Reversible Redox Transitions Research Articles

Adsorption and first stages of polymerization of aniline on platinum single crystal electrodes

The present communication studies the adsorption of aniline on platinum single crystal electrodes and the electrochemical properties of the first layers of polyaniline(PANI) grown on those platinum surfaces. The adsorption process was studied in aqueous acidic solution (0.1M HClO4) and the electrochemical properties of thin films of PANI in both aqueous (1M HClO4) and non-aqueous media (tetrabutyl ammonium hexafluorophosphate (TBAPF6) with additions of methanesulphonic acid in acetonitrile). First of all, it was found that the adsorption of aniline on platinum single crystal surfaces is a surface sensitive process, and even more important that the adsorption features found at low concentrations (5×10−5M) can be directly correlated to the electrochemical properties of thin films of PANI in the very early stages of polymerization. The Pt(110) surface was found to be more suitable to obtain polymers with more reversible redox transitions when studied in aqueous media (1M HClO4). This is in good agreement with the higher polymerization rates found on this surface compared to Pt(100) and Pt(111). Finally the differences in ionic exchange rate were greatly enhanced when they were studied in organic media. The AC 250Hz response in the case of the thin films synthesized on Pt(110) is about twice greater than that obtained in the other basal planes using polymer layers with the same thickness.

Synthesis, photophysical and electrochemical properties of iridium(III) complexes with 2-aryl-1-phenylbenzimidazoles

A series of mixed-ligand cyclometalated iridium(III) complexes with 2-aryl-1-phenylbenzimidazoles (where aryl is phenyl, 4-chlorophenyl, 4-(N,N-dimethylamino)phenyl, 3,4-dimethoxyphenyl, and 2-thienyl) and 11-carboxydipyrido[3,2-a:2′,3′-c]phenazine have been synthesized and characterized by 1H NMR and 31P NMR spectroscopy and mass spectrometry. The electron absorption spectra (EASs) of the complexes are observed to contain charge-transfer bands in the low-energy region (below 600 nm, ɛ ≈ (1–3) × 103 mol/(L cm)). The bands experience appreciable bathochromic shifts as the electron-donating properties of a benzimidazole ligand strengthen. The complexes manifest luminescence in the yellow-red spectral region. According to alternating current voltammetry data, reversible and quasi-reversible redox transitions (E ox ≈ 1.2–1.7 V with respect to SHE, CH3CN) are observed in solutions of the studied complexes.

Nanoporous Platinum Electrodes as Substrates for Metal Oxide-Supported Noble Metal Electrocatalytic Nanoparticles: Synergistic Effects During Electrooxidation of Ethanol

Electrocatalytic oxidation of ethanol in acid medium (0.5 mol dm–3 H2SO4) was significantly enhanced by not only supporting bimetallic PtRu nanoparticles on nanostructured metal oxides (TiO2 or WO3), but also by depositing such catalytic systems on planar nanoporous platinized electrode substrates. Incorporation of TiO2 or WO3 into the electrocatalytic interface was likely to improve proton mobility and to provide –OH groups capable of inducing the removal of poisoning species, such as CO, from the Pt sites in the bimetallic PtRu catalyst. Synergistic interactions between ruthenium and titania were also possible. Regularly porous nanostructured platinum substrate also permitted development of submicro ‘reactors’ where reactant molecules, electrolyte ions, and all active components (TiO2 or WO3, Pt substrate, PtRu catalytic sites) could co-exist and become easily accessible. While WO3 was able to undergo fast reversible redox transitions to non-stoichiometric oxides, efficient utilization of inert (non-electroactive) TiO2 required admixing with carbon nanotubes to ensure easy charge distribution and good conductivity at the electrocatalytic interface.

