Structure studies of the nematic phase formed by bent-core molecules

Nontrivial ultraslow dynamics under electric-field in nematics of bent-shaped molecules

For over decades, nematic liquid crystals have been recognized as highly fluidic materials that respond to electric field on the millisecond scale. In contrast to traditional nematics with fast responsivity, we herein report nontrivial ultraslow electric-driven dynamics in bent-shaped nematic materials. Varying the alkyl chain spacers of bent-shaped cyanobiphenyl dimers (COO_m and OCO_m) shows a 'transition' in the dynamics behavior between the bent-dimeric and bent-core materials. Interestingly, with short alkyl chain spacers, COO₂ exhibits unexpected ultra-slow dynamic pathways, i.e., "quasi-static" electrohydrodynamic convection. A significant observation is that the on/off-electro-switching time of COO₂ is 10 000 times higher than that of typical nematic materials, which is the largest value reported ever in the kilo-second range. In addition, the threshold voltage for inducing the reorientation of the nematic director for COO₂ is higher than 5 V, which is uncommon in traditional N materials. These properties are distinct from those of traditional nematic materials and discussed in terms of dielectric constants and electrohydrodynamic convection.

Stereochemical Rules Govern the Soft Self-Assembly of Achiral Compounds: Understanding the Heliconical Liquid-Crystalline Phases of Bent-Core Mesogens

A series of bent-shaped 4-cyanoresorcinol bisterephthalates is reported. Some of these achiral compounds spontaneously form a short-pitch heliconical lamellar liquid-crystalline phase with incommensurate 3-layer pitch and the helix axis parallel to the layer normal. It is observed at the paraelectric-(anti)ferroelectric transition, if it coincides with the transition from random to uniform tilt and with the transition from anticlinic to synclinic tilt correlation of the molecules in the layers of the developing tilted smectic phase. For compounds with long chains the heliconical phase is only field-induced, but once formed it is stable in a distinct temperature range, even after switching off the field. The presence of the helix changes the phase properties and the switching mechanism from the naturally preferred rotation around the molecular long axis, which reverses the chirality, to a precession on a cone, which retains the chirality. These observations are explained by diastereomeric relations between two coexisting modes of superstructural chirality. One is the layer chirality, resulting from the combination of tilt and polar order, and the other one is the helical twist evolving between the layers. At lower temperature the helical structure is replaced by a non-tilted and ferreoelectric switching lamellar phase, providing an alternative non-chiral way for the transition from anticlinic to synclinic tilt.

Calamitic and discotic liquid crystalline phases for mesogens with triangular cores

Mesogenic compounds having new triangular mesogenic cores containing salicylaldimine and enaminocarbonyl or barbituric moieties have been synthesized and their liquid crystalline, dielectric and optical properties have been studied. Appropriate substitution of such cores with various numbers of lateral chains (alkoxy, perfluorinated or polyether) leads to mesogens forming either lamellar (SmC), nematic or columnar (Colhd and Colho) phases. Some of the materials show an unusual phase sequence with a re-entrant isotropic phase appearing below the columnar (Colhd) phase; others form two columnar hexagonal phases with a weakly first order transition between them. Also a new columnar superstructure with p2gg symmetry (Colros) was found, in which the crystallographic lattice is decorated with a two-molecule motive.

Double helical structure of the twist-bend nematic phase investigated by resonant X-ray scattering at the carbon and sulfur K-edges

The mesogenic dimer displaying nematic and NTB phases was investigated by resonant X-ray scattering at both C and S absorption K-edges and supported by single X-ray crystallography. In the crystal resonant studies revealed the forbidden reflection in non-resonant diffraction similar to that found in the NTB phase. The lack of a second harmonic in both C and S resonant X-ray scattering supports the double helical structure of the twist-bend nematic phase.

Formation and development of nanometer-sized cybotactic clusters in bent-core nematic liquid crystalline compounds

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.

Ferroelectric Nematic and Ferrielectric Smectic Mesophases in an Achiral Bent-Core Azo Compound

Here, we report the observation of ferroelectric nematic and ferrielectric smectic mesophases in an achiral bent-core azo compound consisting of nonsymmetrical molecules with a lateral fluoro substitution on one of the wings. These mesophases are enantiotropic in nature with fairly low transition temperatures and wide mesophase ranges. The liquid crystalline properties of this compound are investigated using polarizing optical microscope, differential scanning calorimeter, X-ray diffraction, and electro-optical studies. As revealed by X-ray diffraction measurements, the nematic mesophase is composed of skewed cybotactic clusters and, in the smectic mesophase, the molecules are tilted with respect to the layer normal. The polar order in these mesophases was confirmed by the electro-optical switching and dielectric spectroscopy measurements. The dielectric study in the nematic mesophase shows a single relaxation process at low frequency ( f < 1 kHz) measured in the range 10 Hz to 5 MHz, which is attributed to the collective motion of the molecules within cybotactic clusters. The formation of local polar order in these clusters leads to a ferroelectric-like polar switching in the nematic mesophase. Of particular interest is the fact that the smectic phase exhibits a field induced ferrielectric state, which can be exploited for designing of the potential optical devices due to multistate switching.

