The fluorescence distinction of chiral enantiomers: a Zn coordination polymer sensor for the detection of cinchonine and cinchonidine

Abstract

Cinchonine (+Ccn) and cinchonidine (−Ccn) are quinoline alkaloid drugs, which are found in natural products of cinchona bark and are used as specific drugs for treating malaria. As a pair of chiral enantiomers, they show different biological activities and performances in pharmacological applications. In this work, we prepared a fluorescence sensor based on a Zn coordination polymer for the sensitive and selective detection of the above enantiomers, namely, [Zn(PT)(H₂O)₂]_n (IMU-Zn1) (H₂PT = 5-(5-pyrimidyl)-1,3-benzenedicarboxylic acid). +Ccn and −Ccn had a “turn-on” effect on the fluorescence of IMU-Zn1, and the ratio of their fluorescence enhancement rate reached 2.22, and the limits of detection (LOD) for +Ccn and −Ccn were 2.10 μM and 1.90 μM, respectively. In addition, infrared spectroscopy, ¹HNMR spectroscopy, X-ray powder diffraction, fluorescence lifetime, Gaussian calculations, and other characterization methods were used to explore the mechanism of fluorescence enhancement. The results showed that there was a photoelectron transfer process and there were electrostatic interactions between +Ccn and IMU-Zn1. The photoelectron transfer process, the electrostatic interactions, and the stronger hydrogen-bonding interactions between −Ccn and IMU-Zn1 are responsible for a more pronounced fluorescence enhancement. This work provides an alternative strategy for the identification of chiral enantiomers using light-emitting coordination polymers as fluorescent probes.

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Introduction

The phenomenon in which an object cannot coincide with its mirror image is called chirality. In the field of chiral chemistry, two enantiomers routinely exhibit different physical and chemical properties. Chirality is essential to many chemical processes in catalysis, pharmaceuticals, life science, etc.¹ Chiral enantiomers may cause dissimilar or fatal effects on living cells on account of distinct drug mechanisms. Therefore, the identification of enantiomers is an important but challenging task for chemists.² As an effective drug component, cinchonine (+Ccn) is used for the prevention and control of malaria and the treatment of fever. In addition, it can promote the process of tumor cell apoptosis and is expected to be applied in the treatment of cancer in the future.³ Cinchonidine (−Ccn) is a chiral enantiomer of +Ccn, and it is commonly utilized as a pharmaceutical intermediate.⁴ In the past 30 years, they have also been as catalysts for chiral asymmetric syntheses due to their structural properties, mild conditions and environmental friendliness.^5,6 Therefore, the identification of chiral isomers is of great importance in the fields of asymmetric synthesis, medicine, and biology.

Coordination polymers (CPs) are constructed by connecting metal ions or metal clusters with bridging ligands to fabricate multi-dimensional structures, which are crystalline materials with periodic network structures formed by a self-assembly process.^7–9 Different from the f–f transitions of lanthanide ions and the antenna effect of ligands, luminescent transition metal CPs are generally based on their organic ligands containing conjugated aromatic rings and/or organic functional groups with some heteroatoms, which may exhibit excellent photophysical properties in the process of fluorescence sensing of analytes. Luminescent transition metal CPs have become a research hotspot in the field of enantiomer recognition in recent years.¹⁰ Chandrasekhar et al. developed a homochiral Zn-MOF based on pyrene-tetralactic acid with specific quenching for D- and L-form enantiomer differentiation of histidine.¹¹ Guo et al. synthesized an achiral metal–organic skeleton based on MIL-53(Al)-NH₂, and its fluorescence quenching was partially restored on account of the chelation between Cu²⁺ and lysine for selective detection of L/D-lysine.¹² Zhang et al. successfully prepared a Cd-MOF using a homochiral pyridinyl precursor, which showed sensitive and enantioselective molecular recognition properties for proline, arginine, etc.¹³ Thoonen et al. reported a chiral Zn-CP, which showed high enantioselectivity for R/S Mosher's acid compared with other chiral analytes.¹⁴

In this work, a Zn coordination polymer was successfully synthesized by self-assembly based on the 5-(5-pyrimidyl)-1,3-benzenedicarboxylic acid ligand (H₂PT). According to the analysis of the single crystal structure of IMU-Zn1, it has been found that IMU-Zn1 contains nitrogen atom sites, which are not involved in coordination with metal centers and exposed to the outside of the complex structure. This exposure can facilitate the interactions between introduced guest molecules and active sites more easily. Chiral enantiomers +Ccn and −Ccn contain hydroxyl groups (Fig. S1, ESI†). It is assumed that the hydrogen-bonding interactions may be induced between the hydroxyl group of +Ccn or −Ccn and the nitrogen sites of IMU-Zn1, which may alter the luminescence behavior of IMU-Zn1 and facilitate the detection of these enantiomers.