A Mechanism for Origin of Reversible Redox Transitions of Molybdenum/Surface Molybdenum Oxides

A model is proposed on the origin of the reversible redox transitions of molybdenum (Mo) and its oxides. The major and minor redox transitions are proposed to be associated with two thermodynamically non-equivalent adsorption sites at the structured surface oxides. Anion interaction/adsorption was found to be a decisive factor. The criterion is based on the combination of evidences and assumptions and is derived from the variations observed in the two redox transitions during a series of cyclic voltammetry (CV), linear sweep voltammetry (LSV), electrochemical impedance spectroscopy (EIS), and open circuit potential (OCP) transient studies. The experiments were conducted using different electrolytes with dissimilar anions (Cl−, SO42− and ClO4−) and spanned a range of different pH values. The results described herein constitute an important step towards understanding the interfacial electrochemical processes of Mo and its oxides, and will be of use for a wide range of their applications.

Redox-active ferrocene-modified pyrimidines and adenine as antitumor agents: structure, separation of enantiomers, and inhihibition of the DNA synthesis in tumor cells

The structures, electrochemical properties, enantiomeric separation of ferrocenyl-(alkyl)pyrimidines and ferrocenyl(ethyl)adenine and their effects on the DNA synthesis in tumor cells were studied. Enantiomeric mixtures were separated by HPLC on modified cellulose as the chiral selector. The electrochemical properties of compounds were studied by cyclic voltammetry. All compounds have reversible single-electron redox transition in the region of 0.52–0.60 V, which belongs to ferrocene—ferrocenium with a positive shift compared to ferrocene (0.52 V). The molecular structure of 1-N-(ferrocenylbenzyl)-5-iodocytosine was studied by X-ray diffraction. 1-N-(Ferrocenylethyl)adenine was studied for ability to inhibit the DNA synthesis in the human ovarian cancer cell culture by the 3H-thymidine test.

Ruthenium carborane complexes: a relationship between the structure, electrochemical properties, and reactivity in catalysis of polymerization processes

The ruthenacarborane complexes of the exo-nido- and closo-structure, namely, diamagnetic exo-nido-5,6,10-[RuCl(PPh3)2]-5,6,10-(μ-H)3-10-H-7,8-(CH3)2-7,8-C2B9H6, 3,3-[Ph2P(CH2)nPPh2]-3-H-3-Cl-closo-3,1,2-RuC2B9H11 (n = 4, 5), paramagnetic 3,3-[Ph2P(CH2)nPPh2]-3-Cl-closo-3,1,2-RuC2B9H11 (n = 2–5), and their some ortho-phenylenecycloboronated derivatives, were studied by cyclic voltammetry. All chelate closo-complexes are characterized by reversible redox transitions, while the exo-nido-complex is liable to irreversible oxidation. Shortening of the methylene link in the diphosphine ligand of closo-ruthenacarboranes and/or the introduction of ortho-phenylenecycloboronated moieties and methyl substituents to the carbon atoms of the {C2B9} ligand lead to a decrease in the redox potential and electron density redistribution to the metal atom. A comparison of the experimental results on methyl methacrylate polymerization in the presence of the catalytic systems based on the studied metallacarboranes with the data on their electrochemical characteristics suggests that the efficiency of using the ruthenium complexes as catalysts is mainly determined by steric factors.

Electrocatalytic properties of nanocomposites based on conducting polymers and titanium dioxide in oxygen reduction process

Electrochemical characteristics and electrocatalytic properties in the oxygen reduction reaction are studied on hybrid nanocomposites based on conducting polymers (polyaniline, polypyrrol) doped with phosphomolybdic acid (PMA), and TiO2 and also their bifunctional analogs containing up to 5 wt % of nanosize platinum. It is found that the obtained nanocomposites in 0.5 M H2SO4 are capable of reversible electro-chemical redox transitions (in the range of potentials from −0.6 to 1.0 V vs. Ag/AgCl), in which the main contribution is provided by the corresponding conducting polymer and dopant (PMA). It is shown that activity of the studied nanocomposites in the oxygen reduction reaction is caused by the joint catalytic effect of all their components: the polymer, TiO2, H3PMo12O40, Pt.

