Quasi-reversible Process Research Articles

Mono- and di-(2,3,5,6-tetrafluoro-4-N,N-dimethylaminophenyl) meso-tetraarylporphyrins: Synthesis, spectral, structural and electrochemical studies

A new series of mono-/di-aminated meso-tetraarylporphyrins has been synthesized and characterized by conventional spectroscopic methods. Crystal structure analysis shows that interactions involving halogens are the major contributors, and the relative contributions are 56% and 53% respectively for MB2c and MB3c. All the compounds were electrochemically analyzed under different reaction conditions and showed a positive shift in the reduction and oxidation potentials. The HOMO–LUMO energy gap was altered with respect to the nature of the supporting electrolyte and reference electrodes used. The linear behavior of Randles-Sevcik plots indicate that the redox processes are diffusion controlled; the first reduction/oxidation is a reversible one-electron step whereas the second reduction/oxidation is a quasi-reversible one-electron process. Results also reveal that the mono-aminated porphyrins are more electron deficient than non-aminated and di-aminated porphyrins.

Comparative electrochemical studies of kinetic and thermodynamic parameters of Quinoxaline and Brimonidine redox process

The electrochemical response of two biologically important benzopyrazine derivative, Quinoxaline and Brimonidine (5-bromo-N-(4,5-dihydro-1H-imidazol-2-yl)quinoxaline-6-amine) was studied at glassy carbon electrode. Investigations were performed in acetate buffer at pH 3.6 where both compounds undergo complex electrode process which begins with quasi reversible redox process at pyrazine ring forming di-hydro derivative, which is further irreversible oxidized to hydroxyl-derivative. Cyclic voltammetry at different scan rates was employed for the determination of heterogeneous electron transfer rate constant and diffusion coefficient of the compounds. The values of electron transfer rate constant at different temperatures were used for the evaluation of thermodynamic parameters like enthalpy, entropy and Gibbs free energy changes, as well as apparent energy of activation.Computational study has been performed in order to evaluate some useful quantum chemical parameters with the aim to confirm and explain the difference in experimentally obtained kinetic and thermodynamic parameters of the investigated QUI and BRIM redox process. Results were compared with studies of similar compounds and pointed out the observed similarities and differences.

Electrochemical Behavior of Sulfur in Aqueous Alkaline Solutions

The kinetics and mechanism of the electrode oxidation-reduction of sulfur on an electrically conductive sulfur–graphite electrode in an alkaline solution was studied by the potentiodynamic method. To examine the mechanism of electrode processes occurring during AC polarization on a sulfur–graphite electrode, the cyclic polarization in both directions and anodic polarization curves were recorded. The kinetic parameters: charge transfer coefficients (α), diffusion coefficients (D), heterogeneous rate constants of electrode process (ks), and effective activation energies of the process (Еа) were calculated from the results of polarization measurements. An analysis of the results and calculated kinetic parameters of electrode processes showed that discharge ionization of sulfur in alkaline solutions occurs as a sequence of two stages and is a quasireversible process.

Catalytic effects of [Ag(H2O)(H3PW11O39)]3− on a TiO2 anode for water oxidation

A [H3AgI(H2O)PW11O39]3−-TiO2/ITO electrode was fabricated by immobilizing a molecular polyoxometalate-based water oxidation catalyst, [H3AgI(H2O)PW11O39]3− (AgPW11), on a TiO2 electrode. The resulting electrode was characterized by X-ray powder diffraction, scanning electron microscopy, and energy dispersive X-ray spectroscopy. Linear sweep voltammetry, chronoamperometry, and electrochemical impedance measurements were performed in aqueous Na2SO4 solution (0.1 mol L−1). We found that a higher applied voltage led to better catalytic performance by AgPW11. The AgPW11-TiO2/ITO electrode gave currents respectively 10 and 2.5 times as high as those of the TiO2/ITO and AgNO3-TiO2/ITO electrodes at an applied voltage of 1.5 V vs Ag/AgCl. This result was attributed to the lower charge transfer resistance at the electrode-electrolyte interface for the AgPW11-TiO2/ITO electrode. Under illumination, the photocurrent was not obviously enhanced although the total anode current increased. The AgPW11-TiO2/ITO electrode was relatively stable. Cyclic voltammetry of AgPW11 was performed in phosphate buffer solution (0.1 mol L−1). We found that oxidation of AgPW11 was a quasi-reversible process related to one-electron and one-proton transfer. We deduced that disproportionation of the oxidized [H2AgII(H2O)PW11O39]3− might have occurred and the resulting [H3AgIIIOPW11O39]3− oxidized water to O2.

