Measurements of optical anisotropy of a calamitic lyotropic liquid crystal

Optical birefringence measurements for a calamitic lyotropic liquid crystal containing sodium lauryl sulphate/decanol/water are presented. A value of the order of 10^‐3 was obtained, much smaller than that of thermotropic liquid crystals. The order parameter temperature dependence is evaluated using an extrapolating method and taking into accounts the influence of the micelle shape. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Biphenylbenzoates: Synthesis and Thermodynamic Properties

Two homologous series of twelve compounds each of biphenylbenzoates have been synthesised and their mesomorphic properties characterised. Heats of transition for both series of compounds have also been measured using differential scanning calorimetry. In the biphenyl 4-p-n-alkoxybenzoate series the first few members are monotropic nematic and the higher ones exhibit enantiotropic nematic as well as smectic phases, whereas in the biphenyl 4-p-n-alkyl benzoate series only the monotropic nematic phase is observed. In both series of compounds the plot of the nematic-isotropic transition temperature against the number of carbon atoms in the alkyl chain exhibit smooth curve relationships.     

Texture transformations and orientational properties of lyotropic nematics in magnetic field

Dynamics of changes of the morphological properties and processes of orientation in lyotropic nematic mesophases by application of external magnetic field have been investigated in detail. The dynamics of changes of the optical birefringence values in nematic‐calamitic mesophase vs. time has been determined. The optical mappings of nonequilibrium magnetically induced textures, which have been observed for this mesophase during orientational process, and changes of birefringence vs. time are presented. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Refracting properties of lyotropic nematic calamitic mesophase under magnetic field

The present study found the time-dependent extraordinary refractive index and birefringence of lyotropic nematic calamitic mesophase for different temperatures during its orientation under magnetic field. Based on the experimental results, the average angle between the mesophase directors and magnetic field was calculated to estimate the orientation dynamics of the mesophase during the director rotation from planar to homeotropic orientations. On the other hand, the textural properties were investigated during the orientation. The ordinary and extraordinary refractive indices were also measured depending on temperature to obtain the orientational order parameter of the mesophase. Using the macroscopic refractive index results, the microscopic polarizability was studied as well.     

Smectic A-Nematic and Nematic-Isotropic Liquid Transitions in 8CB Liquid Crystal: Comparative Analysis

A differential heat flux calorimeter has been used to study the Smectic A-Nematic and Nematic-Isotropic Liquid transitions in 8CB liquid crystal. The Faktor and Hanks model under our working conditions enables us to distinguish first-order from second-order transitions by analysis of differential signal. The critical exponent values obtained justify the proportionality between Cp evolution and the normalized differential signal (differential signal divided by the scanning rate).     

Boron clusters as structural elements of novel liquid crystalline materials

closo-Boranes are characterised by high thermal and oxidative stability due in part to highly delocalised bonding within a σ-framework. These boron clusters when employed as structural elements of liquid crystals give rise to novel structures with unique properties and vast potential in applications. The neutral closo-boranes such as the carboranes provide opportunity to perform fundamental structure–property relationship studies that may provide additional insight on the liquid crystalline state. The anionic closo-borates possessing a highly delocalised negative two charge can be used as structural elements of zwitterionic, quadrupolar liquid crystalline materials. Such materials may be of use for understanding the effects of polarity on the liquid crystalline state. Lastly, the anionic closo-monocarbaborates possessing a single highly delocalised negative charge can be envisioned as structural elements of either highly polar or ionic liquid crystalline materials. The highly polar materials may also serve as models for understanding the effects of polarity on the liquid crystalline state. Additionally, they should have application as positive Δ? additives for nematic liquid crystalline mixture formulations. The ionic materials, where the anionic fragment drives liquid crystalline behaviour, lend themselves as potential electrolytes for anisotropic ion transport in battery technologies. The review presented here introduces these concepts and highlights the current status of boron cluster containing liquid crystalline materials.     

Imidazolium based ionic liquid crystals: structure,photophysical and thermal behaviour of [Cnmim]Br·xH2O (n = 12, 14; x=0, 1)

