Supermolecular Liquid Crystals - Liquid Crystalline Functional Assemblies and Their Supramolecular Structures - page 53

the form of a weak and broad peak at 38.2 ◦ C( H = 0.87 kJ mol -1 ). The only

other thermal event present was a glass transition below room temperature
at -7.9 ◦ C. Only when the sample was left standing at room temperature for
3 weeks did a broad, strong endotherm with onset at 31.6 ◦ Cwithashoulder
at 42.6 ◦ C occur. However, these thermal events were not present in succes-
sive heating and cooling cycles, suggesting that crystallization only occurs on
standing after a long period of time.
Comparison of the phase behavior of compounds 52 and 53 shows clearly
thattheoveralltopologyofthemoleculeinrespecttotheinnercore(i.e.,
which hemisphere carries what mesogen) plays a significant role in deter-
mining the type of mesophase formed, since in both cases the number of
mesogens of each type and the core are the same and simply by placing them
in different hemispheres changes the mesophase exhibited.
The manipulation of the structural fragments (mesogenic units, central
scaffold, and linking units) in the molecular design of such supermolecu-
lar systems potentially allows one to vary mesophase type and therefore the
physical properties and potential applications of materials. Thus the molecu-
lar design of these systems is flexible and potentially capable of incorporating
functional units, thereby allowing us to take some steps towards the molecu-
lar and functional complexity found in living systems.

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3.2
Functional Liquid-Crystalline Supermolecules

The concept of creating functional materials by incorporating a certain func-
tionality within a liquid-crystalline molecule, through covalent attachment,
is a bottom-up approach to self-organizing functional materials. In the fol-
lowing examples, fullerene is incorporated as the functional unit into self-
organizing supermolecular systems. Fullerene is of interest because of its
unusual physical properties, and it is fascinating to see if such a large non-
mesogenic unit affects mesomorphic behavior. Moreover, the synthesis of
fullerene-containing liquid crystals is expected to open new avenues in the
design of self-organized structures containing the fullerene unit [116-122].
This approach to self-organization appears to be particularly interesting for
other functional groups which are not well adapted to being organized in
nanoscale architectures.
The first set of examples described are those which examine the proportion
of mesomorphic groups that are required in the structure of a supermolecule
for it to be mesogenic, and secondly to investigate the incorporation of sec-
ond functional group in the system which might contribute to the activity of
the fullerene unit, see Fig. 64. The parent multipede 54 [123] exhibits a smec-
tic A phase with a clearing point of 168 ◦ C, the functional material 55 clears at
171 ◦ C, and similarly functional multipede 56 clears at 169 ◦ C [124]. All three
materials also exhibit glassy states with transitions to the smectic A phase at