Showing papers in "Molecular Crystals and Liquid Crystals in 1997"
TL;DR: In this paper, the photochemically induced orientation of the photochromic side groups affects also the distribution of non-photochromic but mesogenic moieties, and the combination of both ordering principles causes a significant amplification of the light-induced optical anisotropy.
Abstract: By irradiation with linearly polarized light optical anisotropy is generated in isotropic films of photochromic LCP's due to a photoorientation process. Generally, the efficiency is much higher in films of polymers with a high content of azobenzene moieties. On the other hand, the photochemically induced orientation of the photochromic side groups affects also the distribution of non-photochromic but mesogenic moieties. Furthermore, in initially isotropic films it acts as initializing force, governing the direction of thermotropic self-organization. The combination of both ordering principles causes a significant amplification of the light-induced optical anisotropy. Investigating several polymers of the same general structure but with a different content of azobenzene side groups an interdependent behaviour was found for the photoorientation of the photochromic and the co-operative orientation of non-photochromic side groups.
54 citations
TL;DR: In this paper, a liquid crystalline polymer network in a device cell is formed by the photo-cure of UV photo-currable liquid crystal monomers that are doped in an FLC, where both the rubbed polyimide (PI) and non-rubbed but linearly polarized UV irradiated poly(vinyl cinnamate) (PVCi) films are utilized as surface FLC molecular alignment.
Abstract: Polymer stabilized SSFLCDs have been successfully fabricated. A liquid crystalline polymer network in a device cell is formed by the photo-cure of UV photo-currable liquid crystalline monomers that are doped in an FLC, where both the rubbed polyimide (PI) and non-rubbed but linearly polarized UV irradiated poly(vinyl cinnamate) (PVCi) films are utilized as surface FLC molecular alignment. The actual monomers adopted is acrylate with a mesogenic side chain. The PVCi is one of the photo alignment polymer. The fabrication process of these SSFLC cells is as follows; first the photo-cure is done on a monomers and initiators doped LC medium at the temperature where this material takes SmA phase and then the cell is cooled down to room temperature, at which the medium takes SmC* phase. The layer structure of the liquid crystalline polymer stabilized (LC-PS) FLC cells has been investigated by X-ray diffraction measurement. As the experimental results, it is found that the formed polymer network suppresse...
48 citations
TL;DR: In this paper, the optical properties of perylene microcrystals in wide range of crystal size from 50 to 1 μm were measured, and the interesting behavior of exciton in micro-crystals with less than 200nm was found.
Abstract: The optical properties of perylene microcrystals in wide range of crystal size from 50 ran to 1 μm were measured. The interesting behavior of exciton in microcrystals with less than 200nm was found. For example, with decreasing the crystal size, self-trapped exciton states in microcrystals shifted to the high energy side. Therefore, the strong luminescence from free-exciton was observed, and life time of self-trapped exciton became shorter .
48 citations
TL;DR: In this paper, the authors found three smectic phases, SmX1, SMX2 and SmX3, in achiral compounds with a banana shape and showed that the spontaneous polarization is about 60 nC/cm2 and shows a slight temperature dependence.
Abstract: We found three smectic phases, SmX1, SmX2 and SmX3, in achiral compounds with banana shape. The SmX1 phase has a biaxial single molecular layer, along which polarization exists. The polarization precesses along the layer normal, and is aligned by applying an electric field. The spontaneous polarization is about 60 nC/cm2 and shows a slight temperature dependence. The dielectric constant shows a larger value in SmX1 than those of the other phases, suggesting the Goldstone mode. The relaxation frequency is below 100 Hz.
47 citations
TL;DR: In this paper, a photochemically induced order in an initially isotropic film acts as initializing force, governing the direction of thermotropic self-organization, which causes a significant amplification of the light-induced optical anisotropy.
Abstract: Photoreorientation processes in photochromic LCP's are dependent on the supramolecular order. The efficiency is much higher in films of low order, which have been obtained by a suitable preparation or by an intermediate photochemical decoupling. On the other hand, a photochemically induced order in an initially isotropic film acts as initializing force, governing the direction of thermotropic self-organization. The combination of both ordering principles causes a significant amplification of the light-induced optical anisotropy.
