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.