A new chelating
ligand [4-methyl-2,6-bis-(pyridin-2-yl-hydrazonomethyl)-
phenol] (1) was prepared by the condensation of 2-hydrazinylpyridine with
2,6-diformyl-p-cresol. Compound 1 exhibits weak fluorescence due to intramolecular photoinduced electron transfer (PET). The sensor (1) demonstrates Zn(2+)-specific emission enhancement due to the “PET off” process through a 1:1 binding mode with the
metal ion. The fluorescence quantum yield of
chemosensor 1 is only 0.020, and it increases more than 14-fold (0.280) in the presence of one equivalent of the
zinc ion. Interestingly, the introduction of other
metal ions causes the fluorescence intensity to remain either unchanged or weakened except for Cd(2+). The new sensor showed ‘naked-eye’ detection of Zn(2+)
ions: a color change of the
solution from colorless to yellow. Ratiometric displacement of Cd(2+)
ions from the complex by Zn(2+)
ions supports the formation of a more stable sensor–Zn(2+) complex over the sensor–Cd(2+) complex. The experimental findings have been correlated with theoretical results using the B3LYP functional and 6-31G (d, p), LANL2DZ basis set for Cd(2+) (2) and Zn(2+) (3) complexes, respectively, by the Density Functional Theory (DFT) method. Moreover, the ability of probe 1 to sense Zn(2+) within human
melanoma cancer cells has been explored, and the Zn(2+)-probing process in living cells was found to be reversible with
zinc chelator solution of N,N,N,N-tetrakis(2-pyridylmethyl)ethylenediamine (
TPEN) or
EDTA.