Cybotactic nematic phases of photoisomerisable hockey-stick liquid crystals

New five-ring hockey-stick liquid crystalline materials with 4-bromoresorcinol as the central core unit and an azobenzene-based side arm were synthesised and their mesophase behaviour was investigated by polarising optical microscopy, differential scanning calorimetry, X-ray diffraction and under a triangular wave electric field. Additional structural modification was done by introducing a lateral fluorine atom in the terminal ring of one of the side arms. It is found that regardless of the alkyl chain length or the lateral fluorine substitution, all of the prepared materials are liquid crystalline exhibiting nematic phases composed of cybotactic clusters of the SmC-type (N_CybC) in addition to a monotropic SmC phase for the longest homologue.     

Threshold voltage for purely flexoelectric deformations of conducting homeotropic nematic layers

Elastic deformations induced by an electric field in homeotropic nematic layers with finite anchoring energy were studied numerically. A nematic material possessing flexoelectric properties and characterized by a positive dielectric anisotropy was considered. The ionic space charge and the ion transport across the layer were taken into account. The director orientation, the electric field strength and the ion concentrations were calculated as functions of the coordinate normal to the layer. The calculations show that the electric field distribution, which determines the form of the deformations, is influenced by the ionic current and therefore depends on the ionic content and on the properties of the electrodes. Several types of deformations were distinguished. When the electrode contacts are well conducting or when the ionic content is low, the threshold voltage is very close to the value U _f valid for an insulating nematic. When the electrodes are poorly conducting or blocking at high ion concentration, the threshold voltage decreases much below U _f. At moderate ion concentrations, i.e. between 10¹⁹ and 10²⁰ m^?3, two different behaviours were found depending on the sign of the sum of flexoelectric coefficients e ₁₁+e ₃₃. In the case of e ₁₁+e ₃₃<0, the threshold voltage decreases with the ionic content; in the case of e ₁₁+e ₃₃>0, the deformations occur in two separate voltage regimes. They arise above a certain threshold voltage, disappear at some higher voltage and reappear at an even higher threshold.     

The effects of lateral halogen substituents on the low-temperature cybotactic nematic phase in oxadiazole based bent-core liquid crystals

We have previously demonstrated that the incorporation of lateral methyl groups on oxadiazole-based liquid crystals leads to relatively low-temperature cybotactic nematic phases which, in some cases, supercool to room temperature. We report here the synthesis and phase behaviour of related compounds that possess lateral halogen groups and in some cases, lateral methyl groups as well. Derivatives with three lateral substituents (one halogen and two methyl groups) in a specific pattern supercool in the nematic phase to room temperature. As was the case with the previously reported trimethylated derivatives, the low-temperature nematic phase is glassy in nature. Two of the new trisubstituted derivatives (with bromo and chloro groups) remain in the nematic phase upon subsequent heating until transitioning to the isotropic phase indicating that the low-temperature nematic phase may be more stable than that shown by the trimethylated analogue. Preliminary X-ray diffraction analysis confirms the presence of a tilted cybotactic nematic phase. In addition, the splitting observed in the wide-angle scattering feature is indicative of enhanced local biaxial packing.     

Search for nematic biaxiality in bent-core mesogens: an X-ray diffraction perspective

ABSTRACT

Since its theoretical prediction in 1970, the search for the biaxial nematic phase in thermotropic systems has challenged generations of liquid crystal scientists. Over the last 10 years, bent-core mesogens have drawn much interest as promising candidates for nematic biaxiality. However, despite a number of disputed claims, conclusive evidence of proper (spontaneous and macroscopic) biaxial order in these materials is still missing. By contrast, it is now widely recognised that biaxiality exists on a local scale, in the form of nano-sized clusters of molecules (cybotactic groups) possessing smectic-like positional order and biaxial orientational order. This article provides a review of X-ray diffraction studies on biaxiality and cybotaxis in bent-core nematics, discussing the most relevant issues related to this research field.     

