Four POM-Viologen Color-Changing Materials with Fast Color Response under Various External Stimuli.

Four kinds of polyoxometalate (POM)-viologen compounds were synthesized by hydrothermal method, namely (1-cby)2·[H2(SiMo12O40)]·2H2O (1), (1-cby)2·[H2(SiW12O40)]·2H2O (2), (1-cby)2·(1,1'-bcby)2·{H4[Co4(H2O)2(PW9O34)2]}·12H2O (3), (1-cby)·(1,1'-bcby)·[H(α-PW11O39)CoII(1-cby)]·8H2O (4) (1-cby·Br = 1-Cyclopropylmethyl-[4,4']bipyridinyl-1-ium bromide, 1,1'-bcby·Br = 1,1'-Bis-cyclopropylmethyl-[4,4']bipyridinyl-1-ium bromide). These four POM-viologen compounds exhibit one-dimensional supramolecular network structures. Especially, compound 3 contains a rare sandwich POM subunit {Co4(H2O)2(PW9O34)2}10-. These four compounds can be used as color-changing materials, and they all exhibit noticeable color changes upon exposure to light, heat, and electricity. The discoloration mechanism involves viologen derivatives with electron-deficient properties accepting electrons from POM with electron-rich properties under external stimulation, leading to the formation of viologen free radicals. Among them, compounds 1 and 2 also have good properties for ink-free erasable printing, double anticounterfeiting, and ultraviolet detector because of their rapid color response to ultraviolet (UV) light. In addition, compounds 1-4 also show different color changes in the detection of volatile amines.

Four polyoxomolybdated-based 3D compounds as supercapacitors and amperometric sensors.

Four polyoxomolybdated compounds based on Tetp (Tetp = 4-[4-(2-Thiophen-2-yl-ethyl)-4H-[1, 2, 4]triazole-3-yl]-pyridine), namely [Zn(Tetp)2(H2O)2][(ß-Mo8O26)0.5] (Zn-Mo8), [Co(Tetp)2(H2O)2][(ß-Mo8O26)0.5] (Co-Mo8), [Cu4(Tetp)6(H2O)2]{H3[K(H2O)3](θ-Mo8O26)(Mo12O40)}·8H2O (Cu-Mo20) and [Cu3(Tetp)3][PMo12O40]·H2O (Cu-PMo12) are synthesized by hydrothermal methods and are used as electrode materials for supercapacitors(SCs) and electrochemical sensors. Inserting polyoxometalates (POMs) with redox active sites into transition metal compounds (TMCs) can improve the internal ion/electron transfer rate, thus effectively enhancing the electrochemical performance. Compared with the parent POMs, four compounds exhibit excellent electrochemical properties. In particular, Cu-PMo12 shows an excellent specific capacitance (812.3 F g-1 at 1 A g-1) and stability (94.42%), as well as a wide detection range (0.05 to 1250 µM) and a low detection limit (0.057 µM) for NO2- sensing.

A Series of POM-Viologen Photo-/Electrochromic Hybrids and Hydrogels Acting as Multifunctional Sensors for Detecting UV, Hg2+, and Organic Amines.

In this work, POM anions were introduced into the viologen system in order to synthesize POM-viologen hybrid compounds with excellent properties. Three new POM-viologen compounds, {CdII(tybipy)(DMF)2[ß-Mo8O26]0.5Cl} (1), {CoII(tybipy)2(DMF)2[H2(ß-Mo8O26)]2}·4C2H7N (2), and (tybipy)4·(ß-Mo8O26) (3) (tybipy·Br = 1-thiophen-3-ylmethyl-[4,4']bipyridinyl-1-ium bromide), have been prepared by a solvothermal method, and their structures were characterized. POM anions are modified by mixing organic ligands with transition metals in compounds 1 and 2. However, compound 3 is a supramolecular structure constructed by hydrogen bonding interactions between the dissociative viologen and POM anions. These three compounds have rapid photoresponse and photochromic ability, which can be made into mixed matrix membranes and hydrogels for UV detection. The rigid sandwich devices prepared by compounds 1-3 have achieved ultrafast electrochromism and recovery. In addition, photochromic hydrogels based on compounds 1-3 can achieve ultrafast photochromic recovery. Compounds 1-3 can be used in ink-free printing and Hg2+ fluorescence detecting. Compounds 1 and 2 can also be used as organic amine detectors. Combined with photochromism and fluorescence detection of Hg2+, visual test papers for Hg2+, Cu2+, and Co2+ were successfully realized, which can improve the portability and detection speed of heavy metal ions in the actual environment.

