  Equations of motion for weakly coupled excitonic complexes are derived. The
description allows to treat the system in the basis of electronic states
localized on individual chromophores, while at the same time accounting for
experimentally observable delocalization effects in optical spectra. The
equations are show to be related to the well-known Foerster type energy
transfer rate equations, but unlike Foerster equations, they provide a
description of the decoherence processes leading to suppression of the
resonance coupling by bath fluctuations. Linear absorption and two-dimensional
photon echo correlation spectra are calculated for simple model systems in
homogeneous limit demonstrating distinct delocalization effect and reduction of
the resonance coupling due to interaction with the bath.
