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High-Nuclearity Lanthanide−Titanium Oxo Clusters as Luminescent Molecular Thermometers with High Quantum Yields Dong-Fei Lu, Zi-Feng Hong, Jing Xie, Xiang-Jian Kong,* La-Sheng Long,* and Lan-Sun Zheng Collaborative Innovation Center of Chemistry for Energy Materials, State Key Laboratory of Physical Chemistry of Solid Surface, and Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China

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S Supporting Information *

ABSTRACT: Three heterometallic lanthanide−titanium oxo clusters (LnTOCs) formulated as Eu2Ti4(μ3-O)4(tbba)12(acac)2 (Eu2Ti4, 1, Hacac = acetylacetone), Eu 5 Ti 4 (μ 3 -O) 6 (tbba) 20 (Htbba)(THF) 2 (Eu5Ti4, 2), and Eu8Ti10(μ3-O)14(Ac)2(tbba)34(H2O)4(THF)2(Htbba)2 (Eu8Ti10, 3) were prepared through the reactions of 4-tert-butylbenzoate (Htbba), rare-earth salts, and Ti(OiPr)4. The solution luminescence investigation discovered a size-dependent quantum yield phenomenon in solution. A solid-state luminescence study showed that these three LnTOCs display temperature-dependent photoluminescent properties. Interestingly, the Eu5Ti4 cluster exhibited the highest quantum yield of 94.9% in the solid state among the reported 3d−4f clusters.

1. INTRODUCTION The accurate measurement of temperature in a non-invasive and non-contact way is currently a very active research field for widespread applications in nanotechnology and biomedicine.1−4 Among different kinds of temperature sensors, luminescent temperature sensors have received particular attention because of their high detection sensitivity, rapid response time, fast-moving components, and contactless cells.2 To date, a variety of temperature-dependent luminescent materials, such as phosphors, organic dyes, metal complexes, and metal−organic frameworks (MOFs), have been reported as luminescent thermometers.3 However, a major concern of these luminescent materials is their relatively low quantum yields. In general, the luminescence quantum yield significantly decreases with an increase in temperature because the increasing temperature leads to an increase in the level of thermal activation of radiationless processes. Therefore, developing luminescent molecular thermometers with high quantum yields over a wide temperature range is very important for improving thermosensing performances. Among the proposed molecular thermometers, lanthanide complexes exhibit unique characteristic luminescence properties with long lifetimes and high luminescence quantum yields. As a new species of heterometallic 3d−4f clusters, the lanthanide−titanium clusters have attracted continuous attention over the past few years for their potentially versatile properties, such as photocatalytic, photoelectric catalysis, and fluorescence properties.5,6 In particular, when Ti4+ is linked with the Eu3+ by oxo(hydroxo) bridges, the Eu−Ti−O species usually possess a strong red luminescence because of the energy transfer process.5 Herein, we report the syntheses, crystal structures, and luminescence properties of three heterometallic © 2017 American Chemical Society

lanthanide−titanium oxo clusters (LnTOCs), Eu2Ti4(μ3O)4(tbba)12(C5H8O2)2 (Eu2Ti4, 1, Htbba = 4-tert-butylbenzoate), Eu5Ti4(μ3-O)6(tbba)20(Htbba)(THF)2 (Eu5Ti4, 2), and Eu8Ti10(μ3-O)14(CH3COO)2(tbba)34(H2O)4(THF)2(Htbba)2 (Eu8Ti10, 3). Significantly, the quantum yield of Eu5Ti4 reached 94.9% in the solid state, the highest quantum yield reported for 3d−4f clusters so far.7 Interestingly, investigation of the solution luminescence revealed a size-dependent quantum yield phenomenon, suggesting that the nanosized LnTOCs are excellent candidates for luminescent molecular temperature sensors.

2. EXPERIMENTAL SECTION 2.1. Materials and Physical Measurements. All reagents were of commercial origin and were used as received. The C, H, and N microanalyses were performed with a CE instruments EA 1110 elemental analyzer. The infrared spectrum was recorded on a Nicolet AVATAR FT-IR 330 spectrophotometer with pressed KBr pellets. A thermogravimetric analysis (TGA) curve was prepared on a SDTQ600 thermal analyzer. The lifetime and quantum yield were measured on a Horiba Jobin Yvon Fluoromax-4P-Tcspc spectrometer. Room-temperature fluorescence and temperature-dependent fluorescence were measured on an Edinburgh instruments FLS 980-STM apparatus. The absolute emission quantum yields of the compounds were measured at room temperature using a calibrated integrating sphere as a sample chamber, and specpure BaSO4 was used as a reflecting standard. 2.2. Syntheses of Compounds 1−3. 2.2.1. Synthesis of Eu2Ti4(μ3-O)4(tbba)12(acac)2 (Eu2Ti4, 1). 4-tert-Butylbenzoate (0.173 g, 0.97 mmol) and Eu(acac)3·xH2O (0.0449 g, 0.1 mmol) were added to a mixture of 2.25 mL of acetonitrile and 0.75 mL of tetrahydrofuran Received: June 15, 2017 Published: September 28, 2017 12186

