Viologen System Research Articles

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[β-Mo8O26]0.5Cl} (1), {CoII(tybipy)2(DMF)2[H2(β-Mo8O26)]2}·4C2H7N (2), and (tybipy)4·(β-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.

Revisiting magnetic field effects in homogeneous medium and bio-mimicking environments with emphasis on acridine derivatives

A weak external magnetic field, very close to the hyperfine interactions of the system, can acts as a tool to monitor spin dynamics and assess distance between the components of the spin-correlated transient radical pair or radical ion pair (RIP). The present review focuses on the magnetic field effect (MFE) on the photo-induced electron-transfer (PET) reactions among acridine derivatives and classical as well as biological electron acceptor or donor moieties, which produce spin-correlated RIPs, in homogeneous solvents, heterogeneous micellar media and in biological nanocavities of proteins. Although a confined medium is preferred to observe prominent MFE, yet unanticipated MFE on PET between acridine derivatives [Acridone (AD) and Acridine Yellow (AY)] and classical electron donors is obtained even in homogeneous medium when it consists of impurities like water molecules. In a comparative study of interaction of another acridine derivative, Proflavin (PF+) with two electron donors which are amines of aromatic nature, MFE on PET reveal that the bulk and the structure of the electron donor govern the mechanism as well as the spin dynamics of PET. While studying interaction of PF+ with a different amine which is aliphatic in nature, MFE on PET implies that it is the nature of the solvent matrix which determines the spin dynamics of PET. The cause of discrepancy in the experimental and calculated values of B1/2 for 9-amino acridine – methyl viologen system has been delineated. Apart from micellar medium, prominent MFE on PET is also observed while studying the interaction of PF+, AY and AD with tryptophan residues present in the nanocavities of serum albumins since the inter-radical distance within primary geminate RIP is enough to make exchange interaction negligible.

Theory for admittance voltammetry of reversible two step electron transfer process with DC bias at rough and fractal electrodes

We developed a phenomenological theory for DC-bias dependent electrochemical impedance response and admittance voltammetry of reversible two-step electron transfer (EE) process at randomly rough and fractal electrodes. The power spectrum of roughness is used to incorporate the statistical information of surface randomness. For the finite fractal model, statistical morphological parameters of roughness, viz., fractal dimension (DH), topothesy length (ℓτ) and lower cut-off length (ℓ), significantly influence the admittance response. The magnitude of admittance increases with an increase in the roughness of fractal electrode (e.g. increase in the value of DH or ℓτ or decrease in ℓ) for both the electron transfer steps. The magnitude of admittance with the variation of DC potential at fixed temporal frequency unravels the multi-electron transfer process. The two step electron transfer process has two characteristic peaks in their plot. The enhanced roughness of electrode increases the height of the admittance peaks and the regions around it (obtained at different fixed frequencies). Our theory is validated with experimental data for the ethyl viologen system at a moderately rough Pt electrode. Finally, our current methodology for admittance voltammetry has the dual advantage of impedance as well as of voltammetry.

Electrochromic Properties of Metal Oxide Nanoparticles/Viologen Composite Film Electrodes

Electrochromic substances supported on metal oxide nanoparticle layers have found application in electrodes for electronic paper and smart windows. In addition to applications of technological importance, the nanospaces created by the nanoparticles are scientifically important as they provide specific electrochemical reaction fields. Herein, the electrochromic properties of the composite film electrodes consisting of viologen and a metal oxide (SnO2, Al-doped ZnO, and ZnO) on an indium tin oxide (ITO)-coated glass plate were investigated. All the electrodes exhibited electrochromism ranging from purple to white (or yellowish-white). However, the coloring uniformity, coloring concentration, and response times of coloring/decoloring depended on the nature of the nanoparticles, the reason for which is discussed on the basis of the difference in the mixing state of the nanoparticle–viologen system and the difference in the hydrophilicity of the nanospaces. The SnO2–viologen/ITO electrode demonstrated the most...

Enhanced efficiency of the visible-light photocatalytic hydrogen generation by the ruthenium tris(2,2′-bipyridyl)-methyl viologen system in the presence of cucurbit[n]urils

Cucurbiturils are cyclic oligomers of glycoluril units whose molecular shape defines an internal hollow space accessible through carbonyl portals. It is known that methyl viologen forms strong host-guest complexes with CB[7] and CB[8]. In the latter case even a dimer can be hosted inside CB[8]. Classical solution systems for visible light H(2) generation use methyl viologen as an electron relay. In this work we have found that the efficiency for H(2) generation of the photocatalytic system comprising EDTA-ruthenium tris(2,2'-bipyridyl)-methyl viologen-Pt increases when cucurbiturils are present in the solution. The enhancement follows the order CB[6] < CB[7] < CB[8]. By means of laser flash photolysis we have established that this efficiency increase arises from the combination of an increase in the relative quantum yield of charge separation (higher efficiency in the formation of MV*(+)) and a decrease of MV*(+) lifetime.

