Iron(II)–Schiff Base Complexes as Photocatalysts for Controlled Radical Photopolymerization under Light Emitting Diode Irradiation

Photocatalysts, particularly metal-based complexes, have gained significant attention in photoredox catalysis, enabling controlled radical photopolymerization (CRP2) under mild irradiation conditions, such as sunlight or LED bulbs. In this study, three Fe(II) complexes−FeIISalen, FeIISaloex, and FeIISalophen−bearing non-symmetrical Schiff base ligands were synthesized and designed for potential application in CRP2. The complexes were characterized using FTIR, UV-Vis, and fluorescence spectroscopy, MALDI-TOF mass spectrometry, and elemental analysis. Additionally, density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations were conducted to extract key structural, electronic, and excited-state properties. The catalytic performance of these Fe(II) complexes in the CRP2 of methyl acrylate (MA) was evaluated under different conditions, using ethyl α-bromophenylacetate (EBPA) as the initiator and triethylamine (TEA) as the electron donor. The photopolymerizations proceeded efficiently, yielding polymers with controlled molecular weights and narrow molecular weight distributions (MWD). Among the tested photocatalysts, FeIISaloex exhibited the best performance, achieving high conversion and low polydispersity under LED irradiation. Furthermore, a mechanism was proposed based on spectroscopic analyses and literature data.