Showing papers in "Molecular Crystals and Liquid Crystals in 1994"
TL;DR: In this article, a model is presented to explain the formation and morphologies of 3D periodic surfactant-silicate mesostructures, including the structures of lamellar, hexagonal tubular and a minimal surface cubic liquid crystal/silicate phase.
Abstract: A model is presented to explain the formation and morphologies of 3-d periodic surfactant-silicate mesostructures. The structures of lamellar, hexagonal tubular and a minimal surface cubic liquid crystal/silicate phase are described.
134 citations
TL;DR: In this article, a head-to-tail coupling in 3-substituted polythiophene was shown to have a high chirality with a strong conformational rigidity.
Abstract: The synthesis and (chir)optical properties of a highly regioregular optically active 3-substituted polythiophene are reported. Using the McCullough method a regioselectivity of more than 98% is found for the head-to-tail coupling in 3-[2-((S)-2-methylbutoxy)ethyl]polythiophene 1. Contrary to the small effects observed for the regiorandom analogue 2, made by FeCl3-polymerization, 1 shows a strong induced optical activity in its π-π* transition, provided the polymer is in its almost coplanar associated form. This association with a strong conformational rigidity, a prerequisite for a stable chirality from a helical structure of 1, is found in poor-solvents as well as in the solid state. In a good solvent like CHCl3, the disordered non-planar structure is optically inactive in its backbone. The transfer to an associated form is accompanied with solvatochromism and thermochromism in absorption, while significant changes in fluorescence behaviour and in the circular dichroism spectra are observed. All...
130 citations
TL;DR: In this article, a review of the reduced symmetry aspects of calamitic liquid crystal phases with reference to point symmetry, space symmetry, and helicity is presented, and it is suggested that many novel effects found in chiral liquid crystals are a result of conflicts between the effects of reduced symmetry and the desire to form normal liquid-crystalline structures.
Abstract: Chirality in liquid crystal systems is a complex and sometimes difficult concept to understand and appreciate. In this article we review some of the reduced symmetry aspects of calamitic liquid crystal phases with reference to point symmetry, space symmetry, and helicity. We utilize these concepts in the discussion of new results obtained on Twist Grain Boundary phases, inversions in chiral dependent properties such as the helicity in cholesteric and smectic C* phases and the spontaneous polarization in ferroelectric liquid crystals, and antiferroelectric behaviour in low molar mass systems. Our observations suggest that many novel effects found in chiral liquid crystals are a result of conflicts between the effects of reduced symmetry and the desire to form normal liquid-crystalline structures, i.e., a form of frustration exists.
121 citations
TL;DR: In this paper, a simple model is proposed to describe the electrooptic response of polymer dispersed liquid crystals and a hierarchy of order parameters is introduced to describe orientational order on different length scales.
Abstract: A simple model is proposed to describe the electrooptic response of polymer dispersed liquid crystals. A hierarchy of order parameters is introduced to describe orientational order on different length scales. External fields reorient the liquid crystal in the inclusions, and thus alter the orientational order. The resulting changes in the dielectric and optical properties of the sample are taken into account via changes in the order parameters. The transmittance of PDLC films is considered as a function of static and time dependent magnetic and electric fields.
91 citations
TL;DR: In this article, the principles of a holographic storage process based on the liquid crystalline state of organic polymeric materials are described, and the physical processes involved in the storage process and the capability of such materials to store holographic information is discussed.
Abstract: The review describes the principles of a holographic storage process based on the liquid crystalline state of organic polymeric materials. Such materials are capable of forming anisotropic glasses, which can be obtained as thin films. By suitable means one is able to align the optical axis of the uniaxial system within the film along a given direction or parallel to the film normal. A storage process is made possible by incorporating into the polymer a suitable dye—such as azobenzene—characterized by the fact that it is able to undergo a light-induced isomerization process even in the solid glassy state. This in turn leads to a reorientation of the optical axis within the film and thus to strong modifications of the optical properties. The information written-in in this way can be erased either by heating to temperatures above the glass transition temperature or by light. The paper describes the physical processes involved in the storage process and the capability of such materials to store holog...
