A series of viologen/POM materials with discoloration properties under the stimulation of X-ray, UV, electricity, and organic amines

At present, viologen-based compounds can undergo reversible chemical/physical changes under external stimuli such as light and electricity. This makes these compounds have potential applications in smart windows, displays, and sensors. In order to obtain such materials, three viologen-POM inorganic-organic hybrid compounds have been successfully synthesized by a hydrothermal method, namely {[Cu2(cybpy)8(α-P2W18O62)2}·18H2O (1), (Hbpy)·(cybpy)·[H4(α-P2W18O62)]}·32H2O (2) and {(Hcybpy)2(β-Mo8O26)}·2H2O (3) (cybpy·Br = 1-cyclobutylmethyl-[4,4']bipyridinyl-1-ium bromide, bpy = 4,4'-bipyridine). Three compounds exhibit good discoloration behaviors under various external stimuli, especially under the stimulation of X-ray, UV, electricity, and organic amines. In addition, in order to promote the compounds in the actual production of more applications, they were doped into the polymer matrix to construct hybrid films, which not only have the same response to external stimulation but also increase the repeatability of the photochromic process. Moreover, 1-3 powder samples in ethanol solution were ultrasonic treated and deposited on filter paper, which can be successfully used in erasable inkless printing.

A viologen-based Cd(II) coordination polymer as a multifunctional platform for photochromism, chemochromism and a broad range of fluorescence pH sensing

A multi-responsive Cd(II) coordination polymer (1) has been constructed by introducing a viologen derivative as both the framework backbone and ligand side pendant. Notably, compound 1 exhibits intriguing properties, including photochromism, methanol-assisted photochromism and chemochromism to ammonia. Furthermore, compound 1 also displays fluorescence pH sensing ability in a wide pH range.

Enhancement of Long-Lived Persistent Room-Temperature Phosphorescence and Anion Exchange with I- and SCN- via Metal-Organic Hybrid Formation

An in-depth understanding of structure-property relationships and the construction of multifunctional stimuli-responsive materials are still difficult challenges. Herein, we discovered a 4,4'-bipyridinium derivative with both photochromism and dynamic afterglow at 77 K for the first time. A one-dimensional (1D) Cd(II) coordination polymer (1) assembled by only a 4,4'-bipyridinium derivative and cadmium chloride showed photochromism, room-temperature phosphorescence (RTP), and electrochromism. Interestingly, we found that 1 underwent single-crystal-to-single-crystal transformation during the anion exchange process, and the color of the crystal changed from colorless to yellow (1-SCN^-) within 10 min. Complex 1 exhibited photochromism, whereas 1-SCN^- did not. The difference in the photochromic behavior between the two complexes was ascribed to the electron transfer pathway between the carboxylate groups and viologen. The DFT calculation based on the crystal structure of 1-SCN^- indicated that the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) were mainly located on bipyridine and cadmium atoms, eliminating the possibility of electron transfer, whereas for complex 1, electron transfer was probable from O and Cl atoms to pyridinium N atoms in viologen as demonstrated by density of states (DOS) calculations. In addition, complex 1 was successfully made into test paper for the rapid detection of I^- and SCN^- and displayed potential applications in inkless printing, multiple encryption, and anticounterfeiting.

One-Dimensional Chain Viologen-Based Lanthanide Multistimulus-Responsive Materials with Photochromism, Photoluminescence, Photomagnetism, and Ammonia/Amine Vapor Sensing

In this work, a series of novel multistimulus-responsive lanthanide coordination polymers {[LnL(H₂O)₄]Cl₃·3H₂O}_n (Ln = Dy, Tb, Eu) constructed using a dicarboxylic acid viologen derivative L (L = N,N'-4,4'-bipyridiniodipropionate) and LnCl₃·6H₂O were prepared. All materials showed positive responses to UV light, and the photochromic phenomena accompanied by significant photoquenching of photoluminescence could be observed through a photoelectron transfer mechanism. Strikingly, the Dy analogue displayed photomagnetic behavior, as well as responded positively to small molecules of inorganic ammonia/organic amines. Furthermore, the good photoresponsive and ammonia/amine vapor-responsive properties of the Dy-based material were further fulfilled in dual-function papers involving erasable inkless printing and visual amine detection applications. This work aims to advance the development of multistimulus-responsive multifunctional materials incorporating viologen derivates and versatile lanthanide ions and further enriches the research in this field.

