English

• 0.   Introduction

  Growing attention has been devoted to metal-organic 1D-coordination polymers based on lanthanides owing to their wide applications in the fields of luminescence, catalysis, magnetism, biological activities, and so on^[1-3]. Among these fields, great interest has been aroused in the outstanding fluorescence characteristics of Ln(Ⅲ)-based coordination polymers^[4], i.e., high quantum yield, narrow emission band, large displacement, and long fluorescence lifetime, which make them a forward position in fluorescent probe research^[5]. Compared with the conventional screening techniques, such as chemical method, hydrogel method, polarography, and spectrophotometry, fluorescent probes have been assigned great significance for the response of various analytes owing to their high sensitivity and selectivity, reliability, convenient utilization, and fast response^[6-7]. Fluorescent probes can detect various analytes, such as amino acids^[8], antibiotics^[9], anions^[10], and cations^[11].

  The selection of a suitable ligand coordination with suitable lanthanide ions is crucial for the assembly of luminescent metal-organic coordination polymers. 2, 5-dihydroxyterephthalic acid (H₄dhtp) is of special interest because it possesses two sets of oxygen donors coming from both carboxylic and phenol functional groups^[12], which can be used to construct the expected complexes owning some unique characteristics based on the following considerations: (ⅰ) the carboxylic acid and phenol of H₄dhtp could be deprotonated displaying diverse coordination modes^[13]; (ⅱ) H₄dhtp exhibits strong coordination abilities, which can easily coordinate with the oxophilic lanthanide ions through O donor atoms^[14]; (ⅲ) H₄dhtp is a highly luminescent organic molecule, and it is easy to form luminescence materials^[15]. The Yb³⁺, Tm³⁺, and Er³⁺ ions can be used as optical fiber amplification materials for optical communication because of their excellent spectral properties. Xu et al. have assembled a near-IR-luminescent octanuclear Yb(Ⅲ) complex^[16]. Shi et al. have constructed one Yb₄₂ nanowheel, which shows interesting near-infrared (NIR) lanthanide luminescence sensing towards anions, especially to fluoride at the μmol·L^-1 level^[10].

  Generically speaking, the luminescence center may come from the lanthanide ion itself emission, organic ligand emission, guest molecule emission, and the synergistic interactions of the above mechanisms. And the luminescent mechanisms in coordination polymers stem from ligand-to-metal charge transfers (LMCT), metal-to-ligand charge transfers (MLCT), or ligand-to-ligand charge transfers (LLCT)^[17]. When a target analyte interacts with the coordination polymers, it can cause a fast change in photophysical properties in the material. These changes might include alterations in the absorption or emission spectra, changes in luminescence intensity, or shifts in the luminescence lifetime. To enrich the theoretical basis and luminescent material database, as part of our current focus on the metal-organic luminescence coordination polymers, we have successfully assembled and investigated one lanthanide-based coordination polymer, {[Yb(H₂dhtp)_1.5(H₂O)₄]·3H₂O}_n (1), which exhibits a 1D chain structure. The synthesis, crystal structure, and fluorescence properties have been presented. The finding provides new insights into the synthesis of interesting 1D Ln(Ⅲ)-based linear chain materials showing luminescence properties.

  1.   Experimental

  1.1   Reagents

  All reagents were used as commercially obtained without further purification. Ytterbium(Ⅲ) nitrate hexahydrate (Yb(NO₃)₃·6H₂O, AR) was purchased from Alfa Reagent (Zhengzhou Alfa Chemical Co., Ltd.). The H₄dhtp ligand (AR) was purchased from Macklin Reagent (Shanghai Macklin Biochemical Co., Ltd.). Ligand 4-[tris(hydroxymethyl)methyl]pyridine (4-thmpyH₃, AR) was purchased from Tensus Biotech (Shanghai Tensus Biotechnology Co., Ltd.). Distilled water (H₂O) was prepared in our laboratory. The configuration of the H₄dhtp and thmpyH₃ ligands is shown in Scheme S1 (Supporting information).

