Bipyridinium derivative-based coordination polymers: From synthesis to materials applications

Directed self-assembly of viologen-based 2D semiconductors with intrinsic UV-SWIR photoresponse after photo/thermo activation

Extending photoresponse ranges of semiconductors to the entire ultraviolet–visible (UV)–shortwave near-infrared (SWIR) region (ca. 200–3000 nm) is highly desirable to reduce complexity and cost of photodetectors or to promote power conversion efficiency of solar cells. The observed up limit of photoresponse for organic-based semiconductors is about 1800 nm, far from covering the UV–SWIR region. Here we develop a cyanide-bridged layer-directed intercalation approach and obtain a series of two viologen-based 2D semiconductors with multispectral photoresponse. In these compounds, infinitely π-stacked redox-active N-methyl bipyridinium cations with near-planar structures are sandwiched by cyanide-bridged Mn^II–Fe^III or Zn^II–Fe^III layers. Radical–π interactions among the infinitely π-stacked N-methyl bipyridinium components favor the extension of absorption range. Both semiconductors show light/thermo-induced color change with the formation of stable radicals. They have intrinsic photocurrent response in the range of at least 355–2400 nm, which exceeds all reported values for known single-component organic-based semiconductors.

Developing new materials with broadband photoresponse is highly desirable for realizing commercial photodetectors with extended detection ranges. Here, the authors report a cyanide-bridged layer-directed intercalation approach to design viologen compounds with enhanced broadband photoresponse.

Metalloporphyrinic metal-organic frameworks: Controlled synthesis for catalytic applications in environmental and biological media

Recently, as a new sub-family of porous coordination polymers (PCPs), porphyrinic-MOFs (Porph-MOFs) with biomimetic features have been developed using porphyrin macrocycles as ligands and/or pillared linkers. The control over the coordination of the porphyrin ligand and its derivatives however remains a challenge for engineering new tunable Porph-MOF frameworks by self-assembly methods. The key challenges exist in the following respects: (i) collapse of the large open pores of Porph-MOFs during synthesis, (ii) deactivation of unsaturated metal-sites (UMCs) by axial coordination, and (iii) the tendency of both coordinated moieties (at peripheral meso- and beta-carbon sites) and the N₄-pyridine core to coordinate with metal cations. In this respect, this review covers the advances in the design of Porph-MOFs relative to their counterpart covalent organic frameworks (Porph-COFs). The potential utility of custom-designed porphyrin/metalloporphyrins ligands is highlighted. Synthesis strategies of Porph-MOFs are also illustrated with modular design of hybrid guest@host composites (either Porph@MOFs or guest@Porph-MOFs) with exceptional topologies and stability. This review summarizes the synergistic benefits of coordinated porphyrin ligands and functional guest molecules in Porph-MOF composites for enhanced catalytic performance in various redox applications. This review shed lights on the engineering of new tunable hetero-metals open active sites within (metallo)porphyrin-MOFs as out-of-the-box platforms for enhanced catalytic processes in chemical and biological media.

A viologen-based multifunctional Eu-MOF: photo/electro-modulated chromism and luminescence

A novel viologen-based multifunctional Eu-MOF integrating photochromism, photomodulated fluorescence, and electrochromic and electroluminochromic properties was investigated.

Photochromic inorganic–organic complex derived from low-cost deep eutectic solvents with tunable photocurrent responses and photocatalytic properties

A photochromic inorganic–organic complex |C₁₀H₁₀N₂|[GaF(C₂O₄)₂] derived from low-cost deep-eutectic solvents possesses tunable photocurrent responses and photocatalytic activities.

Photochromism and photocatalysis of organic–inorganic hybrid iodoargentates modulated by argentophilic interactions

A series of organic–inorganic hybrid iodoargentates exhibit photochromic and photocatalytic properties, which could be effectively modulated by argentophilic interactions.

The impact of positional isomerism on electronic and photochromic properties of 1D zinc-based naphthalene diimide coordination polymers

The positional isomeric DPNDI ligands can effectively modulate the interfacial contacts of electron donors/acceptors and the efficiency of electronic/photochromic properties.

A zwitterionic ligand-based water-stable metal–organic framework showing photochromic and Cr(vi) removal properties

A water-stable In-MOF based on the zwitterionic viologen ligand was synthesized. It exhibits photochromic property and Cr(vi) removal performance.

