Cholesteric Mesophase Research Articles

Structural Color Out of the Blue: A Quantitative Framework for the Self-Assembly Kinetics of Cholesteric Cellulosic Mesophases.

Cholesteric mesophases based on cellulose ethers, such as ethyl cellulose and hydroxypropyl cellulose, have been studied widely for their remarkable ability to display macroscopic structural color. However, the typical time scales involved in the multiscale self-assembly of cholesteric liquid crystals, from individual nanoscale helical arrangements to discrete microscopic domains, and their dependence on the gel's viscoelastic properties remain underexplored. Here, we establish a quantitative relationship between the kinetics of structural color formation after shear deformation and cholesteric order development at the nano- and microscales. Utilizing rheology in tandem with static and time-resolved reflectivity measurements, we underscore the strong influence of polymer diffusivity and chain elasticity on self-assembly kinetics in cholesteric cellulose ether gels. We show that our phenomenological model can be employed to assess the structure-property relationships of multiple polysaccharide systems, elucidating key design guidelines for the development and processing of structurally colored cholesteric mesophases.

Polymer dispersed cholesteric liquid crystals with combined photo- and mechanochromic response

The development of stimuli-sensitive systems is one of the newest and most promising directions in the field of modern materials science. The combination of various types of sensitivity in the same material opens the possibility of creating complex devices with a wide range of potential applications. This work deals with the design and preparation of new composite materials based on plasticized polyvinyl alcohol (PVA) films containing droplets of cholesteric liquid crystals (CLC). Mechanical stretching of elastic composite films results in a change in optical properties, namely selective light reflection (SLR), from CLC droplets. This in turn transforms the structure and optical properties of the entire polymer matrix. Such a process represents a certain mechano-optic response of the polymer matrix to the influence of mechanical deformation. The introduction of a chiral photochromic dopant into a composite film makes it possible to transform such a film into a photochromic material with the ability to control its optical properties using, for example, UV irradiation. Therefore, mechanical deformation of a polymer composite leads to a change in its optical properties (SLR) caused by a change chiral supramolecular structure of cholesteric mesophase. At the same time, UV irradiation of a polymer film induces E-Z isomerization process in the chiral photochromic dopant, altering its molecular shape and helical twisting power. This enables the using of this process for remote and contactless control of the composite optical properties. In addition, this feature allows one to implement the photopatterning and the recording of a latent image, which appears under the influence of a mechanical field.

A confined self-assembly approach towards chiral photonic materials with circularly polarized structural colors for information storage and encryption

Chiroptical materials with tunable circular polarization performances have gained wide interest because of promising applications in optical devices and information communication. However, limited by their intrinsic chirality, such materials suffer from serious deficiencies in dynamical manipulation on circularly polarized light (CPL). Here, we demonstrate a kinetic trapping approach to capture multi-domain cholesteric mesophases for polarization-dependent structural coloration. The resultant photonic structures depicted by limited solvent evaporation display light control from selective left-handed circularly polarized light reflection to dual reflection, as a result of forming highly tilted and distorted domains with a phase-retardation effect. Notably, a positive linear correlation is established between the circular polarization behavior and structural orientation regulated by the solvent evaporation. The tunable circular polarization structural colors enable the application of the photonic materials for complex encryption in terms of polychromatic coding arrays. The confined-anisotropic-fabrication method of photonic structures paves a foundation toward advanced functional materials with promising applications in light science and technology.

Spontaneous Co-Assembly of Cellulose Nanocrystals and TiO2 Nanorods Followed by Calcination to Form Cholesteric Inorganic Nanostructures.

Chiral nanomaterials possess unique electronic, magnetic, and optical properties that are relevant to a wide range of applications including photocatalysis, chiral photonics, and biosensing. A simple, bottom-up method to create chiral, inorganic structures is introduced that involves the co-assembly of TiO2 nanorods with cellulose nanocrystals (CNCs) in water. To guide experimental efforts, a phase diagram was constructed to describe how phase behavior depends on the CNCs/TiO2/H2O composition. A lyotropic cholesteric mesophase was observed to extend over a wide composition range as high as 50 wt % TiO2 nanorods, far exceeding other examples of inorganic nanorods/CNCs co-assembly. Such a high loading enables the fabrication of inorganic, free-standing chiral films through removal of water and calcination. Distinct from the traditional templating method using CNCs, this new approach separates sol-gel synthesis from particle self-assembly using low-cost nanorods.

