Elusive Interplay of 3D Structural Similarity and Twinning in Mechanical Flexibility of Luminescent Organic Crystals

The properties of molecular crystalline solids are critically dependent on the spatial arrangement of molecules and underlying noncovalent interactions. Here, two new isoelectronic cocrystals of a substituted carbazole-based emitter are presented, with bipyridyl ethylene and azene molecules, namely, cocrystal 1 and cocrystal 2, respectively. Though both isoelectronic cocrystals are also three-dimensional (3D) isostructural at the supramolecular level, they show similar photoluminescence properties as anticipated, but irreconcilable macroscopic mechanical properties. Upon applying external stress on their respective major crystal faces, cocrystal 1 is elastically flexible, while the 3D isostructural cocrystal 2 exhibits brittle fracture. Remarkably, elastic flexibility in cocrystal 2 can be induced through twinning-mediated crystal face modification, without any change in crystal structure.