Showing papers on "Liquid crystal published in 2012"
TL;DR: In this paper, the authors review the physics aspects of the new research thrusts, in which liquid crystals often meet other types of soft condensed matter, such as polymers and colloidal nano- or microparticle dispersions.
587 citations
TL;DR: In this paper, the authors show how measurement of the divergent nematic susceptibility of the iron pnictide superconductor Ba(Fe(1-x)Co(x))(2)As (2) distinguishes an electronic nematic phase transition from a simple ferroelastic distortion.
Abstract: Within the Landau paradigm of continuous phase transitions, ordered states of matter are characterized by a broken symmetry. Although the broken symmetry is usually evident, determining the driving force behind the phase transition can be complicated by coupling between distinct order parameters. We show how measurement of the divergent nematic susceptibility of the iron pnictide superconductor Ba(Fe(1-x)Co(x))(2)As(2) distinguishes an electronic nematic phase transition from a simple ferroelastic distortion. These measurements also indicate an electronic nematic quantum phase transition near the composition with optimal superconducting transition temperature.
439 citations
TL;DR: Novel cholesteric-liquid-crystalline architectures with the related fabrication procedures must be developed to broaden the bandwidth around a central reflection wavelength, do without the polarization-selectivity rule and go beyond the reflectance limit.
Abstract: The cholesteric-liquid-crystalline structure, which concerns the organization of chromatin, collagen, chitin, or cellulose, is omnipresent in living matter. In technology, it is found in temperature and pressure sensors, supertwisted nematic liquid crystal displays, optical filters, reflective devices, or cosmetics. A cholesteric liquid crystal reflects light because of its helical structure. The reflection is selective - the bandwidth is limited to a few tens of nanometers and the reflectance is equal to at most 50% for unpolarized incident light, which is a consequence of the polarization-selectivity rule. These limits must be exceeded for innovative applications like polarizer-free reflective displays, broadband polarizers, optical data storage media, polarization-independent devices, stealth technologies, or smart switchable reflective windows to control solar light and heat. Novel cholesteric-liquid-crystalline architectures with the related fabrication procedures must therefore be developed. This article reviews solutions found in living matter and laboratories to broaden the bandwidth around a central reflection wavelength, do without the polarization-selectivity rule and go beyond the reflectance limit.
393 citations
TL;DR: This review focuses on different classes of light-driven chiral molecular switches or motors in liquid crystal media for the induction and manipulation of photoresponsive cholesteric liquid crystal systems and their consequent applications.
Abstract: The ability to tune molecular self-organization with an external stimulus is a main driving force in the bottom-up nanofabrication of molecular devices. Light-driven chiral molecular switches or motors in liquid crystals that are capable of self-organizing into optically tunable helical superstructures undoubtedly represent a striking example, owing to their unique property of selective light reflection and which may lead to applications in the future. In this review, we focus on different classes of light-driven chiral molecular switches or motors in liquid crystal media for the induction and manipulation of photoresponsive cholesteric liquid crystal systems and their consequent applications. Moreover, the change of helical twisting powers of chiral dopants and their capability of helix inversion in the induced cholesteric phases are highlighted and discussed in the light of their molecular geometric changes.
388 citations
TL;DR: Rise or fall: Complex-structured freestanding polymer films with molecular order in three dimensions were prepared through photoalignment of polymerizable liquid crystals to deform into cone and saddle shapes upon heating.
Abstract: Rise or fall: Complex-structured freestanding polymer films with molecular order in three dimensions were prepared through photoalignment of polymerizable liquid crystals. The resulting films deform into cone and saddle shapes upon heating.
307 citations
TL;DR: It is argued that it is crucial for a better understanding of liquids to recognize that a liquid generally has the tendency to have a local structural order and its presence is intrinsic and universal to any liquid.
