Showing papers in "Molecular Crystals and Liquid Crystals in 1999"
TL;DR: In this article, a transition from ground-state zwitterion to an excited-state conformer was shown to be due to a transition of the ground-State Zwitterions to a neutral conformer.
Abstract: Macroscopic and nanoscopic current−voltage measurements reveal asymmetries in the DC electrical conductivity through Langmuir−Blodgett multilayers and even monolayers of γ-(n-hexadecyl)quinolinum tricyanoquinodimethanide, C16H33Q-3CNQ (5). These asymmetries are due to a transition of the ground-state zwitterion to an excited-state conformer which is probably neutral. Unimolecular electrical rectification by monolayers of 5 is unequivocally confirmed.
77 citations
TL;DR: In this paper, three Miesowicz viscosity coefficients ηa ηb and ηc were determined by dynamic light scattering (DLS) measurements and a coupling constant of 3.14kBTc was obtained, which is smaller than the mean field theory value.
Abstract: Visco-elastic properties of 5CB as a function of temperature in the nematic region are studied. Three Miesowicz viscosity coefficients ηa ηb and ηc and rotational viscosity γ1 are determined by dynamic light scattering (DLS) measurements. By assuming α3 ≅ 0, other Leslie viscosity coefficients are also calculated. Fitting our γ1 data to Osipov and Terentjev's theory, a coupling constant of 3.14kBTc is obtained, which is smaller than the mean field theory value. Temperature dependence of order parameter is obtained from refractive index measurement using Haller's method. To obtain accurate viscosity parameters from DLS, accurate values of elastic constants are required. Three different techniques are used to determine K11, K22 and K33, respectively: fitting the electric field dependence of capacitance to nematic continuum theory, ac electric field DLS and magnetic Freedericksz transition.
64 citations
TL;DR: In this paper, two derivatives of [Wv(CN)8]3- are presented to study the interaction between nearest metallic neighbours and explore also the influence of diffuse and high energy magnetic orbitals.
Abstract: Prussian blue molecule-based magnets can be designed to exhibit various magnetic properties. One of them based on chromicyanide and vanadium is a magnet at room temperature. Its properties are very sensitive to the synthetic reaction conditions, solvent effects and counterions which can be used to tune the Tc. To study the interaction between nearest metallic neighbours we explore also the influence of diffuse and high energy magnetic orbitals, i.e. 4d or 5d metallic orbitals. Two derivatives of [Wv(CN)8]3- are presented.
63 citations
TL;DR: In this article, the crystal structures and magnetic properties of a series of isostructural magnetic compounds, MII{N(CN)2}2, where M ˭ Cu (1), Ni (2), Co (3), and Fe (4) are presented.
Abstract: The crystal structures and magnetic properties of a series of isostructural magnetic compounds, MII{N(CN)2}2, where M ˭ Cu (1), Ni (2), Co (3) and Fe (4) are presented. The dicyanamide bridges the metal ions in 1–4 to form infinite 3-D metal-organic frameworks with a rutile-type structure. Magnetic data for 1 obey the Curie-Weiss law (Θ= −2.1 K). 2 and 3 are ferromagnets with Curie temperatures (Tc) of 21 and 9 K; 4 is a canted antiferromagnet (TN= 18.8 K). The coercive field of 4 is 17800 Oe, the largest observed for any metal-organic compound. The maximum energy product (BH) is the highest for 2 and is comparable to alloys of SmCo5. We attribute the large coercive field to a combination of single ion and particle shape anisotropies.
50 citations
Abstract: Terminally alkyl- and alkyloxysubstituted 4,6-dichloro-1,3-phenylene bis[4-(phenyliminomethyl)benzoates] have been synthesized and studied by optical microscopy, differential scanning calorimetry, NMR spectroscopy, X-ray diffraction and dielectric measurements. Surprisingly, nematic and smectic phases are observed, although comparable bisesters of resorcinol and monochlororesorcinol exhibit B phases typical for banana-shaped mesogens. It could be proved by NMR investigations in the liquid crystallinen state that the molecules have a nearly rod-like shape. Dielectric measurements indicate a rotation around the molecular long axis as known from calamitic mesogens, which is clearly distinguished from the behaviour of bent molecules. Obviously, substitution of the central phenyl ring of the bent 1,3-phenylene bis[4(4-n-alkyloxyphenyliminomethyl)benzoates] by two chlorine atomas causes a markedly change of the molecular shape resulting in nearly calamitic mesogens. These results are supported by singl...
