A Temperature‐Responsive Smart Europium Metal‐Organic Framework Switch for Reversible Capture and Release of Intrinsic Eu 3+ Ions

Abstract

Stimuli‐responsive structural transformations are emerging as a scaffold to develop a charming class of smart materials. A EuL metal‐organic framework (MOF) undergoes a reversible temperature‐stimulated single‐crystal to single‐crystal transformation, showing a specific behavior of fast capture/release of free Eu³⁺ in the channels at low and room temperatures. At room temperature, compound 1a is obtained with one free carboxylate group severing as further hook, featuring one‐dimensional square channels filled with intrinsic free europium ions. Trigged by lowering the ambient temperature, 1b is gained. In 1b, the intrinsic free europium ions can be fast captured by the carboxylate‐hooks anchored in the framework, resulting in the structural change and its channel distortion. To the best of our knowledge, this is the first report of such a rapid and reversible switch stemming from dynamic control between noncovalent and covalent Eu–ligand interactions. Utilizing EuL MOF to detect highly explosive 2,4,6‐trinitrophenol at room temperature and low temperature provides a glimpse into the potential of this material in fluorescence sensors.

1

Introduction

Flexible crystalline solids are endowed with the ability to change their structures in response to external stimuli, and have recently become appealing to materials scientists. This so‐called dynamism might be key to developing a charming class of smart materials because their structurally dependent physical, chemical, and mechanic properties could be cooperatively moderated. For example, single‐crystal devices generally require a crystal to tolerate nondestructive internal changes to perform a specific function. However, single crystals are usually brittle and tend to crack because of long‐range strain from their disrupted internal periodicity. Single‐crystal to single‐crystal (SCSC) transformations, which could preserve the molecular‐level continuity and retain their single‐crystal character after an atomic solid‐state reorganization, allow the internal changes to be uniformly effected, and the resultant crystals can be readily characterized via single crystal X‐ray diffraction. In some cases, this characterization could unveil the desired dynamic process for the transformation, whereas normally solid‐state reactions involve amorphous phases and are often more difficult to determine. SCSC transformations provide a unique approach for directly observing molecular rearrangements, and offer new materials that cannot be achieved under conventional conditions. Although the SCSC processes occur accidentally, interest in them has increased for chemists working in the field of metal‐organic crystalline materials.

Porous metal‐organic frameworks (MOFs) and their related materials have attracted escalating attention due to their promising applications in chemistry and materials science. The metal‐centered guests, including kinds of simple, hydrated, solvent‐surrounded and ligand‐complexed, could impart or enrich the MOF materials with various functional properties from luminescence to catalysis. Particularly, the host–guest interactions between the metal‐relied guests and parent framework usually focus on noncovalent effects, such as electrostatics, H‐bonding and so on. However, recently in the work of Bu and co‐workers, they showed a new mechanism for controlling the structural topology through covalent interactions between the internal metal ions/clusters and hooks originating from the spare functional groups on the polytopic organic building blocks. Thus, it indicated that metal ions/clusters could be covalently captured by the remaining groups in a concerted manner. This idea likely holds great promise for modifying the structural properties of the framework. However, the captured metal ions/clusters are always tightly fastened in the pores via stable covalent interactions, and cannot handle flexible modifications. In dynamic chemistry, the term “dynamic bond” can be defined as a class of bond that can selectively undergo reversible breaking and reformation upon exposure to certain environmental factors. Reversible or dynamic covalent chemistry involving such “dynamic bond” has a long history in polymer science, and a wide range of stimuli‐responsive materials with many different mechanisms to “read” and respond to the input stimulus have been exploited. The use of dynamic covalent chemistry to program a fundamental response has been a new trend in designing adaptive materials. This could be a strategic approach to harvesting MOF‐based smart materials, which require dynamic control between their noncovalent and covalent interactions.

SCSC conversions in MOFs have emerged as an interesting phenomenon for exploring new MOF materials and enhancing their functions. Most reported SCSC transitions involve solvation and desolvation, ligands and solvent exchanges as well as cation exchanges in the MOFs, whereas structural conversions related to metal guest–ligand interactions are rarely reported. As an extension of our work on lanthanide MOF materials, a new three‐dimensional (3D) EuL MOF (1a) entrapping disordered Eu³⁺ was obtained. It can quickly capture and release Eu³⁺ in its channels at low and room temperatures, respectively, through SCSC transformations. To the best of our knowledge, this is the first report of such a rapid and reversible switch stemming from dynamic Eu‐ligand bonds.