Nematic to Cholesteric Transformation in the Cellulose Nanocrystal Droplet Phase

Nucleation, growth, and transformation of chirality in nanomaterial systems is a growing research topic with broad interest in tunable and configurable chiroptical materials. Similar to other one-dimensional nanomaterials, cellulose nanocrystals (CNCs), which are nanorods of naturally abundant biopolymer cellulose, display chiral or cholesteric liquid crystal (LC) phases in the form of tactoids. However, the nucleation and growth of the cholesteric CNC tactoids to equilibrium chiral structures and their morphological transformations are yet to be critically assessed. We noticed that the onset of liquid crystal formation in CNC suspensions is characterized by the nucleation of a nematic tactoid that grows in volume and spontaneously transforms into a cholesteric tactoid. The cholesteric tactoids merge with the neighboring tactoids to form bulk cholesteric mesophases with various configurational palettes. We applied scaling laws from the energy functional theory and found suitable agreement with the morphological transformation of the tactoid droplets monitored for their fine structure and orientation by quantitative polarized light imaging.

Macroscopic Spiral Patterns of Cholesteric Cellulose Nanocrystals Induced by Chiral Doping and Vortex Flowing.

Negatively surface-charged sulfate cellulose nanocrystals (CNCs) are always slowly self-assembled into left-handed cholesteric mesophases. In this work, macroscopic spiral patterns induced by counterclockwise vortex flowing or chiral doping were investigated. Results show that iridescent patterns of the arithmetic spiral, rose spiral, or latitude ripples were generated under the vortex rotation, indicating a severe microphase separation of CNCs. Moreover, the spiral pattern and rotational symmetry were highly correlated to the twisting and flowability of CNCs as well as chiral dopants. Alternatively, the cholesteric pitch and maximum reflective wavelength (λmax) of CNCs were strongly increased by sinistral dopants other than the dextral ones, indicating an enhanced torsion of left-handed CNC mesophases by the dextral dopants. In addition, macroscopic spiral patterns distinctly existed in dextrally doped CNCs owing to a synergistic chiral enhancement. Therefore, the mechanochiral or chemical chiral transition from microscopic twisting to macroscopic spiral provides a potential inspiration for chiral self-organization of biological macromolecules.

Mn(iii)-mediated C-P bond activation of diphosphines: toward a highly emissive phosphahelicene cation scaffold and modulated circularly polarized luminescence.

Transition metal mediated C-X (X = H, halogen) bond activation provides an impressive protocol for building polyaromatic hydrocarbons (PAHs) in C-C bond coupling and annulation; however, mimicking both the reaction model and Lewis acid mediator simultaneously in a hetero-PAH system for selective C-P bond cleavage faces unsolved challenges. At present, developing the C-P bond activation protocol of the phosphonic backbone using noble-metal complexes is a predominant passway for the construction of phosphine catalysts and P-center redox-dependent photoelectric semiconductors, but non-noble metal triggered methods are still elusive. Herein, we report Mn(iii)-mediated C-P bond activation and intramolecular cyclization of diphosphines by a redox-directed radical phosphonium process, generating phosphahelicene cations or phosphoniums with nice regioselectivity and substrate universality under mild conditions. Experiments and theoretical calculations revealed the existence of the unusual radical mechanism and electron-deficient character of novel phosphahelicenes. These rigid quaternary bonding skeletons facilitated versatile fluorescence with good tunability and excellent efficiency. Moreover, the enantiomerically enriched crystals of phosphahelicenes emitted intense circularly polarized luminescence (CPL). Notably, the modulated CPL of racemic phosphahelicenes was induced by chiral transmission in the cholesteric mesophase, showing ultrahigh asymmetry factors of CPL (+0.51, -0.48). Our findings provide a new approach for the design of emissive phosphahelicenes towards chiral emitters and synthesized precursors.

Mechano‐Optical Response of Novel Polymer Composites Based on Elastic Polyurethane Matrix Filled with Low‐Molar‐Mass Cholesteric Droplets

AbstractAmong the various types of liquid crystalline (LC) structures, the cholesteric mesophase attracts special attention in view of helical supramolecular structure determining their specific optical properties. The helix pitch of cholesteric liquid crystals is very sensitive to mechanical stress, resulting in change of their optical properties and position of selective light reflection peak. Here, films of polymer‐dispersed cholesteric liquid crystals based on polyurethane elastomer are prepared and studied. Liquid crystal mixtures based on chiral cholesterol derivatives are uniformly dispersed in the form of micrometer‐sized droplets in an elastomeric matrix. A significant mechano‐optical response of the composites under the action of deformation is demonstrated, and the change in the position of the selective light reflection band during mechanical stress is studied in detail. A shift in the selective light reflection peak by 100 nm or more is found. This process is accompanied by color transformations of brightly colored films. Such materials can be promising for creating stress and deformation sensors, ripple and bending sensors, and other technical devices.

