A Eu/Tb-codoped coordination polymer luminescent thermometer

Abstract

A mixed-lanthanide coordination polymer Eu_0.0618Tb_0.9382L as a self-referenced luminescent thermometer based on the intensity ratio of the two emissions (Tb³⁺ at 544 nm and Eu³⁺ at 613 nm) is more reliable than EuL/TbL based on one emission.

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Coordination polymers (CPs), which are organometallic polymer structures containing metal cation centers linked by organic ligands, have now occupied an important position in the porous-materials area owing to their particular intriguing architectures and potential applications in gas storage,¹ chemical separation,² heterogeneous catalysis,³ drug delivery,⁴ and sensing.⁵ Among the diverse CPs, the lanthanide coordination polymers (Ln-CPs) display outstanding optical properties such as relatively long luminescence lifetime, high color purity, and sharp line emission originating from characteristic 4f electron transitions, which allow them to have potential application in light-emitting devices, electro-optic devices, and fluorescent probes.^5a,6 Up to now, most of the luminescent Ln-CPs have been developed as fluorescent probes for sensing of organic small molecules, metal cations, anions, explosives and vapors,^5a,b however, luminescent Ln-CPs used for recognition and probing for temperature have seldom been reported.⁷

Temperature is a fundamental thermodynamic variable, which is the most frequently measured physical parameter in numerous fields. Compared with the traditional methods (thermocouples, thermistors, liquid-filled and bimetallic thermometers) for temperature measurements, luminescence-based measurements have unique advantages, such as, fast response, noninvasive, non-contact accurate, and nanometric precision and are able to work even in strong electric or magnetic fields.⁸ However, most luminescence methods for measuring temperature explored until now are mainly based on the temperature-dependent luminescence intensity of one transition. Their accuracy can be influenced by excitation power, the concentration of the sensor, and the drifts of the optoelectronic systems such as lamps and detectors.⁹ The mixed-lanthanide coordination polymers (M′Ln-CPs), utilizing self-referenced signals, can circumvent many of these drawbacks, and are expected to be more accurate luminescent thermometers.¹⁰ Herein, to dope Eu³⁺ into an isostructural Tb³⁺-CP, we obtained a mixed-lanthanide coordination polymer [(Eu_0.0616Tb_0.9382)(L)₂(NO₃)₂]·Cl·2H₂O (Eu_0.0618Tb_0.9382L; L = 1,4-bis(pyridinil-4-carboxylato)-1,4-dimethylbenzene). It displays two main narrow emission bands (Tb³⁺ at 544 nm and Eu³⁺ at 613 nm), each of which has its own temperature dependence. The luminescence intensity ratio of the two emission bands I_Tb/I_Eu affords a self-referenced signal used for more accurate probing for temperature.

Solvothermal reaction of Ln(NO₃)₃·6H₂O with [H₂L]Cl₂ in DMF solvent at 85 °C for 72 h afforded colorless tetragonal-shaped crystals of Ln-CPs, which were formulated as [Ln(L)₂(NO₃)₂]·Cl·2H₂O (Ln = Eu, Tb, and Gd; L = 1,4-bis(pyridinil-4-carboxylato)-1,4-dimethylbenzene). The three lanthanide compounds are isostructural and crystallize in the tetragonal space group I4/m. Herein the EuL compound is a representative for structural description. Single-crystal X-ray diffraction analysis reveals that the compound contains a europium ion, two L ligands, two coordinated nitrate ions, one free chloride ion, and two free water molecules. The central europium ion adopts distorted dodecahedron geometry and is coordinated by eight oxygen atoms from four L ligands and two nitrate ions (Fig. S1†). The average Eu–O distance is 2.367 Å, which is within the expected range (2.353–2.717 Å).

It is noteworthy that the ligand coordinates to two europium ions with the monodentate coordination mode, and two europium ions together with two ligands generate a 38-atom ring-like structure (Fig. 1a). As shown in Fig. 1b, the adjacent rings form a one dimensional chain by sharing the europium ion. By means of the hydrogen bonds (C(10)–H(10)⋯O(4) distance of ca. 3.30(2) Å; C(7)–(H7)⋯O(3) distance of ca. 3.38(2) Å), a two dimensional supramolecular layer forms (Fig. 1c). With the aid of hydrogen bonds (C(10)–H(10)⋯O(4); C(7)–(H7)⋯O(3)) and π–π stacking interactions between aromatic rings (face-to-face distance of ca. 3.49 Å), the layers stack in parallel along the b-axis, and lead to the generation of a three dimensional supramolecular architecture (Fig. S2†). Fig. S3† shows the three dimensional supramolecular structure viewing in the c direction, with the free water molecules and the free chloride ions filling in the interspace. All LnL CPs were isostructural, which was further certified by powder X-ray diffraction (Fig. S4†).

Fig. 1 (a) Ring-like structure unit. (b) One dimensional chain structure and the simplified diagram. (c) Two dimensional supramolecular layer. All the hydrogen atoms, chlorine atoms, and water molecules have been omitted for clarity (blue dotted line: hydrogen bond).

