Radical anions from some photochromic nitrocompounds. An electron paramagnetic resonance and electrochemical study

Five 6′-nitrospiroindole-benzopyrans] have been reduced electrochemically in ACN to the corresponding radical anions. Their reduction potentials were in the range -1.32 to -1.40 V versus SCE, that is in the range expected for substituted nitrobenzenes. The EPR spectra of the species resulting from chemical reduction of the same compounds were also recorded, and suggested that the structure of the observed radicals varies with the reduction media: thus, while the spectra in DMSO could be attributed to the radical anions structurally similar to the starting compounds, those observed in THF under UV irradiation were assigned to the anions of the open merocyanines. It appears that also the species observed by reduction in DMF ought to be assigned the open structure.