Chiral Liquid Crystals Research Articles

Helicity Control of Circularly Polarized Luminescence from Aromatic Conjugated Copolymers and Their Mixture Using Reversibly Photoinvertible Chiral Liquid Crystals.

We synthesized cyclic chiral compounds [(R)/(S)-D2s] by linking a photoresponsive bisbenzothienylethene (BTE) moiety with an axially chiral binaphthyl moiety. Chiral nematic liquid crystals (N*-LCs) were prepared by adding chiral compounds as dopants to host N-LCs. These N*-LCs exhibited reversible chirality inversion upon photoisomerization between the open and closed forms of the BTE moiety. Here, the mechanism underlying chirality inversion in photoresponsive N*-LCs was investigated by comparing the helical twisting powers (HTPs) of (R)-D2s with those of analogous compounds. It was found that the helical inversion of N*-LCs containing (R)-D2s is governed by a delicate balance between two types of opposite helicity, i.e., the right-handed helicity of the inherently chiral binaphthyl moiety and the left-handed helicity of the BTE moiety bearing intramolecularly induced chirality. Namely, (R)-D2s induced chirality of the BTE moiety, which is attributed to intramolecular chirality transfer from the axially chiral binaphthyl moiety to the BTE moiety. Thus, (R)-D2s are chiral compounds with double chirality consisting of an intrinsically chiral moiety and an intramolecularly induced chiral moiety. Photocontrol of the helical senses and reversible photoinversion of the N*-LCs are achieved by utilizing UV and visible light irradiation and the steric effects of the substituents at the binaphthyl rings in (R)-D2s. In addition, photocontrol of the induced circularly polarized luminescence (CPL) was achieved using the photoinvertible N*-LC. The achiral aromatic conjugated copolymers that exhibited red, green, and blue fluorescence were dissolved and mixed in the present N*-LC, and they exhibited left- and right-handed white CPL with large dissymmetry factors (|glum|) ranging from 0.2 to 1.0. The CPLs were reversibly photoswitched due to photoisomerization between the open and PSS forms of the chiral compounds through UV and visible light irradiation.

Helical-caging enables single-emitted large asymmetric full-color circularly polarized luminescence

Colorful circularly polarized luminescence materials are desired for 3D displays, information security and asymmetric synthesis, in which single-emitted materials are ideal owing to self-absorption avoidance, evenly entire-visible-spectrum-covered photon emission and facile device fabrication. However, restricted by the synthesis of chiral broad-luminescent emitters, the realization and application of high-performing single-emitted full-color circularly polarized luminescence is in its infancy. Here, we disclose a single-emitted full-color circularly polarized luminescence system (spiral full-color emission generator), composed of whole-vis-spectrum emissive quantum dots and chiral liquid crystals. The system achieves a maximum luminescence dissymmetry factor of 0.8 and remains an order of 10−1 in visible region by tuning its photonic bandgap. We then expand it to a series of desired customized-color circularly polarized luminescence, build chiral devices and further demonstrate the working scenario in the photoinduced enantioselective polymerization. This work contributes to the design and synthesis of efficient chiroptical materials, device fabrication and photoinduced asymmetric synthesis.

Optical diffraction in chiral smectic-C liquid crystals

Optical diffraction in chiral smectic-C liquid crystals

Mesomorphic and dielectric properties of strategically designed chiral bent-core liquid crystals displaying wide temperature range dark conglomerate phase

Chiral bent-core molecules exhibit spontaneous symmetry breaking, forming opposing chirality domains. Molecular chirality influences supramolecular domain preferences, offering potential for chiral templating applications.

Laser mode control based on chiral liquid crystal microcavities

This article introduces a FRET system and achieves FRET for different laser modes by changing the pump position.

Light-controllable liquid crystal platform for microparticle oscillations and transport.

Liquid crystal colloids manifest complex motion caused by external stimuli, but tunable and addressable control of microsized objects remains a challenge. This study aims to demonstrate light-driven trapping, transport, and sustained periodic motions of microparticles by employing liquid crystal films as a light-controllable colloidal platform. The diverse motions of microscopic particles result from Marangoni convection coupled with elastic deformations in free-surface liquid crystal films subjected to light beam heating. The specific mode of particle motion, including damped and sustained oscillations, also combined with sustained rotation, is defined by the liquid crystal chirality, particle surface treatment, film thickness, and the power of the tightly focused light beam. The results reveal that free-surface liquid crystals provide a unique platform for the indirect optical manipulation of microscopic objects, paving the way for novel applications in microfluidic tools, particle sorting and transport, micropatterning, and various micromachines.

