Charge localization in a 17-bond mixed-valence quinone radical anion

Optical spectra in dimethylformamide are reported for the radical anions of benzoquinone, its tetramethyl and tetrachloro analogues, and tetra-ortho-alkyl derivatives of biphenyl, stilbene, terphenyl, quadriphenyl, and 1,4-bis(2-phenylethenyl)benzene quinones. The first absorption bands for all but the quadriphenyl quinone show vibrational fine structure, demonstrating that they are delocalized (Class III) mixed-valence compounds. The quadriphenyl quinone radical anion shows a wide Gaussian-shaped band having a band maximum that is strongly dependent on solvent, typical of localized (Class II) mixed-valence compounds. The simple O charge-bearing unit of these compounds maintains charge delocalization in examples with unusually large bridges.