Cholesteric Liquid Crystal System Research Articles

Light-Driven Sign Inversion of Circularly Polarized Luminescence Enabled by Dichroism Modulation in Cholesteric Liquid Crystals.

Stimuli-responsive circularly polarized luminescence (CPL) materials show great promise in applying information encryption and anticounterfeiting. Herein, light-driven CPL sign inversion is achieved by combining a photoresponsive achiral negative dichroic dye (KG) and a static achiral positive dichroic dye (NR) as dopants at the 0.5:0.5 weight ratio into the cholesteric liquid crystal (CLC) host. The side chains of KG undergo trans/cis isomerization after 365nm UV light irradiation, leading to the dichroism (SF) decrease. The |glum| value of CLC doping with KG (CLC-KG) weakens from 0.67 to 0.28 in response to the order degree change. Taking advantage of its unique CPL response property, the light-driven CPL sign inversion is achieved (from -0.20/0.14 to 0.02/-0.04) by incorporating NR (0.5:0.5) into the CLC-KG with helical superstructure static. Based on the synergistic use of circular polarization and responsiveness state as cryptographic primitives, the multidimensional information encryption CLC system can be realized.

Thermoresponsive cholesteric liquid–crystal systems doped with terpenoids as drug delivery systems for skin applications

Stimuli-responsive and tunable soft-matter systems are an advanced class of materials applicable for drug delivery. Liquid crystals (LCs) are promising candidates as multifunctional materials that can respond to temperature, light or magnetic field. Particularly, ordering and physical properties of thermoresponsive LCs depend predominantly on temperature as external trigger. The current work addresses an elegant strategy to implement the anisotropic properties of thermoresponsive LCs with a view to extending their application for drug delivery. We firstly fabricated novel compositions with a thermotropic core based on natural products – cholesteryl esters and mono-/bicyclic terpenoids. The distinctive feature of aforementioned systems is their temperature-induced switchability of drug release by transition to the LC state, depending on the skin temperature. Their mesomorphic and optical behavior was characterized via differential scanning calorimetry and polarizing optical microscopy. Furthermore, we describe the dependence of helical pitch on LC formulation for various ternary cholesteric systems doped with terpenoids, suggesting that these stimuli-responsive chiral dopants are nominally untwisting. Data from fluorescence probe technique indicate that cholesteryl esters and terpenoids as essential components of those LC systems jointly disrupt the tight structure of phospholipid bilayer packing enabling the facilitated penetration of drugs. The potential of LC formulations was explored for several model drugs with diverse physicochemical properties by in vitro and ex vivo penetration tests using artificial membranes and full human skin. Our findings confirm the potential of LC systems for various applications in skin drug delivery.

Molecular Engineering for Chiroptical Smart Paints: Photo‐Tunable and Polymerizable Helical Nanostructures by Cyanostilbene‐Based Reactive Mesogens

AbstractMany studies on the basis of cholesteric liquid crystal (CLC) have focused on dynamic optical changes utilizing photochemical reactions raising helical pitch variations. However, the conventional dyes doped in CLC system often show reversible isomerization such as trans‐cis, E‐Z, and open‐close forms that hampers the fixation of desired color cues. Here, a cyanostilbene‐based reactive mesogen (CSRM) for phototunable and subsequently polymerizable smart CLC paints is newly designed and synthesized. The structural color of the CLC paint can be precisely manipulated in remote controllable manner by inducing a photochemical reaction of CSRM. Due to the irreversible isomerization behaviors of CSRM, the constructed periodic structures with different helical pitches are intactly fixed via in situ photopolymerization under 460 nm light. In addition, the combination of helical nanostructures and cyanostilbene luminophore produces a circularly polarized light emission. The dissymmetry factor of circularly polarized luminescence is dependent on the photoregulated helical nanostructures in CLC films. The decoupled system of triple photofunctionality in a single component of chirophotonic crystalline material is applied to develop advanced optical information storages.

