Mesogenic Core Research Articles

Characterization and comparability study of a series of novel fluorinated polyacrylates with a rigid cyclohexane mesogenic core

Surface reconstruction has always been a pain point in the study of short-chain perfluoroalkyl acrylate polymers, and how to maintain the balance between environmental protection and good performance is the key issue. We exploited the high efficiency and selectivity of the reaction between the isophorone di-isocyanate group and the hydroxyl group, and the rigid cyclohexane structure contained in their molecule to efficiently prepare a series of novel fluoro acrylate monomers (IFAM) containing rigid cyclohexane as mesogenic core and their homopolymers (PIFAM) with different lengths of short-chain perfluoroalkyl side chains. Their structures, liquid crystal behavior, thermal properties, surface chemical compositions, surface morphologies, and hydrophobicity were characterized and comparatively studied by FTIR, NMR, XRD, POM, DSC, TGA, XPS, AFM, and static/dynamic contact angle tester and dodecafluoroheptyl methacrylate (DFMA) and its homopolymer as a contrast. The results demonstrated that both the synthesized monomers and homopolymers possessed the expected chemical structures with thermotropic liquid crystalline behavior. Furthermore, the thermal stability and hydrophobicity of the polymers increased with the extension of the fluorocarbon chain length. The incorporation of the mesocrystalline motif and appropriate length of fluoroalkyl tails in the PIFAM polymers would enhance hydrophobic stability, as it promoted the crystallinity of the side chains and limited the structural rearrangement of the fluorocarbon chains. This study offers a straightforward method for synthesizing fluoro acrylate polymers with stable surface properties, highlighting their potential application in hydrophobic coatings.

Tunable Phase Structure of Side-chain Liquid Crystalline Polymers Enabled by Molecular Engineering of Dual Mesogenic Cores

Tunable Phase Structure of Side-chain Liquid Crystalline Polymers Enabled by Molecular Engineering of Dual Mesogenic Cores

Formation of Layered Liquid Crystal by Cooperation of Sidechain Array and Rigid Polyacetylene Mainchain

ABSTRACT An acetylene derivative monomer with a pyrimidine – type sidechain with three rings was synthesized by Steglich esterification. Polymerization of the acetylene derivative was then carried out with a rhodium complex. The acetylene monomer and mono-substituted polyacetylene derivative showed liquid crystals (LCs) because their tricyclic parts worked as a mesogenic core. The monomer as an acetylene derivative showed smectic with a tilted structure in the LC aggregation form. The mono-substituted polyacetylene bearing mesogen via short alkyl spacer showed lyotropic and thermotropic LCs of the smectic phase, although generally flexible long alkyl spacer from the main chain connected to sidechain mesogen is required for exhibiting liquid crystal in sidechain type polymer liquid crystals. In the present study, rigid mainchain aligns the sidechain mesogen array to form non-tilted LC. Properties of LCs were evaluated by various analytical techniques, including polarizing optical microscopy. Structural properties were evaluated by nuclear magnetic resonance spectroscopy, gel permeation chromatography, infrared spectroscopy, and ultraviolet – visible absorption spectroscopy.

Biphenyl Benzoate Derived Polymorphic Self-Assembling Chalcones: Influence of Mesogenic Core Variations And Terminal Alkyl Chains

The construction of multifunctional molecular self-assembly is one of the fascinating areas of current research. Herein we reported a series of biphenyl benzoate derived mesogenic material EBPCH-(2–16), containing ester-imine and chalcone as connecting units. Compounds were substituted by decyloxy chain at one end while the other end has been systematically varied from C2-C16. By using various spectroscopic techniques, and elemental analysis, the structures of the biphenyl benzoates were determined; the results are consistent with the suggested structure. The mesomorphism of new molecules has been studied by differential scanning calorimetry and polarizing optical microscopy. Initial members of the series up to C8 exhibited enantiotropic Smectic-A (SmA) as well as nematic mesophase with good thermal stability, whereas compounds above C10 up to C16 revealed only smectogenic behaviour with enantiotropic SmA as well as SmC mesophase. Apart from these findings, additional variables like the alkyloxy groups in the molecules and the polarity and polarizability of the linking units may also have an impact on these mesogens. Further, the structure–property relationship was carried out by thorough investigations and comparison with structurally related literature on the target compounds’ mesomorphism. In addition, Density functional theory simulations verify the potential conformer of biphenyl benzoates, whereas studies conducted by frontier molecular orbitals and molecular electrostatic potential delve into the molecular surroundings and quantum chemical behaviour of these compounds.

