Mono- and di-quaternized 4,4'-bipyridine derivatives constitute a family of heterocyclic compounds, which in recent years have been employed in numerous applications. These applications correspond to various disciplines of research and technology. In their majority, two key features of these 4,4'-bipyridine-based derivatives are exploited: their redox activity and their electrochromic aptitude. Contemporary materials and compounds encompassing these skeletons as building blocks are often characterized as multifunctional, as their presence often gives rise to interesting phenomena, e.g., various types of chromism. This research trend is acknowledged, and, in this review article, recent examples of multifunctional chromic materials/compounds of this class are presented. Emphasis is placed on solvent-/medium- and environment-responsive 4,4'-bipyridine derivatives. Two important classes of 4,4'-bipyridine-based products with solvatochromic and/or environment-responsive character are reviewed: viologens (i.e., N,N'-disubstituted derivatives) and monoquats (i.e., monosubstituted 4,4'-bipyridine derivatives). The multifunctional nature of these derivatives is analyzed and structure-property relations are discussed in connection to the role of these derivatives in various novel applications.

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Mono- and Di-Quaternized 4,4'-Bipyridine Derivatives as Key Building Blocks for Medium- and Environment-Responsive Compounds and Materials

Among the recent developments in metal-organic frameworks (MOFs), porous layered coordination polymers (CPs) have garnered attention due to their modular nature and tunable structures. These factors enable a number of properties and applications, including gas and guest sorption, storage and separation of gases and small molecules, catalysis, luminescence, sensing, magnetism, and energy storage and conversion. Among MOFs, two-dimensional (2D) compounds are also known as 2D CPs or 2D MOFs. Since the discovery of graphene in 2004, 2D materials have also been widely studied. Several 2D MOFs are suitable for exfoliation as ultrathin nanosheets similar to graphene and other 2D materials, making these layered structures useful and unique for various technological applications. Furthermore, these layered structures have fascinating topological networks and entanglements. This review provides an overview of different aspects of 2D MOF layered architectures such as topology, interpenetration, structural transformations, properties, and applications.

Two-Dimensional Metal-Organic Framework Materials: Synthesis, Structures, Properties and Applications.

Exploitation of room temperature (RT) photochromism and photomagnetism to induce single-molecule magnet (SMM) behavior has potential applications toward optical switches and magnetic memories, and remains a tremendous challenge in the development of new bulk magnets. Herein, a series of chain complexes [Ln3(H-HEDP)3(H2-HEDP)3]·2H3-TPT·H4-HEDP·10H2O (QDU-1; Ln = Dy (QDU-1(Dy)), Gd (QDU-1(Gd)), and Y (QDU-1(Y)); HEDP = hydroxyethylidene diphosphonate; TPT = 2,4,6-tri(4-pyridyl)-1,3,5-triazine) were synthesized by solvothermal reactions. All the compounds exhibited reversible photochromic and photomagnetic behaviors via UV light irradiation at RT, induced by the photogenerated radicals via a photoinduced electron transfer (PET) mechanism. More importantly, the PET process induced significant variations in magnetic interactions for the Dy(III) congener. Strong ferromagnetic coupling with remarkably slow magnetic relaxation without applied dc fields was observed between DyIII ions and photogenerated O• radicals, showing SMM behavior after RT illumination. For the first time, we observed the reversible RT photochromism and photomagnetism in the lanthanide-based materials. This work realized the radicals-actuated on/off SMM behavior via RT light irradiation, providing a new strategy for constructing the light-induced SMMs.

Manipulating On/Off Single-Molecule Magnet Behavior in a Dy(III)-Based Photochromic Complex

Metal–organic frameworks (MOFs), with diverse framework architectures, have evolved as next-generation utility multifunctional hybrid materials. One of the key features of MOFs is their luminescence properties, which can be generated from the building ligands, emissive metal ions, guest ions, or molecules used to construct them as well as from their catalytic activities. MOFs with luminescent properties have been used as excellent platforms for designing luminescent sensors. Their chemically tailorable framework with specific host–guest interactions plays an important role in selectively sensing metal ions, small organic molecules, and biomolecules. This review intends to summarize the recent advances in the construction of MOF-based sensors for chemical sensing and biosensing. Specially, we focus on the fabrication strategies of MOF-based luminescent sensors, and summarize their sensing mechanisms in detail. Also, the major challenges and constraints for this research field are discussed.

