Rates of conformational changes in the anion radicals of bianthrones

Bianthrone and its 3,3'-dimethyl, 3,3'-dimethoxy, and 1,1'-dimethyl derivatives were studied by pulse radiolysis in i-PrOH-water solutions. One-electron reduction of the low-temperature A form of each of these bianthrones leads to the formation of the anion radicals A- which then undergo a conformational changes into B-, the anion radical of the high-temperature B form. Subsequently, B- undergoes second-order disproportionation to yield B and BH- (bianthronyl). The absorption spectra of A- and B- exhibit maxima around 560 and 460 nm, respectively. The rates of the conformational change A- → B- were found to be 7 x 104s-1 for bianthrone and 3,3'-dimenthylbianthrone, 1.7 x 104s-1 for the 3,3'-dimethoxy derivative, and only 1.1 x 103s-1 for the sterically hindered 1,1'-dimethylbianthrone