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MO: Fachverband Molekülphysik

MO 21: Femtosecond Spectroscopy II

MO 21.7: Vortrag

Dienstag, 20. März 2007, 12:15–12:30, 6D

Excited State Reaction Pathways of a Photo-induced Pericyclic Ring Opening and Closure — •Björn Heinz¹, Stephan Malkmus¹, Stefan Laimgruber¹, Steffen Dietrich², Christine Schulz², Karola Rück-Braun², Markus Braun¹, Wolfgang Zinth¹, and Peter Gilch¹ — ¹Department für Physik, Ludwig-Maximilians-Universität, Oettingenstr. 67, D-80538 München, Germany — ²Institut für Chemie, Technische Universität Berlin, Str. d. 17. Juni, D-10623 Berlin, Germany

Many chemical reactions involving cyclic transition states obey the Woodward-Hoffmann rules. They make predictions on the occurrence and the stereochemistry of such reactions and can be applied for either direction of a chemical reaction – for instance a cyclisation and a cycloreversion. This suggests that both directions follow the same reaction path on the potential energy surface. For a photochemical cyclisation/cycloreversion of a fulgimide we present experimental evidence that this is not the case. We have recorded femtosecond fluorescence data for either direction and have observed pronounced temporal and spectral differences: The cyclisation proceeds with a characteristic time of ∼ 400 fs whereas the cycloreversion is about six times slower. Further, the transient fluorescence spectra of the photo-excited isomers significantly differ thoughout the reaction. We discuss the possible involvement of other excited states and a two-dimensional model to account for our observations.