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

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 Hssc2- 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 bts3- 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.

Introduction

In the last few years, design of artificial receptors to specifically target DNA, Rn class="Chemical">NA, and related important small molecules in aqueous solutions has been attracting great interest due to their potential application in the mimic systems for enzymes and clinical trials.[1−4] The recognition process for nucleobases has obtained particular attention because they play major roles in increasing genetic diversities and genome function and, thus, are critical for the survival and adaptability of the species.[5] Therefore, the detailed recognition of nucleobases is necessary for the complete understanding of the genetic and epigenetic regulation.[6,7] The abnormal change in the base of the organisms suggests the mutation and deficiency of the immunity system, which can lead to various diseases.[8,9] So, the quantitative detection of thymine has great significance in biological sciences and clinical medicine. To date, many methods have been used to detect thymine molecule, including high-performance liquid chromatography (HPLC), HPLC–mass spectrometry, monoclonal antibodies, and electrochemical analysis.[10−12] However, some drawbacks such as low sensitivity, long analysis time, and expensive equipment also exist in these above-mentioned methods.[13] So, the fluorescence probe method has obtained wide attention due to their high selectivity and sensitivity compared with other methods mentioned above.[14,15] On the other hand, vanadium has received considerable attention due to its dual physiological properties as a nutrient at lower concentrations and as toxic substance at higher concentrations. And, in general, the content of vanadium in natural samples is often lower, so it is of urgent need to obtain some highly sensitive and reliable measurement methods.[16] For instance, vanadium ion is one of the most important trace elements in human body that is involved in glucose metabolism and diabetes treatment and can decrease the blood glucose and blood lipid in diabetic patients.[17] So far, some analytical methods about sensing of vanadium (V) ions have been found, including atomic absorption spectroscopy (AAS), HPLC, spectrophotometry, and electrochemical methods.[18−20] However, the main disadvantages of the above-methods are that they are laborious and require expensive equipment. Therefore, it is vital to prepare a variety of sensing material based on readily obtained novel materials.[21−23]

In recent decades, metal phosphonates have been extensively investigated, as they have great influences on the catalysis, porosity and separation, magnetism, ion exchange, and proton conductivity.[24−29] Some n class="Chemical">metal phosphonates have been used to prepare fluorescent-sensing material because the component unit of metal phosphonates, including the metal and organic ligand, can generate the luminescence to be used to probe molecules or ions.[30−32] At present, there are a few studies on ions or molecular recognition properties based on metal phosphonates, which are mainly focused on the sensing of the Fe3+, Cu2+ cations, CrO42–/Cr2O72– anions, amino acids, nitroaromatic explosive molecules, etc. For example, Fu et al. reported a terbium phosphonate, with a rapid and selective sensing of nitroaromatic explosives, that has a low detection limit (66 ppb).[33] Yang et al. reported two terbium phosphonates used as luminescent probe to detect Cu2+ cations in aqueous solution, which exhibited the photoluminescence switching behavior modulated by the photoinduced electron-transfer reaction.[34] Our group also constructed three cadmium(II) carboxyphosphonates with mixed ligands, which showed the highly selective sensing of the tryptophan molecule.[35] Although a few investigations on recognition properties of luminescent metal phosphonates have been reported recently, there are still no reports for detecting thymine molecule and VO3– anion. Therefore, it is still a challenge to achieve the sensing of the environmental and biological ions or molecules based on these materials. As an expansion of our work, by employing O(CH2CH2)2NCH2PO3H2 (H2L1) and H2PO3CH2NH(C2H4)2NHCH2PO3H2 (H4L2) as the phosphonate ligand and 5-sulfosalicylic acid (H3ssc) or 5-sulfoisophthalic acid sodium salt (NaH2bts) as the mixed ligand, two luminescent lead phosphonate 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) have been obtained. Furthermore, the two compounds have the potential applications as the highly selective fluorescent probes for sensing of the thymine molecule or VO3– anion through the fluorescent quenching effect. To the best of our knowledge, it is the first example of the sensing of thymine molecule or VO3– anion utilizing luminescent metal phosphonate materials.

