Efficient photopolymerization activated by nonharmful near-infrared (NIR) light is important for various biological applications. Here we propose a NIR light free-radical photoinitiator (PI) fabricated by incorporating oxime-ester coumarin functionality on the surface of upconversion nanoparticles (UCNPs). The coumarin groups of PI absorb the light emitted from the UCNP core, whereas the oxime ester groups undergo cleavage to form radicals. Upon irradiation at 980 nm, the mobile radicals, formed in a manner similar to that of dandelion seed release, initiate both free-radical and thiol-ene photopolymerizations. The superior efficiency of dandelion-like PIs assisted photopolymerizations can be attributed to the reduction of energy loss and increased local PI concentration due to Förster resonance energy transfer process and confinement effect, respectively. Moreover, the proposed PI system can initiate polymerization under low-power NIR laser and reduces the thermal side effects. The possibility of its potential use in deep curing applications was also demonstrated.

Thiol–ene photopolymerization has been widely applied in polymer synthesis and the fabrication of optical devices via 3D printing due to its unique features such as low shrinkage stress and yielding homogeneous networks. However, the poor storage stability and unpleasant odor of conventional primary thiols have limited their application. Detailed evaluation of the photocuring properties of secondary thiols with low odor and good shelf-life stability is lacking. In this study, we systematically discussed the thermal storage stability, photopolymerization kinetics, and mechanical, thermal, and physical properties of three types of multifunctional secondary thiols and one type of primary thiol. The results show that all secondary thiol–ene systems exhibit better thermal storage stability (>7 days) at 60 °C than the primary thiol–ene system (1.54) show great potential for the fabrication of 3D optical devices with high resolution.

Development of efficient unimolecular visible light photoinitiators (PIs) with photobleaching capabilities, which is essential for various biomedical applications and photopolymerization of thick materials, remains an emerging challenge. Based on the photochemistry of heteroaromatic compounds, unimolecular visible light oxime ester PIs (2-O, 2-S, 3-O, and 3-S), which contain coumarin derivatives as visible light chromophores, were designed and synthesized. Photophysical and photochemical properties of these PIs have been investigated using UV-Vis spectroscopy, fluorescence spectroscopy and electron spin resonance (ESR). The oxime-esters photolyse rapidly under 450 nm LED light irradiation, especially 2-S, and have good photobleaching capabilities. Also, we have, for the first time, shown that heterocyclic radicals efficiently initiate free radical polymerization of acrylate monomers. Notably, these photobleaching initiators can be used to induce thiol-based click photopolymerization to prepare materials with a thickness of up to 10 mm.