Electrochemical Reduction Potentials Research Articles

Light-Stimulated Generation of Free Radicals by Quinones-Chelators

Abstract The role of metal ions in the mechanism of light-stimulated redox activity of potential anticancer agent 2-phenyl-4-(butylamino)naphtha[2,3-h]quinoline-7,12-dione (Qc) has been studied by CIDNP (chemically induced dynamic nuclear polarization) and EPR methods. The photo-induced oxidation of NADH and its synthetic analog – substituted dihydropyridine (DHP) – by quinone Qc was used as a model. The Qc capability of producing chelating complexes with divalent metal ions of Fe, Zn and Ca was studied quantitatively by optical absorption spectroscopy. A significant decrease of electrochemical reduction potential of Qc (ΔE=0.4−0.6 eV for ACN and ACN/PBS solutions) in chelating complexes and in protonated form of Qc was observed. A pronounced increase in efficiency of DHP oxidation in chelating complexes with Zn2+ and Ca2+ ions compared with free Qc was demonstrated. The yields of free radicals, including reactive oxygen species (ROS) and reaction products, were a few times higher than those in the absence of metal ions. Application of such chelating compounds to enhance ROS generation looks very promising for anti-cancer therapy, including the photodynamic therapy.

Synthesis and electrical properties of copolymers of o‐phenylenediamine with aniline, 3,4‐ethylenedioxythiophene and 2,3,5,6‐tetrafluoroaniline

AbstractCopolymers (P(PDA/Ar)) of o‐phenylenediamine with aniline (Ar = ANi), 3,4‐ethylenedioxythiophene (Ar = EDOT) and 2,3,5,6‐tetrafluoroaniline (Ar = TFANi) were synthesized via polycondensation initiated by ammonium persulfate. The NH2 group content in the copolymers was determined by analyzing the 1H NMR spectra of the N‐acetylated copolymers. Copolymers crosslinked by viologen (1,1'‐disubstituted 4,4'‐bipyridinium dichloride) were obtained by reaction involving the reactive NH2 groups in the copolymers. The absorption wavelengths of solutions of the copolymers and the electrochemical oxidation and reduction potentials of cast films of the copolymers were affected by the electrical properties of the Ar unit. © 2016 Society of Chemical Industry

Synthesis, structure, electrochemistry, and photophysics of 2,5-dibenzylidenecyclopentanones containing in benzene rings substituents different in polarity

A series of cross-conjugated dienones was synthesized to study the dependence of physicochemical characteristics on the nature of substituents in the aromatic groups of symmetric cyclopentanone dibenzylidene derivatives. The structure of compounds was established by electronic, IR, and NMR spectroscopy and X-ray diffraction study. All the compounds obtained possess the E,E-geometry. In the crystalline state, the arrangement of the dienone molecules is unfavorable for the intermolecular [2+2] photocycloaddition to take place. The low-temperature phases transition for unsubstituted diphenyl derivative of cyclopentanone was detected using variable-temperature X-ray diffraction and differential scanning calorimetry. Oxidation and reduction potentials of the dienones were measured by cyclic voltammetry. Their dependence on the nature and placement of substituents in the benzene rings was demonstrated. A linear correlation (R = 0.9343) between the difference of electrochemical oxidation and reduction potentials and the energy of the long-wavelength absorption maximum was found, that allows us to recommend the use of the data obtained in the correlation analysis of other compounds of this class.