DOI: 10.1021/acs.inorgchem.7b01522 Inorg. Chem. 2017, 56, 12186−12192

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Figure 1. Ball-and-stick view of the crystal structures of (a) Eu2Ti4(μ3-O)4(tbba)12(C5H8O2)2, (b) Eu5Ti4(μ3-O)6(tbba)20(Htbba)(THF)2, and (c) Eu8Ti10(μ3-O)14(CH3COO)2(tbba)34(H2O)4(THF)2(Htbba)2 (blue for coordination H2O molecules). Hydrogen atoms have been omitted for the sake of clarity. (THF). Then the Ti(OiPr)4 (36.0 μL, 0.12 mmol) was added to the solution described above. After half a day, the yellowish crystals were obtained and then washed with acetonitrile [50% yield based on Eu(acac)3·xH2O]. Anal. Calcd for Eu2Ti4C142O32H170 (FW = 2884.30): C, 59.13; H, 5.94. Found: C, 59.00; H, 5.51. IR (KBr, cm−1): 2964 (s), 2931 (s), 2904 (s), 2870 (s), 1410 (s), 1383 (s), 1361 (s), 713 (s), 786 (s). 2.2.2. Synthesis of Eu5Ti4(μ3-O)6(tbba)20(Htbba)(THF)2 (Eu5Ti4, 2). 4-tert-Butylbenzoate (0.088 g, 0.49 mmol), Eu(Ac)3·3H2O (0.0192 g, 0.05 mmol), and Ti(OiPr)4 (18.0 μL, 0.06 mmol) were added to a mixture of 2.25 mL of acetonitrile and 0.75 mL of THF in a glass vial. The resulting solution was stirred for 30 min and then heated to ∼80 °C for 24 h. The product was obtained as colorless block-shaped crystals and then washed thoroughly with acetonitrile [45% yield based on Eu(Ac)3·3H2O]. Anal. Calcd for Eu5Ti4C228O48H277 (FW = 4736.89): C, 57.81; H, 5.89. Found: C, 57.15; H, 5.84. IR (KBr, cm−1): 2964 (s), 2931 (s), 2904 (s), 2870 (s), 1410 (s), 1383 (s), 1361 (s), 713 (s), 786 (s). 2.2.3. Synthesis of [Eu8Ti10(μ3-O)14(tbba)34(Ac)2(H2O)4(THF)2]· 2Htbba (Eu8Ti10, 3). Ti(OiPr)4 (18.0 μL, 0.06 mmol) was added to a solution containing 4-tert-butylbenzoate (0.088 g, 0.49 mmol) and Eu(Ac)3·3H2O (0.0192 g, 0.05 mmol) in a mixture of 2.25 mL of acetonitrile and 0.75 mL of THF in a glass vial. The resulting solution was stirred for 30 min and then heated to approximately 70 °C for 600 min. After the mixture cooled to room temperature, colorless rodshaped crystals were obtained in 45% yield [based on Eu(Ac)3·3H2O]. The products were washed thoroughly with acetonitrile and then heated to 30 °C in a vacuum drying oven, where they were dried for 12 h. Anal. Calcd for Eu8Ti10C408O96H500 (FW = 8634.66): C, 56.75; H, 5.83. Found: C, 55.67; H, 5.51. IR (KBr, cm−1): 2964 (s), 2904 (s), 2870 (s), 1410 (s), 1383 (s), 1361 (s), 786 (s), 713 (s). 2.3. X-ray Crystallography. Data collections were performed on an Agilent Technologies Super Nova Micro Focus single-crystal diffractometer using Cu Kα radiation (λ = 1.54184 Å) at 100 K. Absorption corrections were applied by using the multiscan program. The structures were determined and refined using full-matrix leastsquares based on F2 with SHELXS-97 and SHELXL-978 within Olex2.9 CCDC 1545427, 1545428, and 14707446 for Eu2Ti4, Eu5Ti4, and Eu8Ti10, respectively, containing the supplementary crystallographic data for this paper. The data can be obtained free of charge from The Cambridge Crystallorgaphic Data Centre via www.ccdc.cam. ac.uk/data_request/cif.

