L-Lysine-derived fluorescent salicylidenes: synthesis, characterization and turn-off Cu2+ and Co2+ selective cation sensing

Two L-lysine-derived salicylidene ligands have been synthesized under ultrasound irradiations and characterized by several spectroscopic techniques. Both ligands exhibited a blue fluorescence emission in ethanolic solutions. (S, E)-3-Amino-7-((2-hydroxybenzylidene) amino) heptanoic acid (A) and (S, E)-3-amino-7-((4-chloro-2-hydroxybenzylidene) amino) heptanoic acid (B) showed a turn-off sensing of Cu²⁺ and Co²⁺ cations, respectively, over several tested transition metal cations such as Fe²⁺, Zn²⁺, Cr³⁺, Co²⁺, Ni²⁺ and Cu²⁺ in aqueous media. The ability of both tested L-lysine-derived ligands (A) and (B) to distinguish Cu²⁺ and CO²⁺, respectively, has been verified by optical studies, showing new absorption bands around 304 and 391 nm for (A)-Cu²⁺ and a new band around 455 nm for (B)-Co²⁺ along with the disappearance of the original bands supporting thereby the noticed Turn–Off results.

Graphical abstract