Bent-core Liquid Crystals Research Articles

Continuously adjustable period optical grating based on flexoelectric effect of a bent-core nematic liquid crystal in planar cells

The structures of flexodomains, which are similar to optical gratings and can be controlled by the amplitude of applied voltage and temperature, were verified through polarizing microscopy and light diffraction techniques. The properties of the optical grating induced by a bent-core nematic liquid crystal in planar cells with varied cell gaps and pretilt angles were studied. The period of optical grating decreases with the increase in the amplitude of the applied voltage and pretilt angle. In addition, the period increases with the increase in cell gap and temperature. The period of optical grating has a linear relationship with temperature. The continuously adjustable period has the potential to become an important and extended application of optical grating.

Theoretical investigation of nonlinear optical and vibrational properties of two liquid crystalline compounds

This article is mainly focused on studying the different physical properties of liquid crystalline compounds using density functional theory (DFT) method. The compounds were found to be bent-core calamitic liquid crystals. The theoretical calculation has been used to find the different parameter such as reactivity parameters and structural parameters of these compounds. Dipole moments and Non-Linear Optical (NLO) properties are essential to understanding the physical behaviour of the liquid crystals and also reported in this article. The NLO properties of both the compounds possess a large number of activities which might be useful for NLO based applications. Theoretically, generate a FT-IR spectrum of both compounds helps to analyze the different vibrational modes. Vibrational Energy Distribution Analysis (VEDA), the software was used to interpret the Potential Energy Distribution (PED) function of FT-IR spectra. Detailed vibrational assignments of the wave numbers using VEDA were carried out on the basis of potential energy distribution with good agreements.

Impact of terminal polar substitution on elastic, electro-optic and dielectric properties of four-ring bent-core nematic liquid crystals.

Here we report the influence of terminal –F, –Cl and –NO2 substitution on the elastic, dielectric and polar switching behavior of four-ring bent-core liquid crystals (LCs). Elastic constants of nematic liquid crystals are the key parameters in determining the threshold voltage and sensitivity to electro-optical response in a device. The elastic properties of bent-core liquid crystal systems show atypical temperature dependence and there is no hard-core theory to explain the behavior. However based on molecular simulation and atomistic calculations it is found in earlier studies that the bend angle dominates the behavior of elastic constants and the terminal or lateral substitutions have very little effect. Here we have studied three bent-core compounds which are differentiated only by their terminal polar substitution. The bend angle is identical (∼146°) for all the three compounds yet they show dramatically different elastic properties. In the fluoro-substituted compound K11 > K33, while for the other two compounds K33 > K11. Thus it is evident that the terminal polar substitution plays vital role in determining the elastic properties of bent-core systems. Correlating the mesophase ranges with the respective dipole moments of the samples it is observed that the fluoro-substituted compound (11-2M-F) with lowest dipole moment favours only nematic phase with smallest mesophase range (46.1 °C), compound 11-2M-Cl with moderate dipole moment favours short range nematic, broad range smectic with moderate mesophase range (53.1 °C), whereas the compound 11-2M-NO2 possesses the widest mesophase range (99.8 °C) with a very narrow nematic and a broad smectic phase amongst the three studied compounds.

Thermal and nonlinear optical studies of newly synthesized EDOT based bent-core and hockey-stick like liquid crystals

The synthesis and characterisation of novel bent-core and hockey-stick like liquid crystals from EDOT are reported. All the compounds exhibit an enantiotropic mesophase with a wide temperature range. These mesogens show good photophysical and nonlinear optical properties.

Tuning the phase transition temperature of ferronematics with a magnetic field.

The influence of magnetic field on the isotropic-to-nematic phase transition temperature is investigated in neat bent-core and calamitic liquid crystals, in their mixture, and in samples doped with spherical magnetic nanoparticles for two different orientations of the magnetic field. A magnetic-field-induced negative or positive shift of the transition temperature was detected depending on the magnetic field orientation with respect to the initial orientation of the nematic phase, and on the type of liquid crystal matrix.

Effect of terminal chain length on the helical twisting power in achiral bent-core molecules doped in a cholesteric liquid crystal.

