Positive Oxidation Potential Research Articles

Structure and Redox Behavior of Iron Oxophlorin and Role of Electron Transfer in the Heme Degradation Process

Iron-oxophlorin is an intermediate in heme degradation, and the metal oxidation number can alter spin, electron distribution, and the reactivity of the metal and the oxophlorin ring. The role of electron transfer in the structure and reactivity of [(Py)(2)Fe(III)(PO)] (PO is the oxophlorin trianion) in different redox states has been investigated using the B3LYP and OPBE methods with the 6-31+G* and 6-311+G** basis sets. A computation study has shown that [(py)(2) Fe(III)(PO)] loses one electron from its a(2u) orbital. Thus the oxidized species, [(Py)(2)Fe(III)(PO(•))](+) (where PO(•) is the oxophlorin dianion radical), has an open-shell-singlet ground state with a d(xy)(2) d(xz)(2) a(2u)(1) d(yz)(1) electronic configuration with closely lying triplet and quintet states which are populated at ambient temperature. The aforementioned complex is highly reactive toward O(2). The reduced species [(Py)(2)Fe(II)(POH)] (where POH is the hydroxyheme) has the closed-shell-singlet ground state (π(xz) π(yz))(4) a(2u)(2) d(xy)(2) electronic configuration in which pyridines have a more π-accepting character and, thus, are tightly bound to iron. This reduced form is considerably less reactive toward O(2). The axial ligands effects (Im, t-BuNC) have also been studied in redox reactions of iron oxophlorin complexes. Complex [(Im)(2)Fe(III)(PO)] shows facile oxidation to form a cation radical and a reduction to form hydroxy while the [(t-BuNC)(2)Fe(II)(PO(•))] has high positive oxidation potential.

Electrogenerated chemiluminescence energy transfer for the label-free detection of DNA

Abstract Photobleaching or DNA damage from more positive oxidation potential existed in DNA sensors using intercalators as fluorescent or electrogenerated chemiluminescent (ECL) probes. Herein, to solve the above problems, a new ECL energy transfer system was developed. Crystal violet (CV) intercalated into double strand DNA (dsDNA) was used as the energy receptor with excited luminol as the energy donor. ECL reaction of luminol was triggered by step pulse signal and excited luminol molecules transferred their energy to CV molecules. Excited CV molecules while returning to ground state will release energy, thereby emitting photons. The hybridization event was recognized by the different ECL energy transfer efficiencies due to different interaction abilities of single-stranded DNA (ssDNA) and dsDNA with CV. Thus, the ECL signal of CV-ssDNA was different from that of CV-dsDNA. Accordingly, the target DNA sequence could be identified and the target DNA concentration could be detected. The prominent advantage of this developed ECL energy transfer method was that the photobleaching effect in fluorescence and the high potential damage to DNA in electrochemical and ECL methods were avoided. Furthermore, probe DNA did not need to be labeled, which made the measurement of DNA simple, cheap and quick.

Formation, surface characterization, and electrocatalytic application of self-assembled monolayer films of tetra-substituted manganese, iron, and cobalt benzylthio phthalocyanine complexes

Molecular thin films of manganese (SAM-2), iron (SAM-3), and cobalt (SAM-4) phthalocyanine complexes, non-peripherally tetra-substituted with benzylmercapto, were formed on polycrystalline gold disc electrode by self-assembly technique. Surface characteristics of the films were interrogated by cyclic voltammetry. Significant passivation of voltammetry processes associated with bare gold surface (gold oxidation and underpotential deposition of copper) confirmed formation of the films. Electrocatalytic property of the films was evidenced from better voltammetry responses (less positive oxidation potential and better current signal) of the insecticide, carbofuran, on these films, relative to that on bare gold electrode. In terms of less positive oxidation potential, the FePc derivative (3) gave the best response, while the best current signal was observed on SAM-2-modified gold electrode. The average heterogeneous rate constant, k, for the oxidation of carbofuran was 3.6 × 10−2 cm s−1 on the SAM film with the best current signal (SAM-2).