Abstract: There are at least three fundamental states of matter, depending upon temperature and pressure: gas, liquid, and solid (crystal). These states are separated by first-order phase transitions between them. In both gas and liquid phases a complete translational and rotational symmetry exist, whereas in a solid phase both symmetries are broken. In intermediate phases between liquid and solid, which include liquid crystal and plastic crystal phases, only one of the two symmetries is preserved. Among the fundamental states of matter, the liquid state is the most poorly understood. We argue that it is crucial for a better understanding of liquids to recognize that a liquid generally has the tendency to have a local structural order and its presence is intrinsic and universal to any liquid. Such structural ordering is a consequence of many-body correlations, more specifically, bond angle correlations, which we believe are crucial for the description of the liquid state. We show that this physical picture may naturally explain difficult unsolved problems associated with the liquid state, such as anomalies of water-type liquids (water, Si, Ge, ...), liquid-liquid transition, liquid-glass transition, crystallization and quasicrystal formation, in a unified manner. In other words, we need a new order parameter representing a low local free-energy configuration, which is a bond orientational order parameter in many cases, in addition to a density order parameter for the physical description of these phenomena. Here we review our two-order-parameter model of liquid and consider how transient local structural ordering is linked to all of the above-mentioned phenomena. The relationship between these phenomena is also discussed.
294 citations
TL;DR: In this paper, the authors derived and analyzed the effective low-energy model for collective magnetic excitations in the iron pnictides and showed that the stripe magnetic order is generally preempted by an Ising-nematic order, which breaks lattice symmetry but preserves spin-rotational symmetry.
Abstract: Starting from a microscopic itinerant model, we derive and analyze the effective low-energy model for collective magnetic excitations in the iron pnictides. We show that the stripe magnetic order is generally preempted by an Ising-nematic order, which breaks ${C}_{4}$ lattice symmetry but preserves $O(3)$ spin-rotational symmetry. This leads to a rich phase diagram as function of doping, pressure, and elastic moduli, displaying split magnetic and nematic tricritical points. The nematic transition may instantly bring the system to the verge of a magnetic transition, or it may occur first, being followed by a magnetic transition at a lower temperature. In the latter case, the preemptive nematic transition is accompanied by either a jump or a rapid increase of the magnetic correlation length, triggering a pseudogap behavior associated with magnetic precursors. Furthermore, due to the distinct orbital character of each Fermi pocket, the nematic transition also induces orbital order. We compare our results to various experiments, showing that they correctly address the changes in the character of the magnetostructural transition across the phase diagrams of different compounds, as well as the relationship between the orthorhombic and magnetic order parameters.
272 citations
TL;DR: The results indicate that the amount of applied ultrasound energy affects the microstructure of the suspensions and the pitch of the chiral nematic domains, and the Cox-Merz rule was found to fail after a critical concentration, thereby implying significant structural formation.
Abstract: The rheological properties and microstructure of nanocrystalline cellulose (NCC) aqueous suspensions have been investigated at different concentrations. The suspension is isotropic up to 3 wt %, and phase separates to liquid crystalline and isotropic domains at higher concentrations where the samples exhibit a fingerprint texture and the viscosity profile shows a three-region behavior, typical of liquid crystals. The suspension behaves as a rheological gel at even higher concentrations where the viscosity profile shows a single shear thinning behavior over the whole range of shear rates investigated. The effects of ultrasound energy and temperature on the rheological properties and structure of these suspensions were studied using polarized optical microscopy and rheometry. Our results indicate that the amount of applied ultrasound energy affects the microstructure of the suspensions and the pitch of the chiral nematic domains. The viscosity profile is changed significantly at low shear rates, whereas the...
269 citations
TL;DR: The homotopy theory of topological defects is a powerful tool for organizing and unifying many ideas across a broad range of physical systems as mentioned in this paper, and it has been used for controlling and measuring colloidal inclusions in liquid crystalline phases.
Abstract: The homotopy theory of topological defects is a powerful tool for organizing and unifying many ideas across a broad range of physical systems. Recently, experimental progress was made in controlling and measuring colloidal inclusions in liquid crystalline phases. The topological structure of these systems is quite rich but, at the same time, subtle. Motivated by experiment and the power of topological reasoning, the classification of defects in uniaxial nematic liquid crystals was reviewed and expounded upon. Particular attention was paid to the ambiguities that arise in these systems, which have no counterpart in the much-storied $XY$ model or the Heisenberg ferromagnet.
