Dinuclear Lanthanide Compound as a Promising Luminescent Probe for Al³⁺ Ions

Abstract

Luminescent probes have wide applications in biological system analysis and environmental science. Here, one novel luminescent dinuclear europium compound with a crown ether analogous ligand was synthesized through a solvent–thermal reaction. Through transformation, upon the addition of Al³⁺ ions to the N,N′-dimethyl formamide solution of the europium compound, the luminescent intensity of the characteristic emission of Eu³⁺ decreased, and a new emission peak appeared at 346 nm and increased rapidly. The luminescent investigation indicated that it could act as a highly sensitive and selective luminescent probe for Al³⁺ ions. Moreover, mass spectrometry and single-crystal X-ray diffraction confirmed the formation of a new more stable trinuclear aluminium compound during the sensing process.

1. Introduction

In past decades, interest in the photophysical properties of lanthanide-based compounds has been strongly stimulated [1,2,3,4]. Benefiting from the narrow characteristic emissions resulting from f–f transition of lanthanide ions and few perturbations from environments, lanthanide-based compounds have wide applications in biological system analysis and environmental science [5,6,7]. Therefore, lanthanide luminescent probes have become one of the most important methods for sensing ions due to their excellent monochromaticity and high sensitivity [8,9]. Various lanthanide luminescent probes have been reported, which mainly focus on H⁺ [10], F⁻ [11], K⁺ [12], Ag⁺ [13], Ca²⁺ [14], Zn²⁺ [15], Mg²⁺ [16], O₂ [17], H₂O₂ [18], ATP [19], etc.

Moreover, due to the rapid development of metal–organic frameworks (MOFs) [20], several lanthanide metal–organic frameworks (Ln-MOFs) as important luminescent sensors have been widely investigated due to their advantages such as controllable pore sizes, and the diversity of functional organic ligands for interaction recognition [21,22,23,24,25]. However, due to poor solubility, the applications of MOF-based probes are still limited, especially for biologic imaging. Thus, soluble discrete lanthanide metal–organic assemblies would be quite appropriate for this application. However, due to the unpredictable coordination behavior and the lability of the lanthanide coordination bonds, the controllable synthesis of functional polynuclear discrete lanthanide metal-organic assemblies remain challenging [26]. Thus far, relevant reports on discrete polynuclear lanthanide metal–organic assemblies for luminescent probes are still rare [27,28,29,30].

On the other hand, Al³⁺ ions are harmful to the human brain and nervous system, causing Parkinson’s and Alzheimer’s disease [31,32], and they also adversely affect the growth of plants [33]. Thus, the development of a method for the effective detection of Al³⁺ ions is urgent. To date, several chemical sensors based on organic compounds [34,35,36,37,38] and MOFs [39,40,41] for Al³⁺ ion detection have been reported. However, sensors based on discrete lanthanide metal–organic assemblies are rarely reported. In this study, using crown ethers analogous carboxylic ligand 2,2′-(((ethane-1,2-diylbis(oxy))bis(ethane-2,1-diyl))bis(oxy))dibenzoic acid (H₂TEBA), a novel dinuclear lanthanide compound (1, [Na₄Eu₂(TEBA)₄(H₂O)₄]·[CuCl₂]·Cl·H₂O) was synthesized. Luminescent investigations revealed that it is a promising luminescent probe for Al³⁺ ions. Moreover, the sensing mechanism was studied using mass spectrometry and single-crystal X-ray diffraction and a transformation process was confirmed.

