English

• In industry, palladium is one of the most important metal elements for application in aviation, jewellery, automobiles, electronics, chemicals, and other fields [1-4]. Especially in the field of the chemical industry, palladium is an excellent transition metal catalyst to promote the formation of C—C bonds, such as Suzuki coupling [5,6], Heck reaction [7,8], Stille coupling [9], Sonogashira coupling [10,11]. However, it also produces several problems such as environmental pollution, recycling, and the impact of residues. For example, in organic synthesis, the trace amount of Pd²⁺ remaining in the product will affect the yield of the next step of the synthesis, making it difficult to give the correct conclusion [12,13]. Therefore, the development of an excellent sensor or an efficient scavenger for palladium is of great significance for the palladium industry.

  In recent years, fluorescent materials or sensors have received huge attention [14,15]. The fluorescent sensor is a useful method for detecting Pd²⁺due to its advantages of simple synthesis, convenient operation, and low cost [16-18]. The general strategy is to prepare some Pd²⁺ fluorescence sensors through the coordination of Pd²⁺ to the luminescent ligands containing heteroatoms (C, N, S, O) [16,17,19-21], and the Pd²⁺ will quench the fluorescence of the organic fluorophore. However, most of these sensors are organic small molecule probes, which have problems such as low selectivity and sensitivity [16]. Therefore, the design of a highly efficient and sensitive fluorescent sensor for palladium ions is still a challenge.

  Cage molecules (including organic cages and metal-organic cages) can provide a new platform for the design of novel Pd-fluorescence sensors due to their tuneable confined space and multiple-interacting sites [22-26]. The multiple-interacting sites can probably make the cages provide more possibilities to bind Pd²⁺ ions, which can enhance both the sensitivity and selectivity. Based on this view, we hypothesize that a more sensitive Pd²⁺ sensor can be prepared by introducing luminescent groups into a cage molecule, which contains multiple interacting sites and a coordination environment suitable for a Pd²⁺ centre (e.g., a square planar geometry).

  In this work, we report a covalent organic cage templated by zinc ions by subcomponent self-assembly of TPE-PyCHO (5, 5′, 5″, 5′″-(ethene-1, 1, 2, 2-tetrayltetrakis(benzene-4, 1-diyl))tetrapicolinal dehyde), 1, 3-propanediamine, and Zn²⁺ in a mixture solvent (CH₂Cl₂/CH₃CN_, denoted as TPE-Zn₄, Scheme 1), which features a quadrangular prismatic cage structure. The structure of TPE-Zn₄ was confirmed by nuclear magnetic resonance (NMR), mass spectrum (MS), and single-crystal X-ray diffractions (SCXRD). TPE-Zn₄ emits orange light (λ_em = 620 nm) at an excitation wavelength of 395 nm in the solution state. Its fluorescence can be selectively quenched by Pd²⁺ ions by replacing its Zn²⁺ ions, which is confirmed by NMR and MS studies. By using TPE-Zn₄ as the fluorescent probe for detecting Pd²⁺ in DMSO, a detection limit as low as 62.3 nmol/L was achieved.

  Scheme 1

  

  Scheme 1.  Schematic diagram of the subcomponent self-assembly of the organic cage TPE-Zn₄.

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  The subcomponent TPE-PyCHO was synthesized by Suzuki coupling of 5-bromopicolinaldehyde with tetraphenylethylenetetraboronic acid-pinacol (see Supporting information for details). Cage TPE-Zn₄ was obtained by reacting TPE-PyCHO (2 equiv.), 1, 3-propanediamine (4 equiv.), and Zn(OTf)₂ (4 equiv.) in a mixture of solvent (CH₂Cl₂/CH₃CN, v/v = 1:1) at 70 ℃ for 12 h with a high yield (95%, see Supporting information for details). However, when Zn²⁺ was not presented in the reacting mixture, no organic cage was formed under the same conditions. This result indicates that the Zn²⁺ plays a structure-direct role in the formation of the quadrangular prismatic cage. Single crystals of TPE-Zn₄ suitable for X-ray diffraction were produced by diffusing the ethyl acetate into its DMSO solution.

