Exciton character in picene molecular solids

We have studied the low-energy electronic excitations of solid picene at 20 K using momentum-dependent electron-energy-loss spectroscopy. Our results demonstrate the presence of five excitonic features below the transport energy gap of picene, which are all characterized by a negligible dispersion. One of these excitons has not been observed in the optical-absorption spectrum of picene molecules in solution and thus is assigned to a (solid-state-induced) charge-transfer exciton. This conclusion is supported by the momentum-dependent intensity variation of this exciton, which clearly signals a significant dipole forbidden contribution, in contrast to the other low-energy excitations.