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

A new banana-shaped achiral molecule, 1,3-phenylene bis[4-(3-chloro-4-n-oct yloxyphenyliminomethyl)benzoate] (PBCOB) has been synthesized, and its ferr oelectric properties and homeotropic alignment investigated. The presence o f a lateral chloro-substituent in the Schiff's base moiety prevents the reg ular stacking of molecules and results in lowering the transition temperatu re and the degree of crystallinity of the switchable banana phase. Their sm ectic mesophases, including a switchable banana phase B-7, were characteriz ed by differential scanning calorimetry, X-ray scattering and polarizing op tical microscopy. Both the left- and right-handed helical domains are spont aneously formed upon cooling from the isotropic liquid to the switchable ba nana phase B-7. By X-ray study, the smectic phases showed a layer spacing o f 38.1 Angstrom, compatible with the end-to-end distance of the molecule wi th a bent conformation. Significantly, the smectic B-7 phase exhibited a pe riodicity of 292 Angstrom that corresponds to a helical structure with peri odicity about 7.5 times 38.1 Angstrom. The spontaneous polarization for PBC OB is about 50 nC cm(-2) and shows a temperature dependence. The ferroelect ric lyomesophase of PBCOB showed a ferroelectric electro-optical switching range extending more than 50 degreesC, switchable at room temperature.