Light‐harvesting function of carotenoids in photo‐synthesis: The roles of the newly found 11B state

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Abstract
This minireview article highlights the energetics and the dynamics of the 11B and 31A states of carotenoids discovered very recently. Those “hidden” covalent states have been revealed by measurements of resonance-Raman excitation profiles of crystalline carotenoids. The dependence of the energies of the low-lying singlet states, including the 11B, 31A, 11B, and 21A states, on the number of conjugated double bonds (n) is in agreement with the extrapolation of those state energies calculated by Tavan and Schulten for shorter polyenes (P. Tavan and K. Schulten, Journal of Chemical Physics, 1986, vol. 85, pp. 6602–6609). It has also been shown that the internal-conversion processes among those singlet states take place in accord with the state ordering, i.e., 11B → 11B → 21A → 11A (the ground state) for carotenoids having n = 9 and 10, whereas 11B → 31A → 11B → 21A → 11A for carotenoids having n = 11–13. Radiative transitions of 11B → 21A and 11B → 21A as well as a branching into the triplet manifold of 11B → 13Ag → 13Bu have also been found. Those low-lying singlet states of all-trans carotenoids can facilitate multiple channels of singlet-energy transfer to bacteriochlorophyll in the LH2 antenna complexes of purple photosynthetic bacteria. Thus, the newly found 11B and 31A states of carotenoids need to be incorporated into the picture of carotenoid-to-bacteriochlorophyll singlet-energy transfer. © 2004 Wiley Periodicals, Inc. Biopolymers, 2004

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