Reversible photoreduction of Cu(II)-coumarin metal-organic polyhedra

Abstract

As a novel class of inorganic-organic hybrid materials, metal-organic polyhedra (MOPs) are discrete cage-like supramolecules that consist of organic linkers coordinated to metal ions or metal clusters by self-assembly. In the recent decades, MOPs have been tremendously studied owing to their designability, tailora-bility, solubility and intrinsic porosity through choosing metal sources and designing organic ligands. In addition, MOPs have been applied to several applications such as guest molecule storage, separation, cataly-sis, drug delivery, sensing. In order to develop the functional MOP materials for application, the various techniques have been widely studied such as post-synthetic modification, direct self-assembly, assembly with other materials. We report a new post-synthetic modification for controlled photoinduced reduction of Cu(II) in MOPs that will be a valuable contributor to the development of Cu+/Cu0-based catalysts. To realize controlled Cu2+ reduction, coumarin as a triplet quencher of excited benzophenone was tethered to Cu(II)-metal-organic pol-yhedra (MOPs). In addition, coumarin functionalized MOPs stabilized Cu+ state which has high reduction potential through perturbing the direct electron transfer from benzophenone to Cu paddlewheel clusters by quenching. Moreover, the Cu2+ to Cu+ reduction was reversible by alternating UV irradiation and O2 expo-sure because coumarin groups allowed the MOPs to be soluble in optimized solvent. The photoinduced cata-lytic activity of the coumarin-MOPs was also examined in a Cu(I)-catalyzed azide-alkyne cycloaddition (CuAAC).