English

• 0.   Introduction

  Coordination polymers, especially metal-organic frameworks (MOFs), have recently attracted much attention due to their distinctive structural characteristics and adjustable properties^[1-2]. These features have positioned them as promising candidates for various applications, including luminescence-based thermometry^[3-4]. The employment of coordination polymers in luminescent thermometry combines the advantageous characteristics inherent to both metal centers and organic linkers, resulting in pioneering sensing mechanisms that are capable of functioning across a broad temperature spectrum^[5]. Temperature-dependent luminescence properties were employed as the principal metrics in temperature measurements and sensing, with changes in lifetime, intensity, and color serving as key variables. For example, Zhao et al. developed luminescent thermometers based on Ln-MOF that exhibit high sensitivity and stability within the physiological temperature range^[6]. Guan et al. synthesized Tb-MOFs luminescent thermometers that were effective within a temperature range of 107-393 K^[7]. He and colleagues prepared Eu_x/Tb_1-x(phen)@MOF-808, achieving a sensitivity of up to 8.65%·K^-1 across the 293-383 K range^[8].

  Calixarenes, a kind of cyclic oligomer composed of phenolic units linked by methylene bridges or others, are good ligands for the construction of coordination polymers and adopt a number of different conformations^[9-11]. The supramolecular chemistry community has shown considerable interest in calixarene complexes due to the distinctive structural features and adaptable luminescent properties exhibited by these molecules. Kajiwara et al. altered the photophysical characteristics of Tb/Eu clusters by modifying the pinched cone and cone configuration of p-tert-butylsulfonylcalix[4]arene^[12]. Lu et al. enhanced the photoluminescent characteristics through the alteration of the lanthanide core cluster species encapsulated within tert- butylthiacalix[4]arene frameworks^[13]. This property not only enhances their utility in sensing applications but also exerts an influence on their emission characteristics^[13-15].

  In this study, a metal-calixarene coordination poly- mer, [Tb₄(HTC4A)(TC4A)(OBBA)₂(CH₃OH)₄(μ₄-OH)]_n, featuring a 1D chair-like structure, was obtained by solvothermal synthesis, where H₄TC4A=p-tert-butylthiacalix[4]arene, and H₂OBBA=4, 4′-oxybisbenzoic acid. The objective of this work is to investigate the relationship between temperature and luminescence intensity, with the ultimate goal of developing an effective and responsive temperature sensor. It may have potential applications in environmental monitoring, medical diagnostics, and smart materials, where accurate temperature management and measurement are crucial.

  1.   Experimental

  1.1   General methodology and materials

  Unless otherwise specified, no further specification is required when commercially available reagents are used for all reactions. H₄TC4A was prepared according to the literature methods^[16].

  1.2   Preparation of complex 1

  A mixture of H₄TC4A (18.1 mg, 0.025 mmol), Tb(AcO)₃·6H₂O (16.8 mg, 0.05 mmol), H₂OBBA (12.9 mg, 0.05 mmol), CH₃OH (3 mL), DMF (0.5 mL), and a few drops of triethylamine was added to a 20 mL Teflon-lined stainless steel, heated to 130 ℃ in 90 min, kept for 3 d, and slowly cooled to room temperature in one day. Isolation of the canary yellow crystal blocks was achieved through filtration, followed by a CH₃OH wash and air drying. Yield: ca. 68% concerning H₄TC4A. Elemental analysis Calcd. for C₁₁₂H₁₂₀O₂₃S₈Tb₄·3.75C₃H₇NO(%): C, 49.34; S, 8.55; N, 1.75; H, 4.91. Found(%): C, 47.76; S, 8.65; N, 1.60; H, 5.01.

  1.3   Crystal structural determination

  Crystallographic information was gathered using a Bruker APEX-Ⅱ CCD diffractometer with a graphite-monochromated Cu Kα radiation (λ=0.154 178 nm) at 180 K. The structure of the complex was solved via direct methods with refinement through the full-matrix least-squares technique on F ² using the SHELXTL and Olex2 packages^[17-18]. The E-map was used to locate the heavy atoms, and the differential Fourier synthesis was used to find additional non-hydrogen atoms. All hydrogen atoms of organic ligands were theoretically generated on the specific atom and anisotropically refined using isotropic thermal parameters, whereas all non- hydrogen atoms underwent anisotropic refinement. The method of "SQUEEZE" implemented in PLATON was used to subtract the contribution of the disordered solvent molecules (about 3.75 DMF molecules per formula unit) from the reflection data. The calculated solvent content (10% of the sum of molecular formula mass and solvent mass) aligns with the mass loss in thermogravimetric analysis (TGA). The molecular formula stands for the framework. The various constraints were attached to those disordered groups, and their geometries and atomic displacements were approximated to the theoretical values.

