Nematic Phases in 1,2,4‐Oxadiazole‐Based Bent‐Core Liquid Crystals: Is There a Ferroelectric Switching?

Abstract

Four series of new 1,2,4-oxadiazole derived bent-core liquid crystals incorporating one or two cyclohexane rings are synthesized and investigated by optical polarizing microscopy, differential scanning calorimetry (DSC), X-ray diffraction (XRD), electro-optical, and dielectric investigations. All the compounds exhibit wide ranges of nematic phases composed of tilted smectic (SmC-type) cybotactic clusters with strongly tilted aromatic cores (40-57 ° ) and show a distinct peak in the current curves observed under a triangular wave fi eld. Dielectric spectroscopy of aligned samples corroborates the previously proposed polar structure of the cybotactic clusters and the ferroelectric-like polar switching of these nematic phases. Hence, it is shown that this is a general feature of the nematic phases of structurally different 3,5-diphenyl-1,2,4-oxadiazole derivatives. In these uniaxial nematic phases there is appreciable local biaxiality and polar order in the cybotactic clusters. As a second point it is shown that electric fi eld induced fan-like textures, as often observed for the nematic phases of bent-core liquid crystals, do not indicate the formation of a smectic phase, rather they represent special electro-convection patterns due to hydrodynamic instabilities.

3,5-diarylisoxazoles: A New Entry to Soft Crystal Phase

“…Thus, 2,5-disubstituted 1,3,4-oxadiazoles with a bending angle of ca 134 • is one of the 5-membered heterocyclic ring that have gained attention of LC community because they are at the borderline between classical rod-like LCs and the bent-core mesogens [3]. Even the nonsymmetric 3,5-disubstituted 1,2,4-oxadiazoles with a molecular bend of 140 • have received attention due to their ferroelectric and biaxility behavior [6].…”

“…Among them, bent-core liquid crystals can be classified as unusual liquid crystals and they emerge as interesting class of LC with amazing behavior [2]. Usually bent-core mesogens have 1,3-phenylene unit connected by aromatics rings providing a 120 • bent in the middle of the aromatic core [3]. Bent-shaped molecules are also founded in 5-membered heterocyclic if the bending angle varies between about 135 • and 160 • .…”

Flexoelectric polarization studies in bent-core nematic liquid crystals

“…We measure the sum of these coefficients for an ozadiazole systemss which do exhibit nematic phase over a wide range of temperatures. These are known to form clusters of various types and these systems have been extensively investigated over recent years using X-rays and N.M.R [26,27]. Figure 5 shows P f and a corresponding value of the pyroelectric coefficient γ. shows that these values range from 10 to 15 pC/m across the nematic region [27].…”

Electric‐ and Light‐Responsive Bent‐Core Liquid Crystals: From Molecular Architecture and Supramolecular Nanostructures to Applications

“…There are many examples of bent-core molecules with five-membered aromatic central BUs (Fig. 6.3c): 1,3,4-oxadiazole [49][50][51][52][53][54], 1,2,4-oxadiazole [55,56], 1,3-oxazole [57], 1,3-thiadiazole [58], and 1,3,4-thiadiazole [59]. Most of these molecules exhibit thermotropic biaxial nematic phases with a skewed cybotactic supramolecular structure.…”