Mesogenic Core Research Articles

Third-order nonlinear optical properties of a novel series of D-π-A pyrene-aldehyde derivatives

A series of mono-substituted pyrene derivatives having a pyrene mesogenic core and aldehyde group peripherals were designed and synthesized, which had typical D-[Formula: see text]-A structures. Photophysical properties of the compounds were characterized by UV/Vis spectroscopy and the third-order nonlinearities of the compounds were studied by “open aperture” and “closed aperture” [Formula: see text]-scan techniques. Both of the compounds showed typical nonlinear absorptions and refractions, and the third-order nonlinear susceptibilities have been found to be very high in all cases. The significant nonlinear optical response combined with the possibility to further modify it, by changing the electron acceptor and the electron-donor groups, suggests that these systems can prove to be important candidates for a variety of photonic/optoelectronic applications.

Improved nematic mesophase stability of benzoxazole-liquid crystals via modification of inter-ring twist angle of biphenyl unit

ABSTRACTA series of nematic heterocyclic liquid crystals, 2-(2ʹ-fluoro-4ʹ-alkoxy-1,1ʹ-biphenyl-4-yl)-benzoxazole derivatives (coded as nPF(2)PBx) bearing hydrogen (nPF(2)PBH), methyl (nPF(2)PBM) and nitro (nPF(2)PBN) terminal groups, respectively, are developped and investigated. The results show that the nematic mesophase stability is obviously improved via a slightly increase in the inter-ring twist angle between the planes of two phenyl rings in the biphenyl unit, where modification of the twist angle can be achieved by adjusting the position of the lateral monofluorine substituent from the outer to the inter ring of the biphenyl mesogenic core unit. Meanwhile, the inter-ring lateral fluorine substituent results in a decrease in both melting and clearing points but only a nematic mesophase with an acceptably wide mesophase range for nPF(2)PBx, and thus a potential application in liquid crystal display mixtures is expected, especially for the compounds nPF(2)PBM and nPF(2)PBN. The reduced π–π interaction/conjugation caused by the increased twist angle is considered as one of the possible reasons for the formation of a nematic mesophase.

Influence of polar substituent on central bending unit of bent core mesogens: Synthesis, photophysical, mesomorphism and DFT studies

New five ring bent core mesogens derived from substituted 1,3-phenylenediamine (4-nitro-1,3-phenylenediamine, 4-chloro-1,3-phenylenediamine) were synthesized. Their molecular structures, photophysical properties and mesogenic behaviors were investigated. The molecular structures and the purity of the bent core molecule have been characterized by spectroscopic studies and elemental analysis respectively. Photophysical properties of bent core compounds were investigated in chloroform by using UV–visible and fluorescence spectroscopic studies. The phase transition temperatures were detected by differential scanning calorimetry analysis and the phases are confirmed by polarizing optical microscopy. The polar substituents on bent core unit of bent shaped molecule influence the mesomorphic behaviors of the bent core mesogens. The polar nitro group at 4-position of the bent core unit displays tilted smectic C phase and unknown smectic X phase whereas chloro group at 4-position exhibits orthogonal smectic A phase. The bent core mesogens are fluorescent in nature. The density functional theory calculation was carried out to obtain the stable molecular conformation and chemical reactivity of the bent core molecules. Orbitals involved in the electronic transitions and their corresponding energies together with oscillator strengths have been reported.

Mesophase behaviour of 1:1 mixtures of 4-n-alkoxyphenylazo benzoic acids bearing terminal alkoxy groups of different chain lengths

ABSTRACTThe thermal and mesophase behaviour of all possible 1:1 mixtures (In/Im) formed from any two homologues of 4-n-alkoxyphenylazo benzoic acids bearing different alkoxy chain lengths (n and m vary between 6 and 16 carbons), were investigated by differential scanning calorimetry (DSC) and phases identified by polarised light microscopy (PLM). Phase transitions were investigated as a function of the average alkoxy chain length (=(n + m)/2) of the mixed acids. A comparison, made between the present series and previously investigated simpler analogues, 4-n-alkoxy benzoic acids, revealed that increasing the length of the mesogenic core increased the stabilities of both the smectic C and nematic phases.

