Fluorescent Carbazole-derived Aza[5]helicenes: Synthesis, Functionalization, and Characterization.

Abstract

5,8-Dihydroindolo[2,3-c]carbazole (ICz), 9H-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 S1-T1 gaps were calculated with CnCz showing a small singlet-triplet inversion. Relative pKa values of 6.65-9.55 were estimated for the different types of azahelicenes.