Self-assembled Liquid Crystals Research Articles

Thermotropic Ionic Liquid Crystals.

The last five years’ achievements in the synthesis and investigation of thermotropic ionic liquid crystals are reviewed. The present review describes the mesomorphic properties displayed by organic, as well as metal-containing ionic mesogens. In addition, a short overview on the ionic polymer and self-assembled liquid crystals is given. Potential and actual applications of ionic mesogens are also discussed.

Self-assembled liquid crystal film from mechanically defibrillated chitosan nanofibers

Wet-grinding and high-pressure homogenization were combined to defibrillate chitosan particles into nanoscaled fibrils, and the obtained nanofiber was made into a high strength liquid crystal film by self-organization at relatively low temperature. Mechanically disassembled chitosan nanofibers showed an average diameter of 50nm. The fabricated transparent liquid crystal film had a high tensile strength of 100.5±4.0MPa and a Young's modulus of 2.2±0.2GPa due to its ordered, layered, and porous-free structure. A green, efficient and reliable approach for producing chitosan nanofibers and liquid crystal film has been developed.

Modeling Textural Processes during Self-Assembly of Plant-Based Chiral-Nematic Liquid Crystals

Biological liquid crystalline polymers are found in cellulosic, chitin, and DNA based natural materials. Chiral nematic liquid crystalline orientational order is observed frozen-in in the solid state in plant cell walls and is known as a liquid crystal analogue characterized by a helicoidal plywood architecture. The emergence of the plywood architecture by directed chiral nematic liquid crystalline self assembly has been postulated as the mechanism that leads to optimal cellulose fibril organization. In natural systems, tissue growth and development takes place in the presence of inclusions and secondary phases leaving behind characteristic defects and textures, which provide a unique testing ground for the validity of the liquid crystal self-assembly postulate. In this work, a mathematical model, based on the Landau-de Gennes theory of liquid crystals, is used to simulate defect textures arising in the domain of self assembly, due to presence of secondary phases representing plant cells, lumens and pit canals. It is shown that the obtained defect patterns observed in some plant cell walls are those expected from a truly liquid crystalline phase. The analysis reveals the nature and magnitude of the viscoelastic material parameters that lead to observed patterns in plant-based helicoids through directed self-assembly. In addition, the results provide new guidance to develop biomimetic plywoods for structural and functional applications.

Free-standing mesoporous silica films with tunable chiral nematic structures

Chirality at the molecular level is found in diverse biological structures, such as polysaccharides, proteins and DNA, and is responsible for many of their unique properties. Introducing chirality into porous inorganic solids may produce new types of materials that could be useful for chiral separation, stereospecific catalysis, chiral recognition (sensing) and photonic materials. Template synthesis of inorganic solids using the self-assembly of lyotropic liquid crystals offers access to materials with well-defined porous structures, but only recently has chirality been introduced into hexagonal mesostructures through the use of a chiral surfactant. Efforts to impart chirality at a larger length scale using self-assembly are almost unknown. Here we describe the development of a photonic mesoporous inorganic solid that is a cast of a chiral nematic liquid crystal formed from nanocrystalline cellulose. These materials may be obtained as free-standing films with high surface area. The peak reflected wavelength of the films can be varied across the entire visible spectrum and into the near-infrared through simple changes in the synthetic conditions. To the best of our knowledge these are the first materials to combine mesoporosity with long-range chiral ordering that produces photonic properties. Our findings could lead to the development of new materials for applications in, for example, tuneable reflective filters and sensors. In addition, this type of material could be used as a hard template to generate other new materials with chiral nematic structures.