2

Results and Discussion

2.1

Structures of Two Compounds

X‐ray analysis at 293 K reveals EuL MOF (1a) crystallizes in the monoclinic space group C2/c. The framework is constructed by one Eu³⁺, one‐half Na⁺, one L molecule, and one coordinated DMF molecule. The Eu³⁺ center is nine‐coordinated by eight oxygen atoms from five carboxyl groups in L and one oxygen atom (O13) from incomplete DMF in a tricapped trigonal prism coordination geometry (Figure S1a, Supporting Information). The Eu–O distances range from 2.381(7) to 2.564(6) Å. The Na⁺ is six‐coordinated by four oxygen atoms from four carboxyl groups in L and two aqua ligands, connecting adjacent Eu³⁺ to Eu₂Na units through the L bridging carboxyl group (Figure S2a, Supporting Information). The Na–O distances range from 2.387(6) to 2.846(7) Å. The ligand shows a slightly distorted exo‐conformation with two side benzene rings (called a and b) up and one (called c) down relative to the bridging triazine core. The dihedral angles between the side benzene rings (a, b, and c) and the central triazine ring are 9.216°, 6.316°, and 8.439°, respectively. The distances between the side rings' centroid and the central ring's plane are 0.421(3), 0.251(1), and 0.246(8) Å. Three carboxyl groups adopt the chelated mode and two adopt the monodentate fashion to coordinate with five Eu³⁺, the last carboxyl group was left and uncoordinated (Figure S5a, Supporting Information). The overall framework built from Eu₂Na units and L ligands is a 3D porous framework that has a one‐dimensional (1D) large square channel along the c axis (Figure 1a). The Eu₂Na units behave as the corner and the ligands as the edge. Considering the van der Waals radius, the channel dimension is approximately 14.038(9) × 14.038(9) Å (calculated from the shortest Eu…Eu distance). The distances between two adjacent uncoordinated carboxyl groups are 4.825(7) and 4.878(3) Å (corresponding to opposite oxygen atoms).

SCSC transformations of the porous framework between 1a (left) and 1b (right) induced by the temperature. Top: the overall view of the 3D structures along the c axis. Bottom: the local enlarged view of the structural difference.

We successfully collected the single‐crystal diffraction data at 193 K, and found that the space group of EuL transformed from C2/c to P2₁/n, making a new EuL MOF (1b), with no apparent difference between 1a and 1b in color, shape, and size of the crystals. There are three crystallographically independent Eu centers in 1b. Eu1 and Eu2 remain the coordination fashion of Eu1 in 1a and Eu3 is eight‐coordinated by three oxygen atoms from two carboxyl groups in L, three oxygen atoms from three DMF molecules and two aqua ligands, furnishing a trigonal dodecahedron geometry (Figure S1b, Supporting Information). The Eu₂Na units in 1b are almost identical to those in 1a (Figure S2b, Supporting Information). Compared with the L in 1a, the ligands exhibit two conformations due to their benzene rings rotations, and the spare carboxyl groups adopt chelated and monodentate coordination with Eu3 (Figure S5b, Supporting Information). For the chelated L, the three side rings all rotate around the bridging C–N bonds down from the triazine core in an endo‐conformation, with dihedral angles of 23.891°, 17.781°, and 12.055° for a, b, and c, respectively, which is a more severe distortion than that observed for 1a (Figure 2, middle). The distances between the side rings' centroid and the central ring's plane are 0.464(4), 0.715(3), and 0.463(1) Å. The monodentate L shows a slight distortion, with two side rings angled down and one angled up from the central ring in an exo‐conformation (Figure , right). The dihedral angles are 8.044°, 3.929°, and 7.579°, and the corresponding distances are 0.457(3), 0.259(5), and 0.230(5) Å, respectively. The overall framework of 1b is nearly identical to that of 1a, however, the most significant structural change is the additional Eu3 ions in the pores coordinated to the L in the framework, which distort the entire framework (Figure b). The dimension of the elliptical‐like distorted channels containing Eu3 in 1b changes to 9.056(2) × 16.096(3) Å (corresponding to the Eu3…Eu3 distance at the pore). In compound 1a, each L anion connects three Eu₂Na secondary building units (SBUs), thus the L anion can be defined as a 3‐connected node. The inorganic trinuclear Eu₂Na SBU can be reduced to a 6‐connected node, linking six L anions. On the basis of this simplification, compound 1a possesses a rare binodal (3,6)‐connected ant‐type topology, with the Schläfli symbol of (4²·6)₂(4⁴·6²·8⁸·10). When the additional Eu3 ion was captured, L anion connects each other through the Eu3 center to serve as a 4‐connected node. Thus, the resultant framework of 1b is a binodal (4,6)‐connected new topology with the Schläfli symbol of (4²·5²·6²)₂(4⁴·5⁷·6²·7²), as shown in Figure S6, Supporting Information.

