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

In the present study we found by observing with a polarizing microscope tha t 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 posit ive or negative polarity. In order to investigate the mechanism of the elec tric-field-induced layer rotation of FLCUSP, polarization angle-dependent i nfrared spectra were measured before and after the layer rotation. The pola rization angle-dependence of infrared spectra of FLCUSP suggests that the l ayers of FLCUSP rotate by about 40 degrees. This result is in good agreemen t with the observation by the polarizing microscope. It was also found from the polarization angle-dependence that the CF3 and C=O groups of the chira l dopants are subjected to the hindered rotation around the molecular long axis. It is very likely that both the CF3 and C=O groups are responsible fo r the large polarization. The obtained polarized infrared spectra are compl ex 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) c orrelation spectroscopy and curve fitting to analyze the complex spectra. T he 2D correlation analysis reveals that before the layer rotation the direc tions 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 abs ence 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 la yer rotation.