Mesogenic Substituents Research Articles

Novel Mesogenic Vinyl Ketone Monomers and Their Based Polymers

In the present research, we have synthesized new vinyl ketone monomers with mesogenic substituents, namely, 8-(3'-chloro-4'-pentyl-[1,1'-biphenyl-4-oxy)oct-1-en-3-one (BVK) and 8-[2'-chloro-4‴-octyl-[1,1':4',1″:4″,1‴-quaterphenyl-4-oxy]oct-1-en-3-one (QVK). The comparison of BVK, QVK, and previously synthesized 8-((4″-((1R,4S)-4-butylcyclohexyl)-2'-chloro-[1,1',4',1″-terphenyl]-4-yl)oxy)oct-1-en-3-one (TVK) has revealed that all of them are able to form crystals, while their ability to exhibit liquid crystalline behavior depends on the number of phenyl substituents attached to the para-position of the phenoxy group and is observed for TVK and QVK only. All of the monomers are able to achieve self-polymerization upon heating and free radical polymerization in bulk or in solution under the action of the common radical initiator AIBN. We have also succeeded in the RAFT polymerization of the synthesized vinyl ketones BVK and TVK using asymmetrical trithiocarbonates. The synthesized poly(vinyl ketones) exhibit LC behavior and are able to undergo photodegradation upon UV irradiation.

Synthesis of well‐defined block copolymer composed of flexible amphiphilic poly(ethylene glycol) and hydrophobic liquid crystalline segments by living coordination polymerization of allene derivatives and its application to thin film with perpendicularly oriented cylindrical nanostructure

AbstractA block copolymer composed of a flexible polar poly(ethylene glycol) (PEG) and a less polar liquid crystalline poly(allene) segments is prepared by the living coordination polymerization of an allene derivative possessing trans‐azobenzene‐containing mesogenic substituent by the use of a π‐allylnickel macroinitiator bearing PEG segment. The thin film of the block copolymer is prepared by the spin coating of its solution onto mica or silicon wafer which proves to possess perpendicularly oriented nanocylindrical microphase separated structures as supported by the differential calorimetric, polarized optical microscopic, grazing‐incidence small‐angle X‐ray scattering, transmission electron microscope, and atomic force microscope measurements.

Redox-driven molecular switches consisting of bis(benzodithiolyl)bithienyl scaffold and mesogenic moieties: synthesis and complexes with liquid crystalline polymer.

Molecular switches composed of a benzodithiolylbithienyl scaffold and biphenyl or terphenyl mesogenic substituents were designed and synthesized. The molecular switches could undergo redox-triggered interconversion between the cationic form and cyclized neutral form, and this was confirmed using cyclic voltammetry and UV-vis spectroscopy. Binary complexes consisting of the molecular switches and a liquid crystalline polymer (LCP) were prepared to investigate the function of these redox-active molecular switches as actuating dopants. X-ray diffraction measurements were performed to determine the differences between the layer spacings of the complexes in the liquid crystalline phase with the oxidized and reduced states of the molecular switches. The LCP that was doped with the oxidized cationic form of the molecular switch had layer spacings that were up to 4% larger than the layer spacings in the polymer that was doped with the reduced cyclized molecular switch. Our approach will allow stimulus-responsive deformable materials to be constructed and give an impetus for fabricating redox-driven soft actuators.

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.

Modeling of Free Radical Polymerization of Azobenzene-based Linear Polymers

ABSTRACTModeling of free radical polymerizations of the liquid-crystalline monomer 6-[4-(4-heptyloxyphenylazo)phenoxy]hexylacrylate using the PREDICI software package is reported. The model accounts for all elemental reactions that were identified to be important for radical polymerizations of acrylate-type monomers. On the basis of butyl acrylate kinetic data a remarkable agreement between number average molar masses from modelling (Mn,sim) and from experiments (Mn,exp) is observed: Mn,sim = 17800 g·mol−1 and Mn,exp = 17400 g·mol−1. Similarly, dispersity values of 1.8 and 1.6 were determined via modelling and experiments, respectively. It is shown that the assumption of butyl acrylate kinetics provides a reasonable approximation even for acrylate-based monomers having mesogenic substituents.

