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

Laser flash photolysis of 4-benzoylpyridine N-oxide (BPyO) at 308 nm in aqueous solutions generates a triplet excited state (BPyO)-B-3* that absorbs strongly in the visible, lambda max 490 and 380 nm. (BPyO)-B-3* decays with the rate law k(decay)/S-1 = (3.3 +/- 0.9) x 10(4) + (1.5 +/- 0.2) x 10(9) [BPyO] to generate a mixture of isomeric hydroxylated benzoylpyridines, BPy(OH), in addition to small amounts of oxygen atoms, O(P-3). Molecular oxygen quenches (BPyO)-B-3*, k(Q) = 1.4 x 10(9) M-1 s(-1), but the yields of O(P-3) increase in O-2-saturated solutions to 36%. Other triplet quenchers have a similar effect, which rules out the observed (BPyO)-B-3* as a source of O(P-3). It is concluded that O(P-3) is produced from either (BPyO)-B-1* or a short-lived, unobserved, higher energy triplet generated directly from (BPyO)-B-1*. (BPyO)-B-3* is reduced by Fe2+ and by ABTS(2-) to the radical anion BPyO center dot- which exhibits a maximum at 510 nm, epsilon = 2200 M-1 cm(-1). The anion engages in back electron transfer with ABTS(center dot-) with k = 1.7 x 10(9) M-1 s(-1). The same species can be generated by reducing ground state BPyO with C-center dot(CH3)(2)OH. The photochemistry of BPyO in acetonitrile is similar to that in aqueous solutions.