Tunable Synthesis of Multi-Responsive NH-Containing Helical Nanographenes With a Chiroptical Switching Function

Helical nanographenes (HNGs) have attracted much attention owing to their red-region chiroptical properties. These HNGs, typically based on hexa-peri-hexabenzocoronene (HBC), are commonly synthesized via the Diels-Alder reaction followed by the Scholl reaction; however, the former generally requires harsh conditions (> 200°C), thereby limiting modular tuning and structural diversity. Here, we report the tunable synthesis of NH-containing HNGs based on carbazole frameworks that enable π-extension without protecting the NH group. Multifold Scholl reactions of 1,3,6,8-tetrakis(oligophenyl)-appended carbazoles, derived from a common 1,3,6,8-tetrabromocarbazole platform, enabling uniform installation of arene units for π-extension, afford HNGs through the formation of up to 12 C─C bonds at defined positions. The use of identical arenes suppresses regioisomer formation, while the symmetric framework enforces a single reaction pathway, avoiding regioisomeric scrambling. The resulting HNGs were successfully resolved into their enantiopure forms by chiral HPLC and exhibit distinct chiroptical properties. Notably, the unprotected NH groups enable a pronounced redshift and reversible on/off switching of circularly polarized luminescence (CPL) upon treatment with base and acid, respectively, indicating pH-driven chiroptical switching. Furthermore, excited-state proton transfer in dimethyl sulfoxide generates anionic species showing redshifted fluorescence and CPL even under neutral conditions.