N-naphthalimide ester derivatives as Type Ⅰ photoinitiators for LED photopolymerization

In this study, a series of N-naphthalimide ester derivatives (noted NPIE1-NPIE9) with different substituents were designed as visible light photoinitiators (PIs). These structures exhibited good light absorption properties in the visible range. Photoinitiation abilities of NPIEs were evaluated upon exposure to a light-emitting diode at 405 nm. All the structures demonstrated good performances in free radical polymerization of acrylate monomers. Markedly, the function conversion for acrylate monomers in the presence of NPIE1 was higher than that of the benchmark PI diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide (TPO). Upon exposure to a light-emitting diode at 455 nm, NPIE1 could also initiate the polymerization of acrylate monomers effectively, i.e., the benchmark photoinitiator (TPO) was not efficient for these long irradiation wavelengths. Through the calculations of the N–O bond dissociation energy and the excited state energy, the cleavage of the N–O bond from the singlet state proved to be favorable. Interestingly, CO2 was detected during the polymerization experiments, which indicated that there was a decarboxylation reaction during irradiation. Based on the obtained results, a mechanism of Type Ⅰ PI behavior can be proposed. Due to the good performance, NPIE1 was successfully used in 3D printing experiments. In addition, high thermal stability of NPIEs in monomers was found. This can be a great improvement for elaborating stable formulations.