  The metal salt and solvent used for luminescent detection were commercially available and used as received. ZrCl₄ (AR), FeCl₃·6H₂O (AR), and DMF (AR) were purchased from Macklin Reagent (Shanghai Macklin Biochemical Co., Ltd.). ZnCl₂ (GR) and MgCl₂ (GR) were purchased from Chengdu Aikeda Chemical Reagent Co., Ltd. CaCl₂ (GR) and MnCl₂ (GR) were purchased from Shandong Xiya Chemical Technology Co., Ltd. EtOH (AR) was purchased from Xilong Scientific Co., Ltd. MeOH (AR) was purchased from Sinopharm Chemical Reagent Co., Ltd. KCl (AR) and NaCl (AR) were purchased from Sigma-Aldrich Chemie GmbH.

  1.2   Synthesis of complex 1

  A mixture of H₄dhtp (1.2 mmol, 0.237 8 g) and 4-thmpyH₃ (2 mmol, 0.366 4 g) in 10 mL DMF was continuously stirred for 30 min and then heated for 10 min. After that, a deionized water solution (30 mL) of Yb(NO₃)₃·6H₂O (3 mmol, 1.401 5 g) was added to the above solution for being stirred further for 20min at ambient temperature. The resulting suitable for X-ray diffraction, yellow block-shaped single crystals of 1 were obtained in about 20% yield based on the metal salt. The reaction was repeated to achieve the desired amount of material for further characterization and applications. Elemental analysis Calcd. for C₁₂H₂₀O₁₆Yb(%): Yb 29.16, C 24.29, H 3.40. Found(%): Yb 29.02, C 24.37, H 3.85.

  1.3   Fluorescent sensing

  1.3.1   Fluorescent properties of complex 1 in different solvents

  The powder of 1 (2.9 mg) was dispersed in 50 mL of different solvents (including H₂O, EtOH, MeOH, DMF) with ultrasonication for 30 min to form uniform suspensions. The luminescent spectra of the suspension (2 mL) were recorded. All results of luminescent spectra were repeated five times, and their average values were taken.

  1.3.2   Fluorescent properties of complex 1 in the presence of different metal cations

  The powder of 1 (2.9 mg) was dispersed in 50 mL aqueous solution with ultrasonication for 30 min to form uniform suspensions. The structure of complex 1 exhibited excellent stability after ultrasonication, as confirmed by the powder X-ray diffraction (PXRD) analysis (Fig.S4c). The selectivity experiment of 1 was conducted by introducing a series of cations that are both abundant in nature and easily accessible in the laboratory (Fe³⁺, Zn²⁺, Mg²⁺, Mn²⁺, K⁺, Na⁺, and Ca²⁺, 0.1 mol·L^-1, 5 mL) into an aqueous suspension of 1 (5 mL). The resulting mixtures (2 mL) were then used for luminescence testing. All results of luminescent spectra were repeated five times, and their average values were taken.

  1.4   Details of X-ray crystallography

  A regular and suitable size single crystal (0.31 mm×0.23 mm×0.12 mm) of complex 1 was selected for the X-ray diffraction analysis. The data were obtained through an XtaLAB Synergy Dualflex with HyPix diffractometer by using mirror-monochromated Cu Kα radiation (λ=0.154 184 nm) at 100 K by using the ω scan technique. The structure was solved by direct methods and refined on F² by using full-matrix least-squares with SHELXL and Olex2 software programs. All non-hydrogen atoms were refined with anisotropic thermal parameters. H atoms bonded to C atoms were placed in geometrically calculated positions. H atoms bonded to O atoms were refined in a riding mode. Due to the disorder of the free H₂O molecule, the SQUEEZE procedure in the PLATON software was used for 1. Crystallographic data and crystal structure refinement parameters for complex 1 are summarized in Table S1. The selected bond lengths and angles are presented in Table S2 for 1.

  1.5   Physical techniques

  IR spectrum was observed in the range 4 000-400 cm^-1 using KBr pellets at room temperature using an FTIR spectrophotometer (ThermoFisher Nicolet iS50). Elemental analysis (C and H) for the prepared samples was performed using an Elementar Vario EL Ⅲ elemental analyzer. The Yb content was determined on an Inductively Coupled Plasma-Optical Emission Spectrometry (iCap 7000, ThermoFisher, USA). Thermogravimetric analysis (TGA) of the sample was performed from room temperature to 800 ℃ with a heating rate of 10 ℃·min^-1 in a flowing nitrogen atmosphere using a TA SDT-Q600 analyzer. PXRD measurement was performed at room temperature on a Shimadzu XRD-6100 diffractometer (Cu Kα, λ=0.154 18 nm) with 2θ ranging from 5° to 60° with a scan speed of 5 (°)·min^-1. The accelerating voltage and current were 40 kV and 30 mA, respectively. The luminescent spectra were recorded using a Spectrofluorometer FS5 (Edinburgh Instruments Ltd.). The UV-Vis spectra were recorded using a Jasco V-780 UV-Vis/NIR spectrophotometer from JASCO Corporation.