Paramagnetic nitroxide radical liquid crystalline compounds with methyl di(ethylene glycol) chain

ABSTRACTWe have synthesized a racemic and enantiomerically enriched all-organic paramagnetic nitroxide radical (NR) liquid crystalline (LC) compound, which contains a chiral NR unit in the mesogen core and a methyl di(ethylene glycol) chain as one of their terminal units. These new compounds showed remarkable substituent effects of the methyl di(ethylene glycol) chain on the phase transition behaviors and hydrophilic properties in contrast to structural analogs of the previous reported NR-LC compounds containing alkoxy chains.

Chiral hydrogen-bonded complex with different mesogens length and its effect on the performances of blue phase

A series of chiral hydrogen-bonded liquid crystals (H-bonded LCs) and the analogous covalent-bonded (C-bonded) LCs with similar structures were synthesized to study the effect on the mesogenic behaviours and the performance for extending the temperature range of blue phases (BPs). The length of mesogenic core and the terminal chains were also considered to investigate these effects on the LC behaviours. Additionally, the influence factors of polymer-stabilized H-bond-doped BPLC on electro-optical performances were orthogonally explored.

Synthesis and mesomorphic behaviour of unsymmetrical tetracatenar [1,2,3]-triazole derivatives

ABSTRACTNew nonsymmetric four-chained and three-chained [1,2,3]-triazole derivatives were synthesised. Their phase behaviours were investigated by differential scanning calorimetry, polarised optical microscopy and X-ray diffraction analyses. The tetracatenar derivatives show a columnar mesophase and a lamellar crystalline phase. The effects of the length of the terminal alkoxy chain and the nature of the terminal group attached to the side phenyltriazole unit of the mesogenic core are discussed.

Synthesis, structure, and mesomorphism of novel liquid crystalline acrylate monomers and polymers

ABSTRACTA series of novel liquid crystalline monomers (M1−M8) and side chain polymers base polyacrylate backbone were synthesized. The chemical structures were characterized by FT-IR and 1H-NMR spectra. The mesomorphism and thermal behavior was investigated by polarizing optical microscopy, differential scanning calorimetry, and thermogravimetric analysis. The relationships of structure and mesomorphism are discussed in detail. The eight monomers and their corresponding polymers all show enantiotropic nematic phase. With increasing the spacer length or flexibility of the terminal group, the melting temperature (Tm) and isotropic temperature (Ti) of the corresponding monomers and polymers all decreased. However, with increasing the rigidity of the mesogenic core, Tm and Ti of the corresponding monomers and polymers all increased. TGA showed that all the polymers obtained in this study had excellent thermal stability.

Semiflexible thermotropic polyazomethines based on o-dianisidine mesogenic core

A new series of thermotropic polyazomethines (PAMs) containing flexible spacers in the main chain and side methoxy groups on the mesogenic core has been synthesised by solution polycondensation reaction of o-dianisidine with α,ω-bis(4-formylphenoxy)-alkanes. A model compound was also synthesised by condensation reaction of o-dianisidine with p-butoxybenzaldehyde, in order to accomplish the structural characterisation and for a better understanding of the polymer properties. Chemical structure of the studied polymers was confirmed by Fourier transform infrared spectroscopy and proton nuclear magnetic resonance spectroscopy, and the supramolecular organisation was determined by powder wide angle X-ray diffraction measurements. The thermal properties of the polymers and model compound were investigated by thermogravimetry, polarised light microscopy and differential scanning calorimetry. The resulting data indicated high thermostability without significant weight loss up to 380°C and nematic liquid crystalline behaviour with large stability range (around 100°C) of the mesophase. Electronic UV–Vis absorption and light emitting ability were also investigated; these PAMs presented broad emission bands in the blue domain.

