Mesogenic Groups Research Articles

Dynamic Gel Formation in Solutions of Associating Macromolecules with Mesogenic Groups

Dynamic Gel Formation in Solutions of Associating Macromolecules with Mesogenic Groups

The effect of dynamic cross-links and mesogenic groups on the swelling and collapse of polymer gels

The polymer gels containing dynamic cross-links and mesogenic groups are among the key candidates for the development of soft actuators and detectors, displays, sensors and other programmable and self-healing materials. In this article, firstly, the collapse of polymer networks with irreversible cross-links in the presence of a dynamic cross-linker is investigated by means of Flory-type theory. It is shown that, at not a very poor solvent quality (near the theta-conditions), the swelling ratio for a gel containing irreversible and dynamic cross-links is less than for a gel with irreversible cross-links only, whereas at poor or good solvent qualities sizes of these gels are close to each other. The number of dynamic cross-links monotonically increases with worsening the solvent quality. The gel contraction can be also achieved by increasing the dynamic cross-linker concentration, which allows one to change the transition point in a wide range of solvent conditions. Secondly, polymer networks containing irreversible and dynamic cross-links and incorporating mesogenic side groups is studied using the Maier–Saupe theory. With decreasing in the temperature, the continuous transition from a swollen to collapsed state occurs. With further increase in the temperature, the swelling ratio discontinuously decreases and the nematic order parameter sharply increases from zero to a value ​​close to one. By increasing the dynamic cross-linker concentration, the swelling-to-collapse transition and, then, the isotropic-nematic transition are observed. A phase diagram of the gel is constructed and, depending on the dynamic cross-linker concentration and temperature, the swollen gel with the zero nematic order parameter, the collapsed gel with the zero nematic order parameter, and the collapsed gel with the nematic ordering can be formed. A growth of the length of the mesogenic side group leads to an increase in the area of ​​existence of the collapsed gel with the nematic ordering in the phase diagram. The obtained theoretical results are in agreement with corresponding experimental data from the literature.

MR-TADF liquid crystals: towards self assembling host-guest mixtures showing narrowband emission from the mesophase.

Creating (room temperature) liquid crystalline TADF materials that retain the photophysical properties of the monomolecular TADF emitters remains a formidable challenge. The strong intramolecular interactions required for formation of a liquid crystal usually adversely affect the photophysical properties and balancing them is not yet possible. In this work, we present a novel host-guest approach enabling unperturbed, narrowband emission from an MR-TADF emissive core from strongly aggregated columnar hexagonal (Colh) liquid crystals. By modifying the DOBNA scaffold with mesogenic groups bearing alkoxy chains of different lengths, we created a library of Colh liquid crystals featuring phase ranges >100 K and room temperature mesomorphism. Expectedly, these exhibit broad excimer emission from their neat films, so we exploited their high singlet (S1 ∼2.9 eV) and triplet (T1 ∼2.5 eV) energies by doping them with the MR-TADF guest BCzBN. Upon excitation of the host, efficient Förster Resonance Energy Transfer (FRET) resulted in almost exclusive emission from BCzBN. The ability of the liquid crystallinity of the host to not be adversely affected by the presence of BCzBN is demonstrated as is the localization of the guest molecules within the aliphatic chain network of the host, resulting in extremely narrowband emission (FWHM = 14-15 nm). With this work we demonstrate a strategy for the self-assembly of materials with previously mutually incompatible properties in emissive liquid crystalline systems: strong aggregation in Colh mesophases, and narrowband emission from a MR-TADF core.

Dielectric Study of Liquid Crystal Dimers: Probing the Orientational Order and Molecular Interactions in Nematic and Twist-Bend Nematic Phases.

