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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 analytically derive and numerically simulate a two-dimensional energy functional modelling the effects of a constant electric field on a thin sample of a bent-core liquid crystal in the ferromagnetic SmA-like phase. We start from a three-dimensional domain and show that under proper rescaling and in the limit of small thickness the electric self-interactions term gives rise to boundary terms. We compare our results to previously proposed models. [ABSTRACT FROM AUTHOR]

The thermal stability of some symmetric and asymmetric bent-core liquid crystals derived from 2,7-dihydroxynaphthalene as well as the degradation mechanism by the TGA (thermogravimetric analysis) and TG/MS/FTIR technique (simultaneous mass spectrometry and Fourier transform infrared spectroscopy of off-gases from thermogravimetric analyzer), in inert atmosphere (nitrogen and helium) is reported. The TGA of the three classes of compounds in the nitrogen atmosphere allowed the stability series to be established. TG/MS/FTIR technique indicated that the initiation of thermal decomposition probably that occurs at –N = N– groups continues with the fragmentation of both aromatic units and alkyl terminal chains and removal of CO2 at temperatures above 400 °C. In order to gain insight into the mechanism of thermal decomposition and the factors influencing the different transition temperatures of liquid crystals, a series of quantum calculations were performed. Values of the dipole moment, total energy and energy of the highest occupied/lowest unoccupied molecular orbitals of the mesogens were calculated by the density functional theory method. These along with different shape and size descriptors of the mesogens were used as input data for statistical analysis to find the relationship between these values and mesophase behavior. In addition, the Fukui indices for the radical attack, also determined by quantum calculation, showed a decomposition mechanism similar to the experimental thermal decomposition one. [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]

We report on the synthesis and characterization of bent-core liquid crystal (LC) compounds and the preparation of mixtures that provide an optically isotropic antiferroelectric (OI-AFLC) liquid crystal display mode over a very wide temperature interval and well below room temperature. From the collection of compounds synthesized during this study, we recognized that several ternary mixtures displayed a modulated SmCaPA phase down to below -40 °C and up to about 100 °C on both heating and cooling, as well as optical tilt angles in the transformed state of approximately 45° (optically isotropic state). The materials were fully characterized and their liquid crystal as well as electro-optical properties analyzed by polarized optical microscopy, differential scanning calorimetry, synchrotron X-ray diffraction, dielectric spectroscopy, and electro-optical tests.

[Display omitted] • Binary mixtures of bent-core liquid crystals show columnar and smectic phases with good miscibility. • All mixtures show mesomorphic behaviour, with reductions in their clearing points and liquid crystalline ranges. • The mixtures show strong dielectric response mainly promoted by the presence of ester groups in the bent-core molecules. • Reversible trans -to- cis photoisomerization of azobenzenes is applied to tune the molecular dipole moment, assessed by DFT. • Irradiation increases the conductivity of the equimolar mixture by two orders of magnitude. We report the mesomorphic behaviour, dielectric and ferroelectric properties, and computational modelling of binary mixtures containing two bent-core liquid crystals: the so-called NG75-COO (3,4́- Bis[4-(4-n-tetradecyloxybenzoyloxy)benzoyloxy]biphenyl), which forms smectic phases, and IP31- AzB (3,4′ - Bis[4-(4-n-tetradecyloxyphenylazo)benzoyloxy]biphenyl), which forms columnar phases. The phase diagram, assessed by polarised optical microscopy, shows that the binary mixtures retain the mesophase behaviour of the major component, whilst the equimolar mixture displays smectic-type phases. Dielectric and ferroelectric analyses were carried out on samples containing 10%, 50%, and 90% of IP31-AzB (molar %), and we also investigated structure–property correlations by differential functional theory. The NG75-COO/IP31-AzB mixtures undergo strong dielectric and ferroelectric response due to the presence of highly polarisable groups in the bent-core components, particularly at the ester groups. All the mixtures under study exhibit light-responsiveness induced by reversible E -to- Z photoisomerization (trans -to- cis) of the azobenzene group in IP31-AzB, together with an increase in the molecular dipole moment. The potential to tune the phase behaviour of the mixtures, as well as their dielectric and ferroelectric responses, are investigated by light irradiation under different conditions of intensity and temperature. [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]