Photo-, Thermo-, Electrochromic, Erasable Inkless Printing, Ions Detection, and UV Detector Properties of Viologen Compounds Based on Homomolybdate/Keggin POMs.

Three POMs-based viologen compounds with different structures were successfully constructed under solvothermal and hydrothermal conditions, [Cu(1,4-cby)2(H2O)0.5(ß-Mo8O26)0.5]·C3H7NO·3H2O (1), [H2(1,4-cby)2]·(ß-Mo8O26) (2) (1,4-cby·Cl = 1-(4-carboxybenzyl)-4,4'-bipyridine chloride), [H2(1,4-cbyy)2]·(SiMo12O40) (3) (1,4-cbyy·Cl = 1-(4-cyanobenzyl)-4,4'-bipyridine chloride). Compound 1 is a structure with the number "eight-like" metal-organic chain with Cu as the nodes, and compounds 2 and 3 are fascinating structures connected by hydrogen bonding interactions. More importantly, compounds 1-3 exhibit a good response to both light and electricity and the thermal response of compound 1 was also studied. The reasons for the response of compounds 1-3 to external stimuli were analyzed through methods such as UV-Vis, EPR, and XPS. In addition, the transient photocurrent response results of compounds 1-3 are the same as those obtained from kinetic calculations. Meanwhile, the coated filter paper based on compound 3 has been successfully applied in erasable inkless printing and anti-counterfeiting, the test paper of 3 can also detect metal ions, and the films based on compounds 1-3 are a flexible and portable ultraviolet (UV) detector.

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, {MII(1,4-cby)2[H2(γ-Mo8O26)]}·nH2O (1: M = Ni, n = 4; 2: M = Co, n = 6), and [NiII(1,3-cby)(H2O)4(ß-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.

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 [AgI (bmypd)0.5 (ß-Mo8 O26 )0.5 ] (1) (bmypd ⋅ 2Cl=1,1'-[Biphenyl-4,4'-bis(methylene)]bis(4,4'-bipyridyinium)dichloride), [AgI 2 (bypy)4 (HSiW12 O40 )2 ] ⋅ 14H2 O (2) and [AgI (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.

Anderson polyoxometalates with intrinsic oxidase-mimic activity for "turn on" fluorescence sensing of dopamine.

Anderson-type polyoxometalate containing Fe3+ and Mo6+, (NH4)3[H6Fe(III)Mo6O24] (FeMo6), was found to work as an oxidase-mimicking nanoenzyme for the first time, exhibiting the ability of catalytic oxidation of o-phenylenediamine (OPD), 2,2'-azino-bis(3-ethylbenzthiazoline-6-sulfonic acid) (ABTs), and 3,3',5,5'-tetramethylbenzidine (TMB), which features easy synthesis, low cost, simple operation, and low consumption. Attributed to the nature of FeMo6 and Fenton-like effect, a novel sensor based on two consecutive "turn on" fluorescence was developed for detecting dopamine (DA) by employing the FeMo6-OPD system, and the linear range was from 1 to 100 µM with the detection limit 0.0227 µM (3σ/s). Moreover, to increase oxidase-mimic activity of FeMo6, reduced graphene oxide (rGO) loading FeMo6 composites (FeMo6@rGO (n), n = 5%, 10%, 15%) was fabricated, and results show that oxidase-like activities of FeMo6@rGO (n) are dependent on the mass ratio of FeMo6/rGO, and FeMo6@rGO (10%) exhibits the highest oxidase-mimic activity and the fastest respond time (4 min) among all reported oxidase mimic of DA to date. Graphical abstract Anderson-type Mo-POMs FeMo6 was found to work as an oxidase-mimicking nanoenzyme for the first time and was used to detect DA for two consecutive "turn on" fluorescence sensor modes.

