A pulse radiolysis study of coumarin and its derivatives

Abstract

The kinetics and the transient absorption spectra from the reactions of •OH, O•^-, e_aq^- and SO₄^- radicals with coumarin, 4-methyl-7-hydroxycoumarin, 7-methoxycoumarin and 6,7-dimethoxycoumarin in aqueous solutions were studied by pulse radiolysis with optical detection. The second-order rate constants for the reaction of the OH radical with coumarin was found to be 6.4×10⁹ dm³ mol^-1 s^-1 and the introduction of different substituents to coumarin did not have any significant effect on the rates. The rate constants for the reaction of the hydrated electron, however, decreased with the introduction of the electron donating groups. The transient absorption spectra of the •OH adducts of coumarin and its derivatives have λ_max at 360-370 and 425-460 nm. In the case of 4-methyl-7-hydroxycoumarin, the •OH adduct undergoes dehydration leading to phenoxyl radical. The radical cation of coumarin formed by electron transfer in the SO₄•^- reaction is hydrolysed to yield the •OH adducts. The radical cation is stabilised in methoxy derivatives or undergoes deprotonation to give phenoxyl radical in 4-methyl-7-hydroxycoumarin. The initially formed radical anion of coumarin and its derivatives formed in the reaction of the hydrated electron is protonated very fast (k=10⁷ s^-1). The neutral electron adduct of coumarin (λ_max=360 nm) is finally transformed by 1,2-H shift (k=2.7×10⁴ s^-1) into a thermodynamically more stable oxygen centred radical.