Phenolic-type stabilizers as generators and quenchers of singlet molecular oxygen [formula omitted]. Part I: methyl salicylate, salicylic acid and some related compounds

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 O 2( 1Δ g) generators upon direct (337 nm) irradiation, and that in contrast, the phenol-methylated ester methyl 2-methoxy benzoate produces O 2( 1Δ g) 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 O 2( 1Δ g) scavengers, with rate constants for the interaction with O 2( 1Δ g) in the range 10 7−10 8 M −1 s −1. Both overall quenching and chemical reaction with O 2( 1Δ g) 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 O 2( 1Δ g) . With the exception of methyl 2-methoxybenzoate, the rest of the compounds studied here can be used as efficient dopant agents against O 2( 1Δ g) -mediated degradation of photoprotected materials.