Electrocatalytic and Hg2+ Fluorescence Identifiable Bifunctional Sensors for a Series of Keggin Compounds.

By using a 2,2'-dimethyl-4,4'-bithiazole (dm4bt) ligand, Keggin polyanions, and different metal ions, nine Keggin-based compounds, namely, {[Ag(dm4bt)2][Ag2(dm4bt)3]}2(PW12O40)(H2PWV2WVI10O40) (1), [CuI(dm4bt)2][CuII(dm4bt)2(PW12O40)] (2), [CuI(dm4bt)2]4(SiW12O40) (3), {[Zn(dm4bt)2]2(SiW12O40)} (4), [Zn(dm4bt)2(H2O)]2(HPMoV2MoVI10O40)·2H2O (5), [Zn(dm4bt)2(Mo2O7)] (6), [Cd(dm4bt)3][Cd(dm4bt)2(H2O)(PMo12O40)]2·2H2O (7), [Cd(dm4bt)3](PMo12O40)·(Hdm4bt) (8), and [Cd(dm4bt)2(H2O)2]2(HPMoV2MoVI10O40)·2H2O (9), have been hydrothermally synthesized and characterized by single-crystal X-ray diffraction analysis, IR spectroscopy, and elemental analyses. Compounds 1-9 are zero-dimensional structures except compound 6, which exhibits a one-dimensional structure. In compound 1, there are three isolated subunits: Keggin anions, binuclear [Ag2(dm4bt)3]2+, and mononuclear [Ag(dm4bt)2]+ clusters. The binuclear [Ag2(dm4bt)3]2+ cluster has a Ag-Ag bond. In compound 2, a monosupporting anion {[CuII(dm4bt)2](PW12O40)}- and an isolated [Cu(dm4bt)2]+ cluster exist. By changing the transition metal ions, we obtained two different structures: a supramolecular 3 and a bisupporting anion in 4. By a one-pot method, we successfully obtained compounds 5 and 6, 7 and 8, respectively. In compound 5, the [Zn(dm4bt)2(H2O)]2+ subunit links adjacent PMo12 anions via S···O interactions to form a one-dimensional (1D) supramolecular chain. In compound 6, some PMo12 ions have transformed to [Mo2O7] n2 n- chains. The [Zn(dm4bt)2]2+ clusters buckle up and down the chain. Compound 7 has a monosupporting anion and an isolated [Cd(dm4bt)3]2+ cluster. Compound 8 has isolated anions and [Cd(dm4bt)3]2+ clusters. By changing the pH of 7 and 8, a distinct supramolecular compound 9 was obtained. Additionally, the optical band gap, electrochemical, and photocatalytic properties of 1-9 have been investigated in detail. The carbon paste electrodes can be used as bifunctional amperometric and fluorescence sensors for recognition of Hg2+. The n-CPEs as electrochemical sensors can show accurate selectivity for NO2- ions in some common ions. In the fluorescence sensor experiment, fluorescence intensities decrease more than 80% when quenched by Hg2+.

Systematic Investigation of Reaction-Time Dependence of Three Series of Copper-Lanthanide/Lanthanide Coordination Polymers: Syntheses, Structures, Photoluminescence, and Magnetism.

