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. Herein, three Fe(II) complexes—Fe^IISalen, Fe^IISaloex, and Fe^IISalophen—bearing nonsymmetrical Schiff base ligands are synthesized and designed for potential application in CRP2. The complexes were characterized using fourier transform infrared spectroscopy (FTIR), UV-Vis, and fluorescence spectroscopy; matrix-assisted laser desortion ionization time-of-flight (MALDI-TOF) mass spectrometry; and elemental analysis. Additionally, density functional theory and time-dependent density functional theory calculations are 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) is evaluated under different conditions, using ethyl α-bromophenylacetate as the initiator and triethylamine as the electron donor. The photopolymerizations proceed efficiently, yielding polymers with controlled molecular weights and narrow molecular weight distributions. Among the tested photocatalysts, Fe^IISaloex exhibits the best performance, achieving high conversion and low polydispersity under LED irradiation. Furthermore, a mechanism is proposed based on spectroscopic analyses and literature data.