Phenols from singlet

The above considerations bear on the ambient chemiluminescence from polypropylene, although at 150°C one would still expect that the fluorescence emission from singlet carbonyls would be relatively unaffected by quenching from stabilizers in our samples. One can also infer that physical quenching alone is not sufficient to explain the decrease in light from the polypropylene sample with both phenol and thioether (Table II, comb), since the initial reduction in intensity is more than can be accounted for by the combined effects of the additives separately. 

Addition to arene oxides (5, 480). Photosensitized oxygenation of benzene oxide (1) gives mainly phenol. However, singlet oxygen generated from... 

Figure 6.15 Influence of pH on the rate of oxidation of aqueous solutions of phenols by singlet oxygen. [Reproduced from F. E. Scully and J. Hoigne, Rate constants for reactions of singlet oxygen with phenols and other compounds , Chemosphere 16, 681. Copyright 1987, with permission from Elsevier.]...

Products from the photolysis of the cyclopropyl ketone (44) are dependent on the pH of the solvent.(42) In aqueous dioxane only the 2,3-diphenyl-phenol (45) is formed along with the photoacid (46). Bond cleavage at c takes place via both the singlet and triplet states, whereas bond cleavage at a with phenol formation takes place in the triplet state ... 

Example Ethyl loss clearly predominates methyl loss in the El mass spectrum of 2-(l-methylpropyl)-phenol. It proceeds via benzylic bond cleavage, the products of which are detected as the base peak at m/z 121 and m/z 135 (3 %), respectively (Eig. 6.34a). The McLafferty rearrangement does not play a role, as the peak at m/z 122 (8.8 %) is completely due to the isotopic contribution to the peak at m/z 121. From the HR-El spectrum (Fig. 6.34b) the alternative pathway for the formation of a [M-29] peak, i.e., [M-CO-H]", can be excluded, because the measured accurate mass of this singlet peak indicates CgHgO". HR-MS data also reveal that the peak at m/z 107 corresponds to [M-CHs-CO]" and that the one at m/z 103 corresponds to [M-C2H5-H20]. Although perhaps unexpected, the loss of H2O from phenolic fragment ions is not unusual. 

Singlet excited state acid dissociation constants pK can be smaller or greater than the ground state constant pK by as much as 8 units. Phenols, thiols and aromatic amines are stronger acids upon excitation, whereas carboxylic acids, aldehydes and ketones with lowest >(71, ) states become much more basic. Triplet state constants pKr are closer to those for the ground state. Forster s cycle may be used to determine A pK =pK —pK) from fluorescence measurements if proton transfer occurs within the lifetime of the excited molecule. 

Photochemical rearrangement of 1, 4, and 64 has been examined in dichloromethane by steady irradiation and nanosecond transient spectroscopy. The ring enlargement of 1 to 292 and 4 to 293, respectively, has been postulated to occur via the singlet excited state. Phenolic derivatives 294 and 295 are formed from 1 and 4, respectively, via their triplet state. Product sensitization is proposed for the conversion of 1 and 4 to 294 and 295, respectively.167 Van Duuren et al. have found that on UV irradiation (254 nm) in methylene chloride, 4 rearranges to oxepin 293,4-hydroxypyrene, pyrene, and other unidentified products.168... 

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