Star-shaped oligobenzoates: non-conventional mesogens forming columnar helical mesophases

Star-shaped mesogens with a phloroglucinol or a trimesic acid core and oligobenzoate arms with up to five repeating units have been synthesised. These non-conventional mesogens form various columnar mesophases over a broad temperature range. The liquid-crystal phases were characterised by optical microscopy, differential scanning calorimetry, X-ray diffraction, dilatometry and solid-state NMR spectroscopy. In addition to the high-temperature hexagonal columnar phases, the columnar self-assemblies undulate upon cooling and consequently form higher-ordered body-centred orthorhombic columnar 3D structures. A model of E-shaped folded conformers helically displaced along the columns is proposed. Helical preorganisation in the hexagonal phase precedes the transition to the low-temperature phases. Space filling and nano-segregation compete in the self-organisation process, thus aliphatic chains and the polar oligobenzoate scaffold are not perfectly separated in these star-shaped mesogens.     

Tetrabenzocyclododecatetraene A new core for mesogens exhibiting columnar mesophases

Octasubstituted derivatives of tetrabenzocyclododecatetraene are found to exhibit thermotropic mesophases. The molecules of these mesogens are highly flexible with an average four-fold symmetry. Optical and differential scanning calorimetry measurements are presented on five ester and ether derivatives. All mesophases appear to be columnar with a negative optical anisotropy. From the five compounds studied the four which are derived from aliphatic ester and ether are optically uniaxial, while the mesophase of the decylbenzoyloxy derivative could be biaxial. Miscibility measurements performed with some discotic and pyramidic mesogens of similar molecular dimensions suggest that the mesophases of the tetrabenzocyclododecatetraene core form new families of columnar mesophases.     

Tetrabenzocyclododecatetraene A new core for mesogens exhibiting columnar mesophases

Abstract

Octasubstituted derivatives of tetrabenzocyclododecatetraene are found to exhibit thermotropic mesophases. The molecules of these mesogens are highly flexible with an average four-fold symmetry. Optical and differential scanning calorimetry measurements are presented on five ester and ether derivatives. All mesophases appear to be columnar with a negative optical anisotropy. From the five compounds studied the four which are derived from aliphatic ester and ether are optically uniaxial, while the mesophase of the decylbenzoyloxy derivative could be biaxial. Miscibility measurements performed with some discotic and pyramidic mesogens of similar molecular dimensions suggest that the mesophases of the tetrabenzocyclododecatetraene core form new families of columnar mesophases.     

Ionic interaction induced novel ordered columnar mesophases in asymmetric discotic triphenylene salts

Asymmetric triphenylene imidazolium salts with different spacer lengths were successfully synthesized through quarternization of ω-bromo-substituted triphenylenes with 1-methyl imidazole. The asymmetry in ω-bromo-substituted triphenylenes tended to destroy liquid crystallinity in the sample. However, highly ordered columnar mesophases with a lamellar microphase segregation were induced by ionic interactions among the imidazolium salts, and the lamellar morphology was visualized by transmission electron microscopy. On the basis of an X-ray diffraction study on shear oriented samples, a novel rectangular columnar phase with a plane group of pm was observed for a triphenylene imidazolium salt with a spacer length of C₁₁, while an oblique columnar phase was determined for a triphenylene imidazolium salt with a C₈ spacer. Due to the asymmetric molecular shape and ionic interactions in the triphenylene imidazolium salts, the columnar liquid crystalline phase was extended to below room temperature (c. -20°C) for samples with spacer lengths of C₈ and C₁₁.     

A liquid-crystalline silsesquioxane dendrimer exhibiting chiral nematic and columnar mesophases

A hexadecamer, first-generation, octasilsesquioxane liquid-crystalline dendrimer was synthesized by a platinum-catalyzed hydrosilylation reaction of the parent first-generation vinyl octasilsesquioxane dendrimer with a modified, laterally substituted mesogen. The structure and purity of the octasilsesquioxane substrate was confirmed by 1H, 13C, and 29 Si NMR spectroscopy, microanalysis, and size exclusion chromatography (SEC). The mesogenic substrate was found to exhibit only a chiral nematic phase, whereas the resulting hexadecamer dendrimer displays enantiotropic chiral nematic, disordered hexagonal columnar, and disordered rectangular columnar phases, with a glass transition below room temperature. The lateral or side-on attachment of the mesogen to the dendritic core was found to be a key design feature in the formation of the mesophases.     

Dielectric study of the tilted columnar mesophase in three new chiral metallorganic discotic mesogens

Chiral oxovanadium(IV), copper(II) and palladium(II) beta-diketonates show a room temperature tilted columnar mesophase (rectangular P21). The dielectric response of these compounds was measured in the frequency range 10-2-109Hz. Three different relaxation processes have been observed in the columnar phase. One of them is only present in this mesophase and we suggest that it is a Goldstone-like mode, similar to the one present in the helical SmC* phases. In our case the presence of the helix was confirmed by circular dichroism studies.     

Effect of functionalization on glass formation by columnar mesophases of substituted triphenylene mesogens

A new kind of discotic liquid crystal has been investigated where one out of the six ether side chains of a triphenylene core was replaced by an ester side chain. We have found greatly increased clearing temperatures and suppressed crystallization thus leading to glass forming discotics. Here we report the synthesis, X-ray and dielectric studies concerning the nature of the glass transition. Furthermore we give arguments as to why a 'mixed' substitution can lead to such drastic effects.     

