Selective Recognition of Zn 2+ Ion Using 2,4‐Bis(2‐pyridyl)‐5‐(4‐pyridyl)imidazole: Spectra and Molecular Structure

A trisubstituted imidazole, 2,4‐bis(2‐pyridyl)‐5‐(4‐pyridyl)imidazole ( LH ) in presence of other cations, selectively binds Zn 2+ ion by exhibiting characteristic absorption and fluorescence spectral behaviors. Upon gradual addition of ZnCl 2 to DMSOsolution of LH , intensity of peak at 318 nm decreased. A new peak appeared at 364 nm along with an isosbestic point at 337 nm. The DMSOsolution of LH upon irradiation with λ=320 nm exhibited an emission peak at 391 nm, which decreased gradually with increase in Zn 2+ ion concentrations. A new emission peak also appeared at 445 nm along with a quasi isoemissive point at 413 nm. The HEPES buffered aqueous DMSO solution (2:8 v/v, pH=7.4) of free LH , upon excitation with 320 nm radiation gave emission at 396 nm which shifted to 445 nm during incremental additions of Zn 2+ ion. The red shift is due to binding of Zn 2+ ion to LH . The binding is accompanied by deprotonation of acidic hydrogen in imidazole ring and orientation of two 2‐pyridyl rings in near‐planarity with imidazolate ring. The Zn 2+ ion can be detected with a calculated limit of 6.5×10 −7 M. The Job's plot revealed that ligand binds Zn 2+ ion in 1:2 ratios and was further confirmed by single crystal X‐ray diffraction method.