Surface properties of self-assembled monolayer films of tetra-substituted cobalt, iron and manganese alkylthio phthalocyanine complexes

Self-assembled monolayer (SAM) films of iron (SAM- 1), cobalt (SAM- 2) and manganese (SAM- 3) phthalocyanine complexes, tetra-substituted with diethylaminoethanethio at the non-peripheral positions, were formed on gold electrode in dimethylformamide (DMF). Electrochemical, impedimentary and surface properties of the SAM films were investigated. Cyclic voltammetry was used to investigate the electrochemical properties of the films. Ability of the films to inhibit common faradaic processes on bare gold surface (gold oxidation, solution redox chemistry of [Fe(H 2O) 6] 3+/[Fe(H 2O) 6] 2+ and underpotential deposition (UDP) of copper) was investigated. Electrochemical impedance spectroscopy (EIS), using [Fe(CN) 6] 3−/4− redox process as a probe, offered insights into the electrical properties of the films/electrode interfaces. Surface properties of the films were probed using atomic force microscopy (AFM) and scanning electron microscopy (SEM). The films were employed for the electrocatalytic oxidation of the pesticide, carbofuran. Electrocatalysis was evidenced from enhanced current signal and less positive oxidation potential of the pesticide on each film, relative to that observed on the bare gold electrode. Mechanism of electrocatalytic oxidation of the pesticide was studied using rotating disc electrode voltammetry.

Structural, Spectroscopic, and Multiconfigurational Quantum Chemical Investigations of the Electron-Rich Metal−Metal Triple-Bonded Tc2X4(PMe3)4 (X = Cl, Br) Complexes

The compounds Tc(2)Cl(4)(PMe(3))(4) and Tc(2)Br(4)(PMe(3))(4) were formed from the reaction between (n-Bu(4)N)(2)Tc(2)X(8) (X = Cl, Br) and trimethylphosphine. The Tc(II) dinuclear species were characterized by single-crystal XRD, UV-visible spectroscopy, and cyclic voltammetry techniques, and the results are compared to those obtained from density functional theory and multiconfigurational (CASSCF/CASPT2) quantum chemical studies. The compound Tc(2)Cl(4)(PMe(3))(4) crystallizes in the monoclinic space group C2/c [a = 17.9995(9) A, b = 9.1821(5) A, c = 17.0090(9) A, beta = 115.4530(10) degrees ] and is isostructural to M(2)Cl(4)(PMe(3))(4) (M = Re, Mo, W) and to Tc(2)Br(4)(PMe(3))(4). The metal-metal distance (2.1318(2) A) is similar to the one found in Tc(2)Br(4)(PMe(3))(4) (2.1316(5) A). The calculated molecular structures of the ground states are in excellent agreement with the structures determined experimentally. Calculations of effective bond orders for Tc(2)X(8)(2-) and Tc(2)X(4)(PMe(3))(4) (X = Cl, Br) indicate stronger pi bonds in the Tc(2)(4+) core than in Tc(2)(6+) core. The electronic spectra were recorded in benzene and show a series of low intensity bands in the range 10 000-26 000 cm(-1). Assignment of the bands as well as computing their excitation energies and intensities were performed at both TD-DFT and CASSCF/CASPT2 levels of theory. Calculations predict that the lowest energy band corresponds to the delta* --> sigma* transition, the difference between calculated and experimental values being 228 cm(-1) for X = Cl and 866 cm(-1) for X = Br. The next bands are attributed to delta* --> pi*, delta --> sigma*, and delta --> pi* transitions. The cyclic voltammograms exhibit two reversible waves and indicate that Tc(2)Br(4)(PMe(3))(4) exhibits more positive oxidation potentials than Tc(2)Cl(4)(PMe(3))(4.) This phenomenon is discussed and ascribed to stronger metal (d) to halide (d) back bonding in the bromo complex. Further analysis indicates that Tc(II) dinuclear species containing pi-acidic phosphines are more difficult to oxidize, and a correlation between oxidation potential and phosphine acidity was established.

