Excited states of carotenoid in LH2: an ab initio study

In order to understand the origin of ultrafast carotenoid bandshifts upon p hotoexcitation of bacteriochlorophyll (BChl) molecules in antenna complexes of photosynthetic purple bacteria, we have performed sa-CASSCF(8,7)/6-31G land 6-31G*) calculations for the low-lying excited states of the carotenoi d rhodopin glucoside (RG). The LH2 environment was simulated via atomic poi nt charges to represent the electrostatic field of the ground and the Q(y) excited states of BChls near the carotenoid. The calculations showed that e xcitation of the B800 BChls yields a significant blueshift of the carotenoi d S-2 energy in good agreement with experiments. We also found that the low est triplet state energy of the RG is below 0.78 eV, which is lower than th e energy of O-1(2)* (0.97 eV), in accord with the role of carotenoids as ph otoprotectors. (C) 2001 Elsevier Science B.V. All rights reserved.