Photochemistry of Diaryliodonium and Triarylsulfonium Salts

Many early workers reported that diaryliodonium salts slowly decomposed on standing More recent investigations, however, have shown that these compounds are completely stable when stored in the dark It would seem likely that the previous observations were the result of inadvertent photolysis caused by their exposure to light during storage. In the past few years, a number of workers have investigated the photolysis of diaryliodoniiun salts and have elucidated the structure of their primary photoproducts Crivello and Lam have proposed the following mechanistic pathway which accoimts for the photolysis reaction products which have been observed. 

In the first step of this mechanism, light is absorbed producing an electronically excited diaryliodoniiun salt. Rapid decay of the photoexcited species then occurs with resultant cleavage of a carbon-iodine bond giving an aryliodonium cation-radical and an aryl radical. 

A similar mechanistic pathway has been proposed for the photolysis of triarylsulfonium salts 

The presence of aromatic free radicals as the other primary photoproducts of the photolysis of diaryliodonium and triarylsulfonium salts has been conflrmed by the isolation of their reaction products, namely, aromatic hydrocarbons and biaryls from the photolysis mixtures. Cross coupling experiments in which the simultaneous photolysis of two diaryliodonium salts bearing different subtituents on their aromatic rings are carried out, yield a mixture of biaryl products including those in which the aromatic rings are derived from both starting materials. Clearly, the aryl radicals which are produced on photolysis have sufficient lifetimes to diffuse from the reaction site and to couple with one another. 

A final piece of evidence for the formation of radical species, i.e., aryl radicals and cation-radicals, in the photolysis of diaryliodonium and triarylsulfonium salts is the ability of these onium salts to photoinitiate typical radical polymerizations Indeed, using such photoinitiators, it is uniquely possible to simultaneously initiate both cationic and free radical polymerizations. 

---

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. 

---