Supermolecular Liquid Crystals - Liquid Crystalline Functional Assemblies and Their Supramolecular Structures - page 18

scaffold, thereby providing for a more complete study. The synthetic path-
way to the various carbosilazane scaffolds is shown in Fig. 19. Derivatization
of the double bonds with hydrosiloxane derivatized mesogens results in the
formation of multipedal and dendritic liquid crystals [50-53].
Figure 20 shows a list of the transition temperatures for the various
polypedes and dendrimers that were prepared. It is interesting that the sys-
tems bearing four aromatic rings, 22 to 26 exhibited polymorphism, whereas
the three-ring systems, 17 to 21 , did not. All of the materials, however, ex-
hibited the nematic phase, in keeping with the earlier description of lateral
versus terminal appendages of the mesogens to the central scaffold. The four-
ring systems also exhibited crystalline phases whereas the three-ring systems
essentially formed glassy states at low temperature. This effect is probably re-
lated to the fact that the systems with four-ring mesogens are behaving as
though they are a collection of individual mesogens, ie, the system is being
dominated by the liquid crystal units because the length to breath ratio is
quite high. The three-ring systems on the other hand have greater flexibility
due to the lower length to breath ratios, and the relative dynamics will ensure
that these materials will only exhibit nematic phases. Overall it is also very
interesting that the three-ring systems are in their nematic phases at room
temperature down to temperatures well below 0 ◦ C!
Fig. 19 Synthetic scheme to prepare carbosilazane-based dendritic scaffolds