Methyl Viologen Radical Research Articles

Effects ofL-tyrosine esters on quenching and photosensitized charge separation using polymers containing ruthenium(II) complex

A series of ruthenium(II) complex-containing partially quaternized poly(1-vinylimidazole)s with various alkyl side chains such as hexyl (C6RuQPIm), dodecyl (C12RuQPIm), and hexadecyl (C16RuQPIm) were synthesized. The effects of L-tyrosine esters with hexyl (C6Tyr), octyl (C8Tyr), and dodecyl (C12Tyr) on the quenching with methylviologen and photosensitized charge separation reactions were investigated using these metallopolymers as polymer photosensitizers. The quenching reaction took place through both a dynamic quenching process and a static quenching one mediated by the L-tyrosine esters. The kinetic parameters for these processes were obtained from a computed curve fitting using a Stern–Volmer equation derived from a combination of dynamic quenching and static quenching. The parameters had a significant dependence on the lengths of the alkyl groups in the L-tyrosine esters and the alkyl side chains on these metallopolymers. During photosensitized charge separation, the reaction proceeded through these quenching processes, referred to as direct and mediated processes. The initial rates of methylviologen radical formation also depended on these lengths; they increased when the lengths of the alkyl side chain on these metallopolymers and alkyl groups in the L-tyrosine esters were long. Such a dependence was caused by a change in the balance of some effects such as the electrostatic and steric effects. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 4360–4367, 1999

Effects of L‐tyrosine esters on quenching and photosensitized charge separation using polymers containing ruthenium(II) complex

A series of ruthenium(II) complex-containing partially quaternized poly(1-vinylimidazole)s with various alkyl side chains such as hexyl (C6RuQPIm), dodecyl (C12RuQPIm), and hexadecyl (C16RuQPIm) were synthesized. The effects of L-tyrosine esters with hexyl (C6Tyr), octyl (C8Tyr), and dodecyl (C12Tyr) on the quenching with methylviologen and photosensitized charge separation reactions were investigated using these metallopolymers as polymer photosensitizers. The quenching reaction took place through both a dynamic quenching process and a static quenching one mediated by the L-tyrosine esters. The kinetic parameters for these processes were obtained from a computed curve fitting using a Stern–Volmer equation derived from a combination of dynamic quenching and static quenching. The parameters had a significant dependence on the lengths of the alkyl groups in the L-tyrosine esters and the alkyl side chains on these metallopolymers. During photosensitized charge separation, the reaction proceeded through these quenching processes, referred to as direct and mediated processes. The initial rates of methylviologen radical formation also depended on these lengths; they increased when the lengths of the alkyl side chain on these metallopolymers and alkyl groups in the L-tyrosine esters were long. Such a dependence was caused by a change in the balance of some effects such as the electrostatic and steric effects. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 4360–4367, 1999

Hydrogen-Bond Promoted Intramolecular Electron Transfer to Photogenerated Ru(III): A Functional Mimic of TyrosineZ and Histidine 190 in Photosystem II

As a model for redox components on the donor side of photosystem II (PS II) in green plants, a supramolecular complex 4 has been prepared. In this, a ruthenium(II) tris-bipyridyl complex which mimics the function of P680 in PS II, has been covalently linked to a tyrosine unit which bears two hydrogen-bonding substituents, dipicolylamine (dpa) ligands. Our aim is to mimic the interaction between tyrosineZ and a basic histidine residue, namely His190 in PSII, and also to use the dpa ligands for coordination of manganese. Two different routes for the synthesis of the compound 4 are presented. Its structure was fully characterized by 1H NMR, COSY, NOESY, 13C NMR, IR, and mass spectrometry. 1H NMR and NOESY gave evidence for the existence of intramolecular hydrogen bonding in 4. The interaction between the ruthenium and the substituted tyrosine unit was probed by steady-state and time-resolved emission measurements as well as by chemical oxidation. Flash photolysis and EPR measurements on 4 in the presence of an electron acceptor (methylviologen, MV2+, or cobalt pentaminechloride, Co3+) showed that an intermolecular electron transfer from the excited state of Ru(II) in 4 to the electron acceptor took place, forming Ru(III) and the methylviologen radical MV+ or Co2+. This was followed by intramolecular electron transfer from the substituted tyrosine moiety to the photogenerated Ru(III), regenerating Ru(II) and forming a tyrosyl radical. In water, the radical has a g value of 2.0044, indicative of a deprotonated tyrosyl radical. In acetonitrile, a radical with a g value of 2.0029 was formed, which can be assigned to the tyrosine radical cation. In both solvents the electron transfer is intramolecular with a rate constant kET > 1 × 107 s-1. This is 2 orders of magnitude greater than the one for a similar compound 3, in which no dpa arm is attached to the tyrosine unit. Therefore the hydrogen bonding between the substituted tyrosine and the dpa arms in 4 is proposed to be responsible for the fast electron transfer. This interaction mimics the proposed His190 and tyrosineZ interaction in the donor side of PS II. (Less)