46 citations
TL;DR: In this article, the magnetic properties of oxygen physisorbed in Cu-trans-1,4-cyclohexanedicarboxylic acid having one-dimensional micropores are studied.
Abstract: Magnetic properties of oxygen physisorbed in Cu-trans-1,4-cyclohexanedicarboxylic acid having one-dimensional micropores are studied. O2 is well known as a magnetic molecule with S=1. In the case of the low adsorption, temperature dependence of the susceptibility shows the Schottky-type broad peak and the high-field magnetization process has a step at around 34T with saturation moment of 2 μ B/O2. These results are interpreted by the dimer model of S=½ with an antiferromagnetic exchange interaction. Broadening of the step of magnetization with increasing the adsorption may be due to the inter-dimer interaction or the distribution of the intra-dimer exchange interaction. Linear magnetization process which is characteristic of one-dimensional Heisenberg antiferromagnet has not been observed even in the full pore region.
46 citations
TL;DR: In this paper, structural modifications to photochromic 3H-naphtho[2,1 -b]pyrans and 2H naphthoe[1,2-b] pyrans have produced compounds that have found utility in plastic ophthalmic lens applications.
Abstract: Structural modifications to photochromic 3H-naphtho[2,1 -b]pyrans and 2H-naphtho[1,2-b]pyrans have produced compounds that have found utility in plastic ophthalmic lens applications. The influence of substituents on color, optical density, and fade rate are reported. Synthetic routes to products and key intermediates are also outlined.
44 citations
TL;DR: A series of novel heterocyclic esters has been prepared based on selenophene, thiophene and furan units; for comparison the parent, phenyl analogue was also prepared as discussed by the authors.
Abstract: A series of novel heterocyclic esters has been prepared based on selenophene, thiophene and furan units; for comparison the parent, phenyl analogue was also prepared. All the materials except the least linear furan analogue are mesogenic and exhibit the SA, SC*, SC*ferri and SC*anti liquid crystalline phases. In addition, the selenophene material exhibits a higher ordered antiferroelectric phase which has been designated the antiferroelectric smectic I (SI*anti) phase and below this the SI* phase was generated. The synthesis, mesomorphic behaviour and physical properties of the materials are reported.
44 citations
TL;DR: In this article, a columnar discotic phase is reported for several triphenylene derivatives, characterized by a three-dimensional positional order of the molecules, as in a crystalline state.
Abstract: A plastic columnar discotic phase is reported for several triphenylene derivatives. It is characterized by a three-dimensional positional order of the molecules, as in a crystalline state, while maintaining the rotational mobility of the molecules. Only very minute changes in structure and dynamics occur at the phase transition from the normal discotic hexagonal phase to the discotic plastic phase.
43 citations
TL;DR: In this article, Me4Z [Pd(L)2]2 (Z = N, P, As, and Sb; L = dmit and dmise) is described and the behavior of resistivity under pressure depends on the Se-substitution and the choice of the cation.
Abstract: Crystal and electronic structures and physical properties of the molecular conductors Me4Z [Pd(L)2]2 (Z = N, P, As, and Sb; L = dmit and dmise) are described Every salt has very similar crystal structure with “solid-crossing” columns The behavior of resistivity under pressure depends on the Se-substitution and the choice of the cation Tight-binding band calculations indicate that these salts form a unique two-band system The narrow and two-dimensional HOMO (anti-bonding) band is thought to form the half-filled conduction band Magnetic properties suggest that the insulating state under ambient pressure comes from the strong electron-electron correlation The one-dimensional LUMO (bonding) band is located below the HOMO band The application of pressure induces an overlap of these two bands and changes the band filling, which brings forth the metallic state The system under higher pressure, however, shows non-metallic behavior probably due to the one-dimensional instability associated with th
41 citations
TL;DR: In this paper, a thought experiment was conducted to analyze the effect of the tilt plane on tristable switching in three compounds and concluded that tilting directions in adjacent layers are correlated with the biased directions of molecular rotation about the long axes.