Synthesis, structures, optical and electrochemical properties, and complexation of 2,5-bis(pyrrol-2-yl)phospholes

Abstract 2,5-Bis(pyrrol-2-yl)phosphole derivatives were prepared using the reaction of titanacycles, generated in situ from a TiII reagent and pyrrole-capped 1,6-heptadiynes, with dichloro(phenyl)phosphine. The 2,5-bis(pyrrol-2-yl)phosphole derivatives were found to possess narrower optical HOMO–LUMO gaps and less positive oxidation potentials than those of previously reported 2,5-diarylphosphole analogs. This demonstrates that the intrinsic nature of the electron excessive pyrrole subunits as well as the effective π-conjugation over the coplanar heterole rings. The σ3-P type 2,5-bis(5-phenylpyrrol-2-yl)phosphole underwent complexation with AuCl(SMe2) and PtCl2 to yield the respective AuCl–monophosphine and PtCl2–bisphosphine complexes. In the square planar PtII complex, the pyrrolic NH protons were found to form intramolecular hydrogen bonds with the chlorine atoms that gave rise to symmetrically split, parallel π-chromophores linked by two Pt–P bonds. Density functional theory calculations on a PtII model complex suggested that this cooperative interaction induces a significant split of the original LUMOs of the symmetrical π-conjugated ligands.

Theoretical study of indoline dyes for dye-sensitized solar cells

Indoline dye sensitizers were designed and studied theoretically to increase molar extinction coefficients in the visible to near infrared region for solar-cell devices. To gain insight into dye sensitizers' structural, electronic, and optical properties, DFT/TDDFT calculations were performed on a series of dye sensitizers derived from the D149. The good agreement between the experimental and TDDFT calculated absorption spectra of the D149 sensitizer allowed us to provide a detailed assessment of the main spectral features of a series of dye sensitizers. Increase in the conjugation length resulted in a more red-shifted spectral response and less positive oxidation potential than that of the D149. The dye with the dimethylfluorene group showed stronger absorption bands due to a large dipole moment. The calculated dipoles for the dye series correlate well with the observed strong absorption bands of the electronic spectra. These results provided useful clues for the molecular engineering of efficient organic dye sensitizers.

Metal-free indoline dye sensitized solar cells based on nanocrystalline Zn2SnO4

Zn2SnO4 nanocrystals were synthesized and first used as the electrode materials for the metal-free indoline dyes sensitized solar cells (DSSCs). The highest efficiency of 3.08% was achieved for a D131 DSSC. This might be attributed to the fact that the D131 dye has a greater positive oxidation potential, which can lead to rapid dye regeneration, avoiding the geminate charge recombination between oxidized dye molecules and injected electrons in the Zn2SnO4 film. The efficiency can be improved significantly using a mixture solution of D131 and N719 dyes for which an efficiency of 3.6% was obtained.

Determination of p-phenylenediamine and its metabolites MAPPD and DAPPD in biological samples using HPLC-DAD and amperometric detection

A sensitive and selective HPLC method using a diode array detector (DAD) and an electrochemical detector (ECD) in series has been developed and validated for the quantitative measurement of p-phenylenediamine and its acetylated metabolites N-acetyl-p-phenylenediamine (MAPPD) and N,N′-diacetyl-p-phenylenediamine (DAPPD) in biological samples. The separation was carried out on a hydrophilic modified AQUA C 18 column and the mobile phase was composed of acetonitrile: ammonium acetate solution (5:95, 25 mM, v/v). Spectrophotometric detection was performed at 240 or 255 nm and amperometric detection was carried out using a positive oxidation potential of 400 mV. The quantification of the three analytes was validated in the range of 0.05–50 μM and the established limits of determination were 0.5 μM for PPD and MAPPD and 1 μM for DAPPD. The standard deviations ( N = 9) were lower than 7.5% at a concentration of 1 μM. The samples were stabilised with ascorbic acid to prevent PPD from oxidizing. Pretreatment of samples or analyte enrichment before sample injection is not required. The method proved to be accurate, sensitive and sufficiently specific. It was applied to the ecotoxicological study of the kinetics of the PPD N-acetylation in cell lysates in two different media.