Complex free-energy landscapes in biaxial nematic liquid crystals and the role of repulsive interactions: A Wang-Landau study

General quadratic Hamiltonian models, describing the interaction between liquid-crystal molecules (typically with D_{2h} symmetry), take into account couplings between their uniaxial and biaxial tensors. While the attractive contributions arising from interactions between similar tensors of the participating molecules provide for eventual condensation of the respective orders at suitably low temperatures, the role of cross coupling between unlike tensors is not fully appreciated. Our recent study with an advanced Monte Carlo technique (entropic sampling) showed clearly the increasing relevance of this cross term in determining the phase diagram (contravening in some regions of model parameter space), the predictions of mean-field theory, and standard Monte Carlo simulation results. In this context, we investigated the phase diagrams and the nature of the phases therein on two trajectories in the parameter space: one is a line in the interior region of biaxial stability believed to be representative of the real systems, and the second is the extensively investigated parabolic path resulting from the London dispersion approximation. In both cases, we find the destabilizing effect of increased cross-coupling interactions, which invariably result in the formation of local biaxial organizations inhomogeneously distributed. This manifests as a small, but unmistakable, contribution of biaxial order in the uniaxial phase. The free-energy profiles computed in the present study as a function of the two dominant order parameters indicate complex landscapes. On the one hand, these profiles account for the unusual thermal behavior of the biaxial order parameter under significant destabilizing influence from the cross terms. On the other, they also allude to the possibility that in real systems, these complexities might indeed be inhibiting the formation of a low-temperature biaxial order itself-perhaps reflecting the difficulties in their ready realization in the laboratory.

A combined experimental and theoretical approach to study SmC → NcybC phase transition in a four-ring bent-core liquid crystal

Spectroscopic evidences for the analysis of phase transition and hydrogen bonding in a newly synthesised four-ring bent-core liquid crystal.

Reexamination of the mean-field phase diagram of biaxial nematic liquid crystals: Insights from Monte Carlo studies

Investigations of the phase diagram of biaxial liquid-crystal systems through analyses of general Hamiltonian models within the simplifications of mean-field theory (MFT), as well as by computer simulations based on microscopic models, are directed toward an appreciation of the role of the underlying molecular-level interactions to facilitate its spontaneous condensation into a nematic phase with biaxial symmetry. Continuing experimental challenges in realizing such a system unambiguously, despite encouraging predictions from MFT, for example, are requiring more versatile simulational methodologies capable of providing insights into possible hindering barriers within the system, typically gleaned through its free-energy dependences on relevant observables as the system is driven through the transitions. The recent paper from this group [Kamala Latha et al., Phys. Rev. E 89, 050501(R) (2014)], summarizing the outcome of detailed Monte Carlo simulations carried out employing an entropic sampling technique, suggested a qualitative modification of the MFT phase diagram as the Hamiltonian is asymptotically driven toward the so-called partly repulsive regions. It was argued that the degree of (cross) coupling between the uniaxial and biaxial tensor components of neighboring molecules plays a crucial role in facilitating a ready condensation of the biaxial phase, suggesting that this could be a plausible factor in explaining the experimental difficulties. In this paper, we elaborate this point further, providing additional evidence from curious variations of free-energy profiles with respect to the relevant orientational order parameters, at different temperatures bracketing the phase transitions.

Fredericks transitions in biaxial nematics

Much of our understanding (and applications) of biaxial nematic liquid crystals requires the study of the textural transformations in external electric or magnetic fields. To that end, one should employ theoretical approaches which could have bearing on the minimization problem of the multi-parametric free energy. The immediate shortcoming of the direct free-energy minimization (widely used for uniaxial nematics) is the need to resolve several non-linear constraints. To overcome this difficulty, in what follows we shall use the "angular velocity", which describes space rotations of the order parameter, and is in fact a vector internal curvature of the texture. This method provides a means to resolve the constraints imposed on the order parameter. Thus, we have obtained the set of equations to find all possible one-dimensional textures of biaxial nematics in the external field. To illustrate our method, we calculate the critical fields corresponding to some basic configurations for textural transitions in the biaxial nematics. We feel that this result could be useful to determine the intrinsic degree of biaxiality for liquid crystalline materials.