An X-ray diffraction study of the different nematic mesophases of liquid-crystalline poly(urethane-ester)s

An x-ray investigation on powder specimens and stretched oriented fibers of poly(urethaneester)s TDI-CmCn, derived from various mesogenic alkylene di[4-(ω-hydroxyalkyloxy-4-oxybenzoyl)oxybenzoate]s (CmCn; m = 2, 4, or 6, and n = 4, 6, 8, or 10) and 2, 4-toluenediisocyanate (TDI), is reported. Evidence is provided for the formation of two different nematic mesophases in the polymers, namely a cybotactic nematic and a conventional nematic mesophase. Whereas samples TDI-C2C6, TDI-C6C4, and TDI-C6C10 formed one cybotactic nematic mesophase, samples TDI-C4C6, TDI-C6C6, and TDI-C6C8 exhibited both cybotactic nematic and conventional nematic mesophases in a sequence with increasing temperature, which were connected by a first-order transition. The analysis of the various features of the small-angle x-ray diffraction patterns indicates that two structural arrangements, namely smectic C-like and conventional nematic structures, coexist inside the cybotactic nematic mesophase of these poly(urethane-ester)s. © 1995 John Wiley & Sons, Inc.     

Enhancement of the nematic order of polar liquid crystals in thin cells

Experimental observations show that the nematic order parameter (S) is significantly enhanced as the thickness of the cell is reduced. However, calculations based on earlier theories are not in good agreement with the experimental data. We earlier developed a molecular mean field model that assumes that the medium with polar molecules consists of inter-converting anti-parallel and parallel pairs, to explain the molecular origin of the ‘two lengths’ model. The model has been used to explain effects such as double re-entrance and the effect of electric field on the nematic–isotropic transition temperature, T _NI. We have recently extended the Maier–Saupe (M–S) theory using an empirical variation of surface potential, to account for the enhancement in which a molecule near the surface is assumed to feel the mean field potential (M–S type) and also the surface-induced potential. Variation in the density of the medium due to variation of S has been ignored. In the present paper, the change in the relative proportion of the parallel pairs due to changes in the density of the medium is included in the M–S theory with the surface potential. This results in improved agreement with the experimental data.     

Forces in nematic liquid crystals: from nanoscale interfacial forces to long-range forces in nematic colloids

In this paper we give an overview of experiments that provided an insight into the nature of forces between surfaces and objects in a nematic liquid crystal. These forces, also called ‘structural forces’, are the consequence of the long-range orientational order and orientational elasticity of nematic liquid crystals. Owing to their fundamental as well as technological importance, forces between objects in liquid crystals have been a subject of growing interest during the last decade. Experimental observations and studies of structural forces are described from nanoscale interfacial forces, measured by an atomic force microscope, to the micro-scale forces between colloidal particles in nematics, studied by laser tweezers and optical video microscopy.     

Photoresponsive Stripe Pattern in Achiral Azobenzene Liquid Crystals

A periodic stripe pattern is found in the nematic phase close to the smectic phase of photoresponsive achiral liquid‐crystalline compounds. The origin of the stripe patterns can be ascribed to an extremely large bent elastic constant K₃₃. In addition, we succeeded in controlling the pattern by the following two methods: 1) the stripe disappears by a trans–cis photoisomerization upon UV light irradiation and reappears upon light termination, and 2) the stripe pattern is stabilized over the whole nematic phase, at approximately 10 °C, by polymerization of the compounds.     

Visco-elastic properties of reentrant nematic liquid crystalline mixtures

The visco-elastic properties for binary mixtures of 4-n -hexyloxy-4′-cyanobiphenyl (6OCB) and 4-n-octyloxy-4′-cyanobiphenyl (8OCB) are investigated in detail by a light scattering technique. The mixtures exhibit a reentrant nematic (RN) phase between the smectic A (SmA) and crystal phases in the range 22.0–29.5 wt % of 6OCB. The viscosity and the elastic constant increase with cooling in both the nematic and reentrant nematic phases. It is also found that the mixtures exhibit anomalously large values of viscosity and elastic constant near the phase transition, i.e. a pretransitional phenomenon can be observed. The activation energy for viscosity is smaller in the RN phase than in the ordinary nematic phase appearing at higher temperatures, while the normalized elastic constant (defined as the ratio of the elastic constant to the square of the dielectric anisotropy) is larger. Moreover, the visco-elastic behaviour depends on the specific time during which the sample has been kept in the SmA phase. These results confirm that the SmA phase has a strong influence on the formation of molecular aggregates, which plays an important role in the reentrant phenomenon of these systems.     

