Ultrafast spectroscopy of dark states in solid state sexithiophene

Femtosecond time resolved photoinduced transmission studies are carried out in nanocrystalline dihexylsexithiophene films with long range structural o rder. The results are compared with those obtained for sexithiophene in sev eral states of aggregation. We explain the lack of radiative recombination in the solid phase with the formation of two nonradiative excitations, dark excitons and charge transfer states. The dark exciton dynamics is studied in detail in dihexylsexithiophene films; thermalization occurs within 200 f s, then decay takes place by mutual annihilation in the first ps, and by mo nomolecular recombination at longer times. Optical dynamics indicates that within the pump pulse a second excitation is formed, and we assign it to ch arge transfer states. The latter decay monomolecularly in the hundred ps ti me scale. (C) 1999 American Institute of Physics. [S0021-9606(99)50937-9].