Cyanine borate salts

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. 

In the initial description of the cationic dye-borate system [24, 76], it was postulated that electron transfer was possible because, in nonpolar solvents, dye/borate salts exist predominantly as ion pairs. Since the lifetime of the cyanine singlet excited state is quite short [24, 25], this prerequisite is crucial for eflfective photo-induced electron transfer. Recently initiator systems in which neutral dyes are paired with triarylalkylborate anions have appeared in the literature [77]. In the latter case, the borate ion acts as the electron donor while neutral merocyanine, coumarin, xanthene, and thioxanthene dyes act as the electron acceptors. It is obvious that these initiating systems are not organized for effective electron transfer processes. The formation of an encounter complex (EC) between excited dye and electron donor is required. 

A novel intra-ion-pair electron(i) transfer cleavage of the —C-B— bond is reported by Chatterjee et al. [98] from the photoreaction of triphenyl alkyl borate salts of cyanine dye (110) to generate a carbon-centered radical (113) whose utility for initiating a free radical polymerization reaction has been demonstrated (Scheme 26). An intramolecular concerted bond cleavage/coupling process is described [99] from phenyl anthracene sulphoniiun salt derivatives as a part of designing a novel Photoacid system. 

Dye = benzophenone, isopropyl thioxanthone, merocyanine, etc. Dye/amine/maleimide widi Dye = benzophenone, etc. Dye/amine/diphenyl iodonium salt with Dye = eosin, ketocoumarin, methylene blue, thioxanthene dye, fluorone dye, etc. Dye orate salt/triazine derivative with Dye = cyanine dye, styiylquinolinium dye, etc. Dye/borate salt/alkoxypyridinium salt with Dye = carbocyanine dye, etc. Pl/amine/triazine derivative with Dye = pyrromethene dye. Rose Bengal, etc. Dye/hexabisimidazole derivative (e.g. Cl-HABI)/thiol with PI = styiylnaphtothiazole dye, diethylamino benzophenone, julolidine dye, etc. Dye/amine (or... 

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... 

Another type of ion pairs, called penetrated ion pairs [402], has been found by studying the conductivity of tetraalkylammonium tetrafluoroborates (with variable alkyl-chain lengths) [399, 403], and the UV/Vis spectroscopic behaviour of salts with a trimethinium cyanine cation and the tetrakis(phenylethynyl)borate anion [404], in nondissociating solvents of low relative permittivity. Clearly, in solutions of such low relative permittivity any ionic species will be highly associated. However, it has been found that the ion pairs formed can be smaller than the sum of the van der Waals radii of the components. Clearly, the ions of the ion pair interpenetrate each other depending on their molecular structure in the first case, the BF4 ion penetrates into the voids between the alkyl chains of the tetraalkylammonium ion, and in the second case the cyanine cation penetrates into the crevices of the borate ion. 

Examples of well-known PISs working under laser lights in this blue-green area include the Eosin/amine, Eosin/amine/bromo compound, ketocoumarin/amine/iodonium salt, thioxanthene dye/amine/iodonium salt, erythrosin/amine, julolidine dye/Cl-HABEthiol, cyanines and related compound/borate/additive systems, pyrromethine/amine/triazine or styryl naphtothiazole dye/Cl-HABEthiol systems and Cl-HABI-based systems. A lot of novel Dye/iodonium salts/silanes can also work (e.g. [XIA 15] see sections 1.3.5 and 1.3.6). 

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