Recent progress in the development of highly active dyeing photoinitiators based on 1,3-bis(p-substituted phenylamino)squaraines for radical polymerization of acrylates

Photopolymerization is a very popular technique used in the production of various polymeric materials. The key role in light-induced polymerization processes is played by a photoinitiator. One of the most common and desirable activators of photopolymerization is a dye-based photoinitiating system. Most often, these systems are composed of photosensitizer and other additives, such as co-initiators. The dye (photosensitizer) absorbs the irradiation selectively and interacts with other molecules to form reactive species that initiate a chain reaction. For this reason, design and development of novel photosensitizers, and hence photoinitiators, is crucial to improving the efficiency of photochemically initiated polymerization reactions. In recent years, an interesting group of light absorbers, i.e. squaraines, has been proposed as highly efficient photosensitizers. These dyes show excellent spectral properties, such as high molar extinction coefficients and a wide absorption range, which is one of the most important requirements of an effective photosensitizer. Therefore, it is worth paying attention to the high potential of squaraines for photoinitiation of polymerization reactions. In this review we summarized our recent reports on photoinitiating systems containing squarylium dyes for radical polymerization of acrylate monomers. We focused on the radical photoinitiators consisting of 1,3-bis(p-substituted phenylamino)squaraines in the role of sensitizers. The article highlights the development and progress of squaraine-based photoinitiators.