Design of Functional Disorder in Charge-Transfer Cocrystals

In molecular crystals, disorder is often avoided or ignored as a defect; however, fundamental electronic phenomena, such as dielectricity and ferroelectricity, rely on motion present in the solid state for functionality. In these materials, crystallographic disorder can be an indicator of utility. Here, we explore the dynamics, electronic performance, and origin of whole-molecule disorder in a model charge-transfer (CT) cocrystal formed between 4,6-dimethyldibenzoselenophene (DMDBS) and 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ), DMDBS-DDQ, to investigate the underlying energetics for design guidelines toward functional disorder in molecular crystals. DMDBS-DDQ was selected from the Cambridge Structural Database (CSD) for this investigation because DDQ is disordered over two positions related by a 180° rotation, coupling the disorder of DDQ to the inversion symmetry of the lattice. We prepare DMDBS-DDQ via single-crystal-to-single-crystal desolvation and demonstrate that the electrical performance and anisotropic thermal expansion behavior of the cocrystal are consistent with in-plane dynamic disorder above 33 °C (∼306 K). Importantly, we find that DMDBS-DDQ does not adhere to previous design principles regarding functional disorder in molecular systems that target size-mismatched molecular coformers to access high void/cavity space materials that may favor dynamic disorder. Instead, our findings suggest that design strategies toward dynamic disorder should be informed by interaction enthalpy surfaces of short-range intermolecular interactions.