecular segments in the Sm-${C}^{*}$ phase have been obtained. Contrary to the usual expectations, the average alkyl chain axis does not coincide with the mesogen axis and is less tilted with respect to the layer normal than the mesogen. On the other hand, motion of the carbonyl groups is found to be strongly hindered and the distribution function for the $\mathrm{C}\mathrm{=}\mathrm{=}\mathrm{O}$ bonds is not cylindrically symmetric with respect to the long molecular axis. Time-resolved infrared measurements at different temperatures and voltages have revealed that, on switching the polarity of the electric field, the hydrogen-bonded $\mathrm{C}\mathrm{=}\mathrm{=}\mathrm{O}$ group moves with the mesogen which reaches equilibrium orientations at a faster rate compared to the alkyl chain at the temperatures and voltages employed in this study.