- Electron-transfer studies of a peroxide dianion
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A peroxide dianion (O22-) can be isolated within the cavity of hexacarboxamide cryptand, [(O2)∪mBDCA-5t-H 6]2-, stabilized by hydrogen bonding but otherwise free of proton or metal-ion association. This feature has allowed the electron-transfer (ET) kinetics of isolated peroxide to be examined chemically and electrochemically. The ET of [(O2)∪mBDCA-5t-H6] 2- with a series of seven quinones, with reduction potentials spanning 1 V, has been examined by stopped-flow spectroscopy. The kinetics of the homogeneous ET reaction has been correlated to heterogeneous ET kinetics as measured electrochemically to provide a unified description of ET between the Butler-Volmer and Marcus models. The chemical and electrochemical oxidation kinetics together indicate that the oxidative ET of O22- occurs by an outer-sphere mechanism that exhibits significant nonadiabatic character, suggesting that the highest occupied molecular orbital of O 22- within the cryptand is sterically shielded from the oxidizing species. An understanding of the ET chemistry of a free peroxide dianion will be useful in studies of metal-air batteries and the use of [(O 2)∪mBDCA-5t-H6]2- as a chemical reagent.
- Ullman, Andrew M.,Sun, Xianru,Graham, Daniel J.,Lopez, Nazario,Nava, Matthew,De Las Cuevas, Rebecca,Mueller, Peter,Rybak-Akimova, Elena V.,Cummins, Christopher C.,Nocera, Daniel G.
-
supporting information
p. 5384 - 5391
(2014/06/09)
-
- Structure and function of quinones in biological solar energy transduction: A high-frequency d-band EPR spectroscopy study of model benzoquinones
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Quinones are utilized as charge-transfer cofactors in a wide variety of reactions that are crucial for photosynthesis and respiration. In photosynthetic protein complexes, both Type I and Type II, including oxygenic and anoxygenic reaction centers contain quinone cofactors that are known to participate in electron- and proton-transfer processes. Type II reaction centers, purple bacterial reaction centers, and photosystem II utilize benzoquinone molecules, ubiquinone, and plastoquinone, respectively, to facilitate proton-coupled electron transfer reactions. Here, we report a systematic study of the principal components of the g-tensor of an extensive library of model benzosemiquinone anion radicals in both protic (2-isopropanol) and aprotic (dimethyl sulfoxide) solvents using high-frequency EPR spectroscopy. A detailed comparison of the experimental g-values of the benzosemiquinone models at D-band EPR frequency allows for the discrimination of substituent effects and solvent hydrogen bonds on the principal components of the g-tensor. Further, we compare the primary plastosemiquinone, QA-, of photosystem II with the substituent and solvent hydrogen bond effects of benzosemiquinone models in vitro. This study significantly extends the experimental basis for elucidating the role of both molecular structure and interactions with environment on the functional tuning of quinone cofactors in biological solar energy transduction.
- Chatterjee, Ruchira,Coates, Christopher S.,Milikisiyants, Sergey,Poluektov, Oleg G.,Lakshmi
-
experimental part
p. 676 - 682
(2012/04/10)
-
- Investigation of the oxidation of hydroquinone at the liquid/liquid interface
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The oxidation of hydroquinone (QH2) was investigated for the first time at liquid/liquid (L/L) interface by scanning electrochemical microscopy (SECM). In this study, electron transfer (ET) from QH2 in aqueous to ferrocene (Fc) in nitrobenzene (NB) was probed. The apparent heterogeneous rate constants for ET reactions were obtained by fitting the experimental approach curves to the theoretical values. The results showed that the rate constants for oxidation reaction of QH2 were sensitive to the changes of the driving force, which increased as the driving force increased. In addition, factors that would affect ET of QH2 were studied. Experimental results indicated ion situation around QH2 molecule could change the magnitude of the rate constants because the capability of oxidation of QH2 would be affected by them.
- Lu, Xiao Quan,Dong, De Fang,Liu, Xiu Hui,Yao, Dong Na,Wang, Wen Ting,Xu, Yu Mei
-
experimental part
p. 225 - 228
(2010/12/24)
-
- Quenching of triplet-excited flavins by flavonoids. Structural assessment of antioxidative activity
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(Figure Presented) The mechanism of flavin-mediated photooxidation of flavonoids was investigated for aqueous solutions. Interaction of triplet-excited flavin mononucleotide with phenols, as determined by laser flash photolysis, occurred at nearly diffusion-controlled rates (k~1.6x10 9 Lmol-1 s-1 for phenol at pH 7, 293 K), but protection of the phenolic function by methylation inhibited reaction. Still, electron transfer was proposed as the dominating mechanism due to the lack of primary kinetic hydrogen/ deuterium isotope effect and the low activation enthalpy (-1) for photooxidation. Activation entropy worked compensating in a series of phenolic derivatives, supporting a common oxidation mechanism. Anortho-hydroxymethoxy pattern was equally reactive (k~2.3x109Lmol-1 s-1 for guaiacol at pH 7) as compounds with ortho-dihydroxy substitution (k~2.4x109 L mol-1 s-1 for catechol at pH 7), which are generally referred to as good antioxidants. This refutes the common belief that stabilization of incipient phenoxyl radicals through intramolecular hydrogen bonding is the driving force behind the reducing activity of catechol-like compounds. Instead, such bonding improves ionization characteristics of the substrates, hence the differences in reactivity with (photo)oxidation of isolated phenols. Despite the similar reactivity, radicals from ortho-dihydroxy compounds are detected in high steady-state concentrations by electron paramagnetic resonance (EPR) spectroscopy, while those resulting from oxidation of ortho-hydroxymethoxy (or isolated phenolic) patterns were too reactive to be observed. The ability to deprotonate and form the corresponding radical anions at neutral pH was proposed as the decisive factor for stabilization and, consequently, for antioxidative action. Thus, substituting other ionizable functions for the ortho- or para-hydroxyl in phenolic compounds resulted in stable radical anion formation, as demonstrated for para-hydroxybenzoic acid, in contrast to its methyl ester. 2009 American Chemical Society.
