A reinvestigation of the interaction between triplet states of cyclohexenones and amines

1988 ◽  
Vol 66 (10) ◽  
pp. 2595-2600 ◽  
Author(s):  
D. Weir ◽  
J. C. Scaiano ◽  
D. I. Schuster
Keyword(s):  
Triplet State ◽  
Rate Constant ◽  
Flash Photolysis ◽  
Laser Flash Photolysis ◽  
Radical Cations ◽  
Laser Flash ◽  
Triplet States ◽  
New Evidence

Laser flash photolysis studies lead to the conclusion that the short-lived triplet states of cyclohexenones are readily quenched by amines. For example, in the case of 2-cyclohexen-1-one (1) its triplet state (τT = 40 ns in acetonitrile) is quenched by triethylamine with a rate constant of (9.0 ± 0.8) × 107 M−1 s−1. Cyclohexenone triplets are also quenched efficiently by DABCO and by triphenylamine leading to the formation of the corresponding amine radical cations. The new evidence reported rules out the involvement of long-lived detectable exciplexes.

Mechanism of acid-catalyzed photoaddition of methanol to 3-alkyl2-cyclohexenones

1995 ◽  
Vol 73 (11) ◽  
pp. 2004-2010 ◽  
Author(s):  
David I. Schuster ◽  
Jie-Min Yang ◽  
Jan Woning ◽  
Timothy A. Rhodes ◽  
Anton W. Jensen
Keyword(s):  
Acetic Acid ◽  
Triplet State ◽  
Rate Constant ◽  
Flash Photolysis ◽  
Laser Flash Photolysis ◽  
Laser Flash ◽  
Product Formation ◽  
Triplet States ◽  
Quenching Rate ◽  
Acid Catalyzed

Contrary to a previous report, it is concluded that formation of methanol adducts to 3-methyl-2-cyclohexenones and of deconjugated enones on irradiation of the enones in acidified solutions proceeds via protonation of the intermediate enone π,π* triplet excited state and not by protonation of a relatively long-lived ground state trans-cyclohexenone. A rate constant for protonation of the triplet state of 3-methyl-2-cyclohexenone by sulfuric acid of 1.7 × 109 M−1 s−1 was determined by laser flash photolysis in ethyl acetate. Based on quantum efficiencies of product formation, a rate constant of ca. 108 M−1 s−1 was estimated for protonation of the enone triplet by acetic acid, which is too small to cause measurable reduction in the triplet state lifetime in the mM concentration range used in the preparative studies. The intermediate carbocation can be trapped by methanol, or revert to starting enone or the exocyclic deconjugated enone by loss of a proton. Since products revert to starting materials in an acid-catalyzed process, there is an acid concentration at which the yields of products are optimal. This concentration is ca. 6 mM for acetic acid, but is only 0.1 mM for p-toluenesulfonic or sulfuric acids. Product formation could be quenched using 1-methylnaphthalene and cyclopentene as triplet quenchers; in the latter case, formation of [2 + 2] photoadducts was observed to compete with formation of methanol adducts. Quenching rate constants were determined by laser flash studies. Keywords: laser flash photolysis, kinetic absorption spectroscopy (KAS), photoacoustic calorimetry (PAC), protonation of triplet states, trans-cyclohexenones.

Kinetic study of the reactions of methoxy-substituted phenacyl radicals

1995 ◽  
Vol 73 (2) ◽  
pp. 223-231 ◽  
Author(s):  
S.V. Jovanovic ◽  
J. Renaud ◽  
A.B. Berinstain ◽  
J.C. Scaiano
Keyword(s):  
Rate Constant ◽  
Flash Photolysis ◽  
Laser Flash Photolysis ◽  
Chemical Reactivity ◽  
Laser Flash ◽  
Carbonyl Oxygen ◽  
Peroxyl Radicals ◽  
Quantum Yields ◽  
Triplet States ◽  
Singlet States

The photochemistry of various mono- and dimethoxy-substituted α-bromoacetophenones has been investigated by laser flash photolysis in organic solvents. The short-lived excited singlet states cleave to yield bromine atoms and the corresponding methoxyphenacyl radicals with quantum yields ranging from 0.13 to 0.35. With the exception of 4-methoxy-α-bromoacetophenone (6), all other substrates yield readily detectable triplet states; these have π,π* character and are very poor hydrogen abstractors. Triplet decay does not contribute to methoxyphenacyl radical formation. While methoxyphenacyl radicals may have a spin density as high as 0.3 at the carbonyl oxygen, which accounts for the alkoxy-like visible band in their absorption spectrum, their chemical reactivity is dominated by the radical character at the carbon site. Methoxyphenacyl radicals are modest hydrogen abstractors; for example, reaction of 6 with 1,4-cyclohexadiene occurs with a rate constant of 2.6 × 105 M−1 s−1, while for addition to the double bond in 1,1-diphenylethylene the rate constant is 9.4 × 107 M−1 s−1. Additions to other double bonds are likely to be slower (e.g., 12 + 1,3-cyclohexadiene, k ≤ 105 M−1 s−1). In contrast, reaction with oxygen occurs with kq = 2.5 × 109 M−1 s−1. Under the conditions relevant for their participation in paper yellowing, the methoxyphenacyl radicals will be converted to peroxyl radicals. These probably play a key role in the oxidative photodegradation of lignin. Keywords: methoxyphenacyl radicals, α-bromoacetophenones, laser flash photolysis.

