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In this paper we report how the confining surfaces and the ionic effects of different concentration of guanosine solution can be used to vary the alignment of liquid crystal phases of guanosine nucleotides. Liquid crystal phases of guanosine 5'-monophosphate ammonium salt and guanosine 5'-monophosphate free acid in pure water, with and without silver sulphate, were studied by polarized optical microscope. A periodic modulation of the texture was observed. This modulation depends on both on the concentration and on the presence of silver ions in the liquid crystal phase. We demonstrate that, according to the surface energy of the alignment layers, it is possible to homeotropically align the guanosine chromonic phase without applying any external magnetic field. Finally, we report the formation of spherical, vesicle-like guanosine 5'-monophosphate aggregates, when the solution was confined between two hydrophobic surfaces containing exposed Si groups., (Copyright © 2014 Elsevier B.V. All rights reserved.)

A new type of flexible cholesteric liquid crystal mirror is presented. The simple and effective method for the deposition of a cholesteric mixture on a paper substrate and the particular design of the device give a homogeneous alignment of the cholesteric texture providing mirrors with an intense and uniform light reflectance. A desired polarization state for the reflected light, linear or circular, can be easily obtained varying the thickness and optical anisotropy of the polymer cover film. By using non-azobenzene based photosensitive materials a permanent array of RGB mirrors with high reflectivity can be obtained on the same device. Paper like reflective mirrors are versatile and they can find applications in reflective displays, adaptive optics, UV detectors and dosimeters, information recording, medicine and IR converters.

Liquid crystal films play a key role in advancing next-generation optical and photonic devices that require a precise in-plane modulation of optical anisotropy. This study employs multiphoton direct laser writing, a high-resolution three-dimensional (3D) printing method, to fabricate pseudoperiodic patterns of lines and grooves on glass surfaces for the in-plane alignment of liquid crystal films. Single layers of lines with submicron thickness and line spacing were fabricated in less than half an hour and forced the in-plane alignment of a liquid crystal film with a thickness of about 10 μm. We validate the method on patterns with singular topologies designed to induce the nucleation of disclination defects with a predetermined spatial arrangement, orientation, and topological strength. Compared to other surface patterning methods, high-resolution 3D printing provides the unique advantage of direct surface fabrication, enabling the creation of nonflat geometries such as terraces and lenses and expanding the design and functionalities of liquid crystal devices. We anticipate that this method will be used to create thin-film devices such as polarization gratings, beam steerers, and q-plates for manipulating polarized and structured light. [ABSTRACT FROM AUTHOR]

Bent-core liquid crystals, a class of mesogenic compounds with non-linear molecular structures, are well known for their unconventional mesophases, characterized by complex molecular (and supramolecular) ordering and often featuring biaxial and polar properties. In the nematic phase, their unique behavior is manifested in the formation of nano-sized biaxial clusters of layered molecules (cybotactic groups). While this prompted their consideration in the quest for nematic biaxiality, experimental evidence indicates that the cybotactic order is only short-ranged and that the nematic phase is macroscopically uniaxial. By combining atomic force microscopy, neutron reflectivity and wide-angle grazing-incidence X-ray scattering, here, we demonstrate that multilayer films of a bent-core nematic, deposited on silicon by a combined Langmuir–Blodgett and Langmuir–Schaefer approach, exhibit macroscopic in-plane ordering, with the long molecular axis tilted with respect to the sample surface and the short molecular axis (i.e., the apex bisector) aligned along the film compression direction. We thus propose the use of Langmuir films as an effective way to study and control the complex anchoring properties of bent-core liquid crystals. [ABSTRACT FROM AUTHOR]

Chromonic liquid crystals have recently received a lot of attention due to their spontaneous self-assembly in supramolecular columnar structures that, depending on their concentration in water, align to form a nematic liquid crystalline phase. The chirality may be induced in chromonics by adding chiral moieties to the nematic phase or enhanced by confining them in curved geometrical constraints. This review summarizes the recent research developments on chiral chromonic liquid crystals confined in spherical geometry, relating the results to what was observed for thermotropic liquid crystals in the same conditions. The review focuses on the studies carried out on commercially available nematic chromonics, investigating the effects on their topologies in different anchoring conditions and different chiral dopants and suggesting an application in the sensor field. [ABSTRACT FROM AUTHOR]

Bent-core mesogens (BCMs) are a class of thermotropic liquid crystals featuring several unconventional properties. However, the interpretation and technological exploitation of their unique behavior have been hampered by the difficulty of controlling their anchoring at surfaces. To tackle this issue, we report the nanoscale structural characterization of BCM films prepared using the Langmuir–Blodgett technique. Even though BCMs are quite different from typical amphiphilic molecules, we demonstrate that stable molecular films form over water, which can then be transferred onto silicon substrates. The combination of Brewster angle microscopy, atomic force microscopy, and X-ray reflectivity measurements shows that the molecules, once transferred onto a solid substrate, form a bilayer structure with a bottom layer of flat molecules and an upper layer of upright molecules. These results suggest that Langmuir–Blodgett films of BCMs can provide a useful means to control the alignment of this class of liquid crystals. [ABSTRACT FROM AUTHOR]

In the last decade, the possibility to use liquid crystal droplets as optical micro-cavities and lasers has attracted much attention since it paves the way for many applications in the field of sensors or tunable photonics. Several techniques can be used to obtain small micro-resonators as, for example, dispersing a cholesteric liquid crystal inside an immiscible isotropic fluid to create an emulsion. Since liquid crystals are extremely sensitive to external factors as temperature or external fields, laser tuning can be easily achieved. Here, we report on the possibility to tune the laser emission from dye doped cholesteric liquid crystals microdroplets dispersed in a glycerol matrix in presence of nitric acid molecules in the emulsion. Using a fluorescent dye with pH dependent optical properties, the emitted laser wavelength can be tuned in a range of 60 nm. This effect could find applications for the development of spectroscopy based sensors. [ABSTRACT FROM AUTHOR]