A Simple Procedure to Fabricate Paper Biosensor and Its Applicability—NADH/NAD+ Redox System

A simple device which incorporates three electrodes (working electrode, counter electrode and reference electrode) was constructed to be used currently in laboratories without elevated cost. It does not need more than 2 µL of electrolyte, sample or working solution, his support material is paper, and the working electrode which is based on carbon ink can incorporate enzymes and cofactors. To test this concept we started this investigation using the NADH/NAD+ redox couple which is an omnipresent coenzyme in living systems but is also a challenge to electrochemistry. The paper sensor fabrication was simple, rapid and cheaper. NADH was incorporated in the carbon ink by mixing both and, this mixture was used to print the working electrode. The direct electrochemical system NADH/NAD+ signal obtained, using this device, appeared at low potentials. A quasi-reversible diffusional redox process was achieved and regeneration of the NADH after oxidation was reached. This small paper device was not only used to study the redox process of NAD+/NADH, but also its behavior in the presence of electroactive (ascorbic acid) and non-eletroactive species (glucose).

Electrochemical Kinetics of Diffusion Problems with a Moving Phase Boundary

The growth and dissolution of nuclei and a deposit film on a solid electrode are considered. The diffusion problems of the growth and dissolution of hemispherical nuclei and a thin deposit film are solved. Equations are derived to describe the I–V characteristics and chronoamperograms of reversible, irreversible, and quasi-reversible electrode processes, and the dependences of the peak current, the amount of deposit on an electrode, the potential of the peak current, and the full width at half-maximum of an I–V characteristic on the preelectrolysis parameters are determined. The formation of a new phase at the solid–solution interface and the mass-exchange processes at the weakly soluble film–solution interface are discussed. Anodic films are experimentally studied. In the case of nuclei or a thin anodic film, the amount of compound formed on the electrode surface linearly depends on the ionic concentration and the preelectrolysis time.

Water‐Processable Multiwalled Nanotube: Phenol‐Induced Reversible Oxidation Process and Ambipolar Charge Transport Property

AbstractDue to the fascinating electronic, thermal, and mechanical properties of single‐walled carbon nanotubes (SWCNTs), extensive efforts have been devoted to the development of SWCNT‐based materials. These materials' semiconducting properties and related applications, such as field‐effect transistors (FETs), have been investigated by researchers for many years. However, despite the significant progress achieved, it remains challenging to separate semiconducting and metallic nanotubes from the mixtures of as‐grown SWCNTs. In a few studies, composites of water‐processable phenol formaldehyde resin/multiwalled carbon nanotubes (MWCNTs) have been found to exhibit a quasireversible oxidation process and to behave as semiconductors or field‐effect transistors. This finding has rarely been reported for MWCNTs, and it differs greatly from findings regarding intrinsic semiconductive SWCNTs. Significantly, field‐effect transistors fabricated with MWCNT composites as their semiconductor active layers have shown ambipolar charge transport characteristics. The results provide a high value‐added application pathway for the application of polymer/MWCNTs as the FET materials for electronic devices that offer higher performance at a lower cost.

Cyclic voltammetric study of the influence of porosity on electrochemical response of nickel-alumina modified glassy carbon electrode

Pure and nickel-modified alumina powders with different porosity were synthesized and applied on the glassy carbon electrode by means of Nafion polymer. The data obtained from the nitrogen adsorption-desorption isotherm confirmed that the pore structures in these materials are complex and tend to be made up of interconnected networks of pores of different size and shape. The addition of Ni2+ ions caused the changes in the textural properties of the samples. The influence of porosity on the electrochemical behavior of modified electrodes in quasi-reversible process was tested by cyclic voltammetry. Numerical correlations between electrochemical responses of GCE modified with alumina samples and textural properties have been established.