229 citations
TL;DR: These findings outline a general strategy for the assembly of reconfigurable chiral materials that can easily be moved, stretched, attached to one another and transformed between multiple conformational states, thus allowing precise assembly and nanosculpting of highly dynamical and designable materials with complex topologies.
Abstract: From determining the optical properties of simple molecular crystals to establishing the preferred handedness in highly complex vertebrates, molecular chirality profoundly influences the structural, mechanical and optical properties of both synthetic and biological matter on macroscopic length scales. In soft materials such as amphiphilic lipids and liquid crystals, the competition between local chiral interactions and global constraints imposed by the geometry of the self-assembled structures leads to frustration and the assembly of unique materials. An example of particular interest is smectic liquid crystals, where the two-dimensional layered geometry cannot support twist and chirality is consequently expelled to the edges in a manner analogous to the expulsion of a magnetic field from superconductors. Here we demonstrate a consequence of this geometric frustration that leads to a new design principle for the assembly of chiral molecules. Using a model system of colloidal membranes, we show that molecular chirality can control the interfacial tension, an important property of multi-component mixtures. This suggests an analogy between chiral twist, which is expelled to the edges of two-dimensional membranes, and amphiphilic surfactants, which are expelled to oil-water interfaces. As with surfactants, chiral control of interfacial tension drives the formation of many polymorphic assemblages such as twisted ribbons with linear and circular topologies, starfish membranes, and double and triple helices. Tuning molecular chirality in situ allows dynamical control of line tension, which powers polymorphic transitions between various chiral structures. These findings outline a general strategy for the assembly of reconfigurable chiral materials that can easily be moved, stretched, attached to one another and transformed between multiple conformational states, thus allowing precise assembly and nanosculpting of highly dynamical and designable materials with complex topologies.
215 citations
TL;DR: A ferroelectric response is reported for a columnar liquid crystal adopting a core–shell architecture that accommodates an array of polar cyano groups confined by a hydrogen-bonded amide network with an optimal strength.
Abstract: Ferroelectric liquid crystals are materials that have a remnant and electrically invertible polar order. Columnar liquid crystals with a ferroelectric nature have potential use in ultrahigh-density memory devices, if electrical polarization occurs along the columnar axis. However, columnar liquid crystals having an axial nonzero polarization at zero electric field and its electrical invertibility have not been demonstrated. Here, we report a ferroelectric response for a columnar liquid crystal adopting a core–shell architecture that accommodates an array of polar cyano groups confined by a hydrogen-bonded amide network with an optimal strength. Under an applied electric field, both columns and core cyano groups align unidirectionally, thereby developing an extremely large macroscopic remnant polarization.
TL;DR: This work has chosen the illustrative example of photopolymerizable hydrogen-bonding mesogens to show that a wide variety of functional materials can be made from a relatively simple set of building blocks.
Abstract: Functional organic materials are of great interest for a variety of applications. To obtain precise functional properties, well-defined hierarchically ordered supramolecular materials are crucial. The self-assembly of liquid crystals has proven to be an extremely useful tool in the development of well-defined nanostructured materials. We have chosen the illustrative example of photopolymerizable hydrogen-bonding mesogens to show that a wide variety of functional materials can be made from a relatively simple set of building blocks. Upon mixing these compounds with other reactive mesogens, nematic, chiral nematic, and smectic or columnar liquid-crystalline phases can be formed that can be applied as actuators, sensors and responsive reflectors, and nanoporous membranes, respectively.
TL;DR: In this article, a SEM examination of the fracture surface of a slowly-dried cellulose film showed a surprisingly regular fan-like pattern which was shown to be characteristic of cross-sections of the left-handed helicoidal arrangement of nanocrystals, where the helicoidal axis was almost perpendicular to the film surfaces.