- Huvaere, Kevin,Olsen, Karsten,Skibsted, Leif H.
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experimental part
p. 7283 - 7293
(2010/01/16)
-
- EPR spectroscopic investigation of radical-induced degradation of partially fluorinated aromatic model compounds for fuel cell membranes
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EPR spectroscopic investigations of reactions between monomeric model compounds representing typical structural moieties of poly(aryl) ionomers and photochemically generated hydroxyl radicals are reported. Deoxygenated solutions of the model compounds (in
- Schoenberger, Frank,Kerres, Jochen,Dilger, Herbert,Roduner, Emil
-
experimental part
p. 5782 - 5795
(2010/04/29)
-
- Oxidative and photochemical stability of ionomers for fuel-cell membranes
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To predict hydroxyl-radical-initiated degradation of new proton-conducting polymer membranes based on sulfonated polyetherketones (PEK) and polysulfones (PSU), three nonfluorinated aromatics are chosen as model compounds for EPR experiments, aiming at the identification of products of HO.-radical reactions with these monomers. Photolysis of H2O2 was chosen as the source of HO. radicals. To distinguish HO .-radical attack from direct photolysis of the monomers, experiments were carried out in the presence and absence of H2O2. A detailed investigation of the pH dependence was performed for 4,4′-sulfonylbis[phenol] (SBP), bisphenol A (-4,4′- isopropylidenebis[phenol]; BPA), and [1,1′-biphenyl]-4,4′-diol (BPD). At pH > pKA of HO. and H2O 2, reactions between the model compounds and O2 . or 1O2 are the most probable ways to the phenoxy and 'semiquinone' radicals observed in this pH range in our EPR spectra. A large number of new radicals give evidence of multiple hydroxylation of the aromatic rings. Investigations at low pH are particularly relevant for understanding degradation in polymer-electrolyte fuel cells (PEFCs). However, the chemistry depends strongly on pH, a fact that is highly significant in view of possible pH inhomogeneities in fuel cells at high currents. It is shown that also direct photolysis of the monomers leads to 'semiquinone'-type radicals. For SBP and BPA, this involves cleavage of a C-C bond.
- Mitov, Svetlin,Delmer, Olga,Kerres, Jochen,Roduner, Emil
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p. 2354 - 2370
(2007/10/03)
-
- Involvement of semiquinone radicals in the in vitro cytotoxicity of cigarette mainstream smoke
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Free radicals in cigarette smoke have attracted a great deal of attention because they are hypothesized to be responsible in part for several of the pathologies related to smoking. Hydroquinone, catechol, and their methyl-substituted derivatives are abundant in the particulate phase of cigarette smoke, and they are known precursors of semiquinone radicals. In this study, the in vitro cytotoxicity of these dihydroxybenzenes was determined using the neutral red uptake (NRU) assay, and their radical-forming capacity was determined by electron paramagnetic resonance (EPR). All of the dihydroxybenzenes studied were found to generate appreciable amounts of semiquinone radicals when dissolved in the cell culture medium employed in the NRU assay. Hydroquinone exhibited by far the highest capacity to form semiquinone radicals at physiological pH, even though it is not the most cytotoxic dihydroxybenzene. Methyl-substituted dihydroxybenzenes were found to be more cytotoxic than either hydroquinone or catechol. The formation of semiquinone radicals via auto-oxidation of the dihydroxybenzenes was found to be dependent on the reduction potential of the corresponding quinone/semiquinone radical redox couple. The capacity to generate semiquinone radicals was found to be insufficient to explain the variance in the cytotoxicity among the dihydroxybenzenes in our study; consequently, other mechanisms of toxicity must also be involved. The observed interactions between 2,6-dimethylhydroquinone and hydroquinone in the cytotoxicity assay and EPR analysis suggest that care needs to be taken when the bioactivity of cigarette smoke constituents is evaluated, i.e., the effect of the cigarette smoke complex matrix on the activity of the single constituent studied must be taken into consideration.
- Chouchane, Salem,Wooten, Jan B.,Tewes, Franz J.,Wittig, Arno,Mueller, Boris P.,Veltel, Detlef,Diekmann, Joerg
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p. 1602 - 1610
(2008/12/22)
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- Kinetic isotope effect for hydrogen abstraction by .OH radicals from normal and carbon-deuterated ethyl alcohol and methylamine in aqueous solutions
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Selective H/D kinetic isotope effects (KIEs) have been determined for the various routes by which .OH radicals react with ethanol (CH3CH2OH and CD3CD2OH) and methylamine (CH3NH2 and CD3NH2) in H2 Osolutions. The KIEs have been evaluated from overall rate constants and the yields in which the individual primary radicals were generated in the .OH reaction with these compounds. The analytical method applied for the yield determinations was redox titration with suitable scavengers, namely, methyl viologen and Fe(CN)63- for the reducing radicals (CH3C.HOH/CD3C.DOH and .CH2NH2/.CD2NH2 ) and I- and hydroquinone for the oxidizing radicals (CH3CH2O./CD3CD2O. and .NHCH3/.NHCD3). The numerical results obtained also include, besides yields relative to total available .OH, absolute rate constants for most of these scavenging reactions. For the alcohol, the major process (almost 90%) is H/D abstraction from the Ca bond with KIE = 1.96. For methylamine, abstraction from C?± H/D occurs with only 37% (H) and 26% (D) but at a similar KIE = 1.86. The remainder, denoting the major process in this case, accounts for the formation of aminyl radicals. The secondary KIEs for N-H cleavage, and O-H cleavage in the case of the alcohol, are close to unity, reflecting the expected negligible influence of C-H/D substitution in the attached alkyl groups. Abstraction of ?2-C-attached H/D in ethanol shows, also in qualitative agreement with expectation, a larger KIE = 3.4 than that for ?±-C H/D. An interestingly high KIE a?? 50 was obtained for the 1,2 hydrogen shift that converts the CH3CH2O. and CD3CD2O. oxyl radicals into the corresponding ?±-hydroxyethyl radicals CH3C.HOH and CD3C.DOH. All of the results are discussed in light of existing literature data on relevant H/D isotope effects, the influence of solvent relative to gas phase, the selectivity of the .OH attack, and other mechanistic considerations. Specifically, the mechanism by which the .OH reacts with the amine seems likely to involve a transient, caged aminium/hydroxide ion pair.