Abstract: We have first reviewed (1) the extremely bent molecular shape, (2) in-layer spontaneous polarizations near smectic layer boundaries parallel to the tilt plane in SCA*, (3) a difference in the molecular states of rotation about their long axes between SCA* and SC*, and (4) the ANNNI model with J 3 by Yamashita (C1P.12 (P-124) at this Conf.). Then we have observed the pretransitional effect in the tristable switching in three compounds and analyzed it through a thought experiment based on the above review; tilting directions in adjacent layers are concluded to be correlated with the biased directions of molecular rotation about the long axes. Finally we have presented evolution of switching characteristics from tristable to V-shaped in the first component compound of the mixture reported to show the thresholdless antiferroelectricity (Inui et al., J. Mater. Chem., 6, 671 (1996)), and discussed it in terms of the above thought experiment.
TL;DR: In this paper, a high-resolution x-ray scattering study reveals that light irradiation controls the smectic spacing of 8CB and diheptylazobenzene 7AB at T = 24.00°C.
Abstract: The smectic-A phase of octylcyanobiphenyl (8CB) doped with a photosensitive azo-compound has been reported to exhibit structural changes and formation of periodic layers distortions upon exposure to low-level radiation. It is caused by changes in the smectic layer spacing triggered by light-induced molecular photoisomerization of the dopant azocompound. Our in-situ high-resolution x-ray scattering study reveals that the light irradiation controls the smectic spacing. The exposure to UV light increases the layer spacing by as much as 0.5% (0.155A for the studied mixture of 8CB and diheptylazobenzene 7AB at T = 24.00°C). Irradiation by He·Ne laser at 633 nm causes the contraction of the layers.
TL;DR: In this paper, the relative thermodynamic stability of the spatially uniform phases of binary mixtures of hard rods and discs was investigated using the variational cluster approximation and the factors promoting the stability of biaxial nematic phase were identified and discussed.
Abstract: We use the variational cluster approximation to study the relative thermodynamic stability of the spatially uniform phases of binary mixtures of hard rods and discs. The factors promoting the stability of the biaxial nematic phase are identified and discussed. The results suggest that a stable thermotropic nematic biaxial mixture cannot be obtained from molecules of the sizes and electric dipole interaction strengths commonly encountered in real calamitic and discotic thermotropic phases.
TL;DR: In this article, the angular transmission of PDLC films has been experimentally investigated as a function of droplets dimension, refractive indices and thickness of sample, and the experimental data have been interpreted in terms of the Anomalous Diffraction Approximation slightly modified in order to take into account extra-scattering phenomena generated by some of the peculiar PDLC characters.
Abstract: The angular transmission of Polymer Dispersed Liquid Crystals (PDLC) films has been experimentally investigated as a function of droplets dimension, refractive indices and thickness of sample. The experimental data have been interpreted in terms of the Anomalous Diffraction Approximation slightly modified in order to take into account extra-scattering phenomena generated by some of the peculiar PDLC characters. The dimension of the droplets and refractive index difference between polymer and liquid crystal are found to be the most important factors affecting the light transmission as well as extra scattering factors.
TL;DR: In this article, the structures and characterization of dodecanuclear complexes that exhibit out-of-phase magnetic susceptibility signals at low temperatures (<8 K) are described, which are indicative of slow relaxation of the magnetization, unable to keep in phase with the oscillating AC magnetic field, and are taken as evidence for single-molecul
Abstract: The structures and characterization are described of complexes that exhibit an out-of-phase AC magnetic susceptibility x m” The dodecanuclear complexes [Mn12O12(O2CR)16(H2O)4]z contain 8MnIII, 4MnIV or MnII, 7MnIII, 4MnIV and possess ground state spins of S = 9 or 10 (z = 0) or S = 19/2 (z = −1) The distorted-cubane complexes [Mn4O3X(O2CR)3(dbm)3] contain 3MnIII, MnIV and have been prepared with a variety of X groups; in every case, the ground state is S = 9/2 The anion [Mn11O10X6(O2CPh)11(H2O)4]2− (9MnIII, 2MnIV; X = NCO−, N3 −) contains two [Mn4O3X] distorted-cubanes linked by a [Mn3O4]+ unit; the ground state is S = 5 The cation [V4O2(O2CEt)7(bpy)2]+ (4VIII) has a [V4O2]8+ butterfly core and a S = 3 ground state All these complexes exhibit out-of-phase AC magnetic susceptibility signals at low temperatures (<8 K) These signals are indicative of slow relaxation of the magnetization, unable to keep in phase with the oscillating AC magnetic field, and are taken as evidence for single-molecul
TL;DR: In this article, the authors measured third-harmonic susceptibilities in various one-dimensional conjugated polymer films at a fundamental wavelength of 1.907 μm and found that the relationship between polymer structure and the thirithmic susceptibilities was not well understood.