Travelling colourful patterns in self-organized cellulose-based liquid crystalline structures

Cellulose-based systems are useful for many applications. However, the issue of self-organization under non-equilibrium conditions, which is ubiquitous in living matter, has scarcely been addressed in cellulose-based materials. Here, we show that quasi-2D preparations of a lyotropic cellulose-based cholesteric mesophase display travelling colourful patterns, which are generated by a chemical reaction-diffusion mechanism being simultaneous with the evaporation of solvents at the boundaries. These patterns involve spatial and temporal variation in the amplitude and sign of the helix´s pitch. We propose a simple model, based on a reaction-diffusion mechanism, which simulates the observed spatiotemporal colour behaviour.

Mesophase characteristics of cellulose nanocrystal films prepared from electrolyte suspensions

Cellulose nanocrystals (CNCs) exhibit a cholesteric mesophase above a critical concentration in aqueous suspensions. Above this concentration, CNCs self-organize into left-handed helicoidal structures that can be preserved in dried, stratified films. In this systematic study, we have prepared optically-active CNC films cast from different electrolyte suspensions and investigated, via circular dichroism and other techniques, the effects of counterion type (six mono/divalent salts, including those responsible for promoting “salting-out” and “salting-in” in the Hofmeister series) and ionic strength on mesomorphic behavior and cholesteric arrangement. The presence of electrolytes influences CNC colloidal stability by compressing the electric double layer and altering interactions among neighboring CNCs and water, thereby affecting the extent to which the CNCs form a mesophase. Interestingly, mesomorphic behavior and CNC alignment appear to be sensitive to cationic radius and charge valence, in which case the optical properties of CNC films can be adjusted for targeted sustainable applications. Such heuristic rules can be valuable for predicting the stability and characteristics of CNC microstructure in designer coatings and thin films prepared by introducing suitable cations prior to film formation.

Cholesteric mesophase based 1D photonic materials from self-assembly of liquid crystalline block and random terpolymers containing chromonic molecules.

We describe the influence of competing self-organizing phenomena on the formation of cholesteric mesophase in liquid crystalline brush block terpolymers (LCBBTs) and liquid crystalline random brush terpolymers (LCRBTs) containing chromonic molecules. A library of LCBBTs and LCRBTs are synthesized using ring-opening metathesis polymerization (ROMP) of norbornene side-chain functionalized monomers comprising cholesteryl mesogen (NBCh9), chromonic xanthenone (NBXan), and poly(ethylene glycol) (NBMPEG). Compression molded films of LCRBTs containing chromonic molecules display multilevel hierarchical structure in which cholesteric mesophase co-exists with π–π stacking of the chromonic mesophase along with PEG microphase segregated domains. This is unexpected as conventional LCBCPs and LCBBCs that lack chromonic molecules do not form cholesteric mesophases. The presence of π–π interactions modifies the interface at the IMDS so that both chromonic and cholesteric mesophases coexist leading to the manifestation of cholesteric phase for the first time within block architecture and is very reminiscent of previously published LCRBCs without chromonic molecules. The key to the observed hierarchical assembly in these LCBBTs containing chromonic molecules lies in the interplay of LC order, chromonic π–π stacking, PEG side chain microphase segregation, and their supramolecular cooperative motion. This unique “single component” polymer scaffold transforms our capacity to attain nanoscale hierarchies and optical properties from block architecture similar to nanoscale mesophases resulting in random architecture.

The cholesteric and TGB phases under the applied electric field

ABSTRACT New lactic acid derivatives were prepared and their mesomorphic properties presented in comparison with analogous compounds we pursued previously. Studied materials exhibit cholesteric (Ch) and twist grain boundary (TGB) phases on cooling from the isotropic phase (Iso). We found the helical pitch in Ch phase was rather short, less than the visible light wavelength, and grew quickly on cooling, so at the Ch-TGBA phase transition, we were able to observe the selective reflection in visible range. For the longest homologue, the Ch-TGBA-TGBC phase sequence was detected. Due to the positive dielectric anisotropy, the applied electric field reoriented molecules along the field and the effect analogous to Frederiks transition was observed in cholesteric and TGBA mesophases. X-ray diffraction measurements were performed to unambiguously confirm the phase identification.