Results and Discussion

Structure Description of Compound 1

Single-crystal X-ray diffraction (XRD) analysis indicated that compound 1 crystallizes in the triclinic spn class="Chemical">ace group P1̅ (Table S1). There are three crystallographically independent Pb2+ ions, one Hssc2– ligands, two L12– ligands, and two coordinated water molecules in the structure unit of compound 1 (Figure ). The corresponding coordination environment of the three Pb2+ centers are different, with the Pb1 and Pb3 ions being four-coordinated environment, whereas Pb2 ion being seven-coordinated environment. Three of the four coordination sites for Pb1 or Pb3 ions are filled with three phosphonate oxygen atoms (O1, O2A, and O8A for Pb1 or O3, O7, and O9 for Pb3) from three separate L12– anions and the remaining one-coordination site is occupied by the one oxygen atom (O5) from one coordinated water molecule for Pb1 or one oxygen atom (O14) from Hssc2– ions for Pb3 to form the PbO4 tetrahedral geometry. However, the coordination sites for Pb2 ions are surrounded by seven oxygen atoms from four L12– ligands (O1, O2, O3A, O7, and O8A), one Hssc2– ligands (O11), and one coordinated water molecule (O6).

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.

The phosphonate molecule is described as a tridentate ligand, whereas the Hssc2– anion can n class="Chemical">act as a pentadentate ligand, and they both show only one coordination mode (Figure a). Notably, each phosphonate oxygen atom works as a μ2-type bridge linking to two Pb(II) ions and each Hssc2– anion combines one Pb2 ion and one Pb3 ion through carboxylate oxygen atom (O11) and sulfonate oxygen atom (O14) (Figure b). The coordination mode and the structural characteristics of 5-sulfosalicylic acid imply that compound 1 may be considered as a candidate for selective probing of organic small molecules and anions. As shown in Figure c, the Pb(1)O4, Pb(2)O7, and Pb(3)O4 polyhedra are interlinked to CPO3 tetrahedra via edge and corner sharing to form a one-dimensional inorganic chain in the a-axis, with the morpholinyl groups of L12– grafted in this chain. The neighboring chains are further constructed into a 2D layer by sharing the Hssc2– (Figure d).

Figure 2

(a, b) Coordination modes of the ligands L12– and Hssc2– 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.

(a, b) Coordination modes of the ligands L12– and Hssc2– 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 n class="Chemical">ac-plane.

Structure Description of Compound 2

Single-crystal XRD analysis indicated that compound 2 crystallizes in the triclinic space group P1̅ (Table S1). There are two crystallographically independent n class="Chemical">Pb2+ ions, one L24– ions, one bts3– ions, two coordinated water molecules, and one lattice water molecule in the structure unit of compound 2 (Figure ). The Pb1 is a six-coordinated environment, whereas Pb2 is a nine-coordinated environment. Two of the six coordination sites for Pb1 ions are filled with two phosphonate oxygen atoms (O1A and O2) from two L24– ligands and the remaining four coordination sites are occupied by the four oxygen atom (O8B, O9F, O10C, and O4) from four bts3– ligands to form the PbO6 octahedral geometry. However, the coordination sites for Pb2 ions are surrounded by nine oxygen atoms from two L24– ligand (O3A, O1D, and O2D), two coordinated water (O12 and O13), and three bts3– ligands (O7D, O8D, O7B, and O4).

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.

The phosphonate molecule is described as a hexadentate ligand, whereas the n class="Chemical">bts3– anion can act as a heptdentate ligand, and they both show only one coordination mode (Figure a). Notably, each phosphonate oxygen atom (O3) from the L24– ligand and each carboxylate oxygen atom (O9 and O10) from the bts3– ligand behave as a μ1 metal-linker linking to each Pb(II) ions, whereas each phosphonate oxygen atom (O1 and O2) from the L24– ligand, each sulfonate oxygen atom (O4), and each carboxylate oxygen atom (O7 and O8) from the bts3– ligand work as a μ2-type bridge linking to two Pb(II) ions (Figure b). As shown in Figure c, two Pb(1)O6 octahedra and two Pb(2)O9 polyhedra are interlinked to CPO3 tetrahedra via edge and corner sharing to form a tetramer, which is further assembled into a 2D layer by sharing bts3– in the ac-plane, and the neighboring layers are finally constructed in a 3D structure by sharing the L2– ligand in the ab-plane (Figure d).

Figure 4

(a, b) Coordination modes of the ligands L24– and bts3– 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.

(a, b) Coordination modes of the ligands L24– and n class="Chemical">bts3– 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.

Thermal and Chemical Stability

Thermal gravimetric analyses (TGA) are used to study the thermal stabilities of the two compounds. There exist two steps of weight losses in compounds 1 and 2 (Figure S1). The first step started at 50 °C for 1 and 2 and was completed at 108 °C for 1 (90 °C for 2), corresponding to the loss of two coordinated n class="Chemical">water molecule. When the temperature was increased, the curve did not show weight loss until 333 °C for 1 and 358 °C for 2. Above that temperature, it gradually lost weight until 631 °C for 1 and 548 °C for 2, which can be attributed to the decomposition of the two compounds. The final residues of the thermal decomposition of the two compounds are Pb3(PO4)2 based on powder XRD (Figure S2). To further investigate the thermal stability of the two compounds, powder XRD is achieved at 25–290 °C for 1 and 25–280 °C for 2. The results show that the retention of the framework is up to 270 °C for 1 and 240 °C for 2 (Figure ).

Figure 5

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

In addition, the chemical stability of compounds 1 and 2 were examined under different pH conditions. The solid samples were immersed in the solution of specified pH for 24 h at room temperature, and the XRD tests were carried out to prove the integrity of the frameworks after washing and drying. The material keeps intn class="Chemical">act in an aqueous solution within an extensive pH range of 3–13 for 1 and 4–12 for 2 (Figure ). The above experiments show that the two compounds have high thermal and chemical stability. Moreover, to determine the leaching amount of Pb2+ ions from the lead phosphonates, the atomic absorption spectroscopy (AAS) test was carried out on the two compounds. And, the experimental result indicates that there does not exist Pb2+ ions in the filtrate, so the two compounds are nontoxic in an aqueous solution, which can been used to detect some environmental and biological ions or molecules.

Figure 6

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

Luminescent Properties

The luminescent properties of compound 1, H2L1, and n class="Chemical">H3ssc were examined at the excitation wavelength of 325 nm at room temperature (Figure S3). The H2L1 exhibits weak emission bands at λmax = 428 nm, whereas H3ssc exhibits a strong emission band at λmax = 372 and 439 nm, which can be due to intraligand n → π* and π → π* charge transfer (Figure S3c). In the case of compound 1, a large red-shifted photoluminescence with the main emission at 405 and 452 nm are observed, which are red-shifted about 33 and 13 nm compared to that of free H3ssc (Figure S3), which indicates that the luminescence behavior is attributed to ligand-to-metal charge transfer between the delocalized π bonds of the H3ssc group and the p orbit of Pb2+ ions, and the red-shifted effect may be due to both coordination or excited metal-perturbed intraligand state.[36] The solid luminescent properties of compound 2, H4L2, and NaH2bts were investigated at the excitation wavelength of 325 nm at room temperature (Figure S4). H4L2 exhibits weak emission bands at λmax = 442 nm and NaH2bts exhibits a strong emission band at λmax = 438 nm, which can be due to the π → π* transition for NaH2bts. Compound 2 shows two fluorescent emission centered at 380 nm and 400 nm, which can be due to ligand-to-metal charge transfer between the delocalized π bonds of the NaH2bts group and the p orbit of the Pb2+ ions. Blue-shifts of the emission band of 2 is observed when compared to the emission band of NaH2bts, which indicates that the luminescence behavior is closely related to the coordinated environment of the Pb2+ ions.[37]

