Insights into Biaxial Ordering of Bent-Core Nematics: X-Ray Diffraction Evidence

Recently we have reported the first X-ray diffraction evidence of biaxial order in the fluid cybotactic nematic phase of a pair of trimethylated bent-core mesogens. This evidence is based on the splitting of the wide-angle diffuse scattering in the plane normal to the nematic director. The additional experimental data presented here provide further insights into this unprecedented behavior observed over the entire nematic phase. Although we could not determine the spatial extent of biaxial order—intracluster or intercluster—our observations reveal an enhanced biaxial orientational correlation in the transverse molecular packing, possibly reflecting stronger anisotropic interactions between nearest-neighbor mesogens.     

Biaxial Ordering of Deuterochloroform in a Smectic G Phase

The biaxial symmetry of the low temperature smectic phase of N-(4-n-hexyloxybenzylidene)-4-n-hexylaniline is demonstrated through observation of the deuterium NMR resonance of CDCl₃ probe molecules dissolved in this phase. The biaxial ordering is revealed in the observed powder spectrum as well as in a complete rotation study of a uniformly aligned sample.     

Conoscopic image of a biaxial nematic phase in a sodium decyl sulfate – decanol – D2O mixture

ABSTRACT

The nematic phases of a lyotropic system NadS/ decanol/ heavy water are investigated using optical conoscopy and image processing. The phase diagram obtained from these lyotropic materials predicts the occurrence of a direct phase transition, which does not present the biaxial nematic phase, between the discotic (N_D) and calamitic (N_C) nematic phases. A biaxial nematic (N_B) phase is optically characterized and confirmed through conoscopic image, inside the biaxial range, between the two uniaxial nematic phases. Also, their respective transition points are determined by means of image processing. The N_B phase observed here is discussed as part of the nature of the micellar configuration of lyotropic materials which exhibit uniaxial nematic phases.     

Induction of B1, B6 and Biaxial Smectic A Phases in Binary Mixtures of Compounds with Rod-Like and Bent-Core Molecules

Abstract

We have studied a binary mixture of compounds, one of which has rod-like molecules and the other bent-core molecules. The rod-like molecules are biphilic in nature with a long alkoxy chain attached to an aromatic moeity only at one end, and exhibit the bilayer SmA₂ phase, even though they do not have the highly polar cyano or nitro groups. The compound with bent-core molecules exhibit the B₂ phase which has polarized layers with tilted molecules. Three liquid crystalline phases are induced in the binary mixtures, depending on the concentration: the 2-dimensionally ordered B₁ phase for compositions with 15 to 63 mol% of the rod-like molecules and the B₆ phase for 63 to 87 mol%. The sequence obtained on increasing the concentration of rod-like molecules is the same as that seen on shortening the chain length of the BC molecules. Between 87 to 95.5 mol% of the rod-like molecules, a new biaxial smectic A₂ (SmA_2b ) phase is induced. On the basis of several observations, we argue that it corresponds to a structure in which the BC molecules are reoriented with their arrow directions pointed along the layer normal. The SmA₂ to SmA_2b transition corresponds to an orientational transition of the bent-core molecules in the anisotropic SmA₂ background medium.     

Un Organométallique Disquogěne Thermotrope

The first thermotrophic organometallic discogen is synthesized: the bis(p-n-decyl benzoyl) methanato copper (II) These molecules having a 2/m symmetry form a stable discophase with binary or tetragonal symmetry. An intermediate of the synthesis. the bis(p-n décyl benzoyl) methane has an optically biaxial stable mesophase: the clearing enthalpy is greater than he melting enthalpy.     