Semiconductive donor promoted photochromism of iodoplumbate hybrids

The entirely different photochromic behaviors of two iodoplumbate hybrids indicate the driving role of excitation of the PbI₃⁻ chain donor and the negative role of charge transfer in photoinduced electron transfer and photochromism.

A heterometallic D–A hybrid heterostructural framework with enhanced visible-light photocatalytic properties

A heterometallic D–A hybrid heterostructural framework from the ternary combination of CuI, lanthanide cations and viologen tectons exhibited enhanced photocatalytic activities for degradation of organic dyes.

Photochromic and luminescent switchable iodoargentate hybrids directed by solvated lanthanide cations

A series of photochromic and luminescent switchable iodoargentate hybrids have been constructed by using solvated lanthanide cations as SDAs.

Coordinate bond- and hydrogen bond-assisted electron transfer strategy towards the generation of photochromic metal phosphites

The introduction of conjugated dipyridine-derivative units into metal phosphite system produces two hybrid zincophosphites driven by the coordinate bond- and hydrogen bond-assisted electron transfer.

A multifunctional photochromic metal–organic framework with Lewis acid sites for selective amine and anion sensing

A bipyridinium-based MOF was prepared, which exhibits reversible photochromic properties, good luminescence sensing ability for Cr₂O₇²⁻, and can be considered an excellent colorimetric sensor for the selective detection of amine vapors.

Selective chemochromic and chemically-induced photochromic response of a metal–organic framework

The MOF provides unique confined space furnished with electron acceptor sites, and exposure to amines/alcohols causes specific and size-selective direct/UV-assisted color change owing to spontaneous/photoinduced electron transfer.

Functional Metal Organic Framework/SiO2 Nanocomposites: From Versatile Synthesis to Advanced Applications

Metal organic frameworks (MOFs), also called porous coordination polymers, have attracted extensive attention as molecular-level organic-inorganic hybrid supramolecular solid materials bridged by metal ions/clusters and organic ligands. Given their advantages, such as their high specific surface area, high porosity, and open active metal sites, MOFs offer great potential for gas storage, adsorption, catalysis, pollute removal, and biomedicine. However, the relatively weak stability and poor mechanical property of most MOFs have limited the practical application of such materials. Recently, the combination of MOFs with inorganic materials has been found to provide a possible strategy to solve such limitations. Silica, which has excellent chemical stability and mechanical properties, shows great advantages in compounding with MOFs to improve their properties and performance. It not only provides structured support for MOF materials but also improves the stability of materials through hydrophobic interaction or covalent bonding. This review summarizes the fabrication strategy, structural characteristics, and applications of MOF/silica composites, focusing on their application in chromatographic column separation, catalysis, biomedicine, and adsorption. The challenges of the application of MOF/SiO2 composites are addressed, and future developments are prospected.

Structural Diversity of Bipyridinium-Based Uranyl Coordination Polymers: Synthesis, Characterization, and Ion-Exchange Application

As well-known functional groups with excellent electro/photochromic and ion-exchange properties, bipyridinium motifs have been used in functionalized metal-organic coordination polymers, but they are still rarely applied to construct actinide coordination polymers. In this work, we utilized a bipyridinium-based carboxylic acid, 1,1'-bis(4-carboxyphenyl)-4,4'-bipyridinium bis(chloride) ([H₂bcbp]Cl₂), as the organic ligand to assemble with uranyl cations. By the introduction of different kinds of auxiliary ligands and adjustment of the pH, five novel uranyl coordination compounds, 1-5, have been synthesized through hydrothermal reactions. Starting from uranyl ions and terephthalic acid (H₂TP) and H₂bcbp ligands, [(UO₂)₂(bcbp)(TP)₂]·3H₂O (1) has a wave-shaped two-dimensional (2D) structure consisting of dinuclear units connected by terephthalate linkers and further supported by the longer H₂bcbp ligands. [(UO₂)₂(bcbp)(PA)₂]·4H₂O (2) has a zigzag chain of dimeric uranium units, and [(UO₂)₂(bcbp)(bpdc)₂]·5H₂O (3) forms a one-dimensional ribbonlike structure. The 2D structures of [(UO₂)(bcbp)(OH)(H₂O)]·Cl (4) and [(UO₂)(bcbp)Cl]·Cl (5) are similar, both of which are constructed from dinuclear uranyl units and bcbp^2- ligands. Furthermore, the performance for perrhenate removal of compound 4 with a cationic framework is assessed, and we found that compound 4 can efficiently remove ReO₄^- from an aqueous solution in a wide range of pH values. This work extends the library of viologen derivative-based uranyl coordination polymers, provides to some extent broader insights into actinide coordination chemistry of functionalized ligands, and may facilitate the ion-exchange applications of related coordination polymers.