1D, 2D and 3D liquid crystalline phases formed by bent-core mesogens

The type of the mesophase is altered by a small change in the molecular architecture, i.e. increasing the number of alkyl chains attached to the mesogenic core at terminal positions. Lamellar (1D) and columnar (2D) phases are formed for molecules with one and three terminal chains, respectively. For those with two chains 3D phases are observed with either cubic or triclinic symmetry structure.

Viscoelastic behavior of a binary system of strongly polar bent-core and rodlike nematic liquid crystals

We have investigated the permittivity and viscoelastic behavior of a binary system comprising bent-core and calamitic compounds, both of which are polar, the calamitic being more strongly so, and exhibiting only the nematic mesophase. The permittivity data in the nematic as well as the isotropic phase indicate strong polar interactions between the molecules, even for mixtures with a significant content of the bent-core compound. The thermal dependence of both the splay and bend elastic constants exhibit features different from the literature. The splay constant displays a large increase with increasing concentration of bent-core material, before undergoing a precipitous drop for small calamitic content materials. Upon lowering the temperature, certain mixtures exhibit a convex-shaped feature for the bend elastic constant; that is, the value of the elastic constant is maximum at a specific temperature in the nematic phase, diminishing when the temperature is either increased or decreased. Surprisingly, the pure compounds, especially the bent-core one, show only a monotonically increasing trend for the bend elastic constant. We present two arguments to explain these features: one of these is based on coupling between the molecular shape and director distortion presented in the literature. Then we put forth a new concept of frustration in the packing between the two types of molecules and the polar interactions as an alternative.

The cybotactic nematic phase of bent-core mesogens: state of the art and future developments

The molecular clustering observed in the fluid nematic phase of nonlinear liquid crystal molecules underlies exaggerated field effects that portend unique technological advances in next-generation liquid crystal displays. However, the detailed nature of the molecular organization within the clusters and the temporal and spatial persistence of the organization remain unclear. Herein we review the evolution of structural studies of this unique nematic phase. The mounting experimental evidence points to a converging picture of the microscopic nature of this relatively new class of liquid crystals.

Detection of an intermediate biaxial phase in the phase diagram of biaxial liquid crystals: entropic sampling study

We investigate the phase sequence of biaxial liquid crystals, based on a general quadratic model Hamiltonian over the relevant parameter space, with a Monte Carlo simulation which constructs equilibrium ensembles of microstates, overcoming possible (free) energy barriers (combining entropic and frontier sampling techniques). The resulting phase diagram qualitatively differs from the universal phase diagram predicted earlier from mean-field theory (MFT), as well as the Monte Carlo simulations with the Metropolis algorithm. The direct isotropic-to-biaxial transition predicted by the MFT is replaced in certain regions of the space by the onset of an additional intermediate biaxial phase of very low order, leading to the sequence N(B)-N(B1)-I. This is due to inherent barriers to fluctuations of the components comprising the total energy, and may explain the difficulties in the experimental realization of these phases.

Highly elastic liquid crystals with a sub-nanonewton bending elastic constant mediated by the resident molecular assemblies

A surprisingly high bending elastic constant (K33 ) is obtained in a newly synthesized compound shown here. Mixtures containing a few percent of this compound confirm the influence of the unusual K33 values, and show an improved electro-optic response. The origin of the huge K33 is discussed based on the formation of a smectic cluster structure formed in the nematic phase.

Evidence of Cybotactic Order in the Nematic Phase of a Main-Chain Liquid Crystal Polymer with Bent-Core Repeat Unit

We report the synthesis and structural characterization of a main-chain liquid crystal polymer constituted by a 1,2,4-oxadiazole-based bent-core repeat unit. For the first time, a liquid crystal polymer made of bent mesogenic units is demonstrated to exhibit cybotactic order in the nematic phase. Coupled with the chain-bond constraints, cybotaxis results in maximized molecular correlations that make this material of great potential in the search for the elusive biaxial and ferroelectric nematic phases. Indeed, repolarization current measurements in the nematic phase hint at a ferroelectric-like switching response (upon application of an electric field of only 1.0 V μm^-1) that, albeit to be definitely confirmed by complementary techniques, is strongly supported by the comparative repolarization current measurements in the nematic and isotropic phases. Finally, the weak tendency of this polymer to crystallize makes it possible to supercool the cybotactic nematic phase down to room temperature, thus, paving the way for a glassy phase in which the biaxial (and possibly polar) order is frozen at room temperature.

