Two uranyl heterocyclic carboxyl compounds with fluorescent properties as high sensitivity and selectivity optical detectors for nitroaromatics

Abstract

The two compounds UO₂(C₁₆H₇N₄O₄)₂·2H₂O (1) and UO₂(C₂₀H₁₂N₅O₂)₂ (2) have been synthesized, based on the different heterocyclic carboxyl ligands 2,3-pyrazino[1,10]phenanthroline-2,3-dicarboxylic acid (H₂L₁) and 2,6-bis(2-pyrazinyl)pyridine-4-benzoic acid (HL₂). The two compounds were characterized by elemental analysis, IR spectroscopy, PXRD, thermogravimetric analysis and UV-visible spectroscopy. Single crystal X-ray diffraction analysis revealed that both compounds 1 and 2 exhibited 2D sheet structures. Moreover, the study of fluorescence quenching properties of compounds 1 and 2 showed that luminescence intensity decreases for both 1 and 2 were especially obvious with increasing of nitroaromatics concentration, even if the nitroaromatics were at a very low concentration (for 1 it is 15 ppm, for 2 it is 20 ppm), which can also be detected. The experimental results suggest a high selective quenching of initial fluorescence intensity in the presence of nitroaromatic compounds. And by calculating the quenching constants of the nitroaromatics using the SV equation (I₀/I) = K_SV[A] + 1, we can see the K_SV values of the quencher 2,4,6-trinitrophenol are both largest for the two compounds, and the values are 1.6 × 10⁶ and 8.5 × 10⁵ for 1 and 2, respectively. This means that the two compounds have the most sensitive luminescence. In addition, compound 2 exhibits enhanced fluorescence behavior towards aldehyde compounds, showing a turn-on fluorescence responsive behavior.