摘要:

Four discotic liquid crystals (LCs) from the hexa-alkoxytriphenylene series (R= CnH2n+ 1,n= 5, 7, 9, 11) classified as Dhophases were investigated with respect to their photoconducting properties. Under irradiation, all samples show photoconductivity within the mesophase, whereas in the isotropic phase the photocurrent drops to zero. All of the observed effects, the phase dependence of the photocurrent, the increasing values of the photocurrent with decreasing length of the side chains, and the higher photoconductivity during the cooling process, can be explained in terms of the transition temperatures, the intercolumnar distances and the orientation behaviour. Comparison of the photocurrents of the investigated homologues at a reduced temperature leads to an activation energy of 0·67 eV for the charge transport.

ISSN:0267-8292    

DOI:10.1080/02678299308036478

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

We have recently analysed the crystal and molecular structures of six metalloorganic discogens with substitutedβ-diketone ligands. The molecules consist of a rigid 11 atom core and a fringe made up of four phenyl rings substituted with alkyl/alkoxy chains. In complex (i), with four octyloxy chains, there are four oxygen atoms around the core. Complex (ii) is asymmetrically substituted with two heptyloxy and two heptyl chains and therefore has two oxygen atoms and complexes (iii) to (vi) have only alkyl chains and hence no oxygen atoms around the core. The metal atom used for coordination has been chosen as Cu/Pd/Ni. Determination of the crystal and molecular structures of these discogens has led to the identification of the following similarities: (1) All the six discogens crystallize in the triclinic space group P1. The recurrence of the space group may be correlated with the structural requirements for efficient packing of the molecules in the crystal lattice. (2) The coordination around the metal atom is square planar. (3) The 11 atom core is only nearly planar. (4) The phenyl rings and the chains are tilted with respect to the core. (5) The molecular conformation in the crystal confers a nearly rectangular shape to these discogens. (6) The chains are fully extended in an alltransconformation. (7) The molecular arrangement is tilted columnar except for the crystal structure of complex (ii).

ISSN:0267-8292    

DOI:10.1080/02678299308036479

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Information on molecular dynamics of the nematic liquid crystal 4-n-pentyl-4′-cyanobiphenyl (5CB) doped with the dyes DMANS and DMANAB has been obtained by luminescence and Raman measurements. Temperature dependences of reorientational correlation times are not the same for different molecular motions. Mechanisms responsible for the marked deviation of the time values obtained by different optical methods are discussed.

ISSN:0267-8292    

DOI:10.1080/02678299308036480

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The cubic phases Q230(space group Ia3d) of type I (oil in water) and type II (water in oil) have been studied by freeze-fracture electron microscopy. The preservation of the sample structure during cryofixation was verified by low temperature X-ray diffraction. Three types of fracture planes were clearly identified in the type I system and two in the type II system. These fracture planes showed two-dimensionally ordered domains, each subdivided into subdomains related to each other by displacements and rotations compatible with the symmetry of the space group. The images were filtered using correlation averaging techniques and the filtered images were compared with the corresponding sections of the electron density map. Several conclusions were drawn: (i) The structures of the samples had been preserved after quenching, fracturing and replicating processes, (ii) The electron microscope images were fully compatible with the postulated space group symmetries. (iii) The fracture planes coincide with the planes giving rise to the most intense X-ray reflections and containing the rod-like structure elements that seem to facilitate the propagation of the fractures. This approach constitutes a new and powerful tool of general interest for the study of liquid crystals.

ISSN:0267-8292    

DOI:10.1080/02678299308036481

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Electro-optic properties of liquid crystal phase gratings (LCPGs), regarding groove depth, temperature and a cell gap, have been studied. The off response times are proportional to the square of the groove depth. The transmitted light colour depends on temperature. When there is a gap between the grating and the opposite substrate, their transmittance–voltage curve has a maximum and a minimum. The authors have also simulated the LCPG properties using a homogeneous alignment model. The simulating results coincide with experimental results; therefore the simulation method is considered useful to predict LCPG properties.

ISSN:0267-8292    

DOI:10.1080/02678299308036482

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The temperature dependences of surface tension, γ=f≠ (t), are experimentally studied and discussed for homologous series of liquid crystalline 4-alkyl-4′-cyano biphenyls, 4-alkyl-4′-cyanophenylbenzoates, 4-alkyloxy-4′-cyanophenylbenzoates and also for several nematic liquid crystals of other chemical classes. The surface energy anisotropy of 5CB is measured at fixed temperature.

ISSN:0267-8292    

DOI:10.1080/02678299308036483

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The optical activity and light transmission of the blue phases and isotropic phase of 4″-(2-methylbutylphenyl)-4′-(2-methylbutyl)-4-biphenylcarboxylate (CE2) are investigated under high pressure. The temperature ranges of the first (BP I) and third (BP III) blue phases show only the slightest change, if any, with increasing pressure. This is the first time BP III, a phase which is not understood at all, has been investigated under pressure. In addition, an alternative analysis to the one reported previously is reported which confirms that the pretransitional optical activity in the isotropic phase changes very slightly due to pressure, and even this small change seems to be due to small changes in pitch and index of refraction. Taken together, these observations indicate that high pressure has little effect on both the BP III phase and the transition to the isotropic phase in highly chiral systems, even though the BP III phase differs significantly from the other blue phases and strong short range fluctuations make the transition appear continuous optically.

ISSN:0267-8292    

DOI:10.1080/02678299308036484

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The existence of an exact solution for axial propagation in a dielectric cholesteric medium has been often remarked upon. Here, we obtain the exact solution for axial propagation in a magnetic-dielectric cholesteric medium, using the 4 × 4 matrix method of optics. Our solution procedure also enables us to identify the specific features that make axial propagation in cholesteric media remarkable in having an analytical solution.

ISSN:0267-8292    

DOI:10.1080/02678299308036485

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The molecular orientational states of homogeneously aligned, helix unwinding, chiral smectic C liquid crystals placed in a thin cell (surface-stabilized ferroelectric liquid crystals [SSFLC]) were studied. They were classified by the optical viewing conditions and the relationship between the directions of the chevron layer structure and the surface pretilt. The molecular orientational models of the states were considered and illustrated with regard to the experimental results. The models of molecular orientation give us a total understanding of the orientational states which appear in SSFLCs with parallel rubbing. Furthermore, the effect of the surface pretilt angle on the orientational and optical properties of SSFLCs is discussed.

ISSN:0267-8292    

DOI:10.1080/02678299308036486

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Two series of potentially cross-linkable liquid crystal materials derived fromp-phenylenebisacrylic (seriesI,n(P.FB.P),n= 2–10) andp-hydroxycinnamic acid (seriesII,n(HC.T.HC),n= 2–10) have been synthesized and their thermal and mesomorphic properties studied. All these compounds show enantiotropic mesomorphism over a wide range of temperatures. Compounds with short terminal chains are nematic and when the terminal chain length is increased they show smectic polymorphism-smectic A and C. Most of the compounds are thermally stable over their mesophase ranges.

ISSN:0267-8292    

DOI:10.1080/02678299308036487

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor