Electrochemical and UV-spectrophotometric study of oxygen and superoxide anion radical interaction with anthraquinone derivatives and their radical anions

One-electron reduction of dihydroxyanthraquinone derivatives was studied in dimethylsulfoxide solution by means of electrochemical and spectrophotomet ric methods. It was found that dihydroxyanthraquinone reduction in the pres ence of oxygen occurs according to an EC mechanism, the second step being o xygen reaction with semiquinone anion radical. This mechanism was confirmed by the dependence of CV curves on dihydroxyanthraquinone and oxygen concen tration. Supplementary spectrophotometric and electrochemical studies of th e dihydroxyanthraquinone-potassium superoxide system have shown that no cov alent compound is formed between seminquinone anion radical and molecular o xygen and therefore the reaction following dihydroxyanthraquinone reduction involves electron and proton transfer from semiquinone anion radical to mo lecular oxygen to form hydroperoxyl radical and deprotonated dihydroxyanthr aquinone. The rate constants of the electron-transfer reactions from semiqu inone anion radical to molecular oxygen were determined from the dependence of anthraquinone cathodic reduction peak potential and the corresponding a nodic peak current on oxygen concentration. It was found that the rate cons tants increase with decreasing pK(a) of the hydroxyanthraquinones, which co nfirms the postulated mechanism. (C) 2000 Elsevier Science Ltd. hll rights reserved.