Spectroscopy of dimerization in organic mixed-stack compounds and tetramethyl-p-phenylene-diamine chloranil

The onset of dimerization in organic mixed-stack compounds allows the appearance of the otherwise symmetric phonons in the absorption spectra. A general theoretical framework shows that the most sensitive effect arises when symmetric and antisymmetric modes have nearby frequencies; the growth of dimerization then leads to gradual transfer of absorption intensity, from the antisymmetric mode to the symmetric one. Absorption data on tetramethyl-p- phenylene-diamine chloranil show this feature and thereby demonstrate the onset of dimerization below 250 K. The transition involves a small and continuous change in ionicity.