Abstract
<jats:p>Crystal polymorphism offers an ideal platform for interrogating structure–property relationships, while isostructural crystals of different compounds, despite differing in molecular identity, tend to exhibit similar properties. Here, we present a compelling case of two polymorphs of a photoisomerizable hydrazone derivative, obtained by crystallization from different solvent systems. Despite adopting isostructural packing with only modest differences in lattice parameters and intermolecular interaction strengths, single-crystal X-ray diffraction reveals that the two polymorphs exhibit markedly divergent mechanical behavior. One polymorph undergoes two-dimensional elastic bending along both its major and minor faces, while the other displays plastic deformation along the major face and brittle fracture along the minor face. Nanoindentation measurements corroborate this contrast, revealing distinct elastic moduli and hardness values between the two forms. This mechanical divergence extends to their photomechanical behavior as well: the elastic polymorph undergoes controlled, light-driven bending, whereas the plastic polymorph exhibits photoinduced disintegration and splintering. FTIR and 1H NMR spectroscopy quantify the extent of photoisomerization in each case and, in conjunction with crystallographic analysis, elucidate how subtle variations in intermolecular interactions dictate the propagation of photogenerated strain through the crystal lattice. This work demonstrates that even minute structural variations within isostructural polymorphs can profoundly shape mechanical and photomechanical responses, offering new insights into structure–property relationships and informing the rational design of dynamic molecular crystals with tunable actuation modes.</jats:p>