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.     

Second-harmonic generation and the influence of flexoelectricity in the nematic phases of bent-core oxadiazoles

Second-harmonic generation (SHG) in the nematic phase of bent-core oxadiazole-based liquid crystals (LCs) was studied and compared to that for the rod-like compound 4-cyano-4?-n-octylbiphenyl (8CB). Weak, isotropically scattered second-harmonic (SH) light was observed for all materials, consistent with SHG by nematic director fluctuations. The SH intensity produced by the bent-core materials was found to be up to ~ 3.4 times that of 8CB. We discuss this result in terms of the dependence of SH intensity on temperature, elastic constants and flexoelectric coefficients. We have calculated the latter by using a molecular field approach with atomistic modelling, thus demonstrating how molecular parameters contribute to the flexoelectric coefficients and illustrating the potential of this method for predicting the flexoelectric behaviour of bent-core LCs. We show that the increased SH signal in the bent-core compounds is partly due to their nematic phases being at a much higher temperature, and also potentially due to them having greater flexoelectric coefficients, up to ~1.5 times those of 8CB. These estimates are consistent with reports of increased flexoelectric coefficients in bent-core compounds in comparison to rod-like compounds.     

Polar interactions between bent–core molecules as a stabilising factor for inhomogeneous nematic phases with spontaneous bend deformations

ABSTRACT

It is generally accepted that the transition into the twist–bend nematic phase (N_TB) is driven by an elastic instability related to the reduction of the bend elastic constant. Here we use a molecular–statistical theory to show that sufficiently strong polar interactions between bent-shaped molecules may lead to experimentally observed reduction of the bend elastic constant in the nematic phase even if electrostatic dipole–dipole interactions are not taken into account. We propose a simple model of bent–core particles and derive explicit analytical expressions which enable one to understand how polar molecular shape affects the elastic constants, and, in particular, the important role of the bend angle. Numerical graphs showing temperature variations of all elastic constants are also presented including the variation of the bend and splay elastic constants before and after the renormalisation determined by local polar order of molecular steric dipoles and the corresponding polar correction to the one-particle distribution function.     

Chiral ordering in the nematic and an optically isotropic mesophase of bent-core mesogens with a halogen substituent at the central core

Two new homologous series of bent-core compounds have been synthesized. Their mesophase behaviour has been investigated by polarizing microscopy, differential scanning calorimetry, X-ray diffraction, NMR spectroscopy, and by dielectric and electro-optical measurements. It was found that, with one exception, all the chlorine-substituted compounds form a nematic phase and an optically isotropic ‘banana phase’. The latter phase shows spontaneously chiral domains of opposite handedness. This phase may be considered as a type of smectic blue phase. The mesophase behaviour of the homologous bromine-substituted compounds is more complicated. Depending on the chain length, B₆, columnar, nematic or the isotropic ‘banana phase’ occur.     

Determination of elastic constants of a binary system (7CPB+9.CN) showing nematic,induced smectic Ad and re-entrant nematic phases

The temperature variation of the splay and bend elastic constants of a binary system exhibiting nematic-induced smectic A_d and re-entrant nematic phases measured by electric field-induced Freedericksz transition method has been reported. As bend deformation is not permitted in the smectic A phase, bend elastic constant (K₃₃) could only be determined in the nematic and re-entrant nematic phases. In both the nematic phases, the splay elastic constant has the same order of magnitude and does not show any pretransitional effect. However, in the induced smectic A_d phase, the value of K₁₁ is about one to two orders higher than that in the nematic phases. The bend elastic constant shows a strong pretransitional effect near the nematic–smectic and smectic–re-entrant nematic phase transitions. The influence of the presence of the induced smectic phase is observed even in those mixtures which have no induced smectic phases. As the smectic phase is approached, the ratio K₃₃/K₁₁ increases rapidly and diverges to infinity.     