The long chain imidazolium halides [C_nmim]Br·xH₂O (n = 10, 12; x = 0, 1) have been synthesized and their structural and thermal behaviour together with their photophysical properties characterized. X‐ray structure analyses of the monohydrates ([C₁₂mim]Br·H₂O: triclinic, P1, no. 2, Z = 2, Pearson code aP112, a = 550.0(5) pm, b = 779.4(5) pm, c = 2296.1(5) pm, α = 81.89(5)°, β = 83.76(5)°, γ = 78.102(5)°, 3523 unique reflections with I_o > 2σ(I_o), R₁ = 0.0263, wR₂ = 0.0652, GooF = 1.037, T = 263(2) K; [C₁₄mim]Br•H₂O: triclinic, P1, no. 2, Z = 12, Pearson code aP11, a = 549.86(8) pm, 782.09(13) pm, c = 2511.3(4) pm, α = 94.86(2)°, β = 94.39(2)°, γ = 101.83(2)°, 2063 unique reflections with I_o > 2σ(I_o), R₁ = 0.0429, wR₂ = 0.0690, GooF = 0.770, T = 293(2) K) show for both compounds similar bilayered structures. Sheets composed of hydrophilic structure regions constituted by positively charged imidazolium head groups, bromide anions and hydrogen bonded water alternate with hydrophobic areas formed by interdigitated long alkyl chains belonging to imidazolium cations with different orientation. Combined differential scanning calorimetry and polarizing optical microscopy shows that the monohydrates as well as the anhydrous imidazolium salts are thermotropic liquid crystals which adopt smectic mesophases. The mesophase region is larger in case of the monohydrates when compared to the anhydrous compounds indicating that water obviously stabilizes the mesophase. All compounds show an intense whitish photoluminescence with short lived (¹π←¹π*) and long lived (¹π←³π*) transitions. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Guest-Host Type Liquid Crystal Displays

Eight modes of the guest-host cells (GH-cells), in which types of dye, liquid crystal and their alignments are adequately combined, are surveyed, and the color contrast of each mode is calculated.

Three modes among them are of positive type display, that is, colored patterns on the colorless background contrary to the initially proposed fundamental GH-cell. Another three modes are the GH-cells without polarizer, by which bright display can be obtained.

Light stability of dyes are also discussed and lifetimes of various dyes are estimated by accelerated life test. As a result, it is found that diazo-and axomethine-dayes without thiazole groups as well as anthraqunone-dyes have satisfying lifetimes as long as they are used indoors.     

Hall Effect in Lyotropic Liquid Crystals

It is known that Hall effect takes place in different media. The Hall effect is well studied in semiconductors. It is known quantum, quantum spin, the anomalous Hall effect, as well as the Hall effect in ionized gases. However, up to date, as evidenced from the review of scientific literature, the study of this effect in lyotropic liquid crystals (LLC) was given insufficient attention. In this paper the peculiarities of the Hall effect in the LLC were studied. It was shown that in liquid crystal systems Hall potential is approximately 100 times greater than in semiconductors. Also it was shown that after removal of the electric and magnetic fields, the Hall potential in LLC remained intact, decreasing over time, and periodically reversing its sign.     

Temperature dependence of the density of some liquid crystals in the alkyl cyanobiphenyl series

Experimental results for the temperature dependence of the density of propyl – cyanobiphenyl (3CB), butyl – cyanobiphenyl (4CB) and hexyl – cyanobiphenyl (6CB) are presented. The results are compared with previous results for temperature dependence of other members of the alkyl cyanobiphenyl series (nCB). The deviation of the density from linear temperature dependence is discussed in terms of the recently discovered corresponding rule for nematic liquid crystals. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

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.     

Deformations Periodiques Induites par une Forte Agitation Hydrogynamique dans un Nematique en Regime de Conduction

On montre que dans une couche de M.B.B.A. soumise à une tension d'excitation très supérieure au seuil V_s de l'linstabilité électrochydrodynamique de type Carr-Helfrich-Orsay (V/V_s > 2.4) ilya persistanc des féformations périodiques de l'alignement moléculaire. De plus, une nouvelle dé-formation associée à un écoulement hydrogynamique dans le plan jperpendiculaire au champ appliqueé a été mise en évidence. Cet écoulement et la déformation correspondante peruvent etre attribués aux jphénomènes électroconvectifs des phases isotroes.     

Magnetic Order in Organic Superconductors

We have undertaken an electron spin resonance study at both low and high magnetic field on the superconducting phase of the ambient pressure superconductor (BEDT-TTF>₂I₃, (ET)₂I₃, over the temperature range 1-300 K and hydrostatic pressure range from 0 to 2 kbar. At ambient pressure the ESR results are consistent with the picture of (ET)₂I₃ as a metal. Superconductivity is observed at 1.6 K via low field ESR. Application of modest pressures strongly suppresses the superconducting transition temperature. At pressures above about 0.3 kbar the superconductivity is suppressed in favor of an as-yet-unidentified magnetic state whose onset is at 7K. Through an analysis of the microwave ESR lineshape we find that the microwave conductivity over the temperature range 5-50 K is in agreement with dc measurements.     

Spectres de vibration du Cristal de Biphényle à Basse Température

Abstract

Low-temperature infrared and Raman spectra of Crystalline biphenyl have been investigated in the 3100–25 cm^?1 range, and those of biphenyl-D₁₀ between 200 and 25 cm^?1. The infrared dichroism of an Oriented crystal at 77°K has been measured in the 3100–400 cm^?1 region. The assignment for the internal modes V ₅(B_2u), v ₆ (B_2u), v ₁(B_1u), v ₁₀(B_1g), v ₂(B_2g) et v ₃ (B_3g) is given.

