Photochemistry of cation radicals in solution : photoinduced oxidation by the phenothiazine cation radical
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Laser flash photolysis and EPR studies were performed to elucidate the mechanism of photoinduced step polymerization of thiophene by using diphenyliodonium (Ph2I+) and triphenylsulphonium (Ph3S+) ions as photoinitiators. Photoexcitation of these ions generated phenyliodinium (PhI•+) and diphenylsulphinium (Ph2S•+) radical cations, which were readily quenched by thiophene with rate constants of kq = 1.26 × 1010 and 1.7 × 105 M-1 s-1, respectively. The transient absorption spectra of the corresponding thiophene radical cations were not directly detectable because of the spectral overlap with the precursor salts. However, the related electron-transfer reaction was confirmed by quenching of the PhI•+ radical cation with bithiophene to form the radical cation of bithiophene, which absorb strongly at 420 nm. EPR studies also confirmed the proposed electron-transfer mechanism through the direct detection of the radical cation of thiophene.
Radical ion
Flash photolysis
Photoinduced electron transfer
Photoexcitation
Onium
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[structure: see text] A trimeric phenothiazine and its radical cation were prepared, and their structures were elucidated. In contrast to a largely twisted structure in the neutral species, the radical cation had a unique structure deformation that allowed charge-transfer-type conjugation from the outer phenothiazine rings to the central phenothiazine radical cation.
Trimer
Radical ion
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2,2,6,6-Tetramethyl-4-acetyloxypiperidine oxoammonium hexachloroantimonate (1) was used as an one electron oxidant to prepare single crystals of N-methyl, N-ethyl, N-n-butyl, N-phenyl and N-p-nitrophenyl-phenothiazine radical cation hexachloroantimonates (3a-3e) respectively. Molecular structure of 3a and 3b were evaluated by X-ray difraction analysis. It is shown that the configuration of the radical cations are quite different from those of their neutral molecules, indicating that strong conjugative and hyperconjugative effects are present in the radical cations.
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Abstract Photoreactions of aqueous solutions of synthetic water‐soluble porphyrins were studied by the 1 H and 13 C CIDNP technique. Strong polarizations, which were very sensitive to the presence of added acid, were observed on the cationic porphyrins (TMePyPH 2 ‐TAPPH 2 ) when irradiated through continuous UV‐visible light. They resulted from the reverse electron transfer between the semi‐oxidized and the semi‐reduced species of the derivative. When the experiments were carried out in the presence of nucleobases, guanine (and its derivatives) was the only residue that was polarized. This is thoroughly interpreted in terms of a reversible electron transfer reaction leading to guanine photooxidation by the porphyrin excited triplet state. It was shown to be drastically pH‐dependent and was correlated to the redox potential of the porphyrin. It was not affected by the incident wavelength. The reaction proceeded through the intermediate formation of the correlated radical‐ion pair: porphyrin radical anion‐guanine radical cation. This study suggested that a Type I (free radical) reaction could be one of the primary processes in DNA photosensitization by porphyrins.
Radical ion
CIDNP
Photoinduced electron transfer
Nucleobase
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14N/15N ISOTOPE EFFECT ON THE ELECTRON TRANSFER PROCESS BETWEEN PHENOTHIAZINE AND ITS RADICAL CATION
An appreciable equilibrium isotope effect has been observed for electron transfer from phenothiazine (PT) to the radical cation of its 15N-substituted analogue ([15N]PT+·), i.e. PT+[15N]PT+· ⇋ K PT+·+[15N]PT via electron paramagnetic resonance analysis of the mixed radical cations formed from mixing the [15N]phenothiazine radical cation hexachloroantimonate and phenothiazine in acetonitrile (K=0·77±0·10 at 25 °C), and by physical separation of the neutral phenothiazines from the radical cation salts in the equilibrium mixture (K=0·83±0·10 at 25 °C). Infrared and Raman spectra of [14N]- and [15N]phenothiazines and their radical cations were measured to assign the vibrational frequency shifts caused by the heavy-atom substitution and radical cation formation, which gave an estimate of the enthalpy change of 441·7 J mol−1 for the electron transfer process. These results reveal that 15N substitution of phenothiazine decreases appreciably the ionization potential of the molecule, making it easier to lose an electron to form the corresponding radical cation in solution. © 1997 by John Wiley & Sons, Ltd.
