Photochemie der Farbstoffe, IV: Die Quantenausbeute der Photoreduktion von Anthrachinonderivaten mit Isopropylalkohol; der reaktive angeregte Zustand

Abstract

The quantum yields for the photoreduction of sodium anthraquinone-2-sulfonate and the 3-chloro and 3-bromo derivatives by isopropanol, on excitation with light of the wave length δ = 365 nm were determined at 25 °C. It is shown that the limiting quantum yield of semiquinone formation is two within experimental error. Of the two semiquinoneradical-ions formed for each light quantum absorbed, one is produced by the direct photochemical reaction, while the other one is the product of a follow-up reaction of the ketyl radical, derived from isopropanol. From the dependence of the quantum yield on the concentration of the substrate the rates for the desactivation of the excited states were estimated approximately. They are of the order of 2 • • • 20•10⁷ s^-1. Substitution by a chlorine atom increases this rate by a factor of 1.1, substitution by a bromine atom by a factor of 1.8. From the order of magnitude of the rate of désactivation and from the effect of heavy atoms it is suggested that the reactive excited state is the lowest triplet state of the anthraquinone derivatives.