Viologen as a Colour Modulator in Photochromism of Polyoxomolybdate-Viologen Hybrids

Zinc tungstate encapsulated into a scarce Zn(II)-viologen framework with photochromic, electrochromic and chemochromic properties

Smart chromic materials reacting to physicochemical stimuli are widely applied in optical switches, smart windows, and chemical sensors. Currently, most materials only respond to a single stimulus, but those that respond to multiple external stimuli are still in the minority. Herein, we report a novel porous zinc tungstate@metaloxoviologen framework [Zn3(Bcbpy)6(H2O)2]-[ZnW12O40]·6H2O (ZnW12@MV, H2BcbpyCl2 = 1,1'-bis(3-carboxybenzyl)-4,4'-bipyridinium dichloride), which shows multiple stimulus-responsive properties due to a combination of different functional motifs, namely, viologen electron acceptors, luminescent zinc-oxygen-clusters, porous cationic frameworks, and ZnW12O406- electron donors. Generally, the large-sized polyoxometalate (POM) anions serving as structure-directing agents can easily direct the formation of the oligomeric metaloxoviologen cations, mainly because POMs may break down some linkages leaving larger spaces for themselves. The large ZnW12O406- anions in ZnW12@MV are encapsulated into three-dimensional (3D) metaloxoviologen frameworks built up from the linkages of trinuclear zinc-oxygen clusters and Bcbpy viologens, which offer the first example of a 3D metaloxoviologen framework induced by large-sized POM anions. ZnW12@MV shows a reversible chromic response to X-ray/UV and electricity via different stimulus-induced electron transfers between electron-rich POM anions and electron-deficient metaloxoviologen frameworks, whereas the coloration changes are ascribed to the formation of radical and mixed-valence colored state ZnW12O406- species. The photochromic behavior is accompanied by photoluminescence quenching. The discriminative response to different-sized amines is attributed to the formation of viologen radicals through host-guest electron transfer. These results indicate that the multi-stimulus response ZnW12@MV can be applied in electrochromic devices, inkless erasable printing, and the detection of amines.

A series of isopolymolybdate-viologen hybrids with photo-, thermo- and electro-chromic properties

The combination of electron-deficient viologen ligands with electron-rich POMs is a typical acceptor-donor system that has recently received much attention. Under solvothermal and hydrothermal conditions, by introducing three symmetric viologen ligands into POM-based hybrid materials, we successfully constructed four POMs-viologen inorganic-organic hybrid compounds, namely (1,3-bcbpy)2·(δ-Mo8O26) (1) (1,3-bcbpy·2Cl = 1,1'-bis(3-carboxybenzyl)-4,4'-bipyridine dichloride), {CoII(1,4-bcbpy)2(H2O)2[H2(β-Mo8O26)]}·2H2O·2CH2O (2), (1,4-bcbpy)2·(δ-Mo8O26)·2H2O (3) (1,4-bcbpy·2Cl = 1,1'-bis(4-carboxybenzyl)-4,4'-bipyridine dichloride, CH2O = formaldehyde), and {CuII(1,1-pmbby)2(H2O)[H2(β-Mo8O26)2]}·5H2O·C2H7N (4) (1,1-pmbby·2Cl = 1,1'-[1,4-phenylbis(methylene)]bis-(4,4'-bipyridine)dichloride, C2H7N = dimethylamine). These four compounds exhibit different fascinating structures, especially compound 4 is a typical metal-organic framework. Compounds 1-4 exhibit good discoloration behaviors under various external stimuli. For example, compounds 1-4 showed a positive response to the irradiation from a 300 W Xe lamp. When a positive voltage was applied to the ECD based on compounds 1-4, 1/2/3/4-ECD underwent a significant color conversion. What's more, compound 4 also showed obvious discoloration results after heating. In a word, 1-4 are multifunctional discoloration materials under different external stimuli. In addition, the coated filter paper prepared based on compound 3 can be used as a new printing material medium and can be successfully applied in erasable inkless printing and dual anti-counterfeiting.

Controlling Electron Delocalization in Vanadium-Based Hybrid Bronzes through Molecular Templation

We show that the conductivity of hybrid vanadium bronzes-mixed-valence organic-inorganic vanadium oxides-can be tuned over six orders of magnitude through judicious choice of molecular component. By systematically varying the steric profile, charge density, and propensity to hydrogen bond across a series of eight diammonium-based molecules, we engender multiple distinct motifs of V-O connectivity within the two-dimensional vanadium oxide layers of a family of bulk crystalline hybrid materials. A combination of single-crystal and powder X-ray diffraction analysis, variable-temperature electrical transport measurements, and a range of spectroscopic methods, including UV/Visible diffuse reflectance, X-ray photoelectron, and electron paramagnetic resonance are employed to probe how vanadium oxide layer topology correlates with electron localization. Specifically, alkylammonium molecules yield hybrids featuring more corrugated layers that contain V-O tetrahedra as well as a higher ratio of corner-sharing to edge-sharing polyhedra and that exhibit highly localized electronic behavior, while alkyl bipyridinium molecules yield more regular layers with polyhedral edge-sharing that show substantially delocalized electronic behavior. This work allows for the development of design principles based on structure-property relationships and brings the charge transport capabilities of hybrid vanadium bronzes to more technologically relevant levels.