2. Results

2.1. Synthesis and Structure of Compound 1

Compound 1 was prepared under solvent-thermal condition. Details of the synthesis are presented in the Materials and Methods. Single-crystal X-ray diffraction analysis revealed that compound 1 belongs to the triclinic P$\overline{1}$ space group. The asymmetric unit in compound 1 consists of two dinuclear [Na₄Eu₂(TEBA)₄(H₂O)₄]²⁺ units, two [CuCl₂]⁻ anions, two disorder Cl⁻, and one lattice water (Figure S1). Each Eu³⁺ ion is coordinated by eight O atoms from different TEBA²⁻ ligands. The adjacent two Eu³⁺ ions form a dinuclear unit through the bridges of carboxyl groups from four TEBA²⁻ ligands (Figure 1). The distance between the two Eu³⁺ ions is 4.13 Å. The Na⁺ ion located in the analogous crown ether structure is formed by one TEBA²⁻ ligand, which affords four ether O atoms and two carboxyl O atoms to chelate the Na⁺ ion. Moreover, with one coordinated H₂O molecule and one carboxyl O atom from another TEBA²⁻ ligand, the Na⁺ ion is eight-coordinated. The Eu–O and Na–O distances are in the range of 2.30–2.58 Å and 2.30–3.01 Å, respectively. The [Cl–Cu–Cl]⁻ anions and the lattice water fill in the space among the [Na₄Eu₂(TEBA)₄(H₂O)₄]²⁺ units (Figure S1).

2.2. Luminescent and Sensing Properties of Compound 1

To investigate the luminescent properties of compound 1 in N,N′-dimethyl formamide (DMF) solution (1 × 10⁻⁴ mol/L), emission spectrum measurements were performed at room temperature and excited by a UV light with a wavelength of 292 nm. As shown in Figure 2, typical emission peaks of Eu³⁺ ions can be observed, which can be attributed to ⁵D₀→⁷F₁ (594 nm), ⁵D₀→⁷F₂ (618 nm), and ⁵D₀→⁷F₄ (700 nm) transitions (black curve in Figure 2). The luminescent lifetime of the ⁵D₀→⁷F₂ transition is 0.33 ms (Figure S3). The intensity of the ⁵D₀→⁷F₂ transition (electric dipole) is much stronger than the intensity of the ⁵D₀→⁷F₁ transition (magnetic dipole), which indicates that the coordination environment of the Eu³⁺ ion is asymmetric, in agreement with the results from the crystallographic analysis (Figure 1).

Upon the addition of different cations (Al³⁺, Ca²⁺, Cd²⁺, Co²⁺, Cu²⁺, Fe³⁺, K⁺, Mg²⁺, Ni²⁺, and Pb²⁺) to compound 1 in the DMF solution (1 × 10⁻⁴ mol/L), the emission intensities of Eu³⁺ ions (such as the peak at 618 nm) become weaker to some extent (Figure 2 and Figure S4). Interestingly, after the addition of Al³⁺ ions to the solution, the emission intensity at 346 nm increases rapidly. Up to 2 equiv Al³⁺ ions with respect to compound 1, the intensity of the peak at 346 nm becomes around 43 times stronger than that of the original peak. This sensing process containing both a new increasing luminescent peak and a decreasing characteristic emission of Eu³⁺ is a typical OFF-ON and ON-OFF mode. The difference between two peaks is 272 nm, which improves the sensitivity of the sensing process. The emission intensity at 346 nm exhibits a very good linear relationship with the equivalent addition of Al³⁺ ions with a correlation coefficient r = 0.999 (Figure S5). When other metal ions were added, there were no significant increase at 346 nm (Figure 3). This implies that compound 1 can determine the concentration of Al³⁺ ions within a certain concentration range. Furthermore, additional sensing characterizations for lower concentrations of Al³⁺ ions were performed to determine the lowest limit of detection (Figure S6). The intensity of emission at 346 nm was almost the same when the concentration of Al ions was below 1 × 10⁻⁶ M. However, when the concentration of Al³⁺ ions reached 5 × 10⁻⁶ M, an increase in intensity could clearly be observed. These results show that the detection limit of 1 for sensing Al³⁺ ions was about 5 × 10⁻⁶ M, exhibiting a good sensitivity for Al³⁺ ions [40].