Field-Induced Order in Nematic Liquid Crystals

The effect of an external magnetic field on the orientational order of a nematic liquid crystal has been examined using both Landau-de Gennes and Maier-Saupe theories. In the Maier-Saupe approach a rotationally invariant form of the pseudo-potential is introduced, which in the absence of an external field leads to three degenerate isomorphic solutions for the order parameter, corresponding to alignment along three principal axes; a similar result is obtainable from the Landau-de Gennes theory. Application of a magnetic field lifts the degeneracy of these solutions, and for materials having a positive diamagnetic susceptibility anisotropy, the uniaxial solution with alignment along the field direction is always energetically favorable. For materials with a negative susceptibility anisotropy, a biaxial solution minimizes the free energy at low temperatures, but on increasing the temperature there is a transition from a biaxial phase to an uniaxial phase. The field dependence of the transition temperatures is evaluated, and for positive materials there is a critical field, corresponding to a second order transition above which the nematic and isotropic phases are indistinguishable. A contrasting behavior is predicted for negative materials, and above a certain critical field the biaxial/uniaxial transition changes from first order to second order. For weakly ordered systems it is shown that the Landau-de Gennes expression for the free energy is identical to that obtained from the Maier-Saupe theory. However, for more ordered systems, the results of the two approaches differ, and in particular the Maier-Saupe theory predicts a susceptibility divergence temperature T* which increases with applied field, in agreement with recent experiments.     

Landau Theory of Biaxial Nematic Liquid Crystals

The orientational order of molecules in a liquid crystal is traditionally described by spatial or temporal averages of functions that are quadratic in the direction cosines of molecular axes with respect to laboratory fixed axes. The free energy describing the nematic phase must be independent of the choice of coordinate systems and therefore rotationally invariant with respect to both laboratory and molecular frames. A Landau theory for biaxial and uniaxial nematics based on six fundamental invariants is developed which is properly rotationally invariant. Four possible nematic phases, two uniaxial and two biaxial, result. NMR experiments should be able to identify which of these actually occur in specific materials.     

The dawn in the area of thermotropic biaxial nematics Symposium on Biaxial Liquid Crystals,Kent, 27–28 October 2008

Thermotropic biaxial nematic phases of low molar mass mesogens are highly sougth materials not only for liquid crystal display technology. Recent progress in molecular design, synthesis, experimental methods to study biaxiality and theory gave occassion to organise a symposium on biaxial liquid crystals in Kent. The stimulating interdisplinary discussion shows that this challanging field of research is in his dawn.     

Chiral Gay–Berne model for molecular dynamics computer simulations

Abstract

We have extended the chiral Gay–Berne (GB) pair potential of Memmer and co–workers [Liq. Cryst. (15), 345 (1993)] to the general case of biaxial ellipsoids, and we provide explicit expressions for the gradient and the torques to be used in molecular dynamics (MD) codes. To test our results against published results we have performed MD simulations of samples formed by N?=?1024 uniaxial GB ellipsoids with constant volume, and we have studied how the thermotropic behavior is affected by particle chirality. We have run temperature scans starting from isotropic samples and found clear signatures of the spontaneous formation of both lower temperature blue and cholesteric phases.     

Oriented Thin Films of the Low-Band-Gap Polymer PTB7 by Friction Transfer Method

Low-band-gap polymers are promising materials for organic photovoltaic application. We prepared an oriented thin film of a low-band-gap polymer, PTB7, by friction transfer method. The oriented film showed a strong dichroic absorption in the whole visible range. The orientation of molecular chain was characterized by two-dimensional grazing incident X-ray diffraction (2D-GIXD) with synchrotron radiation. 2D-GIXD results showed PTB7 molecular plane parallel to the substrate plane, i.e., “face on orientation.”     

Phase Diagram Behaviors for Rod/Plate Liquid Crystal Mixtures

We present a simple theory of rod/plate liquid-crystal mixtures in which the angle-dependent pair interactions are assumed to be of second-rank form. Calculations are reported for varying relative anisotropies of the “rods” and “plates”. Temperature vs. mole-fraction phase diagrams show successive isotropic (I) → uniaxial ( U) and U → biaxial ( B) transitions which are first- and second-order, respectively. For each pair of species there exists a special composition ( x _rod*) for which cooling of the isotropic phase leads directly (and continuously) to a biaxial liquid. As X _rod approaches X _rod* from either side, the first-orderness of the I → U transition (i.e. discontinuities in volume, order parameter, etc.) becomes vanishingly small. Furthermore the transition temperature of the rod-(plate-) solvent is found to be depressed when “doped” by not-too-anisotropic plates (rods) and elevated when doped by sufficiently anisotropic plates (rods). These behaviors are explained in terms of simple excluded volume considerations and compared with recent experimental data on rod/plate mixtures.     

