PHOTOCHEMISTRY OF 1,2-DIBROMOETHYL ARENES

UV laser photolysis of 2-(1,2-dibromoethyl)naphthalene in benzene and acetonitrile results in C-Br homolytic bond cleavage via the singlet a nd possibly the triplet manifolds. Cleavage produces a bromine atom, B r-., and the 2-bromo-1-(2-naphthyl)ethyl radical. In benzene, the pres ence of Br-. is indicated by formation of the Br-.-benzene pi-complex, while in acetonitrile with added Br, Br-2(.-) - is produced. In addit ion to these two transient probes, the formation of acidic solutions f ollowing photolysis in the presence of a H atom donor is also an indic ation of Br-. generation. The debromination quantum yield was determin ed by quantifying the formation of both Br-2(.-) Phi(Br). = 0.87 +/- 0 .10, and acid Phi(H)+ = 0.85 +/- 0.14. The difference between these va lues and those obtained for other vicinal dibromides (Phi(Br). > 2) wh ich undergo photochemical debromination followed by facile thermal C-B r cleavage is attributed to the greater stability of the 2-bromo-1-(2- naphthyl)ethyl radical relative to 2-vinylnaphthalene, the product for med by loss of the second Br-.. Photolysis of the 2-bromo-1-(2-naphthy l)ethyl radical using a second laser pulse leads to further C-Br cleav age resulting in enhanced production of 2-vinylnaphthalene. UV laser p hotolysis of 2-bromo-9-(l,2-dibromoethyl)anthracene also causes C-Br c leavage resulting in the production of Br-. and the 2-bromo-1-(2-bromo -9-anthryl)ethyl radical. The efficiency of debromination was lower in this compound (Phi(Br). = 0.45 +/- 0.10; Phi(H)+ = 0.48 +/- 0. 1 1) t han for the naphthalene compound because of competition between cleava ge from the singlet manifold and intersystem crossing to a low-energy unreactive triplet state. Like the naphthalene analog, laser photolysi s of the anthrylethyl radical leads to further debromination. Both rad icals exhibit unusual stability in the presence of oxygen.