Uncatalyzed and ruthenium(III)‐catalyzed reaction of acidic chlorite with methylene violet

The kinetics and mechanism of the uncatalyzed and Ru(III)‐catalyzed oxidation of methylene violet (3‐amino‐7‐diethylamino‐5‐phenyl phenazinium chloride) (MV⁺) by acidic chlorite is reported. With excess concentrations of other reactants, both uncatalyzed and catalyzed reactions had pseudo‐first‐order kinetics with respect to MV⁺. The uncatalyzed reaction had first‐order dependence on chlorite and H⁺ concentrations, but the catalyzed reaction had first‐order dependence on both chlorite and catalyst, and a fractional order with respect to [H⁺]. The rate coefficient of the uncatalyzed reaction is (5.72 ± 0.19) M^?2 s^?1, while the catalytic constant for the catalyzed reaction is (22.4 ± 0.3) × 10³ M^?1 s^?1. The basic stoichiometric equation is as follows: 2MV⁺ + 7ClO₂^? + 2H⁺ = 2P + CH₃COOH + 4ClO₂ + 3Cl^?, where P⁺ = 3‐amino‐7‐ethylamino‐5‐phenyl phenazinium‐10‐N‐oxide. Stoichiometry is dependent on the initial concentration of chlorite present. Consistent with the experimental results, pertinent mechanisms are proposed. The proposed 15‐step mechanism is simulated using literature; experimental and estimated rate coefficients and the simulated plots agreed well with the experimental curves. © 2003 Wiley Periodicals, Inc. Int J Chem Kinet 35: 294–303, 2003     

Photoinduced reaction on quantized GaAs nanocrystals prepared by wet process

Gallium arsenide nanocrystals of 1.5 to 9.0 nm were prepared in triethylene glycol dimethylether (triglyme), and photoinduced reduction of methylviologen (MV²⁺) on the nanocrystals was investigated. The rate of MV+ production determined for an initial stage of photoinduced reduction of MV²⁺ was found to be low compared to that determined for bulk GaAs particles of 0.4 mm, if the rate was evaluated for unit surface area of the semiconductor particles. To account for this finding, the apparent association constant of MV²⁺ to GaAs was determined, which suggested that molecular species which worked as stabilizing agents for the GaAs nanocrystals retarded the adsorption of MV²⁺ onto the particle surfaces.     

Transient Raman detection of excited state electron,transfer to methyl viologen from photoexcited molecular reagents in fluid solution and anchored to SiO2

The lowest electronic excited state of the complexes [Ru(2,2′-bipyridine)₃]²⁺, fac-[ClRe (CO)₃(2,2′-bipyridine)], and fac-[(pyridine) Re (CO)₃(2,2′-bipyridine)]⁺ can be quenched by methyl viologen, MV²⁺, N,N′-dimethyl-4,4′-bipyridinium, in fluid solutions. The quenching obeys Stern—Volmer kinetics as deduced from plots of relative luminescence quantum yield vs [MV²⁺], and the data are consistent with a quenching process that is essentially diffusion controlled. Pulsed laser excitation (18 ns, 354.7 nm frequency tripled Nd: YAG) of the metal complexes in the presence of MV²⁺ shows that a detectable fraction of the quenching results in net electron transfer to form MV⁺. The MV⁺ is detectable by resonance Raman scattering from the trailing portion of the excitation pulse. Excited state electron transfer to MV²⁺ from a photo-excited complex anchored to SiO₂ has also been detected by transient Raman spectroscopy. High surface area SiO₂ was functionalized by reaction with 4-[2-(trimethoxysilyl)ethyl]pyridine to give [SiO₂]-SiEtpyr. Reaction of [SiO₂]-SiEtpyr with [(CH₃CN)Re(CO)₃(2,2′-bipyridine)]⁺ then yields [SiO₂]-[(SiEtpyr) Re (CO)₃ (2,2′-bipyridine)]⁺. Electron transfer quenching of the photo-excited immobilized Re complex occurs when suspended in CH₃CN solutions of MV²⁺ to yield MV⁺ as detected by resonance Raman scattering and by lifetime attenuation in the presence of MV²⁺.     

