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

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Abstract
The S0(ground singlet‐state) Raman spectra of theall‐trans, 9‐cis, 13‐cis, 9,9‐cis, 9, 13‐cis,9,13′‐cisand13,13′‐cisisomers of canthaxanthin as well as the T1(lowest‐excited triplet‐state) Raman spectra of theall‐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 S0and T, states), the carbon‐carbon stretching frequencies were compared between isomeric canthaxanthin and p‐carotene: in the S0state, the C=C stretching frequencies of canthaxanthin were lower by3–10 cm−1than 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 frequenciesvsthe 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 S0state. Both observations support the idea that the “triplet‐excited region” of canthaxanthin is extended over the entire double‐bond system.

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