Evidence for a transition between singlet and triplet states in the electrochemical reduction of 2,2′-4,4′-tetranitrobiphenyl

Two different dianionic species, close in energy, result from the electroreduction of 2,2′-4,4′-tetranitrobiphenyl, 1. The first-formed species has biradical characteristics, such as a triplet EPR spectrum, but it slowly evolves to another EPR-silent species, essentially described as a quinoid form. DFT calculations also support the existence of two dianionic states of the dianion, singlet and triplet, differing by less than 10 kcal mol-1 (in the gas phase); the quinoid singlet state is more stable.