X-ray and UV Dual Photochromism, Thermochromism, Electrochromism, and Amine-Selective Chemochromism in an Anderson-like Zn7 Cluster-Based 7-Fold Interpenetrated Framework

Smart materials are highly desirable over the recent decade due to the growing demand of complicated nature. Stable stimuli-responsive smart materials exhibit widespread potential for applications in smart windows, sensors, separators, chemical valves, and release platforms but are rare. Despite being good candidates, viologen-based multifunctional smart materials are still a challenging task for chemists. To obtain such materials, the judicious strategy is to introduce polynuclear metal-carboxylate clusters as electron donors into a stable framework to increase chromic sensitivity. Toward this endeavor, we have synthesized a novel viologen-based polymer with a unique Anderson-like metal-carboxylate cluster, [Zn₇(bpybc)₃(o-BDC)₆]·2NO₃·6H₂O (bpybc = 1,1'-bis(4-carboxyphenyl)-4,4'-bipyridinium, o-BDC = o-benzenedicarboylic acid) (1), which is a particular 7-fold interpenetrated framework with a 3D pcu network in which bpybc ligand as the linker and Zn₇O₃₀C₁₂ as the second building unit (Zn₇ SBU) were used as 6-connected nodes. More importantly, it shows excellent chromic behavior in response to multiple external stimuli especially soft X-ray and UV dual light, temperature, electricity, and organic amines, which stand out in the viologen-based polymers. Interestingly, the coloration process of 1 from "core" to "edge" is observed upon heating at the appropriate temperature, which has not yet been found in other reported thermochromic materials. Of particular interest for 1 is the couple of quaternary stimuli-sensitive abilities because it simultaneously meets the following conditions: (i) the capability of withstanding high light, higher temperature, extreme pH, and other harsh conditions; and (ii) the high sensitivity to external stimuli keeping away from photodegradation, thermal relaxation, side reactions, and so on. To be noted, 1 has high thermal stability and chemical stability, which are excellent advantages as smart materials. To further develop possible practical utilization, 1 has been doped into the polymer matrixes to construct a hybrid film, which not only keeps the response to external stimuli but also significantly improves the repeatability of the photochromic process, indicating that a new smart device with multi-stimuli-responsive functions will emerge successively in the future.

Hydrogelation Landscape Engineering and a Novel Strategy To Design Radically Induced Healable and Stimuli-Responsive Hydrogels

Herein, we report the versatile ways to prepare both low-molecular weight hydrogels and polymeric hydrogels based on various types of supramolecular interactions, starting from a simple amphiphilic terpyridine-based molecule TPYA. Notably, we report that stable terpyridine-based radicals can be generated by light or heat irradiation in polymeric hydrogels based on hydrogen bonding interactions between -COOH of PAA and the terpyridine motif of TPYA for the first time. The generation of radicals is confirmed by EPR and UV-vis experiments, and the process is accompanied by significant color changes from white to dark purple. The stable radical hydrogels prepared by the supramolecular strategy are self-healing, stretchable, and self-supporting and can be molded into different geometrical shapes. It is deduced that the generation of terpyridine-based radicals enhances the intermolecular hydrogen bonding and π-π interaction of molecules in a hydrogel matrix, which is responsible for the self-healing ability. Finally, we also show that the radical gels can selectively respond to ammonia and stretch with reversible color changes based on the reversible hydrogen-bonding interaction.

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.

