Recently, there has been large interest in organic materials due to promising device applications. Particular attractive are organic molecules which form quasi-1D crystals, with close stacking in one direction: The resulting strong inter-molecular interaction favors generation and transport of free carriers. The optical properties of such organic semiconducting materials, which are dominated by excitonic excitations, are much less understood than those of their inorganic counterparts: For strongly coupled quasi-1D materials, first band structure models for free excitons have only recently been proposed and compared to absorption spectra.1,2 Energy relaxation is even more puzzling: Typical quantum efficiencies of the fluorescence are very low, preventing application in emission devices. A possible reason for the low efficiency is the relaxation of free excitons into self-trapped states,’ a process which is not understood. Therefore, a detailed investigation of the initial exciton relaxation and of the exciton-phonon coupling is needed.

© 2001 Optical Society of America

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