Abstract

Electronic absorption spectra of molecular crystals are dominated by intramolecular (Frenkel) excitons. However, overlap-limited terms couple Frenkel excitons to the manifold of charge-transfer (CT) states where the electron and the hole are located at different molecules. As intermediates between free charge carriers which mediate the coupling of the system to external electric field and charge reservoir, and Frenkel excitons which couple the system to light waves, the CT states play a pivotal role in optoelectronic processes.

In spite of the interest they deserve, the CT states are rarely studied; their small transition moments make them difficult to address experimentally. The presentation will focus on the confrontation of the calculated electroabsorption and absorption spectra of sexithiophene with their experimental counterparts available in the literature, with special emphasis on the peculiarities resulting from the presence of CT states and from their influence on Frenkel excitons, causing substantial deviations from the behavior decreed by conventional molecular crystal theory.