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MO: Fachverband Molekülphysik

MO 5: Photochemie I

MO 5.1: Invited Talk

Tuesday, March 11, 2008, 08:30–09:00, 3F

The Photochemistry of N-Oxides - A White Spot in Femtosecond Spectroscopy — Thorben Cordes, Nadja Regner, Elina Borysova, Björn Heinz, and •Peter Gilch — Lehrstuhl für BioMolekulare Optik, Fakultät für Physik, LMU München, Oettingenstr. 67, D-80538 München

Organic compounds containing an imino N-oxide function (=N⁺-O⁻) have been known to be photo-reactive for a more than a century. This photo-reactivity has been observed e.g. for azoxy (N=N⁺-O⁻) compounds, nitrone (C=N⁺-O⁻) derivatives, and heterocyclic N-oxides and proves to be very general. Numerous examples have been compiled [1,2]. These photo-reactions pave the way to products difficult to access otherwise. But although photo-reactivity is a general feature of N-oxides there are various possible outcomes of these reactions. For instance, upon illumination the N-oxide of isoquinoline transforms into a lactam, whereas its cyano substituted derivative yields an oxazepine [3]. The difference in product formation has been associated with the intermediacy of a radical pair in the first and an oxaziridine in the second case. In some cases such oxaziridines - three membered rings containing the N and O atoms of the former N-oxide - have been isolated [1]. Yet, their intermediacy in most reactions has until now only been postulated. In fact, the whole photochemistry of N-oxides has not been paid much attention by femtosecond spectroscopists.

We have started a research program trying to fill this "white spot". As a starting point, the complex photo-rearrangement of a heterocyclic N-oxide, 2-benzoyl-3-phenylquinoxaline-1,4-dioxide, in solution phase has been addressed by means of femtosecond spectroscopy. This N-oxide transforms into an imidazolone with a quantum yield of 10 % [4]. Transient absorption spectroscopy in the femto- to microsecond range reveals slightly solvent dependent kinetic compounds with time constants (in ethanol) of 1 ps, 10 ps, 200 ps, and 200 ns. The slowest component represents the final product formation. Femtosecond fluorescence spectroscopy associates the 1 ps and 10 ps time constants with excited state relaxation and depletion processes, respectively. For the 10 ps, 200 ps, and 200 ns processes indications for a kinetic branching involving ground state recovery and population of intermediates are observed. The measurements are complemented by femtosecond IR experiments which aim at a structural assignment of the intermediates. Based on these data a kinetic model for the photo-reaction will be derived and prospects of our research on N-oxides are given.
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