Cholesteric Liquid Crystal Materials Research Articles

Formylation boosts the performance of light-driven overcrowded alkene-derived rotary molecular motors.

Artificial molecular motors and machines constitute a critical element in the transition from individual molecular motion to the creation of collective dynamic molecular systems and responsive materials. The design of artificial light-driven molecular motors operating with high efficiency and selectivity constitutes an ongoing fundamental challenge. Here we present a highly versatile synthetic approach based on Rieche formylation that boosts the quantum yield of the forward photoisomerization reaction while reaching near-perfect selectivity in the steps involved in the unidirectional rotary cycle and drastically reducing competing photoreactions. This motor is readily accessible in its enantiopure form and operates with nearly quantitative photoconversions. It can easily be functionalized further and outperforms its direct predecessor as a reconfigurable chiral dopant in cholesteric liquid crystal materials.

Comprehensive Characterization of Organic Light-Emitting Materials in Breast Milk by Target and Suspect Screening.

Organic light-emitting materials (OLEMs) are emerging contaminants in the environment and have been detected in various environment samples. However, limited information is available regarding their contamination within the human body. Here, we developed a novel QuEChERS (quick, easy, cheap, effective, rugged, and safe) method coupled with triple quadrupole/high-resolution mass spectrometry to determine OLEMs in breast milk samples, employing both target and suspect screening strategies. Our analysis uncovered the presence of seven out of the 39 targeted OLEMs in breast milk samples, comprising five liquid crystal monomers and two OLEMs commonly used in organic light-emitting diode displays. The cumulative concentrations of the seven OLEMs in each breast milk sample ranged from ND to 1.67 × 103 ng/g lipid weight, with a mean and median concentration of 78.76 and 0.71 ng/g lipid weight, respectively, which were higher compared to that of typical organic pollutants such as polychlorinated biphenyls and polybrominated diphenyl ethers. We calculated the estimated daily intake (EDI) rates of OLEMs for infants aged 0-12 months, and the mean EDI rates during lactation were estimated to range from 30.37 to 54.89 ng/kg bw/day. Employing a suspect screening approach, we additionally identified 66 potential OLEMs, and two of them, cholesteryl hydrogen phthalate and cholesteryl benzoate, were further confirmed using pure reference standards. These two substances belong to cholesteric liquid crystal materials and raise concerns about potential endocrine-disrupting effects, as indicated by in silico predictive models. Overall, our present study established a robust method for the identification of OLEMs in breast milk samples, shedding light on their presence in the human body. These findings indicate human exposure to OLEMs that should be further investigated, including their health risks.

Achiral Dichroic Dyes‐mediated Circularly Polarized Emission Regulated by Orientational Order Parameter through Cholesteric Liquid Crystals

AbstractIt is noteworthy that cholesteric liquid crystal (CLC) platforms have been witnessed in high‐performance circularly polarized luminescence (CPL) behaviors through the highly organized chiral co‐assembled arrangement of achiral dyes. However, most CPL‐active design strategies are closely relative to the helix co‐assembly structure of CLC rather than achiral dyes. Herein, we developed an intriguing regulation strategy for CPL‐active CLC materials. They were regulated using the orientational order parameter (SF) of achiral dichroic dyes as an incisive probe for the order arrangement degree of achiral dyes in CLC media. The I‐shaped phenothiazine derivative PHECN dye (SF=0.30) emitted a strong CPL signal (|glum|=0.47). In contrast, the T‐shaped derivative (PHEBen) dye (SF=0.09) showed a weak circular polarization level (|glum|=0.07) at similar CLC textures. Most interestingly, this kind of dichroic PHECN dye with a higher SF could greatly improve the contrast ratio of CPL (Δglum=0.47) and emission intensity (ΔFL=46.0 %) at direct‐current electric field compared with the T‐shaped PHEBen (Δglum=0.07 and ΔFL=1.0 %) in CLC. This work demonstrates that an induced CPL emission can be mediated using achiral dichroic dye, which will open a new avenue for developing excellent CPL‐active display materials.

Achiral Dichroic Dyes-mediated Circularly Polarized Emission Regulated by Orientational Order Parameter through Cholesteric Liquid Crystals.

