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.