Structure, thermal properties and molecular mobility in cholesteryl hydrogen phthalate: Different approaches to the crystal, the glassy crystal and the mesophase

Cholesteryl hydrogen phthalate (CHP) is a good glass-former: cooling the isotropic liquid forms the cholesteric mesophase, and subsequent cooling leads to the formation of an orientationally ordered glass. The thermal behavior of CHP was studied by DSC and it was found that, once melted, the commercial crystalline form cannot recover through heat treatments: cold crystallization gives rise to different polymorphic forms; in contrast, recrystallization of the commercial crystal from an ethanol solution leads to the same crystalline form.The structure of the stable ordered polymorph (commercial) was resolved by single-crystal X-ray diffraction (SXRD) at room and low temperature (167 ± 2 K); it belongs to the monoclinic space group P21.The glass transformation of CHP was studied by DSC and thermally stimulated depolarization currents (TSDC). The values obtained for the glass transition temperature and the dynamic fragility indicate that CHP is a moderately fragile glass former.Concerning secondary relaxations, the results suggest that the mobility detected by TSDC has a Johari-Goldstein nature. On the other hand, the interpretation of the results above Tg raises difficulties eventually due to the non-isothermal nature of the TSDC technique together with the temperature dependent structure of the cholesteric mesophase.

Thermotropic cholesteric liquid crystals. Thermo-morphologic, optical refracting and birefringent properties: Direct and reverse phase transitions

Abstract Temperature dependences of the average refractive index 〈n〉, refractive indices of the ordinary ray no and the extraordinary ray ne, and the birefringence Δn have been investigated in two cholesteric liquid crystals. Investigations have been carried out for the heating – cooling reversible cycle. Thermal hysteresis for the ne = ne(T), no = no(T) and ∣Δn ∣ = ∣ Δn ∣ (T) and also shift of the reverse isotropic liquid state – cholesteric mesophase phase transition to lower temperatures have been found. Comparative estimations of temperature changes of the 〈n〉, no, ne and ∣Δn∣ with the thermo-morphologic properties of the heterophase regions of the direct cholesteric mesophase – isotropic liquid state and reverse isotropic liquid state – cholesteric mesophase phase transitions have been carried out. Results of investigations exhibit connection of temperature behavior of the optical properties and the thermo-morphologic peculiarities of the heterophase regions, and indicate that phase transition between cholesteric mesophase and isotropic liquid state is the first order.

Mesophase transition of cellulose nanocrystals aroused by the incorporation of two cellulose derivatives

Cellulose nanocrystals (CNCs) per their twisting structure and high aspect ratio and charged surface property are increasingly receiving great attention in chiral photonic crystal and pigment fabrication. However, the cholesteric mesophases of CNCs is unstable and easily destroyed by the additives with high Mw. In this work, hydroxypropyl cellulose (HPC) and carboxymethyl cellulose (CMC) are incorporated into CNCs for a continuous mesophase transition monitoring. We investigated the effects of HPC and CMC on the properties of CNCs with respect to the morphology, mesophase, rheology, and structure-color properties. Our results showed that the addition of CMC (≥ 1 wt%) prevented the formation of a continuous cholesteric phase but resulting in a fast gelation due to the strong repulsion between CMC and CNCs. Alternatively, the cholesteric phase was well-preserved in the CNC/HPC in which HPC (< 10 wt%) served as an efficient tuner of phase transition, color hue and rheology properties.

Computational method to determine the pitch length in cholesteric liquid crystals

Cholesteric liquid crystals are consisted of nematic layers, with a small rotation of the director as we pass from one to the other. This rotation produces a helical structure and the distance to perform one full rotation of the director is known as the pitch length. The pitch length characterizes the cholesteric mesophase and, for this reason, in the present article, we propose a computational method to determinate this parameter with high precision. We use the texture of the cholesteric phases obtained via polarizing light microscopy and an Object Pascal language to write our program.

Structural-induced lubricity of liquid crystal nanomaterials of cholesterol at metal friction

Experimentally, it was found that with the change in temperature there is a structural-induced increase in the lubricating capacity of liquid-crystal nanomaterials induced by transformations in the cholesteric mesophase. It is shown that in this temperature range, the minimum values of the friction coefficient practically coincide with the peak values of the dynamic viscosity, which together indicates in favor of the ordered state of the cholesterol liquid-crystal structures at these temperatures. As a result, it can be assumed that in this temperature range, spirally twisted layers of liquid-crystal cholesterol molecules with high antifriction action are formed in the friction zone. It is noted that the thickness of the friction-implemented cholesteric liquid-crystal films is sensitive to temperature changes in the contact area, changing the color and energy loss.

