Berlin 2014 – wissenschaftliches Programm
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MO: Fachverband Molekülphysik
MO 19: Femtosecond Spectroscopy 2
MO 19.6: Vortrag
Donnerstag, 20. März 2014, 17:45–18:00, BEBEL HS213
Pyrene: A textbook paradigm for excimer formation with some surprises — •Bastian Baudisch, Mario Masetto, Igor Pugliesi, and Eberhard Riedle — LS für BioMolekulare Optik, LMU München
Pyrene is the textbook example for excimers and their formation. It relies largely on on nanosecond fluorescence measurements by Birks in the 60s [1]. After excitation 370-425 nm fluorescence occurs out of the S1 state. Due to the long S1 life time the excited pyrene molecules diffusionally encounter ground state pyrenes and form excimers which give rise to red shifted fluorescence around 475 nm. We have investigated pyrene with extended pump-probe capabilities [2]. After 334 nm excitation into the S2 state, we observe 85 fs relaxation to the S1 state. In the subsequent dynamics an approximate doubling of the GSB in 6.6 ps occurs. In the ps and ns regime we observe clear signatures of anionic and cationic pyrene. Contrary to the commonly accepted mechanism our results indicate the presence of pre-associated dimers: After the optical excitation of one of the monomers a charge separation/transfer happens and leads to the bleaching of a second monomer. The resulting excited dimer then fluoresces and finally crosses into the triplet manifold. Observation of the dynamics is obscured by an intrinsic impurity identified by excitation wavelength dependent ns fluorescence decay dynmics. The impurity is found even for highly purified samples and seems to originate from a photochemical process.
[1] J. B. Birks et al., Proc. Roy. Soc. A 1963, 275, 575.
[2] E. Riedle et al., Faraday Discuss. 2013, 163, 139.