Intersystem crossing in oligothiophenes studied by fs time-resolved spectroscopy

High triplet quantum yields of more than 90% for bithiophene and terthiophene have to be connected with very fast and effective formation of triplets after excitation. We studied these processes with fs pump-probe spectroscopy. The time behaviour of transient optical spectra within the singlet and triplet manifold was examined for bi- and terthiophene (2T and 3T) in solution. For 2T we used two-photon absorption for excitation. We found transient spectra of the excited singlet state, the triplet state and that of radical cations. The kinetics of the excited-state absorption was described by a bi-exponential function. Additionally we observed formation and recombination of radical cations. The recombination is connected with triplet formation. Both processes could be described by a time constant of 62 ps +/- 9 ps. For 3T we found a dependence of the processes on excitation energy using one-photon absorption. The triplet quantum yield increased with higher excitation energy. The kinetics be comes bi-exponential with increasing amplitude of the short time constant of 2 ps at increasing excitation energy. The main reasons for the effective intersystem crossing (ISC) in both oligothiophenes are - besides the high spinorbit coupling factor introduced by the sulphur atom - the almost isoenergetic positions of the S-1 and T-2 states, detected by PD-PES [1]. At higher photon excitation energy for 3T above the band gap an additional channel for ISC was detected. We believe that during the geometric change from the non-relaxed non-planar to the relaxed planar excited state S1, ultrafast intersystem crossing takes place.