Iodonium borates

The absorption spectra of the iodonium borates depend on the solvent [53, 54]. In acetonitrile, the absorption spectra are equal to the additive spectra obtained from the components (up to 300 nm) [55, 56]. In less polar solvents onium borates exhibit a weak, extended absorption tail in the 320-450 nm region that is attributed to an intra-ion-pair ground-state charge transfer transition from the borate anion to the iodonium cation. Photopolymerization using the iodonium borates can be effectively initiated only by UV irradiation and by violet light of the visible region. 

Stable iodonium borate complexes show minor, but real, absorptions that differ from the sum of absorptions of compounds containing the individual ions when the... 

Feng isolated photoproducts from the reactions of several of the isolated iodo-nium borate salts when irradiated in the presence of simple acrylates. The conclusion was that aryl radicals in iodonium borates, regardless of whether or not there are aryl groups in the borate, owe their formation to the iodonium salt. To prove this point the classical physical organic method, study of a mixed system, was used. This work is important in that it confirms that products such as biphenyl find their origin only in the borate (Scheme 5). These products do not arise from free phenyl radicals such as might be formed from such borates. 

Scheme 5. Photoreaction of iodonium borate in the presence of methyl acrylate. There are no products formed via a phenyl radical (0) path. The biphenyl that forms occurs via an intramolecular reaction and originates in the borate.

The iodonium borate provides a very rapid cure compared with the corresponding antimony product. The difference in reactivity is even greater for the sulfonium salt system (Fig. 6). 

Dye-borate systems were disclosed to initiate radical polymerization [P C 08, PAC 01, KAB 98] but the sensitivity significantly increases by addition of iodonium borates to a dye-borate system [SIM 09]. This combination avoids ion exchange in systems comprising a cationic polymethine dye and an iodonium ion because both comprise the same counter ion. Ion exchange can lead to undesired events (i.e. crystallization) affecting sensitivity of the lithographic material. Further studies on dye-borate systems discuss the efficiency of electron transfer from the borate to the acceptor excited [SCH 90]. Moreover, oxidation of the (anilino)acetic acid results in fast generation of the aminyl radical and the release of CO2 as shown in equation [7.5]. 

Aryl and heteroaryl (furyl, thienyl) boronic acids are especially suitable for the preparation of their iodonium salts, having the added advantage of better yields and lack of toxicity [108]. Tetraarylborates (sodium or potassium) reacted with (diacetoxyiodo)arenes in acetic acid to afford diaryliodonium salts in excellent yield (Scheme 37). It appears that triarylboranes formed upon reaction of the borates with acetic acid serve actually as the real arylating agents [109]. 

All of these species (XIV, XV) have been for the most part applied towards function in the olefin polymerization arena use of these novel anions for the stabilization of other electrophilic species remains to be explored. Recently, the imidazolide anion XVI, as well as the perfluorinated tetraaryl borate derived from the diborane IX of Chart 2, have been used to stabilize iodonium cations.222 These cations are used as photoinitiators for cationic polymerization of epoxy resins in photolithography applications. While use of the [B(C6F5)4] led to a breakthrough in this area of research,223 higher activities are observed for more WCAs. 

The simultaneous addition of iodine and fluorine to cyclohexene leads to frrw.v-l-fluoro-2-iodocyclohe.xanc. The use of different reagents has been described, for example silver(I) fluoride/ iodine184 1 17 224-278 (yield 52-64%), hydrogen fluoride/A -iodosuecinimide217-220 221-274 284 (yield 52-75%), fluorine/iodine223 227 (yield 64%) and bis(pyridine)iodonium tetrafluoro-borate/tetrafluoroboric acid219 (yield 89%). Some further examples of the simultaneous addition of iodine and fluorine to cyclic alkenes are listed in Table 25. 

Using only 500 ppm of sulfonium borate, curing is faster than for the iodonium salt. Under the same conditions the sulfonium hexafluoroantimonate produces no curing. The commercially available photoinitiator UVI 6974 (SbFe anion), even when used at a concentration of 1.5 %, does not allow curing at such thicknesses. 

Ditolyliodonium chloride 3 and ditolyliodonium hexafluoroantimonate 2 have comparable UV spectra, characterized by a molar extinction coefficient a = 40000 at, 1 = 205 nm and s = 18000 at A = 245 nm. The borate iodonium has a different spectrum due to the nature of the borate anion. Between 200 nm and 240 nm the molar extinction coefficient is twice that of the photoinitiator with chloride or hexafluoroantimonate anions. Between 240 and 290 nm, there is little difference between the spectra. The same phenomena have been observed with other iodonium cations. 

As with the iodonium salt, the differences between the two photoinitiators lie between 200 and 240 nm. The absorption at around 300 nm is the same when the antimonate anion is replaced by the borate anion. 

Another chemical filter B was used in order to mask the UV spectra between 200 and 240 nm as previously done with the iodonium salt. This filter has a molar extinction coefficient very close to that of the borate sulfonium salt. Several stoichiometric mixtures have been prepared, as shown in Fig. 15. 

Fig. 7. Polymerization rate and percentage conversion vs time profiles for photopol5nner-ization of phenyl glycidyl ether initiated using 8.86 mM of iodonium tetris pentafluo-rophenyl borate salt at 50°C and light intensity of 25 mW-cm (see reference 150).

Examples of additives are the following EA = iodonium salt, e.g. diphenyliodonium hexafluorophosphate (more rarely, a sulfonium salt) and related derivatives [113], alkyl halide, e.g. phenacyl bromide [1.14] and triazine, e.g. 2,4,6-tris(trichloromethyl)-l,3,5-triazine ED = borate disulfide, group IVb dimetal [1.15] HD = alcohol, THE, thiol, benzoxazine, aldehyde, acetal, silane (e.g. tris(trime1hylsilyl)silane = (TMS)3Si-H) [1.16]), germane, borane, stannane, alkoxyamine, silyloxyamine, polymer substrate, etc. EPD = amine [1.17], thiol, etc. The generated radicals (e.g. Ph, R, RsSi, RR C R R in [1.13]-[1.17] ) are the initiating species. Efficient novel or newly modified dye structures in the Dye/amine, Dye/iodonium salt or Dye/silane two-component PISs have been proposed within the last 4 years (see section 1.3.5). 

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... 

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). 

Borates (VII) function well as donors in systems needing the same counter ion for both the eationic sensitizer and iodonium salt [SIM 09]. This avoids ion exchange between Sens and IV because both components bear the same counter ion. Initiator systems with different counter ions may... 

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