Core-halogenated, helical-chiral triphenylene-based columnar liquid crystals

It is shown that chlorination and even bromination of the mesogenic hexahexyloxytriphenylene 1 can take place at the sterically congested tetracyclic core delivering the respective monohalogenated, sevenfold substituted enantiotropic mesogens 3 and 4 in 32 and 17% yields, respectively, giving mesophases over greater temperature ranges. Miscibility tests among all mesogenic compounds of this study prove formation of the columnar hexagonal type of phase. The steric and electronic effects of those substituents, different from the six equal alkoxy groups, on the mesophase are discussed in detail. In our hands and to our surprise, the sterically hindered acceptor (nitro or halogeno) substituents of 2, 3, or 4 could not be replaced by an alkylthio group in nucleophilic aromatic substitution reactions using the aprotic polar solvent DMEU. Semi-empirical calculations on numerous hexaethers with one or two substituents, e.g. halogens or others, in one of the three bay regions of the triphenylene core reveal that these molecules are helically deformed due to the crowding in the molecule. In this new case of atropisomerism, the 1,12-disubstituted derivatives should offer the possibility of resolution into enantiomers and, therefore, of studying disc-like mesomorphic compounds possessing for the first time the origin of chirality within their molecular centre.     

New switchable columnar liquid crystals

Penta- and hexa-substituted phenanthrenes and penta-substituted benzo[c]-phenanthrenes have been synthesized; these exhibit columnar phases and are electrically switchable when bearing chiral side chains. The liquid crystalline and electro-optical properties are described. A strong tri-state switching behaviour is observed with the phenanthrene derivatives. A helical phase structure, responsible for this switching behaviour, is proposed and the possible mechanism of the columnar phase switching process is discussed.     

Recent developments in the chemistry of triphenylene-based discotic liquid crystals

Triphenylene-based discotic liquid crystals, which have already been used commercially in phase compensation films to improve the viewing angle of liquid crystal display devices, also have application potential as one-dimensional charge carrier systems useful in electrical conduction, photoconduction, electroluminescence, photovoltaic solar cells, gas sensing, optical data storage and other devices. Over the past 25 years, more than 500 triphenylene derivatives have been synthesized to explore these possibilities. Cammidge and Bushby reviewed the chemistry and physical properties of about 100 triphenylene-based discotic liquid crystals prepared up to 1995. This review summarizes advances in the chemistry of triphenylene-based discotic liquid crystals since late 1995.     

Recent developments in the chemistry of triphenylene-based discotic liquid crystals

Triphenylene-based discotic liquid crystals, which have already been used commercially in phase compensation films to improve the viewing angle of liquid crystal display devices, also have application potential as one-dimensional charge carrier systems useful in electrical conduction, photoconduction, electroluminescence, photovoltaic solar cells, gas sensing, optical data storage and other devices. Over the past 25 years, more than 500 triphenylene derivatives have been synthesized to explore these possibilities. Cammidge and Bushby reviewed the chemistry and physical properties of about 100 triphenylene-based discotic liquid crystals prepared up to 1995. This review summarizes advances in the chemistry of triphenylene-based discotic liquid crystals since late 1995.     

First carbohydrate liquid crystals of columnar structure

S, S-Dialkylacetals of aldoses, tripodal in structure and recently synthesized in large numbers by three groups, do not exhibit thermotropically a smectic, but a columnar hexagonal mesophase (Hx) as we have proved by X-ray diffraction. The molecular organization in this mesophase is comparable with a similar one known for phasmidic molecules. These various multiols, although different in stereo-chemistry, form only one type of hydrogen-bonded disc-shaped multimer. Its mesophase structure is made up of about five molecules placed in columns with a skeleton of hydrogen-bridged sugar parts surrounded by thioalkyl groups in the periphery. Since three 6-deoxy sugar dithioacetals were shown to be non-thermomesomorphic the terminal hydroxyl function is essential for this molecular arrangement which seems not to be true for missing ones in the inner part of a sugar chain, as we have found in one case.     

Luminescent columnar liquid crystals based on tristriazolotriazine

Five discotic molecules comprising a tris[1,2,4]triazolo[1,3,5] triazine core were designed and synthesized to obtain luminescent and charge-transporting columnar liquid crystalline materials. With the exception of one compound containing terminal hydroxyl groups all compounds presented a wide thermal range and stable columnar liquid crystalline phase, characterized by differential scanning calorimetry, polarized optical microscopy, and X-ray diffraction (XRD) techniques. The phase formation appeared to be associated to some extent with interdigitation of the alkoxy and benzylalkoxy portion, as suggested by the XRD results. All compounds have a strong blue luminescence in solution and solid phase. At the temperature at which the compounds enter in the mesophase the luminescence decreases significantly. This result suggests that entrance into the Col(h) phase is accompanied by a better π-stacking of the peripheral phenyl rings compared to the solid phase, consistent with the intramolecular distances (3.5 ?) observed in the XRD analysis. These compounds based on tristriazolotriazine are quite robust with good optical and thermal properties for application as solid state emitters, and we anticipate that they may provide an interesting alternative to other discotic molecules based on N-heterocycles, which generally present a high-temperature Col(h) phase.     

