Amphiphilic N-benzoyl-1-amino-1-deoxy-D-glucitol derivatives forming thermotropic lamellar, columnar and different types of cubic mesophases

Novel amphiphilic glucamine derivatives have been synthesized. These are N-benzoyl-1-deoxy1-methylamino-D-glucitols and N-benzoyl-1-amino-1-deoxy-D-glucitols carrying one, two or three aliphatic chains (CnH2n 1O- with n 3, 6 and 12) grafted to the benzamido group. The thermotropic mesophases of these compounds were studied by thermal polarizing optical microscopy and differential scanning calorimetry, and some also by X-ray scattering. Depending on the number and the length of the alkyl chains lamellar, bicontinuous cubic, hexagonal columnar or inverted micellar cubic mesophases were detected by analogy with lyotropic systems. In the contact region between lamellar phases of the single chain amphiphiles and micellar cubic phases of the mesomorphic triple chain compounds, hexagonal columnar phases can be induced. A hexagonal columnar phase was also induced in the contact region between a bicontinuous and a micellar cubic mesophase. The lyotropic liquid crystalline behaviour of the dodecyloxy substituted N-benzoyl-1-deoxy-1-methylamino-D-glucitols was investigated by the solvent penetration method using ethylene glycol as protic solvent. On increasing the solvent content, the double chain compound forms a cubic and a lamellar mesophase and the triple chain compound forms a hexagonal columnar lyomesophase. The dodecyloxy substituted compounds were also investigated with respect to their behaviour as thin films at the air-water interface using a Langmuir film-balance. Different types of pi/Aisotherms were observed whereby the molecular areas at collapse were determined either by the size of the carbohydrate head group (single chain compounds) or by the number of alkyl chains (double and triple chain compound).     

Solidification-induced and shear-induced band texture in thermotropic liquid crystalline polymer films

Two phase diagrams of a six-ring double-swallow-tailed compound are presented where the mixing components involve electron-acceptor compounds. In the mixed phase region, nematic, smectic A and C, cubic and columnar phases are induced. In this way transitions between lamellar, cubic and columnar phases can be realized by variation of the con- centration. The mesophases occurring in these systems have been characterized by X-ray investigations.     

Hydrogen-bonded block mesogens derived from semiperfluorinated benzoic acids and the non-mesogenic 1,2-bis(4-pyridyl)ethylene

Thermal properties of benzoic acids carrying one or two semiperfluorinated alkoxy tails on the aromatic core have been investigated in binary mixtures with the non-liquid crystalline bidirectional trans-1,2-bis(4-pyridyl)ethylene. The hydrogen bonded complexes built from the complementary molecular species show a significantly enhanced mesophase stability compared with the fluorinated acids in their pure states. The mesophase morphologies of the complexes are governed mainly by the number of the partially fluorinated chains grafted to the acid component. Mixed systems comprising the one-chain acids exhibit a smectic C phase followed by a smectic A phase at more elevated temperatures. Incorporation of a second semiperfluorinated chain into the acid leads to the formation of columnar mesophases. These columnar phases of the H-bonded complexes should represent ribbon phases resulting from the collapse of the smectic layers.     

Synthesis and characterization of liquid crystalline monofunctionalized 'two chain' diols and corresponding low molecular weight siloxane derivatives

The selective synthesis and the thermal behaviour of some cis, cis-(3,5-dihydroxycyclohexyl) 3,4-(alkenyloxy, alkyloxy)benzoates (monofunctionalized 'two chain' diols) are described. Thus, several 3-(alkyloxy)-4-(undecenyloxy)benzoic acids and 4-(decyloxy)-3-(undecenyloxy)benzoic acid have been obtained. The monofunctionalized 'two chain' diols form a hexagonal columnar mesophase through hydrogen bonding. Subsequently, low molecular weight liquid crystalline siloxanes, model compounds for polymers, i.e. two twins and one cyclic product, were synthesized via a hydrosilylation reaction. With respect to the 'two chain' diols, the observed hexagonal columnar mesophase was stabilized and the intercolumnar distance was extended by the siloxane moieties. Remarkably, the thermal behaviour of the cyclosiloxane differs from that of the twins. A cubic mesophase, which can be observed very rarely in thermotropic mesogens, was formed at temperatures below the hexagonal columnar phase.     

