Electronic absorption and Raman studies of the radical anion and dianion of a polyene molecule (19,19′,20,20′-tetranor-β,β-carotene)

The electronic absorption and Raman spectra of the radical anion and dianion of 19,19′,20,20′-tetranor-β,β-carotene, a polyene molecule with eleven conjugated C=C bonds, have been studied. The radical anion has an intense absorption at 1.46 eV and a weak absorption at 1.00 eV, while the dianion has an intense absorption at 1.59 eV. The C=C stretching (v₁) band of the radical anion is 26 cm^-1 lower, whereas the v₁ band of the dianion is 15 cm^-1 higher, than that of the neutral species. This upshift observed for the dianion suggests large structural changes such as inversion of bond alternation. The CH in-plane-bending (v₂) bands of the radical anion and the dianion, both observed at 1275 cm^-1, are 15 cm^-1 lower than that of the neutral species and may be used as a marker of negatively charged polyenes.