Polyoxometalate-Viologen Thermochromic Hybrids for Organic Amine Detectors and Memristors with Temperature-Regulating Resistance Switching Characteristics

There are relatively few reports on the combination of viologen and polyoxometalates (POMs). Herein, we successfully synthesized three viologen-POM-based compounds by in situ transformation of ligands under hydrothermal conditions, namely, {M^II(1,4-cby)₂[H₂(γ-Mo₈O₂₆)]}·nH₂O (1: M = Ni, n = 4; 2: M = Co, n = 6), and [Ni^II(1,3-cby)(H₂O)₄(β-Mo₈O₂₆)_0.5]·2H₂O (3) (1,4-cby·Cl = 1-(4-carboxy-benzyl)-[4,4']bipyridinyl-1-ium, 1,3-cby·Cl = 1-(3-carboxy-benzyl)-[4,4']bipyridinyl-1-ium). Isostructural compounds 1 and 2 exhibit two-dimensional (2D) layer structures with POMs as linking nodes, while compound 3 shows a one-dimensional (1D) metal-organic chain with dissociative POM anions. When the temperature increases, compounds 1-3 show good reversible thermochromism properties and also have a fluorescence quenching effect. Moreover, compounds 1-3 can also be used as detectors for organic amines, especially in the atmosphere of ammonia, ethylenediamine, and diethylamine with an obvious discoloration effect. In addition, compound 1 was used as a material for the preparation of memristors with superior properties (distinct temperature-adjusted resistive switching properties). It shows bipolar resistive switching (RS) behavior at different temperatures of 20, 50, and 100 °C. The results show that the 1-based memristor has good thermal stability, which is important for high-temperature environment applications. It also shows that crystalline viologen-POM-based compounds are ideal candidates for making memristors.

Applications of Viologens in Organic and Inorganic Discoloration Materials

Viologen derived from 4,4'-bipyridine has attracted much attention because of its color changing properties with electron transfer, unique redox stability and structural diversity. These characteristics have led to its successful use in various applications, in particular in color-changing materials. In the past few years, researchers have been working on the syntheses of viologen-based color-changing functional materials, and such materials have been widely used in many fields. In photochromic materials, it is used as anti-counterfeiting material; in thermochromic, it is used as memory storage material, and in electrochromic, it is used as a battery material. This Review discusses the progress of viologen in organic and inorganic discoloration materials in recent years. The syntheses of viologen and its derivatives are summarized, and its application in the field of discoloration materials is introduced.

An inclusion complex of cucurbit[7]uril with benzimidazolyl benzyl viologen exhibits fluorescence and photochromic properties

A self-assembled supramolecular inclusion complex of Q[7] with benzimidazolyl benzyl viologen exhibits interesting fluorescence emission and reversible photochromism.

Reversible Photochromic Coordination Polymer by Phototriggered Subtle Molecular Conformation Variations

Photochromic materials are constructed with molecules accompanied by structural change after triggering by light, which are of great importance and necessity for various applications. However, because of space-confinement effects, molecule stacking of these photoresponsive chromophores within coordination polymers (CPs) always results in an efficiency decrement and a response delay, and this phenomenon will lead to a poor photochromic property. Herein, a CP (named CIT-E) with a 3-fold-interpenetrating network structure, which was prepared with (Z)-1,2-diphenyl-1,2-bis[4-(pyridin-3-ylmethoxy)phenyl]ethene (1Z) and a CuI cluster, showed fast reversible photochromic behavior. Under UV-light illumination, the color of CIT-Z changed from pale yellow to reddish brown. With the illumination of green light, the polymer could return to its initial color within 10 s. To reveal the mechanism of reversible photochromic behavior of CIT-Z, single-crystal structures of each color state were fully studied, and other scientific study methods were also used, such as time-dependent density functional theory calculation and control experiments. It was found that, with light illumination, this behavior of CIT-Z was the result of a ligand-to-metal charge-transfer process, and this process was triggered by subtle molecular conformation variation of tetraphenylethylene. It should be noted that CIT-Z has high thermal and chemical stability, which are excellent advantages as smart photoresponsive materials. As a proof of concept, a uniform thin film with such a fascinating photochromic property allows applications in invisible anticounterfeiting and dynamic optical data storage. Overall, the present study opens up a new avenue toward reversible photochromic materials.

Electron transfer photochromism of solid-state supramolecules constructed by cucurbit[n]uril (n = 5–8) and 1-(4-carboxybenzyl)-4-[2-(4-pyridyl)-vinyl]-pyridinium chloride

Solid-state supramolecular complexes of a bispyridinium ethylene derivative with cucurbit[n = 5–8]uril exhibit fast responsive photochromic properties, which are caused by the electron transfer from the host to the guest.

