Achiral Bent-core Molecules Research Articles

Chiroptical Performances in Self-Assembled Hierarchical Nanosegregated Chiral Intermediate Phases Composed of Two Different Achiral Bent-Core Molecules

In this paper, chiral intermediate phases composed of two achiral molecules are fabricated by utilizing nanophase separation and molecular hierarchical self-organization. An achiral bent-core guest molecule, exhibiting a calamitic nematic and a dark conglomerate phase according to the temperature, is mixed with another achiral bent-core host molecule possessing a helical nanofilament to separate the phases between them. Two nanosegregated phases are identified, and considerable chiroptical changes, such as circular dichroism and circularly polarized luminescence, are detected at the transition temperatures between the different nanophase-separated states. The nanosegregated chiral phase-wherein the helical nanofilament and dark conglomerate phases are phase-separated-exhibits the highest chiroptical intensities. The luminescence dissymmetry factor, |glum|, in this phase is amplified by an order of magnitude compared with that of another nanosegregated phase, wherein the helical nanofilament and nematic phases are phase-separated.

Chiroptical Characteristics of Nanosegregated Phases in Binary Mixture Consisting of Achiral Bent-Core Molecule and Bent-Core Base Main-Chain Polymer.

In this paper, a binary mixture system consisting of an achiral bent-core molecule and a bent-core base main-chain polymer is described. The mixture exhibits an intriguing nanosegregated phase generated by the phase separation of the helical nanofilament B4 phase (originating from the bent-core molecule) and the dark conglomerate phase (originating from the bent-core base main-chain polymer). This nanosegregated phase was identified using polarized optical microscopy, differential scanning calorimetry, and X-ray diffraction analysis. In this nanosegregated phase, the enantiomeric domains grew to a few millimeters and a giant circular dichroism was observed. The structural chirality of the helical nanofilament B4 phase affected the conformation of the bent-core base main-chain polymer embedded within the helical nanofilament networks of bent-core molecules.

Preferential Circularly Polarized Luminescence from a Nano-Segregated Liquid Crystalline Phase Using a Polymerized Twisted Nematic Platform.

In this study, a polymerized twisted nematic (TN) network was used as an extrinsic chiral platform to overcome the heterogeneity during spontaneous symmetry breaking in a mixed system comprising an achiral bent-core molecule and rod-like mesogen. The TN platform was prepared by photopolymerizing a reactive mesogen dispersed in a low molecular weight liquid crystal with TN orientation. The use of TN orientation to correct the degeneracy in bent-core molecular systems has been previously reported; however, to the best of our knowledge, this is the first study that uses an extrinsic chiral platform of a polymerized TN network. The heterogeneity in the nano-segregated phase of the achiral mixture was suppressed using the extrinsic TN platform with a twisted angle θ of ≥ |±30°|. When an achiral mixture doped with a luminescent guest molecule was refilled into the extrinsic chiral platform, preferential deracemization with one-handedness occurred, corresponding to the handedness of the TN platform. Therefore, circularly polarized luminescence with a preferential handedness can be achieved using this extrinsic chiral platform.

Enhancement of Luminescence Dissymmetry Factor in Nano-Segregated Phase Generated by Phase Separation between Helical Nanofilaments and Liquid-Crystalline Smectic A Phase

Although several methods exist for the synthesis of circularly polarized luminescent (CPL) materials, the methods are extremely complex and tedious. In recent years, the chiral host-achiral luminescent guest method and the achiral host-achiral luminescent guest method have been employed to fabricate CPL materials; however, the main disadvantage of the latter is the small luminescence dissymmetry factor (glum) that limits the practical applications of the method. Therefore, this study reports on the enhancement of glum in a nano-segregated phase system, generated by the phase separation between helical nanofilaments (HNFs; originating from an achiral bent-core molecule) and a liquid-crystalline (LC) smectic A (SmA) phase (originating from an achiral rod-like mesogen). The observed glum value in the nano-segregated phase between the HNFs and LC SmA phase was larger than that in the nano-segregated phase between the HNFs and LC nematic (N) phase. The enhancement of the glum value was attributed to the order parameter (S) of the dye molecules in the SmA phase being larger than that in the N phase. Therefore, we concluded that the S value of the fluorescent dye molecules, doped into the embedded LC phase between the HNFs, strongly influenced the glum value.