  2.   Results and discussion

  2.1   Synthesis

  Heating a solvent mixture solution of H₂O (30 mL) and DMF (10 mL) of Yb(NO₃)₃·6H₂O, H₄dhtp, 4-thmpyH₃ in the molar ratio of 3∶1.2∶2 afforded yellow block-shaped single crystals of complex 1. In our recent work, the structure possessing a similar reaction system for Yb³⁺ differs from that previously found for Dy³⁺ and Gd^{3+ [18]}. Moreover, there are only H₄dhtp and water molecules within the structure for 1. The DMF and the 4-thmpyH₃ ligands are absent in the crystal structure of 1. Therefore, the reaction was optimized employing pure solvent H₂O conditions. Unfortunately, single crystals could not be obtained. When thmpyH₃ was absent in the reaction system, no crystals were obtained. Therefore, the presence of DMF and 4-thmpyH₃ in the reaction mixture is essential to generate the crystal product of good quality. Additionally, we changed the molar ratios in our experiment, which provided different quality single crystals validated by X-ray structure analysis (Table S3). Experimental results verified that the molar ratio of 3∶1.2∶2 can achieve the single crystals possessing the highest yield and are best suitable for single-crystal X-ray crystallography.

  2.2   Crystal structure

  Complex 1 crystallizes in the triclinic space group P1. As shown in Fig. 1, the asymmetric cell unit of 1 consists of one Yb(Ⅲ) ion, one and a half H₂dhtp^2- ligands, four coordinated water molecules, and three uncoordinated water molecules, i.e., [Yb(H₂dhtp)_1.5(H₂O)₄]·3H₂O. The center Yb1 ion is connected by eight oxygen atoms (O3, O4, O5, O6, O1W, O2W, O3W, and O4W), containing four oxygen atoms of three H₂dhtp^2- ligands, and four oxygen atoms of four coordinated water molecules. The continuous shape measurement (CShM)^[19], which was analyzed with SHAPE 2.1 software, confirms biaugmented trigonal prism geometry (C_2v, CShM=1.035) around the Yb(Ⅲ) ion in complex 1 (Table S4). This result is common for 8-coordinated lanthanide species^[20]. The deprotonated H₄dhtp (H₂dhtp^2-) shows two coordination modes: A and B (Fig.S1). As shown in Fig. 2, the H₂dhtp^2- ligand in mode A (μ₂-η¹∶η¹∶η¹) possesses three-dentate coordination, connected in a pattern with two Yb³⁺ ions, resulting Yb…Yb distance of 0.498 5 nm. The H₂dhtp^2- ligand in mode B (μ₂-η¹∶η¹) possesses two-dentate coordination, connected in a pattern with two Yb³⁺ ions, resulting Yb…Yb distance of 1.191 9 nm. Significantly, two H₂dhtp^2- linkers (mode A) and two H₂dhtp^2- linkers (mode B) extend along four different directions, and such connectivity is repeated periodically forming a 1D chain structure. The distances of the Yb—O bond fall in a range of 0.221 1-0.245 3 nm, which are comparable to the reported Yb(Ⅲ) complexes^[16]. The Yb ion is in a +3 oxidation state determined from a combination of bond length, charge balance, and bond valence sum (BVS) calculations^[21].

  Figure 1

  

  Figure 1.  Molecular structure of complex 1, with the probability being 50%

  Hydrogen atoms and crystalline H₂O are omitted for clarity.

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  Figure 2

  

  Figure 2.  View of the framework of complex 1

  Some atoms are omitted for clarity; Symmetry codes: ⁱ-1+x, y, 1+z; ⁱⁱ-x, 1-y, 1-z; ⁱⁱⁱ 1-x, 1-y, -z; ^iv 1+x, y, -1+z; ^v 2-x, 1-y, 1-z.

