Construction of Site-Specific Vacancies by Organic Small-Molecule Cocrystals

Crystal vacancies play a crucial role in modulating material properties, yet achieving precise control over vacancy formation remains a significant challenge. Intriguingly, conventional characterization techniques such as X-ray diffraction analyses (powder XRD, single-crystal XRD) typically show negligible differences before and after vacancy generation in crystalline materials. In this work, a novel strategy is designed to construct crystal structures with tailored vacancies through multivariate cocrystallization. First, the polymorphs of two organic small molecules HMX and MNX with analogous electron cloud distributions were identified and summarized, and a new crystal form of MNX was discovered. Second, seven binary MNX-based cocrystals were synthesized, along with analysis of disorder characteristics and comparative studies of their HMX counterparts. Among them, HMX–DMF and MNX–DMF cocrystals were selected based on their nearly identical XRD patterns. Third, computational simulations of charge distributions in these cocrystals confirm the feasibility of ternary cocrystal formation. Finally, through simple cooling crystallization, the HMX–MNX–DMF ternary cocrystal was successfully obtained, which exhibited the anticipated oxygen vacancies. This systematic screening and construction methodology provides a powerful tool for designing vacancy-engineered crystals tailored for potential specialized applications.