Showing papers in "Molecular Crystals and Liquid Crystals in 1992"

Generation of Pretilt Angles in Nematic Liquid Crystal, 5CB, Media Aligned on Polyimide Films Prepared by Spin-Coating and LB Techniques: Effect of Rubbing

Abstract: The pretilt angles generated in nematic liquid crystal (5CB) media that are disposed in sandwich type cells and oriented by using various orientation films are shown to depend both on the nature of the orientation films and the rubbing strengths in the following way: first, in a cell with alkyl branched polyimide (PI (1)) prepared by spin-coating, the obtained prettily angle 6P shows a giant peak reaching 51 degrees at the low rubbing strength (RS) region and then it decreases monotonically with the increase of the RS; second, with another alkyl branched PI (PI (2)), the obtained value of Q p is around 4 degrees for a wide range of the RS; third, with branchless spin-coated PI (PI (3)) that is featured by its high electrical polarization, the θ P increases monotonically with the RS from zero level and tends to saturate and level-off as θ P to be 4.5 degrees; fourth, with as stacked PI-Langmuir-Blodgett (LB) films using the same precursors as those used for above mentioned PI (1, 2, and 3), the θ ...

An X-Ray Diffraction Study

Abstract: X-ray oscillation photographs reveal both diffuse and discrete scattering for alkanone guest molecules within inclusion compounds of 5-undecanonehrea grown from methanol. Guest molecules that give rise to diffuse scattering are ordered only along the channel axis, whereas those guest substructures that give rise to discrete scattering are ordered in three dimensions. Precession photographs and diffractometry show that within domains containing three dimensionally ordered guests, the offset (denoted δg) between the heights of guest molecules in adjacent channels is exactly zero. These guests therefore share overall hexagonal symmetry with the host, as do a large number of n-alkanones of varying chain length and symmetry properties. Although side by side ordering optimizes dipole-dipole interactions between alkanones in adjacent channels, the large inter channel distance (8.22A) and the lack of symmetry in 5-undecanone suggest that other factors could influence guest molecule ordering. A careful di...

Erasable Optical Storage in Bistable Liquid Crystal Cells

Abstract: In this paper, we present a new kind of bistable liquid crystal display (Filled Nematic) using static light scattering, and its application as a high resolution laser-addressed display. The new material consists of small inorganic particles dispersed in a nematic liquid crystal. In dispersion the particles form internal interfaces with a large specific surface making it possible to stabilize different director configurations. We report the key parameters for reversible optical data storage in this medium and give a first explanation of the physical mechanism of the laser-writing process.

Electrically Induced Light Scattering from Anisotropic Gels with Negative Dielectric Anisotropy.

Abstract: Homeotropically aligned gels were obtained by photo-induced cross linking of a liquid crystalline (LC) mixture containing LC diacrylates and conventional LC molecules with negative dielectric anisotropy The gels produced in this way were very clear as observed from all directions When an electric field was applied across the gel, its appearance changed and a light scattering texture was formed The intensity of the scattered light increased non-linearly with increasing voltage in a reversible way Light scattered by the gels was not polarised and, especially at high voltages, the scattering efficiency of the gels was found to be independent of the direction of the incident light In the case of gels containing dye molecules, scattering was combined with absorption In this way grey levels for dye absorption could be obtained

Multi-Nuclear Palladium Organyls-Novel Structures and Properties of Disc-Like Metallomesogens1

Abstract: The first non-calamitic/disc-shaped metal organyl series 3 and 6 containing two or even four palladium atoms and eight or twelve flexible side chains presented here have been synthesized via the novel roof-shaped/bent dimers 2 or (the thermomesomorphic) 5, respectively. In addition, from the chemical point of view the dodecaethers 5 and 6a-c are the first derivatives of four novel, uniquely structured tetrapallado-macroheterocycles. Whereas the type of mesophase of 5 is still not distinctly clear yet those ones of 6a-c are enantiotropic discotic hexagonal columnar (Dh) over a wide range of temperature. On the other hand, the dinuclear palladocompounds 3a-d represent the first (monotropic) cases of metallomesogens exhibiting the nematic-discotic (ND) phase. This thermotropic nematic type of phase clearly gets stabilized by doping 3c, d with a strong electron acceptor; under such circumstances mesophase conversions/inductions (ND→Dh) are observed in the cases of 3a-c. Finally, it is important to em...

Starburst Molecules for Amorphous Molecular Materials: Synthesis and Morphology of 1,3,5-Tris(diphenylamino)benzene and Its Methyl-substituted Derivatives

Abstract: Methyl-substituted derivatives of 1,3,5-tris(diphenylamino)benzene are found to constitute a novel class of amorphous molecular materials, as characterized by differential scanning calorimetry and X-ray diffraction. These compounds readily form stable amorphous glasses having glass-transition temperatures of ca. 50 °C on cooling from the melt. The methyl substituent exerts a great influence on the formation of the glassy state.