50 citations
TL;DR: In this article, two new series of achiral banana-shaped molecules have been prepared and their mesomorphic, electrooptical and structural properties have been investigated, and the relation between molecular structure and mesomorphic properties is discussed.
Abstract: Two new series of achiral banana-shaped molecules have been prepared. Their mesomorphic, electrooptical and structural properties have been investigated. The first series of hydrogenated molecules shows the nematic phase with short chains (n=8–10), N and SmC phases with intermediate chain lengths (n = 11–13) and SmC plus an unknown smectic phase with long chains. The second series of fluorinated molecules is more peculiar, exhibiting four disordered smectic phases. Two of these have broken fan-shaped or schlieren optical textures like the SC phase, whereas the two others show slightly different textures with more or less mosaic domains. These phases display various switching characteristics, including but not limited to the “antiferroelectric” one reported for the Niori series. The relation between molecular structure and mesomorphic properties is discussed.
48 citations
TL;DR: In this paper, the optically active hydroxyesters: 1-hydroxy-4-(1-methylheptyloxy carbonyl)benzene and 4-hydrox-4-hydro-carbonyl biphenyl have been elaborated.
Abstract: Methods for preparing optically active hydroxyesters: 1-hydroxy-4-(1-methylheptyloxycarbonyl)benzene and 4-hydroxy-4′-(1-methylheptyloxy-carbonyl)biphenyl, 4-(1-methylheptyloxycarbonyl)phenyl 4′-hydroxybiphenyl-4-carboxylate and 4′-(1-methylheptyloxy carbonyl)biphenyl-4-yl 4-hydroxybenzoate have been elaborated. These compounds were used as intermediates for preparing liquid crystalline homologous series of di- and triesters exhibiting tilted smectic phases CA* and C*. The phase transitions have been measured by DSC and thermomicroscopic methods and the smectic phases have been identified by microscopic textures and miscibility studies.
47 citations
TL;DR: A series of ω-thiol-substituted 2,3,6,7,10,11-hexaalkoxytriphenylenes was synthesized for the purpose of forming self-assembled monolayers (SAMs) on gold in which the self-organizing, discogenic headgroups have a known orientation as discussed by the authors.
Abstract: A series of ω-thiol-substituted 2,3,6,7,10,11-hexaalkoxytriphenylenes was synthesised for the purpose of forming self-assembled monolayers (SAMs) on gold in which the self-organizing, discogenic headgroups have a known orientation. In principle, these headgroups can be oriented either with their short axes perpendicular (disc “face-on”) on their short axes parallel (disc “edge-on”) to the surface, depending on the number and position of the tethering points. To obtain a “face-on” orientation of the headgroups a trithiol (three points of attachment) was synthesized and to obtain an “edge-on” orientation both monothiol and ortho-dithiol derivatives (one or two points of attachment) were made. Within the SAMs it was expected that the headgroups of the “face-on” discogen would self-organize into a hexagonal array and those of the “edge-on” discogens into columns that would run parallel to the surface. The kinetics of formation and the structures of the monolayers were characterized by contact angle measuremen...
46 citations
TL;DR: The electro-optic properties of such Holographic polymer Dispersed Liquid Crystals (H-PDLCs) are strongly dependent on polymer morphology and the degree of separation of the liquid crystal/polymer mixture.
Abstract: Photopolymerization induced phase-separation of liquid crystal/polymer mixtures in holographic gratings show promise as switchable Bragg reflectors for optical communication and reflective display applications. The electro-optic properties of such Holographic Polymer Dispersed Liquid Crystals (H-PDLCs) are strongly dependent on polymer morphology and the degree of separation of the LC/polymer mixture. We report scanning electron microscopy, optical polarizing, and electro-optic studies on H-PDLC materials.
