Hydrogen-bonded supramolecular polymer networks for effective removal of perchlorate in water via clustered hydrogen-bonding

Abstract

Perchlorate is a toxic, explosive, and water-soluble pollutant but while efficient and low-cost removal of ClO₄⁻ from water is an important challenge, suitable methods for precipitation of ClO₄⁻ from water are underdeveloped. Here, we demonstrate a hydrogen-bonded supramolecular polymer network crystal (HBPC) for efficient complexation of ClO₄⁻. The HBPC network is constructed by an A,A’-bis-pyridyl-hydrazone-phenyl conjugate-functionalized pillar[5]arene (PYP5), which self-assembles via the formation of clustered hydrogen-bonds. Two-pyridyl-hydrazone moieties provide coordination sites while the eight ethoxyl moieties on the pillar[5]arene enable multiple hydrogen bonding motifs to tightly bind to the ClO₄⁻. The HBPC network therefore adsorbs ClO₄⁻ ions and facilitates the separation of these ions from water.

Perchlorate is a toxic, explosive pollutant, and is difficult to precipitate due to its water solubility. Here, the authors report a pillar[5]arene-based supramolecular polymer network that can bind with perchlorate by clustered hydrogen-bonds for removal from water.

Introduction

Perchlorate (ClO₄⁻) is a persistent toxic inorganic pollutant, while, unlike most other anions which can be precipitated out from aqueous solution by a suitable precipitator, ClO₄⁻ is freely water-soluble and there is no suitable precipitator for ClO₄⁻ to precipitate it from water. Therefore, separating ClO₄⁻ from water is a huge challenge^{1, 2, 3, 4, 5–6}. However, ClO₄⁻ is widely used as an oxidizing agents or combustible sources in rocket propellants, explosives, fireworks, and airbag inflation systems with fast diffusion, high stability, and difficult degradation^{7, 8, 9, 10, 11–12}. Once ClO₄⁻ enters the water systems and food chain, it will serious threat to human health and environmental security¹³. For instance, low concentrations of ClO₄⁻ inhibit the uptake of iodide by the thyroid gland, thus interfering with the normal functioning of the human thyroid gland and affecting the development of embryos, pregnant women, breastfeeding women, children and adolescents, resulting in serious harm to human health^14,15. Therefore, the treatment methods for ClO₄⁻ pollution have become an research hotspot. By far, the reported methods for abatement ClO₄⁻ pollution mainly include biotechnology methods¹⁶, electrochemical reduction methods¹⁷, chemical catalysis methods¹⁸, ion exchange methods¹⁹ and activated carbon adsorption²⁰ or other materials adsorption methods²¹, etc. Although some progress has been made, how to treat ClO₄⁻ pollution quickly, efficiently, and at low-cost without causing secondary pollution still remains a huge challenge. Due to the non-volatility, high solubility, and kinetic stability of ClO₄⁻, it is difficult for conventional treatment techniques to remove ClO₄⁻ in the water effectively. However, removing pollutants from water through adsorption and separation is still a simple, low-cost, and easy application method^{22, 23–24}. Therefore, the development of materials that can efficiently bind and separate ClO₄⁻ from water has important needs.

With the rapid development of supramolecular chemistry, adsorption and separation materials based on macrocycle-assembled supramolecular polymer^29,34 have attracted widespread attentions because of their excellent host-guest complexation properties^{25, 26, 27, 28, 29, 30, 31, 32, 33, 34–35}. Among various macrocyclic hosts, pillar[n]arenes, first reported by Ogoshi^{36, 37, 38, 39, 40, 41, 42, 43, 44–45}, possesses the merits of easy synthesis and modification, adjustable cavity size, capable of providing a variety of supramolecular interactions, and strong complexing ability to the guest^{38,39,46, 47, 48, 49, 50–51,53}. For instance, perethylated pillar[5]arene can encapsulate linear alkanes^52,53, and per-hydroxylated pillar[6]arene can well complex highly toxic paraquat⁵⁴, etc. These examples were achieved by taking advantage of the electron-rich cavities of pillar[n]arenes and based on the C-H···π interactions. It is worth mentioning that taking the common per-alkoxy-pillar[5]arene as an example, there are ten alkoxyl groups on the two sides of its cavity, which can provide abundant C-H···X (X = O, N, F, etc.) hydrogen bond donors and probably can effectively complex the target guest. However, the report on the utilizing the multi-alkoxy groups at both sides of the pillar[5]arene cavity to bind target guest by forming multiple hydrogen bonds is still very rare. Therefore, how to rationally and efficiently utilize these multi-alkoxy groups to enhance the binding ability of pillar[5]arene to target guests become an interesting opportunity for developing efficient macrocycle-assembled supramolecular polymer materials for binding and separation of specific guests such as ClO₄⁻.

