Chiral Nematic Phase Research Articles

Enantioselective Synthesis of 3-Aryl-3-hydroxypropanoic Esters as Subunits for Chiral Liquid Crystals.

Chiral liquid crystals (LCs) with their unique optical and mechanical properties are perspective functional soft materials for fundamental science and advanced technological applications. Herein, we introduce the chiral 3-aryl-3-hydroxypropanoic ester moiety as a versatile building block for the preparation of LC compounds. Three chiral subunits differing in the aromatic part were obtained through asymmetric transfer hydrogenation using Ru(II) complexes with ee from 98% to >99%. Chiral LC compounds of diverse topologies were further prepared without deterioration of the ee during the synthesis. The mesomorphic behavior of rod-shaped, bent-shaped flexible dimeric, and polycatenar LCs is consistent with their topology─chiral nematic and smectic phases were identified as well as the rarely observed twist grain boundary A and blue phases. The utilization of synthetic chiral building blocks offers the possibility of fine tuning the intermolecular interactions by subtle changes in the molecular structure as well as the preparation of the corresponding racemic forms. This paves the way for the study of self-organization and the structure-property relationship in chiral soft materials.

Ferroelectric chiral nematic liquid crystals: new photonic materials with multiple bandgaps controllable by low electric fields

ABSTRACT We have carried out a spectroscopic study on a prototype ferroelectric nematic material (RM734) doped with a non-polar chiral compound. The mixture presents two chiral nematic phases, one conventional (N*) and the other polar (NF*), whose behaviours under electric fields E are totally different. On the one hand, the N* phase shows a single reflection band if E is not very high, while fields perpendicular to the helix of a few V/mm are already sufficient to produce multiple bands in the NF* phase. On the other hand, the pitch of the NF* phase grows notably on increasing fields, however remaining practically constant in the N* phase. Both effects have been explained in terms of the different type of interaction with E in each phase (ferroelectric in NF* and dielectric in N*). We argue that the NF* phase behaves as a photonic material with multiple gaps tunable by small fields, which presents important potentials for applications. A study of the bandgaps has been carried out through the analysis of the dispersion relation of the optical eigenmodes in the NF* phase under field. Finally, an example of the potentials of the NF* phase within the field of nonlinear optics is briefly presented.

Structure and optical properties of twist-bend nematic liquid crystals doped with chiral dopants.

Twist-bend nematic liquid crystals (N_{TB} LCs), although consisting of achiral molecules, possess a spontaneous conic helix. They have been intensively studied and utilized in many applications in recent years. Herein we add chiral molecules to N_{TB} LCs and study their effects on the structure of the conic helix. We observe that the system is in the regular chiral nematic phase at high temperature and is still in the twist-bend nematic phase at low temperature. The addition of the chiral molecules does not induce a twist of the conic helical axis. The main effect of the chiral molecules is increasing the cone angle of the conic helix. We show that the structural chirality parameters in the chiral nematic phase and the twist-bend chiral nematic phase can be calculated from the same intrinsic chirality parameter, which only depends on the molecular structure and concentrations of the chiral molecule. We also observe a pretransitional phenomenon that the helical pitch of the chiral nematic phase increases dramatically when temperature is decreased toward the chiral nematic to twist-bend nematic phase transition temperature.

Characterisation of cellulose nanocrystals by rheology and small angle X-ray scattering (SAXS)

HypothesisThe combination of rheology and SAXS measurements can be used to great effect to effectively characterise and select the type of CNCs materials for a particular application. ExperimentsTwo commercial sources of CNCs were characterised to investigate their potential applications. Polarised optical imaging, rheology and SAXS measurements were used to determine the structure and behaviour of the suspensions. We compared these to the more typical characterisation methods of TEM, AFM, DLS, conductometric titration and zeta potential. FindingsCNC1 is smaller in length and dispersity than CNC2. Though CNC1 has a higher sulfur content, the zeta potential of both CNCs is similar. CNC1 formed more highly ordered structures including the chiral nematic phase. The gel point of CNC2 was lower than CNC1 and allowed more paste-like materials to be generated. This demonstrates CNC1 to be more suited to applications where highly ordered structures are vital, such as optical materials; CNC2 can be used when viscosity control is important, such as 3D printing. This highlights the need to effectively characterise CNCs prior to use to select the most appropriate properties for the desired applications.

