Electric-Field-Induced Reorientation of a Ferroelectric Liquid Crystal Molecule without a Carboxylate Group near the Stereocenter Studied by Time-Resolved Infrared Spectroscopy Combined with Normalized Sample−Sample Two-Dimensional Correlation Spectroscopy

Abstract

The orientation dynamics of a ferroelectric liquid crystal (FLC) with a naphthalene ring and without a carboxylate group near the stereocenter (FLC-2) during the electric-field-induced switching has been investigated by time-resolved infrared (IR) spectroscopy combined with normalized sample−sample two-dimensional (2D) correlation spectroscopy. To reveal the orientation dynamics, we have proposed a normalized sample−sample 2D correlation spectroscopy that is calculated by normalizing a sample−sample 2D correlation spectrum over every column or row. An important advantage of the normalized sample−sample 2D correlation spectroscopy over sample−sample 2D correlation spectroscopy is that the excessive information is deleted and more precise information therefore can be extracted even from spectra with a rather low signal-to-noise ratio. The normalized sample−sample 2D correlation spectroscopy allows us to explore the differences in the dynamics of each segment more clearly than the usual sample−sample 2D correlation spectroscopy. Time-resolved IR spectra of FLC-2 in a planar-aligned cell were measured over two delay time ranges from 0 to 150 μs and from 200 to 350 μs at an interval of 10 μs with the polarization angle of 45.0° under a rectangular electric field of ±3 V with a 2.5 kHz repetition rate in the smectic-C* (Sm-C*) phase at 115 °C. The normalized sample−sample correlation spectroscopy was applied to these time-resolved spectra. We explored the reorientation process of the alkyl chains, the core, and the carboxylate group of FLC-2 in the positive and negative half-periods of the electric-field-induced switching. It has been found from the present study that the reorientation track of the core in the positive half-period is different from that in the negative half-period, while the reorientation tracks of the CO dipole moment and alkyl chains are very similar between the positive and negative half-periods. The present study has also revealed that the orientation profile of the core moiety is different from those of the alkyl chains and the CO dipole moment, and the difference in the profile depends on the change of dc electric field from −3 to +3 V and from +3 to −3 V at an interval of 0.25V . It seems that the core moiety functions in the presence of memory properties. Furthermore, a tilt angle for FLC-2 with a naphthalene ring and without a carboxylate group near the stereocenter is smaller than other similar FLCs with a naphthalene ring and a carboxylate group near the stereocenter. It has been suggested from the analysis of the polarization-angle dependent spectra under the external dc electric field of two polarities that the carboxylate group near the stereocenter plays an important role in forming a large tilt angle and that the carboxylate group in the core moiety is responsible for the emergence of ferroelectricity.