Visualisation methods for complex nematic fields

In analysis of numerically simulated or experimentally obtained nematic textures, data visualisation plays an important role in interpretation and comparison of results. We review both commonly used, and more recently developed visual presentation methods that expose different aspects of the textures, from director field patterns and topological content of nematic defects, to deformation modes and simulated optical effects. We outline the theoretical background and potential uses, with examples that highlight the features of each of the described techniques.     

Synthesis and mesomorphic properties of four-ring liquid crystals containing cyclohexyl,phenyl and pyridyl units

Two series of chiral and achiral 4-[(3-n-alkoxycarbonylpyridyl)-6-ethynyl]phenyl 4-(trans-4-n-alkylcyclohexyl)benzoate liquid crystals have been synthesized. Their mesomorphic properties were observed and measured by polarizing optical microscopy and DSC. A broad chiral smectic C phase was observed when terminal chains contained a chiral centre.     

Random lasing in unbounded dye-doped nematic liquid crystals

Random lasing in dye-doped nematic liquid crystals (NLCs) contained in a quartz cuvette was studied. With the pumping energy rising, random lasing with nonresonant feedback and resonant feedback emerge successively with the lasing threshold of 20 and 40 μJ, respectively. The lasing properties including fluctuation of the emission spectrum, divergence angle of emission, independence of the emission on the polarisation of the pump beam and the depolarisation of emission are distinct from those of dye-doped nematics confined in the sandwich structure cell or the capillary tube. The distinctions are associated with the thermal fluctuation of dielectric tensor since there is negligible surface anchoring for the nematics in the pump volume. This indicates that the anchoring state of NLCs is an important factor determining the random lasing properties.     

Random nematic structures in the absence of inherent frustrations

We study the impact of anisotropic nanoparticles (NPs) on the nematic liquid crystal (LC) order. Within a mesoscopic Flory–Huggins-type approach we have estimated regimes where LC–NP mixtures are essentially homogenous. Using a lattice Lebwohl–Lasher type approach we have also studied the impact of anisotropic NPs on LC ordering. We analysed the cases where the orientations of NPs are either frozen-in or could be varied, to which we refer as random field mixtures and annealed mixtures, respectively. In the latter case we have demonstrated the existence of qualitatively different regimes. In particular, we determined the concentration regime, where LC configurations resembling a transparent nematic phase could be observed. Such domain patterns are stabilised by NPs hindering the annihilation of topological defects in LC order.     

Flow and anchoring effects on nematic fluctuations in confined geometry

Light scattering observation of nematic director fluctuations in confined geometries can be used to obtain interaction parameters of liquid crystals with surfaces. We present the basics of the method and some examples of the results in planar and cylindrical geometries. These results were obtained after neglecting the coupling of the director motion to flow. We give analytical and numerical results of flow effects on director fluctuations in a slab. The backflow contribution to the effective viscosity is strongly suppressed so that the results for the anchoring energy remain valid. Modal dispersion relations show an interesting behaviour of avoiding crossings.     

Wide nematic phases induced by hydrogen-bonding

New series of hydrogen-bonded supramolecular complexes, In/IIm, were prepared and investigated for their mesophase behaviour. The complexes were prepared through hydrogen-bonding between equimolar amounts of 4-alkoxyphenylazo-benzoic acids as the proton donors and non-mesomorphic 4-alkoxyphenylazo pyridines as the proton acceptors. The length of the terminal alkoxy chains was varied systematically on both terminals of the supramolecular complexes. The formation of 1:1 hydrogen-bonded complexes was confirmed by differential scanning calorimetry (DSC) and FTIR spectroscopy. All of new complexes were characterised by DSC and polarised light microscopy (PLM). It was found that the prepared complexes are dimorphic, possessing smectic C (SmC) and nematic (N) phases with relatively wide ranges. A comparison was made between the present series of complexes and the previously investigated isomers of the simple 4-alkoxybenzoic acids with the 4-(4?-pyridylazophenyl)-4??-alkoxybenzoates, revealed the stability of the nematic phase is dependent on the length of the acid component.     

