Liquid crystalline materials for light-emitting diodes

Liquid crystalline materials display unique properties which can be exploited in organic light-emitting diodes. Characteristic features of liquid crystals are the anisotropy of electronic properties, a strong coupling to external fields as well as a tendency to form spontaneously homogeneous monodomain films. It is found that liquid crystalline materials can be used in light-emitting diodes to control the state of polarization of the emitted light, the magnitude of the onset field for emission as well as the quantum efficiency. Both low molar mass and polymeric liquid crystals have been introduced with great success in single as well as in multilayer devices. © 1998 John Wiley & Sons, Ltd.     

光致变色液晶材料研究进展

本文介绍了液晶相态和光致变色的基本原理,液晶材料在光学领域中的应用,光致变色化合物的光异构化反应对液晶光学性质的影响,以及光致变色液晶材料制备的进展。参考文献53篇。     

Modelling of shear flow in liquid crystalline materials

The flow behaviour of liquid crystalline polymers (LCPs) is quite complex and these materials exhibit varied and complicated textural patterns when subject to a flow field. The complexity arises from two general factors, the first that they are long chained and thus have long relaxation times, and second that they are liquid crystalline, and thus there is co-operative motion of the molecules. In both thermotropic and lyotropic LCPs subject to low shear flows, it is known that defects and disclinations influence the microstructure and rheology, but it is not clear by what mechanisms these distortions shrink or multiply during flow. In this work, a model is developed to examine the behaviour of defects in shear flows. The simulations based on the model show a spectrum of microstrucural development as a function of applied shear rate: reorientation of domains of different alignment associated with disclinations at low shear strains; the multiplication of wall type defects and the orientation of these normal to the shear gradient axis at intermediate shear rates, and the tendency towards disclination annihilation; the generation of a flow-aligned monodomain at higher shear rates.     

Modelling of shear flow in liquid crystalline materials

The flow behaviour of liquid crystalline polymers (LCPs) is quite complex and these materials exhibit varied and complicated textural patterns when subject to a flow field. The complexity arises from two general factors, the first that they are long chained and thus have long relaxation times, and second that they are liquid crystalline, and thus there is co-operative motion of the molecules. In both thermotropic and lyotropic LCPs subject to low shear flows, it is known that defects and disclinations influence the microstructure and rheology, but it is not clear by what mechanisms these distortions shrink or multiply during flow. In this work, a model is developed to examine the behaviour of defects in shear flows. The simulations based on the model show a spectrum of microstrucural development as a function of applied shear rate: reorientation of domains of different alignment associated with disclinations at low shear strains; the multiplication of wall type defects and the orientation of these normal to the shear gradient axis at intermediate shear rates, and the tendency towards disclination annihilation; the generation of a flow-aligned monodomain at higher shear rates.     

Inclusion of thiazyl radicals in porous crystalline materials

The incorporation of benzodithiazolyl (BDTA) and methylbenzodithiazolyl (MBDTA) radicals into porous hybrid frameworks via gas phase diffusion revealed that inclusion appeared selective for the MIL53(Al) framework against a range of other potential hosts. Both PXRD and EPR studies are consistent with retention of a π*-π* dimer motif for BDTA in MIL53(Al)@BDTA whereas MBDTA in MIL53(Al)@MBDTA appears to be monomeric. The guests are readily released by the addition of solvent (CH(2)Cl(2)).     

Self-assembled liquid crystalline materials with fatty acids

Two new series of liquid crystalline materials with non-mesomorphic fatty acids, viz. nonanoic (C₉), capric (C₁₀), undecanoic (C₁₁), tridecanoic (C₁₃), myristic (C₁₄), palmitic (C₁₆) and stearic acids (C₁₈), are prepared with non-mesomorphic proton acceptors, viz. (4-pyridyl)-benzylidene-p-n-alkylanilines (PyBnA; n = 12 and 16). The smectic phase structures formed between the proton donor (fatty acid) and proton acceptor moieties are found due to the intermolecular hydrogen bonding (HB) and are monotropic. The presence of HB is confirmed by Fourier transform infrared spectroscopy in all the compounds. The characteristic optical textures of smectic phases are observed through polarizing optical microscope, provided with a hot stage and a camera. The phase transition temperatures and the enthalpy changes across the phase transitions are determined by differential scanning calorimetry. The smectic phase exhibited by the HB complexes is confirmed by miscibility studies as smectic-B. The π–π stacking interactions in layers are found to influence the mesomorphism in these HB complexes.     

Liquid crystalline materials with complex mesophase morphologies

Liquid crystals, which combine order and mobility on a molecular and supramolecular level are increasingly accepted as a fourth state of matter. Besides the well-established nematic, smectic and columnar mesophases, more complex mesophase morphologies attracted increasing interest during the recent years. These are bicontinuous and discontinuous cubic mesophases and other two- and three-dimensionally ordered intermediate phases, superstructures induced by molecular chirality or by polar order of bent core molecules, novel biaxial smectic phases, and novel mesophase morphologies of polyphilic block molecules and dendrimers.     

Anomalous phase sequence in new chiral liquid crystalline materials

We have prepared and studied a series of rod-like liquid crystalline compounds with the molecular core laterally substituted by bromine atom and with the lactate unit in the chiral terminal chain. Microscopic observation under the polarising microscope, x-ray and dielectric spectroscopy results enable us to establish the TGBA–SmA–TGBA_RE–TGBC phase sequence. The presence of the re-entrant TGBA phase below the regular SmA phase, found for three homologues, is quite uncommon. Results are discussed and compared with the previously studied compounds with regular phase sequences.     

