On Packing in Smectics: X-Ray Diffraction Study on the Homologous Series 4, 4′-di-n-alkyl-azoxybenzenes

The smectic layer spacings of the homologous series 4,4′-di-n-aLkyl-azoxybenzenes with alkyl chain length from m=6 to m=10 were determined and a mean layer spacing increment of 0.109 nm per methylen group was found.

The results are discussed in relation to two different models:

(I) The diffuse cone model of A. de Vries. Differences between layer spacing and molecular length are attributed to orientational disorder leading to mean tilt angles up to 18° of the molecules.

(II) The second model differentiates between the directions of the rigid aromatic core and that of the alkyl chains. Orthogonality of the rigid core long axis to the smectic layer planes and therefore tilting of the alkyl chains results in coincidence between calculated layer thickness and measured smectic layer spacings.

X-ray film erposures of oriented samples clearly demonstrate the S_A -type of the phases and confirm a slight odd-even effect of layer spacing increments.     

X-ray structure of 14-phenyldiquino[3,4 b;4′,3′-e][1,4]thiazine<Superscript>(1)</Superscript>

The title compound 1 (C₂₄H₁₅N₃S) crystallizes in an orthorhombic unit cell, with a = 8.2355(3), b = 19.1499(8), and c = 11.3520(5) Å, Z = 2, and space group Pnma. The pentacyclic ring system is folded mainly along the S–N(2) axis and slightly along the C(3)–C(4) and C(3′)–C(4′) axes. The butterfly folding angle between the two quinoline planes is 48.96(3)^∘ and the angle between the planes of the two halves of the thiazine ring is 43.46(7)^∘. The central thiazine ring is in a boat conformation. The phenyl substituent is unexpectedly in an axial location with respect to the thiazine ring with the C–N⋅ < eqid1 > ⋅S angle of 114.0(1)^∘ and it is perpendicular to the bisecting plane of the pentacyclic ring system.     

Superconductivity in the Organic Charge Transfer Salts: (TMTSF)2X and (TMTTF)2X

Abstract

We report pressure dependent studies of the a-axis resistivity as a function of temperature for several members of the isostructural families of organic charge transfer salts, (TMTSF)₂X and (TMTTF) ₂X. For a typical (TMTSF)₂X material the low temperature metal-insulator transition seen at 1 bar is suppressed above some critical pressure, P_c, where a superconducting transition is observed near 1 K. We find a correlation between P_c and the ambient pressure c lattice parameter which reflects the anion size. The (TMTTF) _cX salts exhibit very different ambient pressure behaviour but we find that with the application of sufficiently high pressures (～30 kbar) their behaviour resembles that seen in the (TMTSF)₂X family but at lower pressures. In particular we find evidence of a possible superconducting transition near 4 K in (TMTTF)₂Br at 25 kbar. At this pressure the conductivity near 4 K is extremely high with a value approaching 10⁶ (Ωcm)^?1 and the resistivity ratio is about 400.     

Modification of the Interlayer in HTiNbO5 and its Physicochemical Characterization

Abstract

A layered titanoniobate, in which the interlayer of the metal oxide based on octahedral framework is pillared with silica, has been prepared from HTiNbO₅ by employing alkylamines (C_nH_2n+1NH₂, n = 4, 6, 8, 10) and tetraethylorthosilicate as interlayer exchange guests and a pillar precursor, respectively. The incorporated amine had bilayer arrangements in the interlayer space and their ammonium head groups pointed to the host (TiNbO5₅ ^?) layers. While the basal spacing of the compounds increased with the chain length of alkylamine. the inclination angle also increased. The intercalated structure facilitated the incorporation of tetraethylorthosilicate into the interlayers, and silica-pillared layered titanoniobate was obtained after calcination. The physicochemical properties of the resultant pillared compounds were characterized on the interlayer microstructure, nitrogen adsorption and UV-VIS absorption spectra.     