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  2.3   Thermal stability, IR, and PXRD analysis

  To investigate the thermal stability of complex 1, TGA data have been obtained, which are shown in Fig.S2. The continuous weight loss of 21.01% between 99 and 243 ℃ can be attributed to the loss of three free H₂O and four coordinated H₂O molecules (Calcd. 21.24%). Then, 1 was slowly decomposed, and the final decomposed product was Yb₂O₃. The IR absorption of 1 is shown in Fig.S3. The asymmetric ν_as(COO) and symmetric ν_s(COO) stretching vibrations of 1 were located at 1 559 and 1 428 cm^-1, respectively. And the vibration of the Ar—OH group at 1 238 cm^-1 was observed, which denotes the presence of the H₂dhtp^2- ligand^[22]. The phase purity was corroborated by PXRD at room temperature. The experimental pattern of 1 was in good agreement with the simulated ones generated from single-crystal diffraction data (Fig.S4).

  2.4   Luminescent properties

  2.4.1   Solid-state luminescent spectra of complex 1

  The solid-state luminescent spectra of 1 displayed an emission band with the maximum at 508 nm (λ_ex=408 nm, Fig.S5), which should be attributed to the intraligand charge transfer of the H₄dhtp ligand. In contrast to the free ligand that showed an emission band with the maximum at 471 nm (λ_ex=380 nm), 1 demonstrated a slight red shift and wider emission band, due to the effect of deprotonation and coordination^[5]. Generally speaking, the luminescence center may originate from the emission of the lanthanide ion itself, the emission of the organic ligand, the emission of the guest molecule, and the synergistic interactions among these mechanisms. In our study, the characteristic NIR emission bands of Yb(Ⅲ) complex have not been observed; therefore, the luminescence center mainly comes from organic ligand emission^[23].

  2.4.2   Fluorescent properties of complex 1 in different solvents

  Upon excitation at 330 nm, the luminescence of 1 in various solvents was also studied. The emission peaks of 1 in all solvents were observed at 554, 554, 500, and 557 nm for H₂O, DMF, EtOH, and MeOH, respectively (Fig.S6). Experimental results indicate that the fluorescence intensity of 1 in water was the strongest. Therefore, the suspension of 1 was prepared with water in the subsequent experiments. It is obvious that the emission peaks of 1 in different organic solvent suspensions were slightly shifted compared with the solid state emission peaks (508 nm for 1), which may primarily be due to the interactions between the guest solvent molecules and the host frameworks. The variation in the luminescence intensity of 1 across various solvents is attributed to the subtle interactions between the framework of 1 and solvents of differing polarities, as well as steric hindrance and electron transfer effects^[24].

  2.4.3   Fluorescent properties of complex 1 in the presence of different metal cations

  To investigate the detection of metal ions by 1, the luminescent response in the presence of different metal cations (Fe³⁺, Zn²⁺, Mg²⁺, Mn²⁺, Zr⁴⁺, K⁺, Na⁺, and Ca²⁺) in aqueous solution under 330 nm excitation was investigated (Fig. 3 and S7). Interestingly, the addition of Zr⁴⁺ resulted in a blue shift and an increase in intensity. It was found that the degree of the luminescence blue-shift of 1 gradually increased as the concentrations of the added Zr⁴⁺ increased (Fig.S8 and S9). Furthermore, the addition of most of the cations causes an increase in the luminescence intensity, but the exception being the decrease in luminescence with Fe³⁺ (Fig. 3). To further illustrate the luminescence sensing property of 1, a luminescence titration of 1 was executed by adding different concentrations of Fe³⁺. It was found that the luminescence intensities of 1 gradually decreased as the concentrations of the added Fe³⁺ increased. The Stern-Volmer equation can be used to calculate the luminescence enhancement and quenching efficiencies (K_SV) of 1 in response to the addition of a cation. The equation is given by (I₀-I)/I=K_SVc_m, where I₀ is the luminescence intensity before the addition of the cation, I is the luminescence intensity after the addition of the cation, and c_m is the concentration of the added cation^[10]. As shown in Fig.S10, the K_SV for 1 detecting Fe³⁺ was 9 132.6 L·mol^-1, indicating the high sensitivity of 1 to this cation.