Surface alignment, anchoring transitions, optical properties, and topological defects in the thermotropic nematic phase of organo-siloxane tetrapodes

We perform optical, surface anchoring, and textural studies of an organo-siloxane "tetrapode" material in the broad temperature range of the nematic phase. The optical, structural, and topological features are compatible with the uniaxial nematic order rather than with the biaxial nematic order, in the entire nematic temperature range -25 °C < T < 46 °C studied. For homeotropic alignment, the material experiences surface anchoring transition, but the director can be realigned into an optically uniaxial texture by applying a sufficiently strong electric field. The topological features of textures in cylindrical capillaries, in spherical droplets and around colloidal inclusions are consistent with the uniaxial character of the long-range nematic order. In particular, we observe isolated surface point defects - boojums and bulk point defects - hedgehogs that can exist only in the uniaxial nematic liquid crystal.

Twisted quasiperiodic textures of biaxial nematic liquid crystals

Textures (i.e., smooth space nonuniform distributions of the order parameter) in biaxial nematics turned out to be much more complex and interesting than expected. Scanning the literature we find only a very few publications on this topic. Thus, the immediate motivation of the present paper is to develop a systematic procedure to study, classify, and visualize possible textures in biaxial nematics. Based on the elastic energy of a biaxial nematic (written in the most simple form that involves the least number of phenomenological parameters) we derive and solve numerically the Lagrange equations of the first kind. It allows one to visualize the solutions and offers a deep insight into their geometrical and topological features. Performing Fourier analysis we find some particular textures possessing two or more characteristic space periods (we term such solutions quasiperiodic ones because the periods are not necessarily commensurate). The problem is not only of intellectual interest but also of relevance to optical characteristics of the liquid-crystalline textures.

Free-energy power expansion for orientationally ordered phases: energy and entropy

We propose an approach to the description of orientational phase transitions that utilizes the specific features of orientational energy and entropy. The approach is applied to building a model for nematic phases in materials with nonpolar parallelepiped-type molecules with symmetry D(2h). The model operates with complex order parameters, generalizes the Landau-de Gennes theory, and predicts the existence of a biaxial nematic phase for the fourth-order expansion of free energy.

Biaxial nematic order induced by smectic fluctuations

We report on a series of measurements on the microscopic structure and the magneto-optical properties of a calamitic liquid crystalline compound in its nematic phase. Structural studies show the existence of short-range, tilted smectic order consistent with pretransitional effects above an underlying smectic phase. Concomitantly, magneto-optical results exhibit the existence of an optic axis not collinear with the uniaxial director. This apparent biaxial nature is discussed within the context of coupling between the tensor nematic and the smectic order parameters.

Liquid-crystalline heterodimesogens and ABA-heterotrimesogens comprising a bent 3,5-diphenyl-1,2,4-oxadiazole central unit

Three new types of terminally connected ABA-heterotrimesogens and heterodimesogens, composed of a bent 3,5-diphenyl-1,2,4-oxadiazole central unit and one or two rod-shaped 4-cyanobiphenyl cores or one 2-phenyl-1,3,4-thiadiazole core, connected by flexible spacers, have been synthesized, and their mesomorphic behavior was studied by optical polarizing microscopy (PM), differential scanning calorimetry (DSC) and X-ray diffraction (XRD). All dimesogens exhibit broad ranges of cybotactic nematic phases (N(cybA) and N(cybC)), in some cases accompanied by additional mesophases (CybA or SmC) at lower temperature. The combination of the 3,5-diphenyl-1,2,4-oxadiazole unit with one cyanobiphenyl core leads to the removal of tilted smectic and cybotactic nematic phases (SmC, N(cybC)), which are replaced by the nontilted CybA phases and nematic phases composed of SmA-type clusters (N(cybA)). The orthogonal cybotactic nematic phases of bent-core mesogens are of special interest for achieving biaxial nematic phases of the orthorhombic type. The orthogonal (N(cybA)) and skewed (N(cybC)) cybotactic nematic phases were distinguished by XRD and optical observations.

