Diaryliodonium salts sensitization, electron transfer

Figure 5. Sensitization of diaryliodonium salts via electron transfer results in formation of the sensitizer radical cation (S ). Hydrogen abstraction produces a proton. Both species may initiate cationic polymerization. Note The nonnucleo-philic anion (AsFo ) has been deleted for clarity.

Several dyes have been found to sensitize the cationic polymerization of cyclohexene oxide, epichlorohydrin, and 2-chloroethyl vinyl ether initiated by diaryliodonium salts (109,110). Acridinium dyes such as acridine orange and acridine yellow were found to be effective sensitizers. One example of a benzothiazolium dye (setoflavin T) was also reported, but no other class of dye nor any other example of a dye absorbing at longer wavelengths were discovered. Crivello and Lam favored a sensitization mechanism in which direct energy transfer from the dye to the diaryliodonium salt occurred. Pappas (12,106) provided evidence that both energy transfer and electron transfer sensitization were feasible in this system. 

Figure 7. Example calculation of AG. Free-energy calculations ( G) show that electron transfer from triplet excited thioxanthane to the diaryliodonium salt is exothermic, and therefore, sensitization is observed. In contrast, electron transfer from triplet excited thioxanthane to the triaryl-sulfonium salt is endothermic thus, sensitization is not observed.

Photosensitization of diaryliodonium salts may occur through energy transfer or electron transfer. The triplet sensitizer transfers energy to the diaryliodonium molecule, which undergoes homolytic cleavage from its triplet state, resulting in a radical pair within the solvent cage (Scheme 12.6). 

Triplet sensitization of sulfonium salts proceeds exclusively by the homolytic pathway, and that the only arene escape product is benzene, not biphenyl or acetanilide. However, it is difficult to differentiate between the homolytic or heterolytic pathways for the cage reaction, formation of the isomeric halobiaryls. Our recent studies on photoinduced electron transfer reactions between naphthalene and sulfonium salts, have shown that no meta- rearrangement product product is obtained from the reaction of phenyl radical with diphenylsulfinyl radical cation. Similarly, it is expected that the 2- and 4-halobiaryl should be the preferred products from the homolytic fragments, the arene radical-haloarene radical cation pair. The heterolytic pathway generates the arene cation-haloarene pair, which should react less selectively and form the 3-halobiaryl, in addition to the other two isomers. The increased selectivity of 2-halobiaryl over 3-halobiaryl formation from photolysis of the diaryliodonium salts versus the bromonium or chloronium salts, suggests that homolytic cleavage is more favored for iodonium salts than bromonium or chloronium salts. This is also consistent with the observation that more of the escape aryl fragment is radical derived for diaryliodonium salts than for the other diarylhalonium salts. 

Chen et al. reported efficient photosensitization of onium salts by various compounds containing a carbazole nucleus. Both diaryliodonium and triarylsulfonium salts are photosensitized by sueh eompounds. Thus, the polymer of N-vinylcarbazole was found by them to be an excellent electron-transfer photosensitizer for various onium salts. They also found that poly(9-vinylcarbazole) yields similar results. Poly(2-vinyl carbazole) turned out to be the most efficient photosensitizer among various polymers with carbazole tested. In addition, Chen et al., concluded that the redox photosensitization by the carbazole molecule or its N-alkylated derivatives occurs predominantly from the singlet excited states. On the other hand, the carbazole derivatives with carbonyl substituents sensitize onium salts via triplet excited states. This follows... 

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. 

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