Cellulose liquid crystal templated TiO2 chiral nematic foams

Aerogels with chiral nematic structure are promising candidates for structural coatings and optical devices. Cellulose nanocrystals (CNCs) are excellent photonic building blocks for the fabrication of chiral nematic aerogels. Conventional bottom-up fabrication process usually is complicated, energy and time consumption. Herein, a simple method for fabricating CNCs and polyethylene glycol (PEG) based foams with chiral nematic structure was proposed. After CNCs-PEG based solid films were swollen with N-methylmorpholine N-oxide (NMMO) aqueous solutions, the obtained hydrogels were freeze-dried, resulting in chiral nematic foams. During the process, both macropores and mesopores were created with chiral nematic phase retained inside foam walls. Owning numerous macropores and chiral nematic walls of varying axial directions, the chiral nematic foams exhibited a broad-band reflection phenomenon. Moreover, the representative Titanium diisopropoxide(bis-2,4-pentanedionate) was permeated into foam walls, and transformed into chiral nematic TiO2 foams after annealing. We envisage that the chiral nematic foams may have a promising application in the fields of chiral separation or chiral catalysis.

Lyotropic isotropic to columnar phase transition in RNA solutions

ABSTRACT RNA oligomers exhibit lyotropic chiral nematic and columnar liquid crystal phases. The lyotropic chiral nematic and columnar phases with special emphasize on the isotropic to columnar phase transition in RNA solutions are described based on Landau theory. A free energy is constructed based on the order parameters to describe the chiral nematic and columnar liquid crystal phases and isotropic to columnar phase transition. The lyotropic isotropic to columnar phase transition is found to be always first order. The birefringence has been calculated in the isotropic phase of isotropic to columnar phase transition. Theoretical observation of the existence of the lyotropic isotropic to columnar phase transition in RNA solutions is confirmed with experimental results.

Interplay of self-assembly and viscoelasticity in chiral liquid crystal gels

Electro-optic and photonic devices that are robust against external stresses and deformation offer several technological advantages. A variety of device applications based on chiral liquid crystals (LCs) exhibiting blue phases (BPs), chiral nematic (N*), and ferroelectric Smectic (SmC*) phases have already been demonstrated. Chiral LC-gels are, therefore, among the best soft materials that can further extend device performance and functionality by combining electro-optic and photonic properties with mechanical stability. In view of this, a composite material composed of two highly chiral calamitic LCs and a low-molecular-weight organogelator was developed and investigated using a variety of experimental techniques. Optical microscopy, selective reflection, and x-ray diffraction studies show that the LC characteristics are very well retained in the chiral LC-gels, namely, BPIII-gel, BPII-gel, BPI-gel, N*-gel, twist grain boundary A phase (TGBA)-gel, and SmC*-gel, which form sequentially as the sample is gradually cooled from the isotropic phase. Rheological studies establish that the chiral LC-gels exhibit several features of a soft viscoelastic solid. This has been understood in terms of the mode of self-assembly of the gelator fibers and the ubiquitous defects pertinent to different LC phases. The major highlights of the study are the formation of a defect-mediated strong N*-gel with enhanced viscoelastic moduli and the induction of a TGBA-gel. The stretchable nature of a free-standing film of the BPI-gel that retains its shape over time is another interesting aspect of the present work.

Self-consistent field theory of chiral nematic worm-like chains.

Many macromolecules of biological and technological interest are both chiral and semi-flexible. DNA and collagen are good examples. Such molecules often form chiral nematic (or cholesteric) phases, as is well-documented in collagen and chitin. This work presents a method for studying cholesteric phases in the highly successful self-consistent field theory of worm-like chains, offering a new way of studying many biologically relevant molecules. The method involves an effective Hamiltonian with a chiral term inspired by the Oseen-Frank (OF) model of liquid crystals. This method is then used to examine the formation of cholesteric phases in chiral-nematic worm-like chains as a function of polymer flexibility, as well as the optimal cholesteric pitch and distribution of polymer segment orientations. Our approach not only allows for the determination of the isotropic-cholesteric transition and segment distributions, beyond what the OF model promises, but also explicitly incorporates polymer flexibility into the study of the cholesteric phase, offering a more complete understanding of the behavior of semiflexible chiral-nematic polymers.