Linear free energy relationships of electrochemical and thermodynamic parameters for the electrochemical reductive dechlorination of chlorinated volatile organic compounds (Cl-VOCs)

Chlorinated volatile organic compounds (Cl-VOCs) belong to a group of the most ubiquitous contaminants in the environment that can be widely found in all kinds of polluted media. Electrochemical reductive dehalogenation is regarded as one of the most promising remediation methods toward these pollutants, but despite all the works that have been done on the kinetics or the dehalogenation mechanisms, few studies on the linear free energy relationships (LFERs) for explaining or predicting the electrochemical dehalogenation reactivity and dehalogenation mechanisms of Cl-VOCs have been conducted. In this study, we successfully developed several LFERs based on the electrochemical parameter (electrochemical reduction potential, Ep) with the four thermodynamic parameters associated with molecular structure (lowest un-occupied molecular orbital (LUMO) energy, C-Cl bond dissociation energy (BDE), one- (E1) and two- (E2) electron reduction potentials) to describe the dehalogenation reactivity and dehalogenation mechanisms of Cl-VOCs. Results clearly demonstrate that a good linear correlation of Ep with LUMO energy is obtained regardless the classes of Cl-VOCs, while separate correlations of Ep against BDE or E2 can give rise to three satisfactory linear relationships according to the dehalogenation mechanisms. Hence, the developed LFERs in this study may provide a new insight in predicting the dehalogenation reactivity and dehalogenation mechanisms for Cl-VOCs and other halocarbons.

Substituent Effects on Reduction Potentials of Meta-substituted and Para-substituted Benzylideneanilines

Effects of meta-substituent of 3,4′/4,3′/3,3′-substituted benzylideneanilines (XBAYs) on the electrochemical reduction potentials (E(Red)) were investigated, in which 49 samples of target compounds were synthesized, and their reduction potentials were measured by cyclic voltammetry. The substituent effects on the E(Red) of target compounds were analyzed and an optimality equation with four parameters (Hammett constant σ of X, Hammett constant σ of Y, excited-state substituent constant σCCex of X, and the substituent specific cross-interaction effect ΔσCCex 2 between X and Y) was obtained. The results show that the factors affecting the E(Red) of 3,4′/4,3′/3,3′-substituted XBAYs are different from those of 4,4′-substituted XBAYs. For 3,4′/4,3′/3,3′-substituted XBAYs, σ(X) and σ(Y) must be employed, and the contribution of ΔσCCex 2 is important and not negligible. Compared with 4,4′-substituted XBAYs, X group contributes less to 3,4′/4,3′/3,3′-substituted XBAYs, while Y group contributes more to them. Additionally, it was observed that either para-substituted XBAYs or meta-substituted XBAYs, the substituent effects of X are larger than those of Y on the E(Red) of substituted XBAYs.

The impact of electrochemical reduction potentials on the electrocatalytic activity of graphene oxide toward the oxygen reduction reaction in an alkaline medium

We report the synthesis of graphene via the electrochemical reduction of graphene oxide (GO). In this study, GO nanosheets from aqueous dispersion were pre-assembled on a glassy carbon (GC) electrode and then electrochemically reduced in 1M KOH under various constant reduction potentials in the range of −0.6V to −1.5V (vs. Ag/AgCl). X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy analyses revealed that the graphitic structure was substantially restored in the resulting electrochemically reduced graphene oxide (ERGO). The ERGO electrodes exhibited significantly enhanced catalytic activity toward the oxygen reduction reaction (ORR) in an alkaline medium compared with the initial GO electrode. Of the ERGO electrodes produced at various cathodic potentials, the ERGO-1.2V electrode, which was produced at a reduction potential of −1.2V, demonstrated the best catalytic activity toward the ORR in an alkaline medium. The ORR on GO and ERGO electrodes was shown to proceed via a two-electron mechanism at low overpotentials. The agreement between the spectroscopy results and electrochemical measurements provide strong evidence that the enhanced ORR catalytic activity is mainly attributed to the restoration of GO’s graphitic structure. Furthermore, the ERGO-1.2V electrode showed excellent tolerance to the methanol poisoning effect compared with a Pt/C catalyst electrode.