Recognition Properties toward Thymine (T)

Recently, the recognition of biological small molecules by metal phosphonates has attrn class="Chemical">acted considerable attention. To study the possibility of compound 1 for sensing of the biological molecules, five nucleobases as analytes are investigated. In the five kinds of nucleobases, the guanine (G) and adenine (A) belong to the purine family with a double-loop structure, whereas uracil (U), cytosine (C), and thymine (T) belong to the pyrimidine compounds with six-membered heterocyclic structures (Figure ). To begin with, five kinds of different nucleobases were soaked in NaOH aqueous solutions, and the suspensions were obtained by adding 2.0 mg powders of 1 into 2.00 mL NaOH aqueous solutions. Then, 0.5 mL of the prepared nucleobases aqueous solutions (1 × 10–2 M) was added into the above suspensions and the luminescence spectra were measured. Interestingly, the luminescence intensities of compound 1 show a degree of quenching upon interaction with five nucleobases, whereas thymine causes the most obvious changes (Figure a). Above 78% of QE (quenching efficiency: (I0 – I)/I0 × 100%, I0 and I represent the luminescence intensity of 1 before and after the addition of nucleobases, respectively) can be obtained with the addition of only 5 × 10–6 mol of T.

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.

(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 n class="Chemical">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.

To assess the sensitivity of compound 1 toward T in detail, the luminescence quenching of compound 1 shows a regular variation following the piecemeal addition 1 × 10–2 M of T from 0 to 500 μL (Figure b), and other four analytes produce no obvious changes (Figure S5). To further study the correlation between the degree of fluorescence quenching and the T concentration, the quenching effect is determined by the Stern–Volmer (SV) equation: I0/I = 1 + Ksv[M] (n class="Chemical">Ksv and [M] represents the quenching rate constant and molar concentration of nucleobases, respectively), and the Ksv is calculated from the curve of relative luminescence intensity (I0/I) against the concentration of nucleobases.[38] For T molecule, the Ksv value is calculated as 1.75 × 103 M–1, the regularity of concentration and fluorescence intensity for the other nucleobases was not obvious (Figure c). Interference experiments for compound 1 are performed by the addition of thymine into the mixed solutions of compound 1 and other four analytes (Figure d). As a result, the quenching efficiency of T in the presence of other four analytes remained nearly the same (Figures S6a–S9a). The Ksv value are calculated as 1.49 × 103, 1.43 × 103, 1.67 × 103, and 1.58 × 103 M–1 for T into the mixed solutions of compound 1 and other four analytes by the above formula (Figures S6b–S9b). The results reveal that the selective detection of T in the existence of other analytes solutions makes compound 1 as a sensitive material for T. The change in the luminescence intensity is linear related to the T amount at low concentration, with the detection limit of the material calculated to be 8.26 × 10–7 M (Figure S10).

Mechanism of Luminescence Quenching (T)