Towards the biaxial nematic phase through molecular design

In this article, the search for the elusive biaxial nematic phase (NB) in liquid crystals is considered. The structure of the phase is described along with theoretical and computational work which suggests how it might be realised. An overview of the work of the Exeter group in this area is then given showing the different approaches adopted and illustrating how one of these has led to a new type of amphiphilicity based on shape.     

Azobenzene-based liquid crystal dimers and the twist-bend nematic phase

ABSTRACT

The synthesis and characterisation of two new sets of non-symmetric liquid crystal dimers is reported, the 1-(4-substitutedazobenzene-4′-yloxy)-6-(4-cyanobiphenyl-4′-yl)hexanes (CB6OABX) and 1-(4-substitutedazobenzene-4′-yloxy)-6-(4-cyanobiphenyl-4′-yloxy)pentanes (CBO5OABX). The terminal substituents are methyl, methoxy, butyl, butyloxy, nitrile and nitro. All the CB6OABX dimers exhibit twist-bend nematic (N_TB) and nematic (N) phases. The CBO5OABX dimers also all show an N phase but only the butyl and butyloxy homologues exhibit the N_TB phase. The transitional behaviour of the non-symmetric dimers is compared to that of the corresponding symmetric dimers, the 1,5-bis(4-substitutedazobenzene-4′-yloxy)pentanes (XABO5OABX) and either 1,7-bis(4-cyanobiphenyl-4′-yl)heptane or 1,5-bis(4-cyanobiphenyl-4′-yloxy)pentane. The XABO5OABX dimers all show a nematic phase and in addition, the butyl homologue exhibits a smectic A phase. The difference in transitional behaviour between the CB6OABX and CBO5OABX dimers is attributed to the difference in their molecular shapes arising from different bond angles between the para axis of the cyanobiphenyl unit and the first bond in the spacer. Specifically, the all-trans conformation of a CBO5OABX dimer is more linear than that of the corresponding CB6OABX dimer. Differences within each set of dimers are attributed to changes in the average molecular shape and the strength of the mixed mesogen interaction on varying the terminal group. Crystal structures are reported for CB6OABOMe, CBO5OABNO₂ and MeOABO5OABOMe.     

Interaction of electromagnetic waves in planar waveguide with a nematic layer

This work is a theoretical study of energy exchange between two coupled TE-wave modes on director diffraction grating in a planar waveguide containing a layer of nematic liquid crystal. The diffraction grating is produced by an external electric field in the nematic layer with spatial periodic anchoring energy between director and waveguide surface. The intensity of a signal mode at the output of the nematic layer has been calculated in dependence of anchoring energy amplitude and modulation period, the size of nematic layer and electrical field value. The cases of co-propagating and oppositely propagating modes have been analysed. The analytical expressions that describe the maximum values of signal mode intensity have been derived. The maximum intensity value output from the nematic has been shown to depend monotonously on the anchoring energy parameters in the case of oppositely propagating wave modes and non-monotonously in the case of co-propagating wave modes. In both cases, the maximum value of signal mode intensity grows with the increase in electric field.     

Monosubstituted ferrocene liquid crystals containing click triazole with a wide nematic phase temperature range

New monosubstituted ferrocenomesogens of the 1,4-substituted-1,2,3-triazole series 3a-3d and the 5-halogen-1,4-substituted-1,2,3-triazole series 4a-4d were successfully synthesised through a facile and versatile synthetic route using click reactions with moderate yields, in different atmospheres, and catalysed by different amounts of CuX (X = Br, I). Two series of compounds exhibited a typical nematic liquid crystal texture, and the 5-halogen-1,4-substituted-1,2,3-triazole series 4 had a wider mesomorphic temperature range than the corresponding series 3 due to the effect of the halogen atom substituent on the triazole. These compounds showed similar absorption spectra, but the fluorescence emission spectra of 4 were obviously redshift relative to 3. Cyclic voltammetry investigations revealed that the redox-active ferrocenyl groups of 3 and 4 follow one-electron transfer processes.