Doping Gd₁₆ nanoclusters for expanded optical properties and thermometry applications

Abstract

Lanthanide metal clusters are composed of rigid multinuclear metal cores encapsulated by organic ligands, which have become one of the most interesting research frontiers because of their fantastic architecture, intriguing physical and chemical properties, and potential applications. However, very little attention has been paid to exploring their potential as highly efficient optical materials. Gd₁₆ clusters are a new cluster structure that has a rich and varied coordination environment, which is highly conducive to doping and thus controlling luminescence and luminescence color modulation. We achieved green emission by doping Tb³⁺ ions and red emission by doping Eu³⁺ ions in the Gd₁₆ cluster structure. Meanwhile, we achieved red–orange–yellow color-tunable luminescence by controlling the composition of Tb³⁺ and Eu³⁺ ions. Studies on the PL properties show that Gd₁₆ clusters as the host can be used for doping and efficiently photosensitizing Tb³⁺ ions and Eu³⁺ ions. The existence of energy transfer from the ligand to Tb³⁺ ions and Eu³⁺ ions in the co-doped Ln₁₆ clusters was sufficiently demonstrated by time-resolved photoluminescence spectroscopy tests, and the energy transfer efficiency in the clusters was calculated. Furthermore, the temperature-dependent photoluminescence properties of these clusters were investigated to determine their potential as luminescent thermometers.

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Zhennan Wu

Zhennan Wu joined Jilin University in 2021 and is currently a full professor at the College of Electronic Science and Engineering. He obtained his Ph.D. degree (2016) from the State Key Laboratory of Supramolecular Structure and Materials at Jilin University under the supervision of Prof. Hao Zhang. Thereafter, he joined the team of Prof. Osman. M. Bakr in KAUST (2016–2018) and Prof. Xie Jianping in NUS (2018–2020) for post-doctoral research. His research interests are colloidal synthesis dynamics, self-assembly chemistry, optical physics, and photoelectric devices of metal nanoclusters.

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Introduction

Lanthanide metal clusters are composed of rigid multinuclear metal cores encapsulated by organic ligands. They perfectly inherit the excellent characteristics of a single lanthanide ion, such as a long luminous life, high luminous color purity, and a large Stokes shift.^1–3 Additionally, they can produce many interesting physical and chemical properties due to the advantages of their structure. For example, they can be used as chemical detection sensors for the fluorescence quenching of some specific functional groups and can be used as optical barcode and temperature sensors by precisely controlling the composition and energy transfer within the clusters.^4,5 Therefore, lanthanide clusters are promising in the field of materials research. However, lanthanide clusters have some disadvantages, such as low luminescence efficiency due to the vibrations of their internal molecules, and their applications need to be further expanded and developed.^6–8

The emerging Gd₁₆ clusters can significantly compensate for these shortcomings.⁹ Trifluoroacetylacetone (Tfac) with a suitable triplet-state energy level (22 700 cm⁻¹) can effectively sensitize the luminescence of terbium (Tb³⁺) and europium (Eu³⁺) ions.^10,11 Secondly, the C–F bond in trifluoroacetylacetone can effectively inhibit the molecular vibrations in the clusters and improve the luminous efficiency.¹² More importantly, Gd₁₆ has multiple sites and a rich coordination environment, which provides abundant sites for doping. Thus, Gd₁₆ clusters are an excellent substrate material. On this basis, red light emission is realized by doping Gd₁₆ clusters with Eu³⁺ ions. Green emission is realized by doping Gd₁₆ clusters with Tb³⁺ ions. By co-doping Gd₁₆ clusters with Tb³⁺ and Eu³⁺ ions and precisely regulating the content of terbium and europium ions, the emission of clusters from red light to yellow light can be tuned. The energy transfer from Tb³⁺ ions to Eu³⁺ ions was confirmed by measuring the decay lifetime of Tb³⁺ ions in the co-doped structure.^13,14

Due to the tunable luminescence in the co-doped structure, we further investigated its temperature-dependent photoluminescence properties.^15–17 These were evaluated using variable-temperature spectroscopy, revealing that the emitted colors exhibit notable temperature sensitivity. By fitting the variable-temperature spectra, the relative sensitivity (S_r) was determined to be 4.6% K⁻¹, which is among the highest reported in the literature. This indicates promising potential for its application in luminescent thermometers.