Redox Behavior of Yttrium and Electrochemical Formation of Y-Al Alloys in Molten Chlorides

The electrochemical behavior of yttrium and its co-deposition with aluminum were investigated by several transient electrochemical techniques on a tungsten electrode at 973K in NaCl-KCl eutectic melts. The results reveal that the reduction of Y(III) in NaCl-KCl-YCl3 melts is a one-step process with three-electron exchanged and the reaction is a quasi-reversible diffusion-controlled process at low scan rates (0.05∼0.5 V/s). The calculated diffusion coefficient is approximately 2.8 × 10−5cm2/s. After AlCl3 was introduced into the melts, cyclic voltammetry and open circuit chronopotentiometry showed the formation of two Y-Al intermetallic compounds, indicating that under-potential deposition of yttrium occurred on tungsten electrode covered with liquid Al. The electromotive force was measured at 973K to determine the thermodynamic properties of Y-Al intermetallic compounds, such as the activity of Y in the two-phase coexistence state, relative partial molar Gibbs energies, as well as the standard Gibbs energies of Y-Al intermetallic compounds. Finally, potentiostatic electrolysis was conducted to prepare Y-Al alloys from molten NaCl-KCl-YCl3 (1.5 mol%)-AlCl3 (1.5 mol%) by the co-reduction method. The cathodic alloys were characterized using X-ray diffraction (XRD) and scan electron micrograph (SEM)-energy dispersive spectrometry (EDS) and the results indicated that the obtained alloys were mainly composed of YAl2, as well as YAl3 and YAl phases. The Y-rich phase intermetallic compound YAl, formed in the later period of electrolysis just when the concentration of AlCl3 is fairly low.

Synthesis, Spectroscopic, Structural Characterization, Conductivity and Electrochemical Studies of a Schiff Base Ligand and Its Copper Complexes

Schiff base ligand (L) derived from glyoxal and 4-aminoantipyrine was synthesized. The ligand (L) has been characterized by IR, NMR, electronic spectral studies and electrochemical studies. Cu(II) complexes of a Schiff base ligand (L) from 4-aminoantpyrine and glyoxal having the composition [CuL1]X2 where X = Cl¯ or NO3¯ have been prepared and characterized by elemental analysis, electrical conductivity in non-aqueous solvent, infrared and electronic, as well as cyclic voltammetric studies. L acts as a neutral tetradentate ligand coordinating through both the carbonyl oxygen and azomethine nitrogen. On both the complexes both the anions are not coordinated. A square planar geometry is assigned for complexes. The electrochemical studies of ligand show a typical cyclic voltammogram for an irreversible process. While copper(II) complexes show the typical cyclic voltammograms for quasi reversible process.

Easily reduced bis-pincer (NS2)2molybdenum(iv) to (NHS2)2Mo(ii) by alcohols vs. redox-inert (NS2)(NHS2)iron(iii) complexes.

Iron and molybdenum complexes supported by a pincer-type dianionic [NS2]2- donor were prepared to compare their structural, spectroscopic, and electrochemical properties. The versatility of the [NS2]2Mo(iv) complex (2) to access different oxidation states was evidenced in the activation of methanol and isopropanol, oxidising them to formaldehyde or acetone with concomitant reduction and protonation to afford [NHS2]2Mo(ii), complex (3). This redox behaviour contrasts with the null reactivity observed for the analogous ferric complex [NS2][NHS2]Fe(iii) (1). Complex 2 presents a quasi-reversible process at E1/2 = -0.80 V relative to the ferrocenium/ferrocene couple (Fc+/Fc), which is attributed to the Mo(iv)/Mo(v) redox couple. Two irreversible cathodic processes were observed at Ecp = -1.59 and -2.20 V, which are attributed to the Mo(iv)/Mo(iii) and Mo(iii)/Mo(ii) redox couples. Cyclic voltammetry and solid-state structures obtained by X-ray crystallography support a 2H+ and 2e- process, whereby the Mo(iv) centre in 2 is reduced sequentially to Mo(iii), and finally to Mo(ii) in 3. These redox events were observed at Ecp = -1.22 and -2.15 V (vs. Fc+/Fc) in the anodic cyclic voltammograms of 2 in THF in the presence of acid. A new reduction peak was detected under these conditions at Ecp = -2.30 V, consistent with electrocatalytic proton reduction. This was corroborated for 2 as a catalyst precursor in the presence of increasing amounts of p-toluenesulfonic acid, with the addition of 2 to 14 equivs resulting in an increase of the current measured.

Comparing Isomeric Tridentate Carbazole-Based Click Ligands: Metal Complexes and Redox Chemistry.

Two novel bis(triazolyl)carbazole ligands Hbtc1 (3,6-di(tert-butyl)-1,8-bis[(1-(3,5-di(tert-butyl)phenyl)-1,2,3-triazol-4-yl)]-9H-carbazole) and Hbtc2 (3,6-di(tert-butyl)-1,8-bis[(4-(3,5-di(tert-butyl)phenyl)-1,2,3-triazol-1-yl)]-9H-carbazole), differing in the regiochemistry of triazole attachment, have been synthesized by Cu-catalyzed azide-alkyne cycloaddition, the so-called "click-reactions". Metalation with Ru, Zn, and Ni precursors led to the formation of M(btc)2 complexes (M=Ru, Zn, Ni), with two deprotonated ligands coordinating to the metal center in tridentate fashion, forming almost perfectly octahedral coordination spheres. The redox properties of M(btc)2 complexes have been investigated by cyclic voltammetry, UV/Vis spectroscopy, spectroelectrochemistry, and chemically. The CV of the ruthenium complexes revealed three quasi-reversible one-electron oxidation processes, one assigned as the RuII/III couple and two originating from ligand-based oxidations. The CVs of both Zn and Ni complexes contained only two oxidation waves corresponding to the oxidation of the two ligands. The oxidation potentials of complexes derived from Hbtc1 ligands were found to be 300-400 mV lower than those of the corresponding complexes derived from Hbtc2, reflecting the significant difference in donation through the N(2) or N(3) atom of the triazole moiety.

Voltammetric Characterisation of Anticancer Drug Irinotecan

AbstractElectrochemical reduction of irinotecan was investigated on a static mercury drop electrode using square‐wave voltammetry. The mechanism of irinotecan electroreduction is a complex, pH‐dependent, quasireversible process and includes the transfer of two electrons and two protons. In acidic medium, the first electron transfer reaction is followed by the chemical reaction, and the product of this chemical reaction undergoes further electrochemical reduction at more negative potentials. Both irinotecan and the product of its reduction adsorb on the mercury electrode surface. Based on the adsorptive character of irinotecan, a new adsorptive stripping square‐wave voltammetric method for its electroanalytical determination has been proposed. The voltammetric response could be used to determine irinotecan in the concentration range from 1×10−7 mol/L to 1.5×10−6 mol/L and from 5×10−9 mol/L to 1.2×10−7 mol/L, if the accumulation time is 20 s and 300 s, respectively. The calculated limit of detection for irinotecan was found to be 8.7×10−9 mol/L (if tacc=300 s).

Voltammetric determination of thujone in herbal matrices in the presence of Triton X-100

A simple, sensitive and rapid method of low concentration of thujone determination using Controlled Growth Mercury Drop Electrode and Differential Pulse Voltammetry was described. The reduction of thujone in phosphoric acid or potassium nitrate, involves a quasi-reversible and adsorption-controlled two-electron process. In voltammetric experiments the addition of ethanol and Triton X–100 is recommended.Experimental conditions, such as composition of supporting electrolyte, concentration of Triton X-100 and operation parameters were established. The optimal electrolyte should contain: 0.04 mol L−1 phosphoric acid (pH = 1.9) or 0.05 mol L−1 potassium nitrate (pH = 6.4), 0.0002% Triton X-100 and 9.6% ethanol. The calibration graph was linear from 0.7 to 140 mgL−1 of thujone in H3PO4 as supporting electrolyte and from 0.7 to 115 mgL−1 of the analyte in KNO3. The detection limit was 0.4 mgL−1 in H3PO4 and 0.6 mgL−1 in KNO3, with the correlation coefficient of 0.998. The influence of interfering substances, such as Al(III), Bi(III), Cd(II), Cr(III), Cu(II), Pb(II), Tl(I), V(III) and Zn(II) was studied. The proposed method was validated by the thujone recovery study from the specially prepared spiked herbal matrices and alcohol extract. The considered real sample was thuja oil, in which thujone was successfully determined. All measurements were performed without sample preparation.

Visible-Light Photoactive, Highly Efficient Triplet Sensitizers Based on Iodinated Aza-BODIPYs: Synthesis, Photophysics and Redox Properties.

