Photochem. Photobiol., in press.

Allen Press, Inc., 2005

Bimolecular Hydrogen Abstraction from Phenols by Aromatic Ketone Triplets

Edward C. Lathioor and William J. Leigh,*

Contribution from the Department of Chemistry, McMaster University, 1280 Main Street West, Hamilton, ON L8S 4M1 Canada

Absolute rate constants for hydrogen abstraction from 4-methylphenol (para-cresol) by the lowest triplet states of 24 aromatic ketones have been determined in acetonitrile solution at 23 oC, and the results combined with previously reported data for roughly a dozen other compounds under identical conditions. The ketones studied include various ring-substituted benzophenones and acetophenones, a,a,a-trifluoroacetophenone and its 4-methoxy analogue, 2-benzoylthiophene, 2-acetonaphthone, and various other polycyclic aromatic ketones such as fluorenone, xanthone, and thioxanthone, and encompass n,p*, p,p*(CT), and arenoid p,p* lowest triplets with (triplet) reduction potentials (E*red) varying from ca -10 to -38 kcal mol-1. The 4-methylphenoxyl radical is observed as the product of triplet quenching in almost every case, along with the corresponding hemipinacol radical in most instances. Hammett plots for the acetophenones and benzophenones are quite different, but plots of log kQ vs. E*red reveal a common behavior for most of the compounds studied. The results are consistent with reaction via two basic mechanisms: a simple electron-transfer mechanism, that applies to the n,p* triplet ketones and those p,p* triplets that possess particularly low reduction potentials, and a coupled electron- / proton-transfer mechanism involving the intermediacy of a hydrogen-bonded exciplex, that applies to the p,p* ketone triplets. Ketones with lowest charge-transfer p,p* states exhibit rate constants that vary only slightly with triplet reduction potential over the full range investigated; this is due to the compensating effect of substituents on triplet state basicity and reduction potential, which both play a role in quenching by the hydrogen-bonded exciplex mechanism. Ketones with arenoid p,p* states exhibit the fall-off in rate constant that is typical of photoinduced electron transfer reactions, but it occurs at a much higher potential than would be normally expected due to the effects of hydrogen-bonding on the rate of electron-transfer within the exciplex.

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