- Bonifacì?icì?, Marija,Armstrong, David A.,Sì?tefanicì?, Igor,Asmus, Klaus-Dieter
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p. 7268 - 7276
(2007/10/03)
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- Intramolecular hydrogen bonding in hydroxylated semiquinones inhibits semiquinone-Mg2+ complex formation
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Complex formation between the semiquinones of 5,8-dihydroxynaphtho-1,4-quinone, NZQ-, 1,4-dihydroxy-anthracene-9,10-dione, QNZ-, benzo-1,4-quinone, BQ-, and phenanthraquinone, PHQ-, and Mg2+ was studied utilizing EPR spectroscopy. Weighted average EPR spectra between those corresponding to the uncomplexed and complexed semiquinones were observed for NZQ-, QNZ- and BQ- while these species were observed simultaneously for PHQ-. The semiquinones NZQ-, QNZ- and BQ- behave as weak Mg2+ chelators while PHQ- chelates this cation much more strongly (binding constant = (1.1 ± 0.5) × 103 dm3 mol-1). The weak binding of Mg2+ by NZQ- and QNZ- is in contrast with the large complex formation constants between the parent quinones NZQ and QNZ and different metal cations. This apparent paradox is explained by the strong intramolecular hydrogen bonding existing in NZQ- and QNZ-.
- Alegria, Antonio E.,Garcia, Carmelo,Santiago, Glyssette,Collazo, Gladys,Morant, Julio
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p. 1569 - 1573
(2007/10/03)
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- Reaction of OH radicals with benzoquinone in aqueous solutions. A pulse radiolysis study
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Hydroxyl radicals have been generated by pulse radiolysis in N2O-saturated aqueous solutions. Their addition to 1,4-benzoquinone BQ (k3 = 6.6 × 109 dm3 mol-1 s-1 by competition with thiocyanate) in neutral solution leads to a build-up of optical absorption that shows different rates at wavelengths at around 330 and at >400 nm. At 330 nm the rate of build-up is proportional to the benzoquinone concentration, and its rate constant agrees with the value (k3) obtained by competition. At the longer wavelengths, it becomes independent of benzoquinone concentration beyond 4 × 10-4 mol dm-3 (k6 = 6.9 × 105 s-1). Kinetic analysis in the ns time-range show that the primarily-generated benzoquinone-OH-adduct radical 1 undergoes rapid (k4 = 2.5 × 106 s-1) keto-enol tautomerization yielding the 2,4-dihydroxyphenoxyl radical 2. To gain support for this proposed reaction, radical 2 [pKa(2) ≈ 4.9 ± 0.2] has been independently generated by one-electron oxidation of 1,2,4-trihydroxybenzene using ·OH in acidic or N3· in neutral and basic solution. Its absorption characteristics compare favourably with those observed in the benzoquinone system. On the basis of spectrophotometric and conductometric data it is proposed that in neutral solution the radical anion 2a is rapidly oxidized by benzoquinone itself (k15 ≥ 2 × 109 dm3 mol-1) into the end product 2-hydroxy-1,4-benzoquinone anion 4a [pKa(4) = 4.1 ± 0.1; λmax(4) = 380 nm; λmax(4a) = 482 nm] and the semibenzoquinone radical anion 3a. The latter decays bimolecularly into benzoquinone and hydroquinone (2k16 = 3.1 × 108 dm3 mol-1 s-1). In acidic solution the rate of oxidation of 2 by benzoquinone is considerably slower (k13 ≤ 2.4 × 107 dm3 mol-1 s-1). The assignment of the final product to 2-hydroxy-1,4-benzoquinone 4 has been confirmed by the absorption characteristics and pKa value of the authentic material obtained by the two-electron electrochemical oxidation of 1,2,4-trihydroxybenzene.
