Solute induced liquid crystalline behaviour of a quaternary ammonium salt and its application to structure determination

A new liquid crystalline phase, induced by the addition of small amounts of a non-mesogenic solute (such as dimethyl sulphoxide or methyl iodide) to a quaternary ammonium salt, N -methyl- N, N, N-trioctadecylammonium iodide (MTAI), has been detected by NMR and optical microscopic studies. In some cases, there is a coexistence of nematic and smectic phases. Information on the ordering of the phases in the magnetic field of the spectrometer has been derived from NMR spectra of a dissolved molecule, 13C-enriched methyl iodide. The low order parameter of the pure thermotropic nematic phase of the salt provides firstorder spectra of the dissolved oriented molecules. Analyses of spectra of cis , cis -mucononitrile exemplifies the utility of the MTAI nematic phase in the determination of structural parameters of the solute.     

Solute-Solvent Interactions in the Nematic and Smectic A Phases of 4 4-di-n-Heptylazoxybenzene Studied by Deuteron NMR

The orientational order parameters have been measured by deuteron N.M.R. of both solute (at low dilution) and solvent in various binary mixtures involving the liquid crystal 4,4'-di-n-heptylazoxybenzene (HAB). The solutes studied were azoxybenzene-d₁₀ and n-heptylbenzene-d₇ which are fragments of HAB, azobenzene-d₁₀ because of its similarity to azoxybenzene, and anthracene-d₁₀ because of its known structure and symmetric shape. The major and biaxial order parameters of the solutes are analysed in terms of a molecular field model for the potential of mean torque for biaxial particles. The behaviour of the solute order parameters on approaching and entering the smectic A phase is interpreted in terms of a temperature and phase dependent partitioning of the solute between aromatic and aliphatic regions of the solvent.     

NMR determination of smectic ordering of probe molecules

The NMR spectra of the three solutes ortho-, meta-, and para-dichlorobenzene in the nematic and smectic A phases of the liquid crystals 8CB and 8OCB are analyzed to yield two orientational order parameters for each solute. Extrapolation of the asymmetry in the energy parameters that describe the orientational ordering in the nematic phase are used to provide estimates of the strength of the nematic potential in the smectic A phase. The experimentally determined asymmetry of the orientational order parameters in the smectic A phase is then used in conjunction with Kobayashi-McMillan theory applied to solutes to give information about the smectic A layering and the nematic/smectic A coupling. In both smectic A solvents, the solute smectic coupling constant, tau, is negative (with the origin fixed at the center of the smectic layer) for all solutes. The signs and relative values of tau indicate that the ortho and para solutes favor the interlayer region while the meta solute is more evenly distributed throughout the layers.     

A microscopic model considering the effect of molecular biaxiality on ferroelectric ordering of liquid crystalline smectic C* phases

On the basis of a mean-field approach, a microscopic model is presented to explain some ferroelectric properties of induced smectic C* phases of chiral dipolar guest molecules in an achiral smectic C host phase. The molecular biaxiality of the chiral dopants has been taken into account, resulting in a polar and quadrupolar ordering of the molecular short axes in the rotational distribution function of the guest molecules. This model explains the dependence of the magnitude and sign of the spontaneous polarization on the molecular structure of a series of cyclo-hexanone derivatives used as chiral dopants, as well as the effect of a local field at higher dopant concentrations.     

Induced smectic phases in phase diagrams of binary nematic liquid crystal mixtures

To elucidate induced smectic A and smectic B phases in binary nematic liquid crystal mixtures, a generalized thermodynamic model has been developed in the framework of a combined Flory-Huggins free energy for isotropic mixing, Maier-Saupe free energy for orientational ordering, McMillan free energy for smectic ordering, Chandrasekhar-Clark free energy for hexagonal ordering, and phase field free energy for crystal solidification. Although nematic constituents have no smectic phase, the complexation between these constituent liquid crystal molecules in their mixture resulted in a more stable ordered phase such as smectic A or B phases. Various phase transitions of crystal-smectic, smectic-nematic, and nematic-isotropic phases have been determined by minimizing the above combined free energies with respect to each order parameter of these mesophases. By changing the strengths of anisotropic interaction and hexagonal interaction parameters, the present model captures the induced smectic A or smectic B phases of the binary nematic mixtures. Of particular importance is the fact that the calculated phase diagrams show remarkable agreement with the experimental phase diagrams of binary nematic liquid crystal mixtures involving induced smectic A or induced smectic B phase.     

