A pyridinium-carboxylate compound undergoes reversible color change under pressure owing to the formation of radicals via electron transfer; dehydration and hydration can also trigger electron transfer.

An electron-deficient and potentially chromic ligand has been utilized to impart redox activity, photo- and hydrochromism, and solvotomagnetism to metal-organic frameworks (MOFs). A pair of MOFs were constructed from the flexible zwitterionic viologen-tethered tetracarboxylate linker N,N'-bis(3,5-dicarboxylatobenzyl)-4,4'-bipyridinium (L): [Co(L)(N)] (1) and [Mn(L)(N)(HO)]·3HO (2). Both compounds show three-dimensional frameworks in which mixed azido- and carboxylato-bridged chains are connected through the electron-deficient viologen moieties. The chain in 1 is built from alternating bis(azide) and (azide)bis(carboxylate) bridges, while that in 2 contains uniform (azide) (carboxylate) bridges. The MOFs shows the characteristic redox properties of the viologen moieties. The redox activity affords the MOFs with different chromic properties, owing to subtle differences in chemical environments. 1 shows reversible photochromism, which is related to the radical formation through photoinduced electron transfer from azide-carboxylate to viologen according to UV-vis, X-ray photoelectron, and electron spin resonance spectroscopy and DFT calculations. 2 is nonphotochromic for lack of appropriate pathways for electron transfer. Unexpectedly, 2 shows a novel type of solid-state hydrochromism. Upon the removal and reabsorption of water, the compound shows remarkable color change because of reversible electron transfer accompanying a reversible structural transformation. The radical mechanism is distinct from those for traditional hydrochromic inorganic and organic materials. Magnetic studies indicate ferro- and antiferromagnetic coupling in 1 and 2, respectively. What's more, 2 shows marked magnetic response to the removal of water molecules owing to the formation of radicals. The compound illustrates a unique material exhibiting dual responses (color and magnetism) to water.

Metal-organic frameworks (MOFs) are versatile platforms to design switchable and sensory materials responsive to external stimulus. Copuling the electron-deficient bipyridinium chromorphore with the pore structures of MOFs is a nice strategy to design multiresponsive MOFs. Here we present a proof-of-concept study. Postsynthetic N,N'-cycloalkylation of UiO-67-bpy (bpy = 2,2'-bipyridyl) leads to a novel ionic MOF (UiO-67-DQ) functionalized by the electron-deficient diquat (DQ) chromophore. The combination of porosity, cationic character and electron deficiency imparts UiO-67-DQ with versatile responsive properties. It readily undergoes anion exchange, with selective ionochromism associated with charge-transfer (CT) complexation; it is electrochemically active and shows anion-dependent photochromism associated with radical formation through electron transfer (ET); the iono- and photochromism cause efficient luminescence quenching because of energy transfer (EnT) to CT complexes or radicals. The properties of UiO-67-MQ (MQ = N,N'-dimethyl-2,2'-bipyridylium) are also presented for comparison. The CT and ET effects and consequently the EnT efficiency in UiO-67-MQ are weaker than those in UiO-67-DQ because the electron-deficient character is weakened by the severe interannular twist in MQ. On the basis of the rich responsive properties, the MOFs are used as sensory and switching materials for facile discrimination of a range of anions, for quantitative detection of I, and for mimicking of logic operations ranging from simple logic gates to complex integrated logic circuits.

A novel Co(ii) coordination polymer, [Co(HO)][Co(bpybdc)(N)(HO)]·8HO (bpybdc = 1,1'-bis(3,5-dicarboxylatophenyl)-4,4'-bipyridinium), has been synthesized from a rigid zwitterionic tetracarboxylate ligand and azide. In this compound, hexacobalt clusters with mixed μ-1,1-azide, μ-1,1,1-azide and μ-1,3-carboxylate bridges are linked into chains by μ-1,3-azide bridges, and the chains are interlinked into 2-fold interpenetrated three-dimensional frameworks through the organic ligand and hydrogen bonds mediated by hexaaquacobalt(ii) complex ions. Magnetic analysis suggested that intracluster ferromagnetic and intercluster antiferromagnetic interactions work together to give overall antiferromagnetic ground states for the azide and carboxylate bridged chain. When applied as an anode for lithium-ion batteries, the coordination polymer changes into an amorphous phase and exhibits a relatively high reversible capacity of 510 mA h g with stable cycling behavior and rate performance.

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A sensitive and simple electrochemical sensor for determination of norepinephrine (NE) was developed. The electrode (ERG-UiO-67-bpy/GCE) was fabricated by modifying the glassy carbon electrode with a Zr (IV) metal-organic framework (MOF) with 2,2 -bipyridyl-5,5 -dicarboxylate (UiO-67-bpy) and graphene oxide (GO), followed by electrochemical reduction of GO to ERG (electrochemically reduced graphene). The ERG and UiO-67-bpy components of the composite electrode show a synergic electrocatalytic effect, leading to much enhanced voltammetric response to the oxidation of NE. Comparative studies indicate that the hydrogen bonds between the bipyridyl moiety of the MOF and the 2-aminoethanol moiety of NE play an important role in promoting electron transfer on the electrode surface. Differential pulse voltammetry using the composite electrode shows a wide linear-response concentration range with a submicromolar detection limit of 0.026 μM. The sensor shows good reproducibility and stability and also be used for sensitive simultaneous detection of NE and uric acid.