CHLOROPHYLL PHOTOSENSITIZED ELECTRON TRANSFER IN PHOSPHOLIPID VESICLE BILAYERS: INSIDE VS OUTSIDE ASYMMETRY*

Abstract— We have determined triplet quenching efficiencies. radical yields and radical recombination kinetics in mixed chlorophyll (Chl)-egg phosphatidylcholine vesicle suspensions in the presence of electrically-charged electron acceptors located either in the external. continuous aqueous phase or within the internal aqueous volume of the vesicles. There was a marked asymmetry between these processes as to whether they occurred at the outer or inner bilayer-water interfaces. With methyl viologen (MV²⁺) as acceptor, 52 ± 4% of the total Chl triplet could be quenched from the inside. whereas only 16 ± 2% was quenchable from the outside. Approximately 35% of the triplet population was inacccssible to quenching by MV²⁺ from either inside or outside. Ouenching rate constants were higher from the outside than from the inside (2 × 10⁶M^?lS-^Ivs 1 × 10⁶M^?Is^?1). A similar pattern was obtained when anthraquinone disulfonate or ferricyanide were used as acceptors. Radical yields and recombination kinetics also displayed asymmetric behaviour. From the inside. only 4 ± 2% of the quenched triplets gave rise to separated radicals using MV²⁺ as acceptor, whereas from the outside the conversion yield was 32 ± 2%. The halftime for the Chl⁺ MV⁺ reaction was approximately 100 times longer at the outer surface than at the inner surface. We conclude the following: (a) Chl is distributed asymmetrically within the bilayer such that more triplet Chl is located within quenching distance of the interface at the inner surface than at the outer surface. Furthermore, an appreciable fraction of the triplet Chl is located sufficiently far from either interface so that quenching is not possible. (b) The mobility of Chl and quencher molecules is greater at the outer surface of the vesicles than at the inner surface.     

Determination of paraquat in biological and environmental samples by a photokinetic method

A fast and sensitive photokinetic method for the determination of paraquat (MV²⁺) (1?27×10^?5M) is described, based on the rate of photoreduction of MV²⁺ by EDTA, sensitized by acridine yellow in the absence of oxygen. The rate of photoreduction, which is a linear function of the concentration of MV²⁺ is monitored polarographically by recording the limiting current of p- benzoquinone, which is reduced by the radical monocation MV^+· generated in the photochemical reaction. The results obtained by the application of the fixed-time, fixed-concentration change and initial-rate kinetic methods are evaluated. An alternative method for monitoring the rate of the process is by measuring the time necessary for the total reduction of p-benzoquinone. The end-point is detected with two platinum electrodes at an applied voltage of 100 mV. The procedure has been successfully applied to the determination of paraquat in commercial herbicides, waters, and flowers and in spiked soils and blood sera.     

MECHANISM OF THE PHOTOSENSITIZED REDOX REACTIONS OF ACRIDINE ORANGE IN AQUEOUS SOLUTIONS-A SYSTEM OF INTEREST IN THE PHOTOCHEMICAL STORAGE OF SOLAR ENERGY†

-We have carried out a very detailed study, using fluorescence and optical flash photolysis techniques, of the photoreduction of methyl viologen (MV²⁺) by the electron donor ethylene diamine tetraacetic acid (EDTA) in aqueous solution sensitized by the dye acridine orange (AOH⁺). A complete mechanism has been proposed which accounts for virtually all of the known observations on this reaction. This reaction is novel in that both the triplet and the singlet state of AOH⁺ appear to be active photochemically. We have shown that mechanisms previously proposed for this reaction are probably incorrect due to an artifact. At pH 7 the fluorescence quantum yield φ_s of AOH⁺ is 0.26 ± 0.02 and the fluorescence lifetime is 1.8 ± 0.2 ns. φ_s is pH dependent and reaches a maximum of 0.56 at pH 4. The fluorescence of AOH⁺ is quenched by MV²⁺ at concentrations above 1 mM and the quenching obeys Stern-Volmer kinetics with a quenching rate constant of (1.0 ± 0.1) × 10¹⁰M^?1 s^?1. The quenching of the AOH⁺ excited singlet state by MV²⁺ almost certainly returns the AOH⁺ to its ground state with no photochemistry occurring. EDTA also quenches the fluorescence of AOH· with Stern-Volmer kinetics but with a smaller rate constant (6.4 ± 0.5) × 10⁸M^?1s^?1 at pH 7. In this case the quenching is reactive resulting in the formation of semireduced AOH. In the presence of MV²⁺, flash irradiation of AOH⁺ does result in the reversible formation of the semireduced MV? which absorbs at 603 nm. We attribute this to a photochemical reaction of the triplet state of AOH⁺ with MV²⁺. The initial quantum yield for formation of MV? (φ_MV:)₀ was found to be constant at 0.10 ± 0.05 for [MV²⁺] from 5 × 10^?5 to 1.0 × 10^?3 with [AOH⁺] = 8 × 10^?6M. Previous workers had found that (φ_MV:)₀ appears to decrease with decreasing [AOH⁺]; however, on careful investigation, we found this was most probably due to quenching of the triplet state of AOH⁺ by trace amounts of oxygen. When EDTA is added to a mixture of AOH ⁺ and MV²⁺ at pH 7, the photochemical formation of MV? becomes irreversible as the [EDTA] is increased. The quantum yield for the irreversible formation of MV? exceeds 0.10 becoming as large as 0.16 for [EDTA] = 0.014M. This fact requires that an alternative photochemical process must be operative and we present evidence that this is a reaction of EDTA with the excited singlet state of AOH⁺ to produce the semi-reduced AOH- which then reacts with MV²⁺ to produce MV?. The full kinetic scheme was tested by computer simulation and found to be totally consistent. This also enabled the processing of a full set of rate constants. When colloidal PtO₂ was added to the optimal mixture [EDTA] = 3.4 × 10^?2M; [MV²⁺] = 5 × 10^?4M; [AOH⁺] = 4 × 10^?5M; pH6 H₂ gas was produced at a rate of 0.2μmol H₂h^?1. Thus, acridine orange should serve as an effective sensitizer in reactions designed to use solar energy to photolyze water.     

