Abstract:
As an emerging class of flexible materials, mechanically bendable molecular crystals are broadly classified as
elastic or plastic. Nevertheless, flexible organic crystals with mutually exclusive elastic and plastic traits, with
contrasting structural requirements, co-existing under different stress settings are exceptional; hence, it is
imperative to establish the concurring factors that beget this rare occurrence. We report a series of
halogen-substituted benzil crystals showing elastic bending (within 2.45% strain), followed by
elastoplastic deformation under ambient conditions. Under higher stress settings, they display
exceptional plastic flexibility that one could bend, twist, or even coil around a capillary tube. X-ray
diffraction, microscopy, and computational data reveal the microscopic and macroscopic basis for the
exciting co-existence of elastic, elastoplastic, and plastic properties in the crystals. The layered molecular
arrangement and the weak dispersive interactions sustaining the interlayer region provide considerable
tolerance towards breaking and making upon engaging or releasing the external stress; it enables
restoring the original state within the elastic strain. Comparative studies with oxalate compounds,
wherein the twisted diketo moiety in benzil was replaced with a rigid and coplanar central oxalate
moiety, enabled us to understand the effect of the anisotropy factor on the crystal packing induced by
the C]O/C tetral interactions. The enhanced anisotropy depreciated the elastic domain, making the
oxalate crystals more prone to plastic deformation. Three-point bending experiments and the
determined Young's moduli further corroborate the co-existence of the elastic and plastic realm and
highlight the critical role of the underlying structural elements that determine the elastic to plastic
transformation. The work highlights the possible co-existence of orthogonal mechanical characteristics
in molecular crystals and further construed the concurrent role of microscopic and macroscopic
elements in attaining this exceptional mechanical trait.