Reactions of dimethylsulfoxide reductase from Rhodobacter capsulatus with dimethyl sulfide and with dimethyl sulfoxide: complexities revealed by conventional and stopped-flow spectrophotometry.

Improved assays for the molybdenum enzyme dimethylsulfoxide reductase (DMSOR) with dimethyl sulfoxide (DMSO) and with dimethyl sulfide (DMS) as substrates are described. Maximum activity was observed at pH 6.5 and below and at 8.3, respectively. Rapid-scan stopped-flow spectrophotometry has been used to investigate the reduction of the enzyme by DMS to a species previously characterized by its UV-visible spectrum [McAlpine, A. S., McEwan, A. G., and Bailey, S. (1998) J. Mol. Biol. 275, 613-623], and its subsequent reoxidation by DMSO. Both these two-electron reactions were faster than enzyme turnover under steady-state conditions, indicating that one-electron reactions with artificial dyes were rate-limiting. Second-order rate constants for the two-electron reduction and reoxidation reactions at pH 5.5 were (1.9 +/- 0.1) x 10(5) and (4.3 +/- 0.3) x 10(2) M-1 s-1, respectively, while at pH 8.0, the catalytic step was rate-limiting (62 s-1). Kinetically, for the two-electron reactions, the enzyme is more effective in DMS oxidation than in DMSO reduction. Reduction of DMSOR by DMS was incomplete below approximately 1 mM DMS but complete at higher concentrations, implying that the enzyme's redox potential is slightly higher than that of the DMS-DMSO couple. In contrast, reoxidation of the DMS-reduced state by DMSO was always incomplete, regardless of the DMSO concentration. Evidence for the existence of a spectroscopically indistinguishable reduced state, which could not be reoxidized by DMSO, was obtained. Brief reaction (less than approximately 15 min) of DMS with DMSOR was fully reversible on removal of the DMS. However, in the presence of excess DMS, a further slow reaction occurred aerobically, but not anaerobically, to yield a stable enzyme form having a lambdamax at 660 mn. This state (DMSORmod) retained full activity in steady-state assays with DMSO, but was inactive toward DMS. It could however be reconverted to the original resting state by reduction with methyl viologen radical and reoxidation with DMSO. We suggest that in this enzyme form two of the dithiolene ligands of the molybdenum have dissociated and formed a disulfide. The implications of this new species are discussed in relation both to conflicting published information for DMSOR from X-ray crystallography and to previous spectroscopic data for its reduced forms.