A new disc-shaped mesogenic compound with olefinic linkage derived from triphenylamine: synthesis, mesogenic behavior and fluorescence properties

A new disc-like triphenylamine containing mesogenic compound has been synthesized by the implementation of the Heck and ring-closing metathesis-based reactions in good yield. The designed and synthesized compound showed rectangular columnar mesophase and this is the first report of liquid crystalline phase of the triphenylamine-based compound with an olefinic linkage. The disc-shaped compound exhibited excellent fluorescence properties.     

A propeller-like uranyl metallomesogen

Uranyl triflate forms with three imidazo[4,5-f]-1,10-phenanthroline ligands a propeller-like complex that exhibits a hexagonal columnar phase. The ligand is not liquid-crystalline, but a mesophase is induced upon complex formation with the uranyl salt. The thermal behavior has been investigated by polarizing optical microscopy and by high-temperature X-ray diffraction. A model of the stacking of the molecules in the mesophase is proposed.     

Structure of gyroid mesophase formed by monodendrons with fluorinated alkyl tails

The structure of a three-dimensional mesophase has been studied by the methods of small-angle X-ray diffraction and reconstruction of electron-density maps of the cubic lattice. In the oriented samples based on monodendrons with partially fluorinated alkyl tails, this mesophase has been shown to coexist with a two-dimensional columnar phase through a wide temperature interval. Epitaxial relationships between (10) planes of the hexagonal lattice and (211) planes of the cubic lattices lead to the ten-point pattern of azimuthal intensity distribution for the first X-ray 211 reflection and to the six-point intensity distribution for the second 220 reflection. The observed 12-point pattern of the 220 reflection is due to the presence of twin “crystallites” of the three-dimensional phase, and their [110] axis is parallel to the axis of cylinders in the columnar phase. The reconstructed electron-density maps show that the regions with increased electron density, which are composed of fluorinated aliphatic tails, form a bicontinuous gyroid structure.     

Tuning the thermotropic and lyotropic properties of liquid-crystalline terpyridine ligands

A rational synthetic strategy is developed to provide compact and simple terpyridine (terpy) mesogens that show liquid-crystallinity both as pure compounds and in organic solution (amphotropic compound). The use of a central 4-methyl-3,5-diacylaminophenyl platform equipped with two lateral aromatic rings, each bearing three appended aliphatic chains, allows connection of a 2,2':6',2'-terpyridine fragment through a polar group such as an ester, amide, or flat conjugated alkyne linker. For the T(12)ester and T(12)amide scaffolds, the mesophase is best described as a lamellar phase, in which the molecules self-assemble into columnar stacks held together in layers. In the T(12)amide case, the additional amide link results in significant stabilization of the lamellar phase. The driving forces for the appearance of columnar ordering are the hydrogen-bonding interactions of the amide groups, which induce head-to-tail pi-stacking of the terpy subunits. Replacing the polar linker by a nonpolarized but linear alkyne spacer, as in the T(12)ethynyl compound, provides a columnar mesophase organized in a rectangular lattice of p2gg symmetry. In this arrangement, two nondiscotic molecules arranged into dimers by hydrogen bonding and pi-pi stacking pile up in a head-to-tail manner to form columns. In addition, the T(12)amide compound proves to be an excellent gelator of cyclohexane, linear alkanes, and DMSO. The resulting robust and transparent gels are birefringent and formed by large aggregates that are readily aligned by shear-flow. TEM and freeze-fracture microscopy reveal that the gels have an original layered morphology made of fibers.     

Investigation of the Structures of the Crystalline and Columnar Phases of Linear Chain Copper(II) Alkanoates

Binuclear copper(11) complexes of fatty acids crystallize at room temperature in a lamellar lattice that has been characterized by X-ray diffraction. A transition to a thermotropic columnar mesophase is observed at about 110-120°C for each compound of the series n = 12 to n = 22, n being even and equal to the number of carbon atoms in the corresponding fatty acid. This columnar mesophase has been investigated by polarizing optical microscopy, differential scanning calorimetry and X-ray diffraction. Columns of polar copper carboxylate groups are surrounded by disordered aliphatic chains, and form a two-dimensional hexagonal lattice. The repeating unit in a column is a binuclear dicopper tetracarboxylate complex.     

