Photosensitization of onium salts

There are also several mechanisms by which photosensitization of onium salts can take place. Of these, electron-transfer photosensitization appears to be the most efficient and generally is the applicable process. Here, absorption of light by the photosensitizers is followed by formation of exiplexes in the intermediate step. This is followed by reductions of the onium salts through electron-transfer and collapse of the exiplexes. Subsequent rapid decomposition of the resulting unstable diaryliodonium free radical prevents back electron-transfer. The overall process is irreversible. Generally, due to their lower reduction potentials, diaryliodonium salts are more easily photosensitized by an electron-transfer process than are triarylsulfonium salts. The mechanism is illustrated below as follows ... 

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... 

Fig. 2 Various types of photoinitiators (1) peroxides, (2) azo compounds based on AIBN, (3) benzoin ethers, (4) triplet photosensitizers, (5) onium salts for cationic polymerization, and (6) controlled free radical polymerization with photoiniferters...

Quantum yields of acid, and iodobenzene or phenyl sulfide were measured by irradiating. 03 molar acetonitrile solutions of diphenyl iodonium hexafluoroarsenate and triphenyl sulfonium hexa-fluoroarsenate. Photosensitizers were included at identical optical density (1.0) at the wavelength of irradiation (366 nm). Relative polymerization rates were calculated as a function of the gelation time of a solution of 50% acetone, and 50% cycloaliphatic diepoxide resin ARALDITE CY-179 (v/v) containing the photosensitizer and onium salt as above. Experimental details were published previously.(8,15) IRGACURE and ARALDITE are registered trademarks of the Ciba-Geigy Corporation. 

Typical photosensitizers for diaryliodonium salts are condensed ring aromatic hydrocarbons, diaryl ketones, and acridinium dyes. Condensed ring aromatic hydrocarbons are particularly effective photosensitizers for triarylsulfonium salts. The use of photosensitizers in onium salt photolysis permits the photoimaging processes induced by these compounds to be optimally fitted to the specific irradiation source used for their exposure. 

DeVoe et al. have reported quantitative aspects of photosensitization of diphenyliodonium salt and bis(4-dimethylaminobenzylidene)acetone (DMBA) [101]. This ketone is a bis-vinylog of Michler s Ketone, which is a well-known sensitizer for onium salt initiated free radical polymerizations [102,103], The reaction with DMBA is an example of electron transfer sensitization gated by conformational relaxation of the sensitizer. The ratio of iodonium salt consumption to aminoketone consumption is two, the second iodonium salt equivalent is consumed by a second reducing equivalent from the aminoalkyl radical on the oxidized photosensitizer. 

A variety of other ketones have been shown to photosensitize iodonium and sulfonium salts from triplet excited states by the electron transfer mechanism [102,103], Rates of electron transfer from triplet 3-(2-isoxazolinyl)-phenyl ketone, XI, to a series of onium salts in aq. acetonitrile have been shown to scale with the free energy change according to the Weller equation (Eq. (28)) [105], In the same study, the rates of triplet quenching of a series of ketones by diphenyliodonium cation in the same solvent were evaluated the results were not tractable in terms of usual linear free energy... 

Over the past several years, there have been developed several new classes of onium salt photoinitiators capable of initiating cationic polymerization. The most significant of these are aryldiazonium salts, diaryliodonium salts, triarylsulfonium salts, and dialkylphenacyl-sulfonium salts. The mechanisms involved in the photolysis of these compounds have been elucidated and will be discussed. In general, on irradiation acidic species are generated which interact with the monomer to initiate polymerization. Using photosensitive onium salts, it is possible to carryout the polymerization of virtually all known cationically polymerizable monomers. A discussion of the various structurally related and experimental parameters will be presented and illustrated with several monomer systems. Lastly, some new developments which make possible the combined radical and cationic polymerization to generate interpenetrating networks will be described. 

Over the past ten years the development of onium salt and other cationic photoinitiators has moved from the realm of speculative investigation to the point today at which they are being employed in numerous commercial applications. Much work still needs to be done in this fields particularly to improve our understanding of the relationship between the structure and the photosensitivity of these photoinitiators. As the field advances, one can expect still other new classes of onium salt photoinitiators to be developed as well as continued improvements to be made in the efficiency of the present systems. An understanding of the mechanism of photosensitization should lead to discovery of more efficient photosensitizers and a further broadening of their spectral response in photoinitiated cationic polymerization. 

