Discotic Triphenylene Twins Linked through Thiophene Bridges: Controlling Nematic Behavior in an Intriguing Class of Functional Organic Materials

Substituted triphenylenes and similar discotic molecules
have a
strong tendency toward columnar organization. Nematic mesophases are
much less commonly observed in discotic systems. We have demonstrated
a general strategy whereby discotic triphenylenes can be twinned to
form stable, boardlike materials that display only nematic mesophases.
The dominant structural feature that leads to nematic behavior is
an enforced void region in the center of the macrocycle that results
from bridging through the triphenylene 3,6-positions. This precludes
simple columnar assembly because it would lead to free space through
the center of each stack. Selection of appropriate bridging units
allows materials to be designed which combine molecular features,
such as the optoelectronic properties of electron-rich triphenylenes
and conjugated thiophene units, with the processability, self-healing,
and alignment features inherent in nematic mesophases. In addition,
communication across twinned structures can lead to additional enhancement
of optoelectronic behavior. This is particularly apparent in fully
conjugated, planar twin 12 which is formally expected
to have some antiaromatic character. This character is manifested
in its spectral properties, and particularly noteworthy is its strong,
Stokes shifted emission at around 500 nm.