Photochromic and electrochromic properties of a viologen-based multifunctional Cd-MOF

A pair of novel multifunctional MOF materials [Cd₂(L)₂(D/L-Lm)(H₂O)₂]·5H₂O (denoted as D/L-Cd-MOF) has been synthesized by combining the viologen-functionalized ligand H₂L⁺Cl^- and chiral D/L-tartaric acid (H₂Lm) with a simple solvothermal method. Due to the unique photoelectric response properties of the viologen units, reversible photochromic and electrochromic properties have been combined in D/L-Cd-MOF, which points to a new way of designing and constructing multifunctional photoelectric materials.

Multistimuli-Responsive Materials Based on Zn(II)-Viologen Coordination Polymers and Their Applications in Inkless Print and Anticounterfeiting

Recently, stimuli-responsive materials have attracted great attention, while most of them respond to single or two stimuli. Thus, it is essential to design multifunctional stimuli-responsive materials and develop their applications. The strategy that constructing high-dimensional coordination polymers facilitates the application scope of a viologen-based photochromic system is put forward and confirmed for the first time. Herein, a novel multistimuli-responsive viologen-based Zn-MOF with a two-dimensional framework has been successfully designed and synthesized. Complex 1 exhibits chromic behavior under a variety of external stimuli such as 365 nm UV, X-rays, heat, electricity, and ethylamine. More interestingly, the crystal state of complex 1 displays dual fluorescence and room-temperature phosphorescence (RTP) emission and emits a yellow afterglow when turning off the UV lamp. In addition, Eu(III)-functionalized hybrids, Eu³⁺@Zn-MOF, were prepared by coordinated postsynthetic modification based on viologen complexes for the first time. The sample of Eu³⁺@Zn-MOF inherits the photochromic characteristics of the viologen complexes and gives the distinctive fluorescence of the europium ions. Based on the multicolor switching of 1 and Eu³⁺@Zn-MOF, their possible practical utilization was successfully developed in the fields of inkless, erasable print media, electrochromic information tag printing, information encryption, and anticounterfeiting.

Ultrafast visible-light photochromic properties of naphthalenediimide-based coordination polymers for visual detecting/filtering blue light

Two novel naphthalenediimide-based (NDI-based) coordination polymers (CPs) [Cd∙(3-PMNDI)∙(HNDC)2∙(DMF)] (1) and [Cd∙(4-PMNDI)∙(NDC)]∙DMF (2) (H2NDC = 1,4-naphthalenedicarboxylic acid, 3-/4-PMNDI = N,N′-bis(3-/4-pyridylmethyl)-1,4,5,8-naphthalenediimide, DMF = N,N′-dimethylformamide), have been designed and synthesized, which are constructed...

Two new photochromic supramolecular compositions based on viologen: photocontrolled fluorescence, aniline detection and inkless erasable printing performance

Two multifunctional supramolecular components synthesized by two different viologen ligands and pyromellitic acid.

A Series of Polyoxometalate-Viologen Photochromic Materials for UV Probing, Amine Detecting and Inkless and Erasable Printing

