Bis(2-pyridyl)diimine as a naked eye colorimetric fluorescence turn off probe selectively for Fe(II) ions as a consequence of energy changes in the electronic states upon complexation

Abstract

Bis(pyridyl)diimine, (1E,1′E)-N,N'-(ethane-1,2-diyl)bis(1-(pyridin-2-yl)methanimine) (L) has coordinating nitrogen atoms in effective positions to form stable chelates with metal ions and is capable of capturing a range of transition metal ions. However, the selective colorimetric response of Fe(II) by using L was observed. Colourless to purple coloured transition resulted on complexation of L with Fe(II) ions selectively and was not affected by the presence of other transition metal ions including Fe(III) ions. Photoluminescence studies have shown the quenching of emission maxima corresponding to L in the presence of Fe(II) ions along with appearance of new peaks at higher wavelength regions. Appearance of a specific color change, which can be visualized by naked eye and an added photophysical feature involving quenching of emission spectra, particularly for Fe(II) are associated with the changes in the electronic state energies of L upon complexation with Fe(II). Thermodynamic estimation of complex stability was done by determining the binding constant of the complex, which was found to be 7.2 × 1010 M−2 using Yoe Jones method. A binding ratio of 1:2 for L and Fe2+ was determined experimentally with help of mole ratio method, which hinted at the geometrical possibilities of the complex of L with Fe(II).). Limit of detection (LOD) for the identification of Fe(II) ions with L was determined using colorimetric naked eye method and was found to be 25 μM. Further L was also used to detect Fe(II) in real sample i.e. river water. NMR analysis supported the low spin state of Fe(II) in the Fe(II)-L complex. Anticipated geometry of the Fe(II)-L complex was optimized using Gaussian09 software through the B3LYP method, which upon investigation of the energies of frontier molecular orbitals, provided the details related to the electronic properties associated with the complexation of Fe(II) and L.