Fluorinated ethylene propylene copolymer coating for the stability enhancement of electroactive and photoactive systems

The effectiveness of radio frequency sputtered fluorinated ethylene propylene copolymer (FEP) for the stability enhancement of electroactive and photoactive systems was investigated. Two kinds of electroactive polymer systems, polyaniline (PANI) coated low density polyethylene (LDPE) film and PANI-viologen assembly, were tested. In both cases, a sputtered FEP coating of <10 nm in thickness significantly enhanced the electrical stability of the films in water. The enhancement of the electrical stability of the PANI-LDPE film was also achieved in basic aqueous solution of pH up to 12 with a FEP coating of 40–50 nm in thickness. The deposition of a FEP coating on the photoactive viologen system (viologen grafted on LDPE film) prolonged its photochromic effect by inhibiting the diffusion of oxygen, and hence the reoxidation of the highly colored viologen radical cations to the dication state.

Long-lived photoinduced charge separation in carbazole–pyrene–viologen system incorporated in Langmuir–Blodgett films of substituted diazacrown ethers

Diaza-18-crown-6 ethers appending two pyrenyl (Py) or two carbazolyl (Cz) groups were synthesized. These macrocyclic compounds form 1:1 host–guest complexes with methyl viologen chloride (MV2+), and these complexes were assembled into monolayers by Langmuir–Blodgett technique. The generated assembly involves the general structure of donor–sensitizer–acceptor (Cz–Py–MV2+) in space, although any of the photo- and redox-active components are not covalently bonded. Photoirradiation of the pyrenyl group resulted in the charge-separated pair Cz+–Py–MV+ which survived up to hours in a well anaerobic atmosphere. An electrode was fabricated by transferring the L–B film on an ITO glass. The photoinduced voltage of this electrode was measured with a saturated calomel reference electrode in hydroquinone (H2Q) solution to be ca. 168mV when the light intensity was 218mW/cm2. This electrode was also used as the light electrode to construct a photogalvanic cell with a platinum electrode as the dark electrode. Irradiation of the light electrode with visible light results in anodic photocurrent, and there is no net chemical change associated with the function of the cell which converts light to electricity.

Electrocatalytic Reduction ofS-Nitrosoglutathione at Electrodes Modified with an Electropolymerized Film of a Pyrrole-Derived Viologen System and Their Application to CellularS-Nitrosoglutathione Determinations

The preparation, electrochemical characterization, and analytical applications of glassy carbon (GC) electrodes modified with electropolymerized films of the cationN,N′-di(3-pyrrol-1-yl-propyl)-4,4′-bipyridine (DPPB) are described. Electropolymerized films of DPPB on GC electrodes exhibit two one-electron redox processes centered at −0.45 and −0.85 V, respectively.S-Nitrosoglutathione (GSNO) can be electrocatalytically reduced at electrodes modified with electropolymerized films of DPPB at approximately −0.4 V vs sodium-saturated calomel electrode, which represents a dramatic diminution of about 600 mV in the overpotential in comparison with the reaction carried out at a bare GC electrode. The kinetics of the catalytic reaction have been characterized using cyclic voltammetry and rotated disk electrode techniques from which a value of (1.3 ± 0.2) × 103M−1s−1was obtained. Using electrodes modified with an electropolymerized film of DPPB we have carried out preliminary studies of the determination of intracellular GSNO concentrations in two strains of the bacteriumRhodobacter sphaeroides.

Flow-force relationships in lettuce thylakoids. 2. Effect of the uncoupler FCCP on local proton resistances at the ATPase level.

