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Quantum yield studies of the photolyses of various tetrahydro-1,4-naphthoquinones

University of British Columbia

The 366 nm photolysis of 2,3,4a,6,7,8a-hexamethyl-4aβ,5,8,8aβ-tetrahydro-l,4-naphthoquinone (27) in benzene yielded 28 with a quantum yield of 0.066 ± 0.003 and 29 with a quantum yield of 0.089 ± 0.003. The formation of 28 was suggested to occur via a C₈ hydrogen abstraction by C₁ oxygen in a five membered transition state (β-hydrogen abstraction), and the formation of 29 was believed to occur via a C₈ hydrogen abstraction by C₃ carbon in a six membered transition state (γ-hydrogen abstraction). A Stern-Volmer analysis (effect of triplet quencher concentration on quantum yield) showed that 28 was formed from a singlet excited state, whereas 29 was formed from a triplet excited state. The 366 nm photolysis of 6,7-dimethyl-4aβ,5,8,8aβ-tetrahydro-1,4-naphthoquinone (10) in benzene yielded 12 with a quantum yield of 0.0080 ± 0.0008 and 13 with a quantum yield of 0.0164 ± 0.0012. The 366 nm photolysis of 10 in tert-butanol yielded 11 with a quantum yield of 0.0081 ± 0.0008. All three photoproducts of 10 were suggested to occur via β-hydrogen abstraction by oxygen. Stern-Volmer analyses showed that both photoproducts 12 and 13 arise via a triplet and a singlet excited state, whereas 11 only arises via a singlet state. A mechanism is proposed explaining why products similar to 11 and 12 are not observed in the photolysis of 27. This mechanism is based on conformational control of the biradical intermediate by the bridgehead substituents. The effect of methyl substituents on the chromophore of 27 are implicated in shifting down the energy of the (π,π*) triplet. It is argued that 29 arises via γ-hydrogen abstraction by this (π,π*) triplet. [Chemical Diagrams]

Text

2010-01-28

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http://hdl.handle.net/2429/19272

Chemistry

University of British Columbia

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