Crystals are usually thought of as being naturally beautiful with straight edges and flat sides that that can be cut cleanly across, cleaving them into geometric shapes like precious stones and diamonds.
So, it came as a surprise to Professor Jagadese J. Vittal and his team from NUS Chemistry when they recrystallised an organic compound — trans-4-phenylazobenzoic acid — they obtained normal straight crystals, but also elastically bendable crystals and permanently bent crystals.
This is known as ‘polymorphism’, and is when the same chemical substance has more than one crystal structure — just like diamond, graphite, and graphene are all different arrangements of carbon.
The team’s results were published in the scientific journal Chemical Communications on 14 November 2019.
The crystal packing with anisotropic weak supramolecular interactions is responsible for the elastically bendable nature. When the solution containing the organic compound was left for crystallisation after being subjected to UV-light irradiation, a small amount of the cis-form was incorporated into the crystal, thus making bent crystals in several beautiful artistic shapes.
There are two types of mechanically bendable crystals known. The first type is elastically or reversibly bendable and the second type is plastically bendable, which retains its shape even after the mechanical force is withdrawn. The permanently bent crystals are formed when there is a problem with the packing of molecules in the solid state.
Trans-4-phenylazobenzoic acid is a potential ligand to be incorporated into metal complexes, coordination polymers, and metal-organic frameworks (MOFs) for making a new series of photo-responsive solids. The properties of these bent crystals are yet to be evaluated.