Hindered Amiie Lighl Stabilizers (HALS) are know to inhiba the photooxidation of polymers. A key reaction in their stabilkation mechanism is believed to be the conversion of a hindered aminoether into a nitroxyl radii. Several different possible mechanisms for this conversion were explored. One, the elimination of the am'noether to form an olefin and hydroxylamine (an intermediate in the formation of a nitroxyl), while possible at high temperatures, cannot account for the inhibitory activity we observed for secondary and primary aminoethers. Direct radical displacement by peroxy radicals was atso considered. However, the products predicted by this reaction pathway were nd observed.Finally, oxidation of the nitrogen by a pemxy radical, by either electron transfer or a radical attack on the nitrogen, was investigated. While electron transfer was shown to be unlikely, direct oxidation of the aminoether nitrogen was supported by our results. A detailed mechanism for the reaction of both alkyl-and acyl-proxy radcals with aminoethers is proposed.
The photochemistry and photophysics of the title compound (2), which finds application as a photoacid generator in photoresist formulations, has been investigated using a combination of laser flash photolysis work and emission spectroscopy, as well as evaluation of acid formation. The studies were carried out in polar (acetonitrile) and nonpolar (cyclohexane, benzene) solvents, employing both direct excitation and sensitized conditions using isopropylthioxanthone (ITX) and acetone as sensitizers. In nonpolar medium, photolysis of 2 follows a conventional mechanism involving C-Cl cleavage. In contrast, direct excitation of 2 in acetonitrile leads to C-Cl heterolysis with formation of the cation, which can be readily identified by its rapid quenching by nucleophiles such as halide anions, azide, and methanol. Interestingly, 2 + must rearrange for its spectroscopic and kinetic parameters to be consistent with carbocation structures. Several possible structures are proposed for 2 + and its rearranged isomers. Photolysis of 2 in the presence of ITX or acetone also leads to the carbocation, but the yields increase by more than 1 order of magnitude, indicating a higher efficiency of the triplet reaction compared to the singlet reaction. No evidence for electron transfer between 2 and ITX could be found; i.e., the sensitization is entirely due to T-T energy transfer. The quantum yield of HCl generation upon direct excitation (monitored in the microsecond time scale) is very low (0.007); sensitization increases it to 0.111 (acetone) and 0.074 (ITX). In nonpolar solvents, the primary photochemical step in the direct photolysis of these compounds is the homolysis of one of the carbon-chlorine bonds; the chlorine atoms formed in this reaction can be detected by complexation with benzene. Contrary to the ionic pathway, the quantum yield of the homolysis is rather insensitive to sensitization.
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