Polypyridyl-based Co(III) complexes of vitamin B₆ Schiff base for photoactivated antibacterial therapy

Herein, five novel polypyridyl-based Co(III) complexes of Schiff bases, viz., [Co(dpa)(L₁)]Cl (1), [Co(dpa)(L₂)]Cl (2), [Co(L₃)(L₂)]Cl (3), [Co(L₃)(L₁)]Cl (4), and [Co(L₄)(L₁)]Cl (5), where dpa (dipicolylamine) = bis(2-pyridylmethyl)amine; H₂L₁ = (E)-2-((2-hydroxybenzylidene)amino)phenol; H₂L₂ = (E)-5-(hydroxymethyl)-4-(((2-hydroxyphenyl)imino)methyl)-2-methylpyridin-3-ol; L₃ = 4'-phenyl-2,2':6',2''-terpyridine (ph-tpy); and L₄ = 4'-ferrocenyl-2,2':6',2''-terpyridine (Fc-tpy), were synthesized and characterized. Complexes 1, 3, and 4 were structurally characterized by single-crystal XRD, indicating an octahedral Co^IIIN₄O₂ coordination core. The absorption bands of these complexes were observed in the visible range with a λ_max at ∼430-485 nm. Complex 5 displayed an extra absorption band near 545 nm because of a ferrocene moiety. These absorptions in the visible region reflect the potential of the complexes to act as visible-light antimicrobial photodynamic therapy (aPDT) agents. All of these complexes showed reactive oxygen species (ROS)-mediated antibacterial effects against S. aureus (Gram-positive) and E. coli (Gram-negative bacteria) upon low-energy visible light (0.5 J cm^-2, 400-700 nm) exposure. Additionally, 1-5 did not show any toxicity toward A549 (Human Lung adenocarcinoma) cells, reflecting their selective bacteria-killing abilities.