- Schuchmann, Man Nien,Bothe, Eberhard,Von Sonntag, Justus,Von Sonntag, Clemens
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p. 791 - 796
(2007/10/03)
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- Light-induced formation of 2,5-dihydroxy-p-benzoquinone from hydroquinone in photoirradiated silver-loaded zirconium phosphate suspension
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Silver-loaded zirconium phosphate [Ag1-xHxZr2(PO4)3] has shown photocatalytic activity concerning the generation of OH? and the hydroxylation of hydroquinone (HQ) to 2,5-dihydroxy-p-benzoquinone (DHQH2). HQ was initially oxidized to p-benzoquinone (BQ) by Ag+ on the surface of Ag1-xHxZr2(PO4)3 in the dark and by photogenerated OH? during visible-light irradiation. As an intermediate, a semiquinone radical BQH? was detected by EPR measurements in both cases. Furthermore, under visible-light irradiation, DHQH2 was identified by its absorption spectrum and thin layer chromatography. The amount of DHQH2 increased and BQ decreased with irradiation time. The total amount of BQ and DHQH2 under visible-light irradiation agreed with that of BQ in the dark. Consequently, DHQH2 appeared to form from BQH? generated by the oxidation of HQ and the reduction of BQ by photogenerated e-. From the XPS and FT-Raman technique analyses, it was found that the addition of the photogenerated OH? to BQH? occurred at the surface of Ag1-xHxZr2(PO4)3. This indicated that both the OH? and HBQ? were stabilized on the surface of Ag1-xHxZr2(PO4)3.
- Miyoshi, Hirokazu,Kourai, Hiroki,Maeda, Takuya
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p. 283 - 287
(2007/10/03)
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- Oxidation mechanism of NAD dimer model compounds
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The oxidation of a dimeric N-benzyldihydronicotinamide with various oxidants such as quinones, triphenyl carbenium ions and a triplet exited tris(bipyridine) ruthenium(II) complex occurs via initial outer-sphere electron transfer followed by fast C-C bond cleavage and second electron transfer. The kinetic studies allow the determination of the oxidation potential of this compound.
- Patz, Matthias,Kuwahara, Yoshihiro,Suenobu, Tomoyoshi,Fukuzumi, Shunichi
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p. 567 - 568
(2007/10/03)
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- Electron transfer barriers for ground- and excited-state redox couples: trans-dioxo(1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane) osmium(VI)/osmium(V)
-
Pulse radiolysis methods were used to measure rate and equilibrium constants for the reaction of the OsVI(TMC)(O)22+/ OsV(TMC)(O)2+ couple with semiquinone/quinone couples, OsVI(TMC)(O)22+ + Q- ?caret; OsV(TMC)(O)2+ + Q, where TMC = tetramethylcyclam and Q is 1,4-benzoquinone or 2-methyl-1,4-benzoquinone. This work yields a self-exchange rate constant of 1.1 × 106 M-1 s-1 and a reduction potential of +0.048 V versus NHE for the dioxoosmium ground-state couple at 0.1 M ionic strength and 25°C. [Os(TMC)(O)2](PF6)2 crystallizes in the monoclinic space group, P21/c, with a = 6.712(2) A, b = 17.756(6) A, c = 10.150(2) A, β = 95.59(2)° and Z = 2. Single crystal X-ray diffraction results for the PF6- salt of the OsVI(TMC)(O)22+ indicate that the dominant isomer present in the crystal is the RSSR isomer in which one pair of N-methyl groups is 'up' while the other is 'down'. The average Os-O and Os-N distances are 1.735(6) and 2.126(8) A, respectively.
- Kelly, Craig,Szalda, David J.,Creutz, Carol,Schwarz, Harold A.,Sutin, Norman
-
-
- Formation of radical anions on the reduction of carbonyl-containing perfluoroaromatic compounds in aqueous solution: A pulse radiolysis study
-
Radical anions are formed on addition of hydrated electrons to pentafluoroacetophenone (PFA) and pentafluorobenzaldehyde (PFB) in aqueous solutions. On the other hand, addition of hydrated electrons to pentafluorobenzoic acid (PFBA) leads to rapid fluoride elimination. The spectrum of the radical anion of PFA has λmax at 300 and 440 nm with absorption coefficient at 440 nm ε440 = 2100 L mol-1 cm-1. PFA?- decays with a rate constant of (7 ± 3.0) × 103 s-1. It has a pKa = 7.5 and the spectrum of the conjugate acid has λmax at 270 and 460 nm with ε460 = 900 L mol-1 cm-1. The spectrum of the radical anion of PFB has λmax at 285 and 430 nm with ε430 = 800 L mol-1 cm-1. PFB?- decays with a rate of (4 ± 2) × 103 s-1. It has a pKa = 7.2 and the spectrum of the conjugate acid has weak absorption at 330 nm. Evidence for the formation of the radical anion was obtained from intermolecular electron transfer from the radical anions of PFA and PFB top-benzoquinone (Q), methyl viologen (MV2+), and 9,10-anthraquinone-2-sulfonate (AQS-). Strong reductants derived from reduction of 2,2-bipyridine (BpyH?) and 1,10-phenanthroline (PhenH?) can reduce both PFA and PFB. From the kinetics of these electron transfer reactions the reduction potentials of PFA and PFB have been determined to be -0.86 ± 0.1 and -0.75 ± 0.1 V vs NHE at pH 9.4. Addition of OH? radical to the aromatic ring of these fluorinated compounds led to rapid HF elimination and the formation of phenoxyl radicals, and addition of H? atoms led to the formation of cyclohexadienyl radical.
- Shoute, Lian C. T.,Mittal, Jai P.
