Intensity and lifetime ratiometric luminescent thermometer based on a Tb(III) coordination polymer

Abstract

A three-dimensional terbium(III) coordination polymer of formula [Tb(bttb)_0.5(2,5-pzdc)_0.5]_n (1) [H₄bttb = 1,2,4,5-tetrakis(4′-carboxyphenyl)benzene and H₂-2,5-pzdc = 2,5-pyrazinedicarboxylic acid] was obtained under hydrothermal conditions. The bttb⁴⁻ tetraanion in 1 adopts the bridging and chelating–bridging pseudo-oxo coordination modes while the 2,5-pzdc²⁻ dianion exhibits a rather unusual bis-bidentate bridging pseudo-oxo coordination mode, both ligands being responsible for the stiffness of the resulting 3D structure. Solid-state photoluminescent measurements illustrate that 1 exhibits remarkable green luminescence emission, the most intense band occurring in the region of 550 nm (⁵D₄ → ⁷F₅) with lifetimes at the millisecond scale. Thermometric performances of 1 reveal a maximum relative sensitivity (S_m) of 0.76% K⁻¹ at 295 K (δT = 0.05 K), constituting a Tb^III ratiometric solid luminescent thermometer over the physiological temperature range. Variable-temperature static (dc) magnetic susceptibility measurements for 1 in the temperature range 2.0–300 K show the expected behavior for the depopulation of the splitted m_J levels of the ⁷F₇ ground state of the magnetically anisotropic terbium(III) ion plus a weak antiferromagnetic interaction through the carboxylate bridges. No significant out-of-phase magnetic susceptibility signals were observed for 1 in the temperature range 2.0–10.0 K, either in the absence or presence of a static dc magnetic field.

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Introduction

The self-assembly between the metal center, especially with lanthanide ion, and organic bridging linkers has generated metal–organic frameworks and/or coordination polymers with promising light-emitting compounds.^1–3 They have been effectively explored in the field of functional materials such as stable temperature sensors.^4–6 Since the introduction of lanthanide-containing molecular compounds for temperature sensing in 2004,⁷ and the development of the first mixed-lanthanide MOF luminescent thermometer (S_m = 1.15% K⁻¹ at 200 K) in 2012,⁸ research efforts focused on Ln³⁺-based molecular thermometers have been reported in the literature, spanning temperatures from the cryogenic (T < 100 K) (S_m = 3.26% K⁻¹ at 35.5 K)⁹ to medium (100–300 K) (S_m = 3.27% K⁻¹ at 300 K),¹⁰ and physiological temperature ranges (293–353 K)⁴ (S_m = 0.37% K⁻¹ at 318 K),^11–13 as well as to high-temperature domains (>400 K) (S_m = 0.11% K⁻¹ at 450 K).^4,14

In general, one strategy to synthesize these compounds and establish structure–property relationships consists of initially selecting appropriate nodes (metal centers) and linkers (organic components), which make possible to design luminescent materials, whose potential property extends to nearly every trivalent lanthanide ion.³ Specifically, terbium(III) holds special significance in optical applications because of its strong phosphorescence within the visible region, emitting a green color under the irradiation of a UV lamp.¹⁵ Furthermore, to obtain lanthanide coordination polymers (Ln-CPs), aromatic ligands that tend to enhance the brightness of the lanthanide cation are of particular interest such as the polycarboxylic derivatives 1,2,4,5-tetrakis(4′-carboxyphenyl)-benzene (H₄bttb)^16,17 and 2,5-pyrazinedicarboxylic acid (H₂-2,5-pzdc)^18–22 that can undergo partial or complete deprotonation (see Table S1, in the ESI†). Moreover, as far as we know, there is no research report in the literature concerning the two polycarboxylic acids in the same reactional system and their coordination behavior with Ln^III centers. Therefore, the first goal that we set out for this study is the search for the ideal synthetic parameters to obtain a terbium(III) luminescent coordination network based on the mixed-polycarboxylate ligands and determine its properties. X-ray quality crystals of formula [Tb(bttb)_0.5(2,5-pzdc)_0.5]_n (1) were grown by combining H₄bttb and H₂-2,5-pzdc with Tb^III ion under hydrothermal conditions. Its crystal structure, along with thermogravimetric analysis, vibrational spectroscopy, Powder X-ray diffraction (PXRD), cryomagnetic behavior, and luminescence properties significantly affected by temperature are detailed in this report. Accordingly, this approach offers the advantage of obtaining a new Tb^III ratiometric luminescent nanomolecular thermometer, exhibiting a relative thermal sensitivity of 0.76% K⁻¹ at 295 K.

Experimental section

Materials

Terbium(III) nitrate pentahydrate, 2,5-pyrazinedicarboxylic acid dihydrate, 1,2,4,5-tetrakis(4′-carboxyphenyl)benzene were purchased from Sigma-Aldrich and used as received.