Interlayer Reaction of Polyamine/α-Zirconium Phosphate Intercalation Compounds with Some Aldehydes

Abstract

The intercalation compound between α-zirconium phosphate and N,N′-bis-(3-aminopropyl)-1, 3-propanediamine has two different modifications with respect to the conformation of the guest amine molecules: one is “bent form” and the other “straight form”. Reactivity of each phase with a series of aldehydes was examined. ¹³C CP/MAS NMR indicated that the tetraamine reacts stereoselectively with aldehydes to form cis-imine in the interlayer spacing. Depending on the molecular size of aldehyde, it is found that a significant difference can be recognized in the yields of the imine products for the two phases of intercalation compounds. It is also noted that residual water molecules in the interlayer spacing play an important role in the imine formation reaction.     

(TMTSF)2(2,5-Tcnqbr2): Structure and Physical Properties

The 2:1 charge-transfer salt (TMTSF)₂(2,5-TCNQBr2) has been prepared and its physical properties investigated. Its crystal structure consists of segregated stacks of TMTSF donors (ring-over-bond overlap pattern; mean interplanar spacing of 3.6A) and chains of edge-on and disordered 2,5-TCNQBr2 acceptors. Infrared data are suggestive of unit charge on the 2,5-TCNQBr2 molecule and, therefore, half charge on the TMTSF donor. Resistivity data are successfully interpreted on the basis of a percolation construction. Magnetic data are also presented.     

Electronic Properties of Ba-Intercalated Fullerides

Abstract

Raman spectra have been measured on Ba_xC₆₀ binary systems for x=3, 4 and 6. The pentagonal pinch Ag(2) mode exhibits a softening in an approximately proportional manner to the formal valence of C₆₀ molecule. This result suggests that the valence of C₆₀ in Ba doped systems are understood by the naive ionic crystal model based on Ba²⁺. The broader Raman peaks in superconducting Ba₄C₆₀ indicates that the electron-phonon interaction is considerably large comparing to those for Ba₃C₆₀ and Ba₆C₆₀.     

Highly Electrical Conductivity of Hybrid Langmuir-Blodgett Films of Transition Metal Dichalcogenide and Amphiphilic Compounds

Abstract

Hybrid alternate layered films of transition metal dichalcogenides and amphiphilic compounds were prepared by Langmuir-Blodgett (LB) technique. The conductivity at room temperature depended on the transition metal dichalcogenide species, showing the highest value for the hybrid LB films of MoS₂ system. This methodology was successfully applied to the fabrication of the hybrid LB films using various organic amphiphiles such as cyanine dyes, phthalocyanines, azobenzene, and ferrocene derivatives. The conductivity depended on the interlayer spacing: the conductivity decreased with increasing interlayer spacing of the film. The highest electrical conductivity of the hybrid LB films was over 100 Scm^?.     

Paracrystalline Nature of Poly (p-Phenylene)

The electron diffraction pattern of oriented, annealed poly (p-phenylene) is explained on the basis of an orthorhombic unit cell: a = 0.7781, b = 0.5520 and c(fiber axis) = 0.4300 nm. and exhibits the following features:

(1)The reflections on the equator and meridian are sharp. Only a few weak diagonal reflections can be observed.

(2) Strong diffuse scattering is seen on the third layer line.

(3)The reflections on the meridian (00l-reflections) have a characteristic line profile. in which the intensity increases suddenly at the Bragg angle and decreases gradually.

From (1) to (3), it can be concluded that the material is paracrystalline like a nematic structure in liquid crystals. Molecular chains are laterally packed in a rather regular way. However, the disorder by the shift of chains in the direction parallel to their axes is remarkable.     