  Figure 3

  

  Figure 3.  (a) Emission spectra of complex 1 dispersed in different cations in H₂O at excitation of 330 nm; (b) Fluorescence intensities of 1 in the presence of different cations in H₂O

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  Generally, the recognition mechanism includes photoinduced electron transfer (PET), intramolecular charge transfer (ICT), intramolecular charge transfer proton transfer (ESIPT), and aggregation-induced emission (AIE). To delve into the potential mechanism underlying the luminescent response of complex 1 by Fe³⁺ and Zr⁴⁺, the infrared spectra of the recovered solid samples of 1 following treatment with Fe³⁺ solution (denoted as Fe/1) and Zr⁴⁺ solution (denoted as Zr/1) were meticulously investigated. As shown in Fig.S3, Fe/1 and Zr/1, compared with the original 1, exhibited slight changes in partial vibration peaks, indicating that 1 and Fe³⁺/Zr⁴⁺ may interact, consistent with the literature^[25-26]. Furthermore, the UV-Vis absorption spectra, excitation spectra, and emission spectra of the solution containing Fe³⁺, Zr⁴⁺, H₄dhtp ligand, and 1 were also measured and compared (Fig.S11). The UV-Vis spectrum of Fe³⁺ exhibited significant overlap with the excitation spectrum of 1, suggesting that the luminescence quenching of 1 by Fe³⁺ is likely due to competitive energy absorption^[27-30]. The emission spectrum of 1 was slightly red-shifted compared to that of H₄dhtp, also indicating that the luminescence of 1 mainly comes from the emission of the ligand. According to the structure of the complex, one Yb(Ⅲ) ion is coordinated with four water molecules. At the same time, the H₄dhtp ligand is a highly luminescent organic molecule. The synergistic effects of ligand field perturbation and solvent interactions may alter the electronic structure of Yb(Ⅲ), leading to efficient energy transfer or competitive absorption between the ligand and Yb(Ⅲ). As a result, the characteristic emission of Yb(Ⅲ) is significantly suppressed^[31-32]. The observed blue shift and enhancement in luminescence upon addition of Zr⁴⁺ ions can be attributed to coordination-induced rigidity. Specifically, Zr⁴⁺ may bind to electron-rich groups on the ligands (e.g., the carboxylic acid moieties of H₄dhtp), forming a more rigid Zr-ligand composite structure. This structural rigidification suppresses non-radiative decay pathways (such as intramolecular vibrations or rotations), thereby enhancing the ligand-centered fluorescence^[33].

  3.   Conclusions

  In summary, we have successfully assembled one 1D chain containing the Yb(Ⅲ) ion and H₄dhtp ligand, which shows luminescent response. The luminescence sensing behaviour of 1 to Fe³⁺, Zn²⁺, Mg²⁺, Mn²⁺, Zr⁴⁺, K⁺, Na⁺, and Ca²⁺ cations has been investigated, and the results indicate that 1 shows high sensitivity to Fe³⁺ and Zr⁴⁺ ions.

  
  Acknowledgements: This work was supported by the Natural Science Foundation of Fujian Province (Grant No.2024J08342), Xiamen Medical College Student Innovation and Entrepreneurship Training Plan in 2024 (Grant No.202412631081), and Science and Technology Program of Xiamen Medical College (Grant No.K2022-06) and is gratefully acknowledged. There are no conflicts to declare.
  Conflicts of interest
  Chen Wanting: formal analysis, validation, visualization, methodology, and investigation. Miao Chufei: formal analysis, validation, visualization, methodology, and investigation. Liu Yan: formal analysis and investigation. Zheng Bobi: formal analysis and investigation. Zheng Xiaoyu: investigation and methodology. Xu Han: investigation and methodology. Tian Jumei: conceptualization, supervision, project administration, data curation, funding acquisition, validation, visualization, writing-original draft, and writing-review & editing.
  Author Contributions
  Supporting information is available at http://www.wjhxxb.cn
  
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  Figure 1  Molecular structure of complex 1, with the probability being 50%

  Hydrogen atoms and crystalline H₂O are omitted for clarity.

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  Figure 2  View of the framework of complex 1

  Some atoms are omitted for clarity; Symmetry codes: ⁱ-1+x, y, 1+z; ⁱⁱ-x, 1-y, 1-z; ⁱⁱⁱ 1-x, 1-y, -z; ^iv 1+x, y, -1+z; ^v 2-x, 1-y, 1-z.

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  Figure 3  (a) Emission spectra of complex 1 dispersed in different cations in H₂O at excitation of 330 nm; (b) Fluorescence intensities of 1 in the presence of different cations in H₂O

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