Given all this, and as our interests in host-guest recognition and separation^{27,55, 56, 57, 58, 59, 60, 61, 62–63}, herein, to achieve efficient complexing and separation ClO₄^- from water, as shown in Fig. 1 and 2, we developed a pillar[5]arene-based hydrogen-bonded supramolecular polymer networks crystal (HBPC), which can efficiently bind ClO₄⁻ through clustered hydrogen-bonding based on two-armed-macrocycle cooperation effect and improve the complexation and separation ability of the ClO₄⁻. Firstly, to enhance the binding property of pillar[5]arene, we rationally conjugate-functionalized pillar[5]arene on its A, A’ sites by bis-pyridyl-hydrazone groups. The bis-pyridyl hydrazone groups can act as two arms of pillar[5]arene host (PYP5) and serve as hydrogen bond donors and acceptors, which can collaborate with the eight surrounding ethoxyl groups on the pillar[5]arene cavity to form clustered hydrogen-bonding with ClO₄⁻. Secondly, the conjugated functionalization model can enhance the rigidity of the two-arm of the PYP5 and help PYP5 molecules assemble to hydrogen-bonded supramolecular polymer networks crystal (HBPC) through multiple hydrogen bonds by the two arms of adjacent PYP5 molecules. As we expected, the HBPC-based crystal materials can efficiently bind and remove ClO₄⁻ from water.

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Fig. 1

Assembly and Single crystals structure of hydrogen-bonded supramolecular polymer networks crystal (HBPC).

a PYP5 dimer and one-dimensional catenulate supramolecular polymers. b Pore structures of HBPC. c Hydrogen-bonds in HBPC. d Single crystals structure of HBPC⊃ClO₄⁻.

Results

Synthesis and characterization of PYP5

Synthetic details of PYP5 are shown in Supplementary Fig. 1, where the key intermediates 1 and p-Q were synthesized by previously reported methods^47,48. Firstly, p-Q was obtained by the classic Suzuki coupling reaction of 1 with 4-formylphenylboronic acid in 55% yield. Subsequently, A,A’-bis-pyridyl-hydrazone-phenyl conjugate-functionalized pillar[5]arene macrocyclic molecule PYP5 was obtained by imine condensation reaction between p-Q and 2-hydrazinylpyridine under reflux conditions in 80% yield. The obtained PYP5 was fully characterized by ¹H NMR, ¹³C NMR, high-resolution mass spectrometry (Supplementary Figs. 3–5), and single-crystal X-ray analysis (Supplementary Fig. 8).

Crystal structure of hydrogen-bonded supramolecular polymer networks HBPC

High-quality colorless transparent single crystals of PYP5 suitable for single-crystal X-ray diffraction were obtained by slowly evaporating the mixed solution of dichloromethane and acetone of PYP5 within 3–4 days at room temperature. X-ray crystallographic analysis revealed that PYP5 single crystals belong to the triclinic crystal system. In the crystal, as shown in Fig. 2a and Supplementary Fig. 8b, the two pyridyl-hydrazone arms are open toward the outside. Interestingly, a pair of N-H···N hydrogen bonds were formed between the -NH group on the hydrazone and the N atom on adjacent molecule of pyridyl group, with bond lengths of 2.235 Å and 2.125 Å, respectively (Fig. 1a). Similarly, a pair of N-H···N hydrogen bonds were also formed between another pyridyl-hydrazone arm with other adjacent molecules, thereby forming one-dimensional catenulate supramolecular polymers (Fig. 1a). More interestingly, adjacent one-dimensional catenulate supramolecular polymers were further self-assembled to form hydrogen-bonded supramolecular polymer networks crystal (HBPC) via C-H···π interactions (Fig. 1a, c and Supplementary Fig. 8c). As shown in Fig. 1b, the networks show pore structures, so, the N₂ gas adsorption-desorption isotherm (Supplementary Fig. 9) was measured at 77.3 K, and the apparent surface area of HBPC was obtained to be 4.756 m² g⁻¹ using the Brunauer-Emmett-Teller (BET) model. Meanwhile, the free volume (FV) based on the calculation from single crystal data is 1840.128 ų, and the free volume fraction (FVF)⁷¹ is up to 46.09% (Supplementary Fig. 10), which supplied suitable pore structures for adsorbing ClO₄⁻.

