Class Of Liquid Crystals Research Articles

Synthesis and examination of alkoxycyanobiphenyl mesogens with a single fluorine atom at specific locations in the tail

ABSTRACT Selective fluorination is a well-established technique to tailor and improve many of the physical properties of liquid crystals. Cyanobiphenyls are among the best-known and understood thermotropic liquid crystals and have served admirably as a testbed for structural modifications that can eventually be extended to other classes of liquid crystals. As such, and expanding on our earlier work, in the present study of the alkoxycyanobiphenyl M series, the individual tail sites of both 4’-(butoxy)[1,1’-biphenyl]-4-carbonitrile (M12) and 4’-(pentyloxy)[1,1’-biphenyl]-4-carbonitrile (M15) were selectively monofluorinated in order to examine the influence on the phase behavior. These fluorinated compounds were synthesized using epoxidation, regioselective epoxide opening and deoxyfluorination chemistry. In all the fluorinated compounds in both series a monotropic nematic phase was found in the cooling cycle. It was observed that as the fluorine atom on the tail moves closer to the core, the compounds exhibited a metastable room temperature nematic mesophase, but recrystallization invariably occurred. Both enantiomers of 4’-(2-fluorobutoxy)[1,1’-biphenyl]-4-carbonitrile were synthesized in high enantiomeric purity by regioselective opening of chiral epoxide intermediates followed by deoxyfluorination, as established by chiral high-performance liquid chromatography, and their phase behaviors were determined. Moreover, the average dipole moments of the synthesized fluorine substituted derivatives were also calculated.

SYNTHESIS OF POTENTIAL LIQUID CRYSTALS WITH CHOLESTEROL FRAGMENT BY WITTIG REACTION

Liquid crystals are substances that owing to the features of their structure and physical properties are of interest not only as objects for theoretical research, but also significantly important practically due to the possibilities of their effective application in various brunches of industry, medicine, in household etc. Among the known classes of liquid crystals, substances known as cholesterics are an important group.
 Cholesteric liquid crystals demonstrate very high optical activity, that significantly exceeds the optical activity of most other known classes of organic compounds. Their ability for appreciable change of color at change of the temperature and environment composition is also practically important.
 Among all compounds belonging to the class of cholesterics an important place possess cholesterol derivatives, especially cholesterol esters. Therefore, the elaboration of new methods of their synthesis and the introduction of new functional groups into their molecules are an urgent tasks indeed.
 This work is devoted to the investigation of the possibility of synthesis of new cholesterol derivatives, namely cholesteryl esters of unsaturated acids by the Wittig reaction - the interaction of various classes of aldehydes with phosphonium salts, with the intermediate formation of phosphorus ylides - alkylidene phosphoranes. We found that the Wittig reaction is a convenient method for the synthesis of cholesteryl esters of unsaturated carboxylic acids. We have developed methods for the synthesis of potential cholesteric liquid crystals by the Wittig reaction, which use a phosphonium salt containing a cholesterol fragment and corresponding aldehydes. The process proceed without the release of an intermediate compound – alkylidene phosphorane, which reduces the labor intensity of the method.
 The application of various aldehydes enables the easily obtaining of cholesteryl esters of unsaturated acids containing various aliphatic, aromatic and heterocyclic fragments in the acid radical. The reaction takes place in mild conditions, without using of high temperatures or aggressive environments. The resulting esters show signs of mesophase formation, which is a confirmation of their liquid crystalline properties.

Modularizable Liquid-Crystal-Based Open Surfaces Enable Programmable Chemical Transport and Feeding using Liquid Droplets.

Droplet-based miniature reactors have attracted interest in both fundamental studies, for the unique reaction kinetics they enable, and applications in bio-diagnosis and material synthesis. However, the precise and automatic feeding of chemicals, important for the delicate reactions in these miniaturized chemical reactors, either requires complex, high-cost microfluidic devices or lacks the capability to maintain a pinning-free droplet movement. Here, the design and synthesis of a new class of liquid crystal (LC)-based open surfaces, which enable a controlled chemical release via a programmable LC phase transition without sacrificing the free transport of the droplets, are reported. It is demonstrated that their intrinsic slipperiness and self-healing properties enable a modularizable assembly of LC surfaces that can be loaded with different chemicals to achieve a wide range of chemical reactions carried out within the droplets, including sequential and parallel chemical reactions, crystal growth, and polymer synthesis. Finally, an LC-based chemical feeding device is developed that can automatically control the release of chemicals to direct the simultaneous differentiation of human induced pluripotent stem cells into endothelial progenitor cells and cardiomyocytes. Overall, these LC surfaces exhibit desirable levels of automation, responsiveness, and controllability for use in miniature droplet carriers and reactors.

Micellar Lyotropic Nematic Gels.

