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

The photochemical reactivity of β-lapachone (1), nor-β-lapachone (2) and β-lapachone 3-sulfonic acid (3) has been examined by laser flash photolysis. Excitation (λ = 266 nm) 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 μs, were quenched by β-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 β-lapachones 1–3 using 2-propanol and 1,4-cyclohexadiene as quenchers led us to conclude that their triplet excited states show ππ* character.