Dielectric spectroscopy in frequencies that range from 10 Hz to 1 GHz has been used to study the molecular orientational dynamics of the two types of dimers that form the twist-bend nematic phase over a wide range of temperatures for both nematic and twist-bend nematic phases. The symmetrical and asymmetrical liquid crystal dimers with the cyanobiphenyl mesogenic groups were investigated. The results were analyzed within the framework of the molecular theory of dielectric permittivity for nematogens. The two molecular processes can be assigned to the reorientation of the monomeric unit: the high frequency one to the precessional rotation of the longitudinal components of the cyanobiphenyl groups (CN) and the second (low frequency) to the end-over-end rotation of the CN dipole around the short molecular axis. The precession mode, which is determined by the local order and is almost unaffected by the phase transition from the nematic to the twist-bend phase, while the end-over-end rotation clearly slowed down at the transition, as it is affected by the growth of the intermolecular interactions. The latter corresponds well to the fact revealed by IR spectroscopy about the longitudinal correlation of the molecular dipoles.

Photo-Optical Processes in Photochromic Comb-Shaped Polymer Complexes with Hydrogen-Bounded Azobenzene Containing Mesogenic Groups

Photo-Optical Processes in Photochromic Comb-Shaped Polymer Complexes with Hydrogen-Bounded Azobenzene Containing Mesogenic Groups

Mesogenic Groups Control the Emitter Orientation in Multi‐Resonance TADF Emitter Films**

AbstractThe use of thermally activated delayed fluorescence (TADF) emitters and emitters that show preferential horizontal orientation of their transition dipole moment (TDM) are two emerging strategies to enhance the efficiency of OLEDs. We present the first example of a liquid crystalline multi‐resonance TADF (MR‐TADF) emitter, DiKTa‐LC. The compound possesses a nematic liquid crystalline phase between 80 °C and 110 °C. Importantly, the TDM of the spin‐coated film shows preferential horizontal orientation, with an anisotropy factor, a, of 0.28, which is preserved in doped poly(vinylcarbazole) films. Green‐emitting (λEL=492 nm) solution‐processed OLEDs based on DiKTa‐LC showed an EQEmax of 13.6 %. We thus demonstrate for the first time how self‐assembly of a liquid crystalline TADF emitter can lead to the so‐far elusive control of the orientation of the transition dipole in solution‐processed films, which will be of relevance for high‐performance solution‐processed OLEDs.

Mesogenic Groups Control the Emitter Orientation in Multi-Resonance TADF Emitter Films.

The use of thermally activated delayed fluorescence (TADF) emitters and emitters that show preferential horizontal orientation of their transition dipole moment (TDM) are two emerging strategies to enhance the efficiency of OLEDs. We present the first example of a liquid crystalline multi-resonance TADF (MR-TADF) emitter, DiKTa-LC. The compound possesses a nematic liquid crystalline phase between 80 °C and 110 °C. Importantly, the TDM of the spin-coated film shows preferential horizontal orientation, with an anisotropy factor, a, of 0.28, which is preserved in doped poly(vinylcarbazole) films. Green-emitting (λEL =492 nm) solution-processed OLEDs based on DiKTa-LC showed an EQEmax of 13.6 %. We thus demonstrate for the first time how self-assembly of a liquid crystalline TADF emitter can lead to the so-far elusive control of the orientation of the transition dipole in solution-processed films, which will be of relevance for high-performance solution-processed OLEDs.

Using Lateral Substitution to Control Conformational Preference and Phase Behaviour of Benzanilide-based Liquid Crystal Dimers.

The inclusion of secondary and tertiary benzanilide-based mesogenic groups into liquid crystal dimers is reported as a means to develop new materials. Furthermore, substitution at the nitrogen atom is shown to introduce an additional synthetic 'handle' to modify the molecular structure of the tertiary materials. The design of these materials has proved challenging due to the strong preferences of 3° benzanilides for the E amide conformation. In this work, lateral substitution is used to modify the conformational preferences of the amide linkage and promote liquid crystallinity for a series of N-methyl benzanilide dimers. As the proportion of the E conformer decreases, the nematic-isotropic transition temperatures increase, and enantiotropic nematic behaviour is observed. We also report the synthesis and characterisation of the analogous 2° benzanilide-based materials, which show nematic and twist-bend nematic behaviour. This approach highlights the effects that seemingly small structural modifications, such as the inclusion and position of a methyl group, can have on molecular shape and hence, liquid crystalline behaviour.