A new high temperature mesophase of a double-swallow-tailed compound with weak optical anisotropy

A new six-ring double-swallow-tailed compound is presented which shows four liquid crystalline phases. In addition to a smectic C phase and two columnar phases (oblique and hexagonal), a high temperature mesophase was observed which exhibits a very weak birefringence and which shows unusual optical textures.     

Hydrogen Bonded Supramolecular Liquid Crystalline Complex of 2,4,6-Triarylamino-1,3,5-triazines with Semiperfluorinated Benzoic Acids

Hydrogen bonded supramolecular complexes of 2,4,6‐triarylamino‐1,3,5‐triazines ( Tn ) with semiperfluorinated benzoic acids have been prepared. The mesophase behaviors of such complexes were investigated with POM (polarized optical microscopy), DSC (differential scanning calorimetry) and X‐ray diffraction. Columnar mesophase was found in the equimolar mixtures of the triazines with the semiperfluorinated benzoic acids. While the equimolar mixture of triazines with the nonfluorinated three fold alkoxy‐modified carboxylic acid was not mesogen. The mesophase formation is analyzed as the hydrogen bonding between the Tn and benzoic acid which leads to discrete dimeric supermolecule and enhances the polarity of the core region of the dimeric supermolecule; simultaneously, an increased intramolecular polarity contrast upon replacing alkyl chains by semiperfluorinated chains, which favors a microsegregation, and leads to the fromation of the columnar phase.     

Liquid Crystals Based on the N-Phenylpyridinium Cation—Mesomorphism and the Effect of the Anion

Families of symmetric, ionic, tetracatenar mesogens are described based on a rigid, N-phenylpyridinium core, prepared as their triflimide, octyl sulfate and dodecyl sulfate salts for a range of terminal chain lengths. The mesomorphism of the individual series is described before a comparison is drawn between the different families and then more broadly with (i) neutral tetracatenar materials and (ii) related bis(3,4-dialkoxystilbazole)silver(I) salts. For the octyl and dodecyl sulfates and the related triflates reported earlier, a SmA phase is seen at shorter chain lengths, giving way to a Col_h phase as the terminal chain lengthens. For the alkyl sulfate salts, an intermediate cubic phase is also seen and the terminal chain length required to cause the change from lamellar to columnar mesophase depends on the anion. Furthermore, there is an unexpected and sometime very large mesophase stabilisation seen on entering the columnar phase. All of the triflimide salts show a rectangular columnar (ribbon) phase.     

Glasses of new 2,3,9,10,16,17,23,24-octasubstituted phthalocyanines forming thermotropic and lyotropic discotic mesophases

The synthesis and mesogenic properties of a new class of liquid crystalline metal-free and metalated phthalocyanines with different numbers of alkylthio side chains are presented. The effect of the substitution pattern in a series of differently octasubstituted phthalocyanines on the width and order of the thermotropic mesophase is investigated. The exchange of the alkylthio groups from chlorine atoms alters the mesophase width and the melting behaviour. Glassy phthalocyanines, without any tendency to crystallize, can be synthesized in this way. Furthermore, mixtures of differently 4,5-disubstituted benzenedicarbonitriles are tetramerized statistically to the corresponding mixtures of substituted phthalocyanines. The effect of decreasing symmetry at the molecular level on the properties of the liquid crystalline materials has been investigated. Highly oriented columnar systems can be obtained with these mixtures. For all compounds containing less than eight alkylthio groups, lyotropic mesomorphism in hexadecane solution is also found.     

