A combined laser flash photolysis, density functional theory and atoms in molecules study of the photochemical hydrogen abstraction by pyrene-4,5-dione

The photochemical hydrogen abstraction reaction of pyrene-4,5-dione (3) has been investigated by laser flash photolysis. Excitation (λ = 355 nm) of a degassed solution of 3 in acetonitrile resulted in the formation of a detectable transient with absorption maxima at 380 and 470 nm. This transient decays with a lifetime of around 4.8 μs and is quenched by oxygen. This transient is most probably a triplet state of 3. Addition of hydrogen donors, such as 2-propanol; 1,4-cyclohexadiene or 4-methoxyphenol led to the formation of a new transient with λmax at 380, 500 nm and a broad absorption at 640 nm. This new transient slowly decays with second order kinetics and was assigned to the semiquinone radical obtained from the hydrogen abstraction reaction. Using DFT and AIM calculations the reactivity of 3 and 9,10-phenanthrenequinone (1) is best interpreted as a proton coupled electron transfer like mechanism for the hydrogen abstraction from 2-propanol.