Structure Of Cholesteric Liquid Crystal Research Articles

Electrically Induced Photonic Structures in Cholesteric and Nematic Liquid Crystals

Electrically Induced Photonic Structures in Cholesteric and Nematic Liquid Crystals

Sustainable Cellulose-Derived Organic Photonic Gels with Tunable and Dynamic Structural Color.

Structural color is a fascinating optical phenomenon arising from intricate light-matter interactions. Biological structural colors from natural polymers are invaluable in biomimetic design and sustainable construction. Here, we report a renewable, abundant, and biodegradable cellulose-derived organic gel that generates stable cholesteric liquid crystal structures with vivid structural colors. We construct the chromatic gel using a 68 wt % hydroxypropyl cellulose (HPC) matrix, incorporating distinct polyethylene glycol (PEG) guest molecules. The PEGs contain peculiar end groups with tailored polarity, allowing for precise positioning on the HPC helical backbone through electrostatic repulsion between the PEG and HPC chains. This preserves the HPC's chiral nematic phase without being disrupted. We demonstrate that the PEGs' polarity tunes the HPC gel's reflective color. Additionally, gels with variable polarities are highly sensitive to temperature, pressure, and stretching, resulting in rapid, continuous, and reversible color changes. These exceptional dynamic traits establish the chiral nematic gel as an outstanding candidate for next-generation applications across displays, wearables, flexible electronics, health monitoring, and multifunctional sensors.

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.

Photonic eigenmodes and transmittance of finite-length 1D cholesteric liquid crystal resonators

Cholesteric liquid crystals exhibit a periodic helical structure that partially reflects light with wavelengths comparable to the period of the structure, thus performing as a one-dimensional photonic crystal. Here, we demonstrate a combined experimental and numerical study of light transmittance spectra of finite-length helical structure of cholesteric liquid crystals, as affected by the main system and material parameters, as well as the corresponding eigenmodes and frequency eigenspectra with their Q-factors. Specifically, we have measured and simulated transmittance spectra of samples with different thicknesses, birefringences and for various incident light polarisation configurations as well as quantified the role of refractive index dispersion and the divergence of the incident light beam on transmittance spectra. We identify the relation between transmittance spectra and the eigenfrequencies of the photonic eigenmodes. Furthermore, we present and visualize the geometry of these eigenmodes and corresponding Q-factors. More generally, this work systematically studies the properties of light propagation in a one-dimensional helical cholesteric liquid crystal birefringent profile, which is known to be of interest for the design of micro-lasers and other soft matter photonic devices.

Preparation of magneto-electro-optically active polymer using Fe ionic liquid as electro-magnetic dopant

In this research, we report the synthesis of conjugated polymers with magnetism. Electrochemical polymerization in liquid crystal was conducted with 1-butyl-3-methylimidazolium tetrachloroferrate (BMITF), which is a paramagnetic character having a large magnetic susceptibility, as a supporting salt. The polymer films thus obtained had magnetism because BMITF remains in the films as a dopant. Poly(2,2′-bithiophene) and poly(bis-(3,4-ethylenedioxythiophene)) were prepared in nematic or cholesteric liquid crystal with BMITF by electrochemical polymerization. Transcription of the liquid crystal structure was succeeded with a small amount of BMITF. Blush or fingerprint structures were confirmed by polarizing optical microscopy observation. The addition of the excess amount of BMITF broke the structure of the liquid crystal. Poly(2,2′-bithiophene) transcribed the cholesteric liquid crystal structure showed a negative Cotton effect, indicating with left-handed helical structure. Poly(bis-(3,4-ethylenedioxythiophene)) transcribed the cholesteric liquid crystal structure showed a positive Cotton effect, indicating the polymer forms a right-handed helical structure. The X-ray photoelectron spectroscopy measurements, confirming the existence of Fe element in the polymers thus obtained. From the electron spin resonance measurements, the polymers thus obtained showed magnetic activity. This method affords facile synthesis of conjugated polymers with magnetism.

