Intersystem crossing in oligothiophenes studied by fs time-resolved spectroscopy |
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Authors: | W Paa J-P Yang S Rentsch |
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Institution: | (1) Institute for Optics and Quantum Electronics, Friedrich-Schiller-University Jena, Max-Wien-Platz 1, 07743 Jena, Germany, DE |
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Abstract: | 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 becomes 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 spin-orbit 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
S
1, ultrafast intersystem crossing takes place.
Received: 6 December 1999 / Published online: 2 August 2000 |
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Keywords: | PACS: 42 62 Fi 78 40 Me 78 55 Bq 79 47 +p |
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