Three series of copper-lanthanide/lanthanide coordination polymers (CPs) Ln(III) Cu(II) Cu(I) (bct)3 (H2 O)2 [Ln=La (1), Ce (2), Pr (3), Nd (4), Sm (5), Eu (6), Gd (7), Tb (8), Dy (9), Er (10), Yb (11), and Lu (12), H2 bct=2,5-bis(carboxymethylmercapto)-1,3,4-thiadiazole acid], Ln(III) Cu(I) (bct)2 [Ln=Ce (2 a), Pr (3 a), Nd (4 a), Sm (5 a), Eu (6 a), Gd (7 a), Tb (8 a), Dy (9 a), Er (10 a), Yb (11 a), and Lu (12 a)], and Ln(III) 2 (bct)3 (H2 O)5 [Ln=La (1 b), Ce (2 b), Pr (3 b), Nd (4 b), Sm (5 b), Eu (6 b), Gd (7 b), Tb (8 b), and Dy (9 b)] have been successfully constructed under hydrothermal conditions by modulating the reaction time. Structural characterization has revealed that CPs 1-12 possess a unique one-dimensional (1D) strip-shaped structure containing two types of double-helical chains and a double-helical channel. CPs 2 a-12 a show a three-dimensional (3D) framework formed by Cu(I) linking two types of homochiral layers with double-helical channels. CPs 1 b-9 b exhibit a 3D framework with single-helical channels. CPs 6 b and 8 b display visible red and green luminescence of the Eu(III) and Tb(III) ions, respectively, sensitized by the bct ligand, and microsecond-level lifetimes. CP 8 b shows a rare magnetic transition between short-range ferromagnetic ordering at 110 K and long-range ferromagnetic ordering below 10 K. CPs 9 a and 9 b display field-induced single-chain magnet (SCM) and/or single-molecule magnet (SMM) behaviors, with Ueff values of 51.7 and 36.5 K, respectively.

Unprecedented application of flexible bis(pyridyl-tetrazole) ligands to construct helix/loop subunits to modify polyoxometalate anions.

By introducing the unprecedented and flexible isomeric bis(pyridyl-tetrazole) ligands into a polyoxometalates (POMs) system, three POM-based compounds, {Ag2(4-bptzb)2(H2O)2[H2PMo12O40]2}·4-bptzb·5H2O (1), [Ag4(3-bptzb)2(PMo(V)Mo(VI)11O40)]·2H2O (2), and Ag3(3-bptzb)2.5(H2O)2[H3P2W18O62] (3) [4-bptzb = 1,4-bis(5-(4-pyridyl)tetrazolyl)butane and 3-bptzb =1,4-bis(5-(3-pyridyl)tetrazolyl)butane], were synthesized under hydrothermal conditions and structurally characterized by single-crystal X-ray diffraction analyses. Compound 1 exhibits a dimeric structure constructed from two Keggin [PMo12O40](3-) anions and a binuclear [Ag2(trans-4-bptzb)2](2+) subunit in which the trans-4-bptzb acts as a bidentate bridging ligand with one tetrazolyl group. In 2, the 3-bptzb acts as a tetradentate bridging ligand with the tetrazolyl and pyridyl groups linking Ag(I) ions to generate a 3D metal-organic framework (MOF), which contains charming meso-helix chains. The Keggin anions acting as bidentate inorganic ligands reside in the distorted tetragonal channels of the MOF. In compound 3, the 3-bptzb adopts versatile coordination modes linking Ag(I) ions to first construct loop connecting loop 1D chains, which are linked by {Ag[P2W18O62]}n zigzag chains to form a scarce hamburger-style 2D sheet. These adjacent sheets are further fused by 3-bptzb ligands to construct a 3D framework. The influences of isomeric bptzb ligands and POMs on the construction of Ag-bptzb subunits and the whole structures of the title compounds are discussed. The electrochemical behaviors and electrocatalytic activities of compounds 2 and 3 and their corresponding parent POMs as well as the fluorescent properties of the title compounds have been studied in detail. In addition, the photocatalytic activities of compounds 2 and 3 and their corresponding parent POMs for decomposition of methylene blue, rhodamine B, and methyl orange under UV irradiation have also been investigated.