Two homologue achiral bent-core liquid crystals (LCs), BCN66 and BCN84, in their nematic phases are studied by dielectric spectroscopy in the frequency range 10 Hz–10 MHz. In each of these compounds, two relaxation processes are identified and assigned to (i) collective dynamics of molecules in nanometer-sized cybotactic clusters and (ii) individual molecular relaxations, in the ascending order of frequency of the probe field. The temperature and the bias electric field dependence of the dielectric strength and relaxation frequency for these processes are shown to give rise to sharpness in cluster boundaries, increased size and volume fraction in the LC nematic phase. The effect of the bias field on the LC cell is similar to reducing its temperature; both variables increase the cluster size and volume fraction and give rise to sharp cluster boundaries. The findings confirm that dielectric spectroscopy is a powerful and an extremely useful technique to provide a deeper understanding of the mechanism of cybotactic cluster formation in the isotropic liquid and the nematic phase of LCs as a function of temperature and the bias field.

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]

Photosensitive liquid crystals represent an important class of functional materials that experience rapid development. Hereby, we present novel bent-core liquid crystals bearing a lateral substitution on the central core and in the vicinity of the photosensitive unit—an azo group. The azo group enables fast (E)-to-(Z)-isomerization upon irradiation with UV-light and visible light, while the substitution facilitates the high stability of the photochemically formed (Z)-isomer. The effectiveness of the irradiation and the composition of photostationary states was determined by UV/Vis and 1H NMR spectroscopy. A nematic phase formed by the materials was characterized by differential scanning calorimetry and optical polarizing microscopy. We show that the materials easily change their relative configuration of the N=N double bond not only in solution, but also in the mesophase, which leads to fast isothermal phase transition from the nematic phase to isotropic liquid.

Bent-core liquid crystals (BCLC) have been widely studied as a result of their unusual polar and chiral properties. Similar to calamitic and discotic molecules, BCLC molecules also exhibit nematic phases, besides other higher order mesophases. The aim of this work is to comparatively analyze the mesomorphic behavior of some bent-core 1,3-disubstituted benzene core compounds derived from resorcinol and isophthalic acid. Thus, the two classes of compounds differ in the nature of the orientation of the ester bond between the benzene central core and the two branches attached to the core. The mesomorphic behavior was elucidated by polarized light optical microscopy and differential calorimetry. Given the relatively high isotropic points of the compounds, confirmation of the thermal stability in the domains manifesting liquid crystalline properties was performed by thermogravimetric studies. The theoretical explanation of the difference in mesomorphic behavior for the two classes was based on molecular modeling studies.

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]

Abstract We report the influence of intermolecular and intramolecular hydrogen bonding on the excited state proton transfer (ESPT) emission behavior of two bent core liquid crystal (BLC) molecules, C 54 H 63 NO 9 (BLC3) and C 60 H 75 NO 9 (BLC4), having a Schiff-base and two long alkyl chains at its two ends. Fluorescence spectra of these BLC molecules dispersed in different solvents show dual emission (at ∼ 365 nm and ∼425 nm) from the keto and enol tautomers. We observed that the population of these keto and enol tautomers and the corresponding intensities of fluorescence emission are strongly influenced by the solvent polarity. In protic solvents, formation of intermolecular hydrogen bond with the Schiff-base of the BLC molecules is highly favoured than the intramolecular hydrogen bonding. This intermolecular hydrogen bonding drastically reduces the population of the keto tautomers in the excited state, resulting in enhanced enol fluoroscence band along with a weak keto emission band. The observed intensity of the enol fluorescence band is the highest for the most studied polar solvent (methanol). On the other hand, in aprotic solvents, the intramolecular hydrogen bonding is highly favoured, which leads to the formation of keto tautomers in the excited state. Hence, an intense keto emission band is observed for the aprotic solvents along with a weak enol emission band. From the time resolved fluorescence studies we observed a longer life time for the keto band than that for the enol band. This is also related to the delayed emission associated with the vibrational bands resulting from the bulky alkyl chains attached to the ends of the BLC molecules. Graphical abstract The intensity of fluoroscence emission can be tuned through solvent polarity via formation of intramolecular or intermolecular hydrogen bonds. Image 1 Highlights • Excited State Proton Transfer emission of the LCs can be tuned through the solvent polarity. • Intensity of keto emission increases with increase in polarity of the protic solvents. • Intra-molecular (inter-molecular) hydrogen bonds prevail in aprotic (protic) solvents. • The intensity of keto - (enol-) emission is stronger in aprotic (protic) solvents. [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]

