Antiferroelectric Switchable Mesophases of Nonchiral Bent-Core Liquid Crystals Containing Fluorinated Central Cores

A new series of liquid-crystalline bent-core molecules comprising a central 3,4‘-disubstituted biphenyl unit (biphenyl 3,4‘-bis-4-[4-dodecyl(oxy)benzoyloxy]benzoates) at which one, two, or even three H atoms in various positions were replaced by F atoms has been synthesized. The biphenyl cores were built up by orthometalation, followed by palladium-catalyzed cross-coupling reactions. The compounds were characterized by polarizing microscopy, DSC, X-ray scattering, and electro-optical investigations. Most compounds exhibit broad regions of antiferroelectrical switchable SmCPA phases, but also rectangular columnar mesophases have been observed for some representatives. Extremely large values of the spontaneous polarization (up to 2300 nC cm-2) have been found for some representatives. Additionally, for some smectic compounds a third peak was detected in the polarization current curve beside the two peaks corresponding to the antiferroelectric switching process. The coexistence of ferroelectric and antiferroelectric domains is discussed as one possible explanation, but requires further investigations.