ELECTRONIC EXCITATIONS IN AGGREGATES OF BACTERIOCHLOROPHYLLS

Photosynthetic organisms have developed antenna systems to enlarge the ir cross section for capturing sunlight. These systems involve in some cases aggregates of bacteriochlorophylls (BChls). The structure of on e such aggregate, a tightly coupled circular hexadecamer of BChls and a loosely coupled octamer of BChls, has recently been solved. In this paper we investigate the electronic excitations in these two aggregate s as well as in monomeric BChl and BChl dimers by means of INDO/S-CI c alculations. The results provide a detailed description of the propert ies of electronic states in the BChl aggregates (energies, dipole and transition dipole moments) that are relevant for the biological functi on of BChls, to absorb light and transfer energy on the subpicosecond time scale. The lower-energy excitations of the BChl hexadecamer are o f exciton type, i.e., experiencing a strong coupling that leads to exc itation delocalization over the entire aggregate. An effective Hamilto nian is provided which reproduces these exciton states and which can b e readily generalized to other aggregates.