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We report on observations of optical switching in bent-core liquid crystals possessing azocinnamoyl groups in both elongated side arms. UV-activated switching was observed in two of the synthesised materials, both of which were studied optically and with dielectric spectroscopy. Polarising microscopy was used to measure the changes in optical transmittance, and while no illumination effect was seen in the nematic phase, visible changes were observed in the smectic phase, most pronounced in the close vicinity of the smectic to nematic-phase transition. Switching dynamics was characterised, and we found that the average switching time varies strongly with temperature. The relaxation process is faster (below $200\,\,{\rm{ms}}$ 200 m s ) and exhibits no significant temperature dependence. Dielectric measurements have shown that the observed smectic C phases were not polar with the molecules possessing a rather small electric dipole moment. Upon illumination, small differences in the dielectric permittivity were observed only in the smectic phase, suggesting that the illumination effect can be attributed to changes in the molecular conformational and orientational order. [ABSTRACT FROM AUTHOR]

We discuss the liquid crystal phases of a compound consisting of bent-core hockey-stick-shaped molecules. The compound exhibits two smectic phases with an in-plane pseudo-polar order in addition to the tilt order in the layers. The transverse shape anisotropy and dissimilar arm lengths of these bent-core molecules stabilise these pseudo-polar tilted smectic phases. The higher temperature smectic phase has an anti-clinic organisation of the molecules in successive layers. An intermediate organisation between the synclinic and anti-clinic structures is found in the lower temperature smectic phase. We have developed a simple phenomenological model to account for these phases. In the higher temperature smectic phase, a field-induced Freedericksz type transition is observed beyond a threshold electric field. We also account for this field-induced transition using our theoretical model. [ABSTRACT FROM AUTHOR]

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]

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 SmCSPA 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. [ABSTRACT FROM AUTHOR]

A series of asymmetric bent-core liquid crystals, bearing the stilbene moiety in the side wing, was synthesised and characterised. Structural modifications of the non-stilbene wing resulted in formation of various mesophases. Compounds 1 and 3 with an ester linking group between the phenyl rings in this wing have an enantiotropic non-polar smectic-type mesophase, while compound 2, incorporating the biphenyl moiety, has a ferroelectric SmCaPF phase. The molecules of 2 undergo photoisomerisation in solution under UV irradiation, while the mesophases are unstable and UV irradiation leads to irreversible changes. The mesophases of 1 and 3 do not show any electro-optical switching and polarisation current. Applying low frequency AC voltage on a planarly aligned film of compound 2, smectic domains grow, while the birefringence and an image flickering increase. The observed features of these compounds identify stilbene as an attractive moiety in the design of advanced materials based on bent-core molecules. [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]

New bent-core liquid crystalline dimers with W-shaped molecular geometry have been prepared and studied. We have modified the dimer shape by variation of the connecting part between two bent-core units and changing the terminal chains (perfluoroalkyl, siloxanealkyl). Additionally, we have altered the inner bend angle value (120°, 60° and 148°) by utilization of different aromatic units. Mesomorphic properties of new dimers were established based on the texture observation in the polarizing microscope and DSC measurements. Moreover, x-ray structural analysis has been performed for selected dimers to confirm phase identification. For most of the studied dimers, nematic or columnar phases have been detected, for several compounds appearing in a nematic-columnar phase sequence on cooling from the isotropic phase. The studied dimers showed richer polymorphism than their monomeric counterparts. [ABSTRACT FROM AUTHOR]

We have prepared liquid crystalline materials based on hydroxynaphthalene carboxylic acids. The mutual location of both functional groups has been varied, which resulted in compounds possessing a bend angle of 120° and 60°, therefore in bent-core and V-shaped compounds, respectively. The mesomorphic properties have been established with polarizing optical microscopy, DSC measurements and X-ray investigation. The properties of the studied have been compared with analogous derivatives to establish the role of ester orientation in the vicinity of the central naphthalene core. [ABSTRACT FROM AUTHOR]

Research on low-temperature polar bent-core nematogens having lower molecular weight has gathered appreciable momentum by virtue of their significance in potential applications. However, the lack of availability and easy-to-perform synthesis processes appears to be the bottleneck towards their fabrication and thereby limiting their possible device applications. Hence, we have designed a new class of achiral symmetrical three-ring-based-bent shaped molecules incorporating an imine and ester linkage at the molecular bend with highly polar nitro/cyano terminal moiety exhibiting low-temperature nematic mesmorphism. The occurrence of disordered nematic mesomorphism has been confirmed by optical texture, differential scanning calorimetry scan and X-ray diffraction measurement. Dielectric spectroscopy and electro-optical investigation has also been carried out intending towards the potential applicability of the materials. Density functional theory analyses at the molecular level provide valuable information regarding the formation of the nematic mesophase and various parameters of the molecular spatial arrangement. Polarising optical microscopy study reveals the easy of alignment of these types of polar bent-core materials upon glass surface suitable for liquid-crystal-based sensing applications. Formation of mesophase with such a small bent molecule is rather difficult but we have successfully demonstrated the existence of disordered nematic mesomorphism at relatively low temperature. [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]

