Dynamics of adiabatic electron transfer reactions at metal electrodes

We have investigated the dynamics of adiabatic electron transfer reactions at metal electrodes using a Hamiltonian suggested by Schmickler (J. Electroanal. Chem., 204 (1986) 31). We show that in the adiabatic limit the problem reduces to that of dynamics of a single variable, the shift of the ionic orbital caused by its interaction with the solvent. This variable is identified as the reaction co-ordinate for the problem and we show that in certain limits, it obeys a non-linear Volterra type integral equation, with a stochastic inhomogeneous term. For an inhomogenous term with the autocorrelation function decaying exponentially, this may be converted into a differential equation for Brownian motion. This equation can be analysed to obtain the rate, through the associated Fokker-Planck equation. The rate so obtained, has a correction to the pre-exponential factor obtained by Schmickler. A possible extension to inner sphere reactions is also discussed.     

Photoinduced electron transfer reactions of rose bengal and selected electron donors

The photoinduced electron transfer reactions of the triplet state of rose bengal (RB) and several electron donors were investigated by the complementary techniques of steady state and time-resolved electron paramagnetic resonance (EPR) and laser flash photolysis (LFP). The yield of radicals varied with the light fluence rate, RB concentration and, in particular, the electron donor used. Thus for L-dopa (dopa, dihydroxyphenylalanine) only 10% of RB anion radical (RB√−) was produced, with double the yield observed with NADH (NAD, nicotinamide adenine dinucleotide) as quencher and more than three times the yield observed with ascorbate as quencher. Quenching of the RB triplet was both reactive and physical with total quenching rate constants of 4 × 10⁸ mol⁻¹ dm³ s⁻¹ and 8.5 × 10⁸ mol⁻¹ dm³ s⁻¹ for ascorbate and NADH respectively. The rate constant for the photoinduced electron transfer from ascorbate to RB triplet was 1.4 × 10⁸ mol⁻¹ dm³ s⁻¹ as determined by Fourier transform EPR (FT EPR). FT EPR spectra were spin polarized in emission at early times indicating a radical pair mechanism for the chemically induced dynamic electron polarization. Subsequent to the initial electron transfer production of radicals, a complex series of reactions was observed, which were dominated by processes such as recombination, disproportionation and secondary (bleaching) reactions.

It was observed that back electron transfer reactions could be prevented by mild oxidants such as ferric compounds and duroquinone, which were efficiently reduced by RB√−.     

Direct electron transfer reactions between human ceruloplasmin and electrodes

In an effort to find conditions favouring bioelectrocatalytic reduction of oxygen by surface-immobilised human ceruloplasmin (Cp), direct electron transfer (DET) reactions between Cp and an extended range of surfaces were considered. Exploiting advances in surface nanotechnology, bare and carbon-nanotube-modified spectrographic graphite electrodes as well as bare, thiol- and gold-nanoparticle-modified gold electrodes were considered, and ellipsometry provided clues as to the amount and form of adsorbed Cp. DET was studied under different conditions by cyclic voltammetry and chronoamperometry. Two Faradaic processes with midpoint potentials of about 400 mV and 700 mV vs. NHE, corresponding to the redox transformation of copper sites of Cp, were clearly observed. In spite of the significant amount of Cp adsorbed on the electrode surfaces, as well as the quite fast DET reactions between the redox enzyme and electrodes, bioelectrocatalytic reduction of oxygen by immobilised Cp was never registered. The bioelectrocatalytic inertness of this complex multi-functional redox enzyme interacting with a variety of surfaces might be associated with a very complex mechanism of intramolecular electron transfer involving a kinetic trapping behaviour.     

Kinetics of multiple electron transfer reactions at porous electrodes under stationary and flow conditions

The kinetics of consecutive two-electron transfer reactions at porous flooded electrodes are investigated under both stationary and flow conditions, where mass transfer is due respectively to diffusion and forced convection. The current-polarization relations were calculated for both modes of mass transfer as a function of the specific surface area of the electrode, the rates of the respective steps of the electron transfer reaction and the appropriate mass transfer coefficients. The computed solutions degenerate to the known limiting cases of single electron transfer control under conditions of very high or very low polarizations. Thus, at high anodic polarization, the electrochemical reaction is controlled by a single electron transfer step, the other step being too fast. Under conditions of 0.1<i/i_L<1, the overall reaction rate is controlled by both mass transfer and electrochemical activation. For flooded diffusion electrodes, the current-voltage curves follow the Tafel equation with a slope of double the normal value. This is attributed to mass transfer control in agreement with previous work. Experimental results, obtained on the porous flow-through electrode, agreed well with the theoretical predictions. The calculations presented here enable a quantitative evaluation of the relative influence of the rate of any step on the overall behaviour of the electrode under the appropriate experimental conditions.     

