Recent advances on visible light pyrrole-derived photoinitiators of polymerization

Photopolymerization is the focus of intense research efforts due to the recent need to develop visible light photoinitiating systems activable under low light intensity and in the visible range. This effort is notably supported by the recent development of 3D/4D printing and light cured dental composite resins requiring new polymerization approaches. With aim at developing new formulations combining fast polymerization kinetics, high final monomer conversions, low energy consumption and activation in the visible range, pyrrole has been identified as an interesting scaffold for the design of various dyes. Indeed, numerous derivatives such as porphyrins, diketopyrrolopyrroles, chalcones, push-pull dyes can be prepared with pyrrole and related derivatives. As the main advantages, pyrrole is a five-membered ring heterocycle characterized by an easiness of chemical modification, low oxidation potential, high thermal and chemical stability that make pyrrole an interesting scaffold for the design of photoinitiators. More precisely, pyrrole has been used for the design of numerous Type II photoinitiators. In this review, an overview of the different dyes based on pyrrole and related derivatives, and reported to date is presented. Comparisons with benchmark photoinitiators will be established to evidence the interest of these new structures.