Synthesis of long wavelength absorbing photoinitiators

Nevertheless, syntheses of several diaryliodonium and triarylsulfonium salts that incorporate long wavelength absorbing chromophores have been successfully accomplished and their structures 7-9 are presented below. The replacement of one of the phenyl rings in a diphenyliodonium salt with a fluorenone moiety as shown in structure 7 results in the generation of a diaryliodonium salt with two absorption bands at 294 and 378 nm [FOU 94, HAR 01]. Both the absorption and the fluorescence spectra of 7 closely resemble those of fluorenone. Unfortunately, despite the long wavelength absorption of 7, photopolymerization studies showed that it was not more efficient as a photoinitiator than simple diphenyliodonium salts that do not possess the fluorenone chromophore. 

The synthesis of 5-arylthianthrenium salts, 8, [CRI 02a, CRI 04] can be carried out by the condensation of thianthrene-S-oxide with various aromatic compounds bearing electron-donating substituents, R, as depicted in equation [2.1], A mixture of methanesulfonic acid and phosphorous pentoxide serves as an acid catalyst and dehydrating agent for this reaction. Replacement of the methanesulfonate anion by a metathetical reaction with NaMtXn affords the desired active photoinitiator, 8. Depending on the specific R substituent, 5-arylthianthrenium photoinitiators have UV absorption maxima in the range of 300-400 nm. 

The synthesis of a sulfonium salt bearing a photosensitizing anthracene moiety in the same molecule was described by Pappas et al. [PAP 03]. Details of the synthesis of 9 are presented in equation [2.2]. The objective of this work was to improve the efficiency of electron-transfer photosensitization by an intramolecular interaction by incorporating the anthracene within the same sulfonium salt molecule. Indeed, 9 bearing the tetrafluoroborate anion was observed to show excellent activity in the 300 nm region due to the presence of the strongly absorbing anthracene chromophor. 

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