Interpretation of the solid-state spectra of tetrathiabenzoquinone derivatives from the standpoint of intermolecular interactions

The solid-state spectra of two tetrathiabenzoquinone derivatives have been investigated from the standpoint of intermolecular interactions, while focusing on the bathochromic shift upon crystallization. These derivatives form a characteristic molecular arrangement consisting of a one-dimensional molecular column and a two-dimensional molecular sheet. Interactions in the excited state which depend on the molecular arrangement, exciton coupling effects (interactions between transition dipoles), were found to be small in both compounds. A small bathochromic shift of the trimethylenedithio derivative is attributed to these effects. The temperature dependence of the solid-state spectra is also in good agreement with the calculated result. However, an extremely large bathochromic shift of the ethylenedithio derivative cannot be explained in terms of the exciton coupling effects. Intermolecular interactions caused by the close atomic contacts between intermolecular sulfur atoms in single crystals were found to play an important role in the solid-state spectra of the ethylenedithio derivative.