Radiolysis of Aqueous Solutions with Pulsed Helium Ion Beams. 3. Yields of OH Radicals and the Sum of eaq- and H Atom Yields Determined in Methyl Viologen Solutions Containing Formate

The production of methyl viologen radical cation (MV•+) in the radiolysis of aqueous solutions of 0.5 mM methyl viologen (MV2+) containing formate and tertiary butanol (t-BuOH) was studied using fast electron (integral yields) and 21 MeV 4He ion beam pulses (track segment yields). The formate concentration was varied in the range of 10-3−2 M. It was confirmed that MV•+ is formed within microseconds and that it is stable in the measured time regions if the solution contains neither t-BuOH nor oxygen. Tertiary butanol radical formed through the scavenging of OH radical or H atom by t-BuOH reacts with MV•+, and this rate constant was determined as (8 ± 2) × 108 M-1 s-1. The effect of formate concentration on the yield of methyl viologen radical cation was measured in deaerated or N2O-saturated solutions. The radical yields in deaerated solutions agreed well with those measured by continuous ion beam radiolysis, which confirmed that this system can be used as a chemical dosimeter for ion beam pulse radiolysis. The yields of OH radicals were derived from the yields of G(MV•+) in the presence and the absence of t-BuOH. In the radiolysis with low linear energy transfer (LET) radiations, the calculated OH radical yields agree well with the values determined by other OH radical scavengers, formate and formic acid, except at high scavenging capacity. In 4He ion beam radiolysis, the yields have a discrepancy in the measured scavenging capacity range. The sum of the contributions of eaq- and H atoms to form MV•+ was also determined. This value was lower than the yields estimated from other studies with 4He ion beams. Yields are highly dependent on the nature of the solutes and combination of solutes because of intratrack reactions of the products.

Self-Repairing Photosensitized Electron Transfer from Thiones to Methyl Viologen in Aqueous Media

Reversible formation of the persistent methyl viologen radical cation (MV•+) was observed by the steady-state photoillumination of thiones in the presence of MV2+ in deaerated aqueous solution. Formation yield and lifetime of MV•+ are favorable at a pH higher than 3.00. By laser flash photolysis, it was observed that fast electron transfer occurs from triplet states of thiones (3PT*) to MV2+, yielding the radical cation of thiones (PT•+) and MV•+. The initial rapid decay of MV•+ is attributed to back electron transfer from MV•+ to PT•+. On the other hand, the slow decay part of MV•+ can be attributed to endothermic electron transfer from MV•+ to disulfide, which is formed by the coupling of the thio radical produced after deprotonation of PT•+. The ratio of slow decay component and fast decay component of MV•+ in the short time scale is a good measure of the yields of persistent MV•+ with changing thiones and pH. A cyclic mechanism interpreting reversible electron transfer was proposed in which thiones act as self-repairing photosensitizers.

Studying Heterogeneous Catalysis by the Scanning Electrochemical Microscope (SECM): The Reduction of Protons by Methyl Viologen Catalyzed by a Platinum Surface

The feedback mode of the scanning electrochemical microscope (SECM) was used for studying a surface-catalyzed electron-transfer reaction. As a model system we examined the reduction of protons by reduced N,N‘-dimethyl-4,4‘-bipyridinum, methyl viologen radical cation, MV+•, catalyzed by a platinum surface. The experimental configuration comprises a biased amalgamated Au ultramicroelectrode (UME) that approaches an unbiased Pt UME substrate of the same radius. MV+• that is electrochemically generated at the amalgamated UME is oxidized at the Pt surface as a result of reducing protons. This novel configuration permits the simultaneous measurement of the catalytic current and the mixed potential (open-circuit potential) attained at the Pt surface. The results are interpreted by an electrochemical model which was previously suggested; however, our approach makes it possible to measure, for the first time, the current−potential relation predicted by the theory. Our results indicate that methyl viologen behaves ...

Solvent effects on redox properties of radical ions 1

The one-electron reduction potentials of the radical cations of 1,4-diazabicyclo[2.2.2]octane (DABCO) and N,N,N′,N′-tetramethylphenylene-1,4-diamine (TMPD) in propan-2-ol, ethanol, methanol, acetone, acetonitrile and dimethyl sulfoxide have been measured by cyclic voltammetry. Furthermore, the one-electron reduction potentials of 1,4-benzoquinone, 1,4-benzoquinone radical anion, methyl viologen dication and methyl viologen radical cation also have been measured in a number of solvents. The present results, together with previously published data on radical anions, have been used to evaluate solvent effects in view of the Kamlet–Taft relationship. The main factors affecting the magnitude of the solvent effects are the gas-phase redox properties of the corresponding neutral molecule (ionization potential and electron affinity) and the charge. In general, the magnitude of the solvent effects on solution redox properties of radical ions decreases with increasing redox stability of the radical ion, reflected by low ionization potential of the corresponding neutral molecule for radical cations and by high electron affinity of the corresponding molecule for radical anions.

