Triplet-state photoexcitations of oligothiophene films and solutions

We present studies of steady-state photoinduced absorption (PIA) spectroscopy on photoexcitations in a series of well-defined α-oligothiophene (Tn, n=6, 7, 9, and 11) films and solutions. The PIA spectra and the excited state lifetimes are consistent with the signatures of a photoexcited triplet state. The PIA spectra consist of a strong vibronically resolved subgap absorption, which is readily observed in solid-state films and in solutions at ambient and cryogenic temperatures. The transition energy is linearly dependent on the reciprocal chain length and shifts to lower energy for longer oligomers. Variation of the modulation frequency and the pump intensity under matrix-isolated conditions reveals that the photoexcitation is created via an intrachain mechanism and decays nonradiatively with monomolecular kinetics. In solid films we find a significant contribution of a bimolecular decay process to the relaxation rate.