Angle resolved fluorescence depolarization experiments on smectic mesophases

Second and fourth rank order parameters (<P₂> and <P₄>) have been determined for some smectics from measurements of the angle-resolved fluorescence depolarization profile of a probe embedded in the systems. It is shown that the degree of order can be separated from the preferential molecular orientation when both <P₂> and <P₄> are known. The degree of order could be described by an extended mean field potential.     

Thermally stable azo-substituted bent-core nematogens: observation of chiral domains in the nematic mesophases composed of smectic nano clusters

In an effort to synthesise thermally stable bent-core nematogens with fairly low transition temperatures and wide nematic mesophase range, we have synthesised five new series of azo-substituted bent-core compounds without Schiff’s base unit. Here, we studied the effect of different lateral substituents (–CH₃ and –Cl), at two different positions in one of the arms, on the mesogenic properties. We found that such variation in the molecular architecture has a clear-cut impact on the transition temperatures as well as the liquid crystalline properties. The mesogenic properties of these compounds were studied using polarising optical microscopy and differential scanning calorimetry. The exact nature of different mesophases is investigated using X-ray diffraction studies. The results obtained indicated the presence of smectic nano clusters in the nematic mesophases of these compounds. We observed domains of opposite handedness in these nematic phases. The unusual properties of these mesophases were investigated by electro-optical studies. The electro-convection pattern study showed that these nematic mesophases are of negative dielectric anisotropic in nature. The compounds synthesised here exhibit photo-switching both in solution and in their nematic mesophases, a property that can be exploited for practical applications.     

New inductions of columnar mesophases in nematic discotic multiyne mesogens [1]

In examples of two large naphthalene- and triphenylene-centred hexaynes exhibiting only the nematic discotic (N_D) type of mesophase, a new kind of phase induction was observed. These disc-shaped nematogens were doped with numerous dipolar carbocyclic compounds leading to the induction of two types of columnar mesophase (i.e. D_ro and D_ho). The phase behaviour of these binary systems, depending on the structure of the dopant applied, was studied by polarizing microscopy.     

Two unusual mesophases in chiral side chain polymers

Abstract

We have reported in a previous paper the synthesis, characterization, structural and electro-optic properties of a new family of chiral mesomorphic side chain polyacrylates. The most salient finding was that many of these polymers presented two S*_C phases of similar structure, but differing in their switching properties. In addition, two unusual mesophases called U1 and U2 were also discovered in this family. We describe here in more detail the symmetries and molecular organizations for these U1 and U2 phases. We also discuss their relation, on the one hand, to the newly discovered antiferroelectric chiral S*_CA and S*_o phases, and, on the other hand, to the 2-dimensional fluid smectic à and smectic °C phases displayed by strongly polar low molar mass mesogens. The occurrence of chevron-like ordering in the U1 and U2 phases may give a clue to an understanding of this peculiar S*_C polymorphism.     

Electrohydrodynamic instabilities in chiral smectic C cells

Experimental results are presented on the optical response of chiral smectic C cells of various thicknesses (between 2 and 9 μm) on AC voltages. For part of the voltage and frequency range the electric field induces patterns of disclination lines. The threshold voltages for these electrohydrodynamic instabilities turn out to be independent of the cell thickness for 4, 6, and 9 μm cell gaps. In the 2 μm cells, structural changes of the chiral smectic C texture are found at voltages below the threshold for the instabilities. The applicability of the Carr-Helfrich model for electrohydrodynamic instabilities in nematics to these chiral smectic C layers is discussed.     

Formation of smectic mesophases in binary systems of short chain alkanoic acid salts

The formation of thermotropic ionic smectic liquid crystals in binary systems of lithium and caesium acetates, propionates and butyrates has been discovered. Mesomorphism of mixed melts of non-mesogenic components results from the formation of complex anions. Correlations between anion mesogenic properties and its size, shape, anisotropy in the Coulomb charge distribution as well as the nature of the surrounding cations are discussed.     

Twin-like domains in chiral smectic C liquid crystals

In the chiral smectic C phase of liquid crystals with the phase transition N*-SmC*, texture development depending on the sample thickness is reported. In very thin samples, domains of rectangular-like shape are observed. As two possible tilts of smectic layers are possible for one anchoring direction, smectic layers inside a domain, called twin-like domains, are tilted with respect to layers in outer regions, similarly to crystalline planes in solid crystalline twins. An elastic model of such a twin domain is proposed and its energy determined.     

A chiral metallacyclophane for asymmetric catalysis

Chiral metallacyclophanes were self-assembled from cis-(PEt3)2PtCl2 and enantiopure atropisomeric 1,1'-bina-phthyl-6,6'-bis(acetylenes) and used in highly enantioselective catalytic diethylzinc additions to aldehydes to afford chiral secondary alcohols.     

