A Temperature‐Responsive Smart Europium Metal‐Organic Framework Switch for Reversible Capture and Release of Intrinsic Eu3+ Ions

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.


Other Supplementary Materials for this manuscript include the following:
Video titled Transformation Mechanism Submitted to 2

Photoluminescent sensing experiments.
Text S2. X-ray crystallography. Table S1. Crystal data and structure refinements of compounds 1a and 1b. Figure S1. Coordination geometries of Eu ions in compounds 1a and 1b. Figure S2. View of the coordination geometry of Na ions and the Eu 2 Na units.

References
Submitted to 3 Text S1. Experimental Details.

Synthesis of (H
A mixture of EuCl 3 ·6H 2 O (0.0183 g, 0.05 mmol), Na 6 L (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 HNO 3 (1 M) and NaOH (1 M) under stirring. And then, it was transferred to a 10 mL Teflon-lined reactor and heated at 65 ºC 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 EuCl 3 ·6H 2 O. Elem. anal. of The fluorescence emission spectra upon excitation at 345 nm were recorded with a Hitachi F-4500 spectrophotometer equipped with a 150 W Xenon lamp as an excitation source. The photomultiplier tube (PMT) voltage was 700 V, the scan speed was 1200 nm/min. The slit widths of excitation and emission were set the same all over sensing experiments.
(1) Quenching percentage determination: At room temperature: The fine grinding sample of EuL (1 mg) was immersed in THF (2 mL), treated by ultrasonication for 30 min to form the stable emulsion. The emission spectra of the emulsions before the addition of nitro-compounds were recorded. And then, the emission spectra were recorded again after that identical quantities (280 μL, 1 mM) of different nitro-compounds were added to the above emulsions.
(2) Stern-Volmer plots determination (Fluorescence quenching titrations) At room temperature: In typical experimental setup, the fine grinding sample of EuL (1 mg) was dispersed in 2 mL THF to form the stable emulsion, which was then added to quartz cuvette. The fluorescence upon excitation at 345 nm was measured in-situ after incremental addition of freshly prepared TNP solutions (1 mM). The emulsion was stirred at constant rate during experiment to maintain homogeneity. Additionally, the fluorescence measurement was carried out under regular intervals after analyte was added.
At low temperature: In addition to the above experimental procedures, the quartz cuvette was placed in a mixture of liquid nitrogen and acetone (-94.6 ºC). After the emulsion with TNP was stirred at given regular interval, the quartz cuvette was taken out and without any delay mounted to the sample holder of the fluorescence spectrophotometer and the fluorescence spectrum was recorded.
The formula used to calculate the final concentration of analyte in the cuvette was listed as followed: The X-ray intensity data for the two compounds (1a and 1b) were collected on a Bruker SMART APEX-II CCD diffractometer with graphite monochromatized Mo-Kα radiation (λ = 0.71073 Å) operating at 1.5 kW (50 kV, 30 mA) at 293 K and 193 K, respectively. Data integration and reduction were processed with SAINT software. [S2] Multiscan absorption corrections were applied with the SADABS program. [S3] Both structures were solved by direct methods and refined employing full-matrix least squares techniques based on F 2 using the SHELXTL-97 crystallographic software package. [S4] All non-hydrogen atoms were refined with anisotropic temperature parameters except the coordination DMF molecule, aqua ligands and lattice solvent molecules in the two compounds. The disordered C and O atoms of the carboxyl groups in 1a and coordinated water molecule in 1b were refined isotropically using the atoms split over two or three sites with equal occupancy. Because guest solvent molecules of the two compounds were seriously disordered, it was impossible to refine by using conventional models appropriately. The contribution of the electron density associated with disordered solvent molecules was removed by the SQUEEZE subroutine in PLATON. [S5] All hydrogen atoms attached to carbon and nitrogen atoms were not generated and not taken into the molecular formula consideration. "ISOR" commands were used to solve the NPD and ADP problems arising from the poor quality of the diffraction data. In compound 1b, in order to rationalize the geometries of coordinated DMF molecules, "DFIX" comment was used to refine the related atoms. The detailed crystallographic data and structure refinement parameters for these compounds are summarized in Table S1.          Figure S13. View of the quenching percentage by different nitro-compounds. Figure S14. The quenching effects on TNP at room temperature with gradually increasing concentration of TNP.