The characteristics of the twist-bend nematic (NTB) phase of an achiral asymmetrical rigid bent-core liquid crystal (LC), the ends of which are terminated by symmetric alkyl chains, are reported. The nematic–nematic phase transition and its properties are studied by differential scanning calorimetry (DSC), polarising microscopy and the electro-optic techniques. Large domains of opposite handedness are observed in the absence of the external field in the NTBphase. Another set of periodic striped pattern consisting of domains with sharp boundaries is formed when a high-frequency electric field with a magnitude above its threshold is applied across a planarly aligned cell. The neighbouring domains are of opposite chirality. The temperature dependence of the heliconical angleθ0is determined from the birefringence measurements using Haller’s extrapolation technique. This material shows lower values of the heliconical angle (~9.3° at a temperature of 155°C within the NTBphase) when compared with the previously reported dimer-based twist-bend nematic LCs (31°±3°). [ABSTRACT FROM AUTHOR]

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]

We have prepared and studied new series of bent-core liquid crystals based on laterally substituted 3-hydroxybenzoic acid. The physical properties of the materials were tuned by the type of lateral substitution in the vicinity of the carboxylic unit. Further modification was achieved by reorientation of connecting ester linkages in the elongating side arms and by changing the length of terminal alkyl chains. The physical properties of the materials were determined by differential scanning calorimetry (DSC), polarising optical microscopy and for selected compounds by x-ray diffraction analysis. The introduction of fluorine as lateral substituent leads to the appearance of a lamellar or columnar mesophase. On contrary, methyl- or chlorine-substituted compounds are mostly crystalline. We have found a strong correlation between the size of the lateral substituent and the mesogenicity of the studied materials. [ABSTRACT FROM PUBLISHER]

Second-harmonic generation (SHG) in the nematic phase of bent-core oxadiazole-based liquid crystals (LCs) was studied and compared to that for the rod-like compound 4-cyano-4ʹ-n-octylbiphenyl (8CB). Weak, isotropically scattered second-harmonic (SH) light was observed for all materials, consistent with SHG by nematic director fluctuations. The SH intensity produced by the bent-core materials was found to be up to ~ 3.4 times that of 8CB. We discuss this result in terms of the dependence of SH intensity on temperature, elastic constants and flexoelectric coefficients. We have calculated the latter by using a molecular field approach with atomistic modelling, thus demonstrating how molecular parameters contribute to the flexoelectric coefficients and illustrating the potential of this method for predicting the flexoelectric behaviour of bent-core LCs. We show that the increased SH signal in the bent-core compounds is partly due to their nematic phases being at a much higher temperature, and also potentially due to them having greater flexoelectric coefficients, up to ~1.5 times those of 8CB. These estimates are consistent with reports of increased flexoelectric coefficients in bent-core compounds in comparison to rod-like compounds. [ABSTRACT FROM AUTHOR]

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]

Isoxazole-based bent-core liquid crystals (LCs) derived from naturally occurring curcumin were synthesised and their LC properties were investigated by polarising optical microscopy, differential scanning calorimetry and X-ray diffraction techniques. Five compounds, including a branched alkyl chain derivative, were prepared and characterised. These derivatives exhibit enantiotropic mesophases. While lower homologues display wide-temperature-range nematic phase, a longer-chain derivative3dshows smectic C phase in addition to the nematic phase. The bent angle in these compounds is in between calamitic LCs and banana LCs. Therefore, the molecules escape from polar order packing observed in typical bent-core LCs. Increasing the length of alkyl chain reduces both melting and isotropic temperatures in the series. However, the compound with branched alkyl chains exhibits significant reduction in the nematic-isotropic temperature only. Detailed XRD experiments confirm the presence of the N phase in the lower homologues and SmC phase in a higher homologue. [ABSTRACT FROM AUTHOR]

Three new series of bent-shaped molecules with 4-chlororesorcinol, 4-bromoresorcinol or 4-fluororesorcinol as the central unit, and azobenzene with different alkoxy chain length as side arms were synthesised. The mesophase behaviour was investigated by polarising optical microscopy, and differential scanning calorimetry. A representative example has also been characterised by X-ray diffraction (XRD) studies. It is found that almost all of the materials prepared are monotropic liquid crystalline. Depending on the substituent at the central unit and on the chain length nematic phases, B6phases, a B4-like dark conglomerate phase and a modulated/undulated anticlinic SmC phase were found. As a unique feature, upon reducing the chain length a transition from nematic to B6-type smectic phases was observed, which is reverse to usually observed phase sequences. The UV–vis absorption spectroscopy was also performed to study the effect of light-inducedtrans–cis-isomerisation on the prepared compounds. [ABSTRACT FROM AUTHOR]

