Absorption and dichroism spectra of cylindrical J aggregates and chlorosomes of green bacteria

We study the absorption and linear and circular dichroism spectra of molecular aggregates having the shape of a cylinder. Examples are the chlorosomes of green bacteria and recently synthesized cyanine J aggregates, called amphipipes. We use a Frenkel exciton model and exploit the cylinder symmetry to separate the exciton states into bands with different transverse wavenumbers. Only three of these bands are optically active. The delicate interference between the contributions of these three bands to the CD spectrum, make this spectrum very sensitive to system parameters, such as the cylinder length. This offers a natural explanation for the observed strong variation of the CD spectra of chlorosomes with sample preparation technique.