Triarylsulfonium salts, photochemistry

Photochemistry of Triarylsulfonium Salts. A similar mechanism was proposed for the photolysis of triarylsulfonium salts having nonnucleophilic anions. The arylsulfonium and to a lesser extent, the aryliodonium salts have improved thermal stability as compared to the diazonium salts, however, their absorption maximum occurs at a much shorter wavelength since the aromatic rings are isolated by the heteroatom, and therefore not conjugated. 

Under conditions of nanosecond laser-flash photolysis, a long-lived transient absorption assigned to VI, A 465 nm, analogous to V, the intermediate proposed in formation of iodobiphenyls from iodonium salts, is observed [83], The diphenylsulfinyl radical cation, /Lmai340, 750 nm, is similarly observed in acetone sensitized laser-flash photolysis. Photo-CIDNP observations, namely emissive polarization for benzene, suggest that the homolytic cleavage pathway also operates under conditions of direct photolysis, accounting, in part, for the diphenylsulfide product [83], as well as the photochemistry of polymeric triarylsulfonium salts [81] (see above). 

Crivello and J.FI.W. Lam,. Polym. Set Polym. Lett. Ed. 17, 759 (1979) J.V. Crivello and J.L. Lee, Photosensitized cationic polymerizations using dialkylphenacylsulfonium and dialkyl(4 hydro xyphenyl)sulfonium salt photoinitiators, Macromolecules, 14, 1141 (1981) S.P. Pappas, Photo generation of acid Part 6 A review of basic principles for resist imaging applications, J. Imaging Technol. 11, 146 (1985) J.L. Dektar and N.P. Hacker, Triphenylsulfonium salt photochemistry. New evidence for triplet excited state reactions, J. Org. Chem., 53, (1988) J.L. Dektar and N.P. Hacker, Photochemistry of triarylsulfonium salts, J. Am. Chem. Soc. 112, 6004 (1990) G. Pohlers, J.C. Sciano, R.F. Sinta, R. Brainard, and D. Pai, Mechanistic studies of photoacid gen eration from substituted 4,6 bis(trichloromethyl) 1,3,5 triazines, Chem. Mater. 9, 1353 (1997). 

The major portion of the article will be devoted to a discussion of the synthesis and mechanistic aspects of photoinitiation by individual photoinitiator systems. Cationic polymerizations induced by these photoinitiators are dark, i.e., non-photo-chemical, processes which are governed by the same parameters which must be taken into account in polymerizations which occur in the presence of conventional initiators such as Lewis and Bronsted acids. Accordingly, cationic polymerizations induced by halogen and sulfur based onium salt photoinitiators will be discussed only from the context in which they are influenced by factors which have their origin in the photoinitiator. Since the photochemistry of diaryliodonium and triarylsulfonium salts is similar, these two types of photoinitiators will be discussed together. The photolysis of dialkylphenacylsulfonium and dialkyl-4-hydroxyphenylsulfonium salts proceeds by a different mechanism, and they will be discussed separately. 

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