Abstract: We have measured third-harmonic susceptibilities in various one-dimensional conjugated polymer films at a fundamental wavelength of 1.907 μm. The relationship between polymer structure and the thir...
TL;DR: In this paper, the multikinetics of fluoro-substituted chromenes are studied with a polychromatic light source in different conditions, and the results obtained in our laboratory, on a U.V-Visible spectrometer at the Photostationary State in toluene and ethanol solutions.
Abstract: : This paper presents the first results obtained in our laboratory, on a U.V-Visible spectrometer at the Photostationary State in toluene and ethanol solutions. Here, the multikinetics of fluoro-substituted chromenes are studied with a polychromatic light source in different conditions. The fluoro-substitution on the aryl moiety always leads to an increase of the fast rate constant.
TL;DR: In this paper, photochromic-doped sol-gel gel-glasses can be prepared by adding a spiropyrane photo-chromic dye to a solution of ethoxy silane monomers containing non-reacting ethyl radicals.
Abstract: Among the developments associated with sol-gel materials, of relevance is the entrapment of photochromic molecules. Photochromic-doped sol-gel gel-glasses can be prepared by adding a spiropyrane photochromic dye to a solution of ethoxy silane monomers containing non-reacting ethyl radicals. After polymerization, normal photochromism (i.e., colored material upon UV irradiation) is obtained in the resulting matrix. The sol-gel matrix strongly affects the photohhermal reactions of the embedded organic molecule. If these materials are attached to optical fibers, the properties of the light throughput may be modified. An approach to optical memory applications by writing and erasing on newly developed photochromic and thermochromic materials is reported. In this report we emphasize on some of the applications of doped sol-gel glasses, reviewing past results and describing recent progress, mainly in our laboratories.
TL;DR: In this paper, crude or gross molecular shapes are used to investigate local packing arrangements of chiral molecules in layered mesophases, and the formation of helical macrostructures is readily described using this form of modelling.
Abstract: Physical theories concerning the structures and properties of chiral smectic liquid crystals abound, however, very few are of practical use in the design and subsequent synthesis of materials. The development of macroscopic theories, although very useful in the explanation of physical phenomena, are not readily transferable to the microscopic state. Thus, the design of materials depends more on a qualitative approach supported by property-structure correlations than it does on quantitative deliberation. However, viewing the formation and stabilisation of mesophase structures from the microscopic molecular aspect can lead to some very interesting perspectives. In this article, crude or gross molecular shapes are used to investigate local packing arrangements of chiral molecules in layered mesophases. The formation of helical macrostructures is readily described using this form of modelling, and in turn the formation of antiferroelectric and twist grain boundary phases can be rationalised in terms ...
TL;DR: In this article, 1H and 13C NMR and broadband dielectric spectroscopy were used to detect local nematic order in cavities of 2.5, 5.0 and 7.5 nm diameter.
Abstract: Orientational order and mobility of nematogens adsorbed in nanoporous glass are investigated by 1H and 13C NMR and broadband dielectric spectroscopy. For the mesogen 5CB, we detect local nematic order in cavities of 2. 5, 5. 0 and 7. 5 nm diameter. Quantitative measurements of the order parameter are reported. The nematic-isotropic transition temperature is lowered by approximately 16K compared to the bulk phase. The N-I transition is smooth, of second or weakly first order. At low temperatures, order parameters of ≈ 0. 7 are reached. Crystallisation (N-Cr) is suppressed considerably, at least to below 220K. A surface layer of mesogenic molecules at the pore walls with considerably retarded molecular dynamics is detected.
