Sensing Zn2+ in Aqueous Solution with a Fluorescent Scorpiand Macrocyclic Ligand Decorated with an Anthracene Bearing Tail

Abstract

Synthesis of the new scorpiand ligand L composed of a [9]aneN₃ macrocyclic ring bearing a CH₂CH₂NHCH₂-anthracene tail is reported. L forms both cation (Zn²⁺) and anion (phosphate, benzoate) complexes. In addition, the zinc complexes of L bind these anions. The equilibrium constants for ligand protonation and complex formation were determined in 0.1 M NaCl aqueous solution at 298.1 ± 0.1 K by means of potentiometric (pH-metric) titrations. pH Controlled coordination/detachment of the ligand tail to Zn²⁺ switch on and off the fluorescence emission from the anthracene fluorophore. Accordingly, L is able to sense Zn²⁺ in the pH range 6-10 down to nM concentrations of the metal ion. L can efficiently sense Zn²⁺ even in the presence of large excess of coordinating anions, such as cyanide, sulphide, phosphate and benzoate, despite their ability to bind the metal ion.

Keywords: anion binding; azamacrocycles; chemosensor; fluorescence; scorpiand; supramolecular interactions; zinc.

Figure 1

The ligand L.

Figure 2

Emission spectra of ligand L at different pH values (λ_exc 362 nm). Inset: distribution diagrams of the species formed by L as a function of pH and emission intensities (dots) at 416 nm. [L] = 1 × 10⁻⁵ M.

Figure 3

Calculated conformations for H₂L²⁺; H-bonds shown as dashed lines (distances in Å). (a) Lower energy conformer. (b) Higher energy conformer. Energies calculated with respect to structure a.

Figure 4

Distribution diagram of the species formed in the system Zn²⁺/L as a function of pH. [Zn²⁺] = [L] = 1 × 10⁻³ M. Charges omitted for simplicity.

Figure 5

Emission spectra of Zn²⁺/L at different pH values (λ_exc 362 nm). Inset: distribution diagrams of the species formed as a function of pH and emission intensities (dots) at 416 nm. [L] = [Zn²⁺] = 1 × 10⁻⁵ M.

Figure 6

Emission spectra of L and Zn²⁺/L complexes, at pH 9, for: (a) [Zn²⁺] = [L] = 1 × 10⁻⁶ M, (b) [Zn²⁺] = [L] = 1 × 10⁻⁹ M. λ_exc 362 nm.

Figure 7

Calculated conformation for the [ZnL(H₂O)₂]²⁺ complex; bond distances in Å.

Figure 8

pH dependence of the emission intensity at 416 nm (λ_exc 362 nm) of the systems: (a) L, L/benzoate, L/phosphate; (b) Zn²⁺/L, Zn²⁺/L/benzoate, Zn²⁺/L/phosphate. [L] = [Zn²⁺] = 1 × 10⁻⁵ M, [benzoate] = [phosphate] = 3 × 10⁻⁵ M.

Figure 9

(a) Variation of the emission spectra of Zn²⁺/L at pH 9 upon addition of cyanide (a) and sulphide (b). [L] = [Zn²⁺] = 1 × 10⁻⁵ M. λ_exc 362 nm. Insets: Emission intensity at 416 nm versus the added equivalents of cyanide (a) and sulphide (b).