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. 2023 Jun 29;8(27):24477–24484. doi: 10.1021/acsomega.3c02419

Self-Assembly of Triple-Stranded Lanthanide Molecular Quasi-Lantern Containing 2,2′-Bipyridine Receptor: Luminescence Sensing and Magnetic Property

Fan Yin †,‡, Zhi Liu †,‡, Jian Yang ‡, Li-Peng Zhou ‡, Chong-Bin Tian †,‡,*, Qing-Fu Sun †,‡,*
PMCID: PMC10339439  PMID: 37457487

Abstract

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Ln2L3-type supramolecular architectures have received significant attention recently due to their unique magnetism and optical properties. Herein, we report the triple-stranded Ln2L3-type lanthanide molecular quasi-lanterns, which are fabricated by the deprotonation self-assembly of a linear ligand featuring a β-diketone chelating claw and 2,2′-bipyridine (bpy) moiety with lanthanide ions (Ln = Eu3+ and Dy3+). The crystal structure analysis indicates that Eu3+ and Dy3+ ions are all coordinated by eight oxygen donors but in different coordination geometries. The eight oxygen donors in Eu2L3 and Dy2L3 are arranged in a square antiprism and triangular dodecahedron geometry, respectively. Taking into account the fact that the bpy moiety has a strong coordination affinity for transition metal ions, luminescence sensing toward Cu2+ ions has been demonstrated with Eu2L3, bearing a detection of limit as low as 2.84 ppb. The luminescence sensing behavior of Eu2L3 is ascribed to the formation host–guest complex between Eu2L3 and Cu2+ ions with a 1:2 binding ratio. Dynamic AC susceptibility measurements for Dy2L3 reveal the relaxation of magnetization in it. This work provides a potential way for design and fabrication of lanthanide-based molecular materials with functions endowed by the ligands.

Introduction

Over the past few decades, discrete architectures generated by the coordination-directed self-assembly approach, including macrocycles, planes, knots, and polyhedra, have experienced rapid and violent development and have been one of the most active research areas of supramolecular chemistry.1−9 Among these architectures, the dinuclear M2L3-type (M, metal; L, ligand) triple-stranded helicates or lantern-like metalla-cages formed by the self-assembly of C2-symmetric ligands with metal centers are particularly appealing due to their potential applications in the field of magnetism,10−12 luminescence,13−16 bionics,17,18 catalysis,19,20 encapsulation,21,22 selective separation and recognition,23 and so on. Since Lehn and co-workers introduced the term helicate into supramolecular chemistry in 1987,24 a considerable number of M2L3-type architectures bearing triple-stranded helicate or lantern-like morphologies have been developed.25−28 As far as we are aware, most of such M2L3-type architectures were fabricated by transition metals and only a few cases by lanthanides.27,29 This is not surprising if we consider that lanthanides usually have high kinetic lability, weak coordination ability, variable coordination numbers, and lack of stereochemical affinity compared with the transition metals containing a strong coordination-directed feature. All those shortcomings make it difficult to construct Ln2L3-type (Ln, lanthanide) architectures. Despite this, the construction of Ln2L3-type assemblies deserves to be explored as the intrinsic properties of lanthanides, such as magnetism and optics, have shown to be excellent candidates as single molecular magnets and luminescence sensors.30−34 In the efforts of Piguet et al.,25,35,36 Bünzli et al.,37,38 Hamacek et al.,39 Hooley et al.,40,41 Duan et al.,42,43 Yan et al.,44,45 our group,21,46,47 and others,48−51 several Ln2L3-type assemblies had been reported. To date, such reports are mainly focused on the investigation of chiral, luminescence, structure evolution, and aggregation of Ln2L3, while other functional explorations are still in their initial stage, in particular for the function stemming from the ligands.

It had been realized that the 2,2′-bipyridine (bpy) moiety is an excellent metal chelator in coordination polymer or metal–organic framework synthesis and can be adopted as a sensor when it is on its own as a luminescence emitting center or in combination with other luminescence emitting centers.52−56 Additionally, the β-diketone chelating claw can well sensitize the luminescence emission of Eu3+ ions15 and once complexation with Dy3+ ions is likely to afford the single-molecular magnet (SMM) behavior.31,57 By combining the bpy moiety and β-diketone chelating claw into one ligand, the recognition toward transition metals and possible SMM properties can be anticipated upon complexation with corresponding lanthanides. Bearing this in mind, herein, a new Ligand H2L was designed by incorporating the bpy moiety into a C2-symmetric linear shape ligand with two β-diketone chelating claws located at the two ends. After assembly of H2L with Ln (Ln = Eu3+ and Dy3+) ions, the triple-stranded Ln2L3 assemblies bearing an elongated lantern-like architecture were obtained. The luminescence probe for sensing Cu2+ ions has been demonstrated on Eu2L3 assembly, which is attributed to the coordination of Cu2+ ions with free bpy sites. Moreover, alternating current susceptibility measurements under a zero dc field indicate that Dy2L3 shows magnetic relaxation.