Facile Stratification-Enabled Emergent Hyper-Reflectivity in Cholesteric Liquid Crystals.

Cholesteric liquid crystals (CLCs) are chiral photonic materials with selective reflection in terms of wavelength and polarization. Helix engineering is often required in order to produce desired properties for CLC materials to be employed for beam steering, light diffraction, scattering, and adaptive or broadband reflection. Here, we demonstrate a novel photopolymerization-enforced stratification (PES)-based strategy to realize helix engineering in a chiral CLC system with initially one handedness of molecular rotation throughout the layer. PES plays a crucial role in driving the chiral dopant bundle consisting of two chiral dopants of opposite handedness to spontaneously phase separate and create a CLC bilayer structure that reflects left- and right-handed circularly polarized light (CPL). The initially hidden chiral information therefore becomes explicit, and hyper-reflectivity, i.e., reflecting both left- and right-handed CPL, successfully emerges from the designed CLC mixture. The PES mechanism can be applied to structure a wide range of liquid crystal (LC) and polymer materials. Moreover, the engineering strategy enables facile programming of the center wavelength of hyper-reflection, patterning, and incorporating stimuli-responsiveness in the optical device. Hence, the engineered hyper-reflective CLCs offer great promise for future applications, such as digital displays, lasing, optical storage, and smart windows.

Cholesteric liquid crystal films with adjustable wavelength band and reflectance by using wash-out/refill technique and light-responsive compounds

ABSTRACT Cholesteric liquid crystal (CLC) film with adjustable high reflection wavelength of light response is prepared by refilling the light-response CLC system into the polymer network with broadband reflection memory effect, which is the washing out/refilling method. The film is reusable with some treatment. The adjustable range of high reflection wavelengths in the near-infrared region and visible region were 1270–1750 nm and 640–1110 nm, respectively. In addition, a highly reflective film with broadband reflection has been successfully prepared in the wavelength range of 1238–1826 nm, and the waveband can reach 588 nm. This research has excellent application prospects in building energy conservation, radiation protection, aerospace and other fields.

Green synthesized TiO2 nanoparticles dispersed cholesteric liquid crystal systems for enhanced optical and dielectric properties

Green synthesis of nanoparticles (NPs) is a simple, low-toxic, and cost-effective way to make NPs. The decreased optical band gap attributable to oxygen vacancies in green synthesized TiO2 NPs is responsible for the unique features for photocatalytic applications as well as increased optical properties. To explore optical and dielectric characteristics, green synthesized TiO2 NPs were dispersed in the cholesteric liquid crystal (CLC). By adding 27 wt% R-811 chiral dopant to E7 pure nematic liquid crystal, CLC was formed. The reflection color in the red region was observed in the polarizing optical microscope (POM) at ambient temperature, and the alignment was improved as the dispersion concentration was increased. Pure and TiO2 NPs dispersed CLC samples were subjected to UV–Visible absorbance, FE-SEM, EDAX, XRD, TEM, photoluminescence (PL), and FTIR analysis. As compared to pure CLC, the PL intensity increased for the NPs dispersed CLC samples without any substantial change in the emission peak position and the UV absorbance decreased as the NPs concentration increased. Furthermore, when compared to pure CLC, reduction in the optical band gap for TiO2 NPs dispersed CLC was observed. FTIR analysis revealed the impact of nanoparticle dispersion on CLC molecular dynamics. Dielectric spectroscopy results show a decrease in the relaxation frequency for TiO2 dispersed CLC system which is a positive outcome of this investigation. The findings reveal that TiO2 NPs when dispersed in CLC had significantly improved optical and dielectric properties. As a result, the optimal TiO2 NPs concentration in CLCs may have a variety of credential applications.