De Vries smectic A phase formed by a liquid crystal side chain copolymer? A 13C NMR study

We study the orientation and order parameter of a liquid crystalline random side chain copolymer by ¹³C NMR. Evidence has previously been presented that this material forms a de Vries smectic A phase. The NMR data show that the molecular tilt angle in the smectic A phase is very small or zero and the smectic A–smectic C* transition is attributed predominantly to a change of the molecular tilt rather than azimuthal ordering. We discuss the NMR results in the context of earlier X‐ray and elastic characterizations of structurally similar materials.     

Local high-density distributions of phospholipids induced by the nucleation and growth of smectic liquid crystals at the interface

Amphiphilic molecules adsorbed at the interface could control the orientation of liquid crystals (LCs) while LCs in turn could influence the distributions of amphiphilic molecules. The studies on the interactions between liquid crystals and amphiphilic molecules at the interface are important for the development of molecular sensors. In this paper, we demonstrate that the development of smectic LC ordering from isotropic at the LC/water interface could induce local high-density distributions of amphiphilic phospholipids. Mixtures of liquid crystals and phospholipids in chloroform are first emulsified in water. By fluorescently labeling the phospholipids adsorbed at the interface, their distributions are visualized under fluorescent confocal microscope. Interestingly, local high-density distributions of phospholipids showing a high fluorescent intensity are observed on the surface of LC droplets. Investigations on the correlation between phospholipid density, surface tension and smectic LC ordering suggest that when domains of smectic LC layers nucleate and grow from isotropic at the LC/water interface as chloroform slowly evaporates at room temperature, phospholipids transition from liquid-expanded to liquid-condensed phases in response to the smectic ordering, which induces a higher surface tension at the interface. The results will provide an important insight into the interactions between liquid crystals and amphiphilic molecules at the interface.     

De Vries smectic A phase formed by a liquid crystal side chain copolymer? A 13C NMR study

We study the orientation and order parameter of a liquid crystalline random side chain copolymer by ¹³C NMR. Evidence has previously been presented that this material forms a de Vries smectic A phase. The NMR data show that the molecular tilt angle in the smectic A phase is very small or zero and the smectic A-smectic C* transition is attributed predominantly to a change of the molecular tilt rather than azimuthal ordering. We discuss the NMR results in the context of earlier X-ray and elastic characterizations of structurally similar materials.     

EPR anisotropy induced by a magnetic field in the smectic A phase of a mesogenic Cu(II) complex derived from a Schiff's base

The Q-band electron paramagnetic resonance (EPR) spectrum of the frozen smectic phase of the mesogenic compound bis[N -(4'-n-pentoxyphenyl)-4-n-decyloxysalicylaldiminate] copper(II) is studied. An anisotropic EPR spectrum is observed if freezing from the SmA phase is done in the presence of a magnetic field; the sample tends to orient with the director perpendicular to the magnetic field. The relative intensities of the EPR signal of this exchange coupled system are discussed in relation to the orientational distribution of the molecules. The results appear to be compatible with a biaxial magnetic susceptibility in the SmA phase. This suggests that an ordering of the short axes of the molecules in the plane perpendicular to the director in the SmA mesophase may take place.     

Stochastic model for the reorientation of molecules around their long axes in a smectic C phase

In smectic phases molecules can reorientate around their long and short axes. The motion of a molecule around its long axis is non-cooperative and influenced by both stochastic and deterministic forces exerted by neighbouring molecules. Solving the Smoluchovsky equation for such a reorientation process, two-time correlation functions are calculated which are related to frequency-dependent susceptibilities. The results are used for investigating how deterministic forces in higher ordered smectic phases have an effect on microwave spectra. It can be concluded that the quadrupolar ordering produces a splitting of the relaxation frequency. This splitting, which should occur at the transition from the smectic A to the smectic C phase, is only weakly influenced by an additional dipolar (ferroelectric) ordering.     

CdSe quantum dots in chiral smectic C matrix: experimental evidence of smectic layer distortion by small and wide angle X-ray scattering and subsequent effect on electro-optical parameters

CdSe quantum dots (QDs) dispersed ferroelectric liquid crystal (FLC) has been subjected to small and wide-angle X-ray scattering and atomic force microscopy to understand the molecular organization in chiral smectic C (SmC*) phase. SAXS indicates that the presence of QDs causes enhancement in the smectic layer separation. The smectic order parameter for neat FLC and FLC–QDs mixtures is obtained in the range of 0.6 to 0.85. Both smectic order parameter and structural tilt are found to be lesser for FLC–QDs mixtures as compared to neat FLC. The insertion of QDs in SmC* matrix causes localized smectic layer distortion in such a way that spontaneous polarization remains almost the same but the electro-optic switching of molecules becomes faster. We have outlined the superiority of FLC–QDs mixtures for electrical energy storage and their suitability in electronic devices.     