two Eu3+, four Ti4+, 4 μ3-O2−, two acetylacetone, and 12 4-tertbutylbenzoate ligands, as shown in Figure 1a. The metal−oxo core of [Eu 2 Ti 4 (μ3 -O) 4 ] 14+ consists of two triangular [EuTi2(μ3-O)]9+ trinuclear units linked by two μ3-O2− atoms (Figure 2a). The planar [Eu2Ti4(μ3-O)4]14+ metal core is

Figure 2. Ball-and-stick view of the assembly of the metal−oxo core of (a) [Eu2Ti4(μ3-O)4]14+ for Eu2Ti4, (b) [Eu5Ti4(μ3-O)6]15+ for Eu5Ti4, and (c) [Eu8Ti10(μ3-O)14]36+ for Eu8Ti10. Color code: purple for Eu3+, green for Ti4+, and red for O atoms.

stabilized by 12 tbba− and two acac− ligands. Each Eu atom displays a distorted triangular dodecahedron geometry (Figures S1 and S4) and is eight-coordinated by five 4-tert-butylbenzoate groups and three oxo−metal atoms (μ3-O). The titanium atoms are six-coordinated and show distorted octahedral geometries. The Eu−O bond distances are 2.336(6)−2.513(6) Å. As shown in Figures 1b and 2b for Eu5Ti4, five Eu3+ and two 4+ Ti ions are connected by four μ3-O2− atoms, forming an Eucentered planar hexagonal structure of [Eu5Ti2(μ3-O)4]15+. Both sides of the hexagonal structure are connected to two Ti4+ ions through two μ3-O2− atoms, producing the [Eu5Ti4(μ3O)6]15+ metal−oxo core. Interestingly, the nine metal atoms are nearly planar. The nine-metal core is protected by 20 tbba−,

3. RESULTS AND DISCUSSION 3.1. Crystal Structures. Heterometallic lanthanide− titanium oxo cluster Eu2Ti4 (1) was prepared through the reaction of 4-tert-butylbenzoic acid (Htbba), Eu(acac)3, and Ti(OiPr)4 in acetonitrile and THF. Clusters 2 and 3 were prepared in a manner similar to that described for 1. Singlecrystal X-ray diffraction shows that cluster Eu2Ti4 consists of 12187

DOI: 10.1021/acs.inorgchem.7b01522 Inorg. Chem. 2017, 56, 12186−12192

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Figure 3. Excitation and emission spectra of (a) the Htbba ligand, (b) Eu2Ti4, (c) Eu5Ti4, and (d) Eu8Ti10 in toluene.

Table 1. Luminescence Lifetimes, Quantum Efficiencies, and Radiative and Nonradiative Rate Constants of Three Lanthanide− Titanium Oxo Clusters in Toluene and in the Solid State cluster

τ (ms) (aq)

η (%) (aq)

kr (s−1) (aq)

knr (s−1) (aq)

η (%) (s)

τ (ms) (s)

kr (s−1) (s)

knr (s−1) (s)

Eu2Ti4 Eu5Ti4 Eu8Ti10

1.09 1.11 1.27

7.1 29.6 45.3

64.72 267.78 357.20

883.36 635.96 430.79

17.6 94.9 73.1

1.19 1.20 1.30

147.15 788.04 560.89

690.37 42.52 205.98

(5D0 → 7F2), 650 (5D0 → 7F3), and 700 nm (5D0 → 7F4), As shown in Figure 3b−d, respectively. Obviously, no emission peak of the Htbba ligand is observed, suggesting that efficient intramolecular energy transfer from the Htbba ligand to Eu3+ occurs in these lanthanide−titanium oxo clusters.10 The characteristic emission peaks of the Eu3+ ion correspond to 4f−4f transitions from the resonating level 5D0 to the groundstate multiplet 7FJ (J = 0, 1, 2, 3, and 4) of the Eu3+ ion. The 5 D 0 → 7 F 1 transition (magnetic dipole transition) is independent of the local environment around the Eu3+ ion, while the 5D0 → 7F2 transition (electric dipole transition) varies strongly with the local symmetry around the Eu3+ ion.11 The different intensity ratios (I1/I2 = 5D0 → 7F2/5D0 → 7F1) of 6.1 for Eu2Ti4, 5.7 for Eu5Ti4, and 4.4 for Eu8Ti10 are attributed to the different symmetry environments.12 To better understand the luminescence properties, the lifetimes and quantum yields in toluene were measured at room temperature. As shown in Table 1, the phosphorescent lifetimes (τ) of Eu2Ti4, Eu5Ti4, and Eu8Ti10 are 1.09, 1.11, and 1.27 ms, respectively, which are consistent with thermally activated nonradiative rate constants. The long lifetime values of the three LTOCs indicate the ligands suppress the nonradiative-decay pathway among the various transition levels and effectively transfer energy to the metal ions.12a The quantum yield is an important parameter for evaluating the efficiency of an emission process. As shown in Table 1, the quantum yields of Eu2Ti4, Eu5Ti4, and Eu8Ti10 in toluene are 7.05, 29.6, and 45.3%, respectively. The difference in the quantum yields of three clusters is attributed to the different sizes of the clusters (2.0 nm for Eu2Ti4, 2.6 nm for Eu5Ti4, and 3.4 nm for Eu8Ti10). The observed size-dependent quantum

