Thioxanthone photosensitizers

The use of isopropylthioxanthone as a PS for two-photon sensitized cationic polymerizations was described by Boiko et al. [BOI 01], The Yagci research group [TAS 08] has also been active in this area and has published the results of their study of the use of benzophenone and benzodixinone as two-photon sensitizers for the diaryliodonium salt polymerization of cyclohexene oxide. These polymerizations are of growing interest since they can be used to spatially define three-dimensional images for the production of solid polymeric objects. 

On irradiation in the presence of an epoxy monomer and a diaryliodonium salt photoinitiator, it has been observed that the initial bright yellow color of curcumin shifts to deep orange. On continued irradiation or simply standing in ambient light, the photopol mer is bleached resulting ultimately in a pale-yellow-colored cross-linked epoxy polymer. 

---

Type 2 initiators are bimolecular systems and consist of a photosensitizer and a hydrogen donor. The most well known system is benzophenone/tertiary amine. Because of the relatively weak absorption of benzophenone at 360 nm the efficiency of this system is rather low. A more efficient aromatic ketone is thioxanthone (TX) and its derivatives simply because of the increased at the exposure wave length. 

A further interesting feature of the polymeric photoinitiators is based on the possibility of anchoring different photosensitive moieties to the same macromolecule in order to provide a synergistic effect on activity due to their interaction along the polymer chain. In this context, the synthesis of several copoly-meric photoinitiators bearing side-chain thioxanthone and hydroxyalkylphenone or morpholino ketone moieties has been reported recently [156]. In particular, the free radical copolymerization of 2-[2-(acryloyloxy)ethylthio] thioxanthone (AETX) with either [4-(2-acryloyloxyethoxy)phenyl]-2-hydroxy-2-propyl ketone (HPA) or [4-(2-acryloyloxyethylthio)phenyl]-2-(iV-morpholino)-2-propyl ketone (APMK) [poly(AETX-co-HPA) and poly(AETX-co-APMK), respectively] and of 4-[2-(methacryloyloxy)ethoxycarbonyl] thioxanthone (METX) with APMK [poly(METX-co-APMK)] has been performed. 

Several types of photosensitizers have been tested in the silicone epoxy polymer in association with the iodonium salt described above. Benzophenone was found to give poor performance (Fig. 7) whereas 2-ethyldimethoxyanthracene (Fig. 8), chlorothioxanthone (Fig. 9) and isopropyl-thioxanthone (Fig. 10) proved to be highly efficient. One of the major problems with epoxy-silicones is the poor solubility of these photosensibilizors. This problem is currently being studied. 

Photosensitizer-Linked Photoinitiator or Coinitiator-Based Systems Attempts have been made to incorporate the dye and the energy or the electron donor in the same molecule. An old example was apolymeric system where a morpholinoketone and a thioxanthone derivative were grafted [215]. A first recent example corresponds to a dye linked to a triazine derivative (10.61) where an intramolecular electron transfer yields a radical anion on the triazine moiety that results in the production of a radical [216,217],... 

J.D. Cho and J.W. Hong, Curing kinetics of UV-initiated cationic photopolymerization of divinyl ether photosensitized by thioxanthone. J. Appl. Polym. Sci. 2005, 97(3), 1345-1351. 

Very recently, it was shown that mixing substituted thioxanthones A or 3 with the morpholine ketone 3 extends the photosensitivity towards the near visible part of the spectrum and accelerates the curing of pigmented coatings [24],... 

Table 2 Role of the addition of a photosensitizer of the thioxanthone series. Reactivity obtained for the curing of a white pigmented lacquer (in m/mn). Rate of polymerization (in a.u.) in solution MMA in toluene (a) and acrylamide in water (b) at A > 400 nm [29]...

As expected, photosensitization of the decomposition of 22 in the presence of 2-methyl thioxanthone was recently demonstrated [32]. However, in phenacyl phenyl sulfide 21i the triplet energy level remains high (73,5 Kcal M [33]) ... 

Liska also investigated several benzophenone- and thioxanthone-based photosensitizers that were eovalently bonded to hydroxyalkylphenone and amino-aUcylphenone based photoinitiators. This was done to enhanee the rate of the exeitation-transfer effeet due to the elose vieinity of the photosensitizer to the photoinitiator. Seleetive exeitation of the photosensitizer ehromophore revealed that the energy transfer signifieantly inereases in eovalently bonded initiators by eomparison to their physieal mixtures. This effeet, however, is most pronouneed in hydroxyalkylphenones that are sensitized by suitable benzophenone derivatives, espeeially at low photoinitiator eontent. 

Various derivatives of thioxanthone are often used as photosensitizers with substituted morpholino ketones as energy aeeeptors. Aetually, the two materials by themselves are effieient initiators. It was shown that when eombinations of thioxanthone derivatives with morpholino ketones are irradiated with light, two types of reaetions ean take plaee. If the triplet-triplet energy transfer oeeurs when the energy level of the thioxanthone is higher than that of the morpholine ketone, eleavage of the morpholino ketone is the result ... 

Another approach, described in a Japanese patent, is to attach a photosensitizer, a thioxanthone molecule to the sulfonium salt ... 

In the electron transfer mechanism, which is more common, the photosensitizer becomes excited upon illumination and forms an excimer with the photoinitiator. Electron transfer from the photoinitiator to the photosensitizer then occurs, with a subsequent proton transfer. This results in the formation of two radicals. The photosensitizer radical is capable of initiating polymerization, while the photoinitiator radical may not unless it undergoes further reaction (3,13,17). Figure 3 demonstrates this for a representative a-amino ketone photoinitiator and a thioxanthone being used as a photosensitizer. 

Scheme 8.22 A Kemp acid derivative functions both as photosensitizer (through the thioxanthone moiety) and as complexing agent...

CHO 05] Cho J.-D., Hong J.-W., Photocuring kinectics of UV-initiated cationic polymerization of a cycloaliphatic diepoxide system photosensitized by thioxanthone , European Polymer Journal, vol. 41, no. 2, pp. 367-374, 2005. 

Therefore the second method of increasing photopolymerization is to use a photosensitizer, which absorbs strongly across the spectrum and transfers energy efficiently to the photoinitiator by means of spin exchange [20]. Thioxanthones are very efficient photosensitizers (Fig. 2.15). 

A wide assortment of electron transfer photosensitizers for diaryiiodonium salts have been described in the journal and patent literature. Among these are polynuclear aromatic hydrocarbons with three or more rings such as anthracene, alkoxyanthracenes, pyrene, and perylene heterocyclic compounds of low basicity such as carbazoles " and phenothiazines aromatic ketones such as benzophenone, Michler s ketone, and thioxanthone and substituted thiox-anthones coumarins phenanthrene-9,10-quinone Mannich bases and (dimethylamino)benzylidyne compounds. In addition, the use of dyes such as eosine, acridine orange, acridine red, and benzoflavin has been employed to provide photosensitization in the visible region of the spectrum. 

The reduction potentials for S,S-dialkyl-S-(hydroxyphenyl) sulfonium salts do not appear to have been measured. These sulfonium salts do not undergo electron transfer photosensitization. Rather, photosensitization has been achieved using typical diaryl ketones such as benzophenone, Michler s ketone, and thioxanthone as photosensitizers. Strongly acidic solutions are produced during photosensitized photolysis, suggesting the formation of a protonic add as one of the products. 

---