EXCITON DYNAMICS IN CIRCULAR AGGREGATES - APPLICATION TO ANTENNA OF PHOTOSYNTHETIC PURPLE BACTERIA

A theoretical model of exciton dynamics in circular molecular aggregat es of tight-harvesting bacteriochlorophyll of photosynthetic bacteria is proposed. The spectra and anisotropy of photoinduced absorption cha nges in the femto- and picosecond time domain are under its scope. The excited state of aggregate was treated due to the standard exciton th eory, taking into account a pigment inhomogeneity. Dephasing processes via the exciton-phonon interactions were described by means of the Ha ken-Strobl equation. it was shown that only two exciton levels are dip ole-allowed in the case of homogeneous circular aggregate. The pigment inhomogeneity results in the appearance of several weak transitions t o higher exciton levels. it was proposed that the minor band (B896) in an absorption spectrum of the B875 complex as well as the similar min or band in spectra of B800-850 complex correspond to electron transiti on from the ground to the lowest exciton level, whereas the major band corresponds to transition to the higher exciton level. The proposed m odel shows the subpicosecond decay of anisotropy at the short-waveleng th side of absorption band and a high degree of anisotropy at the long -wavelength side, even at high temperatures.