Positive cooperative binding, a phenomenon prevalent in biological processes, holds great appeal for the design of highly sensitive responsive molecules and materials. It has been demonstrated that metal–organic frameworks (MOFs) can show positive cooperative adsorption to the benefit of gas separation, but potential binding cooperativity is largely ignored in the study of sensory MOFs. Here, we report the first demonstration of positive cooperative protonation of a MOF and the relevant pH response in fluorescence and proton conduction. The MOF is built of Zr–O clusters and bipyridyl-based tetracarboxylate linkers and has excellent hydrolytic stability. It shows a unique pH response that features two synchronous abrupt turn-off and turn-on fluorescent transitions. The abrupt transitions, which afford high sensitivity to small pH fluctuations, are due to cooperative protonation of the pyridyl sites with a Hill coefficient of 1.6. The synchronous dual-emission response, which leads to visual color change, is ascribable to proton-triggered switching between (n, π*) and (π, π*) emissions. The latter emission can be quenched by electron donating anion-dependent through photoinduced electron transfer and ground-state charge transfer. Associated with cooperative protonation, the proton conductivity of the MOF is abruptly enhanced at low pH by two orders, but overhigh acid concentration is adverse because excessive anions can interrupt the conducting networks. Our work shows new perspectives of binding cooperativity in MOFs and should shed new light on the development of responsive fluorescent MOFs and proton conductive materials.

A novel metal−organic framework (MOF), formulated as [Cd 2 (TTVTC)Cl 2 (H 2 O) 3 ]•2H 2 O (1), was synthesized from a tetracarboxylate ligand ([TTVTC] 2− ) functionalized with the thiazolothiazole extended viologen (TTV 2+ ) fluorophore. The MOF features three-dimensional (10,3)-d frameworks with 6-fold interpenetration. The MOF exhibits reversible photochromism, due to photoinduced electron transfer from carboxylate to TTV 2+ . The photoactivity benefits from the electron donor−acceptor contacts enabled by mutual interpenetration of the frameworks. This is the first demonstration of photochromism in TTV 2+ derivatives. In addition, the fluorescence arising from the TTV 2+ fluorophore can be reversibly modulated during the photochromic process. The work demonstrates the great potential of extended viologen based ligands in the construction of MOFs with dual photomodulable optical properties, which could find future applications in photoelectronics.

Smart materials that respond to chemical stimuli with color or luminescence changes are highly desirable for dailylife 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 [Eu 4 (μ 3 -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.

The MOF shows charge-transfer sensitized Eu(iii) emission and spontaneous/photoinduced guest-to-host electron transfer, which allow chromic and luminescent sensing of NH3 (luminescence turn-off) and O2 (luminescence turn-on).

Novel 2D → 2D entanglement affords close donor–acceptor contacts for electron transfer based photochromism and photomodulable fluorescence of a MOF, which also serves as a regenerable and sensitive luminescent sensor for nitrofuran antibiotics.