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In this account, bent-core molecules involving the polar cyano-group are reviewed with a focus on the intensely investigated class of 4-cyanoresorcinol bisbenzoates. Cybotaxis and phase biaxiality of their nematic phases, the transition from nematic to smectic phases and the development of tilt and polar order in their smectic phases are discussed. Of special interest are heliconical (twist–bend) smectic intermediate phases occurring at the transition from paraelectric to antiferroelectric polar smectic phases if coinciding with the transition between anticlinic and synclinic tilt correlation. For some compounds, a reentrant polar SmA phase (SmALTPF) is observed below SmC and a silylated compound forms a new leaning-type phase. The formation of these achiral phases instead of the polar SmC phases involving layer chirality is discussed under the aspect of enantiophobic vs. enantiophilic organization. The effects of core structure and chain branching and, in addition, the effects of the intrinsic transient molecular, superstructural, and permanent molecular chirality on LC self-assembly are discussed. [ABSTRACT FROM AUTHOR]

The remarkable ability of certain molecules with a bent molecular shape to form hierarchically self-assembled helical building blocks with negative Gaussian or cylindrical curvature featuring in-layer hexatic ordering– so-called B4 phases – has quickly transformed these molecules into desirable building blocks for a variety of potential applications in optics, energy harvesting, and metamaterials. We here demonstrate that these building blocks, helical nanofilaments, helical microfilaments, and heliconical-layered nanocylinders, can to some degree be predictably blended. Supported by scanning as well as transmission electron microscopy and variable angle x-ray scattering data, the three bent-core compounds, each forming exactly one of these B4 morphologies, in binary mixtures at a 1:1 molar ratio, form either the third missing, or one of the two but with opposite handedness, or a random mixture of the two selected morphologies with larger overall dimensions. Furthermore, for two of the mixtures the a priori predicted handedness of the chiral filaments was experimentally confirmed. The third mixture with a priori predicted antagonistic handedness of the initial morphologies forms a combination of the two, one apparently achiral and the other one with opposite handedness. [ABSTRACT FROM AUTHOR]

The splay, twist and bend elastic constants (K11, K22 and K33) have been measured as a function of temperature in bent-core/calamitic mixtures based on three different calamitic materials (5CB, 8CB and ZLI1132) and two bent-core dopants. The behaviour of the splay and bend constants are as expected; a reduction in K33 of ~20%, in line with predictions from mixing rules and other observations. Interestingly, no change is seen in the splay constant, K11 of the calamitic hosts. Surprisingly though, the twist elastic constant exhibits a reduction of 30 – 40% in all mixtures across the nematic range, an effect not previously reported and much larger than mixing rules can explain. The elastic behaviour is universal in our mixtures. We explain part of the reduction in the twist deformation by considering the influence of the chiral conformer fluctuations of the bent-core molecules on the twist elastic constants of the mixtures. However, the dramatic reduction can only be fully explained by also including contributions from chiral conformer fluctuations of the calamitic host, a form of chiral amplification. [ABSTRACT FROM AUTHOR]

Development of new liquid crystalline materials exhibiting interesting properties and phases continues to be an enabling enterprise in the forward march of their successful display and non-display applications. The design and synthesis of a homologous series of liquid crystalline bent-core compounds derived from the oxadiazole bisaniline moiety and the phase behavior of three members of the series that exhibit nematic, smectic C, and dark conglomerate phases is reported. The liquid crystalline phases exhibited by these mesogens are characterized using polarized optical microscopy, differential scanning calorimetry and x-ray scattering techniques. All three homologs prepared exhibit the nematic phase. Interestingly, the homolog with short hexyl terminal chains exhibits only the nematic phase that is stable over a very broad, nearly 100 K wide, temperature range. The compound with terminal octyl chains shows the chiral dark conglomerate phase below the nematic phase despite the bent molecules being achiral. The homolog with dodecyl alkyl chains is found to possess the smectic-C and two additional lamellar phases besides the nematic phase. These compounds enrich the library of achiral bent-core materials capable of exhibiting chiral and nematic phases. [ABSTRACT FROM AUTHOR]

We report the synthesis and study of the mesomorphic behaviour for a series of bent-core materials based on 2-substituted 3-hydroxybenzoic acid as a central unit. Properties of the studied compounds are tuned by lateral substitution (F, Cl, CH3, NO2and CN) in the position 2 of 3-hydroxybenzoic acid and by the length of the terminal alkyl chain. All studied homologues exhibit at least one mesophase. Mesomorphic properties are established and compared with other 3-hyhroxybenzoic derivatives studied previously, with analogous orientation of ester linkages, laterally substituted at different position on the central core. The role of the substitution and the length of the terminal alkyl chain are discussed. [ABSTRACT FROM PUBLISHER]

