Stable dimeric aromatic cation-radicals. Structural and spectral characterization of through-space charge delocalization

The spontaneous assembly of aromatic cation-radicals (D(+)(*)()) with the parent donor (D) to afford the paramagnetic dimer (D)(2)(+)(*)() is accompanied by a dramatic color change. For example, spectral (UV-vis and ESR) and X-ray crystal structure analyses establish the molecular association of octamethylbiphenylene cation-radical with its neutral counterpart to produce the mixed-valence or dimeric cation-radical in which the positive charge is completely delocalized over both aromatic moieties. The use of the sterically hindered cation-radicals confirms the new spectral or charge-resonance (CR) band to result in dimeric cation-radicals in which the intermolecular separation occurs at an optimum distance allowed by van der Waals contacts. The striking similarities between the classical donor/acceptor (EDA) complexes and the dimeric cation-radicals (D)(2)(+)(*)() (both in terms of the geometrical requirement as well as the appearance of new absorption bands) suggest that the latter can be considered as particular examples of Mulliken's charge-transfer complexes in which the positive charge is completely (equally) delocalized over both donor (D) and acceptor (D(+)(*)()).