Potential and performance of anisotropic 19F NMR for the enantiomeric analysis of fluorinated chiral active pharmaceutical ingredients.

Controlling the enantiomeric purity of chiral drugs is of paramount importance in pharmaceutical chemistry. Isotropic 1H NMR spectroscopy involving chiral agents is a widely used method for discriminating enantiomers and quantifying their relative proportions. However, the relatively weak spectral separation of enantiomers (1H Δδiso(R, S)) in frequency units at low and moderate magnetic fields, as well as the lack of versatility of a majority of those agents with respect to different chemical functions, may limit the general use of this approach. In this article, we investigate the analytical potential of 19F NMR in anisotropic chiral media for the enantiomeric analysis of fluorinated active pharmaceutical ingredients (API) via two residual anisotropic NMR interactions: the chemical shift anisotropy (19F-RCSA) and dipolar coupling ((19F-19F)-RDC). Lyotropic chiral liquid crystals (CLC) based on poly-γ-benzyl-L-glutamate (PBLG) show an interesting versatility and adaptability to enantiodiscrimination as illustrated for two chiral drugs, Flurbiprofen® (FLU) and Efavirenz® (EFA), which have very different chemical functions. The approach has been tested on a routine 300 MHz NMR spectrometer equipped with a standard probe (5 mm BBFO probe) in a high-throughput context (i.e., ≈10 s of NMR experiments) while the performance for enantiomeric excess (ee) measurement is evaluated in terms of trueness and precision. The limits of detection (LOD) determined were 0.17 and 0.16 μmol ml-1 for FLU and EFA, respectively, allow working in dilute conditions even with such a short experimental duration. The enantiodiscrimination capabilities are also discussed with respect to experimental features such as CLC composition and temperature.

Response of helielectric nematics under an in-plane electric field.

In traditional chiral nematic liquid crystals, the apolar cholesterics, the dielectric effect is the main driving force for responding to an electric field. The emerging polar chiral nematics, dubbed helielectric nematics, are the polar counterparts of the cholesterics. The head-to-tail symmetry breaking of the new matter state enables it to respond sensitively to the polarity of an electric field. Here, we report on the observation of a sequential polar winding/unwinding process of polarization helices under an electric field applied perpendicular to the helical axes, which behaves distinctly from the unwinding of the apolar cholesteric helices. Understanding the helix-unwinding behaviors provides insights for developing switchable devices based on helielectric nematics.

Extended free-energy functionals for achiral and chiral ferroelectric nematic liquid crystals: theory and simulation.

Polar nematic liquid crystals are new classes of condensed-matter states, where the inversion symmetry common to the traditional apolar nematics is broken. Establishing theoretical descriptions for the novel phase states is an urgent task. Here, we develop a Landau-type mean-field theory for both the achiral and chiral ferroelectric nematics. In the polar nematic states, the inversion symmetry breaking adds three new contributions: an additional odd elastic term (corresponding to the flexoelectricity in symmetry) to the standard Oseen-Frank free energy, electrostatic effect and an additional Landau term relating to the gradient of local polarization. The coupling between the scalar order parameter and polarization order should be considered. In the chiral and polar nematic state, we reveal that the competition between the twist elasticity and polarity dictates effective compressive energy arising from the quasi-layer structure. The polarization gradient is an essential term for describing the ferroelectric nature. Besides, we successfully simulate an experimentally reported structural transition in ferroelectric nematic droplets from a concentric-vortex-like to a line-disclination-mediated topology based on the developed theory. The approaches provide theoretical foundations for testing and predicting polar structures in emerging polar liquid crystals.

Confinement twists achiral liquid crystals and causes chiral liquid crystals to twist in the opposite handedness: cases in and around sessile droplets.