Liquid crystalline materials with complex mesophase morphologies

Liquid crystals, which combine order and mobility on a molecular and supramolecular level are increasingly accepted as a fourth state of matter. Besides the well-established nematic, smectic and columnar mesophases, more complex mesophase morphologies attracted increasing interest during the recent years. These are bicontinuous and discontinuous cubic mesophases and other two- and three-dimensionally ordered intermediate phases, superstructures induced by molecular chirality or by polar order of bent core molecules, novel biaxial smectic phases, and novel mesophase morphologies of polyphilic block molecules and dendrimers.     

Solute–solvent interactions of acid–1,4-dioxane mixtures—By dielectric, FTIR, UV–vis and 13C NMR spectrometric methods

Results of the dielectric studies carried out on the binary mixture of n-butyric and caprylic acids with 1,4-dioxane over the entire composition range and at temperatures 303 K, 308 K, 313 K and 318 K, and FTIR, UV–vis and 13C NMR spectral studies are presented in this paper. The excess permittivity and excess free energy were fitted with the Redlich–Kister polynomial. The variation of Kirkwood correlation factors, excess permittivity and excess free energy of mixing with the concentration and temperature has been investigated in view of understanding the ordering of dipoles of solute and solvent molecules. The FTIR, UV–vis and 13C NMR spectral analysis reveals the formation of complex between solute and solvent molecules. The parallel alignment of electric dipoles of the complex predicted by dielectric studies is well supported by UV–vis spectral analysis. The structure of the complex molecule present in the clusters has been deduced.     

Biaxial smectic phases in non-linear optical properties and phase behaviour of an oxdiazole liquid crystal

A symmetric, low-molecular-mass liquid crystal based on the oxadiazole ring was synthesized and characterized. The symmetric position of the heterocyclic group in the centre of the liquid crystal, bis(4-hexyloxyphenyl)4,4-(1,3,4-oxadiazole-2,5-diyl)dicarboxylate, yields a very rigid and non-linear mesogenic core. Despite this non-linear structure, a broad liquid crystalline range with a smectic C and a smectic A phase was found. Conoscopic experiments on freely suspended films revealed the existence of two optical axes in the smectic A phase, indicating a phase symmetry anticipated for either a 'McMillan' biaxial smectic phase or a biaxial (achiral) ferroelectric smectic phase.     

Liquid crystal phases of achiral banana-shaped molecules: a computer simulation study

The phase behaviour of achiral banana-shaped molecules was studied by computer simulation. The banana-shaped molecules were described by model intermolecular interactions based on the Gay-Berne potential. The characteristic molecular structure was considered by joining two calamitic Gay-Berne particles through a bond to form a biaxial molecule of point symmetry group C 2v with a suitable bending angle. The dependence on temperature of systems of N =1024 rigid banana-shaped molecules with bending angle ϕ=140° has been studied by means of Monte Carlo simulations in the isobaric-isothermal ensemble ( NpT ). On cooling an isotropic system, two phase transitions characterized by phase transition enthalpy, entropy and relative volume change have been observed. For the first time by computer simulation of a many-particle system of banana-shaped molecules, at low temperature an untilted smectic phase showing a global phase biaxiality and a spontaneous local polarization in the layers, i.e. a local polar arrangement of the steric dipoles, with an antiferroelectric-like superstructure could be proven, a phase structure which recently has been discovered experimentally. Additionally, at intermediate temperature a nematic-like phase has been proved, whereas close to the transition to the smectic phase hints of a spontaneous achiral symmetry breaking have been determined. Here, in the absence of a layered structure a helical superstructure has been formed. All phases have been characterized by visual representations of selected configurations, scalar and pseudoscalar correlation functions, and order parameters.     

