The Orientational Order of a Re-entrant Nematic Phase. an Electron Resonance Investigation

We have investigated the orientational order in a re-entrant nematic and its preceding phases using steroidal and alkyl nitroxide spin probes. The order parameter for the steroidal spin probe in the re-entrant nematic phase is found to be an extension of its value in the preceding smectic A phase. In contrast the gradient of the order parameter for the alkyl spin probe exhibits a marked discontinuity at the smectic A - re-entrant nematic transition.     

Order Parameters and Distribution Function of 4'-(N-(p-Pentyloxy Carbonyloxy-Benzylidene)Amino) Valerophenone in Smectic Phase from X-ray Diffraction Measurements

Abstract

X-ray diffraction studies of 4'-(N (p-pentyloxycarbonyloxy-benzylidene)amino)valerophenone (PCBAV in short) oriented by a magnetic field is reported here. The compound has a smectic A phase which is identified by X-ray studies and texture studies. Orienta-tional order parameters (p ₂) and (P ₄) have been determined from the calculation of f(β), the orientational distribution function. An approximate expression for the angular part of the potential of the mean field is obtained from the temperature variation of the orientational distribution function. The value of d, the layer thickness, and D, the intermolecular distance, have been calculated. An approximate value for the translational order parameter τ₁ and average value of the root mean square displacement ?z²?½ are given. We could not get an accurate estimate of the translational order parameter due to the inaccuracy in the measurement of the intensity of the inner ring.     

Cotton-Mouton Effect in Monodisperse Suspensions of Liquid Crystals

Abstract

The collective Cotton-Mouton (C-M) effect in the system of micron-size liquid-crystal droplets freely suspended in the isotropic liquid is studied experimentally and theoretically. The collective C-M constant in liquid-crystal suspensions is found to be from 10⁴ to 10⁸ times higher than the respective “molecular” C-M constant in the isotropic phase of the same substance. These large values cause the saturation in the C-M effect at relatively low magnetic fields. Using the generalized nth order Langevin's functions L, _n the statistical therory is presented for the collective C-M effect in the monodisperse liquid-crystal suspensions. As a result, the induced birefringence is obtained in the form Δn = 1/2(Δn)∞ (3L ₂ – 1), where (Δn)∞ is the induced birefringence at saturation. The collective C-M constant is C _c = CN _s S _g where N ₈ is the number of molecules in the droplet, S ₈ is the orientational order parameter of the long molecular axes in the droplet, and C is the “molecular” C-M constant. The resultant (measured) C-M constant is C ₈ = C(1 + N ₈ S ₈). The relaxation time τ depends on the rotational (Brownian) diffusion constant D and after removing the field B the orientational order parameter of the optical axes of droplets in the suspension S _s = 1/2(3L ₂ – 1) vanishes exponentially as S _s = exp(?6Dt). Hence, the relaxation time τ = (6D)^?1.

The collective C-M effect and its saturation can be used as methods for the analysis of the presence and change of the orientational order within colloidal grains and the derivation of several parameters characterizing the aggregation, self-organization, and morphogenesis processes in multicomponent complex systems.     

Refractive Indices,calculated orientational order Parameters,and Polarizability Anisotropies of Liquid Crystalline 3-(4-Alkyl-phenyloxy)-propane-1,2-diols

For a novel variant of smectic structures the S_A⁺ phase of the homologous 3-(4-alkyl-phenyloxy)-propane-1,2-diols the refractive indices n_e and n_o were measured in dependence on the temperatue. The polarizability anisotropy, the orientational order parameter, and the packing fraction were calculated using a mean field fitting method. The components of the internal field tensor were included in the fitting procedure. The increasing mobility of the alkyl chain for higher homologues reduces the orientational order in the smectic layers and also the birefringence which is very small. The molecular polarizabilities are increasing nearly linearly with the elongation of the alkyl chain. The small alternation of the polarizabilities corresponds to the alternating change of the length-to-breadth ratio. The calculated packing fractions are enhanced for higher homologues.     

Structure Effect of Solute on Orientational Order in Binary Mixture by 2H-NMR and Molecular Simulations

Abstract

The orientational order parameters of six solutes with different shapes and sizes dissolved in two nematic solvents have been measured as a function of temperature. The parameter, S^xx-S^yy is correlated with the molecular geometrical structure, while S^zz is affected by not only the geometry but also the anisotropy in the polarizability. The molecular dynamics simulations are carried out using a realistic atom-atom potential for evaluating the ordering properties in the nematic phase. The results provide reasonable values of S^zz corresponding to experimental ones for solutes and solvents, and also a prediction of uniaxiality for the nematic phase.     

