Cholesteric Research Articles

A Single-Enantiomer Emitter Enabled Superstructural Helix Inversion for Upconverting and Downshifting Luminescence with Bidirectional Circular Polarization.

The helical twisting tendency of liquid crystals (LCs) is generally governed by the inherent configuration of the chiral emitter. Here, we introduce the multistage inversion of supramolecular chirality as well as circularly polarized luminescence (CPL) by manipulating the ratio of single enantiomeric emitters (R-PCP) to LC monomers (5CB). Increasing the content of R-PCP from 1 wt % to 3 wt % inverted the helix of LCs from left-handed to right-handed, accompanying a CPL sign changed from positive to negative. The biaxiality of chiral emitters, as well as the steric effect of chiral-chiral and chiral-achiral interaction, were identified as the reasons for helical sense inversion. Due to the strong helical twisting power, 4 wt % R-PCP drove the photonic band gap (PBG) of chiral LCs to match up with their emission range, leading to an inversion of the CPL again with a high dissymmetry factor (≈1.2). Directly adjusting the PBG using chiral emitters is seldom achieved in cholesteric LCs. On this basis, an achiral sensitizer PtTPBP was assembled into the helical superstructure. The generation of triplet-triplet annihilation-induced upconverted CPL from R-PCP and the downshifting CPL from PtTPBP with opposite rotation was achieved in a single chiral LC system by tuning the position of the PBG.

Emergence and inversion of chirality in hierarchical assemblies of CdS nanocrystal fibers.

Arranging semiconducting nanocrystals into ordered superstructures is a promising platform to study fundamental light-matter interactions and develop programmable optical metamaterials. We investigated how the geometrical arrangement of CdS nanocrystals in hierarchical assemblies affects chiroptical properties. To create these structures, we controlled the evaporation of a colloidal CdS nanocrystal solution between two parallel plates. We combined in situ microscopy and computational modeling to establish a formation mechanism involving the shear-induced alignment of nanocrystal fibers and the subsequent mechanical relaxation of the stretched fibers to form Raman noodle-type band textures. The high linear anisotropy in these films shares many similarities with cholesteric liquid crystals. The films deposited on top and bottom surfaces exhibit opposite chirality. The mechanistic insights from this study are consequential to enable future advances in the design and fabrication of programmable optical metamaterials for further development of polarization-based optics toward applications in sensing, hyperspectral imaging, and quantum information technology.

Photoisomerization dynamics of a light-sensitive chiral dopant in a nematic medium

ABSTRACT Azobenzene-based chiral dopants in cholesteric liquid crystals are of interest since the properties they induce in the liquid crystal could be tuned photochemically. Here, we use a substituted binaphthyl with a halogenated azobenzene as a chiral dopant to induce a photoswitchable cholesteric phase in the nematic 4-n-pentyl-4’-cyanobiphenyl. The azobenzene group chemically attached to the chiral dopant undergoes isomerisation from trans to cis upon irradiation with green light (wavelength 535 nm), and from cis to trans upon irradiation with blue light (wavelength 450 nm). The transition between the two isomers causes helicity inversion of the cholesteric, with a left-handed trans isomer and a right-handed cis isomer. We report on the kinetics of photoisomerisation of both processes (trans-to-cis and cis-to-trans) in the nematic host by following the pitch evolution over time. We show that the kinetic mechanism corresponds to a two-step process: a first-order isomerisation followed by a second-order autocatalytic isomerisation. This mechanism differs from the typical first-order kinetics for cis-to-trans or trans-to-cis isomerisation in azobenzenes. The autocatalytic process is attributed to interactions between the chiral dopant and the nematic host.

