Supermolecular Liquid Crystals - Liquid Crystalline Functional Assemblies and Their Supramolecular Structures - page 34

Fig. 39 Synthesis of supermolecule 39
dendrimers, which has been interpreted as evidence of a globular conform-
ation in solution.
Multipede 39 shows, see Fig. 40, a phase sequence of g -17.5 SmC 63.1
SmA 1 91.7 ◦ C Iso Liq. Thus the effective doubling of the number of meso-
genic units results in the lowering of the clearing point, the SmC to SmA
transition and the glass transition relative to 37 . This thermal behavior is
in contrast with that normally found for liquid crystal dendrimers based on
flexible dendritic cores, where the clearing points increase and the glass tran-
sitions remain almost constant with generation number [87-91].
The mesophase behavior of 39 implies that the dendrimer must, like su-
permolecule 37 , also have a rod-like shape in order that it can pack in layers,
where the molecules within the layers are disordered and the layer structure
is diffuse, revealing that the mesogenic state deforms the globular environ-
ment of the dendrimer. This is remarkable given that the number of meso-
genic units is doubled relative to the number of octasilsesquioxane scaffolds,
see Fig. 41, and where as a consequence one would expect curvature in the
packing of the mesogenic units to occur due to their bunching about the scaf-
folds. Again, unlike the parent mesogenic system, the smectic A phase of the
dendrimer forms a monolayer smectic A phase (SmA 1 ). As with other den-
drimers bearing terminal nitrile groups, a smectic C phase is also formed,
which for the parent system is almost unheard of.
The point of bifurcation, so that 16 mesogenic units can be more eas-
ily located around the octasilsesquioxane core, can be achieved by moving
the bifurcation point away from the central part of the scaffold as shown for