Abstract Nano-doping of bent-core liquid crystals are of recent interest because the blending of bent-core liquid crystals with nanoparticles opens a new avenue both for fundamental research and the development of smart materials with desired properties for different types of applications. A four-ring achiral bent-core liquid crystal possessing only the nematic phase was doped with titanium dioxide (TiO 2) nanoparticles and its dielectric and electro-optical properties are investigated. The experimental result shows both an increase in the moving ion concentration and a drastic increment in the dielectric permittivity of the doped sample. The large increase in the dielectric permittivity at lower frequency is due to the combined effect of space charge polarization and the change in the viscosity of material due to dilution effect. Due to dilution effect, the transition temperature of all the doped samples is decreased by about 10 °C–13 °C. Highlights • Addition of TiO 2 nanoparticles changes the transport properties of bent-core LCs. • Modification of transport properties is due to increase in ions concentration. • The transition temperature of the doped sample decreases by about 10 °C–19 °C. • The decrease in transition temperature is attributed to dilution effect. • Useful for display and non-display based application depending on ions in LCs. [ABSTRACT FROM AUTHOR]

[Display omitted] • Bent-core liquid crystals containing tetra(ethylene-oxide) moieties were doped with lithium triflate salts. • The doped materials display smectic mesophases inside indium tin oxide cells. • Lithium ions are selectively solvated in the ethylene-oxide blocks. • The presence of lithium ions promotes direct current conductivity at low frequencies. • The materials show potential as new nanostructured electrolytes, controllable by light irradiation. We report the dielectric and conductivity response of three materials containing bent-core and tetra(ethylene-oxide) moieties, and their complexes doped with lithium triflate salts, as new potential nanostructured electrolytes. Whilst the pristine bent-core compounds do not show mesomorphism, the doped materials display smectic mesophases inside indium tin oxide cells assisted by the selective solvation of the lithium ions in the ethylene-oxide blocks. The dielectric response of the materials in the high-frequency range is controlled by the chemical composition of the bent-core structure, and the presence of lithium ions promotes direct current conductivity at low frequencies, in the σ dc ∼ 10-5 S cm−1 range, which can be enhanced to σ dc ∼ 10-4 S cm−1 via trans -to- cis photoisomerization of azobenzene groups. The dynamic and dual character of these materials (responding to low and high frequency electrical fields), the formation of ferroelectric crystals capable to store energy, and their interactions with light, will be applied to develop new energy devices. [ABSTRACT FROM AUTHOR]

[Display omitted] • New symmetrical fluorurated bent-core molecules are reported. • Compounds organize into SmC s P A and SmC A P A antiferroelectric structures. • High values of polarisation (900–600 nC/cm2) were obtained. • Bent-core compounds are good candidates for high power capacitors. The synthesis of several symmetrical bent-core compounds with two or four fluorine atoms as substituents on the calamitic units is reported. The influence of fluorine substitution concerning its effect upon the mesophase structure and switching behavior was investigated. The synthesized compounds are built from resorcinol as central core and contain seven phenyl rings linked by ester, imine and azo units with terminal decyloxy/dodecyloxy chains. The mesophases were characterized by polarizing optical microscopy, differential scanning calorimetry, electro-optical investigations and X-ray diffraction (XRD) studies. The photochemical characterisation of bent-core azobenzene compounds confirmed the structure of E isomer. The temperature range of mesophases increases upon the introduction of fluorine substituents, as compared with the corresponding unsubstituted compound. All the compounds exhibit antiferroelectric properties. Depending on the number of the F substituents, racemic SmC S P A mixtures and SmC A P A homochiral structures were identified. High values of spontaneous polarization up to 900 nC/cm2 were recorded. DFT studies were used to calculate the minimum energy molecular structures, the dipole moments and the electrostatic potential surfaces of the compounds. [ABSTRACT FROM AUTHOR]

