Fluorescent Selective Chemosensor for Fluoride Ion with Pyrene Based on Triphenylphosphine

Results and Discussion

The 1H NMR spectrum of 1 showed a doublet at δ 5.34 ppm (J = 14.4 Hz) for the methylene protons due to the coupling of the phosphorus atom and a mixture of multiplets at δ 7.40 to δ 8.31 ppm for the phenyl and pyrene aromatic protons. To investigate the anion binding properties, a series of anions such as tetrabutylammonium (TBA) fluoride, chloride, bromide, iodide, acetate, benzoate, hydrogen sulfate, and dihydrogen phosphate were studied using fluorescence titration with chemosensor 1 in CH3CN. In the absence of anions, pyrene triphenylphosphine derivative 1 exhibits nearly weak fluorescence in acetonitrile solution.

Upon addition of fluoride ion, the fluorescence intensity emission of the dosimeter at 375 and 395 nm was increased 5-fold. Only in the presence of fluoride ion, a strong fluorescence was observed at 375 and 395nm. However, other anions such as chloride, bromide, iodide, acetate, benzoate, hydrogen sulfate, and dihydrogen phosphate did not cause any significant changes in the fluorescence emission intensity, even at excesses guest ions as shown in Figure 1.

Figure 1.Emission spectra of 1 (2 μM) upon the addition of anions to 5 equivalents in CH3CN. (The excitation wavelength is 340 nm).

The crystal structure of chemosensor 1 for X-ray analysis was grown by slow evaporation of acetonitrile solution. Figure 3 showed X-ray structure of chemosensor 1. Table 1 showed the detail data for X-ray structure of chemosensor 1. The crystal system is triclinic and P2(1)2(1)2(1) space group. Unit cell dimension are a = 8.8364(13) Å, b = 15.950(2) Å and c = 20.304(3) Å. The final R indices are R1 = 0.0608 wR2 = 0.1148.

Table 1.Crystal data and structure refinement for 1

Figure 2.Emission spectra of 1 (2 μM) upon the addition of F- to 5 equivalents in CH3CN. (The excitation wavelength is 340 nm).

Figure 3.Crystal structure of the chemosensor 1.

A quite drastic 1H NMR spectral change was observed when treating with fluoride ions as showed in Figure 4. A methylene doublet at δ 5.34 was disappeared and several aromatic peaks at δ 8.31-7.40 ppm were moved slightly downfield at δ 8.78-7.40 ppm upon addition of fluoride ions. The stability constant could not be determined due to the completely disappearance of observing peaks at δ 5.34 with even small addition of anions. But, a new triplet at 16.1 ppm was appeared over 1 equivalents of fluoride addition. It is the typical characteristics of hydrogen fluoride ion (F-H-F).

Figure 4.The partial 1H NMR spectra of compound 1 with fluoride ions.

Color and fluorescence change were easily observed on mixing the compound 1 and anions, as shown in Figure 5. A receptor solution was simply treated with various anions such as tetrabutylammonium (TBA) fluoride, chloride, bromide, iodide, acetate, dihydrogen phosphate, and hydrogen sulfate. The colorless solution became orange when fluoride ion was added to compound 1 in acetonitrile, but no color changes were observed on the additions of chloride, bromide, and hydrogen sulfate ions. Figure 6 showed the UV-visible spectra of 1. In the presence of fluoride ions a new peak at around 500 nm appeared as color indicated in Figure 5.

Figure 5.(a) Color change of 1 on the addition of fluoride in CH3CN. (b) fluorescence change of 1 on the addition of fluoride in CH3CN.

Figure 6.Uv-visible spectra of 1 and 1+F- in CH3CN.

 

Conclusion

In conclusion, pyrene triphenylphosphine derivative 1 was synthesized successfully by the reaction of triphenylphosphine with 2, which only showed a dramatic color and fluorescence change when treated with fluoride ions. The high selectivity for fluoride can be attributed to deprotonation of the methylene protons.

 

Experimental

Compound (1). To a solution 0.295 g (1.0 mmol) of 1-bromomethyl pyrene 2 in 30 mL of CH3CN was added 0.524 g (0.5 mmol) of triphenylphosphine and the reaction mixture was refluxed for 1days under the nitrogen atmosphere. After cooling down to room temperature, the pale yellow solid was obtained by filtration. The filtered solid was added in 20 mL MeOH with NaPF6. The yellow solid was obtained by filtration to yield 0.5 g (95%) of 1. 1H NMR (CD3CN, 300 MHz) δ 8.31-7.48 (m, 24H, aromatic). 8.27 (d, 2H, Ar-CH2, J = 14.4 Hz), 13C NMR (CD3CN, 300 MHz) δ 136.36, 136.32, 135.42, 135.29, 132.84, 132.80, 132.25, 132.23, 131.62, 131.54, 131.24, 131.23, 131.20, 131.03, 130.26, 130.20, 129.84, 129.45, 129.42, 128.88, 128.86, 128.33, 128.30, 127.78, 127.07, 127.06, 126.72, 126.71, 125.96, 125.91, 125.67, 124.92, 123.22, 121.39, 121.27, 118.87, 117.73, 29.01, 28.36.