Azulene in a Bowl: A Nonalternant Buckybowl With 5/7 Topology, Azulene-Derived Properties, and Fullerene Complexation

Despite the intriguing potential of azulene-embedded nanocarbons, their integration into curved buckybowl architectures has remained unexplored due to synthetic and stability challenges. Herein, we report an azulene-containing buckybowl 6, synthesized via a concise four-step route where the azulene core remains stable without rearrangement to naphthalene. Single-crystal X-ray analysis revealed that 6 adopts a nonalternant, bowl-shaped geometric configuration. Unlike classical buckybowls (e.g., corannulene, dibenzo[a,g]corannulene, and summane), 6 exhibits distinct azulene-derived properties: red-shifted UV-vis absorption, anti-Kasha photoluminescence, and reversible proton-responsive behavior. Theoretical studies reveal unique aromaticity in its central pentagon, diverging from known pentagon-containing buckybowls. Furthermore, it was found that buckybowl 6 can assemble with fullerene C₆₀ or C₇₀ in a 1:1 stoichiometry. This work establishes 6 as the unique azulene-based buckybowl and pioneers a design strategy for nonalternant, positively curved π-systems with 5/7 ring topologies and tunable optoelectronic properties.