Elastic properties of bent-core nematic liquid crystals: the role of the bend angle

ABSTRACT

The nematic phase formed from bent-core liquid crystals has been the focus of intensive research for more than a decade. With the potential of biaxial nematic phase and other interesting features, such as high Kerr constants, large flexoelectric coefficients and anomalous elastic constants, these bent-core materials have been in the limelight of research. This paper presents a mini-review of the interesting elastic behaviour reported in various bent-core compounds. The review further focusses on two different types of bent-core compounds as exemplars: one derived from an oxadiazole and other a thiadiazole, highlighting the importance of bend angle in bent-core compounds. The universality of the unique behaviour of bend elastic constants via molecular field theory and atomistic modelling has also been summarised.     

Optical measurement of flexoelectric polarisation change in liquid crystals doped with bent-core molecules using hybrid-aligned structure

We proposed an optical measurement method for determination of flexoelectric polarisation change in liquid crystals (LCs), which can be induced in highly distorted LC geometries. A hybrid-aligned nematic LC (NLC) mode was introduced to evaluate the direction and magnitude of the flexoelectric polarisation. We measured the DC offset amounts for equivalent brightness levels between forward and reverse bias vertical electric fields to estimate the sign and magnitude of e_s?e_b of flexoelectric coefficients. Additionally, the optical incident angle (α_max) for the maximum effective birefringence was investigated to predict the depth distribution of the LC director affecting the magnitude of the flexoelectric polarisation. The relationship between the variations of the DC offset and α_max by the flexoelectric polarisation changes was examined using the NLC mixtures doped with three selected bent-core LCs.     

Anomalies in the twist elastic behaviour of mixtures of calamitic and bent-core liquid crystals

ABSTRACT

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.     

Synthesis and characterisation of novel isoxazole-based banana liquid crystals from naturally occurring curcumin

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 derivative 3d shows 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.     

Ethylenedioxythiophene as a novel central unit for bent-core liquid crystals

We have discovered that ethylenedioxythiophene (EDOT) can be used as a central unit for the synthesis of bent-core liquid crystals (BC LCs). Two series of EDOT-based BC LCs are prepared via Sonogashira coupling reaction. The mesophase behaviour of all the compounds was characterised using a combination of polarising optical microscopy, differential scanning calorimetry and X-ray diffraction measurements. EDOT-based three-ring compounds were found to be nonliquid crystalline, while all the four derivatives of five-ring series, including a branched alkoxy chain derivative, display enantiotropic nematic phase over wide temperature range. The bent angle of these compounds is about 153°, which falls in between typical rod-like and banana liquid crystals. The transition temperature of branched alkoxy chain compound is lower than straight alkoxy chain compounds. The detailed XRD investigations of all the mesogens corroborate the presence of nematic phase.     

Polar Order,Mirror Symmetry Breaking,and Photoswitching of Chirality and Polarity in Functional Bent-Core Mesogens

In recent years, liquid crystals (LCs) responding to light or electrical fields have gained significant importance as multifunctional materials. Herein, two new series of photoswitchable bent-core liquid crystals (BCLCs) derived from 4-cyanoresorcinol as the central core connected to an azobenzene based wing and a phenyl benzoate wing are reported. The self-assembly of these molecules was characterized by differential scanning calorimetry (DSC), polarizing light microscopy (POM), electro-optical, dielectric, second harmonic generation (SHG) studies, and XRD. Depending on the direction of the COO group in the phenyl benzoate wing, core-fluorination, temperature, and the terminal alkyl chain length, cybotactic nematic and lamellar (smectic) LC phases were observed. The coherence length of the ferroelectric fluctuations increases continuously with decreasing temperature and adopts antipolar correlation upon the condensation into superparaelectric states of the paraelectric smectic phases. Finally, long-range polar order develops at distinct phase transitions; first leading to polarization modulated and then to nonmodulated antiferroelectric smectic phases. Conglomerates of chiral domains were observed in the high permittivity ranges of the synclinic tilted paraelectric smectic phases of these achiral molecules, indicating mirror symmetry breaking. Fine-tuning of the molecular structure leads to photoresponsive bent-core (BC)LCs exhibiting a fast and reversible photoinduced change of the mode of the switching between ferroelectric- and antiferroelectric-like as well as a light-induced switching between an achiral and a spontaneous mirror-symmetry-broken LC phase.     

Chiral and orientationally ordered fluid mesophases formed by oxadiazole bisaniline based achiral bent mesogens

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.     