We study the chiral symmetry breaking and metastability of confined nematic lyotropic chromonic liquid crystals (LCLCs) with and without chiral dopants. The isotropic-nematic coexistence phase of the LCLC renders two confining geometries: sessile isotropic (I) droplets surrounded by the nematic (N) phase and sessile nematic droplets immersed in the isotropic background. In the achiral system with no dopants, LCLC's elastic anisotropy and topological defects induce a spontaneous twist deformation to lower the energetic penalty of splay deformation, resulting in spiral optical textures under crossed polarizers both in the I-in-N and N-in-I systems. While the achiral system exhibits both handednesses with an equal probability, a small amount of the chiral dopant breaks the balance. Notably, in contrast to the homochiral configuration of a chirally doped LCLC in the bulk, the spiral texture of the disfavored handedness appears with a finite probability both in the I-in-N and N-in-I systems. We propose director field models explaining how chiral symmetry breaking arises by the energetics and the opposite-twist configurations exist as meta-stable structures in the energy landscape. These findings help us create and control chiral structures using confined LCs with large elastic anisotropy.

Liquid crystal-mediated self-assembly of copper nanoclusters with induced circular dichroism and amplified circularly polarized luminescence.

Elucidating the mechanism of chiral transfer is key to regulating chiral expression and generalizing the structure-property relationship of chiral functional systems. However, it is still an important challenge to select novel building blocks to achieve chiral induction, chiral transfer and chiral modulation. Liquid crystals (LCs) can be considered as promising smart soft materials due to their responsiveness and adaptability. Confining chiral metal nanoclusters (NCs) in an achiral LC phase to construct chiral LCs provides an expanded strategy for the self-assembly of chiral metal NCs in different matrices. Herein, chiral glutathione-stabilized copper NCs (G-SH-Cu NCs)/polyoxyethylene tert-octylphenyl ether (TX-100) LCs were constructed and systematically investigated. The results showed that the introduction of G-SH-Cu NCs into TX-100 LCs induced the generation of supramolecular chirality. More interestingly, the circular dichroism (CD) handedness can be controlled by changing the amount of TX-100 or G-SH-Cu NCs; when the ratio of G-SH-Cu NCs and TX-100 was proportionally matched, the strength of the noncovalent interactions was sufficient to induce chiral inversion. Meanwhile, TX-100 LCs provide effective confinement of G-SH-Cu NCs, which improves the expression of asymmetry at the aggregation level and induces a 2-fold enhancement of the circularly polarized luminescence (CPL) signal. This work realizes the spatial amplification of chirality through dopants in LCs, which provides an effective method for accurately regulating the supramolecular chirality of metal NCs in the LC phase.

Chiroptical Properties of Helical Aromatic Conjugated Polymers Synthesized by Electrochemical Polymerization in Chiral Nematic Liquid Crystals

We synthesized several highly fluorescent fluorene derivatives, in which thiophene, biphenylthiophene, ethylenedioxythiophene (EDOT), or biphenyl EDOT moieties were attached at both ends of the fluorene moiety, and polymerized them via electrochemical polymerization in chiral nematic liquid crystals (N-LCs) providing asymmetric reaction environments. The resulting aromatic conjugated polymers showed induced chirality due to the formation of interchain helically stacked structures and hence exhibited circular dichroism (CD). The absorption dissymmetry factors (|gabs|) were in the range 10-4–10-2 and depended strictly on the moieties in the polymer chains. Some of the polymers exhibited circularly polarized luminescence (CPL) with luminescence dissymmetry factors (|glum|) of approximately 10-3. This is the first example of CPL from polymer films prepared by asymmetric electrochemical polymerization.

Influence of the polymer network on the stability of the heliconical structure in the ferro- and antiferroelectric liquid crystalline phases

The use of polymer networks to stabilise chiral liquid crystalline materials (ChLC), which canfind applications in electro-optical and photonic devices, is becoming increasingly common. Stabilisingthe heliconical structure of a ChLC using a polymer network involves forming a spatial polymer matrix ina liquid crystal medium. The polymer network acts as a kind of scaffold for the liquid crystal molecules,resulting in the helix being stiffened in a particular desired state. This method of helix stabilisation hasbeen mainly used in chiral nematic liquid crystals so far. Effective stabilising the helix with a polymerin chiral smectic liquid crystals could eliminate the main drawback of these materials, i.e., the changein their performance, in electro-optical effects based on the deformation of the helix, with a changein temperature. The main parameter characterising the heliconical structure is its pitch, whose valuechanges rapidly with temperature change, in most smectic liquid crystals, which in turn translates intothe above-mentioned drawbacks. Stabilisation with the polymer should translate into temperaturestability of the helical pitch and the rest of the material parameters. This paper presents the results ofthe temperature dependence of the helical pitch before and after polymerisation of a liquid crystalmaterial with an antiferroelectric phase. A tetrafunctional monomer structurally compatible to thecomponents of the antiferroelectric mixture and, for comparison, a commercially available monomerwith a different structure were used in the study. The results relevant that stabilisation of the heliconicalstructure is easier after the use of a monomer more structurally compatible to the components ofparent liquid crystal material, while maintaining homogeneity of the heliconical structure orderingand a wide temperature range of the antiferroelectric phase occurrence.Keywords: liquid crystals, antiferroelectric phase, helical pitch, polymerization