Results and discussion

Structural description of IMU-Zn1

The structure of IMU-Zn1 crystallizes in the space group of Ama2, which belongs to the orthorhombic system. An asymmetric unit contains a Zn²⁺ ion, a PT²⁻ anion and two coordinated H₂O molecules in the complex. Fig. 1a shows that the Zn²⁺ ion is tetra-coordinated with two oxygen atoms (O1, O1A) from coordinated water molecules, and other two oxygen atoms (O3, O3A) from the monodentate coordination of PT²⁻ ligands, which together constitute the tetrahedral geometry of the Zn²⁺ ion (Fig. 1b). The Zn–O bond lengths of IMU-Zn1 range from 1.917–1.977 Å, which is close to those reported in the literature.¹⁵ Each Zn²⁺ ion is connected by two ligands, thus forming a one-dimensional chain of staggered ligands that extends infinitely (Fig. S2a, ESI†). At the same time, the chains interact with each other through the hydrogen-bonding to shape a two-dimensional layered structure (Fig. 1c). The hydrogen-bonding distances are measured to be 1.923 Å and 1.994 Å, which are close to the values reported in the literature.¹⁶ The π⋯π interactions between PT²⁻ ligands are present between the layers, and ultimately a three-dimensional supramolecular structure is constructed (Fig. 1d). The π⋯π interaction distance between the pyrimidyl group and the benzene ring is 3.8275 Å, which is close to the values reported in the literature (Fig. S2b, ESI†).¹⁷ The crystal structure data of IMU-Zn1 are listed in Table S1 (ESI†), and the detailed data of bond lengths and bond angles are listed in Table S2 (ESI†). Hydrogen bonds are listed in Table S3 (ESI†).

Fig. 1 (a) Coordination diagram of Zn²⁺ ions; (b) tetrahedral geometry of Zn²⁺ ions; (c) two-dimensional layered structures and hydrogen-bonding interactions; and (d) three-dimensional supramolecular structure of IMU-Zn1.

Hirshfeld surface analysis of IMU-Zn1

Hirshfeld surface analysis is based on CIF files obtained by X-ray diffraction of single crystals using Crystal Explorer software to study intermolecular interactions.^18,19 The normalized contact distance (d_norm) is defined as a function of the distance from a point on the surface to the nearest nucleus inside/outside the surface (d_i/d_e) and the van der Waals radius of the atom. The red area indicates that the distance is less than the sum of the van der Waals radii, and the blue area demonstrates that the distance is longer than the sum of the van der Waals radii. The fingerprint in Fig. 2 shows that the O⋯H/H⋯O interaction ratio is 31.6%, and the N⋯H/H⋯N interaction ratio is 10.9%. It is further shown that the existence of a large number of hydrogen-bonding interactions in the structure provides the basis for the formation of more stable complexes. The π⋯π interaction accounted for 5.8% of the weak interactions.²⁰ It can be seen from Fig. S3 (ESI†) that π⋯π interaction mainly comes from the pyrimidine ring and the benzene ring, and the O⋯H/H⋯O interaction mainly originates from coordinated water molecules and uncoordinated carboxylic oxygen atoms. The N⋯H/H⋯N interaction mainly stems from the nitrogen atoms of the pyrimidine ring in the ligand. A large number of hydrogen-bonding interactions with uncoordinated oxygen and nitrogen atoms provide the basis for specific identification.

Fig. 2 (a) Surface distribution of IMU-Zn1 mapped by d_norm; (b) the 2D-fingerprint of IMU-Zn1 and the contribution proportion of the interaction.

