Plastically Deformable and Exfoliating Molecular Crystals of a 2-D Coordination Polymer and Its Ligand

Abstract

4-n-Hexyloxybenzoic acid and the corresponding potassium salt were synthesized and crystallized, and their force-induced mechanical response was studied. 4-n-Hexyloxybenzoic acid is crystallized as large single crystals of the reported monoclinic polymorph, whereas, its potassium salt forms a 2D polymeric structure. When a localized mechanical force is applied, crystals of both materials undergo plastic deformation to the extent that they can be folded to resemble characters of the English alphabet. With few cycles of bending and unbending, the crystals can be exfoliated, and crystalline layers can be separated. In both structures, van der Waals interactions of the hexyl chains contribute toward the formation of slip plane parallel to the wider face of the crystals, which is necessary for their plastic deformability. Reported crystal structures of 4-n-alkoxybenzoic acids with alkyl chain length ranging from C1 to C10 and their inorganic salts were analyzed, and it was found that packing of alkyl chains suitable for the formation of a slip system prevails in some of these crystal structures.