Controlling the formation of heliconical smectic phases by molecular design of achiral bent-core molecules

A heliconical lamellar liquid crystalline phase occurs at the paraelectric–(anti)ferroelectric transition if it coincides with a change of the tilt correlation between the layers.

The rheology property of organogels based on 3D helical nanofilament bnetworks self-assembled by bent-core liquid crystals

In the B4 phase of bent-core liquid crystals, smectic layers of tilted achiral bent-core molecules are chiral and polar, which, driven by intra-layer structural mismatch, eventually twist into helical nanofilaments. We design a NOBOW/hexadecane organogel system, which is different from traditional organogel system, and the studied organogels show reversible gel-liquid transitions under temperature cycles. At high temperature, the NOBOW molecules dissolve in hexadecane and the storage modulus and viscous modulus show typical liquid characteristics. At low temperature, the mobility of NOBOW molecules decreases and the storage modulus of the organogels increases as the temperature decreases. We conduct a rheology experiment to systematically investigate the viscoelasticity of the organogel to understand the property of the organogel and develop the application in soft matter. The viscoelastic studies of the organogels reveal that the helical nanofilaments are internally strained and their 3D networks are relatively stiff, which provides an in-depth insight into the properties of the organogels and paves the way for their applications in soft matter.

Structural organization and molecular self-assembly of a new class of polar and non-polar four-ring based bent-core molecules

We have studied two new series of polar and non-polar four-ring based achiral bent-core molecules containing CN and OCH3 moiety respectively at the tip of the molecular short arm and their molecular self-assembly in crystal and liquid crystal state. Crystal structure analysis revealed the bent molecular geometry of the compounds with bent angle 151° along with fundamental intermolecular interactions such as antiparallel arrangement of the polar molecules, intermolecular hydrogen bonding, different types of π–π interactions and entangled/helical molecular arrangement in 3D space. Due to the dimeric antiparallel molecular arrangement, lower homologues of the polar compounds of series 1-n (1-8, 1-10) exhibited long range nematic phase with cybotactic clustering (Ncyb). However, the lower homologues of non-polar compounds of series 2-n (2-6, 2-8) did not form any type of clustering exhibiting normal nematic phase. Dielectric spectroscopic investigations and X-ray diffraction measurements of compound 1-8 confirmed the presence of nematic cybotacticity. Thus, the formation of clusters in nematic phase of bent-core molecules was not only due to an underlying smectic phase but also due to the combined effect of the molecular structural parameters including bent angle, molecular dipole moment and its spatial arrangement. Negative bend-splay (K33-K11) elastic constant behavior was observed in the entire nematic range.

Nematic and switchable intercalated phases in polymerizable bent-core monomers with naphthalene moiety in the side arms of the aromatic core

Liquid crystals (LCs) formed by achiral bent-core (BC) molecules associated with chirality and polar order exhibit electro-optically switchable lamellar and columnar phases, and nematic phases which could be biaxial or polar. As polymeric LCs could have application in photonics and non-linear optics, BC monomers having naphthalene moiety in the side arms and linked with terminal double bonds as polymerizable functional groups were synthesized. 6B2 with a central phenyl ring and 6N2 with a central naphthalene moiety were investigated by optical, x-ray diffraction, dielectric and electro-optic studies. Possible reasons that can account for the exhibited phase transitions will be described.

Clusters of B7 Fibers Reveal Origin of Blue Phase Stability in a Binary Mixture of Chiral Rod-like and Achiral Bent-Core Molecules.

Liquid crystal (LC) blue phases (BPs) have gained relevance because of their potential applicability as tunable photonic band gap materials. However, their narrow temperature range often restricts technical usage. Doping with an LC made of achiral bent-core (BC) molecules is one of the strategies employed to increase BP stability. It is now shown that mixing a BCLC exhibiting the polarization-modulated lamellar B7 phase, with a calamitic chiral LC made of rod-like (R) molecules, enhances the BP range considerably. The special feature in this system is the spontaneous expulsion of clusters of B7 fibers in the chiral nematic (N*) phase occurring below the BPs. This gives a clear indication that islands rich in BC molecules lie interspersed between the R molecules. Based on several experimental studies, it is shown that the BP stability may be attributed to an interplay of conformational and intrinsic chirality of the BC and R molecules across the interface of these islands. This study provides new insights from a molecular point of view and provides a novel technique for designing stable-induced BPs. The additional novelty is the occurrence of a phase transition within the fibers. Further, the electro-responsive fibers may also have a potential to form new materials.

