Flash photolysis and time-resolved electron spin resonance studies of triplet benzophenone quenching by hindered amine light stabilizers (HALS). A comparison of HALS amines and aminoethers as electron and hydrogen atom donors

Abstract

Transient absorption spectroscopy and time-resolved electron spin resonance (TR-ESR) were employed to understand the electron and hydrogen atom transfer quenching of triplet benzophenone ( 3BP) in acetonitrile and benzene by amine and aminoether derivatives of 2,2,6,6-tetramethylpiperidine (hindered amine light stabilizers, HALS). The amines ( I and II) are efficient quenchers of 3BP. The observed quenching rate constants were found to be in the range 6 × 10 8–6 × 10 9 M −1 s −1 and depend on the structure of the amine and the solvent. On the other hand, the aminoethers ( III and IV) do not exhibit any quenching at all. Reactions of 3BP with amines I and II in benzene yield ketyl radicals (detected by transient absorption spectroscopy) and the corresponding aminyl or aminomethyl radicals (detected by TR-ESR). Neither experimental method provided any evidence for products of the reactions of 3BP with aminoethers III and IV. The experimental results are discussed from the viewpoint of the mechanism of polymer stabilization by HALS.