It is noteworthy that cholesteric liquid crystal (CLC) platforms have been witnessed in high-performance circularly polarized luminescence (CPL) behaviors through the highly organized chiral co-assembled arrangement of achiral dyes. However, most CPL-active design strategies are closely relative to the helix co-assembly structure of CLC rather than achiral dyes. Herein, we developed an intriguing regulation strategy for CPL-active CLC materials. They were regulated using the orientational order parameter (SF) of achiral dichroic dyes as an incisive probe for the order arrangement degree of achiral dyes in CLC media. The I-shaped phenothiazine derivative PHECN dye (SF =0.30) emitted a strong CPL signal (|glum| = 0.47). In contrast, the T-shaped derivative (PHEBen) dye (SF =0.09) showed a weak circular polarization level (|glum| = 0.07) at similar CLC textures. Most interestingly, this kind of dichroic PHECN dye with a higher SF could greatly improve the contrast ratio of CPL (Δglum = 0.47) and emission intensity (ΔFL = 46.0%) at direct-current electric field compared with the T-shaped PHEBen (Δglum = 0.07 and ΔFL = 1.0%) in CLCs. This work demonstrates that an induced CPL emission can be mediated using achiral dichroic dye, which will open a new avenue for developing excellent CPL-active display materials.

Fabrication of high-transmittance and fast-response bistable dual-frequency CLC cells

Liquid crystal devices are popularly used in the design of smart windows. However, the continuous electric field required for their operation may cause unnecessary energy loss and there is a risk of state transition when the electric field disappears. To overcome these problems, bistable devices were developed that exhibit electro-optic memory. In this study, we demonstrate the synthesis of a novel liquid crystalline acrylic chiral dopant. The polymerizable chiral dopant is expected to disperse and be fixed in liquid crystals, homogeneously revealing higher chiral efficiency. Incorporation of the synthesized chiral dopant into the dual frequency cholesteric liquid crystal results in a bistable display performance with a high transmittance of 97.6% and a fast response time of 1.5 ms. Each of the transparent “ON” and opaque “OFF” states was maintained without the bias of a continuous electric field. The achieved results show the possible application of the synthesized acrylic chiral dopant on a bistable dual frequency cholesteric liquid crystal display (BDFCLC) for smart windows and other devices. BackgroundThe continuous electric field required for their operation may cause unnecessary energy loss and there is a risk of state transition when the electric field disappears. MethodsTo fabricate a smart optical device with fast transition between “ON” and “OFF” states, and with continuously electric field addition, a new acrylic chiral dopant was synthesized and mixed with commercially available dual frequency nematic liquid crystal as a new cholesteric liquid crystal material possessing both perpendicular and planer arrangements with and without one-time electric field addition. Significant findingsIn this study, we demonstrate the synthesis of a novel liquid crystalline acrylic chiral dopant. The polymerizable chiral dopant is expected to disperse and be fixed in liquid crystals, homogeneously revealing higher chiral efficiency. Incorporation of the synthesized chiral dopant into the dual frequency cholesteric liquid crystal results in a bistable display performance with a high transmittance of 97.6% and a fast response time of 1.5 ms. Each of the transparent “ON” and opaque “OFF” states was maintained without the bias of a continuous electric field.

Room-Temperature Cholesteric Liquid Crystals of Cellulose Derivatives with Visible Reflection.

Hydroxypropyl cellulose (HPC) derivatives with alkanoyl side chains have attracted attention as bio-based cholesteric liquid crystal (CLC) materials with reflection colors. By taking advantage of the ability to change the reflection color in response to external stimuli, the thermotropic CLCs can be applied to a wide variety of photonic devices for a sustainable society of future generations. However, the thermotropic CLCs of HPC derivatives substituted with only one kind of alkanoyl group are not suitable for such applications because they do not exhibit visible reflection at room temperature. In this report, we describe a promising strategy to control the reflection colors of HPC derivatives at room temperature by introducing two kinds of alkanoyl groups with different lengths into the side chains of HPCs, which also enables the fine control of temperature dependence on the reflection wavelength. By chemically optimizing the side chain, we successfully prepared room-temperature thermotropic CLCs of HPC derivatives with visible reflection. This report would contribute toward the development of versatile photonic applications by CLCs produced from biomass.