Sheet-shaped mesogens based on 1,3,5-triazines: variation of columnar mesophases through intermolecular hydrogen bonding

The mesomorphic behaviour of a liquid crystalline sheet-shaped 2,4,6-triarylaminothe 1,3,5-triazine bearing six peripheral decyloxy chains has been investigated in binary mixtures with two-fold alkoxy substituted non-mesomorphic benzoic acids. Each investigated equimolar mixture exhibits an enantiotropic columnar phase with either a hexagonal or a rectangular lattice. The two-dimensional lattice parameters depend on length and positions of the flexible alkoxy chains of the aromatic carboxylic acids. The structure formation of the mixed systems giving rise to variations of the mesophase structure of the pure triarylaminotriazine results from intermolecular hydrogen bonding between the complementary melamine and acid components.     

Chiral ordering in the nematic and an optically isotropic mesophase of bent-core mesogens with a halogen substituent at the central core

Two new homologous series of bent-core compounds have been synthesized. Their mesophase behaviour has been investigated by polarizing microscopy, differential scanning calorimetry, X-ray diffraction, NMR spectroscopy, and by dielectric and electro-optical measurements. It was found that, with one exception, all the chlorine-substituted compounds form a nematic phase and an optically isotropic 'banana phase'. The latter phase shows spontaneously chiral domains of opposite handedness. This phase may be considered as a type of smectic blue phase. The mesophase behaviour of the homologous bromine-substituted compounds is more complicated. Depending on the chain length, B₆, columnar, nematic or the isotropic 'banana phase' occur.     

Mesomorphic properties of copolyesters of 3,6-linked triphenylene-based units and polymethylene spacers

Main chain discotic liquid crystalline polymers consisting of triphenylene-based units and alkyl spacers (C8, C10 and C12), connected by ester linkages in the 3- and 6-positions of triphenylene, have been synthesized and their mesomorphic properties were studied by DSC, polarizing optical microscopy and X-ray diffraction. It was found that these polymers exhibit a hexagonal columnar (Col h ) mesophase with intracolumnar order over a wide temperature range. The clearing temperature decreases on increasing the spacer length. It was found that the clearing temperatures are rather higher than that of the corresponding triphenylene monomer having six hexyloxy chains. These polymers form an ordered columnar mesophase, while the corresponding monomeric mesogen shows a disordered columnar phase. In the polymeric system, the fluctuations of the disc-like units in the mesophase are restricted by the connection of the mesogenic units, which stabilizes the columnar mesophase.     

Transitions induced by solubilized fat into reverse hexagonal mesophases

Lyotropic liquid crystals of glycerol monooleate (GMO) and water binary mixtures have been extensively studied and their resemblance to human membranes has intrigued many scientists. Biological systems as well as food mixtures are composed of lipids and fat components including triacylglycerols (TAGs, triglycerides) that can affect the nature of the assembly of the mesophase. The present study examines the effect of TAGs of different chain lengths (C(2)-C(18)) at various water/GMO compositions, on phase transitions from lamellar or cubic to reverse hexagonal (L(alpha)-H(II) and Q-H(II)). The ability of the triglycerides to promote the formation of an H(II) mesophase is chain length-dependent. It was found that TAG molecules with very short acyl chains (triacetin) can hydrate the head groups of the lipid and do not affect the critical packing parameter (CPP) of the amphiphile; therefore, they do not affect the self-assembly of the GMO in water, and the mesophase remains lamellar or cubic. However, TAGs with medium chain fatty acids will solvate the tails of the lipid, and will affect the CPP of the GMO, and transform the lamellar or cubic phases into hexagonal mesophase. TAGs with long chain fatty acids are very bulky, not very miscible with the GMO, and therefore, kinetically are very slow to solvate the lipid tails of the amphiphile and are difficult to accommodate into the lipophilic parts of the GMO. Their effect on the transitions from a lamellar or cubic phase to hexagonal is detected only after months of equilibration. In order to enhance the effect of the TAG on the phase transitions in the GMO/triglyceride/water systems, temperature and electrolytes effects were examined. In the presence of short and medium chain triglycerides, increasing temperature caused a transition from lamellar or hexagonal to L(2) phase (highest CPP value). However, in the presence of long chain TAGs, increasing temperature to ca. 40 degrees C caused a formation of H(II) mesophase. In addition, it was found that in tricaprylin/GMO/water systems, the increase in temperature caused a decrease in the lattice parameter. The effect of NaCl on the H(II) mesophase revealed interesting results. At low concentration of tricaprylin (5 wt%), the addition of only 0.1 wt% of NaCl was sufficient to cause the formation of well-defined H(II) mesophase, while further addition of electrolyte increased the hexagonal lattice parameters. At higher TAGs concentrations (10 wt%), addition of electrolyte resulted in the formation of H(II) with modifications of the lattice parameter. All the examined effects were more pronounced with increasing water content.     

