A new fluorescent chemosensor for fluoride anion based on a pyrrole-isoxazole derivative

Abstract

Molecules containing polarized NH fragments that behave as anion-binding motifs are widely used as receptors for recognition and sensing purposes in aprotic solvents. We present here a new example of a receptor, 3-amino-5-(4,5,6,7-tetrahydro-1H-indol-2-yl)isoxazole-4-carboxamide (receptor 1), which contains pyrrole, amide and amino subunits. This receptor shows both changes in its UV-vis absorption and fluorescence emission spectra upon the addition of F(-), resulting in highly selectivity for fluoride detection over other anions, such as Cl(-), Br(-), I(-), HSO(4) (-), H(2)PO(4) (-) and AcO(-) in CH(3)CN. (1)H NMR titration, time-dependent density functional theory (TDDFT) calculations and other experiments confirm that the sensing process is brought about by deprotonation of the pyrrole-NH in receptor 1.

Keywords: deprotonation; fluorescent chemosensor; fluoride anion recognition.

Figure 1

(a) Receptor 1. (b) ORTEP drawing of receptor 1. Thermal ellipsoids are drawn at the 30% probability level.

Figure 2

The absorption spectra of receptor 1 (5 μM) in the absence or presence of a 50 equiv of F⁻, Cl⁻, Br⁻, I⁻, HSO₄⁻, H₂PO₄⁻ and AcO⁻ anions in CH₃CN.

Figure 3

The fluorescence spectra of receptor 1 (5 μM) in the absence or presence of a 50 equiv of F⁻, Cl⁻, Br⁻, I⁻, HSO₄⁻, H₂PO₄⁻ and AcO⁻ anions in CH₃CN (excited at 340 nm).

Figure 4

Comparison of fluorescence emission of 1 (5 μM) in CH₃CN after the addition of 50 equiv of tetrabutylammonium salts.

Figure 5

UV–vis absorption changes of 1 (5 μM) upon the addition of TBAF in CH₃CN.

Figure 6

Fluorescence emission changes of 1 (5 μM) upon the addition of TBAF in CH₃CN (excited at 340 nm).

Figure 7

The fit of the experimental data of fluorescence emission of 1 (5 μM) upon the addition of F⁻ at 400 nm to a 1:2 binding profile (excited at 340 nm). Inset: the partial enlarged curve when less than 5 equiv of F⁻ was added.

Figure 8

Fluorescence emission changes of 1 (5 μM) upon the addition of F⁻ and OH⁻ (5 equiv) in CH₃CN (excited at 340 nm).

Figure 9

Partial ¹H NMR (400 MHz) spectra of receptor 1 in the presence of 0, 0.2, 0.6, 1.0, 1.4, 1.6, 2.0, 3.0 and 4.0 equiv of TBAF in DMSO-d₆.

Figure 10

Anionic form a and b of receptor 1.