During heating of Form I (Compound A) on the (1 0 1)/(−1 0 −1) faces (Fig. 3 ▸), we observed a mechanical response of the material by jumping (see the video in the supporting information). After heating Form I crystals for 5–10 min at 200°C, it converted to Form IV with a small change in unit-cell volume (ΔV = 20 Å3), after correcting for Z′ being 1 and 2 in Form I and IV, respectively. Similarly Forms II and III also converted to Form IV after heating for 5 min with a much larger change in volume (ΔV = 64, 41 Å3). Examination of the individual cell axes (Table 4 ▸) suggests that there is a decrease in the cell length along the a-axis and increase in the b- and c-direction of the triclinic cells of Forms I and II. Form III crystals also showed jumping (see the video in the supporting information) when heated on the (0 1 1)/(0 −1 −1) faces with a slight increase in cell lengths in a, b and c directions. In contrast, Form II crystals have irregular morphology (Table S2), so we were not able to identify specific faces in which the crystal shows a response to heating. Heating Form II crystals beyond 180°C caused blasting (see the video in the supporting information). To understand the reason behind the different mechanical responses (jumping and blasting) of polymorphic molecular crystals to heating, we compared all four polymorphic structures in terms of variation in conformation, hydrogen-bonding interactions and changes in crystal packing (Table S2). During the transition all hydrogen/halogen bonds were retained in the structure with slight changes in the distances along with slight changes in conformation in the molecule, and significant changes in packing of molecules were observed. Forms I and III molecules are arranged in a layered structure (Figs. 3 ▸ and 5 ▸), whereas in form II symmetry-independent molecules are arranged in a corrugated wave motif (Fig. 4 ▸). We hypothesize that upon heating crystals of Form I and Form III, the heat is transferred uniformly from the face resulting in the transmission of thermal stress largely in a single direction, thereby causing a thermosalient effect of jumping. In Form II, however, due to the corrugated wave-like arrangement of molecules, heat transmission is non-uniform resulting in a sudden blast of the crystal. The most likely explanation for these effects is due to the sudden release of accumulated strain energy during phase transition (Etter & Siedle, 1983 ▸), and anisotropy in the cell parameters. However, such thermosalient effects in molecular crystals is still not fully understood due to the limited number of examples in the literature for structure–property correlation. Thermochromic effects in Compound A polymorphs are described in the supporting information (Fig. S21).