Molecular orientation and the infrared dichroism of a chiral smectic liquid crystal in a homogeneously aligned cell at different temperature and bias fields

Abstract

The molecular orientation and the dichroic behavior of the vibrational bands of a homogeneously aligned helical cell containing chiral smectic liquid crystal (R)-(-)-1-methylheptyl 4-(4(')-dodecyloxybiphenyl-4-ylcarbonyloxy)-3-fluorobenzoate are studied at various temperatures as a function of the bias field. These temperatures correspond to the various phase states of the sample at zero field. For those bands that exhibit significant dichroism, the field dependencies of the dichroic parameters (the dichroic ratio and the polarization angle of maximum absorbance) are found to be dependent on temperature, phase state, and helical unwinding. For the SmA* and SmC(*)(alpha) phases, the phenyl band dichroic ratio and the corresponding orientational order parameter are found to be almost independent of the bias field. The temperature dependence of the orientational order for zero field is discussed by taking into account the structures of the phases and the molecular tilt angles. The field dependencies of the phenyl band dichroic parameters for the SmC(*)(A) and SmC(*)(gamma) phases yield results about the distribution of directors in the layers of their unit cells and the state of helical unwinding. The azimuthal orientational distribution function of the carbonyl transition moments with respect to the long molecular axis has been determined. It is found that the degrees of the polar and quadrupolar biasing increase with decrease in temperature and the azimuthal biasing angle for the chiral carbonyl group increases significantly with a reduction in temperature.

Probing field-induced submolecular motions in a ferroelectric liquid crystal mixture with time-resolved two-dimensional infrared spectroscopy

“…The C-O-C stretching mode at 1252 cm −1 exhibits similar but less distinctive variation. Although the variations of IR dichroic ratio with dc voltage had also been observed on a neat FLC material [19], our result shows that the variations of dichroic ratio not only occur during the voltage-holding period but also in the field-free period. Considering that each observed IR peak can originate from molecular fragments belonging to different species, our result suggests that the C=C fragments of different species of the mixture will undergo mutual movement during the field-induced reorientation process.…”

Chiral smectic- A and smectic- C phases with de Vries characteristics

“…Minimum absorbance (A ⊥ ) is proportional to the average value of the squares of the projections of transition dipole moments of the phenyl ring in a direction perpendicular to the directors in the tilt plane. The direction of the tilt angle ( o ) starts moving to the direction of the molecular tilt and with the unwinding of the helix leads to a decrease in A ⊥ and a consequent increase in R. This is quite contrary to the normal Sm-A * to Sm-C * transition where the values of R and S especially in the Sm-A * do not depend on voltage and stay almost constant with field [24]. A simple simulation was performed using Maple software to elucidate our experimental results.…”

Probing Molecular Binding Effect from Zinc Oxide Nanocrystal Doping in Surface-Stabilized Ferroelectric Liquid Crystal with Two-Dimensional Infrared Correlation Technique

“…To devise an effective scheme for layer stabilization without sacrificing the fast response of FLC, insight into intermolecular interaction is indispensable. To probe into the intermolecular interaction and the resulting structural changes of FLC molecules during the field-induced reorientation, − time-resolved Fourier transform infrared (FTIR) absorption spectroscopy had been widely employed. It was noted that upon the reversal of the sign of an applied electric field on a FLC film all of the molecules switch by rotation about a tilted SmC* cone.…”