Chiral Liquid Crystalline Materials Research Articles

Ferroelectric and antiferroelectric chiral multilactate liquid crystalline materials with negative dielectric anisotropy

In this work, the mesomorphic, electro-optic and dielectric properties have been discussed in the light of molecular structure-property correlations of two chiral multilactate liquid crystalline materials possessing the orthogonal paraelectric Smectic A* phase, tilted ferroelectric Smectic C* phase and the tilted antiferroelectric Smectic CA* phase, over a substantially broad temperature range. Interestingly, a reasonably rare re-entrant Smectic C* phase (SmCre*) has also been identified in one of the investigated materials. These materials differ in their linkage groups (keto or ether) and an additional chiral unit in the terminal chain. The phase transition temperatures and transition enthalpies were determined from Polarizing Optical Microscopy and Differential Scanning Calorimetry (DSC) measurements. The compounds exhibit negative dielectric anisotropy (Δε) throughout the mesomorphic range (maximum −18 and −4 in SmA* for both the compounds) and moderately high values of spontaneous polarization (∼153 nC/cm2 in SmCre* phase). The temperature dependence of the response time (τ), bulk viscosity (η) and the activation energies (Ea) throughout the mesomorphic phase have been determined from the spontaneous polarization measurements. To emphasize the structure-property correlations in more detail, dielectric spectroscopy measurement has also been performed to measure the dielectric strength, dielectric loss, frequency dependent permittivities, relaxation time and relaxation frequencies. The clear evidence of the relatively rare SmCre* phase has also been confirmed from the temperature and frequency dependence of the dielectric permittivity. These results shed important light on the emergence of these materials as a smart alternative for their application in multicomponent mixtures targeted for advanced electro-optic and photonic devices.

Synthesis, chiral separation and physical properties of the cinnamic acid derivatives exhibiting short-pitch cholesteric and TGB phases

ABSTRACT Two new photosensitive chiral liquid crystalline materials derived from cinnamic acid were synthesised in (R) and (S) enantiomer forms. The materials were characterised by NMR spectroscopy, purified by preparative liquid chromatography, and the chemical purity of the final products was determined by high performance liquid chromatography (HPLC). High optical purity was reached for all enantiomers. The enantiomer elution order was confirmed by the analysis of non-racemic (R) and (S) mixed samples. The cinnamic acid derivatives are capable of undergoing photoisomerisation. Sample irradiation was performed to realise E-Z isomerisation and investigate its effect on the enantioseparation. The conditions of the enantioselective HPLC method were optimised to achieve a baseline separation for all studied materials both before and after irradiation by the UV light. The mesogenic properties of the studied enantiomers and their racemic mixtures were obtained from differential scanning calorimetry measurements, optical textures observed under the polarising light microscope and X-ray diffraction measurements. Cholesteric and TGB phases have been found for the enantiomers and, analogously, nematic and smectic phases have been confirmed for their racemates. The temperature dependence of the optical birefringence has been studied for the racemates in the temperature range of the mesophases.

Properties of Antiferroelectric Mixtures Differing in the Amount of Added Racemate

Novel three-component liquid crystalline mixtures composed of chiral and achiral (racemic) liquid crystalline materials were designed and studied by polarizing optical microscopy, differential scanning calorimetry, and UV–VIS spectroscopy. The compositions of liquid crystalline mixtures were developed based on the composition of a two-component (binary) mixture marked as W-1000 with the following phase sequence: Cr ↔ SmCA* ↔ SmC* ↔ SmA* ↔ Iso. This mixture has an antiferroelectric (SmCA*) phase over a wide temperature range and exhibits a helical pitch inversion in this phase. All newly obtained mixtures occur in a wide temperature range of the SmCA* phase, while the ferroelectric (SmC*) phase and the orthogonal (SmA*) phase occur in a narrow temperature range. The new mixtures also have a very long helical pitch in the antiferroelectric phase and a short helical pitch in the ferroelectric phase.