Mesophase behaviour of binary mixtures of bent-core and calamitic compounds

The mesophase behaviour of binary mixtures of bent-core and calamitic liquid crystals is presented. The nematogenic 4,6-dichloro-1,3-phenylene bis[4′-(10-undecen-1-yloxy)-1,1′-biphenyl-4-carboxylate] (I) was the banana-shaped component. As the calamitic compound ethyl 4′-(9-decen-1-yloxy)-1,1′-biphenyl-4-carboxylate (II), similar to one arm of the bent-core molecule, was used which exhibits smectic phases in a wide temperature range. A total of six mixtures with different compositions were prepared and studied by polarising optical microscopy, differential scanning calorimetry and X-ray diffraction on non-oriented samples. In the mixtures, a nematic phase is not concomitant with smectic A phase, and the temperature range of both phases highly depends on the concentration of the comprising compounds. Lowered melting temperatures have been observed for all mixtures with respect to that of the pure compounds. Unforeseen finding is the induction of a monotropic SmC phase in mixtures with lowest concentration of the bent-core compound. Semi-empirical quantum-chemical calculations have also been performed. Based on the calculated molecular conformation, as well as on collected X-ray diffraction data, a model for a possible self-assembly of the banana-shaped and calamitic compounds is proposed.     

Bent-core V-shaped mesogens consisting of salicylaldimine mesogenic segments: synthesis and characterization of mesomorphic behaviour

A series of bent-core V-shaped mesogens consisting of salicylaldimine mesogenic segments have been synthesized and their mesomorphic behaviour characterized. In an attempt to understand structure–property relationships, the lengths of the terminal alkoxy chains have been varied from C₂ to C₁₂, C₁₆ and C₁₈, resulting in 13 new bent-core V-shaped molecules. The thermal behaviour of these new compounds has been investigated by optical microscopy, calorimetry and X-ray diffraction studies. In general the compounds show conventional mesophases similar to those shown by calamitic LCs. The materials exhibit good thermal stability, even though their melting and clearing transition temperatures are high as a result of the presence of intramolecular hydrogen bonding between the H-atom of the hydroxyl group and the N-atom of the imine functionality. The phase appearing in the first member of the series with ethoxy chains is a nematic, while the C₃ to C₆ derivatives exhibit a smectic A phase as well as a nematic phase. The higher homologues, C₇ to C₁₂, C₁₆ and C₁₈, show only the smectic A phase. X-ray studies reveal that the SmA phase has a partially bilayer (interdigitated) structure. Remarkably, in some cases, the smectic A phase supercools well below room temperature. It is apparent from our studies that increasing the length of the alkoxy chains promotes smectic behaviour, in agreement with the general observation made for such bent-core molecules.     

Synthesis,mesomorphic, photophysical and computational studies of new achiral four-ring unsymmetrical bent-core mesogens and their Copper(II) complexes

New achiral four-ring unsymmetrical bent-core mesogens derived from 2,5-dihydroxybenzaldehyde and their copper(II) complexes have been synthesised as a new design with an imine and ester linkage. These new bent-core molecules resemble hockey-stick shape, which possesses 4-n-alkyloxy chain (4-n-hexyloxy and 4-n-decyloxy) at one end and methyl or methoxy group at the other end of the molecule. The synthesis, spectroscopic characterisation, phase transition temperature and characterisation of phase behaviour are reported. The bent-core molecules exhibited monotropic nematic and smectic A phase depending on the terminal chain length. Interestingly, copper(II) complexes of bent-core molecules displayed monotropic nematic phase. This is the first report on copper(II) complexes of bent-core molecules that exhibited nematic phase. The four-ring bent-core molecule exhibited fluorescence with large stoke shift. The density functional theory calculations of bent-core molecules and their copper(II) complexes are carried out using Gaussian 09 program at B3LYP level to obtain the stable molecular conformation, dipole moment, highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO) energies and bending angle of the compounds. The natural atomic charges and electronic configurations of the atoms of free ligands as well as the complexes have been evaluated.     