Synthesis of [Tb(bttb)_0.5(2,5-pzdc)_0.5]_n (1)

A mixture of Tb(NO₃)₃·5H₂O (0.043 g, 0.10 mmol), H₄bttb (0.015 g, 0.027 mmol), and H₂-2,5-pzdc·2H₂O (0.015 g, 0.07 mmol) was dissolved in 10 mL of distilled water. The resulting solution was stirred for one hour at room temperature, sealed in a 50 mL Teflon-lined stainless steel autoclave, and heated at 185 °C for three days under autogenous pressure. The reaction system was then cooled to room temperature for two days. Yellow cubic crystals of 1 and a colorless amorphous solid were obtained. 1 was isolated by washing with the mother liquor, separated from this subproduct, and collected. Yield: (0.020 g, 40%). ATR-FTIR (ν/cm⁻¹): 1687 [ν_asym(COO⁻)], 1607, 1579, 1523 [ν(CC/CN)], 1406, 1392 [ν_sym(COO⁻)], 1263 [ν(C–O)], 1180, 1149 [δ_ring + ν(CC)], 1053, 1006 [δ(CH)]; 860, 776 [γ(CH)] and 715 [δ(COO⁻)]. Anal. calcd for C₂₀H₁₀TbNO₆ (1): C, 46.26; H, 1.94; N, 2.70. Found: C, 46.00; H, 1.91; N, 2.68%.

Physical measurements

IR spectra were recorded on a Thermo Scientific iS50 USA spectrophotometer coupled to Pike Gladi FTIR in the wavenumber range 4000–400 cm⁻¹ with an average of 144 scans and 4 cm⁻¹ of spectral resolution using an ATR apparatus. Elemental analyses were carried out with a CHNS/O Elemental Analyzer PerkinElmer 2400 USA. Thermal analyses (TGA) were performed with the simultaneous thermal analyzer Netzsch STA 449 F3 Jupiter in the temperature range 30–1100 °C by using alumina crucibles and around 10 mg of the sample. A dinitrogen flow of 100 cm³ min⁻¹ with a heating rate of 10 °C min⁻¹ was used. Powder X-ray diffraction data were collected on crushed crystals of 1 using a Rigaku Ultima IV with Cu-Kα radiation (λ = 1.5418 Å at 40 kV and 30 mA) and 2θ from 3 to 50°, the step size being 0.02°. Variable-temperature (1.8–300 K) direct current (dc) magnetic susceptibility measurements under an applied dc field of 1 kOe (as well as variable field (0–7.0 T)) magnetization measurements at 2.0 K were carried out on crushed crystals of 1 with a cryogenic SQUID magnetometer. Dynamic (ac) magnetic susceptibility data were collected with a Quantum Design PPMS using applied dc fields up to 5 kOe in the temperature range 2.0–8.0 K. The experimental susceptibilities were corrected for the diamagnetism of the constituent atoms and the sample holder (a plastic bag).

Steady-state photoluminescence

The photoluminescence spectra of the solid sample were obtained in a Fluorolog-3 spectro-fluorometer (Horiba FL3-22-iHR320), with double-gratings (1200 g mm⁻¹, 330 nm blaze) in the excitation monochromator and double-gratings (1200 g mm⁻¹, 500 nm blaze) in the emission monochromator. An ozone-free xenon lamp of 450 W (Ushio) was used as a radiation source. The excitation spectra were corrected in real-time according to the lamp intensity and the optical system of the excitation monochromator using a silicon diode as a reference. The emission spectra were carried out using the front-face mode. All of them were corrected according to the optical system of the emission monochromator and the photomultiplier response (Hamamatsu R928P). Time-resolved spectroscopy (microsecond range) was obtained by using a 150 W xenon lamp and a Time-Correlated Single Photon Count (TCSPC) system equipped in the Horiba FL3-22-iHR320 spectrofluorometer at 298 K. All emission decay curves were measured in triplicate and adjusted by a monoexponential decay function, whereas the emitting state lifetimes were determined as an average of measurements. Excitation and emission spectra were measured using the aforementioned Fluorolog 3 equipment. A THMS600 Linkam system (0.01 °C accuracy) with liquid dinitrogen was used to control the temperature, and optic fibers conducted the excitation and emission signals. Time-dependent emission decay curves (nanosecond range) were acquired using an Edinburgh Analytical Instruments FL 900 spectrofluorimeter with an MCP-PMT (Hamamatsu R3809U-50) and a pulsed diode operating at the λ_exc = 335.2 nm (model ELED-340, FWHD 14.4 nm and a pulse width of 815.2 ps) using a He cryostat (ADP Cryogenics) as an accessory.

X-ray crystallography

All reflections for single crystal X-ray diffraction of 1 were collected on a Rigaku XTaLab mini diffractometer with a CCD detector and Mo-Kα radiation (λ = 0.71073 Å) at room temperature. Data collection, cell refinement, and data reduction were performed with CrysAlisPro 1.171.41.105a program (CrysAlisPRO, Oxford Diffraction/Agilent Technologies UK Ltd, Yarnton, England). The structure of 1 was solved and refined by using SHELXT2018 and SHELXL2018^23,24 software hosted on the OLEX2 suite.²⁵ All non-hydrogen atoms were identified and refined by least-squares full matrix F² with anisotropic thermal parameters. The crystallographic tables were generated by OLEX2 and the structure representations were done through Mercury.²⁶ A summary of the crystallographic data and structure refinement is given in Table 1.