Effect of Aggregation on Bacteriochlorin a Triplet-state Formation: A Laser Flash Photolysis Study¶

2002 ◽  
Vol 76 (5) ◽  
pp. 480 ◽  
Author(s):  
Xavier Damoiseau ◽  
Francis Tfibel ◽  
Maryse Hoebeke ◽  
Marie-Pierre Fontaine-Aupart
Keyword(s):  
Triplet State ◽  
State Formation ◽  
Flash Photolysis ◽  
Laser Flash Photolysis ◽  
Laser Flash

Laser flash photolysis studies of dibenzosuberenyl cations and radical cations

1989 ◽  
Vol 93 (21) ◽  
pp. 7370-7374 ◽  
Author(s):  
L. J. Johnston ◽  
J. Lobaugh ◽  
V. Wintgens
Keyword(s):  
Flash Photolysis ◽  
Laser Flash Photolysis ◽  
Radical Cations ◽  
Laser Flash

Photochemical Properties of Excited Triplet State of 6H-Purine-6-thione Investigated by Laser Flash Photolysis

1998 ◽  
Vol 102 (8) ◽  
pp. 1338-1344 ◽  
Author(s):  
Maksudul M. Alam ◽  
Mamoru Fujitsuka ◽  
Akira Watanabe ◽  
Osamu Ito
Keyword(s):  
Triplet State ◽  
Flash Photolysis ◽  
Laser Flash Photolysis ◽  
Laser Flash ◽  
Excited Triplet State ◽  
Excited Triplet ◽  
Photochemical Properties

Photochemical reaction kinetics and mechanism of bisphenol A with K2S2O8 in aqueous solution: a laser flash photolysis study

2021 ◽  
Vol 99 (1) ◽  
pp. 43-50
Author(s):  
Yongchao Zhu ◽  
Mengyu Zhu ◽  
Jingjing Xie ◽  
Yadong Hu ◽  
Ying Liu ◽  
...  
Keyword(s):  
Bisphenol A ◽  
Rate Constant ◽  
Photochemical Reaction ◽  
Flash Photolysis ◽  
Laser Flash Photolysis ◽  
Phenoxyl Radical ◽  
Laser Flash ◽  
Kinetics And Mechanism ◽  
Main Reaction ◽  
Specific Rate

The photochemical reaction kinetics and mechanism of bisphenol A (BPA) with potassium persulfate (K2S2O8) were investigated by using 266 nm laser flash photolysis and gas chromatography mass spectrum (GC-MS) technique. Sulfate radical (SO4•−), generated upon K2S2O8 photolysis, reacted with BPA with the overall rate constant of (1.61 ± 0.15) × 109 L mol−1 s−1, and two main reaction mechanisms were involved. One was addition channel to generate BPA–SO4•− adduct with a specific second-order rate constant of (1.09 ± 0.15) × 109 L mol−1 s−1. Molecular oxygen was involved in the decay of the BPA–SO4•− adduct with a rate constant of (1.28 ± 0.14) × 108 L mol−1 s−1. Another channel was the formation of BPA’s phenoxyl radical, likely derived from a deprotonation of the cation radical (BPA•+) generated from single electron transfer reactions. The specific rate constant of BPA’s phenoxyl radical formation was determined to be (6.16 ± 0.08) × 108 L mol−1 s−1. The overall rate constant was in line with the sum of aforementioned two specific rate constants for two main reaction channels. By comparing these rate constants, it was indicated that SO4•− addition channel accounted for ∼65% (1.09/1.61) to the overall reaction, and phenoxyl radical formation accounted for only ∼35% (0.62/1.61). The transformation products of BPA were identified by using GC-MS including 4-isopropylphenol, 4-isopropenylphenol, and 2,4-di-tert-butylphenol, and the reaction mechanism was proposed. These results may provide microscopic kinetics and mechanism information on BPA degradation using SO4•−-based advanced oxidation processes.

LASER FLASH PHOTOLYSIS STUDIES OF CHLORIN AND PORPHYRIN SYSTEMS. I. ENERGETICS OF THE TRIPLET STATE OF BACTERIOCHLOROPHYLL

1973 ◽  
Vol 206 (1 The Chemical) ◽  
pp. 649-669 ◽  
Author(s):  
John S. Connolly ◽  
Donald S. Gorman ◽  
G. R. Seely
Keyword(s):  
Triplet State ◽  
Flash Photolysis ◽  
Laser Flash Photolysis ◽  
Laser Flash
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