Effect of lateral substituents on the mesophases formed by some achiral banana-shaped molecules

Several compounds composed of banana-shaped molecules and exhibiting mesophases are reported. The effect of different lateral substituents on the type of mesophase formed is examined and a comparison made with unsubstituted compounds. The mesophases have been characterized using techniques such as polarizing optical microscopy, differential scanning calorimetry, and X-ray diffraction. The B₂ phase shows antiferroelectric switching behaviour.     

Effect of lateral substituents on the mesophases formed by some achiral banana-shaped molecules

Several compounds composed of banana-shaped molecules and exhibiting mesophases are reported. The effect of different lateral substituents on the type of mesophase formed is examined and a comparison made with unsubstituted compounds. The mesophases have been characterized using techniques such as polarizing optical microscopy, differential scanning calorimetry, and X-ray diffraction. The B₂ phase shows antiferroelectric switching behaviour.     

Phases and phase transitions in a nematic lyotropic system with a biaxial phase

NMR spectroscopy and optical microscopy have been used to study phase transitions and structure in nematic lyotropic mesophases formed by potassium laurate, decylammonium hydrochloride and water. The different mesophases obtained in a well-defined composition range have been characterized, by deuterium NMR, following the evolution of the D₂O spectra as a function of temperature and of the orientation of the samples with respect to the magnetic field. Wide ranges of biaxiality have been found and the asymmetry parameter of the averaged electrical field gradient tensor on the deuterium of the D₂O molecule has been determined. The presence of the different mesophases has always been confirmed by observing oriented samples under the polarizing optical microscope.     

Elliptical diffusion of dye in hexagonal columnar polycatenar mesophases

We have studied polycatenar compounds which exhibit hexagonal columnar mesophases. In the planar orientation of these mesophases, the elliptical diffusion of the dissolved dyes is visualized by taking several pictures of the sample. The diffusion ratio D// /D is deduced for these columnar mesophases. Furthermore, using a classical optical absorption technique, we present measurements of dye diffusion in the same mesophases. The diffusion constants are measured in two geometries, along and perpendicular to the columns. The diffusion anisotropy ratios are in agreement with those deduced from the ellipse axes. The structure of these new columnar mesophases exhibited by rod-like mesogens is compared with that of disk-like mesogens.     

Enhancement of the stability and temperature range of the mesophases in the binary mixtures of cholesteryl myristate and 4-n-decyloxy benzoic acid

Thermodynamical, optical, and electrical properties of the binary mixtures of cholesteryl myristate (ChM) and 4-n-decyloxybenzoic acid (DOBA) have been carried out by the differential scanning calorimeter, polarized optical microscopy (POM), and impedance spectroscopy. Through thermodynamic study, various phase transition temperatures, transition enthalpies, and transition entropies have been determined to investigate temperature range and stability of the mesophases of the mixtures. Phase diagrams in the heating and cooling cycles have been drawn for the ChM–DOBA binary system. Optical textures of different mesophases have identified with the help of POM. Dielectric permittivity has been determined in planar and homeotropic alignments of the mixtures having DOBA concentrations 30.0 and 92.3 mol %. The experimental dielectric data in the frequency range of 1 Hz to 35 MHz do not show any relaxation mode in both the alignments of these mixtures. Dielectric anisotropy has been determined for the various observed phases of the mixtures.     

Elliptical diffusion of dye in hexagonal columnar polycatenar mesophases

We have studied polycatenar compounds which exhibit hexagonal columnar mesophases. In the planar orientation of these mesophases, the elliptical diffusion of the dissolved dyes is visualized by taking several pictures of the sample. The diffusion ratio D// /D is deduced for these columnar mesophases. Furthermore, using a classical optical absorption technique, we present measurements of dye diffusion in the same mesophases. The diffusion constants are measured in two geometries, along and perpendicular to the columns. The diffusion anisotropy ratios are in agreement with those deduced from the ellipse axes. The structure of these new columnar mesophases exhibited by rod-like mesogens is compared with that of disk-like mesogens.     

An interplay between molecular pairing,smectic layer spacing,dielectric anisotropy and re-entrant phenomena in ω-alkenyloxy cyanobiphenyls

In this article, we report on the liquid-crystalline properties of the 4-ω-alkenyloxy-4′-cyanobiphenyl series of compounds up to a total aliphatic chain length of eleven. When compared to the analogous fully saturated compounds, we find that the smectic layer spacing is significantly larger for the alkene-terminated materials; conversely the dielectric anisotropy in the nematic phase was found to be significantly smaller. The ability to manipulate bulk properties of nematic and smectic mesophases may have future relevance for display applications.     