Conference Reports

Abstract

The 1995 Society for Information Display (90) International Symposium is the premier international gathering of scientists, engineers, technologists, manufacturers, and users of flat panel displays, where virtually all aspects of information displays are presented. The organizers, Richard H. Bruce (Symposium Chair) and Hugo Steemers (Programme Chair) of Xerox Palo Alto Research Center, did a magnificent job in arranging a programme that covered a wide range of technology and applications' from high definition flat panel displays of both emissive and liquid crystal display (LCD) technology t o the latest in CRT development and large area display projection systems. The symposium featured over 180 papers from around the world in 45 technical sessions; the conference papers are published in the SID International Symposium Digest of Technical Papers Vol. XXVI. It is very apparent just by paging through the Digest of Technical Paper that LCDs have entrenched themselves in the flat panel display community, and as the need for high information content continues t o grow, LCDs are expected to play an even bigger role. From the liquid crystal materials perspective, the conference was heavily weighted towards reflective displays, polymer dispersed liquid crystal displays for projection, alignment and viewing angle issues, and active matrix technologies.     

Fabrication of two-dimensional MoS2 thin-film transistors using a reactive thermal evaporation method combined with an annealing step

ABSTRACT

Two-dimensional (2-D) MoS₂ films were fabricated by reactive thermal evaporation combined with thermal annealing. The 2-D nature of the MoS₂ films is demonstrated by observation of direct transition and a Van Hove singularity in the absorbance curve. The 1T phase MoS₂ is confirmed by X-ray photoelectron spectroscopy. MoS₂ thin-film transistors (TFTs) are fabricated using the MoS₂ active layer transferred onto an oxidized Si wafer from a sapphire wafer. The MoS₂ TFT demonstrates a threshold voltage of 37.6 V, a field-effect mobility of 6.94 cm²V^?1s^?1 a sub-threshold swing of 29.2 V/dec and a switching ratio of 10³.     

The effect of Zn-acetate molar variation on phase formation and photocatalytic degradation activity of Fe3O4/ZnO core-shell nanocomposite

Abstract

The aim of this research is to investigate the photodegradation of Methylene Blue with Fe₃O₄/ZnO core-shells. Fe₃O₄/ZnO core-shells were synthesized by a simple two-step chemical method (co-precipitation and precipitation) using a molar variation of Zn-acetate precursor. We found that the presence of Fe₂O₃ plays an important role in enhancement of photocatalytic activity due to the existence of higher concentration of surface oxygen vacancies and the suppressing effect of the Fe²⁺ ions on the recombination of photoinduced electron-hole pairs. The core-shell photocatalyst can be easily separated by using a commercial magnet without reduces photocatalytic efficiency within three times of recycling process.     

Two Frequency Addressing of a DTN-Cell

The authors have already reported that a DTN-cell (depolarization in a twisted nematic-cell) had several advantages to a matrix display such as a sharp threshold and wide viewing angle. But if the usual amplitude selection method is applied to a large scale matrix display using the DTN-mode, multiplex capability is limited by the cutoff frequency F_c, of a liquid crystal. Therefore, the authors investigated a two frequency addressing method, that is, simultaneous application of a constant low-frequency voltage and a variable high-frequency voltage. In this method, a liquid crystal with lower-F_c can be used. In addition, some advantages of extremely sharp threshold, high contrast and relatively fast response and recovery can be obtained by using a liquid crystal with large negative dielectric anisotropy. These advantages are useful for a large scale matrix display.     

Intermolecularly Associated Carboxylic Acid Dimers in the Supramolecular Assembly of (R)-1,4-Benzodioxane 2-Carboxylic Acid