  Crystalline sample of cage TPE-Zn₄ was firstly characterized by ¹H NMR and infrared (IR) spectrum. As shown in Fig. 1a and Fig. S5 (Supporting information), the ¹H NMR spectrum of the product showed mainly a single set of signals (Fig. 1a and Fig. S5). The peaks of a minor part may be due to the substitution of partial coordination water molecules by deuterated DMSO molecules. The ¹H-¹H correlation spectroscopy (COSY) data allowed the assignment of the signals (Fig. 1a and Fig. S6 in Supporting information). Moreover, the diffusion coefficients were analysed by two-dimensional (2D) ¹H diffusion-ordered spectroscopy (¹H-DOSY). The results showed the ¹H signals mainly belonged to a single species in DMSO (Fig. 1b). The logD was about −10.1, and the hydrodynamic radius (~13.8 Å) was calculated via the Stokes-Einstein equation. The IR spectrum study showed that the original C═O vibration at 1710 cm⁻¹ of the subcomponent TPE-PyCHO disappeared, while a new characteristic vibration peak of C═N was observed at 1654 cm⁻¹ (Fig. S7 in Supporting information), suggesting the Schiff base reaction was successful.

  Figure 1

  

  Figure 1.  (a) ¹H NMR spectrum and (b) 2D DOSY spectrum of TPE-Zn₄.

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  Mass spectrometry is an essential analytical tool for the characterization of multiply charged supramolecular cages [27]. MS study found that a series of signals corresponding to [TPE-Zn₄(OTf)_x(OH)_5-x · (5-x)H₂O]³⁺·(Fig. S8 in Supporting information, peaks 1–4) and [TPE-Zn₄(OTf)_x(OH)_6-x · (6-x)H₂O]²⁺·species (Fig. S8, peaks 5–9) were presented on the spectrum. For instance, peak at m/z = 888.4008 (calcd. = 888.4043) corresponds to [TPE-Zn₄(OTf)₅]³⁺, peak at m/z = 1350.0905 (calcd. = 1350.0995) corresponds to [TPE-Zn₄(OTf)₅(OH)·H₂O]²⁺. All assigned peaks were isotopically resolved, which were in good agreement with the calculated theoretical value distribution (Fig. S9 in Supporting information). The presence of hydroxide may be due to the ionization of water molecules during the MS measurement.

  Single-crystal X-ray diffraction (SCXRD) analysis revealed that TPE-Zn₄ crystallized in the I2/c space group. As shown in Fig. 2a, two TPE-PyCHO molecules are condensed with four 1, 3-propanediamines to form a pumpkin-shaped quadrangular prismatic organic cage, which is templated with four Zn²⁺ ions. Each Zn²⁺ ion coordinates with four N atoms and one O atom (from water molecule) and adopts a pyramid-shaped penta-coordination geometry (Fig. 2a). The Zn-N bond distances are in the range of 2.123–2.169 Å, while that of Zn-O bonds are slightly longer (2.212–2.226 Å), which is in good agreement with the Zn-N/O bond distances of the reported Zn^Ⅱ complexes with similar coordination geometry [28]. As shown in Fig. 2a, the distance between the upper and bottom ethylene groups of the TPE-Zn₄ cage is about 5.649 Å, and its H···H distances between the upper and bottom phenyl groups are about 2.439 Å, suggesting that this cage does not have enough cavity to accommodate the guest molecule (Fig. 2a). The size of TPE-Zn₄ is about 27.62 Å (H···H distance, Fig. 2b), which is consistent with its kinetic radius (13.8 Å) in solution.

  Figure 2

  

  Figure 2.  Crystal structure of TPE-Zn₄: View along with axis (a) a and (b) b, respectively.

