Supermolecular Liquid Crystals - Liquid Crystalline Functional Assemblies and Their Supramolecular Structures - page 19

Fig. 20 Effect of density of mesogens and number generation in multipedal and dendritic
carbosilazanes
2.2
Effect of Spacer Length
The effect of the length of the linking chain between the mesogenic units and
the scaffold were investigated using simple model tetrahedral systems with
cubic symmetry that were created by reacting tetrakis -(dimethylsiloxy)silane
with a variety of alkenyloxy-cyanobiphenyls [77, 78], thereby generating a set
of tetrahedrally substituted mesogenic supermolecules, see Fig. 21 where four
mesogenic units are tethered to a central point. The figure shows that the
tetramers exhibit smectic A liquid crystal phases which are preferred over
the nematic phase exhibited by the mesogenic monomers themselves. In
this sense, these supermolecules are similar in mesophase behavior to con-
ventional side-chain liquid crystal polysiloxanes which have cyanobiphenyl
moieties as the mesogenic side group.
In general, the smectic A phase has a structure where rod-like molecules
pack in diffuse layers where there is no positional ordering of the molecules
in the plane and out of the plane of the layers [79]. The only way in which
the tetramers can form such a phase is by molecular distortion away from
a spherical shape to give a rod-like conformational structuring. Molecular