IR, PXRD, and TGA of IMU-Zn1

In the IR spectrum of IMU-Zn1, the peak at 1447 cm⁻¹ corresponds to the characteristic stretching vibration peak of the C=N group. The peaks at 1616 cm⁻¹ and 1407 cm⁻¹ are attributed to the anti-symmetric and symmetric stretching vibrations of the carboxyl group in the PT²⁻ ligand, respectively. Compared with the –COOH characteristic peak at 1702 cm⁻¹ of the H₂PT ligand, there is a noticeable red shift, indicating the successful coordination of the carboxyl group in the PT²⁻ ligand with Zn²⁺ ions (Fig. 3a).^21–23

Fig. 3 (a) IR spectra of IMU-Zn1 and H₂PT; (b) the PXRD pattern of IMU-Zn1 and its simulated pattern.

The crystal phases of the as-synthesized and the simulated IMU-Zn1 were identified by the PXRD tests (Fig. 3b). The PXRD pattern of the as-synthesized IMU-Zn1 matched well with the simulated one of the IMU-Zn1 model, confirming that the structure of IMU-Zn1 was synthesized as expected with high phase purity.

The thermogravimetric analysis (TGA) of IMU-Zn1 was carried out in the range of 30–1400 °C under a nitrogen atmosphere (Fig. S4a, ESI†). The first weight loss occurred in the range of 146–212 °C with 9.89%, which could be attributed to the loss of two coordinated water molecules (theoretical value: 10.47%). The second weight loss was about 64.0% when the temperature reached 370 °C, corresponding to the decomposition of the framework in IMU-Zn1. Moreover, the PXRD pattern of IMU-Zn1 showed that the positions of the corresponding diffraction peaks before 200 °C remained largely unchanged compared with those of the initial sample. This indicates that the framework of IMU-Zn1 exhibits considerable stability at temperature up to 200 °C (Fig. S4b, ESI†).

pH fluorescence stability and solvent stability of IMU-Zn1

The fluorescence and corresponding PXRD patterns of IMU-Zn1 with different pHs were obtained. The powdered samples of IMU-Zn1 were immersed in aqueous solutions of pH 1.0–14.0, respectively, and the samples were filtered out and dried overnight after 72 hours. The tested PXRD pattern showed that the complex had good structural stability in the range of 3.0–11.0 (Fig. 4a). At the same time, the fluorescence intensity of IMU-Zn1 also maintained good stability in a wide pH range, indicating that IMU-Zn1 has excellent luminescence consistency (Fig. 4b).

Fig. 4 (a) PXRD patterns of IMU-Zn1 dispersed in aqueous solution with pH = 1.0–11.0; (b) fluorescence stability under different pH conditions; (c) PXRD patterns of IMU-Zn1 dispersed in common organic solvents.

A series of commonly used organic solvents were selected such as ethanol, N,N-dimethylacetamide (DMA), N,N-dimethylformamide (DMF), dichloromethane (CH₂Cl₂), acetonitrile (CH₃CN) and water (H₂O). The powdered samples of IMU-Zn1 were dispersed in above organic solvents, respectively; and the PXRD results showed that the framework of the complex remained intact in different organic solvents after three days, respectively (Fig. 4c).

Luminescent behaviors of IMU-Zn1

The solid-state fluorescence spectrum of IMU-Zn1 was measured using a fluorescence spectrophotometer. As shown in Fig. S5 (ESI†), the strongest emission peak of IMU-Zn1 appeared at 434 nm when excited at 350 nm. A similar emission peak at 446 nm for the H₂PT ligand is due to the transitions of π* → π and π* → n (Fig. S6, ESI†). In addition, the maximum emission wavelength of IMU-Zn1 is slightly blue shifted with respect to H₂PT ligand's emission at 446 nm, which can be attributed to the H₂PT ligand's coordination with the Zn²⁺ ions.²⁴

Fluorescence detection of +Ccn and −Ccn

The application of IMU-Zn1 in the recognition of enantiomers was considered on account of its stable photoluminescence and structural features. As mentioned above, IMU-Zn1 contains uncoordinated nitrogen sites, which are exposed on the outside of the complex. Based on the structures of chiral +Ccn and −Ccn, two enantiomers contain hydroxyl groups, which may interact with nitrogen sites of IMU-Zn1 (Fig. S1, ESI†).