Results and discussion

The lanthanide metal clusters Gd₁₆ were synthesized according to previous work.⁹ Analysis of single-crystal structures showed that the compound Gd₁₆ crystallizes in the C₂/m space group within a monoclinic crystal system. The Gd₁₆ cluster comprises 16 Gd³⁺ ions coordinated to 20 trifluoroacetylacetone (TFac) ligands, 8 methanol molecules, 24 μ₃-OH bridging groups, and 2 μ₆-O bridging groups. The Gd³⁺ ions are interconnected through μ₃-OH and μ₆-O bridging groups, facilitating the formation of two distinct structural units. In particular, six Gd³⁺ ions are linked to eight μ₃-OH groups, resulting in the formation of the structural unit [Gd₆(μ₃-OH)₈]¹⁰⁺ (Fig. 1a). The incorporation of μ₆-O bridging ligands further generates the second structural unit, [Gd₆(μ₃-OH)₈(μ₆-O)]⁸⁺(Fig. 1b). Two units are further integrated into the Gd₁₆ cluster through the sharing of a central Gd³⁺ ion. The coordination sphere of the cluster is completed by the coordination of 20 TFac ligands and 8 methanol molecules (Fig. 1c). The Gd1 and Gd6 ions exhibit a nine-coordinate capped square antiprism geometry, whereas Gd2/Gd3/Gd4/Gd5 ions adopt an eight-coordinate biaugmented trigonal prism geometry (Fig. 1d). The coordinating oxygen atoms are derived from TFac ligands, μ₃-OH bridging groups, and μ₆-O bridging groups. The Gd–O bond lengths range from 2.420(1) to 2.450(3) Å, aligning well with previously published studies. Notably, the TFac ligands coordinate with individual Gd³⁺ ions in a chelating bidentate mode and, together with the methanol molecules, exhibit three distinct coordination environments (Fig S1†). Crystal data are summarized in Table S16.† In Fig S2,† the powder X-ray diffraction (PXRD) patterns for the Gd_16−xEu_x, Gd_16−xTb_x and Gd₁₅Tb_1−xEu_x (x = 0.2, 0.4, 0.6, 0.8) samples are shown. The atomic content in these samples were proved via ICP-OES (Table S1†). The observed diffraction peaks predominantly correspond to the Gd₁₆ phase, indicating that the incorporation of dopant ions did not induce significant alterations in the crystalline structure. Furthermore, the subtle variations in peak positions among the different clusters are indicative of minor changes in the crystal lattice parameters, attributed to the differing concentrations of the doped lanthanide ions.

Fig. 1 (a) Ball-and-stick representation of [Gd₆(μ₃-OH)₈]¹⁰⁺. (b) Ball-and-stick representation of [Gd₆(μ₃-OH)₈(μ₆-O)]⁸⁺ (c) Ball-and-stick representation of [Gd₁₆(tfac)₂₀(CH₃OH)₈(μ₃-OH)₂₄(μ₆-O)₂]. (d) The coordination geometries of Gd³⁺.

