Mechanical Flexibility in Halogen-Substituted Niacin and Isonicotinamide

Abstract

Understanding the role of molecular packing and intermolecular interactions in determining mechanical properties is significant for the development of mechanically flexible organic crystals. Here, we obtain a sequence of plastically twisted crystals (3) and elastically bent crystals (2) based on derivatives of nicotinic acid and isonicotinamide. The relationship between the macroscopic mechanical properties and microscopic crystal packing is discussed. Weak and dispersed weak interactions are found to play a significant role in controlling molecular movement to dissipate the strain. The introduction of a combination of noninterfering halogen bonds and weak hydrogen-bonding and van der Waals (vdW) interactions among π–π columns is considered to play a crucial role in achieving irreversible twist deformation. For the two elastically bent crystals, the molecules assemble themselves into a herringbone packing arrangement with weak and strong interactions in their interlayers, respectively, to allow a reversible rearrangement of molecules, compressing along the inner arc and extending along the outer arc. We believe that our work will provide guidance to understand and design mechanically flexible crystals and will be helpful to broaden it to a more generalized system.