The fluorescence spectra were tested at room temperature by adding 300 μL analytes to the suspensions of the IMU-Zn1, respectively. As shown in Fig. S7 (ESI†), the fluorescence intensity of IMU-Zn1 was enhanced by about 1 and 4 times after appending +Ccn or −Ccn solution, respectively. At the same time, it was found that the emission wavelength had undergone a slight blue shift. Fig. S8 (ESI†) shows that the luminescence of IMU-Zn1 was almost instantly enhanced within 40 seconds upon the addition of +Ccn or −Ccn, respectively, and the luminescence intensity remained basically unchanged over next 3 minutes, indicating that the fluorescence response of IMU-Zn1 to the enantiomer analytes is a rapid process.

The fluorescence intensity of IMU-Zn1 showed a gradient of enhancement with increasing concentration of +Ccn (Fig. 5a). According to the Stern–Volmer formula: I/I₀ = K_EC × [C] + 1 (I₀ and I denote the fluorescence intensity of the complex before and after addition of the analytes, respectively; K_EC is the constant; and [C] is the concentration of analytes), the emission intensity of IMU-Zn1 can be obtained as a linear relationship with the concentration of +Ccn with a fluorescence enhancement constant of K_EC = 3.08 × 10³ M⁻¹ and R² of 0.970 (Fig. 5b). The final calculated limit of detection (LOD) for +Ccn is 2.10 × 10⁻⁶ M according to the following equations: LOD = 3 σ/K and σ = 100 S/I₀ (K represents the slope of the S–V curve and S is the standard deviation of I/I₀ – 1).^25,26 Similarly, the fluorescence intensity of IMU-Zn1 displayed a good positive correlation with the concentration of −Ccn, and the enhancement constant K_EC = 6.83 × 10³ M⁻¹ and the LOD for −Ccn is 1.89 × 10⁻⁶ M (Fig. 5c and d). The ratio of K_EC for +Ccn and −Ccn reaches 2.22. This result indicates that there is a significant difference in the fluorescence enhancement rate of two enantiomers. The LOD values are not in the lowest order of magnitude reported in the literature (Table S4, ESI†). It is worth pointing out that this provides an easy procedure for fabricating fluorescence sensors that detect both +Ccn and −Ccn.²⁷

Fig. 5 (a) Fluorescence spectra obtained with gradual addition of different concentrations of +Ccn; (b) S–V fitting diagram at different concentrations of +Ccn; (c) fluorescence spectra at different concentrations of −Ccn; and (d) S–V fitting diagram at different concentrations of −Ccn.

The identification and determination of +Ccn and −Ccn in human urine is of great importance. Since some compounds in human urine may cause the fluorescence effect of IMU-Zn1, it is necessary to conduct anti-interference experiments. The different analytes were separately supplemented to the complex suspensions containing common components of human urine, including creatine, glucose, urea, NaCl, KCl, CaCl₂, and Na₂SO₄. The results illustrated that +Ccn and −Ccn had a corresponding fluorescence enhancement for IMU-Zn1, which was not basically affected by another co-existing component in urine. Therefore, the probe has good selectivity for fluorescence sensing of +Ccn and −Ccn molecules (Fig. S9, ESI†).

The recyclability of IMU-Zn1 as a fluorescent probe for +Ccn/−Ccn was further investigated. As shown in Fig. 6a and c, the luminescence intensity of IMU-Zn1 was consistent with that of the initial one, and its steady frameworks were confirmed by the PXRD patterns after four cycles of experiments (Fig. 6b and d). This result demonstrates that IMU-Zn1 has regeneration ability and structural stability.

Fig. 6 (a) Fluorescence assay of cyclic properties of IMU-Zn1 for detecting +Ccn; (b) PXRD patterns of IMU-Zn1 after four cycles of detecting +Ccn; (c) fluorescence assay of cyclic properties of IMU-Zn1 for detecting −Ccn; and (d) PXRD patterns of IMU-Zn1 after four cycles of detecting −Ccn.