Mediated electrochemical synthesis of copper nanoparticles in solution

Environmentally friendly mediated electrochemical synthesis of copper nanoparticles in the solution using a copper anode as a source of copper ions has been realized for the first time. It is shown that at the potential of the redox pair MV²⁺/MV?⁺ methylviologen MV²⁺ is able to mediate a reduction of Cu²⁺ ions in 60% aqueous DMF/0.1 M Bu₄NBF₄. Copper nanoparticles build large aggregates (200—250 nm) in the absence of a stabilizer. The use of polyvinylpyrrolidone as a stabilizer makes it possible to obtain smaller copper nanoparticles (20—50 nm) of spherical and oval shape and to characterize them by physicochemical methods.     

Use of ring-disc electrodes and viologens for the titration of cytochrome C and oxygen and for the study of their reduction kinetics

The use of ring-disc electrodes enable the electrogeneration of viologen cation radicals V⁺ when the disc potential is controlled in the range ?0.3 to ?0.65 and ?0.2 to ?0.45 V/NHE respectively in MV²⁺ and BV²⁺ solutions. We confirmed the occurrence of very fast reactions between V^.+ and horse-heart cytochrome c or oxygen and of slow or no reaction between V^.+ and β-nicotinamide adenine dinucleotide with xanthine oxidase, riboflavine and ferricyanide. Ring current versus dise current curves enable the titration of cytochrome c and oxygen and the estimation of the reaction rate between cytochrome c and MV^.+ [(6.2±2)10⁵M^?1 s^?1].     

Catalysis of the Sodium Sulfide Reduction of Methylviologene by CuS Nanoparticles

Feasibility was demonstrated for the catalysis of the sodium sulfide reduction of methylviologene (MV²⁺) in aqueous solution using cupric sulfide nanoparticles. The catalytic activity of the nanoparticles depends on their size. The basic features were found for the formation of the MV^+· radical–cation in the reduction of MV²⁺ by HS^– anions in the presence of CuS nanoparticles as the catalyst. This is an equilibrium reaction.     

Formation of silver nanoparticles in formamide:water mixtures: a radiolytic study

The pulse radiolysis of FA and FA:water solutions was studied in the absence and presence of redox indicator 1,1′-dimethyl-4,4′-bipyridinium dichloride (methyl viologen, MV²⁺). The experiments performed in the presence of MV²⁺ have provided strong support to the idea that the first species obtained from the reaction of e_sol⁻ and ^•OH with FA produces radicals that show reactivity towards the MV²⁺. Both the radicals on reaction with MV²⁺ results in the appearance of the well-known intense blue MV^•+ radical absorption signal (λ_max = 395 nm, λ_max = 605 nm). The intermediate radicals formed during radiolysis were used to generate silver nanoparticles.     

DIRECT OBSERVATION OF ELECTRON TRANSFER ACROSS A LIPID BILAYER: PULSED LASER PHOTOLYSIS OF AN ASYMMETRIC VESICLE SYSTEM CONTAINING CHLOROPHYLL,METHYL VIOLOGEN AND EDTA*