A Calorimetric Determination of Fundamental Properties of Polymer-Dispersed Liquid Crystals

Abstract: We have developed a combined calorimetric/electron microscopic technique for determining several fundamental properties of polymer-dispersed liquid crystal (PDLC) films. Calorimetry allows us to determine the solubility limit of liquid crystal in the polymer matrix as well as the fraction of liquid crystal which is dispersed in the form of microdroplets. By combining the calorimetry results with electron microscopy measurements of D, the microdroplet diameter, we are able to calculate the functional relationship between the droplet number density and D. All of these parameters are of considerable importance in assessing the behavior of PDLCs. The usefulness of the method is illustrated by applying it to thermally-cured epoxy-based systems.

Further Low Mass Liquid Crystal Systems with Nematic Columnar (Nc) Phase [1]

Abstract: A number of new disc-like pentaynes 1–4 with super-disc cores are presented. These new non-mesogenic electron donors doped with 2,4,7-trinitrofluorenone (5) form charge transfer complexes showing enantiotropic nematic and hexagonal columnar mesophases.

Mesophases of Alkylammonium Salts. I. First-Order Transitions

Abstract: Thermal properties of symmetric tetra-n-alkylammonium salts are interpreted based on the concept of conformational motion and disorder (condis crystals). A special effort is made to separate the we...

Nematic Homogeneous Alignment Regulated by the Polarization Photochromism of Surface Azobenzenes

Abstract: Alignment modulation of a nematic liquid crystal was performed by a polarization-selective photochromic reaction of surface azobenzenes. The alignment was changed from an initial perpendicular state to a homogeneous planar state by exposure to the linearly polarized UV light. The induced homogeneous axis was perpendicular to the electric polarization of the reaction light. Rotation of the homogeneous axis was demonstrated by changing the light polarization, and the corresponding relaxation time was measured by laser pulse experiments.

Mott-Hubbard state in one-dimensional iodo-bridged binuclear metal dithioacetato complexes, M2(dta)4I (M=Pt and Ni)

Abstract: Polarized reflection, electrical conductivity, magnetic susceptibility measurements and X-ray crystal structure predetermination have been carried out on one- dimensional iodo-bridged binuclear metal dithioacetato complexes, M2(dta)4I (M=Pt and Ni; dta=CH3CS2 −) in order to elucidate their electronic property. The chain structure consists of -M2-5+-M2,5+-I-M2-5+-M2-5+- with no Peierls distortion, and two metal atoms in binuclear moieties are bridged by four dithioacetato ligands to give a cage structure. The magnetic behavior of these compounds revealed very strong anti ferromagnetic coupling between electronic spins (S=1/2) localized on the M-M dimers. They show semi conducting behavior with high conductivities and small activation energies compared with those of the MX compounds. From these results, it may be concluded that the compounds are one-dimensional Mott-Hubbard insulators, in contrast to Peierls insulators of R4[Pt2(pop)4X].nH20 complexes (…M2+-M2+…X-M3+ -M3+-X‥).

Atomic Force Microscopy in Organic Gas/Solid Reactions: How Do the New Phases Build Up?

Abstract: Numerous gas/solid reactions are studied on microscopically smooth surfaces by atomic force microscopy (AFM) Five basic types for solid phase to solid phase transformations have been found, as well as mixed types These are discussed in some detail, exemplified, and compared to some solid state photoreactions in order to provide a basis for a real understanding of non is typical solid state transformations, which are preparatively useful

Wavelength Dependence of Scattering in PDLC Films: Droplet Size Effects

Abstract: We have measured the optical transmittance of UV-cured epoxy PDLC films at normal incidence as a function of applied voltage for a number of different wavelengths. By controlling the cure rate of e...

Spectroscopy and Mechanism in Triboluminescence

Abstract: The presence of C3πu → B3πg emissions of dinitrogen and photoluminescence in triboluminescence spectra provides information about the mechanism of light production.

Computer simulations of nematic droplets with toroidal boundary conditions

Abstract: We present Monte Carlo simulations of nematic droplets with toroidal boundary conditions (TBC) and various anchoring strengths and we investigate the orientational order and the molecular organizations in these systems that mimic polymer dispersed liquid crystals (PDLC). PACS: 02.50 Monte Carlo studies PACS: 61.30.Jf Defects in liquid crystals PACS: 61.30.Gd Orientational order of liquid crystals PACS: 64.70.M Liquid Crystals phase transitions

Blue-Shifted Electroluminescence From a Stable Precursor to Poly(p-Phenylene Vinylene)

Abstract: A partially conjugated polymer precursor to poly(p-phenylene vinylene) (PPV) is used to produce stable electroluminescence which enhanced efficiency over PPV in a simple device configuration with a blue-shift in both the electroluminescence and photoluminescence emission spectra

Observation of the surface structure on as stacked and rubbed polyimide(PI)-LB films with an atomic force microscope and their anchoring capability for nematic liquid crystal

Abstract: We have observed the surface structure of stacked and rubbed polyimide(PI)-Langmuir-Blodgett(LB) films with an atomic force microscope. It is shown that no grooves are formed on the PI-LB films sta...