Synthesis and Structure–Activity Relationships of Ferrocenyl Tamoxifen Derivatives with Modified Side Chains

We report here the synthesis and cell-proliferation properties of derivatives of the breast cancer drug tamoxifen, in which the -O(CH(2))(2)N(CH(3))(2) side chain, responsible for the drug's antiestrogenic properties, has been modified by a ferrocenyl moiety. We recently reported the diphenol compound 5, in which this amino chain had been replaced with an acyl-ferrocenyl (-O(CH(2))(2)C(O)[(eta(5)-C(5)H(4))FeCp]) group, and which showed antiproliferative effects against both the hormone-dependent MCF-7 and -independent MDA-MB-231 breast cancer cell lines. We now report the results of a structure-activity relationship (SAR) study, in which the lateral chain length has been varied, the ketone group has been omitted, and the number of phenol groups has been varied. Compounds 1-4, with a side chain lacking the carbonyl function (-O(CH(2))(n)[(eta(5)-C(5)H(4))FeCp], n = 1-4) and which show a decreasing affinity for ERalpha (ER = estrogen receptor) with increasing chain length, act as estrogens on MCF-7 cells, and mild cytotoxics on PC-3 prostate cancer cells, with IC(50) values around 10 microM. The two monophenolic derivatives of 2, 2 a and 2 b, which show a reduced affinity for ERalpha compared to 2, are also estrogenic, but are only slightly cytotoxic. Finally, we have reexamined compound 5 and discovered that its antiproliferative effect against the MCF-7 cell line does not arise from antiestrogenicity as we had originally suspected, but by means of a cytotoxic pathway. This compound is also sensitive to the number of phenol groups as cell death is diminished when one of the hydroxyl groups is omitted (5 a and 5 b). Molecular modeling studies of the ligand-ERalpha binding stability are broadly consistent with the experimental binding affinity results for compounds 2, 2 a, 2 b, 5, 5 a, and 5 b. Electrochemical experiments show that 1-4, 2 a, and 2 b are stable to oxidation on the electrochemical timescale, unlike 5, 5 a, and 5 b, and that cytotoxicity is related to less positive phenol oxidation potentials. The SAR study shows that the presence of a ketone group and two phenol groups is necessary for strong receptor binding and cytotoxic effects, and that all compounds are estrogenic, despite the presence of a bulky side chain.

Oxidatively induced reactions of a diimine platinum(IV) tetramethyl complex

The oxidation of the Pt(IV) tetramethyl complex [Ar N CH CH N Ar]PtMe 4 (Ar = 2,6-Me 2C 6H 3) has been investigated in acetonitrile and dichloromethane. Cyclic voltammetry demonstrates that the irreversible oxidation of [Ar N CH CH N Ar]PtMe 4 occurs at a slightly less positive oxidation potential than the irreversible oxidation of the analogous Pt(II) species [Ar N CH CH N Ar]PtMe 2. The product distribution arising from the oxidation depends strongly on the reaction conditions and includes cationic Pt(IV) species (acetonitrile, dichloromethane solvents) and Pt(II) species (dichloromethane only). Evidence is presented that suggests that homolytic cleavage of a weakened Pt C bond in [ ArN CH CH NAr ] PtMe 4 + is involved in the oxidatively induced reactions.

Electrochemistry of Late Transition Metal Complexes Containing the Ligand 1,1‘-Bis(diisopropylphosphino)ferrocene (dippf)

The chemically reversible oxidation of 1,1‘-bis(diisopropylphosphino)ferrocene (dippf) was investigated using cyclic voltammetry. In addition, seven new compounds with general formulas of [(MCln)m(dippf)] (M = Co, Ni, Pt, Zn, Cd or Hg, n = 2, m = 1; M = Au, n = 1, m = 2) have been prepared. The Ni and Co compounds were found to be paramagnetic, and the Evans method was used to determine the magnetic moment for these compounds. The remaining compounds were characterized by 1H, 13C, and 31P NMR. As an additional means of characterization, the molecular structures of [PtCl2(dippf)] and [ZnCl2(dippf)] were determined. The electrochemistry of the new compounds and the previously synthesized [PdCl2(dippf)] was investigated using cyclic voltammetry. The results of this investigation show that coordination of the dippf ligands leads to more positive oxidation potentials for the ferrocene backbone.

