Studies of Photochemical Reactions of 7,8-Digermabicyclo[2.2.2]octadienes by Steady-State and Laser Flash Photolysis Techniques

The photochemistry of 1,4-diphenyl-2,3-benzo-7,7,8,8-tetraalkyl-7,8-digermabicyclo[2.2.2]octadienes in solution has been studied in detail by steady-state, nanosecond laser flash photolysis, and matrix isolation techniques. Photolysis of 7,8-digermabicyclo[2.2.2]octadienes in solution resulted mainly in the formation of tetraalkyldigermenes and 1,4-diphenylnaphthalene in the triplet excited state. The 7,8-digermabicyclo[2.2.2]octadiene also photochemically isomerized to 1,4-diphenyl-2,3-benzo-6,6,7,7-tetraalkyl-6,7-digermatricyclo[3.3.0.0]octane as minor product. The reactions of tetraalkyldigermenes with butadienes (2,3-dimethylbuta-1,3-diene and 2-methylbuta-1,3-diene), carbon tetrachloride, and methanol gave the corresponding digermene trapped products:  1,2-digermacyclohex-4-enes, 1,2-dichlorodigermanes together with hexachloroethane, and methoxydigermanes, respectively, in good yields. The transient bands of tetraalkyldigermenes using nanosecond laser flash photolysis appeared at ca. 380 nm. The rate constants for the quenching reactions of the digermene with oxygen, polyhalomethanes, alcohols, 2,3-dimethylbuta-1,3-diene, 1-hexyne, and triethylsilane were also measured under various conditions. The quenching reactions of digermenes with carbon tetrachloride and ethanol (k = ~10⁷ M^-1 s^-1) are 100 times faster than that with 1,3-butadiene (k = ~10⁵ M^-1 s^-1). The digermenes were very rapidly quenched with oxygen (k = ~10⁸ M^-1 s^-l), but very slowly with 1-hexyne and triethylsilane (k < 10⁴ M^-1 s^-l) under reaction conditions. The UV absorption bands of tetraalkyldigermenes generated from 7,8-digermabicyclo[2.2.2]octadienes in matrix at 77 K were also observed at 370−380 nm. The mechanism of digermene formation was also discussed.