Electron-transfer reactions and conformational changes associated with the reduction of substituted bianthrones

The electrochemical reduction of several substituted bianthrones is similar to that of the parent compound. 3,3′-Dimethylbianthrone (II), 3,3′-di-n-heptylbianthrone (III). 3,3′-dimethoxybianthrone (IV) and 1,1′-dimethylbianthrone -(V) were studied in dimethylformamide using cyclic voltammetry and transmission mode spectroelectrochemistry. For each compound the low temperature A form is reduced in a two-electron irreversible reaction to a twisted dianion, B^2?. Upon oxidation, B^2? forms first B, then B, whose spectral properties are identical to those of the high-temperature thermochromic form of the bianthrones. Rate constants for the B-A reaction were determined for each compound. The reduction of 2,3,2′,3′-dibenzo-7,7′-dimethylbianthrone (VI) showed somewhat different features which were tentatively interpreted in terms of redox catalysis.