Compressing a Non‐Planar Aromatic Heterocyclic [7]Helicene to a Planar Hetero[8]Circulene.

This work describes a synthetic approach where a non‐planar aromatic heterocyclic [7]helicene is compressed to yield a hetero[8]circulene containing an inner antiaromatic cyclooctatetraene (COT) core. This [8]circulene consists of four benzene rings and four heterocyclic rings, and it is the first heterocyclic [8]circulene containing three different heteroatoms. The synthetic pathway proceeds via a the flattened dehydro‐hetero[7]helicene, which is partially a helicene and partially a circulene: it is non‐planar and helically chiral as helicenes, and contains a COT motif like [8]circulenes. The antiaromaticity of the COT core is confirmed by nucleus independent chemical shift (NICS) calculations. The planarization from a helically π‐conjugated [7]helicene to a fully planar heterocyclic [8]circulene significantly alters the spectroscopic properties of the molecules. Post‐functionalization of the [7]helicenes and the [8]circulenes by oxygenation of the thiophene rings to the corresponding thiophene‐sulfones allows an almost complete fluorescence emission coverage of the visible region of the optical spectrum (400–700 nm). [ABSTRACT FROM AUTHOR]