THE S0AND Ti STATES OF ISOMERIC CANTHAXANTHIN AS COMPARED WITH THOSE OF β-CAROTENE: EFFECT OF THE TERMINAL CARBONYL GROUPS DETECTED BY RAMAN SPECTROSCOPY

Abstract

The S0 (ground singlet-state) Raman spectra of the all-trans, 9-cis, 13-cis, 9,9-cis, 9, 13-cis, 9,13′-cis and 13,13′-cis isomers of canthaxanthin as well as the T1 (lowest-excited triplet-state) Raman spectra of the all-trans, 9-cis, 13-cis and 9,13-cis isomers were recorded. In order to reveal the effect of the carbonyl groups at both ends on the carbon-carbon conjugated system in-between (in both the S0 and T, states), the carbon-carbon stretching frequencies were compared between isomeric canthaxanthin and p-carotene: in the S0 state, the C=C stretching frequencies of canthaxanthin were lower by3–10 cm−1 than those of β-carotene, indicating increased conjugation in the former. In the T, state, the “C=C” stretching frequencies of canthaxanthin were lower by12–15 cm−1, indicating a large decrease in the “C=C” bond order in the central part of canthaxanthin. Further, the relations of the C=C (S0) and “C=C” (T1) stretching frequencies vs the number of carbon-carbon double-bonds were examined for the above two and additional five carotenoids. The result indicated that the terminal carbonyl groups of canthaxanthin are incorporated in the carbon-carbon conjugated system in the T, state, but that they are almost independent of it in the S0 state. Both observations support the idea that the “triplet-excited region” of canthaxanthin is extended over the entire double-bond system.