Borromean-Entanglement-Driven Assembly of Porous Molecular Architectures with Anion-Modified Pore Space

A charge-decorated porous framework with polar pores and open O donor sites for CO2/CH4 and C2H2/C2H4 separations

Developing efficient adsorbent materials towards energy gas purification, e.g. CO₂ removal from natural gas or hydrocarbon separation, is an important but extremely challenging task. Herein, taking advantage of a cationic bipyridinium ligand in competition with a multicarboxylate ligand for binding with metal ions, a porous material with open carboxylate oxygen atoms exposed on the pore surface has been demonstrated as an efficient adsorbent for gas separation. The polar environment arising from the cationic pyridinium moiety and the negative carboxylate group endows the title compound with selective affinity to CO₂ over CH₄. Moreover, the rich open O donor sites on the channel surface enable the resultant coordination polymer to selectively adsorb C₂H₂ over C₂H₄ through H-bonding interactions. The separation mechanism has been revealed by theoretical studies. This work provides a specific guidance for the design of applicable porous materials toward energy resource purification.

Incorporating Electron-Deficient Bipyridinium Chromorphores to Make Multiresponsive Metal-organic Frameworks

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 MQ²⁺. 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.

Vapour-driven crystal-to-crystal transformation showing an interlocking switch of the coordination polymer chains between 1D and 3D

A rare crystal-to-crystal transformation occurs between 1D coordination polymer chains and 3D mechanically interlocked structures via reversible opening and closing of the homogeneous chains without any change in the coordination mode and composition.

Distinct Chromic and Magnetic Properties of Metal-Organic Frameworks with a Redox Ligand

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^2-): [Co₃(L)(N₃)₄] (1) and [Mn₂(L)(N₃)₂(H₂O)₂]·3H₂O (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.

Post-cycloaddition modification of a porous MOF for improved GC separation of ethanol and water

An olefin-containing porous framework undergoes [2 + 2] cycloaddition to give post-modified pore space, accompanied by turn-on luminescence upon light irradiation. The photochemical modification reinforces the supramolecular network and improves GC separation performance of ethanol and water.

A robust viologen and Mn-based porous coordination polymer with two types of Lewis acid sites providing high affinity for H2O, CO2 and NH3

A water stable PCP with two kinds of Lewis acid sites, open Mn²⁺ cations and viologen, shows high affinity for H₂O, CO₂ and NH₃.

A novel coordination polymer based on Co(ii) hexanuclear clusters with azide and carboxylate bridges: structure, magnetism and its application as a Li-ion battery anode

A Co(ii) coordination polymer with azide and a viologen-based tetracarboxylate ligand shows a relatively high reversible capacity with good cycling and rate performance as lithium-ion battery anode.

The structural diversity and properties of nine new viologen based zwitterionic metal–organic frameworks

A viologen based ligand has been explored as a functional building block to synthesize a series of nine new zwitterionic metal–organic frameworks with this contribution reporting on their structural, thermal and photochromic properties.

Platforms Formed from a Three-Dimensional Cu-Based Zwitterionic Metal-Organic Framework and Probe ss-DNA: Selective Fluorescent Biosensors for Human Immunodeficiency Virus 1 ds-DNA and Sudan Virus RNA Sequences

We herein report a water-stable three-dimensional Cu-based metal-organic framework (MOF) 1 supported by a tritopic quaternized carboxylate and 4,4'-dipyridyl sulfide as an ancillary ligand. This MOF exhibits unique pore shapes with aromatic rings, positively charged pyridinium and unsaturated Cu(II) cation centers, free carboxylates, tessellating H2O, and coordinating SO4(2-) on the pore surface. Compound 1 can interact with two carboxyfluorescein (FAM)-labeled single-stranded DNA sequences (probe ss-DNA, delineated as P-DNA) through electrostatic, π-stacking, and/or hydrogen-bonding interactions to form two P-DNA@1 systems, and thus quench the fluorescence of FAM via a photoinduced electron-transfer process. These P-DNA@1 systems can be used as effective fluorescent sensors for human immunodeficiency virus 1 double-stranded DNA and Sudan virus RNA sequences, respectively, with detection limits of 196 and 73 pM, respectively.

Functional metal–bipyridinium frameworks: self-assembly and applications

Metal–organic frameworks as newly emerged materials have experienced rapid development in the last few years. This perspective highlights recent progress of study on the self-assembly of metal–bipyridinium frameworks and their intriguing properties for various applications.

Ammonia detection by using flexible Lewis acidic sites in luminescent porous frameworks constructed from a bipyridinium derivative

A flexible bipyridinium molecule with Lewis acidic sites has been introduced into isoreticular porous metal–organic frameworks to yield anion-tunable luminescence and exhibit different selective response to ammonia.

A series of interdigitated Cd(ii) coordination polymers based on 4,6-dibenzoylisophthalic acid and flexible triazole ligands

Different bridging ligands with various flexibilities and lengths were utilized to assemble interdigitated coordination polymers with Cd(ii) cations. The length and flexibility of the ligands could significantly influence the interdigitation level of the crystal structure.

Coordination assembly of Borromean structures

The state-of-art advances in the assembly of coordination supramolecular structures featuring Borromean topological character have been introduced.