Table 1 Crystal data and refinement details for 1

Empirical formula	C20H10NO6Tb
Formula weight/g mol^−1	519.21
Temperature/K	293(2)
Crystal system	Monoclinic
Space group	C2/c
a/Å	32.3745(18)
b/Å	7.3369(4)
c/Å	17.0457(9)
α/°	90
β/°	98.753(5)
γ/°	90
Volume/Å^3	4001.7(4)
Z	8
ρ calc./g cm^−3	1.724
μ/mm^−1	3.569
F(000)	2000.0
Crystal size/mm^3	0.095 × 0.072 × 0.049
Radiation	Mo-Kα (λ = 0.71073 Å)
2Θ range for data collection/°	5.092 to 51.5
Index ranges	−39 ≤ h ≤ 39, −8 ≤ k ≤ 8, −20 ≤ l ≤ 20
Reflections collected	26 845
Independent reflections	3816 [Rint = 0.0485, Rsigma = 0.0333]
Data/restraints/parameters	3816/165/253
Goodness-of-fit on F^2	1.081
Final R indexes [I ≥ 2σ(I)]	R 1 = 0.0234, wR2 = 0.0557
Final R indexes [all data]	R 1 = 0.0299, wR2 = 0.0594
Largest diff. peak/hole/e Å^−3	1.06/−0.83
CCDC deposit number	2215049

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Results and discussion

Synthesis, IR spectroscopy, thermal study, and Powder X-ray diffraction (PXRD)

During the synthesis of 1, several experiments were conducted. This is because it is widely recognized that factors such as temperature, molar ratio of reagents, geometric characteristics, and coordination sites of the organic ligands^27–30 can have a significant impact on crystal growth. Moreover, the choice of synthetic strategies, such as stirring,^9,31 diffusion,³² solvothermal (particularly when mixed organic solvents are employed),⁸ and hydrothermal (used when water is the only solvent),^22,33 can also influence the process. Regarding the synthetic approach, it is noteworthy that the solvothermal method is more commonly employed compared to the hydrothermal one for the rational design and synthesis of coordination compounds derived from H₄bttb or H₂-2,5pzdc acids (as indicated in Table S1†). This trend holds for the preparation of lanthanide MOF thermometers as well.^5,8 However, drawing inspiration from earlier work conducted by one of our team members,²² we opted for using clean synthetic conditions to obtain 1, specifically through the hydrothermal reaction of Tb(NO₃)₃·5H₂O and the two polycarboxylic acids (H₄bttb and H₂-2,5-pzdc).

Regarding the FTIR analysis of the carboxylate group coordination in 1, a relevant aspect is the difference between the stretching vibrations of ν_as(COO⁻) and ν_s(COO⁻), which is referred to as Δν.^34,35 The absorption peaks of ν_as(COO)/ν_s(COO⁻) at 1650/1375, 1644/1390, 1687/1406 and 1392 cm⁻¹ in the infrared spectra of Na₄bttb, Na₂2,5-pzdc and 1 respectively (as shown in Fig. S1, ESI†), led to Δν values of 275 (Na₄bttb), 254 (Na₂2,5-pzdc), and 281/295 cm⁻¹ (1). The presence of two different Δν values for 1 suggests the coexistence of bridging and chelating–bridging pseudo-oxo coordination modes for the fully deprotonated bttb⁴⁻ and the bis-bidentate bridging pseudo-oxo one for the 2,5-pzdc²⁻ dianion (as illustrated in Chart 1). These coordination modes in 1 were further confirmed by its crystal structure (see below).

Chart 1 Coordination modes of the bttb⁴⁻ and 2,5-pzdc²⁻ ligands in 1.

The thermogravimetric analysis of compound 1 reveals an initial mass loss in the temperature range of 35–370 °C (obsd 11.9%), followed by a rapid weight loss of approximately 24.9% until 611 °C (Fig. S2a†). The first mass loss was attributed to the almost complete decomposition of the 2,5-pzdc²⁻ into pyrazinecarboxylate (calcd 11.7%), and the second may be due to the loss of bis(carboxyphenyl) (calc. 23.2%) from the remaining carboxylate of the 2.5-pzdc²⁻ and initial decomposition of the bttb⁴⁻. This attribution was based on the thermal stability of the ligands (see Fig. S2b†). An amount of approximately 63.2% was observed as residue, which can be attributed to the ¼Tb₄O₇ and the remaining bttb⁴⁻ ligand.

Powder X-ray diffraction (PXRD) experiments were conducted on crushed crystals of 1 to check the purity of the bulk. The good agreement between the experimental and calculated patterns (see Fig. S3 in ESI†) confirms that the structure derived from the single crystal X-ray analysis corresponds to the synthesized bulk material.

Description of the structure. Compound 1 crystallizes in the monoclinic system with the C2/c space group, where the asymmetric unit contains one Tb^III ion and half of both fully deprotonated ligands (bttb⁴⁻ and 2,5-pzdc²⁻). As depicted in Fig. 1(a), each Tb1 center is eight-coordinate with seven oxygen atoms [O1, O3, O1ⁱⁱ, O4ⁱⁱ, O5ⁱⁱⁱ, O5^iv, O6^iv] plus one nitrogen atom [N1] from four different (bttb⁴⁻) and two distinct (2,5-pzdc²⁻) anions building a triangular dodecahedron shape (Fig. S4, ESI†) after the SHAPE software.^36,37 Selected bond lengths and angles are given in Table S2.† The distorted triangular dodecahedron geometry around the Tb^III center is related to a D_2d symmetry, with the site symmetry approximation derived from continuous shape measures (CShM derived via SHAPE).³⁶ The CShM value of 3.162 is a better match than those found for the biaugmented trigonal prism (CShM of 3.696), biaugmented trigonal prism J50 (CShM of 3.796), and square antiprism (CShM of 5.324) (see Table S3, ESI†). It deserves to be noted that CShM values lower than 1.0 indicate minimal deviations from the ideal shape, while values up to 3.0 indicate significant distortions.³⁸ Hence, the observed deviation in 1 can be attributed to the distinct coordination modes of the carboxylate groups.^38,39 This leads to variations in the Tb–O distances which are illustrated by values like 2.279(3) and 2.383(2) Å for d_Tb–O3 and d_Tb–O1ⁱⁱ, respectively (Table S2†). Additionally, the chelate rings of the bttb⁴⁻ and 2,5-pzdc²⁻ ligands exhibit distorted conformations (as depicted in Fig. S5, ESI†). Perspectives views of the connectivity of the Tb^III ion in 1 where bttb⁴⁻ and 2,5-pzdc²⁻ act as linkers are shown in Fig. 1(b–d).