Results and Discussion

Ligand Design and Synthesis

The ligand H2L containing the 2,2′-bipyridine functional unit was prepared by a multi-step procedure, as shown in Scheme S1, and characterized by NMR. Compound 5,5′-bis(bromomethyl)-2,2′-bipyridine was synthesized according to the reported procedures.58 The 5,5′-bis(bromomethyl)-2,2′-bipyridine was reacted with p-hydroxyacetophenone via nucleophilic substitution reaction, leading to the formation of intermediate 5,5′-bis-4-acetylphenoxymethyl-2,2′-bipyridine. The ligand H2L was then obtained after Claisen condensation of this intermediate with methyl trifluoroacetate. The experimental details can be found in the Supporting Information (Figures S1–S5).

Self-Assembly and Characterization of Eu2L3

The Eu2L3 was synthetized by the assembly of ligand H2L and Eu(OTf)3 in a molar ratio of 3:2 with the trimethylamine (Et3N) as the base in methanol (Scheme 1). The 1H NMR spectrum suggests the formation of a single complex with a high degree of symmetry. Compared with the free ligand, all signals arising from the triple-stranded Eu2L3 (Figure 1B) assembly are minutely shifted downfield and broadening due to the coordination with weak paramagnetic Eu3+ ions, which can be attributed to lanthanide-induced shifts (LIS) and relaxation rate enhancement (LIR) effect.13,59 The high symmetry of the Eu2L3 is further confirmed by 1H–1H COSY spectra (Figure S7), which indicates that three ligands on the Eu2L3 share identical magnetic-chemical environments in DMSO-d6.

Scheme 1. Synthetic Route of the Ln2L3.

Scheme 1

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Light red: coordination oxygen atoms, light blue hand: bpy, sky blue: the ligand backbones, green: lanthanide ions.

Figure 1.

Figure 1

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(A) Molecular structure diagram of Eu2L3. (B) 1H NMR (400 MHz, 298 K) of H2L and Eu2L3 in DMSO-d6.

Besides the observed triple-stranded Eu2L3 architecture, another possible architecture constructed by such bidentate linear ligand H2L is quadruply stranded Eu2L4. To gain the possible species Eu2L4, the ratio of H2L to metal ions (Eu3+) increased from 3:2 to 4:2. Unfortunately, the assembly of H2L, Eu(OTf)3, and Et3N in 4:2:8 stoichiometry afforded Eu2L3 other than Eu2L4, as suggested by its 1H-NMR spectrum, which is identical with the Eu2L3 (Figure S8). This result is significantly different with the observation of Yan et al.14 that the quadruply stranded lanthanide structure can be formed by mixing the similar ligand with Ln3+ in 4:2 stoichiometry. The difference between H2L and Yan et al.’s ligand lies in their linker. In comparison to the flexible 1,2-propanediol linker in Yan et al.’s ligand, the linker in H2L was relative rigid bpy, which gives rise to the decreased flexibility of H2L over Yan et al.’s ligand. Such decreased flexibility should be responsible for the formation of the Eu2L3 architecture instead of the four-stranded Eu2L4 architecture. Therefore, it can be inferred that the flexible feature of the ligand linker plays an important role in determining the formation of triple-stranded Ln2L3 or quadruply stranded Ln2L4.