Poly(l-Lactide) Liquid Crystals with Tailor-Made Properties Toward a Specific Nematic Mesophase Texture.

This paper presents the liquid crystal (LC) properties of poly(l-lactide) (PLLA). Mesophase behavior is investigated using polarized optical microscopy, X-ray diffraction, and differential scanning calorimetry. The performed analyses confirm that pressed PLLA films exhibit the unique capability of self-assembling into a nematic mesophase under the influence of mechanical pressure, temperature, and time. It was originally demonstrated that the chiral nematic mesophase can be obtained by introducing fine powders into the polymer. Based on the research conducted, it was proved that the pressed PLLA films have a chiral nematic mesophase with a nematic-to-isotropic phase transition and a large mesophase stability range overlapping the temperature of the human body, which can persist for years at ambient temperature. The obtained films show tailor-made properties toward a nematic mesophase with a specific texture, including colored planar texture of the chiral nematic mesophase and blue-phase (BP) LC texture. The BP, described for the first time in plain PLLA, occurred over a wider than usual temperature range of stability between isotropic and chiral nematic thermotropic phases (ΔT ≈ 9 °C), which is an advantage of the obtained polymer material, in addition to ease of preparation. This opens up new prospects for advanced photonic green applications.

Experimental and DFT Studies on Well-Defined Odd-Even Effect in Organic Liquid Crystal Tetramers Incorporating Two Chiral Centers

Introduction: A new series of organic liquid crystal tetramers containing two symmetrical mesogenic units of 4-(4’-phenoxylimino)methyl)phenol joined by methylene –(CH2)mspacer wherein m varied from 5 to 10 have been synthesized. Methods: The length for each of the mesogenic units was extended further via connection with two chiral entities of (S)-2- methylbutyl-4-(4’-bromobutyloxyphenyl)benzoate situated at both terminal ends. The structures of these tetramers were elucidated using elemental analysis and spectroscopic techniques (FT-IR and 1H-NMR) whilst the texture, and the phase transition temperatures were studied using polarizing optical microscopy (POM) along with differential scanning calorimetry (DSC). Results: All the homologues in this series are enantiotropic mesogens exhibiting oily streak texture or fan-shaped texture characteristic of the chiral nematic phase. The odd-even effect can be observed upon increasing the methylene spacer length. The structure-properties connectivity has demonstrated that the phase transition temperature for tetramers with even-numbered methylene units in the spacer is relatively higher in comparison with the odd-numbered members. Conclusion: The computational study via molecular constraint dynamics performed at the DFTB level of theory has further supported that more energy is required for the even-numbered tetramer than the odd-numbered member to undergo the structural transition from folded to extended geometries.

Heliconical-fluctuation-induced compensation point in the diluted cholesteric phase of mixtures containing the flexible dimer CB7CB.

We show experimentally and theoretically that the heliconical fluctuations that develop in a cholesteric phase (Ch) close to a transition to a chiral twist-bend nematic phase (N_{TB}) may lead to the appearance of a compensation point. At this point, the equilibrium twist of the cholesteric phase vanishes and changes sign. Mixtures of the flexible dimer CB7CB and the rodlike molecules 8CB or 5CB, doped with a small amount of the chiral molecules R811, S2011, CC, or CB15, are used in experiments to determine the conditions for the appearance of a compensation point.