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.

Photonic Liquid Crystal Polymer Absorbent for Immobilization and Detection of Gaseous Nerve Agent Simulants

Detection and sequestration of chemical warfare agents (CWAs), such as poisonous organophosphates, are highly desirable for both personal security and environmental protection. However, both sensing and absorption in a single device have been rarely reported. In this study, we describe a photonic absorbent based on a cholesteric liquid crystal polymer as a dual sensing and decontamination device for gas-type CWAs. Dimethyl methylphosphonate (DMMP) was used as a simulant compound. A blue reflective photonic polymer was fabricated that was able to detect DMMP vapor through absorption. Hydrogen bond interactions between DMMP and mesogenic carboxylic groups of the polymer allow selectivity and capture. A distinct optical change of the film from blue to bright green indicates the absorption of DMMP vapor molecules and confirms when full absorption of the polymer is achieved. The diffusion of DMMP vapor into the material was observed by the formation of a sharp boundary between swollen and unswollen material, as evidenced by scanning electron microscopy images and the structural color changes. In ambient conditions, DMMP molecules are retained in the photonic absorbent without release to the environment. Heating above approximately 60 °C releases the absorbed DMMP, leading to a reusable optical device. These results confirm the ability of photonic polymers to sense and immobilize dangerous vapor, paving the way for the realization of simple, battery-free optical devices able to simultaneously warn and protect.

Liquid Crystal Dimers and Smectic Phases from the Intercalated to the Twist-Bend

In this review we consider the relationships between molecular structure and the tendency of liquid crystal dimers to exhibit smectic phases, and show how our application of these led to the recent discovery of the twist-bend, heliconical smectic phases. Liquid crystal dimers consist of molecules containing two mesogenic groups linked through a flexible spacer, and even- and odd-membered dimers differ in terms of their average molecular shapes. The former tend to be linear whereas the latter are bent, and this difference in shape drives very different smectic behaviour. For symmetric dimers, in which the two mesogenic groups are identical, smectic phase formation may be understood in terms of a microphase separation into distinct sublayers consisting of terminal chains, mesogenic units and spacers, and monolayer smectic phases are observed. By contrast, intercalated smectic phases were discovered for nonsymmetric dimers in which the two mesogenic units differ. In these phases, the ratio of the layer spacing to the molecular length is typically around 0.5 indicating that unlike segments of the molecules overlap. The formation of intercalated phases is driven by a favourable interaction between the different liquid crystal groups. If an odd-membered dimer possesses sufficient molecular curvature, then the twist-bend nematic phase may be seen in which spontaneous chirality is observed for a system consisting of achiral molecules. Combining the empirical relationships developed for smectogenic dimers, and more recently for twist-bend nematogenic dimers, we show how dimers were designed to show the new twist-bend, heliconical smectic phases. These have been designated SmCTB phases in which the director is tilted with respect to the layer plane, and the tilt direction describes a helix on passing between layers. We describe three variants of the SmCTB phase, and in each the origin of the symmetry breaking is attributed to the anomalously low-bend elastic constant arising from the bent molecular structures.

Effect of disubstitution pattern of the terminal alkyl chains on the mesophase of liquid crystals based on lanthanide(III) complexes: A study of the thermal, emission and dielectric behavior

