When Reality Defies Prediction: Polymorphism, Twinning, and Accordion Crystals

The ability to understand crystallization and predict the resulting solid form of a system is not always easily achieved, but it is critical, particularly in the field of materials science. Intriguing (and previously unreported) crystallization behavior is observed with terephthalic dihydrazide (TeDi) as it rapidly forms two concomitant crystalline polymorphs upon cooling in solution. The crystal morphology of Form I (FI) has not been seen before in organic systems and involves impressive, accordion-like stacks, composed of numerous twin domains and remains stable in solution for years. Form II (FII) exists as large needles that disappear in solution after 20 h. All experimental methods employed reveal that FI is the most stable polymorph. Conversely, all computational methods utilized (conformational analyses, lattice energy calculations, and crystal structure prediction) suggest that FII is the most stable polymorph. Isolation of FII was achieved by the crystallization of TeDi powder with a supramolecular mimetic gelator, as the gel fibers act as a template for the preferential crystallization of FII, due to the comparable crystal packing of FII and the gelator. This work highlights the impact of crystallization behavior in a real laboratory and the defects, disorder, and twinning that lead to remarkable crystal morphologies that may not be accounted for with idealized calculations, and also explores approaches for controlling and directing crystallization outcomes.