Effects of Molecular Structure and Aggregated Structure on Photoluminescence Properties of Liquid-crystalline Gold(I) Complexes with Various Aromatic Rings

Rod-like gold(I) complexes with phenyl, biphenyl, or naphthyl rings in a mesogenic core were synthesized by complexation of ethynyl-substituted aromatic derivatives with (tetrahydrothiophene)AuCl, followed by ligand exchange with alkyl isocyanide, to investigate the effects of molecular and molecular-aggregated structures on their liquid-crystalline (LC) behavior and photoluminescence properties. The gold complexes exhibited enantiotropic liquid crystallinity. Introduction of multi-ring systems into the mesogenic core effectively expanded the LC temperature range. A comparison of the photoluminescence properties in the crystalline phase showed that the molecular-aggregated structure plays a crucial role in the luminescence of these gold complexes.

Influence of mesogenic core polarity and position of chains attachment on columnar phase stability

Series of new Ni(II) metalomesogens of triangular molecular shape and forming Colh liquid crystalline (LC) phase were synthesised and described. Using in the molecular core the barbituric moieties that contain carbonyl or thiocarbonyl groups causes strong polarisation of the molecules and creates a permanent dipole moment μ, which was confirmed by quantum mechanical calculations. The relationship between molecular dipole moment and self-organisation of molecules into the columnar phase was considered. The position of alkyl and alkoxy chains substituted at phenyl ring that affects LC phase formation seems to be connected with planar conformation of the attached chains. These can broaden the mesogenic core and stabilise the Colh mesophase.

Synthesis and Characterization of a New Series of Paramagnetic Ferroelectric Liquid Crystalline Nitroxide Radicals

A new series of all-organic liquid-crystalline (LC) chiral radical compounds, trans-1-alkoxyphenyl-4-[(4-(4-alkoxyphenyl)-2,5-dimethylpyrrolidine-1-oxy-2-yl)-benzylideneamino]benzenes (1) with various alkyl chains, containing a chiral nitroxide unit in the mesogen core were synthesized and their LC properties were fully characterized. The enantiomerically enriched compounds (2S,5S)-1 showed N*, TGBA*, SmA* and SmC* phases, while the racemates (±)-1 exhibited N, SmA and SmC phases. The phase transition behavior and the ferroelectric properties of the SmC* phase of (2S,5S)-1 differed from those of previously reported analogues with an ester group as a substitute for the imino group of (2S,5S)-1. Some of the (2S,5S)-1 series showed a temperature-dependent spontaneous polarization PS (θ) inversion. We discuss the origin of this difference in terms of their molecular structures optimized by molecular orbital calculations. The phase transition behavior supports the hypothesis that the large ϕ, which is defined as the angle between the molecular long axis (one of the principal axes of inertia) and the direction of the dipole moment in each molecule, results in the stabilization of the SmA* (SmA) phase.

Terminal Fluorinated Nitroxide Radical Liquid Crystalline Compounds

We have synthesized a new series of all-organic paramagnetic liquid crystalline (LC) compounds, which include a five-membered ring nitroxide radical (NR) moiety in the mesogen core and fluorine atoms as terminal substituents. These new compounds are the structural analogs of the previously reported NR-LC compounds, and showed remarkable fluorine substituent effects. The introducing fluorine atom into terminal unit stabilizes nematic phase. However, an excess of fluorination leads to the destabilization of nematic phase.

Apolar bimesogens and the incidence of the twist-bend nematic phase.