Structures and structure-related electronic properties of new C60(CF3)10 isomers

Abstract The largest collection of well-characterized homoleptic fullerene derivatives C60(X)n is for X = CF3 and n = 2–18. The largest subset of these with a given composition is the set of 27 isomers of C60(CF3)10. The X-ray structures of five of them were reported previously. In this paper we report the X-ray structures of, and extensive DFT calculations on, eight additional C60(CF3)10 isomers and the electrochemical reduction potentials of eight of the (now) 13 structurally-characterized isomers. The connections between their molecular structures (i.e., their addition patterns), the positions, extents of delocalization, and relative energies of their DFT-predicted LUMOs, and their first reduction potentials are compared and contrasted with each other and with the previously reported isomers. It is shown that addition patterns that produce the greatest number of non-terminal double bonds in fullerene pentagons are the best electron acceptors and tend to exhibit relatively small HOMO–LUMO gaps. A detailed analysis of the nearest-neighbor packing patterns in the 13 X-ray structures reveals significant differences in the number of nearest neighbor fullerene molecules and differences in the ranges of fullerene centroid⋯centroid distances even when the number of nearest neighbors is constant. Nevertheless, ten solvent-free structures exhibit nearly equally-efficient lattice packing arrays as far as crystal density is concerned. When the addition pattern results in two sterically-accessible C(sp2)6 hexagons on the fullerene surface, a one-dimensional chain of electronically-coupled nearest-neighbor C60(CF3)10 molecules is formed.

Incremental Tuning Up of Fluorous Phenazine Acceptors.

In a simple, one-step direct trifluoromethylation of phenazine with CF3 I we prepared and characterized nine (poly)trifluoromethyl derivatives with up to six CF3 groups. The electrochemical reduction potentials and gas-phase electron affinities show a direct, strict linear relation to the number of CF3 groups, with phenazine(CF3)6 reaching a record-high electron affinity of 3.24 eV among perfluoroalkylated polyaromatics.

Effect of Sterical Shielding on the Redox Properties of Imidazoline and Imidazolidine Nitroxides.

The oxidant properties of the series of 2,2,5,5-tetraalkyl imidazoline and imidazolidine nitroxides were investigated. An increase in the number of bulky alkyl substituents leads to a decrease in the rate of reduction with ascorbate, which makes the electrochemical reduction potential more negative and shifts the equilibrium in the mixture of nitroxide and reference hydroxylamine (3-carboxy-1-hydroxy-2,2,5,5-tetramethylpyrrolidine-1-oxyl-1-(15)N) toward the starting compounds. The effect of structural factors on these reactions was analyzed by means of multiple regression using the Fujita steric constant Es and the inductive Hammett constant σI. Satisfactory statistical outputs were obtained in all of the biparameter correlations, denoting that the oxidant properties of the nitroxides are determined by steric and electronic effects of the substituents. The data imply that bulky substituents can stabilize nitroxide and/or destabilize hydroxylamine.

Simple colorimetric chemodosimeters for selective sensing of cyanide ion in aqueous solution via termination of ICT transition by Michael addition

Three simple ferrocene based chemodosimeters for cyanide ion in aqueous solution have been prepared by one step condensation of commercially available starting materials and characterized spectroscopically. The receptors were designed in such a way that the intra molecular charge transfer (ICT) transition existing between the donor (ferrocene) and acceptor (cyano olefine) moieties imparts an intense pink color to the receptors (logɛ=2.92–3.06). The results of UV–vis, fluorescence and 1H and 13C NMR spectral studies indicated that the sensing mechanism involves Michael addition of CN− to the electropositively polarized C-atom of the CC bond. Such an addition reaction leads to “turn-off” of ICT transition and thus leading to a striking color change from pink to colorless. The observed variation in the electrochemical reduction potential (Epc) for the reduction of ferrocene moiety and the formation constants of the 1:1 receptor–CN− ion complexes could be explained using the Taft's polar substituent constants (σ*) of the substituents attached to the acceptor moiety. Moreover, the test strips fabricated using these receptors could instantaneously sense cyanide ion present in aqueous NaCN solution.

HOMO-LUMO energy gap control in platinum(II) biphenyl complexes containing 2,2'-bipyridine ligands.