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.     

Chiral ordering in the nematic and an optically isotropic mesophase of bent-core mesogens with a halogen substituent at the central core

Two new homologous series of bent-core compounds have been synthesized. Their mesophase behaviour has been investigated by polarizing microscopy, differential scanning calorimetry, X-ray diffraction, NMR spectroscopy, and by dielectric and electro-optical measurements. It was found that, with one exception, all the chlorine-substituted compounds form a nematic phase and an optically isotropic 'banana phase'. The latter phase shows spontaneously chiral domains of opposite handedness. This phase may be considered as a type of smectic blue phase. The mesophase behaviour of the homologous bromine-substituted compounds is more complicated. Depending on the chain length, B₆, columnar, nematic or the isotropic 'banana phase' occur.     

Dielectric,viscous and elastic properties of nematogenic 1-(4- trans -propylcyclohexyl)-2-(4-cyanophenyl)ethane

This paper presents the results of measurements of the static and dynamic electric permittivity, the shear viscosity of a freely flowing sample, and the splay and bend elastic constants for 1-(4-trans -propylcyclohexyl)-2-(4-cyanophenyl)ethane (C3H7-CyHx-CH2CH2-Ph-C=N) (3CCPE). The static permittivity of the isotropic phase shows pre-nematic critical behaviour which is discussed in the frame of the fluid-like model of Mukherjee. From the temperature dependence of the dielectric relaxation time (corresponding to the molecular rotation around the short axis) and the shear viscosity, the strength of the nematic potential and the effective length of the 3CCPE molecule (in the isotropic phase), were estimated. The splay and bend elastic constants were determined from the voltage dependence of the capacitance of a planar nematic cell.     

Dielectric and elastic properties of a nematic phase induced by charge transfer interaction in a binary system containing disc-shaped molecules

Abstract

The dielectric constants and the elastic coefficients for splay (K ₁) and bend (K ₃) of the charge transfer induced nematic (N_c) phase of tridecyl pentakis(phenylethynyl)phenyl ether (1) doped with different amounts of 2,4,7-trinitrofluorenone (2) were determined by studying the electric field induced bend deformation using the capacitance method. A negative dielectric anisotropy was observed. For the bend elastic constant K ₃ values up to 22 × 10^?12 N are found which are one order of magnitude higher than the respective values of discotic nematic (N_D) phases. Values of 0·6–0·8 are obtained for the ratio K ₁/K ₃; these show a minimum for the equimolar complex.     

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.     

Monte Carlo lattice simulations of the elastic behaviour of nematic liquid crystals

A pairwise additive potential, which approximately reproduces the free energy density for the elastic deformations of a nematic liquid crystal, originally proposed by Gruhn and Hess, has been investigated by simulating the three Freedericksz transitions as well as that of the Schadt-Helfrich cell. The pair potential depends on the three elastic constants K₁, K₂ and K₃ for the splay, twist and bend deformations, respectively. The results of the simulations are compared with the analytical solutions obtained from continuum theory in order to test the accuracy of the model potential at a quantitative level. This comparison is also made for different temperatures to explore the influence of director fluctuations on the elastic behaviour.     

Elastic,dielectric and optical constants of the nematic mixture E49

The Merck nematic mixture E49 exhibits a large nematic interval (0–100 °C) and a large dielectric anisotropy. Both of these features make E49 interesting for applications and basic physics. Unfortunately, no systematic measurements of the material constants of this mixture and their temperature dependence have been reported in the literature. In this paper we report experimental measurements of the splay and bend elastic constants (K ₁₁ and K ₃₃) of the ordinary and the extraordinary refractive indices (n _ort and n _par) at the wavelength λ?=?632.8 nm and of the two elastic constants parallel and orthogonal to the director (ε_par and ε_ort) at the frequency ν?=?5?kHz. The temperature dependence of all of these parameters is found in the temperature range 25–99 °C. The measurements of the elastic constants are performed using both a dielectric and an optical method simultaneously on the same nematic sample. The results obtained using the two methods are in a satisfactory agreement between them within the estimated experimental uncertainty. The ordinary and the extraordinary indices are measured using the prism method.     

