Novel Norrish type I flavonoid photoinitiator for safe LED light with high activity and low toxicity by inhibiting the ESIPT process

Abstract In this study, flavonoid with N,N-diphenylamino group, 2-(4-(diphenylamino)phenyl)-3-hydroxy-4H-chromen-4-one (3HF-OH) was designed and synthesized as low-toxicity photoinitiator for photopolymerization under safe visible LED sources, however, low photoinitiation activity of 3HF-OH was found due to the excited state proton transfer (ESIPT) process. Hence, by replacing the 3-position hydroxyl group of 3HF-OH with benzoyl chloride and benzenesulfonyl chloride, flavonol carboxylate (3HF-C) and flavonol sulfonate (3HF-S) were synthesized, respectively. 3HF-C and 3HF-S showed not only one-component photoinitiating activity but also two-component photoinitiating activity when they were used with triethanolamine (TEOA) and iodonium salt (ONI) under safe, long-wavelength LED soft light sources. Photopolymerization kinetics reveals that photoinitiator 3HFs at low concentration exhibits the high polymerization rate and conversion. Results also showed that 3HF-S has much higher photoinitiation activity during photopolymerization compared with 3HF-OH and 3HF-C. The high extinction coefficient and aggregation-induced emission (AIE) characteristics of 3HF-S were observed, which proved that reducing the photoinitiator usage can effectively progress the photopolymerization activity. At the same time, the low usage of 3HF-S further reduces the toxicity of the cured material. Through electron spin resonance (ESR) experiment, the photoinitiation and sensitization mechanisms of these photoinitiators were observed. The school badge structure of hydrogels could be successfully 3D printed with 3HF-S and the stiffness of the structure was sufficient for further processing without breaking the structure. Moreover, the use of the 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyl tetrazolium bromide (MTT) method confirmed the lower toxicity for the final photopolymer provided.