Polarized infrared spectroscopic study on the orientation of the moleculesin the smectic-C* phase of a ferroelectric liquid crystal with a naphthalene ring: Alternative theory for the analysis of polarization-angle-dependent intensity changes - art. no. 031704

A theory to explain the polarization-angle dependence of polarized infrared spectra of a ferroelectric liquid crystal in the surface-stabilized ferroe lectric liquid crystal state is proposed, It describes the relationship bet ween the intensity of the absorption bands and the polarization angle of th e infrared radiation. Using this theory the polarization-angle dependence o f the infrared band intensities was analyzed for a ferroelectric liquid cry stal with a naphthalene ring and two phenyl rings with a stacked layer stru cture in the smectic-C* phase. The polarization-angle-dependent spectra wer e measured at 137 degreesC under external dc electric fields of +40 and -40 V to investigate the orientation of the molecules. Plots of the infrared a bsorbance versus polarization angle for representative bands were subjected to a curve fitting procedure by a least squares method. From the curves ob tained the orientation of the transition dipole moments with respect to the molecular long axis and the orientation of the Molecular long axis with re spect to the rubbing direction of the cell were estimated based upon the su ggested theory. The polarization-angle-dependent infrared spectra obtained were also analyzed by two-dimensional (2D) correlation spectroscopy. The 2D correlation analysis clearly detects a slight phase difference in the pola rization-angle dependence which is hardly recognized in ordinary plots of t he intensity changes in the infrared bands. The 2D correlation analysis all ows us to separate asymmetric and symmetric stretching bands due to the chi ral methyl group from those arising from other methyl groups in the alkyl c hains.