89 citations
TL;DR: The thermal stability of poly(p-phenylenevinylene) (PPV), along with its ability to form flexible devices, has sparked a considerable amount of research in the area of polymer light emitting diodes as discussed by the authors.
Abstract: The thermal stability of poly(p-phenylenevinylene) (PPV), along with its ability to form flexible devices, has sparked a considerable amount of research in the area of polymer light emitting diodes...
82 citations
TL;DR: A series of 3,3-diaryl-3H-naphtho[2,1-b]pyrans were prepared and their photochromic properties measured in a plastic matrix as mentioned in this paper.
Abstract: A series of 3,3-diaryl-3H-naphtho[2,1-b]pyrans were prepared and their photochromic properties measured in plastic matrix. The influence of a variety of substituents on color, intensity, and fade were studied.
72 citations
TL;DR: In this paper, the stability of organic electroluminescent (EL) diode during operation and storage using SEM, XPS, cyclic voltammetry, etc.
Abstract: We conducted an investigation of the stability of organic electroluminescent (EL) diode during operation and storage using SEM, XPS, cyclic voltammetry, etc. We found that one must pay attention to the following points; 1) morphological stability of organic thin films, 2) electrochemical stability of organic materials themselves, 3) oxidation and/or peeling-off of the cathode, 4) fabrication process. Uniformity of the organic layers made from molecular materials is essentially important to attain a long-term stability of the EL diode. It improved the contact with the cathode resulting in suppression of dark spots and stable operation
69 citations
TL;DR: Amorphous, optically isotropic films of photochromic liquid crystalline copolymers have been prepared and irradiated with linearly polarized light as discussed by the authors, and high values of dichroism and birefrin have been obtained.
Abstract: Amorphous, optically isotropic films of photochromic liquid crystalline copolymers have been prepared and irradiated with linearly polarized light. In this way high values of dichroism and birefrin...
66 citations
TL;DR: In this paper, high values of optical anisotropy have been induced in glassy films of amorphous copolymers containing azobenzene moieties and rod-like side groups by angular-dependent photoselection.
Abstract: High values of optical anisotropy have been induced in glassy films of amorphous copolymers containing azobenzene moieties and rod—like side groups by angular—dependent photoselection. The photoinduced reorientation of the photochromic groups causes a reorientation of the non—photochromic groups due to a co-operative effect.
61 citations
TL;DR: A review of the decisive role of amphiphilicity in inducing the formation of thermotropic smectic and columnar mesophases is discussed in this article, where the emphasis is placed to amphiphiles bearing multiple hydroxy groups and primarily to those with molecularly recognizable moieties able to form a system of hydrogen bonds.
Abstract: It is well-established that the presence in amphiphilic molecules of two distinct and incompatible segments leads, in general, to the formation of molecular organizates certain of which exhibit ther-motropic liquid crystalline phases. In the present review the decisive role of amphiphilicity in inducing the formation of thermotropic smectic and columnar mesophases is discussed. Following a brief discussion of the main classes of ionic and nonionic amphiphiles that exhibit thermotropic liquid crystalline character, the emphasis is being placed to amphiphiles bearing multiple hydroxy groups and primarily to those with molecularly recognizable moieties able to form a system of hydrogen bonds. The discussion of this topic, primarily seen from the organic chemistry scope, is hoped to trigger further interest towards the synthesis of novel and unconventional amphiphiles with liquid crystalline properties.
TL;DR: In this article, the authors summarized the properties of archiral and chiral liquid crystalline (LC) elastomers and investigated the formation of new LC elastomer by a radical polymerization of acrylate groups covalently linked to LC polymers.
Abstract: This paper summarises the properties of archiral and chiral liquid crystalline (LC) elastomers, the latter of which have found interest due to their piezoelectric properties. In addition the formation of new LC elastomers by a radical polymerization of acrylate groups covalently linked to LC polymers was investigated. This was done concerning the progress of the crosslinking reaction, concerning the influence of the crosslinking on the LC phases and concerning the network properties (swelling ratio and elastic modulus) of the resulting elastomers. It turns out that the networks prepared in this way are very soft and their crosslinking density is low. This can be explained assuming that the radical polymerization of the acrylates leads only to short oligomers, due to steric restrictions. Photo crosslinking initiated by a photoinitiator in the LC phase creates a memory of the orientation before crosslinking.
