A pulse radiolysis study of coumarin and its derivatives

The kinetics and the transient absorption spectra from the reactions of OH, O−, eaq− 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 λmax 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 (λmax=360 nm) is finally transformed by 1,2-H shift (k=2.7×104 s−1) into a thermodynamically more stable oxygen centred radical.