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  The molecules containing (TPE) group are interesting luminogens, which show aggregation-induced emission (AIE) behaviour [29,30]. We investigated the photoluminescence (PL) properties of TPE-Zn₄. In the DMSO solution (concentration: 2 × 10⁻⁵ mol/L), the emission spectrum of TPE-Zn₄ showed a broad band centred at ca. 620 nm with excitation at 395 nm at room temperature, and the PL quantum yield was about 3% (Fig. 3a). We also tested PL of TPE-Zn₄ in the solid-state. The PL measurement showed TPE-Zn₄ had maximum emission at 572 nm upon excitation at 446 nm. The redshift in the maximum emission spectrum for the solution state is probably due to the solvent effect of DMSO. The PL quantum yield (17%) of TPE-Zn₄ in the solid-state was larger than that of its solution state, indicating an AIE behaviour (Fig. 3b). The emission lifetime of TPE-Zn₄ was less than 1 ns, suggesting it emitted fluorescence. For comparison, we also tested the emission spectrum and lifetime of TPE-PyCHO. The maximum emission wavelength of TPE-PyCHO λ_em = 554 nm (λ_ex = 393 nm) was observed in DMSO, and its emission lifetime is also less than 1 ns. TPE-Zn₄ showed a significant red shift in the maximum emission wavelength (Figs. S10 and S11 in Supporting information) in comparison to TPE-PyCHO, suggesting that its emission is probably original from a ligand to metal charge transfer (LMCT) [31].

  Figure 3

  

  Figure 3.  Excitation and emission spectra of TPE-Zn₄ (a) in DMSO solution and (b) solid-state.

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  We further studied the AIE behaviour of TPE-Zn₄. The water was added to the TPE-Zn₄ solution of DMSO (2 × 10⁻⁵ mol/L) as a poor solution. While an amount of water was added to the DMSO solution (f_w = 10%, where f_w is the volume fraction of water in the mixed solvent), the emission intensity was enhanced (~2.2-fold, Fig. S12 in Supporting information). However, when the f_w was increased from 10 to 90%, the emission intensity was gradually quenched (more than 10-fold) (Fig. S12). The fluorescence quenching of TPE-Zn₄ is probably due to the increasing amount of water, which can quench the fluorescence through proton or electron transfer between the excited chromophore and water [32].

  The fluorescence of TPE-Zn₄ was affected by zinc ions, indicating that the luminescence of the TPE-Zn₄ will be changed by replacing the zinc ions with other metal ions. Thus, we tested a series of metal ions to understand the effect of metal ions on the luminescence behaviour of TPE-Zn₄. In a typical experiment, the TPE-Zn₄ was dissolved in DMSO at a concentration of 2 × 10⁻⁴ mol/L. Then, 2 equiv. of Pd²⁺, Fe³⁺, Cr³⁺, Fe²⁺, Cd²⁺, Mg²⁺, Co²⁺, Ba²⁺, Ca²⁺, Pb²⁺, Pt²⁺, Na⁺ and Ni²⁺ were added into this DMSO solution of TPE-Zn₄, respectively, and their emission spectra were recorded at room temperature. It was found that the emission intensity of TPE-Zn₄ did not change significantly after adding Fe³⁺, Cr³⁺, Fe²⁺, Cd²⁺, Mg²⁺, Co²⁺, Ba²⁺, Ca²⁺, Pb²⁺, Pt²⁺, Na⁺ (Fig. 4, yellow columns 2–12), while Pd²⁺ quenched its fluorescence (> 99%, Fig. 4, column 1). These results indicated that TPE-Zn₄ had a good sensing selectivity for Pd²⁺. To further verify the selectivity for Pd²⁺, 2 equiv. of Pd²⁺were added into the solutions of TPE-Zn₄ presenting with Fe³⁺, Cr³⁺, Fe²⁺, Cd²⁺, Mg²⁺, Co²⁺, Ba²⁺, Ca²⁺, Pb²⁺, Pt²⁺, and Na⁺, respectively. The emission measurement showed their fluorescence was immediately quenched by Pd²⁺ (Fig. 4, blue-grey columns 2–12). Interestingly, Ni²⁺ ions boosted the emission of TPE-Zn₄ (intensity increasing about 2.74-fold, Fig. 4, yellow column 13). However, when the Pd²⁺ was further added to its solution, its fluorescence was also quenched immediately (Fig. 4, blue-grey column 13). This result further demonstrated that TPE-Zn₄ showed high selectivity for Pd²⁺ions. Moreover, we studied the selectivity of TPE-Zn₄ to detect Pd²⁺ in a solution containing mixed interfering metal ions. A mixture of four different valence metal ions (Fe³⁺, Zn²⁺, Na⁺ and Cr³⁺) was added to the DMSO solution of TPE-Zn₄, and its fluorescence intensity was quenched by about 30% (Fig. 4, column 14). However, when additional 2 equivalents of Pd²⁺ were added to the mixture solution, the fluorescence was wholly quenched. These results indicate that TPE-Zn₄ is an excellent luminescent probe for Pd²⁺ ions with good antijamming ability.

