Aza[6]- and Aza[7]helicenes: Synthesis and Reversible Protonation-Induced Tuning of Photophysical and Chiroptical Properties

The development of molecular switches represents a versatile approach to the design of smart optical materials. Here, we report a protonation-induced reorientation of the transition dipole moments in a series of newly designed azahelicenes, establishing a distinctive molecular mechanism for chiroptical switching. To access these systems, we developed an efficient synthetic strategy based on intramolecular ortho cyclization, enabling a modular platform for carbazole- and phenanthridine-containing hexa- and heptahelicenes. The synthesized azahelicenes exhibit remarkable fluorescence properties, including quantum yields up to 30% and striking protonation-induced red-shifts of up to 151 nm (0.98 eV). Upon enantiomeric separation of representative derivatives by chiral HPLC, the azahelicenes show significant chiroptical activity with luminescence dissymmetry factors glum reaching 3.5 × 10–3. Detailed photophysical and quantum-chemical analysis reveals that protonation of the basic nitrogen centers triggers a reversible electronic reconfiguration, modulating both emission efficiency and chiroptical response. Notably, the mechanistic origin of this switching was traced back to a protonation-induced redirection of the transition dipole moments, as confirmed by our vector-based analysis. Supported by single-crystal X-ray diffraction and quantum-chemical calculations, this work provides a basis for developing new tunable azahelicenes.