The band splitting is analized and hte components due ot the correlation effect in the fundamentals are separated from the components due to combinations. Isotopic shifts are used to assign the nine external vibrations as well as the torsional mode. The temperature effect on the frequencies occuring below 500 cm^?1 is discussed.     

Electrical and Optical Properties of Ferroelectric Liquid Crystals and Influence of Applied Pressure

Much larger dielectric constants and spontaneous polarizations are evaluated in the smectic-C phase of p-alkoxybenzylidene-p'-amino-2-chloropropyl-cinnamate (HOBACPC, OOBACPC and DOBACPC) which have dipole moments at the chiral part compared with those of p-alkoxybenzylidene-p'-amino-2-methylbutyl-cinnamate. In the mixture of D-DOBAMBC and L-DOBACPC with inverse chirality, the pitch of the helical structure is longer compared with those of original compounds. On the other hand, the dielectric constant and the spontaneous polarization of the compounds are nearly linearly dependent on the concentration of L-DOBACPC.

The phase transition temperature from smectic-A to chiral smectic-C phases in DOBAMBC decreases remarkably with the mixing of organic molecules (PAA, eicosane, etc.) but increases with applied pressure.

The threshold field of electro-optical effects (transmission increase, colour switching, memory effects) in DOBACPC and HOBACPC etc. is lower than that in DOBAMBC etc. in accordance with the increase of the spontaneous polarization in the former.     

Nematic Defects and Colloids as Photonic Elements

Birefringent structures in liquid crystalline fluids, such as colloidal assemblies or topological defects, show high potential for use as photonic elements. Here, we present a brief overview of two photonic phenomena originating from coupling light fields with complex birefringent nematic profiles: (i) the generation of vector laser beams from simple Gaussian beams by propagating light along nematic discliantions, and (ii) tunable photonic crystals from blue phase colloidal crystals conditioned by the different underlying symmetries of the particle lattice and the blue phase birefringence. The polarization profile of initially simple linearly polarized Gaussian beams is shown to change into a defect structure at distinct distances travelled along the disclination with the topological invariant (winding number) of the light field and nematic director distinctly coupled. Upon pulsed laser illumination, the nematic discliantions are also shown to split the light pulse into multiple intensity regions. Blue phase I face centred cubic colloidal crystals are shown as examples of tunable photonic crystals, where local band-baps can open by differently combining the symmetries of the two components, e.g. by changing the particle size. The spatial profiles of selected photonic bands in the blue phase colloidal crystals are shown, finding the particles and blue phase double twist cylinders as possible carriers of high-light-intensity regions.     

Surface Order Parameter of 4-n-Heptyl-4′-Cyanobiphenyl

We measured the order parameter near the interface between a liquid crystal and the solid substrate treated by several methods. The interfacial order parameter is different according to the surface treatment, and is larger than the bulk order parameter. Moreover, in the temperature above the clearing point, the interfacial order still has a finite value.     

Thermodynamics of Solutions of Various Nonmesomorphic Solutes in the Nematic and Isotropic Phases of 7CB

Thermodynamic results from gas-liquid chromatography, (GLC) are reported for thirty two nonmesomorphic solutes, at infinite dilution, in the nematic and isotropic phases of p-n-heptyl-p'-cyanobiphenyl (7CB). The solutes fall into five categories; normal alkanes, branched alkanes, alkenes, hexadienes and aromatics. The results are examined within the framework of statitical thermodynamic models. The observed trends are interpreted on the basis of solute structure (e.g. shape, size, flexibility, polarity and polarizability). The results conclusively show that the presence of one or more double bonds in the solute structure increases its compatibility with the nematic and isotropic phases of the liquid crystalline solvent. This effect is more drastic when the double bonds are conjugated. Other implications of this study include shape selective GLC separation of geometric isomers and an examination of the effect of solute structure on the stability of the nematic phase at finite solute concentrations.     

Order Parameter and the Performance of Nematic Guest-Host Displays

A simple model of a nematic guest-host display is presented in which the limiting “on” and “off” states are considered to be uniform arrays of molecules oriented perpendicular and parallel (or at a small tilt angle), respectively, to the plane of the display. Calculated contrast ratio and brightness curves are presented for several angles of view and of all planes of view as functions of dye order parameter, pretilt andploarizer efficiency. The effects of host refractive index and front surface reflectivity are discussed.     

Detection of Photoemission of Electrons into Anthracene Crystals by Studying Recom ination Radiation

Evidence for photoemission of electrons from a calcium contact into anthracene crystals is presented. It is based on the observation of recombination radiation resulting from mutual annihilation of photoinjected electrons with holes which are supplied by an ohmic counter-electrode. The method avoids spurious effect's due to optical release of trapped electrons. The problem of photoemission of electrons from alkali-metals into anthracene crystals is discussed.