New Fluorine Substituted Liquid Crystal Containing Bicyclo[2.2.2]Octane Unit

We have designed and synthesized a new series of liquid crystalline compounds with hybridized three-ring system based on bicyclo[2.2.2]octane unit together with fluorine substituted phenyl and biphenyl fragments. The key stages of the synthesis are alkylation of substituted benzene in concentrated sulfuric acid by bicyclo[2.2.2]octanoles and further cross-coupling between 3,4-difluorophenylboronic acid and the alkylation product. The influence of bicyclo[2.2.2]octane unit position in mesogenic core and fluorine substituents onto a phase transitions of the liquid crystals is discussed.

Addressing fluorescence and liquid crystal behaviour in multi-mesogenic BODIPY materials

Two series of BODIPYs (4,4′-difluoro-3a,4a-diaza-s-indacene) bearing mesogenic substituents have been synthesised and their fluorescent and self-assembly behaviour has been observed and characterised. In order to induce a strong preference for liquid crystallinity, two mesogenic units have been attached to each fluorophore. The compounds were prepared by microwave-assisted Suzuki couplings in moderate yields. Two different, but related, mesogens were employed but only one series was found to exhibit the formation of liquid crystal mesophases. Monotropic nematic phases, observed by differential scanning calorimetry (DSC) and optical polarising microscopy (OPM), were adopted over varying temperature ranges depending on the degree of alkyl substitution on the fluorophore. Increasing alkyl substitution caused a gradual increase in fluorescence quantum yield due to rotational restriction of the BODIPY 8-phenyl ring, however, this also disrupted the formation of the nematic phase causing a reduction in the temperature range over which the mesophase was exhibited. The large, branched structure of the compounds prepared caused them to have a high viscosity in both the nematic and isotropic liquid phase. Due to the high viscosity, the Schlieren texture of the nematic phase could only be observed after several hours annealing.

ChemInform Abstract: Syntheses of Ferrocene Derivatives with Potentially Mesogenic Functional Substituents.

ChemInform Abstract: Syntheses of Ferrocene Derivatives with Potentially Mesogenic Functional Substituents.

Introduction of unusual properties into polymers by the use of liquid‐crystalline moieties

AbstractThe introduction of mesogenic substituents on polymer backbones facilitates the formation of materials with anisotropic properties. At the same time the polymer chains are connected each other through these substituents in a transient network. The properties of the polymer material, such as the mechanical response or conductivity, are modified by the presence of the liquid‐crystalline groups and can be tuned by changing the organisation of the mesogenic groups. Crosslinking the polymer in the liquid‐crystalline state induces the manifestation of some memory effect. These phenomena were used for improving the mechanisms of recognition in molecular imprinted materials. Copyright © 2006 Society of Chemical Industry

Syntheses of dialkenes and diepoxides, and the influence of their structural parameters on liquid crystalline properties

Liquid crystalline dialkenes and diepoxides having different structural parameters were prepared and characterized. The compounds contained two or three aromatic rings in the mesogenic core. The base structure for the compounds containing two aromatic rings is the 4‐(ω‐oxiranylalkoxy)benzoic acid 4‐(ω‐oxiranylalkoxy)phenyl ester and for the compounds containing three aromatic rings the 4‐{[4‐(ω‐oxiranylalkoxy)benzoyl]oxy}phenyl 4‐(ω‐oxiranylalkoxy)benzoate. The hydroquinone moiety is either unsubstituted or contains polar nitrile or fluorine groups as substituents. The flexible spacers consist of different numbers of methylene groups terminated with double bonds or epoxide groups. Some of the diepoxides undergo thermal polymerization of the epoxide group during the determination of the transition temperatures, and therefore the dialkenes were also investigated because of their almost analogous phase behaviour, but with higher thermal stability. Most of the compounds studied show liquid crystalline behaviour. A systematic study is presented of the influence of structural parameters such as the number of aromatic rings in the mesogenic core, substituents and length of the flexible side chain, on the transition temperatures. The diepoxides substituted with polar nitrile or fluorine groups are suitable precursors for ordered network polymers.

Highly Dichroic Triphenodioxazine Dyes for Guest Host Liquid Crystalline Mixtures

Novel triphenodioxazines have been synthesized and evaluated. The dyes were not only highly dichroic in guest-host liquid crystal mixtures but also vivid because of a sharp peak in absorption spectra. Mesogenic substituents to the 2,9- position of the chromophore improve the order parameter in the mixture. Moreover, the mixtures also showed significant polarized photoluminescence. These results would reveal a few advantages of the novel dyes for some possible applications.

