A quasielastic neutron scattering study of the self-diffusion coefficients for the homologous series of 4,4'-di-n-alkyloxyazoxybenzenes

Incoherent quasielastic neutron scattering measurements performed with high resolution (∼ 1 μcV) have provided the diffusion coefficients for translation of four members of the 4,4'-di-n-alkyloxyazoxybenzene series in their nematic phases. An odd-even effect in these coefficients was observed. The results were compared with those previously obtained by Noack via the NMR technique. There is fairly good agreement between his NMR and our QNS results.     

Molecular reorientation in the nematic and rotatory phases of 4,4'-di-n-pentyloxyazoxybenzene

Results of dielectric relaxation, quasielastic neutron scattering, calorimetric D.S.C. and preliminary X-ray measurements on the fifth member, POAOB, of the 4,4'-di-n-alkoxyazoxybenzene homologous series are presented. It has been found that POAOB exhibits two mesophases: a nematic (N) and an intermediate crystalline phase (CI) just below it. From comparison of the dielectric relaxation and quasielastic neutron scattering studies we can conclude that in the nematic phase the molecule as a whole performs rotational diffusion around the long axis (ω ~ 150ps) and at the same time the two moieties perform faster independent reorientations around the N-δ bonds (ω denotes a benzene ring) with δ ~ 5 ps. The intermediate crystal phase is identified as a solid uniaxial rotational phase in which fast molecular reorientations exist. It seems that the fast reorientations observed in the nematic phase to some extent survive to the crystal I phase. A model of molecular arrangements in the crystal I phase is proposed, and it explains the reduction of the dielectric increment observed on passing from the nematic phase to this phase.     

Temperature dependence of the molecular reorientational dynamics in nematic and isotropic EBBA and 5CB by fluorescent probe depolarization spectroscopy

Information on molecular reorientational dynamics in nematic and isotropic phases has been obtained by a simple combination of time resolved and steady state fluorescence measurements on the dyes DMANS and DPH dissolved in homeotropically aligned samples of EBBA (4-ethoxybenzylidene-4'-n-butylaniline) and 5CB (4-n-pentyl-4'-cyanobiphenyl) respectively. In particular, two reorientational correlation times τ^r₀ and τ^r₂ have been identified. Their evolution with temperature, both in the nematic and well within the isotropic phase, has been studied and evaluated using stochastic models for the molecular reorientational dynamics.     

QNS investigation of restricted molecular reorientation in the nematic phase. I. The incoherent scattering law for MBBA

On the basis of the solution of the random flight problem of a particle between two perfectly reflecting potential barriers the incoherent scattering law for the model of the uniaxial, molecular stochastic reorientation within an arbitrary apex angle Ф₀ has been calculated. In conjunction with the published NMR results evidence is presented, that the along the long axis uniaxial rotation, can be described in terms of the model with an apex angle φ₀ of 57°. The model of the restricted uniaxial reorientation, which is expected to be of interest in nematic phases of biaxial molecules, has been applied to the investigation, by quasi elastic incoherent cold neutron scattering, of the rotational dynamics of the rigid benzylidene central part of a magnetically oriented MBBA sample in the nematic phase at 303 K.     

Study of internal ring rotation and molecular reorientation in the liquid crystal 5O.7 by deuterium N.M.R. spectroscopy

The spectral densities of motion were determined by deuterium N.M.R. relaxation measurements in the nematic, smectic A and smectic C phases of 4-n-pentyloxybenzylidene-d₁-4'-heptylaniline and 4-n-pentyloxybenzylidene-4'-heptylaniline-2,3,5,6-d₄. By examining two atomic sites on a 5O.7 molecule, we were able to gain information on the reorientation motion and internal rotation of the aniline ring. It was also found that director fluctuations make some contribution to the spectral density J₁ (ω). We use the superimposed rotations model to account for the internal ring motion and the small step rotational diffusion model for the molecular reorientation. The derived rotational diffusion constants for the spinning and tumbling motions appear to give physically plausible activation energies in the mesophases of 5O.7.     