Investigation of the Homologous Series of 4-Acetyl-4′-n-Alkanoyloxyazobenzenes by X-ray Diffraction

This paper reports an investigation by X-ray diffraction of the homologous series of liquid crystalline 4-acetyl-4′-n-alkanoyloxyazobenzenes. By using two independent parameters, namely the temperature and the paraffinic chain length, it was possible to obtain some structural information on two solid phases for comparison with a previously reported model for the S_A phase of the same series. In particular, the molecular layer thickness and, by using previously reported dilatometric data, the aromatic sublayer thickness, the methylene group thickness, the molecular area and the aromatic stem inclination angle were obtained for the different phases.     

An X – Ray Study of the p-n-Alkoxycinnamic Acids. Part III.1

The effect upon molecular packing of increasing chain length in a homologous series of mesogens has been examined in the case of the p-n-alkoxycinnamic acids by determination of the crystal structures of the four nematogenic acids having two, four, six, and eight carbon atoms in the alkyl chain. The chains have the all-trans extended conformation, and the molecules exist in the crystal as planar hydrogen-bonded dimers. The dimers are arranged in end-to end fashion in parallel rows. Similar side-to-side packing of pairs of dimmers is found in each crystal structure giving a good fit between adjacent aromatic cores, but the amplitudes of thermal vibration of the chain carbon atoms increase markedly with increasing chain length, indicating a low packing efficiency for these moieties.     

Structure and molecular properties of (1)-Centbutindole. Comparison with Haloperidol

The structure and conformation of (1)-Centbutindole, a newly marketed neuroleptic compound, has been investigated by X-ray crystallography. It crystallizes in the monoclinic system and the non-centrosymmetric space group P2₁ (Z=2) with cell dimensionsa=8.434(6),b=6.620(3),c=18.419(9)Å, and =95.07(6)°. The chain conformation istrans extended. The embedded ³ piperidine ring exists in ahalf-chair conformation whereas the embedded piperazine ring exists in achair conformation. The propylene side chain is equatorial relative to the piperazine. A systematic conformational analysis of centbutindole and of a related molecule, Haloperidol, followed by a Monte Carlo search and a stochastic dynamics simulation, have been performed. Electronic and lipophilic properties have been computed and, molecular electrostatic potential (MEP) maps and molecular lipophilicity potential (MLP) maps, have been displayed for two selected conformers satisfying a reported pharmacophore. The two molecules exhibit very similar molecular properties.     

Remarks on the Proton NMR of Di-Heptyloxyazoxybenzene in the Smectic-C Phase

It is suggested that the ideal smectic-C reorientation model be modified to include both some rotational slipping of the smectic-C domains and hindered reorientation of a small percentage of molecules to account for the behaviour of proton NMR in smectic-C di-heptyloxyazoxybenzene. The calculated poly-domain line shapes are rather insensitive to the characteristic biaxial order parameter predicted by molecular models of the smectic-C phase.     

Local Field Anisotropy in Nematic Liquid Crystals

The anisotropy of the local electric field in the interior of ordered fluids provides a measure of the anisotropy of the molecular distribution. In nematics, both the orientational order parameter and the anisotropy of the local field can be calculated from refractive index and density measurements; results are presented for the liquid crystal butyl p-(p-ethoxyphenoxycarbonyl) phenyl carbonate. It is found that the anisotropy of the local field is very nearly proportional to the orientational order parameter. A simple model incorporating steric intermolecular repulsion predicts this behaviour. The ratio of the local field anisotropy to the uniaxial and biaxial order parameters is simply related to the length-to-breadth ratio of the effective molecular hard core.     