A series of platinum(ii) biphenyl 2,2'-bipyridine complexes containing electron-donating and electron-withdrawing moieties on the 4 and 4' positions of the bipyridine ligand exhibit emission from excited states in the 600 nm region of the spectrum upon excitation in the metal-to-ligand charge transfer transition located near 450 nm. These complexes are distorted from planarity based on both single crystal structure determinations and density functional theory (DFT) calculations of isolated molecules in acetonitrile. The DFT also reveals the geometry of the lowest-lying triplet state (LLTS) of each complex that is important for emission behavior. The LLTS are assigned based on the electron spin density distributions and correlated with the singlet excited states to understand the mechanism of electronic excitation and relaxation. Time-dependent DFT calculations are performed to compute the singlet excited state energies of these complexes so as to help interpret their UV-Vis absorption spectra. Computational and experimental results, including absorption and emission energy maxima, electrochemical reduction potentials, LLTS, singlet excited states, and LUMO and HOMO energies, exhibit linear correlations with the Hammett constants for para-substituents σp. These correlations are employed to screen complexes that have not yet been synthesized. The correlation analysis indicates that the electronic structure and the HOMO-LUMO energy gap in Pt(ii) complexes can be effectively controlled using electron-donating and electron-withdrawing moieties covalently bonded to the ligands. The information presented in this paper provides a better understanding of the fundamental electronic and thermodynamic behavior of these complexes and could be used to design systems with specific applications.

Effect of Fluorosurfactant Additive during Cu-Sn Codeposition from Methanesulfonic Acid

Methanesulfonic acid (MSA) is an interesting supporting electrolyte with many desirable properties such as high salt solubility, high conductivity, low corrosivity and toxicity. Various additives or complexing agents such as brighteners, antioxidants, and surfactants are required for deposition of Sn and Cu-Sn alloys. It has been shown that the simplest electrolyte for successful Cu-Sn deposition can contain just an antioxidant and a fluorosurfactant. In this work we have further examined the role of fluorosurfactant in shifting the electrochemical reduction potential for the more noble metal. The surface adsorption and desorption processes of the surfactant has been inferred through the use of an electrochemical quartz crystal nanobalance (EQCN). Cu and Sn have been deposited individually and simultaneously from MSA electrolytes. Mass changes at the quartz crystal have been compared against those calculated from charge consumption. These data show that in MSA electrolytes the adsorption and desorption of the fluorosurfactant to the surface is potential dependent and it suppresses Cu deposition but facilitates Cu-Sn alloy deposition.

Controlling the Catalytic Efficiency on the Surface of Hollow Gold Nanoparticles by Introducing an Inner Thin Layer of Platinum or Palladium

The efficiency of heterogeneous catalysis of electron-transfer reactions on the surface of gold nanoshells was changed by adding an inner platinum or palladium nanoshell in the double-shell nanocatalysts. The reduction of 4-nitrothiophenol (4NTP) by borohydride was studied as a model reaction. To confirm the heterogeneous catalytic mechanism, the nanocatalysts were assembled into a monolayer on the surface of a quartz substrate using the Langmuir-Blodgett technique, and the 4NTP was allowed to bind to the surface of gold through a strong thiol bond. The stages of the reduction reaction of 4NTP on the surface of gold were successfully followed by time-resolved surface-enhanced Raman spectroscopy. Palladium was found to increase the catalytic efficiency of the gold surface due to the presence of a new Fermi level of the palladium-gold alloy, while platinum decreased its catalytic efficiency due to the electron-withdrawing effect of platinum atoms, which resulted from the difference in their electrochemical reduction potentials.

1-Phenylselanylazulenes: synthesis and selenium atom oxidation

The phenylselanylation of azulene with PhSeCl or PhSeBr was reported. Clean electrophilic substitution of azulene in positions 1 and/or 3 occurred with the first reagent whereas with PhSeBr a complex mixture of products was generated suggesting a radicalic mechanism. The influence on the reaction route of the substituent attached at position 1 of azulene, or of the alkyl positions in alkylated azulenes was investigated. 1-Phenylselanylazulene and 1,3-bis(phenylselanyl)azulene were oxidized using sodium metaperiodate. The oxidation took place at selenium atom and afforded azulenes substituted with PhSe(O) and/or PhSe(O2) groups in positions 1 and/or 3. The magnetic and electronic spectra as well as the electrochemical oxidation and reduction potentials of the obtained products were recorded and the results were briefly discussed. .