Orientation of Liquid Crystalline N-Substituted Polypyrroles

Liquid crystalline polypyrroles having a mesogenic substituent with oxobiphenyl and cyanobiphenyl mesogenic cores at the N-position were synthesized by Ni(cod)2 method. The polymers showing stable enantiotropic smectic phases with large domains and good reproducibility were macroscopically oriented by mechanical alignment techniques of rubbing and friction-transfer, respectively. The ordered structure was characterized from dichroic ratio of polarizing IR and PL spectra. In case of orientation by the rubbing method, arranged mesogenic side chains on the rubbed substrate induced a vertical alignment of polymer backbone to the rubbing direction. In contrast, the friction-transfer method selectively aligned rigid polymer backbone parallel to the friction direction, which successively induced a vertical arrangement of mesogenic side chains.

Polyacetylenes Bearing Mesogenic Side Groups: Synthesis and Properties, 1. Mesogenic Substituents with a Short Flexible Spacer

We report on the synthesis and polymerization of novel acetylenic monomers derived from 3-butyn-1-ol and bearing different potentially mesogenic substitutuents. Polymerization is carried out in solution with typical metathesis catalysts based on Mo and W and yields polyacetylenes with fairly high molecular weight and soluble in common organic solvents. Polymers are fully characterized by the point of view of the molecular structure, by GPC, FT-IR, NMR and UV/VIS techniques, and of the thermal and morphological behavior, by TGA, DSC, POM and X-ray diffraction experiments. Polymers with different amounts of cis and trans units are obtained, depending upon polymerization conditions. Although the monomers do not show any liquid crystalline behavior, polymers P2 and P3, containing in the side-chain respectively biphenyl and benzoyloxy benzoate as mesogenic cores, exhibit an enantiotropic smectic phase.

Photoinduced electron transfer in Langmuir–Blodgett films between the donor [5,10,15,20-tetra(pentyl-oxy-biphenyl-oxy- m-phenyl)-porphyrinato]zinc(II) and quinone acceptors, either separated or linked by a cyclohexylene-bridge

Complexes of zinc porphyrins with mesogenic substituents and quinones, as well as the corresponding mixtures of unlinked porphyrins and quinones, have been investigated in Langmuir–Blodgett (LB) mono- and multilayer films by means of steady-state as well as time-resolved fluorescence spectroscopy. The films were deposited onto quartz substrates. They consist of one or more layers of cadmium behenate (cadmium salt of docosanoic acid) containing either the donor (a zinc porphyrin), the acceptor (a quinone), or both. In the latter case, a covalently linked zinc porphyrin–quinone and mixtures of a zinc porphyrin and a quinone have been investigated. Steady-state S 2-fluorescence of zinc porphyrins was utilized as a reference to determine the rates of photoinduced electron transfer (PET), in order to characterize the intramolecular and intermolecular PET. This approach was found to be particularly useful for monolayer films. For PET of covalently linked zinc porphyrin–quinone compounds in monofilms, the same PET rates as in ethanol were observed (5.3×10 10 s −1). In the case of unlinked zinc-porphyrin/quinone mixtures, the PET rates depend on the concentration of quinones in the acceptor layers. Furthermore, the competition of energy transfer (between zinc porphyrins) and PET (between zinc porphyrins and quinones) in the cadmium behenate matrix is discussed.

Tri-substituted s-Triazines as Mesogenic Cores

Two series of equivalently- and inequivalently-tri-substituted s-triazines (as ESTs and ISTs, respectively) were synthesized and characterized in this study. More than one mesogenic biphenyl substituent were required to display mesomorphic properties, and two or three biphenyl substituents resulted in liquid crystalline textures under polarizing light. The mesophase transition temperatures for both IST and EST series are quite close despite their molecular weight difference. With longer sidechains, s-triazine for IST series formed a columnar hexagonal disordered mesophase (D hd); while for the analogous compound in EST series, a perfectly aligned layered structure was found to be co-existed with the D hd phase. With shorter sidechains, the s-triazines exhibited nematic mesomorphism.