Electron localization function in full-potential representation for crystalline materials

The electron localization function (ELF) is implemented in the first-principles, all-electron, full-potential local orbital method. This full-potential implementation increases the accuracy with which the ELF can be computed for crystalline materials. Some representative results obtained are presented and compared with the results of other methods. Although for crystal structures with directed bonding only minor differences are found, in simple elemental metals, there are differences in the valence region, which give rise to different ELF topologies.     

Structure-fluorescence contrast relationship in cyanine DNA intercalators: toward rational dye design

The fluorescence enhancement mechanisms of a series of DNA stains of the oxazole yellow (YO) family have been investigated in detail using steady-state and ultrafast time-resolved fluorescence spectroscopy. The strong increase in the fluorescence quantum yield of these dyes upon DNA binding is shown to originate from the inhibition of two distinct processes: 1) isomerisation through large-amplitude motion that non-radiatively deactivates the excited state within a few picoseconds and 2) formation of weakly emitting H-dimers. As the H-dimers are not totally non-fluorescent, their formation is less efficient than isomerisation as a fluorescent contrast mechanism. The propensity of the dyes to form H-dimers and thus to reduce their fluorescence contrast upon DNA binding is shown to depend on several of their structural parameters, such as their monomeric (YO) or homodimeric (YOYO) nature, their substitution and their electric charge. Moreover, these parameters also have a substantial influence on the affinity of the dyes for DNA and on the ensuing sensitivity for DNA detection. The results give new insight into the development and optimisation of fluorescent DNA probes with the highest contrast.     

Side‐chain liquid crystalline polyacrylates containing 3,4‐disubstituted pyrrole moiety

Polymerization of liquid crystalline bifunctional monomer N‐{{ω‐{4‐[4‐(11‐acryloyloxy)undecanoxybenzoyl]biphenyleneoxy} alkyl}}‐3,4‐dimethylpyrrole gave a side‐chain liquid crystalline polyacrylate containing 3,4‐dimethylpyrrole group. Liquid crystallinity was determined by DSC and optical polarizing microscope measurements. The monomers having pentylene, hexylene, and decylene as the alkylene spacer group between the mesogenic unit and the 3,4‐dimethylpyrrole group exhibited smectic and nematic phases on the heating and cooling stages. The radically polymerized polyacrylate derivatives containing the 3,4‐dimethylpyrrole group showed nematic phases on the heating and cooling stages. These polymers are the first example of side‐chain liquid crystalline polymers containing 3,4‐dimethylpyrrole as functional group for further reaction. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 1214–1221, 2000     

Morphology-rheology relationships in incompatible blends based on thermotropic liquid crystalline polymers

In this paper the effect of the inclusion of two different thermotropic liquid crystal polymers (TLCP) on the processing of a polyetherimide (ULTEM 1000) is studied.     

Simulation of indentation fracture in crystalline materials using mesoscale self-assembly

A new physical model based on mesoscale self-assembly is developed to simulate indentation fracture in crystalline materials. Millimeter-scale hexagonal objects exhibiting atom-like potential functions were designed and allowed to self-assemble into two-dimensional (2D) aggregates at the interface between water and perfluorodecalin. Indentation experiments were performed on these aggregates, and the stresses and strains involved in these processes were evaluated. The stress field in the aggregates was analyzed theoretically using the 2D elastic Hertz solution. Comparison of the experimental results with theoretical analysis revealed that fracture develops in regions subjected to high shear stress and some, albeit low, tensile stress. The potential for the broader application of the model is illustrated using indentation of assemblies with point defects and adatoms introduced at predetermined locations, and using a two-phase aggregate simulating a compliant film on a stiff substrate.     

Assembly of crystalline halogen-bonded materials by physical vapor deposition

Polycrystalline halogen-bonded assemblies fabricated by physical vapor deposition (PVD) exhibit controllable morphologies and microstructures. Although the solid-state packing may vary going from a solution crystal growth process (used for chromophore single-crystal determination) to a vapor-phase deposition process (used for PVD film fabrication), the corresponding film microstructures are independent of the substrate surface chemistry.     

Phase relationships and materials design in the Ln-Si-Al-O-N system

Subsolidus phase relationships in the system Ln2O3-Si3N4-AlN-Al2O3, where Ln represents Nd, Sm and Dy, were summarized, with emphasis on the region involving α-sialon, β-sialon and AlN-polytypoid phases. This information is further used in designing the compatible matrix phases of sialon materials with desirable properties. Examples were provided to illustrate the advantage of such a basic approach to materials design.     

A single-tilt TEM stereomicroscopy technique for crystalline materials.

A new single-tilt technique for performing TEM stereomicroscopy of strain fields in crystalline materials has been developed. The technique is a weak beam technique that involves changing the value of g and/or s g while tilting across a set of Kikuchi bands. The primary benefit of the technique is it can be used with single-tilt TEM specimen holders including many specialty holders such as in situ straining, heating, and cooling holders. Standard stereo-TEM techniques are almost always limited to holders allowing two degrees of rotational freedom (i.e., double-tilt or tilt/rotation holders). An additional benefit of the new technique is that it eliminates the need to focus with the specimen height control. These advantages make it useful for stereo viewing or for quantitative stereomicroscopy provided necessary consideration is given to errors that may result from the technique.     

Photochemical stability of nonlinear optical chromophores in polymeric and crystalline materials

We compare the photochemical stability of the nonlinear optical chromophore configurationally locked polyene 2-{3-[2-(4-dimethylaminophenyl)vinyl]-5,5-dimethylcyclohex-2-enylidene} malononitrile (DAT2) embedded in a polymeric matrix and in a single-crystalline configuration. The results show that, under resonant light excitations, the polymeric compound degrades through an indirect process, while the DAT2 crystal follows a slow direct process. We show that chromophores in a crystalline environment exhibit three orders of magnitude better photostability as compared to guest-host polymer composites.