Surface transition in a nematic layer with reverse pretilt

Abstract: Recently, a surface induced transition from homeotropic to planar alignment in a nematic layer and the corresponding optical response has been reported/K. Flatischler, L. Komitov, S. T. Lagerwall, B. Stebler, and A. Strigazzi, Mol. Cryst. Liq. Cryst., 198, 119 (1991)/. In the case of surface conditions with reverse pretilt a similar transition from quasi-homeotropic bent alignment to quasi-planar splayed state can be obtained. As known, these two states are topologically incompatible, and therefore the transition is accompanied by nucleation of π-disclination lines, and even is reversible, it exhibits a big hysteresis. Moreover, it can be driven not only by temperature but also by an applied electric field, using a nematic with negative dielectric anisotropy. A simple theoretical description of such an alignment transition is presented.

Storage of erasable laser induced holographic gratings in low molar mass cholesteric liquid crystals

Abstract: We succeeded in storing optical information with low-molecular-weight cholesteric liquid crystals. Nematic mixtures of glassy compounds which are chiralized by the addition of small amounts CB15 (BDH) have been used. Reversible optically induced holographic gratings were stored both below the glass transition temperature and above the glass transition in the temperature region of high viscosity. Erasure can be achieved by local heating of the sample making possible to use the material for multiple storage processes. A mechanism to explain the formation of the grating is discussed.

UV-Photoelectron Spectroscopy of One- and Two Component Organic Crystals

Abstract: For potential applications of organic materials as active components In electronics, the energetic position of their valence and conduction levels are important parameters. For a given parent molecule, e.g. an aromatic hydrocarbon, the positions of these levels can be shifted substantially by attaching donor- or acceptor-type side group units. This will be demonstrated for the perylene skeleton in perylene-3,4,9,10-tetra-carboxylic-dianhydride. N,N′-diphenyl-perylene-3,4,9,10-tetracarboxylic-diimide, and diindeno[1,2,3.-c,d:1′,2′,3′-Imlperylene. Energy level manipulation by introducing donor: acceptor ground state interactions is another possible concept which has been checked with single crystals of the stoichiometric donor: acceptor complexes tetrathiafulvalene: p-chloranil, phenothiazine: tetracyanoquinodimethane. phenothazine: pyromellitic-dianhydride, perylene: pyromellitic-dianhydride and perylene: tetracyanoquinodimethane. where a certain dark conductivity or light-induced photoconductivit...

Dielectric Relaxation Processes in a Model Ferroelectric Liquid Crystal

Abstract: The soft mode, Goldstone mode and a new ferroelectric mode have been detected in the Sm C* phase of a fluorinated compound. The dielectric increment of the Goldstone mode found in the Sm G* phase is by about one order of magnitude tower in comparison with that found in the Sm C* phase. In addition to collective processes, in both chiral phases a biased reorientation of molecules around their long axes contributes to the dielectric increment.

Large optical nonlinearity in pseudoisocyanine j-aggregates

Abstract: The resonant optical nonlinearity in pseudoisocya‐nine J‐aggregate is investigated. Large degenerate four wave mixing signals and negative absorption changes at the wavelengths corresponding to J‐aggregate absorption band are observed. It is concluded that the origin of the large optical nonlinearity is mainly due to the absorption saturation of J‐band. The mechanism of the absorption saturation is discussed on the bases of exciton delocalization effect.

Rheo-optical properties of a thermotropic liquid crystalline polymer

Abstract: Rheo-optical studies on aligned and unaligned melts of a main chain thermotropic polyester with flexible spacers are discussed in terms of the predictions of the Ericksen-Leslie equations. Experiments show that the material exhibits two different shear-flow regimes depending on the temperature at which the experiments are performed. It is shown, using conoscopy, that the director is flow-aligning at temperatures close to TNI and nonflow-aligning close to a smectic to nematic (TSN) phase transition. The implications of these results are discussed in terms of the change in sign of one of the Leslie viscosity coefficients and interpreted in the framework of the Ericksen-Leslie Theory.