  1.4   Physical measurements

  The Thermo Fisher Scientific iS5 spectrometer was used to collect KBr pellets and get FTIR spectra. Elemental analysis (C, H, S, and N) was performed with the VarioEL instrument. TGA was conducted on a STA 449 F3 thermogravimetric analyser. The photoluminescence properties were studied on the Edinburgh analytical instrument (FLS1000 fluorescence spectrometer). The powder X-ray diffraction (PXRD) patterns were recorded on an XRD (Bruker, D8 ADVANCE) with Cu Kα radiation (λ=0.154 18 nm) at room temperature (2θ=5°-50°, U=40 kV, I=40 mA). The simulated PXRD pattern was derived from the single crystal data.

  2.   Results and discussion

  2.1   Synthesis and characterization

  The 1D chair-like terbium complex 1 was synthesized by a 2∶1∶2 molar ratio reaction of Tb(AcO)₃·6H₂O, H₄TC4A, and H₂OBBA in a mixed solution of CH₃OH, DMF, and TEA. Single-crystal X-ray diffraction (SCXRD) analysis revealed that complex 1 crystallizes in the space group P2₁/n of a monoclinic crystalline system, displaying a 1D coordination chain as shown in Fig.1. Details of the crystallographic data and structural refinements for 1 are summed up in Table S1 (Supporting information). In its structure, there are four crystallographically independent Tb³⁺ in each unit. All Tb³⁺ ions are nine-coordinated, including two S-atom bridges, four phenolic O-atoms, one O-atom of OBBA^2-, one CH₃OH ligand, and a μ₄-O located at the center of a Tb₄ square. Bond valence sum calculation suggests the μ₄-O to be monovalent, that is, it is a μ₄-OH. The square Tb₄ (Tb1-Tb2-Tb1^ⅰ-Tb2^ⅰ) was created by joining four neighboring Tb³⁺ with equally complicated surroundings (Fig.1a) via the μ₄-OH. Two deprotonated H₄TC4A molecules with the cone conformation entrap the square Tb₄ and adopt tail-to-tail mode to form a sandwich-like entity [Tb₄(HTC4A)(TC4A)(μ₄-OH)]⁴⁺. Two OBBA^2- link each Tb₄-(TC4A)₂, creating a 1D chain-like structure. The distance between two μ₄-OH is 1.638 5 nm, and the angle between the plane in Tb₄-(TC4A)₂ located and OBBA^2- is 34.90° (Fig.S2). In the square [Tb₄(μ₄-OH)] cluster, two Tb ions joined by a μ₂-O_phenolic have similar distances (Tb1…Tb2 for 0.359 2 nm, and Tb1…Tb2ⁱ 0.359 5 nm, respectively), and the diagonally to the square are 0.499 6 nm (Tb1…Tb1ⁱ) and 0.516 7 nm (Tb2…Tb2ⁱ) respectively. All Tb—O bond lengths are comparable (0.233 6-0.239 6 nm), except Tb—O(μ₄-OH) (0.249 8-0.258 4 nm). It should be noted that the OBBA^2- and the CH₃OH have an extra intermolecular hydrogen bond (C—H…O). The thermal stability of 1 was investigated. Under an argon atmosphere with a heating rate of 10 ℃·min^-1 (Fig.S3), the approximate 10% weight loss before 320 ℃ might be the release of solvent molecules and coordination methanol. The TG results show that 1 was thermally stable and did not break down until 440-470 ℃, respectively. The residual value was 55.5% due to the formation of Tb₂O₃.

  Figure 1

  

  Figure 1.  (a) Geometric configuration of Tb³⁺ and sandwich-like Tb₄-(TC4A)₂ secondary building units (SBUs); (b) Bowl diagrams of connection modes of Tb₄ units; (c) Extended supramolecular structure of 1

  Hydrogen atoms are omitted for clarity; Symmetry code: ⁱ 1-x, 1-y, 1-z.

