Generation of free oxygen atoms O(3P) in solution by photolysis of 4-benzoylpyridine N-oxide

Abstract

Laser flash photolysis of 4-benzoylpyridine N-oxide (BPyO) at 308 nm in aqueous solutions generates a triplet excited state ³BPyO* that absorbs strongly in the visible, λ_max 490 and 380 nm. ³BPyO* decays with the rate law k_decay/s⁻¹ = (3.3 ± 0.9) × 10⁴ + (1.5 ± 0.2) × 10⁹ [BPyO] to generate a mixture of isomeric hydroxylated benzoylpyridines, BPy(OH), in addition to small amounts of oxygen atoms, O(³P). Molecular oxygen quenches ³BPyO*, k_Q = 1.4 × 10⁹ M⁻¹ s⁻¹, but the yields of O(³P) increase in O₂-saturated solutions to 36%. Other triplet quenchers have a similar effect, which rules out the observed ³BPyO* as a source of O(³P). It is concluded that O(³P) is produced from either ¹BPyO* or a short-lived, unobserved, higher energy triplet generated directly from ¹BPyO*. ³BPyO* is reduced by Fe²⁺ and by ABTS²⁻ to the radical anion BPyO˙⁻ which exhibits a maximum at 510 nm, ε = 2200 M⁻¹ cm⁻¹. The anion engages in back electron transfer with ABTS˙⁻ with k = 1.7 × 10⁹ M⁻¹ s⁻¹. The same species can be generated by reducing ground state BPyO with ˙C(CH₃)₂OH. The photochemistry of BPyO in acetonitrile is similar to that in aqueous solutions.