A Curious Case of Two‐Photon Absorption in n‐Helicene and n‐Phenylene, n=6–10: Why n=7 is Different?

Abstract

Abstractn‐Helicenes and n‐Phenylenes are interesting examples of twisted molecules, where although the atoms are connected through conjugated ‐bonds, the ‐conjugation is largely hindered by the twisted nature of the bonds. Such structures provide a unique opportunity to study the effect of twisted ‐system on non‐linear optical properties. In this work, we studied the two‐photon absorption in donor‐acceptor substituted n‐helicenes and n‐phenylenes employing the state‐of‐the‐art RI‐CC2 method and reported a unique feature we observed in n=7 systems. We found that both 7‐helicene and 7‐phenylene systems exhibit largest two‐photon absorption than other members in their respective classes. Furthermore, using generalized few‐state model, we provided a detailed microscopic mechanism of this unique observation involving participation of different transition dipole moment vectors and their relative orientations.