Abstract— Suspensions of vesicles composed of chlorophyll a (Chi) and phospholipid that were asymmetric with respect to aqueous solutions of methyl viologen (MV₂₊), an electron acceptor, and EDTA, an electron donor, were investigated using both flash and steady-state photolysis techniques. It was shown that Chl-photosensitized electron transfer occurred across the walls of the vesicles from EDTA to MV₂₊. Flash photolysis indicated that MV₂₊ dissolved in the interior aqueous compartments of the vesicles oxidized only those triplet excited state Chi molecules that were dissolved in the inner monolayers of the vesicle walls. The resultant radical products, Chi₊ and MV₊, recombined with a halftime of the order of 10_-4s. EDTA, added externally to the vesicles, competed effectively with MV₊ as a reducing agent for Chl₊. This places a lower limit of 10₄ s_-1 on the rate constant for transmembrane electron transfer. Compartmentalization by the vesicle wall of the competing pathways for the reduction of Chi₊ resulted in a nonlinear dependence of the rate constant of Chl₊ decay on EDTA concentration. The magnitude of the rate constant of electron transfer through the membrane and the way that the kinetics of Chl₊ decay depended on the concentration of Chi in the membrane strongly suggest that the electron transfer occurred by electron exchange between Chi and Chl₊.     

Counterion Binding to Ionic Micelles: Effects of Counterion Specificity

The effects of counterion specificity on the properties of sodium dodecyl sulfate micelles containing photochemically reactive solubilizates were studied by laser flash photolysis and light scattering, potentiometric, spectroscopic and microelectrophoretic measurements. The counterions investigated were an amphiphilic ion (cetyltrimethylammonium, CTA⁺) and two divalent ions (cupric, Cu²⁺, and methylviologen, MV²⁺). Cu²⁺ and MV²⁺ showed lower effect than CTA⁺ in promoting changes of micelle size and electrostatic potential at the micelle/solution interface. This can be attributed to the complex interplay between electrostatic and hydrophobic interactions, which determine the average location of counterions, i.e., prevailing interfacial or intercalated binding to the micelle. Laser flash photolysis showed that Cu²⁺ enhanced the rate of decay of biphenyl triplet, while MV²⁺ did not show any effect. The differences between the quenching cations were attributed to the average location of MV²⁺ ions in the Stern layer further away from the micelle core than Cu²⁺.     

Pulse radiolysis investigations of one-electron reduction of terephthalonitrile in aqueous solutions

Pulse radiolysis of aqueous solutions of terephthalonitrile (TPN) was carried out to study the redox properties of anions of TPN, by determining the rate constants for the reaction of TPN with specific one-electron reducing species formed on reaction of OH radicals with methyl,ethyl,isopropyl alcohols,tetrahydrofuran and cytosine. Formation of anions of TPN, which absorb at wavelength 340 nm, was followed. From the graph of rate constants and maximum absorbance vs reduction potential of reducing species, reduction potential of TPN was found out to be —0.85 V vs NHE. Reduction of thionine (TH⁺) and methyl viologen (MV²⁺) with the help of radical anions of TPN was carried out to establish its high reduction potential.     

Inelastic electron scattering in the adsorbed system

The study revealed additional channels of inelastic electron scattering, which accompany the threshold excitation of the substrate Pt4d level — ionization of the valent states of adsorbed particles chemically bonded to the excited atom, and excitation of the surface plasmon vibrations. The conjugate excitation of this type shows up as a series of typical satellites in the spectra of disappearance potentials, which reflects the structure of valent states of adsorbed particles. Analysis of the satellite structure revealed the intermediate formation of NH _x,ads particles in the reaction NO_gas + H_ads on the surface of Pt(100) single crystal and, taking into account the earlier data, made it possible to formulate a general mechanism of selfoscillations in the NO + H₂ reaction on platinum metals. Mathematical modeling of reaction kinetics on the Pt(100) surface within the suggested mechanism demonstrated the presence of regular self-oscillations of the reaction rate at invariable values of the step constants.     

The dependence of the kinetics of some simple outer-sphere electrode reactions on the nature of the electrode material

The kinetics of a number of simple inorganic electrode reactions that are known or expected to follow outer-sphere pathways have been examined at mercury, silver, platinum, and gold-aqueous interfaces in order to explore the effects of varying the electrode material on outer-sphere reactivity. The electroreduction kinetics of Co(III) ammine complexes exhibited only mild dependences on the nature of the electrode material which were compatible with the expected variations in double-layer effects. However, the electrooxidation of Cr²⁺ proceeded at strikingly higher overpotentials on the solid surfaces compared with mercury electrodes. Similar effects were also seen for the electrooxidation of V²⁺, Eu²⁺ and Ru²⁺ in the presence of Cr²⁺. Much larger rate constants were observed for these aquo reactions at solid surfaces in the absence of Cr²⁺, although Cr²⁺ had no influence on Co(NH₃)₆³⁺ electroreduction, or any reaction at mercury electrodes. It is speculated that the very large substrate effects upon the electrode kinetics of aquo couples arise from the influence of the inner-layer water structure on the reactant-solvent interactions experienced by these “structure-making” reactants at their plane of closest approach. The inhibiting influence of Cr²⁺ may be due to its ability to efficiently remove adsorbed catalytic contaminants by incorporation into a substitutionally inert Cr(III) electrooxidation product by means of a ligand-bridge mechanism.     