Self‐Assembly of Imidazolium‐Based Rodlike Ionic Liquid Crystals: Transition from Lamellar to Micellar Organization

By using aryl-amination chemistry, a series of rodlike 1-phenyl-1H-imidazole-based liquid crystals (LCs) and related imidazolium-based ionic liquid crystals (ILCs) has been prepared. The number and length of the C-terminal chains (at the noncharged end of the rodlike core) and the length of the N-terminal chain (on the imidazolium unit in the ILCs) were modified and the influence of these structural parameters on the mode of self-assembly in LC phases was investigated by polarizing microscopy, differential scanning calorimetry, and X-ray diffraction. For the single-chain imidazole derivatives nematic phases (N) and bilayer SmA2 phases were found, but upon increasing the number of alkyl chains the LC phases were lost. For the related imidazolium salts LC phases were preserved upon increasing the number and length of the C-terminal chains and in this series it leads to the phase sequence SmA-columnar (Col)-micellar cubic (CubI /Pm3n). Elongation of the N-terminal chain gives the reversed sequence. Short N-terminal chains prefer an end-to-end packing of the mesogens in which these chains are separated from the C-terminal chains. Elongation of the N-terminal chain leads to a mixing of N- and C-terminal chains, which is accompanied by complete intercalation of the aromatic cores. In the smectic phases this gives rise to a transition from bilayer (SmA2) to monolayer smectic (SmA) phases. For the columnar and cubic phases the segregated end-to-end packing leads to core-shell aggregates. In this case, elongation of the N-terminal chains distorts core-shell formation and removes CubI and Col phases in favor of single-layer SmA phases. Hence, by tailoring the length of the N-terminal chain, a crossover from taper-shaped to polycatenar LC tectons was achieved, which provides a powerful tool for control of self-assembly in ILCs.

Remarkable effect of pre‐organization on the self assembly in chiral liquid crystals

In this article, liquid crystal phases possessing a helical molecular assembly, including frustrated three dimensional (3D) structures, are overviewed. Then, the chirality-originated superstructures in liquid crystals studied by the author are reviewed. The importance of the concept of "pre-organization" is highlighted, thus, molecular design producing a strong chiral effect has been proposed. Dichiral twin materials have been prepared systematically based on this concept, and correlation between molecular architectures and resulting frustrated liquid crustal phases, such as smectic blue, cubic, tetragonal smectic Q, and sponge phases, has been investigated. An electrically induced anisotropic birefringent structure in the chiral isotropic phase and a photoinduced 3D-3D phase transition in the smectic Q phase are introduced as possible application on the basis of the frustrated chiral 3D structured liquid crystal phases. A new type of chiral effect inducing the structural anisotropy in the 3D cubic structure of soft material is also described.

Spectral Tuning of Organic Nanocolloids by Controlled Molecular Interactions

The controlled self-assembly of molecules and interactions between them remain a challenge in creating tunable and functional organic nanostructures. One class of molecular systems that has proven useful for incorporating tunable functionality at different length scales is liquid crystals (LCs) due to its ability to inherently self-organize. Here we present a novel approach to utilize the self-assembly of polymerizable liquid crystals to control the molecular aggregation of stable fluorescent chromophores and create a unique class of organic fluorescent nanocolloids. By adjusting the ratio between the dye and LC molecules inside the nanocolloids, we demonstrate the ability to control the molecular interactions and tune the fluorescent emission spectra of nanocolloid populations under single wavelength excitation. The single absorption spectrum and multiple emission spectra are highly desirable and reminiscent of the spectroscopic signature of quantum dots. These novel fluorescent nanocolloids have broad potential applications in fluorescent imaging and biological labeling.

Macroscopic-scale carbon nanotube alignment via self-assembly in lyotropic liquid crystals

By dispersing carbon nanotubes (CNTs) in a lyotropic liquid crystalline matrix, uniaxial alignment of the nanotubes can easily be achieved over macroscopic areas. We briefly describe the principles behind the technique and then show that it can be applied to multiwall as well as single-wall nanotubes and that a variety of different dispersing materials can be used, from industrial surfactants to DNA. We also present a new microfluidics-based method for transferring the liquid crystal-dispersed CNTs to a substrate, maintaining a fair control of tube direction.