The Polarization of Laser Generation from the Cholesteric Liquid Crystal–Dye-Doped Polymer Layer–Cholesteric Liquid Crystal System

The Polarization of Laser Generation from the Cholesteric Liquid Crystal–Dye-Doped Polymer Layer–Cholesteric Liquid Crystal System

Electrically controlled switching of mixed mode laser within the band-gap of cholesteric liquid crystal

ABSTRACT Both random laser and multimode band-edge laser have been found in cholesteric liquid crystals (CLCs). In this work, the laser emission in CLC with negative dielectric anisotropy under electric fields with different frequencies is investigated. Awide multi-wavelength lasing is achieved via the formation of multiple domains induced by low-frequency alternating current (AC) in the CLC system. The photonic bandgap with aweak reflection for aspecific band can also be observed in the multiple domains structure. Furthermore, two types of liquid crystal hosts with different birefringences are used to obtain different lasing cover ranges. The range of the multi-wavelength lasing is within the CLC bandgap and positively correlated to the bandwidth of the bandgap. In addition, ahigh-frequency AC or direct current (DC) can switch it to aband-edge laser based on the regular planar helical structure. The wide range switchable multi-wavelength laser may provide various applications, such as sensing and communication.

Effect of the position of cyano group in α-cyano-functionalised diarylethene chiral fluorescent photoswitches on phototunable behaviours of cholesteric liquid crystals

ABSTRACT Two novel chiral fluorescent photoswitches bearing unsymmetric α-cyanothienylethene as emissive/photosensitive scaffold and bridged binaphthyl moieties as chiral entities are synthesised. The effects of the position of cyano group in switch 8 and 9 on the photoluminescence and photoresponsive properties of them in organic solvent and cholesteric liquid crystals (Ch-LCs) are explored. Both of these two photoswitches undergo partial reversible Z/E photoisomerisation accompanied by changes in fluorescence intensity under the light irradiations in both organic solvent and LC media. However, fluorescence quantum yields of switch 8 are much higher than those of switch 9. Meanwhile, the initial helical twisting power (HTP) value of switch 8 in LC media is much higher than that of switch 9, which is ascribed to their distinct geometries arising from different positions of cyano group on the C=C bond in them. Moreover, the switch 8-doping Ch-LC system exhibits more fast response and better reversible switching behaviours compared to Ch-LC system containing switch 9. Finally, simultaneous photoswitching of the reflection colour and fluorescence intensity in the Ch-LC sample is demonstrated. This work provides an idea for designing chiral fluorescent photoswitches with advanced photoluminescence and photoswitching characteristics.

Octadecylamine-capped CdSe/ZnS quantum dot dispersed cholesteric liquid crystal for potential display application: Investigation on photoluminescence and UV absorbance

ABSTRACT In the present investigation, we have undertaken the photoluminescence and UV absorbance study of a newly synthesised cholesteric liquid crystal (CLC) and CLC– octadecylamine-capped CdSe/ZnS quantum dots (QDs) composites. Three different CLC–QDs composites are prepared by varying the concentration of QDs in CLC. The thermal behaviour of both the pure CLC as well as three different CLC–QDs composites are performed by using Polarising Optical Microscopic (POM) textural studies. The photoluminescence (PL) study indicates that the PL intensity increases after the dispersion of QDs into the pure CLC. Decrement in the FWHM parameter also supports this increased PL intensity. However, the UV absorbance decreases for the QD dispersed CLC system when compared to the pure CLC. Optical band gap (by the Tauc plot method) suggests that the observed band gap reduces after the dispersion of QDs into CLC. The outcome of these investigations may be useful in the liquid crystal displays (LCDs) and other opto-electronic devices, which require less band gap materials.