Proton activity in nafion films: probing exchangeable protons with methylene blue

A simple approach to monitor the H(+) activity of a proton-exchange membrane (Nafion) is introduced by incorporating methylene blue as an indicator dye. The dye exhibits characteristics absorption maxima at 665 and 745 nm corresponding to its singly and doubly protonated forms, respectively. The apparent proton activity of Nafion as monitored from the appearance of doubly protonated methylene blue absorption is equivalent to 1.2 M H2SO4. By monitoring the spectral changes associated with the protonation equilibrium of the dye, it is possible to probe the rate and the exchangeable proton sites within the Nafion film. For the Nafion 117 film, we estimate the total exchangeable proton sites to be 2.5 x 10(19) sites/cm(2) or 4.2 x 10(-5) mols/cm(2). The equilibrium constant for the H(+)/Na(+) exchange for the bound sites is determined to be 2.2. The feasibility of methylene blue as a probe to monitor proton activity during the operation of a direct methanol fuel cell has been explored.     

Electrochemical behavior of methylene blue at bare and DNA-modified silver solid amalgam electrodes

Journal of Solid State Electrochemistry - Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) were used for a voltammetric study of methylene blue (MB) at a mercury meniscus-modified...     

亚甲蓝修饰电极推动的血红素蛋白质直接电子转移反应

本文研究了几种血红素蛋白质包括牛血红蛋白, 人肌红蛋白和马心细胞色素C在亚甲蓝修饰电极上的非均相电子转移反应, 采用光透薄层光谱电化学法监测了血红素蛋白的直接电化学反应过程, 并进行了动力学研究。     

Energy transfer from chemically attached rhodamine 101 to adsorbed methylene blue on microcrystalline cellulose particles

Rhodamine 101 (R101) was chemically attached onto microcrystalline cellulose and methylene blue (MB) was adsorbed to a sample bearing nearby 6 × 10^?7 mol R101 (g cellulose)^?1. The system was studied by reflectance and emission spectroscopy in the solid state. R101 shows no aggregation in these conditions and, while pure MB builds up dimers on cellulose even at 2 × 10^?8 mol g^?1, in the presence of R101 no evidence on selfaggregation or heteroaggregation is found up to around 10^?6 mol g^?1. No exciplex formation is found as well. The overall fluorescence quantum yield measured on thick layers, once re‐absorption effects are accounted for, amounts to 0.80 ± 0.07 for pure R101 and decreases steadily on increasing the concentration of MB. Results demonstrate the occurrence of radiative and nonradiative singlet energy transfer from R101 to MB. For thick layers of particles, the combined effect of both kinds of energy transfer amounts to nearly 80% at the highest acceptor concentration, while nonradiative transfer reaches 60% both for thin and optically thick layers. The dependence of nonradiative energy transfer efficiencies on the acceptor concentration is analyzed and the origin of departures from Förster behavior at low acceptor concentration is discussed.     

Photoinduced electron transfer reactions of pyrylium derivatives with organic sulfides in acetonitrile

The photochemistry and photophysics of pyrylium derivatives with organic sulfides in acetonitrile medium are investigated. A steady decrease in the fluorescence intensity and fluorescence lifetime of the dyes was observed with increase in the quencher concentration. Bimolecular quenching constants were evaluated and correlated with the free energy of electron transfer. Laser flash photolysis investigations on the dyes in presence of quenchers were done. Observation of pyranyl radical and sulfide cation radicals as intermediates clearly illustrates the electron transfer mechanistic pathway for this reaction. The radical pair energies were calculated and found to be lower than the triplet energy of the sensitisers and hence we do not see any triplet induction in the present system.     

Photoinduced electron transfer in bisporphyrin-diimide complexes

The bisporphyrin host ZnH was synthesized, and its complexation with two aromatic diimide guest molecules, bis(pyridyl)naphthalenediimide NIN and bis(pyridyl)phenyldiimide PIN, was investigated by (1)H NMR and UV/Vis spectroscopy. The diimide guests were complexed simultaneously with both metalloporphyrins of the host, with association constants on the order of 10(8)M(-1). The processes occurring in the complex after excitation of the porphyrinic host were studied by steady-state and time-resolved emission and transient absorption spectroscopy. Complexation alters the photophysical properties of the host ZnH; the luminescence bands shift to the red by 30 nm in the complexed forms, while the emission quantum yield and the lifetime decrease. Comparison of a complex between ZnH and a model guest unable to undergo photoinduced processes allowed us to establish that, in the diimide complexes, quenching of the porphyrinic luminescence occurs with a rate of 1.1 x 10(10)s(-1). The process is identified as an electron transfer from the excited singlet of the porphyrinic host to the imide guest, which yields charge-separated states with a lifetime of 710 ps for ZnH(+)-NIN(-) and 260 ps for ZnH(+)-PIN(-).     

