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.