Viologen-based redox-active ionic liquid crystals forming columnar phases

Viologens possessing three alkoxy chains at each terminal self-organize into columnar liquid-crystalline phases through nanophase segregation and electrostatic interactions. These viologens are redox-active and susceptible to two consecutive electrochemical reductions.     

Novel supramolecular columnar liquid crystals based on thiosemicarbazides

Novel liquid crystal materials based on 3,4-di-n-alkoxybenzoylthiosemicarbazides (3a–h, n = 5–10, 12, 14) were synthesised. The mesomorphic properties of these compounds were characterised and studied by differential scanning calorimetry, polarising optical microscopy and X-ray diffraction. Compound 3a did not show mesomorphic properties; 3b shows a monotropic hexagonal columnar (Col_h) phase. Compounds 3c–h display an enantiotropic Col_h phase. The mesomorphic properties were found to be dependent on the length of alkoxy side chains. In N,N-dimethylformamide solution, all the compounds displayed a room temperature emission with λ_max at 361–332 nm. A thermogravimetric analysis also was performed.     

Influence of acetylene-containing peripheral chains on the mesomorphic properties of triphenylene-based liquid crystals

Three series of novel acetylene-containing triphenylene-based liquid crystals have been prepared. One class of material is symmetrical with the acetylenic unit located at the terminus of a peripheral alkynyloxy chain in a peripheral position at the terminus of an alkoxy chain. These symmetrical hexa-substituted materials possess high melting points which renders mesomorphism monotropic. A second class comprises materials where only one of the six peripheral units contains a terminal alkyne chain. The asymmetry confers relatively low melting points and an enantiotropic hexagonal columnar mesophase is exhibited for all materials, albeit to a lower temperature than the known parent systems. The third series of materials also comprises unsymmetrical structures, here containing one acetylenic unit conjugated with the triphenylene core and five alkoxy chains. This third series of materials show enhanced π–π interactions and enhanced space-filling effects of the acetylenic unit close to the core which enhance mesophase stability markedly over the hexaalkoxy analogues. This result shows that rigidity of peripheral chains, as opposed to molecular symmetry, is the significant factor for the reduction of mesophase stabilities.     

Structural factors controlling the self-assembly of columnar liquid crystals

A series of disc-shaped molecules were prepared by the condensation of 1,2-diamines with 2,3,6,7-tetrakis(hexyloxy)phenanthrene-9,10-dione to investigate the relationship between changes in molecular structure and the self-assembly of columnar liquid crystalline phases. A comparison of compounds with different core sizes indicated that molecules with larger aromatic cores had a greater propensity to form columnar phases, as did compounds substituted with electron-withdrawing groups. In contrast, molecules with electron-donating substituents were nonmesogenic. The clearing temperature of columnar phases increased linearly with the electron-withdrawing ability of the substituents, as quantified by Hammett sigma-values. The observed trends can be rationalized in terms of the strength of pi-pi interactions between aromatic cores in the liquid crystalline phases and suggest that both electrostatic interactions and dispersion forces play important roles in the self-assembly of these materials.     

Hexagonal columnar mesophases of polycatenar para-phenylene connected bis-oxadiazole-based liquid crystals

In this article polycatenar liquid crystals containing two 1,3,4-oxadiazoles interconnected by a para-substituted benzene ring as the central linking unit and three alkoxy chains at each terminal have been synthesised, and investigated by polarising microscopy, DSC and XRD scattering. Molecules with medium chain length form exclusively hexagonal columnar liquid crystalline phases.     

Heterogeneous dynamics in columnar liquid crystals of parallel hard rods

In the wake of previous studies on the rattling-and-jumping diffusion in smectic liquid crystal phases of colloidal rods, we analyze here for the first time the heterogeneous dynamics in columnar phases. More specifically, we perform computer simulations to investigate the relaxation dynamics of a binary mixture of perfectly aligned hard spherocylinders. We detect that the columnar arrangement of the system produces free-energy barriers that the particles should overcome to jump from one column to another, thus determining a hopping-type diffusion. This phenomenon accounts for the non-Gaussian intercolumn diffusion and shows a two-step structural relaxation that is remarkably analogous to that of out-of-equilibrium glass-forming systems and gels. Surprisingly enough, slight deviations from the behavior of simple liquids due to transient cages is also observed in the direction perpendicular to this plane, where the system is usually referred to as liquidlike.     

Evidence of nematic, hexagonal and rectangular columnar phases in thermotropic ionic liquid crystals

Dithiolium salts, with amphipathic character, are compounds of choice for investigations of the influence of an ionic feature upon mesomorphic properties. In this way, salts bearing a branched chain have been studied by SAXS. In spite of their rod-like shape, they exhibit only columnar mesophases, the supramolecular organization of which is close to that of cylindrical inverted micelles. Moreover, the nematic columnar phase, characterized by the loss of lateral positional correlations of the columns themselves, is one of the first examples of such behaviour in the case of thermotropic liquid crystals.     

