Re-examination of 7,7′-dimethyl-7-germanorbornadiene photocleavage in solution and in organic glasses
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Dimethylgermylene and its Ge=Ge doubly bonded dimer, tetramethyldigermene, have been characterized directly in solution by 308-nm laser flash photolysis in n-hexane solution, as well as 254-nm photolysis in hydrocarbon glasses at t = 77 K. An absorption band maximum of λ max ≈ 430 nm and molar absorption coefficient of ε ≈ 2,700 M−1 cm−1 have been shown to be attributable to low-temperature glasses, while the absorption band maximum of λ max ≈ 480 nm and molar absorption coefficient of ε ≈ 2,400 M−1 cm−1 have been shown to be related to dimethylgermylene in n-hexane solution. The molar absorption coefficient of tetramethyldigermene (λ max ≈ 380 nm) was determined to be ε ≈ 84,000 M−1 cm−1. The germylene is formed via (formal) cheletropic photocycloreversion of 7,7′-dimethylgerma-1,4,5,6-tetraphenyl-2,3-benzo-norbornadiene. Tetramethyldigermene and 1,2,3,4-tetraphenylnaphthalene in the triplet state were formed, together with dimethylgermylene. We attempted to explain the various contradictory interpretations of experimental data existing in the literature on this reaction.
Keywords7,7′-dimethyl-7-germanorbornadiene Dimethylgermylene Tetramethyldigermene Laser flash photolysis Organic glasses
The authors would like to thank Dr. I. V. Krylova and Prof. M. P. Egorov (N.D. Zelinsky Institute of Organic Chemistry, the Russian Academy of Sciences) for providing the 7,7′-dimethylgerma-1,4,5,6-tetraphenyl-2,3-benzo-norbornadiene. The authors are also very grateful to Dr. E. M. Glebov and Dr. V. I. Borovkov (Institute of Chemical Kinetics and Combustion, Siberian Branch of the Russian Academy of Sciences).
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