Recent advances on photochroms used as visible light photoinitiators of polymerization

Photopolymerization is now regarded as a more ecological approach than the traditional polymerization techniques for elaborating polymers. Besides, the design of highly reactive systems remains a challenge, especially due to the lower energy of visible light compared to UV light. Among chromophores that can be used for light harvesting and thus to act as photoinitiators and/or photosensitizers for onium salts, photochroms constitute a unique class of push-pull dyes by their abilities to change of color and to modify their UV-visible absorption spectra by mean of an external stimulus, namely light. Thus, in order photochroms to be used as photoinitiators/ photosensitizers, an isomerization step was most of the time required prior to the polymerization process in order to generate "in situ" the colored form of photochroms, what is unusual in photopolymerization. Considering that the photoinduced isomerization of photochroms and photopolymerization are two concurrent processes during irradiation, photochroms were thus carefully selected in order the isomerization process to be slower than the photopolymerization process, enabling to maintain the colored form during the whole polymerization experiments. By chemical engineering, photochroms could be modified, converting these photoswitches as performant photoinitiators and the covalent linkage to well-known photoinitiators such as benzophenone or polyoxometalates were achieved. In this review, the different families of photochroms reported to date as photoinitiators of polymerization and activable under low light intensity is provided.