The mechanism of IMU-Zn1 for detecting +Ccn and −Ccn

The different rates of fluorescence enhancement may be due to the distinctions in the interaction between the analytes and the H₂PT ligand.²⁸ With the addition of +Ccn or −Ccn, it gradually caused a blue shift of the fluorescence emission for IMU-Zn1. This occurs because the formed hydrogen bonds restrict the intramolecular rotation of the H₂PT ligand, thereby reducing the degree of conjugation and resulting in a blue shift (Fig. S10, ESI†).²⁹

The fluorescence lifetimes before and after immersing +Ccn or −Ccn in the suspension of IMU-Zn1 (+Ccn@IMU-Zn1 or −Ccn@IMU-Zn1) were measured at an excitation wavelength of 350 nm. Compared with the fluorescence lifetime of IMU-Zn1, the fluorescence lifetime of +Ccn@IMU-Zn1 increased to different degrees (Fig. S11, ESI†). The fluorescence lifetime reaches 2.55 ns after the interaction of +Ccn with IMU-Zn1, and there is an enlargement of the fluorescence lifetime for −Ccn@IMU-Zn1, which reaches 2.65 ns. This confirms that different interactions of IMU-Zn1 after adding +Ccn or −Ccn eventually lead to varying degrees of fluorescence enhancement (Fig. 7a and b).^30,31

Fig. 7 The fluorescence decay and fit curves of (a) +Ccn@IMU-Zn1 and (b) −Ccn@IMU-Zn1. (c) The PXRD pattern of IMU-Zn1 and IMU-Zn1 in +Ccn/−Ccn solution. (d) HOMO and LUMO levels of the H₂PT ligand and +Ccn/−Ccn.

To further understand the fluorescence enhancement of IMU-Zn1 for +Ccn/−Ccn, the PXRD data of +Ccn@IMU-Zn1 and −Ccn@IMU-Zn1 were measured (Fig. 7c). It has been found that the skeleton structure of +Ccn@IMU-Zn1 or −Ccn@IMU-Zn1 remains consistent compared with that of the original one. Additionally, the IR data in Fig. S12 (ESI†) present a significant change in the intensity of the C=N stretching vibration at 1447 cm⁻¹ after immersing the analyte, which may be attributed to the formation of hydrogen bonds.^32,33 The Zeta potentials of IMU-Zn1, +Ccn and −Ccn were tested. It has been found that IMU-Zn1 is positively charged while +Ccn and −Ccn are negatively charged; and then the addition of +Ccn and −Ccn solutions separately into the suspension of IMU-Zn1 shows that the corresponding potential remains between two values, which indicates that there may exist electrostatic interactions between IMU-Zn1 and +Ccn or −Ccn, respectively (Fig. S13, ESI†).

In order to deeply reveal the luminescence mechanisms of IMU-Zn1, for distinguishing +Ccn and −Ccn, the energy levels of two analytes and the H₂PT ligand were calculated. The LUMO energy level of H₂PT (−2.0509 eV) is significantly lower than those of +Ccn (−1.1478 eV) and −Ccn (−1.3015 eV). It indicates that the photoelectrons on the LUMO of the analytes will be transferred to the complexes, i.e, the photo-induced electron transfer process (PET), resulting in the enhancement of the fluorescence of IMU-Zn1 (Fig. 7d).³⁴

¹HNMR spectra of H₂PT, +Ccn, −Ccn, the mixture of +Ccn and H₂PT (+Ccn + H₂PT), and the mixture of −Ccn and H₂PT (−Ccn + H₂PT) were obtained for studying the interactions of hydrogen-bonding of +Ccn and −Ccn with H₂PT. Fig. 8 shows that ¹HNMR peaks of +Ccn + H₂PT and −Ccn + H₂PT show a significant shift compared to those of the analytes. As shown in Fig. 8a, the chemical shift of the −Ccn + H₂PT methylene group moves to a high field from 1.55 ppm to 1.34 ppm compared with that of −Ccn, which is attributed to the increase in electron density caused by the shielding effect.³⁵ The hydrogen atom at 7.07 ppm on the pyridine ring on the same side as the hydroxyl group moves to 7.29 ppm due to the de-shielding effect and thus the density of the electron cloud around the hydrogen atom decreases. The ¹HNMR spectra of +Ccn + H₂PT exhibit similar results, and the shift at 1.55 ppm moves to 1.40 ppm in a high field; the shift at 7.07 ppm moves to 7.19 ppm in a low field (Fig. 8b). Compared with the peak position of +Ccn + H₂PT, the hydrogen atoms (o) and (n) of −Ccn + H₂PT are obviously different. After adding the analytes into H₂PT, the H atom (n) in the +Ccn molecule moves 0.17 ppm to a higher field due to the steric hindrance (Fig. 9), which is caused by the shielding effect since its proximity to the hydroxyl group. The H atom (o) moves 0.22 ppm towards a lower field, which is caused by the de-shielding effect of the hydrogen atom being further away from the hydroxyl group. The corresponding hydrogen atoms in the −Ccn molecule are on the opposite side of the hydroxyl group, so the position of the peak does not change. The hydrogen atom (g) of the hydroxyl group is active, and its position may be random on the spectrum.³⁶ This confirms that there is a significant distinction in the intensity of hydrogen-bonding between H₂PT and +Ccn or −Ccn.