Specific features of the substitution of Fe<Superscript>3+</Superscript> impurity ions for Zr<Superscript>4+</Superscript> in NaZr<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript> single crystals

The EPR spectra of Fe³⁺ impurity ions in NaZr₂(PO₄)₃ single crystals at 300 K are investigated, and the spin Hamiltonian of these ions is determined. A comparative analysis of the spin-Hamiltonian and crystal-field tensors is performed using the maximum invariant component method. It is demonstrated that Fe³⁺ impurity ions substitute for Zr⁴⁺ ions with local compensator ions located in cavities of the B type. It is revealed that the invariant of the spin-Hamiltonian tensor B₄ and the crystal-field tensor V ₄ ⁴⁴ depend substantially on the mutual arrangement of ions in the first and second coordination spheres. The corresponding dependences are analyzed.     

2H-NMR Spectral Patterns in Magneto-Aligned Smectic E and G Phases

Unique ²H-NMR spectral patterns can be obtained from biaxially ordered smectic E and G phases, where only one director of the phase is aligned by the magnetic field. Two effects on the shape of the deuterium spectral pattern are demonstrated: (1) The effect of a motionally induced asymmetry parameter in the time averaged deuterium quadrupole interaction, and (2) the effect when the principal z-axis of the time averaged interaction is not parallel to the aligned director. Spectral patterns for the S_G phase of the compound 4-n-pentoxybenzylidene-4-n'-heptylaniline selectively deuterated on one of the aromatic rings (50.7-d ₄) were obtained for both unaligned and magneto-aligned samples. Both of these spectral patterns were consistent with that expected for an asymmetry parameter, n < 0.2. Spectral patterns in the S_G phase of the compound 50.6 (24.7 wt.%) + 90.4-αd ₂-γd ₂ (75.3 wt.%) were studied showing a different behavior with respect to the spectral patterns obtained in 50.7. We believe that a shift in the orientation of the z principal axis relative to the sample c axis has occurred in this S_G phase. Spectral patterns were also recorded for the S_E phase in magneto-aligned samples of the mixture of compounds 4-cyano-4'-octyloxybiphenyl (80CB) with 50 wt.% 40.8-d ₄. These data also show a small asymmetry parameter but, in addition, a distribution in the orientation of the principal z-axis indicating that the time scale of the motion of the long molecular axis is in a regime where it is of the order of the ²H-NMR measurement. Models for the smectic E and G phases based on these observations are discussed.     

Thermal expansion of Sm-Tb orthoferrites in the region of the spin reorientation transition

A new model of spin reorientation is discussed on the basis of the analysis of experimental data on thermal expansion of samarium-terbium orthoferrite and the earlier studies of its magnetic and crystallooptical properties. During the orientational phase transitions, the zero values of the thermal and optical parameters of rare earth orthoferrites correspond to the alignment of the magnetization vector along the [110] crystallographic directions. For orthorhombic Sm-Tb orthoferrite crystals, these directions coincide with the twofold axes in the plane.     

Theory of Systems of RodIike Particles: II. Thermotropic systems with orientation-dependent interactions

Abstract

A system of rigid, inpenetrable, rodlike molecules subject to orientation-dependent mutual attractions is treated by extension of the theory presented in the preceding paper. This energy is formulated, for a system at constant volume, by considering interactions between pairs of segments in contact, rather than in terms of interactions between entire molecules. The orientation-dependent energy between a pair of segments is taken to be proportional to cos² Ψ_ij, where Ψ_ij is the angle between the principal axes of their polarizability tensors, assumed to be cylindrically symmetric with respect to the molecular axis. A characteristic temperature T? measures the intensity of these interactions. The orientational energy of the system as a whole is of the form derived by Maier and Saupe. The orientation distribution with respect to the domain axis and the partition function are formulated with T? and the axial ratio x as parameters, Steric effects of molecular shape asymmetry, embodied in x, are of foremost importance. The reduced temperature [Ttilde]_ni = T_ni/xT? at which the nematic-isotropic transition takes place in the neat liquid decreases with decrease in x below its athermal limit x_erit = 6.417 for [Ttilde]^–1 _ni = 0. Both the entropy difference between isotropic and nematic phases and the orientational heat capacity C^p, are monotonic through the transition; C_p, diverges at a temperature appreciably above T_nl, where the metastable anisotropic state becomes unstable. Comparison of theory with experiments on those nematogens whose molecules can be approximated by rigid rods lends encouragement to the prospect of relating characteristics of the nematic-isotropic transition to molecular structure.     