Nitrate Biosensor Based on the Ultrathin-Film Composite Membrane Concept

This paper describes the construction and characterization of an electrochemical nitrate biosensor based on the ultrathin-film composite membrane concept. The composite membrane is prepared by electropolymerization of a thin anion-permselective coating of 1-methyl-3-(pyrrol-1-ylmethyl)pyridinium across the surface of a microporous support membrane. This film separates the analyte solution from an internal sensing solution which contains the enzyme nitrate reductase and an electrocatalyst (methyl viologen). The ultrathin anion-permselective film prevents loss of these components from the internal sensing solution, yet allows the analyte, NO3-, to enter the internal solution. A glassy carbon working electrode is immersed into the internal solution. The nitrate is reduced enzymatically, yielding the oxidized form of the enzyme, which is reduced again by electrogenerated methyl viologen radical cation. The nitrate concentration in the analyte solution is proportional to the catalytic methyl viologen reduction...

Electron transfer photosensitized by a tin lipoporphyrin in solution, micelles, and at water—organic solvent interfaces

Electron transfer photosensitized by a tin lipoporphyrin [Sn(IV) octakis((methoxycarbonyl-methyl)- meso-tetrakis-(((eicosanyloxy)carbonyl) phenyl)-porphyrin (SnLipoP)] is investigated under various solution conditions using a donor—SnLipoP—methylviologen (MV 2+) ternary system, where the donor is triethanolamine (TEA) or ethylenediaminetetraacetic acid (EDTA). The photoreaction of SnLipoP is compared with the photoreactions sensitized by common Sn porphyrins like tin protoporphyrin IX (SnPP) and octaethylporphyrin (SnOEP). A constant photoreaction rate is observed in a water/organic solvent (hexane, benzene) two-phase system in which the porphyrin (SnLipoP, SnOEP) is in the organic solvent and MV 2+ is in the aqueous phase. The rate is monitored by the change in the UV—visible absorption spectra produced by aqueous methylviologen radical MV ·+. In contrast with the two-phase system, macroscopically homogeneous solutions (aqueous SnPP and micellar solutions of SnLipoP, SnPP and SnOEP) give pseudo-logarithmic rates. These electron-transfer processes are completely consistent with reductive primary electron transfer to the tin porphyrin and optical shielding effects. Differences in the rates for SnLipoP and the other Sn porphyrins are explained by structural differences in the porphyrins. In particular, the structure of the porphyrin influences the phase in which the porphyrin resides, its location relative to interfacial regions, and the way it interacts with itself and other system components.

Effect of Formate Concentration on Radical Formation in the Radiolysis of Aqueous Methyl Viologen Solutions

The yields of methyl viologen radical cation (MV•+) in the radiolysis of nitrogen-saturated 0.5 mM methyl viologen (MV2+) solutions containing formate were measured using 60Co γ-rays, 1H ions of 2−15 MeV, and 4He ions of 5−20 MeV. The concentrations of formate were 1, 10, 100, and 1000 mM, which correspond to OH radical scavenging capacities of 3.2 × 106 to 3.2 × 109 s-1. Methyl viologen radical cation yields should be stoichiometrically equivalent to the sum of the hydrated electron, H atom, and OH radical yields. This outcome is found in γ-ray radiolysis except at very high or low formate concentrations. At all formate concentrations, the 1H and 4He ion radiolyses give yields of methyl viologen radical cations that are significantly lower than expected from the results of other aqueous solutions. Possible reasons for the differences are discussed.

