Supermolecular Liquid Crystals - Liquid Crystalline Functional Assemblies and Their Supramolecular Structures - page 6

Fig. 4 Systematic study of the change in linking groups in dimeric and trimeric liquid
crystals
the two mesogenic units is odd, the overall structure is bent, and therefore
antiferroelectric phases are stabilized.
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 behav-
ior. 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 meso-
genic 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 methy-
lene 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 ne-
cessary to have all of the aromatic/rigid units being supportive of mesophase
formation for a supermolecular material to be mesomorphic. Odd par-