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p. 14022 - 14027
(2007/10/03)
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- Reduction potentials and kinetics of electron transfer reactions of phenylthiyl radicals: Comparisons with phenoxyl radicals
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The reduction potentials relative to the standard hydrogen electrode (SHE) for a number of para-substituted phenylthiyl radicals (Eo(p-XC6H4S./p-XC6H 4S-)) have been derived from pulse radiolytic studies of electron transfer equilibria which compare their values to those of radicals of known reduction potentials. A ladder combining the reduction potentials for both phenylthiyl and phenoxyl radicals has been established. These reduction potentials have been shown to be self-consistent and are intermediate between those of p-benzosemiquinone radical anion at 0.02 V and phenoxyl radical at 0.79 V. The reduction potential decreases as the electron donating power of the para substituent rises. The substituent effect is, however, much weaker for the phenylthiyl radicals than for their oxygen analogs. These observations demonstrate that the electronic interaction between the sulfur atoms and the aromatic ring system is much less than that which occurs with oxygen atoms. Examination of the rates of electron transfer in terms of the Marcus theory indicates that the reorganization energies of both p-XC6H4O. and p-XC6H4S. radicals are similarly affected by H, CH3, and CH3O substitution. However, the reorganization energies increase substantially for H2N and O- para substituents with the effect being much less for the p-XC6H4S. radicals than for the p-XC6H4O. radicals. These observations are in accord with structural information from spectroscopic and theoretical studies of the radicals which show that in the latter system the substituent groups interact strongly with the aromatic π system.
- Armstrong,Sun, Qun,Schuler
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p. 9892 - 9899
(2007/10/03)
-
- Determination of the Quantum Yield for the Photochemical Generation of Hydroxyl Radicals in TiO2 Suspensions
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The generation of hydroxyl ((.)OH) radicals plays a key role in the heterogeneous photocatalytic degradation of organic pollutants in aqueous suspensions of TiO2.The quantum yield of this process is thus an important parameter; however, it is not easy to measure in a particulate system arising from problems caused by light scattering from the particles.In this work, a reliable method for the determination of the quantum yield of hydroxyl radical production in heterogeneous systems has been developed, based on measurements of (.)OH radical generation rates and the photon flux absorbed by TiO2 suspensions.In this procedure, a modified integrating sphere method was used to determine the true fraction of light absorbed by TiO2 suspensions.A ferrioxalate chemical actinometer was used to measure the incident photon flux.As a check on the quantum yield method, good agreement with known literature values was obtained for quantum yield measurements of the photochemical generation of the p-benzosemiquinone (BQ(.-)) radical in the photolysis of p-benzoquinone and of the (.)OH radical generation in the photolysis of hydrogen peroxide, respectively.Accordingly, the quantum yield for the (.)OH radical production in the TiO2 suspension was determined to be 0.040 +/- 0.003 at pH 7.Effects on the quantum yield of suspension loading, photon flux, and electron-acceptor addition (H2O2 and O2) were explored.
- Sun, Lizhong,Bolton, James R.
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p. 4127 - 4134
(2007/10/03)
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- Mobilities of Radical Cations and Anions, Dimer Radical Anions, and Relative Electron Affinities by Times of Flight in n-Hexane
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The mobilities of several radical cations and anions are measured in n-hexane using a thin-sheet time-of-flight (TOF) technique.We observe the radical cations of N,N,N',N'-tetramethyl-p-phenylenediamine, zinc tetraphenylporphine, and pyrene and the radical anions of perfluorobenzene, p-benzoquinone, anthraquinone, chloranil, buckminsterfullerene (C60), and octafluoronaphthalene.For all electron acceptors but C60, the dependence of the anionic TOF on acceptor concentration reveals the appearance of the homodimer radical anion at sufficiently high concentrations.The equilibrium constant for the monomer anion/monomer acceptor association reaction is obtained from the concentration studies.A Born-Haber cycle is then applied to estimate the difference between the electron affinities of the monomer and dimer molecules in the gas phase.
- Lim, S. K.,Burba, M. E.,Albrecht, A. C.
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p. 9665 - 9675
(2007/10/02)
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- Solvent Effects on Photoinduced Electron Transfer; A Fourier Transform-Electron Paramagnetic Resonance Study
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The kinetics of photoinduced electron transfer from zinc meso-tetra phenylporphine (ZnTPP) in ethanol and zinc meso-tetra(4-sulfonatophenyl)porphine (ZnTPPS) in ethanol-water mixtures to benzoquinone has been studied with Fourier Transform-Electron Paramagnetic Resonance.The rates of electron transfer, spin-lattice relaxation of the porphyrin donor in the triplet excited state and of the benzoquinone anion radical have been determined along with the values of spin polarization generated by the Triplet and Radical Pair Mechanisms of Chemically Induced Electron Polarization. - Key Words: Electron transfer / Porphyrin / Quinone / EPR, photochemistry
- Ebersole, Marie,Willigen, Hans van
-
-
- Wide-Band, Time-Resolved Photoacoustic Study of Electron-Transfer Reactions: Photoexcited Magnesium Porphyrin and Quinones
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Wide-band, time-resolved, pulsed photoacoustics has been employed to study the electron-transfer reaction between a triplet magnesium porphyrin and various quinones in polar and nonpolar solvents.The reaction rate constants are near encounter limited.The yield of triplet state is 70percent in both solvents.The yield of ions is 85percent in the former and zero in the latter, in agreement with spin dephasing time and escape times from the Coulomb wells in the two solvents.In methanol the plot of measured heat output versus quinone redox potential is linear.This implies that the entropy of electron transfer is constant through the series, but it may not be negligible.
- Feitelson, Jehuda,Mauzerall, David C.