Effect of functionalization on glass formation by columnar mesophases of substituted triphenylene mesogens

Abstract

A new kind of discotic liquid crystal has been investigated where one out of the six ether side chains of a triphenylene core was replaced by an ester side chain. We have found greatly increased clearing temperatures and suppressed crystallization thus leading to glass forming discotics. Here we report the synthesis, X-ray and dielectric studies concerning the nature of the glass transition. Furthermore we give arguments as to why a ‘mixed’ substitution can lead to such drastic effects.     

Proton conduction in discotic mesogens

In this communication, we show that liquid crystalline phases lower the activation energy barrier for proton transport. The liquid crystalline phases were obtained using a triphenylene core with alkyl chains bearing a triazole moiety at their termini.     

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.     

Induction of a nematic columnar phase in a discotic hexagonal ordered phase forming system

The induction of a nematic columnar phase in a discotic hexagonal ordered phase forming system is achieved by mixing hexakispentyloxytriphenylene 1 with a long chain derivative of trinitrofluorenone 3. The difference in chain length has a strong influence on the packing behaviour due to steric effects. The long hydrocarbon chains of the acceptor introduce a strong asymmetry into the electron donor acceptor complex. It could be shown by differential scanning calorimetry, optical microscopy and X-ray measurements that a nematic columnar phase is formed. In this mesophase the triphenylenes form columns but no hexagonal or orthorhombic lattice is built up. Each column behaves like a rod-like nematic mesogen. To prove that the long hexadecane alkyl chains of the acceptor are responsible for this induction, the acceptor 3 was mixed with the non-liquid-crystalline triphenylene derivative 2 containing six hexadecyloxy side groups. The long alkyl chains of the acceptor dissolve perfectly in the side chain region of the discs. No asymmetry is induced and the columns formed can be arranged on a hexagonal lattice resulting in a D_ho phase.     

Novel mesophases in fluorine substituted banana-shaped mesogens

A new homologous series of achiral banana-shaped mesogens ('Dn') has been synthesized and studied by the classical techniques (optical microscopy, differential scanning calorimetry, X-ray diffraction, miscibility studies and electro-optic investigations). The short homologues (D6-D8) exhibit a two-dimensional phase 'B1_x' different from a B1 phase with a rectangular lattice. The longer homologues (D9-D14) present a mesophase which displays the defects of the B7 phase of the PIMB-NO₂ compounds. Nevertheless the D9-D14 mesophase is not miscible with the B7 phase, and contrary to B7, exhibits a bistable behaviour ('ferroelectric' type) suggesting at least a B7 variant.     

A generalized model for the molecular arrangement in the columnar mesophases of polycatenar mesogens. Crystal and molecular structure of two hexacatenar mesogens

The columnar mesophases of two series of hexacatenar palladium(II) mesogens have been studied in detail by a combination of X-ray diffraction on aligned and unaligned samples and dilatometry. The results of these studies, combined with the results of two single crystal structure determinations, have allowed a model of the molecular arrangement in the columnar phases to be proposed. This model differs in detail from that generally accepted for the arrangement of polycatenar mesogens in columnar phases, and a new model is proposed which accounts for both new and existing data.     

The synthesis of triphenylene-based discotic mesogens New and improved routes

The synthesis of discotic liquid crystal materials based on the triphenylene core normally involves the preparation of hexahydroxytriphenylene. This has traditionally been accomplished by a chloranil promoted trimerization of 1,2-dimethoxy-benzene followed by demethylation of the hexamethoxytriphenylene produced. Hexahydroxytriphenylene is subsequently alkylated to give the required derivative. The overall yield of this three step procedure is low and it is difficult to perform on a large scale. It is herein shown that a modified procedure using iron III chloride instead of chloranil yields hexa-alkoxytriphenylenes in a single step. The reaction is high yielding and can be performed on a large scale. This also provides a better route than those currently available for making the unsymmetrically substituted tri-phenylenes required in the synthesis of polymeric discotic liquid crystals. Some related elaborations of the triphenylene nucleus are also discussed.     

Dibenz[a,c]anthracene derivatives exhibiting columnar mesophases over broad temperature ranges

We report the synthesis and characterization of a series of novel hexaalkoxydibenz[a,c]anthracenes. While the parent compound is not mesomorphic, the introduction of substituents in the 10- and 13-positions yields compounds with columnar liquid crystalline phases over very broad temperature ranges.     

Synthesis of unsymmetrical dibenzoquinoxaline discotic mesogens

Unsymmetrical dibenzophenazine discotic mesogens were prepared via the corresponding tetraalkoxybenzil derivatives. These dibenzophenazines exhibit Colho phases over extremely broad temperature ranges.     

Searching for asymmetric inductions in chiral smectic mesophases

Searching for asymmetric synthesis in smectic phases is, to our knowledge, reported for the first time in this paper. Two different reactions able to lead to optical enrichment were carried out in smectic phases (S) composed of chiral molecules. The first was a thermally promoted 1,3 dipolar cycloaddition of a diazo compound to a prochiral thiocarbonyl derivative run in a chiral smectic C meso-phase (S*_C). The second was a monomolecular process, the photochemical inversion of chiral sulphoxides in a smectic A mesophase. In both cases the asymmetric induction was zero or, in the best run, very very poor. This lack of transfer of chirality between the smectic solvent and the reaction is discussed to understand better the requirements for more successful tailoring of such experiments.     

Hexagonal columnar mesophases in oxygen-bridged dicopper complexes

The synthesis of a series of bidentate Schiff 's bases; 3,5-bis(3 ,4 ,5-trialkoxybenzyloxy)phenyl2- N -(3-hydroxypropy)iminopropyl ketone, and their dicopper complexes are reported. The liquid crystalline behaviour of these dicopper complexes was studied by DSC, polarized optical microscopy and X-ray diffraction. The phases were characterized and identified as hexagonal columnar (D ) phases, which are typical of disc-like molecules. h