The relationship between the steady-state proton gradient (delta pH) and the rate of phosphorylation was investigated in thylakoids under various conditions. Under partial uncoupling by carbonyl cyanide p-(trifluoromethoxy)phenylhydrazone (FCCP), the rate of ATP synthesis was reduced by less than expected from the decrease of delta pH. This was observed in the case of the pyocyanine-mediated cyclic electron flow around photosystem 1, but not with the H2O-->photosystem 2-->cytochrome b6f-->photosystem 1-->methyl viologen system. In state 4, a unique relation was found between delta pH and the "phosphate potential", delta Gp, regardless of whether the energy level was controlled by light input or FCCP. The anomalous effect of FCCP on the rate of ATP synthesis disappeared when the ATPase was partially blocked by the reversible inhibitor venturicidin, but not in the presence of tentoxin, an irreversible inhibitor. These results are consistent with the existence of a small kinetic barrier for protons, limiting their access to the ATPase. This resistance would be collapsed by FCCP.

In situ FTIR studies on the structural mechanism of zwitter - viologen system during electrochemical charge - transfer reactions

In situ Fourier transform infrared (FTIR) spectroscopic studies during electrochemical charge transfer reactions of zwitter - viologen system are reported. In formamide electrolyte two reversible redox couples in the cyclovoltammogram are observed and could be correlated to changes in the molecular structure by studying the differential FTIR spectra. In acetonitrile electrolyte besides the assymetric behaviour in the cyclovoltammogram [1] some changes in the FTIR spectra are observed in a potential regime where no electrochemical reaction occurs. This phenomenon is related to different solubility of the molecule in these two electrolytes. Furthermore different vibrational changes of SO 3 − subunits are observed in the two electrolyte systems.

Sandwich colloids of zinc oxide and zinc sulfide in aqueous solutions

Colloid particles, composed of ZnO and ZnS particles in contact or ZnO colloid particles coated with ZnS, have been investigated. The effect of various amounts of ZnS on the ZnO visible emission was studied. The results show that ZnS quenches the ZnO visible emission by hole transfer; however, fast recombination between the electron and hole takes place at the ZnO-ZnS interface. This is reflected by a decrease in the yield of reduction of methyl viologen upon addition of ZnS to the ZnO-methyl viologen system.

Electrochemical behaviour of viologen-cyclodextrin inclusion complexes. The case of non-alkyl group substituted viologen

The electrochemical behavior of non-alkyl substituted viologen, 4,4′-dibenzyl bipyridinium (BzV), 4,4′-dicyanophenyl bipyridinium (CyV) and α-,β-,γ-cyclodextrin (α, β, γ-CD) was studied using cyclic voltammetry and a spectroelectrochemical method. It was found that BzV and Fe(CN) 6 4− formed a charge-transfer (CT) complex with a ratio of 2∶1 and the colour of the solution faded with the addition of an electrolyte. This behaviour is the same as in then-heptyl viologen and ferrocyanide system [1]. BzV, α-CD and γ-CD formed an inclusion complex only in the reduced state, whilst BzV and β-CD formed an inclusion complex in both the oxidized and the reduced state. An EC scheme in which a chemical reaction follows an electrochemical reaction was considered to predominate in the BzV and α-, γ-CD systems, while a CE scheme in which a chemical reaction preceded an electrochemical reaction predominated in the BzV and β-CD system. On the other hand, CyV was found to form an inclusion complex with α-, β-, γ-CD in both the oxidized and the reduced states. therefore a CE scheme was considered to predominate in the CyV-α-, β-, γ-CD systems.

Crystalline heterocyclic radical cation salts as stable intermediates of new redox-mediating systems

N,N′-Dialkyl-pyrazinium and -quinoxalinium radical cations have been isolated as stable iodide or tetraphenylborate salts. E.s.r. spectroscopy of these materials reveals delocalization of seven π electrons in the 1,4-diazine ring, but no intermolecular spin-pairing in the solid state, results which are supported by structural data for the pyrazinium radical cation salt. The thermodynamic stability and chemical persistence of these intermediates suggest their use as electron-transfer mediators by analogy with the methyl viologen system.

An attempt at the preparative alkyl dibromide reduction using electrodes coated by a polypyrrole film containing the viologen (4,4′-bipyridinium) system

An attempt at the preparative alkyl dibromide reduction using electrodes coated by a polypyrrole film containing the viologen (4,4′-bipyridinium) system

Association phenomena and enhancement of light-induced charge separation in the Ru[(C 13H 27) 2bpy] 2+3-methyl viologen system (bpy ≡ 2,2′-bipyridyl) in large sodium dodecylsulphate micelles

The highly hydrophobic complex tris-(4,4′-di-tridecyl-2,2′-bipyridyl)—Ru II associates in rod-like sodium dodecylsulphate (SDS) micelles formed at high ionic strength. The associate emits at 670 nm with τ = 300 ns and a quantum yield of about 1% (the monomer emits at 608 nm with τ ≈ 1150 ns and a quantum yield of 5% – 10%). Association is not observed in concentrated homogeneous solutions in organic solvents. Concomitant with the association of the complex an enhancement of light-induced charge separation in the Ru[(C 13H 27) 2bpy] 3 2+—MV 2+—edta—SDS system (bpy ≡ 2,2′-bipyridyl; MV 2+ ≡ methyl viologen; edta ≡ ethylenediaminetetraacetic acid) is observed in rod-like micelles.