TL;DR: In this article, the optical properties of polymers and oligomers used in light emitting diodes (LEDs) prepared by solution processing or vacuum deposition are investigated, focusing on the comparison of PPP and its oligomers to planarized forms of PPPP.
Abstract: We report on the optical properties of polymers and oligomers used in light emitting diodes (LEDs) prepared by solution processing or vacuum deposition. Special emphasis is put on the comparison of PPP and its oligomers to planarized forms of PPP.
TL;DR: In this paper, charge transport mechanisms and the complex impedance of liquid crystal displays are shown to have a strong effect on the electro-optical performance of passive and active matrix addressed twisted wires.
Abstract: Charge transport mechanisms and the complex impedance of liquid crystal displays are shown to have a strong effect on the electro-optical performance of passive and active matrix addressed twisted ...
TL;DR: In this article, the spectral response as a function of the absorption coefficient was used to conclude that electron hole generation takes place at the interface between the organic thin film and one or both of the semitransparent cover electrodes.
Abstract: Organic photovoltaic thin film structures made by vacuum vapor deposition have been studied. From the spectral response as a function of the absorption coefficient we conclude that electron — hole generation takes place at the interface between the organic thin film and one or both of the semitransparent cover electrodes. The observed short circuit currents, however, are too large to be explained on the basis of a direct light-induced charge transfer at the organic thin film/electrode interface. Rather, a contribution of the bulk-absorbed photons is necessary to account for the observed quantum yield. Transfer of the energy to the interface sites can be explained by diffusional migration of excitons. The efficiency of charge separation can be improved by combining donor and acceptor type partners in organic double layers. The efficiency of power conversion, however, not only depends on a suitable choice of the absorption spectra and of the ionic energy levels of the materials employed, but also o...
TL;DR: In this paper, the authors estimate the magnitude of this error theoretically and experimentally and propose an improvement of the crystal rotation method by which an accurate value of the pretilt can be obtained even when the cell gap is not uniform.
Abstract: To measure the pretilt angle in a liquid crystal cell, the crystal rotation method is commonly used because it is very precise and the measurement time is short. However, if the cell gap is not uniform, a significant error occurs due to a shift of the measurement point. This shift is caused by refraction at the glass plate when rotating the cell. In this paper, we estimate the magnitude of this error theoretically and experimentally and propose an improvement of the crystal rotation method by which an accurate value of the pretilt can be obtained even when the cell gap is not uniform.
TL;DR: In this paper, the role of aromatic imide moieties as mesogenic building blocks has been discussed and a review on structure-property relationships of LC-main chain polymers containing imide groups is presented.
Abstract: The present review reports on structure-property relationships of LC-main chain polymers containing imide groups. The comments focus on the role of aromatic imide moieties as mesogenic building blocks. All the pertinent literature has been cited. This review is subdivided into six sections dealing: A) with fundamental stereochemical and conformational aspects of aromatic imide groups, B) with structure-property relationships of fully aromatic poly(ester-imide), C) with the properties of LC-poly(ester-imide)s containing regular sequences of aromatic and aliphatic building blocks, D) with synthesis and characterization of cholesteric poly(ester-imide)s, E) with structures and properties of LC-polyimides free of ester groups, and F) with the definition and characterization of layer structures in the solid state regardeless whether the pertinent polyimides form a true LC-phase or not.
TL;DR: In this paper, a previously developed hopping concept for rationalizing charge transport in random organic photoconductors is further elaborated, based on the premise that random molecular packing gives rise to energetic disorder that can be accounted for in terms of a Gaussian-shaped density of hopping states (DOS).