Fig. 1 (a) Coordination environment of the Tb^III ion in 1. (b) Bridging coordination mode of bttb⁴⁻. (c) Chelating–bridging pseudo-oxo coordination mode of bttb⁴⁻. (d) Bis-bidentate bridging, pseudo-oxo coordination mode of 2,5-pzdc²⁻ [symmetry code: (i) = −x + 3/2, y + 1/2, −z + 1/2; (ii) = −x + 3/2, y − 1/2, −z + 1/2; (iii) = −x + 3/2, −y + 1/2, −z + 1; (iv) = x, −y, z − 1/2; (v) = −x + 3/2, −y + 1/2, −z].

One carboxylate group of the of bttb⁴⁻ adopts the syn–syn μ-carboxylate-O′:O′′ bridging mode through the O3 and O4 oxygen atoms [2.280(3) and 2.306(3) Å for Tb1–O3 and Tb1ⁱ–O4, respectively] (Fig. 1(b)) whereas the remaining ones exhibit the same μ-carboxylate-κ²O′′′,O′′′′:κO′′′ chelating–bridging pseudo-oxo coordination mode across the O5 and O6 oxygen atoms [2.382(2), 2.363(3) and 2.518(2) for Tb1–O5ⁱⁱⁱ, Tb1–O6^iv, and Tb1ⁱ–O5ⁱⁱⁱ, respectively] (Fig. 1(c)). As far as the 2,5-pzdc²⁻ dianion is concerned, it adopts the trans-μ₄-κ²N,O′:κO′:κ²N′,O′′′′:κO′′′ bis-bidentate bridging, pseudo-oxo coordination mode through the N1, O1, N1^v and O1^v atoms [2.356(2), 2.621(3) and 2.384(2) Å for Tb–O1, Tb–N1 and Tb1ⁱ–O, respectively] (see Table S3†). In general, when linking four metal ions, the 2,5-pzdc²⁻ dianion adopts the μ₄-κ²N,O:κ²-N′,O′′:κO′:κ′′ coordination mode where it acts as a bis-bidentate bridge through pyrazine-nitrogen and carboxylate-oxygen atoms and anti–syn bridging across the carboxylate group²² (see Table S1†). However, 2,5-pzdc²⁻ in 1 adopts a bis-bidentate and mono-atomic (or pseudo-oxo) bridging mode [Fig. 1(d)] which has been rarely observed in the last decade, with only one reported example as we are aware.¹⁹ The value of the terbium–terbium separation through the oxo(carboxylate) bridge is 3.7557(2) Å (Tb1⋯Tb1ⁱ), whereas that through the pyrazine ring is 7.9697(7) Å (Tb1⋯Tb1^v).

As suggested by the IR analysis, the carboxylate anions of bttb⁴⁻ and 2,5-pzdc²⁻ exhibit distinct coordination modes. Specifically, the carbon–oxygen bonds (C–O) within the carboxylate groups are differently affected by the coordination to the metal center, their values being usually characteristics of the described coordination modes (Table S4, ESI†).³⁵ Consequently, in the context of the bridging and chelating modes observed in 1, the values of the C–O distance are around 1.24 Å (observed for bonds like C₂₁–O₃, C₂₁–O₄, and C₂₀–O₆), indicating that there are no significant differences in the bond order among these bonds. A mean value for the C–O bond length of 1.28 Å (notably observed for C₁–O₁ and C₁–O₅) is identified for the pseudo-oxo (or monoatomic bridging) mode. It's worth noting that both of these observed values are longer than the uncoordinated C–O bond [C₁–O₂ = 1.198(5) Å], which is expected because of their weaker bond order.

The coexistence of the two bridging ligands results in a neutral 3D framework (Fig. 2) which is formed by the bttb⁴⁻ ligands connecting eight Tb atoms along the crystallographic ac plane [Fig. 2(a)] with intralayer distances of 19.585(1) (Tb1⋯Tb1^vi) and 19.890(1) Å (Tb1^iv⋯Tb1^vi) (symmetry code: (vi) = −x + 1, −y + 1, −z + 1) and by the 2,5-pzdc²⁻ anions promoting the polymeric expansion into the bc plane [Fig. 2(b)] with the shortest terbium–terbium distances of 7.9699(7) Å (Tb1⋯Tb1^v).

Fig. 2 Views of fragments of the structure of 1 along the crystallographic ac (a) and bc planes (b) and the resulting 3D packing (c).

