Defining the Structures of Supermolecules

The type of spacer chain can be varied without change to the mesogenic units. For example, the second structure in Fig. 2 has a short dimethylsilox-ane spacer included in the aliphatic linking chain. The inclusion of such groups can affect the overall packing of the molecules leading to reductions in melting points and the stabilization of lamellar smectic phases over nematic phases. 

Not only are there examples of dimers based on mesogens with rod-like structures, there are also examples reported of dimers composed of disclike mesogens, see bottom structure in Fig. 2. Generally for these types of material the pattern of mesomorphic behavior follows that of the analogous monomers. 

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. 

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