Multiresponsive chemosensors based on ferrocenylimidazo[4,5-b]pyridines: Solvent-dependent selective dual sensing of Hg2+ and Pb2+

Abstract

Two new chemosensors, namely, 2-ferrocenyl-5-(pyridin-3-yl)-1H-imidazo[4,5-b]pyridine (L1) and 2-ferrocenyl-6-(pyridin-3-yl)-1H-imidazo[4,5-b]pyridine (L2), were synthesized and characterized. Furthermore, the solid-state structure of L1 was unequivocally established by X-ray diffraction analysis. These nitrogen-rich ferrocene derivatives showed remarkable multiresponsive (fluorescent, UV active and redox) Hg2+/Pb2+ sensing properties. Interestingly, the selectivity of L1 and L2 could be switched from Hg2+ to Pb2+ by changing the solvent from CH3OH/H2O (V/V, 1/1) to CH3CN according to UV–vis and fluorescence measurements. L1 and L2 showed highly selective recognition toward Hg2+ in CH3OH/H2O (V/V, 1/1) over a wide range of tested metal ions with a redshift of the absorption bands at approximately 330 nm in their UV–vis absorption spectra. These sensors exhibited immense brightness and remarkable fluorescence quenching following Hg2+ coordination. Interestingly, similar UV responses were induced by adding Pb2+ rather than Hg2+ to the CH3CN solutions of L1 and L2, and remarkable fluorescence enhancements (CHEF = 25 for L1 and 33 for L2) were observed in the presence of Pb2+. Furthermore, each displayed an anodic shift of the redox wave (ΔE1/2 = 0.04–0.15 V) upon complexation with Hg2+ or Pb2+ in CH3CN/CH2Cl2 (V/V, 1/1). Both L1 and L2 formed a 1:1 complex with Hg2+ or Pb2+ through coordination of the two N atoms of the imidazo[4,5-b]pyridine ring, and the PET mechanism was responsible for their fluorescence changes with Hg2+ and Pb2+. When a 3-pyridyl group was introduced, regardless of its position, the selectivity and sensitivity of the sensors for Hg2+ and Pb2+ detection were obviously improved.