Photoluminescence results. The excitation spectrum of 1 obtained in the solid state at 298 K, was recorded with an emission peak at 545 nm. This spectrum displays a broad excitation band ranging from 250 to 350 nm, which is attributed to the 4f–5d energy transfer combined with a ligand energy transfer. Moreover, narrow and low intensities 4f → 4f excitation bands assigned to the Tb^III ion were identified [Fig. 3(a)]. When exciting the sample at 275 nm or 320 nm, the resulting emission spectra reveal a broad band centered around 400 nm, which can be a result of the π*–π or π–n electronic transitions of the bttb⁴⁻ ligand^40,41 as well as the triplet state of the 2,5-pzdc²⁻ ligand,¹⁸ and narrow bands assigned to ⁵D₄ → ⁷F_6,5,4,3,2 transitions of the Tb^III ion [Fig. 3(b)].

Fig. 3 (a) Excitation (λ_em = 545 nm) and (b) emission (λ_exc = 275 and 320 nm) spectra of 1 monitored at 298 K.

The excitation spectrum in the solid state at 77 K (λ_em = 545 nm) reveals an energy transfer from the ligand to excited states of the Tb^III ion. Additionally, there are narrow and weak intensity bands attributed to the Tb^III ion (4f–4f) transitions [Fig. S6(a), ESI†]. The emission spectra obtained by exciting the sample at 275 nm or 320 nm which are shown in Fig. S6b (ESI†) indicate no significant difference in the emission profile between the ligands (broad band) and Tb^III (narrow bands). However, it is possible to verify a better energy transfer from the ligands to the Tb^III ion since the broad emission band attributed to the ligand has a relatively lower intensity compared to one obtained at 298 K [Fig. 3(b)]. All emission decay curves were adjusted by a biexponential decay function showing two-lifetime values in accordance with the distorted triangular dodecahedron related to a D_2d symmetry obtained by the continuous shape measurements. Moreover, no essential changes in the emission lifetime values as a function of temperature from 298 to 10 K occur and therefore, there is no significant luminescence quenching due to vibrational processes, which evidences the stiffness of the 3D structure (see Fig. S7, S8 and Tables S5, S6 in ESI†). From the emission spectra obtained at 298 and 77 K, it is possible to observe changes in the relative emission intensities of the ligands and terbium, suggesting the potential for using luminescence as a temperature probe. Consequently, a temperature-dependent luminescence analysis was conducted on a sample of 1 to investigate the possibility of having a Tb^III ratiometric luminescent thermometer.

The optical thermometric parameter was determined using the ratio of integrated band intensities of the ligand (I_400(L)) to those of the Tb^III ion emission bands (I_Em(Tb)), expressed as Δ = I_400(L)/I_Em(Tb). The excitation spectra of 1 were recorded within the wavelength range of 250–550 nm varying with the temperature (10–320 K) under 545 nm emission, as illustrated in Fig. S9 (see ESI†). The temperature variations affected the overall intensities, displaying higher intensities at lower temperatures due to the minimal molecular vibrations compared to those present at higher temperatures.

The emission spectra obtained as a function of temperature are depicted in Fig. 4(a). These spectra exhibit a similar pattern as the excitation spectra with higher intensities observed at lower temperatures. This trend can be attributed to the decrease in the rates of quenching mechanisms such as vibrations⁴² and back energy transfer.⁴³

Fig. 4 Temperature-dependent luminescent spectra of 1: (a) emission spectra at λ_exc = 320 nm. (b) Temperature dependence of the thermometric parameter (Δ), relative thermal sensitivity (S_r), and temperature uncertainty (δT) by considering the thermometric parameter as Δ_{400(L)/Em(Tb)}.

From the obtained data, the thermometric parameter [Δ, in Fig. 4(b)] was calculated through eqn (1):

In this expression, I_400(L) and I_545(Tb) are the integrated emission area of the ligand and ⁵D₄ → ⁷F₅ terbium band respectively, as a function of temperature contained in the emission spectra collected after excitation at 320 nm. The values of the optical thermometric parameter show an increasing trend reaching a maximum at 320 K. The relative thermal sensitivity (S_r) allows checking the variation Δ per degree of temperature change through eqn (2):

One of the main advantages of using relative sensitivity (S_r) is that it does not depend on the nature of the thermometer, which can encompass mechanical, optical, or electric types. This property enables direct and quantitative comparisons across various samples in thermometry. In general terms, S_r ≥ 1% K⁻¹ is commonly employed for practical applications in lanthanide complexes.^11,44 For instance, in the work by Sigoli et al.,⁴⁵ using complexes of general formula [Ln(X-bza)₃-(pdppo)₂] functionalized on PDMS utilizing Eu^III, Tb^III, and Yb^III ions as single-center in luminescent thermometry, S_r values ranging between 0.5 and 1.6% K⁻¹ were observed. Herein, the maximum relative sensitivity obtained for 1 is 0.76% K⁻¹ at 295 K.

The temperature uncertainty [δT in eqn (3)] was also calculated. The δΔ/Δ represents the error derived from the measurements of the thermometric parameter, which was calculated using eqn (4). In this equation, I refers to the integrated intensity of the emission band and δI is the error associated with the measured intensity.

Therefore, the temperature uncertainty (δT) is determined experimentally by observing the distribution of temperature measurements for the thermometer at the specific reference temperature. This uncertainty is closely linked with the nature of the detector used in the measurement such as CCDs, photodiode arrays (PDAs), and PMTs where the temperature uncertainty can be less than 0.03 K.⁴⁵ For instance, Guan et al.⁴⁶ developed a type of Ln-MOF material of formula [Tb₂(atpt)₃(phen)₂(H₂O)]_n, (H₂atpt = 2-amino-terephthalic acid, phen = 1,10-phenanthroline monohydrate) which possesses optimal properties for physiological, low and high-temperature sensing, with relative sensitivity values (S_r = 6.17% K⁻¹) and a temperature uncertainty of 0.1 K, which, according to Brites and coworkers,⁴ falls within the temperature range of 0.1–1.0 K, a range that is commonly observed for most luminescent thermometers.

