Unexpected Photoreactivity in a NO₂-Functionalized Aluminum-MOF

The metal–organic framework CAU-10-NO2 [Al(OH)BDC-NO2] (CAU stands for Christian-Albrechts-University; H2BDC-NO2 is 5-nitroisophthalic acid) was observed to exhibit unexpected photochemical reactivity. Upon irradiation of the MOF with UV light with a wavelength of 365 nm (or with sunlight), guest molecules inside the pore system of the MOF can be oxidized and stable radicals are formed from the organic linker molecules. The reactivity toward different alcohols was studied by UV/vis spectroscopy and EPR spectroscopy. The amount of generated radicals depends on the size of the solvent molecules; however, as an exception, methanol shows a much lower reactivity than ethanol. DFT calculations were carried out to gain insights into these photochemical reactions. The results indicate that the nitro group is reduced to form a nitroso moiety. This was confirmed by means of NMR spectroscopy. The exact nature of the radical could not be revealed, but the results indicate that it could be a further reduced anionic nitroso radical. Methanol and ethanol can be distinguished using this photochemical reaction simply by the coloring of the irradiated MOF. Such a property is characteristic for a sensor; therefore, a synthesis procedure was developed to implement the MOF into a device by which the compound was directly grown onto gold substrates.