Crystal Structure Description

Slow volatilization of the Eu2L3 DMF solution at room temperature for 3 months results in the generation of colorless plate-like single crystals Eu2L3, which were suitable for X-ray diffraction measurement. To shorten the crystal growth time, Dy2L3 single crystals were prepared by slow vapor diffusion of CH2Cl2 into the DMSO solution of Dy2L3 for about 2 weeks. Single-crystal X-ray diffraction analysis indicates that both of Eu2L3 and Dy2L3 are isomorphous compounds, crystallizing in the space group P-1 with Z = 2 and featuring an elongated lantern-like triple-stranded core structure with different coordination solvents, i.e., DMF for Eu2L3 and DMSO for Dy2L3. Therefore, the structure description of compound Eu2L3 will be given here as a representation. Its asymmetric unit contains only an Eu2L3 molecule, which consists of two Eu3+ ions, three L2– anions, and four coordinated DMF molecules. Each Eu1 and Eu2 atom is ligated by eight oxygen atoms, among which six oxygen atoms arising from three β-diketone chelating claws and the other two oxygen atoms from two coordinated DMF molecules (Figure S12A). Like the coordination environment of Eu1 and Eu2 in Eu2L3, the Dy1 and Dy2 in Dy2L3 are also coordinated by eight oxygen atoms (Figure S12B). However, the Eu3+ and Dy3+ ions adopt different coordination geometries, that is, square antiprism (Figure 2B) and triangular dodecahedron (Figure S12C) coordination geometry, respectively, as confirmed by the calculations of continuous shape measures (CShM) (Table S4).60 Eu–O bond distances were in the range of 2.33–2.43 Å, which is comparable with other related compounds.61 The important bond lengths and angles of Eu2L3 are summarized in Tables S2 and S3. Every Eu2L3 connects to its neighboring ones through intermolecular π–π interactions in three dimensions, resulting in the formation of a supramolecular framework (Figure S13). Additionally, intermolecular hydrogen bonds are also observed in Eu2L3 (Figure S14). The distances of Eu···Eu within the molecule are 26.14 Å, and the nearest intermolecular Eu···Eu distances are 8.09 Å. Compared with the previously reported examples fabricated by the similar linear β-diketone ligands,44,45 which usually possess triple-stranded helical structure characteristic, the helical feature disappeared in our cases. Given that the bpy groups are coplanar units and easy to form π–π interactions among different molecules, we suggest that the absence of helix may be attributed to the introduction of the bpy unit, which weakens the spiral synergy of the three chains within one molecule.

Figure 2.

Figure 2

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(A) View of the molecular structure of Eu2L3, hydrogen atoms were omitted for clarity. (B) Coordination geometry for Eu1 and Eu2 in Eu2L3 (light red: O, blue: N, pink: Eu, green: F, gray: C).

UV–Vis, Luminescence, and Ion Recognition

The UV–vis absorption spectra of ligand H2L and compound Eu2L3 are shown in Figure 3A. Compared with the free ligand H2L, the maximum absorption wavelength of Eu2L3, assignable to the π–π* charge transfer transition from the aromatic group to the β-diketone units, is blue-shifted by about 27 nm (λ = 352 nm for H2L and 325 nm for Eu2L3). This blue shift can be ascribed to the reduction of β-diketone chelating claw conjugation after complexation with Eu3+ ions, which can be confirmed by the crystal structure of Eu2L3. Upon excitation at 325 nm, Eu2L3 displays characteristic narrow line-like emission bands at 577, 591, 612, 652, and 702 nm, corresponding to 5D0 → 7FJ (J = 0–4) transitions of Eu3+ (Figure 4A), among which the 5D0 → 7F2 transition is a typical electric dipole transition and is very sensitive to the coordination environment of the Eu3+ ion. Due to the strong interaction between deporotonated β-diketonate and Eu3+ ion and the low symmetry of the local coordination environment without an inversion center,62 the intensity of the electric dipole transition was enhanced, and strong red luminescence (visible to the naked eyes under the irradiation of a 365 nm UV lamp) is observed (Figure S15).

Figure 3.

Figure 3

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(A) UV–vis absorption spectra of ligands H2L, compound Eu2L3 and the emission spectrum of compound Eu2L3. (B) Luminescence quenching behaviors at 612 nm after the addition of 3 equiv of different transition metal ions. (C) Luminescence titration spectra (λex = 325 nm) of Eu2L3 (1.02 μΜ) with the addition of 0–3.21 μM of Cu2+ ions with the inset showing luminescence intensity change. (D) Plots of log((F0 – F)/F) as a function of the logarithm of the Cu2+ concentration; the red line represents the best fit with the modified Stern–Volmer equation.

Figure 4.

Figure 4

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(A) Plots of xmT vs T for Dy2L3 with the inset displaying M vs H/T at different temperatures for Dy2L3. (B) Temperature dependence of the out-of-phase susceptibilities for Dy2L3.