Self-assembly of functional columnar liquid crystals

Columnar liquid crystals have recently been developed as dynamically functional anisotropic materials. In this feature article, some recent examples of the materials design and functionalisation of columnar liquid crystals are described. Not only disklike molecules but also molecules with unconventional shapes can form columnar liquid-crystalline assemblies with nanosegregated structures. Moreover use of non-covalent interactions such as hydrogen bonding, ionic and electron donor-acceptor interactions leads to the formation of supramolecular functional columnar materials that are stimuli-responsive. Nanosegregated columnar molecular order can also be used for one-dimensional transportation of electrons, ion and molecules.

Self-assembly of bent-core liquid crystals: formation of a modulated smectic phase with p2gg lattice to escape anticlinic tilt

A new silyl terminated bent-core mesogen was synthesized which shows a randomly tilted SmCR phase and a columnar phase with p2gg lattice as new modes of self-assembly in this class of compounds.

Frustrated self-assembly of dendron and dendrimer-based supramolecular liquid crystals

A new “inverted” topological configuration is demonstrated both experimentally and theoretically for self-assembled dendron and dendrimer-based supramolecular liquid crystals in which the dendrons/dendrimers occupy the continuous domain and the ionically attached pendant chains are confined in discrete domains. All previous studies on dendrimer and dendron-based liquid crystals have reported “normal” liquid crystalline configurations in which the dendritic templates occupy discrete domains (in spherical or columnar phases) or continuous struts (in bicontinuous cubic phases), while the pendant chains occupy the continuous space-filling domain. These surprising results mandate a re-examination of the packing mechanisms for this important class of materials and open new routes to unique nanostructures of possible use in existing and emerging technologies.

Flexible Liquid-Crystal Display Stabilized by Pixel Walls Directly Bonded via Ion-Beam Irradiation

We characterize a flexible self-assembled liquid crystal display (LCD) fabricated from a polyimide (PI) alignment layer with polydimethylsiloxane (PDMS) pixel walls. Ion-beam irradiation aligned liquid crystal molecules in the PI layer and bonded two flexible plastic substrates in a one-step assembly of the pixel walls. X-ray photoelectron spectroscopic analysis, Fourier transform infrared spectroscopy, and scanning electron microscopy provided chemical and physical evidence for the formation of stable chemical bonds between the PI layer and the PDMS pixel walls, in addition to the important maintenance of a uniform gap between the two substrates without the use of any epoxy resins or other polymers. Electro-optical performance of our flexible device was highly reliable, even under deformation conditions, and is comparable to that of conventional rigid glass substrate LCDs.

Multiresponsive self-assembled liquid crystals with azobenzene groups

An optical and electric field-responsive self-assembled complex containing nitril azobenzene groups and 1,3,5-triazine-2,4-diamine was obtained and characterized. Both the azobenzene precursor and the complex form a liquid-crystalline phase in a certain temperature range. The transition temperature from crystalline phase to liquid-crystalline mesophase was obviously decreased in the complex by the self-assembling. The self-assembled liquid crystals revealed good response to both stimuli of light irradiation and electric field, and the induced molecular orientation could be held even after the removal of the stimuli. The structural and mechanical investigation proved that the formation of hydrogen bonds and assembly-induced molecular dipolar change contributed to the multiresponding action. This kind of self-assembled complex thus has potential applications in imaging and data storage.

The development of self-assembled liquid crystal display alignment layers

From simple pocket calculators to mobile telephones and liquid crystal display (LCD)-TV, over the past few decades, devices based on LCD technology have proliferated and can now be found in all conceivable aspects of everyday life. Although used in cutting-edge technology, it is surprising that a vital part in the construction of such displays, namely the alignment layer, relies essentially on a mechanical rubbing process, invented almost 100 years ago. In this paper efforts to develop alignment layers (also called command layers) by processes other than rubbing, namely self-assembly of molecular and macromolecular components will be discussed. Two topics will be presented: (i) tuneable command layers formed by stepwise assembling of siloxane oligomers and phthalocyanine dyes on indium tin oxide surfaces and (ii) command layers formed by self-assembly of porphyrin trimers. The potential use of these layers in sensor devices will also be mentioned.

