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 (ν1) band of the radical anion is 26 cm−1 lower, whereas the ν1 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 (ν2) 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.