Abstract: The chiral nematic self-assembly of aqueous suspensions of cellulose nanocrystals is partially preserved on evaporation of water, but the ordering of the rod-like nanoparticles may become distorted by changes in volume, ionic strength and surface and convective forces during evaporation, thus affecting the morphology and optical properties of the dried film. Proposed applications for these solids with chiral nematic order require confirmation of their structure. A SEM examination of the fracture surface of a slowly-dried film showed a surprisingly regular fan-like pattern which is shown to be characteristic of cross-sections of the left-handed helicoidal arrangement of nanocrystals, where the helicoidal axis was almost perpendicular to the film surfaces. Superimposed on this pattern was what appeared to be a regular porosity, which is postulated to result from pull-out of the nanocrystals oriented orthogonal to the fracture surface.
Book•
29 May 2012
TL;DR: In this article, Chenming Xue and Quan Li described a light-driven chiral molecular switches or motors in liquid crystal media. But they did not specify whether the switches were driven by liquid crystal Lasers.
Abstract: Preface vii Contributors ix 1 Liquid Crystal Lasers 1 Hideo Takezoe 2 Self-Organized Semiconducting Discotic Liquid Crystals for Optoelectronic Applications 29 Chenming Xue and Quan Li 3 Magnetic Liquid Crystals 83 Rui Tamura, Yoshiaki Uchida, and Katsuaki Suzuki 4 Ferroelectric Liquid Crystals for Nonlinear Optical Applications 111 Yongqiang Zhang and Jesus Etxebarria 5 Photo-Stimulated Phase Transformations in Liquid Crystals and Their Non-Display Applications 157 C V Yelamaggad, S Krishna Prasad, and Quan Li 6 Light-Driven Chiral Molecular Switches or Motors in Liquid Crystal Media 213 Yan Wang and Quan Li 7 Liquid Crystal-Functionalized Nano- and Microfibers Produced by Electrospinning 251 Jan P F Lagerwall 8 Functional Liquid Crystalline Block Copolymers: Order Meets Self-Assembled Nanostructures 285 Xia Tong and Yue Zhao 9 Semiconducting Applications of Polymerizable Liquid Crystals 303 Mary O Neill and Stephen M Kelly 10 Liquid Crystals of Carbon Nanotubes and Carbon Nanotubes in Liquid Crystals 341 Giusy Scalia 11 Liquid Crystals in Metamaterials 379 Augustine M Urbas and Dean P Brown 12 Ferroelectric Colloids in Liquid Crystals 403 Yuriy Reznikov 13 Fact or Fiction: Cybotactic Groups in the Nematic Phase of Bent Core Mesogens 427 Bharat R Acharya and Satyendra Kumar 14 Lyotropic Chromonic Liquid Crystals: Emerging Applications 449 Heung-Shik Park and Oleg D Lavrentovich 15 Liquid Crystal-Based Chemical Sensors 485 Jacob T Hunter and Nicholas L Abbott 16 Polymer Stabilized Cholesteric Liquid Crystal for Switchable Windows 505 Deng-Ke Yang 17 Liquid Crystals for Nanophotonics 525 Timothy D Wilkinson and R Rajesekharan Index 569
TL;DR: The lyotropic N*-LC phase of diLCPA could be promising materials possessing CPL functionality for use in next-generation π-conjugated organic optoelectronic devices, displays, and sensors.
Abstract: We synthesized disubstituted liquid crystalline polyacetylene (diLCPA) derivatives bearing 4-nonyloxy phenyl groups with lyotropic and thermotropic LC behavior. The poly(diphenylacetylene) main chain structure of the diLCPAs and the chirality induced with either chiral moieties or chiral dopants allow the formation of a highly ordered lyotropic N*-LC phase. Circular dichroism (CD) spectra of the diLCPAs imply that one-handed intrachain helical structures are formed in solution, while interchain helical π-stacking between the polymer main chains are formed in cast film and in the N*-LC state. Absorption dissymmetry factors (gabs) in the N*-LC state show values on the order of 10–1. The N*-LC state facilitates the formation of helically π-stacked structures with a high degree of helical ordering of the diLCPA and is indispensable for the generation of circularly polarized luminescence (CPL) with high emission dissymmetry factors (gem) on the order of 10–1. To the best of our knowledge, this is the highest r...