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Fig. 2

Clustered Hydrogen-Bonds binding ClO₄⁻: effect and mechanism.

a Single crystal structures of PYP5 and PYP5⊃2ClO₄⁻ and its side view, top view show the Clustered Hydrogen-Bonding based on two-armed-macrocycle cooperation. b ESP maps of PYP5 and PYP5⊃2ClO₄⁻. c Visualization of Clustered Hydrogen-Bonding via the IGMH method based on single crystal data from a different view.

Separation property of HBPC crystal

To study ClO₄⁻ removal properties⁶² of the HBPC in aqueous solution, the sodium perchlorate standard solution was adsorbed using HBPC crystal materials. Uptake efficiency of HBPC crystal materials for ClO₄⁻ (5.0 mg/L) in aqueous solution was measured by Ion Chromatograph (IC) analysis up to about 99.24% (Fig. 3a and Supplementary Table 3). The residual ClO₄⁻ concentration was 37.8 μg/L, which is lower than the World Health Organization standard (WHO, 70 μg/L)⁶⁴ for drinking water quality, indicating that the HBPC crystal materials demonstrate effective removal capabilities on ClO₄⁻ in aqueous solution. Then, we examined the uptake efficiency of HBPC crystal materials for common anions such as F⁻, Cl⁻, Br⁻, NO₃⁻ and SO₄²⁻ in water as shown in Fig. 3b and Supplementary Table 3, and the results showed that HBPC crystal materials have good selectivity to ClO₄⁻. Meanwhile, compared with the reported ClO₄⁻ treatment methods and materials (Supplementary Tables 8, 9), this approach with the advantage of simple operation (only needs filter), low cost, low energy consumption, convenient application, and higher uptake efficiency.

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Fig. 3

Uptake efficiency of HBPC-based crystal materials.

a The uptake effect of ClO₄⁻ in water by HBPC-based crystal materials. b The uptake efficiency of HBPC crystal materials for common anions in water.

For deeply understand the uptake behavior of HBPC crystal materials for ClO₄⁻, we investigated the feed ratio effect between HBPC crystal materials and ClO₄⁻. At feed molar ratios of HBPC crystals to ClO₄⁻ ranging from 10: 1 to 2: 1, the uptake efficiencies ranged from 99.24% to 91.81% (Supplementary Table 4). Subsequently, we also investigated the concentration effect of ClO₄⁻. At concentrations of ClO₄⁻ ranging from 1 to 5 mg/L, the uptake efficiencies ranged from 94.83% to 99.24% (Supplementary Table 5), and the residual ClO₄⁻ concentration after uptake by HBPC crystals was lower than the WHO for drinking water quality standard. In addition, to further verify the role of pillar[5]arene in the clustered hydrogen-bonding based on two-armed-macrocycle cooperation strategy on binding of ClO₄⁻, the unit model (PY) without pillar[5]arene group was designed and synthesized (Supplementary Fig. 2), and its uptake efficiency to ClO₄⁻ was investigated. The results are shown in Supplementary Tables 6, 7, the uptake efficiencies of PY to ClO₄⁻ in aqueous solution were significantly lower than that of HBPC. For example, the uptake efficiency of PY to ClO₄⁻ is 75.40%, with the feed molar ratio of 2: 1, while the uptake efficiency of HBPC to ClO₄⁻ is 91.81%. This fully proved that pillar[5]arene plays a key role in the two-armed-macrocycle cooperation based clustered hydrogen-bonding enhancing host-guest binding strategy, which remarkably improves the complexation ability of ClO₄⁻.

