Boracycle‐Annulation Stabilizes Antiaromaticity and Enhances Diradical Character in s ‐Indacene/Dicyclopenta[ b , g ]Naphthalene‐Cored Organoboron Polycyclic Hydrocarbons

For antiaromatic polycyclic hydrocarbons (PHs), heterocycle-fusion is an efficient approach to alter their electronic structures and physical properties. Herein, we disclose pristine boracycle-annulation of antiaromatic π-systems. Two organoboron PHs designed by fusion of 1-boraphenalenes onto one s-indacene or dicyclopenta[b,g]naphthalene core were synthesized. As studied, this boracycle-annulation remarkably stabilizes the antiaromaticity of s-indacene owing to the B atom with small electronegativity, and moderately enhances diradical character of dicyclopenta[b,g]naphthalene by inducing cross conjugation between the B atoms and radical centers and promoting spin delocalization. These electronic effects are in stark contrast to those of other N-/O-/S-heterocycles, thus demonstrating the unique contributions of the B atoms. Furthermore, they show a few intriguing optoelectronic properties, such as near-infrared absorptions, short excited-state lifetimes and four-step redox activity. In-depth investigations demonstrate that the amphoteric antiaromatic core together with the electron-accepting B-containing rings make significant contributions to their characteristic properties. Thus, these findings may serve as an important basis for the design of functional heterocyclic PHs, especially antiaromatic and diradical materials.