Determination of the electron transfer parameters of a covalently linked porphyrin-quinone with mesogenic substituents — optical spectroscopic studies in solution

The photoinduced electron transfer (PET) of a covalently linked porphyrin-quinone with mesogenic substituents was studied using visible and near-IR (NIR) spectroscopy. Mesogenic substituents were introduced at the porphyrin moiety in order to mimic the anisotropic membrane properties of the native reaction centre of photosynthesis. Photophysical characterization of this system in homogeneous solution is a prerequisite for a better understanding of the effects occurring in anisotropic medium. For this reason, we studied the fluorescence and phosphorescence quenching and lifetime of the charge-separated state. Time-resolved fluorescence measurements indicated an effective singlet PET. The complete set of PET parameters was calculated using Marcus theory of non-adiabatic electron transfer (ET). Steady state measurement of singlet oxygen luminescence, which allows indirect access to phosphorescence quenching, indicated that no triplet PET was involved in the decay processes. Using transient absorption spectroscopy, the lifetime of the charge-separated state was found to be 1.9 ns.

Liquid Crystalline Behaviors of Conducting Polyacetylene Derivative with Mesogenic Substituent and Its Mixture with Ferroelectric Liquid Crystal

Liquid crystalline behavior has been studied in a polyacetylene derivative substituted with long side chains containing phenylcyclohexyl mesogenic units such as poly [5-( p-( trans-4'- n-octylcyclohexyl)phenoxy)-1-pentyne] (PPCH803A). A texture typical of smectic liquid crystal has been observed using an optical microscope. The smectic layer structure has also been confirmed by X-ray diffraction. Anisotropy of the optical absorption spectrum of the smectic phase, which is consistent with the results of the X-ray diffraction measurements, has been observed. The band gap of PPCH803A in the liquid crystalline phase has been evaluated as 2.9 eV, which is smaller than that in the isotropic liquid phase. The dielectric constant of PPCH803A changes stepwise at the phase transition temperatures. A mixture of PPCH803A with a ferroelectric liquid crystal of low molecular weight exhibits a large dielectric constant and unique ferroelectric behavior.

Electropolymerization of thiophene derivatized with a mesogenic substituent

The synthesis of a new mesogenic thiophene derivative is reported. The monomer, which exhibits interesting mesophase properties, contains a p‐cyanobiphenyl group that is electronically and sterically decoupled from the thiophene ring by means of an octyl spacer. Electropolymerization of this monomer produces a polymer with conjgation lenght, conductivity, and electroactivity comparable to those of poly (3‐alkylthiophenes). The use of indirect substitutional effects to control the long‐range order and dimensionality of conducting polymers is discussed.

New fluorinated chiral side-chain polymers with very large tilt angles in the smectic C* phase

Abstract A fluorinated chiral side-chain polysiloxane with very large tilt angles in the ferroelectric phase was studied. The optical response on an applied square wave could be analyzed in detail by an optical/electrical model for the cell, only assuming book-shelf geometry and negligible dielectric director torques. The model yields molecular tilt angles to a high accuracy by a curve-fitting procedure. Comparisons to other mesogenic substituents were also made, and indications were found that small (monatomic) substituents favor large tilt angles.

Liquid-crystalline properties of chiral malolactonate monomers and their comb-shaped polymers

Abstract A series of optically pure mesogenic ester precursors, and the malolactonate monomers containing mesogenic alcohols obtained from them were prepared and polymerized to homopolymers and copolymers. The liquid-crystalline properties of the precursors, monomers and polymers were investigated by differential scanning calorimetry, polarized light microscopy and wide angle X-ray diffraction. The liquid-crystalline properties of the chiral mesogenic precursors and monomers were only slightly influenced by the chemical structure, configuration and the alkyl spacer length of the mesogenic alcohol substituent of the ester. The precursor, 1-[6-(4′-hexyloxy-4-biphenylyl)oxyhexyl] hydrogen (S)-O-mesylmalate (V-(S)-6) formed smectic E and smectic A phases on melting, while the monomer, 6-(4′-hexyloxy-4-biphenylyl)oxyhexyl (R)-malolactonate (II-(R)-6) showed only a crystal-isotropic phase transition. In contrast, the homopolymer of the latter exhibited a chiral smectic C phase. Copolymers from 2-(4′-hexyloxy-4-...