Two Cu-Based Phosphonate Metal–Organic Frameworks as Efficient Water-Splitting Photocatalysts

Two novel three-dimensional metal–organic frameworks (MOFs) based on the photoactive pyrene tetraphosphonate ligand and copper (denoted as IEF-8 and IEF-9) have been hydrothermally synthesized and fully characterized (XRD, FTIR, TGA, SEM, XPS, etc.). Their crystal structures were unveiled by single-crystal X-ray diffraction. Remarkably, these materials exhibit coordinatively unsaturated copper (II) sites, free −PO3H2 and −PO3H acidic groups, and good thermal and chemical stability. Further, their optoelectronic characterization evidenced a photoresponse suitable for photocatalysis. In this sense, the photocatalytic activity of pyrene phosphonate MOFs was evaluated for the first time for the challenging hydrogen evolution reaction. In particular, IEF-8 exhibited a catalytic efficiency higher than that of the benchmarked Ti carboxylate photocatalyst MIL-125(Ti)–NH2, producing 1800 μmol·g–1 after 22 h under UV–vis irradiation in the absence of any co-catalyst. Furthermore, this material presented good reusability (at least up to 4 cycles), preserving its activity and structural integrity.