Fluorescence Recognition of Anions Using a Heteroditopic Receptor: Homogenous and Two-Phase Sensing

Abstract

In contrast to monotopic receptor 3, the anthracene functionalized squaramide dual-host receptor 1 is capable of selectively extracting sulfate salts, as was evidenced unambiguously by DOSY, mass spectrometry, fluorescent and ion chromatography measurements. The receptors were investigated in terms of anion and ion pair binding using the UV-vis and ¹H NMR titrations method in acetonitrile. The reference anion receptor 3, lacking a crown ether unit, was found to lose the enhancement in anion binding induced by the presence of cations. Besides the ability to bind anions in an enhanced manner exhibited by ion pair receptors 2 and 4, changing the 1-aminoanthracene substituent resulted in their exhibiting a lower anion affinity than receptor 1. By using receptor 1 and adjusting the water content in organic phase it was possible to selectively detect sulfates both by "turn-off" and "turn-on" fluorescence, and to do so homogenously and under interfacial conditions. Such properties of receptor 1 have allowed the development of a new type of sensor capable of recognizing and extracting potassium sulfate from the aqueous medium across a phase boundary, resulting in an appropriate fluorescent response in the organic solution.

Keywords: anthracene; crown ethers; fluorescent sensors; ion pair receptors; squaramides; sulfate extraction.

Figure 1

Structures of receptors 1–4.

Scheme 1

Synthesis of receptors 1–3. Reagents and conditions: (i) 3,4-dimethoxy-3-cyclobutene-1,2-dione, methanol, 48 h, room temperature, yield 63% for M1 and 85% for M2; (ii) 4-aminobenzo-18-crown-6 ether, methanol, DIPEA, 12 h, room temperature, yield 60% for 1 and 70% for 2; (iii) aniline, methanol, DIPEA, 12 h, room temperature, yield 77%.

Figure 2

Representative binding behaviour of receptor 1 upon gradual addition of TBACl: (a) UV–vis titration spectra and (b) corresponding experimental (points) and theoretical (lines) binding isotherms (λ = 430 nm) for TBACl alone and with the presence of 1 equivalent of KPF₆.

Figure 3

Image presenting fluorescence of the crystals of 1 with KCl/NaCl and 1 with NaBr salts, together with the molecular arrangements of moieties in the solid state: 1-D polymers in 1 + KCl/NaCl (a) and centrosymmetric dimmers in 1 + NaBr (b). All C-H hydrogen atoms omitted for clarity.

Figure 4

Top: Changes observed in the emission spectra of 1 in CH₃CN (c = 1.05 × 10⁻⁴ M; excitation at 327 nm). A: receptor 1; B: upon addition of 100 equivalents of TBA₂SO₄; C: upon addition of 100 equivalents of TBACl; D: upon addition of 100 equivalents TBAF, and the corresponding changes seen after UV illumination (λ = 254 nm). Bottom: A’: receptor 1 (c = 1.01 × 10⁻⁴ M) in CH₃CN; B’: upon addition of TBABr, TBACl, TBANO₃ (33 equivalents each); C’: upon addition of TBABr, TBACl, TBANO₃, TBA₂SO₄ (25 equivalents each), and the corresponding changes seen after UV illumination (λ = 254 nm).

Figure 5

Changes observed in the emission spectrum of 1 in wet CH₃CN (1% of H₂O; c = 2.4 × 10⁻⁴ M, excitation at 327 nm). A: receptor 1; B: upon addition of 5 equivalents of TBA₂SO₄; C: upon addition of 5 equivalents of TBACl; and the corresponding changes seen after UV illumination (λ = 254 nm).

Figure 6

Fluorescence response of solution of receptor 1 (c = 2.3 mM) in CHCl₃ (under UV light, λ = 254 nm). A: after contact with water; B: after extraction with aqueous K₂SO₄ (50 mM); C: after extraction with aqueous KCl (50 mM).