Novel multifunctional unimolecular initiators built on natural indole featuring fast photobleaching and visible light/thermal double polymerization

Abstract

Four asymmetric D–π-A–π-A′ visible light initiators (IDs) were synthesized with indole as the electron donor (D), and the carbonyl group and oxime ester as the first and second electron acceptors (A and A′). IDs feature type I and type II photoinitiators (PIs) owing to their light induced N–O bond homolysis and intermolecular hydrogen abstraction. In the process of generating free radicals, the carbonyl transformed to hydroxyl alkyl radicals, which disrupted the conjugation systems and resulted in color fading. Real time infrared (RT-IR) showed that IDs effectively initiated the photopolymerization of 1, 6-hexanediol diacrylate (HDDA) under various light sources (395 nm, 405 nm, and 455 nm). According to DFT calculations, the co-planarity of ID₃s in the excited triplet state was superior to that of ID₂s, which was favorable to the electron transfer and radical generation, and then brought about better initiation and bleaching performance. RT-IR displayed that the HDDA conversion initiated by ID₃-2 could reach as high as 84%. Under irradiation, ID₃-2 demonstrated the best photobleaching behavior evidenced by the fact that its acetonitrile solution faded completely within 30 s. Furthermore, a colorless polymer column as high as 50 mm was obtained by ID₃-2 initiation. Interestingly, ID₃-2 also presented thermal initiation behavior in HDDA. Therefore, the highly efficient photobleaching and polymerization properties make IDs have tremendous potential for applications in deep photopolymerization. Simultaneously, the excellent thermal stability of IDs provides a promising prospect for curing applications.