Bio‐Inspired Cholesteric Phase Cellulose Composite with Thermochromic and Circularly Polarized Structural Color for Multilevel Encryption

AbstractThe escalating need for enhanced cryptographic security necessitates the development of advanced materials designed for the secure storage and transmission of information. Drawing inspiration from the unique color‐altering and polarizing capabilities of beetles, this work develops a transparent, thermochromic, and circularly polarized cholesteric phase cellulose composite (CPCC). This is achieved by integrating self‐assembled hydroxypropyl cellulose with cholesteric liquid crystal (CLC) structure in tandem with a crosslinked poly(N‐isopropyl acrylamide) (PNIPAM) network. The crosslinking density impacts the response degree of thermochromism, which can be regulated by modifying the UV exposure time during PNIPAM production. The CLC structure in CPCC uniquely results in reflected right circularly polarized light. When coupled with waveplates, this mechanism inverts the rotation direction of the reflected light, creating orthogonal structural colors of different brightness levels under left and right circularly polarized light. The excellent transparency of CPCC facilitates seamless integration with the environment, offering optimal camouflage. Sophisticated techniques, such as color coding and Morse coding, can further be incorporated within the CPCC to increase encryption security and the complexity of decryption. Collectively, the CPCC's transparency, thermochromism, and chirality present significant potential in the design and development of materials for high‐security information encryption, contributing valuable insights to the field.

Recent Advances in Electro-Optic Response of Polymer-Stabilized Cholesteric Liquid Crystals

Cholesteric liquid crystals (CLC) are molecules that can self-assemble into helicoidal superstructures exhibiting circularly polarized reflection. The facile self-assembly and resulting optical properties makes CLCs a promising technology for an array of industrial applications, including reflective displays, tunable mirror-less lasers, optical storage, tunable color filters, and smart windows. The helicoidal structure of CLC can be stabilized via in situ photopolymerization of liquid crystal monomers in a CLC mixture, resulting in polymer-stabilized CLCs (PSCLCs). PSCLCs exhibit a dynamic optical response that can be induced by external stimuli, including electric fields, heat, and light. In this review, we discuss the electro-optic response and potential mechanism of PSCLCs reported over the past decade. Multiple electro-optic responses in PSCLCs with negative or positive dielectric anisotropy have been identified, including bandwidth broadening, red and blue tuning, and switching the reflection notch when an electric field is applied. The reconfigurable optical response of PSCLCs with positive dielectric anisotropy is also discussed. That is, red tuning (or broadening) by applying a DC field and switching by applying an AC field were both observed for the first time in a PSCLC sample. Finally, we discuss the potential mechanism for the dynamic response in PSCLCs.

Preparation of Structurally Colored Fiber with Cellulose Derivative Exhibiting Cholesteric Liquid Crystal at Room Condition

To prepare structural colored fibers by a cholesteric liquid crystal (CLC), cellulose derivatives forming a CLC structure under a room condition were synthesized through the esterification of hydroxypropyl cellulose (HPC) with propionic an hydride. Obtained cellulose derivatives formed CLC under room conditions within the degree of esterification by propionic acid (PA) from 0.83 to 1.1 per glucose unit. It was also possible to mold a film shape or a fiber shape. The derivative was possible to exhibit structural color under a room condition when it contained moisture more than 13.76 wt% after the preparation and 1.85 wt% at the time of exhibiting color. The glass transition temperature of the HPC derivative was between 5-13 °C. Its CLC structure was reconstructed under a room condition after destroyed the CLC structure by straining. The structural colored fiber was wet spun with the HPC derivative acetone solution to water as a coagulation bath. The specular reflection spectrum of the fiber showed the angle-dependence according to Bragg’s law. The pitch of CLC structure in the fiber was 423 nm, and the size of the CLC domain was 5.13 μm. In the scattering spectrum of the fiber, a reflection peak was observed in the fiber diameter direction regardless of the scattering angle, which suggests that the CLC domains have anisotropy and is radially oriented in the radial direction from the center of the fiber.

(Digital Presentation) Synthesis of Conjugated Polymer Alloy Prepared By Electrochemical Polymerization in Chiral Liquid Crystal

