Organic Molecular Crystals with Dual Stress-Induced Mechanical Response: Elastic and Plastic Flexibility

The envisaged shift in the interest of crystal engineering researchers from crystal structure to mechanical property has sown the seeds of analogous understanding and interpretation of the molecular packing, intermolecular interactions, and their mechanical behavior with the utmost objective of exploiting the knowledge in designing superior materials. Here, we have implemented a previously proven strategy to extend the mechanical flexibility of molecular crystals to new systems by replacing functional groups with strong hydrogen bonding donor and acceptor atoms. For this purpose, we have chosen compounds of four different halogenated derivatives belonging to carboxylic acid, benzohydrazide, and primary amide and examined their crystals in terms of mechanical flexibility and crystal structure. The results showed that crystals of seven derivatives belonging to benzoic acids and benzohydrazides displayed elastic and plastic flexibility coupled within the same molecular crystals among the twelve compounds. In contrast, all the benzamide crystals were found to be brittle. The dual elastic and plastic nature of the crystals is attributed to the presence of flexible π···π interactions and slip systems, respectively. These observations instantiate and encapsulate the valuable insights and impact of molecular packing and intermolecular interactions on the stress-induced mechanical properties of solid crystalline materials.