Electrochemical and UV–visible spectroscopy studies of K6As2W18 − xMoxO62 (x = 0–3) Wells–Dawson heteropolyacid catalysts for oxidative dehydrogenation of benzyl alcohol

Redox behaviors of molybdenum-substituted Wells–Dawson tungstoarsenate heteropolyacid catalysts (α-K6As2W18−xMoxO62 (x=0–3)) were probed by electrochemical and UV-visible spectroscopy measurements. Reduction potential of K6As2W18−xMoxO62 (x=0–3) catalysts shifted to more positive potential with increasing molybdenum content. UV–visible spectroscopy analysis was conducted as a simple diagnostic method for reduction potential. Absorption edge energy determined by UV–visible spectroscopy decreased with increasing reduction potential. Gas-phase oxidative dehydrogenation of benzyl alcohol was carried out as a model reaction. Yield for benzaldehyde increased with increasing electrochemical reduction potential and with decreasing UV–visible absorption edge energy of the catalysts.

Electrochemical reduction of ciprofloxacin at the mercury electrode and its voltammetric determination in tablet and urine

In this study a reduction square wave voltammetric method was developed and validated for the direct determination of ciprofloxacin (CIP) in pharmaceutical formulation and biological fluid using hanging mercury dropping electrode (HMDE) surface. Best results were obtained for the quantitative determination of CIP in 0.02 M Britton-Robinson buffer at pH 2.5 and at a potential of −1300 mV vs. Ag/AgCl reference electrode. Various experimental and instrumental parameters affecting the peak current and potential of CIP electrochemical reduction were investigated and optimized. The monitored peak current was directly proportional to the concentration of CIP, where it exhibited a linear response in the range 3.0 × 10−7–2 × 10−6 M (r = 0.99). The accuracy of the proposed method was concluded based on the value of mean recovery of 98 ± 0.72 % with RSD of 0.181 % at a detection limit of 7 × 10−9 M. Possible interferences by various substances usually present in pharmaceutical formulations have been also evaluated. After validating the proposed method, the applicability of this voltammetric method was demonstrated by estimating CIP in its pharmaceutical formulation and spiked human urine, where values of mean recoveries of 97 ± 1.0% and 108.0 ± 2.0% were obtained, respectively.

Extending Photoinduced Charge Separation Lifetimes by Using Supramolecular Design: Guanine–Perylenediimide G-Quadruplex

We report here a potassium-induced guanine quadruplex as a supramolecular platform for controlled assembly of electron donor-acceptor systems. A monodisperse, C4-symmetric octamer of a guanine-perylene-3,4,9,10-bis(dicarboximide) conjugate (GPDI) was prepared in tetrahydrofuran. The two layers of cyclic guanine tetramers have the same direction of rotation, and the PDI moiety between the layers adopts a nearly eclipsed relationship (H-aggregation), as revealed by small- and wide-angle X-ray scattering, NMR spectroscopy, and steady-state UV/vis absorption. Following photoexcitation of the PDI moiety in the quadruplex, charge separation occurs in τCS = 98 ± 12 ps to give G(+•)-PDI(-•) that recombines in τCR = 1.2 ± 0.2 ns, which is >100 times longer than that in the monomeric GPDI dyad. The transient absorption spectrum of G(+•)-PDI(-•) within the GPDI quadruplex suggests the formation of a radical anion delocalized over the neighboring PDI units, and this result is consistent with the more favorable electrochemical reduction potential for PDIs in the quadruplex relative to the monomer.

