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.具备互连型差异化孔隙域与功能域的结晶三维材料，可实现在三维空间内对分子及纳米尺度物质进行定向排布与输运。本文报道了具备此类结构特征的金属有机框架（Metal-Organic Framework, MOF）原型材料的制备方法。具体而言，针对以bMOF-100为骨架的介孔金属有机框架同构系列，本研究对配体交换法实现孔径扩张的过程展开了探究。研究表明，孔径扩张以小幅增量的方式逐步进行，且在单颗晶体内部遵循“由外至内”的渐进演化路径。该配体交换反应可在完全晶型转化前终止，进而得到中间产物金属有机框架，分别记为bMOF-100/102与bMOF-102/106，此类材料的孔隙率梯度沿晶体外围至核心方向依次递减。作为概念验证，本研究通过阳离子交换法，将金硫醇盐纳米团簇Au₁₃₃(SR)₅₂选择性负载于bMOF-102/106晶体的外围区域，实现了尺寸选择性掺入。上述全新方法为调控多孔金属有机框架材料内部的分子排布与输运过程提供了可行途径。