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-digerma bicyclo[2.2.2]octadienes in solution has been studied in detail by steady-s tate, nanosecond laser flash photolysis, and matrix isolation techniques. P hotolysis of 7,8-digermabicyclo[2.2.2]octadienes in solution resulted mainl y in the formation of tetraalkyldigermenes and 1,4-diphenylnaphthalene in t he triplet excited state. The 7,8-digermabicyclo[2.2.2]octadiene also photo chemically isomerized to 1,4-diphenyl-2,3-benzo-6,6,7,7-tetraalky-6,7-diger matricyclo[3.3.0.0]octane as minor product. The reactions of tetraalkyldige rmenes with butadienes (2,3-dimethylbuta-1,3-diene and 2-methylbuta-1,3-die ne), carbon tetrachloride, and methanol gave the corresponding digermene tr apped products: 1,2-digennacyclohex-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 rea ctions of the digermene with oxygen, polyhalomethanes, alcohols, 2,3-dimeth ylbuta-1,3-diene, 1-hexyne, and triethylsilane were also measured under var ious conditions. The quenching reactions of digermenes with carbon tetrachl oride and ethanol (k = similar to 10(7) M-1 s(-1)) are 100 times faster tha n that with 1,3-butadiene (h = similar to 10(5): M-1 s(-1)). The digermenes were very rapidly quenched with oxygen (h = similar to 10(8) M-1 s(-1)), b ut very slowly with 1-hexyne and triethylsilane (k < 10(4) M-1 s(-1)) under reaction conditions. The UV absorption bands of tetraalkyldigermenes gener ated from 7,8-digermabicyclo[2.2.2]octadienes in matrix at 77 K were also o bserved at 370-380 nm. The mechanism of digermene formation was also discus sed.