Reflectivity enhancement of full color tri-layer electrowetting display with polymer cholesteric liquid crystal films

Electronic paper display has received increasing attention due to its outstanding properties of wide viewing angle, bistable state and lower energy consumption. Among various electronic paper display technologies, electrowetting e-paper (EWD) is the most promising as it can achieve full-color display by using stacked layers or RGB color filter. However, the EWD is faced with several challenges, including poor color brightness, low contrast ratio and small color gamut. To improve the performance of the EWD, we proposed a new configuration of EWD consisting of three independent PCLC reflection films based on the cholesteric liquid crystal materials. We designed and experimentally fabricated three types of reflection films, and then added the PCLC films to the tri-layered EWD device. We experimentally characterized the reflectivity and color gamut of the sample in the dark room. It is found that the experimental results and simulation results match with each other. It is demonstrated that with the PCLC films, the reflectivity is improved by 20%, while the color gamut is improved by 80%, which is sufficient for the daily display demand of E-paper. The proposed EWD device containing PCLC reflection films provide a new strategy to improve the brightness and color gamut of current EWD device, and is promising for realizing the full-color E-paper display.

Spatial Patterning of Fluorescent Liquid Crystal Ink Based on Inkjet Printing.

Fluorescent cholesteric liquid crystal materials (FCLC) with aggregation-induced emission (AIE) properties can effectively solve the contradiction between aggregation-induced quenching (ACQ) and liquid crystal self-assembly when light-emitting materials are aggregated, and they have great application value in the fields of anti-counterfeit detection and information hiding. However, generating a visually appealing design, logo, or image in the application typically requires an intricate fabrication process, such as the use of prefabricated molds and photomasks, which greatly limits the practical application of FCLC materials. Herein is reported a new method for spatially patterned liquid crystal (LC) microdroplet arrays using drop-on-demand inkjet printing technology. Through rational composition design, a spatial array composed of different liquid crystal microdroplets was established, and the array contains two entirely distinct but intact patterns at the same time, which can be reversibly switched under the irradiation of UV and natural light. This study provides a new method for the integrated preparation of different component liquid crystal materials.

The Influence of quantum dots on the optical properties of a room temperature cholesteric liquid crystal

ABSTRACT In the present investigation, we have undertaken photoluminescence, UV–Vis absorbance, transmission and polarised light microscopy of the composites of a cholesteric liquid crystal (CLC) doped with quantum dots (QDs). The locally formed QDs clusters may distort the CLC’s helix to induce an imperfect planar texture and cause broadening of CLC’s photonic band gap. This broadening is accompanied by a change of the wavelength positions of the central and both the long and short band-edges of the photonic band gap as the concentration of the doped QDs increases. A remarkable enhancement in the photoluminescence intensity of CLC materials is observed by doping with QDs. A decrement in the FWHM parameter also supports this increased photoluminescence intensity. The UV absorbance increases for the QDs dispersed CLC material and is followed by a slight red shift when compared to the pure CLC. The optical band gap obtained by the Tauc plot method suggests that the observed optical band gap narrows after the dispersion of QDs into the CLC. The outcome of these investigations proves that the dispersion of QDs in CLC is beneficial and may be useful in liquid crystal displays and other opto-electronic devices, which require less band gap materials.

Research Progress of Cholesteric Liquid Crystals with Broadband Reflection.

Cholesteric liquid crystal (ChLC) materials with broadband reflection are witnessing a significant surge in interest due to their unique ability to self-organize into a helical supra-molecular architecture and their excellent selective reflection of light based on the Bragg relationship. Nowadays, by the virtue of building self-organized nanostructures with pitch gradient or non-uniform pitch distribution, extensive work has already been performed to obtain ChLC films with a broad reflection band. This critical review systematically summarizes the optical background of the ChLCs with broadband reflection characteristics, methods to obtain broadband reflection of ChLCs, as well as the application in this area. Combined with the research status and the advantages in the field, the challenges and opportunities of applied scientific problems in the research direction are also introduced.

Ultra‐Broadband Tunable Bragg–Berry Optical Vortex Generators of a Circularly Symmetric Chiroptic Structure

AbstractNew optical–photonic devices with the capability to manipulate, modify, and even tailor the light characteristics dynamically, such as wave surface, play an increasingly important role in the development of modern and future optics. Optical vortex with spiral wave surfaces is a special type of light with intrinsic orbital angular momentum (OAM). Thus, this vortex can be applied to the development of advanced optics, such as optical tweezers, ultra‐resolution microscope, optical communication, and quantum technology. This study demonstrates an optical device known as Bragg–Berry optical vortex generator (BBOVG) with a circularly symmetric chiral photonic structure using an easy‐to‐implement method by filling a ferroelectric liquid crystal‐doped cholesteric liquid crystal (FLC‐CLC) material into an indium‐tin‐oxide‐coated cell pre‐treated with photoalignment along the azimuthal direction. The BBOVG devices can be operated by dynamically tuning the photonic bandgaps of the opposite‐handed FLC‐CLCs throughout the full‐visible region (400–700 nm) in a low voltage range (≤ 3.0 V) to obtain reflective optical vortices with opposite OAM signs in a full‐visible range near room temperature. The present devices, which can transform geometric phase and have ultra‐wideband manipulability and high operability at room temperature, provide a good example in demonstrating key devices that fit the use in future optical/photonic applications.

