Photoelectrochemical response of methyl viologen cation radical incorporated in coated polysiloxane film

A photoelectrochemical response was found for a methyl viologen pendant polysiloxane film coated on an indium tin oxide (ITO) electrode dipped in an aqueous electrolyte solution containing O₂. Both the current direction (cathodic or anodic) and the extent of the photoresponse depended on the substitutional ratio of methyl viologen on the polymer chain. This behavior was not explained by simple photoinduced electron transfer from the excited cation radical of the methyl viologen to O₂. The mechanism is discussed for steady state O₂ and cation radical concentrations and with respect to O₂ permeability in the polysiloxane film.     

Thin-layer spectroelectrochemical kinetic study of viologen cation radicals reacting at hydrogen-evolving gold and nickel electrodes

The kinetics of viologen cation radicals reacting at hydrogen-evolving gold and nickel electrodes in pH 6–8 electrolytes have been investigated. Visible absorption spectroscopy was used to follow the course of the reaction in an optically transparent thin-layer electrochemical cell under quasi-steady-state conditions. The spectroelectrochemical data were analyzed using classical kinetics and yielded zero-order behavior with respect to the viologen cation radical. For methyl viologen cation radical at gold, a formal zero-order rate constant evaluated at zero hydrogen overpotential was found to be 1.0 × 10^?13 mol s^?1 cm^?2. At nickel the comparable rate constant was nearly two orders of magnitude larger than at gold. Increasing pH from 6 to 8 at gold electrodes shifted both the hydrogen evolution and the methyl viologen cation radical reaction 60–70 mV/pH unit in a negative direction. The diquat cation radical behaved in a similar manner. The proposed mechanism involves a fast, non-rate-limiting, chemical reaction between the viologen cation radical and adsorbed hydrogen atom(s). Results are interpreted in terms of previous proposed hydrogen evolution reaction mechanisms.     

Synthesis of Diazacrown Ethers with Chromophores and Their Photoinduced Charge‐Separation with Methyl Viologen

Two 4,13‐diaza‐18‐crown‐6 ethers with either two pyrenyl or two carbazolyl groups were synthesized. The two crown ethers can form complexes with methyl viologen in methanol solution. Photoirradiation of the complexes resulted in the electron transfer from the excited states of the chromophores to methyl viologen as demonstrated by the quenching of the chromophore fluorescence and the detection of the absorption spectrum of the generated viologen radical cation. The back electron transfer in these systems was inhibited by the electrostatic repulsion between the positively charged viologen radical cation and the generated chromophore radical cation. Long‐lived charge separation states (up to tens of min) were observed.     

ESR spectrometric characterization of the methyl viologen dosimeter in poly(vinyl alcohol) film

A dosimeter of poly(vinyl alcohol) (PVA) film containing methyl viologen dichloride (MV²⁺ (Cl^-)₂) was characterized by means of ESR and u.v. spectrometries. γ-irradiation of the MV²⁺-PVA dosimeter induced one-electron reduction of MV^2+· to thecation radical (MV⁺), thus giving rise to blue coloration. The resulting MV^+· showed an ESR signal with a g-factor of 2.0031. The yield of MV^+· at a given radiation dose was estimated from duplicate integral of the ESR first-derivative spectra by reference to 1,1'-diphenyl-2-picrylhydrazyl (DPPH). The yield of MV^+· thus estimated increased linearly with increasing the radiation dose up to about 1.4 Mrad. The ESR spectrometry of MV^+· showed a linear correlation with the u.v. spectrometric method reported previously.     

A charge‐transfer salt composed of methyl viologen and hexacyanidoferrate(II)

The methyl viologen dication, used under the name Paraquat as an agricultural reagent, is a well‐known electron‐acceptor species that can participate in charge‐transfer (CT) interactions. The determination of the crystal structure of this species is important for accessing the CT interaction and CT‐based properties. The title hydrated salt, bis(1,1′‐dimethyl‐4,4′‐bipyridine‐1,1′‐diium) hexacyanidoferrate(II) octahydrate, (C₁₂H₁₄N₂)₂[Fe(CN)₆]·8H₂O or (MV)₂[Fe(CN)₆]·8H₂O [MV²⁺ is the 1,1′‐dimethyl‐4,4′‐bipyridine‐1,1′‐diium (methyl viologen) dication], crystallizes in the space group P 2₁/c with one MV²⁺ cation, half of an [Fe(CN)₆]⁴⁻ anion and four water molecules in the asymmetric unit. The Fe^II atom of the [Fe(CN)₆]⁴⁻ anion lies on an inversion centre and has an octahedral coordination sphere defined by six cyanide ligands. The MV²⁺ cation is located on a general position and adopts a noncoplanar structure, with a dihedral angle of 40.32 (7)° between the planes of the pyridine rings. In the crystal, layers of electron‐donor [Fe(CN)₆]⁴⁻ anions and layers of electron‐acceptor MV²⁺ cations are formed and are stacked in an alternating manner parallel to the direction of the −2a + c axis, resulting in an alternate layered structure.     

