Azobenzene-Functionalized UiO-66-NH₂: Solid Base Catalysts with Photocontrollable Activity

Smart solid base catalysts were fabricatedby chemicallyanchoring photoresponsive azobenzene motifs onto the metal-organicframework UiO-66-NH2 for the first time. The obtained catalystswere evaluated in the Knoevenagel condensation reaction, and tailorablecatalytic activity can be achieved by tuning the steric hindrancechange of the catalysts via the isomerization of azobenzene motifsupon UV- and visible-light irradiation. The change of the yield isup to 56.2%, displaying excellent photoresponsive regulation efficiency. Heterogeneous solid base catalysts are highly expecteddue to theirhigh activity and environmentally friendly nature in a variety ofreactions. However, the catalytic activity of traditional solid basecatalysts is controlled by external factors (such as temperature andpressure), and regulation of the activity by in situ changing theirown properties has never been reported. Herein, we report a smartsolid base catalyst by chemically anchoring the photoresponsive azobenzenederivative p-phenylazobenzoyl chloride (PAC) ontothe metal-organic framework UiO-66-NH2 (UN) forthe first time, which can regulate the catalytic activity throughremote control of external light. The prepared catalysts have a regularcrystal structure and photoresponsive properties. It is fascinatingthat the configuration of PAC can be isomerized easily during UV-and visible-light irradiation and resulted in regulation of the catalyticactivity. In the Knoevenagel condensation of 1-naphthaldehyde andethyl cyanoacetate to ethyl 2-cyano-3-(1-naphthalenyl)-acrylate, theoptimal catalyst shows up to 56.2% of change after trans/cis isomerization, while the change of the yieldover UN is negligible. The regulated catalytic behavior can be assignedto the steric hindrance change of the catalysts under external lightirradiation. This work may shed light on the design and constructionof smart solid base catalysts with tailorable properties for variousreactions.