Photoinduced electron transfer in photozyme systems

Photozymes are novel water-soluble polymers usually constructed by copolymerization of a mixture of water-soluble and water-insoluble comonomers, some of which contain chromophores capable of absorbing light and transmitting the excitation energy by means of the antenna effect to selected traps. The interactions between the hydrophobic and hydrophilic groups in the polymer with water cause the formation of hypercoiled pseudomicellar conformations of the polymer coil, leading to hydrophobic regions or pockets in the interior of the macromolecular coil. If the water contains hydrophobic organic molecules, they will locate preferentially in these hydrophobic polymer microdomains, and in the presence of light they can be photochemically transformed into useful products with high efficiency and selectivity. This paper reviews some recent results on photochemical reactions initiated by photoinduced electron transfer in these novel systems, and their possible commercial applications to pollution abatement, and solar production of hydrogen from water.     

Photoinduced electron transfer in pyromellitimide-bridged porphyrins

The kinetics of intramolecular photoinduced electron transfer in a series of pyromellitimide-bridge porphyrins have been studied using transient absorption and fluoresence techniques. The dependence of both charge separation and recombination rates on connecting chain length, metallation state, coordination state, conformation, solvent and temperature have been systematically measured and found to be broadly in agreement with theoretical predictions. In particular, the inverted region is observed at large exoergicity. Also, in the inverted region, when the porphyrin to pyromellitimide separation is large the electron transfer rate can be faster than at small separations; this is also explained by theory. At low temperatures, temperature-independent nuclear tunnelling limits the electron transfer rate, while in solvents having a slow dielectric relaxation this solvent reorientation also limits the rate. Fluorescence data provide evidence of multiple conformations in the free base compounds but in the longer-chained Zn and Mg derivatives, where the pyromellitimide oxygen atoms can bond to the metal, molecular conformations are limited. On addition of basic ligands, this metal to oxygen bonding is released and the electron transfer is switched off.     

Electron transfer reactions of cytochrome c at metal electrodes

The heterogeneous electron transfer reactions of cytochromec occurring at platinum, gold and mercury electrodes are shown to be quasi-reversible. In each case the electrodes have not been modified and the cytochromec samples are native. This work extends previous work and demonstrates that biological molecule electron transfer reactions can be studied at clean metal surfaces to gain fundamental knowledge of the mechanisms of these reactions.     

Electron transfer of hemoglobin at electrodes modified with colloidal clay nanoparticles

Nanostructured sodium montmorillonite was prepared via a colloidal chemical approach and deposited onto glassy carbon electrodes (GCE). Subsequently, hemoglobin was spontaneously adsorbed onto the clay membrane-modified electrode. The colloidal clay nanoparticles and the adsorbed protein were characterized by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The electrochemical impedance behavior of the system was studied using a microlithographically fabricated interdigitated microsensor electrode (IME). The interaction of the clay nanoparticles with hemoglobin was investigated by UV-VIS spectroscopy and electrochemical methods. The heme protein adsorbed in this way displayed a well-defined electrode process and the electron transfer was confirmed to originate from its heme site. Furthermore, nitric oxide affects the hemoglobin electrochemistry.     

Photoacoustic spectroscopic studies of methylene blue and malachite green adsorbed on silica

Spectra and calibration curves of methylene blue and malachite green adsorbed on silica show that photoacoustic spectroscopy can be used for the determination of these compounds. The problem of the existence of an equilibrium between monomer and dimer for methylene blue is considered.     

Direct electron transfer reactions of laccases from different origins on carbon electrodes

Electrochemical studies of laccases from basidiomycetes, i.e., Trametes hirsuta, Trametes ochracea, Coriolopsis fulvocinerea, Cerrena maxima, and Cerrena unicolor, have been performed. Direct (mediatorless) electrochemistry of laccases on graphite electrodes has been investigated with cyclic voltammetry, square wave voltammetry as well as potentiometry. For all mentioned high potential laccases direct electron transfer (DET) has been registered at spectrographic graphite and highly ordered pyrolytic graphite electrodes. The characteristics of DET reactions of the enzymes were analysed under aerobic and anaerobic conditions. It is shown that the T1 site of the laccase is the primary electron acceptor, both in solution (homogenous case) and at surface of the graphite electrode (heterogeneous case). A mechanism of ET for the process of the electro-reduction of oxygen at the laccase-modified graphite electrodes is proposed and the similarity of this heterogeneous process to the laccase catalysed oxygen reduction homogeneous reaction is concluded.     

Photoinduced intramolecular electron transfer between fluorescein and carbazole

Several dyads consisting of a fluoreseein covalently linked with a carhazole at site 2 or site 6 have been synthesized and characterized.Studies of absorption spectra,emission spectra and fluorescence lifetime quern hing Indicate that the ground-state interaction between fluorescein and carhazole in dyads is negligible and the intramolecular electron transfer (ET) reactions are mainly of dynamic process.Moreover,the efficiency and raie conslam of lectron transfer reactions in ZFO4 (carbazole linked at site 2'of fluorescein) are larg er than those in 4FOZ (carbazole linked at site 6 of fluorescein) 0 74; KET 11×108S-1),because the mutual orientation of donor and acceptor in ZFO4 is nearly face-to-face,which is more favorable to the process than the shoulder-to-shoulder mutual orientation in 4FOZ.Estimations are also formed of the free energy change of the photomduced electron transfer and the back reactions in the dyads.