Fast photobleachable and low migration one-component green indole visible photoinitiators

Abstract

Six symmetric D-π-A-π-D indole derivatives (IDs) have been investigated as visible light photoinitiators (PIs). The IDs, with different indole as the electron donor (D), C = C bond as the π-bridge and carbonyl group as the electron acceptor (A), are type II PIs owning to their intermolecular hydrogen abstraction. In the process of generating free radicals, the carbonyl transformed to hydroxyl alkyl radicals, which disrupted the large conjugation systems and resulted in the color fading. Real time infrared (RT-IR) indicated that IDs and ID/N-methyldiethanolamine (MDEA) effectively initiated the photopolymerization of 1, 6-hexanediol diacrylate under illumination. ID-HA containing acrylate double bonds is expected to be served as not only a PI but also a crosslinker with extremely high migration stability. The extraction rate of ID-HA from the polymer films following 30 min of illumination was 1.0 %. ID-HA has a lower energy in the excited triplet state (DFT calculations), which is favorable for electron transfer and radical generation. Therefore, the color of ID-HA in acetonitrile solution completely faded within 15 s under illumination, demonstrating the best photobleaching behavior. Meanwhile, the polymerization conversion initiated by it alone could reach as high as 81 %. Furthermore, 47 mm high (with 96.8 % whiteness bleaching ratio) and 55 mm high (with 94.5 % whiteness bleaching ratio) polymer columns were obtained respectively, by using ID-HA and ID-HA/MDEA as PI. Thus, their outstanding photobleaching performance, migration stability and thermal stability endow IDs potential applications in deep photopolymerization, food packaging and biomedicine.