Improvement in the storage stability of free radical photocurable materials under sunlight based on the cis → trans photoisomerization of pyrrole chalcone photoinitiator

A novel pyrrole chalcone photoinitiator, (E)-3-((1H-pyrrol-2-yl)methylene)chroman-4-one (E-PCO), was synthesized and characterized. The photoisomerization progress of E-PCO converting into a mixture of E-PCO/Z-PCO was examined by analysing the time-dependent evolution using 1H NMR spectroscopy and UV–vis absorption spectroscopy. The results demonstrated that E-PCO efficiently initiated LED free radial photopolymerization with the additive TEOA, while the photoequilibrium of E-PCO/Z-PCO ineffectively initiated photopolymerization, endowing photocurable materials containing E-PCO/Z-PCO with high storage stability to sunlight. Moreover, Z-PCO of the photoequilibrium was proven to isomerize back to E-PCO fully upon LED@465 nm irradiation so that photocurable materials containing E-PCO/Z-PCO could restore its photoreactivity for polymerization after illumination. Therefore, an approach for improving photocurable materials' storage stability to sunlight by controlling the initiating system's photoreactivity was proposed in this research, which has notable potential for outdoor photopolymerization technology.