The nematic twist-bend phase (NTB) was, until recently, only observed for polar mesogenic dimers, trimers or bent-core compounds. In this article, we report a comprehensive study on novel apolar materials that also exhibit NTB phases. The NTB phase was observed for materials containing phenyl, cyclohexyl or bicyclooctyl rings in their rigid-core units. However, for materials with long (>C7) terminal chains or mesogenic core units comprising three ring units, the NTB phase was not observed and instead the materials exhibited smectic phases. One compound was found to exhibit a transition from the NTB phase to an anticlinic smectic C phase; this is the first example of this polymorphism. Incorporation of lateral substitution with respect to the central core unit led to reductions in transition temperatures; however, the NTB phase was still found to occur. Conversely, utilising branched terminal groups rendered the materials non-mesogenic. Overall, it appears that it is the gross molecular topology that drives the incidence of the NTB phase rather than simple dipolar considerations. Furthermore, dimers lacking any polar groups, which were prepared to test this hypothesis, were found to be non mesogenic, indicating that at the extremes of polarity these effects can dominate over topology.

Three-Ring-Based Thermotropic Mesogens with a Dimethylamino Group: Structural Characterization, Photophysical Properties, and Molecular Order

Thermotropic liquid crystals exhibiting light-emitting properties are gaining popularity as functional materials in view of their application in organic light-emitting diodes. Such mesogens essentially require active chromophoric moieties in the mesogenic core so that the mutual light-emitting and liquid crystalline properties can be realized. In this work, three-ring-core-based mesogens with a terminal dimethylamino unit are subjected to structural characterization by various techniques. These mesogens exhibit enantiotropic nematic as well as smectic A phase with interdigitated layer organization (SmAd). This is a surprising observation because the SmAd organization is commonly observed for calamitic mesogens with terminal polar groups. Interestingly, the single-crystal structure of the C6 homologue indicates antiparallel packing. Furthermore, the photophysical properties of a representative C12 mesogen in solution disclose yet another exciting feature. The steady-state and time-resolved fluorescence studies indicate negative solvotochromism in solvents with differing polarity. To obtain greater insight, density functional theory (DFT)-based highest occupied molecular orbital–lowest unoccupied molecular orbital studies are carried out which support intramolecular charge-transfer interactions in this class of mesogens. Additionally, the DFT calculations also provide the 13C chemical shifts which are compared with the solution NMR values for the structural assignment of all carbons in the core unit. Furthermore, the two-dimensional separated local field measurements for the C12 homologue in nematic and SmAd mesophases offer 13C–1H dipolar couplings from which the molecular order is determined to be 0.59 and 0.70, respectively.

Molecular engineering room-temperature bent-core nematics

We report the transition temperatures for several series of structural variations on bent-core mesogens (BCMs) derived from the asymmetric oxazole biphenol mesogenic core. Trimethyl-substituted mesogens exhibit nematic phases that can be supercooled to room temperature. Preliminary X-ray diffraction (XRD) results confirm a cybotactic supramolecular organisation in these nematics; moreover, the splitting of the diffuse wide-angle XRD feature observed in trimethylated mesogens points to an enhanced biaxial orientational correlation in the transverse molecular packing compared to other BCMs. Finally, one of the trimethyl-substituted mesogens shows the formation of a room temperature metastable fibre-like crystalline order when supercooled in a magnetic field.

Synthesis of Multiring Azo-Benzoic Acid Liquid Crystalline Molecules and Their Special Photosensitive Property

10 new rod-like azo benzoic acid liquid crystals belonged to three series have been prepared, in which trans-cyclohexyl benzene, biphenyl or bi-trans-cyclohexyl carboxylic acid mesogenic cores with ethyl, n-propyl, n-butyl or n-pentyl substituents were connected with azo benzoic acid. Meantime, a simple method was developed in synthesizing compounds bearing both ester and carboxyl groups in one-step reaction. All these compounds have been characterized on the basis of their spectral data, differential scanning calorimeter (DSC) and hot stage polarizing optical microscope (HS-POM). All these compounds have higher thermo-stability. 8 of these compounds have liquid crystalline properties. Their temperature ranges of mesophase have not been obtained, resulting from all the compounds decomposed before their clearing points. Although these compounds are photosensitive in solution under illumination of UV light (365nm), they can't do trans-cis isomerization in their mesophses, indicating the super stabilities of the trans isomer in mesophase.

