'The photochemistry of various mono-and dimethoxy-substituted a-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-a-bromoacetophenone (6), all other substrates yield readily detectable triplet states; these have .rr,.rre 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 x 10-5 M-I s-I, while for addition to the double bond in I,]-diphenylethylene the rate constant is 9.4 x lo7 M-' s-'. Additions to other double bonds are likely to be slower (e.g., 12 + 1,3-cyclohexadiene, k 5 10' M-' s-I). In contrast, reaction with oxygen occurs with kc] = 2.5 x 10%~-' s-I. 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.Key rvords: methoxyphenacyl radicals, a-bromoacetophenones, laser flash photolysis.
Rksum6: Faisant appel ii la photolyse laser Cclair, on a ttudit la photochimie de diverses mono-ou dimCthoxy-a-bromoacCtophCnones dans des solvants oganiques. Les &tats singulets excitCs de courte durCe de vie se brisent pour donner des atomes de brome et les radicaux mCthoxyphCnacyles correspondants, avec des rendements quantiques allant de 0,13 ii 0,35. A I'exception de la 4-mCthoxy-a-bromoacCtophCnone (6), tous les autres substrats fournissent des ttats triplets facilement dCtectables; ils posskdent un caractkre .rr,.rr:"t une trks mauvaise habilitC ii enlever des hydrogknes. La dCgCnCrescence de 1'Ctat triplet ne contribue pas la formation du radical mCthoxyphCnacyle. Alors que les radicaux mCthoxyphCnacyles peuvent possCder une densite de spin allant jusqu'ii 0.3 au niveau de I'oxygkne du carbonyle, ce qui explique la bande semblable ii celle des alcoxy dans leur spectre d'absorption, leur rCactivitC chimique est dominee par le caractere radicalaire au site carbonC. L'habilitC des radicaux mCthoxyphCnacyles ii enlever des hydrogknes est modeste; par exemple, la rCaction du composC 6 avec le cyclohexa-1,4-dikne se produit avec une constante de vitesse de 2,6 x 10' M-' s-', alors que, pour l'addition ii la double liaison du 1,l -diphCnylCthylkne, la constante de vitesse est de 9,4 x lo7 M-' s-I. 11 est probable que les additions sur d'autres doubles liaisons seront plus lentes (par exemple, pour 12 + cyclohexa-l,3-dikne, k 5 10-5 M-' s-I). Par ailleurs, la rC...