Core-Shell Structures Arise Naturally During Ligand Exchange in Metal-Organic Frameworks.

Examination of the microstructure of metal-organic frameworks (MOFs) after postsynthetic exchange (PSE) reveals that the exchanged ligand is concentrated at the edges of the crystal and decreases in concentration with crystal depth, resulting in a core-shell arrangement. Diffusion studies of the carboxylate ligand into MOF-5 indicate that diffusion is limiting to the exchange process and may ultimately be responsible for the observed core-shell structure. Examination of PSE in UMCM-8 and single crystals of UiO-66 shows a similar trend, illustrating the applicability of PSE as a method for the creation of core-shell MOFs.