Radicals produced from the laser-induced photoionization of acridine in solution

We report on the transient species obtained in the biphotonic ionization of acridine in aqueous solution (pH 12.3) induced by the third harmonic of a neodymium-doped glass laser. The species directly produced during the laser pulse were the hydrated electron and the acridine cation which was detected for the first time in fluid media at room temperature. The hydrated electron and the cation decayed with pseudo-first-order kinetics, leading to microsecond transients which were assigned to two different acridine neutral radicals. The acridine cation was neutralized by the OH⁻ present in the solution (k = 3 × 10⁸ M⁻¹ s⁻¹), leading to a radical (the “N radical”) characterized by an absorption around 600 nm (λ_max = 595 nm; ϵ_max ≈ 5000 M⁻¹ cm⁻¹). The hydrated electron reacted efficiently with unexcited acridine (k = 4 × 10¹⁰ M⁻¹ s⁻¹) to give a hydrogenated radical (the “C radical”) which showed a broad absorption in the visible region around 500 nm (λ_max = 510 nm; ϵ_max ≈ 4000 M⁻¹ s⁻¹).     

Hydrated electron extinction coefficient revisited

The extinction coefficient of the hydrated electron (e(-))aq generated by pulse radiolysis is evaluated relative to the methyl viologen radical cation (*)MV(+), whose extinction coefficient at 605 nm has been carefully measured in the past. We find that the room temperature (e(-))aq extinction coefficients reported in the literature are underestimated by 10-20%. We obtain = 22,700 M(-1) cm(-1) for the 20 degrees C hydrated electron at 720 nm, assuming the (*)MV(+) extinction is 13,700 M(-1) cm(-1) at 605 nm. This has implications both for second-order reaction rate measurements of (e(-))aq and for the estimate of its integrated oscillator strength.     

敏化光学氧化还原反应V:锌四苯基卟啉磺酸钠与柴精的缔合作用及其对光敏还原甲基紫精效率的影响

用吸收光谱和发射光谱法研究了负电荷的锌四苯基卟啉磺酸钠与正电荷的紫精的络合作用,络合常数随紫精的疏水力而增长,在甲基紫精的敏光敏还原时,甲基紫精游离基阴离子平均产率随甲基紫精的起始浓度而增长,与Okura等的报导矛盾.     

A Smart Photochromic Semiconductor: Breaking the Intrinsic Positive Relation Between Conductance and Temperature

Breaking the intrinsic rule of semiconductors that conductivity increases with increase of temperature and realizing a dramatic dropping of conductivity at high temperature may arouse new intriguing applications, such as circuit overload or over‐temperature protecting. This goal has now been achieved through T‐type electron‐transfer photochromism of one organic semiconductor assembled by intermolecular cation???π interactions. Conductivity of the viologen‐based model semiconductor (H₂bipy)(Hox)₂ (H₂bipy=4,4′‐bipyridin‐1,1′‐dium; ox=oxalate) increased by 2 orders of magnitude after photoinduced electron transfer (a record for photoswitchable organic semiconductors) and generation of radical cation???π interactions, and fell by approximately 81 % at 100 °C through reverse electron transfer and degeneration of the radical cation???π interactions. The model semiconductor has at least two different electron transfer pathways in the decoloration process.     

光敏氧化还原反应——四苯基卟啉类化合物光敏还原甲基紫精

在DMSO-H₂O混合溶剂中,用TPP和它的四种金属络合物作为敏化剂,EDTA作为电子给体,经可见光照射,成功地还原了甲基紫精。反应活性顺序为ZnTPP>TPP>MgTPP》CoTPP>CuTPP。对以ZnTPP为敏化剂的反应,我们测得其量子产率为φ_418nm=0.08。我们考察了ZnTPP、MV²⁺和EDTA的浓度以及氧气对反应的影响。在一定浓度范围内,我们发现,反应速度与MV²⁺和EDTA浓度之间的关系符合于从推测的反应机理导出的动力学方程。     

Main‐chain viologen polymers with organic counterions exhibiting thermotropic liquid‐crystalline and fluorescent properties*

