Fluorescent chemosensor for Zn(II) ion by ratiometric displacement of Cd(II) ion: A spectroscopic study and DFT calculation

A molecule having ethylenediamine unit with symmetrically attached aromatic frames has been synthesized, characterized and reported as a highly selective and sensitive chemosensor for Zn2+ and Cd2+ ion. UV–vis absorption and fluorescence emission spectra, quantum yield calculation, fluorescence lifetime measurement and the observed stability constant of metal-sensor complex support 1:1 complex formation for both Zn2+ and Cd2+ ion with the sensor. Enhancement of large Stokes shifted (130 nm for Zn2+, 103 nm for Cd2+) and polarity dependent emission bands were observed for Zn2+ and Cd2+ ions, while other transition metal ions Cr3+, Fe2+, Ni2+, Co2+, Cu2+, Hg2+ quench the fluorescence emission of the sensor. Ratiometric displacement of Cd2+ ion from the complex by Zn2+ ion supports the formation of more stable sensor-Zn2+ complex over the sensor-Cd2+ complex. Scanning electron micrographs show different structural morphology of the complexes. The experimental findings have been correlated with theoretical results using B3LYP functional and 6−311++G(d,p), LANL2DZ basis set for sensor and metal ion, respectively, by Density Functional Theory (DFT) method.

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► Metal ion induced fluorescence enhancement of reduced Schiff base receptor.
► Zn2+ and Cd2+ metal ion fluorosensor.
► Ratiometric displacement of Cd2+ by Zn2+ ion from L-Cd2+ complex.
► Metal mediated conformational switching.