Chiral mesoscopic

This strategy was subsequently extended to produce a series of chiral mesoscopic macrocycles of controllable... 

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... 

Praefcke et al.47 and Boden et al.48 have both synthesized triphenylene derivatives with halogens attached to the core (46). It was assumed that the resulting chirality would express itself more strongly than chirality present in the side chains. These halogenated compounds could be readily modified to give the hepta-alkylated triphenylenes however, in spite of calculations which revealed that the triphenylene core indeed becomes helical, no expression of the helicity at the mesoscopic level could be achieved. 

In this section we will address the issue of chirality in self-assembled discotic molecules in dilute solution. We will show how and why chirality is expressed at the mesoscopic level of the self-assembly and make clear that unique information concerning expression of chirality can be gathered from assemblies of discotics in solution. 

Phthalocyanines provided with crown ethers and chiral alkyl side chains (64) have also been reported to self-assemble in nonpolar solvents.77 These molecules form helical columns as could be deduced from the presence of a Cotton effect upon aggregation. To investigate the expression of chirality at the mesoscopic level, sergeants-and-soldiers experiments75 were performed. Chiral phthalocyanine molecules 64 were used as the sergeants and achiral... 

Jiang, H. and Lin, W. (2006) Directed assembly of mesoscopic metallocycles with controllable size, chirality, and functionality based on the robust Pt-alkynyl linkage. J. Am. Chem. Soc., 128 (34), 11286-11297. 

Terms such as macroscopic, nanoscopic, and mesoscopic chirality are known from studies by Mislow [a) A. B. Buda, T. Auf der Heyde, K. Mislow, Angew. Chem. 1992, 104, 1012-1031 Angew. Chem. Int. Ed. 1992, 32, 989-1007] and Avnir [b)... 

In the domain of mesoscopic objects (a few nm), there are also chiral representatives [6], for instance, the related problems studied by Whitesides [7]. Leaving aside... 

Dyes can also be incorporated into hydrogen-bonded superstructures using the complementarity of the hydrogen bond donors and acceptors. The addition of a perylene bisimide dye to chiral dialkyl melamine derivatives leads to aggregates (Fig. 26) which show induced circular dichroism in the dye part of the assembly in methylcyclohexane as well as the formation of mesoscopic fibres when the solvent is evaporated [227]. The melamine compounds... 

Supramolecular architectures are highly sensitive to chiral perturbations in general, and in systems that form liquid crystals in particular. Small amounts of enantiopure guest molecule added to a nematic host can induce a transition to a cholesteric phase, and the helical organization in the mesoscopic system is very sensitive to the structure of the guest molecule. Chiral amplification was successfully achieved in such liquid crystals, using CPL as the chiral trigger for the phase transition [183]. 

Since the discovery of the first liquid crystalline material in 1888, helicity has proven to be one of the most fascinating topics in this field."" Several liquid crystalline phases with helical structure were reported, such as chlolesteric phase, blue phase, ferroelectric and antiferro-electric smectic phases, and helical smectic A phase. In most of these helical phases, at least a fraction of the constituent molecules have an asymmetric carbon, and it was long believed that chirality at a molecular level is a prerequisite to construct chiral architectures at the mesoscopic level. However, Watanabe et al. reported the first example of spontaneous helix formation in liquid crys-... 

Whilst tubules are hollow fibers, rodlike fibers with no hollow may also reveal the chirality of their molecular components at a mesoscopic scale. However, rodlike fibers are often very thin and structurally more difficult to... 

The foregoing studies show that chiral supramolecular modules could be obtained from self-assembly of achiral molecule components, and further assembly generates supercoil structures in a mesoscopic dimension, just like the behavior of some chiral molecules. Appropriate manipulation and combination of intermole-cular noncovalent interactions can provide new routes to constructing artificial or-... 

In liquid crystals, the chirality from molecular level usually transfers to mesoscopic level by forming helical structures. The way in which chiral information is transferred from stereo-centers to molecular, supramolecular, macromolecular and ultimately to mesoscopic dimensions is an intriguing puzzle. For further reading of the problem, we recommend the papers by Lubensky et al., by Kubal, and by Lemieux. ... 

To relate macroscopic properties, especially the results of chirality measurements, to mesoscopic and further to pseudoscalar molecular properties, experimental data should be available in order to develop and check structure-property relations, mechanisms, and models. A set of usable data for the chiral nematic phase consists of the composition of the phase, p, HTP, V.2, Ki, K2, and K3. On the microscopic scale the (HTP)i as well as the microscopic order paramet S, D, A, B, the helicity tensor Qy or the chirality interaction tensor W y, and a chirality tensor Cy or an equivalent quantity should be known for every component. At the moment, the coordinates W y can only be estimated because a variation of the order of the chiral dopant for a constant host order needs to be known for their measurement. Some data are collected in TABLE 1 in order to give a feeling for the size and sign of available quantities [25-33] but, as can be seen, no complete sets of data for a system have yet beoi given in the literature. 

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