In this research, we synthesized polymer alloy by electrochemical polymerization in chiral liquid crystal. Homopolymers and the copolymer can be synthesized when electrochemical polymerization was conducted in the presence of several types of monomers. The polymer alloy thus prepared contains both blend of homopolymers and copolymers. The polymer alloy shows optical activity. The chirality derived from transcription of liquid crystal during the polymerization reaction results to show optical activity. The polymer alloy can show optical activity with no asymmetric centers. The optical activity is due to existence of charge carriers (polarons) in the main chain as a form of conjugated polymers. This is called chiral charge carrier “chiralions”. This research found that the chiral polymer alloy has chiralions.Polymer alloys have been prepared by blending several kinds of polymers in the reaction. Polymer alloys also can be prepared by melt method. Unsubstituted conjugated polymers have poor solubility and fusibility because of the rigid π-bonds in the skeleton, which is drawback for preparation of π-conjugated polymer-based alloys. We have studied on preparation of chiral conjugated polymer alloys with combination of achiral polymers and chiral polymers.In the present study, conjugated polymer alloys were synthesized with the method described in Figure 1a. Cholesteryl acetate (chiral inducer), 2,7-di(2-thienyl)fluorene (mono1), N-methyl-3,6-di(2-thienyl)carbazole (mono2) and TBAP (supporting salt) were dissolved in 4-cyano-4'-pentylbiphenyl (5CB) as a host liquid crystal to prepare a chiral liquid crystal electrolyte. The electrolyte exhibited a fingerprint structure under observation of polarizing optical microscopy. For the electrochemical polymerization reaction, the electrolyte solution is charged in the cell consisted of two indium-tin-oxide (ITO) glass plates as electrodes and a poly(tetrafluoroethylene) spacer (200 μm thickness). Next, dc voltage of 3.0 V was applied to the ITO glass cell for 30 min for electrochemical polymerization. The polymer was deposited on the anode electrode as a thin film form. After polymerization, the remaining electrolyte solution on the ITO glass electrode was carefully washed with a large amount of hexane, yielding dark blue film. This film is abbreviated as Alloy1. The molecular structure was confirmed by infrared spectroscopy with the KBr method. Alloy1 shows a fingerprint structure under POM observations (Figure 1b). The macroscopic structure of cholesteric liquid crystal was transcribed from the liquid crystal electrolyte solution to Alloy1. Synchrotron XRD measurement was carried out. The peak at 10.3 Å is derived from monomer repeat units. The XRD signal at 5.1 Å and 3.4 Å can be derived from π-stacking of Alloy1. Alloy1 shows UV-vis absorption due to the three signals derived from π-π* transition of the main chain, polarons (radical cations) and bipolarons (radical dications). Electrochemical properties of Alloy1 were evaluated by the cyclic voltammetry (CV). Alloy1 film deposited on the ITO glass was used as a working electrode. An Ag/Ag+ electrode and a platinum wire served as the reference and counter electrodes, respectively. A propylene carbonate solution with 0.1 M of TBAP was employed as an electrolyte solution for the CV measurements. Cyclic voltammograms of Alloy1 shows the oxidation peak and reduction trough (Figure 1c). Plots of the oxidation peak current (I pa) and the reduction peak current (I pc) as a function of the square root of the scan rate show a linear shape of the anodic and cathodic peak current values when scan rates from 10 to 100 mV/s with intervals of 10 mV/s. This result indicated that the redox reaction of Alloy1 was reversible and controlled by electron transfer processes. In-situ circular dichromism (CD) and in-situ optical rotatory dispersion (ORD) spectra of Alloy1 with application of voltages were examined in a propylene carbonate solution with 0.1 M of TBAP. The application voltages were between 0-1.2 V (vs. Ag/Ag+) with intervals of 0.1 V. A negative Cotton effect was observed from the in-situ CD, indicating Alloy1 has left-handed helical structure. The ORD shows negative signals, and changes with applied voltage. The ellipticity signal in the CD and optical rotation in the ORD could control in the electrochemical method, accompanied by electrochemical redox process.We synthesized the polymer alloy by electrochemical polymerization in chiral liquid crystal. The conjugated polymer alloy showed chirality with no chiral centers in the main chain. The CD and ORD bands at NIR range are assigned to doping band as a form of polarons (radical cations). This research evaluated electrochemical control of “chiralions”. Figure 1

Optical characteristics and lasing spectra of a cholesteric liquid crystal with defects of different types

Spectral and laser characteristics of impurity steroidal cholesteric liquid crystals (CLC) with helical structure defects were studied. It is established that in the transmission spectra of defective CLC structures there is a local dip in the center of the selective reflection (SR) band for linear polarization of light. The spectra of laser generation in defective structures of CLC of different types have been studied and it has been shown that they are located within the dip in the transmission spectra, which allows interpreting these defects as a phase jump in the helical structure of CLC. The obtained experimental data are compared with the theoretical model of laser generation in CLC with helical structure defects.

