π-Conjugated molecules identified for reversible and visual detection of F− in aqueous: Effect of heterocycle unit on sensing performance

Abstract To illustrate the impact of molecular structure, especially heterocycle unit, on the sensing performance, two kinds of π-conjugated molecules containing aromatic heterocyclic (Dye 1) and aromatic ring (Dye 2) were identified and compared each other. Even with similar structures, they possessed quite different spectral and colorimetric responses to F−, Cl−, Br−, I−, NO3−, HSO4−, H2PO4−, ClO4− and CH3COO−, etc. The reason might result from the difference in withdraw-electron ability of aromatic and heterocyclic rings, which would lead to different acidity of active H in the target π-conjugated molecules. In acidic aqueous, Dye 1 expressed a reversible ratiometric-colorimetric response to F−, accompanying with a visual color change from bright yellow to purple, a nice linear range of 2.0–35.0 × 10−6 mol/L and a low detection limit of 1.60 × 10−7 mol/L. While Dye 2 did not react with any anion due to its weak acidity of active hydrogen. Under the optimized conditions, Dye 1 was successfully applied for colorimetric or naked-eye detection of F− in environmental water, tea and toothpaste samples with RSD ≤ 3.1%. The recognition mechanism for Dye 1 to F− was confirmed to be deprotonation one with a 1:1 binding stoichiometry.