Boron‐Doped Tri(9,10‐anthrylene)s: Synthesis, Structural Characterization, and Optoelectronic Properties

The reaction of 9,10-dibromo-9,10-dihydro-9,10-diboraanthracene (9,10-dibromo-DBA, 3) with two equivalents of 9-lithio-2,6- or 9-lithio-2,7-di-tert-butylanthracene gave the corresponding 9,10-dianthryl-DBAs featuring two (4) or four (5) inward-pointing tert-butyl groups. Compound 4 exists as two atropisomers, 4 and 4', due to hindered rotation about the exocyclic B-C bonds. X-ray crystallography of 5 suggests that the overall interactions between facing tert-butyl groups are attractive rather than repulsive. Even in solution, 4/4' and 5 are stable toward air and moisture for several hours. Treatment of 3 with 10-lithio-9-R-2,7-di-tert-butylanthracenes carrying phenyl (R=Ph), dimesitylboryl (R=Mes(2)B), or N,N-di(p-tolyl)amino (R=Tol(2)N) groups gave the corresponding 9,10-dianthryl-DBA derivatives 9-11 in moderate to good yields. In these molecules, all four solubilizing tert-butyl groups are outward pointing. The solid-state structures of 4, 5, 9, and 10 reveal twisted conformations about the exocyclic B-C bonds with dihedral angles of 70-90°. A significant electron-withdrawing character was proven for the Mes(2)B moiety, but no appreciable +M effect was evident for Tol(2)N. Compounds 5, 9, and 11 show two reversible DBA-centered reduction waves in the cyclic voltammogram. In the case of 10, a third reversible redox transition can be assigned to the Mes(2)B-anthryl substituents. The UV/Vis absorption spectrum of 5 is characterized by a very broad band at λ(max)=510 nm, attributable to a twisted intramolecular charge-transfer interaction from the anthryl donors to the DBA acceptor. The corresponding emission band shows pronounced positive solvatochromism (λ(em)=567 nm, C(6)H(12); 680 nm, CH(2)Cl(2)) in line with a highly polar excited state. The charge-transfer bands of 10 and 11, as well as the emission bands of 9 and 10, are redshifted relative to those of 5. The Tol(2)N derivative 11 is essentially nonfluorescent in solution, but emits bright wine-red light in the solid state.

Molybdenum, Molybdenum Oxides, and their Electrochemistry

The electrochemical behaviors of molybdenum and its oxides, both in bulk and thin film dimensions, are critical because of their widespread applications in steels, electrocatalysts, electrochromic materials, batteries, sensors, and solar cells. An important area of current interest is electrodeposited CIGS-based solar cells where a molybdenum/glass electrode forms the back contact. Surprisingly, the basic electrochemistry of molybdenum and its oxides has not been reviewed with due attention. In this Review, we assess the scattered information. The potential and pH dependent active, passive, and transpassive behaviors of molybdenum in aqueous media are explained. The major surface oxide species observed, reversible redox transitions of the surface oxides, pseudocapacitance and catalytic reduction are discussed along with carefully conducted experimental results on a typical molybdenum glass back contact employed in CIGS-based solar cells. The applications of molybdenum oxides and the electrodeposition of molybdenum are briefly reviewed.

Eine elektrochemische und strukturelle Studie an den Eisensilylamiden Fe[N(SiMe3)2]2 und Fe[N(SiMe3)2]3 / An Electrochemical and Structural Study of the Iron Silylamides Fe[N(SiMe3)2]2 and Fe[N(SiMe3)2]3

Abstract The bis(trimethyl)silylamido complex Na(THF){Fe[N(SiMe3)2]3} and the disilane tBu3SiSitBu3 were obtained from the reaction of Fe[N(SiMe3)2]3 with the sodium silanide Na(THF)2[SitBu3] in a mixture of benzene and THF. Single crystals of Na(THF){Fe[N(SiMe3)2]3} suitable for X-ray diffraction were grown from the reaction solution at ambient temperature (orthorhombic, C2221, Z = 4). The solid-state structure features a contact-ion pair with two short N-Na contacts. The THF adducts {M(THF)2[N(SiMe3)2]2} reacted with 2,2´-bipyridine to give the corresponding complexes {M(2,2´bipy)[N(SiMe3)2]2} (M= Mn; Fe). Their structures (M= Fe: orthorhombic, Pca21, Z = 8; M = Mn: orthorhombic, Pbca, Z = 8) feature monomeric units. The cyclic voltammogram of Fe[N(SiMe3)2]3 revealed a reversible redox transition with the potential of -0;523 V (E½), which was assigned to the Fe(III)[N(SiMe3)2]3 → Fe(II)[N(SiMe3)2]- 3 redox transition, whereas the compounds {Fe(THF)2[N(SiMe3)2]2} (Eox = -0;379 V) and {Fe(2,2´bipy)[N(SiMe3)2]2} (Eox = -0;436 V) featured irreversible oxidation waves. The related manganese bis(trimethylsilyl)amido complexes {Mn(THF)2[N(SiMe3)2]2} (Eox = -0;458 V) and {Mn(2,2´bipy)[N(SiMe3)2]2} (Eox = -0513 V) also underwent irreversibile electron transfer processes.