We prepared a homologous series of achiral bent-core (BC) liquid crystals with different terminal alkoxy chain lengths, n (BC-n), and evaluated the helical twisting power (HTP) of the BC-n doped in a cholesteric liquid crystal. The BC-n molecules with longer terminal chains showed larger HTPs. To interpret this striking phenomenon, a stochastic dynamics simulation was performed to determine the distribution of the chirality order parameters (χ) for BC molecules with n = 8–16. The distribution of χ for each simulated conformation varied with n, and the variation tendency was different for molecules with n < 12 and n > 12 despite the linear relationship between HTP and n in the experiment.

Anomalously low twist and bend elastic constants in an oxadiazole-based bent-core nematic liquid crystal and its mixtures; contributions of spontaneous chirality and polarity

The splay, twist and bend elastic constants of an oxadiazole based bent core liquid crystal and it's mixtures have been measured across the entire nematic range.

Self-assembling, macroscopically oriented, polymer filaments; a doubly nematic organogel.

Nanoscale phase separation and self-organisation in liquid crystals leads to the formation of remarkable hierarchical structures. There are several examples of heliconical nanofilament structures including in the nematic twist-bend (NTB) phase, the B4 phase and liquid crystal gels formed from the B4 phase. Both the formation of the polymer-like structures that permeate the soft-solids and their hierarchical structures are fascinating, not least because of the analogies that can be drawn with naturally-occurring structures. Here, we report a remarkably simple binary system formed from a non-symmetric BC molecule and the rod-like liquid crystal, 5CB. The pure bent-core system exhibits both nematic and dark conglomerate liquid crystal phases. At very low concentrations of the BC material (5-10%) this binary system spontaneously self-assembles into a soft solid formed from nanoscale filaments that are aligned by their nematic environment. Macroscopically, the soft solid shows behaviour that can be associated with both polymers and gels. Interestingly, the sub-micron scale structure of the filaments appears remarkably similar to some organised fibrous structures in nature (e.g. chitin, cellulose, insect cuticle, plant cell walls) something we attribute to self-assembly and self-organisation in an aligned liquid crystalline environment. The nanoscale structure of the filaments shows no features that can be associated with heliconical ordering down to length scales of tens of nanometers. However, the X-ray data suggest that a metastable rectangular columnar phase which is highly ordered in one dimension initially forms, changing to a hexagonal lattice on a timescale of tens of minutes.

Synthesis and Properties of Thiadiazole Bent-Core Liquid Crystal with Difluorooxymethylene-Bridge

Synthesis and Properties of Thiadiazole Bent-Core Liquid Crystal with Difluorooxymethylene-Bridge

Cybotactic clusters seen in the nematic phase of four-ring achiral bent-core liquid crystal by dielectric spectroscopy

ABSTRACTCybotactic nematic phase of the bent-core liquid crystals are considered as the new functional material for the next-generation liquid crystal display because of its unusual physical properties which has no calamitic counterpart. In this work, the cybotactic nature of the bent-core liquid crystal are confirmed by dielectric spectroscopy. The liquid crystal sample also shows a lower tendency towards crystallization paving the way for room temperature liquid crystal devices.

Elastic and dielectric properties of ferroelectric nanoparticles/bent-core nematic liquid crystal blend.

Bent-core liquid crystals present the first evidence of forming polar superstructures from achiral molecules. The nematic phase is the newest member of the bent-core family and turns out to be extremely interesting owing to its distinct features compared to its calamitic counterpart. Here the investigation of one achiral unsymmetrical 2-methyl-3-amino-benzoic acid (2,6-substituted toluene)-derived four-ring bent-core nematic (BCN) liquid crystals (11-2M-F) is presented after nanodispersion. Ferroelectric nanoparticles significantly affect the phase transition temperature, threshold voltage, dielectric permittivity, elastic constants and splay viscosity of the pristine BCN. In most bent-core nematic liquid crystals the bent elastic constant (K33) is usually lower than the splay elastic constant (K11) owing to the presence of short-range smectic-C-like correlations in the nematic phase. Thus the elastic anisotropy ([Formula: see text]) is usually negative in bent-core nematics unlike in rod-like nematic liquid crystals where K33 is always greater than K11. Here we report a short-core bent-shaped nematic liquid crystal whose negative elastic anisotropy was turned to positive by minute addition of ferroelectric nanoparticles.