Lyotropic liquid crystal (LLC) gels are a new class of liquid crystal (LC) networks that combine the anisotropy of micellar LLCs with the mechanical stability of a gel. However, so far, only micellar LLC gels with lamellar and hexagonal structures have been obtained by the addition of gelators to LLCs. Here, the first examples of lyotropic nematic gels are presented. The key to obtain these nematic gels is the use of gelators that have a non-amphiphilic molecular structure and thus leave the size and shape of the micellar aggregates essentially unchanged. By adding these gelators to lyotropic nematic phases, an easy and reproducible way to obtain large amounts of lyotropic nematic gels is established. These nematic gels preserve the long-range orientational order and optical birefringence of a lyotropic nematic phase but have the mechanical stability of a gel. LLC nematic gels are promising new materials for elastic and anisotropic hydrogels to be applied as water-based stimuli-responsive actuators and sensors.

Lyotropic nematic liquid crystals: interplay between a small twist elastic constant and chirality effects under confined geometries

ABSTRACT There are two classes of liquid crystals (LCs): Thermotropic LCs where the organic molecule represents the anisotropically shaped mesogen and lyotropic LCs, where the mesogens are non-spherical supramolecular assemblies dispersed in a solvent. Various kinds of lyotropic nematic liquid crystals (LLCs) exist, differing in shape and composition of the aggregates: Chromonic, polymeric or micellar LLCs.However, micellar LLCs are the most ubiquitous kind of LLCs, occurring in soaps and biological systems. Although these nematic lyotropics are so different, they exhibit – contrary to thermotropic nematics – spontaneous reflection symmetry breaking under confinement. We discuss spontaneous mirror symmetry breaking under capillary confinement for a standard micellar ND phase and give a detailed director field structure of the twisted polar configuration. Furthermore, we measure for the first time a complete set of viscoelastic properties of a micellar nematic LLC, finding a small twist elastic constant K 22, which is at least one order of magnitude smaller than K 11 and K 33. As demonstrated by the following work, a small K 22 and therefore spontaneous mirror symmetry breaking under confinement is rather the rule than the exception in lyotropic nematics and constitutes a unique difference between lyotropic and typical thermotropic nematics, for which the well-known one-constant approximation holds. Abbreviations: LC: Liquid Crystal; LLC: Lyotropic liquid crystal; TER: twisted escaped radial director configuration; TP: twisted polar director configuration; : director; K 11: splay elastic constant of the nematic phase; K 22: twist elastic constant of the nematic phase; K 33: bend elastic constant of the nematic phase; s: strength of a defect/disclination; ER: escaped radial director configuration; CDEAB: N,N-ethylhexadecylammonium bromide; DOH: decan-1-ol; ND: nematic phase of disk-shaped building blocks; NC: nematic phase of rod-shaped building blocks; H-field: magnetic field; PR: planar radial director configuration; PP: planar polar director configuration; RR: right handed – right handed; LL: left handed – left handed; RL: right handed – left handed; LR: left handed – right handed.

A chiral-racemic lyotropic chromonic liquid crystal system.

The two main classes of liquid crystals are thermotropic (containing no solvent) and lyotropic (containing solvent). Both of these classes possess the nematic phase, the most simple of liquid crystal phases with only uniaxial orientational order. For both of these classes, if the constituent molecules are chiral or if a chiral dopant is added, the preferred direction of orientation rotates in helical fashion in what is called the chiral nematic phase. Recent research has shown that because the ordering entities of the two classes are quite different (molecules versus molecular assemblies), important differences in the properties of the nematic phase can result. While thermotropic chiral nematics have been extensively examined, less is known about lyotropic chiral nematics, especially for the most ideal case, a chiral-racemic system. Furthermore, none of the lyotropic chiral-racemic studies has included lyotropic chromonic liquid crystals, which are solutions of dyes, drugs, and nucleic acids. Inverse pitch measurements are reported for a chiral-racemic system of a chromonic liquid crystal across the entire chiral fraction range and over a 30 °C temperature interval. The inverse pitch depends linearly on chiral fraction and decreases with increasing temperature, indicating that achiral and chiral molecules participate in the assembly structure similarly. The helical twisting power is significantly larger than for other chiral lyotropic liquid crystals due to the very high scission energy of the investigated system.

Chirality of Liquid Crystals Formed from Achiral Molecules Revealed by Resonant X-Ray Scattering.

Intensive research on chiral liquid crystals (LCs) has been fueled by their actively tunable physicochemical properties and structural complexity, comparable to those of sophisticated natural materials. Herein, recent progress in the discovery of new classes of chiral LCs, enabled by a combination of nano- and macroscale investigations is reviewed. First, an overview is provided of liquid crystalline phases, made of chiral and achiral low-weight molecules, that exhibit chiral structure and/or chiral morphology. Then, recent progress in the discovery of new classes of chiral LCs, particularly enabled by the application of resonant X-ray scattering is described. It is shown that the method is sensitive to modulations of molecular orientation and therefore provides information hardly accessible by means of other techniques, such as the sense of helical structures or chirality transfer across length scales. Finally, a perspective is presented on the future scope, opportunities, and challenges in the field of chiral LCs, in particular related to nanocomposites.

