A Stable Carbon-Centered Radical Showing Six Amphoteric Redox States

An air- and moisture-stable hydrocarbon radical with four six- and three five-membered rings alternately fused to a heptacycle was obtained by ortho fusion in a suitably ortho,ortho'-substituted diphenylfluorene and subsequent re-establishment of the conjugation. The radical was obtained in five consecutive steps from commercially available starting materials with a total yield of 34 %; key steps are Suzuki couplings and cyclizing S_EAr reactions. Mesityl substituents at the five-membered rings ensure the stability of the radical. This cyclopenta-fused polyaromatic hydrocarbon (CP-PAH) was characterized by EPR and UV/Vis spectroscopy and by cyclic voltammetry. Quantum chemical calculation disclosed further properties and led to simulated spectra (spin densities, aromaticity, orbital energies, and UV/Vis/NIR). This radical is best described with the unpaired electron centered in the outer five-membered rings; these resonance formulas show the largest number of fully intact benzene rings. Its triradical character was computed to be very small and can be neglected. The five-membered rings show notable antiaromatic character, especially in the central ring. The constitution of the radical could be further substantiated by X-ray crystallographic analysis of its direct precursor.