SPECIAL FEATURES OF THE ELECTROCHEMICAL OXIDATION OF SUBSTITUTED 4-CARBOXY-1,4-DIHYDROPYRIDINES

Electrochemical oxidation potentials of 1,4-dihydropyridines substituted with 4-COOH, 4-COOR, and 4‑CONRR' groups have been determined in aprotic acetonitrile by the rotating ring-disk electrode method (RRDE). The electrochemical reduction potentials of the resulting products were also determined at the ring electrode. It was established that protonated pyridines are formed in the oxidation of derivatives with and without a substituent in position 4 of the heterocycle. In the case of 4-alkoxycarbonyl substituted compounds the substituent at position 4 is generally retained. 1,4-Dihydrogenated derivatives of isonicotinic acid as a rule loose the substituent at position 4 on oxidation, and both types of product were recorded for the corresponding 4-carbamoyl derivatives. The substituent at position 9 of the heterocycle was mainly retained on electrochemical oxidation of the 3,3,6,6-tetramethyl-1,8-dioxo-1,2,3,4,5,6,7,8,9,10-decahydroacridine derivatives studied. English Translation in Chemistry of Heterocyclic Compounds , 2000, 36 (10), pp 1177-1184 http://link.springer.com/article/10.1023/A%3A1002868600011 Authors: J. Stradins, L. Baumane, A. Kalnins, J. Uldrikis, E. Bisenieks, J. Poikans, and G. Duburs

Steric and Electronic Effects on the Configurational Stability of Residual Chiral Phosphorus‐Centered Three‐Bladed Propellers: Tris‐aryl Phosphane Oxides

A series of tris-aryl phosphane oxides existing as residual enantiomers or diastereoisomers with substituents on the aryl rings differing in size and electronic properties were synthesized and characterized. Their electronic properties were evaluated on the basis of their electrochemical oxidation and reduction potentials together with those of the corresponding "blade bromides" (i.e., the naphthalene derivatives displaying the same substitution pattern of the tris-naphthyl phosphane oxide blades, with a bromo substituent where the phosphorus atom is located) determined by CV. The residual stereoisomeric phosphane oxides were isolated in a stereochemically pure state and were found to be highly configurationally stable at room temperature (stereoisomerization barriers of about 27 kcal mol(-1)). The chiroptical properties of the residual stereoisomers and the assignments of absolute configuration are discussed. The configurational stability of residual tris-aryl phosphane oxides was found to be scarcely influenced by the electronic properties of the substituents present on the aromatic rings constituting the blades, while steric effects play the most relevant role. Detailed theoretical calculations are in agreement with the experimental results and also contribute to a rational interpretation of the stereodynamics of these systems.

Physicochemical characterization of environmental mutagens: 3-nitro-6-azabenzo[a]pyrene and its N-oxide derivative

Molecular geometries, nucleus independent chemical shifts, electronic, infrared and Raman spectra, vertical ionization energy and electron affinity, dipole moment, and electronic polarizabilities of 3-nitro-6-aza- benzo(a)pyrene and its N-oxide derivative were calculated for the gas phase using semiempirical and density functional theory. The influence of the N-oxidation on structural, elec- tronic, and spectroscopic properties is discussed. The infrared spectral region around 1,300 cm -1 involving the azabenzene N-O and the NO2 stretching vibrations is potentially useful to distinguish 3-nitro-6-azabenzo(a)pyrene from its N-oxide derivative. The calculated vertical ionization energy for 3-nitro-6-azabenzo(a)pyrene N-oxide is smaller than that of its parent compound, whereas the vertical electron affinity is higher in agreement with the experimental electrochemical reduction potentials. Both dipole moment and polarizability increase on passing from 3-nitro-6-azabenzo(a)pyrene to its N-oxide derivative, the contribution from the N-O bond formation being substantial. The electric property enhance- ments are explained through atomic charges and spectral shifts. Present results support the higher mutagenic activity for the N-oxide derivative in comparison with its parent compound. Keywords 3-Nitro-6-azabenzo(a)pyrene � 3-Nitro-6-azabenzo(a)pyrene N-oxide � Nitro-polycyclic aromatic hydrocarbons � Mutagenic activityDFT calculations