Photoactivated antibacterial activity of Fe(III) complexes via singlet oxygen-mediated bacterial membrane lysis

In this work, four novel Fe(III) complexes viz., [Fe(dpa)(L1)]Cl (Fe1), [Fe(Fc-dpa)(L1)]Cl (Fe2), [Fe(dpa)(L2)]Cl (Fe3), and [Fe(Fc-dpa)(L2)]Cl (Fe4), where dpa = bis(2-pyridylmethyl)amine; Fc-dpa = N-(1-ferrocenyl)-methyl-1-(pyridine-2-yl)-N-(pyridine-2-ylmethyl)methanamine; H₂L₁ = (E)-2(2-hydroxybenzylidene)amino)phenol and H₂L₂ = (E)-1-(2-hydroxyphenyl)imino)methyl)naphthalen-2-ol, have been synthesized and characterized. The light-responsive nature of these complexes was tailored by extending π conjugation or attaching a ferrocene moiety in the ligands. These complexes displayed light absorption within the visible region with λ_max ∼ 450 nm. X-ray structures of Fe3 revealed a distorted octahedral structure around the Fe(III) center with a FeN₄O₂ core. Further, the frontier molecular orbitals (FMOs) and adiabatic energy splitting between excited states were determined by DFT calculations. Fe1-Fe4 did not demonstrate any notable antibacterial action without light. Nevertheless, the antibacterial effect of Fe1-Fe4 was significantly increased due to their ¹O₂ generation ability under light conditions. Interestingly, the MIC value of Fe1-Fe4 was observed to be ca. 0.2 μg mL^-1 against S. aureus and 0.5 μg mL^-1 against E. coli. under light exposure. The light-induced antibacterial activities of these complexes were attributed to bacterial cell membrane damage via reactive oxygen species (ROS) generation. Overall, this study presents the first report of Fe(III) complexes showing antibacterial activity under visible light conditions.