Measurements of the ratio of the Frank constants for splay and bend in nematics consisting of disc-like molecules 2, 3, 6, 7, 10, 11-hexakis(p-alkoxybenzoyloxy)triphenylenes

Measurements are reported of the magnetic Frederiks transition in nematics consisting of disc-like molecules; 2, 3, 6, 7, 10,11-hexakis(p-alkoxybenzoyloxy)triphenylenes. In agreement with theoretical predictions [1] and previous measurements on 2, 3, 7, 8, 12, 13-hexa(alkanoyloxy)truxenes [2], we find that the splay elastic constant, K₁, is larger than the bend elastic constant, K₃. The ratio K₁/K₃ is remarkably constant throughout the nematic regime.     

Elastic constants of uniaxial nematic liquid crystals: A comparison between theory and experiment

Using the unified molecular theory developed in our earlier paper (1992, Phys. Rev. A, 45, 974) we study in detail the influence of molecular interactions on the fundamental elastic properties of uniaxial nematic liquid crystals composed of molecules of cylindrical symmetry. The expressions for the elastic moduli associated with 'splay', 'twist' and 'bend' modes of deformations are written in terms of order parameters characterizing the nature and amount of ordering in the phase and the structural parameters which involve the generalized spherical harmonic coefficients of the direct pair correlation function of an effective isotropic liquid. Numerical calculations are done for a model system, the molecules of which have prolate ellipsoid of revolution symmetry and interact via a pair potential having both repulsive and attractive parts. The repulsive interaction is represented by a repulsion between hard ellipsoids of revolution. The attractive potential is represented by the dispersion and electrostatic interactions. Results for the elastic constants are reported for a range of molecular length-width ratio, temperature, density and molecular parameters and are compared with the experimental values of p-azoxyanisole (PAA) and 4'-n-octyloxy-4-cyanobiphenyl (8OCB). It is found that the inclusion of electrostatic interactions reduces the values of the ratios K₂/K₁ and K₃/K₁. The absolute values of the elastic constants and their ratios are in good agreement with the experimental and computer simulation values. The temperature dependence of the elastic constants and their ratios is studied. It is observed that the twist elastic constant has a weak temperature dependence but a pronounced influence is observed on the bend moduli. We also observed a pronounced increase in the values of the twist and bend elastic constants on approaching the nematic-smectic A transition temperature.     

Anomalous anisotropy of the elastic constants in the induced smectic O film

Surface tension is known to induce smectic O (S_O) films at the free surface of isotropic droplets of 1-methylheptyl terephthalylidene-bis-4-aminocinnamate (MHTAC). The S_O film has its molecules disposed in a herringbone fashion, with the layers parallel to the free surface. It constitutes a model of a 2D polar nematic liquid crystal without any contact to solid substrates. The film is oriented uniformly when applying a uniform electric field, except along disclination walls. By measuring the width of the disclination walls in the two configurations with the electric field parallel and perpendicular, we determine the anisotropy of the elastic constants in the induced S_o films. We find the ratio of the 2D bend to splay elastic constants K_b/K_s to be ≃ 0.25 which is anomalously small when compared to the corresponding 3D ratio (K₂ + K₃)/2K₁ usually found for the bulk nematic phase. This experimental result is comparable to measurements performed with 2D suspended S_c films with, however, a different physics.     

A new series of nematic and smectic liquid crystals with negative dielectric anisotropy: the effect of terminal chain substitution on thermal and electro-optical properties

Two alkyl (1b and 1c) and four fluoroalkyl derivatives (1d-1f) of 4-arylbutyric acid (1c, 1d and 1e) and 4-arylbutanol (1b, 1f and 1g) [aryl = 2',3'-difluoro-4'-(2-(E-4-pentylcyclohexyl)ethyl)-biphenyl-1-yl] were prepared and investigated in the pure form as nematic materials (1b and 1c) and as additives to a ferroelectric liquid crystal (FLC) host (1d-1f). A comparison of 1b and 1c with the decyl analogue 1a demonstrates the effect of terminal chain modification on thermal and electro-optical properties. The substitution of the -CH₂O- (1b) or -COO- (1c) for -CH₂CH₂- in 1a destabilized the N and SmA phases or completely eliminated the smectic behaviour (1c). Dielectric analysis revealed that the chain modification increased the negative Δε, reduced elastic constant K ₁₁ and moderately decreased rotational viscosity γ₁. The temperature dependence of the key electro-optical parameters was analysed for ester 2, the methyl analogue of 1c, which exhibits a 45 K wide N phase. All four fluoroalkyl derivatives 1d-1f showed enantiotropic SmA phases and 1e also exhibited a monotropic SmC phase. Solutions of 1d-1f in a FLC host (0.2 mmol g^-1) increased the tilt angle Θ (up to 45° for 1g), reduced rotational viscosity γ₁ and the risetime τ. The most dramatic changes were observed for 1g, which contains 15 fluorine atoms.     

