- Optimizing electron transfer from CdSe QDs to hydrogenase for photocatalytic H2 production
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A series of viologen related redox mediators of varying reduction potential has been characterized and their utility as electron shuttles between CdSe quantum dots and hydrogenase enzyme has been demonstrated. Tuning the mediator LUMO energy optimizes the performance of this hybrid photocatalytic system by balancing electron transfer rates of the shuttle.
- Sanchez, Monica L. K.,Wu, Chang-Hao,Adams, Michael W. W.,Brian Dyer
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- Reactions of Three Bis(viologen) Tetraquaternary Salts and Their Reduced Radicals
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Monoradical trications X3+. and diradical dications X2+.. were produced by reduction of three bis(viologen) X4+ (1,1''-alkanediylbis(1'-alkyl-4,4'-bipyridinium)) tetraquaternary salts.The spectral properties of X3+. and its disproportionation kinetics were examined by eaq-/CO2- reduction of X4+.Reduction potentials of the X4+/3+ and X3+/2+ couples were determined by cyclic voltametry and spectral examination of equilibrated mixtures of X4+ and X2+.. radicals.These potentials were used to apply successfully the Marcus theory to reduction of X4+ by SO2- and oxidation of X3+. and of X2+.. by O2 and cobalt(III) complexes.The reactivity toward O2 and the comproportionation of X2+.. (with X4+) is believed to be influenced by the tendency of X2+.. to form an intramolecularly associated conformation.
- Atherton, Stephen J.,Tsukahara, K.,Wilkins, R. G.
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- Effect of structural change in viologen acceptors on the rate of single electron transfer from tributylphosphine
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The "flexible" 3 and "rigid" cyclic viologens 4, diquarternary salts of 2,2′-bipyridine and 1,10-phenanthroline, respectively, were treated with tributylphosphine (1) in acetonitrile containing a large amount of methanol under an argon atmosphere. A single electron transfer (SET) easily occurred from the latter to the former, the SET to 4 being 105-106 times faster than the SET to 3. The reorganization energy λ for the latter SET is thought to be larger than that for the former SET, because 3 undergoes a structural change upon the one-electron reduction to its radical cation, whereas the one-electron reduction of 4 takes place without a structural change. Taking into account the difference in λ, and also taking into account the bond formation energy brought about by the follow-up reaction of the phosphine radical cation 1 + with methanol, the observed kinetics were well interpreted in terms of the Marcus theory. The Royal Society of Chemistry 2006.
- Yasui, Shinro,Itoh, Kenji,Ohno, Atsuyoshi,Tokitoh, Norihiro
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- Kinetics of Reduction of Eight Viologens by dithionite Ion
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The rate constants are reported for reduction by dithionite of methyl viologen, diquat, and six other diquaternary salts of 4,4'-bipyridine, 2,2'-bipyridine, and 1,10-phenanthroline.The active reductant is the SO2(1-) radical, and rate constants vary from >5*108, to 8.5*103 M-1s-1 with increasing negative reduction potential of the viologen.It is concluded that self-exchange rate constants for the viologens (X2+/+ couple) are ca. 108 M-1s-1, and it is supported by the results of a cross-reaction involving two viologens, the second-oreder rate constant being measured by pulse radiolytic techniques.
- Tsukahara, K.,Wilkins, R. G.
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p. 2632 - 2635
(2007/10/02)
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- Radical Cations of some Low-potential Viologen Compounds; Reduction Potentials and Electron-transfer Reactions
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The one-electron reduction potentials (E1) of certain pyrazinediium, diazepinediium and diazocinediium viologen compounds substituted with methyl groups, V(2+), have been determined from the position of the one-electron transfer equilibria with reference compounds using pulse radiolysis.E1 ranges from -491 +/- 6 mV (vs NHE) for 6,7-dihydro-2,11-dimethyldipyridopyrazinediium dibromide (V21(2+)) to -832 +/- 11 mV for 6,7,8,9-tetrahydro-2,3,12,13-tetramethyldipyridodiazocinediium dibromide (V42(2+)).The rates of reduction by e(1-)aq were found to be independent of E1 whereas the rates of electron transfer from CO2(1-). and (CH3)2C.OH species do show a dependence on E1 for V(2+) compounds of lowest E1.Marcus-type treatment of the rate-constant data yields a rate constant of electron exchange between propan-2-oxyl radicals and acetone in the range (2-6)E6 dm3mol-1s-1 and between the radical cation of 6,7-dihydro-2,3,10,11-tetramethyldipyridopyrazinediium dibromide (V22.(1+)) and its unreduced form (V22(2+)) of 5E7 dm3mol-1s-1.
- Anderson, Robert F.,Patel, Kantilal B.
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p. 2693 - 2702
(2007/10/02)
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- Magnetic Resonance Studies of Some Bipyridylium Diactions and Cation Radicals
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In an attempt to resolve ambiguities in and contradictions among previous studies, NMR spectra of the methylviologen (MV2+), benzylviologen (BV2+), and diquat (DQ2+) dications and ESR spectra of the corresponding cation radicals were reinvestigated.ESR spectra of the cation radicals in methanol solution had sufficiently good signal-to-noise ratios and resolution to permit completely unambiguous determinations of hyperfine coupling constants.ESR intensity data showed that MV+. and BV+. precipitate as ESR-inactive solids at low temperature and that MV+. associates to micelle-like solution particles at high concentration.The intensity data show no evidence for formation of dimers or other low molecular weight polymers.Literature reports of dimer formation in methanol solution are the result of temperature-dependent saturation of the ESR spectra.Line broadening of NMR spetra of MV2+, BV2+, or DQ2+ in the presence of traces of cation radical was used to provide unambiguous assignments of ESR hyperfine coupling constants, and to give good estimates of the cation radical concentrations and the dication/cation radical electron exchange rate constant.The NMR line broadening experiments also provide an estimate of the rate of conformational change of the ethylene bridge in the diquat cation radical.
- Rieger, Anne L.,Rieger, Philip H.
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p. 5845 - 5851
(2007/10/02)
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- Rates and Mechanism for Oxidation of Paraquat and Diquat Radical Cations by Several Peroxides
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The rates of oxidation of the free radicals MV(+)-radicals and DQ(+)-radicals derived from herbicides Paraquat and Diquat by hydrogen peroxide, peroxodiphosphate species, and peroxodisulfate have been investigated.Where consistent data were obtainable, the reactions are first order each in peroxide and radical.Results for H2O2 and MV(+)-radical are k = 2.0 (M*s)-1 at 25 deg C, ΔH(excit.) = 92 kJ*mol-1, and ΔS(excit.) = 73 J*(mol*K)-1; similar results were found with H2O2 and DQ(+)-radical.Although hydroxyl radicals are likely intermediates, the predicted inhibition when methanol is present did not materialize; a mechanistic rationalization is presented.The rates with peroxodiphosphate were pH dependent and could be interpreted as different contributions from H2P2O8(2-), HP2O8(3-), and P2O8(4-) in the same order as found for peroxodiphosphate and unstable radicals.The rate with S2O8(2-) is fastest, but complications prevented the evaluation of a rate constant.
- Levey, Gerrit,Rieger, Anne L.,Edwards, John O.
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p. 1255 - 1260
(2007/10/02)
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