40 citations
TL;DR: An improved synthesis of 2,3-dihydroxylamino-2,3dimethylbutane (and its monosulphate salt) in a Zn/NH4Cl/THF-H2O system is proposed in this article.
Abstract: The products formed in the course of the reduction of 2,3-dimethyl-2,3-dinitrobutane to 2,3-dihydroxylamino-2,3-dimethylbutane in a Zn/NH4Cl/EtOH-H2O system have been investigated. An improved synthesis of 2,3-dihydroxylamino-2,3-dimethylbutane (and its monosulphate salt) - the key precursor of Ullman's nitroxides - in a Zn/NH4Cl/THF-H2O system is proposed.
38 citations
TL;DR: In this article, the structural and magnetic characterization of a series of sheet-like transition metal polymers of formula [ML2X2] [M ˭ Mn(II), Fe (II), Co(II) and Ni(II): L = pyrazine (pyz) and...
Abstract: The preparation and the structural and magnetic characterization of a series of sheetlike transition metal polymers of formula [ML2X2] [M ˭ Mn(II), Fe(II), Co(II) and Ni(II): L = pyrazine (pyz) and...
TL;DR: In this article, the structural and switching behavior of liquid crystals consisting of bent-shaped molecules with the linkage of an alkylene spacer, where m and n are carbon numbers of end and spacer chains, respectively.
Abstract: Structure and switching behavior were studied in liquid crystals consisting of bent-shaped molecules with the linkage of an alkylene spacer, m(O)AMnAM(O)m, where m and n are carbon numbers of end and spacer chains, respectively. All the compounds exhibit a fan-shaped texture without a fringe structure in their smectic phase, different from the texture of conventional bent (banana)-shaped liquid crystals. In 12AM5AM12, two switching current peaks indicating the antiferroelectric phase were observed. The spontaneous polarization was about 600 nC/cm2. The color change due to the birefringence change is associated with the switching, though the extinction direction between crossed polarizers remains the same. This clearly proves that the bent-molecules do not tilt with respect to the layer normal. In the dielectric measurements, there exist two relaxations at 600 kHz and below 100 Hz and they are suppressed by a biased voltage. Structure and switching in the other systems, 80AM5AMO8, are also describ...
TL;DR: In this article, the X-ray crystal structure of M 3·(1 Br)2 was shown to crystallize in the space group P2, with a = 12.469(2) A, b = 15.278(2), c = 11.602(2); α = 104.59°; β = 111.86°; γ= 88.32°; and Z = 1.32
Abstract: Bis(hexafluoroacetylacetonato)copper(II), Cu(hfac)2 (= M) reacts with 5-bromo-1,3-phenylenebis(N-tert-butylaminoxyl), 1 Br to yield a complex of formula [M 3·(1 Br)2]. The X-ray crystal structure shows that it crystallizes in the space group P2, with a = 12.469(2) A, b = 15.278(2) A, c= 11.602(2) A, α = 104.59(1)°;, β = 111.86(1)°;, γ= 88.32(1)°;, and Z= 1. The crystal contains copper(II) trinuclear structure, M-1 Br-M-1 Br-M. The magnetic susceptibility measurements revealed a structural transition at ca. 48 K accompanied by a temperature hystereisis.
TL;DR: In this article, the rotational motions of the molecules around their short axes in the nematic and isotropic phases were derived (dielectric increments, relaxation times, activation enthalpies, retardation factors, nematic potentials, order parameters).
Abstract: Results of dielectric studies of three homologous series: n-alkyl-cyanobiphenyls - nCB, n-alkyloxy-cyanobiphenyls - nOCB, and trans-4-(4′-n-alkyl-cyclohexyl)-cyanobenzene - nPCH, with n from 5 to 8, are presented. The parameters characterizing the rotational motions of the molecules around their short axes in the nematic and isotropic phases were derived (dielectric increments, relaxation times, activation enthalpies, retardation factors, nematic potentials, order parameters). Comparative analyses of the parameters in respect to the lengths of the terminal groups and the structures of the molecular cores are given.
