Solid-State NMR Provides Location-Specific Information on the Chemical Environment Inside MOF Pores

Solid-state NMR can capture the inequivalence of differently shaped MOF pores. We show that the chemical shifts of reporting substituents that extend a defined distance into a MOF pore reflect the pore shape. Comparison of the framework isomers PCN-222 and PCN-224 demonstrates that the same linker and node provide three distinct chemical environments in the three pores present in the two MOFs. The chemical inequivalence of differently positioned substituents ensures that NMR spectroscopy can reveal into which pore post-synthetically added ligands are facing without a need for single crystals, and even if only 10% of nodes are functionalized. The operational facility with which location-resolved information can be obtained permits NMR to shed light on the thermodynamics and kinetics of mixed-linker MOF growth, impurity phase formation, and the dynamic behavior of added ligands.