Excitons in quasi-one-dimensional organic crystals

We present an electron energy-loss study of the nature and dispersion of excitons in quasi-one-dimensional 3,4:9,10-perylenetetracarboxylic dianhydride molecular crystals. The observed anisotropy of the excitation energies, the spectral shape and the dispersion indicates the presence of two types of excitons near the excitation onset. These are associated with mixed intramolecular Frenkel and intermolecular charge-transfer excitons. In particular, the excitons exhibit a negative dispersion along the molecular stacks which has important consequences for their recombination.