Synthesis and characterization of a highly selective turn-on fluorescent chemosensor for Sn2+ derived from diimine Schiff base

This study is exploratory in proposing a Schiff base (Ligand=L), which was designed and synthesized through one-step condensation reaction using 2,2′-bithiophene-5-carboxaldehyde and 4,4'-diaminotriphenylamine (DATPA), was employed for fluorescent detection of the presence of Sn2+ ions. The ligand was well-characterized by infrared (FT-IR), UV–vis spectroscopy, 1H and 13C nuclear magnetic resonance (NMR) analyses. The chelation of the ligand with Sn2+ ion caused a decrease in the optical band gap in the UV–vis absorption spectrum of the ligand upon the addition of Sn2+ ions. The binding of L to Sn2+ caused a blue shift due to an internal charge transfer (ICT). The synthesized L was conceivable to be an outstanding receptor to detect Sn2+ ions with a remarkable fluorescent enhancement due to a potent chelation enhanced fluorescence (CHEF) effect. Non-fluorescent ligand executed a fast response and highly sensitive turn-on fluorescent sensing behavior to detect Sn2+ ions via 1:2 (ligand:metal) complexation without any interference from other metal cations. The ligand’s specificity was indicated with an observation of no transition in colour with any other metal cations. The selectivity of the probe for Sn2+ was confirmed by the absence of any interference from other competitive cationic species. The calculated limit of detection (LOD) and binding constant (Ka) were found to be 3.14 × 10−7 M and 6.8 × 109 M−2, respectively. The synthesized ligand was of use for the devise of metal sensors based on diimine Schiff base.