Excited states of carotenoid in LH2: an ab initio study

In order to understand the origin of ultrafast carotenoid bandshifts upon photoexcitation of bacteriochlorophyll (BChl) molecules in antenna complexes of photosynthetic purple bacteria, we have performed sa-CASSCF(8,7)/6-31G (and 6-31G*) calculations for the low-lying excited states of the carotenoid rhodopin glucoside (RG). The LH2 environment was simulated via atomic point charges to represent the electrostatic field of the ground and the Qy excited states of BChls near the carotenoid. The calculations showed that excitation of the B800 BChls yields a significant blue-shift of the carotenoid S2 energy in good agreement with experiments. We also found that the lowest triplet state energy of the RG is below 0.78 eV, which is lower than the energy of 1O2* (0.97 eV), in accord with the role of carotenoids as photoprotectors.