Application of double click to prepare near-infrared absorbing dye for photo-thermal tuning of cholesteric liquid crystal

Cholesteric liquid crystal (CLC) exhibits selective reflection, making it a great application in the field of light control. Here, a near-infrared light (NIR) was used to drive the self-organizing spiral superstructure of the cholesteric liquid crystal to adjust the reflection notch. The polymer dye M2 with near-infrared absorption properties was synthesized by thiol-ene click polymerization and [2 + 2] click reaction; and has good solubility in the CLC system. M2 absorbs near-infrared light to increase the temperature of the system and the temperature-sensitive chiral dopant CD can realize the photothermal tuning of CLC system by the spiral twisting force of CLC with the change of temperature. The use of near-infrared light is beneficial compared to ultraviolet (UV) and visible light because of its invisibility, high penetration and ease of remote control. The entire CLC system has good near-infrared absorption, fast light response and a wide adjustable range.

Color tuning in thermo-sensitive chiral photonic liquid crystals based on the pseudo-dielectric heating effect

The ability of electro-thermal tuning in center wavelength (λc) of the Bragg reflection band is promoted in cholesteric liquid crystal (CLC) containing a positive-dielectric-anisotropy liquid crystal host (E7) and a binary chiral dopant (R-811 and R-5011). In the CLC mixtures with a fixed R-811 content of 30 wt%, the transition temperature of the smectic-A phase to CLC phase decreases when the R-5011 loading increases. The CLC systems are temperature-sensitive, each exhibiting a temperature range of about 9–12 °C in the CLC phase. Optimization of the content of R-5011 at 1.5 wt% allows for the remarkably effective thermal tuning in λc from 780 nm to 380 nm in response to a temperature increase of merely 3 °C from the room temperature. Under the mechanism of pseudo-dielectric heating, the tunable λc can shift in the entire visible spectrum by an applied voltage as low as 7 Vrms at a frequency of 2 MHz across the cell gap of a 4.8-μm-thick cell. Furthermore, a multifunctional and operation-reversible device is demonstrated by the control of the applied AC voltage, yielding an optical device in which the reflection band can be tuned and the mode can be switched among the reflection, scattering, and transparent states.

Frequency-modulated textural formation and optical properties of a binary rod-like/bent-core cholesteric liquid crystal

We explore the mechanisms of voltage-induced textural switching and the corresponding electro-optical responses of a binary cholesteric liquid crystal (CLC) composed of the rod-like nematic LC E7 with positive dielectric anisotropy and the bent-core LC dimer CB7CB with large flexoelectric coefficients. Our results indicate that the minimal voltage (VH) required for retaining the CLC in the homeotropic state and the optical transparency of the CLC after treating with decreasing voltage (from VH to zero) are dependent on the voltage frequency. The observed frequency modulated electro-optical properties are characterized by the frequency regimes separated by the critical frequencies of flexoelectric polarization and dielectric relaxation in dielectric dispersion. These unusual features, specific to the CB7CB-doped CLC, are explained by the dielectric and frequency-dependent flexoelectric responses of LC molecules to an external AC voltage. Accordingly, in addition to the known Grandjean planar and focal conic states, the uniform lying helix as a third stable state can be feasibly generated in the binary CLC with 45 wt% CB7CB by two pathways. The first one is to treat the cell directly with a sufficient voltage lower than VH; the other is to decrease the voltage gradually from VH to zero in the low-frequency regime, where the flexoelectric strength is significant. Manifested by its optical tristability, frequency-controllable optical transparency, and fast flexoelectro-optical response, the proposed binary rod-like/bent-core CLC system is promising for developing a variety of memory- and dynamic-mode photonic and optoelectronic devices.

Photostationary RGB Selective Reflection from Self‐Organized Helical Superstructures for Continuous Photopatterning

AbstractLight‐driven cholesteric liquid crystals (CLCs) are intriguing materials due to the dynamic tuning of their selective reflection, which originates from self‐organized helical superstructures, in response to light. However, CLC systems are restricted to displaying static optically addressed images and need to be initialized or refreshed before every new image can be displayed. Herein, a novel tuning mechanism based on a partial photochemical phase transition is proposed to enable continuous patterning of photostationary red, green, and blue (RGB) colors in a CLC system, which contains nonresponsive chiral dopants and o‐fluoroazobenzenes (Fazo) to serve as the photoswitch. Distinct isomer ratios of Fazo result in precise light‐directed RGB colors for the photostationary states, and, thus, a fixed relationship is established between the light stimulus and the reflection color. Accordingly, the RGB color patterns can be continuously erased and rewritten under light irradiation with different wavelengths. A photocontrollable “Tetris game” is demonstrated for the first time according to the programmable modulation of the CLC reflection, providing a new concept for optically rewritable displays.