Spontaneous or induced chiral symmetry breaking in achiral systems is unusual and understanding the origin of such a phenomenon has been an important area of research for several years. The optically isotropic mesophases exhibited by unconventional liquid crystals are one of the most interesting systems to investigate spontaneous chiral symmetry breaking in liquid crystal mesophases formed by achiral moieties. The dark conglomerate (DC) phases are one such optically isotropic family of phases. In this paper, a detailed account of the tendency of bent-core mesogens to form a variety of polar smectic phases, the formation of DC phases due to layers deformations and the general optical, electrical, physical properties of the DC phases are given. An example of a DC phase which exhibit distinct electro-optic properties is described with the nature of dynamics of the response and physical reasons responsible for such behaviour. The challenges and prospects of the DC phases are discussed for their potential applications in novel devices. [ABSTRACT FROM PUBLISHER]

Azo-functionalised materials are of special interest due to their photochromic nature, i.e. reversibletrans–cisisomerisation upon photoirradiation. The combination of photosensitivity and liquid crystalline properties in the same molecule allows the material to be exploited for optical and optoelectronic devices. Azobenzene-based bent-core liquid crystals (BCLCs) have attracted considerable attention in recent years due to their rich mesomorphism. In this review, the main research directions and different molecular structures of bent-core molecules incorporating azobenzene unit and its subtype the so-called hockey-stick molecules are summarised. Additionally, azobenzene-based U-shaped molecules, hydrogen-bonded bent-shaped liquid crystalline materials and some selected examples of two different types of photoswitchable mesogenic dimers are provided. The nature, number and position of the lateral substitutions able to modify the phase behaviour of such BCLCs, affording in turn interesting liquid crystalline phases are discussed. Finally, the isomerisation process of these photosensitive BCLCs in solutions or in mesophases under the effect of UV–visible irradiation is summarised. [ABSTRACT FROM PUBLISHER]

In some liquid crystal (LC) mixtures of bent-core host molecules that form helical nanofilaments (HNFs) and chiral, rod-shaped molecular guests, the spontaneous chirality of the HNFs is not influenced by the guest handedness. In other mixtures, the filaments become homochiral, responding to the handedness of the guest. We show that the important distinction between these two behaviours is the solubility of the guest material in the HNF phase. In our experiments, chiral LC mesogens doped into the HNF phase result in an enantiomeric imbalance and sometimes change the phase sequence on cooling from the isotropic melt. [ABSTRACT FROM PUBLISHER]

Bent-core liquid crystal was studied by polarizing microscopy, and electro-optical measurements. This compound forms a sequence of smectic liquid crystalline phases (SmA, SmCSPR, SmCSPF) with growing tilt, polar coherence length and increasing layer coupling on decreasing temperature. The paper is focused on the first observation of a helix in achiral SmCSPFphase. The helix is parallel to the layer normal and the pitch is much shorter than the optical wavelength. This new phase shows fast (∼30 μs) grey-scale switching due to the deformation of the helix, providing potential use in devices with fast linear and thresholdless electro-optical response. [ABSTRACT FROM AUTHOR]

Brief history of ferroelectric and antiferroelectric liquid crystals is described. To reduce the symmetry of the molecular systems, two ideas were successfully used; introductions of chirality and molecular shape asymmetry. The chirality introduced into rod-shaped and disc-shaped molecular systems led to chiral smectic C and chiral discotic columnar phases. Dogleg from rod and cone or bowl from disc in molecular shapes brought about polar organization by their packing, realizing so-called banana phases and polar columnar phases, respectively. Particularly, bent-core mesogens exhibit a variety of novel polar phases. In addition to such established polar structures, some evidences about polar structures realized in polar nematic and cholesteric phases are given in some detail. Attempts for the application of these polar liquid crystals such as displays are also overviewed. [ABSTRACT FROM AUTHOR]

A new series of bent-shaped mesogens has been synthesized and their mesomorphic properties studied. All compounds exhibit the B2 (SmCP) phase. Compounds with a 3,4'-biphenyldiol central core containing salicylaldimine-based and two terminal tetradecyloxy tails are reported. In addition, the effects of lateral substituent (R=F and Cl) at the biphenyl core into 3'-position are examined. These substituents have a strong influence in reducing the clearing temperatures and increasing temperature range of the SmCP phase. The electro-optical switching characteristics have been investigated using both triangular-wave and DC electric fields. For several compounds, chirality switching from racemic to the homochiral state was seen after application of a low-frequency AC field. We studied spontaneous polarization, switching time, tilt angle, switching time, voltage-transmittance effect, and dielectric parameters (namely dielectric strength, relaxation frequency, distribution parameter) of a 7.5-µm-thick planar-oriented sample of a highly tilted anti-ferroelectric liquid-crystal material and investigated the correlation between their electro-optical properties and the chemical structures of these compounds. [ABSTRACT FROM AUTHOR]