Structural studies of laterally attached liquid crystalline polymers

Structural studies of a series of laterally attached side chain liquid crystalline polysiloxanes have been made by X-ray diffraction. All but one exhibit a nematic phase with S_C-like short range ordering. The diffraction has been interpreted in terms of a model in which the mesogenic units form a 'jacket' around the polymer backbone. The backbone conformation has been studied by small angle X-ray scattering from oriented samples of mesomorphic solutions and by infrared dichroism measurements on oriented samples of the polymers. In both cases the results were consistent with the mesogens being oriented parallel to the backbone which is consistent with the 'mesogen jacket' model.     

Structure of the smectic D phase

Abstract

The so-called ‘smectic’ D phase of 4′-n-octadecyloxy-3′-cyanobiphenyl-4-carboxylic acid (one of only four materials known to exhibit this phase) has been shown unambiguously by X-ray diffraction to be characterized by a primitive cubic space group. The space group is either P23 or Pm3, and the lattice parameter a ₀ is 86 Å. It is shown that data from previous studies of this phase may be re-intepreted to be consistent with these findings. In view of these conclusions it is clearly inappropriate to refer to the phase as smectic.     

Morphological and Thermal Investigations of the ‘Active’ Dimer Hydroxide of Chromium(III)

The ‘active’ dimer hydroxide, [Cr₂(μ-OH)₂(OH)₄(OH₂)₄]·2 H₂O, has been characterised by means of IR spectroscopy, thermal analysis, X-ray powder diffraction, and electron microscopy. Thermogravimetric measurements and the Cr content of the hydroxide are in agreement with this composition. The IR spectrum supports the notion that this hydroxide is composed of unaltered dimer fragments linked through hydrogen bonds of ca. 2.8 Å. The compound is microcrystalline in nature with an X-ray powder diffraction pattern distinctly different from that of the isomeric ‘active’ monomer hydroxide. Electron micrographs of the ‘active’ dimer hydroxide indicate that this material consists of platelets the sie of which is dependent on the pH of precipitation.     

Nematic colloidal suspensions of V2O5 in water—or Zocher phases revisited

Abstract

Vanadium pentoxide gels and sols, V₂O₅, nH₂O, exhibit a ribbon-like structure. They form colloidal suspensions (also called Zocher phases or ‘tactophases’) which are clearly shown here to present a homogeneous lyotropic nematic phase for concentrations larger than ≈0·12mol 1^?1. Classical threaded textures were observed in polarized light and small angle X-ray scattering patterns of oriented samples displayed a diffuse spot quite comparable to that of the Tobacco Mosaic Virus. For a sample of volume fraction φ ≈ 5 per cent, the V₂O₅ ribbons are separated by about 160 Å, so that the cross sectional area of the ribbons is about 1300 Ų, in agreement with previous estimations by electron microscopy.     

Liquid Crystalline Side Chain Polysiloxanes with 4-Cyano- and 4-Alkoxy-Stilbene Mesogenic Units

Abstract

Side chain homo- and copolysiloxanes with 4-cyano- and 4-alkoxy-4′-stilbene mesogens, spaced apart from the backbone by oligomethylene segments of variable lengths, were synthesized via a hydrosilylation coupling reaction of five stilbene-containing α-olefins with four commercial poly(methylhydrosiloxane)s and poly(methyl-hydro-∞-dimethylsiloxane)s. Broad smectic phases were observed for the polysiloxanes with cyanostilbene mesogens, whereas the homologous with alkoxy-terminated stilbenes displayed only narrow mesophases in the high temperature range. Preliminary room tem-perature X-ray diffraction studies on mechanically oriented samples evidenced the occurrence of side chain crystallization, microphase separation and indicated the interdigitated smectic A nature of the mesophases.     

Consequences of the ‘jacketed’ effect on mesomorphic and orientational properties of ‘side-on fixed’ liquid crystalline polymers

Abstract

The mesomorphic properties have been studied as a function of the degree of polymerization for certain ‘side-on fixed’ polyacrylates. A peculiar evolution of the clearing temperature, T _IN, as well as of the glass transition temperatures, T _g, revealed that beyond a certain backbone length, T _IN and T _g decrease as the main chain length increases. The nematic ‘jacketed’ structure of these polymers induces a more or less high anisotropy of the polymer backbone conformation in the nematic phase and this can counterbalance the usual effect of an increase in the degree of polymerization on the thermodynamical properties of these systems. This evolution allows us to explain the unusual diamagnetic anisotropy anomaly observed as a function of temperature for this type of polymer.     