one protonated Htbba, and two terminally coordinated THF ligands. The five Eu3+ atoms display three types of coordination geometries (Figures S2 and S4). Eu1 is eight-coordinated and shows a distorted square antiprism geometry, while Eu2 is ninecoordinated and displays a capped square antiprism coordination configuration. Notably, Eu3 adopts a rare six-coordinated octahedral geometry with Eu−O distances of 2.417(7) and 2.381(6) Å. The crystal structure of Eu8Ti10 was reported in our previous work (Figure 1c). The metal−oxo core of [Eu8Ti10(μ3-O)14]36+ consists of two [Eu3Ti3(μ3-O)3]13+ units connected by one planar [Eu2Ti4(μ3-O)4]14+ unit and four μ3-O2− atoms (Figure 2c). The [Eu3Ti3(μ3-O)3]13+ unit consists of three vertexsharing [Eu2Ti(μ3-O)3]8+ triangular units, which are connected to each other by two shared Eu3+ ions. The planar [Eu2Ti4(μ3O)4]14+ unit, which is assembled from two Eu3+, four Ti4+, and four μ3-O2− atoms, displays the same geometry as the metal core in the Eu2Ti4 cluster. The metal−oxo core of [Eu8Ti10(μ3O)14]36+ is further stabilized by 34 tbba− and two acetate (Ac−) ligands, and the metal coordination sphere is completed with two THF and four water molecules (colored blue in Figure 1c). A detailed structural description is available in our previously reported work.6 3.2. Luminescence Properties. The emission and excitation spectra measured at room temperature in toluene of the three lanthanide−titanium oxo clusters and the Htbba ligand are displayed in Figure 3. The free Htbba ligand exhibits a weak emission at 338 nm, which is attributed to π → π* transitions under 280 nm ultraviolet light excitation (Figure 3a). Eu2Ti4, Eu5Ti4, and Eu8Ti10 exhibit the characteristic transitions of Eu3+ at 580 (5D0 → 7F0), 592 (5D0 → 7F1), 614 12188

DOI: 10.1021/acs.inorgchem.7b01522 Inorg. Chem. 2017, 56, 12186−12192

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Figure 4. (a) Emission spectra of Eu2Ti4. Temperature dependence of the emission intensity of (b) Eu2Ti4, (c) Eu5Ti4, and (d) Eu8Ti10 in temperature ranges of 100−300, 180−240, and 160−240 K, respectively (the red solid lines are linear fits).

Figure 5. Temperature-dependent emission spectra of clusters (a) Eu2Ti4, (b) Eu5Ti4, and (c) Eu8Ti10. (d) Excitation and emission spectra of the ligand in the solid state.

has also been observed in semiconductor nanocrystals.13 To confirm the size-dependent quantum yield phenomenon, the

yield phenomenon is mainly due to the decrease in the number of nonradiative vibrations caused by molecular rotation, which 12189

DOI: 10.1021/acs.inorgchem.7b01522 Inorg. Chem. 2017, 56, 12186−12192

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potential application in luminescent thermometers for different temperature ranges.

