A–π–D–π–D type 2,5-bis(4-nitrophenyl)−1,4-dihydropyrrolo[3,2-b]pyrrole derivatives as radical photoinitiators for LED photopolymerization

Abstract

By using strong electron donors (D) and acceptors (A), intramolecular charge transfer (ICT) bands of dyes could bring new visions for structure-property relationship in photopolymerization systems. Three di(4-nitrophenyl)− 1,4-dihydropyrrolo[3,2-b]pyrrole (named as Py-NO2-H, Py-NO2-CH3 and Py-NO2-OCH3) compounds were synthesized and used as radical photoinitiators. From theoretical calculations, the substitutes of 1,4-phenyl on their A-π-D-π-A structures obviously influence the electron distribution of the pyrrolo[3,2-b]pyrrole core, and transition-state geometry further explains the different sensitivity on 365, 405 and 470-nm LED in the photoinitiation process. The irradiation wavelength can be chosen according to hυ 412 nm (the transition-state energy). Additionally, the initiators in trimethylolpropane triacrylate (TMPTA) under 470-nm light shows complete photobleaching. A dual-stage curing of Py-NO2-H/TMPTA favors rapid photopolymerization rather than directly irradiated under 405 nm, increasing double bond conversions four times. This work provides experimental basis for the structure-property relationships field of strong electron donating fused pyrrole dyes.