Chirality Amplified: Long, Discrete Helicene Nanoribbons

Here
we report the synthesis of two polyhelicene frameworks consisting,
from end-to-end, of 18 and 24 fused benzene rings. The latter exhibits
the largest electronic circular dichroism in the visible spectrum
of any molecule. These shape-persistent helical nanoribbons incorporate
multiple helicenes, a class of contorted polycyclic aromatic molecules
consisting of ortho-annulated rings. These conjugated,
chiral molecules have interesting chemical, biological, and chiroptical
properties; however, there are very few helicenes with extraordinary
chiroptical response over a broad range of the visible spectruma
key criterion for applications such as chiral optoelectronics. In
this report, we show that coupling the polyhelicene framework with
multiple perylene-diimide subunits elicits a significant chiroptic
response. Notably, the molar circular dichroism increases faster than
the absorptivity of these molecules as their helical axis lengthens.
Computational analysis reveals that the greatly amplified circular
dichroism arises from exciton-like interactions between the perylene-diimide
and the helicene moieties. We predict that even greater chiroptic
enhancement will result from further axial elongation of these nanoribbons,
which can be readily enabled via the iterative synthetic method presented
herein.