Prediction of thioredoxin and glutaredoxin target proteins by identifying reversibly oxidized cysteinyl residues

Summary A significant part of cellular proteins undergo reversible thiol-dependent redox transitions which often control or switch protein functions. Thioredoxins and glutaredoxins constitute two key players in this redox regulatory protein network. Both interact with various categories of proteins containing reversibly oxidized cysteinyl residues. The identification of thioredoxin/glutaredoxin target proteins is a critical step in constructing the redox regulatory network of cells or subcellular compartments. Due to the scarcity of thioredoxin/glutaredoxin target protein records in the public database, a tool called Reversibly Oxidized Cysteine Detector (ROCD) is implemented here to identify potential thioredoxin/glutaredoxin target proteins computationally, so that the in silico construction of redox regulatory network may become feasible. ROCD was tested on 46 thioredoxin target proteins in plant mitochondrion, and the recall rate was 66.7% when 50% sequence identity was chosen for structural model selection. ROCD will be used to predict the thioredoxin/glutaredoxin target proteins in human liver mitochondrion for our redox regulatory network construction project. The ROCD will be developed further to provide prediction with more reliability and incorporated into biological network visualization tools as a node prediction component. This work will advance the capability of traditional database- or text mining-based method in the network construction.

Hierarchically‐ordered electroactive silica‐polyaniline nanohybrid: A novel material with versatile properties

AbstractPoly(trimethoxy silylpropylaniline), a nanoporous (pore diameter of 2.4 nm), electroactive (stable reversible redox characteristics), electrochromic (yellow at −0.10 V, blue green at +0.50 V, and dark green at +0.70 V), and pH‐sensitive, silica–polyaniline (PANI) hybrid material (designated as KGM‐1) has been synthesized in powder form by a simple one‐pot chemical synthesis as well as a “thin nanolayered film” by cyclic voltammetry. High‐resolution transmission image of KGM‐1 informs that the particles are spherical, with diameters in the range of 0.5–1.5 μm. X‐ray diffraction pattern of pristine KGM‐1 confirms the combined presence of ordered silica network and PANI chains. The surface area of calcined KGM‐1 is 40 m2/g (∼15 times higher than KGM‐1), and the average pore size is 2.4 nm. The N2 adsorption features also inform that PANI is present as a uniform layer within the pores of silica and because of that the silica pores are not completely blocked. The reversible redox transitions in PANI units and nanoporosity of KGM‐1 are effectively used for the electro‐driven loading/release of DNA or adenosine 5′‐triphosphate. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2010

Activation of carbon-supported platinum nanoparticles by zeolite-type cesium salts of polyoxometallates of molybdenum and tungsten towards more efficient electrocatalytic oxidation of methanol and ethanol

The activity of Vulcan-supported Pt nanoparticles (Pt40%/Vulcan XC-72) towards oxidation of methanol and ethanol is increased by admixing them with zeolite-type cesium salts of heteropolymolybdic and heteropolytungstic acids: Cs 2.5H 0.5PMo 12O 40, Cs 2.5H 1.5SiMo 12O 40, Cs 2.5H 0.5PW 12O 40, and Cs 2.5H 1.5SiW 12O 40. It is apparent from IR measurements that these salts remain the Keggin-type structure. They are electroactive and undergo reversible redox transitions, as well as they are hydrated and contain mobile protons. As evidenced from cyclic voltammetric, stair-case voltammetric and chronoamperometric diagnostic experiments, the electrocatalytic enhancement effect has been most pronounced upon application of phosphododecamolybdate and phosphododecatungstate salts. The overall activation effect may also reflect the micro and mesoporous (zeolite-type) high surface area morphology of polyoxometallate cesium salts. The presence of well-defined polyoxometallate clusters in the vicinity of Pt is expected increase population of reactive oxo groups at the electrocatalytic interface.