Wide temperature-range, multi-component, optically isotropic antiferroelectric bent-core liquid crystal mixtures for display applications

ABSTRACTWe present studies on 21 multi-component mixtures consisting of bent-core liquid crystals to obtain room temperature switching between optically isotropic and birefringent states. Four of the mixtures show a significant enhancement over our previous results that were obtained either only at elevated temperatures, or did not show switching at room temperatures with appreciable contrast or speed. Although the switching of the new mixtures still requires high fields and shows only speeds at ~100 ms, the results appear already useful for specific applications, such as transparent displays, that do not require video-rate switching and fast refresh rates of the content.

Development of ferroelectricity in the smectic phases of 4-cyanoresorcinol derived achiral bent-core liquid crystals with long terminal alkyl chains

Bent-core molecules can form numerous polar and symmetry broken liquid crystalline phases with fascinating properties. Here we report the characteristics of a previously unknown polar synclinic smectic phase, $\mathrm{Sm}{C}_{\mathrm{S}}{P}_{\mathrm{X}}$, found in the phase sequence of an achiral bent-core material with a 4-cyanoresorcinol bisbenzoate core, terminated by long linear alkyl chains ($n=18$) on both ends. This phase exists over a narrow range of temperatures and is sandwiched in between the random polar synclinic smectic phase ($\mathrm{Sm}{C}_{\mathrm{S}}{P}_{\mathrm{R}}$) and polar synclinic ferroelectric ($\mathrm{Sm}{C}_{\mathrm{S}}{P}_{\mathrm{F}}$) phase with ${\mathbit{P}}_{S}\ensuremath{\sim}250\phantom{\rule{0.16em}{0ex}}\mathrm{nC}/\mathrm{c}{\mathrm{m}}^{2}$. In a planar-aligned cell it exhibits only chirality flipping on application of a conventional ac field but it also exhibits optical switching by rotation of the molecular directors on the tilt cone subjected to a modified sequence of bipolar pulses. This changeover is discussed in terms of the model given by Nakata et al. [Phys. Rev. Lett. 96, 067802 (2006)], involving a competition between the two forms of switching: the rotation around the long molecular axis and the switching through rotation of the molecular directors on the tilt cone. The model is modified to take account of the azimuthal pretilt and the molecular tilt angles. In addition to it, characteristics of $\mathrm{Sm}{C}_{\mathrm{S}}{P}_{\mathrm{R}}$ are also discussed.

Phase transition analysis of V-shaped liquid crystal: Combined temperature-dependent FTIR and density functional theory approach

Temperature-dependent Fourier transform infrared spectroscopy (FTIR) combined with density functional theory (DFT) is employed to study the mechanism of phase transitions of V-shaped bent-core liquid crystal. Since it has a large number of flexible bonds, one-dimensional potential energy scan (PES) was performed on the flexible bonds and predicted the most stable conformer I. A detailed analysis of vibrational normal modes of conformer I have been done on the basis of potential energy distribution. The good agreement between the calculated spectrum of conformer I and observed FTIR spectrum at room temperature validates our theoretical structure model. Furthermore, the prominent changes observed in the stretching vibrational bands of CH3/CH2, CO, ring CC, ring CO, ring CH in-plane bending, and ring CH out-of-plane bending at Iso→nematic phase transition (at 155°C) have been illustrated. However, the minor changes in the spectral features observed for the other phase transitions might be due to the shape or bulkiness of molecules. Combined FTIR and PES study beautifully explained the dynamics of the molecules, molecular realignment, H-bonding, and conformational changes at the phase transitions.

Effect of Conformational Chirality on Optical Activity Observed in a Smectic of Achiral, Bent-Core Molecules.

Smectics of achiral, tilted bent-core liquid crystal molecules are chiral, but their optical activity is generally small. Here, we study the effect of conformational chirality on optical activity in smectic phases of an achiral, bent-core mesogen, W513. The neat material has a modulated B4 phase, which appears dark under crossed polarizers and shows no observable optical rotation under decrossed polarizers. However, mixtures of W513 with a rod-like mesogen, 8CB, show a conventional B4 phase, in which distinct left- and right-handed chiral domains with opposite optical activity are observed. The optical behavior of the mixtures is consistent with NMR results, which show a splitting of the carbonyl peak of the bent-core molecules into two, indicating a twisted conformation between the two molecular arms of the bent-core molecules as in conventional B4 materials.