Confined Crystallization in the Self-Assembled Nanostructures of Cellulose Nanocrystals and Polyethylene Glycol

Thin layers of semicrystallized polymers covered on nanoparticles significantly influence the multiple properties of their nanocomposites because of the confined crystallization of polymers. Cellulose nanocrystal (CNC) nanorods have a hydroxy-rich surface that favors the hydrogen-bonding interaction with polymers and a twisted nanostructure, which can construct a chiral nematic liquid-crystal phase scaffold for the filling of polymers. All these make CNCs’ self-assembled nanostructure a unique confined condition for polymers. Understanding the crystallization of polymers in the coassembled CNC/polymer composites presents a challenge in this regard. In this study, we focus on the confinement crystallization in the coassembled composites of CNCs and poly(ethylene glycol)(PEG). The composition-dependent changes in composite morphology and crystallization have been extensively characterized using scanning electron microscopy, Fourier transform infrared spectroscopy, synchrotronic powder X-ray diffraction, synchrotronic grazing incidence wide-angle X-ray diffraction, and differential scanning calorimetry. At low PEG content (10 wt.%), strong confinement has been identified, showing an extremely low crystallinity with tiny PEG crystallites. As the PEG reached 30 wt.%, the PEG (120) plane with the flat-on orientation of the crystal lamella appeared. As the PEG content increased continuously (40 wt.% to 50 wt.%), the intensity of the (120) plane of the PEG crystal became larger, accompanied by an increasing flat-on orientation degree. When the PEG weight percentage was up to 60 wt.%, the crystals grew randomly, losing their preferential orientation. The free growth of PEG crystals also induced a significantly decreased orientation degree of CNCs. The CNC/PEG system presents a mutual influence from two components, which enriches our understanding of the coassembly and confined crystallization of polymer composites. It will benefit the further modulation of CNC-based nanocomposites, both in their structures and properties.

Non-symmetrical bent-core homologous series bearing 1,2,4-oxadiazole core with a cholesterol terminal arm: Synthesis, characterization and their liquid crystalline properties

Nonsymmetric 1,2,4-oxadiazole derivatives with cholesteryl and alkoxy chain as end moieties exhibiting monotropic liquid crystalline behavior have been reported. The molecular structures have been confirmed by elemental analysis and spectroscopic studies. Differential scanning calorimetry (DSC) studies revealed multiple phase transitions and polarized optical microscopy (POM) investigations confirmed smectic A (SmA) and chiral nematic (N*) phases. The importance of alkyl chain length in mesophase formation is discussed. A comparison with other reported mesogenic materials possessing similar molecular structures is also discussed.

Capillary Flow Characterizations of Chiral Nematic Cellulose Nanocrystal Suspensions.

Studying the flow-induced alignment of anisotropic liquid crystalline materials is of major importance in the 3D printing of advanced architectures. However, in situ characterization and quantitative measurements of local orientations during the 3D printing process are challenging. Here, we report a microfluidic strategy integrated with polarized optical microscopy (POM) to perform the in situ characterization of the alignment of cellulose nanocrystals (CNCs) under the shear-flow condition of the 3D printer's nozzle in the direct ink writing process. To quantify the alignment, we exploited birefringence measurements under white and monochromatic light. We show that the flow-induced birefringence patterns are significantly influenced by the initial structure of the aqueous CNC suspensions. Depending on the CNC concentration and sonication treatment, various structures can form in the CNC suspensions, such as isotropic, chiral nematic (cholesteric), and nematic (gel-like) structures. In the chiral nematic phase, in particular, the shear flow in the microfluidic capillary has a distinct effect on the alignment of the CNC particles. Our experimental results, complemented by hydrodynamic simulations, reveal that at high flow rates (Er ≈ 1000), individual CNC particles align with the flow exhibiting a weak chiral structure. In contrast, at lower flow rates (Er ≈ 241), they display the double-twisted cylinder structure. Understanding the flow effect on the alignment of the chiral liquid crystal can pave the way to designing 3D printed architectures with internal chirality for advanced mechanical and smart photonic applications.

Glass-Forming Chiral Liquid Crystalline Dimers Based on an Oligo(phenylenevinylene) Unit Exhibiting Circularly Polarized Photoluminescence

Oligo(p-phenylenevinylene)-based chiral nematic liquid crystals were synthesized. Mixtures of dimeric and monomeric materials exhibited a glassy chiral nematic phase at room temperature, and the reflection band was tunable from near UV to near IR areas by the temperature and mixing ratio of the chiral nematic materials. The reflection band could be tuned to cover the emission band of the oligo(p-phenylenevinylene) chromophores completely, resulting in the circularly polarized fluorescence with the dissymmetric factor exceeding 1.4. The reflection band could be immobilized by cooling film samples of the mixtures in ice water. The dissymmetric factor of the glassy chiral nematic films exceeded 1.3 at room temperature. From the film samples of the S-form of the chiral nematic liquid crystals emitted left-handed circularly polarized light while right-handed circularly polarized light was enhanced at the edge of the reflection band due to the Purcell effect.