2.3. Sensing Mechanism Studies

The sensing behavior of compound 1 may be attributed to the direct transformation from a compound containing Eu³⁺ to a new compound containing Al³⁺ [42,43,44]. To confirm this suspicion, mass spectrometry and single-crystal X-ray diffraction were performed to verify the new aluminum compound. To investigate the ionic state of compound 1, electrospray ionization mass spectrometry (ESI-MS) (Figure S7) and matrix assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS) (Figure S8) were applied. ESI-MS results for compound 1 in DMF show the main peaks of H₂TEBA + Na⁺ ([C₂₀H₂₂O₈Na]⁺ calcd: 413.12; found: 413.12) rather than the peaks of compound 1 (Figure S7). This can be attributed to the structural destruction of compound 1 due to the high energy of electrospray ionization process. When MALDI-TOF MS was applied, it showed a main fragment [Na₂Eu(TEBA)₂]⁺ (C₄₀H₄₀EuNa₂O₁₆ calcd: 975.13; found: 975.13) from compound 1 (Figure S8). After luminescent intensity of the DMF solution of compound 1 no longer increased upon the addition of Al³⁺ ions, ESI-MS measurements of this solution were performed, and a new m/z 1261.28 emerged (Figure S9). Compound 1 was destroyed during the ESI-MS measurement and a new peak appeared after the addition of Al³⁺ ions; thus, we speculate that the new peak resulted from a newly formed aluminum compound, which was more stable than compound 1.

To further determine the origin of the new peak and the structural information of the Al³⁺ compound, we used AlCl₃·6H₂O instead of CuCl₂·2H₂O under the same synthesis conditions as compound 1, and colorless long-stripe-like crystals were obtained. Single-crystal X-ray diffraction confirmed that it was a trinuclear aluminum compound with a molecular structure of [Al₃(μ₃-O)(TEBA)₃(H₂O)₃]₂·[Eu(NO₃)₅]·EtOH·0.5H₂O (2). Compound 2 crystallizes in triclinic P$\overline{1}$ space group. Each asymmetric unit consists of two [Al₃(μ₃-O)(TEBA)₃(H₂O)₃]⁺ units, one [Eu(NO₃)₅]²⁻ anion, one lattice disordered ethanol molecule and half a lattice water molecule (Figure S2). The Al³⁺ ion is six-coordinated by four carboxyl O atoms from four different carboxyl groups, one coordinated water molecule and one μ₃-O²⁻ atom, forming an octahedral geometry (Figure 4). Three Al³⁺ ions form a stable trinuclear cluster via the bridge of the μ₃-O²⁻ atom and three TEBA²⁻ ligands. One water molecule coordinates to each Al³⁺ ion, and is located in the center cave of the TEBA²⁻ ligand. Each coordinated water molecule forms two hydrogen bonds with two ether O atoms. A dissociative [Eu(NO₃)₅]²⁻ anion and two trinuclear Al³⁺ clusters balance the charge. The Eu³⁺ ion is ten-coordinated by ten O atoms from five different NO^3- ions, leading to a dodecahedron geometry. Through π–π interactions and van der Waals forces between two [Al₃(μ₃-O)(TEBA)₃(H₂O)₃]⁺ units and electrostatic interactions among [Eu(NO₃)₅]²⁻ anions, a three-dimensional packing structure forms.

The ESI-MS result of compound 2 in DMF solution displays a main peak at m/z 1261.27 (Figure S10), which is consistent with the results of the mixture of compound 1 and Al³⁺ ions (m/z 1261.28, Figure S9) as well as the theoretical value of [Al₃(μ₃-O)(TEBA)₃]⁺ (C₆₀H₆₀Al₃O₂₅ calcd: 1261.29). The results indicate a transformation process. Upon the addition of Al³⁺ ions to compound 1 in DMF solution, the [Na₄Eu₂(TEBA)₄]²⁺ units decompose and a more stable species [Al₃(μ₃-O)(TEBA)₃(H₂O)₃]⁺ forms with an increasing emission peak at 346 nm. To further explore this sensing behavior, the luminescence of the H₂TEBA ligand and compound 2 were investigated (Figures S11 and S12). When excited at 292 nm in DMF solution, the H₂TEBA ligand exhibits an emission peak at 340 nm, which is close to the emission peak (346 nm) of compound 2 (Figure S12) and the mixture of Al³⁺ ions and compound 1 in DMF (Figure 2). When setting the emission peak at 340 nm, the excitation spectrum of the H₂TEBA ligand shows two peaks at 268 and 312 nm, which are different from the spectra of compound 1 at 292 nm and compound 2 at 296 nm (Figure S13). The results indicate that after being coordinated to Al³⁺ ions, the luminescence of compound 2 exhibits a slight red-shift compared with the ligand.

