Cyanine radical formation

Table 1. Oxidation and reduction potential data, rate constants for electron transfer for cyanine borates in acetonitrile and benzene solution, and efficiency of cyanine radical formation.

Photopolymerization reactions are widely used for printing and photoresist appHcations (55). Spectral sensitization of cationic polymerization has utilized electron transfer from heteroaromatics, ketones, or dyes to initiators like iodonium or sulfonium salts (60). However, sensitized free-radical polymerization has been the main technology of choice (55). Spectral sensitizers over the wavelength region 300—700 nm are effective. AcryUc monomer polymerization, for example, is sensitized by xanthene, thiazine, acridine, cyanine, and merocyanine dyes. The required free-radical formation via these dyes may be achieved by hydrogen atom-transfer, electron-transfer, or exciplex formation with other initiator components of the photopolymer system. 

The evidence for radical formation from the cyanine borates is the following ... 

Figure 3. The absorption of cyanine dye (Cy) radicals monitored at 430 nm following excitation of a benzene solution with an 18 ps laser pulse. The time dependence of the absorption changes of cyanine radical for the benzyltriphenylborate case is faster than its decay. For the vinyltriphe-nylborate, back electron transfer and the reaction that follows electron transfer have competitive rates. For the tetraphenylborate salt, the back electron transfer process dominates after electron transfer, therefore the boron-carbon bond cleavage does not occur and almost no cyanine dye radical formation is observed (data adapted from [25]).

Significantly, if the cyanine dye is incorporated into the system in monomeric-dimeric form, although radical formation can... 

Borate salts are especially useAil in combination with cyanine dyes. Depending on the cyanine used, there are different absorption maxima in the visible region with usually high molar extinction coefficients (e 10 L moP cm ). Radical formation by borate is illustrated in reaction (42). 

In recent years visible photoinitiators for the formation of polymers via a radical chain reaction have also been developed. These absorb light which is blue, green, or red and also cause the polymerization of polyolacrylates, in some instances, such as encapsulated systems, with speed which is near photographic. Some of these photoinitiators provide the photochemical backbone of the nonsilver, near-photographic speed, imaging processes such as the Cycolor processes invented by the Mead Corporation. Cycolor initiators are cyanine dye, borate ion salts (4)—so-called ( +, —) ion pair... 

Aryliodonium salts have been found to be coinitiators for photooxidizable dye sensitization (105). Smith polymerized aerylamide-bis(acrylamide) mixtures using acridine, xanthene, or cyanine dyes and, for example, diphenyllodonium chloride as an electron acceptor. Reduction of the salt results in the formation of phenyl radicals. 

Schuster and co-workers discovered that 1,4-dicyanonaphthalene solutions containing an alkyltriphenylborate salt, when irradiated, yield one-eleetron oxidation of the alkyltriphenylborate leading to carbon-boron bond cleavage and formation of free alkyl radicals [23]. In Gottschalk s hands [24, 25], it was shown that ionic salt pairs formed from cyanine dyes and alkyltriphenylborates (Figure 1) could be used as photoinitiators [26] that were active in the visible region of the spectrum. 

Other examples include acridine dyes (with absorption peaks aroimd 475 nm), xanthene dyes ( 500-550 nm), fluorone dyes ( 450-550 nm), coiunarin dyes (" 350-450 nm), cyanine dyes ( 400-750 nm), and carbazole dyes ( 400 nm) (12,19-21). The oxidation or reduction of the dye is dependent on the co-initiator for example, methylene blue can be photoreduced by accepting an electron from an amine (22) or photo-oxidized by transferring an electron to benzyltrimethyl-stannane (12). Either mechanism will result in the formation of a free-radical active center capable of initiating a growing polymer chain. For a more detailed discussion of the mechanisms, see Reference 12. 

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