A. Soltermann, D. Peña, S. Nonell
1995
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Journal
Polymer Degradation and Stability
Abstract
The generation and quenching of singlet molecular oxygen by the widely employed photostabilizer methyl salicylate have been studied in several fluid solvents at room temperature. For comparative purposes, the following related compounds were also studied: methyl 2-methoxybenzoate, methyl 4-hydroxybenzoate, methyl 3-hydroxybenzoate, salicylic acid, 5-methylsalicylic acid and 5-nitrosalicylic acid. The results indicate that the compounds with a phenol group are inefficient O2(1Δg) generators upon direct (337 nm) irradiation, and that in contrast, the phenol-methylated ester methyl 2-methoxy benzoate produces O2(1Δg) with a quantum yield of 0.24 using MeCN-EtOH 4:1 as a solvent. Under adequate environmental conditions of medium polarity and pH, all the compounds are effective O2(1Δg) scavengers, with rate constants for the interaction with O2(1Δg) in the range 107−108 M−1 s−1. Both overall quenching and chemical reaction with O2(1Δg) are favoured in highly polar solvents or in the presence of bases, confirming the previously proposed mechanism through charge-transfer species. The presence of an intramolecular H-bond in salicylic acid, 5-methylsalicylic acid, 5-nitrosalicylic acid and methyl salicylate does not exert any influence on their interaction with O2(1Δg). With the exception of methyl 2-methoxybenzoate, the rest of the compounds studied here can be used as efficient dopant agents against O2(1Δg)-mediated degradation of photoprotected materials.