TL;DR: In this paper, the V-shaped switching reported by Inui et al. was investigated in a free-standing binary mixture and several subphases were observed in the ordinary sequence, indicating frustration between ferro and antiferro-electricity.
Abstract: To elucidate the V-shaped switching reported by Inui et al., we have studied another binary mixture. In its free-standing film, several subphases were observed in the ordinary sequence, indicating frustration between ferro- and antiferro-electricity; in particular, the antiferroelectric subphase exists in a temperature range as wide as 22 °C and the helical pitch is longer than 2. 5 μm. In a homogeneous cell, however, only two liquid crystal phases were distinguished; one is SA and the other is a phase characterized by the nearly V-shaped switching. We have concluded that the substrate interfaces induce tilting randomization, breaking the reduced intrinsic tilting correlation in adjacent layers.
TL;DR: In this article, the electronic and molecular structures and charge transfer (CT) complex formation of a new electron donor molecule, bis(ethy1enedioxy)-dibenzotetrathiafulvalene (BEDO-DBTTF), were studied from the point of the peculiar ability of ethylenedioxyl group in delivering conductive solid complexes.
Abstract: The electronic and molecular structures and charge transfer (CT) complex formation of a new electron donor molecule; bis(ethy1enedioxy)-dibenzotetrathiafulvalene (BEDO-DBTTF), were studied from the point of the peculiar ability of ethylenedioxy group in delivering conductive solid complexes. The BEDO-DBTTF molecules afforded 37 solid complexes with 40 kinds of electron acceptor molecules belonging to 5 systems; TCNQ, quinone, fluorene, percyano and nitrophenyl systems. The infrared and ultraviolet-visible-near infrared spectra of the complexes were examined to study the ionicity of their ground states in solid. A plot of CT transition energies and the difference of redox potentials; Δ E(DA), of donor and acceptor molecules indicated that 10 complexes have neutral ground state. No definitely identified back CT bands were observed in the 4 complexes of completely ionic ground state. 23 complexy of partially ionic ground state gave CT transitions below 5×103 cm−1 and are highly conductive. The prese...
TL;DR: In this article, a liquid crystalline photoconductor 2-(4′-heptyloxypheny1)-6-dodecylthiobenzothiazole was designed and its carrier transport characteristics was measured with Time-of-Flight (TOF) technique.
Abstract: Liquid crystalline photoconductor 2-(4′-heptyloxypheny1)-6-dodecylthiobenzothiazole was designed and its carrier transport characteristics was measured with Time-of-Flight (TOF) technique. In crystal state, only transient current decay originating from a capturing process of photo-generated carriers by deep traps formed at grain boundaries was observed. In smectic A (SmA) phase, non-dispersive transient photocurrent based on fast hole transport was observed, in which hole mobility was 5×10−3 cm/Vs, independent of electric field. In isotropic phase, ambipolar transport proceeded and the mobilities of both positive charge and negative charge carriers were on the order of 10−5 cm2/Vs.
TL;DR: In this paper, the authors address the topic of the elucidation of magnetic structures by means of the neutron scattering technique and propose two examples of two-and three-dimensional, homo-and heterometallic oxalatebridged coordination compounds.
Abstract: Polymeric two- and three-dimensional, homo- and heterometallic oxalatebridged coordination compounds offer exciting opportunities, mainly in the fields of molecular magnetism and photophysics. Given that a large variety of magnetic phenomena have been reported so far from these molecular magnets, very limited experience is gained from elastic neutron scattering experiments. Therefore, with two examples, we will address the topic of the elucidation of magnetic structures by means of the neutron scattering technique. In addition, due to the possibility of the variation of different metal ions in varying oxidation states, interesting photophysical processes can be observed within the extended three-dimensional host/guest systems.
TL;DR: The photokinetic behavior of a naturally occurring chromene, lapachenole (2,2-dimethyl-6-methoxy-7,8-benzo(2H)chromene, Ch), has been investigated by using monochromatic irradiation in the UV and visible ranges as discussed by the authors.