Layering transitions and metastable structures of cholesteric liquid crystals in cylindrical confinement

Our study of cholesteric lyotropic chromonic liquid crystals in cylindrical confinement reveals the topological aspects of cholesteric liquid crystals. The double-twist configurations we observe exhibit discontinuous layering transitions, domain formation, metastability, and chiral point defects as the concentration of chiral dopant is varied. We demonstrate that these distinct layer states can be distinguished by chiral topological invariants. We show that changes in the layer structure give rise to a chiral soliton similar to a toron, comprising a metastable pair of chiral point defects. Through the applicability of the invariants we describe to general systems, our work has broad relevance to the study of chiral materials.

Multi-stable cholesteric liquid crystal windows with four optical states

ABSTRACT An electrically switchable multi-stable liquid crystal device is demonstrated based on a non-uniform cholesteric liquid crystal (CLC) structure in a single cell. The non-uniform CLC structure consists of a polymer stabilised CLC layer and a non-polymer stabilised CLC layer. By taking advantage of the difference in driving electric field strength of the two layers, the device can be switched electrically to four different states: the reflecting state, the reflecting and scattering co-existing state, the scattering-only state and the transparent state. The optical properties of the four states and the transition conditions between the states were elucidated. Moreover, all states with reflecting and/or scattering properties are stable for an extended period of time after switching off the electric field. Thanks to its multi-stability, this device can be used for energy-saving smart windows of automobiles, buildings and in architectural aesthetics. An electrically switchable multi-stable liquid crystal device is demonstrated based on a non-uniform cholesteric liquid crystal (CLC) structure in a single cell. The device can be switched electrically to four different states. All states with reflecting and/or scattering properties are stable for an extended period of time after switching off the electric field. Thanks to its multi-stability, this device can be used for energy-saving smart windows of automobiles, buildings and in architectural aesthetics.

Optical Response Associated with the Orientation and Structure of Liquid Crystals with Respect to Phosphatidylcholine Concentration

Based on the anchoring effect due to the self-assembling behavior of the phospholipid molecules at the interface between the liquid crystal and water phases on the orientation of liquid crystals, the optical response associated with the orientation and structure of liquid crystals with respect to the concentration of 1,2-didodecanoyl-sn-glycero-3-phosphocholine solution has been investigated. The optical response owing to changes in the orientation and structure of the mixed cholesteric liquid crystals with respect to the change in the concentration of phosphatidylcholine has been obtained. Moreover, the feasibility of using as-prepared mixed cholesteric liquid crystals to measure the phosphatidylcholine concentration has been verified. A methodology to measure the reflectance spectrum by using mixed cholesteric liquid crystals to sensitize the phosphatidylcholine concentration has been further realized. The sensitization effect of the mixed cholesteric liquid crystals on the measurement of phosphatidylcholine concentration was also verified.

Bioinspired Multi-Stimuli Responsive Actuators with Synergistic Color- and Morphing-Change Abilities.

The combination of complex perception, defense, and camouflage mechanisms is a pivotal instinctive ability that equips organisms with survival advantages. The simulations of such fascinating multi‐stimuli responsiveness, including thigmotropism, bioluminescence, color‐changing ability, and so on, are of great significance for scientists to develop novel biomimetic smart materials. However, most biomimetic color‐changing or luminescence materials can only realize a single stimulus‐response, hence the design and fabrication of multi‐stimuli responsive materials with synergistic color‐changing are still on the way. Here, a bioinspired multi‐stimuli responsive actuator with color‐ and morphing‐change abilities is developed by taking advantage of the assembled cellulose nanocrystals‐based cholesteric liquid crystal structure and its water/temperature response behaviors. The actuator exhibits superfast, reversible bi‐directional humidity and near‐infrared (NIR) light actuating ability (humidity: 9 s; NIR light: 16 s), accompanying with synergistic iridescent appearance which provides a visual cue for the movement of actuators. This work paves the way for biomimetic multi‐stimuli responsive materials and will have a wide range of applications such as optical anti‐counterfeiting devices, information storage materials, and smart soft robots.

A Planar Fresnel Lens in Reflection Type Based on Azo-Dye-Doped Cholesteric Liquid Crystals Fabricated by Photo-Alignment.

