Re-examination of 7,7′-dimethyl-7-germanorbornadiene photocleavage in solution and in organic glasses

Abstract

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 k max & 430 nm and molar absorption coefficient of e & 2,700 M -1 cm -1 have been shown to be attributable to low-temperature glasses, while the absorption band maximum of k max & 480 nm and molar absorption coefficient of e & 2,400 M -1 cm -1 have been shown to be related to dimethylgermylene in n-hexane solution. The molar absorption coefficient of tetramethyldigermene (k max & 380 nm) was determined to be e & 84,000 M -1 cm -1 . The germylene is formed via (formal) cheletropic photocycloreversion of 7,7 0 -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.