(Photo)electrochemical behavior of selected organic compounds on TiO2 electrodes. Overall relevance to heterogeneous photocatalysis

Oxidation potentials of resorcinol, 4-chlororesorcinol, 4,6-dichlororesorcinol, catechol, 3,4-dihydroxybenzoic acid and 1,2,4-trihydroxybenzene were measured on particulate TiO2 (Degussa P-25) thin films, immobilized on optically transparent SnO2 conducting glass electrodes, by cyclic voltammetry in 0.5M KCl aqueous electrolyte solutions. The effect of adsorption on oxidation potentials was examined with the compounds adsorbed on the TiO2 particle surface. Scan rate dependencies of oxidation peak currents indicate that adsorbed species are consistently characterized by less positive oxidation potentials compared to those attributed to solution free species; the difference ranges from about 0.2 to 0.8V. Results show that depending on the nature of the working electrode, associating a single oxidation potential to such compounds does not adequately describe their electrochemical behavior. Such observations have relevance in heterogeneous photocatalysis in that predictions of whether a substance will be photooxidized or photoreduced cannot be based on Fermi levels (redox potentials) of the redox couples in homogeneous solutions.

Flow injection amperometric detection of catechol using dual-band poly(3-methylthiophene) electrodes

Abstract This work reports the electrochemical detection of catechol (10 −4 to 10 −6 M) in the presence of ascorbic acid (10 −4 to 10 −6 M) using dual-band poly(3-methylthiophene) (P3MT) electrode flow-injection amperometry without prior separation. The selectivity involved in this method is based on the differences in electrochemical behavior of catechol and ascorbic acid. A variety of dual-band Pt, Au, glassy carbon, and P3MT electrodes were constructed and used as the electrochemical detector. The upstream electrode of the series dual-band electrode unit is used for detection of catechol and ascorbic acid and the downstream electrode for detection of the oxidized catechol. The band dimensions range from 0.1×2.5 to 1×5.5 mm with the interelectrode gaps varying from 0.1 to 0.5 mm. Although this method is effective for dilute solutions of catechol ( −5 M) in the presence of ascorbic acid ( −5 M) under both neutral and acidic conditions, it is not efficient for more concentrated mixtures (>10 −5 M) unless the measurements are made under acidic conditions such as at pH 1.6. A negative deviation from the ideal calibration curve of the oxidized catechol reduction is found in the more concentrated mixtures (>10 −5 M) at physiological pH 7.4. Charge–dipole interaction and hydrogen bonding between the oxidized products of catechol and ascorbic acid in the concentrated neutral solutions are proposed to explain the suppression of the current signals. Variation of the flow rates from 0.5 to 3.0 ml/min has no effect on the performance of the detector. The dual-band P3MT electrode has less positive oxidation potentials for catechol and ascorbic acid oxidation compared to the bare Pt, Au, and glassy carbon electrodes because of the catalytic activity of the P3MT electrode surface. Dual-band P3MT as well as glassy carbon electrodes have the best (highest) collection efficiency for catechol detection. The collection efficiency also remains constant when the interelectrode gaps vary from 0.1 to 0.5 mm.

Synthesis and redox properties of tetraethynyl tetrathiafulvalenes

The synthesis and characterization of protected tetraethynyltetrathiafulvalenes ( 1a and 1b) is reported. The cation radical of 1a has a slightly more positive oxidation potential ( E ox0 1 = 0.63 V vs Ag AgCl ) than TTF. Stable CT solids can be prepared with iodine and TCNQF 4. These compounds are the first members of a series of highly unsaturated TTF derivatives having in-plane π-orbitals.

Solvato-Controlled Doping of Conducting Polymers

Silver(I) is employed to dope poly(3-alkylthiophenes) (P3ATs) and other conjugated polymers. Solutions containing silver(I) salts and P3ATs, which usually undergo spontaneous reaction, doping, and precipitation, were stabilized using coordinating ligands that modify the reduction potential of the Ag(I) species. Upon removal of the coordinating ligand, spontaneous oxidative doping of the polymer occurred. Uniform electronically conducting films were thus obtained by simultaneous evaporation of both solvent and Ag(I)-coordinating liganda process termed “solvato-controlled doping”. Doping was observed with a variety of silver(I) salts consisting of relatively electronegative counterions, whereas a series of poly(3-alkylthiophenes) possessing increasing head-to-head dyad content showed a decreasing tendency to undergo Ag(I)-doping due to the increasingly positive oxidation potential of the polymer. Solvato-controlled doping of polyaniline and poly(3-methoxythiophene) with Ag(I) is also described, thus demonstrating the applicability of this approach to other conducting polymer systems.

