Bipyridinium Array-Type Porous Polymer Displaying Hydrogen Storage, Charge-Transfer-Type Guest Inclusion, and Tunable Magnetic Properties

3D porous polymers for selective removal of CO2 and H2 storage: experimental and computational studies

In this article, newly designed 3D porous polymers with tuned porosity were synthesized by the polycondensation of tetrakis (4-aminophenyl) methane with pyrrole to form M1 polymer and with phenazine to form M2 polymer. The polymerization reaction used p-formaldehyde as a linker and nitric acid as a catalyst. The newly designed 3D porous polymers showed permanent porosity with a BET surface area of 575 m2/g for M1 and 389 m2/g for M2. The structure and thermal stability were investigated by solid 13C-NMR spectroscopy, Fourier-transform infrared (FT-IR) spectroscopy, and thermogravimetric analysis (TGA). The performance of the synthesized polymers toward CO2 and H2 was evaluated, demonstrating adsorption capacities of 1.85 mmol/g and 2.10 mmol/g for CO2 by M1 and M2, respectively. The importance of the synthesized polymers lies in their selectivity for CO2 capture, with CO2/N2 selectivity of 43 and 51 for M1 and M2, respectively. M1 and M2 polymers showed their capability for hydrogen storage with a capacity of 66 cm3/g (0.6 wt%) and 87 cm3/g (0.8 wt%), respectively, at 1 bar and 77 K. Molecular dynamics (MD) simulations using the grand canonical Monte Carlo (GCMC) method revealed the presence of considerable microporosity on M2, making it highly selective to CO2. The exceptional removal capabilities, combined with the high thermal stability and microporosity, enable M2 to be a potential material for flue gas purification and hydrogen storage.

Fluorescence detection platform of metal-organic frameworks for biomarkers

Sensitive and selective detection of biomarkers is crucial in the study and early diagnosis of diseases. With the continuous development of biosensing technologies, fluorescent biosensors based on metal-organic frameworks have attracted increasing attention in the field of biomarker detection due to the combination of the advantages of MOFs, such as high specific surface area, large porosity, and structure with tunable functionality and the technical simplicity, sensitivity and efficiency and good applicability of fluorescent detection techniques. Therefore, researchers must understand the fluorescence response mechanism of such fluorescent biosensors and their specific applications in this field. Of all biomarkers applicable to such sensors, the chemical essence of nucleic acids, proteins, amino acids, dopamine, and other small molecules account for about a quarter of the total number of studies. This review systematically elaborates on four fluorescence response mechanisms: metal-centered emission (MC), ligand-centered emission (LC), charge transfer (CT), and guest-induced luminescence change (GI), presenting their applications in the detection of nucleic acids, proteins, amino acids, dopamine, and other small molecule biomarkers. In addition, the current challenges of MOFs-based fluorescent biosensors are also discussed, and their further development prospects are concerned.

Viologen-functionalized magnetic material for the removal of Iodine and benzanthracene in an aqueous solution

The development of magnetically active adsorbents for effective iodine removal would be highly desirable to address environmental pollution and remediation. Herein, we demonstrated the synthesis of Vio@SiO₂@Fe₃O₄ as an adsorbent via surface functionalisation of electron-deficient bipyridium (viologen) units on the surface of magnetically active silica-coated magnetite (Fe₃O₄) core. This adsorbent was characterised thoroughly using various analytical techniques, such as field emission scanning electron microscopy (FESEM), thermal gravimetric analysis, Fourier transform infrared spectroscopy (FTIR), field emission transmission electron microscopy (FETEM), Brunauer-Emmett-Teller (BET) analysis and X-ray photon analysis (XPS). The removal of triiodide from the aqueous solution was monitored via the batch method. It revealed that the complete removal was achieved upon stirring for 70 min. The thermally stable and crystalline Vio@SiO₂@Fe₃O₄ displayed efficient removal capacity even in the presence of other competing ions and at different pHs. The adsorption kinetics data were analysed following the pseudo-first-order and pseudo-second-order models. Further, the isotherm experiment showed that the maximum uptake capacity of iodine is 1.38 g/g. It can be regenerated and reused over multiple cycles to capture iodine. Further, Vio@SiO₂@Fe₃O₄ displayed a good removal capacity toward toxic polyaromatic, Benzanthracene (BzA) pollutant with an uptake capacity of 2445 μg/g. This effective removal of toxic pollutants iodine/benzanthracene was attributed to the strong non-covalent electrostatic and π-π interaction with electron-deficient bipyridium units.

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.

