Occurrence of Five Different Orthogonal Smectic Phases in a Bent-Core (BC) Liquid Crystal (original) (raw)
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Physical Review Research, 2020
The physical properties and the phase transitions of a smectic liquid crystal material built from achiral bent-core molecules are investigated by electro-optics, polarizing microscopy, birefringence, and spontaneous polarization. This material is shown to exhibit a number of orthogonal and tilted smectic phases. The observation of an anomalous phase transition on cooling from a tilted to orthogonal antiferroelectric smectic phase at lower temperatures is made, and a proof of the evidence is presented. The importance of the biaxial anchoring term in the free energy is modeled and discussed.
2011
The mesomorphic properties of an achiral bent-core liquid crystal derived from 4-cyanoresorcinol are studied by polarizing optical microscopy, x-ray diffraction, and second harmonic electro-optic response. It shows a novel sequence of four nontilted or orthogonal smectic phases on cooling: SmA-SmAP R-SmAP X-SmAP A. Here SmAP X is the new orthogonal polar uniaxial smectic phase. The electric-field-induced transformations in the SmAP X phase give rise to two biaxial states separated by a uniaxial one. The second harmonic electro-optic response in this phase is interpreted in terms of the polar interaction with the electric field. A comparison of the experimental results with the next-nearest-neighbor model for the structure of the SmAP X phase shows it to be an SmAP phase.
Liquid Crystals, 2020
We present a new series of bent-core liquid crystals possessing a lateral substituent in the apex position of the central core and a terephthalate connecting unit in one elongating side arm. The synthesised materials have been studied by polarising optical microscopy, differential scanning calorimetry, dielectric spectroscopy and X-ray diffraction. Electro-optical studies have also been performed to confirm the character of the identified polar smectic phase. We document that materials substituted with fluorine exhibit polar synclininic antiferroelectric phase, for homologues with longer terminal alkyl chains, this SmC S P A phase is accompanied with a high-temperature SmC phase. Our results document that the introduction of the aforementioned structural features triggers significant modification of the mesomorphic behaviour previously observed for similar compounds.
Optically isotropic liquid-crystal phase of bent-core molecules with polar nanostructure
Physical Review E, 2005
We found that the optically isotropic (I M ) mesophase observed recently below the nematic phase of the bent core liquid crystal, 4-chlororesorcinol bis[4-(4-ndodecyloxybenzoyloxy)benzoate], shows ferroelectric-type switching. Polarizing microscopic, electric current, dielectric, and dynamic light scattering studies lead us to propose that the I M phase is composed of interconnected orthoconic racemic smectic (SmC a P F ) nano-domains with random layer orientations. Near the nematic phase, where the polarization can be saturated by electric fields, the system responds in a fashion analogous to the granular structure of a magnetic spin glass -in particular, we observed that the relaxation back to the non-poled structure follows a similar, inverse logarithmic rule.
Advanced Functional Materials, 2012
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.
Chirality and polarity transfers between bent-core smectic liquid-crystal substances
2006
Liquid-crystal phases with both solid and liquid characteristics are unique, since they exhibit chirality on both molecular and mesoscopic levels under special circumstances. We have studied binary composites formed by mixing of a chiral dimer ͑B-Ch͒, in which a bent core is covalently linked to a promesogenic cholesterol unit through a flexible spacer, and an achiral bent-core mesogen having two vicinal fluorine atoms ͑B-2F͒. The pure materials show smectic phases featuring C 2 symmetry. The dimer B-Ch displays conventional chiral smectic C ͑SmC * ͒ phase with a chirality-induced polarization P c ϳ 30 nC/ cm 2 , whereas B-2F possesses synclinic and anticlinic antiferroelectric "banana" smectic ͑SmCP͒ phase with a relatively high-polarization ͑P b ͒ value of about 900 nC/ cm 2. Mixing these two materials we were able to measure P c , and we have verified that the molecular chirality contributes to the polarization of banana phases only in synclinic configurations.
Properties of non-symmetric bent-core liquid crystals with variable flexible chain length
Liquid Crystals, 2010
New non-symmetrical bent core mesogens with a variable flexible chain length have been synthesized. The compounds were derived from 3-hydroxybenzoic acid, which is the central unit of the molecules. The tails on both ends were connected by ester functionality, one tail contains a terminal double bond and the other one is saturated. The compounds represent novel nonsymmetric molecules both in their core and tails. The main features of DSC, polarizing optical microscopy, electro-optical and polarization current measurements for all studied materials indicate a transition between single tilted synclinic ferroelectric and an antiferroelectric smectic phases. However details show important deviations from the simplest picture, which will be the subject of future X-ray studies.
Bent-Core Liquid Crystals: Structures and Mesomorphic Properties
2019
Bent-core (BC) molecules became an attractive liquid crystal class due to their potential use in smart displays and photonic devices. In contrast to calamitic mesogens, bent-shaped mesogens are self-organized superstructures with remarkable properties, given the presence of polar order in mesophases, although the molecules themselves are not chiral. A particular interest represents the biaxial nematic liquid crystal materials that are used in display technology and allow a faster switching response, compared to calamitic liquid crystals, with considerably reduced costs. This chapter briefly reviews the bent-core liquid crystals with three different core units in the structure: (1) 2,5-disubstituted oxadiazole, (2) 1,3disubstituted benzene, and (3) 2,7-disubstituted naphthalene. To the central bent units (BUs) containing reactive functional groups of phenolic or aminic type, various mesogenic groups are symmetrically or asymmetrically connected, via esterification or condensation rea...