Enhanced Aromaticity and Open‐Shell Diradical Character in the Dianions of 9‐Fluorenylidene‐Substituted Expanded Radialenes.

Radialenes and expanded radialenes are cross‐conjugated macrocycles displaying poor aromatic character. In this work, three 9‐fluorenylidene substituted expanded [n]radialenes (ER‐n, n=3–5) with a diacetylene spacer were synthesized and their structures were confirmed by X‐ray crystallographic analysis and NMR spectroscopy. They all can be easily reduced into relatively stable dianions. Detailed experimental measurements and theoretical calculations suggest that their dianions (ER‐n2−, n=3–5) are stabilized by both the aromatic fluorenyl anion substituents and the central aromatic rings with formally [4n+2] delocalized π electrons. In addition, the dianions of the extended radialenes (ER‐42− and ER‐52−) show unique open‐shell diradical character with a small singlet‐triplet energy gap. For comparison, their linear counterparts (L‐3 and L‐4) were also synthesized; their dianions exhibit very different redox and optical properties from their respective macrocycles. [ABSTRACT FROM AUTHOR]