The Origin of Collective Dielectric Relaxation Process in Ferroelectric and Antiferroelectric Phases of Liquid Crystals

Dielectric spectroscopy has proven as an important tool for the characterization of ferroelectric and antiferroelectric phases observed in the thermotropic chiral liquid crystals compounds. These compounds generally have paraelectric smectic A (SmA*) phase, ferroelectric smectic C (SmC*) phase and antiferroelectric smectic C (SmCA*) phase. In some compounds, ferroelectric (SmCg*) phase is also found. Dielectric spectra of these phases under planar alignment of the molecules are sometimes complex but rich in information and are very helpful in the determination of the structure of these phases. In the SmA* phase, a mode in high kHz region is observed. In the SmC* phase, a relaxation mode observed in low kHz region having high dielectric strength. However, in the SmCg* phase observed mode relaxation frequencies lies in same frequency window as SmC* phase but their dielectric strength was less than at least one order magnitude in comparison to observed mode of SmC* phase. In the SmCA* phase, dielectric spectrum contains two relaxation modes observed in kHz and MHz regions respectively. The dielectric strengths of these two modes are very small and less than at least one/two order magnitude in comparison to observed mode of SmA*/ SmC* phases. These observed relaxation processes are discussed as a function of temperature and bias electric field and their origin is also examined in the light of the experimental work. Dielectric parameters of the above phases observed in such material are discussed with regards to their structure property relationship.

Soft-Matter Physics Provides New Insights on Myocardial Architecture: Automatic and Quantitative Identification of Topological Defects in the Trabecular Myocardium.

This article is the third in our series dedicated to the analysis of cardiac myoarchitecture as a nematic chiral liquid crystal (NCLC). Previously, we introduced the concept of topological defects (disclinations) and focused on their visual identification inside the compact myocardium. Herein, we investigate these using a mathematical and automated algorithm for the reproducible identification of a larger panel of topological defects throughout the myocardium of 13 perinatal and 11 early infant hearts. This algorithm identified an average of 29 ± 11 topological defects per slice with a 2D topological charge of m = +1/2 and an average of 27 ± 10 topological defects per slice with a 2D topological charge of m = -1/2. The excess of defects per slice with a 2D topological charge of m = +1/2 was statistically significant (p < 0.001). There was no significant difference in the distribution of defects with a 2D topological charge of m = +1/2 and m = -1/2 between perinatal and early infant hearts. These defects were mostly arranged in pairs, as expected in nematics, and located inside the trabecular myocardium. When isolated, defects with a 2D topological charge of m = +1/2 were located near the luminal extremity of the trabeculae and those with a 2D topological charge of m = -1/2 were located at the anterior and posterior part of the interventricular septum. These findings constitute an advance in the characterization of the deep cardiac myoarchitecture for application in developmental and pathological studies.

Electro-optical-coloration active polymers: Electrochemical polymerization in chiral liquid crystals supported by an α-tocopherol derivative

Electrochemical polymerization was performed in a cholesteric liquid crystal (CLC) with an α-tocopherol (vitamin E) derivative as a chiral inducer to yield electroactive polymers exhibiting dynamic electrochemical-driven changes in optical activity. Materials possessing multiple asymmetric centers with long alkyl chains can be conveniently obtained using vitamins as chiral substances. First, an α-tocopherol derivative was synthesized as a chiral dopant (chiral inducer) to induce a CLC from a nematic liquid crystal. Thereafter, π-conjugated polymers were electrochemically prepared in the CLC with the α-tocopherol derivative. The fingerprint texture derived from the CLC was successfully transcribed to the polymers, and molecular asymmetric imprinting polymerization was achieved. Furthermore, the oxidized (doped) and reduced (dedoped) forms of the polymers were subjected to ultraviolet–visible and circular dichroism analyses. Electrochemical polymerization in the helical CLC resulted in molecular aggregation–imprinted chirality in the polymers. The chiral information was transferred from vitamin E to the nematic liquid crystal and resulting polymers.

