Exciton dispersion and momentum dependent exciton spectral weight in diindenoperylene (DIP): Interference of Coulomb and charge-transfer exciton coupling

A combination of optical absorption and electron energy-loss spectroscopy has been used to investigate the exciton in highly ordered diindenoperylene films. The exciton dispersion is shown to be negative, and the data allow the determination of the total inter-molecular exciton coupling. This coupling is H-aggregate-like, which explains the variation of the spectral profile when going from diindenoperylene (DIP) molecules in solution to the crystal. Momentum dependent spectral weight changes indicate a sizable contribution from charge-transfer exciton coupling, which reduces the total coupling to some extent. We derived a consistent set of model parameters for the description of the exciton behavior in DIP.