View of the different conformations of L in compounds 1a and 1b. Top‐to‐down: top view, side view, and schematic view. Left‐to‐right: L in 1a (L1), two different L anions in 1b (L2 and L3). Color codes (bottom): blue (triazine core), brown (ring a), pink (ring b), and green (ring c).

2.2

Single‐Crystal to Single‐Crystal Transformation Process and Mechanism

To clarify that the above structural conversion upon exposure to room and low temperatures occurs in a SCSC manner, monitoring the single‐crystal diffractions of the same single crystal in the low temperature region is thus highly desirable. Keeping the crystal at −80 °C within approximately 10 min yielded the cell parameter for 1b. This result indicated the SCSC transformation from 1a to 1b is rapid. The same single‐crystal was kept at room temperature for approximately 30 min, and the cell parameter for 1a was collected again, which confirmed that the SCSC process is fast and reversible (Scheme S1, Supporting Information). Although similar low temperature‐induced SCSC transformations have been previously reported in MOFs, they were slow or irreversible, whereas a fast and reversible SCSC conversion has not been reported to date. To more clearly observe the reversibility, the diffractions of the same single crystal from −80 °C back to room temperature were monitored. The cell parameters for 1b gradually changed to that of 1a (Table 1). A temperature‐down DSC analysis from 20 °C to −90 °C showed a pair of narrow, sharp peaks at about −75.8 °C, which demonstrates the fast phase transformation for EuL. An endothermic peak followed by an exothermic one immediately without any plateau separation indicates a drastic two‐step phase transition with a transient intermediate state. The reversibility of this special phase transformation was verified by a similar profile of DSC curve with the temperature increased from −90 °C to 20 °C (Figure 3). The simulated PXRD patterns for 1a and 1b are quite different (Figure S9, Supporting Information). Additionally, the EuL compounds were further characterized by thermogravimetric analysis and Fourier transform infrared spectrum (Figures S7 and S8, Supporting Information).

The cell parameters of the same single‐crystal with the temperature gradually increased from 193 to 293 K

Temperature	193 K	203 K	223 K	248 K	293 K
Crystal system	Monoclinic P	Monoclinic P	Monoclinic P	Monoclinic P	Monoclinic C
a [Å]	26.06	26.11	26.08	26.10	23.80
b [Å]	21.63	21.73	21.90	22.13	24.53
c [Å]	31.25	31.27	31.30	31.32	31.20
β [º]	106.75	106.81	107.04	107.21	105.86
V [Å3]	16 869	16 984	17 095	17 281	17 519

Low temperature DSC analyses of EuL from 20 to −90 °C (blue line), and from −90 to 20 °C (red line).

To further demonstrate the transformation mechanism, we compared the structures of 1a and 1b. Three Eu³⁺ were found in framework 1b, but only two were found in framework 1a (taking the symmetry into consideration). Because the number of Eu³⁺ ions in the structure is constant before and after the transformation, there must be one additional disordered Eu³⁺ in the channel of 1a, further confirmed by ICP analysis (Eu/Na ratio = 2.78); however, single‐crystal analysis could not locate the exact position because of its intrinsically high disorder. Thus 1a can be formulated as (H₃O⁺)Eu_0.5[EuNa_0.5L(DMF)(H₂O)]·(solvent)_x, with H₃O⁺ to balance the charge. Keeping the crystal of 1a at low temperature quickly slows the disordered and rapid Eu³⁺ movement in the pores. When the ion crosses the channel edge, it is rapidly and completely captured by two free carboxyl groups in the framework to form Eu–OOC covalent bonds. This process requires the ligand molecules to simultaneously orient and cooperatively distort in a concerted fashion to suit the capture of Eu³⁺, and act as hooks that form covalent metal–ligand bonds. The ligands conformations changed as the benzene ring rotates around the triazine core, which distort the framework (Figure ). When the temperature is gradually increased to room temperature, the Eu³⁺ vibration becomes increasingly violent, the captured Eu³⁺ finally escapes the hooks into the channels and the structure is regained (Figure 4). As documented, the Ln–ligand bond has ever shown the dynamics to some extent, and could be reversibly rearranged under pressure or broken and reformed by heating. This case is intriguing because the metal cation guests could be controlled, captured, and released through smart Eu–L interactions in a SCSC manner. The synergistic effects of the metal guests, carboxyl hooks, and dynamic interactions contribute to the SCSC processes. After the transformation, 1b could be formulated as (H₃O⁺)₂[Eu₃NaL₂(DMF)₅(H₂O)₂].(solvent)_y.

