Supermolecular Liquid Crystals - Liquid Crystalline Functional Assemblies and Their Supramolecular Structures - page 29

Fig. 32 A nematogenic supermolecular octasilsesquioxane, 35 , with laterally appended
mesogenic groups
The structure of the nematic phase is proposed to be one where the cen-
tral cores are surrounded by the mesogenic groups, with the long axes of the
mesogens pointing in roughly the same direction, as shown in Fig. 33.
Terminally appended systems on the other hand tend to have more pro-
nounced stability of the smectic phases, and indeed in some cases poly-
morphism is also observed. For example, the substitution of silsesquiox-
anes with mesogenic groups that are conducive to smectic mesophase for-
mation, yields supermolecular materials that as expected exhibit smectic
phases. For the many materials studied, the formation of smectic phases
and mesophase temperature ranges are considerably enhanced for the super-
molecular material over those of the individual mesogenic parents. In the
case of substitution of the hexasilsesquioxane scaffold with the 4 -(2-methyl
butylbenzoylxoy)biphenyl-4-carboxylate mesogenic unit, 36 , see Fig. 34, [81]
which favors the formation of orthogonal smectic A and tilted smectics C,
I, and F phases, the supermolecule exhibits only the tilted smectic C phase.
Smectic polymorphism is suppressed, which is important for uses of such
materials in various applications. For compound 36 the smectic C phase
is present from 11.5 to 191.5 ◦ C, i.e., a temperature range of over 180 ◦ C,
whereas it is only present over a temperature range of 22-23 ◦ Cforthemeso-
genic monomer unit. The coupling of the potential stability of the smectic C
phase with chirality built into the mesogenic units allows for the possibility
of synthesizing supermolecular ferroelectric materials, which may have inter-