  Figure 4

  

  Figure 4.  Fluorescence intensity of TPE-Zn₄ (2 × 10⁻⁴ mol/L) in DMSO after the addition of 2.0 equiv. of metal ions (yellow columns) and further addition of 2.0 equiv. of Pd²⁺ (blue-grey columns).

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  To understand the sensing performance of TPE-Zn₄ for Pd²⁺, we further carried out the fluorescence titration experiment of Pd²⁺with TPE-Zn₄. By increasing the isogradient concentrations of Pd²⁺ from 1 × 10⁻⁵ mol/L to 2 × 10⁻⁴ mol/L (DMSO solution), the emission intensity of TPE-Zn₄ (DMSO, 2 × 10⁻⁴ mol/L) was gradually quenched (Fig. 5a). In addition, the quenching of the emission showed a good linear relationship between the intensity and amount of Pd²⁺, which was observed from 0.15 equiv. to 0.5 equiv. under the tested conditions (Fig. S13 in Supporting information). The limit of detection (LOD) was estimated by 3σ method, (LOD = 3σ/K_sv, where σ is the standard deviation of blank solutions, K_sv is the slope of calibration curve) [33], which was 62.3 nmol/L (Fig. S14 in Supporting information). To the best of our knowledge, TPE-Zn₄ was the first metal-templated organic cage for application as a sensor for detecting Pd²⁺ ions. Its LOD for Pd²⁺ ions was lower than that of the reported MOF materials [34,35] and some organic molecular sensors (Table S2 in Supporting information) [16].

  Figure 5

  

  Figure 5.  (a) Fluorescence titration experiment of TPE-Zn₄ (2.0 × 10⁻⁴ mol/L) in DMSO solution with the addition of Pd²⁺. Inset: Fluorescence photographs of TPE-Zn₄ under UV light (excited at 365 nm) before and after the addition of Pd²⁺. (b) ¹H NMR titration experiment of TPE-Zn₄ in deuterated DMSO with the addition of Pd²⁺.

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  The mechanism of quenching fluorescence of TPE-Zn₄ by Pd²⁺is probably due to that its Zn²⁺ ions are replaced with Pd²⁺. The Pd²⁺ ion generally prefers to adopt a planar four-coordination geometry [36]. The four-coordinate environment of the organic cage in TPE-Zn₄ is similar to salen molecules [37]. Thus, it will probably prefer to coordinate with Pd²⁺. When Pd²⁺ enters the DMSO solution of TPE-Zn₄, it will rapidly replace Zn²⁺to form a Pd²⁺ coordinated species. The fluorescence quenching is probably due to the heavy atom effect of the Pd²⁺and the chelation-enhanced quenching effect between the Pd²⁺ and the cage (Fig. S15 in Supporting information) [38-40]. Moreover, the coordinating H₂O molecules of TPE-Zn₄ will be released after replacing with Pd²⁺, and the entropy will increase in this process, favouring the exchange between Zn²⁺ and Pd²⁺.