Schematic mechanism for the SCSC transformation. An additional detailed video is included in the Supporting Information.

2.3

Photoluminescent Properties and Sensing of the Highly Explosive TNP

The Eu³⁺ ions as nodes and luminescent center, the L ligands as linkers and antenna in the framework could impart intriguing luminescent properties. The excitation spectrum of 1a, monitored under the characteristic emission (617 nm) of the Eu³⁺ ion, exhibits a broad band from 250 to 370 nm with a maximum at 345 nm, and a series of sharp lines. The broad excitation band is assigned to the absorption of the organic ligands, while the sharp line at 395 nm and 465 nm could be assigned to ⁷F₀→⁵L₆ and ⁷F₀→⁵D₂ transitions, respectively. The emission spectrum of 1a, upon excitation at 345 nm, reveals well‐resolved magnified luminescence of the f‐f transitions, attributed to the energy transfer from L ligands to Eu³⁺ ions. It shows five major peaks centered at 579, 592, 617, 653, and 700 nm, corresponding to the f‐f electronic transitions (⁵D₀→⁷F_J, J = 0–4) of the Eu³⁺ ion, with the hypersensitive ⁵D₀→⁷F₂ transition dominating the spectrum (Figure S10, Supporting Information). Furthermore, the high intensity ratio ⁵D₀→⁷F₂/⁵D₀→⁷F₁ and the presence of the weak symmetry forbidden ⁵D₀→⁷F₀ emission suggests that the Eu³⁺ ions occupy low‐symmetry coordination sites with no inversion centers, consistent with the result of X‐ray structural analysis.,[13d] By decreasing the temperature from 300 to 10 K, the luminescent intensity of the EuL MOF material gradually increase because that the nonradiative deactivation process arising from the high‐energy OH, NH, and CH oscillations could be minimized at low temperature (Figure S11, Supporting Information). It is further confirmed by a gradual increase fluorescence lifetime of ⁵D₀ decay from 0.8294 ms at 300 K to 0.9267 ms at 10 K (Table S2, Supporting Information).

The 1D channel in our designed EuL MOF with bright‐red luminescence may facilitate the preconcentration and recognition of a targeted analyte. The detection process for electron‐deficient nitro‐compounds is mainly based on fluorescence quenching via donor–acceptor electron transfer mechanism. Under room temperature, 2,4,6‐trinitrophenol (TNP), 4‐nitrophenol (4‐NP), 3‐nitrophenol (3‐NP), ortho‐nitrotoluene (o‐NT), meta‐nitrotoluene (m‐NT), nitrobenzene (NB), and nitromethane (NM) can weaken the photoluminescent intensity of the 1a emulsion to different degrees with the quenching efficiency order of TNP > 4‐NP > 3‐NP >m‐NT >o‐NT > NB > NM (Figure S13, Supporting Information). The highest quenching percentage (QP) of 61.42% for TNP comparing with the other nitro‐compounds at room temperature indicated a high selectivity for TNP. It highlights our material as an excellent candidate for detecting highly explosive TNP with superior selectivity and sensitivity. The presence of two crystallographic phases inspired us to investigate the sensing behaviors towards TNP at room temperature (RT) and low temperature (LT). The fluorescence quenching titration experiments demonstrated that the luminescent intensity decreased continuously upon incremental addition of TNP in THF (1 × 10⁻³ m), with a detectable emissive response at low concentration (4.98 × 10⁻⁶ m, Figure 5a and Figure S14, Supporting Information). To further quantify the quenching efficiency, the Stern–Volmer plots of the relative luminescent intensity (I₀/I) versus the TNP concentration are shown in Figure b, where I₀ and I are the fluorescence intensity of the emulsion in the absence and presence of the analyte, respectively. Nonlinear Stern–Volmer curves (RT and LT) can be well‐fitted by the exponential equations of I₀/I = 0.52e^11446.85 [TNP] + 0.45 and I₀/I = 0.89e^13776.95[TNP] + 0.14 with quenching constants of 5.97 ×10³ and 1.22 × 10⁴ m⁻¹, respectively, in the low‐concentration range. The nonlinear plots versus the typical linear ones may be attributed to the presence of simultaneous static and dynamic quenching. The well‐documented electron transfer process from the conduction band (CB) of MOFs to the lowest unoccupied molecular orbital (LUMO) of the analyte, synergize with the interactions between hydroxyl group of TNP and free Lewis‐base sites (triazine core in L) in the fluorophore via electrostatic interactions to enhance the TNP quenching efficiency. Most likely, the guest TNP molecules are more orderly aligned and vibrate weakly at low temperature, which benefits the ligand and TNP interactions, improves the short‐range electron transfer, and enhances the quenching efficiency. Using EuL MOF to detect highly explosive TNP at room and even low temperatures is innovative, and expands the applicability for this efficient, economic, and portable fluorescent sensor.

