Comparing a photoinduced pericyclic ring opening and closure: differences in the excited state pathways.
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.
Heinz
Björn
B
Department für Physik - Munich Center for Integrated Protein Science, Ludwig-Maximilians-Universität, Oettingenstrasse 67, D-80538 München, Germany.
Malkmus
Stephan
S
Laimgruber
Stefan
S
Dietrich
Steffen
S
Schulz
Christine
C
Rück-Braun
Karola
K
Braun
Markus
M
Zinth
Wolfgang
W
Gilch
Peter
P