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 Å250 K. The transition involves a small and continuous change in ionicity.
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