Induction of a chiral nematic phase in smectic polymers

A number of new photosensitive copolyacrylates of different composition were obtained by the copolymerization of chiral photochromic benzilidene-p-menthane- 3-one acrylic monomers with a smectogenic monomer containing a hexyloxyphenylbenzoate mesogenic group. The chiral, photochromic monomers differ by the length of the aliphatic spacer and the aromatic fragment. It was found that the introduction of a small number of chiral units into the copolymers (5 mol%) leads to the “degeneration” of the smectic C phase, which characterizes the hexyloxyphenylbenzoate homopolymer, and to the formation of the smectic A phase. An unusual effect of chiral nematic phase induction was observed for copolymers containing chiral side groups with two ring aromatic fragments. It should be pointed out that the chiral nematic phase does not occur in the case of the homopolymers of both initial comonomers. An explanation of this effect, based on the consideration of the chemical structure of the chiral and hexyloxyphenylbenzoate units, was suggested. The optical properties of cholesteric copolymers were investigated; the helical twisting power of the chiral groups of different structures was calculated. The possibility of using such copolymers as new photosensitive materials was demonstrated. Received: 16 December 1999/Accepted: 1 February 2000     

Nematic and smectic mesophases in a new series of Cu(II) organometallic complexes

The synthesis and the phase behaviour of a homologous series of copper, bis[N-[[4-[4-(alkoxy)benzoyloxy],2-hydroxyphenyl]methylene]-methanamino] complexes is reported. They all exhibit thermotropic mesomorphism. The thermal stability range of the mesophase decreases slightly with increasing length of the alkoxy terminals. With the onset of smectic mesomorphism, for alkoxy groups containing ten carbon atoms, the stability interval of the nematic phase decreases progressively so that the compound containing 14 carbon atoms in the alkoxy terminal chains exhibits only smectic mesomorphism. Optical texture analysis suggests the smectic C nature of the phase in all cases. The enthalpies for the liquid crystal-isotropic transition have been measured and they are comparatively low. Remarkably low values have also been measured for the melting enthalpies of the smectogenic members of the series. These are associated with solid phase polymorphism which is discussed briefly.     

A liquid-crystalline silsesquioxane dendrimer exhibiting chiral nematic and columnar mesophases

A hexadecamer, first-generation, octasilsesquioxane liquid-crystalline dendrimer was synthesized by a platinum-catalyzed hydrosilylation reaction of the parent first-generation vinyl octasilsesquioxane dendrimer with a modified, laterally substituted mesogen. The structure and purity of the octasilsesquioxane substrate was confirmed by 1H, 13C, and 29 Si NMR spectroscopy, microanalysis, and size exclusion chromatography (SEC). The mesogenic substrate was found to exhibit only a chiral nematic phase, whereas the resulting hexadecamer dendrimer displays enantiotropic chiral nematic, disordered hexagonal columnar, and disordered rectangular columnar phases, with a glass transition below room temperature. The lateral or side-on attachment of the mesogen to the dendritic core was found to be a key design feature in the formation of the mesophases.     

Paramagnetic chiral mesophases of Shiff's base complexes of transition metals

Copper(II), nickel(II) and oxovanadium(IV) complexes of 4-(4-heptyloxybenzoyloxy)-N-(S)-2-methylbutylsalicylaldimine, as well as the parent ligand, were studied by optical and DSC methods. For the first two complexes, there exist between a tightly twisted chiral nematic phase and the isotropic liquid either some blue phases or novel-type amorphous phases, with their temperature ranges depending significantly on the metal centre. For the third complex, direct isotropization takes place.     

Macroscopic properties of mixed chiral smectic liquid crystals

Binary mixtures of chiral and non-chiral smectogens, DOBAMBC and DBHP respectively, exhibit a chiral smectic C phase in two separated concentration regions (about 0-20 and 60-100 mol per cent of DOBAMBC) and a chiral smectic I phase in the region of 60-100 mol per cent of DOBAMBC. For the S*_cphase, the temperature and the concentration dependences of the spontaneous polarization, the tilt angle and the helical pitch were determined. The electric field switching properties of the S*_iphase have been studied. For higher concentrations, the switching is similar to that of the S*_iphase of pure DOBAMBC. When the concentration of DOBAMBC is lowered, bistable switching is observed.     

Novel polymer-supported chiral catalysts for asymmetric synthesis

Hydroxy amino acid monomers 3, 5 and amino acid monomers 10, 11 were prepared. Suspension polymerization of these chiral monomers with styrene and crosslinking agent afforded insoluble chiral polymers which were used as chiral ligands of the enantioselective Diels-Alder reaction catalysts. A continuous flow system using polymeric catalyst was also successfully achieved in the same reaction.     

Liquid-crystalline side chain polymers containing a chiral spacer unit exhibiting chiral smectic phases

The synthesis and mesomorphic properties of two liquid-crystalline side chain polymers with a chiral centre in the α or β position of the α-hydroxy acid representing the spacer unit are described. The chiral α branching leads to a dramatic decrease in the transition temperatures and a strong narrowing of the smectic mesophase (compared with the unbranched model compound I). The chiral β branching results in a chiral smectic phase, a pronounced contraction of the S_c phase, and the loss of the higher ordered S_f phase. The S*_c phase was confirmed by X-ray investigations of oriented samples. Depending on the polymerization conditions samples were obtained which were oriented in melt drawn fibres either with their smectic layers or their mesogenic units in the direction of stress.