Radical ion
Kinetic isotope effect
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Abstract The kinetics and mechanism of the photochemical reactions of the peroxide radicals have been investigated in polytetrafluorethylene by electron paramagnetic resonance. Under irradiation by UV light, with λ < 300 nm, the peroxide radicals ˜CF 2 CF(O 2 )CF 2 ∼ and ∼CF 2 CF 2 O 2 are dissociated and the end fluoralkyl radicals ∼CF 2 CF 2 are produced. The activation effective energy of the photodissociation of the peroxide radicals is equal to approximately 1‐2 kcal/mole. Quantum yield of the dissociation of peroxide radicals is equal to about 1. In the presence of oxygen the photorecombination of the peroxide radicals takes place. The photorecombination of the peroxide radicals bring about a reversal of the reactions of the dissociation of the end peroxide radicals and the addition of oxygen to the end fluoralkyl radical. Due to a dependence upon the initial concentration of peroxide radicals, the quantity of molecules of CF 2 O produced varies from 25 to 45 per one vanished radical. The rate of the photorecombination of the peroxide radical is in linear dependence on light intensity and in the interval 150‐500 torr does not depend upon oxygen pressure.
Peroxide
Organic peroxide
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The mechanism of bacterioviridin photochemical oxidation has been studied by the methods of ESR, flash-photolysis and low-temperature spectrophotometry. ESR spectrum of pigment cation-radical, a singlet line with H=11 G, g = 2.0027, has been recorded. The bands with maxima at 370, 470, 525, 590, 840 nm correspond to bacterioviridin cation -- radical in the absorption spectra. When -- benzoquinone is used as an electron acceptor with excitation light 640 nm the product of bacterioviridin irreversible oxidation is formed with the absorption band maximum 760 nm and absorption between 350 and 370 nm. It is suggested that this product is of double-oxidized non-radical nature and the mechanism of its formation through oxidation of the pigment cation-radical is discussed. The regeneration reaction of double-oxidized bacterioviridin up to cation-radical form in the presence of triphenylamine as a reducing agent has been carried out. The rate constants of cation-radical decay in the dark and desactivation of triplet state have the following values: K1=(1,64+/-0,15)-10(3) sec-1, K2=(13+/-2,0)-10(3) sec-1 correspondingly. The activation energy of the radical decay in the dark is Eact =(13,2-0,5) kcal/mole.
Flash photolysis
Radical ion
Triphenylamine
Hydroxyl radical
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Photodegradation
Hydroxyl radical
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This chapter contains sections titled: Introduction Formation of Phenoxyl Radicals Oxidation of Phenols by Metal Ions Oxidation of Phenols by Free Radicals Oxidation of Phenols by Radical Cations Reaction of Phenols with Hydroxyl Radicals. The Addition/Elimination Mechanism Formation of Phenoxyl Radicals by Oxidative Replacement of Substituents Formation of Phenoxyl Radicals by Intramolecular Electron Transfer Formation of Phenoxyl Radicals from Phenols and Hydroxyphenols by Reaction with O2 and/or O2·− (Autoxidation) Properties of Phenoxyl Radicals Electron Spin Resonance Spectra of Phenoxyl Radicals Phenoxyl and Monosubstituted Phenoxyl Radicals Phenoxyl Radicals with Extended π-systems Kinetic ESR Measurements Comparison with Isoelectronic Radicals Optical Spectra of Phenoxyl Radicals Acid–Base Equilibria of Phenoxyl Radicals Reactions of Phenoxyl Radicals Reduction Potentials of Phenoxyl Radicals References
Autoxidation
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Radical cations generated from thioxanthene and xanthene undergo structural relaxation in low-temperature hydrocarbon glasses upon warming. Under conditions of unrestricted diffusion, deprotonation of the radical cations occurs and results in the formation of the thioxanthyl and xanthyl radicals. Subsequent electron loss leads ultimately to the closed-shell thioxanthyl and xanthyl cations. Using combined steady-state and time-resolved radiolytic and photochemical methods, the radical cations, radicals, and cations of interest were spectroscopically and kinetically characterized.
Xanthene
Radical ion
Solvated electron
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