Liquid Crystal Display Modes in a Nontilted Bent-Core Biaxial Smectic Liquid Crystal

Liquid crystal display (LCD) modes associated with the rotation of the secondary director in nontilted, biaxial smectic phase of an achiral bent-core compound are demonstrated. For LCDs, we find that at least four display modes are possible using SmAPA phase of the studied material, in which the minor directors in adjacent layers are aligned antiferroelectrically. The advantages of these modes include low driving field (1–2 V/ µm), high contrast ratio 1000:1, relatively fast switching time of 0.5 ms and continuous gray scale. The molecular short axis or the polar axis in a negative dielectric, biaxial material is oriented by the in-plane electric field by a combination dielectric biaxiality and polarity at low electric fields and polarity at higher fields

A fast linear electro-optical effect in a non-chiral bent-core liquid crystal

We report the observation of an electro-optical effect for a linear rotation of the optical axis as function of the electric field in an achiral tilted bent-core liquid crystalline compound, in which 4-cyanoresorcinol bisbenzoate core is symmetrically terminated by long linear alkyl chains (n =16) on both ends. The linear electro-optic effect observed in this material is similar to the deformed helical ferroelectric liquid crystal (DHFLC) effect reported previously in a FLC material except that dimensions of the helical pitch differ by two orders of magnitude. In the chiral systems, the helical pitch usually lies on the scale of sub-optical wavelength, ~0.3 m depending on the material and temperature, whereas in the achiral system, the pitch lies on the nm scale. The DHFLC mode shows fast, analog switching due to a gradual distortion of the uniform helical structure by the electric field; the switching speed is faster than for the chiral systems. In contrast to the chiral FLCs, achiral liquid crystalline materials appear as better candidates for applications in displays and the photonic technologies

Electric field induced biaxiality and the electro-optic effect in a bent-core nematic liquid crystal

We report the observation of a biaxial nematic phase in a bent-core molecular system using polarizing microscopy, electro-optics, and dielectric spectroscopy, where we find that the biaxiality exists on a microscopic scale. An application of electric field induces a macroscopic biaxiality and in consequence gives rise to electro-optic switching. This electro-optic effect shows significant potential in applications for displays due to its fast high-contrast response. The observed electro-optic switching is explained in terms of the interaction of the ferroelectric clusters with the electric field

Formation of a Double Diamond Cubic Phase by Thermotropic Liquid Crystalline Self-Assembly of Bundled Bolaamphiphiles

A quaternary amphiphile with swallow-tail side groups displays a new bicontinuous thermotropic cubic phase with symmetry Pn3‾ m and formed by two interpenetrating networks where cylindrical segments are linked by H bonds at tetrahedral junctions. Each network segment contains two bundles, each containing 12 rod-like mesogens, lying along the segment axis. This assembly leads to the first thermotropic structure of the "double diamond" type. A quantitative geometric model is proposed to explain the occurrence of this rare phase

Occurrence of Five Different Orthogonal Smectic Phases in a Bent-Core (BC) Liquid Crystal

Five orthogonal smectic phases (Sm-A, Sm-Ab, Sm-APR, Sm-APAR, and Sm-APA) were observed in a 4-cyanoresorcinol bisbenzoate with twoterephthalate–based wings achiral bent-core(BC) compound. The phase behavior was investigated by polarising optical microscopy, second harmonic electro-optic response(EO2) and dielectric spectroscopy. The field dependent biaxiality of polar and non-polar Sm-A phases was studied. It is obviously of enormous scientific significance and practical interest to find five different orthogonal smectic phases in a low molecular weight bent-core (BC) compound at relatively low temperatures

Dielectric and Optical Study of Biaxial Bent-Core Nematic Liquid Crystal

We report the observation of biaxial nematic phase in a bent-core molecular system in which the biaxiality exists on a microscopic scale using polarizing microscopy, electro-optics and dielectric spectroscopy. An application of electric field induces a macroscopic biaxiality and therefore electro-optic switching. The observed electro-optic switching is explained in terms of the interaction of the ferroelectric clusters with the electric field. Dielectric spectra of the sample shows existence of low-frequency collective processes related to the microscopic polar clusters. The dielectric constant changes its sign from positive to negative and again to positive values