The correlations between the molecular core structure and mesomorphic behaviour for chiral liquid crystals with several (S)-lactate groups

To contribute to better understanding of the structure–property relationship for chiral liquid crystalline materials a series of new liquid crystalline (LC) materials possessing several (S)-lactate groups as chiral centres have been designed and investigated. All the materials possess the orthogonal paraelectric smectic A* phase stable over a broad temperature range. For the specific lengths of the chiral and non-chiral alkyl chains the ferroelectric tilted smectic C* phase have been detected at lower temperatures. The mesomorphic behaviour and structural properties of new multilactate materials have been studied by polarizing optical microscopy, differential scanning calorimetry, small angle X-ray diffraction and dielectric spectroscopy. The obtained results are discussed and compared to that of the structurally similar lactic acid derivatives. The main objective of the present work is to establish the molecular structure – mesomorphic property relationship for this specific class of multilactate-based LC materials, which can be further successfully utilized as chiral dopants for the design of smart multicomponent LC mixtures targeted for application in optoelectronics and photonics.

How mobile phase composition and column temperature affect enantiomer elution order of liquid crystals on amylose tris(3-chloro-5-methylphenylcarbamate) as chiral selector.

A comprehensive study into the effects of mobile phase composition and column temperature on enantiomer elution order was conducted with a set of chiral rod-like liquid crystalline materials. The analytes were structurally similar and comprised variances such as length of terminal alkyl chain, presence of chlorine, number of phenyl rings, and type of chiral center. Experiments were carried out in polar organic and reversed-phase modes using amylose tris(3-chloro-5-methylphenylcarbamate) immobilized on silica gel as the chiral stationary phase. For all liquid crystals, reversal of elution order of enantiomers was observed based on type of used cosolvent and/or its content in the mobile phase; for some of the liquid crystals a temperature-induced reversal was also observed. Both linear and nonlinear dependencies of natural logarithm of enantioselectivity on temperature were found. Tested mobile phases comprised pure organic solvents and binary and tertiary mixtures of acetonitrile with organic solvents and/or water. Effect of acidic/basic mobile phase additives was also tested. Effect of structure of chiral selector is briefly discussed.

Gnomonious projections for bend-free textures: thoughts on the splay-twist phase.

The Hopf fibration has inspired any number of geometric structures in physical systems, in particular, in chiral liquid crystalline materials. Because the Hopf fibration lives on the three sphere, , some method of projection or distortion must be employed to realize textures in flat space. Here, we explore the geodesic-preserving gnomonic projection of the Hopf fibration, and show that this could be the basis for a new liquid crystalline texture with only splay and twist. We outline the structure and show that it is defined by the tangent vectors along the straight line rulings on a series of hyperboloids. The phase is defined by a lack of bend deformations in the texture, and is reminiscent of the splay-bend and twist-bend nematic phases. We show that domains of this phase may be stabilized through anchoring and saddle-splay.

Elucidation of the de Vries behavior in terms of the orientational order parameter, apparent tilt angle, and field-induced tilt angle for smectic liquid crystals by polarized infrared spectroscopy.

We report experimental results of the orientational order parameter, the apparent tilt angle, and the field-induced tilt angle for three chiral smectic liquid crystalline materials investigated using infrared (IR) polarized spectroscopy. The common feature in these materials is use of the core 5-methyl-2- pyrimidine benzoate as the central part of the mesogen. This core is terminated by siloxane or carbosilane chains on one of the ends and by the chiral alkoxy chains on the opposite. These compounds exhibit low concomitant layer shrinkage at the smectic A^{*} (SmA^{*}) to smectic C^{*} (SmC^{*}) transition temperature and within the SmC^{*} phase itself. The maximum layer shrinkage in SmC^{*} is observed as ∼1.5%. We calculate the apparent orientational order parameter, S_{app} in the laboratory reference frame from the observed IR absorbance for homeotropic aligned samples, and the true order parameter, S, is calculated using the measured tilt angle and is also interpolated from Iso-SmA^{*} transition temperature closer to SmC^{*} phase. The apparent tilt angle in the SmA^{*} phase calculated from a comparison of order parameters S and S_{app} is found to be significantly large. A low magnitude of S_{app} found for homeotropic aligned samples in the SmA^{*} phase indicates that the order parameter plays a vital role in determining the de Vries characteristics, especially of exhibiting larger apparent tilt angles. Furthermore there is a significant increase in the true order parameter at temperatures close to SmA^{*} to SmC^{*} transition temperature in all three compounds. The planar-aligned samples are used to study the dependence of induced tilt angle on the applied electric field. The generalized Langevin-Debye model given by Shen etal. reasonably fits the experimental data on the field-induced tilt angle. The results show that the dipole moment of the tilt correlated domain in SmA^{*} diverges as temperature is lowered to the SmA^{*}-SmC^{*} transition temperature. The generalized Langevin-Debye model is also found to be extremely effective in confirming some of the conclusions of the de Vries behavior.

