Mesophase assembly

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. 

Figure 8.9 Schematic representation of a conical monodendron self-assembled into a supramolecular spherical micellar dendrimer, and then into an fm3m cubic mesophase.

Fig. 2. Schematic representation of the supramolecular cylinders of the dendrimer derived from macromonomer 9 (R=OC12H25,n=3) in the Qh mesophase atop view of a cylinder containing six repeat units in a stratum with the alkyl tails melted to match the average column radius determined by X-ray scattering experiments b side view of a cylinder containing 30 repeat units of the polymer assembled with melted alkyl tails. Reproduced with permission from references 5 a...

Thomas, A. Schlaad, H. Smarsly, B. Antonietti, M. 2003. Replication of lyotropic block copolymer mesophases into porous silica by nanocasting Learning about finer details of polymer self-assembly. Langmuir 19 4455 1459. 

Gibaud, A. Grosso, D. Smarsly, B. Baptiste, A. Bardeau, J. F. Babonneau, F. Doshi, D. A. Chen, Z. Brinker, C. I Sanchez, C. 2003. Evaporation-controlled self-assembly of silica surfactant mesophases. J. Phys. Chem. B 107 6114-6118. 

Reinitzer discovered liquid crystallinity in 1888 the so-called fourth state of matter.4 Liquid crystalline molecules combine the properties of mobility of liquids and orientational order of crystals. This phenomenon results from the anisotropy in the molecules from which the liquid crystals are built. Different factors may govern this anisotropy, for example, the presence of polar and apolar parts in the molecule, the fact that it contains flexible and rigid parts, or often a combination of both. Liquid crystals may be thermotropic, being a state of matter in between the solid and the liquid phase, or they may be lyotropic, that is, ordering induced by the solvent. In the latter case the solvent usually solvates a certain part of the molecule while the other part of the molecule helps induce aggregation, leading to mesoscopic assemblies. The first thermotropic mesophase discovered was a chiral nematic or cholesteric phase (N )4 named after the fact that it was observed in a cholesterol derivative. In hindsight, one can conclude that this was not the simplest mesophase possible. In fact, this mesophase is chiral, since the molecules are ordered in... 

Nuckolls and Katz have synthesized discotic liquid crystalline molecules in which the core is a helix in its own right.37 Nonracemic helicene 33 was found to assemble into a columnar mesophase in which the helicenes stack on top of each other. CD spectroscopy showed a strong increase of the Cotton effect upon going from the molecularly dissolved state to the aggregated state, exhibiting an amplification of chirality. These helical columns give rise to a strong expression of chirality because the intrinsic shape of the helicenes... 

Mortensen K (2000) Small angle scattering studies of block copolymer micelles, micellar mesophases and networks. In Alexandridis P, Lindman B (eds) Amphiphilic block copolymers self assembly and applications. Elsevier, Amsterdam... 

Assembly of molecules in a nematic mesophase having a short-range smectic-like array of the constituent molecules. 

Mesophase in which disc-shaped molecules, the disc-shaped moieties of macromolecules or wedge-shaped molecules assemble themselves in columns packed parallel on a two-dimensional lattice, but without long-range positional correlations along the columns. 

Mesophase in which board-shaped molecules assemble in stacks packed parallel to one another on a one- or two-dimensional lattice (see Figs. 17 and 18). 

The mesogenic structures of glycolipids are due to the occurrence, on the same molecule, of a hydrophilic and a hydrophobic moiety often referred to as head and tail respectively. As a result, glycolipids are able to self-organize into a large variety of mesophases also called liquid crystals (Fig. 2) [ 10]. Supramolecular assemblies of mesogenic compounds can be caused by a rise in temperature (thermotropic liquid crystals) or by the addition of water (lyotropic liquid crystals) they result from different responses of the carbohydrate and the alkyl chain to temperature or solvent (water), respectively. 

At higher concentrations, micelles assemble in turn, to form hexagonal or cubic phases while longer chains or multi-chain compounds afford lamellar phases in which the amphiphilic derivative is arranged in parallel bilayers, separated by water. The succession of mesophases depending on temperature and concentration of the amphiphile can be visualized in a phase diagram (Fig. 3 c). 

As its name suggests, a liquid crystal is a fluid (liquid) with some long-range order (crystal) and therefore has properties of both states mobility as a liquid, self-assembly, anisotropism (refractive index, electric permittivity, magnetic susceptibility, mechanical properties, depend on the direction in which they are measured) as a solid crystal. Therefore, the liquid crystalline phase is an intermediate phase between solid and liquid. In other words, macroscopically the liquid crystalline phase behaves as a liquid, but, microscopically, it resembles the solid phase. Sometimes it may be helpful to see it as an ordered liquid or a disordered solid. The liquid crystal behavior depends on the intermolecular forces, that is, if the latter are too strong or too weak the mesophase is lost. Driving forces for the formation of a mesophase are dipole-dipole, van der Waals interactions, 71—71 stacking and so on. 

This study demonstrated that the micro-mesoporous composite materials could be synthesized with two-step treatment by microwave using two different templates system with TPABr and MTAB. This formation was controlled by the self-assembly formation of supramolecular templates between MTA micelles and SiO /TPA gels. As varying microwave irradiation time of micro-mesoporous materials, gradually transition from the mesophase to micro-mesophase was occurred. These materials have higher dm spacing of mesoporous materials and lead to transition from mesophase to micro-microphase by an increment of synthetic time, while the calcined products is formed with bimodal and trimodal pore size distribution under microwave irradiation within 3 h. From TG-DTA and PL analysis, the self-assembly formation of supramolecular templates between MTA+ micelles and SiO /TPA+ gels were monitored. 

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