Push-pull coumarin-based one-component iodonium photoinitiators for cationic nanocomposite 3D-VAT printing

In this article, we described a new group of cationic photoinitiators. The influence of the arrangement of electron-donating substituents in the coumarin chromophore on their photophysical properties and photoinitiating activity is analysed. Five new coumarin-based iodonium salts exhibit two patterns of D-pi-A structure - longer and shorter. They differ in the strength of the push-pull effect, which determines all properties of such compounds. Due to the intense ICT absorption band, new photoinitiators are active at 365 nm, 405 nm, and 415 nm. They exhibit excellent photoinitiating activity toward monomers such as vinyl ethers, epoxides, oxetanes, and glycidyl ethers at room temperature. However, at elevated temperatures, their activity increases dramatically. This effect is caused by the ability of coumarin salts to thermally react with monomers at elevated temperatures. Thus, the new compound exhibits photo-thermal initiating activity, which helps overcome scattering and shadowing problems during composite curing. Due to this phenomenon, coumarin-based iodonium salts proved to be excellent photoinitiators for nanocomposite DLP 3D-VAT printing.