Photosensitization of onium salt cationic photoinitiators

There is mueh preeedent in the literature for this approach and there are several methods of eonducting the photosensitization of a photolysis reaction. In this chapter, the term photosensitization will be applied to any method by whieh a light-absorbing compound induces the onium salt deeomposition resulting in the liberation of a strong Bronsted aeid that is capable of initiating a eationic polymerization. 

A second photochemical process called electron-transfer photosensitization is, in reality, a photoinduced redox reaction [EBE 87, PAP 84a, PAP 84b] and this method of photosensitization has been much more successful for the extension of the spectral sensitivity of onium salt cationic photoinitiators into the long wavelength UV and visible spectral regions. Electron-transfer photosensitization is a well-understood process and a general mechanism for this process as exemplified for diaryliodonium salts is shown in Diagram 2.2. 

Diagram 2.2. Mechanism for the electron-transfer photosensitization ofdiaryliodonium salts 

A particularly graphic example of electron-transfer photosensitization is the interaction of diphenyliodonium salts with perylene. Exposure of a dichloromethane solution of a mixture of these two compounds to UV light for 2 s results in the formation of the intensely blue-colored and relatively stable perylene eation-radical. The blue color remains in solution in air for a considerable time until it slowly fades. When this photolysis reaction is conducted in the presence of a cationically polymerizable monomer, the blue color of the perylene cation-radical is very transient. 

The fact that onium salts 1-5 undergo photosensitization via an electron-transfer mechanism is not surprising. The positively charged iodine, sulfur 

---