Reorganization of Intermolecular Interactions in the Polymorphic Phase Transition of a Prototypical Dithiazolyl-Based Bistable Material

The spin transitions undergone by several molecular crystals of dithiazolyl (DTA) radicals make this type of radical promising candidates for future sensors and memory devices. Here, we present a systematic computational study of the intermolecular interactions existing in the two polymorphs of the neutral radical 1,3,5-trithia-2,4,6-triazapentalenyl in order to elucidate the origin of the difference in energy between those two polymorphs involved in its spin transition and to understand the crystal packing of this prototype of bistable materials. The pi-pi interactions between radicals are the main driving force for the crystal packing of both polymorphs, which comprises pi-stacks of radicals. Among the interstack interactions, the strongest ones are those mediated by six- and four-center S center dot center dot center dot N bridges. The difference in energy between polymorphs, in turn, is mainly controlled also by the intrastack pi-pi intermolecular interactions, and the interstack S center dot center dot center dot S contacts instead of the S center dot center dot center dot N contacts. Since the supramolecular motifs herein identified as important for the crystal packing and/or for the energy difference between polymorphs (and, thus, for the spin transition temperature) are common to other members of the DTA family, our results provide valuable information to understand better the structure and properties of other switchable DTA-based materials.