The complex photo-rearrangement of a heterocyclic N-oxide: Kinetics from picoseconds to minutes

The photo-induced rearrangement of 2-benzoyl-3-phenylquinoxaline-1,4-dioxide yielding 1,3-dibenzoylbenzimidazolone (DBBI) is studied by time-resolved fluorescence, UV/vis absorption, and IR spectroscopy. This complex rearrangement occurs on time scales ranging from 0.1 ps up to minutes. Processes within the excited singlet state(s) (0.1–1 ps) are followed by a multi-phasic depletion of the fluorescent states (time constants of 1–200 ps). During that depletion both a triplet state and a precursor of the photo-product are populated. In absence of oxygen the triplet state decays in 8 μs. It does not participate in the formation of the photoproduct. A process with a time constant of 77 min terminates the product formation. It can be assigned to a hindered rotation of the benzoyl residues in DBBI.