An undecacyclic carbon-centered hydrocarbon radical: synthesis and investigation

A stable, cyclopenta-fused polyaromatic hydrocarbon (CP-PAH) radical, comprising six six-membered and five five-membered rings alternately fused within an undecacyclic framework, was synthesized via ortho-fusion of a suitably o,o'-substituted difluorenylfluorene, followed by establishment of the π-conjugated system. The target radical was obtained in eight consecutive synthetic steps with an overall yield of 7%, with a Suzuki coupling and cyclization via electrophilic aromatic substitution (S_EAr) serving as key steps. Mesityl substituents attached to the five-membered rings provide kinetic stabilization of the radical species. Comprehensive characterization was performed using EPR spectroscopy, UV-vis spectroscopy, and cyclic voltammetry, further supported by quantum-chemical calculations. The computational studies enabled simulation of the UV-vis spectra and provided insights into spin-density distribution, aromaticity, and frontier orbital energies. The radical is best described by the resonance structure in which the unpaired electron is predominantly localized on the outer cyclopentadiene units, maximizing the number of preserved aromatic benzene sextets. Calculations indicate only a minor triradical contribution to the electronic ground state. Notably, the five-membered rings, particularly the central ring, exhibits pronounced antiaromatic character.