Dipolar interactions, molecular flexibility, and flexoelectricity in bent-core liquid crystals

The effects of dipolar interactions and molecular flexibility on the structure and phase behavior of bent-core molecular fluids are studied using Monte Carlo computer simulations. Some calculations of flexoelectric coefficients are also reported. The rigid cores of the model molecules consist of either five or seven soft spheres arranged in a "V" shape with external bend angle gamma. With purely repulsive sphere-sphere interactions and gamma = 0 degrees (linear molecules) the seven-sphere model exhibits isotropic, uniaxial nematic, and untilted and tilted smectic phases. With gamma > or = 20 degrees the untilted smectic phases disappear, while the system with gamma > or = 40 degrees shows a direct tilted smectic-isotropic fluid transition. The addition of electrostatic interactions between transverse dipole moments on the apical spheres is generally seen to reduce the degree of molecular inclination in tilted phases, and destabilizes the nematic and untilted smectic phases of linear molecules. The effects of adding three-segment flexible tails to the ends of five-sphere bent-core molecules are examined using configurational-bias Monte Carlo simulations. Only isotropic and smectic phases are observed. On the one hand, molecular flexibility gives rise to pronounced fluctuations in the smectic-layer structure, bringing the simulated system in better correspondence with real materials; on the other hand, the smectic phase shows almost no tilt. Lastly, the flexoelectric coefficients of various nematic phases--with and without attractive sphere-sphere interactions--are presented. The results are encouraging, but a large computational effort is required to evaluate the appropriate fluctuation relations reliably.     

Nematic nanocomposites with enhanced optical birefringence

The proper performance of electro-optical devices utilising liquid crystals (LCs) requires materials with high diffraction efficiency, i.e. with high optical/dielectric anisotropy, low threshold voltage and fast switching. One can achieve increase of dielectric anisotropy by using chemical synthesis or mixing LC materials. However, in most cases, this causes an increase in the threshold voltage and switching times. Therefore obtaining materials with high dielectric anisotropy and keeping threshold voltage and switching times low is a challenging task. We achieved promising results by making binary mixtures of a polar nematic LC 4'-hexyl-4-biphenylcarbonitrile (HBPCN) with low percentage (1–10% by weight) gold nanoparticles. We report that for the mixtures with 1% and 2% gold the dielectric anisotropy increases by 100% and the birefringence by about 50% of their values for pure nematic. We also report that the increase of the dielectric anisotropy in the mixtures only slightly affects threshold voltage and switching times. We propose that this increase is caused by cluster formation in the mixtures.     

Observation of disordered mesomorphism in three-ring-based highly polar bent-core molecules: design,synthesis and characterisation

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.     

Low-temperature nematic phase in asymmetrical 1,3,4-oxadiazole bent-core liquid crystals possessing lateral methoxy group

Asymmetrical bent-core molecules based on 1,3,4-oxadiazole bent-core unit have been synthesised as a new design with a lateral methoxy group at outer phenyl ring of the molecule. These new asymmetrical bent-core molecules resemble hockey-stick shaped due to the presence of two different arms of different lengths. One arm of these molecules is elongated having two phenyl rings and possesses a 4-n-alkyloxy chain of a different number of carbon atoms (n = 4, 8, 12 and 18) and other arm is short and has one phenyl ring with fixed 4-n-octyloxy chain. The bent-core molecules possess a lateral polar methoxy group at the elongated arm of the molecule. These bent-core compounds exhibited fluorescence emission in the UV wavelength region (~377–386 nm) whereas in acetonitrile and dimethylformamide, solvent displays blue emission peak with a large stoke shift.The bent-core molecules with the number of carbon atoms (n = 4, 8 and 12) at the elongated arm exhibited monotropic nematic phase at low temperature, while the 4-n-octadecyloxy chain at the elongated arm displayed smectic A phase. Dielectric studies were performed in the nematic phase of the bent-core mesogens confirm the formation of the cybotactic cluster in the nematic mesophases.     

Inverse Langmuir–Schaefer films of bent-core molecules

Mesostructures in inversed Langmuir–Schaefer films of each of two contrasting bent-core molecules are explored via atomic force microscopy. One of the molecules forms Langmuir films that are optically isotropic and the other those that are optically anisotropic. Both show suboptical mesostructures in the 100 nm range, in spite of appearing optically uniform. The anisotropic films are effective in aligning nematic bent-core samples, but not rod-like liquid crystals such as 5CB.     

Investigating the Calculation of Rotational Viscosity of the Mixture Comprising Different Kinds of Liquid Crystals:Molecular Dynamics Computer Simulation Approach

Molecular dynamics (MD) computer simulation techniques, as a powerful tool commonly utilized by the liquid crystal display (LCD) community, usually are employed for computing the equilibrium and transport properties of a classical many body system, since they are very similar to real experiments in many respects. In this paper we present molecular dynamics computer simulation results taken for a mixture of the two different kinds of nematic liquid crystals (LCs). We calculated rotational viscosity from Brownian behavior with friction of the mean director of the mixture comprising pentylcyanobiphenol (5CB) and decylcyanobiphenol (10CB) by using molecular dynamics computer simulation, where intermolecular potential parameter is Generalized AMBER force field (GAFF). Our computed results show a good agreement with the experimental results.     

The NTB phase in an achiral asymmetrical bent-core liquid crystal terminated with symmetric alkyl chains

ABSTRACT

The characteristics of the twist-bend nematic (N_TB) 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 N_TB phase. 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 angle θ₀ is 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 N_TB phase) when compared with the previously reported dimer-based twist-bend nematic LCs (31°±3°).     

