Mesogenic Fragments Research Articles

Synthesis and Mesomorphic Properties of Biphenyl Derivatives with Central Unit Based on 1,6-Hexamethylene Diisocyanate Oligomers

A number of compounds with biphenyl as a mesogenic fragment and a structure simulating LC dimers, polymers with mesogenic side chain groups and a star-shaped structure have been synthesized. An approach for the synthesis of such compounds under mild conditions and without a complex purification procedure with a quantitative yield, has been developed. The approach is the implementation of “click chemistry” methods for the synthesis of LC compounds. It has been shown that the LC state is observed only if there are three biphenyls in one molecule, i.e., the compounds appear to be similar to polyesters with biphenyl as a mesogenic moiety. The determining factor in the formation of the LC state and particularly smectic mesomorphism of compounds with such a potentially “weak” mesogen as biphenyl is the formation of intermolecular hydrogen bonds.

High-efficiency circularly polarized emission from liquid-crystalline platinum complexes

Realizing both a high emission efficiency and luminescence dissymmetry factor (glum) in circularly polarized solution processable organic light-emitting diodes (CP-OLEDs) remains a significant challenge. In this contribution, two chiral phosphorescent liquid crystals based on cyclometalated platinum complexes are prepared, in which the chiral s-2-methyl-1‑butyl group is introduced into the cyclometalating ligand and the mesogenic fragment is attached to the periphery of the ancillary ligand. The platinum complexes exhibit both smectic and chiral nematic phases as evidenced by polarized optical microscopy, differential scanning calorimetry and small-angle X-ray diffraction. Remarkably, a high photoluminescent quantum efficiency of over 78% and clear circularly polarized luminescent signal with gPL of about 10–2 are observed for the complexes. Further, solution-processed CP-OLEDs show maximum external quantum efficiencies (EQE) of over 15% and strong circularly polarized electroluminescent signals with a gEL ≈ 10–2. This research demonstrates that both liquid crystallinity and the number of chiral centers play key roles in improving the chiroptical property, paving the way for a new approach for the design of high-efficiency CPL emitters.

Synthesis and radical copolymerization of 2‐(4‐((4‐isocyanophenyl)diazenyl)phenoxy)ethyl‐3‐phenylacrylate with maleic anhydride

AbstractA monomer containing a photochromic group was synthesized by condensation of 4‐(2‐hydroxyethyloxy)‐4‐cyanoazobenzene with cinnamoyl chloride. The structure and composition of the monomer were established by FTIR, UV–Vis, 1H NMR spectroscopy, and elemental analysis. The values of chemical shifts, multiplicity, and integrated intensity of 1H signals in one‐dimensional NMR spectra of the monomer were determined, and the homo‐ and heteronuclear interactions were found. The crystal structure of 4‐(2‐hydroxyethyloxy)‐4‐cyanoazobenzene was established using X‐ray diffraction analysis. A copolymer containing photochromic mesogenic fragment was synthesized by radical copolymerization of 2‐(4‐((4‐isocyanophenyl)diazenyl)phenoxy)ethyl‐3‐phenylacrylate with maleic anhydride.

Phase Structure Recording in a Nematic Side-Chain Liquid-Crystalline Polymer.

Dye-doped nematic side-chain liquid-crystalline polymers possess extraordinary large optical nonlinearity and ability to store the induced orientational deformations in a glassy state, which makes them a very promising material for photonic applications. In this study, the phase structures were generated and recorded in the bulk of a 50-μm layer of a nematic liquid-crystalline side-chain polymer, containing polyacrylate backbone, spacer having five methylene groups, and phenyl benzoate mesogenic fragment. The polymer was doped with KD-1 azodye. The director field deformations induced by the light beam close to the TEM01 mode were studied for different geometries of light–polymer interaction. The phase modulation depth of 2π was obtained for the 18-μm spacing between intensity peaks. The experimental data were analyzed based on the elastic continuum theory of nematics. The possibility to induce and record positive and negative microlenses in the polymer bulk was shown experimentally.

Thermotropic Liquid Crystalline Polyesters with Mesogenic Fragments Based on the p-Oxybenzoate Unit

Main trends in the development of thermotropic liquid crystalline (LC) polymers based on p‑hydroxybenzoic acid are described in this review. The relationship between the structure of the main chain of copolymers and their thermal and mechanical properties is shown. The possibility of creating biodegradable LC polymers is demonstrated.