Multiple Anti-Counterfeiting Composite Film Based on Cholesteric Liquid Crystal and QD Materials.

A composite film with multiple anti-counterfeiting features was demonstrated by superposing quantum dots (QDs) polymer matrix (film A) and cholesteric liquid crystal film (film B) together. The first-line and second-line anti-counterfeiting characteristics were successfully implemented by employing thermochromic, angular photochromic, and circularly polarized discoloration of film B, respectively. By initiatively utilizing the different relative positions between the fluorescence emission peak (λem) of film A and the central selective reflection wavelength (λm) of film B at different temperatures, which resulted in changes in the fluorescence spectra or the different presence of latent patterns, the most important third-line anti-counterfeiting feature was successfully achieved.

Broaden reflection bandwidth of chiral nematic liquid crystals by chiral compound-loaded thermally responsive microcapsules

ABSTRACT Controllable broadband reflection of cholesteric liquid crystal (N*-LC) materials has sparked enormous interests due to its significance in areas such as infrared shielding glass films and LC displays. In this study, we report a novel method to broaden the bandwidth of the chiral nematic liquid crystal (N*-LC) by a thermally responsive microcapsule (TRMC) loaded with chiral compound. Diffusion of chiral compound loaded in the microcapsules triggered by heating resulted in non-uniform pitch distribution, which were locked by UV-radical polymerisation. The results showed that the reflection bandwidth could be broadened by adjusting mass of the microcapsules, heating time, UV intensity, concentration of UV-polymerisable free radical monomer and polymerisation temperature. The reflection bandwidth was broadened from 240 nm to 374 nm by optimising the above factors.

Broadband Reflection in Polymer-Stabilized Cholesteric Liquid Crystals via Thiol-Acrylate Chemistry.

Thiols are prone to react with a multitude of various functional groups in high yields, which has been widely used for surface- and particle-patterning, bioorganic synthesis, polymer modification, imprint nanolithography, the fabrication of optical components, hydrogel synthesis, and the curing of hard protective coatings. In this work, a chiral thiol with a high helical twisting power was synthesized through a novel synthetic route with high selectivity, yield, and cost-effectiveness. It was then used to fabricate a liquid-crystal composite film with ultra-wide broadband reflection via thiol click chemistry. Cholesteric liquid-crystal materials with broadband reflection have many potential applications for broadband polarizers, polarizer-free displays, organic optical data storage media, smart switchable reflective windows, and continuous waveband laser protection.

Small footprint cholesteric liquid crystal laser.

We demonstrate a small optical bench footprint laser assembly based on the small pulsed Nd:YAG laser head SSY-1 for pumping cholesteric liquid crystal (CLC) lasers and illustrate its performance using low molecular weight CLC samples doped with the fluorescent dye PM597. A low lasing threshold, narrow laser line, and far-field interference patterns of the CLC laser were observed using the SSY-1-based laser assembly as the pump. The emission characteristics of the CLC laser are similar to those observed with comparable CLC materials pumped by an order of magnitude physically larger and many orders of magnitude more expensive commercial Nd:YAG laser systems. The small footprint CLC laser demonstrated in this work provides an opportunity for significant size and cost reduction of CLC lasers and fostering their practical applications.

Cholesteric Liquid Crystal Materials for Tunable Diffractive Optics

AbstractModern optics and photonics constantly require break‐through materials and designs in order to achieve miniature, lightweight, highly tunable, and effective optical devices. One of the basic optical components is the diffraction grating (DG), widely used for the dispersion of light, beam steering, etc. This review gathers research efforts on diffractive optical elements based on cholesteric liquid crystal (CLC) materials with a supramolecular helical architecture. All main types and fabrication approaches of periodic diffractive structures from CLCs are classified and described. Key optical properties of DGs, their advantages and drawbacks are considered. Special attention is paid on the tunability of DGs including design principles and prospective chiral materials. The review consists of three parts divided according to the formation mechanism of diffractive structures: i) the spontaneously formed periodic structures from CLCs confined in cells with hybrid or homeotropic boundary conditions; ii) DGs generated by external electric field applied to CLCs layers; iii) light‐generated DGs (e.g., obtained by holography, mask exposure, photoalignment). The review also aims to initiate and gain collaborations between physicists, engineers and organic chemists to combine novel chiral photoswitches and molecular motors with sophisticated optical design paving the way towards novel smart optical materials.