Design Strategies and Redox-Dependent Applications of Insoluble Viologen-Based Covalent Organic Polymers

Dicationic quaternary salts of 4,4'-bipyridine, also referred to as the viologen family, are well-known for their interesting redox chemistry, whereby they can be reversibly reduced into radical cationic and neutral moieties. Because of this ability to switch between different redox states, viologens have frequently been incorporated into covalent organic polymers (COPs) as molecular switches to construct stimuli-responsive materials. Although many viologen-based COPs have been reported, hyper-conjugated insoluble COPs started to emerge fairly recently and have not been comprehensively reviewed. In this review, we investigate the design strategies employed in the synthesis of insoluble viologen-based COPs, which can be broadly classified as those with viologen in the backbone and those with viologen as pendant groups. Chemical reactions used in the synthesis of each category, including Sonogashira-Hagihara cross-coupling, Menshutkin and Zincke reactions, are highlighted. Diverse applications of these COPs are discussed with particular reference to the redox state of viologen in each material. Uses of these materials for gas adsorption, organic and inorganic pollutant removal, catalysis, sensing and film fabrication are explored.

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.

Hydrothermal generation, structural versatility and properties of metal(ii)-organic architectures driven by a pyridine-tricarboxylic acid

A new series of metal(ii) coordination compounds driven by a pyridine-tricarboxylate block was generated, their structural features, magnetic, luminescent or photocatalytic properties were explored.

Metal carbonyl cluster-based coordination polymers: diverse syntheses, versatile network structures, and special properties

This highlight surveys recent progress in groups 6–10 metal carbonyl cluster-based coordination polymers, focusing on diverse synthetic strategies, versatile structures, structural transformations, and semiconducting as well as magnetic properties.

A 3D heterometallic Ni(ii)/K(i) MOF with a rare rna topology: synthesis, structural features, and photocatalytic dye degradation modeling

A new picolinate-driven Ni/K MOF was prepared by different methods, fully characterized, and explored in the photocatalytic degradation of bromocresol green.

A novel photochromic hybrid containing trinuclear [Cd3Cl12]6− clusters and protonated tripyridyl-triazines

A novel organic–inorganic hybrid [H₃TPT]₂[Cd₃Cl₁₂] exhibits excellent photochromic performance with ultrafast photoresponsive rate, obvious coloration contrast and high thermal stability.

A metal–organic framework constructed by a viologen-derived ligand: photochromism and discernible detection of volatile amine vapors

A Co(ii)-MOF based on a viologen-derived ligand was obtained: the Co(ii) compound exhibits photochromism, and allows the visual and differentiable detection of different volatile alkylamines.

Three birds, one stone - photo-/piezo-/chemochromism in one conjugated nanoporous ionic organic network

A nanoporous material bearing a high ion density and inherent organic radical character was synthesized by a facile one-pot process, which exhibits photo-, piezo- and chemochromism, driven by the diverse electron transfer processes between the acceptor framework and different electron donors. The responsive behavior is useful for its sensing application, as demonstrated here for pressure, anion and gas sensing.

Differentiable Detection of Volatile Amines with a Viologen-Derived Metal-Organic Material

The development of selective sensing materials for amine detection has received considerable attentions because amines have high toxicity and exist widely. In this article, we demonstrate for the first time that a degree of discriminative detection of alkylamines can be achieved by a metal-organic coordination material. The material is derived from Cd^II and 4,4'-bipyridinium-1,1'-bis(phenylene-3-carboxylate), shows 1D channels lined with electron-deficient viologen chromophores, and exhibits different colors upon contact with amine vapors of different molecular sizes and types (primary, secondary, and tertiary). The vapochromism is attributable to electron transfer from the amine group to viologen. The discrimination between amines is because the analyte-receptor interactions, which either directly mediate or indirectly affect electron transfer, are influenced by the number of the N-H bonds in the amine molecule, the size of the amine molecule relative to the receptor channel and the steric hindrance for the electron donor-acceptor contacts. The material also shows reversible photo- and hydrochromism owing to stimuli-induced reversible electron transfer. The compound can be deposited in paper simply by spraying the mixture solution of the starting metal salt and the ligand. The paper can be used as portable test strips for visual and differentiable detection of amines and as erasable inkless printing medium.

Five monocyclic pyridinium derivative based halo-argentate/cuprate hybrids or iodide salts: influence of composition on photochromic behaviors

Using novel monocyclic pyridinium derivatives as structural directing agents and electron acceptors, five composition dependent photochromic compounds have been synthesized.

A photochromic zinc-based coordination polymer for a Li-ion battery anode with high capacity and stable cycling stability

A zinc-based coordination polymer with photochromic 4,4′-bipyridinium derivatives as redox active building units is demonstrated as an anode material for Li-ion batteries.

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.