In addition, using the ratio of longer lifetime values of the ligand (400 nm) and the emitting state (⁵D₄) of the Tb^III ion (545), it was also possible to build a lifetime ratiometric probe (Fig. S10†), obtaining a maximum relative sensitivity (S_r) of 1.02% K⁻¹ at 29 K with a temperature uncertainty (δT) of ca. 0.2 K. The thermometric parameter curve shows a profile presenting two different trends giving a second temperature region where the relative sensitivity (S_r) is about 0.6% K⁻¹ at 110 K with a temperature uncertainty (δT) of ca. 0.3 K. This behaviour indicates a limitation for using the lifetime ratio temperature probe, and therefore we indicate using, in this case, the emission integrated intensity as the temperature probe. The emission decay curves and time-dependent lifetimes for the emission of the Tb^III ion at 545 nm and for the ligand at 400 nm are shown in Fig. S11 and Table S7.†

The relative sensitivity achieved in this study, besides being lower than one, is comparable to other thermometric sensors with lanthanide complexes (as shown in Table 2). However, the temperature uncertainty (δT) values are close to those of portable detection systems, and they align with the majority of values reported in the literature (see Table 2). Notably, values of 0.05 K uncertainty at 295 K, that is the range of maximum relative sensitivity, can be observed. To facilitate a comparison between the thermometric performance of our work and other studies in the literature, we have compiled values for the maximum thermal sensitivity, temperature uncertainty, lanthanide ions employed in thermometry, and operating temperature range for both single- and double-center luminescent lanthanide(III) complex systems, all listed in Table 2. These examples underscore the potential applications that the system introduced in this study holds within the field of optical temperature sensors.

Table 2 Maximum values of the thermal sensitivity (S_m) for lanthanide(III)-containing ratiometric systems

System^a	Ln^III	ΔT/K	S m/% K^−1	δT/K	Ref.
a Abbreviations: H4bttb = 1,2,4,5-tetrakis(4′-carboxyphenyl)benzene; H2-2,5-pzdc·2H2O = 2,5-pyrazinedicarboxylic dihydrate; H4qptca = 1,1′:4′,1′′:4′′,1′′′-quaterphenyl-3,3′′′,5,5′′′-tetracarboxylic acid; 3,5-bbza = dibromobenzoato; pdppo = polidimetilsiloxano; PDMS = propyldiphenylphosphine oxide; bpm = 2,2′-bipyrimidine; tfaa = 1,1,1-trifluoroacetylacetonate; tfac^− = 1,1,1-trifluoro-2,4-pentanedionate; DPA = 2,6-pyridinedicarboxylic acid; bzac = tris(1-phenyl-1,3-butanedione); H2FDC = 9-fluorenone-2,7-dicarboxylic acid; hfa = hexafluoroacetylacetonato; tppo = triphenylphosphine oxide; tta^− = 3-thenoyltrifluoroacetonate; pyphen = pyrazino[2,3-f][1,10]phenanthroline; Hcpb = 4-carboxyphenylboronic acid; H2PDC = pyridine-3,5-dicarboxylic acid; bz = benzoate, phen = o-phenanthroline, PS = polystyrene; 2,4-DFBA = 2,4-difluorobenzoic acid; Tp = 1,3,5-triformylphloroglucinol; bpy = 2,2′-bipyridine-5,5′-diamine; Hacac = acetylacetone; 3,5-bbza = dibromobenzoato; pdppo = polidimetilsiloxano; PDMS = propyldiphenylphosphine oxide; TEOS/APTES = tetraethyl orthosilicate/aminopropyltriethoxysilane; bpy-Et-PMO = bipyridine-ethane periodic mesoporous organosilica; 1,3-H2bdc = 1,3-benzenedicarboxylic acid; DMBDC = 2,5-dimethoxy-1,4-benzenedicarboxylate; 1,3-bdc = 1,3-benzene-dicarboxylic acid; H2PIA = 5-(pyridin-4-yl)isophthalic acid; BDC = 1,4-benzenedicarboxylic acid; D-H2cam = D-camphoric acid; H3imdc = 4,5-imidazole dicarboxylic acid.
[Tb(bttb)0.5(2,5-pzdc)0.5]n	Tb	250–320	0.76	0.05	This work
[Eu2(qptca)(NO3)2(DMF)4][CH3CH2OH]3	Eu	293	1.28	—	47
Perylene: ZJU-88⊃perylene
[Dy2(bpm)(tfaa)6]	Dy	298	3.3	0.1	48
Ln(tfac)3·2H2O	Eu or Tb	293	7.1	0.09	49
Ln-DPA (Ln = Eu, Tb)	Eu or Tb	293	1.5	0.3	50
[Eu(bzac)3(H2O)2]	Eu	293	1.4	0.08	51
(Me2NH2)3[Eu3(FDC)4(NO3)4]·4H2O	Eu	170/300	2.7/0.33	0.3	52
Tb(hfa)3(tppo)2	Tb	273	2.8	—	53
Eu(tta)3(pyphen)	Eu	323	1.7	0.001	54
{[Tb(cpbOH)(H2O)2](cpb)}n	Tb	180	1.84	—	55
Eu0.05Tb1.95-PDC	Eu–Tb	293–333	1.37	0.05	56
20Tb0.995Eu0.005(bz)3(phen): 80PS	Eu–Tb	303–473	4.8	—	57
LaNbO4:1.0% Bi^3+/0.5% Tb^3+	Tb–La	303–483	1.47	—	58
β-NaYF4:10% Ce^3+/5% Tb^3+	Tb–Ce	303–563	0.65	—	59
Tb1.976Eu0.0244(2,4-DFBA)6(phen)2	Eu–Tb	77–300	2.3	—	60
TpBpy-Eu/Tb-acac	Eu–Tb	10–360	1.4	0.05	61
Eu,Tb – γ-Fe2O3@TEOS/APTES	Eu–Tb	10–350	4.9	—	62
Gd2O3:Tb^3+,Eu^3+	Eu–Tb	123–473	0.77	—	63
KVO(O2)2–Tb,Sm(acac)-bpy-Et–PMO	Tb–Sm	293–363	1.8	0.06	64
[Tb1−xEux(CH3COO)(1,3-bdc)(H2O)]	Tb–Eu	150–350	0.4	0.53	11
Tb0.9931Eu0.0069DMBDC	Tb–Eu	50–200	1.15	—	4 and 8
Tb0.87Eu0.13(1,3-bdc)3(H2O)2	Tb–Eu	12–101	3.26	0.07	9
[Tb0.9Eu0.1(pia)]	Tb–Eu	100–300	3.27	—	10
[Tb0.99Eu0.01(BDC)1.5(H2O)2]	Tb–Eu	300–320	0.37	—	11 and 13
[Tb0.3Eu0.7(D-cam)(Himdc)2(H2O)2]	Eu–Tb	100–450	0.11	—	4 and 14