The stability diagram of a nematic liquid crystal confined to a cylindrical cavity

Different nematic structures confined to a long cylindrical cavity with homeotropic surface anchoring are studied using a numerical minimization of the free energy of the uniaxial nematic liquid crystal. The stability of escaped radial structures and planar polar structures (with and without line defects) is analysed in terms of the ratio of elastic constants K₂₄/K₁₁, K₃₃/K₁₁, anchoring strength and external magnetic field applied perpendicular to the symmetry axis of the cylinder. We draw the analogy between the stability diagram of the cylindrical structures and structures in a spherical droplet. In particular, a simple way extracting the value of the saddle-splay elastic constant K₂₄ from the stability studies is discussed.     

Static periodic distortion above bend Freedericksz transition in nematics

The linearized mathematical model developed by Allender, Hornreich and Johnson [1987, Phys. Rev. Lett., 59, 2654], for explaining the appearance of the magnetic field induced stripe phase (SP) above the bend Freedericksz threshold in a nematic close to the smectic transition, is generalized to the case of uniform tilt θ₁ of the nematic director n₀ away from the homeotropic with the field H acting normal to n₀. Calculations of SP threshold and domain wave vector Q are presented for different elastic ratios and tilts θ₁, by exact computation of the ground state homogeneous deformation (HD) under the rigid anchoring hypothesis. Approximate estimates based on energetics, explicitly taking into account the modal symmetry of perturbations, agree well with the results of exact calculations based on the solution of torque equations. For homeotropic alignment (θ₁ = 0) calculations predict that the SP domain width should decrease when the sample is heated away from the smectic transition point; at a given temperature when H is rotated through a small angle with respect to the sample planes the domains should grow wider. These points can be verified experimentally. It is also shown that for sufficiently high initial tilt θ₁ away from the homeotropic director alignment, SP may be quenched. Materials, such as nematic polymers, which exhibit static periodic domains (PD) in splay geometry (of the kind discovered by Lonberg and Meyer, 1985, Phys. Rev. Lett., 55, 718) may also show SP for director tilts θ₁ close to the homeotropic. It appears possible to make tentative predictions regarding the effects of weak anchoring and oblique magnetic fields on the SP threshold and domain wave vector.     

Stability constants of some bivalent metal complexes of N-phenyl-o-tolylbenzohydroxamic acid

The stability constants of the Cu²⁺, Zn²⁺, Ni²⁺ and Mu²⁺ complexes of N-phenyl-o-methoxybenzohydroxamic acid at 30° in 50% v/v aqueous dioxan are: log K₁ 10·45, 8·16, 7·52, 6·33; log K₂ 8·90, 6·70, 6·01, 5·59 (for the ions in the order given).     

Elastic deformations in a magnetic field of a liquid-crystalline polymer with mesogenic groups in the main and side chains

Orientational elastic deformations in a magnetic field and phase transition temperatures of a thermotropically mesogenic “combined” aromatic polyester with mesogenic groups, both in the main and in the side chains, have been investigated in the range of 900 to 10,200 mol. wt. It was shown that in this molecular weight range the birefringence of a completely oriented nematic and correspondingly, the degree of its orientational order S are independent on molecular weight. When the relative temperature ΔT changes from −2 to −30°C the orientational order parameter increases from 0.35 to 0.55. The bend elasticity constants K₃ coincide in the order of magnitude with those for high and low molecular weight nematics investigated previously and their dependence on molecular weight has not been detected.     

Kinetics and mechanisms of the redox reaction of hexaaquotitanium(III) and the ethylenediamine tetraacetato titanium(III) complex by aqueous solutions of bromine

At 25°C, I = 1.0 M (CF₃SO₃⁻Li⁺+CF₃SO₃H), [H⁺] = 0.034–0.274 M and λ = 453 nm, the rate equation for the oxidation of Ti(H₂O), ₆³⁺ by bromine was found to be: −d/[Br₂]T/dt=kK/[Br₂][Ti^III]/[H⁺]+K+kK/[Br₃⁻][Ti^III]/[H⁺+K, where k = 9.2 × 10⁻³ M ⁻¹ s ⁻¹ and K = 4.5 × 10⁻³ M. At [H⁺] = 1.0 M, [Br⁻] = 0.05–0.4 M, the apparent second-order rate constant decreases as [Br⁻] increases.