Angular Photochromic LC Composite Film for an Anti-Counterfeiting Label

In the harsh application environment, improving the mechanical properties of liquid crystal materials is a fundamental and important problem in the design of anti-counterfeit materials. In this paper, by a stepwise polymerization of first, photo-polymerization and subsequently thermal-polymerization, a coexistent polymer dispersed network was first constructed in cholesteric liquid crystal materials containing a photo-polymerizable system of urethane acrylate and a thermo-polymerizable system of isocyanate. Results revealed that the coexistent polymer dispersed network exhibited largely enhanced mechanical performance, and the networks obtained by different methods had different contributions to the enhancement of the peel strength and toughness of the composite films. Then an angular photochromic anti-fake label based on a coexistent polymer dispersed network with enhanced mechanical and apparent angular discoloration characteristics, suitable for practical applications, was successfully achieved.

Transition of vertically aligned liquid crystal driven by fan-shaped electric field

Interdigital electrodes are implemented in many commercial and novel liquid crystal devices to align molecules. Although many empirical principles and patents apply to electrode design, only a few numerical simulations of alignment have been conducted. Why and how the molecules align in an ordered manner has never been adequately explained. Hence, this investigation addresses the Fréedericksz transition of vertically aligned liquid crystal that is driven by fishbone electrodes, and thereafter identifies the mechanism of liquid crystal alignment. Theoretical calculations suggest that the periodic deformation that is caused by the fan-shaped fringe field minimizes the free energy in the liquid crystal cell, and the optimal alignment can be obtained when the cell parameters satisfy the relation p/2d=k11/k33, where p is the spatial period of the strips of the electrode; d denotes the cell gap; and k11 and k33 are the splay and bend elastic constants of the liquid crystal, respectively. Polymer-stabilized vertical alignment test cells with various p values and spacings between the electrodes were fabricated, and the process of liquid crystal alignment was observed under an optical microscope. The degree of alignment was evaluated by measuring the transmittance of the test cell. The experimental results were consistent with the theoretical predictions. The principle of design, p/2d=k11/k33, greatly improves the uniformity and stability of the aligned liquid crystal. The methods that are presented here can be further applied to cholesteric liquid crystal and other self-assembled soft materials.

Multiple-wavelength surface patterns in models of biological chiral liquid crystal membranes.

We present a model to investigate the formation of surface patterns in biological materials through the interaction of anisotropic interfacial tension, bending elasticity, and capillarity at their free surfaces. Focusing on the cholesteric liquid crystal (CLC) material model, the generalized shape equation for anisotropic interfaces using the Rapini-Papoular anchoring and Helfrich free energies is applied to understand the formation of multi-length scale patterns, such as those found in floral petals. The chiral liquid crystal-membrane model is shown to be analogous to a driven pendulum, a connection that enables generic pattern classification as a function of bending elasticity, liquid crystal chirality and anchoring strength. The unique pattern-formation mechanism emerging from the model here presented is based on the nonlinear interaction between bending-driven folding and anchoring-driven creasing. The predictions are shown to capture accurately the two-scale wrinkling of certain tulips. These new findings enable not only to establish a new paradigm for characterizing surface wrinkling in biological liquid crystals, but also to inspire the design of functional surface structures.

Hybrid anchoring for a color-reflective dual-frequency cholesteric liquid crystal device switched by low voltages

Cholesteric liquid crystal (CLC) materials used in electro-optical (EO) devices are characterized by high operating voltage and slow response speed, which hinders their further development in display applications. Dual-frequency CLCs (DFCLCs) can solve the problem of slow bistable transition, but the operating voltage is still high, especially in color-reflective DFCLC cells. Here we report a simple approach to lowering the switching voltage as well as to shortening the response time. This technique adopts hybrid surface treatment to modulate the structural arrangement of CLC molecules. Both planar- and vertical-alignment layers are employed and coated on one and the other substrates separately to improve the electro-optical properties of DFCLCs. We show that the threshold voltage for switching can be decreased to as low as 5 V and the shortest response time is measured to be 0.8 ms, which renders CLC EO devices including displays more practical for commercial purpose.