Electro-Optical Properties of a Polymer Dispersed and Stabilized Cholesteric Liquid Crystals System Constructed by a Stepwise UV-Initiated Radical/Cationic Polymerization

Polymer-dispersed liquid crystal (PDLC) and polymer-stabilized liquid crystal (PSLC) are two typical liquid crystal (LC)/polymer composites. PDLCs are usually prepared by dispersing LC droplets in a polymer matrix, while PSLC is a system in which the alignment of LC molecules is stabilized by interactions between the polymer network and the LC molecules. In this study, a new material system is promoted to construct a coexistence system of PDLC and PSLC, namely PD&SChLC. In this new material system, a liquid-crystalline vinyl-ether monomer (LVM) was introduced to a mixture containing cholesteric liquid crystal (ChLC) and isotropic acrylate monomer (IAM). Based on the different reaction rates between LVM and IAM, the PD&SChLC architecture was built using a stepwise UV-initiated polymerization. During the preparation of the PDS&ChLC films, first, the mixture was irradiated with UV light for a short period of time to induce the free radical polymerization of IAMs, forming a phase-separated microstructure, PDLC. Subsequently, an electric filed was applied to the sample for long enough to induce the cationic polymerization of LVMs, forming the homeotropically-aligned polymer fibers within the ChLC domains, which are similar to those in a PSLC. Based on this stepwise UV-initiated radical/cationic polymerization, a PD&SChLC film with the advantages of a relatively low driving voltage, a fast response time, and a large-area processability is successful prepared. The film can be widely used in flexible displays, smart windows, and other optical devices.

Photoresponsive Chiral Dopants: Light‐Driven Helicity Manipulation in Cholesteric Liquid Crystals for Optical and Mechanical Functions

AbstractThis Review focuses on recent advancements regarding the design of photoisomerizable chiral dopants which can switch the helical orientation in cholesteric liquid crystals upon light stimuli. Chiral dopants exhibiting conformational change in response to an external stimulus can provide dynamic control of the cholesteric pitch. Development of such responsive systems has been attempted since the 1970s. However, major advances on efficient chiral dopants exhibiting either high helical twisting power or large photoswitching have been achieved only in recent years. This Review covers photoresponsive low molecular weight cholesteric liquid crystal systems based on chiral dopants capable of photochemically “switching power to twist” surrounding liquid crystalline host molecules, “inverting” their handedness or even “inducing” helical organization from the racemic nematic phase.

Piecewise Phototuning of Self-Organized Helical Superstructures.

Cholesteric liquid crystals (CLCs) exhibit selective reflection that can be tuned owing to the dynamic control of inherent self-organized helical superstructures. Although phototunable reflection is reported, these systems hitherto suffer from a limitation in that the tuning range is restricted to one narrow period and the optically addressed images have to sacrifice one color in the visible spectrum to serve as the background, resulting from the insufficient variation in helical twisting power of existing photoresponsive chiral switches that are all bistable. Here, delicate patterns of three primary red, green, and blue (RGB) colors with a black background are presented, which is realized based on piecewise reflection tuning of the CLC induced by a newly designed photoresponsive tristable chiral switch. Three stable configurations of the chiral switch endow the CLC with two continuous and adjacent tuning periods of the reflection, covering not only entire visible spectrum, but also one more wide period within near-infrared region. Therefore, the concept of piecewise tuning in CLC system demonstrates a new strategy for phototunable RGB and black reflective display.