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p. 8410 - 8413
(2007/10/02)
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- Radiolytic Reductions and Oxidations in Dimethyl Sulfoxide Solutions. Solvent Effects on Reactivity of Halogen Atom Complexes
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Radiolysis of dimethyl sulfoxide (DMSO) solutions containing various additives was used to achieve clean one-electron reduction or oxidation of solutes.Pulse radiolysis of benzoquinone in DMSO solutions containing acetone and triethylamine permitted conversion of all primary radicals into reducing species.The total yield of reduction in the γ-radiolysis of methyl viologen solutions was found to be 0.37 μmol/J.In the pulse radiolysis of TMPD and triphenylamine in aerated DMSO containing LiCl and/or CCl4, all the primary radicals were converted into oxidizing species and gave a maximum yield of 0.39 μmol/J.In the latter systems, oxidation was partly by halogen atom complexes.The reactivity of complexes of DMSO (DMSO*Cl, DMSO*Br) and of halide ions (Br2.1-, I2.1-) was examined for several organic compounds.DMSO*Cl oxidizes chlorpromazine, triphenylamine, and zinc porphyrin with rate constants of the order of 1E7-1E8 M-1 s-1, and the rates increase upon addition of CH2Cl2 as well as upon addition of water and formamide.DMSO*Cl also reacts with olefins by addition of Cl to the double bond; the rate constants increase upon increasing the electron-donating properties of the substituents on the double bond.The rate constants for oxidation of chlorpromazine by Br2.1- and I2.1- increase by more than 2 orders of magnitude upon changing the solvent from DMSO gradually to water.The change was less with acetonitrile/water mixtures, and the difference is probably due to differences in ion solvation.
- Kumar, M.,Neta, P.
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p. 3350 - 3354
(2007/10/02)
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- ESR, ENDOR and TRIPLE Resonance Studies on Radical Reactions of 1,4-Benzoquinone and 1,4-Hydroquinone in Alkaline Methanol and Liquid Ammonia
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The relative signs and absolute values of the hyperfine coupling constants of different methoxy-1,4-benzoquinones were measured by ESR, ENDOR and TRIPLE resonance spectroscopy.Reactions of 1,4-benzoquinone and 1,4-hydroquinone with methanol took place in alkine methanol, giving rise to methoxy-substituted 1,4-benzoquinones.The same substitution reaction occured in a mixture of ammonia and methanol.The reactions depend on temperature and the alkali concentration.Coupling constants were assigned by the modified additivity relationship method.An extremely small linewidth of 0.014 G was measured in the ESR spectrum of deuterated 2,5-dimethoxy-1,4-benzoquinone.
- Joela, H.,Kasa, S.,Maekelae, R.,Salo, E.,Hannonen, K.
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p. 261 - 267
(2007/10/02)
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- Ligand-Assisted Electron Transfer from the Triplet State of Zinc Tetraphenylporphyrin to 1,4-Benzoquinone
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Effects of axial ligands on the electron transfer from excited triplet state of zinc tetraphenylporphyrin (ZnTPP) to benzoquinone (BQ) have been studied by usung a laser flash technique.For the reaction of pyridinate complex of ZnTPP (Py-ZnTPP) as well as ZnTPP, quantum yield of the porphyrrin cation radicals resulting from the electron transfer, Φion, were determined in various solvents of different dielectric constant (ε).Remarkable enhancement in Φion due to axial ligation of pyridine has been found in the solvents having relatively moderate dielectric constant (9A comparison of the ε dependence of Φion for the reaction of the triplet ZnTPP to that for the triplet Py-ZnTPP has demonstrated that the axial pyridine causes partial neutralization of the effective charge of Py-ZnTPP cation radical.When the axial pyridine is replaced by several kinds of the substituted pyridines having different pKa, chloride ion, or ethanol, a significant change in Φion is revealed in dichloroethane.The change in Φion was found to depend on the ability of the ligand to transfer negative charge toward the porphyrin ring via the zinc ion.The ligand ability should cause reduction of effective charge on the porphyrin ring moiety of the ligand-bound ZnTPP cation radical which interacts with the BQ anion radical.Cosequently it has been concluded that the enhancement in Φion due to axial ligation is ascribed to the reduction of the effective charge of the cation radical which results in easier separation of the cation radical from its complex paired with BQ anion radical.
- Seki, Hiroshi,Hoshino, Mikio,Shizuka, Haruo
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p. 3630 - 3634
(2007/10/02)
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- Altered Condensed-Phase Electron Affinities of Carbonyl- 13C-, 14C-, and 17O-Substituted Ketones
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Electron spin resonance experiments have shown that the solution electron affinities of both benzoquinone (BQ) and benzophenone (BZO) in liquid ammonia are diminished when a 13C replaces the 12C in the carbonyl position.For the reaction *R+R.-=*R.-+R, where *R represents the 13C-substituted material (either BZO-13C or BQ-13C), the equilibrium constants (Keq) are 0.80 and 0.50 at -75 deg C for the BQ and BZO systems, respectively.The reduction of radioactive samples of benzophenone (mixtures of BZO and BZO-14C, 14C substitution at the carbonyl carbon) with deficient amounts of sodium metal in liquid ammonia followed by removal of the ammonia leaves a solid mixture of benzophenone and benzophenone ketyl.Sublimation of the neutral benzophenone from the anion radical salt produces benzophenone that is enhanced in radioactivity relative to the starting BZO/BZO-14C mixture.This enhancement in radioactivity is consistent with the equilibrium constant again being less than unity when *R represents the 14C-substituted benzophenone.In contrast to these results, substitution of the oxygen atom with 17O results in an increase in the relative solution electron affinity.This is explained in terms of the increase in "bonding" involving the oxygen upon reduction, due to ion association.
- Lauricella, Toni L.,Pescatore, John A.,Reiter, Richard C.,Stevenson, Randal D.,Stevenson, Gerald R.
