Field-induced Colour Change of Liquid Crystalline Dyes

Planar oriented nematic layers and also partially oriented smectic C phases of certain diphenyl-1,2,4,5-tetrazine compounds due to their positive dielectric anisotropy can be oriented by an electric field (Freedericksz-transition). The orientation is connected with a distinct change of absorption respectively colour. The mechanism of the effect is discussed. For both liquid crystalline phases the response times and decay times were measured.     

Highly Dichroic Triphenodioxazine Dyes for Guest Host Liquid Crystalline Mixtures

Abstract

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.     

The Use of Azo Dyes in Guest-Host Displays

Azo dye molecules are cylindrically shaped, and therefore, due to their resemblance to liquid crystal molecules, would seem to be suitable for use in guest-host mixtures. The dichroic ratio of a dye in a given nematic host depends on the molecular length of the dye molecules, the order parameter of the liquid crystal host and its molecular dimensions. It is possible to obtain a guest-host mixture with an order parameter which is higher than that of the pure host. Lateral substitution of mono-azo dyes lowers the order parameter, whilst lateral substitution of the corresponding bis-and tris-azo molecules results in only a slight change in their order parameter.

The stability of these compounds to light is poor, and depends on the wavelength for maximum absorption, and on the host material. Whilst lateral substitution increases the fastness of azo compounds when used for textile dying, this is not the case when they are employed with liquid crystal mixtures. The kinetics of degradation are zero order in dye concentration.

It has been found that azo dyes are not in general suited to applications where prolonged exposure to sunlight may be encountered, e.g. watch applications.     

The Crystalline state and the Mesophases of Cholesteryl Oleyl Carbonate

The preparation of pure crystalline cholsteryl oleyl carbonate is described. The melting point, the mesophase transition temperatures, and the enthalpy of the melting process are recorded, and a method of maintaining cholesteryl oleyl carbonate in a pure state over extended periods of time is suggested.     

Dynamic Mechanical Behaviour of Some Cholesteryl Liquid Crystals

Dynamic mechanical studies of cholesteric liquid crystalline substances in the solid state have been reported using Du Pont's Dynamic Mechanical Analyser, DMA-981. All the substances showed multiple transitions. There is no common transition though the parent structure was similar in the compounds. These weak transitions are not detectable by DSC and hence appear to be second order transitions possibly arising due to rotation of the molecules around the long axis. Alternatively the lattice defects may also be responsible for such transitions.     

Liquid Crystalline Catalysis, 31, Monomolecular Rearrangements of Terpene Derivatives in Liquid Crystalline Solvents

The ability of limonene and linalool to rearrange in mesomorphic media is apparently determined by the constraints exerted by the solvent structure on the translational diffusions of the reactant solute molecules. Selective conversions seem to be promoted by the different media. Alkylcyclohexyl- and alkylbicyclohexyl-carboxylic acids and their mixtures with toluic acid have been used as solvents. The B structure of their smectic phases has been demonstrated by X-ray diffraction studies.     

Liquid Crystalline Compositions as Gas Sensors

Droplets and films of liquid crystalline mixtures were studied as promising detectors of volatile organic compounds (VOCs) in the air. Under increasing concentration of VOC in the air the detection may rely on each of the following effects sequentially observed one after the other: i. slight changes in orientation and order parameter of liquid crystal, ii. formation of bubbles on the top of the liquid crystalline droplet, iii. complete isotropisation of the liquid crystal. All three stages can be easily monitored by optical microscopy. Detection limits corresponding to the first stage are typically lower by a factor of 3–6 than detection limits corresponding to isotropisation. The qualitative model is presented to account for the observed changes.     

State of Order in Liquid Crystalline Elastomers

The orientation behavior of crosslinked liquid crystal-line side chain polymers is determined by IR-dichroism measurements. In stretched samples the conformation of the polymer main chain and the chemical constitution of the rodlike side chain determine the position of the director in relation to the axis of stress. The order parameter S has the same temperature dependence and is of the same magnitude as in the corresponding uncrosslinked liquid crystalline polymers. The order parameter and the nematic to isotropic phase transformation temperature are independent of the applied mechanical stress under the experimental conditions (elongation up to 50%).     

Conformational Analysis of Thermotropic Liquid Crystalline Polyesters

PCILO conformational calculations have been carried out on several model compounds of thermotropic liquid crystalline polyesters [—OC—φ—φ—φ—CO—Os—R—O—] where R = (—CH₂—)₂; (—CHCH₃—)₂; (—CHCH₃—CH₂—). Several conformations corresponding to the trans and gauche states of the C-C single bonds are preferred, the TGT form being of lower energy than the TTT form. The replacement of—CH₂—group by—CH(CH₃) group results in a decrease of the number of the energy minima, an increase in the energy of the molecule in all the preferred conformations, compared to that of the TGT form, a sharpening of the potential energy wells and an increase in the energy barriers. The presence of a methyl group also shifts the aliphatic chain away from the ideal trans and gauche positions.     

