From Brittle to Elastic: Substituent Effects on Mechanical Flexibility in Aromatic Amide Crystals

ABSTRACT Mechanical flexibility in molecular crystalline materials represents a compelling paradigm shift from the long‐held perception of crystals as inherently brittle solids. Herein, we demonstrate a brittle‐to‐elastic transition by subtle molecular modification in a pair of structurally analogous aromatic amides; N‐[(4‐methoxyphenyl)methyl]formamide ( N4MFA , Crystal 1 ) and N‐benzylformamide ( NBFA , Crystal 2 ). Despite their close structural similarity, Crystal 1 exhibits brittle fracture under minimal stress, whereas Crystal 2 shows 1D elastic flexibility with reversible bending. Structural, computational, and mechanical analyses reveal that this contrast arises from substituent‐controlled supramolecular packing. In Crystal 1 , the methoxy (–OCH 3 ) group promotes dense, anisotropic packing, leading to rigidity and fracture under stress. Removing the substituent in Crystal 2 enhances isotropy, π–π stacking, and interlocked packing, enabling reversible strain during elastic bending. Nanoindentation, energy framework, and elastic tensor analyses confirm this transition: Crystal 2 shows near‐isotropic stiffness ( E max / E min = 1.65) and interconnected energy networks, whereas Crystal 1 exhibits pronounced anisotropy ( E max / E min = 3.95) and 1D cohesion. Hirshfeld surface analysis supports more balanced contacts in the elastic crystal. This work establishes a direct structure–mechanical correlation, showing that minor chemical modifications can tune flexibility and provide insights to guide the development of adaptive crystalline materials.