PHENOLIC-TYPE STABILIZERS AS GENERATORS AND QUENCHERS OF SINGLET MOLECULAR-OXYGEN (O-2((1)DELTA(G))) .1. METHYL SALICYLATE, SALICYLIC-ACID AND SOME RELATED-COMPOUNDS

The generation and quenching of singlet molecular oxygen by the widely employed photostabilizer methyl salicylate have been studied in sever al fluid solvents at room temperature. For comparative purposes, the f ollowing 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((1) Delta( g)) generators upon direct (337 nm) irradiation, and that in contrast, the phenol-methylated ester methyl 2-methoxy benzoate produces O-2((1 ) Delta(g)) with a quantum yield of 0.24 using MeCN-EtOH 4:1 as a solv ent. Under adequate environmental conditions of medium polarity and pH , all the compounds are effective O-2((1) Delta(g)) scavengers, with r ate constants for the interaction with O-2((1) Delta(g)) in the range 10(7)-10(8) M(-1) s(-1). Both overall quenching and chemical reaction with O-2((1) Delta(g)) are favoured in highly polar solvents or in the presence of bases, confirming the previously proposed mechanism throu gh charge-transfer species. The presence of an intramolecular H-bond i n salicylic acid, 5-methylsalicylic acid, 5-nitrosalicylic acid and me thyl salicylate does not exert any influence on their interaction with O-2((1) Delta(g)). With the exception of methyl 2-methoxybenzoate, th e rest of the compounds studied here can be used as efficient dopant a gents against O-2((1) Delta(g))-mediated degradation of photoprotected materials.