Ion pairing in radical cations: the example of 9,9 '-bianthryl

9,9'-Bianthryl (1) was one-electron oxidized under several different condit ions. Depending on the counter anion and the solvent, two structures of the corresponding radical cation were established. The temperature behaviour o f the EPR spectra is interpreted in terms of a loose ion pair formed prefer entially at low temperatures with the spin and the charge delocalised withi n the entire pi system of 1(.+). At increased temperatures and with CF3COO- as the counterion and trifluoroacetic acid or 1,1,1,3,3,3-hexafluoropropan ol as the solvent, a tight ion pair is formed which resembles the anthracen e radical cation. The distinction between the two species can also be drawn from the optical spectra. Quantum mechanical calculations indicate that th e formation of the radical cation is guided with a change of the twist angl e between the two anthracene planes from 90 degrees to ca. 74 degrees.