Abstract: A previously developed hopping concept for rationalizing charge transport in random organic photoconductors is further elaborated. It rests on the premise that random molecular packing gives rise to energetic disorder that can be accounted for in terms of a Gaussian-shaped density of hopping states (DOS). Computer simulations confirm that an array of randomly positioned dipoles generates a Gaussian DOS whose width is in excellent agreement with experiment. The existence of energetic disorder can explain in a quantitative manner the scaling behavior of the shape of time of flight signals with respect to sample length and electric field as well as the transition to dispersive transport at low temperatures.
TL;DR: Linkage isomerization in the crystals of some Co(III)-ammine complexes is chosen as an example to illustrate the effect of the crystalline surroundings on an intramolecular process, and vice versa as mentioned in this paper.
Abstract: Linkage isomerization in the crystals of some Co(III)-ammine complexes is chosen as an example to illustrate the effect of the crystalline surroundings on an intramolecular process, and vice versa.
TL;DR: In this paper, a more complicated ladder C model is proposed, which still considers only the two backbone hybrid orbitals on each silicon atom, but includes the transfer integrals between all pairs of orbitals in a pair of neighboring silicons.
Abstract: The standard Sandorfy C model of electronic structure of polysilanes is inherently incapable of providing a physical interpretation of the observed conformational effects on UV spectra. We describe a more complicated ladder C model, which still considers only the two backbone hybrid orbitals on each silicon atom, but includes the transfer integrals between all pairs of orbitals on a pair of neighboring silicons. Already at the Huckel level, this model accounts for the general features of the observed spectra both as a function of chain length and of conformation. Results of our ab initio calculations suggest, however, that for general non-planar conformations the backbone orbitals should not be treated separately from those that carry the substituents, at least not in relatively short chains. The simplest level at which the conformational effects on the spectra of oligosilanes of all lengths can be accounted for simultaneously will probably be the even more complicated ladder H model, in which al...
TL;DR: In this article, electrical conductivity and thermoelectric power measurements of pressed pellets of (PEO)0.92MoS2 and (Polyaniline) 0.35MoS 2 showed p-type metallic behavior.
Abstract: Polyaniline, poly(ethylene oxide), poly(propylene glycol), poly-(vinylpyrrolidinone), methyl cellulose, polyethylenimine, polyethylene, and Nylon-6, were encapsulated into MoS2. Electrical conductivity and thermoelectric power measurements of pressed pellets of (PEO)0.92MoS2 and (Polyaniline)0.35MoS2 show p-type metallic behavior. Below 14 K and 9 K respectively both show a metal to insulator transition. The thermal stability of these materials is reported.
TL;DR: The photochromes rather behave, in the spiro and merocyanine form, as O2 (1 Δg) as discussed by the authors, whereas the photo-oxidation of some representative spiropyrans and spiro-oxazines does not seem to involve singlet oxygen O 2 ( 1 Δg).
Abstract: Photo-oxidation of some representative spiropyrans and spiro-oxazines does not seem to involve singlet oxygen O2 (1 Δg). The photochromes rather behave, in the spiro and merocyanine form, as O2 (1 ...
TL;DR: In this article, a new synthetic method for the preparation of PcLuNPc was proposed and the electrochemical and electrical properties of thin films of this compound show it is a new molecular semiconductor.
Abstract: We propose a new synthetic method for the preparation of PcLuNPc. The electrochemical and electrical properties of thin films of this compound show it is a new molecular semiconductor.
TL;DR: Several alkoxybenzamides have been synthesized and characterized by spectroscopic and chromatographic methods as discussed by the authors, including 3,4,5-Tris(alkoxy)benzosamides, which exhibit a mesophase of columnar or stack-like type.
Abstract: Several alkoxybenzamides are synthesized and characterized by spectroscopic and chromatographic methods. 3,4,5-Tris(alkoxy)benzamides have been found to exhibit a mesophase of a columnar or stack-like type.
TL;DR: In this article, the authors report the results of dynamic grating experiment with a planar dye-doped liquid crystal and the dye used is D2 (from EM chemicals).