Studying Molecular Rearrangement of P1 Dye at a Passivating Alumina Surface Using Vibrational Sum-Frequency Generation Spectroscopy: Effect of Atomic-Level Roughness

The effect of roughness and thickness of alumina layers, mimicking the passivation layer commonly used in dye-sensitized photoelectrodes, on the molecular adsorption of P1 dye, 4-(bi(4-(2,2-dicyano-vinyl)-thiophene-2-yl]-phenyl]-aminobenzoic acid) has been studied using surface-sensitive vibrational sum frequency generation(VSFG) spectroscopy. The VSFG spectra reveal the formation of poorly ordered dye layers on relatively rough surfaces where XPS measures a higher dye loading. Furthermore, these poorly ordered dye molecules are responsible for the generation of trapped electronic states as probed by successive photoluminescence (PL) measurements. Surface sensitive VSFG spectroscopy in combination with XPS and PL measurements provide complementary spectral information on ordering of the adsorbed dyes, their density on the surface and electronic states of the adsorbed monolayer which are prerequisite for improving our understanding of molecularly functionalized photoelectrodes and their further development.     

Carborane-containing liquid crystals: a comparison of 4-octyloxy-4-(12-pentyl-1,12-dicarbadodecaboran-1-yl)biphenyl with its hydrocarbon analogues

4-Octyloxy-4-(12-pentyl-1,12-dicarbadodecaboran-1-yl)biphenyl ( 1BC ) has been synthesized along with three hydrocarbon analogues in which the 1,12-dicarbadodecaborane is replaced by a phenyl ( 1PH ), trans -cyclohexyl ( 1CH ) or bicyclo\[2.2.2]octyl ( 1BO ) ring. The mesogenic properties of these materials have been compared and contrasted in both their pure states and as binary mixtures. The binary phase diagrams for the liquid crystal 1BC , with its hydrocarbon analogues 1CH and 1BO exhibit excellent miscibility of the smectic A phase while the more highly ordered smectic phases (SmB and SmE) for the hydrocarbons are suppressed by 1BC . In contrast the binary mixture of 1BC with the terphenyl analog ( 1PH ) exhibits complex behaviour in which the thermal stability of the smectic E phase is enhanced. X-ray diffraction data for the 1PH - 1BC binary mixture suggest a strong in-plane molecular ordering which might be attributed to intermolecular associations stabilizing the smectic E phase in preference to other smectic modifications.     

Molecular field theory for polar,biaxial bent-core nematics

We have studied a polar, biaxial nematic liquid crystal formed from bent-core molecules using molecular field theory. The model includes a simple Heisenberg-form dipolar intermolecular interaction in addition to the usual quadrupolar nematic interaction, and mimics a system consisting of nematogenic bent-core molecules with a large transverse dipole along the bisector of the two molecular arms. Such systems are regarded as good candidates for biaxial nematic liquid crystals. In principle, the molecular dipoles can align, thus stabilising the ordering of the minor axes. Our calculations predict that, for suitable values of the bent-core interarm angle, the biaxial nematic phase can be stabilised at higher temperatures than in the absence of the transverse dipole. In general, the transverse macroscopic polar order stabilises the biaxial nematic phase. In particular, for a large enough dipolar interaction, the Landau point in the pure biaxial nematic develops into a line of first-order polar biaxial nematic-to-isotropic phase transitions.     

Dielectric properties of a highly polar liquid crystalline material showing various types of layered structures

Liquid crystal compound N-(4-n-pentyloxybenzalidene) 4′-n-pentylaniline is very interesting because of its rich phase variant, i.e. nematic, smectic A, smectic C, smectic F and smectic G phases. In the present work, temperature- and frequency-dependent dielectric studies on this compound in the homeotropic and planar orientation of molecules have been carried out in the frequency range of 1 Hz–10 MHz. The compound shows negative dielectric anisotropy (i.e. Δε < 0). Dielectric studies have shown three relaxation modes of different origin in various phases of the material. Relaxation frequencies of the observed modes follow Arrhenius behaviour. The relaxation due to interfacial polarisation, i.e. Maxwell–Wagner–Sillars effect has been observed in the nematic and isotropic phases. A relaxation mode due to the fluctuation of molecular tilt resembling the soft mode has been detected in the smectic A phase with pre- and post-transitional effects. Another mode due to the fluctuation of molecules about their short axes has been detected in the nematic and smectic A, C, F and G phases. Activation energies and ionic conductivity of these phases have also been determined.     

Biaxial smectic A liquid crystal in a pure compound

The synthesis and characterisation is reported of a low molecular weight organic compound to exhibit the biaxial smectic A (SmAb) phase, which shows a transition from the partial bilayer uniaxial SmAd phase to the SmAdb phase as the temperature is lowered.