To better understand the fluorescence quenching effect of compound 1 on thymine, the quenching mechanism was studied. First, the powder XRD pattern of compound 1 after sensing of the T was very well consistent with the parent compound 1, ruling out that the collapse of the main framework of compound 1 induces variation in the luminescence intensity (Figure S11). Second, considering that n class="Chemical">uracil, cytosine, and thymine have smaller sizes than guanine and adenine, the nucleobases with smaller size may enter into the interlayer of compound 1 more easily, thus showing fluorescence quenching ability to compound 1. The sizes of 5 kinds of nucleobases were measured with Gaussian 09 (Figure ). Third, UV–vis absorption spectra of compound 1 were observed to appear as a new absorption band at 290 nm after adding pyrimidine analytes (uracil, cytosine, and thymine), which indicates that they form a ground-state complex between compound 1 and pyrimidine analytes (Figure S12).[39,40] However, when the concentration of thymine increases, no new emission peaks appear, further indicating that the ground-state complex does not emit luminescence (Figure S13). Compound 1 and thymine formed a ground-state complex, reducing the concentration of compound 1, resulting in a decrease in the fluorescence intensity of the system.[41,42] The luminescent quenching caused by T may be due to three actions: (1) the uncoordinated oxygen atoms in sulfosalicylic acid may form hydrogen bonds with thymine;[43] (2) thymine has a planar structure that may form a π–π stacking interaction with benzene ring in sulfosalicylic acid;[44] and (3) it is noteworthy that T has more one methyl than U, so C–H/π plays a very important role in molecular recognition. Compared with other pyrimidine molecules, the methyl of thymine molecule may form a C–H/π interaction with the benzene ring of 5-sulfosalicylic acid (Figure ).[45−47]

Figure 9

Structures of nitrogenous bases and the possible weak interaction mechanisms of Hssc2–.

Recognition Properties toward VO3–

The powder sample of compound 2 (2 mg) was immersed in 2 mL of deionized water and 0.5 mL n class="Chemical">salt solutions of sodium (1 × 10–2 M) (OH–, NO3–, F–, SO42–, HCO3–, Ac–, C6H5COO–, Cl–, Hbts2–, SiO32–, H2PO4–, S2O42–, CO32–, WO42–, and VO3–) was added into the above suspensions, which were then tested by luminescence analyses. The luminescent measurements indicate that different anions have different influence on the luminescent intensity of compound 2. The fluorescence intensity of compound 2 did not change significantly for fifteen anions (OH–, NO3–, F–, SO42–, HCO3–, Ac–, C6H5COO–, Cl–, Hbts2–, SiO32–, H2PO4–, S2O42–, CO32–, and WO42–), but the decrease in the luminescence intensities could be observed in the presence of VO3– anion. Compared with the initial compound 2, the luminescence intensity of VO3– @ compound 2 is reduced by 92% in the same test condition (Figure a).

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.

(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 n class="Chemical">VO3– concentration; and (d) the luminescence intensity of compound 2 upon the addition of different ions followed by VO3– anion.

The fluorescence quenching of compound 2 is significantly affected by gradual addition of VO3– (Figure b). To further study the correlation between the degree of fluorescence quenching and the n class="Chemical">VO3– concentration, the quenching effect is determined by the Stern–Volmer (SV) equation: I0/I = 1 + Ksv[VO3–] (Ksv and [VO3–] represents the quenching rate constant and molar concentration of VO3–, respectively), and the Ksv is calculated from the curve of the relative luminescence intensity (I0/I) against the concentration of VO3–. The Ksv value is calculated as 7.35 × 103 M–1 for compound 2, which has a strong quenching effect (Figure c).[48] To further study the interaction between compound 2 and VO3– anion, we added compound 2 to the other fifteen kind of ion solutions and then added VO3– anion to the mixed solutions. As a result, the intensity of emission peak for compound 2 quickly reduces upon further addition of VO3– (Figure d). The above results reveal that the highly selective sensing of VO3– in the existence of other analytes solutions make compound 2 a sensitive sensor for VO3–.

