Helically Locked Tethered Twistacenes

Twisting
linear acenes out of planarity affects their electronic
and optical properties, and induces chirality. However, it is difficult
to isolate the effect of twisting from the substituent effect. Moreover,
many twistacenes (twisted acenes) readily racemize in solution. Here,
we introduce a series of twistacenes having an anthracene backbone
diagonally tethered by an n-alkyl bridge, which induces
a twist of various angles. This allows us to systematically monitor
the effect of twisting on electronic and optical properties. We find
that absorption is bathochromically shifted with increasing twist,
while fluorescence quantum efficiency drops dramatically. The tethered
twistacenes were isolated to their enantiomerically pure form, displaying
strong chiroptical properties and anisotropy factor (g-value). No racemization was observed even upon prolonged heating,
rendering these tethered twistacenes suitable as enantiopure helical
building units for π-conjugated backbones.