Flexible organic crystals. Understanding the tractable co-existence of elastic and plastic bending

Abstract

As an emerging class of flexible materials, mechanically bendable molecular crystals are broadly classified as either elastic or plastic. Nevertheless, flexible organic crystals having elasticity and plasticity coexisting 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 at ambient conditions. At higher stress settings, they display exceptional plastic flexibility that one could bend, twist, or even coil them around a capillary tube. Structural, computational, and microscopy studies reveal the microscopic and macroscopic basis for the exciting coexistence of elastic, elastoplastic, and plastic properties in the crystals. The isotropic distribution of weak interactions and the dispersive interactions that stitch the interlayer region enables the system to adopt partial molecular movements, restoring the original state within the elastic strain. The novel polymorph of F-derivative (BZF) shows exceptional mechanical deformation characteristics vis-à-vis the brittle nature of the known form (BZF_O). Comparative studies with oxalate compounds, wherein the twisted diketo moiety in benzil replaced with a rigid and coplanar central oxalate moiety, enabled us to understand the effect of the anisotropic factor in the crystal packing induced by the C=O···C tetral interactions. The enhanced anisotropy reduced the elastic regime, making the oxalate crystals more prone to plastic deformation. The work highlights the possible coexistence of orthogonal mechanical characteristics in molecular crystals and further construed the concurrent effect of the microscopic and macroscopic elements in attaining this exceptional mechanical trait.