PHASE RELATIONS BETWEEN THERMALLY INDUCED STRAIN–HYSTERESIS AND BENDING DEFORMATIONS IN ELASTIC CRYSTALS OF CAFFEINE COCRYSTAL SOLVATE

More than a decade ago, the discovery of elastic bending in single crystals of caffeine cocrystal solvates opened up a new area of research in mechanical properties of molecular crystals. Here we report its structural transition at T > 320 K that is accompanied by negative area expansion of the planes normal to the bending axis; and non-linear increase of intermolecular distances within π···π stacks and the unit cell volume. Upon applying heating–cooling cycles from 360 K ≤ T ≤ 380 K to T = 100 K, three structurally irreversible states are discovered associated with hysteresis of the area of the planes and intermolecular distances. During these various thermal processes, the acid-base dimers rotate as well as move apart. The ratio between variation in rotations and distances (η) reveals that its order in the high T phase as well as those of irreversible structural states are similar to those known for mechanical bending at T = 100 K.