Mechanochemical synthesis of -salicylidene-aniline: thermosalient effect of polymorphic crystals.

Polymorphs of the dichloro derivative of -salicylideneaniline exhibit mechanical responses such as jumping (Forms I and III) and exploding (Form II) in its three polymorphs. The molecules are connected the amide N-H⋯O dimer synthon and C-Cl⋯O halogen bond in the three crystal structures. A fourth high-temperature Form IV was confirmed by variable-temperature single-crystal X-ray diffraction at 180°C. The behaviour of jumping exhibited by the polymorphic crystals of Forms I and III is due to the layered sheet morphology and the transmission of thermal stress in a single direction, compared with the corrugated sheet structure of Form II such that heat dissipation is more isotropic causing blasting. The role of weak C-Cl⋯O interactions in the thermal response of molecular crystals is discussed.