KINETIC STUDIES ON ANTHRALIN PHOTOOXIDATION

Abstract— The photooxidation of the antipsoriatic drug anthralin (1,8-dihydroxy-9-anthrone) has been studied by several kinetic techniques, including direct observation of ¹O₂ (¹Δ_g) luminescence at 1.27 μm. The rate of deactivation of ¹O₂ increases at higher pH, demonstrating that the trihydroxy-anthracene anion is the reactive species. Direct determination of the rate constant of ¹O₂ deactivation (k_R+k_Q) in deuterated buffer systems by luminescence quenching gave a value of 3.0 times 10⁸M⁻¹⁻¹ for the anion; the neutral anthrone is unreactive. The rate constant for the neutral anthrone in benzene-d₆ is 2.8 times 10⁴ M⁻¹ s⁻¹ Competition experiments with tetramethylethylene in acetonitrile gave a rate constant for reaction alone (k_R) of 2.1 times 10⁸ M⁻¹ s⁻¹ for the anion.