The luminescence of lanthanide ions is typically excited via the antenna effect of the coordinating ligands. However, for Gd³⁺, efficient excitation via the ligand's antenna effect is challenging due to its high resonance energy levels. To enhance the luminescence of Gd³⁺-based clusters, doping with optically active ions provides a viable strategy. In the case of Gd_15.2Eu_0.8, the excitation spectrum reveals a significant absorption band around 335 nm (Fig. 2a), which can be attributed to the ligand ¹π → ¹π*/³π* charge transfer transition. Here, ¹π, ¹π*, and ³π* represent the ground state, the first excited singlet state, and the first excited triplet state, respectively.¹⁸ Upon excitation at 336 nm, the emission spectrum exhibits four distinct peaks at 595 nm, 615 nm, 655 nm, and 695 nm, corresponding to the ⁵D₀ → ⁷F₁, ⁵D₀ → ⁷F₂, ⁵D₀ → ⁷F₃ and ⁵D₀ → ⁷F₄ transitions of Eu³⁺ (Fig. 2b). This indicates that the tfac⁻ ligand effectively facilitates the sensitization of Eu³⁺ in these clusters.^19–21 The clusters exhibit a characteristic red emission, as depicted in the CIE chromaticity diagram (Fig. S4†). Additionally, the dependence of photoluminescence on doping concentration was explored (Fig. S5†). The intensity of the Eu³⁺ intrinsic transitions increases significantly with the doping concentration, suggesting that higher doping levels promote more efficient energy transfer from the ligand to Eu³⁺ ions.

Fig. 2 (a) The excitation spectra of Gd_15.2Eu_0.8. (b) The emission spectra of Gd_15.2Eu_0.8. (c) Trend curve of I_e/I_m in different Eu³⁺ratios. (d) Diagram of the energy transfer mechanism.

The coordination environment significantly influences the physical properties of compounds, and Gd₁₆, with its atomically defined structure, offers a well-controlled system for investigating these effects. Among the four characteristic transitions of Eu³⁺ the ⁵D₀ → ⁷F₁ transition is a magnetic dipole transition, which is insensitive to the symmetry of the Eu³⁺ coordination sphere.^22,23 In contrast, the ⁵D₀ → ⁷F₂ transition is an electric dipole transition, which is highly dependent on the symmetry of the Eu³⁺ local lattice environment. From the emission spectra, the intensities of the electric dipole transition (I_e) at 615 nm and the magnetic dipole transition (I_m) at 595 nm were derived for the samples Gd_15.8Eu_0.2, Gd_15.6Eu_0.4, Gd_15.4Eu_0.6, Gd_15.2Eu_0.8 (Fig. 2c). As the doping concentration increases, the ratio of I_e/I_m progressively increases, which suggests a reduction in the symmetry of the Eu³⁺ coordination sites. This reduction in symmetry is further linked to changes in energy transfer efficiency, and theoretical calculations were performed to quantify these effects (Fig. 2d and Table S2–12†).²⁴ The analysis of the forward rate (W^T) and backward rate (W^T_B) constants for the two representative samples, as shown in Table S3,† reveals that the difference between W^T and W^T_B for Gd_15.4Eu_0.6 is smaller than for Gd_15.2Eu_0.8, suggesting that the latter exhibits more efficient luminescence. These results indicate that the emission intensity of the clusters is influenced by both the symmetry of the lattice and the dynamics of energy transfer, with the latter exerting a more dominant influence. In a similar manner, the Tb³⁺ doped Gd₁₆ clusters also exhibit notable sensitization characteristics, which further highlight the versatility of the coordination environment in modulating the photoluminescence properties. The emission spectra, obtained under 336 nm excitation, show four characteristic peaks at 492 nm, 547 nm, 587 nm, and 623 nm, which correspond to the ⁵D₄ → ⁷F₆, ⁵D₄ → ⁷F₅, ⁵D₄ → ⁷F₄, and ⁵D₄ → ⁷F₃ transitions of Tb³⁺ (Fig. S6 & S7†).²⁵ The corresponding CIE chromaticity diagrams indicate that the emission is predominantly in the green region (Fig. S8†). As with Eu³⁺ doped Gd₁₆, the photoluminescence properties of Tb³⁺ doped samples exhibit a dependence on doping concentration and the coordination environment, showing trends similar to those observed for Eu³⁺ doping (Fig. S9†). These results further support the notion that the ligands in Gd₁₆ possess strong sensitization ability, which can be leveraged to expand the optical properties of the material.

