Cyanine borate

More recently, Schuster [25] has demonstrated that cyanine dyes, i.e., cyanine borates or cyanine dye-borate mixtures, provide visible light activated initiation of free radical polymerization [26]. The photoexcited cyanine dye oxidizes alkyltriphenylborates by PET to produce the bleached reduced cyanine along with an alkyl radical. The alkyl radical can then initiate free radical polymerization [27], This visible light activated PET bond cleavage is of considerable importance in photoimaging and photocuring [28]. 

Furthermore, an intramolecular PET in ion-pairs involving dye cations is possible. So, cyanine borates exist in ester solution as ion pairs [145]. The irradiation of that species initially creates cyanine radicals and alkylboranyl radicals. Latter species fragment to triphenylborane and alkyl radicals see Eq. (13). In support of the assumption of an intra-ion-pair electron transfer, no cyanine radicals have been obtained by irradiation in acetonitrile solution. 

Cyanine borates exist as tight ion pairs certain of the salts have even been shown to form ion pairs in which the center-to-center distance between the ions is less than the sum of the individual radii [27-29]. Since the lifetime of the excited singlet state of cyanine dye is quite short, too short to allow an efficient encounter at an achievable concentration of borate in a situation in which diffusion is necessary, the self-association of the cyanine cation and borate anion is a prerequisite for efficient photoinduced reaction. 

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

The absorption maximum of the cyanine dye can be changed by altering the number of conjugated alkene units linking the cyanine chromophores. This makes the cyanine borate photo-redox pair a so-called tunable photoinitiator, in that compounds which absorb throughout the visible and infrared spectrum can be obtained. Recently, Kabatc et al. [35] described the important features of cyanine borate photo-redox pairs (Table 2). The structures of dyes tested are shown in Figure 5. 

The quantum efficiency of these cyanine borates when they are used as photoinitiators depends strongly on structure (Figure 6). However, in general, cyanine borates are not particularly efficient photoinitiators. For example the RBAX-iV-phenylglycine photo-redox pair [36] is much more efficient. The main value of the cyanine borates is that they give substantial wavelength flexibility so they can be used in cases where polychromaticity is required (Figure 7). 

The data summarized in Table 2 indicate that the series of cyanine borates with the n-butyltriphenylborate counterion possess a negative AGei value, i.e., they fulfill the basic Rehm Weller requirements for effective photoinduced electron transfer processes. Extending this to a practical application of the Marcus equation (see... 

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.

Figure 5. General formula of the cyanine borates tested by Kabatc et al. [35], For Z, R, R, and R see Table 2.

Section 4.4) on the photoinitiation process, one can anticipate that under certain conditions (identical free radicals formed), the rules regulating the primary processes can also be applied for the secondary processes. The results presented in Figure 8 confirm this expectation. It is clear from the data (Figure 8) that the rate of polymerization as initiated by the series of cyanine borates in Table 2 increases as the driving force of the electron transfer increases. This behavior is predicted by the classical theory of photoinduced electron transfer. 

Schuster s original measurements were performed in benzene, where the cyanine borates are know to exist as tight ion pairs. In practice, the monomers used in most photopolymerization systems possess higher dielectric constants than benzene. Therefore the degree of dissociation of the cyanine borate in monomer or mixtures of monomers is expected to be higher than it is in benzene. As an example, the data... 

Figure 9. A cyanine borate absorbing in the infrared region [37],...

Cyanine borates belong to a specific group of photoinitiating systems. Both components exist as ions, so in nonpolar sofvents they exist predominantly as ion pairs. After electron transfer the [Cy j [R j pair of radicals diffuses from the initial solvent cage giving free radicals because it is no longer solvent-stabilized. We have shown... 

Chatterjee S, Gottschalk P, Davis PD, Schuster GB (1988) Electron-transfer reactions in cyanine borate ion pairs photopolymerization initiators sensitive to visible light. J Am Chem Soc 110 2326-2328... 

KAB 98] Kabatc J., Pietrzak M., Paczkowski J., Cyanine borates revisited. Application of the Marcus equation for the description of the kinetics of photoinitiated free radical polymerization . Macromolecules, vol. 31, pp. 4651-4654, 1998. 

In contrast to many other initiating systems based on dye sensitization, cyanine-borate complexes are ionic before electron transfer and are transformed into neutral species. Other systems are neutral before electron transfer and get ions thereafter. 

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