Abstract: We report the results of dynamic grating experiment with a planar dye-doped liquid crystal. The dye used is D2 (from EM chemicals). It was found that for this particular DDLC system, the excited state dye molecules caused interesting transient reorientations of the liquid crystal director axis in directions orthogonal to the initial molecular alignment.
TL;DR: In this paper, the effects of side chain size on the structure and optical properties of PPV were investigated and it was shown that increasing the size of the solubilizing side chains increases the intrinsic persistence length of the PPV backbone.
Abstract: Light scattering and optical spectroscopy experiments were performed on [2-methoxy, 5-(2′-ethyl-hexyloxy-p-phenylene-vinylene](MEH-PPV), poly[2-butoxy, 5-(2′-ethyl-hexyloxy-p-phenylene-vinylene](BEH-PPV), and poly[2-dicholestanoxy-p-phenylene-vinylene](BCHA-PPV) in solution with p-xylene to determine the effects of side chain size on the structure and optical properties of PPV. We find that increasing the size of the solubilizing side chains increases the intrinsic persistence length of the PPV backbone and this change in rigidity has dramatic effects on the photoluminescence of PPV. Quantum yield determination relative to Rhodamine 6G shows the luminescence efficiency of the rigid BCHA-PPV is 0.66 ± 0.05 and that the yield decreases to 0.20 ± 0.05 for MEH-PPV, the most coiled derivative. Excitation profiles show an increase in non-radiative decay at high energies when the backbone is more flexible.
TL;DR: In this article, a highly saturated Li graphite intercalation compound synthesized under high pressure, was investigated after pressure relaxation by Li NMR spectroscopy, which confirmed short metal-metal bondlengths and possible Li-Li covalent bonds.
Abstract: A highly saturated Li graphite intercalation compound synthesized under high pressure, was investigated after pressure relaxation by Li NMR spectroscopy. At low temperature the sample exhibits a doublet at 256 ppm downfield from Li+. The lineshape agrees with the simulation of an island of 7 Li atoms with nearest neighbours at 2.46° distances, in good agreement with a local LiC2 structure. This confirms short metal-metal bondlengths and possible Li-Li covalent bonds which were shown by IR Spectroscopy.
TL;DR: In this article, the wavelength dependence of the optical reorientation of nematic liquid crystals was investigated in the presence of two dyes, using the z-scan technique, and a strong correlation was found between the magnitude of the dyeinduced amplification of optical torque and the dichroism of the system.
Abstract: The wavelength dependence of the optical reorientation of nematic liquid crystals is investigated in the presence of two dyes, using the z-scan technique. A strong correlation is found between the magnitude of the dyeinduced amplification of the optical torque and the dichroism of the system.
TL;DR: In this paper, the authors discuss the molecular design of selected examples of structural units containing taper shaped exo-receptors and various crown ether, oligooxyethylenic, and H-bonding based endo-Receptors, which self-assemble into cylindrical channel-like architectures via principles resembling those of tobacco mosaic virus (TMV).
Abstract: The first part of this paper discusses the molecular design of selected examples of structural units containing taper shaped exo-receptors and various crown ether, oligooxyethylenic, and H-bonding based endo-receptors, which self-assemble into cylindrical channel-like architectures via principles resembling those of tobacco mosaic virus (TMV). The ability of these structural units to self-assemble via a delicate combination of exo- and endo-recognition processes will be presented. A comparison between various supramolecular (generated via H-bonding, ionic, and electrostatic interactions) and molecular “polymer backbones” will be made. The formation of columnar hexagonal (Φh), nematic and re-entrant isotropic phases by selected self-assembled systems will be discussed. The present limitations concerning the ability to engineer the structural parameters of these supramolecular channel-like architectures and some possible novel material functions derived fiom them will be briefly mentioned. The seco...
TL;DR: In this paper, electron donating/withdrawing substituents and electronegative centers have been successfully employed in substantially widening the range of photo-generated colours available in the spiroindolinonaphthoxazine class of photochromic materials.
Abstract: Electron donating/withdrawing substituents and electronegative centres have been successfully employed in substantially widening the range of photo-generated colours available in the spiroindolinonaphthoxazine class of photochromic materials.