Efficient red light polymerization facilitated by low energy excitation based on a single-atom substituted naphthalimide

Abstract

Photoinduced polymerization with low-energy, longer wavelength light has attracted enormous attention in material/biological applications owing to its unique advantages of greener and deep curing compared to conventional harsh UV-triggered polymerization. However, the low-energy polymerization methods often rely on heavy atom-containing sensitizers or suffer from the slow solidification process, which restricts their applicability in several areas. Herein we construct a novel efficient photoinitiating system based on a single-atom substituted pure organic triplet photosensitizer, prepared from a common naphthalimide chromophore through a simple one-pot process, and exemplified its potential to enable the efficient and rapid polymerization (just a few tens of seconds) of acrylate upon low-intensity (0.36 W/cm2) red-light illumination. We show that the single atom replacement (oxygen with sulfur) can redshift the absorption profile of naphthalimide chromophore over 150 nm and also effectively promotes its intersystem crossing efficiency (from 0 to 86.4%). For the first time, the thionaphthalimide (NIS) was introduced in triplet-triplet annihilation upconversion, showing a high upconversion quantum yield of (4.4–12.9%) with a large anti-Stokes shift (0.65–0.80 eV) in various solvents. We successfully employed the upconverted annihilator's singlet to trigger the photoinitiator via Förster resonance energy transfer (FRET) for polymerization. Furthermore, this photoinitiating system is found to be effective enough to be operable under sunlight illumination and useful for the fabrication of high-quality flexible products via photolithography approach. Thus, our results are beneficial for the future design of eco-friendly, low-energy activated chemical conversion systems.