Electron-transfer reactions and conformational changes associated with the reduction of 2,3,2'3'-dibenzo-7,7'-dimethylbianthrone and 5,6,5',6'-dibenzo-2,2'-dimethylblanthrone

The electrochemical reduction of the title compounds, I and II respectively, was investigated by cyclic voltammetry in dimethylformamide as solvent. Each was shown to behave as do other bianthrones for which large differences in the reactivity of A and B conformations is observed. The A form is reduced to B^2- in an ECE process and on the return scan B^2- is oxidized sequentially and reversibly to B^- and B. The B form can be detected on the third half cycle but its conversion back to A causes diminution of its reduction peak permitting evaluation of k_BA, the rate constant for the B → A conversion. For I, k_BA was 50 s^-1 at 293 K whereas for II it was 4.5 s^-1. The activation enthalpy for the reaction of I was found to be 67.8 kJ mol and ΔS‡ is 18 kJ mol. A second reduction peak observed in an earlier study, of I is suggested to have been caused by an impurity which served as a homogeneous redox catalyst.