Pentacene films which are model systems for organic electronics exhibit contrary to the pentacene monomer extremely little photoluminescence. This points to an ultrafast relaxation mechanism. We apply pump-probe absorption spectroscopy with a time resolution of 30 fs to microcrystalline pentacene films. It is found that the primarily excited Frenkel excitons decay within 70 fs to a non fluorescing singlet species. Contrary to expectations fission into triplets is only a minor channel. We propose that the ultrafast relaxation of the photoexcited excitons leads to a species very similar to excimers. The radiative transition of the excimer to the ground state is electric dipole forbidden due to symmetry reasons. The excimer formation provides therefore an efficient mechanism to turn off the emission.
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