Synthesis and Characterization of Carbazole-Containing Aza[7]helicenes.

Azahelicenes have attracted significant interest due to their intrinsic axial chirality and tunable chiroptic and chiral recognition properties. Herein, we present an efficient synthetic route for the preparation of two aza[7]helicene scaffolds. Commercially available carbazole could be converted into a diamine precursor by double Suzuki coupling. Excellently proceeding double ortho fusion reactions via diazotization and twofold intramolecular azo coupling or Morgan-Walls cyclization then led to either 9H-dicinnolino[3,4-c:4',3'-g]carbazole or 9H-pyrrolo[2,3-k:5,4-k']diphenanthridine. XRD measurements and quantum chemical calculations confirm their unique screw-shaped geometries. The enantiomers of the pyrrolodiphenanthridine were separated by chiral HPLC, and electronic circular dichroism (ECD) spectra were recorded. Both compounds exhibit strong acidochromic behavior with emissions up to 614 nm in the orange visible range. Calculated singlet-triplet energies may allow for singlet exciton fission (SEF) for the dicinnolinocarbazole.