The effect of molecular structure on the chiral random grain boundary phase

A chiral random grain boundary (RGB) phase was recently observed in a hockey-stick biphenyl-substituted 1,3,4-oxadiazole derivative. In the RGB phase, achiral molecules self-assemble into randomly oriented blocks of chiral smectic layers and the motif of the unique microstructure is attributed to the similarity of hockey-stick molecules both to bent-core and to rod-like molecules. In order to explore the effect of molecular structure on the RGB phase in detail, we systematically change the molecular design. When the flexible tail decreases, the high-temperature Smectic A (SmA) phase is replaced by a nematic phase, showing a phase sequence of Iso-N-SmA-RGB-Cryst in 2-(4-dodecyloxyphenyl)-5-biphenyl-1,3,4-oxadiazole. However, when we replace the 1,3,4-oxadiazole group by the 1,3,4-thiadiazole group, the bending angle increases in the 1,3,4-thiadiazole derivatives and the RGB phase disappears. Or when the length of the arm becomes short in naphthalene-substituted 1,3,4-oxadiazole, these molecules exhibited only normal rod-like molecules’ mesomorphism. These results will provide deep insights on the relationship of molecular structure and mesophase structural property.     

Selective and direct detection of free amino acid using the optical birefringent patterns of confined nematic liquid crystals

Detection of amino acid (AA) is an essential step to understand various biological processes. In this study, we used innovative optical birefringent patterns of nematic liquid crystals (LCs) for the detection of several AAs. We attempted to use capillary-confined nematic LCs as sensor for AA analysis because their three-dimensional micro-scale architecture of LCs allowed better performance than that of mainly reported LC thin film sensors owing to the effect on the formation and dynamics of point defect. The sensing system was built by disrupting the alignment of a nematic LC, 4-cyano-4′-pentylbiphenyl (5CB), using the dopant of dodecyl aldehyde. Detection principle is based on the chemical analytical method of aldehyde titration for AAs, wherein the reaction between AAs and aldehyde group generates Schiff bases that could alter the configuration of nematic LCs at the aqueous/LC interface. The patterns generated in the reaction are captured by polarising optical microscopy (POM) and are visible to the naked eye. The functionalised LCs detected glycine at concentrations as low as 1 pM. There was a surprising result that it can selectively detect D-AAs against their L-isomers, however, further efforts are required to explain the mechanism.     

W-shaped mesogens and variations of their molecular structure

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.     

Study of pentyl-cyanobiphenyl nematic doped with gold nanoparticles

We studied the dispersion of up to 20 wt% gold nanoparticles (GNP) in a room temperature nematic: cyanobiphenyl homologue 4′-pentyl-4-biphenylcarbonitrile (PBPCN or 5CB). At 15% GNP, the threshold voltage decreases to 1.2 times its value for the pure nematic and the switching-off time decreases by a factor of three. We show that this decrease of display parameters is caused by a 1.8 times decrease in dielectric anisotropy, a 1.3 times change in the average elastic parameter, and a change of about 4.6 times the rotational viscosity. The doped GNP did not affect the thermal stability of the material's nematic phase and the material parameters behaviour versus gold concentration was different from the reported earlier analogical behaviour for another cyanobiphenyl homologue, 4′-hexyl-4-biphenylcarbonitrile (HBPCN or 6CB). We propose that these observed differences between the properties of two homologues doped with gold are due to differences in the number of carbon atoms in the alkyl fragment of liquid crystal molecules.     

Rod-shaped dopants for flexoelectric nematic mixtures

We report the synthesis and liquid crystalline behaviour of two series of para-substituted terphenyls as dopants with a rigid rod-like shape, rather than a wedge-, pear- or banana-shape, for guest–host nematic mixtures with flexoelectric properties. One series of liquid crystalline dopants is of low-to-strongly negative dielectric anisotropy and the other is of low-to-strongly positive dielectric anisotropy. The usefulness of apolar and polar rod-like dopants as components of flexoelectric nematic mixtures of positive dielectric anisotropy for use in LCDs is investigated in general and the dependence of the flexoelectric properties of the doped nematic mixtures on the polarity of the dopants is studied in particular. The correlation between the concentration of the dopant and the magnitude of the flexoelastic ratio of several guest–host nematic mixtures is investigated.     

A new family of four-ring bent-core nematic liquid crystals with a highly polar end-group

A new family of four-ring achiral bent-core compounds derived from 2-methyl 3-amino benzoic acid with the methyl group in the bent direction incorporated into the central core have been designed and synthesised. These compounds possess an alkoxy chain attached at only one end of the bent core molecule, while the other arm consists of a biphenyl moiety possessing a highly polar cyano-group. The molecular structure has been confirmed by elemental analysis and spectroscopic data, and the thermal behaviour and phase characterisation has been investigated by differential scanning calorimetry and polarising microscopy. All the compounds exhibit a wide-range enantiotropic nematic phase. A comparison with non-mesomorphic unsubstituted and 4-methyl-substituted homologues is also presented.