A study of the enolization of some alkyl ketones in lyomesophases formed by alkylammonium surfactants

The kinetics of enolization of acetone, 3-pentanone, and 6-undecanone have been studied in lyotropic systems in the isotropic, nematic, and lamellar phases. The data, analysed in terms of spontaneous and acid-catalyzed rate constants, do not show any relevant influence of the physical nature of the phase. Effects are instead observed in relation to the length of the alkyl chains of the ketones and the type of the amphiphilic molecule. 6-undecanone has a surprising effect in stabilizing ordered mesophases.     

Self‐Assembly of Aminocyclopropenium Salts: En Route to Deltic Ionic Liquid Crystals

Aminocyclopropenium ions have raised much attention as organocatalysts and redox active polymers. However, the self‐assembly of amphiphilic aminocyclopropenium ions remains challenging. The first deltic ionic liquid crystals based on aminocyclopropenium ions have been developed. Differential scanning calorimetry, polarizing optical microscopy and X‐ray diffraction provided insight into the unique self‐assembly and nanosegregation of these liquid crystals. While the combination of small headgroups with linear p‐alkoxyphenyl units led to bilayer‐type smectic mesophases, wedge‐shaped units resulted in columnar mesophases. Upon increasing the size and polyphilicity of the aminocyclopropenium headgroup, a lamellar phase was formed.     

A Novel Type of Liquid Crystals Based on Axially Fluorinated Cyclohexane Units

Decidedly higher clearing temperatures and often also broader mesophases are exhibited by axially fluorinated cyclohexane derivatives 1 and 2 relative to their nonfluorinated analogues. A convenient method for the multigram synthesis of 1 and 2 , which represent a promising new type of liquid crystals with positive or negative dielectric anisotropy, is presented.     

Enhancement of the properties and mesophases stability after the electron beam irradiation on a racemic anti-ferroelectric liquid crystalline mixture

The display parameters and the stability of the various mesophases of a newly synthesised racemic mixture of anti-ferroelectric liquid crystals (AFLCs) have been boosted with the help of the electron beam irradiation. The mixture has phase sequence of isotropic–smectic C–smectic C_A crystal. The effective macroscopic polarisation and transition temperatures of the various mesophases of the irradiated mixture have increased as compared with the pure mixture. The macroscopic polarisation of the mixture has increased from 21 to 27 nC/cm² due to the irradiation. From various studies, we have observed that the temperature range of the smectic C_A phase has increased by 20.6°C with marginal decrease in the temperature range of the smectic C phase.     

Novel polymerizable bent‐shaped monomeric molecules

Novel bent‐shaped mesogens based on substituted naphthalene‐2,7‐diol, possessing a double bond at the end of terminal branches, have been synthesized and characterized. A variety of banana‐mesophases has been found, depending on the type of lateral substitution. Increasing the length of the alkyl chain leads to the appearance of the SmC_AP_A phase for two of the compounds synthesized. Under an electric field a transition into the SmC_SP_F phase takes place and a tilt angle close to 45° has been found. These materials are appropriate for the preparation of polymeric materials.     

Design and synthesis of nematic liquid crystals with negative dielectric anisotropy

Axially fluorinated cyclohexane derivatives are a promising new type of liquid crystals with negative dielectric anisotropy. Up to now, the technical application of these materials in LCDs was impeded by their strong tendency to form smectic B mesophases. We present a convenient synthetic pathway leading to new homologues carrying alkenyl side chains which induce nematic phases.     

The optical anisotropy and orientational order parameter of two mesogens having slightly different flexible side chain – a comparative study

Variation of the phase transition temperatures and existence of different number of mesophases, due to the changes of -CH₂ groups at the oxygen containing flexible side chain between two Schiff’s base liquid crystals compounds p-ethoxybenzylidine p-heptylaniline (EBHA) and p-butoxybenzylidine p-heptylaniline (BBHA) have been studied by using POM and DSC. Refractive indices as a function of temperature of these two compounds have also been measured by the well-known thin prism technique. To get the best accuracy of refractive indices data a comparison between the experimental data and theoretical values which are obtained from modified four-parameter model based on Vuks’ hypothesis have been done. A comparative study of some important parameters such as, optical anisotropy, orientational order parameters of these two liquid crystal compounds have also been conducted.     

Cholesterol-based dimeric liquid crystals: synthesis and mesomorphic behaviour

Chiral non-symmetric dimeric liquid crystals consisting of a cholesteryl ester moiety as chiral entity and a biphenyl aromatic core, interconnected through n-butyl (C₄) or n-pentyl (C₅) parity alkylene spacers, have been synthesized and investigated for their liquid crystalline properties. All the dimers exhibit enantiotropic mesophases. The first member of the dimers having the C₄ central spacer exhibit only the chiral nematic (N*) mesophase, while the higher homologues also show smectic A (SmA) and twist grain boundary (TGB) mesophases. The dimers of the other series containing the C₅ central spacer also have stable SmA, TGB and N* mesophases, except for the first which does not show the TGB phase. Both series of compounds show a weak odd-even effect with terminal alkyl chain substitution, while the spacer length has a marked influence on the phase transition temperatures.     

