Establishing\nPorosity Gradients within Metal–Organic\nFrameworks Using Partial Postsynthetic Ligand Exchange

Abstract

Crystalline\n3-D materials bearing interlinked domains of differential\nporosity and functionality offer the potential for organizing and\nshuttling molecular and nanoscale matter to specific locations within\n3-D space. Here, we present methods for creating prototype MOF materials\nthat have such structural features. Specifically, the process of pore\nexpansion via ligand exchange was studied for an isoreticular series\nof mesoporous MOFs based on <b>bMOF-100</b>. It was found that\npore expansion occurs incrementally in small steps and that it proceeds\ngradually in an “outside→in” fashion within individual\ncrystals. The ligand exchange reaction can be terminated prior to\ncomplete crystal conversion to yield intermediate product MOFs, denoted <b>bMOF-100/102</b> and <b>bMOF-102/106</b>, which bear descending\nporosity gradients from the crystal periphery to the crystal core.\nAs a proof of concept, size-sensitive incorporation of a gold–thiolate\nnanocluster, Au₁₃₃(SR)₅₂, selectively in the <b>bMOF-102/106</b> crystal periphery region was accomplished via\ncation exchange. These new methods open up the possibility of controlling\nmolecular organization and transport within porous MOF materials.