Recent advances on core-extended thioxanthones as efficient photoinitiators of polymerization

Photopolymerization is an unavoidable polymerization technique used in plethora applications. If benzophenone or thioxanthone have been popular UV photoinitiators, these two structures are unsuitable for performing polymerization experiments in the visible region due to insufficient absorption. With regards to thioxanthone and in order to redshift its absorption towards the visible range, numerous core-extended thioxanthones have been proposed, enabling to shift their absorptions at longer wavelengths and thus to elaborate visible light photoinitiators. In this field, thioxanthonation of dyes has been extensively studied but the major drawback of this approach was the access to polyaromatic structures exhibiting a low solubility in monomers, adversely affecting the monomer conversion. To address this issue, innovative strategies were envisioned to extend the π-conjugation of thioxanthone while providing an excellent solubility to photoinitiators both in common organic solvents and in monomers. In this review, the different strategies developed to access to these core-extended thioxanthones are detailed and the photopolymerization performance discussed.