Photoexcitations in oriented tetrahexyl-sexithiophene thin films

We report the photoexcitation properties of alkyl substituted sexithiophene thin films grown on a single crystal of potassium acid phtalate by organic molecular-beam deposition. These films possess a layered sugramolecular structure with anisotropic optical properties due to the preferential molecular orientation. The poorly structured photoluminescence spectra retain the polarization properties of the absorption, which are consistent with the emission originating from the weakly interacting bulk molecules as well as from molecules at the grain boundaries rather than from immobile x traps. The layered card pack structure combined with the presence of lateral alkyl chains results in a very weak electronic coupling between oligothiophene molecules in all but one direction, namely the card pack axis. This is clearly confirmed by the photoluminescence time decay, which is very well accounted for by a one-dimensional diffusion process towards nonradiative quenching centers within the oriented crystalline domains.