Highly Oriented Liquid Crystal Semiconductor for Organic Field-Effect Transistors.

We report a mesogenic compound which introduces nematic liquid crystal (LC) ordering into the benzothienobenzothiophene (BTBT) family of LCs, creating a new class of LC semiconducting materials which respond in a facile way to anisotropic surfaces, and can, thereby, be effectively processed into highly oriented monodomains. Measurement on these domains of the electrical conductivity, with in situ monitoring of domain quality and orientation using LC birefringence textures in electroded cells, brings a new era of precision and reliability to the determination of anisotropic carrier mobility in LC semiconductors.

Computational Chemistry‐Guided Design of Selective Chemoresponsive Liquid Crystals Using Pyridine and Pyrimidine Functional Groups

AbstractComputational chemistry‐guided designs of chemoresponsive liquid crystals (LCs) with pyridine or pyrimidine groups that bind to metal‐cation‐functionalized surfaces to provide improved selective responses to targeted vapor species (dimethylmethylphosphonate (DMMP)) over nontargeted species (water) are reported. The LC designs against experiments are tested by synthesizing 4‐(4‐pentyl‐phenyl)‐pyridine and 5‐(4‐pentyl‐phenyl)‐pyrimidine and quantifying LC responses to DMMP and water. Consistent with the computations, pyridine‐containing LCs bind to metal‐cation‐functionalized surfaces too strongly to permit a response to either DMMP or water whereas pyrimidine‐containing LCs undergo a surface‐driven orientational transition in response to DMMP without interference from water. The computation predictions are not strongly dependent on assumptions regarding the degree of coordination of the metal ions but are limited in their ability to predict LC responses when using cations with mostly empty d orbitals. Overall, this work identifies a promising new class of chemoresponsive LCs based on pyrimidine that exhibits enhanced tolerance to water, a result that is important because water is a ubiquitous and particularly challenging chemical interferent in chemical sensing strategies based on LCs. The work also provides further evidence of the transformative utility of computational chemistry methods to design LC materials that exhibit selective orientational responses in specific chemical environments.

Liquid crystal optrode arrays: a novel approach to neural interfaces

Liquid crystal optrode arrays: a novel approach to neural interfaces

Front-Rear Polarization by Mechanical Cues: From Single Cells to Tissues.

Directed cell migration is a complex process that involves front-rear polarization, characterized by cell adhesion and cytoskeleton-based protrusion, retraction, and contraction of either a single cell or a cell collective. Single cell polarization depends on a variety of mechanochemical signals including external adhesive cues, substrate stiffness, and confinement. In cell ensembles, coordinated polarization of migrating tissues results not only from the application of traction forces on the extracellular matrix but also from the transmission of mechanical stress through intercellular junctions. We focus here on the impact of mechanical cues on the establishment and maintenance of front-rear polarization from single cell to collective cell behaviors through local or large-scale mechanisms.

Holographic recording in chiral and linear isomers of single-component phototropic liquid crystals: an experiment and theoretical approach

ABSTRACTPhototropic liquid crystals (PtLC) comprising one component are a new class of liquid crystals (LC) which due to the phenomenon of photochemical phase transition are of special interest, especially in view of their potential applications in photonic devices. So far, however, only a little attention has been paid to these materials. In this paper, we discuss holographic recording in chiral and linear single-component PtLCs from the family of 4-alkyl-4ʹ-alkoxyazobenzene and we develop the one-dimensional mathematical model describing the formation of the diffraction grating. The grating formation process and its dynamics, resulting from the photoinduced isotropic-to-nematic phase transition, have been described by the mechanism that assumes the formation and growth of the liquid-crystalline domains of different sizes induced by the sinusoidally modulated Gaussian light intensity distribution. We show that the model is based on one parameter, that is the incubation time which can be easily obtained experimentally and which can be used for the fitting of the experimental data. Reporting new PtLCs materials and the model that describes the processes of holographic recording in these types of materials can be useful in the design of new phototropic LC materials and for optimisation of the experimental conditions.

Orientational order of motile defects in activenematics.

The study of liquid crystals at equilibrium has led to fundamental insights into the nature of ordered materials, as well as to practical applications such as display technologies. Active nematics are a fundamentally different class of liquid crystals, driven away from equilibrium by the autonomous motion of their constituent rod-like particles. This internally generated activity powers the continuous creation and annihilation of topological defects, which leads to complex streaming flows whose chaotic dynamics seem to destroy long-range order. Here, we study these dynamics in experimental and computational realizations of active nematics. By tracking thousands of defects over centimetre-scale distances in microtubule-based active nematics, we identify a non-equilibrium phase characterized by a system-spanning orientational order of defects. This emergent order persists over hours despite defect lifetimes of only seconds. Similar dynamical structures are observed in coarse-grained simulations, suggesting that defect-ordered phases are a generic feature of active nematics.

