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Effects of Ligand and Guest Solvent Molecule on Luminescent Property of Tb:Eu-Codoped Indium-Based MOF
Published on 17 February 2016. Downloaded by Gazi Universitesi on 17/02/2016 12:00:58.
Received 00th January 20xx, Accepted 00th January 20xx
Wenbo Yan, Le Wang, Kete Yangxiao, Zhixing Fu and Tao Wu*
DOI: 10.1039/x0xx00000x www.rsc.org/
Luminescent lantahanid MOF materials are good sensors for analyzing some specific gas or volatile small molecules. However, some potential interference factors coming from material itself, such as bridging ligand or guest solvent molecules entrapped in the channel of MOF, were usually ignored during sensing process. Hereby, two Tb:Eu-codoped Indium-based MOFs with different briding ligands were obtained for exploring effects of ligand and guest solvent molecules on their luminescent properties. The current studies demonstated that ligand with the triplet state locating between the excited states of Tb and Eu and polar guest solvent molecules encapsulated in lanthanide MOF with such type of ligand as linker can interfere sensing process since they can substantially facilitate energy transfer between Tb and Eu.
MOFs are generally regarded as 3D frameworks assembled by metal ions as nodes and organic ligands as linkers. They can be utilized as ideal materials for sensing small molecular analytes since the permanent porosity and large surface area provide proper environment for host-guest interactions, and tunable luminescent properties of most MOF materials make detection signal to be easily readout.1, 2 Generally, photoluminescence of MOF materials stems from the organic linkers or inorganic parts, or both.3 As to the inorganic parts, lanthanide ions with unique luminescent properties have attracted great attention. Doping more than one type of lanthanide ions into MOF materials is common method to modify and control luminescent properties of the resulting materials.4 Among them, terbium (Tb) and europium (Eu) are most commonly used ions since their characteristic peaks are located in visible range and luminescent intensities are comparable.5 By changing the doping ratio of dual lanthanide ions, luminescent properties can be accurately controlled and modified.6 Mixed-lanthanide MOF sensors show several advantages over single-lanthanide MOFs. The detection signal is easier to be read out and detection sensitivity is higher.7 Such type of MOF materials
State and Local Joint Engineering Laboratory for Novel Functional Polymeric Materials, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Jiangsu 215123, China. E-mail: [email protected] †Electronic Supplementary Information (ESI) available: Synthesis, XRD, and TGA data. For ESI and crystallographicdata in CIF or other electronic format see DOI:XXXXXXX
have already been widely applied in sensing. For instances, Chen’s group successfully realized the sensing of temperature by virtue of the europium-doped terbium-based MOF, Eu0.0069Tb0.9931-DMBDC (DMBDC = 2,5-dimethoxy-1,4-benzenedicarboxylate).8 Lately, they also obtained another Ln-MOF (Tb0.9Eu0.1PIA, PIA = 5-(pyridine-4yl)isophthalate) to optimize the detection performance.9 Wu’s group also applied a luminescent mixed-lanthanide-based MOF material, [LnTCM(H2O)2]·3DMF·H2O (TCM = 4,4’,4”-(((2,4,6-trimethylbenzene-1,3,5-triyl)-tris(methylene))-tris(oxy))tribenzoicacid), in sensing volatile organic small molecules with a unique readout. The luminescent intensity ratio of Tb:Eu changes with different substrates through energy transfer process between lanthanide ions.10 The sensing mechanism for the dual-lanthanide MOF sensors relies on the change of luminescent intensity of lanthanide ions, which is usually realized through energy transfer process between lanthanide ions tuned by analytes. However, the bridging ligand sometimes can also serve as energy transporter to facilitate the energy transfer between lanthanide ions, which will cause interferences in sensing some specific analytes. Therefore, ligand selection becomes very important in creating lanthanide-based MOFs as sensor. In addition, most of MOF-involved sensing processes are carried out in liquid environment, in which guest solvent molecules are inevitably entrapped in the channel or pore of MOFs.2 It is generally accepted that polar solvent molecule can affect the dipole moment of organic linkers, which could potentially affect energy transfer process. Unfortunately, the influence of ligand and solvent guest molecules as potential interference factors has always been ignored in some solvent-involved sensing systems.11 Herein, we purposely synthesized two Tb:Eu-codoped indiumbased MOFs with different bridging ligands, Ln-In-TATB (1) and Ln-In-BTC (2) (TATB = 4,4’,4”-s-triazine-2,4,6-triyltriibenzoate, BTC = 1,3,5-benzene tricarboxylic acid, and Ln = Tb3+ and Eu3+), and investigated effects of ligand and guest solvent molecules on their luminescent properties. The current data demonstrated that the TATB ligand with the triplet state locating between the excited state of Tb and Eu and polar guest solvent molecules encapsulated in lanthanide MOF with TATB ligand can interfere sensing process since both of them can substantially facilitate energy transfer between Tb and Eu.