The efficient energy transfer between the ligand and Tb³⁺/Eu³⁺ ions, coupled with their distinct emission colours, offers significant potential for achieving a wide range of colour-tuning through co-doping and modulation of ion concentrations.²⁶ Doping Gd₁₆ clusters with Eu³⁺ and Tb³⁺ respectively can make Gd₁₅Eu_1−x and Gd₁₅Tb_1−x show red or green fluorescence colour (Fig. 3b). The emission spectra of Gd₁₅Tb_1−xEu_x (x = 0.2, 0.4, 0.6, 0.8) confirmed the effective sensitization of the ligand and display both the characteristic emissions of Tb³⁺ ions and Eu³⁺ ions (Fig. 3b). Notably, the emission profile of Gd₁₅Tb_1−xEu_x can be finely tuned through the variation of Eu³⁺ doping, resulting in a shift from yellow to red emission. It is noteworthy that increasing the concentration of Eu³⁺ ions enhances the relative intensity of Eu³⁺ emission relative to that of Tb³⁺, thereby allowing for precise control over the color-tuning of the emission output. In the co-doped Gd₁₆ clusters with Tb³⁺ and Eu³⁺, revealed that the spacing between the Ln³⁺ ions is less than 10 Å, which facilitates effective energy transfer.²⁷ To demonstrate the energy transfer of Tb³⁺ → Eu³⁺, we investigated the emission decay ⁵D₄ → ⁷F₅ transition of Tb³⁺ at 547 nm (Fig. 3c and S10†). A comparison of the emission lifetimes of the donor ion (Tb³⁺) in the presence and absence of the acceptor ion (Eu³⁺) (Table S13†) revealed that the luminescence lifetime of Tb³⁺ decreases in the presence of Eu³⁺, providing strong evidence for the Tb³⁺ → Eu³⁺ energy transfer process. To further illustrate the energy transfer mechanism from Tb³⁺ to Eu³⁺ ions, we have provided a schematic diagram of the process in Gd₁₅Tb_1−xEu_x (x = 0.2, 0.4, 0.6, 0.8) clusters, based on the work of Crosby and Whan (Fig. S11†).²⁸ Upon excitation of the tfac⁻ ligand from the ground state, it undergoes intersystem crossing to the T₁ (first excited triplet state) energy level. The energy in T₁ then relaxes to the ⁵D₄ energy level of the Tb³⁺ ion which subsequently relaxes to the ground state, emitting characteristic Tb³⁺ luminescence. Simultaneously, energy is transferred from the ⁵D₄ level of Tb³⁺ to the ⁵D₁ level of Eu³⁺, which then relaxes to the ground state, producing the characteristic Eu³⁺ emission. This dual emission from both Tb³⁺ and Eu³⁺ ions results in the observed dual-color output.

Fig. 3 (a) Schematic illustration of fluorescence colour modulation in Gd₁₆ clusters upon doping with Eu³⁺ and Tb³⁺ ions. (b) The emission spectra of Gd₁₅Tb_1−xEu_x (x = 0.2, 0.4, 0.6, 0.8) clusters. (c) The emission decay lifetimes of ⁵D₄ → ⁷F₅ transition of Tb³⁺ at 547 nm in Gd₁₅Tb_1−xEu_x (x = 0.2, 0.4, 0.6, 0.8) clusters.

Meanwhile, to calculate the energy transfer efficiency between Tb³⁺ → Eu³⁺ in clusters, we use the formula proposed by Piguet and colleagues to calculate the energy transfer efficiency of Tb³⁺ → Eu³⁺(eqn (1)) (Table. S13†), where τ denotes the luminescence lifetime of Tb³⁺ in the presence of the host ion and τ₀ denotes the luminescence lifetime of Tb³⁺ in the absence of the host ion.¹³ Analysis of the excited-state lifetimes of the cluster within the framework of the Förster theory of energy transfer showed that the transfer has an efficiency of ∼12%.