Kerr Effect Studies in Liquid Crystalline Substances

The static and dynamic pretransitional behaviour was investigated by the electric Kerr effect in three nematic substances above the clearing point. The measured Kerr constants exceed that of nitrobenzene by more than a factor of 50…︁ 80 which are due mainly to pretransition phenomena and the strong dipole moments of the compounds. A formula is given for the calculation of the Kerr constant. The latent heat at the nematic-isotropic phase transition calculated from the temperature dependence of the Kerr constant by means of the Landau theory is in good agreement with the experimental values. Size and relaxation behaviour of pseudonematic domains are determined by Kerr effect measurements.     

X-Ray Diffraction Studies of Liquid Crystalline Polymers

Thermotropic liquid crystalline copolyesters display an ordered fluid phase at elevated temperatures which permits the development of unusually high orientation at ambient temperatures. The transition which occurs upon cooling the high temperature nematic liquid crystalline phase very rapidly (e.g. fiber spinning) results in a polymeric glass with nematic structural order. Annealing increases structural order from the nematic glass toward ideal three dimensional crystalline order. Precision X-ray diffractometry has been used to directly observe changes in structural correlations which occur with annealing. Structural transitions range in character in these copolyesters from no change upon annealing to a transition from fully two dimensional to fully three dimensional structural order. Increased three dimensional order also results in substantially increased first order character of the high temperature phase transition. The chemical structure of co-monomers determines, in part, the final degree of dimensionality.     

Polarizabilities and Related Properties of Azo Benzene Liquid Crystal Compounds

A novel method, molecular vibration approach is presented and used to evaluate polarizabilities, polarizability anisotropies and diamagnetic susceptabilities of Azo Benzene liquid crystal compound. In this approach vibrational frequencies are used to calculate the force constants, mean amplitude of vibrations and bond polarizabilities. In addition, the variation of order parameter with temperature is also studied. The results are compared and discussed with the literature values.     

Blends of a Liquid Crystalline Copolyester with Polyethersulphone

Examination of the rheological properties of blends of polyethersulphone (PES) and a liquid crystalline polymer (LCP) has shown an approximately fourfold drop in viscosity at low shear rates for the addition of only 2% LCP. Optical microscopy of sections of extruded pellets (cut parallel and perpendicular to the extrusion axis) has been carried out. It is found that the LCP orientation varies significantly across the pellet. When the thin sections are annealed at temperatures～300° C the LCP tries to minimise interfacial contact with the PES suggesting a high surface energy.     

Synthesis and Properties of New Liquid Crystalline Materials

In the last years several reviews on liquid crystalline substances were published. These reviews concern different topics, however, mainly aspects of practical use of liquid crystals. Now we try to give a survey on the activities of the Halle Liquid Crystal Group in the last years with respect to synthesis and physicochemical investigation of new materials for liquid crystal displays.     

Resistivity Measurements on Aligned Amphiphilic Liquid Crystalline States

We measured the resistivity of a ternary liquid mixture consisting of decylammoniumchloride, ammoniumchloride and water (45:5:50 wt%). The mixture forms a nematic phase between 41°C and 61°C; below 41°C a neat soap, and above 61°C an isotropic micellar solution. The resistivity decreases with increasing temperature. In the nematic and in the neat soap phases the resistivity is anisotropic, and it is higher for currents parallel to the director. The anisotropy increases strongly with decreasing temperature, due to a much stronger temperature dependence of the resistivity parallel to the director. The changes accompanying the phase transitions are small. The surprisingly small change of the resistivity parallel to the director at the nematic to neat soap transition indicates that the lamellae in the neat soap contain a large number of defects.     

Temperature-dependent Anisotropic Nano-Molecular Orientations of Liquid Crystalline Materials

In the present work, our investigation is to study the optical anisotropic properties of the binary mixture of cyano-benzilidene-p-n-octyl-oxy-aniline (CBOOA), cholesteryl nonanoate (CN), which exhibits a very interesting liquid crystalline Cho-TGB-SmC-SmA-SmB phases sequentially when the specimen cooled from its isotropic phase. These phases have been characterized by using microscopic and optical anisotropic technique. The temperature variations of optical anisotropy and electrical conductivity have been discussed. X-ray studies are supported to calculate the nano-aggregated grain size of the molecules.     

Photodecomposition Rates of Some Anthraquinone Dyes in Liquid Crystals

Photodecomposition rates of five anthraquinone dyes dissolved in liquid crystal hosts were measured by exposure to radiation at 300-800 nm. Depending on structure, the decomposition rates varied by almost 3 orders of magnitude.     

Novel Liquid Crystalline Derivatives of Cubanes and Hydrogenolyzed Cubanes

The N-I transition temperatures for a range of 4-propyl 1-monoesters and several 1,4-diesters of cubane, bicyclo(2.2.2)octane, cydohexane and benzene have been determined. The differences between the values for the monoesters and diesters of each system and between those for the various ring systems for the mono- and di-ester series are discussed. Cubane has a very poor ability to generate nematic phases of high thermal stability, and the relative nematic stability of the benzene derivatives are significantly different in the two series, the diesters with a central benzene ring having the highest N-I transition temperatures. The values for diesters and monoesters of dihydrocubane are compared with those for cubanes and the effect of deviations from colinearity of the substituent bonds and of changes in flexibility are discussed.