Establishing Porosity Gradients within Metal–Organic Frameworks Using Partial Postsynthetic Ligand Exchange

Crystalline
3-D materials bearing interlinked domains of differential
porosity and functionality offer the potential for organizing and
shuttling molecular and nanoscale matter to specific locations within
3-D space. Here, we present methods for creating prototype MOF materials
that have such structural features. Specifically, the process of pore
expansion via ligand exchange was studied for an isoreticular series
of mesoporous MOFs based on bMOF-100. It was found that
pore expansion occurs incrementally in small steps and that it proceeds
gradually in an “outside→in” fashion within individual
crystals. The ligand exchange reaction can be terminated prior to
complete crystal conversion to yield intermediate product MOFs, denoted bMOF-100/102 and bMOF-102/106, which bear descending
porosity gradients from the crystal periphery to the crystal core.
As a proof of concept, size-sensitive incorporation of a gold–thiolate
nanocluster, Au133(SR)52, selectively in the bMOF-102/106 crystal periphery region was accomplished via
cation exchange. These new methods open up the possibility of controlling
molecular organization and transport within porous MOF materials.