Molecular Structure of a Partly Deuterated Chiral Smectic Liquid Crystal Studied by Polarized FTIR Spectroscopy

Abstract

The molecular structure of a partly deuterated chiral smectic liquid crystal 4-(1-methylpentyloxycarbonyl) phenyl 4′-octyloxybiphenyl-4-carboxylate (acronym M5POBC-d ₂₅ ) in a homogeneously aligned sample is studied using polarized Fourier-transform infrared (FTIR) spectroscopy. The relative orientations of the chiral and deuterated achiral alkyl chains of molecules with respect to their long axes and some properties of the orientational distributions of the carbonyl groups are quantitatively analysed for the ferroelectric SmC* phase with a field unwound structure. The analysis is carried out in terms of the absorption anisotropy of several bands related to stretching vibrations of the phenyl C ₆ H ₄ fragment, the deuterated biphenyl C ₁₂ D ₈ fragment, two carbonyl groups situated in different parts of a molecule, the methylene groups CH ₂ and CD ₂ . It has been found that the chiral and achiral alkyl chains make substantially different angles (∼60° and ∼36°, respectively) with the molecular long axis. The rotational orientational distribution of the carbonyl groups about the molecular long axes has been found to have a comparatively low polar biasing.

Keywords:

• Infrared spectra of a deuterated FLC compound
• biasing in a FLC compound
• IR dichroism
• mobility of chains in a FLC compound