Laser Flash Photolysis Study of the Reactions of Carbonyl Triplets with Phenols and Photochemistry of p-Hydroxypropiophenone

Article abstract of DOI:10.1021/ja00404a029

The quenching of aromatic carbonyl triplets by phenols is very fast process for both n,?^* and ?,?^* states.Representative rate constants for benzophenone triplets in benzene are 1.3x10⁹ (phenol) and 8.1x10⁸ (m-fluorophenol) M^-1 s^-1, while for p-methoxypropiophenone the values are 4.9x10⁹ and 5.0x10⁹ M^-1 s^-1 for the same pair of substrates; 18 different phenols were examined.The rate constants are smaller in wet acetonitrile as a result of hydrogen bonding; for example for the benzophenone-phenol system we measured 8.0x10⁷ M^-1 s^-1.Isotope effects are larger for p-methoxypropiophenone than for benzophenone: for example in the case of phenol the values of k_H/k_D are 3.9 and 1.2, respectively (in wet acetonitrile).The quenching reaction leads to the efficient, but not quantitative, formation of phenoxy and ketyl radicals, with the only exception of p-nitrophenol where the process involves electronic energy transfer.The temperature dependence of a few representative systems as well as the quenching by methyl ethers of similar structure, has also been examined.The photochemistry of p-hydroxypropiophenone is characterized by efficient self-quenching involving a head-to-tail hydrogen-transfer reaction.