In this work, by using two kinds of viologen ligands three POM-based Compounds were obtained under hydrothermal conditions, namely [Ag^I (bmypd)_0.5 (β-Mo₈ O₂₆ )_0.5 ] (1) (bmypd ⋅ 2Cl=1,1'-[Biphenyl-4,4'-bis(methylene)]bis(4,4'-bipyridyinium)dichloride), [Ag^I ₂ (bypy)₄ (HSiW₁₂ O₄₀ )₂ ] ⋅ 14H₂ O (2) and [Ag^I (bypy)(γ-Mo₈ O₂₆ )_0.5 ] (3) (bypy⋅Cl=1-Benzyl-4,4'-bipyridyinium chloride). The structures were characterized by Fourier transform infrared spectroscopy (FT-IR), Powder X-ray diffraction (PXRD), X-ray photoelectron spectroscopy (XPS) and single crystal X-ray diffraction. Compounds 1-3 show excellent photochromic ability with fast photoresponse under the irradiation of ultraviolet light with different degrees of color changes. So compounds 1-3 can be used as visible ultraviolet detectors. Compounds 1-3 also possess photoluminescence properties with fast and excellent fluorescence quenching effect. Compounds 1-3 also can be used as inkless and erasable printing materials with suspensions of 1-3 applied to filter paper. Compounds 1-3 can also produce color changes in amine vapor environment, especially in an NH₃ atmosphere. Compounds 1-3 can be used as organic amine detectors.

A Multistimuli Responsive Crystalline Cd(II)-Viologen Coordination Polymer with Single-Crystal-Single-Crystal Transformation

It is necessary to develop stable and fast multistimuli responsive materials due to the growing demand in our daily life. In this work, a new viologen-based Cd-complex (1) exhibits multiple thermochromic and photochromic behaviors through 10 states with 7 colors. For example, it responds to both Cu Kα/Mo Kα X-ray sources and UV dual light quickly with a color change from colorless to dark blue (1X) (Cu Kα/Mo Kα X-ray sources) and cyan (1-UV) (UV light), respectively. Interestingly, it exhibits a three-step coloration phenomenon when heated, which is unprecedented in viologen compounds. Crystal 1 undergoes a color change to pink, blue, and brown under 130, 180, and 240 °C, respectively. In addition, upon fumigation, both 1P and 1Q undergo a decoloration process to colorless (1K) and yellow (1T), respectively. Four more states (1P, 1K, 1T, and 1O) obtained via dehydration-hydration treatment are all photochromic. More importantly, via single-crystal-single-crystal transformation (SC-SC), the photochromic and thermochromic behaviors of 1 were investigated from the molecular level, which is also rather rare for thermochromic species. The detailed electron donor and the pathways for electron transfer were clearly given according to the results of crystal structure. The colorful states upon external stimuli may be attributed to the multiple pathways for electron transfer.

Porous 3,4-di(3,5-dicarboxyphenyl)phthalate-based Cd2+ coordination polymer and its potential applications

Porous coordination polymers with organic aminium as one of the guest species possess a potential application in dye adsorption and white-light material manufacture. Polycarboxylic acid with multiple COOH substituents tends to form this type of porous material (with metal ion). Here the solvothermal self-assembly between Cd²⁺ and a hexacarboxylic acid creates such a porous material [(CH₃)₂NH₂]₆[Cd₃(L)₂]·5DMF·3H₂O (H₆L = 3,4-di(3,5-dicarboxyphenyl)phthalic acid) 1. Total potential guest accessible void volume in 3-D 1 is found to be 4327 Å³. Based on its better porous structure and stability, the ability of 1 to adsorb organic dyes is investigated. It has been proved that (i) 1 can selectively adsorb cationic dyes as Azure A (AA⁺) and/or Methylene Blue (MB⁺), rather than neutral and anionic ones; (ii) the maximum adsorption capacity is 698.2 mg·g^-1 for AA⁺ and 573.2 mg·g^-1 for MB⁺, respectively; and (iii) to the adsorption of AA⁺, it can be recycled for at least five rounds. Also, it is utilized to fabricate the while-light emitting material. Based on the blue-light emission of 1, the trace Eu³⁺ and Tb³⁺ ions are introduced into the pores of 1 successfully, obtaining a white-light emitting material Eu³⁺/Tb³⁺@1 (CIE chromaticity coordinates: (0.33, 0.32)). Meanwhile, Eu³⁺/Tb³⁺@1 is found to be a potential fluorescence photochromic material, showing a yellow-white-blue light emission. According to these investigations, the relationship between material structure and its adsorption property for dyes, the points that should be paid attention to in the construction of white-light emitting materials as well as the potential adsorption mechanism for dyes and rare earth ions are deeply discussed.