Counterion-Induced Nanosheet-to-Nanofilament Transition of Lyotropic Bent-Core Liquid Crystals.

The smart flexibility of phase transitions in liquid crystals (LCs) makes them suitable for various applications and is an important research field in contemporary science, engineering, and technology. Unlike most reports focused on bent-core LCs in the thermotropic situation, in our present study, we designed and synthesized a fully rigid bent-core molecule with the sulfonic acid group replacing conventional flexible chains. A rich variety of counterion-induced supramolecular LC phase behaviors have been systematically investigated. It was found that the smectic phase with nanosheets tends to transform to the hexagonal phase with nanofilaments when the protons of the sulfonic acid group are partially replaced by alkali metal ions. The experimental results show that the nanoaggregate and phase transition are controlled by the displacing ratio of alkali metal ions rather than the molecular concentration. Another interesting feature is that the achiral bent-core molecules self-assemble into columns by helical stacking and present macroscopic chirality, indicating that spontaneous chiral symmetry breaking occurs in the columnar phase. The fully rigid bent-core molecules reveal surprisingly hierarchical molecular self-assemblies with the smectic-to-hexagonal phase transition, which was not previously observed in supramolecular complexes. The findings will provide new possibilities for applications in LC-based photonic devices, biosystem switches, and supramolecular actuators.

Effect of achiral halogen-substituted bent-core molecules on the blue phases

The effects of molecule structure and concentration of achiral bent-core molecules on the blue phase liquid crystal stabilization have been investigated. Blue phase mixtures are formed doping bent-core molecules and BP temperature range is able to widen doping bent-core molecules with different substituents. Bent-core molecules with different substituents are found to cause a decrement in the threshold and turn-on voltages of the bent-core doped blue phase liquid crystal samples, as a result of the difference in their molecular shape and dipole moments. Use of bent core LC as dopants for a blue phase LC mixture led to a reduction of the driving voltage by more than 50%.

Effect of terminal chain length on the helical twisting power in achiral bent-core molecules doped in a cholesteric liquid crystal.

We prepared a homologous series of achiral bent-core (BC) liquid crystals with different terminal alkoxy chain lengths, n (BC-n), and evaluated the helical twisting power (HTP) of the BC-n doped in a cholesteric liquid crystal. The BC-n molecules with longer terminal chains showed larger HTPs. To interpret this striking phenomenon, a stochastic dynamics simulation was performed to determine the distribution of the chirality order parameters (χ) for BC molecules with n = 8–16. The distribution of χ for each simulated conformation varied with n, and the variation tendency was different for molecules with n < 12 and n > 12 despite the linear relationship between HTP and n in the experiment.

Twist-bend nematic phase in cyanobiphenyls and difluoroterphenyls bimesogens

ABSTRACTThe paper reviews assignment of the low-temperature nematic phase observed in simple bimesogenic or dimeric systems based on cyanobiphenyls and difluoroterphenyls to the twist-bend nematic phase, NTB, using a range of experimental techniques. These include DSC, X-rays, Polarising Microscopy, electro-optics, birefringence and measurements of the electroclinic effect arising from flexoelectricity. An emphasis is laid on the observations of the chiral domains of opposite handedness at zero field in an otherwise achiral liquid crystalline system in this phase. These observations are a direct consequence of the structure of the twist-bend phase predicted by Ivan Dozov for achiral bent core molecules. The paper reviews the electro-optic phenomena and the observed electroclinic effect and how these observations assign it as the NTB phase. Results of the nanoscale helical pitch measurements using freeze-fracture microscopy are reviewed and discussed briefly. Results of the measurements of elastic constants especially close to the N–NTB transition are also reviewed.

Spontaneous helix formation in non-chiral bent-core liquid crystals with fast linear electro-optic effect.

Liquid crystals (LCs) represent one of the foundations of modern communication and photonic technologies. Present display technologies are based mainly on nematic LCs, which suffer from limited response time for use in active colour sequential displays and limited image grey scale. Herein we report the first observation of a spontaneously formed helix in a polar tilted smectic LC phase (SmC phase) of achiral bent-core (BC) molecules with the axis of helix lying parallel to the layer normal and a pitch much shorter than the optical wavelength. This new phase shows fast (∼30 μs) grey-scale switching due to the deformation of the helix by the electric field. Even more importantly, defect-free alignment is easily achieved for the first time for a BC mesogen, thus providing potential use in large-scale devices with fast linear and thresholdless electro-optical response.