Photogeneration of NADPH by oligothiophenes coupled with ferredoxin-NADP reductase

The photogeneration of nicotinamide adenine dinucleotide hydrophosphate (NADPH) and its associated reaction were studied in a novel photo-energy conversion system. The system was composed of the oligothiophene, dimethyl-4,4′-bipyridinium (MV 2+), EDTA, and combined with ferredoxin-NADP reductase (FDR, E.C.1.18.1.2). The NADPH was generated by the FDR catalysis via the reduction of MV 2+ by the photocatalysis of oligothiophene and EDTA as the sacrificial electron donor. As a result, the rate of the NADPH generation depended on the concentrations of nicotinamide adenine dinucleotide phosphate (NADP +) and MV 2+. The effect of the intension of applied light showed the same tendency. Rate values of photoreduction of MV 2+ and photogeneration of NADPH systems were investigated as a function of the concentration of MV 2+. The production rates of methyl viologen radical cation (MV ·+) were a 1000-fold more than the NADPH production rates in the oligothiophenes. These results suggest that the electron transfer between FDR and its substrates is the rate-determining step.

Polymer-protected [formula omitted] bimetallic cluster catalysts for visible-light-induced hydrogen generation from water and electron transfer dynamics

Colloidal dispersions of platinum/ruthenium bimetallic clusters, as well as those of platinum and ruthenium monometallic clusters, were prepared by alcohol reduction and applied to visible-light-induced hydrogen generation in the electron transfer system of tris(bipyridine)ruthenium(II) dichloride/methyl viologen dichloride/metal cluster/ethylenediaminetetraacetic acid disodium salt. Hydrogen generation rates are in proportion to the concentration of cluster metal at low concentration (about 10 −6 to 10 −5 mol dm −3), where the platinum/ruthenium bimetallic cluster is more active as a catalyst than platinum and ruthenium monometallic clusters. In addition, even the physical mixture of platinum and ruthenium monometallic clusters is more active than platinum and ruthenium monometallic clusters. Electron transfer rates from the methyl viologen radical cation to metal clusters were measured as well and compared with the hydrogen generation rates. Thus, both rates are in proportion to each other, suggesting that the hydrogen generation rate can be controlled by the electron transfer rate.

Continuous Photoreduction of Methyl Viologen Using Disubstituted Terthiophenes and EDTA in Aqueous Solution

AbstractThe continuous photoreduction of methyl viologen by 5,5″‐bis(aminomethyl)‐2,2′:5′,2″‐terthiophene (AT) and 2,2′:5′,2″‐terthiophene‐5,5″‐dicarboxylic acid (CT), using EDTA as a sacrificial electron donor, has been investigated in aqueous solution at various pH. Apparent rates and efficiencies of production of the methyl viologen radical cation (MV+) were found to be dependent on the pH, the concentrations of all three components and the intensity of the incident light. The highest conversion efficiency (77%) was shown by the bis(aminoniethyI (‐substituted terthiophene AT at pH 7.7. The quantum yield for the formation of MV+ under these conditions was 0.24, which is comparable with other common systems

Applications of the channel flow cell for UV‐visible spectroelectrochemical studies: The kinetics of dimerization of the methyl viologen radical cation

AbstractA channel flow cell specifically designed for ultraviolet‐visible spectroelectrochemical experiments is described. The performance of the cell was characterized using the oxidation of N,N, N′, N′‐tetramethyl‐p‐phenylenediamine in aqueous electrolytes and excellent agreement between theory and experiment was obtained. The technique was applied to study the dimerization rate of methyl viologen radical cations. The mechanistic results confirm that the electrogenerated methyl viologen radical cation dimerizes rapidly in aqueous solution and a previously unknown dimerization rate constant close to 104 M−1 s−1 is reported.

