"Photochemistry with thermal versus optical excess energy: Ultrafast cycloreversion of indolylfulgides and indolylfulgimides
"T. Brust, S. Malkmus, S. Draxler, S. A. Ahmed, K. Rück-Braun, W. Zinth, M. Braun
Journal of Photochemistry and Photobiology A: Chemistry 207 (2009) 209-216
The dependence of the pericyclic ring-opening reaction of indolylfulgides and indolylfulgimides on excess energy is investigated by quantum efficiency measurements and by ultrafast spectroscopy. The ring-opening reaction shows a pronounced improvement of reaction efficiency up to a factor of 6, when excess energy is available either by increasing the temperature or by exciting the molecules above the 0-0 transition. Ultrafast spectroscopy allows to deduce time constants for the ring-opening reaction and leads to a theoretical model, where the redistribution of excess energy among different vibrational modes is considered. The analysis shows that excess energy supplied by optical excitation accelerates the ringopening reaction less efficiently than thermal energy. Apparently vibrational relaxation from highly excited modes to modes promoting the ring-opening reaction is not completed within the ~10 ps duration of the ring-opening reaction.
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