A new series of asymmetric bent-core compounds were synthesised using azo linkages with equal as well as unequal terminal alkyl chains and their photo and electrical switching properties investigated. The mesomorphic properties were characterised using polarised optical microscopy, X-ray diffraction and differential scanning calorimetry. The lower homologues of the series of compounds show a B1 phase whereas higher homologues exhibit a B2 phase. The B2 phase shows an anticlinic-antiferroelectric switching behaviour during electro-optic measurements. The photo-switching properties of the azobenzene containing bent-core molecules were investigated using ultraviolet-visible spectroscopy. The trans to cis photo isomerisation was observed at 25 s whereas reverse processes took 14 h in chloroform. [ABSTRACT FROM AUTHOR]

A series of new bent-shaped liquid crystals based on 3-aminophenol was synthesised and their mesomorphic behaviour studied by differential scanning calorimetry, polarised optical microscopy and X-ray diffraction. Generally, the introduction of an amide group leads to a small increase in the clearing temperatures and stabilisation of the formed mesophases in comparison with the structurally related resorcinol based materials. The role of the linking group orientation on mesomorphic properties was also studied. Compounds with an identical orientation of ester groups in the same direction with respect to the central benzene core exhibited the columnar B1 phase for the shortest length of the terminal chain and the lamellar B2 phase for longer homologues. The reorientation of the ester unit(s) in another series resulted in the appearance of another type of columnar phase (B1REV). [ABSTRACT FROM AUTHOR]

In the continuation of our study of the role of direction of the linking ester group and lateral substitution, we present three series of bent-core liquid crystals based on 7-hydroxynaphthalene-2-carboxylic acid. The mesomorphic properties were investigated by polarised optical microscopy, differential scanning calorimetry, X-ray diffraction and electro-optical methods. For non-substituted and laterally methyl-substituted compounds, columnar phases of broken-layer type and a smectic A phase were found. With the increasing length of the terminal alkyl chain, an increasing stability of the lamellar smectic A phase was observed. Substitution with a methyl group led to substantial narrowing of the mesophase range and the chloro-substituted materials exhibited no mesophases. The results are also discussed in context with the earlier studied naphthalene based mesogens. [ABSTRACT FROM AUTHOR]

We report the synthesis and mesomorphic properties of a homologous series (10a-10g) of bent-core molecules constructed through covalent linkage of structurally non-symmetrical rod-like mesogens connected with a 1, 3-phenylene unit. The study of homologous series underlines the importance of length and nature of terminal chains. The homologues of shorter chains show a typical non-switchable rectangular columnar B1 phase, while the switchable lamellar (B2) phase is induced on moving to higher homologues. X-ray diffraction patterns indicate the presence of B1 and B2 mesophases. Polarised optical microscopy investigations under electric field in the B2 phases revealed the existence of anticlinic antiferroelectric texture. The measured spontaneous polarisation value in one of the compounds is 936 nC cm-2, a high polarisation value in bent-core liquid crystals. [ABSTRACT FROM AUTHOR]

Planar (homogeneous) and homeotropic alignment of some unconventional nematic liquid crystals with both positive (Δε > 0) and negative (Δε < 0) dielectric anisotropies are studied on planar (AL-1254) and homeotropic (JALS-204, SE-1211) alignment layers. Irrespective of the sign of dielectric anisotropy and molecular structure, uniform planar alignment of the director is observed in Al-1254 coated cells whereas no stable homeotropic alignment of bent-core compounds with Δε < 0 is observed in cells coated with JALS-204 or SE-1211. Alignment properties of these two types (Δε > 0 and Δε < 0) of unconventional nematic liquid crystals are explained considering the steric, short-range dipolar and long-range van der Waals interactions at the interface. A bent-core compound (Δε < 0) which has no higher-order liquid crystalline phase except for the nematic exhibits a narrow temperature range of homeotropic state (on JALS-204) while cooling from the isotropic phase followed by an anchoring transition to a planar state as the temperature is further lowered. The measured polar anchoring energy of this compound on JALS-204 is significantly lower than many known calamitic liquid crystals and is responsible to the anchoring transition observed. [ABSTRACT FROM AUTHOR]

The nematic phase occurring in bent-core (or banana-shaped) molecule systems is conducive to the occurrence of biaxial (Nb) phase due to the inherent biaxial molecular shape. The tendency of bent-core molecules to stabilise a layered (smectic) structure poses one of the difficulties in obtaining the thermotropic Nb phase. To investigate the factors favouring the formation of the nematic phase, a number of bent-core nematic liquid crystals derived from 1, 3-phenylene central bent motifs were synthesised. Results of an investigation into the influence of the nature and position of substitutions on the central core and two rigid arms as well as the type of linkage units on the occurrence and stability of the nematic phase reveal important structure-property relationships. Textural and X-ray diffraction measurements have helped us identify molecules with the potential of exhibiting the Nb phase. [ABSTRACT FROM AUTHOR]

New non-symmetrical bent-core mesogens with a variable flexible chain length have been synthesised. 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 is saturated. The compounds represent novel non-symmetric molecules both in their core and tails. The main features of differential scanning calorimetry, polarising optical microscopy, electro-optical and polarisation current measurements for all studied materials indicate a transition between a single tilted synclinic ferroelectric and an antiferroelectric smectic phase. [ABSTRACT FROM AUTHOR]