Decoding Elastic, Plastic, and Elastic-Plastic Deformation in Aryl Benzoate Molecular Crystals via Substituent-Driven Interaction Hierarchies.

Mechanical flexibility in molecular crystals is crucial for their integration into adaptive and flexible technologies, yet it remains challenging to predict and control. We report a systematic study of the structure-mechanical properties of 36 p-substituted aryl benzoate ester crystals. They exhibit a range of mechanical responses-elastic, plastic, elastic-plastic, and brittle. By varying the type and position of substituents, we were able to tune intermolecular interaction hierarchies, packing anisotropy, and deformation pathways. Three-point bending experiments, combined with single-crystal X-ray diffraction (SXRD), molecular electrostatic potential (MESP) analysis, Hirshfeld surface (HS) analysis, surface rugosity, attachment energy calculations, and energy-framework analysis, reveal the structural origins of distinct mechanical behaviors. Elasticity in these crystals is linked to the corrugated packing with balanced, reversible dispersive interactions; plasticity, in contrast, is driven by layered architectures with weak interlayer interfaces and accessible slip planes. Elastic-plastic transitions emerge from a subtle balance between stiff intralayer frameworks and stress-sensitive interlayer contacts. The study provides a comprehensive and predictive framework for understanding the structure-mechanical relationships in molecular crystals. It offers robust design principles for engineering organic solids with tailored elastic and plastic properties.