A series of novel iodinated NO2 -substituted aza-BODIPYs have been synthesized and characterized. Highly desirable photophysical and photochemical properties were induced in NO2 -substituted aza-BODIPYs by iodination of the pyrrole rings. In particular, high values of singlet oxygen quantum yields (ΦΔ ) ranging from 0.79 to 0.85 were measured. The photooxygenation process proceeds via a Type II mechanism under the experimental conditions applied. The compounds studied exhibited an absorption band within the so-called "therapeutic window", with λmax located between 645 nm to 672 nm. They were non-fluorescent at room temperature with excited singlet-state lifetimes within the picosecond range as measured by femtosecond transient absorption. Nanosecond laser flash photolysis experiments revealed T1 →Tn absorption spanning from ca. 400 nm to ca. 500 nm and allowed determination of the triplet-state lifetimes. The estimated triplet lifetimes (τT ) in deaerated acetonitrile ranged between 2.74 μs and 3.50 μs. As estimated by CV/DPV measurements, all iodinated aza-BODIPYs studied exhibited one irreversible oxidation and two quasi-reversible reductions processes. Estimation of the EHOMO gave the value of -6.06 to -6.26 eV while the ELUMO was found to be located at ca. -4.6 eV. Thermogravimetric (TGA) analysis revealed that iodinated aza-BODIPYs were stable up to approximately 300 °C. All compounds studied exhibit high photostability in toluene solution.

Electrochemical and spectroelectrochemical study on novel non-peripherally tetra 1,2,4-triazole substituted phthalocyanines

In the present study novel tetra 4-(4-fluorophenyl)-5-(4-methoxyphenyl)-4H-1,2,4-triazole-3-thio substituted non-peripherally metal free (4), zinc(II) (5), lead (II) (6) and copper(II) (7) phthalocyanines were synthesized. The obtained novel compounds were characterized by a combination of FT-IR, 1H NMR, UV–Vis and MALDI-TOF techniques. The redox properties of the complexes have been investigated via cyclic voltammetry, square wave voltammetry and in situ spectroelectrochemistry. The compounds displayed ring-based, reversible and/or quasi-reversible reduction and oxidation processes and aggregation of the complexes influenced the redox character of the processes. The color changes during the redox processes of metallo phthalocyanine were recorded by in-situ spectroelectrochemical measurements. In situ UV–vis spectroelectrochemical measurements, which was associated with color change of the complexes, showed their applicability in the fields of the electrochemical technologies.

New Cu (II), Co(II) and Ni(II) complexes of chalcone derivatives: Synthesis, X-ray crystal structure, electrochemical properties and DFT computational studies

The reaction of nickel(II), copper(II) and cobalt(II) with 4-hydroxy-3-[(2E)-3-(1H-indol-3-yl)prop-2-enoyl]-6-methyl-2H-pyran-2-one (HL) leads to a series of new complexes: Ni(L)2(NH3), Cu(L)2(DMF)2 and Co(L)2(H2O). The crystal structure of the Cu(L)2(DMF)2 complex have been determined by X-ray diffraction methods. The Cu(II) lying on an inversion centre is coordinated to six oxygen atoms forming an octahedral elongated. Additionally, the electrochemical behavior of the metal complexes were investigated by cyclic voltammetry at a glassy carbon electrode (GC) in CH3CN solutions, showing the quasi-reversible redox process ascribed to the reduction of the MII/MI couples. The X-ray single crystal structure data of the complex was matched excellently with the optimized monomer structure of the desired compound; Hirschfeld surface analysis supported the packed crystal lattice 3D network intermolecular forces. HOMO/LUMO energy level and the global reactivity descriptors quantum parameters are also calculated. The electrophilic and nucleophilic potions in the complex surface are theoretically evaluated by molecular electrostatic potential and Mulliken atomic charges analysis.

