Responsive interlayer spacing in staggered metal-organic framework nanosheet membranes

Precise control over the stacking of two-dimensional (2D) metal-organic framework (MOF) nanosheets enables the fabrication of MOF films or membranes with adjustable interlayer spacing and/or pore size, offering great potential for numerous applications, yet remains highly challenging. We herein propose a postsynthetic modification strategy to functionalize solution-processable NUS-8 nanosheets with photo-responsive monodentate carboxylic acids undergoing a "trans-cis" conformational change upon irradiation or with non-responsive counterparts. This yields azobenzene-functionalized NUS-8 (designated as NUS-8-Azobenzene) and/or tetra-phenylethylene-functionalized NUS-8 (NUS-8-TPE). The 2D nature and excellent solution processability of resultant NUS-8-Azobenzene and NUS-8-TPE facilitate the fabrication of large-area, highly homogeneous films/membranes with (00l) preferential orientation. Responsive interlayer spacing was observed in the NUS-8-Azobenzene stacked membranes, confirmed by multiple characterizations. Subtle variations in interlayer spacing of NUS-8-Azobenzene stacked membranes under irradiation enable a certain control over the ubiquitous trade-off between membrane selectivity and permeability. This study presents a strategy for fabricating 2D MOF membranes with scalable production and new functionalities, even for rigid and non-responsive MOFs, holding significant potential for practical applications in separation.