Mechanical Flexibility in Halogen-Substituted Niacin and Isonicotinamide

Understandingthe role of molecular packing and intermolecularinteractions in determining mechanical properties is significant forthe development of mechanically flexible organic crystals. Here, weobtain a sequence of plastically twisted crystals (3) and elasticallybent crystals (2) based on derivatives of nicotinic acid and isonicotinamide.The relationship between the macroscopic mechanical properties andmicroscopic crystal packing is discussed. Weak and dispersed weakinteractions are found to play a significant role in controlling molecularmovement to dissipate the strain. The introduction of a combinationof noninterfering halogen bonds and weak hydrogen-bonding and vander Waals (vdW) interactions among & pi;-& pi; columnsis considered to play a crucial role in achieving irreversible twistdeformation. For the two elastically bent crystals, the moleculesassemble themselves into a herringbone packing arrangement with weakand strong interactions in their interlayers, respectively, to allowa reversible rearrangement of molecules, compressing along the innerarc and extending along the outer arc. We believe that our work willprovide guidance to understand and design mechanically flexible crystalsand will be helpful to broaden it to a more generalized system. A series of mechanically flexible crystalswere obtainedbased on the halogen substitution of nicotinic acid and isonicotinamide.