Directing the hetero-growth of lattice-mismatched surface-mounted metal–organic frameworks by functionalizing the interface
Heterostructured metal–organic framework materials (MOF-on-MOF or core–shell MOFs) feature synergistic properties of the hetero-architecture. Selective modification of the interface between different MOF crystallites by mimicking a self-assembled organic monolayer (SAM) of suitably functionalized isophthalate (fu-ip) ligands allows for the oriented growth of lattice-mismatched Cu2ndc2dabco@Cu3btc2 (A@B) using a stepwise liquid-phase epitaxial (LPE) deposition approach. Prior to the growth of the hetero-structured surface-mounted MOF (hetero-SURMOF), the binding configuration of a range of fu-ip ligands deposited at the external surface of SURMOF B was explored. Among many possibilities, only the configuration of fu-ip ligands in the “head-on” fashion binding on the external surface of SURMOF B with proper exposure of the nucleation-directing terminal functional groups validly acts as a template for the oriented hetero-growth of SURMOF A on top of SURMOF B. The quartz crystal microbalance (QCM) frequency change profiles, the infrared reflection absorption spectra (IRRAS) and the water contact angle (WCA) measurements confirm the presence of the valid configuration of fu-ip on the external surface of SURMOF B. The fu-ip ligands containing the functional side chain at the fifth position with long flexible aliphatic spacers and coordination-active functional terminal groups are revealed to be the most efficient for growing porous, highly crystalline, lattice-mismatched hetero-SURMOFs. This concept is expected to be a good reference for the fabrication of other types of hetero-SURMOFs.