Biphenyl based schiff base derivatives of 6-aminocoumarin: Design, synthesis, mesomorphic properties and DFT studies

The study focused on synthesizing two series of coumarin Schiff base liquid crystals. These series involved molecules with variations in alkoxy chains at aldehyde ends: one being the Schiff base with 4′‑hydroxy-[1,1′]-biphenyl-4-carbaldehyde (ASBP series) and the other being alkyl 4-formyl-[1,1′]-biphenyl-4-carboxylate (AEBP series). The compounds were characterized using FTIR, NMR, and Mass spectral analyses. All the compounds were studied for their mesomorphic properties, differential scanning calorimetry (DSC) and polarized optical microscopy (POM). In both the homologous series, variation of alkyl chain length showed intriguing phase transitions. In the first homologous series ASBP, compound with hexyloxy chain displayed an enantiotropic nematic (N) phase, while those with chain lengths n = 8–16 exhibited enantiotropic smectic A (SmA) mesophase. Similarly, in the AEBP series, compounds with n = 6,8 chain length showed an enantiotropic nematic (N) phase, while those with higher chain lengths n = 10–16 exhibited enantiotropic smectic A (SmA) phases. SmA mesophase was further confirmed by powdered XRD analysis. The study included DFT theoretical calculations to comprehend the compounds' mesomorphic behavior, comparing them with analogous reported compounds. Additionally, the investigation correlated experimental findings of individual compounds with calculated parameters like polarizability, dipole moment, and aspect ratio. Molecular electrostatic potential projections and analysis of frontier molecular orbitals were also employed to elucidate how polarity variations in terminal chains and mesogenic cores influenced the energy gap of FMOs and the distribution of electrostatic charges on these compounds. It has been found that the ASBP series has lower energy gap and higher dipole moment than the AEBP series.

Design of ionic liquid crystals enabled by [2]rotaxane structure formation

A new molecular design approach for liquid crystals is demonstrated by utilizing [2]rotaxane structure formation integrating a molecular axle as a mesogen core and a molecular ring as flexible tails.

Effect of the linking bridge type on the self-assembly behaviour of 2',3'-difluoroterphenyl derivatives.

The self-assembly behaviour and the crystallization kinetics of two liquid crystal compounds containing 2',3'-difluorosubstituted terphenyl as the mesogenic core have been described. Calorimetric studies show that the linking bridge type affects the polymorphism of smectic phases. The CH2O.3 compound with the -CH2O- linking bridge connected with a longer methylene spacer possesses the chiral smectic phase with antiferroelectric properties (SmCA* phase), while the COO.6 liquid crystal with the -COO- linking bridge connected with a shorter alkyl chain forms the chiral smectic phase with ferroelectric properties (SmC* phase). Both compounds crystallize upon slow cooling, while fast cooling causes the vitrification of the conformationally disordered crystal phase. Dielectric measurements reveal the complex relaxation dynamics in the identified thermodynamic states. DFT calculations allow us to estimate the nature of relaxation processes.

Observation of ferroelectric behaviour in non-symmetrical cholesterol-based bent-shaped dimers.

