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

Abstract

ABSTRACT 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.