The Structural Origins of Intense Circular Dichroism in a Waggling Helicene Nanoribbon

We report the synthesis
of a new perylene-diimide-based helical
nanoribbon, which exhibits the largest molar electronic circular dichroism
in the visible range of any molecule. This nanoribbon also displays
a substantial increase in molar circular dichroism relative to a smaller
helical analogue, even though they share a similar structure: both
nanoribbons incorporate two conformationally dynamic double-[4]­helicene
termini and a rigid [6]­helicene-based core within their helical superstructures.
Using DFT and TDDFT calculations, we find that the double-[4]­helicenes
within both nanoribbons orient similarly in solution; as such, conformational
differences do not account for the disparities in circular dichroism.
Instead, our results implicate the configuration of the double-[6]­helicene
within the larger nanoribbon as the source of the observed chiroptical
amplification.