• Observation of an evident induced chirality in Sunset Yellow (SSY) microspheres. • Explanation of the aggregation mechanism for chiral dopants in SSY. • Polarized Optical images of rich microspheres textures. • Texture modulation of SSY microspheres by ionic salts. The reflection symmetry breaking observed in chromonic liquid crystals confined in different geometries, like tactoids, microspheres or capillaries, has been the focus of researchers in the last few years. Although the assembly mechanism that drives this behavior is common to most chromonic materials, the interface phenomena and the induced chirality are strongly dependent on the chromonic molecule itself. In this work, we report advances in understanding the supramolecular arrangement and induced chirality for the chromonic molecule Sunset Yellow and its interaction with salts and l -peptides. By means of optical microscopy, X-ray diffraction and atomic force microscopy, we demonstrate the rise of enhanced chirality in Sunset Yellow microspheres and propose an explanation of this phenomenon through two possible mechanisms: intercalation of the chiral dopant or binding. Our findings on the capability to control the delicate thermodynamic balance among chromonics, ions, chiral dopants, and water and its kinetics may open new perspectives for using these microspheres in optical and sensing applications. [ABSTRACT FROM AUTHOR]

Induced or spontaneous chirality in natural systems is an intriguing issue. In recent years, a lot of attention has been focused on chirality of chromonic liquid crystals, a class of materials that is able to self-assemble in columnar structures. However, the mechanism involved in the arising of chirality in these materials, that starts at the molecular level and controls the supramolecular structure, is poorly understood; however, it is certainly affected by ionic strength. In this work we present the results obtained doping Cromolyn, a chromonic material, with a strong helical-twisting-power peptide, and confining it in a spherical geometry. We demonstrate, by means of optical polarized microscopy and structural analysis, that both the geometrical constraint and the presence of the chiral dopant enhance the chiral effect; we also demonstrate that they favor the rise of a highly ordered helical superstructure, that may be optimized upon adding an ionic dye to the system. Finally, we report a procedure for the preparation of free-standing polymeric films, embedding and preserving the microspheres, and paving the way for the creation of biocompatible and eco-friendly optical devices to be used in the sensor and anticounterfeiting fields. [ABSTRACT FROM AUTHOR]

Spin coated films of a chiral comb-like liquid crystalline copolymer containing azobenzene chromophores in its side chains are optically isotropic in their twisted smectic-like glassy state. In contact with a nematic liquid crystal (5CB, E7, MBBA) they provide a degenerate planar orientation. When irradiated by unpolarized UV light, they orient the same nematics homeotropically. Treated with linearly polarized UV light they orient nematics homogeneously with the director along the electric vector of the exciting light. After a combined irradiation first with unpolarized UV light and then with linearly polarized visible light, the films again provide a homogeneous liquid crystal orientation, this time with the director perpendicular to the visible light electric vector. The phenomena observed are related to the light induced optical anisotropy. Two main processes are responsible for the anisotropy (1) a UV light depletion of trans-isomers of the azobenzene chromophores from the chosen direction and (2) a reorientation of the chromophores by polarized visible light. [ABSTRACT FROM AUTHOR]

This work describes the observation of lasing in an intermediate chiral phase of a dye-doped cholesteric liquid crystal mixture. This intermediate phase exists between cholesteric and smectic A phase and it presents anomalous selective reflection properties. The lasing was observed at the long-wavelength edge of the photonic band gap. [ABSTRACT FROM AUTHOR]

Three photosensitive cholesteric liquid crystal mixtures have been investigated as UV sensors. Each of them is sensitive to a well definite part of the solar UV radiation spectrum, the UV A or the UV B or the UV C range, with suitable absorption spectra. The photosensitive elements in the mixtures are either photoisomerizable nematic hosts or photoisomerizable optically active dopants. The selective reflection peaks of these cholesteric mixtures are in the visible part of the light spectrum. The UV exposure changes the cholesteric pitch and hence the position of the selective reflection peak. The consequence is that the colour reflected by each mixture varies under influence of the UV and it can be used as an indicator of the UV exposure itself. We present the mechanisms of the observed effects and possible application features. [ABSTRACT FROM AUTHOR]

[Display omitted] • Role of molecular flexibility in chromonic self assembly. • Enhancement of chirality by spherical confinement. • Design of novel materials. Chromonic liquid crystals are a sub-category of lyotropic liquid crystalline materials whose self-assembling mechanisms in complex supramolecular structures are still not fully understood. Chirality can be induced in chromonics using chiral aminoacids with alternating results. Understanding the key factors that lead to maximize this phenomenon is important for the design of novel materials able to self-assemble in chiral macroscopic structures with tailored optical properties, as selective reflection. In this work, we present a comparative study involving a not commercial chromonic, a synthesized metal containing chromonic compound [(Bpy-OH) 2 Ag][CH 3 COO]H 2 O, and two model chromonics that behave in different manner with respect to chiral induction. Spherical confinement provides a quick way to qualitatively evaluate the elasticity properties in new materials. Also, optical textures of the metal compound, after doping the material with a chiral moiety, show that its chiral phase is more similar to one of the two model chromonics. This observation points to a role played by the flexibility of the molecular structure that it's reflected into the resistance of the columnar aggregates to undergo a twist. This last observation can be useful when selecting the most suitable building blocks in designing new materials. [ABSTRACT FROM AUTHOR]