Photosensitization By Free Radical Induced Decomposition

In contrast to the photolysis of diaryliodonium salts in acetonitrile which gives quantum yields for the formation of aryliodides of 1, photolysis in tetrahydrofuran (THF) at 366 nm gives quantum yields of approximetaly Quantum yields greater than unity in this case suggest that a chain induced decomposition of the 

The aryl radicals produced from the photolysis of the diaryliodonium salt abstract a hydrogen atom from THF producing the aryl hydrocarbon and the THF radical. The THF radical is further oxidized by the diaryliodonium salt, resulting in the formation of a stabilized THF cation which may initiate cationic polymerization. Simultaneously, the aryl radical which is the principal chain carrier is regenerated. Because such free radical chain reactions are inhibited by oxygen, this process is most efficiently carried out in the absence of air. 

Triphenylsulfonium salts do not undergo free radical induced chain decomposition. However, Ledwith has reported that diphenyl-4-thiophenoxyphenylsulfonium salts give high quantum yields of diphenyl sulfide when irradiated at 304 nm in THF ( = 1,5) and cyclohexene oxide (4 = 3) indicating that these particular compounds also undergo free radical induced chain decomposition. 

Benzophenone sensitized quantum yields of acid formation in THE and 2-propanol are in the range of 5-8 and 6-12 for air and vacuum, respectively Since oxygen is known to quench the triplet benzophenone as well as intercept the ketyl and aryl radicals, it is not surprising that the quantum yields are lower in the presence of air than in its absence. 

Diaryliodonium salts are also reduced by easily oxidized radicals generated from typical free radical photoinitiators. Ledwith has studied the cationic poly- 

---