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Fig 1. Structure of Ln-In-TATB (1). (a) The [In3Ln2(OH)3O]10+ SBU. (b) TATB ligand. (c) The arrangement of SUB and TATB ligand. (d) View from a direction. (e) View from c direction.
Fig 2. (a) Solid-state PL spectra of 1 with different content ratio of Tb:Eu. (b)
(Table S1). To be noticed, crystal qualities of 1 and 2 View wereArticle not Online good DOI: 10.1039/C5DT04844C enough to get the structure from the single-crystal X-ray diffraction data. However, fortunately, powder XRD data (Fig S1, Fig S2) confirmed that 1 and 2 are isostructural with CPM-19 and CPM-5 (Fig S3), respectively. The TGA data (Fig S4) also showed both compound 1 and 2 have good thermal stability. The location of lanthanide ions in crystal structure has been identified by EDS mapping measurements (Fig S5). As iso-structure with CPM-19, the building unit of compound 1 also consists of three 6-coordinated In3+, one 9-coordinated Ln3+ and one 10-coordianted Ln3+, which can be regarded as the combination of cubic-like M4(μ3-O)4and pyramid-like M4(μ4-O) secondary building unit (SBU) (Fig. 1a-1c). These SBUs then coordinate with nine TATB ligands to form a 3D framework (Fig. 1d and 1e). Solid-state luminescent properties of compound 1 and 2 were firstly studied. All the samples were dried before measurements under vacuum to remove guest solvent molecules entrapped in the cavity. Both 1 and 2 show typical luminescent properties of lanthanide ions, and the emission colour can be well controlled. By changing the raw content ratio of Tb:Eu in both 1 and 2, the emitting colour can change from red to green. From emission spectra with the excitation wavelength of 315nm (Fig. 2a), despite the content ratio of Tb:Eu in the raw materials rising from 1-fold to 9-fold, the emitting colour of 1 remains red, and it turns to yellow only when the ratio is greater than 19-fold (Fig. 2b). Up to 39-fold, compound 1 appears yellow-green emission. The EDS data show that the content ratio of Tb to Eu in the final crystalline products has a linear relationship with that in the starting materials (Fig. 2c). However, the intensity ratio of the characteristic peak of Tb (5D4→7F5 at 545 nm) to Eu (5D0→7F2 at 615 nm) is non-linear to the raw ratio (Fig. 2d). Obviously, there must exist energy transfer between Tb and Eu. In the meanwhile, the emitting colour of 2 gradually changes from red to yellow, then to green with the content ratio of Tb to Eu increasing (Fig. 3a and 3b). From the EDS data, the final real content ratio has a linear relationship with raw content ratio (Fig. 3c). The
Photograph of 1 with different content ratio of Tb:Eu under UV lamp. (c) Plot of the raw content ratio versus real content ratio of Tb:Eu in 1. (d) Plot of the characteristic peak intensity ratio of Tb:Eu versus the raw content ratio in 1.