TL;DR: In this paper, a new dicyanodistyryrylbenzene-based phasmidic molecule, (2Z,2′Z)-2, 2′-(1,4-phenylene)bis(3-(3,4,5-tris(dodecyloxy)phenyl)acrylonitrile), GDCS, is reported, which forms a hexagonal columnar liquid crystal (LC) phase at room temperature (RT).
Abstract: A new dicyanodistyrylbenzene-based phasmidic molecule, (2Z,2′Z)-2,2′-(1,4-phenylene)bis(3-(3,4,5-tris(dodecyloxy)phenyl)acrylonitrile), GDCS, is reported, which forms a hexagonal columnar liquid crystal (LC) phase at room temperature (RT). GDCS molecules self-assemble into supramolecular disks consisting of a pair of molecules in a side-by-side disposition assisted by secondary bonding interactions of the lateral polar cyano group, which, in turn, constitute the hexagonal columnar LC structure. GDCS shows very intense green/yellow fluorescence in liquid/solid crystalline states, respectively, in contrast to the total absence of fluorescence emission in the isotropic melt state according to the characteristic aggregation-induced enhanced emission (AIEE) behavior. The AIEE and two-color luminescence thermochromism of GDCS are attributed to the peculiar intra- and intermolecular interactions of dipolar cyanostilbene units. It was found that the intramolecular planarization and restricted molecular motion associated with a specific stacking situation in the liquid/solid crystalline phases are responsible for the AIEE phenomenon. The origin of the two-color luminescence was elucidated to be due to the interdisk stacking alteration in a given column driven by the specific local dipole coupling between molecular disks. These stacking changes, in turn, resulted in the different degree of excited-state dimeric coupling to give different emission colors. To understand the complicated photophysical properties of GDCS, temperature-dependent steady-state and time-resolved PL measurements have been comprehensively carried out. Uniaxially aligned and highly fluorescent LC and crystalline microwires of GDCS are fabricated by using the micromolding in capillaries (MIMIC) method. Significantly enhanced electrical conductivity (0.8 × 10−5 S•cm−1/3.9 × 10−5 S•cm−1) of the aligned LC/crystal microwires were obtained over that of multi-domain LC sample, because of the almost perfect shear alignment of the LC material achieved in the MIMIC mold.
TL;DR: The authors reviewed recent accomplishments and new insights in this fascinating area of soft matter nanocomposites including work from our laboratory on a series of CdSe and CdTe quantum dots as additives in nematic liquid crystal hosts.
Abstract: Liquid crystal nanoscience, a field exploring the mutually beneficial combination of the unique properties of nanoscale materials and fluid, yet ordered liquid crystalline phases, is increasingly focusing on semiconductor quantum dots. In one major research thrust, the anisotropic properties of the liquid crystal host are sought to facilitate the assembly of quantum dots into arrays, in another, both size- and shape-dependent optical and electronic properties of quantum dots are used to manipulate optical, electro-optical and alignment properties of liquid crystalline materials. This feature article reviews recent accomplishments and new insights in this fascinating area of soft matter nanocomposites including work from our laboratory on a series of CdSe and CdTe quantum dots as additives in nematic liquid crystal hosts.
TL;DR: These materials show replication of structural features found in the silica template on nanometer to millimeter length scales.
Abstract: Anatase TiO(2) nanocrystals have been organized into high-surface-area (150-230 m(2) g(-1)) mesoporous films with long-range chiral nematic ordering. The chiral structure of the anatase films causes them to selectively reflect circularly polarized light and appear iridescent. These materials show replication of structural features found in the silica template on nanometer to millimeter length scales.
TL;DR: A class of naphthopyran-based materials that exhibit photo-induced conformational changes in molecular structure capable of yielding order-increasing phase transitions is reported, which has profound implications in optics, photonics, lasing and displays and will merit further consideration for applications in solar energy harvesting.