Abstract  The title compound, C₉H₈O₄, (I), crystallizes in the monoclinic space group P 21/c with unit cell parameters a = 7.3380(5) ?, b = 9.3790(7) ?, c = 12.3172(9) ?, β = 90.687(7)°, Z = 4. The molecule consists of a benzene ring fused to a 1,4 dioxane ring in a half-chair conformation with a carboxylic acid group bonded at the dioxane 2-position. The carboxylic acid group forms a classic O–H···O hydrogen bonded dimer with an O···O distance of 2.6292(12) ? in a R₂² (8) graph-set motif which links the molecules into pairs around inversion centers in a supramolecular assembly in the unit cell. The dihedral angle between the mean planes of the R₂² (8) graph-set motif’s of two adjacent dimers in the unit cell is 75.7(5)°. The mean plane of an R₂² (8) graph-set motif makes an angle of 88.4(5)° with the mean plane of the benzene ring in the same dimer and 13.1(2)° or 13.4(7)° with the mean plane of a benzene ring from an adjacent dimer. Additional weak Cg π-ring intermolecular interactions significantly influence the bond distances, bond angles and torsion angles of the dioxane ring and attached carboxyl group. Comparison to a MOPAC AM1computational calculation provides support to these observations. Graphical Abstract  The association of carboxylic acid dimers in the supramolecular assembly of (R)-1,4-benzodioxane 2-carboxylic acid is investigated by examination of its crystal structure. Its geometric and packing parameters are described and compared to a MOPAC AM1 computational calculation. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Vibrational Spectra of Liquid Crystals. XII

Abstract

Recent papers by Gray et al.¹ have discussed the synthesis, structure, and properties of the mesomorphic 4-cyano, 4′-alkyl (or alkoxy) biphenyls. The reports on these materials have made mention of the existence of solid state dimorphism for certain homologs. This paper discusses this dimorphism as it manifests itself in the Raman spectrum of the octyloxy homolog (hereinafter referred to as 80CB) in the CN stretching region.

The approach used involves direct measurement of the Raman spectrum as a function of temperature, as well as computation of a correlation function obtained from the Raman band envelope by means of Fourier transformation. The application of this correlation function technique to this system has been discussed by us recently in detail.^2,3

Gray and Mosley,⁴ in a recent study of the Raman spectrum of 4-cyano, 4′-pentylbiphenyl note that the CN stretch of this compound is a doublet in the solid phase (2224 and 2233 cm^?1), but only the 2224 cm^?1 band appears in the nematic phase. They further report that only a single band appears in the CN stretching spectrum of solid 4'-heptyl-, -hexyl-, and -butyl-derivative. They speculate that the doublet might appear in the spectrum of the metastable solid phase of the heptyl homolog.

In this work, we show that for 80CB this is not the case. Rather, a doublet is observed for the stable solid but not for the metastable. The origin of this doublet is discussed, and all of the CN stretches for stable solid, metastable solid, and smectic liquid crystal are analyzed from the viewpoint of the vibrational relaxation correlation function.     

An X-ray double-crystal method utilizing non-parallel setting for measuring local lattice mismatches between epitaxial films and substrates

An X-ray topographic method utilizing non-parallel (+,-) setting for measuring lattice mismatches between epitaxial films and substrates is reported. This method gives quantitative as well as topographical information on lattice distortions in a distorted crystal and allows us to study any reflection from various crystals, in which the fractional variation of lattice spacing, δd/d, is of the order of 10^-4?10^-5. Examples of applications are lattice mismatches between epitaxial magnetic garnet films and (111) gadolinium garnet (GGG) substrates. The spacing of the epitaxial film was slightly larger along the [111] direction than that of the substrate and the apparent crystal symmetry of the epitaxial film is rhombohedral.     

Neutron Scattering by Copolymers

Abstract

Neutron scattering is a very efficient tool for studying polymers in bulk as well as in solution. This is mainly due to the difference of scattering length between hydrogen and deuterium which allows the possibility of changing the contrast without too serious alteration in the conformation and thermodynamics of the systems.

In order to take full advantage of this possibility, one has to remember the general rules which allow the evaluation of the total scattering function S(q) as a function of the partial scattering functions S _pq (q) which describes the contributions of the interferences between scattering center of species p and q.^1.2 In a first part we shall briefly establish these rules for incompressible systems introducing what has been called by J. Koberstein³ the molecular contrast and the phase contrast.