Effect of Terminal Groups on Formation of Crystal Structure in α,γ-Disubstituted n-Alkanes

The crystal and molecular structure of 1,16-dibromohexadecane have been determined by X-ray diffraction. Crystal data; empirical formula, C₁₆H₃₂Br₂, formula weight M_r = 384.24, monoclinic, P2₁/a, a = 31.00(2), b = 5.38(1), c = 5.48(1) Å, β = 90.8(2)°, V = 914(3) ų, Z = 2, D_x = 1.396 g/cm³, δ(CuKα) = 1.5418 Å, μ(CuKα) = 55.41 cm⁻¹, F(000) = 396.00, T = 293 ± 1 K, wR = 0.082, R = 0.054 for 940 unique reflections [I > 3.00σ(I)]. The molecular skeleton has an all trans zigzag hydrocarbon chain. The crystal has a layer structure, but the directions of molecular axes in neighbouring layers alternate. This molecular packing resembles to that of 1,16-hexadecanediol, whereas n-alkanes or α-monosubstituted n-alkanes do not appreciably have these molecular arrangement. The effect of terminal groups at both ends of α,ω-disubstituted n-alkanes on the crystal structure is discussed.     

Molecular and crystal structure of <Emphasis Type="Italic">n</Emphasis>-hexyloxybenzoic anhydride at low and room temperatures

The crystal and molecular structures of n-hexyloxybenzoic anhydride, C₆H₁₃-O-C₆H₄-C(O)-O-C(O)-C₆H₄-C₆H₁₃, at low (120 K) and room (296 K) temperatures have been investigated. The molecule has an asymmetric bent structure. The dihedral angle between the benzene ring planes is 48.5°. The aliphatic chain on one side of the molecule has a transoid orientation with respect to the “internal” C4 atom of the closest benzene ring, whereas the aliphatic chain on the other side has a cissoid orientation with respect to the analogous C(4A) atom. The crystal packing does not exhibit any pronounced separation of the crystal space into closely packed aromatic or loosely packed aliphatic regions. No weak directional interactions are observed in the packing; this fact explains the absence of liquid-crystal properties for this compound.     

Computational analysis of ordering in non-liquid crystalline versus liquid crystalline materials with special reference to <Emphasis Type="Italic">n</Emphasis>BAC

A computational analysis of ordering in non-liquid crystalline p-n-alkyl benzoic acid, having 1 (1BAC), 2 (2BAC) and 3(3BAC) carbon atoms in the alkyl chain has been carried out with respect to translatory and orientational motions, but detailed results are reported only for 3BAC. The evaluation of net atomic charges and dipole moments at each atomic center has been carried out using complete neglect differential overlap (CNDO/2) method. The modified Rayleigh-Schrodinger perturbation theory along with the multicentered-multipole expansion method has been employed to evaluate long-range interactions, while a “6-exp” potential function has been assumed for short-range interactions. On the basis of stacking, in-plane and terminal interaction energy calculations, all possible arrangements of a molecular pair have been considered. A comparative picture of molecular parameters, such as total energy, binding energy, and total dipole moment of 3BAC with higher homologous series liquid crystalline compounds having 4(4BAC), 5(5BAC), and 6(6BAC) alkyl chain carbon atoms, has been given. It is found that, if a suitable functional group is attached to 3BAC, so that the length to breadth ratio is increased, the molecule will show a change in the long-range order, the phase transition temperature and other liquid crystalline properties.     

Chiral phases in a system of biaxial molecules

Chiral phases in a system of biaxial molecules are investigated based on a theory of biaxial liquid crystals, which we have presented in past work. Four chiral phases are identified, corresponding to a uniaxial nematic phase, two biaxial nematic phases, and a discotic phase, respectively. We find that in the chiral uniaxial nematic phase, the pitch of the chiral structures does not depend on temperature. In contrast, in the two chiral biaxial nematic phases, as well as in the chiral discotic phase, the pitch is temperature-dependent.