---

Static (dc) and dynamic (ac) magnetic properties. The dc magnetic properties for 1 in the form of χ_MT vs. T and M vs. H plots [χ_M, M, and H are the molar magnetic susceptibility per formula unit, the molar magnetization, and applied dc magnetic field, respectively] are shown in Fig. 5. The value of χ_MT value at room temperature is equal to 11.6 cm³ K mol⁻¹. It is somewhat smaller than the calculated one for a magnetically non-interacting terbium(III) ion with a ⁷F₆ low-lying state (χ_MT = 11.8 cm³ K mol⁻¹, 4f⁸, J = 6, L = 3, g_J = 3/2, and S = 3). The effects of the crystal field on the ground state issued from the spin–orbit coupling could justify this difference. Upon cooling, the values of χ_MT smoothly decrease from room temperature to 100 K, and they further exhibit an abrupt downturn to attain 6.83 cm⁻³ K mol⁻¹ at 1.8. K. This last decrease can be mainly attributed to the thermal depopulation of the M_J states. The magnetization plot for 1 at 2.0 K exhibits a fast increase below 2 T and then gradually reaches 4.58μ_B at 7 T (see inset of Fig. 5). Such a value is well below the expected saturation one of 9.0μ_B for a single Tb^III ion (considering g_J = 3/2 and J = 6) because of the occurrence of magnetic anisotropy and/or low-lying excited states in 1.

Fig. 5 Temperature dependence of the χ_MT product (○) for 1 under an applied dc field of 1 kOe. The solid and dashed lines are the best fits through eqn (5) with Δ = −17.8 and +12.1 cm⁻¹, respectively (see text). The inset shows the field dependence of the molar magnetization (M) at 2.0 K.

Anyway, they are very close to those observed in the previous reports.^65–70 Finally, weak antiferromagnetic interactions between the Tb^III ions through the different carboxylate bridging modes of the bttb⁴⁻ [Fig. 1(b) and (c)] and 2,5-pzdc²⁻ [Fig. 1(d)] ligands could be involved.^71,72

Although the above-combined effects make difficult to quantify each contribution to the dc magnetic data of 1, in an attempt to obtain more information on the magnetic anisotropy for this compound, we analyzed its magnetic susceptibility data employing the following Hamiltonian [eqn (5)]

H = λLS + Δ[L_z² − L(L + 1)/3] + βH(−κL + 2S) (5)

whose application was discussed in a previous report.³ The first, second, and third terms in this expression correspond to the spin–orbit coupling, the axial ligand field component (x = y ≠ z), and the Zeeman effect, respectively. Moreover, λ, Δ, and κ are the spin–orbit coupling parameter, the energy gap between the M_L components, and the orbital reduction parameter. The values of the best-fit parameters obtained by using the VPMAG program⁷³ are λ = −247.6 cm⁻¹, Δ = −17.8 cm⁻¹, θ = −0.77 K and R = 3.7 × 10⁻⁵ (R is the agreement factor defined as ∑[(χ_MT)_calcd − (χ_MT)_obs]²/∑[(χ_MT)_obs]²). In the fitting process, the value of κ was kept equal to the unit. θ is a Curie–Weiss parameter introduced in the fit to account for the possible weak magnetic interactions between the Tb^III ions through the different carboxylate bridging coordination modes. As shown in Fig. 5, the calculated curve (solid line) matches quite well the experimental data in the whole temperature range investigated.