Lanthanide clusters incorporating both Tb³⁺ and Eu³⁺ ions are theoretically highly sensitive to temperature changes due to their dual-emission features.²⁹ To investigate the potential of Gd₁₅Tb_1−xEu_x (x = 0.2, 0.4, 0.6, 0.8) clusters as luminescent temperature sensors, temperature-dependent emission spectra of Gd₁₅Tb_0.2Eu_0.8 were recorded. In the temperature range of 80–160 K, the intensities of the characteristic emission peaks of both Tb³⁺ and Eu³⁺ remained relatively stable, likely due to the suppression of non-radiative relaxation at lower temperatures (Fig. 4a). However, between 160 and 300 K, the intensities of these emission peaks gradually decreased, suggesting that increased temperature enhances non-radiative relaxation (Fig. 4c). The temperature-dependent emission spectra of Gd₁₅Tb_0.8Eu_0.2 were analyzed, showing similar trends (Fig. 4b). In the 80–160 K range, the emission peaks of Eu³⁺ ions and Tb³⁺ ions remained relatively constant. At higher temperatures (160–300 K), the Tb³⁺ emission intensity decreased continuously, while the Eu³⁺ emission intensity initially increased before also decreasing (Fig. 4d). This behavior can be attributed to energy transfer from Tb³⁺ to Eu³⁺, which boosts the Eu³⁺ emission peak. However, as the temperature continues to increase, the energy transfer becomes insufficient to compensate for the non-radiative relaxation losses in Eu³⁺, causing its emission peak to decline. In the Gd₁₅Tb_0.2Eu_0.8 cluster, although energy transfer from Tb³⁺ to Eu³⁺ also occurs, the low concentration of Tb³⁺ ions in this material is not sufficient to offset the energy loss from non-radiative relaxation in Eu³⁺. To assess the sensitivity of Gd₁₅Tb_0.2Eu_0.8 and Gd₁₅Tb_0.8Eu_0.2 as luminescent thermometers, the relative sensitivity, a key parameter for evaluating performance, was calculated.³⁰ The relative sensitivity was derived by fitting the ratio Δ using the Boltzmann function (eqn (2), Fig. 4e & f),^31,32 with relevant fitting parameters listed in Table S14† (where Δ = I₁/I₂, I₁ represents the peak emission intensity of Tb³⁺ at 547 nm and I₂ represents the peak emission intensity of Eu³⁺ at 615 nm). The relative sensitivities corresponding to the Gd₁₅Tb_1−xEu_x (x = 0.2, 0.8) clusters can be obtained by applying eqn (3) to the fitted Boltzmann function (Fig. 4e & f).³³ The relative sensitivities of these clusters increase with temperature, exhibiting high sensitivities at 300 K (relative sensitivities 3.7% K⁻¹ and 4.6% K⁻¹, respectively), demonstrating their potential as light-emitting thermometers.

Fig. 4 (a) Temperature-dependent photoluminescence spectra of Gd₁₅Tb_0.2Eu_0.8. (b) Temperature-dependent photoluminescence spectra of Gd₁₅Tb_0.8Eu_0.2. (c) The intensity changes of emission peaks for Eu³⁺/Tb³⁺ in Gd₁₅Tb_0.2Eu_0.8. (d) The intensity changes of emission peaks for Eu³⁺/Tb³⁺ in Gd₁₅Tb_0.8Eu_0.2. (e) The temperature dependence of the thermometric parameter (Δ) of Gd₁₅Tb_0.2Eu_0.8 (lines represent the fitted function). (f) The temperature dependence of the thermometric parameter (Δ) of Gd₁₅Tb_0.8Eu_0.2 (lines represent the fitted function).

Conclusions

In conclusion, green emission was achieved by doping Tb³⁺ ions and red emission was achieved by doping Eu³⁺ ions into the Gd₁₆ cluster. By co-doping with Tb³⁺ and Eu³⁺ ions adjusting their concentrations, we obtained red–orange–yellow color-tunable luminescence in Gd₁₆ clusters. Photoluminescence studies demonstrated that Gd₁₆ effectively serves as a host for the efficient photosensitization of both Tb³⁺ and Eu³⁺ ions. The energy transfer from the ligand to Tb³⁺ and Eu³⁺ ions in the co-doped Gd₁₆ clusters was confirmed through temperature-resolved photoluminescence measurements, and the energy transfer efficiency was calculated. Additionally, the temperature-dependent photoluminescence properties of these clusters were examined to assess their potential as luminescent thermometers. Overall, this work significantly advances the field of rare earth metal clusters by enabling tunable color emissions in Gd₁₆ clusters.

