Sensitization diaryliodonium salts

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. 

Diaryliodonium salts, with few exceptions, are stable compounds towards heat, oxygen and humidity they are mildly light-sensitive and should be stored in the dark, without refrigeration. Generally, their reactivity is less pronounced than that of other hypervalent iodine compounds. Indeed, in several of their reactions relatively drastic conditions may be necessary, especially for the least reactive heterocyclic iodonium salts. The search for optimum conditions is often desirable even for well-established reactions, by applying new findings concerning the use of specific... 

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.

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.

Photochemical Sensitization. Photolysis of diaryliodonium salts in the presence of benzoin ethers results in efficient reaction of the iodonium salt [96,97]. Scheme 5 illustrates the mechanism of photolysis according to Ledwith [96] and Timpe [92], Accordingly, photocleavage of benzoin ethers yields easily oxidized ketyl radicals (and acyl radicals which can also initiate radical polymerization). That only ketyl radicals participate in photochemical sensitization of onium salt decomposition was confirmed by ESR spin trapping with benzylidene-tcrt-butylamine-AT-oxide [10b]. As the chemistry... 

Abstraction of labile hydrogen atoms, for example, from tetrahydrofuran (THF), by photoexcited ketones also yields easily oxidized radicals [60a]. Quantum yields for benzophenone sensitized photolysis of diphenyliodo-nium hexafluoroarsenate in acetonitrile with hydrogen atom donors are shown in Table 9. Diaryliodonium salts are capable of oxidizing electron rich radicals from isopropanol and THF but triarylsulfonium salts are not [96]. The oxidation potential of tetrahydrofuranyl radical is —0.35 V versus SCE [109], apparently sufficient for irreversible reduction of diphenyliodonium cation ( red = approx. —0.7 V versus SCE, see above) but not of triphenyl-sulfonium cation ( red = —1.2V versus SCE). 

SCHEME 12.6 Photolysis of diaryliodonium salts—sensitized energy transfer. 

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). 

It should also be mentioned that the absorption maxima of the diaryliodonium and the triarylsulfonium salts lie around 250 nm. These salts have no appreciable absorption above 300 nm. Dyes such as acridine yellow (Xmax 411 nm), benzo-flavin (Xmax 460 nm), acridine orange (X ax 539 nm), etc., can be used to spectrally sensitize iodonium salts. " In contrast, triarylsulfonium salts are not sensitized by acridine derivatives or other common dyes they can be sensitized by perylene and other polynuclear aromatic hydrocarbons. ... 

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. 

The use of isopropylthioxanthone as a PS for two-photon sensitized cationic polymerizations was described by Boiko et al. [BOI 01], The Yagci research group [TAS 08] has also been active in this area and has published the results of their study of the use of benzophenone and benzodixinone as two-photon sensitizers for the diaryliodonium salt polymerization of cyclohexene oxide. These polymerizations are of growing interest since they can be used to spatially define three-dimensional images for the production of solid polymeric objects. 

CIDNP spectroscopy has been applied to both direct and sensitized photoreactions of onium salts (diaryliodonium and triarylsulfonium salts) [134] see Chart XVIII. In a recent investigation [134f] ambiguities of earlier studies on sensitized photolysis of iodonium salts [134a, 134b], where S-T -type polarizations had to be postulated to rationalize the polariz-... 

Commercially important photoinitiators for crosslinking by cationic polymerization are onium salts, particularly the thermally and hydrolytically stable diaryliodonium and triarylsulfonium salts,and mixed ligand arene cyclopentadienyl metal salts with complex metal halide anions. Direct and sensitized photolysis of these systems have been reviewed " and are discussed in detail in Volume 4, Chapter 20. 

Direct and sensitized photolysis of diaryliodonium and triarylsulfonium salts, as well as sensitized photolysis of phenacylsulfonium salts, yields both cationic and radical initiators (see Volume 4, Chapter 20). Accordingly, triarylsulfonium salts have been used for photocrosslinking mixtures of acrylate- and epoxy-functional resins, as well as acrylate- and vinyl ether-functional resins with suppression of air inhibition. 

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