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. Postsynthetic modification of rigid MOFs with photoswitchable azobenzene molecules enables tunable interlayer spacing in stacked 2D MOF membranes, greatly expanding the fabrication of 2D MOF membranes with dynamic interlayer spacing.