Thermotropic liquid crystals based on chito-oligosaccharides. II. Discotic columnar liquid crystals in chitobiose octaalkanoates and chitotriose hendecaalkanoates

Chitobiose octaalkanoates and chitotriose hendecaalkanoates with varying acyl pendant lengths were synthesized and their mesophase properties studied. Both series of derivatives showed an enantiotropic mesophase in a wide temperature region below 200°C. An X-ray diffraction analysis revealed the mesophase to be of a hexagonal columnar type, in which the columns built up by a periodic stacking of chitobiose or chitotriose cores are packed into a two dimensional hexagonal lattice. The mesophase is thus similar to the hexagonal ordered columnar (D_ho) phase in discotics. Compared with cello-oligosaccharide counterparts, the diameter of the column is fairly large and the stacking period somewhat short; these can be interpreted as resulting from the intermolecular hydrogen bonding which is formed between the secondary amide group in the C2 position and the ester group.     

Star-shaped mesogens of triazine-based dendrons and dendrimers as unconventional columnar liquid crystals

Dendrons Gn-Cl and Gn-NH (n = 2-4) and novel dendrimers Gn-N approximately N-Gn (n = 2-4) based on triazine and piperazine units were efficiently prepared in good yields without employing the protection and deprotection processes and are fully characterized by 1H NMR and 13C NMR spectroscopies, elemental analysis, and mass spectroscopy. These compounds are transparent and possess good thermal stability. G4-Cl shows a monotropic columnar phase in a narrow range with a coexisting crystalline phase. Dendron G4-NH shows a rectangular column-phase, and dendrimer G4-N approximately N-G4 exhibits a monotropic hexagonal columnar phase. These identifications were supported by the polarizing optical scope and powder XRD studies.     

Molecular control of macroscopic cubic, columnar, and lamellar organizations in luminescent lanthanide-containing thermotropic liquid crystals

The connection of lipophilic gallic acid derivatives at the 5,5'- or 6,6'-positions of the rigid 2,6-bis(1-ethyl-benzimidazol-2-yl)pyridine core provides two pro-mesogenic tridentate ligands L10 and L12, whose molecular shapes, anisometries, and directional intermolecular pi-stacking can be tuned. X-ray diffraction data in the crystalline state, combined with solution 1H NMR measurements, show that complexation with trivalent lanthanides, Ln(III), produces the neutral hemi-disklike complexes [Ln(Li)(NO3)3] (i = 10, 12), which dimerize to give the rodlike bimetallic complexes [Ln2(Li)2(NO3)6] at lower temperature. The relevant thermodynamic parameters for the latter process depend on the nature of the ligand, the size of the metal ion, and the strength of the intermolecular interactions involved in the condensed phase. These three-dimensional models obtained for the complexes in the crystals and in solution are eventually confronted with small-angle XRD profiles recorded in the intermediate thermotropic liquid crystalline phase, in which the rigidity of the packed polyaromatic cores is maintained, while the alkyl chains are molten. According to the specific geometries and nuclearities of the molecular complexes, three types of mesophases (lamellar, columnar, and cubic) can be induced, which provides a direct correlation between the microscopic arrangements and the macroscopic ordering in lanthanide-containing metallomesogens.     

Characterization by mirage effect technique of dye diffusion in columnar discotic liquid crystals

The anisotropic behaviour of the matter diffusion in columnar discotic liquid crystals is studied by the 'mirage effect' technique. The D_h and D₀ mesophases of C₈HET and C₁₁HET, respectively, are considered. The impurity (a dye, the 1-[4-(xylylazo)xylylazo]-2-naphthol) diffusing in these mesophases is detected by the photothermal deflection technique. Measurements of the diffusion coefficients are performed in two perpendicular directions, along and perpendicularly to the molecular columns. Effects of impurity size, length and type of the branched chains on the discs of triphenylene, and molecule stackings in columns, are presented.     

Spiropyran-based liquid crystals: the formation of columnar phases via acid-induced spiro-merocyanine isomerisation

Hexagonal columnar liquid-crystalline phases are induced for a new fan-shaped spiropyran compound as the result of an acidichromism effect of spiro-merocyanine isomerisation through protonation upon incorporation of 4-methylbenzenesulfonic acid.     

Transfer-matrix theory of the crystal–columnar transition in polymeric liquid crystals

The statistical–mechanical problem of the transition between crystalline and columnar phases in a main-chain liquid–crystalline polymer is treated in a simple model in which only longitudinal motions of the polymer chains are permitted. A mean-field approximation for the interchain potential is used to obtain a self-consistent equation for the crystalcolumnar transition temperature. When applied to typical homopolymers this theory correctly predicts transition temperatures above the degradation temperature; when applied to a crude model of a random copolymer a temperature in the observed range is predicted.