Purification and properties of sulfite reductase from Desulfovibrio vulgaris, Miyazaki F.

The sulfite reductase of Desulfovibrio vulgaris, strain Miyazaki F (MF), was purified by ammonium sulfate precipitation and chromatography on DEAE-cellulose, Ultrogel AcA34, and hydroxylapatite. The molecular weight was estimated to be 180,000 by gel filtration. It had a subunit structure of α2β2; the molecular weight of the α subunit was 50,000 and that of β, 39,000. The absorption spectrum with characteristic peaks at 629 and 409 nm and the amino acid composition resembled those of the sulfite reductase from D. vulgaris, Miyazaki K. The MF enzyme reduced sulfite to trithionate, thiosulfate, and sulfide by hydrogen when coupled with a hydrogenase-methyl viologen system, like other sulfite reductases from Desulfovibrio.

Electrochemical coating of an electrode by a poly(pyrrole) film containing the viologen (4,4′-bipyridinium) system

The electro-oxidation of a pyrrole N-substituted by an aliphatic chain bearing the viologen (4,4′-bipyridinium) group (V2+) produces a film of electroactive poly(pyrrole) containing the V2+ redox center on the electrode surface.

Water photolysis by a photoelectrochemical cell using an immobilized chloroplasts-methyl viologen system.

An immobilized chloroplast film, prepared by immobilizing spinach chloroplasts in 2 wt% agar gel, was attached to a SnO2 optically transparent electrode to obtain the immobilized chloroplast film electrode. The immobilized chloroplast film electrode worked as a photoanode under illumination in the presence of methyl viologen, which was an electron carrier from chloroplasts to the SnO2 optically transparent electrode. Water photolysis for producing hydrogen by a photoelectrochemical cell using the immobilized chloroplasts film electrode was successfully achieved. A smooth platinum electrode was used as a cathode to produce hydrogen. The pH and temperature of the anolyte were kept at 7.8 and 25°C. Optimizations of the concentrations of methyl viologen and chlorophyll in the immobilized chloroplast film were studied. The optimumthickness for the immobilized chloroplast film was about 0.8 mm.The immobilized chloroplasts had higher storage stability than that of isolated chloroplasts and they retained more than 50% of the initial activities of photosystem I and photosystem II after 10 days when they were stored at 4°C in the dark. It was conceived from the relationship between the photocurrent and the photosystem I and II activities that the main cause for the decrease in the photocurrent was the photochemical inactivation of photosystem II.

Influence of electrostatic forces upon the efficiency of charge separation for the zinc porphyrin/methyl viologen system

The bimolecular rate constant for quenching the triplet excited state of a metalloporphyrin by methyl viologen depends upon the electronic charge on the porphyrin ring. The rate constant increases with increased electrostatic attraction between the reactants and this is due to efficient formation of an encounter complex and a high thermodynamic driving force for formation of an ion-pair. However, dissociation of the ion-pair to separate ion products is favoured by strong electrostatic repulsion between the products due to electrostatic factors. In addition the yield of redox products depends upon the amount of electronic energy that must be dissipated in the form of heat, and the rate of internal conversion of the ion-pair decreases significantly with increased energy.

Siroheme as an Active Catalyst in Sulfite Reduction

Siroheme extracted by acetone/HCl treatment of sulfite reductase from yeast and purified by column chromatography catalyzed the reduction of sulfite to thiosulfate and sulfide when coupled with a hydrogen-hydrogenase-methyl viologen system. The activity increased with decrease in pH from 7 to 4, and an apparent Km value of 50 mM for sulfite was obtained. In contrast to sirohydrochlorin plus Fe2+, addition of inorganic iron or 2,2'-bipyridine prior to the reduction reaction had scarcely any effect on the sulfite-reducing activity of siroheme. Hydroxylamine was reduced by siroheme at a much faster rate than sulfite, and the rate increased with increase in pH from 6 to 9. Siroheme extracted from Chromatium vinosum strain D sulfite reductase also reduced sulfite to thiosulfate and sulfide.