Non-symmetrical cholesterol-based dimers have emerged as crucial materials in the field of liquid crystal research, owing to their remarkable ability to stabilize various exotic mesophases, including the blue phases (BPIII, BPII, BPI), cholesteric nematic (N*) phase, smectic blue phase (SmBP), twist grain boundary (TGB) phase, smectic A/smectic A* (SmA/SmA*) phase, and smectic C/smectic C* (SmC/SmC*) phase. These mesophases have garnered considerable attention due to their diverse applications in spatial light modulation, chiro-optical devices, optical switching, thermochromic materials, and more. In this study, we present the synthesis and comprehensive characterization of a series of non-symmetrical cholesterol-based bent-shaped dimers (1/12, 1/14, 1/16) in which the cholesterol unit is intricately linked to an aromatic mesogenic core through a flexible spacer. These novel materials exhibit the intriguing ability to stabilize a variety of mesophases, including the N*, TGBA, SmA, and SmC* phases. The chiro-optical properties of the helical SmC* phase have been meticulously investigated through temperature-dependent chiro-optical measurements, shedding light on their potential for advanced optoelectronic applications. Additionally, we have conducted a thorough examination of the physical characteristics of these cholesterol-based dimers, including static permittivity measurements, dielectric spectroscopy, and electro-optical performance analysis. Remarkably, two homologues (1/14, 1/16) exhibit negative dielectric anisotropy, a crucial parameter for liquid crystal devices. Furthermore, our investigation reveals that these materials exhibit ferroelectric behaviour in the SmC* phase, with compounds 1/14 and 1/16 demonstrating substantial spontaneous polarization (PS) values of approximately 132 nC cm-2 and 149 nC cm-2, respectively. These findings underscore the potential of non-symmetrical cholesterol-based dimers as versatile components for the development of innovative electro-optical devices.

Design and rheological behaviour of lactic acid based cholesteric liquid crystalline materials with hydroquinone unit in the molecular core

In order to contribute to molecular structure – physical property relationship for chiral self-assembling materials, two chiral compounds derived from the lactic acid have been designed. Mesogenic core of materials consists of two benzoate units and one hydroquinone unit connected by the ester linkage groups and two flexible alkyl chains. Different lengths of the chiral alkyl chain were tested, while the length of the achiral alkyl was kept same for all materials. The mesomorphic properties were established by polarizing optical microscopy and differential scanning calorimetry. Both materials possess reasonably broad and stable cholesteric (N*) phase down to room temperature. The rheological characterisation was performed in the N* phase on the compound with shorter chiral alkyl chain using a rotational rheometer in a plate/plate geometry with and without external electric field applied perpendicularly to the flow direction. The results reveal a shear thinning behaviour, even without electric field applied. The N* phase exhibits a slight electrorheological effect, at low shear rates, that continuously decrease with the increase of the shear rate, due to the competition of the flow and electric fields. The mesomorphic behaviour of the designed materials was compared to structurally similar materials. Quite broad temperature range of the N* phase down to room temperature makes studied lactic acid derivatives as suitable chiral dopants for design of new multicomponent mixtures targeted for various applications in photonics and optoelectronics.

Synthesis and performance of novel fluoro acrylate polymer containing rigid cyclohexane group as a mesogenic core

Fluorine-containing liquid crystal polymers have excellent hydrophobic stability, making them attractive for use in the field of stable hydrophobic coatings. To improve the hydrophobic stability of fluoropolymers, a novel fluoro acrylate monomer (FAM) containing a rigid cyclohexane structure was simply synthesized using isophorone diisocyanate (IPDI), hydroxyethyl methacrylate (HEMA), and perfluorohexylethanol (S104) as raw materials and its homopolymer (PFAM) was prepared by radical solution polymerization. The structure, liquid crystal behavior, thermal properties, hydrophobic properties, and surface properties of FAM or PFAM and a reference substance, dodecafluoroheptyl methacrylate (DFMA) and its homopolymer (PDFMA), were studied using various techniques including FT-IR, NMR, GPC, XRD, POM, DSC, TGA, XPS, AFM, and static/dynamic contact angle measurements. The results indicate that FAM and PFAM are thermally liquid crystalline. PFAM's thermal stability is higher than that of PDFMA. The presence of a rigid cyclohexane structure facilitates the dense distribution of fluorocarbon chain tails of the homopolymer and also prevents surface reconstruction when the homopolymer comes into contact with polar liquids, resulting in improved hydrophobicity and hydrophobic stability. Thus, this study provides a straightforward method for the synthesis of fluoro acrylates with surface stability, highlighting their potential application in hydrophobic coatings.