Hua and Crivello found that this polymer funetions as an eleetron-transfer photosensitizer for a wide variety of onium salts, ineluding diaryliodonium, triarylsulfonium, and di-aUcylphenaeylsulfonium salts. The broadening of the speetral response through the use of these photosensitizers aeeounts for the observed rate enhaneement. In addition, alternating eopolymers prepared by the free radieal eopolymerization of N-vinylearbazole with vinyl monomers also exhibit exeellent photosensitization aetivity. 

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. 

CRI 03b] Crivello J.V., Jang M., Anthracene electron-transfer photosensitizers for onium salt induced cationic photopoiymerizations , Journal of Photochemical and Photobiology, vol. 159, no. 2, pp. 173-188, 2003. 

NAR 10] Narewska J., Strzelczyk R., Podsiadly R., Fluoflavin dyes as electron transfer photosensitizers for onium salt induced cationie photopolymerization . Journal of Photochemistry and Photobiolology A Chemistry, vol. 30, pp. 68-74, 2010. 

The electron transfer mechanism best explains the phenomenon of onium salt photosensitization in most cases. This conclusion is supported hy the following observations. Analysis of the products formed by the photosensitized photolysis of diaryliodonium and triarylsulfonium salts are those which are predicted on the basis of the above mechanism. Moreover, these products are identical to those formed by the electrolytic reduction of these same onium salts which give rise to the same diaryliodide and triarylsulfide radical intermediates. 

Using aryl ketones in combination with hydrogen donor solvents or conventional free radical photoinitiators, it is, therefore, possible to effect a new kind of photosensitization of the photolysis of onium salts which results in an expansion in the spectral sensitivity of these cationic photoinitiators. This photosensitization occurs not as a result of a direct interaction between the onium salt and the excited photosensitizer but by a secondary dark nonphotochemical reaction of the onium salt with the radical photoproducts of the photosensitizer. 

Here, the excited triplet ketone abstracts a hydrogen atom from the phenol to give a phenoxy cation-radical. This latter species is further oxidized by the ketyl radical to the resonance-stabilized ylide. The protonated ketone then collapses to regenerate ketone and give the acid, HX,. It is noteworthy that in contrast to the photosensitization of dialkylphenacylsulfonium salts which involves reduction of the onium salts, the above process results in the oxidation of the sulfonium salt. Although formally Scheme 12 is an oxidation, photosensitization closely resembles the non-photosensitized process with respect to the products which are formed, i.e., an ylide and a Bronsted acid. Hence, in this case, photosensitized photolysis is a reversible process and addition of monomers to pre-irradiated solutions does not result in polymerization. 

Timpe, H.-J., Ulrich, S., and Fouassier, J.-P., Photochemistry and use of decahydroacridine-l,8-diones as photosensitizers for onium salt decomposition,/. Photochem. Photobiol., A Chem., 73,139,1993. 

Onium salts have many of the characteristics required of photosensitive initiators for light-curable coatings. Aryl-diazonium, aryliodonium and arylsulfonium salts all initiate rapid cure. Storage life of photosensitized formulations ranges from a few hours to more than six months, depending upon structure of the photoinitiator, composition of the coating and storage conditions. 

There are many commercially available photosensitizers. The overall cost of coating formulation may be reduced by the use of small amounts of a highly absorbing photosensitizer in combination with a reduced concentration of the onium salt. 

An understanding of the theory of electron transfer sensitization is necessary in order to choose sensitizers which can photoreduce the onium salt. For electron transfer to be energetically favorable, the excitation energy of the photosensitizer (E ) must be greater than the net energy required to oxidize the photosensitizer, and reduce the onium salt.( )... 

Photosensitization. The chief absorption bands for onium salts I-III lie below 230 nm in the ultraviolet region. Photolysis at longer wavelengths is, therefore, ineffective. However, the photolysis of these compounds can be readily photosensitized to respond to long wavelength uv and to visible light. The mechanism of photosensitization involves an electron transfer process... 

Acid Catalyzed Hydrolysis. When photosensitive onium salts are incorporated into a film of a hydrolytically sensitive polycarbonate and exposed to ultraviolet irradiation, the acid which is produced catalyzes chain scission at random sites along the backbone (Equation 25). 

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