Emerging Photo-Initiating Systems in Coatings from Molecular Engineering Perspectives

Photoinitiators (PIs) are pivotal in enabling energy-efficient, spatiotemporally controlled photopolymerization for coatings. To address application-specific demands of coatings, diverse systems of Norrish-Type I (e.g., oxime esters, acylphosphine oxides) and Type II (e.g., onium salts, ketones) PIs have been engineered through systematic molecular design strategies. A comprehensive review necessitates highlighting recent achievements in designing PIs by various molecular engineering approaches. The π-conjugation extension, push–pull structures, and auxochrome incorporation boost strong and long-wavelength absorption; unimolecular PI systems with hydrogen-donor modifications improve reactivity and reduce oxygen inhibition; photobleaching via cleavable bonds and blocking conjugation enables colorless coating and deep-penetration curing; polymerizable macromolecular designs enhance migration resistance; organosilicon-functionalized structures optimize monomer compatibility. These strategies bridge molecular innovations with advanced applications in biomedical and deep-cured coatings.