Bent-core Mesogens Research Articles

Probing the impact of bent-core mesogen on the dielectric relaxations of ferroelectric liquid crystal

The modulation in dielectric relaxations of a conventional ferroelectric liquid crystal (FLC, SCE9) material by doping the bent-core mesogen (BCM) derived from the oxadiazole bisaniline moiety has been greatly investigated using frequency-temperature dependent dielectric spectroscopy. The application of probing ac voltage (i.e., oscillating voltage, 800 mV) has clearly shown the presence of partially unwound helical mode (p-UHM) along with Goldstone mode (GM). However, the temperature-dependent dielectric studies further demonstrate that p-UHM gets merged with GM at higher temperatures and hence becomes difficult to observe it separately. It appears that doping of BCM into FLC leads to the suppression of p-UHM at lower temperature, but with the increase in the concentration of dopant beyond a certain point, p-UHM mode reappears. Moreover, the significant decrement (∼9 °C) in SmC*-SmA* phase transition temperature of FLC with increase in the dopant BCM concentration up to 3 wt%. We believe that such studies would be certainly useful for the understanding of the plausible interaction between two different shaped mesogens (i.e., rod and bent-core) present in the mixture and could pave the way forward to the use of one type of mesogen in the modulation of the dielectric properties of other mesogen.

Synthesis of four-ring unsymmetrical bent-core mesogens and cybotactic cluster formation in their nematic phase

The synthesis and characterization of novel achiral unsymmetrical four-ring bent-core molecules resembling a hockey-stick shape are described. The present compounds bearing a methyl substituent in the central phenyl ring exhibit the skewed cybotactic nematic (NcybC), smectic C (SmC) and additional tilted smectic phase with tilted molecular arrangement. The skewed cybotactic nematic phase is formed by the bent-core molecules with long range orientational ordered clusters possessing local smectic C type order and within a cluster of tilted molecular assembly. The previously reported four-ring compounds without the methyl moiety did not exhibit nematic phase but show polarization modulated B1revtilt phase. The x-ray diffraction pattern clearly indicated the existence of SmC-type cybotactic clusters over a wide N temperature range; the layer reflection peak is weak at a higher temperature region and becomes more conspicuous with decreasing temperature. The lower temperature phase is distinctly the SmC phase, and in addition another tilted smectic phase emerges below SmC.

Competing synclinic and anticlinic interactions in smectic phases of bent-core mesogens

Presented molecules combine two features, the bent-core rigidity with the dimeric character, and reveal variety of specific phases. Longer homologues exhibit tilted smectic phases, short homologues manifest a propensity for helical structures.

Novel antiferroelectric materials with resorcinol-based symmetrical fluorinated bent-core mesogens

The synthesis of several symmetrical bent-core compounds with two or four fluorine atoms as substituents on the calamitic units is reported. The influence of fluorine substitution concerning its effect upon the mesophase structure and switching behavior was investigated. The synthesized compounds are built from resorcinol as central core and contain seven phenyl rings linked by ester, imine and azo units with terminal decyloxy/dodecyloxy chains. The mesophases were characterized by polarizing optical microscopy, differential scanning calorimetry, electro-optical investigations and X-ray diffraction (XRD) studies. The photochemical characterisation of bent-core azobenzene compounds confirmed the structure of E isomer. The temperature range of mesophases increases upon the introduction of fluorine substituents, as compared with the corresponding unsubstituted compound. All the compounds exhibit antiferroelectric properties. Depending on the number of the F substituents, racemic SmCSPA mixtures and SmCAPA homochiral structures were identified. High values of spontaneous polarization up to 900 nC/cm2 were recorded. DFT studies were used to calculate the minimum energy molecular structures, the dipole moments and the electrostatic potential surfaces of the compounds.

Substituents role in highly symmetrical 1,2,3-triazoles derivatives toward self-assembly of soft liquid crystals