Theoretical Platform for Liquid-Crystalline Self-Assembly of Collagen-Based Biomaterials

The collagen triple helix is a ubiquitous biomacromolecule used in acidic aqueous solutions as precursor in the fabrication of artificial compact bone and cornea and in tissue engineering. The primary architecture of these highly structured solid tissues is formed during the cholesteric liquid-crystalline stage of their morphogenesis. The theoretical platform that describes the coupled dynamics of phase-ordering and mass transfer developed, implemented and validated here can be used for optimal material design and plays a significant complementary role to future experimental studies. Based on uniaxiality assumption, we have recently developed and validated a theory for the free energy tailored for acidic collagenous dispersions. Here we significantly expand and generalize our previous study, by including biaxiality since cholesteric phases must have a degree of biaxiality. In this work, we first modify the proposed interchain interaction and excluded-volume contribution by use of the addition theorem for spherical harmonics. Then, the Euler-Lagrange minimization followed by expansion around I/N* transition allows us to construct the free energy of ordering in terms of the phenomenological Landau–de Gennes formulation. Finally, we use the time-dependent Ginzburg-Landau equations to study the non-Fickian evolution of a single two dimensional cholesteric tactoid through a shallow quench from the isotropic to biphasic region of the phase diagram. Although equilibrium biaxiality is considerably low for these long-pitch cholesterics, we found that during self-assembly the biaxial order parameter achieves significant larger values than the equilibrium value. Additionally, the relaxed director field becomes both onion-like and defect-less, which is consistent with the twisted bipolar structure observed experimentally. The self-assembly simulations demonstrate that the formulated theoretical platform is not only consistent with previous theoretical and experimental studies but also able to be used to explore new routes for non-equilibrium collagen self-assembly. Taken together, this study deepens our understanding of cholesteric (chiral nematic N*) mesophase in acidic solutions of tropocollagen, and suggests a systematic spatio-temporal model that is capable of being used to extract the engineering principles for processing of these sought-after biomaterials.

Multi-arm ionic liquid crystals formed by cholesteric mesophase and pyridinium groups

ABSTRACTA series of novel star-shaped ionic liquid crystals (ILCs) compounds with various counterions (derived from HBF4, HPF6, CF3COOH, d-MH-SO3H, H3PO4, p-Toluenesulfonic acid) were designed and synthesised starting from precursors of pyridines and liquid crystalline monomer cholesteryl 4-bromobutanoate. Inducing various counterions by use of ionic self-assembly due to electrostatic attraction of various ion clusters was one of the most essential factors for intermolecular separation. The chemical structures, liquid crystalline properties, self-assembly behaviour and ionic conductivity of these compounds were researched during multiple experimental techniques. The star-shaped ILCs showed a smectic A (SmA) mesophase. The d-spacing of star-shaped ILCs increased slightly due to the increase volume of anion. The clearing temperatures of the pyridinium salts suggested that the effect of the stabilisation on the SmA structures was in the order H2PO4−>BF4−>TS−>D-MH-SO3−>CF3COO−>PF6−. All these star-shaped ILCs displayed ionic conductivity in mesophase. It was noted that the conductivity (σ) increased with the increase of the anion size and temperature.

Synthesis, molecular and crystal structure of spirocyclopropyl derivatives of lupane series and their ability to induce cholesteric mesophase in nematic solvents

New spirocyclopropyl derivatives of lupane series were synthesised and their spatial structure was determined by X-ray analysis. These compounds may be used as chiral dopants for cholesteric liquid-crystal compositions which are applied in the surface stabilized cholesteric texture and polymer stabilized cholesteric texture displays with low power consumption. The relationship between stereostructure of the synthesised substances and their ability to induce cholesteric mesophase in different nematic solvents were examined. The highest values of the helical twisting power |β| (213,8 ± 1,1) and (230,5 ± 4,3) mkm-1mol⋅pats-1 in the 4′-pentyl-1,1′-biphenyl-4-carbоnitrile as a nematic solvent were showed by the (1S,2S)-1-(1-phenyl-3-(4-tolyl)-1H-pyrazole-4-yl)-2,2′-spirocyclopropylallobetulone and (1S,2S)-1-(3-(4-methoxyphenyl)-1-phenyl-1H-pyrazole-4-yl)-2,2′-spirocyclopropylallobetulone correspondingly.