Molecular orientation and the infrared dichroism of a chiral smectic liquid crystal in a homogeneouslyusly aligned cell at different temperatures 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- (48-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 a * 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 SmCA* and SmC g * 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.