Triazole bent-core mesogens based on symmetrical bent-shaped of two 1,2,3-triazole ring arms were synthesized and characterized. The the thermail stability of those new synthesised mesogens from the embient temperature to 215 °C giving mesogenic phase transitions over a wide temperature range is investigated. A study of different substituents located on the rigid bentcore was carried out for compact supramolecular innovated architectures which were produced by self-assembly of many triazoles soft crystals with a liquid crystals behaviour. The synthetic procedures, characterizations and polymorphism of four novel bent-core mesogens comprising symmetrical 1,2,3-triazole arms were demonstrated. The preparation process was carried out through‘‘click chemistry’’ via Cu-catalyzed azide-alkyne cycloaddition of nitro- and alkyl-substituted aromatic azides with aromatic compounds, comprising terminal alkyne groups and long alkyl or alkoxy side-chains. The triazole derivatives were fully characterized by 1H/13C NMR, Fourier-transform infrared spectroscopy (FTIR) and elemental analysis. The mesophases and textures were examined by employing polarizing optical microscope (POM), differential scanning calorimetry (DSC) and X-ray Diffraction (XRD). The analogues with shorter alkoxy terminal substituents displayed higher mesomorphic properties with wide mesomorphic temperature ranges compared to analogues with shorter alkoxy terminals. The morphological characteristics of the self-assembled triazoles were investigated by scanning electron microscopy (SEM) which demonstrated arrangement of highly ordered columnar architectures. These orders were driven by layer to next layer adjacent influences and π-π stacking. The effect of 1,2,3-triazole ring and length of terminal aliphatic chains on the liquid crystalline performance were discussed. Through an appropriate choice of lateral structures employing “click chemistry’’, both transition temperature and mesophases ranging from isotropic to columnar or B1/B4-like supramolecular liquid crystalline aggregates can be tuned.

Optical and flexoelectric biosensing based on a hybrid-aligned liquid crystal of anomalously small bend elastic constant

Liquid crystal (LC)-based biosensors rely on the response of the LC molecules to perturbation generated by analytes at the interface, leading to the susceptible change in molecular alignment or orientation. The sensitivity of these biosensors is primarily dependent on the LC's material properties and surface anchoring strength. By incorporation of an unconventional mesogenic compound (CB7CB) coupled with the hybrid-alignment cell configuration, this work presents a binary nematic LC for label-free biosensing, manifesting a novel sensing technology that takes advantage of CB7CB-induced flexoelectricity in the transducer. Herein, we prepared LC mixtures by blending a typical rod-like nematic LC (E7) with the bent-core mesogen CB7CB in various weight ratios and studied the effect of the CB7CB content on E7/CB7CB-based biosensing performance in vertically aligned and hybrid-aligned nematic (HAN) cells. Owing to the anomalously small bend elastic constant K33 in CB7CB, the mixture designated CB45 with the highest CB7CB weight percentage (45 wt% in this study) was best applicable to biosensing in HAN cells. When observed under a polarizing optical microscope, CB45 in the HAN geometry showed the capability of detection of as low as 10−10 g/mL for the protein standard bovine serum albumin (BSA). Moreover, the quantitation of the assay was fulfilled by both dielectric and light transmission measurements of the hybrid-aligned cholesteric CB45/R5011. The limit of detection of 7 × 10−10 g/mL was achieved by spectrometric analysis. To the best of our knowledge, this work is the first to demonstrate flexoelectric biosensing on the basis of flexoelectric polarization associated with giant flexoelectricity in CB7CB partially constituting the LC transducer.

Nanoscale Structure of Langmuir-Blodgett Film of Bent-Core Molecules.

Bent-core mesogens (BCMs) are a class of thermotropic liquid crystals featuring several unconventional properties. However, the interpretation and technological exploitation of their unique behavior have been hampered by the difficulty of controlling their anchoring at surfaces. To tackle this issue, we report the nanoscale structural characterization of BCM films prepared using the Langmuir–Blodgett technique. Even though BCMs are quite different from typical amphiphilic molecules, we demonstrate that stable molecular films form over water, which can then be transferred onto silicon substrates. The combination of Brewster angle microscopy, atomic force microscopy, and X-ray reflectivity measurements shows that the molecules, once transferred onto a solid substrate, form a bilayer structure with a bottom layer of flat molecules and an upper layer of upright molecules. These results suggest that Langmuir–Blodgett films of BCMs can provide a useful means to control the alignment of this class of liquid crystals.

Chiral emergence in multistep hierarchical assembly of achiral conjugated polymers

Intimately connected to the rule of life, chirality remains a long-time fascination in biology, chemistry, physics and materials science. Chiral structures, e.g., nucleic acid and cholesteric phase developed from chiral molecules are common in nature and synthetic soft materials. While it was recently discovered that achiral but bent-core mesogens can also form chiral helices, the assembly of chiral microstructures from achiral polymers has rarely been explored. Here, we reveal chiral emergence from achiral conjugated polymers, in which hierarchical helical structures are developed through a multistep assembly pathway. Upon increasing concentration beyond a threshold volume fraction, dispersed polymer nanofibers form lyotropic liquid crystalline (LC) mesophases with complex, chiral morphologies. Combining imaging, X-ray and spectroscopy techniques with molecular simulations, we demonstrate that this structural evolution arises from torsional polymer molecules which induce multiscale helical assembly, progressing from nano- to micron scale helical structures as the solution concentration increases. This study unveils a previously unknown complex state of matter for conjugated polymers that can pave way to a field of chiral (opto)electronics. We anticipate that hierarchical chiral helical structures can profoundly impact how conjugated polymers interact with light, transport charges, and transduce signals from biomolecular interactions and even give rise to properties unimagined before.