Experimental section

Materials and methods

All chemicals were used as is without further purification. Powder XRD patterns were obtained using a Rigaku Ultima IV diffractometer with a Cu source. Photoluminescence excitation (PLE) and photoluminescence (PL) were measured on a Hitachi F-4700 spectrometer with excitation and emission slits of 2.5 nm. Other parameters: scan speed 240 nm min⁻¹, PMT voltage 700 V, and response 2.0 s. The temperature change test and emission decay life of all samples measurement were conducted on an FLS1000 spectrofluorometer (Edinburgh Instruments Ltd). FTIR spectra were obtained using a Nicolet 6700 FTIR spectrometer in ATR mode (Fig. S3†). ICP-OES analysis was performed with an Agilent 5110 ICP-OES instrument.

Synthesis of Gd_16-xEu_x (x = 0.2; 0.4; 0.6; 0.8)

The synthesis of this series of lanthanide clusters follows a similar procedure.⁹ A mixture of (16-x)/16 mmol of Gd(NO₃)₃·6H₂O and x/16 mmol of Eu(NO₃)₃·6H₂O were dissolved in a mixture of CH₃CN (2 mL) and CH₃OH (1 mL) at ambient temperature with continuous stirring to ensure complete solvation. Trifluoroacetylacetone (2 mmol, 245 μL) was subsequently added, and the solution was stirred for 7–8 minutes. Triethylamine (4.4 mmol, 550 μL) was then introduced as a base, and the mixture was stirred for an additional 4–5 minutes. The reaction mixture was then filtered, stored at 4–8 °C, and allowed to evaporate slowly, yielding crystals within one week. Selected IR peaks (cm⁻¹): 3433(s), 2993(w), 2935(w), 1639(s), 1527(s), 1473(s), 1363(s), 1290(s), 1228(m), 1189(m), 1137(s), 1016(m), 945(m), 854(s), 779(s), 727(s), 673(m), 613(w), 559(s).

Synthesis of Gd_16-xTb_x (x = 0.2; 0.4; 0.6; 0.8)

Compounds Gd_16−xTb_x were prepared using the same procedures as described above for the synthesis of compounds Gd_16−xEu_x but replacing Eu(NO₃)₃·6H₂O with Tb(NO₃)₃·6H₂O. After one week of solvent evaporation at 4–8 °C, colorless crystals were obtained. Selected IR peaks (cm⁻¹): 3433(s), 2993(w), 2935(w), 1639(s), 1527(s), 1473(s), 1363(s), 1290(s), 1228(m), 1189(m), 1137(s), 1016(m), 945(m), 854(s), 779(s), 727(s), 673(m), 613(w), 559(s).

Synthesis of Gd₁₅Tb_1-xEu_x (x = 0.2; 0.4; 0.6; 0.8)

Compounds Gd₁₅Tb_1−xEu_x were prepared using the same procedures as described above for the synthesis of compounds Gd_16−xEu_x but replacing Eu(NO₃)₃·6H₂O with 0.9375 mmol Gd(NO₃)₃·6H₂O, (1 − x)/16 mmol of Tb(NO₃)₃·6H₂O and x/16 mmol of Eu(NO₃)₃·6H₂O. After one week of solvent evaporation at 4–8 °C, colorless crystals were obtained. Selected IR peaks (cm⁻¹): 3433(s), 2993(w), 2935(w), 1639(s), 1527(s), 1473(s), 1363(s), 1290(s), 1228(m), 1189(m), 1137(s), 1016(m), 945(m), 854(s), 779(s), 727(s), 673(m), 613(w), 559(s).

Data availability

The authors confirm that the data supporting the findings of this study are available within the article and its ESI.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (12174151 and 12304448).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4nr04779f

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