An electrochromic film device to teach polymer electrochemical physics

We discuss the background associated with an electrochromic device that can reversibly change its color and optical density at a specific potential. We discuss the underlying science needed to make a new polyaniline (PAN)/polyvinyl alcohol(PVA) electrochromic composite film on an indium-tin oxide (ITO) conducting glass by electropolymerization and describe a reversible redox transition of the PAN. The experiment gives students an opportunity to fabricate an electrochromic device containing PAN, one of the most important conducting polymers. The experimental conditions are flexible so that each group of students can construct their own electrochromic device with particular behavior. Two techniques for polymerizing the PAN and three methods of demonstrating the electrochromism are given, depending on the available apparatus. A sophisticated three-electrode potentiostat or a crude apparatus containing a battery, wire, a variable resistor, and a voltage meter is used to synthesize the PAN deposit. The electrochromic property is repetitively observed by reversibly changing the applied potentials on the device. A potentiostatic apparatus, a single flashlight battery, or a flashlight battery accompanied by a variable resistor allows students to observe multicolor electrochromism. The experiments significantly enhance students’ understanding of polymer chemicophysics principles and their appreciation of novel variable colorful films. The experiments are safe and easy to perform, provided that appropriate precautions are taken.

Synthesis of Polythiophene Derivatives and Their Application for Electrochemical DNA Sensor

In this study, we synthesized thiophene derivatives by the protection of the carboxyl group of 3-thiophene acetic acid(monomer) with different substituted benzyl groups for hybridized electrochemical DNA sensors composed of a conductive polymer and oligo-DNA (ODN). In general, while 3-thiophene acetic acid is not suitable for direct electro-polymerization because of the electron withdrawing effect of the –COOH group in above monomer, our synthesized thiophene monomer derivatives are easily electro-oxidized to form stable electro-active polymer films by the protection of –COOH group. These polymer films showed the specific electrochemical characteristics of poly 3-alkylthiophenes with a reversible redox transition in the range of 0.8–1.6 V in CV (cyclic voltammogram) measurement. Biological recognition was monitored by comparison with the CV signal from the hybridization of single and double strands of ODN. The oxidation current of double strand ODN was less than that of the single strand, which corresponds to a decrease in electrochemical activity of the conducting polymer with an increase in stiffness of the side group of the electroactive polymer.

Capacitance Enhancement of Carbon Fabric Electrodes in Electrochemical Capacitors Through Electrodeposition with Copper

Electrodeposition of copper on activated carbon fabric was conducted in an attempt to promote the capacitance of the carbon fabric employed as electrodes in electrochemical capacitors using KOH solutions as the electrolyte. The deposition was conducted in for different lengths of time at a current density of and this was followed by thermal oxidation that was found to be essential for capacitance enhancement. The capacitance was shown to increase with the degree of deposition and passed through a maximum at a deposition time of 4 h, at which a Cu content of ca. 6 wt % with a capacitance enhancement of ca. 40%, e.g., from 114 to was achieved. The capacitance enhancement may partially result from the reversible redox transitions of Cu in the presence of ions. The Cu-deposited fabrics exhibited an increase in capacitance at the early stage of charge-discharge cycling. Cyclic voltammetric measurements suggest that the capacitance increase may result from the formation of surface complexes that serve as centers for reversible redox reactions. The deposition with Cu resulted in the reduction of electrode resistance, while the leakage current was found to increase upon deposition. The results of the present study have provided a simple and economical means that can substantially enhance the capacitance of carbonaceous electrodes. © 2002 The Electrochemical Society. All rights reserved.