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  2.2   Photophysical properties

  The solid-state luminescence properties of 1 were studied at ambient temperature (Fig.2a and S4). When excited at 364 nm, the emission spectrum of 1 exhibited the characteristic emission peaking at 490, 548, 586, 622, 649, 668, and 679 nm, originating from the ⁵D₄→⁷F_J (J=6-0) f-f transitions of the Tb³⁺ ions. The primary emission was 548 nm (green), belonging to the ⁵D₄→⁷F₅ transition, with the lifetime of 1.20 ms and the quantum yield of 6.5%. The CIE 1931 (International Commission on Illumination) chromaticity map, which combines the three primary colors to create a variety of hues, has been widely used for targeted modulation of emission intensities and wavelengths. The CIE chromaticity map converts the emission spectrum of one to the x and y axes. Evaluating the gross emission of the sample according to the CIE standard chromaticity map, in conjunction with the chromaticity coordinates (x, y) of 1 as (0.38, 0.56), it is easy to display that 1 emits green light.

  Figure 2

  

  Figure 2.  (a) Excitation (dashed line) and emission (line) spectra of 1; (b) Emission spectra of 1 at the temperatures from 298 to 548 K; (c) Decay lifetimes of 1 at different temperatures; (d) Emission spectra of 1 recovered at 473 K

  Inset: the chromaticity coordinates of 1 at 298 K.

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  Notedly, the temperature-dependent luminescence of 1 was investigated. The luminescence and lifetime decay spectra of the complex were measured across a broad thermal range (50-548 K, Fig.2b, 2c, and S5). Remarkably, both emission intensity and decay lifetime exhibited remarkable thermal stability below 298 K, and a distinct transition occurred beyond the threshold temperature (298 K), where the luminescence decreased due to the thermal activation of the non-radiative decay pathway and relaxation, which is almost completely quenched at 548 K. The luminescent properties (emission intensity and lifetime) and structural integrity were recovered by exposing the sample treated at 473 K to a methanol atmosphere for a period of time (Fig.S6), which can be repeated at least five times (Fig.S7).

  As depicted in Fig.2b, the Tb³⁺ emitted a strong intensity (548 nm, ⁵D₄→⁷F₅) at 298 K and subsequently decreased with temperature by 2.36%·K^-1, whereas the intensity at 548 nm at 398 K was quenched by more than half. The performance and stability parameters of the thermometer were determined (Fig.3). The relative thermal sensitivity (S_r), defined as S_r=(∂Δ/∂T)/Δ^{[6, 19]}, is typically used to evaluate the performance of different optical thermometers. Here, T represents the temperature and Δ signifies the thermometric parameter. Within the temperature range of 323-473 K, the emission intensity of ⁵D₄→⁷F₅ (I_{548 nm}) with T demonstrated a linear relationship (I=2.979 29-0.006 15T). It could be calculated that the maximum S_r of 1 was 8.743%·K^-1 at 473 K.

  Figure 3

  

  Figure 3.  (a) Temperature-dependent intensity of the ⁵D₄→⁷F₅ and the fitted curve for 1; (b) Relative sensitivity values of 1 at different temperatures

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  The emission lifetime of 1 also exhibited a similar trend to the emission intensities. To gain further insight into the temperature sensing capability of 1, the decay times of the resultant samples were measured at diverse temperatures. The temperature-dependent decay time curves of 1 are shown in Fig.2c. The lifetimes of 1 presented a decreasing trend with the increase in temperature in the range of 298-548 K. According to the Struck and Fonger′s model, the lifetime and temperature are postulated to maintain the following relationship^[20-21]:

  $ \tau(T)=\frac{\tau_0}{1+A \exp \left(\frac{-\Delta E}{k_{\mathrm{B}} T}\right)} $

  (1)

  In this context, τ₀ is the lifetime of 1 at 0 K, while A is a constant. The Boltzmann constant (8.626×10^-5 eV·K^-1) is represented by k_B; while T denotes the absolute temperature and ΔE is the thermal quenching active energy, respectively. As illustrated in Fig.4a, the temperature-dependent lifetime data exhibited a satisfactory fit to the experimental data using the theoretical model expressed by Eq.1. The A value was determined to be 4 190, and the ΔE value was calculated to be 0.32 eV (2 581 cm^-1).

  Figure 4

  

  Figure 4.  (a) Emission lifetime of 1 as a function of temperature; (b) Relative sensitivity of integrated emission intensity and lifetime decay concerning temperature

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  Relative sensitivity (S_R), which is independent of the characteristics of different optical thermometers, is commonly used to evaluate the temperature measurement performance. It is independent of the specific characteristics of different thermometers and represents the relative change in the parameter Δ per Kelvin of temperature change. This can be determined as^[22-23]:

  $ S_{\mathrm{R}}=\left|\frac{1}{\tau} \cdot \frac{\mathrm{~d} \tau}{\mathrm{dT}}\right| \times 100 \%=\frac{A \exp \left(\frac{-\Delta E}{k_{\mathrm{B}} T}\right)}{1+A \exp \left(\frac{-\Delta E}{k_{\mathrm{B}} T}\right)} \times \frac{\Delta E}{k_{\mathrm{B}} T^2} \times 100 \% $

  (2)

  The S_R values were calculated and described in Fig.4b with the aid of Eq.2 and the fitted parameters. As disclosed, the S_R initially rose and subsequently decreased overall as the temperature increased between 298 and 548 K. 1 showed a decrease rate of 0.68%·K^-1 at 548 nm from 298 to 548 K, and as high as 1.06%·K^-1 at 498 K. The result indicates that 1 has the potential to serve as a promising candidate for non-contact optical thermometers by leveraging lifetime technology in conjunction with intensity. Significantly, the sensitivities that are achieved by these two technologies are different, demonstrating that the thermometric properties of 1 can be efficiently tuned by using different modes.

  Alongside these luminescence properties, it is crucial to investigate the luminescence stability of 1 in various environments. For example, both theoretical and experimental research have proved that solvent molecules (e.g., H₂O) not only cause damage to the crystal structure of the material but also significantly enhance the vibrational bursts in the emission cores. The [Tb₄(μ₄-OH)] unit is protected by a pair of conical configurations of H₄TC4A ligands by tail-to-tail, forming a sandwich structure, while the other coordination sites for Tb³⁺ are provided by methanol and deprotonated H₂OBBA. An extra hydrogen bond between the OBBA^2- and methanol reduces the potential for attack of the [Tb₄(μ₄-OH)] moiety by other solvent molecules, effectively reducing the non-radiative vibrations and stabilizing its luminescent properties.

  Further investigation was done into its luminescent properties in organic solvents. The samples have poor solubility in common organic solvents (CH₃OH, CHCl₃, CH₃COCH₃, DMF, etc.) and water. As demonstrated in Fig.5a and 6a, the PXRD analysis confirmed that 1 retained its structural integrity after 48 h exposure to the aforementioned solutions and aqueous solutions with different pH values (pH=1, 6, 9, and 14). Conversely, the luminescence characteristics of the treated samples exhibited slight variations (Fig.5b and 6b), which are hypothesized to be due to the alkaline conditions or solvent interactions, which may enhance the energy transfer from the ligand to the rare earth ions and consequently affect the fluorescence intensity^[24].

  Figure 5

  

  Figure 5.  PXRD patterns (a) and emission spectra (b) of 1 in different solvents

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  Figure 6

  

  Figure 6.  PXRD patterns (a) and emission spectra (b) of 1 in aqueous solutions with different pH values

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  3.   Conclusions

  Within this work, we fabricated a 1D coordination polymer (1) utilizing SBUs with V-type dicarboxylate ligands. The photoluminescence revealed that complex 1 emitted green emission with the chromaticity coordinate of (0.38, 0.56). As the temperature increased, the emission intensity of 1 concomitantly decreased. Specifically, 1 was an efficient luminescent thermometer over a wide temperature range from 298 to 548 K and with relative sensitivity (S_r) of 8.743%·K^-1. This work might offer an innovative perspective for developing lanthanide-calixarene coordination compounds for thermometric applications.

  
  Supporting information is available at http://www.wjhxxb.cn
  
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• 

  Figure 1  (a) Geometric configuration of Tb³⁺ and sandwich-like Tb₄-(TC4A)₂ secondary building units (SBUs); (b) Bowl diagrams of connection modes of Tb₄ units; (c) Extended supramolecular structure of 1

  Hydrogen atoms are omitted for clarity; Symmetry code: ⁱ 1-x, 1-y, 1-z.

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  Figure 2  (a) Excitation (dashed line) and emission (line) spectra of 1; (b) Emission spectra of 1 at the temperatures from 298 to 548 K; (c) Decay lifetimes of 1 at different temperatures; (d) Emission spectra of 1 recovered at 473 K

  Inset: the chromaticity coordinates of 1 at 298 K.

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  Figure 3  (a) Temperature-dependent intensity of the ⁵D₄→⁷F₅ and the fitted curve for 1; (b) Relative sensitivity values of 1 at different temperatures

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  Figure 4  (a) Emission lifetime of 1 as a function of temperature; (b) Relative sensitivity of integrated emission intensity and lifetime decay concerning temperature

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  Figure 5  PXRD patterns (a) and emission spectra (b) of 1 in different solvents

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  Figure 6  PXRD patterns (a) and emission spectra (b) of 1 in aqueous solutions with different pH values

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