The aggregation of 1,3,4-thiadiazole based Hockey stick shaped Mesogen in Langmuir-Blodgett thin film in comparison to that of 1,3,4-oxadiazole

Bent Core Mesogens are remarkably interesting achiral Liquid Crystals. These grab attention for their utility. The compound, taken for investigation, is a Schiff base addition product, composed of 2-(4β-aminophenyl)-5-(4?β-butyloxyphenyl)-1,3,4- thiadiazole and 4-n-hexadecyloxy salicylaldehyde. The property of this imine derivative is in accordance with liquid crystal and is a Bent Core Mesogen (BCM). The thin layers were prepared with the help of Langmuir-Blodgett apparatus. The morphology and photo-physical characteristics of thin films were examined in comparison to similar BCM derivative of 1,3,4-Oxadiazole. The expectation was the formation of monolayer of molecules on the substrate. Practically there were layer of clusters on the substrate. Both molecules form nano clusters. The typically different aggregates by the thiadiazole moiety in comparison to oxadiazole moiety are revealed. Cluster formation is also supported by the Atomic Force Microscopic (AFM) images.

Helical phases assembled from achiral molecules: Twist-bend nematic and helical filamentary B4 phases formed by mesogenic dimers

The synthesis and characterisation of an homologous series of bent odd-membered mesogenic dimers, the 4′-(6-{4-[(E)-{[4-(alkylthio)phenyl]imino}-methyl]phenoxy}hexyl)[1,1′-biphenyl]-4-carbonitriles (CB6O.Sm) is reported. This general class of materials, despite being achiral, has a strong tendency to form helical structures, and here, for the first time we report three such chiral phases in a single homologous series. Specifically, the heliconical twist-bend nematic (NTB) phase for short terminal thioalkyl chains, and its smectic equivalent - the twist-bend smectic C (SmCTB) phase for longer chains. All the dimers showed the helical filament B4 phase, which is typically seen for rigid bent-core mesogens, but has only rarely been reported for flexible dimeric molecules. In addition, on increasing chain length, smectic behaviour emerges including the smectic A and the smectic CTB phases. We also show that these materials have the potential for their morphology to be controlled through surface interactions. The presence of the little-studied thio-linkage in the terminal chain, and the wide range of properties present in this single group of homologues, with promise of broad applicability in optics, photonics, as well as fundamental significance as a case study to achieve a better understanding of chirality and symmetry breaking in liquid crystals, ensures the importance of this new series of mesogens.

Synthesis and characterization of α-cyanostilbene-based bent-core hexacatenar mesogens with different central groups

Two series of bent-core hexacatenars containing a diphenyl ketone/diphenylmethylene malononitrile as central group and two α-cyanostilbene units as molecular arm were synthesized by Suzuki coupling and Knoevenagel reactions. The effect of the central groups on the liquid crystalline and photophysical behaviors was investigated in detail. POM, DSC and XRD results indicated that all the compounds could self-assemble into hexagonal columnar mesophases and diphenylmethylene malononitrile-based hexacatenars show much wider range of mesophase. The introduction of malononitrile unit into aromatic core could induce more distinct solvatochromic effect. Both series of hexacatenars show aggregation-induced emission enhancement (AIEE) behaviors and the fluorescence intensity is affected by the size of spherical aggregates in the aggregated state. Additionally, the maximum emission bands, fluorescence intensity and fluorescence color in condensed state showed obvious transition upon rising temperature.

From Bend to Splay Dominated Elasticity in Nematics

In the past decade, much evidence has been provided for an unusually low cost for bend deformations in the nematic phase of bent-core mesogens and bimesogens (liquid crystal dimers) having a bent shape on average. Recently, an analogous effect was observed for the splay mode of bent-core mesogens with an acute apical angle. Here, we present a systematic computational investigation of the Frank elastic constants of nematics made of V-shaped particles, with bend angles ranging from acute to obtuse. We show that by tuning this angle, the elastic behavior switches from bend dominated (K33>K11) to splay dominated (K11>K33), with anomalously low values of the splay and the bend constant, respectively. This is related to a change in the shape polarity of particles, which is associated with the emergence of polar order, longitudinal for splay and transversal for bend deformations. Crucial to this study is the use of a recently developed microscopic elastic theory, able to account for the interplay of mesogen morphology and director deformations.

