Recent advances on push-pull organic dyes as visible light photoinitiators of polymerization

Photoinitiators activable under visible light and low light intensity are actively researched as it can advantageously address the safety concerns raised by the traditional UV-light photopolymerization process as well as the light penetration issue. Among dyes strongly absorbing in the visible range, one of the most promising family of photoinitiators are push-pull dyes based on an electron-donating and an electron-accepting moiety connected at both ends of a polyenyl-type spacer. By acting on both the strength of the electron donor and the electron acceptor, position of the intramolecular charge transfer (ICT) band can be easily tuned from the near-UV to the near infrared region. In this review, an overview of the different D-pi-A push-pull dyes used as visible photoinitiators of polymerization is presented. Over the years, a clear evolution in the chemical structure of the electron acceptors can be clearly observed, evidencing the strong activity existing around these structures.