Efficiency of polymer sensitizer in photosensitized reaction of tris(bipyridine)ruthenium(II) complex-containing polymer

Photochemical properties of Ru(bpy)₂(poly-4-methyl-4′-vinyl-2,2′-bipyridine)Cl₂ ( 2 ) were studied and compared with that of Ru(bpy)₃Cl₂. Continuous irradiation of a solution, which contains polymer 2 as a photosensitizer, methylviologen (MV²⁺) or 4,4′-bipyridinium-1,1′-bis(trimethylenesulfonate) (SPV) as an electron acceptor and triethanolamine (TEOA) as a sacrificial donor, resulted in the formation of viologen radical ion (MV⁺ or SPV^?). The rate of formation of MV⁺ or SPV^? for the polymer 2 system was smaller than that for the Ru(bpy)₃ Cl₂ systems. The reason for this fact was kinetically analyzed by quenching experiments of excited Ru(II) complexes by MV²⁺ or SPV, the photosensitized reactions of the TEOA–Ru(II) complex–MV²⁺ or -SPV systems, and the dye laser photolysis of the Ru(II) complex–MV²⁺ or -SPV systems.     

Dependence of the adsorption and catalytic properties of a copper-platinum catalyst on the structure of metal particles and the composition of the catalyst surface

The kinetics of H₂ desorption from the surface of a copper-platinum catalyst deposited on silica gel ([1 wt % Pt + 0.15 wt % Cu]/SiO₂) and the kinetics of C₆H₁₂ dehydrogenation were studied. The effects of copper introduction in a platinum catalyst on the structural characteristics of platinum particles, the composition of their surface, and the effects of plasmochemical treatments on these parameters were studied by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The metal-H atom bond energies (E _Pt-H) and the catalytic activity were found to increase in the presence of Cu. This was explained by the formation of new hydrogen adsorption centers (due to the Cu^+δ adatoms) and catalytic centers composed of Cu^+δ adatoms and carbon atoms. The mean diameter of Pt particles (D) increased twofold. The microstresses (ɛ) in the particles increased after the catalyst was treated with glow discharge plasma in Ar and O₂ and with high-frequency plasma in H₂ (HF-H₂). The observed changes in the bond energy E _Pt-H and kinetic parameters were explained by the increase in microstresses in Pt particles.     

Ultrafast Electron Transfer from Photoexcited CdSe Quantum Dots to Methylviologen

The ultrafast charge separation at the quantum dot (QD)/molecular acceptor interface was investigated in terms of acceptor concentration and the size of the QD. Time‐resolved experiments revealed that the electron transfer (ET) from the photoexcited QD to the molecular acceptor methylviologen (MV²⁺) occurs on the fs time scale for large acceptor concentrations and that the ET rate is strongly reduced for low concentrations. The increase in the acceptor concentration is accompanied with a growth in the overlap of donor and acceptor wavefunctions, resulting in a faster reaction until the MV²⁺ concentration reaches a saturation limit of 0.3–0.4 MV²⁺ nm^?2. Moreover, we found significant QD size dependence of the ET reaction, which is explained by a change of the free energy (ΔG).     

Quasi-solid medium for photoinduced charge separation

New quasi-solid was proposed as a medium for photoinduced charge separation. Photoinduced charge separation and hydrogen production with the new quasi-solid medium using polysaccharide has been investigated. The new medium is a quasi-solid polysaccharide containing a large quantity of water. This medium is tight and elastic solid, obtained by cooling an aqueous solution of a polysaccharide (agarose or κ-carrageenan in this paper) to room temperature after it was dissolved in water by heating. When the quasi-solid (agarose or κ-carrageenan) involving sacrificial electron donor ethylenediaminetetraacetate (EDTA), tris(2,2′-bipyridine)ruthenium complex ([Ru(bpy)₃]²⁺) and methylviologen (MV²⁺) was irradiated with visible light, methylviologen cation radical (MV⁺) was formed. The formation rate of methylviologen cation radical in an agarose solid was faster than that in a κ-carrageenan one. Photoinduced hydrogen production was achieved with this system containing additionally proton reduction catalyst such as platinum black (Pt black), platinum oxide (PtO₂), and ruthenium oxide (RuO₂). The characteristics of this solid medium for photochemical reactions were discussed.