Synthesis and phase characterization of a double-tailed pyrrole-containing surfactant: a novel tecton for the production of functional nanostructured materials

A double-tailed polymerizable (pyrrolylalkyl) ammonium amphiphile has been synthesized, and its interfacial properties and aqueous phase behavior have been studied by polarized optical microscopy and X-ray diffraction. The Krafft temperature is about 27 degrees C, and the critical micelle concentration at 40 degrees C is about 1 mM, as obtained from surface tension measurements, potentiometry, and isothermal titration calorimetry. The lyotropic behavior of the surfactant is found to be of a complex nature. At concentrations higher than the micellar (L1) region, two mesophases have been identified: a second isotropic (L2) phase, which is probably micellar but not fully miscible with water, and a lamellar (L(alpha)) phase, showing interesting alignment properties. Small-angle X-ray scattering analysis of the mesophases has been evaluated in terms of a model of spherical micelles, which describes a mutual arrangement by a structure factor derived from a hard-sphere potential (Percus-Yevick, "PY", approach). Interest in the comprehensive phase behavior of the polymerizable surfactant is based on the desire to integrate the system into a composite material to obtain potentially conducting self-assembled hybrid mesostructures.     

Synthesis and mesomorphic properties of compounds exhibiting the undulated twist grain boundary smectic C* phase

The synthesis and characterization of three homologous series of compounds exhibiting the undulated twist grain boundary smectic C* (UTGB_C*) phase are reported. The chiral mesophases have been obtained using cholesterol as the chiral moiety. Cholestanol and [S]-[+]-octan-2-ol have also been used as the chiral moiety for comparitive purposes. In addition to this novel phase, cholesteric, smectic A, smectic C* and TGB_A phases have also been observed. The mesophases were characterized using a combination of polarizing optical microscopy, differential scanning calorimetry, X-ray diffraction and measurement of helical pitch.     

Characterization and flow properties of lyotropic ultrahigh-molar-mass polysaccharide mesophases

The occurrence of lyotropic mesophases in solutions of various polysaccharides, such as non-ionic schizophyllan and anionic xanthan, was demonstrated by means of polarizing microscopy. In contrast to synthetic polymers where the formation of liquid-crystalline phases has been attributed to the presence of mesogenic groups in the main- or side-chain, here a helical structure is a prerequisite for the formation of mesophases. It was then shown that the stability of the helix is dependent on the chemical structure and the arrangement of the side groups. Apart from their optical anisotropy, some lyotropic mesophases are distinguished by extraordinary viscous and elastic properties (e.g. maximum behaviour of viscosity). The viscoelastic material functions were determined by rheological methods. A precise characterization of the chemical and steric microstructure should provide information on the ability of polysaccharides to form mesophases. Determination of the chemical microstructure (quaternary polymer) was carried out by ¹H NMR spectroscopy after ultrasonic degradation. Low- and multi-angle laser light scattering were employed for the determination of the steric microstructure which indicates an expanded semi-flexible structure. The experimental results were compared with those from molecular modelling.     

Synthesis and characterization of non-symmetrical liquid crystals containing a cholesteryl ester moiety

In this paper,a series of chiral non-symmetrical liquid crystals(nBA-chol)consisting of a cholesteryl ester moiety as chiral entity and a biphenyl aromatic core with different terminal alkyl chain has been synthesized and investigated for their liquid crystalline properties.Effects of numbers of methylene units in the terminal alkyl chain on the phase transition temperatures and on the temperature-dependent pitch lengths of the chiral liquid crystals have been studied.The long terminal alkyl chain tends ...     

Synthesis and mesomorphic properties of some esters of trans-4-n-alkoxycinnamic and trans-4-n-alkoxy-alpha-methylcinnamic acids exhibiting ferroelectric and antiferroelectric phases

The synthesis and mesomorphic properties of several homologues of (S)-(+)-1-methylheptyl 4-[4(trans-4-n-alkoxycinnamoyloxy)benzoyloxy]benzoates (series I) and (S)-(+)-1-methylheptyl 4-[4-(trans-4-n-alkoxy-alpha-methylcinnamoyloxy)benzoyloxy]benzoates (series II) are described. The influence of the alpha -methyl group on the different mesophases and the clearing temperatures has been evaluated. Many homologues of both the series show a ferroelectric chiral smectic C (SmC*) phase while the antiferroelectric smectic C phase (SmC*A) is exhibited by compounds of series I. The mesophases have been identified by optical polarizing microscopy, differential scanning calorimetry and miscibility studies. The spontaneous polarization, the helical pitch and the tilt angle of two of the compounds have also been measured.