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.     

Bent-core V-shaped mesogens consisting of salicylaldimine mesogenic segments: synthesis and characterization of mesomorphic behaviour

A series of bent-core V-shaped mesogens consisting of salicylaldimine mesogenic segments have been synthesized and their mesomorphic behaviour characterized. In an attempt to understand structure-property relationships, the lengths of the terminal alkoxy chains have been varied from C₂ to C₁₂, C₁₆ and C₁₈, resulting in 13 new bent-core V-shaped molecules. The thermal behaviour of these new compounds has been investigated by optical microscopy, calorimetry and X-ray diffraction studies. In general the compounds show conventional mesophases similar to those shown by calamitic LCs. The materials exhibit good thermal stability, even though their melting and clearing transition temperatures are high as a result of the presence of intramolecular hydrogen bonding between the H-atom of the hydroxyl group and the N-atom of the imine functionality. The phase appearing in the first member of the series with ethoxy chains is a nematic, while the C₃ to C₆ derivatives exhibit a smectic A phase as well as a nematic phase. The higher homologues, C₇ to C₁₂, C₁₆ and C₁₈, show only the smectic A phase. X-ray studies reveal that the SmA phase has a partially bilayer (interdigitated) structure. Remarkably, in some cases, the smectic A phase supercools well below room temperature. It is apparent from our studies that increasing the length of the alkoxy chains promotes smectic behaviour, in agreement with the general observation made for such bent-core molecules.     

Elastic properties of bimesogenic liquid crystals

The second nematic phase found in some bimesogenic liquid crystals with an odd flexible spacer has aroused considerable interest due to many unusual properties exhibited by them. However, the reason for such molecules to exhibit transitions to the modulated phase is still unclear. Dozov [Europhys Lett. 2001;56:247] predicted that negative K ₃₃ can lead to a modulated phase where the director has either splay-bend or twist-bend distribution. Though various theoretical studies have suggested that this scenario may be valid, no experimental evidence has so far been given. In order to study the influence of the elastic constants, we measured the splay and bend elastic constants in the ordinary nematic phase of a dimer material, CBC11CB, for temperatures down to 0.6°C above the Nx–Nu transition. Our results show that the bend constant is reduced by a factor of 2 compared with that closer to the I-Nu transition but is positive and the trend does not seem that K ₃₃ will extrapolate to zero or fall below it for the temperature range investigated. Compared to K ₁₁, K ₃₃ is reduced by a factor of 3, close to the Nu–Nx transition temperature.     

A continuum description for cholesteric and nematic twist-bend phases based on symmetry considerations

ABSTRACT

The recently discovered twist-bend nematic phase, N_tb, is a non-uniform equilibrium nematic phase that presents a spontaneous bend with a precession of the nematic director, n, on a conical helix with a tilt angle θ and helical pitch P. The stability of the N_tb phase has been recently demonstrated from the elastic point of view by extending the Frank elastic energy density of the nematic phase to include the symmetry element of the helical axis, t. In the present article, we investigate the influence of an external bulk field (magnetic or electric) on the N_tb phase. Using symmetry arguments we derive the expression for the flexoelectric polarisation in twist-bend nematic phases. We show that, besides the standard contribution related to the spatial variation of the nematic director, two new contributions connected with the existence of the helical axis appear. In the ground state, where the nematic deformation is a pure heliconical deformation, the new contribution vanishes identically, and the total flexoelectric polarisation is perpendicular to the nematic director. Furthermore, as an example, we study the role of an external magnetic field applied parallel to the helical axis for a material with positive magnetic susceptibility anisotropy. We show that the field modifies the range of values of the coupling parameter between the director and the helical axis, thus shifting the interval of values for which this coupling results in the N_tb phase.     

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.