Two Highly Stable Luminescent Lead Phosphonates Based on Mixed Ligands: Highly Selective and Sensitive Sensing for Thymine Molecule and VO3 - Anion

Abstract

Two luminescent lead phosphonates with two-dimensional (2D) layer and three-dimensional (3D) framework structure, namely, Pb₃[(L₁)₂(Hssc)(H₂O)₂] (1) and [Pb₂(L₂)_0.5(bts)(H₂O)₂]·H₂O (2) (H₂L₁ = O(CH₂CH₂)₂NCH₂PO₃H₂, H₄L₂ = H₂PO₃CH₂NH(C₂H₄)₂NHCH₂PO₃H₂, H₃ssc = 5-sulfosalicylic acid, NaH₂bts = 5-sulfoisophthalic acid sodium) have been prepared via hydrothermal techniques. The two compounds not only show excellent thermal stability but also remain intact in aqueous solution within an extensive pH range. Moreover, the atomic absorption spectroscopy analysis experiment indicates that there does not exist the leaching of Pb²⁺ ions from the lead phosphonates, which show they are nontoxic in aqueous solution. In compound 1, the Pb(1)O₄, Pb(2)O₇, Pb(3)O₄, and CPO₃ polyhedra are interlinked into a one-dimensional chain, which is further connected to adjacent chain by sharing the Hssc^2- to form a 2D layer. Interestingly, compound 1 as a highly selective and sensitive luminescent material can be used to detect the thymine molecule with a very low detection limit of 8.26 × 10^-7 M. In compound 2, the Pb(1)O₆ and Pb(2)O₅ polyhedra are interlinked into a dimer via edge sharing, which is further connected to adjacent dimer to form a tetramer via corner sharing, and such a tetramer is then interlinked into a 2D layer through bts^3- ligands; the adjacent 2D layers are finally constructed to a 3D structure by sharing the L₂ ^4- ligand. Compound 2 can be applied as an excellent luminescent sensor for sensing of VO₃ ^- anion. Furthermore, the probable fluorescent quenching mechanisms of the two compounds have also been studied.

Figure 1

Structure unit of compound 1 showing the atom labeling. All H atoms are omitted for clarity. Thermal ellipsoids are drawn at the 30% probability level. Symmetry code: (A) −x, −y, −z + 1 and (B) −x + 1, −y, −z + 1.

Figure 2

(a, b) Coordination modes of the ligands L₁^2– and Hssc^2– in compound 1; (c) the 2D layered structure of compound 1 viewed in the bc-plane; and (d) the 2D layered structure of compound 1 viewed in the ac-plane.

Figure 3

Structure unit of compound 2 showing the atom labeling. All H atoms are omitted for clarity. Thermal ellipsoids are drawn at the 30% probability level. Symmetry code: (A) −x, −y + 1, −z; (B) −x + 1, −y + 1, −z; (C) −x + 1, −y, −z + 1; (D) x + 1, y, z; (E) −x + 1, −y + 1, −z + 1; and (F) x, y – 1, z + 1.

Figure 4

(a, b) Coordination modes of the ligands L₂^4– and bts^3– in compound 2; (c) the 3D framework structure of compound 2 viewed in the ac-plane; and (d) the 3D framework structure of compound 2 viewed in the ab-plane.

Figure 5

Simulated XRD pattern and the experimental PXRD patterns of compound 1 (a) and compound 2 (b) with different treatment temperatures.

Figure 6

Simulated single-crystal XRD and powder XRD for 1 (a) and 2 (b) at different pH values.

Figure 7

Structures and sizes of five kinds of nucleobases.

Figure 8

(a) Quenching efficiency of 1 in five kinds of nucleobases aqueous solutions; (b) the luminescence intensities of suspensions of 1 in the presence of different amounts (50–500 μL) of thymine aqueous solutions; (c) the Stern–Volmer (SV) plot of different nucleobases at different concentrations; and (d) comparison of the luminescence intensity of compound 1 exchanging with 0.01M T in the presence of other nucleobases.

Figure 9

Structures of nitrogenous bases and the possible weak interaction mechanisms of Hssc^2–.

Figure 10

(a) Quenching efficiency of compound 2 in 15 kinds of anions aqueous solutions; (b) fluorescence quenching experiments of standard suspensions of compound 2 with the addition of different concentrations of VO₃^–; (c) the Stem–Volmer (SV) quenching plot presented as I₀/I – 1 versus VO₃^– concentration; and (d) the luminescence intensity of compound 2 upon the addition of different ions followed by VO₃^– anion.