Synthesis and characterization of a novel η 2-iminoacyl monocyclopentadienyl titanium(IV) complex

An unusual complex, [CpTi(η²-(C,N)-2-ArNH–C₆H₄C=NAr)Cl₂] (Ar?=?2,6- ⁱ Pr₂C₆H₃) (1) has been synthesized and characterized by elemental analysis, NMR spectra, and single crystal X-ray diffraction. The ¹³C NMR resonance of the imine carbon atom of 1 at δ?221?ppm is consistent with the η²-(C,N) binding. This was confirmed by single crystal X-ray diffraction study of 1. In the complex, Ti atom is five-coordinate with a η²-bound iminoacyl ligand and one Cp ligand occupying the axial position in a distorted square pyramid.     

Design of liquid crystals with ‘de Vries-like’ properties: the effect of an ethynyl spacer in the core structure

We investigate the origins of ‘de Vries-like’ liquid crystalline behaviour by introducing an ethynyl spacer in the core of the tricarbosilane-terminated 5-phenylpyrimidine mesogens QL16-6 and QL17-6. The rationale for this structural change is based on the assumption that an ethynyl spacer would create more free volume in the core sub-layer and therefore decrease the orientational order parameter S₂ in the SmA phase. The tricarbosilane-terminated mesogens WL41-5 and WL42-6 with a 5-(phenylethynyl)pyrimidine core in either a normal or inverted orientation were synthesised, and their mesomorphic and ‘de Vries-like’ properties characterised using polarised optical microscopy, differential scanning calorimetry, birefringence measurements, small–angle (SAXS) and 2D X-ray scattering. Reduction factors R derived from SAXS and optical tilt angle measurements suggest that neither WL41-5 (R = 0.49) nor WL42-6 (R = 0.80) exhibit ‘de Vries-like’ properties. The S₂(T) profiles show an increase in orientational order with decreasing temperature and a sharp discontinuity at the SmA-SmC transition, which is consistent with ‘de Vries-like’ behaviour. However, the L_eff(T) profiles suggest an increase in interdigitation that reduces the positive effect of increasing S₂ in compensating for the molecular tilt.     

Real-time X-ray scattering study during heating of oriented injection-molded polyethylene

Highly oriented linear polyethylene was prepared by elongational flow injection molding. The changes in crystal orientation were investigated as a function of temperature by real-time wide-angle X-ray diffraction. Additionally, the influence of molecular weight upon the microstructure and the changes in orientation, during heating near the melting point, and after cooling have been examined. A shish-kebab structure is inferred for the high molecular weight samples (M_w≥10⁵) from SAXS observations, while for samples with M_w<10⁵ only an oriented lamellar structure is found. Consequently, a higher thermal stability is shown by the higher molecular weight samples. Furthermore, a recovery of crystal orientation on rapid cooling of the samples from the melt is only observed for samples with M_w≥10⁵. The results are discussed in terms of a preferential recrystallization of chain-folded lamellae, on cooling, onto the shish fibrils which survive at high temperature.     

Electrohydrodynamic instability in some nematic cyanobiphenyls in an a.c. electric field

Abstract

Electrohydrodynamic instability in homeotropically oriented nematic samples of 4′-n-octyl-4-cyano-biphenyl and 4′n-alkyloxy-4-cyanobiphenyl, (n = 8.9) have been studied in an a.c. electric field. The domain patterns during the instability in these compounds in a very low frequency a.c. field are very similar to those in a d.c. field. The domain patterns observed at higher frequencies have been identified as ‘maltese crosses’ or ‘crossed isogyres’. The electro-convective ‘isotropic’ flows near the electrode play an important role in the observed instability.     

A 2D Cd(II)-MOF as a multifunctional luminescencent sensor for nitroaromatics,iron(III) and chromate ions

A Cd(II)-MOF, {[Cd(L)(4,4′-bipy)]·H₂O·DMF}_n (1) (L = nicotinic acid (2,4-dihydroxybenzylidene)-hydrazide and 4,4′-bipy = 4,4′-bipyridine), has been synthesized and characterized by microanalyses, FTIR, TGA, and single-crystal X-ray diffraction. Additionally, powder X-ray diffraction was performed to check the phase purity of the synthesized compound. Single-crystal X-ray diffraction reveals that 1 has a 2D grid network. Photoluminescent sensing of nitrobenzene, Fe(III) and CrO₄^2? ions indicates that 1 could be a candidate for developing selective luminescent sensors for these species. Theoretical calculations have been performed to gain insight into the possible mechanism of quenching effect in emission on addition of nitrobenzene in 1 which supports the mechanism operating through ground state charge transfer between 1 and nitrobenzene.     