Given the metal chelator feature of the bpy moiety, the luminescence sensing properties of Eu2L3 toward transition metal ions were explored. Six typical transition metal ions, including Cu2+, Mn2+, Pb2+, Ni2+, Co2+, and Fe2+, were investigated. After the addition of transition metal ions, luminescence quenching behavior at 612 nm was distinctly observed for the investigated transition metal ions, among which Cu2+ is the best luminescence quenching ion (Figure 3B). Therefore, the Cu2+ ion was selected for luminescence titration experiments to study the mechanism of luminescence quenching by these transition metal ions (Figure 3C). In the range of 0–0.75 μM, the decrease in luminescence intensity at 612 nm displays a linear behavior with the addition of Cu2+ ions, from which the limit of detection (LOD) is evaluated to be 43.46 nM (2.84 ppb, S/N > 3) (Figure S16), lower than the value observed in most MOF probes.53 The luminescence quantum lifetimes are τ0 = 716.43 μs and τ = 711.91 μs for 0 and 3 equiv of Cu2+ ions (Figure S17), respectively. The ratio of τ0/τ is very close to 1, meaning that the quenching mechanism affiliates to static quenching because dynamic quenching gives a considerable decreased lifetime, but static quenching will not change it. For static quenching, it occurs when a ground state complex is formed through the interaction between the fluorophore and quencher. Moreover, the bpy moieties usually exhibit strong chelation preference on Cu2+. Hence, we speculate that the Cu2+ ions were coordinated to the byp unit of Eu2L3 and form a new complexation species, in which Cu2+ ions act as the quenching sites, leading to the luminescence quenching. This luminescence quenching phenomenon may be attributed to the effect of ligand to metal charge transfer (LMCT) or metal to ligand charge transfer (MLCT) on the triplet state energy level of the ligand after the formation of 1:2 host–guest complexation, which leads to the mismatch of the ligand triplet state energy level with the excited state energy level of the Eu3+ ion.

The UV–vis titration experiments were also performed to confirm our speculation (Figure S18). From Figure S18, we can see that the shoulder peak (285 nm) of π–π* transitions belonging to the bpy moiety decreased clearly with gradual addition of Cu2+ ions, and the maximum absorption peak is slightly red-shifted from 325 to 327 nm. Moreover, a new broaden absorption peak at about 355 nm appeared, which can be attributed to the ligand to metal charge transfer (LMCT) or metal to ligand charge transfer (MLCT) generated by complexation of bpy with metal ions.63 In a word, with the reduction of the absorption peak at 285 nm, the red shift of the maximum absorption band together with the appearance of the new absorption band suggests the formation of new species, in which Cu2+ ions are ligated to the bpy unit of Eu2L3, in line with our speculation. According to Cu2+ binding numbers, three possible products, Eu2CuL3, Eu2Cu2L3, and Eu2Cu3L3, can be anticipated, that is, the binding ratios between Eu2L3 and Cu2+ ions are 1:1, 1:2, and 1:3. To identify the possible binding ratio, the curve of log((F0 – F)/F) vs log([Cu2+]) (Figure 3D) was treated by the modified Stern–Volmer equation (see the Supporting Information for details).64−66 The best fit in the range of 0–3.15 equiv gives K = 2.013 × 1014 M–2, n = 2.196 (the n value is near 2 and far less than 3), revealing that the binding ratio between the host (Eu2L3) and guest (Cu2+) is 1:2. In addition, the result of Job’s plot pointed out that the change of luminescence intensity reached the maximum when the molar fraction of (Cu2+) was 0.64 (Figure S19), also corroborating the fact that the binding stoichiometry of Eu2L3 and Cu2+ is 1:2. The formation of the 1:2 host–guest compound was also testified by electrospray ionization time-of-flight mass spectroscopy (ESI-TOF-MS). After the addition of 3.0 equiv of Cu2+ ions, the presence of Eu2Cu2L3 can be obviously observed in ESI-TOF-MS, and the assignments for host–guest peaks are further demonstrated by carefully comparing the simulated ones with experimental data (Figure S20). Therefore, complex Eu2Cu2L3 with 1:2 stoichiometry between the host and guest is probably the new complexation species. The possible binding way of Cu2+ ions in Eu2Cu2L3 was proposed and is shown in Figure S21. The formation of 1:2 complexation rather than 1:1 or 1:3 complexation may be caused by the steric hindrance, strong coordination ability of the bpy moiety, and relatively flexible characteristic of ligand H2L.