Metallo-supramolecular polyelectrolyte multilayers with cobalt(II): preparation and properties

Abstract Metal ion mediated self-assembly of the ditopic ligand 1,4-bis(2,2′:6′,2′′-terpyridine-4′-yl)benzene and Co(II) results in a metallo-supramolecular coordination polyelectrolyte (Co-MEPE), which is analyzed by spectroscopic methods, elemental analysis and cyclovoltammetry. The Co-MEPE shows a reversible one-electron redox transition in the potential range from −0.15 to 0.35 V with a half-wave potential of 87 mV. Thin films of Co-MEPE are fabricated by layer-by-layer self-assembly together with poly(styrenesulfonate) (PSS) on planar substrates and are analyzed by UV/vis spectroscopy, microgravimetry, and electrochemistry. The Co-ions in the multilayer can be removed and displaced by Fe-ions. The electrochemical activity of the Co-ions is fully maintained in the films. Charge transport through the multilayer is consistent with electron self-exchange reactions. The permeability of the multilayers is investigated by electrochemistry using K3[Fe(CN)6] as electroactive probe. Compared to poly(allylamine hydrochloride) (PAH)/PSS films, the electroactive probe does not permeate through PSS/Co-MEPE multilayers. Fluorescence measurements with pyrene as polarity probe indicate that the Co-MEPE films are more hydrophobic than PAH/PSS multilayers.

Reactive groups on polymer coated electrodes: 10. Electrogenerated conducting polyalkylthiophenes bearing activated ester groups

Three new thiophene derivatives containing different activated ester groups covalently attached to the thiophene ring via a linear undecyl spacer have been synthesized. Electropolymerization of these monomers in acetonitrile led to stable electroactive polymers, while activated functional groups withstand the polymerization conditions and were correctly incorporated in the resulting polymers. These polymers show the characteristic electrochemical behavior of poly(3-alkylthiophene)s with the reversible redox transition in the range of 0.7–0.9V. UV/Vis spectra of them exhibit an absorption maximum at around 460nm. Conductivity measurements on oxidized films of these polymers by means of the two-probe method gave values in the range of 10−3–10−2Scm−1. The reactivity of the pendant activated ester groups was demonstrated by the reaction with butylamine, (4-amino-2-oxabutyl) ferrocene and 2-aminoanthraquinone. Spectroscopic studies and electrochemical characterization confirmed that the immobilized amino compounds are covalently bound to the polymer surfaces. These results suggest that the obtained new polymers can be used as electrically conducting carrier materials for the immobilization of biochemically interesting molecules.

Anomalous electrochemical behaviour of palladium in aqueous solution

Palladium electrode surfaces in aqueous sulphuric acid solution were activated or disrupted by repeated growth and reduction of multilayer hydrous oxide films. A very low percentage (<1.0%) of the metal deposited on reduction of such films existed initially in an unusually active state. This active palladium exhibited a reversible redox transition at ca. 0.24 V (RHE) at 25°C and the charge involved in the latter process was directly proportional to the much larger charge value associated with the reduction of the HO2 component in the multilayer hydrous oxide deposit. The charge associated with the active state of the metal decreased gradually, in a first-order manner, subsequent to the generation of the active state. The nature of the active state of metal surfaces, which is assumed to be of considerable importance in heterogeneous catalysis and electrocatalysis, is discussed briefly.

Synthesis and electropolymerization of newp-nitrophenyl-functionalized thiophene derivatives

Electrooxidative polymerization of three new p-nitrophenyl-functionalized thiophene derivatives leads to materials which show the electroactivities of both nitrobenzene and polythiophene. However, other than the reversible redox transition in solution, the nitro groups exhibit a complex nature of the cyclic voltammetric curve in the obtained solid polymeric films. It was found that the electrochemical behavior of the nitro groups has a dramatic effect on the conjugated π-electron systems of the substituted polythiophenes. Upon electrochemical addressing of the nitro groups the electroactivities of the corresponding polymers decrease rapidly compared to their analogous polymers without nitro groups. FTIR studies of these polymers revealed that after the reduction/oxidation process of the nitro groups still a large amount of electrolyte salt remains in the polymer films. As typical of functionalized polythiophenes. however, these new polymers can be switched under controlled conditions between their oxidized and neutral forms and exhibit the typical properties of electroactive polythiophenes, e.g., electrochromism.