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

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 x 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 x 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.