Structural Origins of Elastic and 2D Plastic Flexibility of Molecular Crystals Investigated with Two Polymorphs of Conformationally Rigid Coumarin

Understanding the structural origins of diverse mechanical behaviors of organic crystals is critical for designing functional materials for a number of technological applications. To facilitate this effort, we have examined the mechanical behaviors of two polymorphs of a structurally rigid molecule, coumarin. Surprisingly, form I crystals are highly elastic while form II crystals are two-dimensional (2D) plastic and twistable. The strikingly different mechanical behaviors corroborate with the respective prevailing structural mechanisms, i.e., the high elasticity is enabled by an interlocked layer structure with nearly isotropic dispersive interactions, while permanent twisting requires two orthogonal slip planes. Since molecular conformation does not vary, the strikingly different mechanical behaviors prove that molecular flexibility is not a prerequisite for crystals to exhibit mechanical flexibility. Instead, the differences in coumarin molecular packing and correspondingly different molecular interactions underlie the distinct mechanical behaviors of the two forms, which are systematically probed through crystal bending and nanoindentation, micro-Raman spectroscopy, and energy framework analysis.