A novel multichromic Zn(II) cationic coordination polymer based on a new flexible viologen ligand exhibiting aniline detection in the solid state

In this work, a novel multichromic cationic coordination polymer, named [Zn4(BTC)3(bcbpy)2]·5H2O (1), based on a new flexible viologen ligand 1,1'-bis(3-cyanobenzyl)-[4,4'-bipyridine]-1,1'-diium (H2bcbpy·2Cl), Zn(NO3)2·6H2O and pyromellitic acid (H4BTC) was synthesized. Compound 1 has good photosensitive activity and can respond to sunlight at room temperature. The colour of compound 1 changes rapidly in response to UV light and blue ray irradiation within 5 s. We rarely obtained the crystal structures after irradiation under UV light and blue ray. At the same time, compound 1 shows the hydrochromism phenomenon when heated at 120 °C, and it also shows the ability of detecting aniline and NO2- under low-concentration conditions (10-4 M).

Three-Dimensional Interpenetrating Frameworks Based on {P4Mo6} Tetrameric Clusters and Filled with In Situ Generated Alkyl Viologens

Four novel three-dimensional interpenetrating frameworks based on {P₄Mo₆} units, H[C₁₂H₁₄N₂]₄[TM₄(PO₄)(H₂O)₄Na₆][TM₂(Mo₆O₁₂(HPO₄)₃(PO₄)(OH)₃)₄]·8H₂O (1, 2) and H[C₁₄H₁₈N₂]₄[TM₄(PO₄)(H₂O)₄Na₆][TM₂(Mo₆O₁₂(HPO₄)₃(PO₄)(OH)₃)₄]·8H₂O (3, 4) (TM = Co²⁺ (1, 3), Mn²⁺ (2, 4)) were synthesized and characterized using infrared spectroscopy, elemental analyses, thermogravimetric analysis, and single-crystal X-ray diffraction. In situ generated methyl viologen (compounds 1 and 2) or ethyl viologen (compounds 3 and 4) cations function as templates to induce the generation of 2-fold interpenetrating structures in which the {P₄Mo₆} tetrameric clusters with [TM₄(PO₄)Na₆] (TM = Co²⁺ (1, 3) and Mn²⁺ (2, 4)) as the core were bridged by transition metal ions. Compounds 1-4 possess high thermal stabilities and the decomposition temperature of the inorganic frameworks were all >500 °C. It is worth noting that the four compounds all exhibited the bifunctional catalytic performance that they not only had an excellent photocatalytic activity for the reduction of hexavalent chromium (Cr(VI)) under visible light irradiation but also showed a good electrocatalytic activity for the reduction reaction of hydrogen peroxide.

A novel viologen-based coordination polymer with multi-stimuli responsive chromic properties: photochromism, thermochromism, chemochromism and electrochromism

Increasing interest in chromic materials is due to the growing demand. However, most chromic materials exhibit color changes in response to only one stimulus, but there are multiple stimuli in nature. Therefore, the construction of multistimuli responsive chromic materials still faces great challenges. Herein, a new multi-stimuli responsive coordination polymer [Zn₂(2,3-PDC)₂CV·(H₂O)₂]·H₂O (1) (2,3-PDC = 2,3-pyridine dicarboxylic acid, CV = N,N'-4,4'-bipyridiniodipropionate) has been successfully synthesized, which exhibits photochromism under 300 W xenon lamp irradiation accompanied by an obvious color change from colorless to light blue. Meanwhile, compound 1 displays excellent thermochromic properties with a color change from colorless to light yellow when heated at 106 °C. The product of thermochromism is named 1T and the loss of free water improves the photoresponsive properties of 1T. Moreover, the compound can show differentiable detection of amines because of the electron-deficient nature of the viologen. Finally, 1 shows excellent electrochromic properties and turns from colorless to purple at E = -3 V. In conclusion, compound 1 exhibits multi-chromic behaviors in response to light, heat, amines and electricity, which are prominent in viologen based coordination polymers.

Three viologen-derived Zn-organic materials: photochromism, photomodulated fluorescence, and inkless and erasable prints

Three zinc–viologen materials exhibit reversible photochromism, photomodulated fluorescence and the capability as inkless and erasable print media.

A novel photochromic coordination polymer based on a robust viologen ligand exhibiting multiple detection properties in the solid state

A new photochromic and porous Cd(ii) coordination polymer (CP) exhibiting multiple detection properties for amines and benzenes in the solid state.

Two novel multichromic coordination polymers based on a new flexible viologen ligand exhibiting photocontrolled luminescence properties and sensitive detection for ammonia

Two novel Cd-CPs based on a new viologen ligand exhibiting photochromism, thermochromism, photocontrolled luminescence properties, thermocontrolled luminescence properties and ammonia detection.

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.