radiative rates (kr) and nonradiative rates (knr) were obtained according to the relationship equations shown in the Supporting Information. As shown in Table 1, the radiative rate (kr = 64.7 s−1) of Eu2Ti4 is much lower than the nonradiative rate (knr = 883.4 s−1), which indicates that the nonradiative process dominates with a small molecular size. The values of kr increase to 267.8 s−1 for Eu5Ti4 and 357.2 s−1 for Eu8Ti10 because the competing radiative pathways become more prevalent with an increasing cluster size. Notably, Eu8Ti10 shows the highest quantum yield of 45.3% in solution among reported 3d−4f clusters. The comparison of the quantum yields of these three LTOCs suggests that high-nuclearity LTOCs are ideal luminescent molecular materials with high quantum yields. The quantum yields in the solid state were also studied. As shown in Table 1, the quantum yields in the solid state are 17.6% for Eu2Ti4, 94.9% for Eu5Ti4, and 73.1% for Eu8Ti10. Compared with the quantum yields in toluene, the quantum yields in the solid state are remarkably increased because molecular rotations were confined. The red emissions of these clusters are evident to the naked eye, even in daylight (Figure S6). However, it is surprising that Eu5Ti4 displays the highest quantum yield of 94.9% in the solid state, which is the highest reported value for a lanthanide-containing cluster. Although Eu8Ti10 is the largest, the quantum yield of 73.1% is smaller than that of the Eu5Ti4 cluster. Through detailed structural analysis, the main reason for this observation is the increase in the rate of the nonradiative transition caused by the stretching vibrations of the four coordinated water molecules in Eu8Ti10. To prove the effect of the Ti4+, electron paramagnetic resonance (EPR) spectra were recorded. As shown in Figure S16, the EPR spectra of Eu2Ti4 with irradiation show an enhanced characteristic peak at g = 2.00 for Ti3+ species.13 These results suggest the presence of an energy transfer process in the Eu−Ti−O clusters, and the Ti4+ ions can be reduced to Ti3+ because the light promotes electron transfer.14 Because of the high quantum yields in the solid state, the temperature-dependent photoluminescent (PL) properties of these LnTOCs were investigated to determine their potentials as luminescent thermometers. The temperature-dependent emission spectra of Eu2Ti4, Eu5Ti4, and Eu8Ti10 between 100 and 300 K are illustrated in Figures 4 and 5. The maximum emission intensity of Eu3+ (5D0 → 7F2) in Eu2Ti4 decreases gradually as the temperature increases because of thermal activation of nonradiative-decay pathways.11a,b,15 It is worth mentioning that the emission intensity at 620 nm for Eu2Ti4 linearly increases when the temperature is decreased from 300 to 100 K (Figure 4b). The temperature is linearly related to the emission intensity, suggesting that Eu2Ti4 is an excellent luminescent molecular thermometer in this wide temperature range.16 Over the temperature range of 100−300 K, Eu2Ti4 displays a high sensitivity of 0.36% per kelvin, comparable to the high values of luminescent lanthanide coordination polymers or lanthanide MOF thermometer materials.17,18 However, for Eu5Ti4 and Eu8Ti10, the temperature-dependent fluorescence properties are complicated. As shown in panels c and d of Figure 4, the fluorescence emission intensities of Eu5Ti4 and Eu8Ti10 decrease as the temperature increases from 100 to 140 K because of thermal quenching. When the temperature increases from 180 to 240 K, the intensities linearly increase with sensitivities of 0.31% per kelvin for Eu5Ti4 and 0.74% per kelvin for Eu8Ti10. These results indicate that high-nuclearity lanthanide−titanium oxo clusters have

4. CONCLUSION In summary, we reported three heterometallic lanthanide− titanium oxo clusters (Eu2Ti4, Eu5Ti4, and Eu8Ti10) for use as luminescent molecular thermometers. The temperature-dependent PL properties of Eu2Ti4 displayed a high temperature sensitivity over a wide temperature range of 100−300 K. Interestingly, Eu5Ti4 exhibited the highest quantum yield of 94.9% in the solid state among reported lanthanide-containing clusters. In solution, a size-dependent quantum yield phenomenon was observed, suggesting that the large highnuclearity LTOCs are ideal luminescent molecular materials with high quantum yields. The results obtained in this study will provide new insights for designing LTOCs for developing luminescent molecular temperature-sensing devices.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01522. Synthesis and characterization details for 1−3 (PDF) Accession Codes

CCDC 1545427 and 1545428 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Xiang-Jian Kong: 0000-0003-0676-6923 La-Sheng Long: 0000-0002-0398-4709 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the 973 Project (Grant 2014CB845601) from the Ministry of Science and Technology of China, the National Natural of Science Foundation of China (Grants 21422106, 21371144, 21431005, 21673184, and 21390391), and the Fok Ying Tong Education Fundamental (151013) for support.

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REFERENCES

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DOI: 10.1021/acs.inorgchem.7b01522 Inorg. Chem. 2017, 56, 12186−12192