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

This minireview article highlights the energetics and the dynamics of the 11B-u and 31A-g 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+u , 31A-g , 11B-u , and 21A-g 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. 66026609). 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+u (right arrow)11B-u (right arrow) 21A-g (right arrow) 11A-g (the ground state) for carotenoids having n = 9 and 10, whereas 11B+u (right arrow) 31A-g (right arrow) 11B-u (right arrow) 21A-g (right arrow) 11A-gfor carotenoids having n = 11-13. Radiative transitions of 11B+u (right arrow) 21A-g and 11B-u (right arrow) 21Ag as well as a branching into the triplet manifold of 11B-u (right arrow) 13Ag (right arrow) 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-u and 31A-g states of carotenoids need to be incorporated into the picture of carotenoid-to-bacteriochlorophyll singlet-energy transfer.