The solid-state photoluminescence (PL) properties of [H₂L]Cl₂ and LnL have been investigated at room temperature and given in Fig. S6–S10.† As shown in Fig. S6,† the free ligand [H₂L]Cl₂ shows a broad band with a maximum at 397 nm under excitation at 203 nm, which could be ascribed to π→π* electron transitions. Upon excitation at 352 and 357 nm, the EuL and TbL reveal the typical Eu³⁺ and Tb³⁺ characteristic emission peaks, respectively. EuL displays the emission peaks at 595, 617, 651, and 697 nm, which are assigned to ⁵D₀→⁷F_1–4 (Eu³⁺) transitions (Fig. S7†); while TbL exhibits very strong emission peaks at 488, 544, 590, and 621 nm, corresponding to the ⁵D₄→⁷F_6–2 (Tb³⁺) transitions (Fig. S8†). As expected, the M′Ln-CPs, Eu_0.0311Tb_0.9689L and Eu_0.0618Tb_0.9382L simultaneously display both ⁵D₀→⁷F_1–4 (Eu³⁺) and ⁵D₄→⁷F_6–2 (Tb³⁺) characteristic transitions (Fig. S9 and S10†). The blue light emission for the free ligand completely disappears in these Ln-CPs, suggesting that the [H₂L]Cl₂ ligand is an excellent antenna chromophore for sensitization of both Eu³⁺ and Tb³⁺ ions. It is worth mentioning that the emission of the Eu³⁺ ions in M′Ln-CPs is further sensitized by the Tb³⁺ ions within the same frameworks. As seen in Fig. S11 and S12,† the room temperature emission spectra of Eu_0.0311Tb_0.9689L and Eu_0.0618Tb_0.9382L excited at 488 nm, which exclusively corresponds to the ⁷F₆→⁵D₄ transition of the Tb³⁺ ions, provides the evidence for energy transfer from Tb³⁺ to Eu³⁺ ions.

To investigate their potential as luminescent thermometers, the temperature-dependent photoluminescence (PL) properties of these LnL and M′LnLs were also studied. The emission spectra of EuL and TbL from 25 K to 300 K has been given in Fig. 2a and b, and the intensities of the ⁵D₀→⁷F₂ (Eu³⁺, 617 nm) and ⁵D₄→⁷F₅ (Tb³⁺, 544 nm) transitions have been shown in Fig. 2d (inset). The emission intensity of both Eu³⁺ in EuL and Tb³⁺ in TbL decreases gradually with the temperature increasing, which is expected to be due to the thermal activation of nonradiative-decay processes. As expected, the M′Ln-CP, Eu_0.0618Tb_0.9382L, displays a different temperature-dependent luminescence behavior from those of EuL and TbL (Fig. 2c). The emission intensity of the Tb³⁺ ions in Eu_0.0618Tb_0.9382L decreases with the temperature increasing, while the emission intensity of Eu³⁺ ions increases slowly from 25 K to 150 K and then decreases in the 150–300 K temperature range (Fig. 2d). The emission intensity ratio I_Tb/I_Eu (⁵D₄→⁷F₅ (Tb³⁺, 544 nm) to ⁵D₀→⁷F₂ (Eu³⁺, 613 nm) transition), is commonly used as a self-referenced signal probe for temperature, which can overcome many of the drawbacks. The good linear relationship between the I_Tb/I_Eu ratio and the temperature in the range of 25–200 K can be fitted as a function of

T = 494.30 − 178.25 I_Tb/I_Eu

Fig. 2 Emission spectra of (a) TbL, (b) EuL, (c) Eu_0.0618Tb_0.9382L recorded between 25 and 300 K, and (d) temperature-dependent intensity of the ⁵D₄→⁷F₅ and ⁵D₀→⁷F₂ transition for Eu_0.0618Tb_0.9382L (Inset) temperature-dependent intensity of the ⁵D₄→⁷F₅ transition of TbL and ⁵D₀→⁷F₂ transition of EuL.

The temperature sensitivity of Eu_0.0618Tb_0.9382L is 0.56% per K. This reveals that Eu_0.0618Tb_0.9382L is an excellent luminescent thermometer in the range from 25 K to 200 K (Fig. 3).

Fig. 3 Temperature-dependent intensity ratio of Tb³⁺ (544 nm) to Eu³⁺ (613 nm) and the fitted curve for Eu_0.0618Tb_0.9382L from 25 K to 200 K.

In order to better clarify the mechanism of energy transfer, the temperature dependence lifetimes of the excited states ⁵D₀ (Eu³⁺, 544 nm) and ⁵D₄ (Tb³⁺, 613 nm) were investigated (Fig. S15–S17†). The phenomenon that Eu_0.0618Tb_0.9382L shows shorter ⁵D₄ (Tb³⁺) lifetime than TbL but longer ⁵D₀ (Eu³⁺) lifetime than EuL at the same temperature, suggests the energy transfer from the Tb³⁺ to Eu³⁺ occurs. The energy transfer efficiency can be calculated as follows:

E = 1 − τ₁/τ₀

where τ₀ and τ₁ are the donor lifetime in the presence (τ₁) and absence (τ₀) of the Eu³⁺ acceptors. As shown in Table S1† and Fig. S18,† the energy transfer efficiency from Tb³⁺ to Eu³⁺ in Eu_0.0618Tb_0.9382L is enhanced at the elevated temperature (from 22.64% at 25 K to 34.41% at 200 K), which may be chiefly controlled by the phonon-assisted Förster transfer mechanism.¹¹

In summary, a luminescent thermometer Eu_0.0618Tb_0.9382L has been obtained. The intensity ratio of the two emissions, I_Tb/I_Eu, affords a self-referenced signal and can be a more robust and reliable probe for temperature. In addition, Tb³⁺ to Eu³⁺ energy transfer process exists in the Eu_xTb_1−xL compounds. We expect to design and construct practically useful M′Ln-CPs luminescent thermometers with high sensitivity and further investigation is ongoing.

This work was supported by the financial aid from the National Natural Science Foundation of China (grant nos. 91122030, 21210001 and 21221061), and the National Key Basic Research Program of China (no. 2014CB643802).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Synthetic details, X-ray powder diffraction data, TGA plot, experimental details of the luminescence determination. CCDC 1006519–1006521. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4qi00122b

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