Amphotropic liquid crystal polyacetylene derivative bearing pyrimidine type mesogen

An amphotropic liquid crystal polyacetylene derivative with pyrimidine–type two-ringed mesogenic core, exhibiting both thermotropic and lyotropic liquid crystallinity, was synthesized in this study. The polyacetylene derivative showed a lyotropic liquid crystal in d-limonene. The cis configuration of the main chain was confirmed via 1H NMR. Heat treatment induced cis–trans isomerization of the as-prepared polymer. Gas-phase iodine doping of the polymer initiated a chemical interaction between the polyene and iodine, acting as an electron acceptor, generating charge carriers (radical cations, solitons) detected with the electron spin resonance spectroscopy. Moreover, gas phase doping of the polymer induced cis–trans isomerization.

A hybrid ionic liquid crystal comprising imidazolium surfactant-graphene oxide

ABSTRACT To produce graphene oxide liquid crystal composites with better controllability, imidazolium surfactant with biphenyl mesogenic core (CBphCM) has been synthesised and used to modify the negatively charged surface of graphene oxide (GO) to obtain a hybrid ionic liquid crystal (CBphCM-GO). The process of GO and CBphCM recombination was monitored by various experimental techniques such as FT-IR, Uv-vis, Raman and XPS. The experimental results indicated that the C=O bonds on GO sheets are all reduced and C−O bonds are partially reduced due to the imidazolium units being electrically active and nitrogen-rich, and the presence of the N−O bonds indicates that a new hybrid is formed between CBphCM and GO. The recombination destroys the electrostatic repulsion balance among the GO lattices, and the CBphCM-GO is observed a fluffy appearance by TEM. The liquid crystal properties of CBphCM and CBphCM-GO were investigated by DSC, polarised optical microscopy (POM) and variable temperature X-ray diffraction, respectively. As an ionic liquid crystal, CBphCM has SmA as well as the rare SmC phases exhibits de Vries-like behaviour with a layer shrinkage. The liquid crystal properties of CBphCM-GO are derived from CBphCM, and the layer shrinkage behaviour from SmA to SmC phase has also been observed.

Three Distinct Polar Phases, Isotropic, Nematic, and Smectic-A Phases, Formed from a Fluoro-Substituted Dimeric Molecule with Large Dipole Moment.

A dimeric molecule, di-5(3FM-C4T), with fluoro-substituted mesogenic cores composed of three-aromatic rings and linked by a pentamethylene spacer is prepared. Di-5(3FM-C4T) forms the ferroelectric nematic (NF), ferroelectric smectic-A (SmAPF), and polar isotropic (IsoP) phases. The NF phase is composed of molecules in U-shaped conformation that behave like polar rod-like molecules. The reversal spontaneous polarization (Ps) is approximately 8 μC cm-2, which is extremely large and reflects the huge dipole moment (11.2 D) of the one-side mesogenic core. On the other hand, the SmAPF phase is formed by bent-shaped molecules. The NF-SmAPF phase transition thus follows the conformational change of molecules. The reversal Ps of the SmAPF phase is around 4 μC cm-2, which is half of that in the NF phase, and this is an expected value from the bent shape of the molecules. It is interesting that the highest temperature IsoP phase still exhibits the polar structure and possibly retains some polar aggregation of molecules in small domains. The three polar phases exhibit the dielectric mode due to the collective polarization fluctuation at around 100 Hz, giving the high dielectric constant over 8000.