Preface

Preface

Biocompatible, Hydrophilic, Supramolecular Carbon Nanoparticles for Cell Delivery

Liquid-crystal self-assembly is used to fabricate carbon nanoparticles specially suited for cell-delivery applications (see figure). The nanoparticles possess high-activity surfaces for easy functionalization and aqueous dispersion, and are shown to be rapidly internalized by mesothelial cells, wherein they show no measurable toxicity relative to crystalline silica used as a positive control.

Room-Temperature Ionic Liquids as Template to Monolithic Mesoporous Silica with Wormlike Pores via a Sol−Gel Nanocasting Technique

This paper reports a novel room-temperature ionic liquid (RTIL), [C4mim]+ BF4-, as template to monolithic mesoporous silica with wormhole framework via a convenient nanocasting technique. In contrast with the applied liquid crystal self-assembly of long-chain surfactants on the preparation of mesoporous nanostructures, a new so-called hydrogen bond-co-π−π stack mechanism was proposed to be responsible for the present self-assembly of the RTIL in the reaction system for the formation of the wormlike mesopore, in which both the hydrogen bonds formed between the [BF4]- and silano group of silica gel and the π−π stack interaction of the neighboring imidazolium rings play crucial roles in the formation of the wormhole framework of mesporous silica. The proposed hydrogen bond-co-π−π stack mechanism with the RTIL as template may open a new pathway to prepare mesoporous materials.

Influence of molecular–substrate interaction on the self-assembly of discotic liquid crystals

We compare by scanning tunnelling microscopy the structure of hexakis-2,3,6,7,10,11-alkyloxytriphenylene self-assembled at the liquid–solid interface with either gold (1 1 1) or graphite (HOPG) as the substrate. The symmetry-breaking transition previously observed on graphite, when the length of the side-chains is increased, is observed to take place also on gold. However, it corresponds to a more drastic reduction of symmetry and is associated to a decrease of the molecular area. We interpret this change in terms of a variation in the adsorption energy of the conjugated core with the substrate.

Phase Separation and Liquid Crystal Self-Assembly in Surfactant−Inorganic−Solvent Systems

The behavior of surfactant−inorganic oxide−solvent systems is studied using lattice Monte Carlo simulations. Under no inorganic condensation conditions, these systems phase separate into a liquid crystal phase that contains mainly surfactant and inorganic oxide, in equilibrium with a solvent-rich phase. In the systems studied, the solvent and the inorganic oxide have favorable interactions with the surfactant head, but the inorganic oxide−surfactant interactions are stronger than the solvent−surfactant interactions, which leads to a phase separation, regardless of the oxide−solvent miscibility. The formation of ordered liquid crystal phases is observed in the phase containing a high surfactant concentration, and the structure of this phase depends on the system composition and strength of the interactions. The formation of hexagonal and lamellar structures at different conditions is in qualitative agreement with experimental evidence on the formation of surfactant−silica liquid crystals and the synthesis ...

Guanosine Derivatives: Self-Assembly and Lyotropic Liquid Crystal Formation

The self-assembly and lyotropic mesomorphism of guanosine derivatives are reviewed. Natural and synthetic guanosine nucleotides self-assemble into columnar aggregates based on G-quartets; at the appropriate concentrations these aggregates form, in water, lyotropic mesophases of the cholesteric and hexagonal type. Lipophilic derivatives undergo different types of self-assembly: in the presence of alkali metal ions they form, in organic solvents, columnar structures and lyomesophases similar to those observed for the hydrophilic derivatives; in the absence of ions they form instead ribbon-like aggregates which give rise to new types of lyotropic phases. The ribbon-like aggregates have interesting electrical properties.