Benzothiadiazole-derived double aza [5]helicenes: Synthesis, crystal structure and optical properties

Abstract

Double helicenes have attracted increasing attention of researchers for their unique structures as well as potential application in material science. Most of the double helicene compounds are all-carbon helicenes, and nitrogen-doped double helicenes are rarely synthesized and studied. As polycyclic aromatic hydrocarbons doped with heteroatoms could show specific optoelectronic properties, it is necessary to develop new strategy for the synthesis of double azahelicenes. In this work, three double aza [5]helicene compounds have been designed and synthesized through Bischler-Napieralski cyclization. These molecules have symmetrical saddle-type conformations and P/M configurations. Moreover, these molecules could exhibit positive solvatochromic properties and have large Stokes shifts in various organic solvents. According to the DFT calculation results, the distributions of HOMOs are mainly over bilateral aromatic rings, while the LUMOs are delocalized over the central benzothiadiazole or benzopyrazine moiety. The calculation results are in consistent with the fluorescent properties of these molecules. This work provides a new strategy for the synthesis of double aza [5]helicene molecules with fluorescent properties.