Dicyclopenta[4,3,2,1-cde:4′,3′,2′,1′-pqr]-peri-tetracene: Synthesis and an Example of Annulene-within-an-Annulene Aromaticity in Different Redox States

We report a robust strategy for tuning the electronic structure and chemical stability of π-conjugated polycyclic hydrocarbons (PHs). By fusing two cyclopentadienyl rings in the peri-tetracene bay regions, we introduce antiaromatic character into the π-system, leading to unique photophysical and electronic properties. A stable mesityl-substituted dicyclopenta-peri-tetracene derivative was synthesized through stepwise formylation/intramolecular cyclization at the bay regions of the dihydro peri-tetracene precursor, followed by oxidative dehydrogenation. This compound features an open-shell singlet diradical ground state, global antiaromaticity, exceptional stability under ambient conditions, and amphoteric redox behavior with a small energy gap. Its dication also possesses an open-shell singlet ground state, and its structure was identified by X-ray crystallographic analysis, revealing a unique [14]annulene-within-[22]annulene global aromatic structure. In contrast, the dianion exhibits a closed-shell singlet ground state. This work provides valuable insights for designing and synthesizing novel open-shell PHs with tunable electronic structures and remarkable stability.