Study of the N2 bΠu state via 1 + 1 multiphoton ionization

Medium-resolution spectra of the N₂ b¹Π_u-X¹Σ_g⁺ band system were recorded by 1 + 1 multiphoton ionization. In the spectra we found different linewidths for transitions to different vibrational levels in the b ¹Π_u state: Δν₀ = 0.50 ± 0.05 cm⁻¹, Δν₁ = 0.28 ± 0.02 cm⁻¹, Δν₂ = 0.65 ± 0.06 cm⁻¹, Δν₃ = 3.2 ± 0.5 cm⁻¹, Δν₄ = 0.60 ± 0.07 cm⁻¹, and Δν₅ = 0.28 ± 0.02 cm⁻¹. From these linewidths, predissociation lifetimes τ_ν were obtained: τ₀ = 16 ± 3 ps, τ₁ > 150 ps, τ₂ = 10 ± 2 ps, τ₃ = 1.6 ± 0.3 ps, τ₄ = 9 ± 2 ps, and τ₅ > 150 ps. Band origins and rotational constants for the b ¹Π_uν = 0 and 1 levels were determined for the ¹⁴N₂ and ¹⁴N¹⁵N molecules.     

Dynamics of the Frederiks transition in nematics consisting of disc-like molecules Thermal dependence of a bend viscosity coefficient

We report measurements of the dynamics of the magnetic Frederiks transition in nematics consisting of disc-like molecules. In this paper the results are presented for three 2, 3, 6, 7, 10, 11-hexakis(p-alkoxybenzoyloxy)triphenylenes, which exhibit a normal nematic phase, and for three 2, 3, 7, 8, 12, 13-hexa(alkanoyloxy)truxenes, which exhibit an inverted nematic phase. We find that the thermal dependence of a bend viscosity coefficient (γ*₁) can be accurately described by the expression, γ*₁ ∼ S² exp (E_a/kT). The absolute value of γ*₁ is found to be higher (by a factor of 10-100) than is commonly encountered in nematics consisting of rod-like molecules.     

Dielectric studies of liquid crystals under high pressure III. Low frequency relaxation process in 4-(trans-4'-n-hexylcyclohexyl)isothiocyanato benzene

Dielectric studies of 4-(trans-4'-n-hexylcyclohexyl) isothiocyanatobenzene (6CHBT) were performed in the pressure range 0·1-150 MPa, the frequency range 1 kHz-13 MHz and the temperature range 295-325 K. The temperature and pressure dependencies of the static permittivity ε_0∥ and of the relaxation time τ_∥ are analysed and compared with the analogous data obtained recently for 4-n-pentyl-4'-cyanobiphenyl (5CB) (Parts I and II of this series). Marked differences in the dielectric properties of the nematic phases of the two substances are observed. They are interpreted as a result of varying degrees of molecular association in particular compounds. It is concluded that in the nematic phase of 6CHBT dipole-dipole correlations do not exist or are very weak, whereas for 5CB they are easily broken by a relatively low pressure.     

Calculation of the surface charge and potential and of the structural component of disjoining pressure in an interlayer in KCl solutions between glass samples

The dependence of the Stern potential, ψ₁, of glass samples on the distance between these, H, has been theoretically calculated, while taking into account the Stern isotherm and the electroneutrality equation. Comparison of the theoretical dependences ψ₁(C)_H→∞ with those previously experimentally obtained enables one to calculate the energy of adsorption of OH ions on glass and, further, the dependence ψ₁(H). It has been shown that for pH 4–6 and C_KCl = 10^-2-10^-5 mol/L, the value of ψ₁ practically does not depend on H. The result obtained was used to calculate theoretically the ionic-electrostatic forces and to compute (from the experimental values of the interaction forces) structural forces U_s(H). The dependence thus obtained, U_s(H), is of exponential character.     

Microstructure and dynamics of a mesogenic diol

A number of techniques have been used to elucidate the structure and dynamics of 4,4'-bis(6-hydroxyhexyloxy)biphenyl (BHHBP) in its various phases. X-ray diffraction studies indicate that the molecules pack in a crystalline phase which melts to produce a highly ordered smectic/disordered crystal mesophase. Based on molecular models and the infrared results, the all trans conformation requires a 45°-55° tilt of the molecules in the smectic layers. Infrared spectroscopic results indicate that a predominantly trans chain conformation and hydrogen bonding of the layered crystal structure persists through the mesophase. Additionally, rotational freedom about the biphenyl linkage appears to occur only in the isotropic phase. NMR data indicate that the alkoxy chain is at or near co-planarity with the respect to the phenyl ring in the crystalline phase, with reorientational motion of the biphenyl group becoming allowed in the mesophase in the form of rapid (τ_c ∼ 3 μs at 100°C) small angle liberations and, perhaps, slower (τ_c ∼ 0·5 ms at 100°C) 180° ring flips. The alkyl chains exhibit a progressive increase in mobility with distance from the biphenyl core and achieve considerable mobility at the hydroxy end of the chain despite the fact that hydrogen bonding still occurs in the mesophase.     

