Introducing a photo-switchable azo-functionality inside Cr-MIL-101-NH2 by covalent post-synthetic modification
For the ﬁrst time an azo functionality was covalently introduced into a MOF by post-synthetic modi ﬁ cation. The reaction of Cr-MIL-101-NH 2 with p-phenylazobenzoylchloride (1) and 4-(phenylazo)- phenylisocyanate (2) as the reactants led to the compounds Cr-MIL-101_amide and Cr-MIL-101_urea, with the azo groups protruding into the mesoporous cages. XRPD and N 2 sorption measurements con ﬁ rm the intactness of the framework and the successful covalent modi ﬁ cation was proven by IR- and NMR- spectroscopy. Furthermore, cis/trans isomerisation upon irradiation with light was demonstrated by UV/ Vis spectroscopy. More distinct changes in the UV/Vis spectra were observed for Cr-MIL-101_amide compared to Cr-MIL-101_urea, while the degree of functionalization, i.e. the number of reacted NH 2 -groups, seems to have a less pronounced effect. The variation of the sorption properties due to the cis/trans isomerisation was proven by methane adsorption measurements.
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