Azo-functionalized materials are one of the appealing groups of the functionalized materials owing to their photoswitching behaviour and have been explored for various potential applications viz., optical data storage, sensor, display devices, nonlinear materials and molecular switches. Recently, azo-functionalized bent-core liquid crystals (BCLCs) have gained significant attention because they have dual properties of BCLCs and azobenzene, which enables to generate new multifaceted functional and smart materials. In this report, the recently synthesized azobenzene containing bent-core mesogens and its subclass, the so-called hockey stick and V-shaped molecules are summarized. The mesomorphic behaviour of reported BCLCs affected by the type of central core unit, the nature, number and position of the lateral substituents and the type and length of the terminal chain are discussed. The photoisomerization process of these photoresponsive BCLCs in solid, solution and mesophase, as well as the impact of light on the chemical and electrical properties of them, are discussed.

We present small-angle X-ray scattering, polarized optical microscopy and electric current measurements of a sulfur-containing bent-core liquid crystal material for characterization of the layer and director structures, thermally and electrically driven transitions between antiferroelectric and ferroelectric structures and switching properties. It was found that the material has polarization-modulated homochiral synclinic ferroelectric (SmCsPFmod), homochiral anticlinic antiferroelectric (SmCaPA) and racemic synclininc antiferroelectric (SmCsPA) structures that can be reversibly switched between each other either thermally and/or electrically. High switching polarization combined with softness of the liquid crystalline structure makes this compound a good candidate for applications in high-power capacitors and electrocaloric devices.

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]

Dielectric measurements were carried out on a series of bent-core liquid crystals based on resorcinol and 2, 7-dihydroxinaphtalene cores and presenting B mesophase in between 30-160°C. The dielectric constants in the liquid crystalline phase were measured as a function of frequency and temperature. The dielectric relaxation of bent-core compounds has been analyzed by dielectric constants/frequency curves. The variation of the dielectric strength with temperature has been determined as well. The frequency dependence of dielectric loss has also been analyzed by means of frequency exponent. [ABSTRACT FROM AUTHOR]

[Display omitted] • Triazole bent-core mesogens based on bis -1,2,3-triazoles were synthesized. • The triazoles exhibited mesogenic phase transition over a wide temperature range. • The mesophases and textures were studied by employing POM, DSC, and XRD. • SEM investigation revealed highly ordered columnar architectures of the compounds. • The self-assembly was driven by van der Waals and π-π stacking interactions. Triazole bent-core mesogens based on symmetrical bent-shaped of two 1,2,3-triazole ring arms were synthesized and characterized. The the thermail stability of those new synthesised mesogens from the embient temperature to 215 °C giving mesogenic phase transitions over a wide temperature range is investigated. A study of different substituents located on the rigid bentcore was carried out for compact supramolecular innovated architectures which were produced by self-assembly of many triazoles soft crystals with a liquid crystals behaviour. The synthetic procedures, characterizations and polymorphism of four novel bent-core mesogens comprising symmetrical 1,2,3-triazole arms were demonstrated. The preparation process was carried out through"click chemistry" via Cu-catalyzed azide-alkyne cycloaddition of nitro- and alkyl-substituted aromatic azides with aromatic compounds, comprising terminal alkyne groups and long alkyl or alkoxy side-chains. The triazole derivatives were fully characterized by 1H/13C NMR, Fourier-transform infrared spectroscopy (FTIR) and elemental analysis. The mesophases and textures were examined by employing polarizing optical microscope (POM), differential scanning calorimetry (DSC) and X-ray Diffraction (XRD). The analogues with shorter alkoxy terminal substituents displayed higher mesomorphic properties with wide mesomorphic temperature ranges compared to analogues with shorter alkoxy terminals. The morphological characteristics of the self-assembled triazoles were investigated by scanning electron microscopy (SEM) which demonstrated arrangement of highly ordered columnar architectures. These orders were driven by layer to next layer adjacent influences and π-π stacking. The effect of 1,2,3-triazole ring and length of terminal aliphatic chains on the liquid crystalline performance were discussed. Through an appropriate choice of lateral structures employing "click chemistry", both transition temperature and mesophases ranging from isotropic to columnar or B1/B4-like supramolecular liquid crystalline aggregates can be tuned. [ABSTRACT FROM AUTHOR]