TL;DR: In this article, a series of oligomeric and first generation dendritic materials based on silane-siloxane and silsesquioxane core has been synthesized.
Abstract: A series of oligomeric and first generation dendritic materials based on silane-siloxane and silsesquioxane core has been synthesised. The attachment of cyano-biphenyl mesogens separated by spacers to these cores which were varied systematically, resulted in liquid-crystalline materials. The synthesis of the materials is outlined and the liquid-crystalline phase behaviour of the materials, which are of uniform in size and stereochemistry, is described.
TL;DR: In this article, the effect of the chiral dopants molecular structure on the kind of the temperature dependencies of the induced cholesteric helical pitch has been studied for LC systems on the base of 4-pentyl-4′-cyanobiphenyl or 4-methoxybenzylidene-4-butylaniline.
Abstract: Effect of the chiral dopants molecular structure on the kind of the temperature dependencies of the induced cholesteric helical pitch has been studied for LC systems on the base of 4-pentyl-4′-cyanobiphenyl or 4-methoxybenzylidene-4-butylaniline including chiral S,α-phe-nylethylamine and E-1R,4R-2-(4-phenylbenzylidene)-p-menthane-3-one derivatives. The established correlations and their possible reasons have been discussed.
TL;DR: In this paper, the effects of replacing the Schiff's base moiety with a tolan unit were investigated for dimesogen; cholesteryl 6-[4-n-butyloxyphenylethynyl)phenoxy]hexanoate.
Abstract: Dimesogenic compounds consisting of two non-identical mesogenic units connected through a polymethylene spacer are now of interest in the area of liquid crystal research Previous studies have shown that dimesogens made up of a cholesteryl ester unit and a Schiff's base unit connected through a central alkyl spacer give rich mesomorphism We have taken up investigations to study the effects of replacing the Schiff's base moiety with a tolan unit As an example, the dimesogen; cholesteryl 6-[4-(4-n-butyloxyphenylethynyl)phenoxy]hexanoate was synthesized and studied for its liquid crystalline properties Although this compound shows only one mesophase, viz, the cholesteric phase, the temperature range of the mesophase is very wide, perhaps one of the largest ranges reported to date
TL;DR: Syntheses and mesomorphic properties of dimesogens incorporating dissimilar mesogenic units viz cholesterol and substituted biphenyls are reported in this paper, where the mesomorphism of these units is investigated.
Abstract: Syntheses and mesomorphic properties of dimesogens incorporating dissimilar mesogenic units viz cholesterol and substituted biphenyls are reported
TL;DR: The fine-structure parameters were determined by the angular dependence of resonance fields as discussed by the authors, which indicated that π-spin delocalization effects were hampered by introducing heteroatoms to π spin systems.
Abstract: Heteroatomic high-spin molecules are suitable for a model study of room-temperature stable molecular based magnetics. We studied high-spin nitrenes (S = 1 and 2) with s-triazine skeleton, which were generated by the photolysis of the polyazide precursor. We identified mononitrene 1 (S = 1) and bisnitrene 2 (S = 2) for the first time by single-crystal ESR measurements. The fine-structure parameters were determined by the angular dependence of resonance fields. The determined fine-structure parameters were |D| = 1.402 cm−1 and |D| = 0.280 cm−1 for mononitrene 1 and bisnitrene 2, respectively. The large D values indicate that π-spin delocalization effects were hampered by introducing heteroatoms to π-spin systems. The hyperfine splittings due to the monovalent nitrogen nucleus of 1 were observed with magnetic field along the principal axis of the fine structure tensor. The hyperfine splitting constant was about 0.0018 cm−1. The temperature dependence of ESR signals showed that 1 and 2 were triplet a...
TL;DR: In this paper, a diamagnetic RbFeCo cyanide was synthesized to demonstrate the photomagnetic effect of photo-induced magnetisation in CoFe cyanide, and the conditions required to observe the phenomenon and the mechanism of electron transfer were discussed.