Extraordinary magnetic field effects on the LC phases of homochiral and racemic 4-cyanoresorcinol-based diamagnetic bent-core mesogens

A huge effect of a magnetic field on the layer spacing of smectic and polar smectic liquid crystalline phases of a diamagnetic bent-core mesogen is reported, which is opposite for the uniformly chiral compound and its racemic mixture.

The unique asymmetric nano-cluster formation by the uneven hockey-stick-shaped mesogen based on 1,3,4-oxadiazole in Langmuir–Blodgett thin film

Achiral Bent Core Mesogens (BCMs) are an exciting class of potentially useful material that catch special attention in the last decade. The newly synthetic compound 5-hexadecyloxy-2-[(E)-({4-[5-(4-octyloxyphenyl)-1,3,4-oxadiazol-2-yl] phenyl}imino)methyl]phenol, named as ABC, is a Schiff base derivative. Having both flexible hydrophobic and rigid hydrophilic part it shows a surface-active liquid crystalline property. This study is based on the Langmuir–Blodgett (LB) thin film fabricated with this molecule. The spectral study, showing intense bathochromic shift of characteristic peaks, reveals that the J-aggregation of molecules in the film in comparison to its solution. Atomic force microscopic (AFM) and scanning electron microscopic (SEM) images reveal the occurrence of unique unsymmetrical nano-size cluster formation in pure ABC thin film as well as mixed thin film with Behenic acid (BA) as supporting matrix.

Influences of Central Units and Terminal Chains on the Banana-Shaped Liquid Crystals

Azo-functionalized materials are one of the appealing groups of the functionalized materials owing to their photoswitching behaviour and have been explored for various potential applications viz., optical data storage, sensor, display devices, nonlinear materials and molecular switches. Recently, azo-functionalized bent-core liquid crystals (BCLCs) have gained significant attention because they have dual properties of BCLCs and azobenzene, which enables to generate new multifaceted functional and smart materials. In this report, the recently synthesized azobenzene containing bent-core mesogens and its subclass, the so-called hockey stick and V-shaped molecules are summarized. The mesomorphic behaviour of reported BCLCs affected by the type of central core unit, the nature, number and position of the lateral substituents and the type and length of the terminal chain are discussed. The photoisomerization process of these photoresponsive BCLCs in solid, solution and mesophase, as well as the impact of light on the chemical and electrical properties of them, are discussed.

Nanostructure of Unconventional Liquid Crystals Investigated by Synchrotron Radiation.

The macroscopic properties of novel liquid crystal (LC) systems—LCs with unconventional molecular structure as well as conventional LCs in unconventional geometries—directly descend from their mesoscopic structural organization. While X-ray diffraction (XRD) is an obvious choice to investigate their nanoscale structure, conventional diffractometry is often hampered by experimental difficulties: the low scattering power and short-range positional order of the materials, resulting in weak and diffuse diffraction features; the need to perform measurements in challenging conditions, e.g., under magnetic and/or electric fields, on thin films, or at high temperatures; and the necessity to probe micron-sized volumes to tell the local structural properties from their macroscopic average. Synchrotron XRD allows these problems to be circumvented thanks to the superior diffraction capabilities (brilliance, q-range, energy and space resolution) and advanced sample environment available at synchrotron beamlines. Here, we highlight the potentiality of synchrotron XRD in the field of LCs by reviewing a selection of experiments on three unconventional LC systems: the potentially biaxial and polar nematic phase of bent-core mesogens; the very high-temperature nematic phase of all-aromatic LCs; and polymer-dispersed liquid crystals. In all these cases, synchrotron XRD unveils subtle nanostructural features that are reflected into macroscopic properties of great interest from both fundamental and technological points of view.

The synthesis, mesomorphic and dielectric investigation of new unsymmetrical bent-core mesogens derived from 3-hydroxybenzoic acid

The synthesis, mesomorphic and dielectric investigation of new unsymmetrical bent-core mesogens derived from 3-hydroxybenzoic acid central core with linked two rodlike units carrying (S)-3,7-dimethyloctyloxy group and an n-alkyloxy chain at both terminals are presented. The liquid crystalline properties of the new unsymmetrical bent-core materials have been investigated by polarizing optical microscopy and differential scanning calorimetry. New compounds with the reversed ester linking unit exhibit a monotropic or enantiotropic columnar mesophase at lower temperatures as compared to analogs with a straight chain depending on the presence of the branched (S)-3,7-dimethyloctyloxy terminal chain. Dielectric measurements for one of the bent-core mesogens, 3-[[4-[[4-(octyloxy)benzoyl]oxy]benzoyl]oxy]benzoic acid 4-[[4-((S)-3,7-dimethyloctyloxy)phenoxy]carbonyl]phenyl ester (OBDPE), have been carried out on the angular frequency range from 25.12 to 50265k rad/s at different temperatures. Thanks to dielectric measurements, real and imaginary dielectric constant, conductivity mechanism and dielectric relaxation mechanism of OBDPE were obtained.