Complexation mechanism

For investigating the complexation mechanism of PYP5 on ClO₄⁻, firstly, we performed HR-MS characterization and found that the complexation ratio between PYP5 and ClO₄⁻ was 1: 2 (Supplementary Fig. 11). Fortunately, we obtained the single crystals of HBPC-ClO₄⁻ complex HBPC⊃ClO₄⁻, and the binding mode of PYP5 and ClO₄⁻ in HBPC can be understood more directly through the single crystal structure. In the crystal structure of PYP5 (Fig. 2a, left), the bis-pyridyl-hydrazone stretches towards the outside of the pillar[5]arene, where one of the pyridine N atoms is away from the pillar[5]arene cavity. When combined with ClO₄⁻, in the single crystals of HBPC⊃ClO₄⁻, the pyridyl-hydrazones on the PYP5 underwent adaptive rotation due to the inducing of ClO₄⁻ (Fig. 2a right, and Supplementary Fig. 12), and the two-pyridyl-hydrazone arms and the pillar[5]arene cavity cooperatively bind the ClO₄⁻ with multiple clustered hydrogen bonds. Among them, the pyridine groups on the bis-pyridyl-hydrazone have been protonated by obtaining the proton from solvent and formed N-H···O hydrogen bonds with ClO₄⁻; meanwhile, the methylene and methyl groups on the multiple ethoxyl groups of pillar[5]arene formed multiple C-H···O hydrogen bonds with ClO₄⁻; moreover, the -NH groups on the hydrazone group of adjacent molecule of PYP5 also form N-H···O hydrogen bonds with ClO₄⁻ (Supplementary Fig. 13), all these hydrogen bonds surrounded the ClO₄⁻ and formed Clustered Hydrogen-Bonding to achieve very effective binding for ClO₄⁻ (Fig. 2a, right). In addition, for deeply theoretical study and visualization of the Clustered Hydrogen-Bonding, the hydrogen bonds and weak interaction forces between PYP5 and ClO₄⁻ were calculated based on crystal data through the IGMH method⁷⁰. As shown in Fig. 2c and Supplementary Fig. 14, ClO₄⁻ forms multiple clustered N-H···O and C-H···O hydrogen bonds with -NH groups on pyridyl-hydrazone, as well as the methyl and methylene groups on ethoxyl groups. It was further demonstrated that PYP5 could produce effective complexation of ClO₄⁻ via Clustered Hydrogen-Bonding. Interestingly, the HBPC assembled from PYP5 after complexing ClO₄⁻ produced a single-crystal to single-crystal transformation. As shown in Fig. 5a, the PXRD pattern of HBPC showed observable changes after uptake of ClO₄⁻, which confirmed that the uptake of ClO₄⁻ caused the transformation of crystal structure^30,46. At this time, the HBPC has assembled through π···π stacking interactions (Supplementary Figs. 15–17), C-H···π interactions, and N-H···O hydrogen bonds (Fig. 1d and Supplementary Figs. 18, 19), and the networks structure of the HBPC still maintained, which endow the beneficial property for HBPC on efficient separation of ClO₄⁻.