Thioether-linked liquid crystal dimers and trimers: The twist-bend nematic phase

Systematic synthesis of thioether-linked dimers and trimers was carried out to reveal molecular designs for inducing mesophases and twist-bend nematic (NTB) phases. A five-fold approach based on molecular structural parameters including the terminal substituent, the position of the thioether bond, the nature of the bridge bond linking benzene rings in the mesogenic fragments, the nature of bonds at the linkage on the side oppose to the thioether linkage, and oligomeric effect was evaluated. Dimers with cyano groups at the 4,4′-positions in each mesogenic fragment of the thioether-based dimeric system were found to afford a large long-axis directional dipole moment in the fragments and exhibited potential for application as mesogens, with some molecules indicating the ability to form NTB phase. The dimers containing the thioether bond as a part of the spacer and not at the linkage position to the mesogenic fragments deteriorate as twist-bend nematogens but behave as traditional nematogens. Also, imine-, azo- and triple-bond bridged mesogenic fragments lead the dimers to be twist-bend nematogenic. Notably, all asymmetric thioether-linked cyanobiphenyl dimers with different functional bonds at the linkage on the side oppose to the thioether linkage, such as methylene, ketone, and two-typed esters (COO and OCO) can form NTB phases, with some of them retaining the phase upon cooling to room temperature, which highlights the utility of thioether linkage in the molecular design of twist-bend nematogens. In the trimer system (or oligomeric effect), a cyanobiphenyl-based trimer composed of interior ether and exterior thioether linkers formed an NTB phase. This paper, for the first time, not only reveals the systematic molecular design of thioether-linked dimers but also offers the prospect of the thioether-linked oligomers, for twist-bend nematic liquid crystals.

The influence of a magnetic field on the morphology and thermomechanical properties of a liquid crystalline epoxy carbon composite

The synthesis of a novel ordered liquid crystalline epoxy carbon composite was presented in this article. Anthracite thermally modified at 2,000°C (SV2000) having graphitic structure was used as a filler in the composite with matrix obtained by crosslinking a liquid crystalline diepoxy monomer LCEM containing triaromatic mesogenic group. The curing agent was 4,4′‐diaminodiphenylmethane. The morphology and thermomechanical properties of the final products were characterized by differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), and wide angle X‐ray scattering (WAXS) analysis. It was found by WAXS measurements that alignment of LCEM was effectively induced in the matrix and in the composite filled with SV2000 by applying external magnetic field during cure. Thermosets with locked smectic C structure were obtained in all cases, with mesogenic fragments arranged along the applied field. The experimental results indicated that the presence of SV2000 as the filler of composite partially decreased the degree of order. The oriented composites exhibited anisotropy of the dynamic‐mechanical properties. POLYM. COMPOS., 39:E2573–E2583, 2018. © 2018 Society of Plastics Engineers

Optimization of the GAFF force field to describe liquid crystal molecules: the path to a dramatic improvement in transition temperature predictions

The physical properties and phase transitions of thermotropic liquid crystals are highly sensitive to small changes in chemical structure. However, these changes are challenging to model, as both the phase diagram and mesophase properties obtained from fully atomistic simulations are strongly dependent on the force field model employed, and the current generation of chemical force fields has not proved accurate enough to provide reliable predictions of transition temperatures for many liquid crystals. This paper presents a strategy for improving the nematic clearing point, TNI, in atomistic simulations, by systematic optimization of the General Amber Force Field (GAFF) for key mesogenic fragments. We show that with careful optimization of the parameters describing a series of liquid crystal fragment molecules, it is possible to transfer these parameters to larger liquid crystal molecules and make accurate predictions for nematic mesophase formation. This new force field, GAFF-LCFF, is used to predict the nematic-isotropic clearing point to within 5 °C for the nematogen 1,3-benzenedicarboxylic acid,1,3-bis(4-butylphenyl)ester, an improvement of 60 °C over the standard GAFF force field.

New spirocyclic nitroxides of 2,5-dihydroimidazole series flanked by two mesogenic fragments

N/C-Hydroxylated spirofused derivatives of 2,5-dihydroimidazole which were synthesized by condensation of 4-(4-hydroxyphenyl)cyclohexanone with aryl hydroxylaminoalkyl ketones in the presence of ammonia were determined to be trans-ee-isomers of 1,4-cyclohexane framework by NMR spectroscopy. Oxidation of the cyclic hydroxylamino moiety followed by acylation of the phenolic hydroxyl group with 4-alkyloxybenzoic acid chlorides led to the corresponding spirocyclic nitroxides bearing two mesogenic fragments.