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p. 3687 - 3691
(2007/10/02)
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- Nitrogen Dioxide and Related Free Radicals: Electron-transfer Reactions with Organic Compounds in Solutions containing Nitrite or Nitrate
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The absolute rate constants for the reaction of NO32-. (formed by one-electron reduction of nitrate) with oxygen, benzoquinone, and methyl viologen have been determined by pulse radiolysis.Experiments have shown that such reactions can occur in competition with the hydrolysis reaction leading to NO2. and that the formation of the latter can be catalysed by the presence of hydrogen or ammonium ions.Absolute rate constants for the oxidation of the phenothiazine derivative metiazinic acid, of 2,2'-azinobis-(3-ethyl-2,3-dihydrobenzothiazole-6-sulphonate), of ascorbate, and of dihydroxyfumarate by NO2. have also been measured.
- Forni, Luigi G.,Mora-Arellano, Victor O.,Packer, John E.,Willson, Robin L.
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- SPIN-POLARIZED TRANSIENT RADICALS FROM p-BENZOQUINONE AND DI-t-BUTYL-p-QUINONES BY PHOTO-EXCITATION OF BENZOPHENONE IN A MAGNETIC FIELD
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In the photochemical electron transfer reactions of t-alkylamine and p-benzoquinone in liquid, emissively spin-polarized transient anion radicals from p-benzoquinone (PBQ) and 2,5- or 2,6-di-t-butyl-p-quinone (25Q or 26Q) are exclusively produced by photo-excitation of benzophenone (BP) in a magnetic field.Transient radicals of quinones with different unpaired spin distribution are obtained at the onset of photolysis.
- Kaise, Masahiro,Nishihara, Chizuko,Nozoye, Hisakazu,Someno, Kazuo
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p. 589 - 592
(2007/10/02)
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- Entropy Changes and Electron Affinities from Gas-Phase Electron-Transfer Equilibria: A(-) + B = A + B(-)
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By measuring the electron-transfer equilibria 1, A(-) + B = A + B(-), at 150 deg C with a pulsed electron high-pressure mass spectrometer we determined the ΔGo1 values involving 12 new compounds.Measurements of the temperature dependence of K1 for 21 reactions involving some of the new compounds and many compounds whose ΔGo1 had been determined previously led, via van't Hoff plots, to ΔHo1 and ΔSo1 values.These were interconnecting such that ΔHo and ΔSo continuous scales (ladders) were obtained.These were anchored to SO2 whose electron affinity is accurately known.Available geometries and vibrational frequencies for SO2 and SO2(-) permit the evaluation of So(SO2(-)) - So(SO2).Through the ΔSo scale the So(B(-)) - So(B) for the other compounds B could be obtained also.Certain regularities in the So(B(-)) - So(B) data permitted entropy estimates to be made also for compounds for which no van't Hoff plots were made.In this manner a table of ΔHo, ΔSo, and ΔGo data for the electron capture e + B = B(-) was obtained, which contains some 50 compounds B.Most of the compounds are substituted benzenes, quinones, conjugated acid anhydrides, and perfluorinated organics.
- Chowdhury, Swapan,Heinis, Thomas,Grimsrud, Eric P.,Kebarle, Paul
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p. 2747 - 2752
(2007/10/02)
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- Electron Affinities of Di- and Tetracyanoethylene and Cyanobenzenes Based on Measurements of Gas-Phase Electron-Transfer Equilibria
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The electron affinities of tetracyanoethylene, trans-1,2-dicyanoethylene, and eleven substituted benzonitriles as well as two naphthonitriles were determined by measurement of the electron-transfer equilibria A-+B=A+B- with a pulsed electron high ion source pressure mass spectrometer.Rate constants for exothermic electron transfer involving the cyano compounds were found to be near unit collision efficiency.The EA (tetracyanoethylene)=3.17 eV obtained in the present work is considerably higher than the 2.3 eV photodetachment value of Palmer and Lyons.The electron affinities of benzene and benzonitrile substituted by CN, CHO, and NO2 increase in the given order, while the order for nitrobenzene is CHO, CN, NO2.This reversal of order is explained on the basis of a larger attenuation of the ?-withdrawing effect relative to the field effect of substituents when the electron density in the ?* single-electron orbital is decreased.
- Chowdhury, Swapan,Kebarle, Paul
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p. 5453 - 5459
(2007/10/02)
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- Powder ENDOR Spectra of p-Benzoquinone Anion Radical: Principal Hyperfine Tensor Components for Ring Protons and for Hydrogen-Bonded Protons
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ENDOR spectra for the immobilized p-benzosemiquinone anion radical (BQ.-) in disordered matrices are presented.Hyperfine interactions of the unpaired electron with three different classes of protons are apparent in the spectra and have been investigated: α-proton, hydrogen-bonded proton, and general matrix proton.Dipolar interactions are not averaged in powder samples, and first derivative ENDOR lines are observed for α- and hydrogen-bonded protons at frequencies which correspond to their principal hyperfine tensor values.Interpretation of the spectra has been facilitated by selective deuteration of the parent quinone and of the solvent.The g anisotropy of BQ.-, although weak, allows orientation selection experiments at X-band which have provided information on the axis directions for the tensor components relative to the molecular structure.Hydrogen bonding of the BQ.- carbonyl group to the alcohol hydroxyl group of the solvent is characterized by a purely dipolar interaction exhibiting axial symmetry.The hydrogen bond direction is in the plane of the quinone ring and the O...H bond distance is calculated to be 1.6 Angstroem.The principal hyperfine tensor components of the α-proton interaction are shown to depend critically on the nearest neighbor carbon spin density values which cause the principal values to deviate substantially from those expected for an isolated .C-H fragment.For the unpaired electron spin density distribution in BQ.-, the α-proton hyperfine tensor acquires approximately axial character.In the matrix region, several classes of weakly interacting protons contribute to the structured ENDOR line shape observed; orientation selection and selective deuteration have been used to resolve the origin of some of the lines in this region.The observed ENDOR band shapes for each type of proton-electron interaction indicate that the nuclear relaxation probability (ωn) is independent of orientation.