To further explore the complexation mechanism based on Clustered Hydrogen-Bonding, ¹H NMR titration experiments were carried out based on a constant concentration of PYP5 and varying concentrations of ClO₄⁻ at room temperature (Fig. 4). As the concentration of ClO₄⁻ increased, the signal peaks of protons H_d, H_f, H_b, and H_a on the conjugated bis-pyridyl-hydrazone arms shifted to the low fields (blue dashed arrows), and the signal peaks of protons H_e shown slight upfield shift (red dashed arrows), indicating that N-H···O hydrogen bonds formed between the conjugated bis-pyridyl-hydrazone arms and ClO₄⁻. In the meantime, the signal peaks of protons H_c and H_g on the pyridyl-hydrazone functionalized two-arms shown downfield shifts, indicating that C-H···O hydrogen bonds formed between the conjugated bis-pyridyl-hydrazone arms and ClO₄⁻. And the signal peaks of protons H_j and H_k on the ethoxyl groups at the ends of the pillar[5]arene cavity also displayed obvious chemical shifts, which were attributed to the formation of multiple C-H···O hydrogen bonds between the ethoxyl groups at the ends of the pillar[5]arene cavity and ClO₄⁻. ¹H NMR titration experiments also support the Clustered Hydrogen-Bonding between PYP5 and ClO₄⁻ based on two-armed-macrocycle cooperation effect. Moreover, since PYP5 is a two-armed macrocyclic host, PYP5 and ClO₄⁻ could gradually form 1: 1 and 1: 2 complexes with corresponding complexation constants K_a1 and K_a2⁶³. The complexation constants K_a1, K_a2, and K_a between PYP5 and ClO₄⁻ were obtained by ¹H NMR titration experiments based on the method of non-linear curve-fitting according to the SupraFit program⁶⁵ as (2.7 ± 0.0010) × 10⁴M⁻¹, (2.1 ± 0.01) × 10²M⁻¹, and (5.7 ± 0.03) × 10⁶M⁻¹, respectively, and the ΔG1, ΔG2, ΔG as −25.32 kJ/mol, −13.24 kJ/mol, and −38.56 kJ/mol, respectively, which indicated that the stable complex has been formed between PYP5 and ClO₄⁻ and the binding of PYP5 to ClO₄⁻ was spontaneous. Meanwhile, as shown in Supplementary Table 10, compared with the reported results, the PYP5 shows considerable complexation constants and can firmly bind ClO₄⁻.

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Fig. 4

¹H NMR titration evidence of Clustered Hydrogen-Bonding for ClO₄⁻.

Partial ¹H NMR spectra (DMSO‑d₆, 400 MHz, 298 K) of PYP5 in the presence of various molar equivalents of ClO₄⁻.

To gain a deeper understanding of the Clustered Hydrogen-Bonding based on two-armed-macrocycle cooperation effect, the complexation mechanism of PYP5 with ClO₄⁻ was further studied using density functional theory (DFT)^{66, 67–68}. Firstly, in order to clearly present the surface charge distribution before and after the host-guest complexation, the electrostatic potential surface (ESP) of PYP5, ClO₄⁻ and PYP5⊃2ClO₄⁻ were calculated (Fig. 2b)⁶⁹. The ESP showed that the eight ethoxyl groups on the pillar[5]arene and part of bis-pyridyl-hydrazone arms supply an electron-deficient environment, whereas ClO₄⁻ is electron-rich, which is favorable for the binding of the PYP5 to ClO₄⁻. Meanwhile, the electrons were redistributed during the formation of the host-guest complex PYP5⊃2ClO₄⁻. Subsequently, the complex mechanism also has been investigated by frontier molecular orbitals data for host and host-guest complexes (highest occupied molecular orbital and lowest unoccupied molecular orbitals, HOMO and LUMO). As shown in Fig. 5b, the HOMO orbitals of PYP5 are mainly distributed on the pillar[5]arene cavity, while in the host-guest complexes PYP5⊃2ClO₄⁻, the HOMO orbitals are distributed on the guest ClO₄⁻. Meanwhile, the LUMO orbitals were transferred from the pillar[5]arene functionalized two-arms of PYP5 to the single-arm of PYP5⊃2ClO₄⁻. And the HOMO-LUMO gap after forming the host-guest complexes PYP5⊃2ClO₄⁻ is 0.81 eV, which is less than PYP5 (3.41 eV). These changes are caused by the charge transfer between molecules during the formation of host-guest complex and the formation of multiple hydrogen bonds. Moreover, the binding energy between PYP5 and ClO₄⁻ was calculated to be −183.34 kcal/mol, indicating that PYP5 has strong binding ability to ClO₄^− 72. The above DFT calculation results provide a clear explanation and theoretical basis for the formation of Clustered Hydrogen-Bonding between PYP5 and ClO₄⁻. Therefore, crystal structure analysis, ¹H NMR titration experiments, and DFT calculations fully demonstrated that Clustered Hydrogen-Bonding based on two-armed-macrocycle cooperation effect was formed between PYP5 and ClO₄⁻, which enhanced significantly the adsorption and separation ability of PYP5 to ClO₄⁻.

