Exciton band structure of pentacene molecular solids: breakdown of the Frenkel exciton model.

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.