Ferroelectric Nematic and Ferrielectric Smectic Mesophases in an Achiral Bent-Core Azo Compound

Jitendra Kumar and Veena Prasad
J. Phys. Chem. B, 122, 2998-3007 (2018)

Here, we report the first observation of ferroelectric nematic and ferrielectric smectic mesophases in an achiral bent-core azo compound consisting of non-symmetrical molecules with a lateral fluoro substitution on one of the wings. These mesophases are enantiotropic in nature with fairly low transition temperatures and wide mesophase ranges. The liquid crystalline properties of this compound are investigated using polarising optical microscope, differential scanning calorimeter, X-ray diffraction and electro-optical studies. As revealed by X-ray diffraction measurements, the nematic mesophase is composed of skewed cybotactic clusters and in the smectic mesophase, the molecules are tilted with respect to the layer normal. The polar order in these mesophases was confirmed by the electro-optical switching and dielectric spectroscopy measurements. The dielectric study in the nematic mesophase shows a single relaxation process at low frequency (f  1 kHz) measured in the range 10 Hz - 5 MHz, which is attributed to the collective motion of the molecules within cybotactic clusters. The formation of local polar order in these clusters leads to a ferroelectric-like polar switching in the nematic mesophase. Of particular interest is the fact that the smectic phase exhibits field induced ferrielectric state, which can be exploited for designing of the potential optical devices due to multistate switching.