Assignment of transient species observed on laser flash photolysis of p-benzoquinone and methylated p-benzoquinones in aqueous solution

Ronfard-Haret, J.C.; Bensasson, René V.; Amouyal, Edmond

doi:10.1039/f19807602432

J. Chem. Soc., Faraday Trans. 1

1980

DOI: 10.1039/f19807602432

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Assignment of transient species observed on laser flash photolysis of p-benzoquinone and methylated p-benzoquinones in aqueous solution

J.C. Ronfard-Haret¹,

René V. Bensasson²,

Edmond Amouyal³

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Cited by 44 publications

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“…This has been attributed to the formation of free radicals. [32] Since only one Q concentration had been studied, it escaped attention [32,38] that the radical yield is also a function of the Q concentration ( Figure 6, lower curve, left scale). In a flash photolysis study on duroquinone (2,3,5,6-tetramethyl-1,4-benzoquinone) in aqueous solution, [84] free-radical formation was only observed upon triplet±triplet annihilation, that is, at high triplet concentrations.…”

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confidence: 98%

“…[32,52,53,58,64,70] However, it has been calculated that the reduction potential of the relaxed triplet state of Q is insufficient to oxidize water, and it has been suggested that the water oxidation (formation of 1,4-benzosemiquinone CQH observed by time-resolved Raman spectroscopy) must be due to the reaction of an unrelaxed triplet state. [35] As another potential route to COH, electron transfer from a ground-state quinone to an excited quinone and oxidation of OH À or water by the ensuing quinone radical cation has been considered.…”

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confidence: 99%

“…[32] In this study, [32] where the UV absorption spectrum of the triplet was given in the range between 350 and 500 nm, the transient displayed a maximum at 410 nm, but the much stronger one at 282 nm (cf. Figure 5) had been missed, and also in the more recent study on 2-methyl-1,4-benzoquinone, [60] no attempt has been made to measure the triplet spectrum below 330 nm.…”

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confidence: 99%

“…[39] There are, however, observations that are not compatible with free COH as intermediates. The addition of an excess of tertiary butyl alcohol that would have scavenged all free COH does not affect the spectral developments after a laser flash up to 200 ms. [32] Also a kind of warning is the fact that acetone in its triplet state does not abstract a hydrogen ). [74] In the present paper, we will show that neither a free COH radical nor an COH radical complexed to CQH are intermediates in the photohydroxylation reaction, and evidence for an alternative mechanism will be given.…”

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confidence: 99%

“…The rate of triplet decay has been reported at k = 1.9 î 10 6 s À1 . [32] In [a] Upper limit for the uncatalyzed reaction. For proton and buffer catalysis see text.…”

mentioning

confidence: 99%

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Photohydroxylation of 1,4‐Benzoquinone in Aqueous Solution Revisited

Sonntag

Mvula

Hildenbrand

et al. 2004

Chemistry A European J

129

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In water, photolysis of 1,4-benzoquinone, Q gives rise to equal amounts of 2-hydroxy-1,4-benzoquinone HOQ and hydroquinone QH(2) which are formed with a quantum yield of Phi=0.42, independent of pH and Q concentration. By contrast, the rate of decay of the triplet (lambda(max)=282 and approximately 410 nm) which is the precursor of these products increases nonlinearly (k=(2-->3.8) x 10(6) s(-1)) with increasing Q concentration ((0.2-->10) mM). The free-radical yield detected by laser flash photolysis after the decay of the triplet also increases with increasing Q concentration but follows a different functional form. These observations are explained by a rapid equilibrium of a monomeric triplet Q* and an exciplex Q(2)* (K=5500+/-1000 M(-1)). While Q* adds water and subsequent enolizes into 1,2,4-trihydroxybenzene Ph(OH)(3), Q(2)* decays by electron transfer and water addition yielding benzosemiquinone (.)QH and (.)OH adduct radicals (.)QOH. The latter enolizes to the 2-hydroxy-1,4-semiquinone radical (.)Q(OH)H within the time scale of the triplet decay and is subsequently rapidly (microsecond time scale) oxidized by Q to HOQ with the concomitant formation of (.)QH. On the post-millisecond time scale, that is, when (.)QH has decayed, Ph(OH)(3) is oxidized by Q yielding HOQ and QH(2) as followed by laser flash photolysis with diode array detection. The rate of this pH- and Q concentration-dependent reaction was independently determined by stopped-flow. This shows that there are two pathways to photohydroxylation; a free-radical pathway at high and a non-radical one at low Q concentration. In agreement with this, the yield of Ph(OH)(3) is most pronounced at low Q concentration. In the presence of phosphate buffer, Q* reacts with H(2)PO(4) (-) giving rise to an adduct which is subsequently oxidized by Q to 2-phosphato-1,4-benzoquinone QP. The current view that (.)OH is an intermediate in the photohydroxylation of Q has been overturned. This view had been based on the observation of the (.)OH adduct of DMPO when Q is photolyzed in the presence of this spin trap. It is now shown that Q*/Q(2)* oxidizes DMPO (k approximately 1 x 10(8) M(-1) s(-1)) to its radical cation which subsequently reacts with water. Q*/Q(2)* react with alcohols by H abstraction (rates in units of M(-1) s(-1)): methanol (4.2 x 10(7)), ethanol (6.7 x 10(7)), 2-propanol (13 x 10(7)) and tertiary butyl alcohol ( approximately 0.2 x 10(7)). DMSO (2.7 x 10(9)) and O(2) ( approximately 2 x 10(9)) act as physical quenchers.

