Ferroelectric smectic meso-phase formed by banana-shaped achiral liquid crystals

Abstract

A new banana-shaped achiral molecule, 1,3-phenylene bis[4-(3-chloro-4-n-octyloxyphenyliminomethyl)benzoate] (PBCOB) has been synthesized, and its ferroelectric properties and homeotropic alignment investigated. The presence of a lateral chloro-substituent in the Schiff's base moiety prevents the regular stacking of molecules and results in lowering the transition temperature and the degree of crystallinity of the switchable banana phase. Their smectic mesophases, including a switchable banana phase B-7, were characterized by differential scanning calorimetry, X-ray scattering and polarizing optical microscopy. Both the left- and right-handed helical domains are spontaneously formed upon cooling from the isotropic liquid to the switchable banana phase B-7. By X-ray study, the smectic phases showed a layer spacing of 38.1 Angstrom, compatible with the end-to-end distance of the molecule with a bent conformation. Significantly, the smectic B-7 phase exhibited a periodicity of 292 Angstrom that corresponds to a helical structure with periodicity about 7.5 times 38.1 Angstrom. The spontaneous polarization for PBCOB is about 50 nC cm(-2) and shows a temperature dependence. The ferroelectric lyomesophase of PBCOB showed a ferroelectric electro-optical switching range extending more than 50 degreesC, switchable at room temperature.