Coumarin-functionalized metal-organic frameworks: adsorbents with photo-responsive active sites for adsorptive desulfurization.

Photo-responsive metal-organic frameworks (PMOFs) have great potential in on-demand controllable adsorption processes under distinct light conditions, which is challenging to realize by conventional adsorbents with static frameworks and properties. Here, we report a new type of adsorbent with photo-responsive active sites for adsorptive desulfurization. Coumarin monomers were incorporated in MIL-101(Cr) to construct the photo-responsive adsorbents by coordinating with unsaturated chromium sites. Trigged by UV light (>310 nm), coumarin monomers undergo dimerization to a cyclobutane structure, which forms a conjugation effect with π electrons on the aromatic ring of the sulfur compound, leading to remarkably increased uptake of 4,6-dimethyldibenzothiophene (4,6-DMDBT). Coumarin dimers can transform back to monomers under irradiation of 254 nm UV light, corresponding to decreased adsorption of 4,6-DMDBT. The change in adsorption amount between PMOF and the coumarin monomers and dimers is up to 62.8%. The present study might open up an avenue for the fabrication of adsorbents with on-demand active sites for various applications.