Structural characterization and magnetic properties of the spinel compound CoIn0.5Cr1.5S4

Single crystals of the magnetic semiconductor CoIn_0.5Cr_1.5S₄, belong to the system CoIn_(2‐2X)Cr_(2X)S₄ with x = 0.75, was grown by the chemical transport method. X‐ray powder diffraction characterization by the Rietveld method indicated that CoIn_0.5Cr_1.5S₄ crystallizes in the space group Fd‐3m, Z = 8, with a = 10.0700(6) Å and V = 1021.2(1) ų, in a normal spinel structure. The temperature dependence of the DC magnetization suggests that the studied compound presents a ferromagnetic behavior with a Curie temperature Tc = 220 K. Sharp spin‐glass like behavior was found also. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Anisotropic Atomic Displacement Parameters and Molecular Motions in Mesogenic Crystals

Abstract

In this paper we describe the results of vibrational analysis of molecules in the crystalline phase of three mesogenic compounds of the series n-alkyl cyclohexyl cyanophenyl pyrimidine from experimental atomic displacement parameters using the rigid body model of Cruickshank. The rigid body model has been applied so far for estimation of corrections in bond lengths arising from the librational modes of motion. We have undertaken this vibrational analysis to find out whether we can get any idea about the transition from crystalline to liquid crystalline phase. Two tensors T (translation) and L (libration) are used to describe the motion. These tensors have been diagonalized to obtain the principal axes and amplitudes of translational and librational motions.     

The Orientational Order in Cholesteric Liquid Crystals

The investigation of the orientational order in the mesophase of steroidal cholesterogens has been all but neglected by experimentalists, possibly because of the lack of an appropriate technique. However electron resonance spectroscopy can be used to determine the order parameter of a spin probe dissolved in a cholesteric mesophase. In addition the structure of the spin probe may be made to resemble that of the cholesteric solvent. Here this technique is employed to determine the orientational order for cholesteryl and cholestanyl benzoate, as a function of temperature. The results of these experiments yield order parameters which are comparable, or slightly smaller, than those typically observed for nematics. A comparison of these values with mean field theories suggests that the low order parameters of steroidal cholesterogens stems from deviation of the constituent molecules from cylindrical symmetry.     

Macroscopic properties and their temperature dependence of the mesogen 4 cyano-4/-n-nonyl biphenyl as a function of temperature

The thermal dependence of optical and dielectric properties of one member namely 4 cyano-4^/-n-nonyl biphenyl (code name 9CB) of the mesogenic homologous series of 4 cyano-4^/-n-alkyl biphenyl (nCBs) is reported. The variation of orientational order parameter with temperature is determined and comparison made with theoretical Maier–Saupe values as well as the order parameter values of the other member of the series. The effect of temperature on the effective molecular dipole moment μ and the angle of inclination β of the dipole axis with the preferred direction are investigated.     

Formation of thesyn-isomer of [Cp*MoO(μ−S)]2

The structure of the title compound,syn-[Cp^*MoO(μ?S)]₂,1, has been determined.1 crystallized in the monoclinic space groupC2/c with dimensionsa=11.758(2),b=12.243(2), c = 15.585(3)Å, β=104.63(3)°. The reaction of Cp^* ₂Mo₂(S₂)(S)(S₂O₃) with Me₂PhP producessyn-[Cp^*MoO(μ?S)]₂ and Me₂PhPO. The compounds [Cp′MoO(μ?S)]₂ [Cp′=Cp, CpMe, Cp^*] can exist as either thesyn- oranti-isomers. The present structure,syn-[Cp^*MoO(μ?S)]₂ is compared with the structure of theanti-isomer and with the previously determined structure ofsyn-[Cp^*MoO(μ?S)]₂ in a different space group.     

