Development of ferroelectricity in the smectic phases of 4-cyanoresorcinol derived achiral bent-core liquid crystals with long terminal alkyl chains

Abstract

Bent-core molecules can form numerous polar and symmetry broken liquid crystalline phases with fascinating properties. Here we report the characteristics of a previously unknown polar synclinic smectic phase, $\mathrm{Sm}{C}_{\mathrm{S}}{P}_{\mathrm{X}}$, found in the phase sequence of an achiral bent-core material with a 4-cyanoresorcinol bisbenzoate core, terminated by long linear alkyl chains ($n=18$) on both ends. This phase exists over a narrow range of temperatures and is sandwiched in between the random polar synclinic smectic phase ($\mathrm{Sm}{C}_{\mathrm{S}}{P}_{\mathrm{R}}$) and polar synclinic ferroelectric ($\mathrm{Sm}{C}_{\mathrm{S}}{P}_{\mathrm{F}}$) phase with ${\mathbit{P}}_{S}\ensuremath{\sim}250\phantom{\rule{0.16em}{0ex}}\mathrm{nC}/\mathrm{c}{\mathrm{m}}^{2}$. In a planar-aligned cell it exhibits only chirality flipping on application of a conventional ac field but it also exhibits optical switching by rotation of the molecular directors on the tilt cone subjected to a modified sequence of bipolar pulses. This changeover is discussed in terms of the model given by Nakata et al. [Phys. Rev. Lett. 96, 067802 (2006)], involving a competition between the two forms of switching: the rotation around the long molecular axis and the switching through rotation of the molecular directors on the tilt cone. The model is modified to take account of the azimuthal pretilt and the molecular tilt angles. In addition to it, characteristics of $\mathrm{Sm}{C}_{\mathrm{S}}{P}_{\mathrm{R}}$ are also discussed.