Rod-like Liquid Crystals of Organic Transition Metal Complexes 31: A Reversible Transition between Dimer Smectic E Phase and Monomer Smectic A Phase in the [N,N′-bis(5-alkylsalicylidene) ethylenediaminato nickel(II) Complexes

Abstract: It was found that the [N,N′-bis(5-alkylsalicylidene)ethylenediaminato Ni(II) (abbreviated as (Cn-Salen)2Ni: n = 6, 12) complexes show two smectic liquid crystalline phases, SE and SA The SE ⇆ SA phase transition in the C12-salen)2Ni complex is accompanied by a reversible stretching of the interlayer distances A dimer SE-monomer SA phase transition can be deduced from this phenomenon

Time-Resolved FT-IR Analysis of Electric Field-Induced Reorientation Dynamics in Nematic 4-Pentyl-(4-cyanophenyl)-Cyclohexane Liquid Crystals

Abstract: The application of rapid-scan FT-IR time-resolved spectroscopy to the study of molecular reorientation dynamics in a nematic liquid crystal during the electric field-induced homogeneous-to-homeotropic transition is reported. The average orientation of bond segments in a homogeneously oriented 4-pentyl- (4-cyanophenyl)-cyclohexane liquid crystal is determined from static infrared dichroism measurements. The observed temperature dependence of the molecular orientational order parameter agrees with the prediction of the Maier-Saupe theory. Analysis of time-resolved dichroic spectra shows that the homogeneous-to-homeotropic transition requires an induction period of several milliseconds and is completed in 10-20 ms. The cyanophenyl group reorients as a unit in response to the applied electric field and the reorientation rate increases as temperature is increased.

Octaalkyl Esters of 2,3,9,10,16,17,23,24-(29H,31H)-Phthalocyanineoctacarboxylic Acid: A New Homologous Series of Discotic Liquid Crystals

Abstract: The synthesis of a homologous series of octaalkoxycarbonyl-substituted metal-free phthalocyanines (Pc) is described. The mesomorphic properties of these new materials were studied by differential scanning calorimetry (DSC), optical microscopy and X-ray investigations. All compounds show a discotic mes-ophase in an extremely large temperature interval including room temperature. X-ray diffraction patterns of the mesophases confirm that all compounds form a hexagonal columnar mesophase of the type Dho .

Mesomorphic Metal Complexes Derived from 4-Alkyl-oxystilbazoles

Abstract: The weakly mesomorphic trans-4-alkyloxy-4′-stilbazoles(n-OPhVPy) are readily synthesised from alkyloxyiodobenzenes and 4-vinylpyridine in a palladium-catalysed Heck reaction. While the highly symmetric palladium and platinum complexes trans-[MCl2(n-OPhVPy)2] show mesophases only at very high temperatures, the lower symmetry platinum complexes trans-[PtCl2(η2-olefin) (n-OPhVPy)] melt below 100°C. Similarly, the complexes [MCl(CO)2(n-OPhVPy)] (M = Rh, Ir) are low melting. The silver complexes [Ag(n-OPhVPy)2][X] (X = C12H25OSO3, C8H17OSO3, CF3SO3) can be obtained by direct reaction of the stilbazole with AgX. These ionic materials show a rich thermotropic polymorphism, including nematic and cubic mesophases. Some fluorinated stilbazoles have also been synthesised and complexed to silver; the mesomorphism of these complexes is also reported.

Molecular Structure of the Hedgehog Point Defect in Nematics

Abstract: In view of the limitations inherent to phenomenological approaches, the problem of the internal structure of defects in nematics is here afforded from a molecular viewpoint. By using an extension of the Maier-Saupe mean-field potential which accounts for distorted states of nematics, the general equation applicable inside a defect is obtained by self-consistency. The specific example of the hedgehog point defect is developed in detail. It is shown that the size of the defect core depends on the order parameter characterizing the undistorted nematic phase: the core size decreases when the order parameter increases.

Photochemically induced modification of polymer dispersed liquid crystals with photochromic stilbene

Abstract: Photosensitive polymer dispersed liquid crystals (PDLC) were prepared using the nematic mixture E-5 doped with 2 wt-% photochromic stilbene dye in a gelatin binder The stilbene undergoes E-Z photo

C-H … O hydrogen bonding and the deliberate design of organic crystal structures

Abstract: The C-H…O interaction, though weak, is not van deer Wails in nature but electrostatic and resembles O-H…O and N-H…O hydrogen bonds in its geometrical properties. Spectroscopic evidence hints at some degree of covalent bonding between H and 0 atoms. The long-range, electrostatic character of the C-H…O interaction determines its important role in crystal engineering. Planar aromatic hydrocarbons adopt herringbone structures but if a critical number of O atoms is present, a planar molecule will cross the structural threshold from a C…H stabilised herringbone structure to a C-H…O stabilised layer structure wherein adjacent molecules are parallel and highly overlapped. These ideas have been used to design a family of crystal structures of alkoxyphenylpro-piolic acids that participate in an intermolecular, solid state Diels-Alder reaction. When the number of C-H…O interactions in a structure is unusually large or small, there is a possibility of distortions in the O-H…O network.