The optical response of polymer aggregates has been described by considering coulombic interactions of transition dipoles that can couple in either a side-by-side orientation, (H-aggregates), or aligned along one dimension (J-aggregates). For thin-films of polythiophenes, it has been reported that both optical absorption and the photoluminescence spectra can be well explained by weakly coupled H-aggregates. This is due to dense π-stacking of the polymer chains with short distances between them. According to Kasha’s rule, vibrational relaxation to the lowest excited state takes place on a much shorter time scale than the lifetime of an exciton. The photon emission thus takes place from the lowest energy excited state.