1‐(2‐Pyridyl)‐3‐ferrocenylpyrazoline‐Based Multichannel Signaling Receptors for Co 2+ , Cu 2+ , and Zn 2+ 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 Co 2+ , Cu 2+ , and Zn 2+ 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 Co 2+ ion (Δ E 1/2 = 99–156 mV), while a minimum shift was observed with the Zn 2+ ion (Δ E 1/2 = 72–129 mV) on complexation with these receptors. Disappearance of the high‐energy (HE) band and a red shift (Δ λ = 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 ( λ 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.