Open-Framework Zn Compound with Cationic-π Interaction: Photochromism and Benzene Series Detection

Photochromic open-framework compounds are of potential application in detection/sensors. The issue of improving the detection limits has received much attention. We synthesized a new open-framework Zn compound, namely, compound 1 ([Zn(MQ)(IPA)Cl]·5H₂O) (MQ = N-methyl-4,4'-bipyridinium, IPA = m-phthalic acid), which showed a 1D channel structure by a cationic-π interaction. It is noteworthy that this compound is an effective detector for aniline though luminescence emission, which exhibited unprecedented detection limits in photochromic open-framework compounds.

Two viologen-based photoluminescent compounds: excitation-wavelength-dependent and photoirradiation-time-dependent photoluminescent switches

We synthesized two isostructural multi-coloured photoluminescent coordination polymers. They exhibit excitation-wavelength-dependent photoluminescence emission and photoirradiation-time-dependent photoluminescence emission in solid-state.

A multifunctional colorimetric sensor originating from a cadmium naphthalene diimide-based metal–organic framework: photochromism, hydrochromism, and vapochromism

A new cadmium metal–organic framework with photocontrolled luminescence properties and multiple chromic properties was synthesized for used as a multifunctional colorimetric sensor and provide a new route to design functional MOFs.

A smart sensing Zn(ii) coordination polymer based on a new viologen ligand exhibiting photochromic and thermochromic and multiple solid detection properties

A multi-functional one-dimensional chain coordination polymer synthesized from Zn(ii), m-phthalic acid and a novel viologen ligand.

Tri(pyridinyl)pyridine Viologen-Based Kagome Dual Coordination Polymer with Selective Chromic Response to Soft X-ray and Volatile Organic Amines

The development of new responsive smart materials has been highly desirable in the recent decade due to growing demand in our daily life, and extended viologen-based coordination polymers are regarded as proper and promising candidates for stimuli-responsive study. A tri(pyridinyl)pyridine viologen-based Kagome dual (kgd) topological coordination polymer, [Mn₃Cl₄(tpptb)₂]·Cl₂·(H₂O)₂, (tpptb = N,N',N″-tri(3-carboxybenzyl)-2,4,6-tri(pyridinium-4-yl)pyridine; 1) has been solvothermally synthesized, which can selectively respond to soft X-ray Al-Kα (λ = 8.357 Å) irradiation but not to UV light and hard X-rays of λ < 1.5418 Å at room temperature. Appealingly, 1 is very sensitive and convenient for the visual detection of various volatile amine vapors, especially ethylamine vapors at a low concentration of 100 ppm, and the vapochromic sample can be recovered after exposure in the air at room temperature. The sequence of amines in vapochromism could be rationalized by combined consideration of vapor pressure, the molecular size, and electron-donor ability of various amine molecules as well as the void spaces of 1. In addition, 1 exhibits an obvious hydrochromic transformation upon heating in the air and an anhydrous atmosphere. Combined XPS and EPR confirmed that these physical and chemical stimuli can cause electron transfer from electron-rich groups to quaternary nitrogen atoms of the ligand to generate charge-separated radicals, leading to soft X-ray-induced photochromic and selective vapochromic behavior of 1. Such behavior indicates that it will become a convenient, recyclable, and practical multifunctional material for chemical and environmental sensing. These results provide an effective avenue for the rational design and synthesis of multifunctional chromic materials for potential use in sensing devices.

Near-infrared photothermal conversion of stable radicals photoinduced from a viologen-based coordination polymer

A highly stable radical photoinduced from a viologen-based coordination polymer exhibited a remarkable near-infrared photothermal effect with good recyclability.