Photochemical reaction dynamics of 9,10-phenanthrenequinone and 1,2-naphthoquinone with hydrogen donors in solution

Photoreaction dynamics of 9,10-phenanthrenequinone (PQ) and 1,2-naphthoquinone (NQ) in solution has been studied by laser flash photolysis technique. The real-time transient absorption measurements found out: (1) by excitation at 355 nm in the presence of alcohol, absorption band characteristic to the quinone ketyl radical emerged as the triplet-triplet (T-T) absorption of the quinone submerged, (2) the rise of the absorption of the ketyl radical consisted of two components; the fast and slow ones, where the fast one had the rise rate constant corresponding to the decay rate of triplet quinone, while the slow one rose up much slowly. The experimental fact clearly revealed that the slow reaction should give rise to formation of the ketyl radical following the hydrogen abstraction of triplet PQ and NQ from alcohol, and should be attributed to the hydrogen atom transfer between the parent quinone in the ground state and counter α-hydroxyalkyl radical produced from alcoholic molecule. The reaction mechanism of the ketyl radical formation was thoroughly discussed in the text. The notable reactivity toward hydrogen abstraction of the triplet o-quinones was also discussed.