In the second part of this talk we shall discuss the problem of copolymers. S(o) the intensity scattered at zero angle depends on the heterogeneity in composition of the sample,⁴ for homogeneous composition S(o) = o. This gives the possibility of studying trans-esterification in polyester, for example.

At finite q for a monodisperse system, S(q) is not zero and it will be shown how it depends on the length of the sequences and the architecture of the copolymers. If the number of blocks is large S _pq (q) is practically not affected by the total length of the polymer.⁵

The condition I ^?1(q) = o allows one to write the equation of a q dependent generalized spinodal and therefore to determine the parameters which govern the stability of a multiphase system.

In the last part we shall discuss the problem of the application of this formalism to networks and gels made of different types of units. It will be shown that the scattering is mainly sensitive to the local conformation and that the introduction of translational order between the units does not affect significantly the scattering curves but can explain the occurrence of secondary maxima which have been observed in polyurethane and ionomers.⁶     

Dependence of the Tilt Angle on External Forces for Smectic-C And Chiral Smectic-C Liquid Crystals—Measurement of the Heat Capacity of DOBAMBC

A measurement of the specific heat of DOBAMBC is presented, and it is shown that a Landau expansion of the free-energy density with a sixth-order term is adequate to describe the measurement. The coefficients in the Landau expansion are estimated from this measurement.

The dependence of the tilt angle on applied electric and magnetic fields is discussed for both smectic-C and chiral (ferroelectric) smectic-C liquid crystals. Numerical estimates of the Landau coefficients are used to show that the effect of external forces on the tilt angle is negligible except when being less than approximately 0.2 K from the transition to the smectic-A phase. The effect on the smectic-C-smectic-A phase-transition temperature is also discussed.

Boundary effects can also affect the tilt angle, and an expression for the penetration depth of a boundary-induced disturbance is calculated. For thin, planar samples, a coupling between the two boundaries can induce a transition to the smectic-A phase. It is demonstrated that this transition is analogous to the ordinary Freederickz transition in nematic liquid crystals.     

Formation of a Banded Texture in Solutions of Liquid Crystalline Polymers: 1. Hydroxypropylcellulose in H2O

Abstract

The formation of a banded texture was investigated for sheared anisotropic solutions of hydroxypropylcellulose (HPC) in H₂O. A cone and plate rheometer made of highly polished glass allowed the exploration of the shear rate (γ) range between 0.1 and 150 sec^?1 and the study of the effect of shear time t_s. Texture, monitored with a video-camera, appeared after a time t_b following cessation of shear. t_b is usually a decreasing function of γ and t_s. In most cases a precursor banded texture precedes the banded texture, and t_b attains zero at high enough γ or t_s. However, when a particularly good orientation of the flowing solution is achieved, the precursor texture is not observed, t_b is finite, and the bands are particularly well formed. We speculate that bands are the normal relaxation mode for well oriented solution. For less oriented solutions flow instability may lead to bands even during flow.     

Phonon Side Bands of Specular Optical Transitions in Molecular Crystals

Quantitative investigations of phonon side bands (PSB) in the small radious centers absorption and luminescence spectra of some impurity molecular crystals are presented. Three pairs of specular spectra were considered corresponding to three impurity systems: benzene-d ⁰ in benzene-d ⁶, phenol in benzene-d ⁰ and β-methylnaphthalene-d ¹⁰ in naphthalene-d ⁸. Analysis of the observed spectra revealed such PSB regularities.

1) The most common characterization of PSB spectra is the deviation of the PSB shape from specular symmetry.

2) The non-specularness of the PSB shape does not depend on whether the PSB integral intensities are equal or different.

3) There is an unambiguous correlation between the expression of the PSB non-specularness and the temperature variation of the phononless band half width and shift.

4) For weak transitions, the non-specularness of the PSB integral intensities is observed in addition to the PSB shape non-specularness.

For the first time the observed characteristics were explained quantitatively in terms of the deviation from the Condon approximation on the basis of one-phonon spectra substracted from the PSB observed.