Function Spaces for Liquid Crystals

We consider the relationship between three continuum liquid crystal theories: Oseen-Frank, Ericksen and Landau-de Gennes. It is known that the function space is an important part of the mathematical model and by considering various function space choices for the order parameters $s$, ${\bf n}$, and ${\bf Q}$, we establish connections between the variational formulations of these theories. We use these results to derive a version of the Oseen-Frank theory using special functions of bounded variation. This proposed model can describe both orientable and non-orientable defects. Finally we study a number of frustrated nematic and cholesteric liquid crystal systems and show that the model predicts the existence of point and surface discontinuities in the director.

Solid-State13C NMR Study of Cholesteric Liquid Crystals

We investigated the structural behavior of cholesteric liquid crystals of 4-(hexyloxy)-4'-cyanobiphenyl (6OCB) in an 11.7 T magnetic field by solid-state (13)C nuclear magnetic resonance. Five 6OCB cholesteric liquid crystal systems were prepared with 4-methoxyphenyl 3,4-O-isopropylidene-2,6-bis-O-(4-methylbenzoyl)-β-d-galactopyranoside (CR), in which 6OCB was doped with 0.51, 1.00, 2.34, 5.60, and 6.47 mol % CR, giving products with helical twisting powers (HTPs) of 0.28, 0.54, 1.27, 3.05, and 3.52 μm(-1), respectively. Analyses of the alignment-induced shifts showed that the liquid crystals directors in pure 6OCB and 6OCB doped with 0.51 and 1.00 mol % CR become aligned with the 11.7 T magnetic field direction. However, 6OCB doped with 2.34, 5.60, and 6.47 mol % CR retained their cholesteric structures when the magnetic field was applied. The critical HTP value for resisting realignment by magnetic field was estimated to be approximately 1.27 μm(-1). A biaxiality of the phase was partially considered during spectral simulations, although the molecular shape of 6OCB can be defined as cylinderical when it is rotating rapidly. Our current understanding is that the order parameters in the cholesteric structures are not uniform; i.e., the molecular directors already aligned almost perpendicular to the magnetic field are significantly perturbed by the external field.

Gold nanoparticle self-assembly moderated by a cholesteric liquid crystal

We show that the study of gold nanoparticle self-assemblies induced by a liquid crystal matrix reveals the intimate distorted structure of the liquid crystal existing prior to nanoparticles incorporation. We also show how this intimate structure monitors the spacing between nanoparticles in the self-assemblies. We have created hybrid films of cholesteric liquid crystal (CLC) and gold nanoparticles, the CLC being deformed by competing anchorings at its two interfaces. Whereas previous results have evidenced formation of only slightly anisotropic clusters of large nanoparticles (diameter 20nm), we now demonstrate for smaller nanoparticles (diameter 4.2nm) formation of long needles of length larger than 50 nanoparticles and width smaller than 5 nanoparticles, on average oriented perpendicular to the anchoring direction. The difference between the two kinds of nanoparticle aggregations is interpreted by a modification of the balance between aggregation between nanoparticles and trapping by the defects, favoured by the disorder induced by the alkylthiol molecules grafted around the nanoparticles. This leads to a well-defined, anisotropic Localized Surface Plasmonic Resonance (LSPR) of the 4.2nm embedded nanoparticles. Interpretation of these optical properties using generalized Mie theory allows for a comparison between CLC/gold nanoparticles and the same nanoparticles trapped within smectic topological defects or deposited on the same substrate without liquid crystal. A smaller spacing between nanoparticles is demonstrated in the CLC system with an attraction between nanoparticles induced by the CLC matrix, related to the additionnal disorder associated with the nanoparticles presence. The experimental observations allow us to estimate the disordered size of the liquid crystal shell around the nanoparticles in the CLC to be of some nanometers. They also suggest that the CLC distorted by competing anchorings is characterized by the presence of arrays of defects with topological cores of width smaller than 5nm that act as efficient anisotropic traps for the nanoparticles.