Synchronization of conformational chirality in mixtures of achiral bent-core molecules

SEM images and frequency variation of ε′′ in PDMBC with flat layers, M50 and M20 with respectively weak and strong layer deformation.

Photomodulated Supramolecular Chirality in Achiral Photoresponsive Rodlike Compounds Nanosegregated from the Helical Nanofilaments of Achiral Bent-Core Molecules.

We prepared a nonchiral mixture of achiral bent-core molecules and photoresponsive rodlike liquid crystalline (LC) molecules. With the help of the isothermal photochemical nematic (N)-isotropic (Iso) phase transition of the photoresponsive rodlike LC molecules, the corresponding phase transition from a dark conglomerate BX phase to another distinguishable dark conglomerate B4 phase took place in the mixture. A large circular dichroism (CD) signal originating from supramolecular chirality was detected in the initial BX phase. On the other hand, the detected CD signal was decreased in the B4 phase after UV irradiation. Interestingly, the decreased CD signal could be reverted to the initial CD signal with visible irradiation. This chiroptical process revealed in this work was stable and reversible and thus opens up the possibility of practical applications such as rewritable optical storage.

Temperature-Dependent Behaviours of Blue Phase I Observed for a Bent-Core Molecular System

Temperature-dependent behaviours, such as the Bragg reflection band and the EO performance, of the liquid-crystalline BP I observed for a nematogenic achiral bent-core molecule doped with a small amount of chiral additive was investigated. Our experimental results indicate that the physical behaviours of our sample were highly sensitive to temperature changes.

Enhanced enantiotropic B2 phase by mixing homologous chlorine-substituted molecules

The phase diagram of a binary system was presented by mixing two components: an achiral five-ring bent-core molecule with monotropic B2 phase and a homologous chlorine-substituted bent-core molecule with enantiotropic nematic phase and monotropic X phase. Depending on the mixing concentrations, the enantiotropic B2 phase, B1 phase and nematic phase were induced in the mixed phase region. The mesophases occurring in this system were characterised by optical textures, X-ray diffraction and electro-optical measurements. The results indicated that the formation of the enantiotropic B2 phase occurred even when the mixing concentration reached 60 mol%. The spontaneous polarisation and switching time of the B2 phase are also discussed.

Antiferroelectric Behaviors of the SmCP Phase in Liquid-Crystalline Compounds with Achiral Bent-Core Liquid-Crystalline Molecules Containing Salicylaldimine-Based

A new series of bent-shaped mesogens has been synthesized and their mesomorphic properties studied. All compounds exhibit the B2 (SmCP) phase. Compounds with a 3,4′-biphenyldiol central core containing salicylaldimine-based and two terminal tetradecyloxy tails are reported. In addition, the effects of lateral substituent (R˭F and Cl) at the biphenyl core into 3′-position are examined. These substituents have a strong influence in reducing the clearing temperatures and increasing temperature range of the SmCP phase. The electro-optical switching characteristics have been investigated using both triangular-wave and DC electric fields. For several compounds, chirality switching from racemic to the homochiral state was seen after application of a low-frequency AC field. We studied spontaneous polarization, switching time, tilt angle, switching time, voltage-transmittance effect, and dielectric parameters (namely dielectric strength, relaxation frequency, distribution parameter) of a 7.5-µm-thick planar-oriented sample of a highly tilted anti-ferroelectric liquid-crystal material and investigated the correlation between their electro-optical properties and the chemical structures of these compounds.

Achiral Bent-Core Molecules with a Series of Linear or Branched Carbosilane Termini: Dark Conglomerate Phases, Supramolecular Chirality and Macroscopic Polar Order

New organic−inorganic hybrid materials that combine a bent π-conjugated aromatic core with one linear or branched carbosilane unit have been synthesized and investigated, with respect to their self-assembly in liquid crystalline (LC) phases, by means of polarizing microscopy, differential scanning calorimetry (DSC), X-ray diffraction (XRD), and electro-optical techniques. Most of these achiral compounds show spontaneous symmetry breaking into chiral superstructures that represent conglomerates with macroscopic domains of opposite handedness. These fluid chiral superstructures can be frozen into the glassy state and, for one of the compounds, chirality was switched under the application of a special waveform of an applied external electric field between two enantiomeric states. This flipping of supramolecular chirality occurs between oppositely tilted structures, which represents a new mode of chirality switching. Besides spontaneous chirality, these materials show polar order, leading to ferroelectric (FE...