CONDENSATION REACTION OF GLYCEROL AND CARBONYL COMPOUNDS: SYNTHESIS, CHARACTERIZATION, AND DERIVATIZATION FOR LIQUID CRYSTALS

Condensation reactions of glycerol with aldehydes and ketones were performed under thermal heating and microwave irradiation regimes. Homogeneous and heterogeneous catalysts were tested in both conditions. A silica sulfated (SiO2-SO3H) heterogeneous catalyst demonstrated the best performance relative to a selectivity of >95% in favor of 5-membered ketals. For acetals, preference in favor of 5-membered or 6-membered functional groups depends on the nature of the catalyst. Homogenous catalysts favor the more stable 6-membered acetals, whereas heterogeneous catalysts favor the less stable 5-membered acetals. However, the isomer ratios in the acetalization reaction are too low, and hence the reaction cannot be used in a synthetic plan for functional materials. Ketalization processes mediated by SiO2-SO3H show a high selectivity in favor of a 5-membered ring (1,3-dioxolane). The scope of condensation was tested with different ketones. A mechanism for heterogeneous catalysis related to the selectivity in the cyclization process is presented herein. Solketal, a commercial product, was also obtained by a condensation reaction of glycerol and propanone, and showed a high selectivity in favor of 1,3-dioxolane. It was transformed to potential allylic and chiral intermediates. A mesogenic core was connected to the organic framework of glycerol to produce a monomer liquid crystal material with a stable smectic-C mesophase.

Synthesis of cyclopentyloxy terphenyl liquid crystals with negative dielectric anisotropy

Liquid crystalline materials possessing negative dielectric anisotropy (Δε) have attracted considerable attention because they can be used to formulate eutectic mixture for several display applications. The negative Δε can be achieved by introducing a lateral polar substituent onto the mesogen core of a liquid crystalline material, and fluorine atoms are usually used because of the small size and high electronegativity. 2,3-ortho-Difluoro substituent liquid crystals exhibit stable and profound physical properties such as the low viscosity, strong negative Δε, and high solubility. To avoid the decrease of the mesophase range, 2,3-ortho-difluorophenyl is often placed next to the terminal chain. In this paper, we have synthesised a new series of 2,3-ortho-difluoroterphenyl liquid crystals with a negative dielectric anisotropy. Ether oxygen is introduced to the structure because it can further increase the lateral dipole moment. 3-Alkylcyclopentane end group decreases both the melting point and the viscosity, which makes it an appropriate substituent for negative Δε 2,3-ortho-difluoroterphenyl liquid crystals. The mesomorphic properties as well as the optical anisotropies of the synthesised liquid crystals are discussed in this paper. All compounds in the series are found to possess highly negative dielectric anisotropy, wide mesophase ranges; therefore, they have a great potential to formulate eutectic mixture for liquid crystal displays.

Synthesis and phase transition behaviours of new non-symmetric liquid crystal dimers

A new series of non-symmetric liquid crystal dimers N-(4-(n-(4-(benzothiazol-2-yl)phenoxy)alkyloxy)-4-nitro-benzylideneimine) containing benzothiazole and benzylideneaniline as the mesogenic core units connected by a flexible alkyl spacer, -(CH2)n-, with n ranging from 2 to 6, 8, 10 and 12 have been prepared. Their molecular structures were proposed via spectroscopic techniques. Mesomorphic properties were studied using differential scanning calorimetry and polarising optical microscopy. The results showed that the clearing temperatures decreased upon lengthening of the alkyl spacers. Whilst even member with the shortest alkyl spacer (n = 2) exhibited nematic phase, the rest of even members (n = 4, 6, 8, 10 and 12) possessed smectic and nematic properties. Nematic phase range gradually decreased from n = 2 to n = 12. Odd members are belonged to non-mesogenic compounds except for n = 5 which exhibited monotropic (metastable) nematic phase.