Variously Substituted Cyclopenta[2,1‐b:3,4‐b’]difluorenyl Radicals: A Validation of Clar's Rule

Two new cyclopenta[2,1-b:3,4-b’]difluorenyl radicals were synthesized using Suzuki coupling to form a teraryl precursor, double ortho cyclization and reestablishment of conjugation as key steps. They were thoroughly characterized by electron paramagnetic resonance (EPR) and UV/Vis spectroscopy, as well as cyclic voltammetry, and complemented by quantum-chemical calculations. The latter provided insight into orbital energies and spin densities and gave simulated UV/Vis/NIR spectra. We previously proposed that these radicals are best described by resonance structures with the radical centers located in the outer five-membered rings. This interpretation is supported by calculated spin densities and follows Clar’s rule, favoring resonance formulas containing the maximum number of intact benzene rings. This suggestion is now further supported by experimental results: It turned out that the presumed centers of high spin density have mandatorily to be substituted by bulky aryl substituents, frequently used for the stabilization of carbon-centered radicals (i.e., mesityl or 2,6-dichlorophenyl) due to their orthogonal arrangement, and thus kinetic stabilization. In contrast, simple phenyl groups do not fulfill the needed criteria at these sites but can be introduced at the central ring, where calculated spin densities are lower. These results furthermore support the findings from quantum-chemical calculations that a triradical character of these radicals can be neglected.