Through-Bond versus Through-Space Conjugation and High‑Dissymmetry Chiroptical Switching in Proton–Responsive [8]Helicene Bisbenzimidazoles

Abstract

We report a novel class of bisbenzimidazole-fused [8]helicenes ([8]HBIs) that integrates a configurationally stable helical scaffold with two proton-responsive benzimidazole units. Three distinct regioisomers, generated through twofold imidization of [8]helicene dianhydride, provide a modular platform for systematically tuning through-bond and through-space π-conjugation. Relative to the diimide analogue [8]helicene diimide, the [8]HBIs exhibit significantly enhanced π-delocalization, as reflected in red-shifted absorption and emission, reduced optical and electrochemical gaps, and improved photophysical performance. Crucially, reversible protonation induces helical compression and reorganization of electronic structure, resulting in pronounced bathochromic shifts in circular dichroism (CD) and circularly polarized luminescence (CPL), while preserving large dissymmetry factors (|gabs| and |glum| up to 4.3×10−2, and 3.0×10−2, respectively) in the protonated state, outperforming the majority of previously reported acid-responsive chiroptical switches. Electrochemical data and quantum chemical calculations identify helical pitch modulation and orbital overlap as the molecular basis for these effects. These findings position [8]HBIs as versatile, stimuli-responsive materials for advanced chiroptical applications.