Halogen-Bonded Liquid Crystals

While the halogen bond has been recognised and studied for over a hundred years, it is only in more recent times that chemists have begun to apply it and see its possibilities as another supramolecular interaction that can be deployed in the preparation of materials. This review takes one of those areas, liquid crystals, and considers examples of motifs that have been deployed successfully to generate new mesogens. In particular, rather than attempting to be comprehensive, the article reviews critically data from well-characterised systems and seeks to first make some comparisons with analogous hydrogen-bonded materials, before considering how the lability and flexibility of the halogen bond expresses itself in liquid crystal behaviour.     

梯形聚硅氧烷液晶的研究

梯形聚硅氧烷液晶具有独特的稳定性、透明性及成膜性等优点,是一种性能优异的新型功能材料。本文综述了梯形聚硅氧烷液晶近年来的发展现状。介绍了梯形聚硅氧烷液晶的结构特点,根据液晶基元接枝到梯形高分子主链上的方式,将梯形聚硅氧烷液晶分为鱼骨形和划艇形两种。总结了挂接不同侧基的梯形聚硅氧烷（包括梯形聚苯基倍半硅氧烷、梯形聚甲基倍半硅氧烷、梯形聚氢基倍半硅氧烷、梯形聚乙烯基倍半硅氧烷等）的合成路线。介绍了几种液晶基元不同的梯形聚硅氧烷液晶的合成方法。最后提出几个梯形聚硅氧烷液晶研究工作面临的问题,并对梯形聚硅氧烷液晶的发展前景作了展望。     

D-xylose Derivative Liquid Crystals

We studied the phase transition temperatures of a series of amphiphilic D-xylopyranose and D-xylofuranose derivatives in which the lipophilic part is an alkyl chain R (n-C_nH_2n+1), regiospecifically linked to D-xylose, at different positions, by Z which is an atom or a functional group (O, S, O–(CH₂)₃–S). The alkyl chain was moved from the C-1 to the C-5 position in the xylose moiety, thereby allowing us to compare directly the phase transition temperatures of the individual materials. These compounds give thermotropic and/or lyotropic liquid crystals. In some cases, we also observed solid–solid phase transitions. This revised version was published online in August 2006 with corrections to the Cover Date.     

Polymerizable Ionic Liquid Crystals

Polymerizable vinylimidazolium ionic liquids (ILs) that contain mesogenic coumarin and biphenyl units, respectively, have been synthesized. The N‐alkylation of N‐vinylimidazole with bromoalkylated mesogenic units 7‐(6‐bromohexyloxy)coumarin ( 1 ) and 4,4′‐bis(6‐bromohexyloxy)biphenyl ( 2 ) was then carried out. The thermal behavior of the obtained ILs 3 and 4 was investigated by differential scanning calorimetry and polarizing optical microscopy. These measurements showed that the attached mesogenic units induce the self‐assembly of ILs and, therefore, the occurrence of liquid crystalline phases. Subsequently, the ionic liquid crystals (ILCs) 3 and 4 were polymerized by a free‐radical mechanism.

     

Pyrrolidinium Ionic Liquid Crystals

N‐Alkyl‐N‐methylpyrrolidinium cations have been used for the design of ionic liquid crystals, including a new type of uranium‐containing metallomesogen. Pyrrolidinium salts with bromide, bis(trifluoromethylsulfonyl)imide, tetrafluoroborate, hexafluorophosphate, thiocyanate, tetrakis(2‐ thenoyltrifluoroacetonato)europate(III) and tetrabromouranyl counteranions were prepared. For the bromide salts and tetrabromouranyl compounds, the chain length of the alkyl group C_nH_2n+1 was varied from eight to twenty carbon atoms (n=8, 10–20). The compounds show rich mesomorphic behaviour: highly ordered smectic phases (the crystal smectic E phase and the uncommon crystal smectic T phase), smectic A phases, and hexagonal columnar phases were observed, depending on chain length and anion. This work gives better insight into the nature and formation of the crystal smectic T phase, and the molecular requirements for the appearance of this highly ordered phase. This uncommon tetragonal mesophase is thoroughly discussed on the basis of detailed powder X‐ray diffraction experiments and in relation to the existing literature. Structural models are proposed for self‐assembly of the molecules within the smectic layers. In addition, the photophysical properties of the compounds containing a metal complex anion were investigated. For the uranium‐containing mesogens, luminescence can be induced by dissolving them in an ionic liquid matrix. The europium‐containing compound shows intense red photoluminescence with high colour purity.     

Wettability of Nematic Liquid Crystals

Organic liquids such as nematic liquid crystals should wet solids with high surface energies like mica, yet they generally do not. In the model proposed here, the affinity to wet the solid in form of Hamaker forces is opposed by elastic effects due to nematic order. Results predict correctly that such liquids show small contact angles and the formation of ultrathin liquid films ahead of the bulk drops. Copyright 2001 Academic Press.     

