Employing inelastic electron scattering we demonstrate here the determination of the exciton dispersion in a molecular solid. The failure of the applied tight-binding description provides strong evidence for a necessary reconsideration of the traditional, Frenkel-exciton based, understanding of the lowest-lying electronic transitions in organic semiconductors.

• Received 28 June 2006

DOI:https://doi.org/10.1103/PhysRevLett.98.037402