A series of viologen polymers with bromide, tosylate, and triflimide as counterions were prepared by either the Menshutkin reaction or metathesis reaction in a common organic solvent. Their polyelectrolyte behavior in methanol was determined by solution viscosity measurements, and their chemical structures were determined by Fourier transform infrared and Fourier transform NMR spectroscopy. They were characterized for their thermotropic liquid‐crystalline properties with a number of experimental techniques. Each of the viologen polymers with organic counterions had a low melting transition or fusion temperature above which it formed either a high‐order smectic phase or a low‐order smectic phase. Each of them also exhibited a smectic‐to‐isotropic transition. The ranges of the liquid‐crystalline phase were 80–88 °C for viologen polymers with tosylate as a counterion and 120–146 °C for viologen polymers with triflimide as a counterion. They had excellent thermal stability. The ranges of thermal stability were 288–329 °C for viologen polymers with tosylate as a counterion and 343–350 °C for viologen polymers with triflimide as a counterion. The fluorescence property for all of the viologen polymers in either aqueous or methanol solution was also included in this study. For example, the viologen polymer containing the 4,4′‐bipyridinium and p‐xylyl units along the backbone of the polymer chain with triflimide as a counterion had an absorption spectrum (λ_max = 265 nm), an excitation spectrum (λ_ex values = 357, 443, and 454 with monitoring at 533 nm), and an emission spectrum (λ_em = 536 nm with excitation at 430 and 450 nm) in methanol. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 659–674, 2002; DOI 10.1002/pola.10134     

Thermotropic liquid-crystalline properties of extended viologen bis(triflimide) salts

A series of extended, symmetric viologen triflimides were synthesised by the metathesis reaction of lithium triflimide with the respective viologen tosyalates in methanol. Their chemical structures were characterised by Fourier Transform Infrared, ¹H and ¹³C Nuclear Magnetic Resonance spectroscopy and elemental analysis. Their thermotropic liquid-crystalline (LC) properties were examined by a number of experimental techniques including differential scanning calorimetry, thermogravimetric analysis, polarising optical microscopy and variable temperature X-ray diffraction. The viologen salts containing alkyl chain of two carbon and three carbon atoms were relatively low melting salts. Those of alkyl chains of four carbon and five carbon atoms formed ionic liquids at 88 and 42°C, respectively. Those of alkyl chain of 9, 10 and 11 carbon atoms were high melting salts, as high as 166°C. Those of higher alkyl chains of 16, 18 and 20 carbon atoms showed thermotropic LC phases forming SmC, SmA and an unidentified smectic (SmX) phases, and showed SmA to isotropic transitions at high temperatures. As expected, all the viologen triflimides had excellent stabilities in the temperature range of 338–365°C.     

Properties of semiconductor electrodes coated with living films of cyanobacteria

IntactPhormidium sp. cells, immobilized on a SnO₂ semiconductor electrode, are capable of transferring electrons to SnO₂ in a light-dependent reaction. Drying a “wet” algal electrode at 50°C for 60 min increases photocurrent output capacity by 100-fold. We have studied the effect of various parameters on photocurrent generation. The magnitude of the photocurrent increased with increasing light intensity and depended on the nature of the electrolyte solution. The output, about 8 μA 10 μg Chl^?1 cm⁺², was obtained using 50 mM H₃BO₃?Na₂CO₃?KCl buffer as an electrolyte, an irradiance (>460 nm) of 250 J/m², and potentiostatic conditions (the algal working electrode was poised at +0.6 V vs a saturated calomel electrode). The yield was more than doubled upon addition of an electron carrier, such as methyl viologen, benzyl viologen, or Vitamin K₃, to the electrolyte solution. Maximum photocurrent was obtained at around pH 8 and 45°C, which are optimal conditions for growth of the cyanobacterium. Furthermore, DCMU, an inhibitor of photosynthetic electron flow, drastically decreased the yield, as did heat treatment of the electrode at 110°C for 15 min. The photocurrent action spectrum peak coincided well with the absorption peak of the light-harvesting pigment, phycocyanin. These results support the idea that electron transfer can occur across algal cell walls from the source of the light-induced reactions located within the lamellar membranes to the semiconductor electrode.     

Vibrational and electronic study of the methyl viologen radical cation MV+. in the solid state

A complete vibrational analysis (IR, Raman) of the methyl viologen radical cation MV^+., generated either via thermolysis from the solid MV²⁺(Cl⁻)₂ salt, or via chemical reduction from the dictation intercalated in the layered CdPS₃ compound, is reported fpr different isotopic derivatives. The results are discussed with respect to the conformation and the electronic configuration of the radical species, (MV^+.) and (MV^+.)₂.     

Thermal and photo-induced phenomena in γ-irradiated poly(methyl methacrylate)

ESR spectra and optical absorption spectra of γ-irradiated poly(methyl methacrylate) (PMMA) at 77°K and the photo- and thermal bleaching behavior have been studied. γ-Irradiation of PMMA yields a singles spectrum with a line width of 6 G which is bleachable with visible light. This species is assigned to the polymer cation. The unbleached species consists mainly of a four-line spectrum and is assigned to a polymer radical having a different CH₂ conformation from the usual nine-line spectrum. On subsequent warming to room temperature, the spectrum changes into the nine-line spectrum. γ-Irradiation of PMMA containing biphenyl as an additive at 77°K gives biphenyl cation. Yield of polymer radical is reduced by the addition of biphenyl and 2-methyltetrahydrofuran. A mechanism is proposed which is consistent with the experimental results.     

