ELECTRONIC ABSORPTION AND RAMAN STUDIES OF THE RADICAL-ANION AND DIANION OF A POLYENE MOLECULE (19,19',20,20'-TETRANOR-BETA,BETA-CAROTENE)

The electronic absorption and Raman spectra of the radical anion and d ianion of 19,19',20,20'-tetranor-beta,beta-carotene, a polyene molecul e with eleven conjugated C=C bonds, have been studied. The radical ani on 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 st retching (nu(1)) band of the radical anion is 26 cm(-1) lower, whereas the nu(1) band of the dianion is 15 cm(-1) higher, than that of the n eutral species. This upshift observed for the dianion suggests large s tructural changes such as inversion of bond alternation. The CH in-pla ne-bending (nu(2)) bands of the radical anion and the dianion, both ob served at 1275 cm(-1), are 15 cm(-1) lower than that of the neutral sp ecies and may be used as a marker of negatively charged polyenes. (C) 1997 Elsevier Science B.V.