Generation of free oxygen atoms O((3)P) 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 (3)BPyO* that absorbs strongly in the visible, λmax 490 and 380 nm. (3)BPyO* decays with the rate law kdecay/s(-1) = (3.3 ± 0.9) × 10(4) + (1.5 ± 0.2) × 10(9) [BPyO] to generate a mixture of isomeric hydroxylated benzoylpyridines, BPy(OH), in addition to small amounts of oxygen atoms, O((3)P). Molecular oxygen quenches (3)BPyO*, kQ = 1.4 × 10(9) M(-1) s(-1), but the yields of O((3)P) increase in O2-saturated solutions to 36%. Other triplet quenchers have a similar effect, which rules out the observed (3)BPyO* as a source of O((3)P). It is concluded that O((3)P) is produced from either (1)BPyO* or a short-lived, unobserved, higher energy triplet generated directly from (1)BPyO*. (3)BPyO* is reduced by Fe(2+) and by ABTS(2-) 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 × 10(9) 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.