Effect of Chirality on Molecular Dynamics in Smectic C Liquid-Crystalline Compounds

Abstract

The molecular dynamics of the chiral and achiral smectic liquid crystals S-4-(1-methylhexyloxycarbonyl)phenyl 4'-octyloxybiphenyl-4-carboxylate (S-MH(6)POBC) and 4-(hexyloxycarbonyl)phenyl 4'-octyloxybiphenyl-4-carboxylate (H(6)POBC), respectively, was studied by solid-state 13C-NMR spectroscopy. The full width at half maximum (FWHM) and the spin-lattice relaxation time (T1) of several resonance peaks were analyzed in a wide temperature range from the SmA phase to the crystal phase, and the effect of chirality on the molecular fluctuation was investigated. No effect of chirality on the molecular dynamics was detected in the carbon nuclei belonging to the aromatic core or in most of the carbon nuclei in the flexible chain from the results the FWHM and T1 measurements. However, the 13C-NMR study of the chiral and achiral carbons (^*C23 and C23) revealed that introducing chirality, namely, the introduction of a (-CH3) group to the C23 carbon, affected the intramolecular correlation between the mobility of the carbon and the rotational motion of the aromatic core. The achiral C23 carbon maintained its active mobility in the wide temperature range from the SmA phase to the crystal phase. However, the mobility of the chiral ^*C23 carbon became much lower during the smectic-to-crystal phase transition because the freezing of the molecular motion of the aromatic core affected the mobility of ^*C23. Since the intramolecular correlation might be concerned about the emergence of the spontaneous polarization in the chiral smectic phases, we are investigating further details for the molecular dynamics of chiral liquid crystals by means of a two-dimensional analysis of the solid-state 1H- and 13C-NMR spectroscopy.