Study on synthesis and investigation of supramolecular cuperic (Cu(II)) metallomesogens with Smectic C phase

Functional Bent-core 2D and 3 D copper (II) mesogens with polarizing ability were intended to be synthesized. The bent-core mesogens contain molecular plane directed perpendicular to that of the polarization vector plane. This perpendicular orientation of the planes results in the formation of a clear broken fan phase of the Smectic C phase. Thereby, the functional supramolecular methacrylic Cu (II) metallomesogen with the Smectic phase was synthesized by connecting aromatic two-ring systems with the perpendicular orientational plane. The mesogen designing was fabricated through the ester linkage with the azomethine group by treating 2,4 di hydroxybenzaldehyde with octadecyl amine to obtain [N-4-(1-Bromo-4-octyloxybenzoate) octadecyl imine] Schiff base. The mesogenic (N-4-(methacryloyloxy(4-octyloxybenzoate) octadecylimine)) monomer was further polymerized by free radical polymerization (often referred to as active mesogens) with AIBN as the initiator. The supramolecular Poly (N-4-(4-octyloxybenzoate) octadecylimine)) methacrylate mesogen was further converted to an active metallomesogen by treating methacrylic mesogen with Cu (II)acetate in the presence of ethanolic solution. The bent-core alignment of mesogens reinforces a strong driving force for the macromolecular arrangement of metal-polymer bonding to form supramolecular metallomesogens. The acrylate metallomesogens show extensive thermal stability and can be used in ultra-thin electronic devices. The synthesized supramolecular Cu (II) mesogen compound [Cu(PMA-L1) and Cu(PMA-L2)] was extensively analyzed for the monotropic Smectic C (Sinclinic and anticlinic) transition from an isotropic state by polarized optical microscopy (POM), XRD diffractogram and DSC. The Tetrahedral geometry of the Cu (II) metallomesogens was further confirmed by EPR.

Influence of alkyl and alkoxy groups on photoresponsive behaviour of bent-core azo mesogens: Synthesis, mesomorphic and photoswitching properties

Abstract Two homologous series of 3-hydroxy benzoic acid based bent-core mesogens with an azobenzene wing bearing either alkoxy or alkyl terminal chains are synthesised and their liquid crystalline and photoswitching properties are investigated. The synthesised compounds are characterized using IR, 1H NMR, elemental analysis (EA) and UV–Vis spectroscopy. The liquid crystalline properties of these compounds are studied by using polarizing optical microscopy (POM) and differential scanning calorimetry (DSC). They are found to exhibit B1, B2 and an unidentified mesophase, M. Photoswitching studies suggest that the compounds with alkoxy chain have excellent thermal back relaxation as compared to those having alkyl chains. Therefore, a representative compound from alkoxy series is used to fabricate an optical storage device and tested for the efficiency. The results obtained provide a strong evidence for the structure-property relationship in such bent-core mesogens indicating that even a small change in the chemical structure such as, changing the alkyl to alkoxy chain or even the chain length can influence the light induced state of the molecules, the property that is used for creation of optical storage devices.

Comparative 2H NMR and X-Ray Diffraction Investigation of a Bent-Core Liquid Crystal Showing a Nematic Phase

Bent-core liquid crystals showing a nematic phase stable at low temperatures are very attractive for applicative purposes in view of the inherent biaxial nature of the nematic phase. In this work, a typical five-ring bent-core mesogen was investigated by means of 2H NMR spectroscopy and X-ray diffraction (XRD) methods. These techniques provide complementary information on the structural properties of the nematic phase and the average mesogen conformation: small-angle XRD reveals the presence of short-range positional order in the form of skewed cybotaxis, while a comparison of the orientational order parameters measured by wide-angle XRD and NMR provides an estimate of the molecule bend angle. In addition, 2H NMR puts in evidence the occurrence of an unexpected transition to a low-temperature tilted phase, having a crystalline or smectic-like character. The results were compared with those of previous 13C NMR investigations.