The pH-dependence of the oxidation of Ti^III-edta by bromine is interpreted in terms of the change in identity of the Ti^III-edta species as the pH of the reaction medium changes. The second-order rate constants were fitted using a non-linear least-square computer program with (1/k₀^edta)² weighting into an equation of the form: k₀^edta =k₁+k₂K₁[H⁺]⁻¹+k₃K₁K₂[H⁺]^−2/1+K₁[H⁺[H⁺⁻¹+K₁K₂[H⁺]⁻², with K₁ and K₂ fixed as earlier determined at 9.55 × 10⁻³ and 2.29 × 10⁻⁹ M, respectively, for the oxidation of bromine. k₁=k₂=(3.1±0.32)×10³M⁻¹s⁻¹ k₃=(2.3±0.45)×10⁶N⁻¹s⁻¹.

It is proposed that these electron transfer reactions proceed by univalent changes with the production of Br₂^.− as a transient intermediate. An outer-sphere mechanism is proposed for these reactions. The homonuclear exchange rate for Ti^III-edta+Ti^IV-edta is estimated at 32 M⁻¹ s⁻¹.     

A comparison of mesogenic properties of p-carborane-1,12-dicarbaldehyde schiff's bases with their terephthaldehyde analogues

A homologous series of carborane-containing Schiff's bases 1A[n] (n = 1-10) was prepared and compared with the analogous series 1B[n] derived from terephthaldehyde. An exponential fit of the T _NI values for both series yielded a quantitative assessment of the effect of ring structure on mesophase stability. This includes the T _NI value for n→∞ (86°C for 1A[n] and 209°C for 1B[n]) and steepness of descent (0.135 for 1A[n] and 0.095 for 1B[n]). The difference in behaviour of the two series was attributed, largely, to conformational properties of the central rings A and B. Electronic interactions between the central rings and the π-substituents were investigated by UV spectroscopy and by quantum-mechanical calculations. The effect of replacement of O with CH₂ in the terminal chain of 1[n] on the namatic phase stability was assessed for n = 5-7.     

Determination of stability constants for rare earth elements and fulvic acids extracted from different soils

Fulvic acids (FAs) were extracted by alkali extraction from different soil samples in China, then purified using resins and characterized by Fourier transform infrared spectroscopy. The complexing ability of FAs was investigated by measuring the stability constants of rare earth elements (REEs) (La³⁺, Ce³⁺, Sm³⁺, Gd³⁺, Y³⁺) with FAs by the ion exchange technique. The results indicated that maximum binding ability forY³⁺ (4.414.44) was higher than other REEs (La³⁺, Ce³⁺, Sm³⁺, Gd³⁺) (0.721.03). There were two types of binding sites in the functional groups of fulvic acids. The complexing reaction followed two steps. The stability constants (K₁ and K₂) of REEs with FAs were calculated from experimental data by division of Scatchard plots into two straight-line segments. Y³⁺ (log K₁=5.72±0.05, log K₂=4.83±0.01) also has higher stability constants than the other four REEs (log K₁=4.37±0.16, log K₂=3.62±0.28).     

Light scattering from a nematic monodomain in an electric field Twist elastic constant and viscosity coefficient of nematic polymer-solvent mixtures

The light scattering technique was used to investigate the viscoelastic parameters characterizing director twist distortions in miscible nematic mixtures of 5CB (pentacyanobiphenyl) with two side chain liquid crystal polymers and a main chain liquid crystal polymer. By applying an AC electric field to homeotropically-aligned nematic monodomains of the mixtures, the field-dependent scattering intensities and director orientation fluctuation relaxation rates yield, respectively, the twist elastic constant K₂₂ and viscosity coefficient γ₁. The results directly demonstrate that the addition of liquid crystal polymers causes substantial decreases of the relaxation rates for dynamic light scattering from the twist mode and these changes are due to small decreases in K₂₂ coupled with large increases in γ₁. The decrements in K₂₂ are comparable for both side chain and main chain liquid crystal polymers. The relative increase in the twist viscosity for the side chain liquid crystal polymers is much smaller than those of main chain polymers. A theoretical model is used to qualitatively interpret the difference between the viscous behaviour of the twist mode for both side chain and main chain liquid crystal polymers in a nematic solvent.     

