Visible light-promoted nickel-NHC photocatalysts for free radical photopolymerization and 3D printing application

Abstract

Two Ni( ii )-NHC complexes were investigated as novel photocatalysts for the free radical photopolymerization of TMPETA under LED@405 nm. They showed notable photoinitiation capabilities, with the optimal conditions successfully applied in 3D printing. The evaluation of Ni II complexes bearing bidentate N-heterocyclic carbene (NHC) ligands has shown their great potential as catalysts in various reactions. However, their luminescence properties and photocatalytic applications remain underexplored. Two novel nickel-based NHC complexes, [Ni(py-NHC) 2 ](PF 6 ) 2 (NiC1) and [Ni(ph-NHC) 2 ] (NiC2), were synthesized with ligands N -mesityl- N ′-2-pyridylimidazolium tetrafluorophosphate and 1-mesityl-3-phenyl-1 H -imidazol-3-ium tetrafluorophosphate, respectively. The complexes were thoroughly characterized using spectroscopy techniques, including FTIR, UV-Vis, 1 H NMR, and fluorescence, along with elemental analysis, cyclic voltammetry, and mass spectrometry. NiC1 and NiC2 exhibited emission properties and lifetimes of 3.8 and 9.1 ns, respectively. The photocatalytic properties of NiC1 and NiC2 were investigated for initiating free radical photopolymerization of ethoxylated trimethylolpropane triacrylate under violet light. This process incorporated di- tert -butyl-diphenyl iodonium hexafluorophosphate and ethyl dimethylaminobenzoate as additives. NiC2 achieved the highest monomer conversion under air or in laminate and was effective in on–off LED irradiation. Proposed oxidative and reductive mechanisms were supported by photolysis, fluorescence quenching, and electron spin resonance data. The optimized system was applied in 3D printing, producing smooth-surfaced, high-resolution patterns.