1-(2-Pyridyl)-3-ferrocenylpyrazoline-Based Multichannel Signaling Receptors for Co2+, Cu2+, and Zn2+ Ions

A simple one-pot synthesis, the characterization, optoelectronic, and cation sensing properties of 1-(2-pyridyl)-3-ferrocenylpyrazolines 4-6 are described in this article. Reaction of ferrocenyl chalcones with 2-hydrazinopyridine gave the target compounds in good yield. These compounds were characterized by general spectroscopic techniques and the structure of 4 was determined by means of X-ray crystallography. These ferrocene compounds behave as selective multichannel chemosensors (redox, chromogenic, and fluorescent) in the presence of Co2+, Cu2+, and Zn2+ ions. The binding assay and recognition ability of these receptors towards the metal ions were explained by electrochemical and optical studies. A maximum cathodic shift in the redox potential of the ferrocenium couple was observed towards the Co2+ ion (E-1/2 = 99-156 mV), while a minimum shift was observed with the Zn2+ ion (E-1/2 = 72-129 mV) on complexation with these receptors. Disappearance of the high-energy (HE) band and a red shift ( = 7-13 nm) of the low-energy (LE) band in the absorption spectra of the receptors 4 and 5 was observed upon complexation with these metal ions. This change in absorption was accompanied by a color change from yellow to red/brown, which enabled potential naked eye detection. The emission spectra ((ex) = 350 nm) of receptor 4, in the presence of these cations, showed a 2-7-fold increment in the chelation-enhancement fluorescence (CHEF) and a 4-9-fold increase in the quantum yield.