Modeling of the lyotropic cholesteric liquid crystal based toxic gas sensor using adaptive neuro-fuzzy inference systems

Detection of toxic gases is important in a variety of settings, including industrial facilities, laboratories, and even in homes. In these settings, toxic gas detection can help prevent accidents and protect the health and safety of workers, researchers, and others who may be exposed to these gases. This study evaluates an Adaptive Neuro-Fuzzy Inference System (ANFIS) models in predicting the machining responses in the detection of toxic gases vapor, such as toluene (T), phenol (P) and 1,2 dichloropropane (D) using lyotropic cholesteric crystal (CLC) have been shown to have potential as gas sensors due to their unique optical and liquid crystal (LC) properties, and the ANFIS model may be used to better understand and optimize these properties for toxic gas detection. Experiments were carefully carried out to gather data on the response of a lyotropic CLC toxic gas vapor sensor. The effectiveness of using ANFIS combined with Grid Partitioning (GP) was then carefully studied and evaluated in terms of modeling and predicting the responses of the sensor. The best ANFIS-GP model is chosen from these criteria; RSS, PCC, R2, RMSE, MSE, MAE, and MAPE. In addition, validation was performed between the model and experimental data using the LOOCV method. The results show that the ANFIS-GP5 model with 96 fuzzy inference systems (FIS) rules with high R2 values. According to the ANFIS-GP5 model, R2varied ranges from 0.77 to 1 for train, test, and total data of lyotropic CLC sensor exposed to toluene, phenol and 1,2 dichloropropane toxic gases vapors.

Transition Temperature in Thermochromic Liquid Crystals using Second-Order Features Extraction

<p>Liquid crystals are a type of substance that has solid and liquid properties. One of the types of it is cholesteric. Cholesteric liquid crystals have a characteristic which is called pitch. Pitch is very sensitive to changes in temperature. The pitch will reflect different colors depending on the wavelength at a particular temperature. Thermochromic Liquid Crystals (TLC) is the cholesteric liquid crystals material sold commercially. At the transitional temperature, the texture of TLC changes, so the reflected color will also change. Second-order feature extraction was chosen to determine the change in texture with the transition temperature. The TLC layer was made by thickness of 100 µm. This layer was heated and observed using a polarizing microscope with an angle between the polarizer and analyzer of 90<sup>o</sup>. The obtained result are cross patterns emerged at anisotropic transition temperature and higher temperature on TLC will lead to an isotropic phase.<em></em></p>

Preparation of the colorful retroreflective film based on the polymer-stabilized cholesteric liquid crystal

A cholesteric liquid crystal (CLC) mixture was prepared using a photoisomerizable chiral dopant and acrylates. With extending the irradiation time of a 365-nm UV light, the Bragg reflection band of the CLC mixture shifted to long wavelength. After photopolymerization, a polymer-stabilized CLC (PSCLC) film was obtained. The colorful PSCLC patterns with glass microspheres on the surface were prepared by controlling the competition between the photoisomerization of the chiral dopant and the photopolymerization of the acrylates. Due to the retroreflective property, the patterned PSCLC films can be applied as the traffic signs and for advertisement.

Direct‐Ink‐Written Cholesteric Liquid Crystal Elastomer with Programmable Mechanochromic Response

AbstractCholesteric liquid crystal elastomers (CLCEs) are unique anisotropic rubbers that can change their structural color in response to various stimuli such as heat, chemicals, electric fields, and mechanical stress. Methods such as anisotropic deswelling and surface alignment have been adopted to prepare CLCEs; however, they have limitations in creating spatially controlled CLCE geometries. In this work, a direct ink writing (DIW)‐based 3D‐printable CLCE that can be prepared by extruding viscous CLC ink is developed through a 3D printer nozzle, followed by photopolymerization. Interestingly, the helical axis is inclined to the printing direction by ≈32° due to a combination of the shear‐induced alignment causes during extrusion and the elongational force generated during deposition onto the substrate. This unusual helical axis distortion leads to both blue and red shifts of the reflection color depending on the direction of observation relative to the printing axis. Notably, the printed CLCE exhibits anisotropic mechanochromism upon stretching, because of the stretching‐direction‐dependent variations in the slant angle of the helical axis. This anisotropic mechanochromism can be harnessed to develop a unique CLCE‐based strain sensor displaying intricate color patterns upon stretching, with significant application potential in encryption, anticounterfeiting, and structural health monitoring.