Abstract: A new class of liquid crystals is reported that undergoes light-induced ordering and order-increasing phase transitions; possible applications include ophthalmic devices, such as variable transmission sunglasses. As a rule, photoresponsive liquid crystals such as azobenzenes become structurally disordered on exposure to light. In a few instances, however, the reverse has been reported, so that in certain conditions the crystals can become more ordered. Here, Kosa et al. demonstrate a new class of naphthopyran-based liquid crystals that takes the latter behaviour much further, displaying a variety of ordering transitions: from isotropic to nematic, from isotropic to cholesteric and from nematic to smectic. With appropriate functionalization of the naphthopyran dyes, their isotropic-to-nematic transition results in a clear to strongly absorbing dichroic state. These properties could be useful in a variety of applications, not least in ophthalmic devices such as polarized variable-transmission sunglasses. Liquid crystals are traditionally classified as thermotropic, lyotropic or polymeric, based on the stimulus that governs the organization and order of the molecular system1. The most widely known and applied class of liquid crystals are a subset of thermotropic liquid crystals known as calamitic, in which adding heat can result in phase transitions from or into the nematic, cholesteric and smectic mesophases. Photoresponsive liquid-crystal materials and mixtures can undergo isothermal phase transitions if light affects the order parameter of the system within a mesophase sufficiently. In nearly all previous examinations, light exposure of photoresponsive liquid-crystal materials and mixtures resulted in order-decreasing photo-induced isothermal phase transitions2. Under specialized conditions, an increase in order with light exposure has been reported, despite the tendency of the photoresponsive liquid-crystal system to reduce order in the exposed state3,4,5,6,7. A direct, photo-induced transition from the isotropic to the nematic phase has been observed in a mixture of spiropyran molecules and a nematic liquid crystal8. Here we report a class of naphthopyran-based materials that exhibit photo-induced conformational changes in molecular structure capable of yielding order-increasing phase transitions. Appropriate functionalization of the naphthopyran molecules leads to an exceedingly large order parameter in the open form, which results in a clear to strongly absorbing dichroic state. The increase in order with light exposure has profound implications in optics, photonics, lasing and displays and will merit further consideration for applications in solar energy harvesting. The large, photo-induced dichroism exhibited by the material system has been long sought in ophthalmic applications such as photochromic and polarized variable transmission sunglasses.
TL;DR: In this paper, a micro-lithography system with a digital micro-mirror device as dynamic mask forms arbitrary micro-images on photoalignment layers and further guides the LC molecule orientations.
Abstract: We propose and implement a technique for arbitrary pattern fabrication in liquid crystal (LC) alignments and local polarization control for light wavefront. A micro-lithography system with a digital micro-mirror device as dynamic mask forms arbitrary micro-images on photoalignment layers and further guides the LC molecule orientations. Besides normal phase gratings, more complex 2D patterns such as quasicrystal and checkerboard structures are demonstrated. To characterize the optical performances of the fabricated structures, the electro-optically tunable diffraction patterns and efficiencies are demonstrated in several 1D/2D phase gratings. Compared to other techniques, our method enables the arbitrary and instant manipulation of LC alignments and light polarization states, facilitating wide applications in display and photonic fields.
TL;DR: In this paper, the nonlinear properties of fishnet metamaterials infiltrated with nematic liquid crystals were studied experimentally and it was shown that moderate laser powers result in significant changes of the optical transmission of the composite structures.
Abstract: We study experimentally the nonlinear properties of fishnet metamaterials infiltrated with nematic liquid crystals and find that moderate laser powers result in significant changes of the optical transmission of the composite structures. We also show that the nonlinear response of our structure can be further tuned with a bias electric field, enabling the realization of electrically tunable nonlinear metamaterials.
TL;DR: In this article, the phase behavior of a colloidal hard-rod-like model system consisting of silica rods was studied for aspect ratios ranging from 3.7 to 8.0.
