Manganese(II)-PyNHC Complex as Visible-Light-Triggered Photocatalyst for Photopolymerization of Acrylates and 3D Printing

Transition metals from the first series have recently attracted attention as potential photocatalysts in photopolymerization reactions and 3D printing applications. However, manganese(II) complexes are among the least explored in photopolymerization reactions. Hence, in this study, an organomanganese complex was developed bearing two pyridine-NHC ligands. The complex [MnII(PyImes)2](PF6)2 (Mn-PyImes), where PyImes = N-mesityl-N '-2-pyridylimidazolium, was isolated upon heating a tetrahydrofuran (THF) solution of MnCl2 with a 2-fold excess of PyImes(PF6) in the presence of potassium tert-butoxide (KOtBu). Mn-PyImes was characterized by FTIR, UV-vis, fluorescence (steady state and time-resolved), and ESR spectroscopies, elemental analysis, cyclic voltammetry, and mass spectrometry. Mn-PyImes was applied in the reactions of controlled radical photopolymerization (CRP2) and free radical photopolymerization (FRP). Mn-PyImes showed slight control in the CRP2 of methyl acrylate (MA) using triethylamine (TEA) and phenacyl bromide (PhBr) as additives under LED@365 and LED@390-405. Mn-PyImes also showed a good catalytic ability in the FRP of ethoxylated trimethylolpropane triacrylate (TMPETA) under LED@405 nm, LED@455 nm, and laser diode@532 nm using di-tert-butyldiphenyl iodonium hexafluorophosphate (Iod) and ethyl dimethylaminobenzoate (EDB) as additives in a photoinitiating system. The CRP2 chemical mechanism was studied by electron spin resonance spin trapping, fluorescence, cyclic voltammetry, laser flash photolysis, and steady state photolysis techniques. Although Mn-PyImes demonstrated good light absorption properties under LED@365 nm, it also enabled effective photopolymerization of acrylates in thick films upon exposure to LED@405 nm, LED@455 nm, and laser diode@532 nm. More interestingly, stereoscopic 3D patterns were successfully fabricated by the laser writing technique.