Banana-calamitic Dimers: Further Variations of the Bent-core Mesogenic Unit

Banana-calamitic dimers are a topic in the field of bent-core mesogens. New materials were prepared by chemical variations at the bent-core fragment that is by a cyano group terminally attached to the bent-core fragment, by lateral substitution with a chlorine atom and methyl group, respectively, and by increasing the number of phenyl rings from five to six. The bent-core and the calamitic mesogenic moieties are connected by aliphatic spacers of different length. The materials were investigated by optical microscopy, DSC, X-ray diffraction studies and electro-optical measurements. This way nematic, smectic and various types of polar columnar phases have been characterized. The low temperature phase in a sequence of two nematic phases exhibiting different properties can be classified as a nematic twist-bend phase. Such NTB phases are only formed if a bend exists along the molecular long axes. This prerequisite can be doubly achieved in banana-calamitic dimers: by the intrinsic bend of one of the mesogenic fragments and by an odd-numbered spacer.

Spirocyclic 2,5-dihydro-1H-imidazole 1-oxyl radicals with a mesogenic substituent on C4. Synthesis and crystal structure

Stable spirocyclic 2,5-dihydro-1H-imidazole 1-oxyl radicals containing a mesogenic fragment were synthesized on the basis of 1-[4-hydroxy(alkoxy)phenyl]-2-hydroxyamino-2-methylpropan-1-ones. The crystalline structure of two aminoxyl radicals was determined by X-ray analysis.

Immobilization of quantum dots of cadmium selenide on the matrix of a graft liquid-crystalline polymer

A new luminescent composite based on quantum dots of CdSe immobilized on the polymer LC matrix prepared through the graft polymerization of the monomer of 4-(ω-acryloyloxyhexyloxy)benzoic acid on a fluorocarbon support after its preliminary irradiation with vacuum ultraviolet light is elaborated. The structure, composition, and optical characteristics of the prepared composite are studied via the methods of the FTIR spectroscopy, energy-dispersive and wave-dispersive analyses, and luminescence spectroscopy. The CdSe particles are shown to interact with the carboxyl groups of mesogenic fragments of the LC polymer, and this interaction leads to the integration of quantum dots into the ordered LC structure of the composite. As a result of immobilization, the luminescence peak of the quantum dots is shifted toward lower wavelengths owing to the interaction between the nanoparticles and the polymer LC matrix.

Synthesis and conformation properties of alkylene-aromatic poly(ether esters) with nonlinear hetaryl-containing mesogenic fragments in the backbone

A thermotropic mesogenic alkylene-aromatic poly(ether ester) was prepared by phase-transfer polycondensation of the azomethine derived from 5-amino-2-phenylbenzoxazole and 3,4-dihydroxybenzaldehyde with 1,6-bis(4-chlorocarbonylphenoxy)hexane in the presence of a phase-transfer catalyst, trimethylhexadecylammonium bromide. The intramolecular orientation order parameters and the angle formed by the pendant 5-imino-2-phenylbenzoxazole substituent relative to the backbone were estimated by methods of molecular hydrodynamics and optics.

Callimatic and T-shaped liquid crystalline compounds with fluorinated terminal groups

A series of liquid crystalline polyesters with a rigid T-shaped or a stick-like mesogenic fragment and fluorinated terminal groups of various structures was synthesized and studied by means of polarization optical microscopy, differential scanning calorimetry, and IR and 1H NMR spectroscopy. It was shown that the effect of the fluorinated group on the thermostability of the mesophase depends on the shape of the mesogenic fragment. The thermostability of the mesophase in callimatic liquid crystalline compounds with perfluorinated terminal groups is higher and in those with difluoromethyl terminal groups containing one hydrogen atom is lower compared with their alkyl analogs. This is connected with the possibility of weak hydrogen bonding that destroys the liquid crystalline order. The thermostability of the mesophase in compounds with a T-shaped mesogenic fragment and any fluorinated terminal groups is always higher compared with their alkyl analogs.