Photolytic degradation of aqueous hydrogen sulphide with simultaneous hydrogen production using visible light

Hydrogen was photochemically produced from an aqueous, catalyst-free solution containing Ru(bipy), methyl viologen, and hydrogen sulphide as electron donor. This system was most efficient at pH 5 after 4 h irridiation, but the extrapolated turnover number was higher at a somewhat higher pH because of the sensitizer bleached substantially as a function of pH, most serious at low pH. The behaviour of the methyl viologen radical is different to that in similar systems but with EDTA instead of H 2S as electron donor. In systems with EDTA, the methyl viologen radical forms a CT-complex with a proton with p K a = 4.0 whereas with hydrogen sulphide, competing reactions reduce the formation of methyl viologen radicals at pH > 4. Nevertheless, with H 2S three orders of magnitude more hydrogen was produced than with EDTA in catalyst-free solutions.

Preparation of a photochromic glass doped with methylviologen radical cation via a sol-gel process

Preparation of a photochromic glass doped with methylviologen radical cation via a sol-gel process

Kinetics and Mechanisms of Photoinduced Reduction of Methylviologen by N-Alkyltetraphenylporphyrinatozinc(II) in Methanol

Abstract N-3-Bromopropyl-5,10,15,20-tetraphenylporphyrinato(1-)chlorozinc(II), [Zn(BrPrtpp)Cl], catalyzed the formation of a methylviologen radical cation (MV+•) on steady-state irradiation of degassed methanol solutions with visible light (>420 nm) in the presence of triethanolamine (TEOA). The initial rate (Vi) of the formation of MV+• was of first order in the concentration of MV2+ and in the intensity of light and was independent of the concentration of TEOA above 0.025 M. Plots of Vi vs. the concentrations of [Zn(BrPrtpp)Cl] were curved. The fluorescence of [Zn(BrPrtpp)Cl]*, whose lifetimes are τ1s = 0.7 ns (48%) and τ2s = 2.0ns (52%) in methanol at 25 °C, was not quenched by MV2+. The excited triplet state of [Zn(BrPrtpp)Cl] (τ0t = 3.2 μs in MeOH) was efficiently quenched by MV2+ (kq = (6.0 ± 0.6) × 107 M−1 s−1 at 25 °C, 1 M = 1 mol dm−3). The thermal back electron-transfer reaction between [Zn(BrPrtpp)Cl]+• and MV+• was also observed (kb = (1.2 ± 0.2) × 1010 M−1 s−1). On the basis of the steady-state irradiation kinetics and ns-laser flash photolysis, the quantum yield of the excited triplet state of [Zn(BrPrtpp)Cl] can be evaluated to be 0.98 ± 0.02. [Zn(BrPrtpp)Cl] was found to be a more effective photocatalyst than [Zn(Prtpp)Cl] (HPrtpp = N-propyl-5,10,15,20-tetraphenylporphyrin) or [Zn(tpp)] (H2tpp = 5,10,15,20-tetraphenylporphyrin) under the present experimental conditions.

Formation of Methylviologen Radical Monopositive Cations and Ensuing Reactions with Polychloroalkanes on Silica Gel Surfaces

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTFormation of Methylviologen Radical Monopositive Cations and Ensuing Reactions with Polychloroalkanes on Silica Gel SurfacesYun Mao, Nancy E. Breen, and J. Kerry ThomasCite this: J. Phys. Chem. 1995, 99, 24, 9909–9917Publication Date (Print):June 1, 1995Publication History Published online1 May 2002Published inissue 1 June 1995https://doi.org/10.1021/j100024a038RIGHTS & PERMISSIONSArticle Views265Altmetric-Citations28LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (785 KB) Get e-Alerts

The photoreduction of methylviologen via electron transfer from aluminosilicate zeolites

The photoreduction yields of methylviologen and various alkylated homologues in zeolite-X, Y and A were determined at 77 K in the absence of any reducing counteranion. Upon irradiation at 77K, dehydrated samples turned light blue and a single line ESR spectrum was observed with a g-factor identical to that of the methylviologen radical cation. Samples that were not dehydrated produced ESR spectra about 10% as intense as those that had the water removed. The photoreduction yield was largest in zeolite Y, less in zeolite X which has more cations and hence more crowding in the cages, and very small in zeolite A which has much smaller cages. The electron donor in these systems is believed to be the anionic aluminosilicate framework of the host zeolite.