Rapid measurement of fish freshness indices by an amperometric flow-injection system with a 16-way switching valve and immobilized enzyme reactors

A novel flow-injection system is proposed for the rapid measurement of the fish freshness indices K₁ and K₂: K₁=[([HxR+[Hx])×100/([IMP]+[HxR]+[Hx])] and K₂=[[Hx]×100/([HxR]+[Hx])], where [IMP], [HxR] and [Hx] are inosine-5′-monophosphate, inosine and hypoxanthine concentrations, respectively. For the estimation of index K₁, 5′-nucleotidase immobilized reactor and nucleoside phosphorylase (NP)/xanthine oxidase (XO) coimmobilized reactor were incorporated in series in the flow-injection line made up by a 16-way switching valve with two sample loops. For the estimation of index K₂, NP and XO immobilized reactors were also incorporated in the similar flow-line. Two sample portions passed through the flow-line with different residence times so that two peaks were obtained. The first and second peaks obtained in the K₁-determining system corresponded to the total of HxR and Hx and the total of Hx, HxR and IMP, respectively. Similarly, the first and second peaks obtained in the K₂-determining system corresponded to Hx and the total of Hx and HxR, respectively. Therefore, the indices K₁ and K₂ can be estimated by

where i₁ and i₂ present the peak current of the first and second peaks, respectively, and f the ratio of the peak currents of the first and second peaks for a Hx standard solution. A sea bream was selected as a model fish and it was stored at 4 °C after death. A good correlation was found between the index K₁ and the storage time over a period of 400 h, with a correlation coefficient of 0.992, but no correlation between the index K₂ and the storage time. The measurements could be performed at a rate of 22 samples per hour with satisfactory precision (0.6–1.2% R.S.D.), without calibration for each species, accurate weighing of fish meat and any interferences in fish meat.     

The synthesis and liquid crystalline behaviour of alkoxy-substituted derivatives of 1,4-bis(phenylethynyl)benzene

Despite the prevalence of organised 1,4-bis(phenylethynyl)benzene derivatives in molecular electronics, the interest in the photophysics of these systems and the common occurrence of phenylethynyl moeties in molecules that exhibit liquid crystalline phases, the phase behaviour of simple alkoxy-substituted 1,4-bis(phenylethynyl)benzene derivatives has not yet been described. Two series of 1,4-bis(phenylethynyl)benzene derivatives, i.e. 1-[(4'-alkoxy)phenylethynyl]-4-(phenylethynyl)benzenes (5a-5f) and methyl 4-[(4'-alkoxy)phenylethynyl-4'-(phenylethynyl)] benzoates (18a-18f) [alkoxy = n-C₄H₉ (a), n-C₆H₁₃ (b), n-C₉H₁₉ (c), n-C₁₂H₂₅ (d), n-C₁₄H₂₉ (e), n-C₁₆H₃₃ (f)] have been prepared and characterised. Both series have good chemical stability at temperatures up to 210°C, the derivatives featuring the methyl ester head-group (18a-18f) offering rather higher melting points and generally stabilising a more diverse range of mesophases at higher temperatures than those found for the simpler compounds (5a-5f). Smectic phases are stabilised by the longer alkoxy substituents, whereas for short and intermediate chain lengths of the simpler system (5a-5c) nematic phases dominate. Diffraction analysis was used to identify the SmB_hex phase in (5d-5f) that is stable within a temperature range of approximately 120-140°C. The relationships between the organisation of molecules within these moderate temperature liquid crystalline phases and other self-organised states (e.g. Langmuir-Blodgett films) remain to be explored.     

Broadband dielectric studies of weakly polar and non-polar liquid crystals

Complex dielectric permittivities, for two orientations of the director n, parallel (E || n) and perpendicular (E ⊥ n) to the probing electric field E, of the weakly polar liquid crystals (LCs) 4,4'-dihexylazoxybenzene (D6AOB) and 4,4'-diheptylazoxybenzene (D7AOB) as well as the non-polar LC diheptylazobenzene (D7AB) have been measured in the frequency range 75 kHz to 1 GHz. The measurements were performed in the nematic, smectic and isotropic phases of the LCs. The dielectric anisotropies Δε (=ε_||-ε_⊥) obtained from the values of dielectric permittivities at 100 kHz in the nematic phase were found to increase with decreasing temperature. However, for the DnAOBs, the Δε values are somewhat smaller than that for D7AB which does not have a permanent dipole moment. In the nematic phase two molecular relaxation processes were observed for both DnAOBs in each of the orientations—parallel and perpendicular. The four processes merge into two separate processes in the isotropic phase. For D7AB no orientational relaxations were observed in the experimental frequency range.     