In the present work, new luminescent lanthanide complexes with extended mesomorphic range were prepared by coordination to lanthanide ions (Eu3+, Sm3+ and Tb3+) of the new 4-pyridone based organic ligands (L) with 3,4- (7-n) and 3,5-di(alkyloxy)benzyl (8-n) mesogenic groups and variable length (n = 12 or 14 carbon atoms) onto the benzyl unit. The cumulative results of the elemental analyses as well as the 1H, 13C NMR and IR spectroscopies support the structure of the organic derivatives and their lanthanide complexes [LnL3(NO3)3] (9-n/Ln and 10-n/Ln) described in this work. These complexes show characteristic lanthanide solid state emission, both at room and elevated temperatures corresponding to crystalline, glassy or liquid crystal states. The long range SmA phases displayed by all complexes were supported by a combination of characterization methods, including: differential scanning calorimetry (DSC), polarizing optical microscopy (POM) and variable-temperature powder X-ray diffraction (XRD). This work shows that the number, substitution pattern and length of flexible alkoxy chains are important parameters to control the phase-transition characteristics of the lanthanide complexes. Complexes with 3,4-disubstituted pattern (9-n/Ln) show higher clearing temperatures (nearly 60 °C for Eu3+, 70 °C for Sm3+ and 85 °C for Tb3+ complexes) compared to their counterparts with 3,5-disubstituted pattern (10-n/Ln). Moreover, complexes 9-n/Ln crystallize when cooling their LC phase while complexes 10-n/Ln are stable in glassy state at room temperature as a consequence of the different close interdigitated molecular packing evidenced by XRD measurements. Dielectric spectroscopy was employed to detect the changes of order degree specific to each phases (crystalline, LC or isotropic). The variation of dielectric constant and the electrical conductivity versus temperature shows three transition intervals for selected complexes 10-14/Sm and 10-14/Tb, which delimit the main intervals: 45–60 °C, 90–110 °C; 140–160 °C corresponding to the Cr1-Cr2, Cr2 – SmA and SmA-Iso transitions, and agree very well with the DSC results. The change of characteristic time, obtained by Havriliak-Negami fit function, with temperature also provides a very good correlation with the DSC and POM results.

The application of liquid crystalline epoxy resin for forming hybrid powder coatings

The purpose of the study was to check whether the addition of a liquid crystalline epoxy resin containing rigid mesogenic groups would have a positive effect on the properties of epoxy/polyester hybrid powder coating. The research included the preparation of three hybrid compositions of epoxy/polyester powder coating consisting of polyester resin GS 7371 with the addition of commercial bisphenol A-based epoxy resin E 011 (Composition 1), mixture of epoxy resin E 011 and liquid crystalline diepoxy resin MU22 (Composition 2) and only liquid crystalline diepoxy resin MU22 (Composition 3). By using the DSC method, the course of the cross-linking process of the prepared compositions was analyzed, and the curing conditions were selected. Based on the DSC analysis, it was also found that the addition of diepoxide MU22 reduces the glass transition temperature of the obtained coatings. For coatings cured on a steel substrate, a number of tests were carried out on the basis of which the surface structure was correlated with the chemical structure of the coatings and macroscopic surface behavior, i.e.: color, thickness, gloss, roughness, abrasion and scratch resistance, hardness, elasticity, contact angle, surface free energy, as well as adhesion to steel surface. It has been found that the modification of polyester/epoxy hybrid powder paint with liquid crystalline epoxy resin had a positive effect on its properties.

Effects of Copolymer Composition on Phase Transition Behavior and Mobility of Mesogenic Group for Side-Chain Liquid Crystalline-Amorphous Random Copolymers

Effects of Copolymer Composition on Phase Transition Behavior and Mobility of Mesogenic Group for Side-Chain Liquid Crystalline-Amorphous Random Copolymers

Liquid-Crystalline Thermally Activated Delayed Fluorescence: Design, Synthesis, and Application in Solution-Processed Organic Light-Emitting Diodes.

Realizing both high efficiency and liquid crystallinity in one molecule remains a challenge in thermally activated delayed fluorescence (TADF) emission. Herein, two isomeric compounds─m-DPSAc-LC and p-DPSAc-LC with different connection positions between donor and acceptor moieties─were synthesized and characterized. Diphenylsulfone (DPS) was used as the acceptor, acridine (Ac) was used as the donor, and biphenyl derivatives (LC) were employed as the mesogenic group. Both compounds showed a smectic mesophase evidenced by differential scanning calorimetry (DSC), polarized optical microscopy (POM), and temperature-dependent small-angle X-ray scattering (SAXS). The compound p-DPSAc-LC clearly exhibited thermally activated delayed fluorescence due to the much more distorted geometry, whereas m-DPSAc-LC showed simple fluorescence. Compared to the parent TADF molecules without appended mesogenic groups (DPS-Ac), these liquid-crystalline emitters possessed higher hole mobilities and improved device performance. The OLEDs fabricated via solution processing using the liquid-crystalline compound p-DPSAc showed a maximum external quantum efficiency of ∼15% and as such is the first example of a liquid-crystalline TADF material in an OLED device.