Mesomorphic, structural, electro-optic and dynamic properties of lactic acid derivative and its selectively deuterated isotopomers by means of electro-optics, SAXS, 2H-NMR and neutron spin-echo spectroscopy

ABSTRACT In this work, we present the synthesis of two analogues of a lactic acid derivative selectively deuterated on phenyl and biphenyl moieties together with a detailed investigation of their properties within the Twist Grain Boundary smectic A* (TGBA*), the ferroelectric Smectic C* (SmC*) and the crystal E phases. The main structural features of the two liquid crystalline phases, namely TGBA* and SmC* phases, were determined by polarising optical microscopy, differential scanning calorimetry, electro-optical techniques, Small Angle X-ray Scattering (SAXS) and 2H Nuclear Magnetic Resonance (2H NMR) measurements. Preliminary results from Neutron Spin-Echo (NSE) spectroscopy, a technique rarely applied to study chiral liquid crystalline materials, are reported. The effect of the external magnetic field on the supramolecular structure of the ferroelectric phase was studied by means of 2H-NMR experiments under different external magnetic fields. The supramolecular and orientational ordering properties of this ferroelectric lactic acid derivative and its deuterated homologues are discussed in the frame of actual knowledge in the field of ferroelectric liquid crystal materials.

Synthesis of new chiral mono- and diacrylates for ferro- and antiferroelectric liquid crystals

The synthesis and characterization of new, chiral and reactive single liquid crystals based on biphenyl benzoate core are described. Compounds are terminated with partially perfluorinated alkoxy chain (monoacrylates) or acryloyloxy-alkoxy chain (diacrylates) on the non-chiral side of molecule, while the opposite terminal side is chiral acryloyloxy unit. New families of smectic LCs present alternative molecular approach to the monomer design. Compounds are characterized by mass spectrometry (electron ionization) analysis, proton and carbon nuclear magnetic resonance. They show liquid crystal behavior in broad temperature ranges, confirmed by a polarizing thermomicroscopy and differential scanning calorimetry. Detailed synthetic procedures are attached. Synthesized compounds stand as promising candidates for polymer stabilization of chiral liquid crystalline materials.

Nature-Inspired Emerging Chiral Liquid Crystal Nanostructures: From Molecular Self-Assembly to DNA Mesophase and Nanocolloids.

Liquid crystals (LCs) are omnipresent in living matter, whose chirality is an elegant and distinct feature in certain plant tissues, the cuticles of crabs, beetles, arthropods, and beyond. Taking inspiration from nature, researchers have recently devoted extensive efforts toward developing chiral liquid crystalline materials with self-organized nanostructures and exploring their potential applications in diverse fields ranging from dynamic photonics to energy and safety issues. In this review, an account on the state of the art of emerging chiral liquid crystalline nanostructured materials and their technological applications is provided. First, an overview on the significance of chiral liquid crystalline architectures in various living systems is given. Then, the recent significant progress in different chiral liquid crystalline systems including thermotropic LCs (cholesteric LCs, cubic blue phases, achiral bent-core LCs, etc.) and lyotropic LCs (DNA LCs, nanocellulose LCs, and graphene oxide LCs) is showcased. The review concludes with a perspective on the future scope, opportunities, and challenges in these truly advanced functional soft materials and their promising applications.

