Discotic liquid crystals solution

Chiral Lyotropic Discotic Liquid Crystals and Self-Assembly of Chiral Discotics in Dilute Solution... 

It was quickly recognized that chirality would play an important role in discotic liquid crystals, not only for the possibility of creating cholesteric and ferroelectric liquid crystals but also as a tool for studying the self-assembly of these molecules as a whole, both in solution and in the solid state. However, initial studies revealed that expression of chirality in discotic liquid crystals was not as straightforward as for liquid crystals derived from calamitic molecules. More recently, with the increase in interest in self-assembly and molecular recognition, considerably more attention has been directed to the study of chiral discotics and their assemblies in solution. The objective of this chapter is... 

To understand how chirality is expressed, it is important to first describe the different thermotropic mesophase assemblies which can be formed by chiral discotics. Even though expression of chirality has been observed in thermotropic mesophases, the chiral expression occurs in a rather uncontrolled manner, and systems which are suitable for applications, for example, easily switchable columns/ferroelectric discotic liquid crystals, consequently have not yet been developed. Hence, the assembly of discotics in solution has received considerable attention. Supramolecular assemblies of discotic molecules in solution are still in their infancy and have not yet found commercial application, but they are of fundamental importance since they allow a detailed and focused investigation of the specific interactions that are required to express chirality at higher levels of organization. As such, the fundamental knowledge acquired from supramolecular assemblies in solution might formulate the design criteria for thermotropic chiral discotic mesophases and provide the necessary tools for the creation of functional systems. 

CHIRAL LYOTROPIC DISCOTIC LIQUID CRYSTALS AND SELF-ASSEMBLY OF CHIRAL DISCOTICS IN DILUTE SOLUTION... 

Since their discovery as discotic liquid crystals, both triphenylenes and phthalocyanines have been studied as chiral columns [74]. There are no chirooptic signatures such as circular dichroism or optical rotation changes for the triphenylenes that are substituted with chiral side chains. For the phthalocyanines, Nolte and coworkers have shown that when their exterior is substituted with chiral side chains the phthalocyanine core shows an altered CD spectrum in aggregates, in thin films and in concentrated solutions [75]. These helical stacks, wrapped like ropes, can be seen with electron... 

Finally, phase biaxiality has been studied by dissolving deuterated solute molecules in discotic liquid crystals [3.32]. Since discotic liquid crystals have negative Ax, an aligned sample consisting of one type of deuteron will exhibit a planar powder pattern (Fig. 3.7) without the necessity of sample rotation as in the conventional rod-like liquid crystals. Two discotic phases, Drd and have been studied by spectral pattern simulation using nonzero [3.32]. 

Lyotropic liquid crystal phases are formed by amphiphihc molecules (surfactants, block copolymers) in solution, driven by repulsive forces between hydrophobic and hydrophihc parts. In a polar solvent, the hydrophihc parts associate with the solvent, whereas the hydrophobic parts interact to form the interiors of micelles (as in low-molecular smfactants). Micelles can be spherical, rod-like or discotic in shape. The contour of the micelle is determined by the relative sizes of the hydrophihc and hydrophobic groups. MiceUe shapes are influenced by solvent, concentration and temperatme. 

Both calamitic and discotic liqttid crystals ate also called thermotropic liquid crystals because the liquid crystal phase is stable for a certain temperature interval. Pure compounds or mixtures of compounds fall into this category. There is another type of molecnle, however, which forms liqnid crystal phases only when mixed with a solvent of some kind. For these compounds, the concentration of the solution is just as important, if not more important, than the temperatnre in determining whether a liquid crystal phase is stable. To differentiate these substances from thermotropic liquid crystals (which need no solvent), these compotmds have been given the name lyotropic liquid crystals. 

Thus, supermolecular liquid crystals with a cyclotriphosphazene dendritic core and polycatenar mesogenic units (144) were obtained in three steps by the conventional sequence of substitution (i), derivatization (ii and iii) methods from [N3P3CI6] (Scheme 9). Due to the microsegregation of the alkyl chains and the aromatic central cores and the space-filling properties, compounds (144) adopt a discotic conformation assembled in a columnar mesophase and illustrate the possibilities of using cyclotriphosphazenes for the design of columnar assemblies at room temperature, in the mesophase or in a vitrified solid state with interest for applications in material science. Similarly, the new family of solution processable, photoluminescent, monodisperse nanocomposite dendrimers (145) (Tg > 165 °C, > 465 °C)... 

For UV and fluorescence measurements, the most commonly used liquid crystals are mixtures of the nematic 4 -alkylbicyclo-hexyl-4-carbonitrile s (CCH) (e.g., ZLI 1167 and 1695), which are transparent down to 200 nm and exhibit nematic ranges between =30 and 80 °C (see, for example, [315, 331, 333, 334]), and various cholesteric or compensated nematic phases of cholesteryl chloride/cholesteryl ester mixtures, which are transparent to =240 nm [310, 313, 326, 329]. Some use has also been made of 4 -(4-alkylcyclohexyl)benzo-nitriles (PCH-n), which are transparent to =290 nm [330, 335]. Several other meso-phases, including thermotropic smectics, discotics, and lyotropic phases, have low absorption in the UV region and have been used from time to time as well. The most commonly used liquid crystals in FTIR studies are the CCH-mixtures ZLI 1167 and 1695 [314, 321, 336, 337]. The orientation of the liquid crystalline solution is most commonly achieved either by cell surface treatment or the application of an electric or magnetic field. 

---