Vitamin B₆ Appended Polypyridyl Co(III) Complexes for Photo-Triggered Antibacterial Activity

Three novel polypyridyl-Co(III)-vitamin B₆ complexes viz., [Co(CF₃-phtpy)(SBVB₆)]Cl (Co1), [Co(anthracene-tpy)(SBVB₆)]Cl (Co2), [Co(NMe₂-phtpy)(SBVB₆)]Cl (Co3), where 4'-(4-(trifluoromethyl)phenyl)-2,2':6',2''-terpyridine=CF₃-phtpy, 4'-(anthracen-9-yl)-2,2':6',2''-terpyridine=anthracene-tpy;, 4-([2,2':6',2''-terpyridin]-4'-yl)-N,N-dimethylaniline=NMe₂-phtpy, (E)-5-(hydroxymethyl)-4-(((2-hydroxyphenyl)imino)methyl)-2-methylpyridin-3-ol=H₂SBVB₆ were successfully developed for aPDT (antibacterial photodynamic therapy) applications. Co1-Co3 exhibited an intense absorption band at ca. 435-485 nm, which is attributed to ligand-to-metal charge transfer and was beneficial for antibacterial photodynamic therapy. The distorted octahedral geometry of the complexes with Co^IIIN₄O₂ core was evident from the DFT study. The visible light absorption ability and good photo-stability of Co1-Co3 made them good photosensitizers for aPDT. Co1-Co3 displayed significant antibacterial responses against gram-positive (S. aureus) and gram-negative (E. coli) bacteria upon light exposure (10 J cm^-2 _, 400-700 nm) and showed MIC values between 0.01-0.005 μg mL^-1. The aPDT activities of these complexes were due to their ability to damage bacterial cell membranes via ROS generation. Overall, this study shows the photo-triggered ROS-mediated bacteria-killing potential of Co(III) complexes.