Metal-Organic Framework Magnets

Metal-organic frameworks represent the ultimate chemical platform on which to develop a new generation of designer magnets. In contrast to the inorganic solids that have dominated permanent magnet technology for decades, metal-organic frameworks offer numerous advantages, most notably the nearly infinite chemical space through which to synthesize predesigned and tunable structures with controllable properties. Moreover, the presence of a rigid, crystalline structure based on organic linkers enables the potential for permanent porosity and postsynthetic chemical modification of the inorganic and organic components. Despite these attributes, the realization of metal-organic magnets with high ordering temperatures represents a formidable challenge, owing largely to the typically weak magnetic exchange coupling mediated through organic linkers. Nevertheless, recent years have seen a number of exciting advances involving frameworks based on a wide range of metal ions and organic linkers. This review provides a survey of structurally characterized metal-organic frameworks that have been shown to exhibit magnetic order. Section 1 outlines the need for new magnets and the potential role of metal-organic frameworks toward that end, and it briefly introduces the classes of magnets and the experimental methods used to characterize them. Section 2 describes early milestones and key advances in metal-organic magnet research that laid the foundation for structurally characterized metal-organic framework magnets. Sections 3 and 4 then outline the literature of metal-organic framework magnets based on diamagnetic and radical organic linkers, respectively. Finally, Section 5 concludes with some potential strategies for increasing the ordering temperatures of metal-organic framework magnets while maintaining structural integrity and additional function.

Metal-dependent photosensitivity of three isostructural 1D CPs based on the 1,1′-bis(3-carboxylatobenzyl)-4,4′-bipyridinium moiety

The comparison of the photochromic behaviors of three CPs indicates that metal ions play an important role in regulating their photochromic properties.

Sun, UV and X-ray triple photochromic properties of three coordination polymers based on 1,1′-bis(3-carboxylatobenzyl)-4,4′-bipyridinium ligand

Reversible color changes for three compounds can be observed in the air upon sunlight, UV-light and X-ray irradiation.

Guest-Selective Recognition in a Flexible Bipyridinium-Based Framework in a Reversible Crystal-to-Crystal Fashion

A flexible bipyridinium-linker-based porous host framework with electron-accepting pore surface, namely, [Zn₂ (L)(pmc)_1.5 ]⋅12 H₂ O (1; L⋅Cl₂ =1,1'-[1,4-phenylene-bis(methylene)]bis(4,4'-bipyridinium) dichloride, H₄ pmc=pyromellitic acid) exhibits recognition of phenol and aromatic amine guests based on adsorbent-adsorbate charge-transfer interactions. Significantly, the resultant guest-encapsulated complexes 1@Guests can all be characterized by single-crystal X-ray diffraction. The host framework undergoes a reversible single crystal-to-single crystal transformation in response to the inclusion of different guests with flexible torsional motions of the hexagonal ring and the trapezoid-shaped bipyridinium groups. Such recognition can be visibly monitored and detected by obvious color changes. The host framework could also be recovered, and this suggested that guest sorption/desorption is reversible and that the host framework could be reused in potential applications. This work may provide an effective way to develop porous materials with special emphasis on applications involving guest recognition.

Versatile and Switchable Responsive Properties of a Lanthanide-Viologen Metal-Organic Framework

Metal-organic frameworks (MOFs) provide intriguing platforms for the design of responsive materials. It is challenging to mobilize as many components as possible of a MOF to collaboratively accomplish multiple responsive properties. Here, reversible photochromism, piezochromism, hydrochromism, ionochromism, and luminescence modulation of an ionic Eu(III) MOF is reported furnished by cationic electron-deficient viologen units and exchangeable guest anions. Mechanistically, the extraordinarily versatile responsive properties are owed to electron transfer (ET), charge transfer (CT), and energy transfer, involving viologen as electron acceptor, anion as electron donor, luminescing Eu(III) as energy donor, and anion-viologen CT complex or ET-generated radical as energy acceptor (luminescence quencher). Moreover, guest anions and waters provide flexible handles to control the ET-based responsive properties. Water release/reuptake or exchange with organic solvents can switch on/off the response to light, while reversible anion exchange can disenable or awaken the responses to pressure, light, and water release/reuptake. The impacts of water and anions on ET are justified by the high polarity and hydrogen-bonding capability of water, the different electron donor strength of anions, and the strong I^- -viologen CT interactions. The rich responsive behaviors have great implications for applications such as pressure sensors, iodide detection, and chemical logic gates.

