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MO: Fachverband Molekülphysik
MO 4: Femtosekundenspektroskopie II
MO 4.7: Talk
Monday, March 10, 2008, 18:00–18:15, 3G
Ultrafast Singlet versus Triplet Dynamics in Thin Pentacene Films — •Henning Marciniak1, Stefan Lochbrunner1, and Bert Nickel2 — 1Intstitut für Physik, Universität Rostock — 2Fakultät für Physik und CeNS, LMU München
Thin pentacene films are frequently applied as active layers in prototypical organic electronic devices. Their electronic structure and dynamics determine to a large extent the device behavior. We investigate with femtosecond absorption spectroscopy microcrystalline pentacene films with a thickness of roughly 30 monolayers. The transient spectra show that optically excited singlet excitons decay within 70 fs to a non fluorescing species [1]. The fission of a singlet exciton into two triplet excitons is discussed as potential relaxation path. The process was observed in crystalline tetracene and the time scale is feasible since the total spin is conserved. By tilting the pentacene layers relative to the incident laser beams it is possible to probe singlets and triplets or exclusively singlets since the transition dipole of the lowest triplet-triplet absorption band is orientated along the long axis of the upstanding molecules. A transient absorption band at 525 nm can thereby be identified as triplet absorption. However, it rises with a time constant of about 0.9 ps instead of 70 fs and it is more than a factor of 10 weaker than expected. We conclude that triplet formation is a secondary step and only a small fraction of excitons experiences fission into triplets. The majority transforms probably to excimer excitons.
[1] H. Marciniak, M. Fiebig, M. Huth, S. Schiefer, B. Nickel, F. Selmaier, and S. Lochbrunner, Phys. Rev. Lett. 99 (2007), 176402.