The value of λ for 1 is close to that of the free Tb^III ion and the one found in a previous report for an oxamate-based Tb^III coordination polymer.⁷¹ A critical point in the analysis through the above simple Hamiltonian is that a negative value Δ is extracted through the fit of the dc magnetic susceptibility data in the investigated temperature range. In this respect, the χ_MT data of 1 cannot be well reproduced with a positive value of Δ (dashed line in Fig. 5). A pretty different shape of the χ_MT curve and a different magnetic moment for the ground state would occur for Δ > 0.³ The highest value of M_L (that is M_L = ±3) is the lowest energy state when Δ < 0 suggesting that the wave function representing the ground state will contain an essential contribution of the M_J = ±6.³ Finally, it deserves to be noted that the weak antiferromagnetic interaction between the Tb^III ions in 1 is of the same order as those reported for other carboxylate-bridged terbium(III) complexes.^71,72

To explore the possible dynamics of the magnetization of 1, alternating current (ac) magnetic susceptibility measurements were carried out in the temperature range 2.0–10.0 K under zero and 1 kOe applied dc fields. This study showed the lack of any significant out-of-phase signal for 1 under zero and non-zero applied dc fields, and then, this anisotropic system does not exhibit SMM behavior.

Conclusion

In this study, we utilized two polycarboxylate ligands to synthesize a mixed-ligand Tb^III coordination polymer of formula [Tb(bttb)_0.5(2,5-pzdc)_0.5]_n (1) through a one-step synthetic procedure. From a structural point of view, 1 is a new and stable 3D coordination polymer with the Tb^III ion arranged in a distorted triangular dodecahedron geometry related to a D_2d symmetry. This coordination sphere is built by nitrogen and oxygen atoms from fully deprotonated bttb⁴⁻ and 2,5-pzdc²⁻ carboxylates. Bttb⁴⁻ exhibits bridging and chelating–bridging pseudo-oxo coordination modes and the 2,5-pzdc²⁻ dianion adopts the rather unusual bis-bidentate bridging pseudo-oxo coordination modes. The pseudo-oxo (or monoatomic bridging mode) from both ligands contributes to the expansion of the polymer into a 3D network. The thermogravimetric analysis of compound 1 reveals two mass losses in the temperature range of 35–611 °C, and notably, the complete decomposition of the network of 1 is not observed until 1000 °C, and this seems to be influenced by the presence of bttb⁴⁻, as evidenced by its thermogravimetric analysis. The temperature-dependent luminescence analysis of 1 offers a relative thermal sensitivity of 0.76% K⁻¹ at 295 K, with a maximum temperature uncertainty of 0.05 K, whose value is close to a portable detection system, making it a potential candidate for a ratiometric luminescent nanomolecular thermometer. The investigation of variable-temperature static (dc) magnetic susceptibility showed the coexistence of magnetic anisotropy of the Tb^III ion and a weak antiferromagnetic interaction mediated by different carboxylate bridging pathways. No out-of-phase signals were observed in the latch and the presence of applied dc fields. Furthermore, 1 is stable under open-air conditions, and this outcome underscores that the selection of ligands and the chosen synthetic route were well-suited to achieve a Tb^III coordination polymer with promising prospects for applications in the field of optical temperature sensors. Yet, despite the passage of two decades up to today, there has been only a limited number of publications that explore the use of deprotonated forms of H₄bttb or H₂-2,5-pzdc as ligands towards lanthanide ions, none of them involving both ligands simultaneously.

Author contributions

The manuscript was written through the contributions of all authors, and all authors have approved the final version. A. I. C. synthesized and characterized the coordination polymer; R. M. R. d. S. and L. D. G. B. participated in the characterization by thermal analysis and Powder X-ray diffraction; S. F. N. C. and F. A. S. collected and wrote the photoluminescence results; J. H., F. T. M., and J. E. collected and treated single-crystal X-ray diffraction data; A. M. G., W. N., F. L., and M. J. collected and performed the magnetic data acquisition and processing; M. V. M. participated supervising the synthesis, all analyses and interpretation besides writing and reviewing all the data of the paper.

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

This work was supported by the Fundação de Amparo à Pesquisa do Estado de Minas Gerais (FAPEMIG, CEX – APQ-01597-17 and APQ-00544-23), Spanish MCIN/AEI10.13039/501100011033 (Project PID2019-109735GB-I00), Generalitat Valenciana (AICO/2021/295), CNPq (312505/2021-3) and FAPESP (2021/06326-1, 2021/08111-2, 2017/15850-0, and 2021/04876-4). A. I. C. and R. M. R. d. S. thank CAPES Foundation [Brazil, CAPES, finance code: 001] and L. D. G. B. (PIBICT-FAPEMIG) for their research fellowships. The authors wish to thank PROAP and PRPPG (UNIFAL-MG) and the laboratories of UNIFAL-MG: (IR spectroscopy, TG/DTA analysis (FINEP 01/10/0798/00), Powder XRD Multiuser Laboratory), and USP-São Carlos: (SXRD diffraction) for facilities.

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Footnote

† Electronic supplementary information (ESI) available: Revision (bttb⁴⁻ and 2,5-pzdc²⁻) based compounds (Table S1), crystallographic data (Table S2), geometric analysis of the coordination environment (Table S3), bond lengths of the C–O bond of bttb⁴⁻ (Table S4), thermometric performance (Tables S5 and S6), time-dependent lifetime (Table S7). FTIR spectra (Fig. S1), TG and DTA curves (Fig. S2), PXRD patterns (Fig. S3), coordination polyhedron (Fig. S4 and S5), and photoluminescent data (Fig. S6–S11). CCDC 2215049. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d3dt03555g

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