Mechanism of Luminescence Quenching (VO3–)

As far as we know, the fluorescent quenching mechanisms of detecting heavy metal anions were due to collapse of the main framework, competitive absorption of energy or resonance energy transfer in the n class="Chemical">metal phosphonates. So, we carried out the following experiments of compound 2. First, the XRD of compound 2 after sensing of the VO3– anion was performed; the skeletons of compound 2 remain intact after detecting VO3– anion (Figure S14), ruling out that the collapse of the main framework of compound 2 induces variation in the luminescence intensity. Second, if the absorption band of the analytes overlap effectively with that of compound 2, the competitive absorption of the irradiated light can cause luminescent quenching between the compound and the analytes. So, the UV–vis spectra of various anions have been determined to check this mechanism. The VO3– anion in an aqueous solution shows a broad absorption band from 260 to 310 nm, which overlaps a little with that of compound 2 (200–270 nm), indicating that the VO3– anion could not absorb the excitation energy of compound 2 (Figure S15).[49] So, the competitive absorption is not the main reason for the luminescent quenching of compound 2. Third, the emission spectrum of the fluorophore partly overlaps the absorption spectra of the analytes, so resonance energy transfer may occur between fluorophores and analytes, which can greatly reduce the luminescent intensity.[50,51] As shown in Figure S16, the emission spectra of compound 2 partially overlap with the absorption spectra of the VO3– aqueous solution, which indicates that the resonance energy transfer should be the main reason for the luminescent quenching of compound 2. Moreover, the fluorescence lifetimes of compound 2 were reduced from 1.59 to 0.38 ns in the presence of 200 μL VO3– aqueous solution, further indicating that there are interactions between compound 2 and VO3– anion (Figure S17).

Conclusions

Two luminescent lead phosphonates with 2D layer and 3D framework structures have been prepared via hydrothermal techniques. The two compounds have good thermal stability and excellent chemical stability with high n class="Chemical">acid and base resistance in a wide pH range. Moreover, the AAS 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. Compound 1 emerged as a highly selective and sensitive sensor to detect the thymine molecule with a very low detection limit of 8.26 × 10–7 M. Furthermore, the material also can identify the thymine molecule in the presence of other nucleobases. The quenching mechanisms may be mainly attributed to the hydrogen bonds, π–π stacking interactions, and C–H/π interactions between thymine and H3ssc of compound 1. Meanwhile, compound 2 can be regarded as a highly selective and sensitive luminescent sensor for detecting VO3– through the fluorescent quenching effect, and it can identify VO3– at lower concentration (1.52 × 10–7 M) with the presence of other interfering anions. The quenching mechanisms indicate that the resonance energy transfer should be the main reason for the luminescent quenching of compound 2. This work will encourage us to design and synthesize some novel metal phosphonates and investigate their structures and the recognition properties in the future.

Experimental Section

Materials and Characterizations

O(CH2CH2)2NCH2PO3H2 (n class="Chemical">H2L1) and H2PO3CH2NH(C2H4)2NHCH2PO3H2 (H4L2) were prepared by the Mannich reaction according to methods of the literature.[52] All other chemical reagents were purchased from commercial sources without further purification. The elemental analyses of C, H, and N were operated with the PE-2400 elemental analyzer. The thermogravimetric (TG) analyses were carried out using a PerkinElmer Pyris Diamond TG–differential thermal analysis thermal analyzer in static air on polycrystalline samples with a heating rate of 10 K min–1 from 50 to 1000 °C. IR spectroscopy test was performed on the Bruker AXS TENSOR-27 FT-IR spectrometer using KBr pellets in the range 4000–400 cm–1. The studies for the UV–vis spectroscopic test were achieved on the Lambda 35 spectrophotometer. The Powder XRD data were obtained on a Bruker D8 Advance diffractometer with a Cu Kα radiation (λ = 1.5418 Å). The test for the leaching amount of Pb2+ ions were determined quantitatively by atomic absorption spectroscopy (AAS) on the HITACHI Z-2300 instrument. The luminescent experiments of compounds 1 and 2 in the solid state or aqueous suspension for luminescence analyses were carried out on the HITACHI F-7000 spectrofluorimeter at 300 K.