Low-melting point eutectic mixture of two bismaleimides containing polymethylene flexible and aramide-arylate mesogen groups

Low-melting point eutectic mixture of two bismaleimides containing polymethylene flexible and aramide-arylate mesogen groups

Aligned structures of mesogenic motifs in epoxy resin and their thermal conductivities

The epoxy-based crosslinked polymer with the mesogenic group has been studied as a candidate resin material with high thermal conductivity due to the ordered structure of the mesogenic groups. In this study, we conducted all atomic molecular dynamics simulations with iterative crosslinking procedures on various epoxy resins with mesogenic motifs to investigate the effect of molecular alignment on thermal conductivity. The stacked structure of aromatic groups in the crosslinked polymer was analyzed based on the angle-dependent radial distribution function (ARDF), where the resins were categorized into three groups depending on their monomer shapes. The thermal conductivities of resins were higher than those of conventional polymers due to the alignment of aromatic groups, but no distinct correlation with the ARDF was found. Therefore, we conducted a further study about two structural factors that affect the alignment and the TC by comparing the resins within the same groups: the monomer with an alkyl spacer and functional groups in hardeners. The alkyl chains introduced in the epoxy monomers induced more stable stacking of aromatic groups, but thermal conductivity was lowered as they inhibited phonon transfer on the microscopic scale. In the other case, the functional groups in the hardener lowered the TC when the polar interaction with other polar groups in the monomer was strong enough to compete with the pi–pi interaction. These results represent how various chemical motifs in mesogenic groups affect their alignment on the atomistic scale, and also how they have effects on the TC consequently.

Thiol-acrylate side-chain liquid crystal elastomers.

The Michael addition ‘click’ chemistry was used to graft acrylate-terminated mesogenic groups onto the polysiloxane backbone polymer chain with thiol functional groups, with a constant 15% fraction of diacrylate reacting monomers as crosslinkers. Three different types of mesogens were used, and also their 50 : 50 mixtures, and in all cases we have obtained the smectic-A phase of the resulting liquid crystalline elastomer. Using X-ray diffraction, calorimetry and dynamic mechanical analysis, we investigated the relationship between the molecular structure of mesogenic side groups and the structure and properties of the elastomers. The shape-memory of smectic elastomers was verified. The unusual features were the semi-crystalline nature of elastomers with non-polar mesogens and the clear role of side-by-side rod dimerization of polar mesogens leading to a higher smectic layer spacing. We investigated the evolution of the smectic alignment on uniaxial stretching along the layer normal and identified two distinct ways in which the elastomer responds: the coarsened Helfrich–Hurault zig-zag layer texture and the large-scale stripe domains of uniform layer rotation in the systems with lower order parameter and the associated layer constraints.

High intrinsic thermally conductivity side-chain liquid crystalline polysiloxane films grafted with pendent difunctional mesogenic groups

Herein, the microscopic ordered aggregation morphologies of SCLCP films are investigated, and molecular structures with regular arrangement can increase heat transfer via suppressing the scattering of phonons, thus greatly improving the λ of SCLCPs.

A Review on Synthesis, Properties, of Liquid Crystal Dendrimers

This review provides brief information concerning with the dendrimer. The supramolecular organization of selected examples of liquid-crystalline dendrimers within lamellar, columnar and nematic phases is reported. It is shown that tuning of the mesomorphic structure can be achieved by an appropriate molecular design depending upon the chemical nature of the terminal mesogenic groups, dendritic core and dendrimer generation. The pseudospherical shape of a dendrimer arises from its structure, which consists of an internal region (the core) which is connected to repeating units constituting a radial branching pattern.