Sterically controlled 5-exo-dig cyclization enables synthesis of non-benzenoid polycyclic aromatic hydrocarbons with intriguing (anti)aromaticity and diradical properties

Abstract

Non-benzenoid polycyclic aromatic hydrocarbons (PAHs) containing antiaromatic indacene or pentalene and aromatic azulene subunits emerged as compelling materials, distinguished by their unique electronic configurations, exceptional optoelectronic characteristics, and potential applications in organic electronics. However, their controllable synthesis remains challenging due to inherent instability and stringent electronic requirements. Herein, we present a modular synthetic strategy that enables the construction of stable non-benzenoid PAHs (1, 2, and 3) featuring indacene, pentalene, and azulene motifs through a carefully designed sequence of 5-exo-dig cyclization (with controllable E/Z-selectivity), nucleophilic addition, Friedel–Crafts cyclization and oxidative dehydrogenation. Comprehensive structural and electronic analyses revealed that 1 and 2 exhibit global antiaromaticity and 2 displays a more pronounced open-shell diradical character than 1, while 3 maintains a global aromaticity and a closed-shell structure. Notably, compound 2 demonstrated promising p-type semiconductor behavior with a hole mobility of up to 0.083 cm² V⁻¹ s⁻¹. Additionally, all three compounds demonstrated remarkable stability under ambient conditions, underscoring their potential for practical applications in organic electronics.