Short-range correlations seen in the nematic phase of bent-core liquid crystals by dielectric and electro-optic studies

Three bent-core nematic liquid crystals having the same core but with different terminal groups, short (C4) and long (C7,C9) tails, are investigated by dielectric and electro-optic contrast spectroscopic techniques. C4 shows sign reversal in the dielectric anisotropy Δε' as a function of both temperature and frequency, whereas C9 shows only negative Δε' in the entire mesophasic region. The behavior of C7 is intermediate of the two. Results of a dielectric study show that both C7 and C9 exhibit strong short-range polar correlations normal to the director. The correlation lengths of these interactions are found to be similar to those from the x-ray scattering. An increased hindered rotation for C9 compared to C4 moves the dielectric dispersion for ε(|') to much lower frequencies, such that C9 shows only negative Δε' over the entire temperature range.

Nanostructures of liquid crystal phases in mixtures of bent-core and rod-shaped molecules

We report small angle x-ray scattering (SAXS) studies of isotropic, nematic, and smectic mesophases formed by binary mixtures of bent-core (BC) and rod-shaped (RS) molecules. While optical studies indicate that the components are fully miscible, SAXS reveals fascinating structures that are consistent with segregation on a nanoscopic scale. We find that tilted smectic clusters, which have been previously reported in both the nematic and isotropic states of the pure BC materials, are also present in mixtures with up to 50 wt% of the RS compound; this is consistent with previous dielectric and flexoelectric studies on such mixtures. Unexpectedly in this concentration range the clusters are present in the isotropic and in the induced smectic phase range, as well as throughout the nematic phase. The results in the smectic phase also reveal complex layering phenomena, providing important insight into the interaction between bent and rod-shaped molecules. These studies will be crucial in the design of promising new functional nanomaterials.

Second-harmonic generation in a bent-core nematic liquid crystal

Second-harmonic generation (SHG) is studied in the magnetically aligned nematic phase of a bent-core liquid crystal (BCN) and compared to similar measurements made on a conventional rodlike (calamitic) nematic compound. The second-harmonic (SH) light detected from both materials is predominantly due to scattering and therefore incoherent. Results on the calamitic are consistent with a polarization induced by ordinary director fluctuations in the nematic phase. However, the SH scattering collected in the BCN exhibits a different temperature and angular dependence. We discuss how these differences could arise from the effects of short-range correlated, smectic-C-type molecular clusters, which have been detected in recent studies on various BCN materials.

Surface alignment, anchoring transitions, optical properties, and topological defects in the nematic phase of thermotropic bent-core liquid crystal A131

We study optical, structural, and surface anchoring properties of thermotropic nematic bent-core material A131. The focus is on the features associated with orientational order as the material has been reported to exhibit not only the usual uniaxial nematic but also the biaxial nematic phase. We demonstrate that A131 experiences a surface anchoring transition from a perpendicular to tilted alignment when the temperature decreases. The features of the tilted state are consistent with surface-induced birefringence associated with smectic layering near the surface and a molecular tilt that changes along the normal to the substrates. The surface-induced birefringence is reduced to zero by a modest electric field that establishes a uniform uniaxial nematic state. Both refractive and absorptive optical properties of A131 are consistent with the uniaxial order. We found no evidence of the "polycrystalline" biaxial behavior in the cells placed in crossed electric and magnetic fields. We observe stable topological point defects (boojums and hedgehogs) and nonsingular "escaped" disclinations pertinent only to the uniaxial order. Finally, freely suspended films of A131 show uniaxial nematic and smectic textures; a decrease in the film thickness expands the temperature range of stability of smectic textures, supporting the idea of surface-induced smectic layering. Our conclusion is that A131 features only a uniaxial nematic phase and that the apparent biaxiality is caused by subtle surface effects rather than by the bulk biaxial phase.

Splay bend elasticity of a bent-core nematic liquid crystal

We measured the splay (K11) and bend (K33) elastic constants in the nematic phase of a bent-core liquid crystal. In the vicinity of the nematic-isotropic transition temperature K33 is proportional to the square of the order parameter. In the nematic range K11 increases monotonically with decreasing temperature, whereas K33 is practically independent of temperature and is smaller than K11 . K33 exhibits a pretransitional slow divergence toward the transition temperature to the smectic phase and becomes slightly larger than K11. The small K33 is explained on the basis of strong coupling of the bent shape of the molecules with the bend distortion.

Experimental realization of biaxial liquid crystal phases in colloidal dispersions of boardlike particles

Biaxial nematic and biaxial smectic phases were found in a colloidal model system of goethite (alpha-FeOOH) particles with a simple boardlike shape and short-range repulsive interaction. The macroscopic domains were oriented by a magnetic field and their structure was revealed by small angle x-ray scattering. In accordance with theoretical predictions, biaxiality appears in a system with particles that have a shape almost exactly in between rodlike and platelike. Our results suggest that biaxial phases can be readily obtained by a proper choice of the particle shape.