Mesomorphic, electro-optic and structural properties of binary liquid crystalline mixtures with ferroelectric and antiferroelectric liquid crystalline behaviour

ABSTRACT Two new binary liquid crystalline mixtures composed from the chiral liquid crystalline materials with partially fluorinated non-chiral chain and without lateral substitution at the molecular core have been designed and investigated by polarisation optical microscopy, differential scanning calorimetry, broad-band dielectric spectroscopy, and X-ray techniques. Both mixtures possess the paraelectric orthogonal smectic A*, the tilted ferroelectric smectic C* and the antiferroelectric smectic CA* phases over a reasonably broad temperature range. The gained results reveal a considerable improvement of (i) the broadness of the mesophases temperature range of the ferroelectric and antiferroelectric phases (up to 130 K breadth) and (ii) electro-optical properties, namely the spontaneous polarisation (up to 200 nC/cm2) and tilt angle (higher than 40o), with respect to that of the original pure compounds.

Localised polymer networks in chiral nematic liquid crystals for high speed photonic switching

Self-assembled periodic structures based upon chiral liquid crystalline materials have significant potential in the field of photonics ranging from fast-switching optoelectronic devices to low-threshold lasers. The flexoelectro-optic effect, which is observed in chiral nematic liquid crystals (LCs) when an electric field is applied perpendicular to the helical axis, has significant potential as it exhibits analogue switching in 10–100 μs. However, the major technological barrier that prohibits the commercial realisation of this electro-optic effect is the requirement of a uniform, in-plane alignment of the helix axis between glass substrates. Here, it is shown that periodic polymer structures engineered in the nematic phase of a chiral nematic LC device using direct laser writing can result in the spontaneous formation of the necessary uniform lying helix (ULH) state. Specifically, two-photon polymerization is used in conjunction with a spatial light modulator so as to correct for aberrations introduced by the bounding glass substrates enabling the polymer structures to be fabricated directly into the device. The ULH state appears to be stable in the absence of an externally applied electric field, and the optimum contrast between the bright and dark states is obtained using polymer structures that have periodicities of the order of the device thickness.

Modulated nematic structures induced by chirality and steric polarization.

What kind of one-dimensional modulated nematic structures (ODMNS) can form nonchiral and chiral bent-core and dimeric materials? Here, using the Landau-de Gennes theory of nematics, extended to account for molecular steric polarization, we study a possibility of formation of ODMNS, both in nonchiral and intrinsically chiral liquid crystalline materials. Besides nematic and cholesteric phases, we find four bulk ODMNS for nonchiral materials, two of which, to the best of our knowledge, have not been reported so far. These two structures are longitudinal (N_{LP}) and transverse (N_{TP}) periodic waves where the polarization field being periodic in one dimension stays parallel and perpendicular, respectively, to the wave vector. The other two phases are the twist-bend nematic phase (N_{TB}) and the splay-bend nematic phase (N_{SB}), but their fine structure appears more complex than that considered so far. The presence of molecular chirality converts nonchiral N_{TP} and N_{SB} into new N_{TB} phases. Surprisingly, the nonchiral N_{LP} phase can stay stable even in the presence of intrinsic chirality.

ChemInform Abstract: Chiral Liquid Crystals: Structures, Phases, Effects

AbstractReview: 107 refs.

AFM study of advanced composite materials for organic photovoltaic cells with active layer based on P3HT:PCBM and chiral photosensitive liquid crystalline dopants

Advanced composite materials aimed for construction of organic photovoltaic cells have been studied by atomic force microscopy (AFM). The composites are based on poly(3-hexylthiophene) (P3HT), [6,6]-phenyl C61-butyric acid methyl ester (PCBM) and two different chiral photosensitive liquid crystalline (LC) materials. The objective of the study was to examine the nanoscale morphology of the active layer without and after annealing at specific temperature. The preliminary results of AFM observation of the morphological changes done on the investigated composites revealed an increase in the surface ordering. The surface area ratio decreases for both studied composites, while the basic roughness parameters (Sa and Sq) have been found toughly dependent on the structure of the photosensitive LC dopant.

