A polarized infrared spectroscopic study on electric-field-induced layer rotation of ferroelectric liquid crystal mixtures with ultrashort pitch

Abstract

In the present study we found by observing with a polarizing microscope that the liquid crystal layers of ferroelectric liquid crystal (FLC) mixtures with ultrashort pitch (FLCUSP) rotate in the smectic-C* (Sm-C*) phase under the application of a simple rectangular pulse wave with a voltage of positive or negative polarity. In order to investigate the mechanism of the electric-field-induced layer rotation of FLCUSP, polarization angle-dependent infrared spectra were measured before and after the layer rotation. The polarization angle-dependence of infrared spectra of FLCUSP suggests that the layers of FLCUSP rotate by about 40°. This result is in good agreement with the observation by the polarizing microscope. It was also found from the polarization angle-dependence that the CF₃ and CO groups of the chiral dopants are subjected to the hindered rotation around the molecular long axis. It is very likely that both the CF₃ and CO groups are responsible for the large polarization. The obtained polarized infrared spectra are complex particularly in the 1290–1200 cm⁻¹ region where a band due to the CF₃ stretching mode is expected to appear. Thus, we used two-dimensional (2D) correlation spectroscopy and curve fitting to analyze the complex spectra. The 2D correlation analysis reveals that before the layer rotation the directions of the CF₃ groups in the chiral dopants with respect to the molecular long axis under the applied dc voltage are different from those in the absence of the electric field and that the directions of the CF₃ groups in the absence of the electric field change significantly before and after the layer rotation.