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

9,9'-Bianthryl (1) was one-electron oxidized under several different conditions. Depending on the counter anion and the solvent, two structures of the corresponding radical cation were established. The temperature behaviour of the EPR spectra is interpreted in terms of a loose ion pair formed preferentially at low temperatures with the spin and the charge delocalised within 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-hexafluoropropanol as the solvent, a tight ion pair is formed which resembles the anthracene radical cation. The distinction between the two species can also be drawn from the optical spectra. Quantum mechanical calculations indicate that the formation of the radical cation is guided with a change of the twist angle between the two anthracene planes from 90 degrees to ca. 74 degrees.