Photoreactions of 2,5-dibromo-3-methyl-6-isopropyl-1,4-benzoquinone

The photochemistry of 2,5-dibromo-3-methyl-6-isopropyl-1,4-benzoquinone (Br2TQ) in solution was studied by UV-vis spectroscopy using 20 ns laser pulses at 308 nm. The first transient with a lifetime of 0.06-0.2 μs is assigned to a triplet state. This triplet is converted into a short-lived zwitterion, which is formed after intramolecular H-atom transfer from the isopropyl substituent. The zwitterion has a lifetime of 0.1-2 μs and further conversion into 1,4-hydroquinones without involvement of free radicals is proposed. The properties of Br2TQ differ from those of 2-tert-butyl-1,4-benzoquinone (BuBQ), where no intermediate could be detected under comparable conditions, and of 2-methyl-5-isopropyl-1,4-benzoquinone (thymoquinone: TQ), where the triplet state escapes observation at room temperature. The effects of solvent polarity and other specific properties of photoinduced intramolecular H-atom transfer in Br2TQ are discussed. The mechanisms are specific for Br2TQ, TQ and BuBQ, whereas the quantum yield of photodecomposition is large, e.g. in neat acetonitrile and several other solvents, even in the presence of oxygen.