Laser flash photolysis study of the triplet reactivity of beta-lapachones

摘要

The photochemical reactivity of beta-lapachone(1),nor-beta-lapachone(2)and beta-lapachone 3-sulfonic acid(3)has been examined by laser flash photolysis.Excitation(gamma=266 ma)of degassed solutions of 1-3,in acetonitrile or dichloromethane,resulted in the formation of detectable transients with absorption maxima at 300,380 and 650 nm.These transients,with lifetimes of 5.0 us,were quenched by beta-carotene at a diffusion-controlled rate constant and assigned to the triplet excited states of 1-3.Addition of hydrogen donors,such as 2-propanol,1,4-cyclohexadiene,4-methoxyphenol or indole led to the formation of new transients,which were assigned to the corresponding ketyl radicals obtained from the hydrogen abstraction reaction by the triplets 1-3.In the presence of triethylamine it was observed the formation of the long-lived anion radical derived from 1-3,which shows absorption maxima at 300 and 380 nm.The low values observed for the hydrogen abstraction rate constants for the beta-lapachones 1-3 using 2-propanol and 1,4-cyclohexadiene as quenchers led us to conclude that their triplet excited states show pi pi* character.