• The synthesis of bent-core amphiphiles with a TEG tail and their materials containing [1/1] lithium ions are reported. • TEG-decorated bent-core molecules are versatile amphiphiles to build nanostructured soft materials. • The Li-doped materials stabilize layered bent-core liquid crystal phases such as the SmCP or HNF-like organizations. • The compounds form organogels with nonpolar solvents that provide platelets, helical fibers, and tubular morphologies. • By self-assembly, non-chiral bent core-based materials transfer conformational chirality to nanostructures. The synthesis, supramolecular self-assembly and structural characterization of a new family of tetraethylene oxide (TEG)-based bent-core compounds and their 1/1 lithium-containing complexes are reported. TEG-based bent-core amphiphiles, even joining the TEG/Li+ tandem, are suitable building-blocks to achieve supramolecular nanostructures, in some cases showing chiral features from achiral molecules either in the mesophase or in solvents. The thermal and liquid crystal behavior of these materials studied by polarizing optical microscopy and X-ray diffraction confirmed that Li-based materials stabilized polar smectic C and helical nanofilament-type mesophases, in contrast to the non-liquid crystalline pure TEG-compounds. Alternatively, both the pure amphiphiles and the lithium-doped materials self-assemble into physical gels in non-polar solvents, displaying three-dimensional networks composed of long fibers with lamellar molecular organizations as shown by transmission electron microscopy and X-ray diffraction. Interestingly, biphenyl- and azobenzene-based bent core amphiphiles aggregate in solvent into chiral nanostructured morphologies with supramolecular trends comparable to their molecular arrangement in their liquid crystalline phases. [ABSTRACT FROM AUTHOR]

We have previously reported the first realization of an orthogonal ferroelectric bent-core SmAPF phase by directed design in mesogens with a single tricarbosilane-terminated alkoxy tail. Given the potentially useful electrooptic properties of this phase, including analog phase-only electrooptic index modulation with optical latching, we have been exploring its "structure space", searching for novel SmAPF mesogens. Here, we report two classes of these--the first designed to optimize the dynamic range of the index modulation in parallel-aligned cells by lowering the bend angle of the rigid core, and the second expanding the structure space of the phase by replacing the tricarbosilane-terminated alkyl tail with a polyfluorinated polyethylene glycol oligomer. [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]

Brief overview of polar liquid crystals (LCs) is given with a tutorial manner. They are chiral smectic C ferroelectric LCs (FLCs), chiral smectic CAantiferroelectric LCs (AFLCs), their ferrielectric subphase LCs, chiral columnar FLCs, chiral and nonchiral ferronematic LCs, various bent-shaped polar LCs such as B1, B2, B4, B7, twist-bend nematic, orthogonal polar smectic, and nonchiral columnar FLCs. After summarizing the definition of FLCs and AFLCs, and techniques to certify these polar LCs, their polar structures and characteristics are described together with important review articles. [ABSTRACT FROM AUTHOR]