Phase transitions from density and order parameter from polarisabilities in symmetric dimeric liquid crystals

As a part of our systematic studies on liquid crystal dimers, we present in this article the nature of phase transitions across isotropic–nematic and nematic–smectic-A exhibited by DLCs, α,ω-bis-(4-n-alkylaniline benzylidene-4?-oxy) alkanes. Further, the orientational order parameter in the nematic phase of these DLCs are estimated from the molecular polarisabilities calculated using the experimental refractive indices and density results. The molecular polarisabilities α_e and α_o are obtained for the compounds using the above results for both Vuks and Neugebauer local field models applicable to nematic liquid crystal. α_e and α_o calculated in this way are used to obtain Δα. The polarisability anisotropy in the perfect order (absolute K) is calculated semi-empirically using the δ-function model developed by Lippincott et al. and molecular vibration method. The values of polarisability anisotropy for both local electric field models differ significantly. No criterion is known to decide which value is correct. To avoid the determination of uncertain α and Δα values considering different local field models, a simple procedure developed by Kuczynski et al. was used for evaluation of S, based solely on birefringence δn = (n_e-n_o) and this value of S is compared with those obtained from field models.     

Fractionally Charged Excitations in Quasi-One-Dimensional Systems

Recent theoretical studies have shown that in quasi-one-dimensional conductors having a Peierls distortion of commensurability index n (ratio of the distortion period to lattice specing), there exist excitations whose charge is Q = ω 2e/n or ω 2e/n ± e. These excitations are kinxs in the order parameter ψ describing the lattice distortion. In trans polyacetylene, n = 2 and Q = O, ± e with spin S = 1/2, O respectively. For n = 3 (e.g. TTF-TCNQ at 19Kb) Q = ± 2/3 e, ± 1/3 e, ω 4/3 e with spin S = O, 1/2, O respectively. Electronic states localized at the kink have energies in the Peierls gaps. Properties of these stable fractionally chareged objects are discussed.     

Synthesis and Liquid Crystal Behaviour of Further 4,4″-Disubstituted 2'-Fluoro-1,1':4',1”-Terphenyls

Further to earlier work on the liquid crystal properties of fluoro-1,1':4',1”-terphenyls¹ we have now extended this series to include terminally fluoro-and cyano-substituted 2'-fluoro-1,1':4',1″-terphenyls, chiral 2'-fluoro-1,1':4',1″-terphenyls, and esters derived from 2'-fluoro-1,1':4',1”-terphenyls and incorporating the alkylcyclobutyl group. The preparations and transition temperatures for these series of compounds are presented and their transition temperatures and mesophase types are discussed. An interesting result from this work was the appearance of an S^* _C phase for one chiral homologue and of S _c phases for the esters incorporating the alkylcyclobutyl group. The S _c formation is attributed to the presence of the fluoro-substituent.

Keywords: terphenyls, fluoroterphenyls, smectic C phases, cyclobutyl esters     

Synthesis,Structure, and Fluorescence of a Novel Cadmium Compound

A novel cadmium compound, [(CdI₃)₂(μ₂-I)]₂(Me₂-4,4'-H₂bipy)₃ (1) (Me = methyl; 4,4'-bipy = 4,4'-bipyridine) with the Me₂-4,4'-H₂bipy moiety generated in situ, has been prepared through a solvothermal reaction and structurally characterized by X-ray single crystal diffraction analysis. Compound 1 is characterized by an isolated structural motif, consisting of [(CdI₃)₂(μ₂-I)]^3– anions and Me₂-4,4'-H₂bipy²⁺ cations. The [(CdI₃)₂(μ₂-I)]^3– anions and Me₂-4,4'-H₂bipy²⁺ cations interconnect together via C–H···I hydrogen-bonding interactions to complete a three-dimensional (3D) supramolecular network. Solid state fluorescent spectrum reveals an emission band in the violet region.     

Prediction of Leslie Coefficients for Rodlike Polymer Nematics

Doi's molecular theory for the dynamics of rod-like polymers in concentrated solutions is here extended to include the effect of a molecular field. The resulting expression for the stress tensor is then linearized so as to obtain the Leslie coefficients in the nematic phase. Each of these coefficients takes the form of a basic viscosity - dependent on rod concen tration and rotational diffusion constant - times a different function of the equilibrium order parameter. The predicted ordering of the three Miesowicz viscosi ties is in agreement with the experimental evidence.