Polarized infrared spectroscopic study on the orientation of the molecules in the smectic-C* phase of a ferroelectric liquid crystal with a naphthalene ring: alternative theory for the analysis of polarization-angle-dependent intensity changes.
In: Physical Review E: Statistical, Nonlinear, and Soft Matter Physics, Jg. 64 (2001) ; Nr. 3-1, S. 031704/1-031704/10
Zeitschriftenaufsatz / Fach: Chemie
A theory to explain the polarization-angle dependence of polarized IR spectra of a ferroelec. liq. crystal in the surface-stabilized ferroelec. liq. crystal state is proposed. It describes the relation between the intensity of the absorption bands and the polarization angle of the IR radiation. Using this theory the polarization-angle dependence of the IR band intensities was analyzed for a ferroelec. liq. crystal with a naphthalene ring and 2 Ph rings with a stacked layer structure in the smectic-C* phase. The polarization-angle-dependent spectra were measured at 137 Deg under external d.c. elec. fields of +40 and -40 V to study the orientation of the mols. Plots of the IR absorbance vs. polarization angle for representative bands were subjected to a curve fitting procedure by a least squares method. From the curves obtained the orientation of the transition dipole moments with respect to the mol. long axis and the orientation of the mol. long axis with respect to the rubbing direction of the cell were estd. based upon the suggested theory. The polarization-angle-dependent IR spectra obtained were also analyzed by 2-dimensional (2D) correlation spectroscopy. The 2D correlation anal. detects a slight phase difference in the polarization-angle dependence which is hardly recognized in ordinary plots of the intensity changes in the IR bands. The 2D correlation anal. allows sepn. of asym. and sym. stretching bands due to the chiral Me group from those arising from other Me groups in the alkyl chains.