Modulation of Koelsch Radical Stability and Aromaticity through Nonhexagonal Ring Fusion

The synthesis and properties of a fused Koelsch radical derivative incorporating a seven-membered ring are described. The strategic incorporation of a heptagon into the extended π-system was hypothesized to enable access to multiple persistent redox states by synergistically controlling the local aromaticity. Consequently, three persistent redox states (cation, neutral radical, and anion) of the compound were generated and characterized. The neutral radical was less stable (τ = 28 min in THF under air) than the Koelsch radical, which was ascribed to the 0.4 V lower oxidation potential compared to that of the Koelsch radical. The lower oxidation potential originates from tropylium-type 14π-aromatic stabilization of the incorporated seven-membered ring, which was confirmed by ¹H NMR spectroscopy and quantum chemical calculations. In contrast, tropylide-type 8π-antiaromatic character was found in the anion state. This work provides fundamental insights into the relationship among the geometry, charge delocalization, and conjugation pathways for the topology-based design of functional molecules.