Fig. 8 (a) ¹HNMR spectra of −Ccn and H₂PT; (b) ¹HNMR spectra of +Ccn and H₂PT.

Fig. 9 The structures of (a) −Ccn + H₂PT and (b) +Ccn + H₂PT optimized by Gaussian software.

The XPS tests on IMU-Zn1 soaked in 1.0 × 10⁻² M +Ccn and −Ccn solution for 3 days were performed, respectively, as shown in Fig. S14 (ESI†). The peak of N1s in −Ccn@IMU-Zn1 moves from 399.26 eV to 399.51 eV compared with that of IMU-Zn1, which is due to the formation of hydrogen-bonding interactions, resulting in a decrease in the density of the electron cloud on the N atom in IMU-Zn1. In contrast, the N1s peak position of +Ccn@IMU-Zn1 only moves to 399.32 eV. It is obvious that the hydrogen-bonding interaction of −Ccn@IMU-Zn1 is stronger than that of +Ccn@IMU-Zn1.^37–39

Using Gaussian 09 software, the hydrogen-bonding interactions between H₂PT and +Ccn or −Ccn were optimized at B3LYP/6-31G (d,p) base levels.^40,41 As shown in Fig. 9a, the pyridine N atom of H₂PT tends to be close to the –OH group of −Ccn; the measured distance of O–H⋯N is 1.9185 Å, illustrating that the strong hydrogen-bonding interactions may form after the introduction of −Ccn in IMU-Zn1. As shown in Fig. 9b, the interactions between +Ccn and H₂PT after the optimized structure is not evident, and the measured distance of O–H⋯N is 2.9918 Å, which proves that there is a weaker hydrogen-bonding interaction between +Ccn and the H₂PT ligand.⁴²

In short, there is a PET process and there are electrostatic interactions between +Ccn and IMU-Zn1, which lead to the fluorescence enhancement of IMU-Zn1. The influence of −Ccn can lead to a PET process and electrostatic interactions between −Ccn and IMU-Zn1, and the hydrogen-bonding interactions between them may bring about greater system stability, thereby inducing more fluorescence enhancement of IMU-Zn1.

Conclusions

In summary, a new transition metal coordination polymer IMU-Zn1 was synthesized using a solvothermal method. The structure of IMU-Zn1 is a one-dimensional chain, and its three-dimensional supramolecular assembly is formed by the interactions of hydrogen-bonding and π⋯π stacking. IMU-Zn1 had excellent crystallinity and phase purity, and exhibited good thermal stability and fluorescence sensing performance. The experimental results showed that IMU-Zn1 could be used as a fluorescent probe to distinguish +Ccn and −Ccn, and it displayed excellent anti-interference and recyclability for the detection of analytes. It was found that there was a PET process and electrostatic interactions existed between +Ccn or −Ccn and IMU-Zn1, leading to the fluorescence enhancement mechanism. −Ccn exhibited a stronger hydrogen-bonding interaction with IMU-Zn1, resulting in a more significant fluorescence enhancement effect. Using the above results we have elucidated the structure–response relationship between chemical sensors and analytes and provide meaningful guidance for designing light-emitting coordination polymers for the detection of chiral molecules.

Author contributions

Wenping Hu: investigation and writing – original draft. Nan Wu, Dechao Li, Yefang Yang, and Shaowen Qie: formal analysis. Shuai Su, Ruijie Xu and Wenting Li: validation. Ming Hu: supervision, writing – review and editing, and funding acquisition.

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

This work is financially supported by the National Natural Science Foundation of China (No. 21761024, No. 22161032) and the Natural Science Foundation of Inner Mongolia (No. 2021MS02012).

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Footnote

† Electronic supplementary information (ESI) available: Experimental details, crystallographic details, FT-IR spectra, thermogravimetric analysis, fluorescence spectra and fluorescence lifetime results. CCDC 2361371. See DOI: https://doi.org/10.1039/d4tc03506b

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