Short chiral pitch and blue phase stability in cholesteric liquid crystal mixtures by adding achiral benzoic acid derivative

ABSTRACT Cholesteric liquid crystal (CLC) is anticipated to have applications such as blue light and ultraviolet (UV) light blocking for human skin owing to its selective wavelength reflection against incident sunlight through several hundred nanometres of a periodic helical twist structure. In this study, we investigated short chiral pitch control and enlargement of the blue phase (BP) temperature range by adding an achiral constituent of 4-n-octylbenzoic acid (8 BA) into CLC mixtures composed of three kinds of cholesteryl compounds: cholesteryl chloride, cholesteryl pelargonate and cholesteryl palmitate. Hydrogen bonding dependence of chiral pitch and BP temperature range were also investigated for achiral dopant (8 BA) and CLC mixture through temperature-dependent Fourier-transform infrared analysis. It was found that 40 wt% of 8 BA within the CLC mixture selectively reflected UV and blue light over 360–400 nm with a decrease of chiral pitch. It was also found that the BP temperature range increased by 15.6°C owing to hydrogen bond dimers composed of 8 BA molecules in the CLC mixture.

Crystal structure simulation of cationic cholesteric Liquid-Crystal polyesters. Non-Viral vectors

The crystal structure of six cationic cholesteric liquid crystal polyesters with choline, ammonium, and amide end groups are reported: PTOBEE-Choline [(C26H20O8)n-C5H13N]; PTOBDME-Choline [(C34H36O8)n-C5H13N]; PTOBEE-Ammonium [(C26H20O8)n-C5H13N]; PTOBDME-Ammonium [(C34H36O8)n-C5H13N]; PTOBEE-Amide (C26H19O9N)n and PTOBUME-Amide (C33H33O9N)n, have demonstrated non-viral vector properties in gene therapy by delivering DNA to the cell nucleus. They were synthesized by functionalization of precursor racemic polyesters via a condensation reaction between 4,4′-(terephthaloyl-di-oxydibenzoic chloride) (TOBC) and the racemic mixture of glycol (R-S-1,2-butanediol) or (R-S-1,2-dodecanediol), respectively. The resulting helical macromolecular structures exhibit unexpected optical activity and chiral morphology, previously attributed to the kinetic resolution of one preferable helical conformer of the possible four observed by NMR. Conformational analysis and energy minimization are performed on the four helical conformer monomeric models which are then polymerized to obtain helical chains. Three levels of chirality were observed in these polymer structures. Crystal models are built in triclinic primitive P1cells, with their molecular chains oriented parallel to the Z-axis (c lattice parameter equal to the helical pitch length), the cell parameters are obtained by performing geometry optimization tasks. A raw estimation of the relative weight of the four possible helical conformers on each synthetic cationic polymer has been performed on ideal mixtures considering their approximate known crystal structure with their experimental X-ray powder diffraction by Powder Quantitative Phase Analysis. The results would confirm the theory of the preferable recrystallization of a diastereoisomer, among the four possible, during the synthetic procedures. These findings shed light on the observed optical activity and provide insights into the crystal structure of these cationic polymers. The morphology of the cationic polymers is studied by ESEM in wet samples and crystals after drying the solvent.

Properties of Structural Colored Fiber Prepared by Wet Spinning of Cellulose Derivatives Exhibiting Cholesteric Liquid Crystals

Properties of Structural Colored Fiber Prepared by Wet Spinning of Cellulose Derivatives Exhibiting Cholesteric Liquid Crystals

Improved performance of cadmium selenide quantum dots-doped polymer stabilized cholesteric liquid crystals for light shutter

ABSTRACT In this work, cadmium selenide quantum dots (CdSe QDs)-doped polymer stabilized cholesteric liquid crystals composites (PSCLC) are proposed for low-power light shutters. CdSe QDs with different concentrations, (0.1 and 0.25 wt.%) into PSCLC composites are fabricated and studied. The phase-dependent and morphological studies are investigated in all the samples. The morphological study reveals that doped sample transform from planar to homeotropic type of orientation at low voltage (~15 V), at an optimum concentration 0.1 wt. % of CdSe QDs in PSCLC. Dielectric analysis infers that dispersion of CdSe QDs increases the permittivity (~36%) of the composites. At 0.1 wt.% QDs-doped PSCLC system, the dielectric anisotropy also increases, however a decrease in splay elastic constant could be seen. Voltage dependence on transmission behaviour also shows that the threshold as well as saturation voltages are lower (6.5 V and 19.5 V, respectively) for 0.1 wt.% QDs-doped system than undoped sample. This signifies that desired alignment to the liquid crystal molecules is easily achieved at low electrical energy by the incorporation of CdSe QDs. CdSe QDs may be incorporated into PSCLC to develop novel functional devices operated at low power.