Synthesis

Synthesis of Pb3[(L1)2(Hssc)(H2O)2] (1)

A mixture of H2L1 (0.2 mmol, 0.04 g), n class="Chemical">H3ssc (0.1 mmol, 0.03 g), and Pb(Ac)2·3H2O (0.2 mmol, 0.08 g) was dissolved in 10 mL distilled water with continuous stirring for about 30 min at 300 K. The resulting solution (pH = 4) was put into a 20 mL stainless steel reactor with a Teflon liner and heated at 120 °C for 5 days and then colorless block crystals were collected. The yield of compound 1 was about 80.2% (based on Pb). Anal. calcd for C17H28N2O16P2Pb3S (%): C, 16.57; H, 2.29; N, 2.27; S, 2.60. Found (%): C, 16.60; H, 2.34; N, 2.25; S, 2.69. IR (KBr cm–1): 3398(m), 1568(m), 1447(m),1006(s), 585(w) (Figure S18a). The diffraction peaks of the single-crystal simulated XRD patterns could be well consistent with the powder XRD patterns, which indicated compound 1 was the pure phase (Figure S19a).

Synthesis of [Pb2(L2)0.5(bts)(H2O)2]·H2O (2)

A mixture of H4L2 (0.25 mmol, 0.07 g), n class="Chemical">NaH2bts (0.5 mmol, 0.14 g), and Pb(Ac)2·3H2O (0.75 mmol, 0.28 g) was dissolved in 10 mL distilled water with continuous stirring for about 30 min at 300 K. The resulting solution (pH = 4) was put into a 20 mL stainless steel reactor with a Teflon liner and heated at 140 °C for 5 days and then the colorless block crystals were collected. The yield of compound 2 was about 86.2% (based on Pb). Anal. calcd for C11H16NO13PPb2S (%): C, 15.59; H, 1.90; N, 1.65; S, 3.78. Found (%): C, 15.54; H, 1.85; N, 1.62; S, 3.86. IR (KBr cm–1): 3456(m), 1568(m), 1473(m), 1104(s), 652(w) (Figure S18b). The diffraction peaks of the single-crystal simulated XRD patterns could be well consistent with the powder XRD patterns, which indicated compound 2 was pure phase (Figure S19b).

Crystallographic Studies

X-ray diffraction data for the two compounds were collected on a Bruker AXS Smart APEX II CCD X-diffrn class="Chemical">actometer with graphite-monochromated Mo Kα (λ = 0.71073 Å) at 300 K. The structures were solved by direct methods and refined on F2 by full-matrix least-squares methods by using the SHELX 97 program.[53] All nonhydrogen atoms were refined anisotropically. The H atoms of organic ligands were refined isotropically as a riding mode. Details of crystallographic data, selected bond distances, and angles of the two compounds are listed in Tables S1–S3, respectively. CCDC 1587687 and 1843247 contain the supplementary crystallographic data of this article.

Fluorescence Measurements

In a typical experimental setup, five kinds of different nucleobases were soaked in n class="Chemical">NaOH aqueous solutions, and the suspensions were obtained by adding 2.0 mg powders of 1 into 2.00 mL NaOH aqueous solutions. The fluorescence upon excitation at 325 nm of compound 1 suspension was measured in situ after incremental addition of freshly prepared nucleobases solutions (1 × 10–2 M, 50 μL addition each time).

The powder sample of compound 2 (2 mg) was immersed in 2 mL of deionized water and then the n class="Chemical">salt solution of sodium (1 × 10–2 M) (OH–, NO3–, F–, SO42–, HCO3–, Ac–, C6H5COO–, Cl–, Hbts2–, SiO32–, H2PO4–, S2O42–, CO32–, WO42–, and VO3–) was added into the above suspensions, which then were tested by luminescence analyses. The fluorescence upon excitation at 280 nm of compound 2 suspension was measured in situ after incremental addition of freshly prepared analyte solutions (1 × 10 –2 M, 50 μL addition each time).