Liquid crystalline dimers containing a cholesteryl benzoate unit: smectic phase, chiral nematic phase and blue phase

Multiple mesophases, including the smectic A phase (Sm A), chiral nematic phase (N*), and blue phase (BP I), were seen in a series of non-symmetric bent mesogenic dimers containing a cholesteryl benzoate group.

Azobenzene-decorated cellulose nanocrystals as photo-switchable chiral solutes in nematic liquid crystals

Cellulose nanocrystals, surface-decorated with azobenzenes, are introduced as chiral solutes with molecular and morphological chirality and bestowed with the ability to photoswitch the helical pitch of induced chiral nematic liquid crystal phases.

Optimizing the performance of the near-infrared (NIR) photothermal conversion via modulating the domain size of the chiral nematic phase in co-assembled cellulose nanocrystal composite films

The self-assembled liquid crystal nanostructure of CNCs provides an effective phase-segregation for PEDOT, thus improving the NIR photothermal performance of CNCs/PEDOT:PSS/OS/TA composites significantly.

Anisotropic cellulose nanocrystal hydrogel with multi-stimuli response to temperature and mechanical stress

Conventional hydrogels with isotropic polymer networks usually lack selective response to external stimuli and that limits their applications in intelligent devices. Herein, hydrogels with distinctive anisotropic optical characteristics combined with thermosensitivity were prepared through in situ photopolymerization. Self-assembled cellulose nanocrystals (CNCs) with chiral nematic ordered structure were embedded in polyethylene glycol derivatives/polyacrylamide polymer networks. The arrangement of CNCs showed a strong dependence on the self-assembly angle and standing time, enabling the fabrication of hydrogels with customizable CNCs arrangements. Increasing the self-assembly angle from 0° to 90° changed the CNCs arrangement from chiral nematic to symmetrical nematic order which, together with CNCs dynamic arrangement from isotropic to annealed chiral nematic phase at longer standing time, provided versatile ways to produce CNCs hydrogels with tunable anisotropic properties. In addition, the obtained hydrogel displayed reversible temperature and compression response, showing excellent promise to be used as soft mechanical stress and temperature sensors or novel anti-counterfeiting materials.

Role of local intermolecular contacts in the physical properties of induced helical phases based on nematic disubstituted azobenzene

The mesomorphic, dielectric, orientational and rheological properties of the induced chiral nematic phase on the base of 4-butyl-4′-octanoyloxyazoxybenzene (C4-AB-OCOC7) doped with 1R(+)-1′,7′,7′-trimethylbicyclo[2.2.1]heptane[2′,3′-b]-2,3-dicyanopyrazine ((R + )CDCP) and (2R,3R)(−)2,3 butanediol (R-)BD were studied. The clearing temperatures and the pitch were measured, and the value of the helical twisting power were calculated. A strong influence of the dopants structure on the thermal stability of the spiral phase and the dielectric anisotropy was shown. An increase in the orientational order parameter LC upon doping with a chiral diol and the formation of its H-bond with the ester group of the terminal substituent were established by 1H NMR. Stable structures of dopant - LC solvates, their dipole moments, anisotropy of polarizability and formation energy have been established by DFT simulation. The determining influence of local dipole–dipole contacts and H-bonds on the structure and properties of solvates has been substantiated. The calculated data allowed to establish the reasons for the effect of dopants on the physical properties of the spiral phases. The effect of (2R, 3R) (-) 2,3 butanediol on the kinematic viscosity of two nematic solvents - C4-AB-OCOC7 and a binary mixture of alkoxycyanobiphenyls (CB-2) was investigated. The influence of the dopant was shown to be determined by the geometry of solvates formed due to local H-bonds with electron-donor moieties of nematic solvents.