Infrared Spectroscopic Study of Orientational Order and Phase Transformations in Liquid Crystalline CBOOA

Abstract

An infrared study of the different phases of liquid crystalline CBOOA (N-p-cyanobenzilidene p-n-octyloxyaniline) has been made in the range 30-3100 cm⁻¹. From a study of the -C≡N vibration in aligned samples, the absolute orientational order parameter, S, is determined in the smectic A and nematic phases. The variation of S is compared with the relative values determined earlier from diamagnetic anisotropy, NMR and birefringence studies. The farinfrared spectrum of the solid (Phase I) exhibits a mode whose intensity decreases gradually as the crystal-smectic A transition is approached. Also, a band observed at 296 cm⁻¹ in the solid (Phase I) practically disappears in the fluid phases. These results are discussed in terms of possible pretransition effects and intramolecular changes characteristic of crystal-mesophase transitions. Dichroic effects are evident in the far-infrared absorption of the liquid crystalline phases and these are considered in relation to the influence of molecular structure on the absorption in this region.