The Electronic Spectra of the Anion Radical Salts Derived from Halogen-substituted p-Benzoquinones

The anion-radical salts derived from p-chloranil, 2,5-dibromo-3,6-dichloro-1,4-benzoquinone, and p-bromanil were prepared. The absorption spectra in an acetone solution and the diffuse reflection spectra of the solid salts were examined. These solid salts were then classified into three groups, namely, those showing the spectrum of the anion-radical monomer, those showing a strong charge-transfer band characteristic of the solid salts, and, finally, those showing not only the spectrum of the anion-radical monomer but also an intermolecular charge-transfer band. The mode of intermolecular interaction in these anion-radical salts was discussed according to this classification. Their magnetic behaviour was also speculated upon on the basis of the charge-transfer interaction.