Photoactivated antibacterial activity of Fe(<scp>iii</scp>) complexes <i>via</i> singlet oxygen-mediated bacterial membrane lysis

In this work, four novel Fe(III) complexes <i>viz.</i>, [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 = <i>N</i>-(1-ferrocenyl)-methyl-1-(pyridine-2-yl)-<i>N</i>-(pyridine-2-ylmethyl)methanamine; H₂L₁ = (<i>E</i>)-2(2-hydroxybenzylidene)amino)phenol and H₂L₂ = (<i>E</i>)-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 <i>λ</i>_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 <i>ca</i>. 0.2 μg mL^-1 against <i>S. aureus</i> and 0.5 μg mL^-1 against <i>E. coli</i>. under light exposure. The light-induced antibacterial activities of these complexes were attributed to bacterial cell membrane damage <i>via</i> reactive oxygen species (ROS) generation. Overall, this study presents the first report of Fe(III) complexes showing antibacterial activity under visible light conditions.