The results of a study of the effect of concentration on the electronic and infrared absorption spectra of LiTCNQ and Li[²H₄]TCNQ in dimethylsulphoxide, as well as the Raman and infrared spectra of aqueous solutions of the same salts, are reported. The formation of the self-dimer of the TCNQ radical anion is accompanied by the appearance of some peculiar absorptions in the infrared. They are attributed to a vibronic intensity enhancement of some vibrational modes of the dimer, corresponding to the out-of-phase coupling of totally symmetric modes of the isolated radicals. The phenomenon is interpreted in terms of the Ferguson–Person vibronic model, which embodies the basic concepts of the charge transfer (CT) interaction.

The comparison of the vibronic effects discovered for the TCNQ radical dimer in solution with those reported in the literature for the infrared spectrum of crystalline KTCNQ shows that the dimeric unit essentially contains the same vibronic features induced by the CT interaction in the solid state. The infrared spectra of the two polymorphic forms RbTCNQ(I) and RbTCNQ(II) are also reported. Their comparison shows that the intensity of the vibronic features is sensitive to different types of anion stacking in the two structures.

An overall analysis of the reported data allows us to give the complete list of the infrared absorptions of vibronic origin and to reassign some infrared active fundamentals of the monomeric TCNQ radical anion.