Distinct and selective amine- And anion-responsive behaviors of an electron-deficient and anion-exchangeable metal−organic framework

Smart materials that respond to chemical stimuli with color or luminescence changes are highly desirable for daily-life and high-tech applications. Here, we report a novel porous metal−organic framework (MOF) that shows multiple, selective, and discriminative responsive properties owing to the combination of different functional ingredients [tripyridinium chromogen, Eu(III) luminophore, cationic framework, and special porous structure]. The MOF contains two interpenetrated three-dimensional cationic coordination networks built of a tetrahedral [Eu₄(μ₃-OH)₄] cluster and a tripyridinium-tricarboxylate zwitterionic linker. It shows reversible and discriminative chromic response to aliphatic amines and aniline through different host− guest interactions between electron-deficient pyridinium and electron-rich amines. The size- and shape-selective response to aliphatic amines is ascribed to the radical formation through host−guest electron transfer, whereas the response to aniline is ascribed to the formation of sandwich-type acceptor−donor− acceptor complexes. The MOF is capable of reversible anion exchange with various anions and shows selective and discriminative ionochromic response to iodide, bromide, and thiocyanate, which is attributed to charge-transfer complexation. The above chromic behaviors are accompanied by efficient quenching of Eu(III) photoluminescence. The MOF represents a multi-stimuli dual-output responsive system. It can be used for discrimination and identification of anions and amines. The potential use in invisible printing, reusable sensory films, and optical switches was demonstrated by the ink and the membrane made of the MOF and organic polymers.