Bis(2-pyridylmethyl)amine-functionalized alizarin: an efficient and simple colorimetric sensor for fluoride and a fluorescence turn-on sensor for Al3+ in an organic solution

Abstract

A complexone analog chemosensor, H₂L, bearing chelating bis(2-pyridylmethyl) amine and alizarin groups was synthesized via the Mannich reaction. H₂L chromically responds to OH⁻, F⁻, CH₃COO⁻, and H₂PO₄⁻ in DMF, CH₃CN, and acetone, but not in CH₃OH or H₂O. The addition of F⁻ ions to H₂L selectively induces a significant and visible color change in acetonitrile and shifts both methylene proton signals upfield. H₂L also exhibits visible responses to Mg²⁺, Sr²⁺, Ba²⁺, Tb³⁺, Cu²⁺, Co²⁺, Ni²⁺, Zn²⁺, Mn²⁺, Cd²⁺, and Fe³⁺ in solution. AlCl₃ can form an Al : L = 2 : 3 complex that not only changes the color of the DMF solution, but also significantly increases its fluorescence intensity. The limit of fluorescence turn-on detection for AlCl₃ in DMF is 2.7 × 10⁻⁸ M, which is an order higher than those of other anthraquinone sensors reported in the literature. NMR spectroscopy shows that hydroxyl is not deprotonated upon interacting with Al³⁺, but will be partially deprotonated in the presence of Zn²⁺. Contrary to the complexone, the H₂L–Ce(III) complex does not react chromically to F⁻. However, the H₂L–NiCl₂ complex responds chromically to F⁻, with higher sensitivity (LOD = 1.3 × 10⁻⁶ M F⁻ in acetonitrile) than free H₂L. The spectral changes in the presence of F⁻ are similar to that of OH⁻; however, the spectrum shifts slightly to a longer wavelength and is more sensitive to both H₂L and the H₂L–NiCl₂ complex. Moreover, 4% or less H₂O in the solvent essentially has no influence on the F⁻ sensitivity; however, high water content significantly decreases the F⁻ sensitivity. The spectral changes of the Zn²⁺, Cu²⁺, Fe³⁺, Ce³⁺, and Ni²⁺ complexes in the presence of different NaOH concentrations were also investigated.