Properties of Non-Tilted Bent-Core Orthogonal Smectic Liquid Crystal

We present the properties of different achiral orthogonal polar smectic phases observed in a homologous series of bent-core molecular compounds. The macroscopically uniaxial SmAPR phase transforms to biaxial state by the application of higher electric fields and biaxial SmAPA phase undergoes biaxial-uniaxial-biaxial texture transformation with increasing electric fields,i.e.,initial antiferroelectric structure transforms into ferroelectric state under higher electric fields and it shows three different optically distinguishable states. The SmAPAR phase shows an entirely different response compared to other polar smectic phases under study. The dielectric measurements in the SmAPAR phase is the evidence of antiferroelectric behavior of uniaxial state. The polarizing microscopy texture observation supports the corresponding biaxiality measurements

Structure and Polymorphism of Biaxial Bent-Core Smectic Liquid Crystal

The mesomorphic properties of a homologous series of achiral bent-core compounds are studied by polarizing optical microscopy, electro-optics and polarization measurements. Induction of a new orthogonal smectic phases with the increase of alkyl side chain length was observed. One of the compounds exhibits a unique phase transition between four non-tilted smectic phases (SmAPA–SmAPAR-SmAPR-SmA). The uniaxial but antiferroelectric nature of SmAPAR phase was confirmed by POM, current response, the 2nd harmonic electro-optic response and polarization measurements. The structure of SmAPAR phase was studied theoretically by Next-Nearest-Neighbor model and was identified as SmAPα

Biaxial order parameter in the homologous series of orthogonal bent-core smectic liquid crystals

The fundamental parameter of the uniaxial liquid crystalline state that governs nearly all of its physical properties is the primary orientational order parameter (S) for the long axes of molecules with respect to the director. The biaxial liquid crystals (LCs) possess biaxial order parameters depending on the phase symmetry of the system. In this paper we show that in the first approximation a biaxial orthogonal smectic phase can be described by two primary order parameters: S for the long axes and C for the ordering of the short axes of molecules. The temperature dependencies of S and C are obtained by the Haller's extrapolation technique through measurements of the optical birefringence and biaxiality on a nontilted polar antiferroelectric (Sm-APA) phase of a homologous series of LCs built from the bent-core achiral molecules. For such a biaxial smectic phase both S and C, particularly the temperature dependency of the latter, are being experimentally determined. Results show that S in the orthogonal smectic phase composed of bent cores is higher than in Sm-A calamatic LCs and C is also significantly large

Helical networks of π‐conjugated rods – a robust design concept for bicontinuous cubic liquid crystalline phases with Ia3d and chiral I23 lattice

Bicontinuous cubic liquid crystalline phases of π‐conjugated molecules, representing self‐assembled 3D‐ordered interpenetrating networks with cubic symmetry, are receiving increasing attention due to their capacity for charge transport in all three dimensions and their inherent spontaneous helicity. Herein, a robust general design concept for creating bicontinuous cubic phases is reported. It is based on a nonsymmetric‐substituted π‐conjugated 5,5′‐diphenyl‐2,2′‐bithiophene platform with one end containing three out‐fanning flexible chains and with a range of substituents at the other end (the apex). The cubic phases are stable over broad temperature ranges, often down to ambient temperature, and tolerate a wide range of apex substitution patterns, allowing structural diversity and tailoring of the cubic phase type and application‐relevant properties. With an increasing number and size of apex substituents, a sequence of three different modes of cubic self‐assembly is observed, following an increasing helical twist. Thus, two ranges of the achiral double network Ia 3d phase range can be distinguished, a long pitch and a short pitch, with the chiral triple network I23 cubic phase in the intermediate pitch range. The findings provide a new prospect for the directed design of cubic phase‐forming functional materials based on spontaneously formed helical network liquid crystals with tunable application specific properties