Fluctuating Interfaces in Liquid Crystals

Summary: We review and compare recent work on the properties of fluctuating interfaces between isotropic and nematic liquid-crystalline phases. Molecular dynamics and Monte Carlo simulations have been carried out for systems of ellipsoids and hard rods with aspect ratio 15:1, and the fluctuation spectrum of interface positions (the capillary wave spectrum) has been analyzed. In addition, the capillary wave spectrum has been calculated analytically within the Landau-de Gennes theory. The theory predicts that the interfacial fluctuations can be described in terms of a wave vector dependent interfacial tension, which is anisotropic at small wavelengths (stiff director regime) and becomes isotropic at large wavelengths (flexible director regime). After determining the elastic constants in the nematic phase, theory and simulation can be compared quantitatively. We obtain good agreement for the stiff director regime. The crossover to the flexible director regime is expected at wavelengths of the order of several thousand particle diameters, which was not accessible to our simulations.     

二苯乙炔类液晶的合成

以碘代烷基苯（或联苯）与烷基苯乙炔为中间体，采用二价镍为催化剂的方法合成了二苯乙炔类液晶化合物。其相变过程通过差热分析测试，该类化合物表现出优良的液晶性能。     

含氟液晶研究进展

本文简述了含氟液晶的研究进展。根据小分子含氟液晶中氟原子或含氟基团的位置不同,将其分为3类：末端是氟原子或含氟基团的液晶、苯环上氢原子被氟原子取代的液晶、中心桥键上的氢原子被氟原子取代的液晶。根据小分子含氟液晶特点,归纳了氟原子或含氟基团对液晶分子物理性质的影响。同时对高分子含氟液晶的研究进展也做了介绍。     

聚合物分散液晶从液体桥的组装

We report the organization of polymer-dispersed liquid crystals (PDLCs) into ordered concentric rings over large areas by drying a drop of bound PDLC toluene solution (i.e., confined between a spherical lens and an indium tin oxide (ITO)-coated glass substrate; sphere-on-ITOgeometry). The formation of regular ring-like deposits was a direct consequence of controlled "stick-slip" cycles of three-phase contact line during the course of solvent evaporation, which was effectively regulated through the use of the sphere-on-ITO geometry. This simple approach based on controlled evaporative organization may provide a new means of processing polymer/LC mixture to produce ordered surface patterns in one step, where microscopic LCs are dispersed within the polymer matrix.     

Photo-switchable Fluorescence in Hydrogen-Bonded Liquid Crystals

A series of hydrogen-bonded liquid crystals showing switchable fluorescence is reported. The fluorescence behavior results from the unique combination of hydrogen bonding, liquid crystallinity, and photobasicity. Thus, the molecular mobility in the mesophase is essential for the reversible photo-initiated proton transfer switching on the fluorescence of the assemblies. The application potential of the materials for photo-patterning was demonstrated.     

Liquid Photonic Crystals for Mesopore Detection

Nitrogen adsorption–desorption for mesopore characterization requires the using of expensive instrumentation, time‐consuming processes, and the consumption of liquid nitrogen. Herein, a new method is developed to measure the pore parameters through mixing a mesoporous substance with a supersaturated SiO₂ colloidal solution at different temperatures, and subsequent rapid measurement of reflection changes of the precipitated liquid photonic crystals. The pore volumes and diameters of mesoporous silica were measured according to the positive correlation between unit mass reflection change (Δλ/m) and pore volume (V), and the negative correlation between average absorption temperature (T) and pore diameter (D). This new approach may provide an alternative method for fast, convenient and economical characterization of mesoporous materials.     

Ionic Liquid Crystals with Hemicyanine Chromophores

(E)-N-alkyl-4-(2-(4-dimethylaminophenyl)ethenyl) pyridinium bromides (hemicyanine bromides) with C₁₄, C₁₆ and C₁₈ straight alkyl chains are described. The thermal behaviour of these compounds was investigated by differential scanning calorimetry (DSC) and by optical microscopy. The textures observed in polarised light revealed that these compounds exhibit a smectic A mesophase. A focal-conic fan texture was observed, together with regions showing homeotropic alignment. The phase behaviour of the compound with the C₁₆ chain can be summarized as: Cr₁·99·Cr₂·249·SmA·268·I (dec.). The transition temperatures are rather independent of the length of the alkyl chain. The compounds have a strong tendency to supercool to a glassy mesophase. A structural model for the mesophase is proposed. The bromide anion of the hemicyanine bromides can be replaced by the bulky tetrakis(-diketonato)lanthanide(III) anion. As the -diketone, 2,4-pentanedione (acetylacetone, acac) and 1,3-diphenyl-1,3-propanedione (dibenzoylmethane, DBM) are chosen. None of the lanthanide complexes containing the hemicyanine chromophore shows a mesophase. The complexes either melt directly to an isotropic liquid, or decompose without melting.     

The Synthesis of Cyclotriphosphazene Liquid Crystals

Generally,thermotropicliquidcrystalsareconstructedoforganiccomPoundsofanisotropicshaPe.Inthesearchfornewmoleculardesignsformesogensoforiginalshapes,onecanroughlyfollowtwodirections.Thefirstideaistovarythenumberofalkylchainsgraftedaroundagivencore.Thesecondideaofshapemodificationconsistsintheformationofclustersofafewidenticalmesogens,generallyatwin.Accordingtothesetwoideas,manynewkindsofliquidcrystalshavebeensysthesized.Hexakis(4-(4'-alkoxy)biphenoxy)cyclotriphosphazeneswerethemostinteresting…