A comparative study of the cathodic behaviour of EuCl3 in two imidazolium chloride ionic liquids, the 1-butyl-3-methylimidazolium (C4mimCl) and the 1-ethyl-3-methylimidazolium (C2mimCl), on a glassy carbon electrode

This work presents a comparative study of the electrochemical behaviour, on a glassy carbon electrode, of dissolved EuCl3 in two chlorobasic ionic liquids, the 1-ethyl-3-methylimidazolium chloride, C2mimCl, and the 1-buthyl-3-methylimidazolium chloride, C4mimCl, over the temperature ranges 363–398K and 343–363K respectively. In both media, the electro-reduction of EuCl63− takes place via only one electrochemical step EuCl63−/EuCl64−. The electrochemical system EuCl64−/Eu(0) has not been observed within the electrochemical window of both ILs, due to the prior reduction of the respective imidazolium cation from the solvents, which inhibits the electro-extraction of Eu(0) from the media on the GC electrode.The paper describes how several electrochemical techniques (e.g. cyclic voltammetry (CV), convolutive potential sweep voltammetry (CPSV), steady state voltammetry (SSV), chronopotentiometry (CP) and chronoamperometry (CA)) have been used experimentally: i) verifying assumptions used in their theoretical analysis, and ii) ensuring reproducible conditions at the electrode/electrolyte interface, with the aim to obtain high-precision measurements of the diffusion coefficient of EuCl63−. In order to know if there are discrepancies between the obtained data, a two-way analysis of variance, ANOVA, has been carried out. The analysis has shown that with a 95% confidence level there are no significant differences between the diffusion coefficients obtained by the different techniques. On the other hand, the diffusion coefficient of EuCl63− increases with the temperature following the Arrhenius law, being the activation energy for diffusion 40.4±2.6 and 60.2±1.8kJmol−1 in C2mimCl and C4mimCl respectively. The dimensionless Schmidt numbers, defined as the ratio between solvent viscosity and solute diffusivity (Sc=ν/D), have also been calculated to characterise the solute global mass transport through its environment.On the GC electrode, the electro-reduction of EuCl63− to EuCl64− is a quasi-reversible process. Accurate values of the kinetic parameters (i.e. the intrinsic rate constant of charge transfer, k0, and the charge transfer coefficient, α), as well as the reversible half wave potential, Er1/2, have been obtained for the first time in the mentioned ionic liquids, by simulation of the cyclic voltammograms and logarithmic analysis of the voltammograms and convoluted curves.

Oxidation State-Dependent Intramolecular Electronic Interaction of Carbazole-Based Azacyclophanes with 9,10-Anthrylene Units.

We designed an azacyclophane comprising 3,6-substituted carbazole and 9,10-anthrylene units to investigate the unique electronic properties of the oxidation states arising from the presence of multiple oxidizable azacyclophane. This compound and its analogue containing 1,4-phenylene in substitution of 9,10-anthrylene units were synthesized by Buchwald-Hartwig coupling reactions. Cyclic voltammograms of both the 9,10-anthrylene and 1,4-phenylene derivatives exhibited four quasi-reversible oxidation processes. The absorption spectra of the oxidation states of the 9,10-anthrylene derivative exhibited broad absorption bands in the near-infrared region arising from charge-resonance and intramolecular charge-transfer interactions. The photophysical and electrochemical properties of the 9,10-anthrylene derivative were compared with those of the corresponding 1,4-phenylene derivative on the basis of theoretical calculations to evaluate the intramolecular electronic interactions.

Voltammetric determination of thujone by a renewable silver amalgam film electrode in herbal matrices and thuja oil

Thujone is a ketone and a dicyclic monoterpene which naturally occurs in a variety of common plants, but in large quantities may be toxic for humans. In this work a simple and rapid DPV methodology of thujone determination on renewable silver amalgam film electrode is proposed. It was observed that the reduction of thujone in nitric acid is a quasi-reversible two-electron process, while the reaction was controlled by surface phenomena. Experimental conditions such as composition of the supporting electrolyte and operation parameters were optimized. The promising results were obtained in 0.0115molL−1 HNO3 (pH1.85) and 9.6% ethanol. The calibration graph was linear from 0.7–142mgL−1 with correlation coefficient r=0.9996. Obtained sensitivity was 0.0724μA/mgL−1, while detection limit 0.05mgL−1. In the presence of copper the sensitivity was reduced to 0.0504μA/mgL−1. Procedure was verified using complex herbal matrices spiked by the thujone. Recovery of the low analyte concentration i.e. 0.7–2.1mgL−1 significantly depends on the level of the interferences. A self-referencing strategy of thujone determination in thuja oil was also developed and applied in exploration of thujone without preparation of the sample in commercially available product.