Abstract: Photo-induced magnetisation has been recently evidenced by Hashimoto et al in CoFe cyanide[1] We synthesized a diamagnetic RbFeCo cyanide able to present the same photomagnetic effect The magnetic properties of the excited state, the electronic structure and the local structure of the ground and the excited states have been investigated The conditions required to observe the phenomenon and the mechanism of the electron transfer are discussed
TL;DR: One-dimensional coordination polymers consisting of {Mn[N(CN)2]2}∞ structural units have been characterized in this paper, and incorporation of 2,2′-bipyridine and 4,4''-bibyridine affords zigzag and cylindrical chain structures, respectively; weak antiferromagnetic coupling is also observed.
Abstract: One-dimensional coordination polymers consisting of {Mn[N(CN)2]2}∞ structural units have been characterized. Incorporation of 2,2′-bipyridine and 4,4′-bipyridine affords zigzag and cylindrical chain structures, respectively; weak antiferromagnetic coupling is also observed. 4,4′-bipyridine unexpectedly binds as a monodentate ligand.
TL;DR: In this paper, the magnetic behavior of MII[N(CN)2]2 (M ˭ Co, Ni, Mn and Zn) was summarized, and the authors showed that the magnetic properties of these properties are consistent with high-spin 1b, 2, 3 and 4.
Abstract: We summarize the magnetic behavior of MII[N(CN)2]2 (M ˭ Co, Ni, Mn and Zn). Analyses of the DC magnetization and AC susceptibility data demonstrate that α-CoII[N(CN)2]2 (1a) and NiII[N(CN)2]2 (2) are ferromagnets below Tc, while β-CoII[N(CN)2]2 (1b) and MnII[N(CN)2]2 (3) are canted antiferromagnets below TN. ZnII[N(CN)2]2 (4) is diamagnetic. Specific heat and neutron powder diffraction results clearly define the transition temperatures for 1a, 1b, 2 and 3. Crystal structure determination from high-resolution neutron powder data for 1a, 2 and 3 shows that the three systems are isomorphous and crystallize in the orthorhombic space group Pnnm with Z = 2. Single crystal X-ray diffraction shows that 4 crystallizes in the orthorhombic space group Pnma with Z = 4. We solved the magnetic structures of 1a, 2 and 3 from neutron powder diffraction experiments. All low-temperature measurements are consistent with high-spin 1b, 2, 3 and 4, though 1a exhibits an effective J′ = 1/2 behavior.
TL;DR: In this article, two materials exhibiting proper ferroelectricity are reported; (1) polar order in nematic liquid crystals of aromatic polyesters and (2) ferro electric switching in a lyotropic liquid crystal of polypeptide.
Abstract: After summarizing ferroelectric and antiferroelectric liquid crystals (LCs) in liquid crystals so far discovered, two materials exhibiting a proper ferroelectricity are reported; (1) polar order in nematic LC of aromatic polyesters and (2) ferroelectric switching in a lyotropic liquid crystal of polypeptide. In the polypeptide LC, strong second-harmonic generation (SHG) was observed by applying an electric field, and the phase of the SH field was reversed by reversing the field, indicating the ferroelectric switching of the nonlinear polarization. SHG was also observed in the unperturbed helical state when the incident light with the wavelength the same as the helical pitch is passed through along the helical axis. In the aromatic polyester LC, SHG arises when the degree of polymerization is larger than 20. It is proposed that these polar organization originates from the dipole-dipole interaction, namely the proper ferroelectricity.
TL;DR: In this article, the mesophases of 4-Chloro-1,3-phenylene bis[4-(nalkylphenyliminomethyl)benzoates] have been investigated by optical microscopy, X-ray diffraction measurements and electrooptical methods.
Abstract: 4-Chloro-1,3-phenylene bis[4-(4-n-alkylphenyliminomethyl)benzoates] have been synthesized and the mesophases were investigated by optical microscopy, X-ray diffraction measurements and electrooptical methods. Some derivatives exhibit the electrooptically switchable mesophase B2 between 75–125°C, which can be supercooled additionally by about 20 K. This existence range is very useful for further physical measurements.
