Journal of Physical Chemistry, Vol.98, No.28, 7022-7028, 1994
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 and possibly the triplet manifolds. Cleavage produces a bromine atom, Br-., and the 2-bromo-1-(2-naphthyl)ethyl radical. In benzene, the presence of Br-. is indicated by formation of the Br-.-benzene pi-complex, while in acetonitrile with added Br, Br-2(.-) - is produced. In addition to these two transient probes, the formation of acidic solutions following photolysis in the presence of a H atom donor is also an indication of Br-. generation. The debromination quantum yield was determined 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 values and those obtained for other vicinal dibromides (Phi(Br). > 2) which undergo photochemical debromination followed by facile thermal C-Br cleavage is attributed to the greater stability of the 2-bromo-1-(2-naphthyl)ethyl radical relative to 2-vinylnaphthalene, the product formed by loss of the second Br-.. Photolysis of the 2-bromo-1-(2-naphthyl)ethyl radical using a second laser pulse leads to further C-Br cleavage resulting in enhanced production of 2-vinylnaphthalene. UV laser photolysis of 2-bromo-9-(l,2-dibromoethyl)anthracene also causes C-Br cleavage 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) than for the naphthalene compound because of competition between cleavage from the singlet manifold and intersystem crossing to a low-energy unreactive triplet state. Like the naphthalene analog, laser photolysis of the anthrylethyl radical leads to further debromination. Both radicals exhibit unusual stability in the presence of oxygen.
Keywords:PHOTOINITIATED CATIONIC POLYMERIZATION;ABSOLUTE RATE CONSTANTS;RADICALS;STATE;ATOM;9;10-DIBROMOANTHRACENE;SPECTROSCOPY;ACTINOMETER;ENERGIES;SYSTEMS