3. Materials and Methods

All reagents and solvents were commercially available and used as received without further purification. Analysis of C, H and N were carried out on an elementar vario EL elemental analyzer. The FT-IR spectra were measured with a Bruker Tensor 27 Spectrophotometer (Bruker, Karlsruhe, Germany) on KBr disks. The emission spectra in the visible region were measured on a Cary Eclipse fluorescence spectrophotometer (Agilent Technologies Inc., Santa Clara, CA, USA). The ESI-MS spectra were measured with a VG ZAB-HS spectrometer (VG, Manchester, UK). The MALDI-TOF spectra were measured on a Bruker Autoflex III TOF/TOF200 spectrometer (Bruker, Karlsruhe, Germany) using α-Cyano-4-hydroxycinnamic acid as matrix.

Synthesis of [Na₄Eu₂(TEBA)₄(H₂O)₄]·[CuCl₂]·Cl·H₂O (1). A mixture of H₂TEBA (0.3 mmol, 117.0 mg), Eu(NO₃)₃·6H₂O (0.1 mmol, 44.6 mg), CuCl₂·2H₂O (0.2 mmol, 34.0 mg), NaOH (0.4 mmol, 16 mg) and 10 mL ethanol was sealed in 25 mL Telfon-lined stainless steel container, and heated to 160 °C for 72 h, then cooled to room-temperature (temperature decrease rate: 2 °C/h). The yellow long-stripe-like crystals were obtained in ca. 44% yield based on Eu. Elemental Analysis calcd: C, 43.67; H, 4.08%; found: C, 43.19; H, 4.19%.

Synthesis of [Al₃(μ₃-O)(TEBA)₃(H₂O)₃]₂·[Eu(NO₃)₅]·EtOH·0.5H₂O (2). A mixture of H₂TEBA (0.3 mmol, 117.0 mg), Eu(NO₃)₃·6H₂O (0.1 mmol, 44.6 mg), AlCl₃·6H₂O (0.2 mmol, 48.3 mg), NaOH (0.4 mmol, 16 mg) and 10 mL ethanol was sealed in 25 mL Telfon-lined stainless steel container, and heated to 160 °C for 72 h, then cooled to room-temperature (temperature decrease rate: 2 °C/h). The colorless long-stripe-like crystals were obtained in ca. 32% yield based on Al. Elemental Analysis calcd: C, 46.53; H, 4.45%; found: C, 45.94; H, 4.16%.

Suitable crystals of compound 1 and 2 were selected and mounted on a SuperNova, (Single source at offset) Eos diffractometer equipped with graphite monochromated Mo Kα radiation (λ = 0.71073 Å) under the temperature 120(2) K. Using Olex2 programme [45], the structures were solved with the ShelXS structure solution program using Direct Methods and refined with the ShelXL refinement package using Least Squares minimisation [46]. Both structures were treated as twinning crystal.

Crystallographic data for 1 and 2 reported in this paper have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication no. CCDC 2215277 and 2215275, respectively. This data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif (accessed on 26 October 2022).

4. Conclusions

In conclusion, a novel luminescent dinuclear europium compound with a crown ether analogous ligand was synthesized through solvent–thermal reaction and was structurally characterized. The luminescent investigations indicate that this compound is a promising luminescent probe for Al³⁺ ions. Through transformation, a new, more stable trinuclear aluminum compound was formed. The luminescent intensity of the characteristic emissions of Eu³⁺ decreased, and a new emission peak appeared at 346 nm and increased rapidly as the concentration of Al³⁺ increased. This transformation mechanism provided a novel OFF–ON and ON–OFF luminescent probe, which improved the sensitivity of this sensor. We believe that this novel probe will open a new route to the design of lanthanide luminescent probes.