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.