Asymmetrically substituted 5,5′′-diaryl-2,2′:6′,2′′-terpyridines as efficient fluorescence 'turn-on' probes for Zn2+ in food/cosmetic samples and human urine

The present article describes the synthesis of new asymmetrically substituted 5,5′′-diaryl-2,2′:6′,2′′–terpyridines as prospective chemosensors/probes for the fluorescence “turn-on” detections of Zn2+. After the fluorescence titration experiments two most representative terpyridines, namely 4-phenyl-1-(6-(4-p-tolyl-6,7-dihydro-5H-cyclopenta[c]-pyridin-1-yl)pyridin-2-yl)-6,7-dihydro-5H-cyclopenta[c]pyridine and 5-phenyl-5′′-p-tolyl-2,2′:6′,2′′-terpyridine were selected based on high binding constant up to 106 M−1 for the 1:2 (Zn2+ : ligand) complexes along with the limits of detection (LOD) as low as ∼0.1 × 10-9 M. These ligands have been further used as fluorescence probes for the qualitative and quantitative detection of Zn2+ in real samples, such as food/cosmetic samples and human urine. For all the samples, these probes exhibited an intensive fluorescence enhancement via the effective transduction LE-TICT-switching signal. The stoichiometry and structure of the Zn2+ complexes were supported by DFT calculations and 1H NMR experiments.