Synthesis and Mesomorphic Properties of New Non-symmetric Liquid Crystal Dimers

Abstract: New non-symmetric liquid crystal dimers, N-(4-(n-(4-(benzothiazol-2-yl)phenoxy) alkyloxy) benzylidene)-2,4-difluoroaniline (DSFCn), containing two mesogenic core units of benzothiazole and benzylideneaniline were reported. The core system was connected by a flexible alkyl spacer, -(CH2)n-(where n =2-6, 8, 10, and 12). The spectroscopic techniques were employed to confirm the molecular structure of the title compounds. Differential scanning calorimetry and polarizing optical microscopy were used to study the mesomorphic properties of the synthesized compounds. Observation under crossed polarized light reveals that compounds (n = 4, 5, 6, 8, 10, 12) are purely nematogens. The nematic phase in the dimers are caused by the presence of the lateral fluorine atoms at the benzylideneaniline core. Mesophase thermal stability has been suppressed significantly by the fluorine atom at the lateral position which increased the molecular breadth and subsequently, results in weaker overall lateral intermolecular attraction. The structure-property relationships of the dimers were inferred through comparison of thermal properties of the present dimers with those of the earlier reported analogous that do not possess any lateral substituents.

Imine based Four-Ring Chalcone-Ester liquid Crystals: Synthesis, Characterization, mesomorphic behaviour and DFT approach

In this study, new imine-based chalcone-ester liquid crystals with symmetric and unsymmetrical alkoxy chains were synthesized and their mesomorphic behaviour was examined. The FT-IR, 1H NMR, 13C NMR, and mass spectral analysis have all been used to characterize new mesogens. From the study of mesophase behaviour, the compounds of CHE-(6–18) and 12-CHE-(6–18) series with different alkoxy chains at both the ends exhibited stable enantiotropic smectic C in addition to nematic mesophase for compounds with longer alkoxy chains. Whereas the three compounds of CHAE series were found to be non mesogenic. By comparing new compounds with structurally similar compounds, the impact of inclusion/replacement of various mesogenic units have been discussed. The theoretical calculations were performed to illustrate the mesomorphic behaviour of the studied compounds in terms of aromaticity and π–π stacking. Also, the experimental findings of the individual compounds were correlated with the calculated polarizability, dipole moment, and aspect ratio. Moreover, a projection of the molecular electrostatic potential and the frontier molecular orbitals has been considered to demonstrate the influence of the polarity of the terminal chains and the mesogenic cores on the energy gap of the FMOs and the distribution of electrostatic charges on the investigated compounds.

Two mechanisms for the formation of the ferronematic phase studied by dielectric spectroscopy

A non-chiral ferroelectric nematic compound with a 1,3-dioxane unit in the mesogenic core called 2,3′,4′,5′-tetrafluoro-[1,1′-biphenyl]-4-yl 2,6-difluoro-4-(5-propyl-1,3-dioxan-2-yl) benzoate (DIO) was studied by dielectric spectroscopy in the frequency range 0.1 Hz–10 MHz over a wide range of temperatures. The compound exhibits three nematic phases on cooling from the isotropic phase, i.e., the ordinary paraelectric nematic N; the intermediate nematic NX and the ferroelectric NF phase. The least frequency process is due to the dynamics of ions. The middle frequency relaxation process P1 is like as observed in other ferronematic compounds and this mode is a continuation of the molecular flip-flop motion in the isotropic phase to the collective dynamics of dipoles which are strongly coupled with the splay fluctuations in nematic phases. In addition to this process, DIO shows an additional collective relaxation process P2 at higher frequencies both in the N and the NX phases. This mode originates from the polar/chiral domains of the opposite chirality, these arise from the spontaneous symmetry breaking of achiral mesogens in the N phase. Both collective processes, P1 and P2, show soft mode-like characteristic behavior on cooling from the N to the NX-NF phase transition temperature and are shown to contribute independently to the formation of the ferronematic NF phase.