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Fig. 5

Host-guest study.

a PXRD patterns of single crystalline samples of HBPC: I, before ClO₄⁻ uptake; II, after uptake of ClO₄⁻. b HOMO and LUMO of PYP5 and PYP5⊃2ClO₄⁻ were analyzed by DFT calculations based on crystal data.

Discussion

In conclusion, to achieve convenient removal ClO₄⁻ from water, we successfully developed a hydrogen-bonded supramolecular polymer networks crystal (HBPC) based on reasonable clustered hydrogen-bonding enhancing host-guest binding strategy to improve the complexation and separation ability of the hosts to ClO₄⁻. The clustered hydrogen-bonding is realized through two-armed-macrocycle cooperation effect based on a bis-pyridyl hydrazone functionalized pillar[5]arene PYP5. Interestingly, the PYP5 can form hydrogen-bonded supramolecular polymer networks crystal (HBPC) through hydrogen-bond self-assembly, which can efficiently separate ClO₄⁻ from water with an uptake efficiency of up to 99.24%, and the residual concentration of ClO₄⁻ is below the WHO standard for drinking water quality. Compared with the reported ClO₄⁻ treatment materials, the HBPC-based crystal materials possess considerably higher removal efficiency as well as with the advantage of easy operation and convenient application. The excellent ClO₄⁻ binding and removal properties of the HBPC is originate from the cooperation of the two pyridyl hydrazone arms and the eight ethoxyl groups on pillar[5]arene of PYP5 by forming multiple clustered hydrogen-bonding, the proposed binding mechanism has been carefully investigated and verified by single crystals XRD analysis and theoretical calculation. The proposed clustered hydrogen-bonding based on two-armed-macrocycle cooperation is a reasonable and useful way to enhance the binding property of supramolecular polymer, which not only offers a rational and feasible solution for separating water-soluble pollutants or anions from water but also provides experience and design strategies for macrocycle-assembled supramolecular polymer materials.

Methods

Materials

Starting materials and reagents including ammonium ceric nitrate (AR, 99%), sodium dithionite (AR, 90%), pyridine (99.5%), trifluoromethanesulfonic anhydride (AR, ≥99.5%), tetrakis(triphenylphosphine)palladium (99%), 4-formylphenylboronic acid (98%), 2-hydrazinopyridine (98%), were purchased from commercial suppliers and used without further purification unless stated otherwise.

Synthesis of 1

Compound 1 was prepared according to a previously reported literature. ^47-48 Firstly, EtP5 (1.78 g, 2.00 mmol) was dissolved in 100 mL of dichloromethane (CH₂Cl₂). Diammonium cerium (IV) nitrate (CAN, 2.19 g, 4.00 mmol) in 20 mL of water was added drop by drop. The mixture was stirred at room temperature for 30 min. The organic layer was separated, washed with water three times and dry with Na₂SO_4, then the organic layer was purified by silica gel chromatography to yield EtP[4]Q[1] as a red solid (1.00 g, yield 60%). Secondly, EtP[4]Q[1] (1.67 g, 2.00 mmol) was dissolved in 100 mL of dichloromethane (CH₂Cl₂), and a solution of sodium dithionite (1.74 g, 10.00 mmol) in 40 mL of water was added. The mixture was stirred under a nitrogen atmosphere at room temperature overnight. The red solution turned into colorless. The organic layer was separated and washed with water three times. After concentration, a light-yellow solid was obtained, which was used in the next step immediately. Finally, trifluoromethanesulfonic anhydride (Tf₂O, 1.50 mL, 3.00 mmol) was added dropwise to a mixture of the light-yellow solid obtained in the previous step and pyridine (dry, 5 mL) in CH₂Cl₂ (dry, 50 mL) at 0 °C under a nitrogen atmosphere. The mixture was stirred at room temperature for 24 h before being quenched with water. After washing the organic phase with water three times, the organic layer was concentrated under vacuum. And the crude product was purified via silica gel column chromatography to yield 1 as a white powder (1.80 g, yield 82%).

