Optical properties of molecular aggregates have been an active research area for over 50 years. The special interest in the optical response of these systems arises from collective effects that are caused by the interaction between the molecules within the aggregate. The study of such effects has recently attracted considerable interest in a much broader class of systems known as nanostructures, which include not only molecular aggregates but also polymers and semiconductor nanostructures.
Most of linear optical response of molecular aggregates can successfully be studied by means of the Frenkel exciton. The molecular aggregate is then modeled as a linear chain ot two-level absorber, whose electronic states are described by the Frenkel Hamiltonian. It has been shown that that the low-temperature absorption properties of molecular aggregates embedded in a glassy host can be explained quantitatively by takin into account Gaussian disorder of the transition frequencies of the absorbers within each aggregate (inhomogeneity).
Our group has been mainly concerned with optical properties of disordered mlecular aggregates when defects present some kind of spatial correlations. We have considered two rather different kind of spatial correlations, namely short-range correlations (random dimer lattices) and long-range correlations (aperiodic lattices).