Photosensitized reduction of methyl viologen using eosin-Y in presence of a sacrificial electron donor in water-alcohol mixture

Photoreduction of methyl viologen (MV²⁺) by eosin-Y (EY²⁻) in the presence of triethanolamine (TEOA) has been investigated in water–methanol mixture by means of steady-state photolysis and laser-flash photolysis in the visible/near-infrared regions. The complete conversion to the persistent methyl viologen radical cation (MV^·+) was observed in the presence of lower concentrations of EY²⁻ and excess TEOA. By laser-flash photolysis measurements, electron transfer was confirmed to occur from the triplet state of EY²⁻ [³(EY²⁻)*] to MV²⁺ in the rate constants of ca 2.0 × 10¹⁰ M ⁻¹ s⁻¹. The rates and efficiencies of production of MV^·+ were found to be dependent on solvent compositions and concentrations of MV²⁺ ionic salt and TEOA. The back electron transfer reaction from MV^·+ to EY^·− was retarded in the presence of TEOA, which supports that EY²⁻ is reproduced by accepting an electron from TEOA. In the presence of excess TEOA, the indirect formation of MV^·+ from EY^·3− which was produced by accepting an electron from TEOA, was confirmed. The contributions of both the oxidative and reductive routes of ³(EY²⁻)* for the MV^·+ formation have been confirmed.     

Application of immobilized hydrogenase to h2 storage in concentrated solutions of methyl viologen

It has been found that immobilized cells ofC. pasteurianum possessing hydrogenase activity efficiently catalyze reversible reduction of concentrated (up to 0.5M) solutions of methyl viologen with H₂. A 0.5M aqueous solution of methyl viologen dissolves 240 times as much H₂ as pure water under the same pressure of hydrogen. The experimentally obtained levels of methyl viologen reduction and H₂ evolution are in satisfactory agreement with theoretical calculations. The potential of concentrated solutions of methyl viologen containing immobilized hydrogenase as a H₂ storage medium is discussed.     

Ionic species in irradiated poly(methyl methacrylate). A pulse radiolysis and ESR study

Poly(methyl methacrylate) free of initiators was synthesized by γ-irradiation and cast into transparent films. The samples were investigated by ns pulse radiolysis at various temperatures, and by ESR spectroscopy after γ irradiation at 77°K. Short-lived transients with optical absorptions at 440 and 725 nm were observed. The 440 nm absorption has been ascribed to the cation and the 725 nm absorption (εG = 3000 M^?1 cm^?1 (100 eV)^?1, τ_1/2 = 190 ns at ?13°C, E_a = 6.5 kcal/mol) to the anion. These assignments are based on ESR data of samples of poly(methyl methacrylate) and pivalic acid methyl ester deuterated at the ester deuterated at the ester and α-methyl groups, respectively, and subjected to thermal annealing and photobleaching. The anion decomposes on photobleaching by loss of the ester ·CH₃ radical, and the cation is proposed to decay by loss of the ·CH₃ radical from the α-methyl group. The thermal decay of the anion is discussed.     

Electron transfer from α-aminoalkyl radicals to methyl viologen

The reactions of tert-butoxyl radicals with amines, leading to the formation of α-aminoalkyl radicals, and the reactions of these with the electron acceptor methyl viologen have been examined using laser flash photolysis techniques. For example, the radicals CH₃?HNEt₂ and HOCH₂?H N(CH₂CH₂OH)₂ react with methyl viologen with rate constants equal to (1.3 ± 0.1) × 10⁹ and (2.1 ± 0.4) × 10⁹M^?1 · s^?1, respectively, in wet acetonitrile at 300 K.     

A glucose biosensor using methyl viologen redox mediator on carbon film electrodes

A new methyl viologen-mediated amperometric enzyme electrode sensitive to glucose has been developed using carbon film electrode substrates. Carbon film electrodes from resistors fabricated by pyrolytic deposition of carbon were modified by immobilization of glucose oxidase through cross-linking with glutaraldehyde in the presence of bovine serum albumin. The mediator, methyl viologen, was directly immobilised with the enzyme together with Nafion cation-exchange polymer. The electrochemistry of the glucose oxidase/methyl viologen modified electrode was investigated by cyclic voltammetry and by electrochemical impedance spectroscopy. The biosensor response to glucose was evaluated amperometrically; the detection limit was 20 μM, the linear range extended to 1.2 mM and the reproducibility of around 3%. When stored in phosphate buffer at 4 °C and used every day, the sensor showed good stability over more several weeks.