The temperature effect on dielectric parameters of bent-core liquid crystal (DHB)

In this study, we have been analyzed in detail the dielectric, ac conductivity properties and phase transition temperatures of the bent-core liquid crystal 3′-{4-[4-(3,7-Dimethyloctyloxy)benzoyloxy]benzoyloxy}-4-{4-[4-[11-(1,1,1,3,5,5,5-heptamethyltrisiloxan-3-yl)undec-1-yloxy]benzoyloxy]benzoyloxy}biphenyl (DHB) exhibiting a dark conglomerate mesophase (DC[⁎] phase) by Impedance Spectroscopy measurements at different temperatures.

Temperature-dependent infrared study of phase transitions in bent-core liquid crystalline compound 12O(OH)2MeO10

Temperature-dependent Fourier-transform infrared (FTIR) investigation on a novel four-ring bent-core liquid crystal ((4′-n-decyloxyphenylazo)-phenyl-4-yl-3-[N-(4′-n-dodecyloxy-2-hydroxybenzylidene) amino]-2-methyl benzoate) has been done to study the dynamics of the molecules in mesophases at the molecular level. The spectroscopic analysis revealed that specific modes gave a clear evidence of Cr⇄SmC and SmC⇄NcybC phase transitions and successfully depicts the mechanism at these transitions. One of the important objectives to find the spectroscopic signature of the coexistence of SmC and cybotactic nematic NcybC phases below NcybC→SmC phase transition up to 3°C could not be achieved. FTIR spectroscopy confirmed the phase reversibility. In order to get the theoretical support density functional theory (DFT) has been adopted for conformational analysis that predicted the most stable conformer I. Conformer I might be expected closer to the real system and utilized for vibrational assignment along with potential energy distribution (PED). It has been demonstrated that the combination of FTIR spectroscopy and DFT paved a way to understand a candid picture of structural modification and dynamics of mesophases at the macromolecular level during the phase transitions.

Two-wave energy exchange in photorefractive hybrid cell with bent-core liquid crystal

ABSTRACTEnergy gain of weak signal interacting with a strong pump at the director grating in photorefractive hybrid cell is studied. The cell is filled by bent-core cholesteric LC with small twist/bend elastic constants. The director grating is written by the space-charge field penetrating into LC from the photorefractive substrate. Due to a decreased twist/bend elasticity of the LC, the grating magnitude increases causing an increase of the energy gain. Influence of the elastic constants ratio, flexoelectric coefficients, and cholesteric pitch is studied. We show that the gain coefficient can reach higher values in bent-core cholesterics compared to conventional ones.

Anomalously high dielectric strength and low frequency dielectric relaxation of a bent-core liquid crystal with a large kink angle

We investigated the dielectric dispersion property of a bent-core liquid crystal (BLC) with a large kink angle in the frequency range of 1.0 Hz–5.5 MHz in a planar aligned cell. Single dielectric dispersion was observed in the smectic A, nematic, and isotropic phase of the planar aligned sample. The dielectric strengths, relaxation frequencies, distribution parameters, and dc conductivity were measured as a function of temperature. The dielectric strength of the observed relaxation mode was anomalously high (∼70), whereas the relaxation frequency was low at ∼500 Hz. The relaxation mode observed in the planar aligned cell was attributed to the fluctuations in the polarization direction due to the cooperative motion of the molecules.

Multi-step modeling of liquid crystals using ab initio molecular packing and hybrid quantum mechanics/molecular mechanics simulations

A density functional theory (DFT) based multi-step simulation method is used to characterize the detailed molecular structure and inter/intra- molecular interactions of two benchmark liquid crystals (LC) 5CB, 8CB and a novel tri-biphenyl ring bent core LC material. The method uses hybrid DFT at the B3LYP/6-31G* level to obtain molecular structure and Raman data. These results are fed to a crystal packing simulation to find possible crystal structures. A pico-second quantum mechanics/molecular mechanics (QM/MM) simulation model is built for the selected structures with lower overall energy as well as optimal density. The stabilized crystal structures are then extended into a super cell, heated and simulated using a mixed force field and nano-second molecular dynamics (MD). The described simulation process sequence provides predictions of molecular Raman spectrum, LC density, isotropic depolarization ratio, ratio of differential polarizability, order parameters, molecular structures, and rotating Raman spectrum of the different mesophases. The Raman spectra, order parameters and depolarization ratios all agree well with existing experimental and previous simulation results. The study of the novel tri-biphenyl ring bent core LC system shows that the ratio of differential polarizability depends on intra-molecular interactions. The findings presented in this manuscript contribute to the on-going efforts to establish links between LC molecular structures and their properties, including optical behavior.