a) The quenching effects on TNP at low temperature with increasing TNP concentration. b) Stern–Volmer plots of I0/I versus the TNP concentration at room and low temperatures.

3

Conclusion

In summary, an unprecedented fast, reversible capture and release of intrinsic Eu³⁺ in the porous EuL MOF via a SCSC transformation was observed. The transformation simultaneously involves the cleavage and formation of dynamic Eu–O bonds, ligand conformations change, and the tremendous symmetry change of MOF structures. At low temperature, the Eu³⁺ motion quickly decelerates, and it is fully captured by the spare carboxyl groups, which results in a structural deformation. Gradually increasing the temperature to room temperature causes the captured Eu³⁺ to escape the hooks and regains the structure. This interesting phenomenon represents a rare dynamic control between noncovalent and covalent interactions, and provides a designed synthetic route for new MOF materials. Our ongoing work will focus on exploring stimuli‐responsive structural transformations and switchable transformation‐dependent functionalities.

4

Experimental Section

Synthesis of 1a: A mixture of EuCl₃·6H₂O (0.0183 g, 0.05 mmol), Na₆L (0.0187 g, 0.025 mmol), and DMF (2 mL) was placed in a beaker and stirred for 10 min. The pH value of the mixture was adjusted to 4.3–4.8 using HNO₃ (1 m) and NaOH (1 m) under stirring. And then, it was transferred to a 10 mL Teflon‐lined reactor and heated at 65 °C for 3 days. After it was cooled to room temperature, colorless block crystals were collected by filtration, washed with DMF and EtOH in sequence, and dried in air with a yield of 63% based on EuCl₃·6H₂O.

Synthesis of 1b: When the single‐crystal of 1a was kept at −80 °C within approximately 10 min, it changed to crystal 1b.

Crystal Data of 1a: C₅₄N₁₂O₂₈NaEu₂, M_r = 1591.57, monoclinic, C2/c, a = 24.3176(16) Å, b = 23.9869(18) Å, c = 31.166(2) Å, β = 104.7100(10)°, V = 17583(2) ų, Z = 4, linear absorption coefficient: μ = 0.743 mm⁻¹, radiation wavelength: λ = 0.71073 Å, temperature of measurement: T = 293(2) K, 2θ_max = 52.26°, reflections collected/independent: 77297/17433, R_int = 0.1169. The final R₁ = 0.0871 (I > 2σ(I)), wR₂ = 0.2382 (I > 2σ(I)), GOF = 0.954.

Crystal Data of 1b: C₆₉N₁₇O₃₃NaEu₃, M_r = 2073.73, monoclinic, P2₁/n, a = 25.723(3) Å, b = 21.974(2) Å, c = 31.243(3) Å, β = 107.0240(10)°, V = 16886(3) ų, Z = 4, linear absorption coefficient: μ = 1.149 mm⁻¹, radiation wavelength: λ = 0.71073 Å, temperature of measurement: T = 193(2) K, 2θ_max = 47.68°, reflections collected/independent: 65880/24930, R_int = 0.0863. The final R₁ = 0.0660 (I > 2σ(I)), wR₂ = 0.1723 (I > 2σ(I)), GOF = 0.977. CCDC 1014506 (1a) and 1014507 (1b) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

Acknowledgements

M.Z. and X.‐Z.S. contributed equally to this work. The authors are grateful for the financial aid from the National Natural Science Foundation of China (Grant Nos. 21221061, 51372242, 91122030, and 21210001), the National Key Basic Research Program of China (Grant No. 2014CB643802), and Jilin Province Youth Foundation (20130522122JH). Note: One of the corresponding authors was incorrectly displayed when the PDF was first published in the issue. This was corrected on April 21, 2015.