Recent advances on squaraine-based photoinitiators of polymerization

The development of photoinitiating systems activable in the visible and the near-infrared region is an active research field. Compared to the traditional UV photopolymerization for which the light penetration into the photocurable resin remains limited, a significant enhancement of this latter can be obtained in the visible and the near infrared range so that the scope of applications of photopolymerization can be revolutionized. Among dyes whose absorption can be facilely tuned, squaraines can be cited as relevant examples. Depending on the peripheral groups introduced on both sides of the central four-membered squaric ring, absorptions ranging from 400 to 1000 nm can be obtained. In this review, an overview of the recent development concerning the squaraines-based photoinitiating systems is provided. To evidence the interest of these new photoinitiating systems, comparisons with reference systems will be established.