Access to benzo-fused aza[7]helicene: Via unexpected indolization of alkyne-amine

Abstract

Benzo-fused aza[7]helicene derivatives containing two and three pentagon rings were synthesized via highly efficient cyclization of alkyne-amine to form quinolone-fused indole as the key step. The quinolone-fused indole demonstrated aggregation-induced emission enhancement due to the formation of J-aggregates. The diaza[7]helicene compounds bearing two pentagon rings possessed high racemization energies and could be chirally resolved into their enantiomers by chiral HPLC. The P- and M-enantiomers were further characterized by circular dichroism spectroscopy. Meanwhile, triaza[7]helicene containing three pyrrole rings showed a relatively low enantiomer inversion barrier at room temperature and it was difficult to perform the chiral separation, owing to the influence of the number of pentagon rings on the helical dimension. However, the spectrographic analysis of triaza[7]helicene revealed an improved intramolecular charge transfer process due to the presence of electron-rich pyrrole in the molecular center.