Synthesis of p-Q

Compound p-Q was prepared according to a previously reported literature. ⁴⁸ Under the protection of argon, 1 (1.10 g, 1.00 mmol), 4-formylphenylboronic acid (37.49 mg, 2.50 mmol), Pd(PPh₃)₄ (28.99 mg, 0.25 mmol) and K₂CO₃ (55.28 mg, 4.00 mmol) in 40 mL of a mixed solvent (THF / H₂O, V / V = 3: 1). The mixture was stirred and heated at reflux for 24 h. After the reaction was deemed complete, the volatiles were removed under reduced pressure, and the crude product was purified using silica gel column chromatography to yield p-Q as a white powder (558 mg, yield 55%).

Synthesis of PYP5

In a 100 mL round bottom flask, p-Q^47,48 (0.20 g, 0.20 mmol), 2-hydrazinopyridine (47.48 mg, 0.44 mmol), and anhydrous ethanol (20 mL) were added. Then add the catalytic amount of glacial acetic acid. The reaction mixture was stirred under reflux temperature until p-Q was completely reacted. After the reaction, the solvent was removed by evaporation, and the precipitate was washed with anhydrous ethanol to obtain PYP5 as a white solid (189 mg, yield 80%).

Synthesis of PY

In a 100 mL round bottom flask, 4,4-p-terphenyl dicarboxaldehyde (143.17 mg, 0.50 mmol), 2-hydrazinopyridine (120.05 mg, 1.10 mmol), and anhydrous ethanol (20 mL) were added. Then add the catalytic amount of glacial acetic acid. The reaction mixture was stirred at reflux temperature until 4,4-p-terphenyl dicarboxaldehyde was completely reacted. After the reaction, the solvent was removed by evaporation, and the precipitate was washed with anhydrous ethanol to obtain PY as a yellow solid (182 mg, yield 78%).

Theoretical calculations

All calculations were performed by density functional theory (DFT) using the B3LYP hybrid function combined with 6-31 G (d, p) basis set under Gaussian 09 D.01 program. Using single crystals as input files, visualized by Multiwfn 3.8 program and Visual Molecular Dynamics software.

Acknowledgements

This work was supported by the NSFC (Nos. 22471222, 22461040, 22061039, 22069031, 22001214, 22165027), the Top Leading Talents Project of Gansu Province, the Key R & D Program of Gansu Province (No. 21YF5GA066), the Science Fund for Distinguished Young Scholars of Gansu Province (22JR5RA131), Gansu Province College Industry Support Plan Project (No. 2022CYZC-18), the Fundamental Research Funds for the Central Universities (No. 31920230145), Natural Science Foundation of Gansu Province (Nos. 2020-0405-JCC-630, 20JR10RA088, 21JR1RA220), Gansu Province Science Foundation for Youths (23JRRA690), and Northwest Normal University Young Scholars Research Capacity Improvement Program (NWNU-LKQN2023-05), Northwest Normal University Postgraduate Research Funding Project (No. 2022KYZZ-S162), Gansu Province Outstanding Postgraduate Students “The Star of Innovation” Project (No. 2023CXZX-349).

Author contributions

J.T.: Molecular synthesis, Experiments, Data analysis, Writing – original draft. X.H.: Experiments, Data analysis. H.-R. Y.: Data analysis. J.L.: Funding acquisition. J.-F.C.: Data analysis. T.-B.W.: Experiments, Data analysis. H.Y.: Data analysis. W.-J.Q.: Data analysis. B.S.: Funding acquisition. Q.L.: Conceived and designed the experiments, Data analysis, Supervision, Conceptualization, Funding acquisition, Manuscript writing and revision. All authors discussed and commented on the manuscript.

Peer review

Peer review information

Nature Communications thanks Tomoki Ogoshi, Ying-Wei Yang, and the other anonymous reviewer(s) for their contribution to the peer review of this work. A peer review file is available.

Data availability

All data supporting the findings of this study are available from the article and its Supplementary Information or available from the corresponding author upon request. The X-ray crystallographic coordinates for structures reported in this study have been deposited at the Cambridge Crystallographic Data Centre (CCDC), under deposition numbers 2379654 (PYP5), and 2379655 (HBPC⊃ClO₄⁻). These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Source data are provided with this paper. Atomic coordinates for DFT experiments are provided with the paper.

Competing interests

The authors declare no competing interests.

Supplementary information

The online version contains supplementary material available at https://doi.org/10.1038/s41467-025-61910-y.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

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