Strategies to fabricate metal–organic framework (MOF)-based luminescent sensing platforms

Three-dimensional porous coordination polymers, which are known as metal–organic frameworks (MOFs), have drawn considerable attention owing to their properties, such as highly crystalline and ordered structures, inorganic–organic-hybrid nature, tunability of chemical functionality, high porosity and surface area and moderate-to-high stability. Due to these properties, MOFs are applied extensively as probes for the detection of large varieties of organic molecules. In this line, different MOF-based instrumental and material-based methods along with different strategies have been developed to study and extend our information about the capabilities of MOFs as sensing probes for the detection of organic molecules as well as improving and developing their detection limits, selectivity and sensitivity toward specific analytes. Considering these points, we have presented and classified the content of this review based on the chemical structures of organic analytes in three categories, including (I) nitroaromatic explosives and energetic materials, (II) small organic molecules, such as solvents and volatile organic compounds, and (III) organic amines. For each group of these analytes, different material and instrumental methods using MOFs have been explained, with illustrations of remarkable examples. Finally, we compare the methods for the detection of each group of analytes and discuss which instrumentation is more effective for each group of analytes.

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Sensing organic analytes by metal–organic frameworks: a new way of considering the topic

While viologen derivatives have long been known for electrochromism and photochromism, here we demonstrated that a viologen-carboxylate zwitterionic molecule in the crystalline state exhibits piezochromic and hydrochromic behaviors. The yellow crystal undergoes a reversible color change to red under high pressure, to green after decompression, and finally back to yellow upon standing at ambient pressure. Ultraviolet-visible spectroscopy, X-ray photoelectron spectroscopy, electron paramagnetic resonance X-ray diffraction and DFT calculations suggested that the piezochromism is due to the formation of radicals via pressure-induced electron transfer from carboxylate to pyridinium, without a crystallographic phase transition. It was proposed that electron transfer is induced by pressure-forced reduction of intermolecular donor–acceptor contacts. The electron transfer can also be induced by dehydration, which gives a stable green anhydrous radical phase. The color change is reversible upon reabsorption of water, which triggers reverse electron transfer. The compound not only demonstrates new chromic phenomena for viologen compounds, but also represents the first example of organic mechanochromism and hydrochromism associated with radical formation via electron transfer.

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Piezochromism and hydrochromism through electron transfer: new stories for viologen materials

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 (L2–): [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 electr...

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

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 CdII 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.

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

A 3D metal–organic framework (LVMOF-1) with unique electron-deficient channels was synthesized and its sensing properties for electron-rich benzene derivatives were demonstrated. The MOF is built of robust [Eu(OH)(COO)2]n columns and tetratopic viologen-based crosslinkers and shows excellent chemical stability. The structure integrates Eu(III) centers to luminesce and viologen moieties to accept electrons, and most notably, the electron-deficient viologen moieties, like those in box-like diviologen cyclophanes, are ideally spaced for sandwiching electron-rich aromatic rings. The MOF shows a bimodal response (color and luminescence) to phenols, anilines, benzenediols and aminophenols, with excellent selectivity against a wide range of other organic molecules. The chromogenic phenomena allow facile, quick and naked-eye test-paper detection of these priority contaminants in water, while the luminescence response affords very fast and sensitive quantitative detection. In particular, the detection limits for anilines and benzenediols are as low as 1–9 ppb. The charge transfer and energy transfer mechanisms for the sensing properties were elucidated on the basis of X-ray crystallography after single-crystal-to-single-crystal adsorption and orbital energy analyses according to electrochemical and spectroscopic data and also DFT calculations. The MOF bridges the gap between discrete cyclophanes functioning in solution and extended porous lattices in the solid state and can provide a blueprint for further development of sensory MOFs.

A stable electron-deficient metal–organic framework for colorimetric and luminescence sensing of phenols and anilines

The solvothermal reaction of Mn(ClO4)2, NaN3, and a rigid viologen-tethered tetracarboxylic acid (1,1′-bis(3,5-dicarboxyphenyl)-4,4′-bipyridinium chloride, [H4L]Cl2) led to a coordination polymer of formula [Mn4(L)(N3)6(H2O)2]n. X-ray analysis revealed a 3D coordination structure. The Mn(II) ions are connected by mixed azide and carboxylate bridges to give 2D layers, which are pillared by the viologen tether of the zwitterionic ligand. Magnetic analyses suggested that the compound features antiferromagnetism and field-induced metamagnetism. The compound also shows photochromic and photomagnetic properties. The long-range magnetic ordering is owed to the spin-canting structure of the Mn(II)-azide-carboxylate layer; the photochromism involves the formation of viologen radicals via photoinduced electron transfer, and the photomagnetism is related to the interactions between the metal ion and the photogenerated radicals. The study demonstrates a strategy for the design of new multifunctional materials with ph...

Qi Sui

Papers

Piezochromism and hydrochromism through electron transfer: new stories for viologen materials

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

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

A stable electron-deficient metal–organic framework for colorimetric and luminescence sensing of phenols and anilines

Magnetic and Photochromic Properties of a Manganese(II) Metal-Zwitterionic Coordination Polymer