"Comparing a Photoinduced Pericyclic Ring Opening and Closure: Differences in the Excited State Pathways"B. Heinz, S. Malkmus, S. Laimgruber, S. Dietrich, C. Schulz, K. Rück-Braun, M. Braun, W. Zinth, P. Gilch
J. Am. Chem. Soc. 129 (2007) 8577
The photochromicity of fulgimides rests on the existence of open (E) and closed ring (C) isomers. As predicted by the Woodward-Hoffmann rules both isomers can photochemically be interconverted. This interconversion has been studied by femtosecond fluorescence and transient absorption spectroscopy. For either direction (E -> C cyclization and C -> E cycloreversion) a biphasic fluorescence decay on the 0.1-1 ps time scale is observed. The longer time constants of the decays equal the formation times of the
photoproducts. The time constants retrieved (0.06 and 0.4 ps for E -> C, 0.09 and 2.4 ps for C -> E) and the associated spectral signatures differ substantially. This indicates that no common excited-state pathway for the two directions exists, as one would infer from a simple Woodward-Hoffmann consideration. These findings support recent quantum dynamic calculations on the excited-state topology of fulgimides.
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Ultrafast Fluorescence Spectroscopy - The Kerr-Gate Setup
Femtosekunden Untersuchung ultraschneller molekularer Schalter