Aza[7]helicene Diimide: Backbone-Controlled Excited-State Charge Transfer and Electronic Coupling

A carbazole-embedded aza[7]helicene diimide (aza[7]HDI) was synthesized as a backbone-engineered [7]HDI. Unlike outer-rim donor-substituted push-pull [7]HDIs, aza[7]HDI combines pronounced excited-state charge transfer character with strengthened interimide electronic communication. It exhibits strong solvatochromic fluorescence and circularly polarized luminescence, a large excited-state dipole-moment change, and enhanced fluorescence quantum yields. Electrochemical and quantum-chemical analyses consistently support enhanced through-space coupling. Backbone donor incorporation thus emerges as an effective strategy for tuning excited-state polarization and electronic coupling in chiral donor-acceptor helicenes.