Magnetic Property of Dy2L3

Direct current (dc) magnetic susceptibility measurements were performed on polycrystalline samples of the complex Dy2L3 in the temperature range of 300–2 K under an applied field of 1000 Oe (Figure 4A). The xmT products are 28.90 cm3·K·mol–1 for Dy2L3 at 300 K, which is close to a value of 28.34 cm3·K·mol–1 expected for two isolated Dy3+ ions (J = 15/2 and g = 4/3). Upon cooling, the xmT values exhibit a gradual decrease until 30 K, and then a sharp decrease appears, reaching the minimal values of 22.48 cm3·K·mol–1 at 2 K. The decrease in xmT may be caused by the thermal depopulation of the stark sublevels.67 The M–H curves of Dy2L3 were collected at 2, 3, and 5.0 K in the range of 0–8 T (Figure S22). As shown in the inset of Figure 3A, the M vs H/T plots are not superimposable and increase rapidly at low field and then slowly at high field regions but do not reach saturation even at 8 T, indicating the presence of magnetic anisotropy.

To certificate the presence or absence of relaxation of the magnetization, the dynamics of the magnetization for Dy2L3 was investigated by using alternating current susceptibility measurement under 3.0 Oe. Both the in-phase (Figure S23) and out-of-phase susceptibilities show temperature dependence behavior under a zero dc field (Figure 4B), but no peaks are observed under the measured temperature range, which may suggest the presence of slow magnetic relaxation behavior. However, the contribution of fast quantum tunneling of magnetization (QTM) relaxation cannot be excluded as it can also induce the rapid increase in ac magnetic susceptibilities at low temperature range, as was commonly observed in other lanthanide magnetic relaxation systems.68,69 To determine the energy barrier (Ueff) and relaxation time (τ0), the Debye model based on the following equation was adopted68,70

graphic file with name ao3c02419_m001.jpg 1

The best-fitting affords an energy barrier (Ueff/KB) of 10.03 K and τ0 of 1.56 × 10–5 s for Dy2L3 under a zero dc field (Figure S24). This energy barrier is comparable to previously reported triple-stranded compounds71,72 and is surpassed by two known Dy2L3-type compounds constructed by the same β-diketone chelating claw, that is, Dy2(MBDA)3 (H2MBDA = N-methyl-4,4′-bis(4,4,4-trifluoro-1,3-dioxobutyl)diphenylamine),73 Dy2(BTB)3 (H2BTB = 3,3′-bis(4,4,4-trifluoro-1,3-dioxobutyl)biphenyl),44 and Dy2(pbth)3 (pbth = (3z,3′z)-4,4′-(1,3-phenylene)bis(1,1,1-trifluoro-4-hydroxybut-3-en-2-one)),60 with energy barriers of 28.4, 20.6, and 110.2 K, respectively. Given that both Dy3+ ions in Dy2L3 are in triangular dodecahedron coordination geometry with D2d symmetry, this may not favor QTM suppression.31 Thus, the observed smaller energy barrier in Dy2L3 than those in Dy2(MBDA)3, Dy2(BTB)3, and Dy2(pbth)3 may be mainly attributed to the low symmetric coordination geometry of local Dy3+ sites.

Conclusions

In summary, Ln2L3-type (Ln = Eu3+ and Dy3+) compounds were fabricated by the deprotonation self-assembly of the C2-symmetric linear ligand containing the 2,2′-bipyridine moiety with lanthanide ions. The luminescent Eu2L3 compound exhibits excellent sensing performance for Cu2+ ions with detection limits of 43.46 nM. The luminescence sensing behavior of Eu2L3 was attributed to the generation of 1:2 host–guest complexation between Eu2L3 and Cu2+ ions, where Cu2+ ions act as the quenching sites, leading to the luminescence quenching. The magnetic measurements show that Dy2L3 exhibits magnetic relaxation. The results presented in this contribution offer a potential way for fabrication of lanthanide-based molecular materials with chemosensor function toward transition metal ions.

Acknowledgments

This work was funded by NNSF of China (grants 21971237, 21825107, and 22171264), National Key Research and Development Program of China (grant 2021YFA1500400), and Science Foundation of Fujian Province (grants 2021J02016 and 2022J01507).

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.3c02419.

  • Details of spectroscopic data (1H-NMR, 13C-NMR, PXRD, TG, magnetic data, UV–vis, and mass spectroscopy), experimental sections, optical properties, and crystallographic and structural data (PDF)

  • Crystallographic data of 2251041 and 2251298 (CIF)

Accession Codes

CCDC 2251041 and 2251298 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_Dy2L3 and data_Eu2L3/cif or by emailing data_request@ccdc.cam.ac.uk or by contacting the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +441223 336033.

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

The authors declare no competing financial interest.

Supplementary Material

ao3c02419_si_001.pdf (1.7MB, pdf)
ao3c02419_si_002.cif (11.1MB, cif)

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