Pulse radiolytic one-electron reduction of 2-hydroxy- and 2,6-dihydroxy-9,10-anthraquinones

Abstract

The semiquinone free radicals produced by one-electron reduction of 2-hydroxy-9,10-anthraquinone (2HAQ) and 2,6-dihydroxy-9,10-anthraquinone (26DHAQ) in aqueous formate solution, water–isopropyl alcohol–acetone mixed solvent and isopropyl alcohol have been studied using the pulse radiolysis technique. The absorption characteristics, kinetic parameters of formation and decay, acid–base behaviour and redox characteristics of the semiquinones have been investigated and compared with the corresponding characteristics of a few intramolecularly hydrogen-bonded anthrasemiquinone derivatives. The non-hydrogen-bonded semiquinones show two pK_a values (4.7 and 10.7 for 2HAQ and 5.4 and 8.7 for 26DHAQ, respectively) within the pH range 1–14, whereas other intramolecularly hydrogen-bonded semiquinones show only one pK_a. The one-electron reduction potential (E′₇) values for 2HAQ (–440 mV) and 26DHAQ (–400 mV) are more negative than those of the intramolecularly hydrogen-bonded systems.