Two Modifications Formed by "Sulflower" C16S8 Molecules, Their Study by XRD and Optical Spectroscopy (Raman, IR, UV-Vis) Methods

Sublimation of sulflower, octathio[8]circulene C16S8 (1), on heating under high vacuum (similar to 10(-5) Toff) leads to successive formation of two modifications: a white film (1W) and a red polycrystalline solid (1R). When kept at room temperature for several weeks, 1W spontaneously turns pink, reflecting the monotropic phase transition 1W -> 1R. The accurate molecular and crystal structure of 1R has been studied using low-temperature (100 K) high-resolution single crystal X-ray analysis. The C16S8 molecule in crystal is strictly planar with nearly equalized bonds of each type (C-C, C-S, and C=C). The point symmetry group of the free molecule is D-8h, and the crystal space group is P2(1)/n. These data allowed group-theoretical analysis of vibrational normal modes to be accomplished. Investigation of the charge density distribution of 1R including Bader's AIM approach has revealed rather strong intermolecular S center dot center dot center dot S, S center dot center dot center dot C, and C center dot center dot center dot C interactions of charge transfer and pi-stacking types with overall lattice energy of 28.5 kcal/mol. The charge transfer due to the S center dot center dot center dot S interactions is the reason for the red coloration of 1R. The latter is reflected by its UV-vis spectrum exhibiting absorption bands in the visible region which are absent from that of 1W. Both modifications were studied comparatively by vibrational (Raman, IR) and electronic spectroscopies as well as XRD powder diffraction. All the results obtained are fully consistent and show that 1W is much less ordered than 1R with significantly weakened intermolecular interactions. Rationalizing of these results has led to an-idea that 1W could be soluble, in contrast to 1R. Indeed, 1W appeared soluble in common solvents; this finding opens the way to the study of the chemistry of 1 and investigation of its electrooptical properties.