Morphological phase behaviour under pressure of polycatenar mesogens with a perfluorinated moiety

Two polycatenar materials composed of a four‐aromatic‐ring core with a perfluorinated moiety attached in one terminal position through either butylene‐ or pentylene spacer groups, and three tetradecyloxy chains at the other end (abbreviated as 14PC₄F and 14PC₅F), were investigated to study the effect of pressure on the phase transition behaviour. A polarizing optical microscope equipped with a high pressure optical hot stage, was used for the purpose. The T vs. P phase diagrams of 14PC₄F and 14PC₅F were constructed in the pressure region up to 100 MPa. 14PC₄F showed the stable crystal (Cr₁)–columnar tetragonal (Col_tet)–smectic A (SmA)–columnar hexagonal (Col_h)–isoropic liquid (I) phase transition sequence under all pressures. 14PC₅F exhibited the phase sequence metastable crystal (Cr₂)–cubic (Cub)–Col_tet–SmA–I in a melt‐cooled sample on heating under pressure. But when the melt‐cooled Cr₂ sample was annealed at 52–54°C for 2–3 h, the stable crystal (Cr₁) was formed slowly, giving a stable Cr₁–Cub–Col_tet–SmA–I phase sequence. The temperature region of the stable cubic phase broadened with increasing pressure. Furthermore a new mesophase of 14PC₅F was pressure‐induced between the I and SmA phases on cooling at pressures above about 16 MPa. Since the monotropic mesophase exhibited a texture very similar to that of the high temperature Col_h phase of 14PC₄F with planar orientation, the new phase was assigned at a high temperature columnar hexagonal phase of 14PC₅F.     

The influence of charge-transfer interactions on solid and cholesteric liquid crystalline mixtures of cholesteryl anthraquinone-2-carboxylate and cholesteryl 9,10-dimethoxyanthracene-2-carboxylate

The phase behaviour of 3beta-cholesteryl 9,10-anthraquinone-2-carboxylate (CAQ, an excellent electron acceptor) and 3beta-cholesteryl 9,10-dimethoxyanthracene-2-carboxylate (CMAQ, an excellent electron donor) have been investigated neat and as mixtures by a variety of techniques. Both molecules form thermotropic cholesteric phases. UV-Vis absorption spectra of CAQ:CMAQ mixtures provide strong evidence for the formation of charge-transfer (CT) complexes in the solid state. DSC thermograms of the mixtures reveal several transitions, only some of which are observable by optical microscopy. The mesophase range of CAQ is increased by c. 40 upon addition of as little as 10 wt% of CMAQ as a consequence of increased attractive forces in the CT complexes. X-ray powder diffraction patterns confirm that CAQ forms different crystalline phases upon being cooled from the mesophase and by recrystallization from solvents. The 5:5 mixture can be supercooled to a glass from its cholesteric phase. Some of the phase behaviour of the CAQ:CMAQ mixtures is explained with a model that includes selective crystallization of the two components into eutectics from supercooled liquid crystalline matrices.     

Chiral ordering in the nematic and an optically isotropic mesophase of bent-core mesogens with a halogen substituent at the central core

Two new homologous series of bent-core compounds have been synthesized. Their mesophase behaviour has been investigated by polarizing microscopy, differential scanning calorimetry, X-ray diffraction, NMR spectroscopy, and by dielectric and electro-optical measurements. It was found that, with one exception, all the chlorine-substituted compounds form a nematic phase and an optically isotropic ‘banana phase’. The latter phase shows spontaneously chiral domains of opposite handedness. This phase may be considered as a type of smectic blue phase. The mesophase behaviour of the homologous bromine-substituted compounds is more complicated. Depending on the chain length, B₆, columnar, nematic or the isotropic ‘banana phase’ occur.