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.

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.

Multilayer, Broadband Infrared Reflectors Based on the Photoinduced Preparation of Cholesteric Liquid Crystal Polymers.

This paper focuses on preparing broadband reflective liquid crystal films through the diffusion of monofunctional and bifunctional monomers in a photoinduced trilayer system. By combining the hydrophilic and hydrophobic liquid crystal glass surface treatment technologies, the polymer network of polymer-stabilized cholesteric liquid crystal (PSCLC) itself serves as a diffusion channel to form a trilayer cholesteric liquid crystal composite system containing bifunctional monomers, a nematic liquid crystal composite system, and a cholesteric liquid crystal composite system containing monofunctional monomers. Utilizing the difference in the polymerization rates of monofunctional and difunctional polymerizable monomers, the monomers and chiral compounds diffuse relative to each other, so that the liquid crystal pitch exhibits a gradient distribution, and the broadened reflective width can reach up to 1570 nm. There is no doubt that new and improved processes and technologies offer important possibilities for preparing and applying PSCLC films.

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.

Solid-State Cholesteric Liquid Crystals as an Emerging Platform for the Development of Optical Photonic Sensors.

Over the past decade, solid-state cholesteric liquid crystals (CLCsolid ) have emerged as a promising photonic material, heralding new opportunities for the advancement of optical photonic biosensors and actuators. The periodic helical structure of CLCsolid s gives rise to their distinctive capability of selectively reflecting incident radiation, rendering them highly promising contenders for a wide spectrum of photonic applications. Extensive research is conducted on utilizing CLCsolid 's optical characteristics to create optical sensors for bioassays, diagnostics, and environmental monitoring. This review provides an overview of emerging technologies in the field of interpenetrating polymeric network-CLCsolid (IPN) and CLCsolid -based optical sensors, including their structural designs, processing, essential materials, working principles, and fabrication methodologies. The review concludes with a forward-looking perspective, addressing current challenges and potential trajectories for future research.

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.

3D-Printed Biomimetic Structural Colors.

Resolution control and expansibility have always been challenges to the fabrication of structural color materials. Here, a facile strategy to print cholesteric liquid crystal elastomers (CLCEs) into complex structural color patterns with variable resolution and enhanced expansibility is reported. A volatile solvent is introduced into the synthesized CLC oligomers, modifying its rheological properties and allowing direct-ink-writing (DIW) under mild conditions. The combination of printing shear flow and anisotropic deswelling of ink drives the CLC molecules into an ordered cholesteric arrangement. The authors meticulously investigate the influence of printing parameters to achieve resolution control over a wide range, allowing for the printing of multi-sized 1D or 2D patterns with constant quality. Furthermore, such solvent-cast direct-ink-writing (DIW) strategy is highly expandable and can be integrated easily into the DIW of bionic robots. Multi-responsive bionic butterfly and flower are printed with biomimetic in both locomotion and coloration. Such designs dramatically reduced the processing difficulty of precise full-color printing and expanded the capability of structural color materials to collaborate with other systems.

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.

Extending the capabilities of vectorial ptychography to circular-polarizing materials such as cholesteric liquid crystals.

The problem of imaging materials with circular-polarization properties is discussed within the framework of vectorial ptychography. We demonstrate, both theoretically and numerically, that using linear polarizations to investigate such materials compromises the unicity of the solution provided by this computational method. To overcome this limitation, an improved measurement approach is proposed, which involves specific combinations of elliptical polarizations. The effectiveness of this strategy is demonstrated by numerical simulations and experimental measurements on cholesteric liquid crystal films, which possess unique polarization properties. With the help of Pauli matrices algebra, our results highlight the technique's ability to discern between the different types of circular polarizers, uniform vs. non-uniform, and determine their handedness.