Antimicrobial silver (I) complexes derived from aryl-benzothiazoles as turn-on sensors: Syntheses, properties and density functional studies

Two Ag(I) complexes derived from 2-(pyridyl)benzothiazole (pbt) and 2-(quinolyl)benzothiazole (qbt) namely, [Ag(pbt)2]BF4 (1) and [Ag(qbt)2]BF4 (2) have been synthesized and structurally characterized. The Ag(I) center in both complexes are distorted tetrahedral and both ligands bind silver in the N,N-bidentate mode. Upon complexation, the fluorescence of both ligands is quenched significantly and it is restored gradually upon release of silver under biological conditions. Results from theoretical studies indicate that a facile 1MLCT → 3LC (a 1S → T1 intersystem crossing) transition is responsible for the diminution of the luminescence in these complexes of silver (a heavy metal with high spin-orbit coupling). Both complexes exhibit antibacterial properties toward Gram-positive and Gram-negative bacteria. The bactericidal effect of the complexes is accompanied by an enhancement in fluorescence due to silver delivery to the target bacteria.