Cyclopenta-Fused Polyaromatic Hydrocarbon (CP-PAH) Radicals: Synthesis, Characterization, and Quantum Chemical Calculations.

Two air- and moisture-stable cyclopenta-fused polyaromatic hydrocarbon (CP-PAH) radicals with six six- and three five-membered rings alternately fused to nonacycles were obtained by ortho fusion in suitably ortho,ortho'-substituted dinaphthylfluorenes and subsequent establishment of the conjugation. The radicals were obtained in five consecutive steps with total yields of 38 and 16%, respectively; key steps are Suzuki couplings and cyclizing SEAr reactions. Mesityl substituents at the five-membered rings ensure the kinetic stability of the radicals. They were characterized by EPR and UV/Vis spectroscopy. Quantum chemical calculations led to simulated UV/Vis/NIR spectra and disclosed further properties like spin densities, aromaticity, and orbital energies. Both radicals are best described with the unpaired electron centered in the outer five-membered rings. The respective resonance formulas show the largest number of fully intact benzene rings. A possible triradical character was computed to be small in both compounds. The five-membered rings, especially the central rings show significant antiaromatic character.