Resonance Raman study of carotenoid cation radicals

The resonance Raman (RR) spectra of the neutral and the cation radical of 8'-apo-beta-caroten-8'-al (I), 7',7'-dicyano-7'-apo-beta-carotene (II) and canthaxanthin (III) were measured using 752.6 nm laser excitation. Optical and RR spectra of the electrochemically prepared cation radicals of I-III were measured in situ at different time intervals during the electrolysis. The RR spectra of the cation radicals are considerably different from those of the neutral species. The frequencies of the C=C stretching vibrations are decreased by 30-40 cm(-1), whereas those of C-C stretching vibrations are increased by 15-30 cm(-1). These results suggest delocalization of the unpaired electron density throughout the entire carbon backbone which is consistent with our previous EPR and ENDOR results.