Abstract: Recently, a novel colloidal hard-rod-like model system was developed which consists of silica rods [Kuijk et al., JACS, 2011, 133, 2346]. Here, we present a study of the phase behavior of these rods, for aspect ratios ranging from 3.7 to 8.0. By combining real-space confocal laser scanning microscopy with small angle X-ray scattering, a phase diagram depending on concentration and aspect ratio was constructed, which shows good qualitative agreement with the simulation results for the hard spherocylinder system. Besides the expected nematic and smectic liquid crystalline phases for the higher aspect ratios, we found a smectic-B phase at high densities for all systems. Additionally, real-space measurements on the single-particle level provided preliminary information on (liquid) crystal nucleation, defects and dynamics in the smectic phase.
TL;DR: This work utilizes glassy, liquid crystal polymer networks to spontaneously form 3D shapes that are independent of a user that are autonomously fixed with rapid temperature cycling.
Abstract: Repeatedly forming temporary shapes can be a limitation to the employment of shape memory polymers. This work utilizes glassy, liquid crystal polymer networks to spontaneously form 3D shapes that are independent of a user. These shapes are autonomously fixed with rapid temperature cycling.
TL;DR: The use of NMR spectroscopy is described to determine experimentally whether in reality the phase is chiral or not, and how the phase chirality can be demonstrated with probe molecules which contain deuteriated prochiral methylene groups.
Abstract: One of the defining characteristics of the twist–bend nematic phase, formed by the methylene-linked liquid crystal dimer 1″,7″-bis(4-cyanobiphenyl-4′-yl) heptane (CB7CB), is its chirality. This new nematic phase, predicted by Dozov, is of particular interest because although the constituent molecules are achiral the phase itself is chiral. Here, we describe the use of NMR spectroscopy to determine experimentally whether in reality the phase is chiral or not. The basis of this novel procedure is that the equivalence of the protons or deuterons in a prochiral methylene group in a nematic phase with D∞h symmetry is lost in a chiral phase because its symmetry is reduced to D∞ on removal of the mirror plane. Recording proton-enhanced local field (PELF) NMR experiments shows that in the standard nematic phase all of the methylene groups in the heptane spacer have equivalent pairs of C–H groups but this equivalence is lost for the six prochiral methylene groups with their enantiotopic protons on passing to the t...
TL;DR: Using a magnetic Frederiks transition technique, the temperature and concentration dependences of splay K1, twist K2, and bend K3 elastic constants for the lyotropic chromonic liquid crystal sunset yellow formed through noncovalent reversible aggregation of organic molecules in water are measured.
Abstract: Using a magnetic Frederiks transition technique, we measure the temperature and concentration dependences of splay K 1, twist K 2, and bend K 3 elastic constants for the lyotropic chromonic liquid crystal sunset yellow formed through noncovalent reversible aggregation of organic molecules in water. K 1 and K 3 are comparable to each other and are an order of magnitude higher than K 2. At higher concentrations and lower temperatures, K 1 and the ratios K 1/K 3 and K 1/K 2 increase, which is attributed to elongation of self-assembled lyotropic chromonic liquid crystal aggregates, a feature not found in conventional thermotropic and lyotropic liquid crystals formed by covalently bound units of a fixed length.
TL;DR: Two novel light-driven dithienylethene chiral molecular switches possessing remarkable changes in helical twisting power during photoisomerization as well as very highHelical twisting powers were found to experience photochemically reversible isomerization with thermal stability in both isotropic organic solvents and anisotropic liquid crystal media.
Abstract: Adding external, remote, and dynamic control to self-organized superstructures with desired properties is an important leap necessary in leveraging the fascinating molecular subsystems for employment in applications. Here two novel light-driven dithienylethene chiral molecular switches possessing remarkable changes in helical twisting power during photoisomerization as well as very high helical twisting powers were found to experience photochemically reversible isomerization with thermal stability in both isotropic organic solvents and anisotropic liquid crystal media. When doped into a commercially available achiral liquid crystal host, the chiral switch was able to either immediately induce an optically tunable helical superstructure or retain an achiral photoresponsive liquid crystal phase whose helical superstructure was induced and tuned reversibly upon light irradiation. Moreover, reversible light-directed red, green, and blue reflection colors with thermal stability in a single thin film were demon...