- O'Malley, Padraig J.,Babcock, Gerald T.
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p. 3995 - 4001
(2007/10/02)
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- Electron Affinities from Electron-Transfer Equilibria : A(-) + B = A + B(-)
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Determination of the equilibrium constants K1 for gas-phase electron-transfer equilibria with a pulsed electron beam high ion source pressure mass spectrometer led to the electron affinities of 34 compounds with EA's between 0.5 and 3eV.The compounds are mostly substituted nitrobenzenes, substituted quinones, and conjugated molecules containing oxygen atoms.The EA of smaller molecules like SO2, NO2, CS2, and CH3NO2 also were determined.The method is very well suited for rapid, accurate, routine determinations of electron affinities.A comparison with EA's determined with other gas-phase methods and EA's evaluated from polarographic half-wave reduction potentials and charge-transfer spectra in solution is made.The rate constants for a number of exothermic electron-transfer reactions were determined.Most of these proceed at near collision rates.Electron-transfer reactions involving perfluorinated compounds like perfluoromethylcyclohexane, perfluorocyclohexane, and sulfurhexafluoride do not follow this behavior.While the perfluoro compounds have high thermal electron capture cross sections, they do not accept electrons from A(-) of compounds A with lower electron affinity.The perfluoro anions do transfer electrons to compounds A with higher electron affinity, and the rate constants increase with EA(A) - EA(perfluoro compound).
- Grimsrud, Eric P.,Caldwell, Gary,Chowdhury, Swapan,Kebarle, Paul
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p. 4627 - 4634
(2007/10/02)
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- Einfluss von Lithiumkationen auf die Reduktion von Nitrobenzol und p-Benzochinon an Kohlenstoff-Faser-Elektroden in Acetonitril und Dimethylformamid
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The electrochemical behavior of nitrobenzene and p-benzoquinone in acetonitrile and dimethylformamide and the influence of the Li(1+)-cations on the reduction of these depolarizers are investigated on carbon fibre electrodes.When Et4NClO4 is used as the supporting electrolyte the corresponding radical anion is formed in all cases as the primary reduction product; the radical anion is further reduced to the final product at higher negative potentials.The introduction of small amounts of LiClO4 leads to the formation of ion pairs between the radical anion and the Li(1+)-cations, which produce a significant shift of both reduction waves to higher positive potentials.These effects are more pronounced in acetonitrile, where the ion pairs formed take the additional electrons during a succeeding fast heterogeneous process; consequently only one reduction peak is observed, i.e. the over all reduction of the substrates investigated is substantially facilitated in the presence of the Li(1+)-cations. - Keywords: Nitrobenzene / p-Benzoquinone / Li(1+)-cations / Carbon fibre electrodes / Acetonitrile / Dimethylformamide
- Jannakoudakis, P. D.,Karabinas, P.,Theodoridou, E.
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- Electron Attachment to p-Benzoquinone and Photodetachment from Benzoquinone Anion in Nonpolar Solvents
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Although p-benzoquinone has a high electron affinity (1.9 eV), its reaction with excess electrons exhibits an unusual solvent and temperature dependence.The reaction is fast and has a positive activation energy in n-pentane but is much slower and has a negative activation energy in Si(CH3)4, neopentane, and 2,2,4,4-tetramethylpentane.The results can be explained in terms of an equilibrium with a short-lived excited state of the anion: e- + benzoquinine benzoquinone(-)*.The attachment rate is fast in all solvents.The reverse, autodetachment from the excited anion, is characterized by an activation energy, which depends on the value of V0, the energy level of the electron in the liquid.For Si(CH3)4 for which V0 is the lowest (-0.55 eV), the activation energy is 5.7 kcal/mol.As V0 increases, the activation energy increases and is 9.4 kcal/mol in 2,2,4,4-tetramethylpentane for which V0=-0.35 eV.When V0 is close to zero, as in n-pentane, the activation energy is so large that detachment from the excited anion state is too slow to compete with deactivation and a fast diffusion-controlled forward rate is observed.The spectrum of benzoquinone anion as observed by the laser photodetachment technique is reported, and the threshold for photodetachment is found to be 2.32 eV in Si(CH3)4 and 2.58 eV in 2,2,4-trimethylpentane.The quantum yield of photodetachment reaches a maximum value at 480 nm in Si(CH3)4.
- Holroyd, R. A.
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p. 3541 - 3547
(2007/10/02)
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- Photochemical Primary Processes of Xanthene Dyes. I. Investigations of the Primary Processes of Selenopyronine by Flash Excitation
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The triplet-state of selenopyronine absorbs light in the whole investigated spectral range (λmax = 400 nm, 480 nm, 690 nm).As results of the bimolecular triplet decay a half-reduced (λmax = 430 nm) and a half-oxidized (λmax = 475 nm) form of the dye are observed. p-Benzoquinone quenches the triplet-state (k7 = 1,5*109 l/mol s) and the results are the half-oxidized form and the p-benzosemiquinone ion.For these two products different decay processes exist.The reducing agents DABCO, EDTA and Hydroquinone also quench the triplet state (k10 = 1,2*106 l/mol s, k11 = 1,0*106 l/mol s, k12 = 1,0*109 l/mol s) and as result the half-reduced form is observed.Measurements with thiopyronine give analogous results, which are in good agreement with investigations published in the literature.
- Ortmann, W.,Kassem, A.,Hinzmann, S.,Fanghaenel, E.
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p. 1017 - 1025
(2007/10/02)
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