Transversely graded polarization volume gratings fabricated by freeform holographic photoalignment.

Polarization volume gratings (PVGs) based on chiral nematic liquid crystals offer a great potential as polarization-dependent holographic optical elements, but it is not easy to fabricate PVGs with varying pattern periods in the transverse plane. Here, we fabricate a PVG with an in-plane gradient of the pattern period by performing two-beam interference photoalignment on a flexible polyimide substrate. The pattern period varies depending on the local interference angle, which is controlled by the bent shape of the flexible substrate. We demonstrate fabrication of a PVG with a linearly graded sub-micrometer period, showing the potential of the proposed method to fabricate designer PVGs.

Tailoring Chiral Discotic Liquid Crystals: Mesophase Engineering through Alternative Approaches and Chain Lengths.

Hydrogen (H)-bonding is crucial in constructing superstructures in chemical (such as chiral discotic liquid crystals (DLCs)) as well as in biological systems due to its specific and directional nature. In this context, we achieved the successful synthesis of two branches of heptazine-based H-bonded complexes using distinct strategies. Hpz*-Es-Cn , we incorporated chiral alkyl tails (Hpz-chiral) onto the central C3 symmetric heptazine core, connected to achiral benzoic acid derivatives (Es-Cn acid) through H-bonding. In Hpz-Es-Cn -acid*, we used an achiral heptazine derivative (Hpz-Es-Cn ) linked to a chiral acid via H-bonding. On the other hand, based on the DSC results, we observed that Hpz*-Es-Cn complexes exhibited three distinct phases, whereas Hpz-Es-Cn -acid* complexes displayed only a single mesophase. In polarized optical microscopy (POM) observations, all the complexes displayed birefringence at room temperature, with the color of the POM images changing as the temperature varied. X-ray diffraction (XRD) studies at lower temperatures confirmed that Hpz*-Es-C8 exhibited the columnar rectangular (Colr ) phase, while Hpz*-Es-C10/12 exhibited the columnar oblique (Colob ) phase. However, all the H-bonded complexes exhibited the columnar hexagonal (Colh ) phase at higher temperatures. The chiroptical spectra recorded by Circular dichroism (CD) highlight the specific observations in the columnar phase of two complexes, Hpz*-Es-C10 and Hpz*-Es-C12 . This behavior has potential applications in various fields, including sensors, displays, and responsive materials.

Reflective 2D diffraction grating based on short pitch chiral liquid crystal

Thermotropic liquid crystals have recently become popular for use in optical components for augmented and virtual reality. Chiral liquid crystal (CLC), is of particular interest due to the formation of a photonic bandgap that allows for the creation of polarization-sensitive reflective optical components. Previous investigations have studied 2D diffraction gratings based on long-pitch (5 μm) CLC in combination with a 2D periodic alignment configuration. This yielded 2D diffraction patterns in transmission with the ability to switch between high and low voltage states with hysteresis. In the current study, short-pitch CLC (400 nm and 1 μm) is deposited between two substrates with perpendicularly rotating periodic photoalignment patterns, the shorter pitch resulting in a diffraction grating that produces a 2D diffraction pattern in reflection for wavelengths in the visible range. While a relatively high diffraction efficiency is achieved in the first diffraction order, additional diffraction orders with lower efficiency are present due to the complex CLC configuration in the bulk.

Electrically Tunable Polymer Stabilized Chiral Ferroelectric Nematic Liquid Crystal Microlenses

AbstractTunable optical lenses are in great demand in modern technologies ranging from augmented and virtual reality to sensing and detection. In this work, electrically tunable microlenses based on a polymer‐stabilized chiral ferroelectric nematic liquid crystal are described. The power of the lens can be quickly (within 5 ms) varied by ≈500 diopters by ramping an in‐plane electric field from 0 to 2.5 V µm−1. Importantly, within this relatively low‐amplitude field range, the lens is optically isotropic; thus, its focal length is independent of the polarization of incoming light. This remarkable performance combines the advantages of electrically tuned isotropic lenses and the field‐controlled shape of the lens, which are unique properties of chiral ferroelectric nematic liquid crystals and have no counterpart in other liquid crystals. The achieved lens performance represents a significant step forward as compared to liquid lenses controlled by electrowetting and opens new possibilities in various applications such as biomimetic optics, security printing, and solar energy concentration.