Effect of chiral photosensitive liquid crystalline dopants on the performance of organic solar cells

In order to design a new type of an organic solar cell device and to contribute for its performance optimization, i.e. transport and optical properties, a composite organic solar cell have been designed. It has been fabricated in two modifications from poly(3-hexylthiophene) and [6,6]-phenyl C61-butyric acid methyl ester mixtures (P3HT:PCBM) and two chiral photosensitive liquid crystalline (PCLC) materials used as additives. Doping of the P3HT:PCBM solar cell by a specific PCLCs led to a distinct adjustment (increase or decrease) of power conversion efficiency in comparison to that of an undoped cell and it is also strongly depends on the viscosity of the PEDOT:PSS used. In order to control the properties of the resulting composite P3HT:PCBM solar cells doped by PCLCs, the photovoltaic and impedance spectroscopy studies have been done for active layers annealed at different temperatures. Differential scanning calorimetry (DSC) in combination with polarized optical microscopy (POM) has been used to examine the morphology of an active layer. Thermal annealing of the sample within a mesophase temperature range can remove defects, optimize the morphology of the active layer, and hence might be responsible for the increase of photocurrent and cell efficiency.

Chiral Liquid Crystals: Structures, Phases, Effects

The introduction of chirality, i.e., the lack of mirror symmetry, has a profound effect on liquid crystals, not only on the molecular scale but also on the supermolecular scale and phase. I review these effects, which are related to the formation of supermolecular helicity, the occurrence of novel thermodynamic phases, as well as electro-optic effects which can only be observed in chiral liquid crystalline materials. In particular, I will discuss the formation of helical superstructures in cholesteric, Twist Grain Boundary and ferroelectric phases. As examples for the occurrence of novel phases the Blue Phases and Twist Grain Boundary phases are introduced. Chirality related effects are demonstrated through the occurrence of ferroelectricity in both thermotropic as well as lyotropic liquid crystals. Lack of mirror symmetry is also discussed briefly for some biopolymers such as cellulose and DNA, together with its influence on liquid crystalline behavior.

Effect of a bulky lateral substitution by chlorine atom and methoxy group on self-assembling properties of lactic acid derivatives

Several chiral liquid crystalline materials derived from the lactic acid have been studied with the aim to establish the effect of bulky lateral substituents on their self-assembling properties. A chlorine atom and methoxy group have been used as lateral substituents in ortho position to ether group position on phenyl ring far from the chiral centre. All the studied materials possess tilted ferroelectric smectic C* phase in a broad temperature range. In dependence on the molecular structure namely type of lateral substituent and length of the chiral chain, the cholesteric mesophase, orthogonal paraelectric smectic A* and crystal mesophases have been detected. Lateral chlorine substitution results in decrease of both the clearing point and crystallisation temperature as well as in a distinct increase of spontaneous polarization. Bulky methoxy substitution slightly suppresses the spontaneous polarisation but strongly increases the melting point that results in monotropic peculiarity of the SmC* phase. Mesomorphic, spontaneous, structural and dielectric properties of the substituted compounds were established and compared to those of the non-substituted ones in order to contribute to better understanding of the structure–property relationship for such chiral self-assembling materials.

Anomalous phase sequence in new chiral liquid crystalline materials

We have prepared and studied a series of rod-like liquid crystalline compounds with the molecular core laterally substituted by bromine atom and with the lactate unit in the chiral terminal chain. Microscopic observation under the polarising microscope, x-ray and dielectric spectroscopy results enable us to establish the TGBA–SmA–TGBARE–TGBC phase sequence. The presence of the re-entrant TGBA phase below the regular SmA phase, found for three homologues, is quite uncommon. Results are discussed and compared with the previously studied compounds with regular phase sequences.

Synthesis and Liquid Crystalline Properties of New Chiral Monomers and Polysiloxanes

To study the structure-property relationships of new chiral liquid crystalline materials based on menthol and explore their potential applications, a monomer 4-(4-allyloxybenzoyloxy) phenyl-4-menthyl-oxyacetyloxybiphenyl-4-carboxylate (LCM) and the corresponding homopolymer (LCP) were synthesized. The chemical structure was characterized by FT-IR and 1H NMR. The liquid crystalline properties were investigated by differential scanning calorimetry, thermogravimetric analysis, polarizing optical microscopy, and X-ray diffraction. The monomer LCM formed a cholesteric phase when a flexible linkage chain was inserted between the mesogenic core and the terminal menthyl groups by reducing the steric effect. The homopolymer LCP exhibited a batonnet texture of a smectic A phase.