Mesomorphic [2]Rotaxanes: Sheltering Ionic Cores with Interlocking Components

Two types of liquid crystalline [2]­rotaxanes based on
a conventional
tetracatenar motif (a rod-shaped molecule with two side chains at
each end) have been prepared. Dicationic compounds with ester stoppers
and tetracationic materials with pyridinium stoppers are compared
to each other and their dumbbell shaped analogs. Since the ionic core
contributes about 70% to the overall length and molecular weight of
the molecules, sheltering the ionic cores with an interlocked neutral
macrocycle has considerable effect on the mesomorphism and thermal
stability of the materials. The influence of the sheltering macrocycle,
the numbers of charges on the core and the size and nature of the
side chains (aliphatic vs siloxane) were probed. [2]­Rotaxanes with
linear side chains and minimum ratios of chain-to-core volumes of
about 0.35 and 0.30 for tetra- and dicationic compounds, respectively,
display smectic liquid crystal phases. Larger ratios increase the
temperature range of the smectic A phases beyond the decomposition
temperatures; a disadvantage for processing because no stable isotropic
liquid phase is available. The change from tetra- to dicationic [2]­rotaxanes
increased not only the fluidity of their smectic A phases but also
their thermal and chemical stability. Branched side chains (2-hexyldecyl)
disfavor the formation of lamellar mesophases and, instead, induce
higher ordered soft crystal phases. No liquid crystal phases but soft
crystal phases are observed for the analogous di- and tetracationic
compounds without an ion sheltering interlocked macrocycle (dumbbells).