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"Unraveling the flavin-catalyzed photooxidation of benzylic alcohol with transient absorption spectroscopy from sub-pico- to microseconds" U. Megerle, M. Wenninger, R.-J. Kutta, R. Lechner, B. König, B. Dick, E. RiedlePhys. Chem. Chem. Phys. 13, 8869-8880 (2011) Abstract: Flavin-mediated photooxidations have been described for applications in synthetic organic chemistry for some time and are claimed to be a route to the use of solar energy. We present a detailed investigation of the involved photophysical and photochemical steps in methoxybenzyl alcohol oxidation on a timescale ranging from sub-picoseconds to tens of microseconds. The results establish the flavin triplet state as the key intermediate for the photooxidation. The initial step is an electron transfer from the alcohol to the triplet state of the flavin catalyst with 3kET ~ 2x107 M-1s-1, followed by a proton transfer in ~ 6 µs. In contrast, the electron transfer involving the singlet state of flavin is a loss channel. It is followed by rapid charge recombi-nation (&tau = 50 ps) without significant product formation as seen when flavin is dissolved in pure benzylic alcohol. In dilute acetonitrile/water solutions of flavin and alcohol the electron transfer is mostly controlled by diffusion, though at high substrate concentrations > 100 mM we also find a considerable contribution from preassociated flavin-alcohol-aggregates. The model including a productive triplet channel and a competing singlet loss channel is con-firmed by the course of the photooxidation quantum yield as a function of substrate concen-tration: We find a maximum quantum yield of 3% at 25 mM of benzylic alcohol and signifi-cantly smaller values for both higher and lower alcohol concentrations. The observations in-dicate the importance to perform flavin photooxidations at optimized substrate concentrations to achieve high quantum efficiencies and provide directions for the design of flavin photo-catalysts with improved performance. BMO authors (in alphabetic order): Uwe Megerle Eberhard Riedle Matthias Wenninger Order Hardcopy  WWW-Version |
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Letzte Änderung: 2016-09-14 13:34
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