Structure Property Correlation of a Series of Halogenated Schiff Base Crystals and Understanding of the Molecular Basis through Nanoindentation

Organic molecular crystals were perceived as brittle and inelastic entities; however, very recently there has been a sudden spurt of reports of soft molecular crystals. We describe a family of halogenated Schiff base molecular crystals with a design protocol aimed at achieving incorporation of structural features for a desired mechanical property. We were able to produce five crystals, of which two were elastically bendable and the remaining three were brittle. One of them is dimorphic, which means one form is brittle while the other form is elastically bendable. Delicate rebalancing between weak and dispersive noncovalent interactions along with packing features ultimately gives rise to two different polymorphs having different mechanical properties. Further, the nanoindentation technique was employed to understand the role of weak interactions so that the design of crystals with desired properties can be done more precisely in the future. This combination of elastic bending flexibility and fluorescence optical properties of molecular crystals can be used in various applications in flexible optoelectronics.