ChemInform Abstract: The Cybotactic Nematic Phase of Bent‐Core Mesogens: State of the Art and Future Developments

AbstractReview: 44 refs.

3,4-Difluoropyrrole-, 3,3,4,4-tetrafluoropyrrolidine- and pyrrolidine-based liquid crystals

A new class of liquid crystals was formed with N-heterocycles as the terminal group, such groups as a 3,4-difluoropyrrole, 3,3,4,4-tetrafluoropyrrolidine, or pyrrolidine. Their properties were modified by varying the terminal heterocycles and/or the length of the alkyl chains on the cyclohexane liquid crystal building block. A comparison of the non-fluorinated and fluorinated pyrrolidine and fluorinated pyrrole as terminal group in the cyclohexane liquid crystals was studied. The mesophase behaviour was affected by altering the polarity of the terminal N-heterocycle. Compounds n/m-4H having the terminal pyrrolidine show a nematic phase. Comparatively, compounds 3/2-4F and 5/2-4F, having a fluorinated pyrrolidine, exhibit SmG phase. With 3,4-difluoropyrrole as terminal group, n/m-2F exhibit the broadest nematic phase range and good thermal stability, thus meeting the criterion for high performance liquid crystals materials.

Introduction to Optical Methods for Characterizing Liquid Crystals at Interfaces

This Instructional Review describes methods and underlying principles that can be used to characterize both the orientations assumed spontaneously by liquid crystals (LCs) at interfaces and the strength with which the LCs are held in those orientations (so-called anchoring energies). The application of these methods to several different classes of LC interfaces is described, including solid and aqueous interfaces as well as planar and nonplanar interfaces (such as those that define a LC-in-water emulsion droplet). These methods, which enable fundamental studies of the ordering of LCs at polymeric, chemically functionalized, and biomolecular interfaces, are described in this Instructional Review on a level that can be easily understood by a nonexpert reader such as an undergraduate or graduate student. We focus on optical methods because they are based on instrumentation that is found widely in research and teaching laboratories.

ChemInform Abstract: A New Synthetic Approach Towards 7‐Substituted 2‐Alkyl‐2,3,4,9‐tetrahydro‐1H‐fluorenes.

AbstractA facile synthetic approach towards hitherto unknown fluorene derivatives, key intermediates for the preparation of a novel class of liquid crystals, is presented.

Phototropic liquid crystals

A new class of liquid crystal that is controlled using light has been developed at AlphaMicron, a spin-off company working with the US Department of Defense.

Organic Zeolite Analogues Based on Multi-Component Liquid Crystals: Recognition and Transformation of Molecules within Constrained Environments.

In liquid crystals (LCs), molecules are confined in peculiar environments, where ordered alignment and certain mobility are realized at the same time. Considering these characteristics, the idea of “controlling molecular events within LC media” seems reasonable. As a suitable system for investigating this challenge, we have recently developed a new class of ionic LCs; the salts of amphiphilic carboxylic acids with 2-amino alcohols, or those of carboxylic acids with amphiphilic 2-amino alcohols, have a strong tendency to exhibit thermotropic LC phases. Because of the noncovalent nature of the interaction between molecules, one of the two components can easily be exchanged with, or transformed into, another molecule, without distorting the original LC architecture. In addition, both components are common organic molecules, and a variety of compounds are easily available. Taking advantage of these characteristics, we have succeeded in applying two-component LCs as chiral media for molecular recognition and reactions. This review presents an overview of our recent studies, together with notable reports related to this field.

Cybotaxis dominates the nematic phase of bent-core mesogens: a small-angle diffuse X-ray diffraction study

New temperature dependent X-ray diffraction (XRD) data on the bent-core mesogens based on the nonlinear 2,5-bis(p-hydroxyphenyl)-1,3,4-oxadiazole (ODBP) unit enable a consistent interpretation of the supramolecular structure in this class of liquid crystals. Strong evidence for cybotaxis in the high temperature phase explains the small-angle four-spot pattern and calls into question prior XRD interpretations. We find that the data can be satisfactorily explained by skewed cybotaxis, a stratified arrangement of tilted, bent-core mesogens (BCMs). We also observe the temperature-induced evolution of skewed cybotaxis to normal cybotaxis—strata wherein the long axes of the BCMs are normal to the layer fluctuations. Our XRD interpretation is compatible with the NMR data that exhibit biaxiality in the nematic phase of ODBP mesogens.