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(ß-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 series of viologen complexes containing thiophene and Br- dual fluorescent chromophores for continuous visual sensing of pH and Hg2.

The mercury ion (Hg2+) is a typical high-toxicity substance that can cause severe damage to the environment and human bodies. For the detection of Hg2+, there are still significant challenges in the detection range and limit of detection (LOD). In this study, three viologen-based fluorescent probes are developed, CdCl4(Btybipy) (1), ZnBr4(Btybipy) (2), CdBr4(Btybipy) (3) (Btybipy = bis-1-thiophen-3-ylmethyl-[4,4']-bipyridinyl) through conventional solvent methods for detecting pH and Hg2+. Reversible discoloration and fluorescence response behaviour in the pH range of 4-12.8 is demonstrated by viologen-based fluorescent probes, which exhibit "ON-OFF-ON" signal changes. Compared with complex 1, it is surprising to find that complexes 2-3 display both fluorescence enhancement and fluorescence quenching simultaneously with the addition of different concentrations of Hg2+ (0-20 and 25-400 µM). There is broad linearity in the range of 0-20 and 50-300 µM with LODs of 2.14 and 3.13 nM, respectively. This occurrence of dual-signal modes is attributed to the participation of Br- and the thiophene S atom as dual chromophores in the coordination reaction of Hg2+. Dual-signal mode output, high sensitivity, wide detection range, and low LODs are exhibited by these fluorescent probes. The unique coordination reaction between Br- and the thiophene S atom with Hg2+ can provide a potential strategy for the exploitation of promising sensing platforms for monitoring Hg2+.

The applications of Keggin-based metal-organic compounds in sensing and catalysis.

Environmental pollution and energy problems caused by excessive use of fossil fuels deviate from the theme of green and sustainable development. It is very promising to detect small molecules or catalyze the conversion of pollutants to obtain renewable energy by using photoelectric technology. Therefore, there is an urgent requirement to develop materials with low detection limits and high catalytic performance. Keggin polyoxometalate-based metal-organic compounds (POMOCs) hold great promise for sensing, and catalytic applications due to their controllable structure, remarkable reversible multi-electron transfer capability and multi-component synergistic activity. In this review, the applications of Keggin POMOCs in photocatalytic/electrocatalytic conversion of energy materials and the detection of metal ion/inorganic molecule are introduced. The different mechanisms of Keggin POM units and MOF units in sensors and catalysis are discussed. Additionally, the prospects of the Keggin POMOCs as electrode materials or catalysts for enhancing the performance of sensors and catalysts are discussed, which will provide a platform for further development of advanced Keggin POMOC material-based sensors and catalytic systems.

A lantern-shaped fluorescent probe based on viologen/polyoxometalate for the detection of Ag+ in beverages and daily necessities.

A lantern-shaped viologen/polyoxometalate (POM)-based compound [NiII(MSBP)2(H2O)2]·(ß-Mo8O26)·H2O (Ni-POM) (MSBP = 1-(4-Methanesulfonyl-benzyl)-[4,4']bipyridinyl-1-ium) was successfully synthesized by a hydrothermal method for the efficient detection of Ag+. A strong affinity between Ag+ and SO in the viologen component of the Ni-POM structure made them interact, which led to blue fluorescence quenching. In the concentration range of 0.1-4 µM, a strong linear relationship was observed between the Ag+concentration and the fluorescence intensity ratio of Ni-POM, and the limit of detection (LOD) was 20.4 nM. Considering the widespread presence of Ag+ in various water sources, daily necessities and food preservatives, the utilization of Ni-POM for detecting the concentration of Ag+ in real samples (water, daily necessities and beverages) was proved to be highly effective. Moreover, a remarkable recovery rate ranging from 95.70 % to 103.60 % was achieved, indicating that the monitoring results of practical samples were satisfactory. A fluorescent ink based on Ni-POM was designed for the purpose of information confidentiality. More importantly, the hydrogel intelligent device for visual detection of Ag+ was developed, which could realize visual real-time on-site quantitative detection of Ag+ concentration in beverages and daily necessities. Therefore, Ni-POM provides an effective platform for the development of visually quantitative detection of Ag+ in food and daily necessities.