Highly Substituted Cyclams: Stereoselective Synthesis and Coordination Properties

AbstractWe report here a new synthetic route to highly‐substituted tetraazamacrocycles. Raney nickel hydrogenation of macrocyclic phenylenediamine–acetylacetone condensation products gives cyclams in high yield with complexes all‐cis stereoselectivity. The extensive C‐substitution of the cyclam 2a changes its ligand properties markedly. The X‐ray structure analyses of NiII, CuII, and ZnII complexes of 2a reveals a deviation of the metal ion coordination geometry from the usual square‐planar arrangement. This may account for the differences in the redox properties of the NiII and CuII complexes of 2a leads to more positive oxidation and reduction potentials compared to the analogous unsubstituted cyclam complexes. While the binding constant between Zn(ClO4)2 and 2a is lower by a factor of 105 compared to cyclam, the subsequent binding of uridine to the zinc‐cyclam complexes in methanol/water is slightly stronger in the case of 2a · Zn(ClO4)2].

2,3,7,8,12,13,17,18-Octafluoro-5,10,15,20-tetraarylporphyrins and Their Zinc Complexes: First Spectroscopic, Electrochemical, and Structural Characterization of a Perfluorinated Tetraarylmetalloporphyrin

We report a convenient and general synthesis of β-octafluoroporphyrins bearing meso-tetraaryl substituents, including the first synthesis and full characterization of a perfluorinated tetraarylporphyrin, perfluoro-5,10,15,20-tetraphenylporphyrin, 2. The structural, spectroscopic, and electrochemical data indicate that β-octafluoro-meso-tetraarylporphyrins are a new class of planar, electron-deficient ligands. Particularly impressive is the 0.5 V window over which the formal oxidation potential can be tuned using only aryl substituents. The invariance of the ligand structure with increasingly positive formal oxidation potential is a key advance; electronic effects have been severed from the nonplanar conformations exhibited by all other highly electron-deficient porphyrins.

Visible light induced photo-oxidation of water. Formation of intermediary hydroxyl radicals through the photoexcited triplet state of perfluorophenazine

1,2,3,4,5,6,7,8-Octafluorophenazine (F-Phen) has an absorption spectrum in the longer wavelength extending to the visible-light region and has a more positive oxidation potential than for unfluorinated phenazine. F-Phen has been photolysed with water in acetonitrile under visible-light irradiation to produce stoichiometrically 1,3,4,5,6,7,8-heptafluoro-2-hydroxyphenazine (F-Phen-2-OH). Photolysis of F-Phen with water in the presence of benzene leads to the formation of phenol followed by the disappearance of F-Phen. EPR analysis and laser flash photolysis reveal that the photoexcited triplet state of F-Phen participates in water oxidation to hydroxyl radicals with concurrent conversion to F-Phen-2-OH. The mechanism is discussed with the results of semi-empirical molecular orbital calculations.

Spectra and Reactivity of Methoxyoligothiophene Cation Radicals

A set of α-coupled oligothiophenes, dimer through hexamer, was studied. Each oligomer was terminated with two methyl groups and was substituted with two or four methoxy groups. Electrochemical studies showed that methoxy groups substituted at “inside” β-positions on the terminal rings (positions toward the center of the chain) gave stable cation radicals. The E0 values for the reversible couples were 0.24−0.52 V (SCE). The changes in E0 with structure were explained using PPP molecular orbital calculations. Cation radicals were generated using either chemical or electrochemical oxidation, and in several cases stable salts were isolated. The vis−near-IR spectra of the cation radicals were recorded. In CH2Cl2 solvent the cation radicals were monomeric and showed two π−π*-bands, which shifted to longer wavelength with oligomer length. In CH3CN solvent the cation radicals were primarily in the form of π-dimers, which showed two π−π*-bands and a charge transfer band at longer wavelength in the near-IR. All three bands shifted to longer wavelength with oligomer length. The equilibrium constant for dimerization increased by a factor of 103 on going from a terthiophene to a quinquethiophene. Spectra of oligomers terminated with hydrogens or dimethylphenylsilyl groups showed that steric effects of the small hydrogen and bulky silyl group on π-dimerization were very small. Methyl-terminated oligomers substituted with “outside” methoxy in the terminal rings gave rather positive oxidation potentials and unstable cation radicals. In one case, the cation radical decomposition products were studied and it was shown that reactions had occurred on a terminal methyl group.