Local modes of molecular mobility in comb-shaped liquid crystalline ionomers containing alkaline metal ions

Molecular mobility of the comb-shaped copolymer poly(4-[6-(acryloyloxy)hexyloxy]benzoic acid- co-butyl acrylate) and Li- and Rb-containing LC ionomers based on this polymer was studied at low temperatures by the methods of dielectric spectroscopy and thermally stimulated depolarization current. The above copolymers are shown to experience the γ 2, γ 1, and β-processes, which are related by the reorientation of the end СООН groups, by the mobility of a spacer, and by the reorientation of mesogenic fragments (dimers of oxybenzoic acids) with respect to a long axis, respectively. The formation of multiplet structures in ionomers leads to a suppression of the intensity of the β-process that relates with a partial breakdown of hydrogen-bonded dimmers and with a decrease of their molecular mobility.

Dipole architecture of molecules and mesomorphic behavior of liquid crystals with rigid T-shaped mesogenic fragment

A number of liquid crystalline polyesters having a rigid T-shaped mesogenic fragment and differing in dipole architecture were synthesized and examined by polarizing optical microscopy, differential scanning calorimetry, and IR and 1H NMR spectroscopy. The thermal stability of the mesophase in the given series of compounds was shown to increase with extension of arms in the T-shaped mesogenic fragment, as well as on replacement of the terminal ester groups (COOAlk) therein by ether moieties (OAlk). Such replacement also enhances smectogenic properties.

Mesomorphic compounds with a rigid T-shaped mesogenic fragment

Mesomorphic polyesters with a rigid T-shaped mesogenic fragment were prepared and studied by polarization optical microscopy, differential scanning calorimetry, and IR and 1H NMR spectroscopy. An increase in the length of aliphatic substituents in the mesogenic fragment leads to a change from the nematic type of the mesophase to the smectic type.

Thermal behaviour and degradation of a liquid crystalline alkylene-aromatic polyester: Poly(decamethylene-fumaroyl-bis-4-oxybenzoate)

The thermal behaviour and degradation of an alkylene-aromatic liquid crystalline polyester, poly(decamethylene-fumaroyl-bis-4-oxybenzoate), were studied by thermogravimetric analysis under dynamic conditions and by pyrolysis–gas chromatography and pyrolysis–gas chromatography/mass spectrometry in the temperature range 450–650 °C. Among the degradation products detected, maleic anhydride, phenol, 4-hydroxybenzoic acid and the corresponding decenyl ester were the most abundant. The type and the composition of the pyrolysis products gave useful information about the mechanism of thermal degradation. The polyester decomposition starts with a free-radical scission in the mesogenic fragment and continues with electrocyclic reactions in the spacer. The influence of fullerene C 60 addition and of the deuteration of the decamethylene spacer on thermal behaviour and degradation were investigated.

The effect of external orienting electric field on dielectric relaxation of segmented polyesters in dilute solution

A comparative study of dipole polarization relaxation in the absence and in the presence of an external orienting electric field was performed for linear segmented polyesters with alternating rigid (oxyfumaroylbis-4-oxybenzoates) and flexible (methylene-CH2-, ethylene oxide-CH2CH2-O-, and dimethylsiloxane-Si(CH3)2-O-Si(CH3)2-) fragments in dilute solutions. Polyesters that do not display mesomorphic properties in the bulk show several regions of dielectric absorption with relaxation character. These regions are associated with the motions via the local mobility mechanisms of different polar fragments of the macromolecule. In solutions of polyesters that possess LC properties in the bulk, large-scale dipole polarization relaxation with long relaxation times and high activation energies was revealed along with local dielectric relaxation transitions. This process is associated with the cooperative motion of mesogenic fragments in their associates. In an external orienting electric field, the intensity of dielectric absorption usually increases for all types of dielectric transition; relaxation times and activation energies experience changes only for large-scale processes.

Synthesis and comparative studies of carbosilane liquid crystalline dendrimers with chiral terminal mesogenic groups

A new series of carbosilane liquid crystalline (LC) dendrimers from the first to the third generations with 8, 16 and 32 chiral terminal mesogenic groups, respectively, has been synthesized. The molecular structures and purity of all new compounds were confirmed by 1H NMR spectroscopy and GPC analysis. Data of polarization microscopy and SAX analysis demonstrated that all LC dendrimers synthesized form a chiral smectic SmC* phase at temperatures below 50 °C. It has been found that bistable electrooptical switching is observed for all dendrimers. The influence of chiral mesogenic fragment length on phase behavior and ferroelectric properties of carbosilane LC dendrimers is discussed.