TL;DR: In this article, the influence of a magnetic field on the alignment of a triphenylene derivative was investigated in the discotic nematic (ND) phase of a cell, where the magnetic field was applied parallel to the cell surface during the cooling process.
Abstract: The influence of a magnetic field on the alignment was investigated in the discotic nematic (ND) phase of a triphenylene derivative. The uniform alignment was achieved when a magnetic field of 5T was applied parallel to the cell surface during the cooling process. The director was perpendicular to the surface when the cell was coated with polyimide alignment layers and parallel when the surface was uncoated.
TL;DR: In this paper, the first series of achiral non symmetric banana-shaped mesogens has been synthesized and the mesophases observed for these compounds depend on the whole number of carbons in the alkyl chains.
Abstract: The first series of achiral non symmetric banana-shaped mesogens has been synthesized. The mesophases observed for these compounds depend on the whole number of carbons in the alkyl chains (N). They were investigated by optical microscopy, calorimetry, X-ray diffraction experiments and electro-optical measurements. For intermediate and long chains (N ≥ 20) the liquid crystal phase is smectic (Sm?) and for shorter chains (N < 20) the mesophase appears to be columnar ordered (Col?). Yet the structure of neither one is fully resolved. Both phases exhibit a ferroelectric behaviour.
TL;DR: In this paper, the results of the measurements of thermodynamic properties of phase transitions of some cyclohexane derivatives in the condensed state and provides the analysis of the changes of dynamic behaviour of molecules during transitions from rigid crystal to plastic crystalline state.
Abstract: The present paper summarizes the results of the measurements of thermodynamic properties of phase transitions of some cyclohexane derivatives in the condensed state and provides the analysis of the changes of dynamic behaviour of molecules during transitions from rigid crystal to plastic crystalline state. These results are discussed on the basis of the main trends of thermodynamic properties of solid state transitions in the series of the compounds having the similar structure. The analysis of the data obtained allows to develop new concept of energy states of molecules in plastic crystals. The combination of the results of calorimetric and spectral measurements with the data obtained by the method of statistical thermodynamics for the series of cyclohexane derivatives leads to the conclusion of the existence of internal rotation, ring inversion, and rotation of molecules as a whole in the nodes of the plastic crystal lattice.
TL;DR: In this paper, the magnetic properties of a pentanuclear and nonanuclear Ni(II) cluster are described and the crystal structures and magnetization properties of these two clusters are described.
Abstract: The crystal structures and the magnetic properties of a pentanuclear (1) and a nonanuclear (2) Ni(II) clusters are described. In the former, crystallizing in the cubic I23 space group, the Ni(II) assembly is composed of a regular tetrahedral geometry of four metal ions centered on the fifth. The nonanuclear cluster crystallizes in the hexagonal crystallographic system -R3 space group - the Ni(II) assembly is composed of two regular tetrahedra, like that of the pentanuclear cluster, sharing a common apex. All metal ions in either cluster are six-coordinate. Variable-temperature magnetic susceptibility data show antiferromagnetic interactions for both clusters. Moreover, magnetization data are in line with uneven intermediate-spin ground states [S=1, S=2] and [S=2, S=3] for 1 and 2, respectively. A comparison of both the symmetry and the spin ground state of 1 to those of lower-symmetry pentanuclear Ni(II) clusters, reveals that the ground-state spin value increases with the cluster symmetry.
TL;DR: In this article, the synthesis of a new heterocyclic core has been developed and its standard derivatives afforded new types of FLCs and their physical properties were determined, and a new FLC core was proposed.
Abstract: The synthesis of a new heterocyclic core has been developed. Its standard derivatives afforded new types of FLCs and their physical properties were determined.
TL;DR: In this paper, the spin density distribution in a uracil-substituted nitronyl nitroxide Ur-NN was examined as well as its correlation with the molecular structure, together with a study of the formation of aggregates of ur-NN in solution.
Abstract: We have synthesized and chemically as well as structurally characterized a new α-nitronyl nitroxide. The spin density distribution in this uracil-substituted nitronyl nitroxide Ur-NN was examined as well as its correlation with the molecular structure, together with a study of the formation of aggregates of Ur-NN in solution.