Polypyridyl-based Co(III) complexes of vitamin B 6 Schiff base for photoactivated antibacterial therapy.

Herein, five novel polypyridyl-based Co(III) complexes of Schiff bases, viz. , [Co(dpa)(L 1 )]Cl (1), [Co(dpa)(L 2 )]Cl (2), [Co(L 3 )(L 2 )]Cl (3), [Co(L 3 )(L 1 )]Cl (4), and [Co(L 4 )(L 1 )]Cl (5), where dpa (dipicolylamine) = bis(2-pyridylmethyl)amine; H 2 L 1 = ( E )-2-((2-hydroxybenzylidene)amino)phenol; H 2 L 2 = ( E )-5-(hydroxymethyl)-4-(((2-hydroxyphenyl)imino)methyl)-2-methylpyridin-3-ol; L 3 = 4'-phenyl-2,2':6',2''-terpyridine (ph-tpy); and L 4 = 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 III N 4 O 2 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.