Information Content of Diffraction Experiments on Liquids and Amorphous Solids

Abstract

Starting with simple monatomic liquids such as condensed rare gases, the information that can be extracted from diffraction experiments is discussed. The rich interplay between atomic and electronic structure in liquid metals such as Na and K is emphasized: expanded Rb and Cs taken up the liquid–vapour coexistence curve towards the critical point are also mentioned. A liquid metal like Na is, in fact, a two-component system, Na⁺and electrons. Classical two-component systems are also considered, and specifically BaCl₂ which freezes into a fast-ion conducting phase. The structural theory of two-component liquids is extended to apply to amorphous Si, the ‘second component’ now being the bond charge. The way a combination of X-ray and electron diffraction can elucidate the nature of the local ordering, as well as the directional bonding, is stressed. There is clear evidence here for the bonding electrons, directly from the X-ray and electron scattering intensity.     

An X-ray diffraction study of terminally bromo substituted side chain polysiloxanes

Abstract

X-ray diffraction results are reported for two bromo substituted side chain liquid crystal polymers with polysiloxane backbones. Both exhibit a smectic A phase with the observed lamellar d spacings being consistent with an almost totally overlapped interdigitated structure. Below the S_A phase the biphenyl derivative gives an X-ray pattern with sharp 1, 10, and 210 reflections which can be interpreted as smectic E.     

Chiral ordering in the nematic and an optically isotropic mesophase of bent-core mesogens with a halogen substituent at the central core

Two new homologous series of bent-core compounds have been synthesized. Their mesophase behaviour has been investigated by polarizing microscopy, differential scanning calorimetry, X-ray diffraction, NMR spectroscopy, and by dielectric and electro-optical measurements. It was found that, with one exception, all the chlorine-substituted compounds form a nematic phase and an optically isotropic ‘banana phase’. The latter phase shows spontaneously chiral domains of opposite handedness. This phase may be considered as a type of smectic blue phase. The mesophase behaviour of the homologous bromine-substituted compounds is more complicated. Depending on the chain length, B₆, columnar, nematic or the isotropic ‘banana phase’ occur.     

Hydrothermal synthesis and characterization of a novel polyoxometallate-templated three-dimensional supramolecular network

The well-known polyoxomolybdate anion [Mo₈O₂₆]^4? has been used as a noncoordinating anionic template for the construction of a novel three-dimensional (3D) supramolecular network, [Co(phen)₃]₂[Mo₈O₂₆]·?2.5H₂O (1) (phen?=?o-phenanthroline). Compound 1 has been characterized by elemental analyses, IR spectrum, X-ray photoelectron spectroscopy (XPS), TG analysis and single-crystal X-ray diffraction. Single-crystal X-ray diffraction analysis reveals that [Co(phen)₃]²⁺ coordination complexes are packed together via aromatic?π–π?stacking and hydrogen-bonding interactions, and exhibit an interesting 3D supramolecular network with one-dimensional (1D) box-like channels in which the octamolybdate anions reside.     

Hydrothermal synthesis,crystal structure and fluorescence of a 3D organic–inorganic hybrid coordination network [CuI(4,4′-bipy)]4[ δ -Mo8O26] exhibiting an interesting polymorphism

A new three-dimensional (3D) extended solid [Cu^I(4,4′-bipy)]₄[δ-Mo₈O₂₆] (1) built on {Cu(4,4′-bipy)}⁺ (bipy?=?bipyridine) and isopolymolybdate anion [δ-Mo₈O₂₆]^4? has been synthesized and characterized by elemental analysis, IR, TG, X-ray powder diffraction and single-crystal X-ray diffraction analysis. Compound 1 shows six coppers bonded to [δ-Mo₈O₂₆]^4? to form inorganic planes which are linked by 4,4′-bipy into a 3D network, the first example of [δ-Mo₈O₂₆]^4? six-connected linkage based on an organic-inorganic coordination polymer. A polymorphism of compound 1 was found which has an identical molecular formula but absolutely different structure. The influence of the synthesis conditions of 1 has been studied. Furthermore, the fluorescence of 1 is reported.     