Solubility equilibria in the uric acid-sodium urate-water system

In a constant ionic medium, corresponding to a physiological environment (I_c = 0.15 mol dm⁻³ NaCl), the solubilities of anhydrous uric acid, uric acid dihydrate and monosodium urate monohydrate have been measured as a function of p[H] = −log[H⁺](2-8) and temperature (25°, 32°, 37° and 42°C). The solubility equilibria in the uric acid-sodium urate-water system are discussed on the basis of the solubility constants (K_s) and the first dissociation constant (K₁) of uric acid and the solubility product (K_s0) of monosodium urate. The quantities measured in this work are in good agreement with literature values, however, the present solubility data have a much higher precision.     

Kinetics and mechanisms of homogeneous catalytic reactions: Part 7. Hydroformylation of 1-hexene catalyzed by cationic complexes of rhodium and iridium containing PPh3

Cationic rhodium and iridium complexes of the type [M(COD)(PPh₃)₂]PF₆ (M = Rh, 1a; Ir, 1b) are efficient precatalysts for the hydroformylation of 1-hexene to its corresponding aldehydes (heptanal and 2-methylhexanal), under mild pressures (2–5 bar) and temperatures (60 °C for Rh and 100 °C for Ir) in toluene solution; the linear to branched ratio (l/b) of the aldehydes in the hydroformylation reaction varies slightly (between 3.0 and 3.7 for Rh and close to 2 for Ir). Kinetic and mechanistic studies have been carried out using these cationic complexes as catalyst precursors. For both complexes, the reaction proceeds according to the rate law r_i = K₁K₂K₃k₄[M][olef][H₂][CO]/([CO]² + K₁[H₂][CO] + K₁K₂K₃[olef][H₂]). Both complexes react rapidly with CO to produce the corresponding tricarbonyl species [M(CO)₃(PPh₃)₂]PF₆, M = Rh, 2a; Ir, 2b, and with syn-gas to yield [MH₂(CO)₂(PPh₃)₂]PF₆, M = Rh, 3a; Ir, 3b, which originate by CO dissociation the species [MH₂(CO)(PPh₃)₂]PF₆ entering the corresponding catalytic cycle. All the experimental data are consistent with a general mechanism in which the transfer of the hydride to a coordinated olefin promoted by an entering CO molecule is the rate-determining step of the catalytic cycle.     

Relationship between the pitch and elastic constants of the transient planar state in cholesteric liquid crystals

We investigate experimentally the relationship between the pitch and the elastic constants of a transient planar state in cholesteric liquid crystals. The pitch of the transient planar state is measured by the optical reflection technique. The twist and bend elastic constants of cholesteric materials are determined from the measured values of the threshold voltages for the transitions between a focal-conic state and a homeotropic state. By comparing these values, we experimentally confirm the relationship for the pitch of a transient planar state; P_transient=(K₃₃/K₂₂)P₀, which has been suggested theoretically.     

Preparation of a discotic 2,4,7,9-tetraaryl-6H-dibenzo[c,e][1,2]thiazine and generation of a persistent Radical 1

6H-Dibenzo[d,e][1,2]thiazine substituted with four 3,4-dioctyloxyphenyl groups forms a room temperature discotic phase with a clearing temperature of 104°C. Oxidation of the thiazine 2b with either metal oxides (PbO₂ or AgO) or SO₂Cl₂ gave a moderately stable radical 1b (a _N = 0.95 mT, g = 2.0045). Full conversion of 2b to 1b was observed with AgO in a toluene/MeCN mixture or with SO₂Cl₂. The radical 1b was isolated in the neat form with the latter oxidant. It shows sensitivity to molecular oxygen, which hampered its purification and subsequent thermal and magnetic studies.     

Observation of textures of nematic polymers and estimation of the elastic constants

The texture of a copolyesteramide, Vectra® B950, has been studied by optical microscopy. At rest, the defects are typical of a biaxial nematic phase, but under the influence of a magnetic field or under shear, the nematic transforms into a uniaxial phase. The Freedericksz geometry yields a K₁ elastic constant of the order of 10^-8 N. The diffusivities K_III/_n and K_II/_n are obtained by measuring the light intensity transmitted through a sample during shear start-up and relaxation, and from this we estimate K_III ≅ 10 ^-9N and K_II ≅ 10 ^-10N. Since the biaxiality is small, these uniaxial constants adequately describe the phase at rest.