Photochromic properties of three 2D MOFs based on 1-carboxyethyl-4,4′-bipyridinine

Viologen units have been widely used to impart metal–organic frameworks (MOFs) with photochromic properties. However, construction of a stable photochromic system in viologen MOFs has not been fully explored. Herein, we report three examples of MOFs, namely, {[Cd(CEbpy)(m-BDC)(DMF)]·2H₂O}_n (1), {[Cd(CEbpy)(p-BDC)(H₂O)]·H₂O}_n (2), and {[Zn(CEbpy)(p-HBDC)(p-BDC)_0.5]·H₂O}_n (3) based on benzenedicarboxylic acids (m-H₂BDC = 1,3-benzenedicarboxylic acid, p-H₂BDC = 1,4-benzenedicarboxylic acid) and a viologen-derived ligand 1-carboxyethyl-4,4′-bipyridine (L = CEbpy). As expected, the incorporation of the viologen unit into the frameworks results in the predefined photochromism upon both sunlight and UV-light. Compounds 1–3 feature a two-dimensional (2D) layered structure and are all photochromic due to the formation of CEbpy radicals by photoinduced electron transfer (PET). The aggregates build an interesting stable crystalline framework that exhibits long-lived color constancy in the solid state.

Viologen units have been widely used to impart metal–organic frameworks (MOFs) with photochromic properties.

Photochromism of supramolecular assemblies based on benzenecarboxylate donors and viologen acceptors

Similar viologen acceptors, a regular change in donor moieties and photochromism provide an ideal model for elucidating D–A matching rules.

Effect of counter cations on the photochromic behaviors of three Zn–viologen complexes

Similar viologen cations, regular change in acceptor moieties and photochromism provide an ideal model for elucidating D–A matching rules.

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.

Towards energy efficient separations with metal organic frameworks

The huge energy requirement for industrial separations of chemical mixtures has necessitated the need for the development of energy efficient and alternative separation techniques in order to mitigate the negative environmental impacts associated with greenhouse gas emissions from fossil fuel combustions for energy generation.

Photochromic and photomodulated luminescence properties of two metal–viologen complexes constructed by a tetracarboxylate-anchored bipyridinium-based ligand

Photochromic and photomodulated luminescence properties of two metal–viologen complexes constructed using a tetracarboxylate-anchored bipyridinium-based ligand are reported.

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.

Host-Guest Engineering of Coordination Polymers for Highly Tunable Luminophores Based on Charge Transfer Emissions

Aiming at the targeted construction of coordination polymer luminophores, the engineering of host-guest architectures with charge transfer based emissions is performed by utilizing the interactions between the electron-deficient 2,4,6-tri(pyridin-4-yl)-1,3,5-triazine (tpt) and electron-rich polycyclic aromatic hydrocarbons (PAHs) motifs as acceptors and donors, respectively. Through guest modulation of a prototype coordination polymer [Cd(tpt)(1,4-pda)(H₂O)₂]·(tpt)·(H₂O)₂ (1) (1,4-H₂pda = 1,4-phenylenediacetic acid), a series of coordination polymers with different PAHs as guests, [Cd₂(tpt)₂(1,4-pda)₂]·guest (2-5) (guest = triphenylene for 2, pyrene for 3, coronene for 4, and perylene for 5), are successfully fabricated. Distinct from 1, coordination polymers 2-5 reveal unique bilayer structures with PAHs interlayer and good stability, owing to the enhanced stacking interactions between tpt motifs and PAH guests. Moreover, their emissions cover a wide range of wavelength due to the effective guest to host charge transfer interactions between donor and acceptor motifs. Their readily tunable host-guest charge transfer based emissions make them good candidates as potential luminophores.

Two new photochromic coordination compounds with nonphotochromic ligands and different metal centers

Two new photochromic compounds were obtained by the different metal joint dots coordinated with nonphotochromic ligands. Compound 2 shows better photosensitivity for the short Φ…Φ distance appeared in 2 with only binuclear Zn as metal joint points.

Postsynthetic N-methylation making a metal–organic framework responsive to alkylamines

The postsynthetically created electron-deficient bipyridinium moieties in a Zr-MOF provide charge-transfer sites for selectively recognizing and capturing alkylamines with fast and reversible vapochromism and luminescence quenching.

Anion-controlled photochromism of two bipyridinium-based coordination polymers and nondestructive luminescence readout

The first Zn-based coordination polymer constructed with a bipyridinium-functionalized m-benzenedicarboxyl ligand has been synthesized, which can show dual luminescence responses to light in different spectral regions to render nondestructive luminescence readout.

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.

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.

Novel photo- and/or thermochromic MOFs derived from bipyridinium carboxylate ligands

Two new Zn-MOFs based on 1-(3,5-dicarboxyphenyl)-4,4′-bipyridinium bromide have been synthesized. Compound 1 has a 3D interpenetrated cationic framework with photo- and thermochromism, while compound 2 possesses a 2D layered cationic network with only photochromism.

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.

A porous metal-organic framework based on Zn6O2 clusters: chemical stability, gas adsorption properties and solvatochromic behavior

A highly connected 3D metal-organic framework with tfz-d topology based on Zn(6)O(2) clusters and flexible carboxylate ligands has been synthesized. The obtained Zn-MOF shows solvatochromic behavior for fluorescence sensing of small molecules, gas adsorption properties and exceptional chemical stability and might have applications for separation and detection purposes.