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

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• 作者：Helmut G6 ; rner
• 关键词：Quinones ; Photoreduction ; Intramolecular H-atom transfer ; Zwitterion ; Radical
• 刊名：Journal of Photochemistry and Photobiology A: Chemistry
• 出版年：2005
• 出版时间：2005
• 年：2005
• 卷：175
• 期：2-3
• 页码：138-145
• 全文大小：325 K

文摘

The photochemistry of 2,5-dibromo-3-methyl-6-isopropyl-1,4-benzoquinone (Br₂TQ) in solution was studied by UVȁ3;vis spectroscopy using 20 ns laser pulses at 308 nm. The first transient with a lifetime of 0.06ȁ3;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 0.1ȁ3;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.