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mentioning

confidence: 98%

mentioning

confidence: 99%

mentioning

confidence: 99%

mentioning

confidence: 99%

“…The rate of triplet decay has been reported at k = 1.9 î 10 6 s À1 . [32] In [a] Upper limit for the uncatalyzed reaction. For proton and buffer catalysis see text.…”

mentioning

confidence: 99%

See 3 more Smart Citations

Photohydroxylation of 1,4‐Benzoquinone in Aqueous Solution Revisited

Sonntag

Mvula

Hildenbrand

et al. 2004

Chemistry A European J

129

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Photolytic crosslinking of poly(p‐hydroxystyrene)

Nakabayashi

Schwalm

Schnabel

1992

Angew. Makromol. Chem.

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Light-induced crosslinking of poly(p-hydroxystyrene) (PPHS) is significantly enhanced by 02. This was evidenced by molar mass (light scattering measurements) and by gel content determinations which were performed on various polymer samples before and after continuous irradiation at hinc = 254 nm. The following mechanism CCC 0003-3146/92/$04.00 191 K. Nakabayashi, R. Schwalm, W. Schnabel stand befinden, erzeugt. Triplett-angeregte phenolische Gruppen, die sich auch bilden, desaktivieren nicht durch Spaltung der 0-H-Bindungen. Sie reagieren allerdings sehr effektiv, sofern vorhanden, mit 0,. Bei dieser Reaktion entstehen HOG-und zusatzliche Phenoxylradikale. Alle handelsiiblichen und die meisten im Labor synthetisierten PPHS-Proben enthalten chemisch gebundene, chinoidartige Verunreinigungen. Durch Versuche mit niedermolekularen Modellverbindungen wurde nachgewiesen, daL3 triplett-angeregte chinoide Gruppen einerseits mit phenolischen Gruppen unter Bildung von Phenoxylradikalen reagieren und daB sie andererseits unreaktiv sind beziiglich der Abstraktion aliphatischer Wasserstoffatome. Die Bestrahlung von PPHS bei hinc = 254 nm bewirkt die Bildung chinoider Gruppen, die bei dieser Wellenlange auch stark absorbieren. Die Lichtabsorption dieser Gruppen wird im Laufe der weiteren Bestrahlung zum bestimmenden Faktor hinsichtlich der photochemischen Veranderung des Polymeren.

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Photooxidation von Leukofarbstoffen. XII. Kurzzeitspektroskopische Untersuchungen zur Photooxidation von Bis‐schwefelsäuremonoestern der Leukoformen von Thioindigoverbindungen

et al. 1985

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Photooxidation of Leuco Dyes. XII. Time Resolved Investigations on the Photooxidation of Bis‐sulfuric Acid Monoesters of Leucothioindigoid Compounds The photooxidation of 4, 4′‐dimethyl‐6, 6′‐dichlorothioindigosol (1) was studied by means of laser and conventional flash photolysis. Three transients were detected and assigned to a radical R generated by homolytic splitting of one OS‐bond of the parent compound and to two further radicals which are formed from R by hydrolytic cleavage of the second ester group. The latter two radicals are in a protolytic equilibrium with each other and react, depending on the nature of the solvent by H‐atom abstraction, or by disproportionation forming the leuco‐thioindigoid dye or both dye and leuco dye. A mechanistic scheme is proposed.

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Assignment of transient species observed on laser flash photolysis of p-benzoquinone and methylated p-benzoquinones in aqueous solution

Cited by 44 publications

References 0 publications

Photohydroxylation of 1,4‐Benzoquinone in Aqueous Solution Revisited

Photohydroxylation of 1,4‐Benzoquinone in Aqueous Solution Revisited

Photolytic crosslinking of poly(p‐hydroxystyrene)

Photooxidation von Leukofarbstoffen. XII. Kurzzeitspektroskopische Untersuchungen zur Photooxidation von Bis‐schwefelsäuremonoestern der Leukoformen von Thioindigoverbindungen

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