Four octamolybdate compounds with properties of organic dye adsorption and photocatalytic reduction of Cr(VI).

Hydrothermal synthesis was used to create four different POM-based compounds, namely {[Co(Hptpm)2(ß-Mo8O26)0.5(ξ-Mo8O26)0.5]}·H2O (1), [Co(Hptpm)2(δ-Mo8O26)]·H2O (2), [Co(Hptpm)2(ß-Mo8O26)] (3) and [Zn(Hptpm)2(ß-Mo8O26)] (4) (ptpm = 4-[3-(3-pyridine-2-yl-[1,2,4]triazol-4-yl)-propyl]-morpholine). 1-4 containing different octamolybdate isomers were characterized. Compounds 1-4 showed good electrochemical performance and can be utilized as bifunctional sensors for NO2-, H2O2, Cr(VI) and Fe(III). Taking compound 1 as an example, the detection limits are 0.081 µM for NO2-, 0.072 µM for H2O2, 0.054 µM for Cr(VI) and 0.063 µM for Fe(III), respectively. Compounds 1-4 have good capacitance. Moreover, compounds 1-4 also show good adsorption properties for organic cationic dyes. The cationic dyes include methylene blue (MB), crystal violet (CV) and neutral red (NR). In addition, 1-4 have excellent characteristics that can reduce Cr(VI) to Cr(III) by photocatalytic technology. Within 30 min, the reduction rates were 95.85% for 1, 93.99% for 2, 90.29% for 3 and 88.18% for 4.

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(ß-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(ß-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.

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.

Application of tetrazole-functionalized thioethers with different spacer lengths in the self-assembly of polyoxometalate-based hybrid compounds.

Three metal-organic networks based on Keggin-type polyoxometalates (POMs) have been hydrothermally synthesized by tuning the spacer lengths of bis(tetrazole)-functionalized thioether ligands and structurally characterized: [Cu(4)(bmtm)(4)][SiW(12)O(40)]·2H(2)O (1), [Cu(4)(bmte)(3.5)][SiW(12)O(40)] (2), and [Cu(4)(bmtp)(4)][SiW(12)O(40)] (3) [bmtm = 1,1'-bis(1-methyl-5-mercapto-1,2,3,4-tetrazole)methane, bmte = 1,2-bis(1-methyl-5-mercapto-1,2,3,4-tetrazole)ethane, and bmtp = 1,5-bis(1-methyl-5-mercapto-1,2,3,4-tetrazole)pentane]. The spacer lengths and sulfhydryl of bis(tetrazole)-functionalized thioether ligands play important roles in the final framework formation, as shown by X-ray diffraction analysis. In compound 1, with the connection of a N,S bridge of bmtm, two kinds of binuclear Cu(I) units are formed and linked to construct a one-dimensional (1D) chain. The [SiW(12)O(40)](4-) (SiW(12)) cluster provides four terminal O atoms linking four binuclear units to generate a two-dimensional layer with (8(3))(2)(8(5)·10) topology. In compound 2, centrosymmetric octameric moieties composed of two equivalent tetrameric Cu(I) units are bridged by bmte ligands to form a 1D chain. The SiW(12) clusters show an unusual (2,8)-connected mode to connect with the 1D chain and construct a four-connected three-dimensional (3D) network with 5(3)·6(2)·7 topology. Compound 3 exhibits a rare 3D host framework with a type of large cavity and two types of small windows. The SiW(12) clusters as templates are strongly cemented into the large cavities and completely encircled by small windows. Furthermore, the compound 2 bulk-modified carbon-paste electrode (2-CPE) displays good electrocatalytic activity toward the reduction of nitrite.

Assemblies of copper bis(triazole) coordination polymers using the same keggin polyoxometalate template.

