Tetrafluoro(aryl)sulfanylated Bicyclopentane Crystals that Self-Destruct upon Cooling: An Organic "Cryosalient" Material

Whereas single crystals of organic compounds that respond to heat ("thermosalient") or light ("photosalient") have been reported and studied in detail, crystalline organic compounds that elicit an extreme mechanical response at cryogenic temperatures ("cryosalient") are relatively rare in the chemical literature. During an attempt to measure an X-ray crystal structure of a tetrafluoro(aryl)sulfanylated bicyclopentane synthesized in our laboratory, we discovered that the crystals jump and forcefully disintegrate upon cooling below ~193 K. Accordingly, we investigated the origin of this cryogenic mechanical response (CMR) through NMR, SC-XRD, DSC, Raman, Brillouin, and microscopy experiments. To our surprise, NMR, SC-XRD, and DSC experiments suggest the phenomenon can neither be attributed to a chemical transformation nor a crystalline-crystalline or crystalline-amorphous phase transition. Rather, microscopy, Raman, and Brillouin experiments provide evidence that the built-up strain released from the crystal upon self-destruction may be associated with a phase transition that occurs in amorphous microstructure within the crystal. This study demonstrates that molecular structural changes within organic material microstructure – which may not necessarily be visible by SC-XRD – can have a significant impact on the thermodynamic behavior of the bulk crystal-line material. Thus, the role of microstructure may be considered more heavily in future mechanistic studies on mechanically responsive crystals.