Supermolecular Liquid Crystals - Liquid Crystalline Functional Assemblies and Their Supramolecular Structures - page 47

It is also interesting to consider how the selection process for the helical
organization of material 47 is generated upon cooling from the isotropic li-
quid. As the C 60 core of the material is spherical, and the mesogenic units are
attached by relatively short methylene spacer units, it is not unreasonable to
assume that, in the liquid phase, the mesogenic units are symmetrically dis-
posed about the central core. Cooling into the chiral nematic phase, however,
thehelicalorganizationwasbeexpectedtobearesultoftheorganizedpack-
ing of the dendritic supermolecules, i.e., they are considered to be no longer
spherical in shape. However, it was found that when the diameter of the C 60
core is compared to the length of the mesogenic units, it is clear that flexible,
random packing of the mesogenic units about the core in the liquid crystal
statewasnotpossible,andthatthemesogensarerequiredtobeorganized
in their packing arrangements relative to one another, both on the surface
of the dendrimer and between individual dendrimer molecules. One possi-
bility was postulated where the mesogenic units are oriented parallel to one
another, thus when the material cools into the liquid-crystalline phase direc-
tional order of the mesogens is selected by the external environment, such as
the surface. In doing so, this information is transmitted to the other mesogens
associated with the spherical dendrimer and further to the neighboring den-
dritic supermolecular compounds. Alternatively, for an individual dendrimer
it was proposed that the direction of the mesogens would spiral around the
C 60 core to give poles at the top and bottom of the structure, as shown in
Fig. 57. Thus the spherical dendrimer was projected to have a well-defined
chiral surface, thereby resulting in the creation of a chiral nanoparticle, i.e.,
amolecular“ Boojum ”. When the chiral nanoparticles pack together they
were expected to do so through chiral surface recognition processes, resulting
in the formation of a helical supramolecular structure. Consequently, the chi-
ral supermolecular nanoparticles transmit their local organization through
Fig. 57 Proposed helical structure of a molecular “Boojum”