Supermolecular materials

Fig. 1 Templates for the design of liquid-crystalline supermolecular materials. The meso-genic units are shown as various shapes, however, the materials may be constructed with mesogenic units of the same type. In this case, many of the constructions shown are essentially dendritic...

Further studies by Nishiyama et al. [34-45] showed that when taken in isolation, only one of the aromatic units within a supermolecular system has a propensity to exhibit liquid crystal phases, then the supermolecular material itself could be mesomorphic, see Fig. 5. For example, for the top molecular structure, 5 [45], in Fig. 5, only the biphenyl unit at the center of the structure supports mesophase formation, whereas the benzoate units are too isolated from the biphenyl moiety in order to affect mesomorphic behavior. The second material, 6 [45] has terminal phenyl units, which are only connected by aliphatic chains to the benzoate units. Thus in this case, the material has four aromatic units out of six which are not in positions that can enhance mesophase formation. However, the second material has similar transition temperatures and phase sequences to the first, i.e., both materials exhibit an unidentified smectic phase and a synclinic ferroelectric smectic C phase. If the third material, 7 [38], is examined, it can be seen that the mesogenic unit at the center of the supermolecule is an azobenzene unit which is more strongly supportive of mesophase behavior than the simple biphenyl moiety. Thus the clearing point is higher for this material in comparison to the other two. The attachment of the terminal phenyl unit is by a methylene spacer of odd parity, and as a consequence the smectic C phase has an anticlinic structure rather than synclinic. 

These studies demonstrate that for supermolecular materials it is not necessary to have all of the aromatic/rigid units being supportive of mesophase formation for a supermolecular material to be mesomorphic. Odd par-... 

There have also been a number of examples reported of supermolecular materials composed of different mesogenic units. For example, Yelamaggad et al. [48,49] reported a supermolecular material, 8, that was composed of three different mesogenic groups linked together in a linear fashion, see Fig. 6. This material possesses a chiral steroidal unit, a linear biphenyl moiety and a photoactive azobenzene unit, and remarkably it exhibits unusual frustrated chiral mesophases. 

With trimeric supermolecular materials the situation becomes more complicated because not only are there linearly and laterally linked possible structures, but also there are structures where the mesogenic units could be linked to a central point, creating a molecular knot . In a similar way, tetra-, penta-, etc., substituted supermolecules can be created. 

Fig. 13 Template structure of a smart or functional supermolecular material...

Fig. 16 Templates for supermolecular materials bearing A-B blocks of mesogenic units, some of which are functionalized...

In the following sections we describe the structures of novel polypedal, multipedal and functional supermolecular materials. 

Fig. 17 A nematic supermolecular material which forms a rod-like conformation...

As noted earlier, the incorporation of chiral groups in the liquid crystal moieties can have the effect of inducing non-linear properties, which include thermochromism, ferroelectricity, antiferroelectricity, electrostriction, and flexoelectricity. In a now classical study, Hult [82] demonstrated that it was possible for supermolecular material 34 to exhibit two-state ferroelectric switching. The remarkable material he investigated, shown in Fig. 30, was found to exhibit two hitherto unclassified mesophases between the smectic... 

Fig. 55 Supermolecular material 47 which possesses a boojum-like structure and exhibits a chiral nematic phase...

In this brief review, we have demonstrated the potential for creating a wide variety of novel supermolecular materials with designer structural and functional properties. In our own development of new materials we have borne in... 

In contrast to supermolecular materials, a supramolecular system is a self-assembled, noncovalently bonded entity, in which molecular units are assembled through noncovalent forces to create a complex structureFor large entities, such supramolecular systems are similar in constitution to those of quaternary structures found in proteins. Thus, for a supramolecular system, it is possible to have variations in the constitution depending on how many molecular units are present and the nature of the bonding interactions. The subunits in the case of supramolecular systems need not be mesomorphic as long as the assembled entity is. 

In the following sections, liquid-crystalline supermolecular materials are discussed, followed by supramolecular systems. 

Figure 22 Smectic giant supermolecular material structure is deformed to a cylindrical shape.-...

---