A laser flash photolysis study of aromatic nitrones. Triplet state properties and reactivity with singlet oxygen

Abstract

Results of a nanosecond laser flash photolysis study of a series of aromatic nitrones in solution at room temperature are presented. Direct laser flash excitation (337.1 or 308 nm) of nitrone solutions in benzene or methanol results in ground state bleaching which can be attributed to the photocyclization to oxaziridines. The quantum yields of this process are estimated to be 0.1-0.3 in methanol. From the rate constants of triplet quenching by nitrones, their triplet energies are located in the range 1.4-2.0 eV. With the exception of cyclic nitrogen (2 - in Scheme 1), no transient absorption phenomena due to nitrone triplets are observed as a result of the extensive quenching of sensitizer triplets; thus the triplets of non-cyclic nitrones are very short lived (t_T < 10 ns), possibly due to structural relaxation analogous to that in olefin triplets. With zinc tetraphenylporphyrin as the sensitizer, there is evidence for electron transfer quenching in which the nitrones act as donors. No ground state bleaching is observed in the course of triplet sensitization, suggesting that photocyclization to oxaziridines does not occur from the triplet state. The nitrones are moderately efficient quenchers of singlet oxygen; the corresponding bimolecular rate constants are in the range (0.5-8) × 10⁷ M^-1 s^-1 in various solvents.