A new 2D rhombic network metal-organic framework: (H<Subscript>3</Subscript>O<Superscript>+</Superscript>)[In(Tbip)<Subscript>2</Subscript>] · H<Subscript>2</Subscript>O (H<Subscript>2</Subscript>Tbip = 5-<Emphasis Type="Italic">tert</Emphasis>-butylisophthalic acid)

A new coordination polymer (H₃O⁺)[In(Tbip)₂] · H₂O (I) (H₂Tbip is 5-tert-butylisophthalic acid) has been synthesized under hydrothermal conditions. Complex I has been characterized by elemental analysis, IR, TG analysis, and powder X-ray diffraction. Single-crystal X-ray diffraction studies revealed that I crystallizes in the monoclinic crystal system, space group C2/c. In I the metal centers are linked by bridging Tbip ligand to form an interesting extended two-dimensional rhombic network.     

Thermotropic ionic liquid crystals of pyridinium octylphosphate A N.M.R. and X-ray study

Abstract

A thermotropic ionic lamellar phase from non-stoichiometric pyridinium octyl-phosphates has been investigated by multinuclear N.M.R. and X-ray diffraction. At room temperature and above, this phase is formed for pyridine to octylphosphoric acid molar ratios from 0.2 to 0.8.²H and ¹³C relaxation experiments show that the pyridinium ion undergoes a very anisotropic motion with D_zz > D_xx ? D_yy, z and x being the perpendicular direction to the ring and the c ₂ symmetry axis, respectively. The order parameters given by the ²H quadrupolar splittings and the ¹³C chemical shift anisotropy (CSA) are S_zz = 0.13, S_yy = -0.08 and S_xx = -0.05, showing that the pyridinium ring is preferentially oriented parallel to the lamellar plane. The ³¹P CSA and the C1-P dipolar splitting yield S_zz = 0.33 and S_xx ? S_yy for the octylphosphate anion. The order parameters of alkyl C-H bonds have been obtained from the J resolved two-dimensional ¹³C N.M.R. spectra of oriented samples. Two limiting conformational models have been considered to calculate the S _CH. One of them is reasonably consistent with the structure derived from X-ray experiments and has been used to calculate the dipolar ³¹P relaxation. Taking into account the CSA contribution, the relaxation measurements performed at 36, 121 and 202 MHz show that the octylphosphate anion undergoes a quasi-axial reorientation about the long molecular axis x with D∥/D⊥ = 4 and D⊥ ? 10⁷ rad/s at 300 K.     

The twist-bend nematic phase: molecular insights from a generalised Maier–Saupe theory

ABSTRACT

Bent-shaped liquid crystal dimers exhibit a non-conventional nematic phase, which in the last few years has been actively investigated. A structural model of this phase has been proposed, which is characterised by a helical modulation with a periodicity of the order of few molecular lengths. Such a model, which is generally denoted as twist-bend nematic (N_TB), is consistent with various experimental evidences and is supported by theories and simulations. Here, we will examine in more detail the features and the implications of a generalised Maier–Saupe theory, which was recently proposed to explain the origin and structure of the N_TB phase. In particular we will analyse the relevant molecular order parameters, with special attention to the polar and biaxial ones, and we will discuss the concept of ‘tilted director’, which may give rise to some ambiguity. The phase behaviour will be described as a function of the bend angle between the mesogenic moieties of a dimer, which in the generalised Maier–Saupe theory is the key molecular feature. Extension of the theory to include fluctuations of the bend angle will allow us to examine the effect of the molecular flexibility on the phase diagram.     

Hexakis(dimethylformamide)iron(II) complex cation in hexahalorhenate(IV)-based salts: synthesis,X-ray structure and magnetic properties

Abstract

Two iron(II)-rhenium(IV) compounds of general formula [Fe^II(dmf)₆][Re^IVX₆] [X = Cl (1) and Br (2); dmf = N,N-dimethylformamide] have been prepared and characterized. X-ray powder diffraction measurements on samples of 1 and 2 support the same structure for both systems. The crystal structure of 1 was determined by single-crystal X-ray diffraction. 1 crystallizes in the triclinic system with space group Pī. Each iron(II) is six-coordinate and bonded to six oxygens from six dmf molecules building a distorted octahedral environment. Rhenium(IV) is six-coordinate by six halide anions in an almost regular octahedral geometry. The magnetic properties were investigated from variable-temperature magnetic susceptibility measurements performed on microcrystalline samples of 1 and 2, whose experimental data were reproduced by a model of two isolated paramagnetic centers [S = 2 (Fe^II) and S = 3/2 (Re^IV)] with large values of zero-field splitting (zfs) parameter.