Unusual and Persistent Free Radical Intermediate Production from 2-Pyridyl Ketones via UV Irradiation: A Direct ESR...

Aryl ketones are often used as photosensitizers and photoinitiators. Free radical intermediates have been suggested, but not confirmed, to be generated after photoirradiation. Here we found, unexpectedly, that a persistent radical was produced from di-2-pyridyl ketone after UV irradiation, which was detected by the direct ESR method. Interestingly, the persistent radical was very sensitive to oxygen and the pH of the reaction medium. A similar persistent radical was also observed from phenyl-2-pyridyl ketone, but not from 3-benzoylpyridine, 4-benzoylpyridine, and benzophenone, suggesting that the presence of a carbonyl group connected to the ortho-position of the pyridine ring is critical for such radical production. By complementary applications of ESR, HPLC, and ESI-Q-TOF-MS, the possible chemical structures of the persistent radical and final product were identified, and the possible underlying reaction mechanism was proposed. This represents the first report on UV-induced persistent radical generation from 2-pyridyl ketones, which should be of great significance for future studies.