Oxidation of phenols by triplet aromatic ketones in aqueous solution

Aromatic ketones efficiently mediate the photo-oxidative degradation of phe nols in aerated aqueous solution, a process likely to be relevant in sunlit natural waters. Absolute bimolecular rate constants for the quenching of t hree model ketone triplets by nine phenols bearing various substituents, fr om electron-donating alkyl and alkoxy groups to the electron-withdrawing cy ano group, were measured by nanosecond laser flash photolysis. Tripler benz ophenone (BP) is quenched at nearly diffusion-controlled rates (2.6-5.6 x 1 0(9) M-l s(-1)). Triplet state quenching of 3'-methoxyacetophenone (3'-MAP) and 2-acetonaphthone (2-AN) by the same set of phenols occurs more selecti vely, with rate constants spanning a range of I and more than 2 orders of m agnitude, respectively. Quenching rate constants obey a Rehm-Weller relatio nship to the free energy of electron transfer from the phenol to the ketone tripler. By comparison of the quenching constants with overall photo-oxida tion rates obtained by stationary irradiation in air-saturated aqueous solu tion, phenols bearing electron-donating substituents were found to be deple ted with quantum yields generally exceeding 0.5, whereas parent phenol and, presumably, acceptor-substituded phenols are transformed at only similar t o 0.1 efficiency. The present quenching data were used to interpret the eff iciency of photosensitized oxidation of phenols by dissolved natural organi c matter (DNOM), an important sunlight absorber present in surface waters. The effective reduction potential of reactive excited tripler states in DNO M was estimated to be at least 1.36 V vs NHE.