Novel frequency dependence of dielectric biaxiality of surface stabilized ferroelectric liquid crystals

The frequency dependence of the dielectric biaxiality of surface stabilized ferroelectric liquid crystals (SSFLCs) was studied. The principal values of the dielectric tensor ε₁, ε₂ and ε₃ were measured by the MOM (molecular orientational model) method. Three dielectric permittivities were measured for each of two samples. These were the permittivity of the homeotropic cell and the permittivity of the planar homogeneous cell with and without the DC bias. Then the dielectric tensor components were calculated based on the molecular orientational models. We present the theory and experimental procedure of the MOM method. Measurements have been performed on Merck FLC compound SCE-8. The following novel dielectric behaviour was observed, as the DC bias voltage was increased the dielectric permittivity of the planar homogeneous cell decreased at the low frequencies (∼ 1 kHz) while increased at the high frequencies (10kHz ∼). The sign of the dielectric biaxiality ∂εε (= ε₂ - ε₁) inverted around 1 kHz, being negative at low frequencies and positive at high frequencies. The roles of the biaxiality on the dielectric behaviour of SSFLC cells are discussed.     

Synthesis and thermal properties of dimeric alkoxy-substituted tolans

Four homologous series of dimeric tolans of the general formula RC₆H₄C≏C₆H₄O(CH₂)_m OC₆H₄C'CC₆H₄R (m'6-10; R'H, OC₆H₁₃, OC₁₀H₂₁ and OC₁₄H₂₉), denoted further as TOmOT or nOTOmOTOn, according to the number of aliphatic carbon atoms, have been synthesized. Dimers with terminal chains exhibited liquid crystalline behaviour. Crystal-crystal transitions were also observed. Resulting from the optical, thermal and miscibility studies, the following mesophases have been identified: nematic, smectics A and B, and the tilted smectics C and (probably) F or I. In the 6OTOmOTO6 series, a phase with a strong tendency to self-alignment occurred between the nematic and smectic A phases. This phase resembled the nematic in its viscoelastic properties, but also showed focal-conic textures. A transition between this 'intermediate phase' and the nematic was detected only from microscopic observations. On the contrary, the 'intermediate phase'-smectic A transition was also detected by DSC (δH ∼ 1kJ mol^-1). Both tilted (S_c and S_F/I) and non-tilted (S_A and S_B) smectics were observed in the 10OTOmOTO10 series, while only tilted smectic phases were identified in the dimers with longer terminal substituents (14OTOmOTO14). Double melting behaviour was found in 14OTO8OTO14 and 14OTO10OTO14. An alternation of the transition temperatures and enthalpies with the odd-even alternation of the lengths of bridging groups was clearly observed. A correlation between total enthalpies of transition and solubilities of the dimeric tolans is stated.     

Structural evolutions of the n-heneicosane and n-tricosane molecular alloys at 293 K

A structural study of odd-numbered n-alkane (C_n) binary mixtures (C₂₁ : C₂₃) was carried out on powder samples using a Guinier-de Wolff camera with increasing concentration of n-C₂₃ at 293 K.

Despite the reports in the literature, these molecular alloys do not form an orthorhombic continuous homogeneous solid solution to C₂₁ from C₂₃ at “low temperature”. Instead, as already observed in two even-numbered C_n systems, X-ray diffraction results show the existence of seven solid solutions as the molar concentration of C₂₃ increases: four terminal solid solutions, denoted β₀(C₂₁)β₀(C₂₃), isostructural with the “low temperature” phase of pure C₂₁ and C₂₃ (Pbcm), β′₀(C₂₁) and β′₀(C₂₃), identical to the phase β′₀ which appears in pure C₂₃ above the δ transition, and three orthorhombic intermediate solid solutions, designated β″₁, β′₁ and β″₂.

On the basis of powder X-ray photographs, the phases β″₁ and β″₂ (C₂₁ : C₂₃) are indistinguishable, and they are isostructural with the intermediate solid solution β″ of the even-numbered C_n binary systems (C₂₂ : C₂₄) and (C₂₄ : C₂₆). The phase β′₁(C₂₁ : C₂₃) is also isostructural with the two indistinguishable intermediate solid solutions β′₁ and β′₂ of the molecular alloys (C₂₂ : C₂₄) and (₂₄ : C₂₆).