This paper presents a focusing efficiency and focal length tunable planar Fresnel lens in reflection type based on azo-dye-doped cholesterol liquid crystal film. The Fresnel-like pattern of a pumping beam can be formed by a Sagnac interferometer. When the azo-dye molecules are irradiated by the pumping beam, the photoalignment effect will be induced in the bright (odd) zones due to the trans–cis photoisomerization of azo-dye molecules. Thus, the structures of cholesteric liquid crystals in the odd zones will reorient from the imperfectly planar textures to the perfectly planar textures. The different structures of cholesteric liquid crystals in two adjacent zones will give rise to phase difference for the reflected light and thus function as a Fresnel lens. The focusing efficiency of the proposed Fresnel lens can be controlled by the applied voltages and affected by the polarization state of incident light. Moreover, various focal lengths of the Fresnel lens can be achieved by rewriting a different center radius of the Fresnel-like pattern.

Hierarchical cross-linking for synergetic toughening in crustacean-mimetic nanocomposites.

The twisted plywood structure as found in crustacean shells possesses excellent mechanical properties with high stiffness and toughness. Synthetic mimics can be produced by evaporation-induced self-assembly of cellulose nanocrystals (CNCs) with polymer components into bulk films with a cholesteric liquid crystal structure. However, these are often excessively brittle and it has remained challenging to make materials combining high stiffness and toughness. Here, we describe self-assembling cholesteric CNC/polymer nanocomposites with a crustacean-mimetic structure and tunable photonic band gap, in which we engineer combinations of thermo-activated covalent and supramolecular hydrogen-bonded crosslinks to tailor the energy dissipation properties by precise molecular design. Toughening occurs upon increasing the polymer fractions in the nanocomposites, and, critically, combinations of both molecular bonding mechanisms lead to a considerable synergetic increase of stiffness and toughness - beyond the common rule of mixtures. Our concept following careful molecular design allows one to enter previously unreached areas of mechanical property charts for cholesteric CNC-based nanocomposites. The study shows that the subtle engineering of molecular energy dissipation units using sophisticated chemical approaches enables efficient enhancing of the properties of bioinspired CNC/polymer nanocomposites, and opens the design space for future molecular enhancement using tailor-made interactions.

Knot all that it seems

Liquid Crystals Although for some, knots are merely a frustration caused by poorly tied shoelaces, interest in knots in physical systems spans many disciplines, including fluid and optical vortices, Skyrmion states, liquid crystals, excitable media, polymers, proteins, DNA, and even chemical molecules. Tai and Smalyukh describe the creation of localized knotted structures in cholesteric liquid crystals using electric fields (see the Perspective by Alexander). The knots are topologically distinct from the host medium and diffuse and organize like colloidal particles, forming regular crystalline arrangements. Science , this issue p. [1449][1]; see also p. [1377][2] [1]: /lookup/doi/10.1126/science.aay1638 [2]: /lookup/doi/10.1126/science.aaz0479

Three-dimensional crystals of adaptive knots

Starting with Gauss and Kelvin, knots in fields were postulated to behave like particles, but experimentally they were found only as transient features or required complex boundary conditions to exist and could not self-assemble into three-dimensional crystals. We introduce energetically stable, micrometer-sized knots in helical fields of chiral liquid crystals. While spatially localized and freely diffusing in all directions, they resemble colloidal particles and atoms, self-assembling into crystalline lattices with open and closed structures. These knots are robust and topologically distinct from the host medium, though they can be morphed and reconfigured by weak stimuli under conditions such as those in displays. A combination of energy-minimizing numerical modeling and optical imaging uncovers the internal structure and topology of individual helical field knots and the various hierarchical crystalline organizations that they form.

Sensing Characteristics of Side-Polished Fiber Based on the Alterations in Helical Structure of Thermo-Sensitive Cholesteric Liquid Crystals

Cholesteric liquid crystals (CLCs) are sensitive to environmental temperature changes, and have been employed as a specific intermediary for biosensors. Considering the temperature-dependent structural changes of CLCs, this study aimed to determine the sensing properties of side-polished fibers (SPFs) after coating with CLCs. The experimental results demonstrated that, with regard to the transmitted spectrum, the loss peak of CLC-coated SPFs exhibited a positive linear relationship with temperature changes over a range of 20 to 50 °C. The linear correlation coefficient achieved 97.8% when the temperature increased by 10 °C, and the loss peak drifted by 12.72 nm. The reflectance spectrum of CLCs coated on the polished surface were obtained using optical fiber sensors. The feasibility of measuring the helical structure of CLCs was further verified using SPF transmission spectroscopy. The findings indicated that the transmitted spectrum of SPFs could be adopted to characterize the helical structure of CLCs, which lays a solid foundation for further study on SPF-based biosensors.