TL;DR: The nematic elastic energy promotes the alignment of the flux lines of the nematic director towards geodesics and/or lines of curvature of the surface, which enables the manipulation of nematic alignment for the design of new materials and technological devices.
Abstract: When nematic liquid crystals are constrained to a curved surface, the geometry induces distortions in the molecular orientation. The mechanisms of the geometrical frustration involve the intrinsic as well as the extrinsic geometry of the underlying substrate. We show that the nematic elastic energy promotes the alignment of the flux lines of the nematic director towards geodesics and/or lines of curvature of the surface. As a consequence, the influence of the curvature can be tuned through the Frank elastic moduli. To illustrate this effect, we consider the simple case of nematics lying on a cylindrical shell. By combining the curvature effects with external magnetic fields, the molecular alignment can be reoriented or switched between two stable configurations. This enables the manipulation of nematic alignment for the design of new materials and technological devices.
TL;DR: The present compounds are the first chiral dopants that induce a helical inversion in N*-LC via the photoisomerization between open and closed forms of the dithienylethene moiety.
Abstract: Chirality switching is intriguing for the dynamic control of the electronic and optical properties in nanoscale materials. The ability to photochemically switch the chirality in liquid crystals (LCs) is especially attractive given their potential applications in electro-optic displays, optical data storage, and the asymmetric synthesis of organic molecules and polymers. Here, we present a dynamic photoswitching of the helical inversion in chiral nematic LCs (N*-LCs) that contain photoresponsive axially chiral dopants. Novel photoresponsive chiral dithienylethene derivatives bearing two axially chiral binaphthyl moieties are synthesized. The dihedral angle of the binaphthyl rings changes via the photoisomerization between the open and closed forms of the dithienylethene moiety. The N*-LCs induced by the dithienylethene derivatives that are used as chiral dopants exhibit reversible photoswitching behaviors, including a helical inversion in the N*-LC and a phase transition between the N*-LC and the nematic LC. The present compounds are the first chiral dopants that induce a helical inversion in N*-LC via the photoisomerization between open and closed forms of the dithienylethene moiety.
TL;DR: In this paper, seven members of the bis(ω-(cholesteryloxycarbonyl)alkyl) disulfides, which contain a sulfur−sulfur link in the flexible spacer have been synthesized and their liquid crystal properties characterised.
Abstract: Seven members of the homologous series of the liquid crystal dimers, the bis(ω-(cholesteryloxycarbonyl)alkyl)disulfides, which contain a sulfur−sulfur link in the flexible spacer have been synthesised and their liquid crystal properties characterised. The dimers are referred to using the acronym Chol-n-SS-n-Chol in which n denotes the number of carbon atoms linking the cholesteryl-based groups and the sulfur atoms, and was varied between 3, 5, 6, 8, 10, 11 and 12. All seven homologues exhibit a chiral nematic phase and for the longest three members a smectic A phase was also observed. An odd−even effect is apparent in both the transition temperatures and the values of the entropy change associated with the chiral nematic−isotropic transition, ΔSN*I /R, in which dimers with even values of n show the higher values. This is interpreted in terms of the average molecular shapes in which the C−S−S−C dihedral angle is around 90°. The values of ΔSN*I/R shown by these dimers are very small for liquid crystal dimer...
TL;DR: Two enantiomeric light-driven azo molecular switches with axial chirality and extended conjugation were found to exhibit unprecedented reversible photoisomerization in both organic-solvent and liquid-crystal media only upon visible-light irradiation.
Abstract: Two enantiomeric light-driven azo molecular switches with axial chirality and extended conjugation were found to exhibit unprecedented reversible photoisomerization in both organic-solvent and liquid-crystal media only upon visible-light irradiation. When doped in an achiral liquid crystal with a different concentration, the chiral switch was able either to immediately induce an optically tunable helical superstructure or to retain an achiral liquid-crystal phase whose helical superstructure was induced and tuned reversibly upon visible-light irradiation. Furthermore, reversible dynamic red, green, and blue reflection achieved only by using visible light was demonstrated.