Synthesis, phase transition behaviour and dielectric properties of smectogenic palm-based liquid crystals containing schiff base ester and phenyl ring

Smectogenic liquid crystals (LCs) play an important role in various technological applications due to their interesting morphologies and defects. With incessant effort to add value to palm oil, another two series of palm-based liquid crystals (PBLCs), namely PB1a-PB3a and PB1b-PB3b, utilizing palm fatty acids as flexible alkyl chain have been synthesized. The appearance of smectic A phase for all PBLCs was authenticated by Polarizing Optical Microscopy (POM) and further verified by Differential Scanning Calorimetry (DSC) and Small Angle X-ray Scattering (SAXS). Results revealed that the mesophase range of all synthesized LCs is highly dependent on flexible alkyl chain length, the presence of polar terminal substituent as well as the breath to width ratio of mesogenic core. Dielectric measurements show that the dielectric strength is also affected by the length of the flexible alkyl chain. It is worth noting that, as far as mesophase range is concerned, PBLCs were found to be far superior over other structurally related compounds that can be found in the literature.

Phase-controllable topochemical polymerization of liquid crystalline epoxy according to spacer length

A new liquid crystal epoxies (LCEs) system based on the twin mesogenic core and imidazole curing agent is investigated. The LCEs records high thermal conductivity (0.49 W m−1 K−1) which are highly affected by microscopic orientation.

Ion Transport in 2D Nanostructured π-Conjugated Thieno[3,2-b]thiophene-Based Liquid Crystal.

Leveraging the self-assembling behavior of liquid crystals designed for controlling ion transport is of both fundamental and technological significance. Here, we have designed and prepared a liquid crystal that contains 2,5-bis(thien-2-yl)thieno[3,2-b]thiophene (BTTT) as mesogenic core and conjugated segment and symmetric tetra(ethylene oxide) (EO4) as polar side chains for ion-conducting regions. Driven by the crystallization of the BTTT cores, BTTT/dEO4 exhibits well-ordered smectic phases below 71.5 °C as confirmed by differential scanning calorimetry, polarized optical microscopy, temperature-dependent wide-angle X-ray scattering, and grazing incidence wide-angle X-ray scattering (GIWAXS). We adopted a combination of experimental GIWAXS and molecular dynamics (MD) simulations to better understand the molecular packing of BTTT/dEO4 films, particularly when loaded with the ion-conducting salt lithium bis(trifluoromethanesulfonyl)imide (LiTFSI). Ionic conduction of BTTT/dEO4 is realized by the addition of LiTFSI, with the material able to maintain smectic phases up to r = [Li+]/[EO] = 0.1. The highest ionic conductivity of 8 × 10-3 S/cm was attained at an intermedium salt concentration of r = 0.05. It was also found that ion conduction in BTTT/dEO4 is enhanced by forming a smectic layered structure with irregular interfaces between the BTTT and EO4 layers and by the lateral film expansion upon salt addition. This can be explained by the enhancement of the misalignment and configurational entropy of the side chains, which increase their local mobility and that of the solvated ions. Our molecular design thus illustrates how, beyond the favorable energetic interactions that drive the assembly of ion solvating domains, modulation of entropic effects can also be favorably harnessed to improve ion conduction.

LC-elastomers: structure -property relations and concepts to improve applicability

ABSTRACTThis review deals with liquid crystalline elastomers (LCEs) and focuses thereby on three aspects. At first, (i) structure–property relations to optimise the magnitude of achievable actuation are discussed and then different exemptions and their origin. They apply especially to smectic systems with a strong segregation between mesogenic core and the interlayer region as they are found, e.g., in diluted smectic LC-polysiloxanes. In the next chapter, concepts to make LCEs better applicable as active elements in micromachines are discussed. To allow actuation in a device, which stays – as a whole – at constant temperature, the incorporation of ohmic heaters (electrical actuation) or suitable chromophores (photochemical actuation) is discussed. Finally, a system is presented which can be moved around in the decimetre range by magnetic fields and actuated locally (micrometre range) by a thermally or photochemically induced phase transition.