Abstract: The photokinetic behavior of a naturally occurring chromene, lapachenole (2,2-dimethyl-6-methoxy-7,8-benzo(2H)chromene, Ch), has been investigated by using monochromatic irradiation in the UV and visible ranges. Two colored photoproducts were identified: one is thermoreversible, the other is thermostable and photoreversible. The quantum yields of the color-forming and color-bleaching photoreactions were determined by a spectrokinetic method. The kinetic parameters of the thermal steps were measured (285 – 325 K). The sensitized color-forming reaction was also investigated, using thioxantone as a triplet sensitizer. A plausible reaction mechanism is proposed.
TL;DR: Magnetic properties of two nitronyl nitroxide (4,4,5,5-tetramethyl-4, 5-dihydro-1H-imidazol-1-oxyl 3-oxide) radicals are investigated down to 0.5 K and up to 9 T as discussed by the authors.
Abstract: Magnetic properties of two nitronyl nitroxide (4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-1-oxyl 3-oxide) radicals which show nearly isotropic S=½ alternating linear chains are investigated down to 0.5 K and up to 9 T. Pentafluorophenyl nitronyl nitroxide at low temperatures shows an antiferromagnetic chain with alternation, while the material at room temperature shows a uniform chain structure. 2-Methyl nitronyl nitroxide, which contains an alternating chain with ferromagnetic and antiferromagnetic interactions, exhibits a three-dimensional phase transition at 1.3 K.
TL;DR: In this paper, the birefringence and refractive indices of several liquid crystalline structures were measured throughout the visible wavelength region (400-900 nm) at different temperatures.
Abstract: The birefringence and refractive indices of several liquid crystalline structures were measured throughout the visible wavelength region (400–900 nm) at different temperatures. Mixtures composed of members of the same homologous series were used in order to achieve the nematic phase at room temperature. Data on 28 mixtures, mainly with cyclohexane - phenyl and biphenyl cores, are presented. The set includes widely used structures as well as newly synthesized. Some details of experimental methods, formulas for interpolation of the results and observed regularities are briefly discussed.
TL;DR: In this article, the magnetic structure of graphite-like structures with edge and/or topological defects using the tight-binding model (the Huckel model) was studied.
Abstract: Magnetic structure is studied for several π-networks of graphite-like structures with edge and/or topological defects using the tight-binding model (the Huckel model). Zigzag-edged graphite have ed...
TL;DR: In this article, an interpretation on this unique force titration behavior is made in terms of the rearrangement of the surface carboxylic acid groups when the probe tip and the substrate come into contact.
Abstract: Ω-Mercaptoundecanoic acid self-assembled monolayer (SAM) on gold was titrated with buffered water and chemical force microscopy (CFM) was used to follow the titration process by measuring the adhesion force between the SAM-modified substrate and probe tip The adhesion force vs pH curve thus obtained, which can be named as force titration curve, was found to show a big hump around pH 5. Taking the contact-angle titration result together, an interpretation on this unique force titration behavior is made in terms of the rearrangement of the surface carboxylic acid groups when the probe tip and the substrate come into contact.
TL;DR: In this paper, the transport and optical properties of the metallic reticulate-doped polymer composites based on bis(ethylenedioxy)tetrathiafulvalene (BEDO-TTF) complexes are investigated, and compared with the structural and physical properties of their respective bulk crystals.
Abstract: Transport and optical properties of the metallic reticulate-doped polymer composites based on bis(ethylenedioxy)tetrathiafulvalene (BEDO-TTF) complexes are investigated, and compared with the structural and physical properties of the respective bulk crystals. Exposing the surface of polycarbonate (PC) film containing the molecularly dispersed donor to the solvent/(iodine or bromine) vapor converted the donor into charge-transfer (CT) complexes, which dominate the conducting and thermoelectric properties of the composites. Both film surfaces doped with bromine and iodine are well conducting, showing metallic behavior down to low temperatures. A characteristic CT band for the highly conducting states of the partially oxidized BEDO-TTF molecules appeared in the infrared region. However, compared with the respective bulk crystal, absorptions of the intramolecular excitation of the donor molecules are greatly diminished for the incident light perpendicular to the film surface. The PC film of (BEDO-TTF...