Tailoring Cobalt(II) Schiff Base Photocatalysts for Enhanced LED-Induced Free Radical Polymerization

Co-(II) complexes, despite their potential as cost-effective alternatives to noble-metal systems, remain underexplored as photocatalysts (PCs) in free radical photopolymerization (FRP). In this study, a series of Co-(II) complexes bearing symmetrical Schiff bases (Co-Ph, Co-EtO, Co-Cl, Co-Me, and Co-t Bu) was synthesized and characterized by FTIR, UV-vis, \ fluorescence spectroscopy, MALDI-TOF mass spectrometry, cyclic voltammetry, and advanced density functional theory (DFT/TD-DFT) calculations. Their photocatalytic performance was evaluated in three-component photoinitiating systems with ethyl 4-(dimethylamino)-benzoate (EDB) and diphenyliodonium hexafluorophosphate (Iod) for the FRP of trimethylolpropane ethoxylate triacrylate (TMPETA) under UV, violet, and blue LED irradiation. Co-(II) complexes enabled efficient polymerization under optimized conditions, reaching high conversions without an inhibition period under UV irradiation. Co-EtO demonstrated a superior photocatalytic efficiency across all tested wavelengths relative to that of the other Co-(II) complexes evaluated in this study. This enhanced performance is attributed to a synergistic combination of its unique structural, electronic, and electrochemical properties. The proposed mechanism was supported by photolysis experiments, literature data, and free energy calculations, indicating the involvement of both oxidative and reductive pathways.