From this study and our other laboratory results, the sequences of appearance of the solid solutions and the structural identities between these phases are established at “low temperature” for all the binary molecular alloys of consecutive C_n (odd-odd, even-even or odd-even: 19 < n < 27) when increasing the solute concentration.     

Molecular reorientation in the nematic and rotatory phases of 4,4'-di-n-pentyloxyazoxybenzene

Abstract

Results of dielectric relaxation, quasielastic neutron scattering, calorimetric D.S.C. and preliminary X-ray measurements on the fifth member, POAOB, of the 4,4'-di-n-alkoxyazoxybenzene homologous series are presented. It has been found that POAOB exhibits two mesophases: a nematic (N) and an intermediate crystalline phase (CI) just below it. From comparison of the dielectric relaxation and quasielastic neutron scattering studies we can conclude that in the nematic phase the molecule as a whole performs rotational diffusion around the long axis (ω ~ 150ps) and at the same time the two moieties perform faster independent reorientations around the N-δ bonds (ω denotes a benzene ring) with δ ~ 5 ps. The intermediate crystal phase is identified as a solid uniaxial rotational phase in which fast molecular reorientations exist. It seems that the fast reorientations observed in the nematic phase to some extent survive to the crystal I phase. A model of molecular arrangements in the crystal I phase is proposed, and it explains the reduction of the dielectric increment observed on passing from the nematic phase to this phase.     

Detailed study of a two component smectic 4TPB-8OCB system with a nematic gap. Phase diagram and viscosity study

The phase diagram of a two component system composed of two smectic compounds: 4-octyloxy-4'-cyanobiphenyl (8OCB) and 4-isothiocyanatophenyl 4-butylbenzoate (4TPB) was investigated. Three Miesowicz viscosity coefficients η₁, η₂, η₃ and the refractive indices at different temperatures as well as the enthalpies of the phase transitions were measured. It was stated, that the properties of the induced nematic phase, for example, the nematic phase existing between two smectic regions, are the same as the ones observed in the case of low viscosity nematic mesogens.     

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.     

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 triple fluorescence of benzanilide and the dielectric medium modulation of its competitive excitation

The fluorescence of the benzanilide molecule at 298 K is inferred to consist of three independent electronic transitions associated with the single ground-state molecular species. F₁ (λ_max340 nm), the normal fluorescence is observed weakly and is ascribed to an n,π*,-π,π* mixed state. F′₂ is ascribed to the proton-transfer imidol tautomer fluorescence (previously reported) with unresolved λ_max (inferred at ≈460 nm). F″₂ is ascribed to a charge-transfer state fluorescence to the ground state, and occurs as a resolved CT transition in tetrahydrofuran at λ_max 520 nm. Comparison of the spectra of N-methylbenzanilide exhibiting only F₁ and F″₂ (CT) permitted the analysis of the benzanilide spectra.     

Structure cristalline et conformation en solution aqueuse de la carnosine (β-alanyl-L-histidine)

Carnosine (β-alanyl-L-histidine) is a biologically active molecule involved in muscular metabolism. It crystallises in the C; space group with a = 24.725 Å b = 5,427 Å c = 8,004 Å β = 100,2° (Z = 4)

In the crystal, acid and basic groups are engaged in hydrogen bonds whose strength is evaluated through IR frequencies. Molecular conformation in the solid state is defined by τ₁ = /t-177° τ₂ = −38° φ = −96° ψ = +131° χ₁ = 181° χ₂₁ = 62°

NMR study of carnosine in aqueous solution indicates that rotation about CH₂-CH₂ is free and that the other angles take the following values: Ø −150° or −90° and X₁ = 165° or 315°. Infrared and Raman spectra suggest that τ₂ undergoes small changes when going from crystal to solution while ψ is close to +150°.     

Photomechanically induced phase transitions in ferroelectric liquid crystals

We demonstrate that the addition of small amounts of a novel azo-dye to a ferroelectric liquid crystal and illumination with low intensity (∼ 0.8 mW cm^-2) UV light can result in reversible, isothermal phase transitions and dramatic changes in the properties of the system. In particular we examine light induced transitions between the S*₁, and S*_C phases and the consequent photomechanical regulation of spontaneous polarization.