Homogeneous electron-transfer reaction between anionic species of anthraquinone derivatives and molecular oxygen in acetonitrile solutions: Electrochemical properties of disperse red 60

In this work, the voltammetric behaviour of disperse red 60 (DR60) in acetonitrile solutions in the presence of water and dioxygen (O-2) is reported for the first time. DR60 can be reduced in two oneelectron steps to form first the anion radical and then the dianion. The first reduction process of DR60 is affected by the water content of the acetonitrile, with digital simulations of the cyclic voltammetric waves supporting interactions between the radical anion and water molecules. Additionally, the presence of O-2 modifies the first voltammetric process of DR60, suggesting the occurrence of homogeneous electron transfer reaction between anion radicals of DR60 and O-2. Furthermore, the water content of the acetonitrile also affects the reduction wave of O-2, with digital simulations supporting the occurrence of two association steps with the superoxide intermediate. The homogeneous electron transfer reaction between the quinone and oxygen species was also examined by reacting bulk electrolyzed solutions containing the anions of DR60 and O-2 with neutral species of O-2 and DR60, respectively, and recording the variation in the colour of the electrolyzed solutions. An equilibrium constant value was obtained from the difference between the formal reduction potentials of DR60 and O-2 (-0.9955 and -0.9770, respectively, both vs. Ag in CH3CN with 0.5 M n-Bu4NPF6), indicating the direction of the homogenous electron transfer reaction (K-eq = 2.013). Similarly, to confirm the occurrence of the homogeneous electron transfer reaction between O-2 and another quinone based molecule, cyclic voltammetry and bulk electrolysis experiments were carried out on 9,10-anthraquinone (AQ), and its equilibrium constant value was also obtained (K-eq = 4.392). (C) 2020 Elsevier Ltd. All rights reserved.