In this study, new hockey-stick-like molecules were synthesized and characterized. The desired bent-core molecules consist of two arms with different numbers of aromatic rings, each with its own flexible chain with a different number of carbons: one arm contains four aromatic rings and a terminal propyloxy group, and the other arm contains an aromatic ring with a terminal hexyloxy or dodecyloxy group. To improve the net dipole of the bulk state, fluorine atoms were laterally substituted onto an aromatic ring within each molecule. The structures of the obtained compounds were characterized via FTIR and NMR spectrometry and elemental analysis. Their mesomorphic properties were investigated using DSC and polarizing optical microscopy. [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 review the impact of tetrahedral order on the macroscopic dynamics of bent-core liquid crystals. We discuss tetrahedral order comparing with other types of orientational order, like nematic, polar nematic, polar smectic, and active polar order. In particular, we present hydrodynamic equations for phases, where only tetrahedral order exists or tetrahedral order is combined with nematic order. Among the latter, we discriminate between three cases, where the nematic director (a) orients along a fourfold, (b) along a threefold symmetry axis of the tetrahedral structure, or (c) is homogeneously uncorrelated with the tetrahedron. For the optically isotropic T phase, which only has tetrahedral order, we focus on the coupling of flow with, e.g., temperature gradients and on the specific orientation behavior in external electric fields. For the transition to the nematic phase, electric fields lead to a temperature shift that is linear in the field strength. Electric fields induce nematic order, again linear in the field strength. If strong enough, electric fields can change the tetrahedral structure and symmetry leading to a polar phase. We briefly deal with the T phase that arises when tetrahedral order occurs in a system of chiral molecules. To case (a), defined above, belong (i) the non-polar, achiral, optically uniaxial D2d phase with ambidextrous helicity (due to a linear gradient free energy contribution) and with orientational frustration in external fields, (ii) the non-polar tetragonal S4 phase, (iii) the non-polar, orthorhombic D2 phase that is structurally chiral featuring ambidextrous chirality, (iv) the polar orthorhombic C2v phase, and (v) the polar, structurally chiral, monoclinic C2 phase. Case (b) results in a trigonal C3v phase that behaves like a biaxial polar nematic phase. An example for case (c) is a splay bend phase, where the ground state is inhomogeneous due to a linear gradient free energy contribution. Finally, we discuss some experiments that show typical effects related to the existence of tetrahedral order. A summary and perspective is given. [ABSTRACT FROM AUTHOR]

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]

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]

In this study, dielectric properties and ac conductivity mechanism of the bent-core liquid crystal 3′-{4-[4-(3,7-Dimethyloctyloxy)benzoyloxy]benzoyloxy}-4-{4-[4-[6-(1,1,3,3,5,5,5-heptamethyltrisiloxan-1yl)hex-1-yloxy]benzoyloxy]benzoyloxy}biphenyl ( DBB ) have been analyzed by impedance spectroscopy measurements at different temperatures. According to the polarizing microscopy results, DBB liquid crystal compound exhibits a dark conglomerate mesophase (DC [*] phase) which can be identified by the occurrence of a conglomerate of domains with opposite chirality. The chiral domains of this low-birefringent mesophase become more visible by rotating the polarizer. The variation of the real ( ε ′) and imaginary ( ε ″) parts of dielectric constant with angular frequency and Cole–Cole curves of DBB have been analyzed. The fitting results for dispersion curves at different temperatures revealed that DBB system exhibits nearly Debye-type relaxation except for 125 °C. Moreover, it has been determined that while the relaxation frequencies shift to higher frequencies as the temperature increases from 25 °C to 125 °C, the peak intensities remarkably decrease with increasing temperature. According to Cole–Cole plot and phase angle versus frequency curve, it has been determined that DBB LC may have a possibility of utilizing as a super-capacitor at room temperature. Furthermore, it has been found that the conductivity mechanism of the DBB alters from Correlated Barrier Hoping (CBH) model to Quantum Tunneling Model (QMT) with in increasing temperature at high frequency region. In terms of CBH model, optical band gaps at 25 °C and 75 °C temperatures have also been calculated. Finally, activation energies for some selected angular frequencies have also been calculated. [ABSTRACT FROM AUTHOR]

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]