Tuning Photophysical and Chiroptical Properties of [7]Helicene through Formation of Imidazole-Based Push–Pull Systems

Abstract

Although carbohelicenes are attractive circularly polarized luminescence (CPL) emitters, they often suffer from low fluorescence quantum yields and relatively modest luminescence dissymmetry factor (glum), resulting in limited CPL brightness. Here, we report the modulation of the photophysical and chiroptical properties of [7]helicene through the formation of imidazole-based push–pull systems. A series of diaryl imidazole derivatives bearing electron-donating and electron-withdrawing substituents was synthesized, and selected compounds were obtained in enantiomerically pure form. Imidazole incorporation markedly enhances fluorescence quantum yields and molar absorption coefficients, while preserving blue emission (λem ≈ 452–457 nm). Although absorption and luminescence dissymmetry factors decrease compared to the parent helicene, the combined increase in molar attenuation coefficient at the excitation wavelength (ε) and fluorescence quantum yields (Φ) leads to a substantial enhancement of CPL brightness. The best-performing derivatives exhibit up to a 15-fold increase in CPL brightness (BCPL) relative to unsubstituted [7]helicene. These results identify imidazole-based push–pull functionalization as an effective strategy for improving the overall CPL performance of helicenes.