EXCITONS IN CHAINS OF DIMERS

Excitons in circular aggregates of dimers are discussed with the aim t o understand the possible spectral and energy transfer properties of t he ringlike peripheral complexes of photosynthetic bacteria. The syste m is explicitly heterogeneous (i.e., the difference in transition ener gies of the molecules within the dimer as, well as the difference in t he intra-and interdimer resonance interactions, is accounted for). It is demonstrated that the energy spectrum of such a system exhibits man y of the features as observed in spectrally inhomogeneous circular agg regates. The model is used to illustrate the changes in absorption and circular dichroism spectra that take place upon incorporating a dimer into a circular chain. The exciton dynamics in the aggregate is consi dered in the Haken-Strobl-Reineker approach. When terms are neglected describing the phase relaxation between nonnearest neighbors, the equa tions for the diagonal density matrix elements are obtained containing both coherent exciton motion within the dimer-the building block of t he aggregate-and an incoherent hopping of the excitation between dimer s. It is demonstrated that these equations contain a wavelike soliton solution (if dephasing is absent) as well as a diffusion-like solution (for large dephasing rates).