Fluorescent Carbazole‐Derived Aza[5]Helicenes: Synthesis, Functionalization, and Characterization

Abstract 5,8‐Dihydroindolo[2,3‐ c ]carbazole (ICz), 9 H ‐cinnolino[3,4‐ c ]carbazole (CnCz), and variously alkyl‐, alkenyl‐, and aryl‐substituted indolo[2,3‐ k ]‐ and ‐[3,2‐ a ]phenanthridines (IPs) were synthesized using an ortho fusion strategy with Suzuki cross couplings, intramolecular nitrene insertions, diazo couplings, and Morgan–Walls cyclizations as key reactions. The IPs were additionally transformed into organoboranes and helicene conjugates with tetraphenylethylene derivatives. The compounds fluoresce with large Stokes shifts, exhibit strong acidochromism, and show a good to excellent aggregation‐induced emission. Their helical structure was elucidated by x‐ray crystallographic analysis and by quantum chemical calculations. HOMO–LUMO gaps of 3.96−4.06 eV and S 1 ‐T 1 gaps were calculated, with CnCz showing a small singlet‐triplet inversion. Relative p K a values of 6.65−9.55 were estimated for the different types of azahelicenes.