  To verify this exchanging process, ¹H NMR titration, inductively coupled plasma atomic emission spectroscopy (ICP-AES), and MS measurement were carried out. When Pd²⁺ was gradually added to the TPE-Zn₄ DMSO solution, the signals of H_a1, H_b1 and H_d1 gradually disappeared, while the signals of new H_a2, H_b2 and H_d2 were observed (Fig. 5b). This phenomenon indicated that the chemical environment of the cage was changed, and the Zn²⁺ ions were probably replaced by Pd²⁺ ions. Other signals (H_c1, H_e1, H_f1) also gradually shifted downfield or upfield. When more than four equivalents of Pd²⁺ (e.g., 5 and 6 equiv.) were added, the signal of ¹H NMR of the cage did not change (Fig. 5b). Furthermore, ICP-AES measurement showed that the Pd/Zn ratio after the reaction was 14.28:1, indicating Pd²⁺ ions occupied the most position of Zn²⁺ ions (> 93%). These results indicated that the Zn²⁺ had been exchanged by Pd²⁺ to form a Pd²⁺-based cage TPE-Pd₄. We also used MS to study whether the Zn²⁺ ions can be replaced by Pd²⁺. When TPE-Zn₄ (2.0×10⁻⁴ mol/L) and Pd(NO₃)₂ (2 equiv.) were mixed in DMSO, the formation of the TPE-Zn₂Pd₂ was observed by MS. A series of signals for [TPE-Zn₂Pd₂(OTf)₃(NO₃)·H₂O]⁴⁺ (m/z = 632.3103, calcd. = 632.3114), [TPE-Zn₂Pd₂(OTf)₂(NO₃)·H₂O]⁵⁺(m/z=475.8698, calcd.=475.8682), [TPE-Zn₂Pd₂(OTf)(NO₃) · H₂O]⁶⁺ (m/z=371.9061, calcd. = 371.8987), [TPE-Zn₄(NO₃)₂ · 2H₂O]⁶⁺ (m/z = 346.5800, calcd. = 346.5772) were presented in the spectrum (Fig. S16 in Supporting information), respectively. The MS further proved that the Zn²⁺ was replaced by Pd²⁺ after mixing the solution of TPE-Zn₄ and Pd²⁺ ions. From the results of both ¹H NMR, ICP-AES, and MS, the mechanism of fluorescence quenching of TPE-Zn₄ is indeed due to the fluorescence quenching caused by Pd²⁺, which replaces the Zn²⁺ ions with an exchange process.

  In conclusion, we have successfully synthesized and characterized a zinc-templated quadrangular prismatic covalent cage TPE-Zn₄, which emits orange fluorescence under both solution and solid state. TPE-Zn₄ can be used as a new Pd²⁺ fluorescence sensor with high selectivity and sensitivity, with a detection limit as low as 62.3 nmol/L. The use of AIEgen TPE molecule as a building block to construct a covalent cage provides a new strategy for designing and assembling advanced materials for sensing metal ions and removing heavy metal ions [41]. The design and synthesis of more complex organic cages based on TPE groups templated with metal ions are on the way.

  Declaration of competing interest

  The authors declare no conflict of interest.

  Acknowledgments

  This work was supported by the National Natural Science Foundation of China (Nos. 21731002, 21871172 and 22171106), the Guangdong Major Project of Basic and Applied Research (No. 2019B030302009), the Fundamental Research Funds for the Central Universities (No. 21622103), Guangdong Natural Science Foundation (No. 2022A1515011937), Guangzhou Science and Technology Program (No. 202002030411), the China Postdoctoral Science Foundation (No. 2022M711327) and Jinan University.

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2022.07.029.

  
  
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  Scheme 1  Schematic diagram of the subcomponent self-assembly of the organic cage TPE-Zn₄.

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  Figure 1  (a) ¹H NMR spectrum and (b) 2D DOSY spectrum of TPE-Zn₄.

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  Figure 2  Crystal structure of TPE-Zn₄: View along with axis (a) a and (b) b, respectively.

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  Figure 3  Excitation and emission spectra of TPE-Zn₄ (a) in DMSO solution and (b) solid-state.

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  Figure 4  Fluorescence intensity of TPE-Zn₄ (2 × 10⁻⁴ mol/L) in DMSO after the addition of 2.0 equiv. of metal ions (yellow columns) and further addition of 2.0 equiv. of Pd²⁺ (blue-grey columns).

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  Figure 5  (a) Fluorescence titration experiment of TPE-Zn₄ (2.0 × 10⁻⁴ mol/L) in DMSO solution with the addition of Pd²⁺. Inset: Fluorescence photographs of TPE-Zn₄ under UV light (excited at 365 nm) before and after the addition of Pd²⁺. (b) ¹H NMR titration experiment of TPE-Zn₄ in deuterated DMSO with the addition of Pd²⁺.

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