Four inorganic-organic hybrid compounds, [Cu(I)(4)(bte)(4)(SiW(12)O(40))] (1), [Cu(II)(2)(bte)(4)(SiW(12)O(40))].4H(2)O (2) [bte = 1,2-bis(1,2,4-triazol-1-yl)ethane], [Cu(I)(4)(btb)(2)(SiW(12)O(40))].2H(2)O (3), and [Cu(II)(2)(btb)(4)(SiW(12)O(40))].2H(2)O (4) [btb = 1,4-bis(1,2,4-triazol-1-yl)butane], were hydrothermally synthesized through use of the same Keggin polyoxometalate as the template and tuning the molar ratio of the bis(triazole) ligand to the Cu(II) ion. The ratio of the bis(triazole) ligand to Cu(II) has a crucial influence on the structures of this series. Single-crystal X-ray diffraction analyses indicate that compound 1 is constructed by tetranuclear ring-connecting chains and polymerized [Cu(bte)](+) chains, between which SiW(12) anions are inserted to form a three-dimensional (3D) structure. Compound 2 shows a (4(4).6(2)) two-dimensional grid sheet. The discrete SiW(12) anions are sandwiched by the sheets, just like "hamburgers". Compound 3 presents channel-like [Cu(2)(btb)](2+) polymerized chains, which are further connected by SiW(12) anions to construct a 3D framework. Compound 4 exhibits a (6(6)) 3D Cu-btb framework with hexagonal channels, into which the tetradentate SiW(12) anions are incorporated. The thermal stabilities of the compounds are discussed.

Novel Anderson-type [TeMo6O24](6-)-based metal-organic complexes tuned by different species and their coordination modes: assembly, various architectures and properties.

Four novel Anderson-type polyoxometalates (POMs) [TeMo6O24](6-) (TeMo6)-based metal-organic complexes (MOCs), namely, H2[Zn4(3-Hdpyp)2(TeMo6O24)2(H2O)16]·6H2O (1), [Zn3(3-dpyb)2(TeMo6O24)(H2O)12]·8H2O (2), [Cu2(4-Hdpyp)2(TeMo6O24)(H2O)6]·4H2O (3), and [Cu3(3-dpyp)2(TeMo6O24)(H2O)8]·4H2O (4) (3-dpyp = N,N'-bis(3-pyridinecarboxamide)-1,3-propane, 3-dpyb = N,N'-bis(3-pyridinecarboxamide)-1,4-butane, 4-dpyp = N,N'-bis(4-pyridinecarboxamide)-1,3-propane), were hydrothermally synthesized and structurally characterized by single-crystal X-ray diffraction, IR spectra, powder X-ray diffraction (PXRD) and thermogravimetric analyses (TGA). In complex 1, two TeMo6 polyoxoanions bridge Zn(II) ions to generate a discrete tetranuclear zinc complex H2[Zn4(3-Hdpyp)2(TeMo6O24)2(H2O)16]. In 2, the TeMo6 anions connect the metal-organic units [Zn3(3-dpyb)2](6+) to form an Anderson-type TeMo6-based wave-like metal-organic chain. Both the discrete tetranuclear subunits in 1 and 1D wavy chains in 2 were further extended to 2D supramolecular networks through the hydrogen bonding interactions. In complex 3, the TeMo6 anions link the metal-organic units [Cu2(4-Hdpyp)2](6+) to construct a 2D layer, which is further extended to a 3D supramolecular framework by the hydrogen bonding interactions. In complex 4, the TeMo6 polyoxoanions bridge the Cu(II) ions to generate a 2D Cu-TeMo6 inorganic layer, which is further linked by the µ2-bridging 3-dpyp ligands to form a 3D metal-organic framework. The effects of various coordination modes of TeMo6 polyanions and the bis-pyridyl-bis-amide ligands, as well as the central metals on the structures of the title complexes have been discussed. Besides, the fluorescence, electrochemical properties and photocatalytic activities of the title complexes have been investigated in detail.