Due to the various coordination models and high coordination numbers for lanthanide ions, it is hard to construct well designed LnMOFs.12 Fortunately, partial in-situ doping methods can well solve this problem. Partial doping method is a common way to construct isostructural MOF materials, especially Ln-based MOFs.13 Compound 1 was prepared by heating the mixture of Eu(NO3)3·6H2O, Tb(NO3)3·5H2O, In(NO3)3·4.5H2O and TATB in DMF and H2O solution at 120oC for 5 days. Synthetic method was similar with the preparation of CPM-19, only replacing H3BTB (1,3,5-tri(4-carboxyphenyl)benzene) with H3TATB. The guestaccessible volume is 72.8% of the unit cell and the pore size are 7.4, 7.6, and 9.4 Å.14 Compound 2 was prepared by in-situ doping Tb3+ and Eu3+ into previously reported compound CPM-5. The guestaccessible volume of compound 2 is 47.9% of the unit cell with the pore size in the range of 1.68-4.00 Å as reported.15 By changing the content ratio of Tb and Eu in the raw reactants, series of compound 1 and 2 with different content ratio of Tb:Eu have been obtained. The content of Tb:Eu is characterized by EDS and ICP measurements
Fig 3. (a) Solid-state PL spectra of 2 with different content ratio of Tb:Eu. (b) Photograph of 2 with different content ratio of Tb:Eu under UV lamp. (c) Plot of the raw content ratio versus real content ratio of Tb: Eu in 2. (d) Plot of the characteristic peak intensity ratio of Tb:Eu versus the raw content ratio in 2.
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intensity ratio of the characteristic peaks versus raw content ratio is also linear correlation (Fig. 3d). These results show that there is no obvious energy transfer between Tb and Eu in 2. In addition, for the compound 1 and 2 with the same ratio of Tb:Eu (9:1), the quantum yield of 1 (25%) is quite higher than that of 2 (1.67%), which also indicates the existence of effective energy transfer in 1. The excited triplet state of ligand is key consideration while determining its role in the energy transfer between lanthanide ions. The excited state of Tb3+ and Eu3+ is about 20400 cm-1(5D4) and 17250 cm-1 (5D0), respectively.5, 16 Big energy gap between the excited state of Tb and Eu prevents effective energy transfer between them. The energy level of triplet state of BTC (23200 cm-1) is higher than the excited state of Tb and Eu,16 therefor, BTC ligand can not serve as energy transporter to facilitate energy transfer between Tb and Eu, as observed in compound 1. While the triplet state of TATB (calculated 19500 cm-1, Fig S6) locates in the middle of excited state of Tb and Eu, there occurs energy transfer between them, and finally influences the luminescent intensity ratio of Tb:Eu, as observed in compound 2. The influence of guest solvent molecules on the luminescent properties of 1 and 2 was also studied. To eliminate interference of guest molecules originally embedded in the pore during synthesis, the raw crystalline compounds were grounded, and then evacuated under vacuum at 120oC for 12h, following by dispersing ground vacuumed sample in solvents with different polarity. For compound 1, emission intensity ratio of characteristic peak of Tb:Eu decreased with solvent polarity increasing (Fig 4a, Fig S7). However, for compound 2, intensity ratio almost remains unchanged (Fig 4b). The histogram of characteristic peak intensity ratio of Tb:Eu clearly
shows that emission property of 1 is sensitive to solvent polarity, View Article Online 10.1039/C5DT04844C while emission property of 2 has no responseDOI: to solvent polarity (Fig 4c). So, it is clear that polar guest solvent molecules have influence on luminescent properties of 1 in which there exists ligand-assisted energy transfer between Tb and Eu, and have no effect on luminescent properties of 2 in which no ligand-assisted energy transfer process gets involved. In summary, we purposely synthesized two Tb:Eu-codoped indium-based MOF sand investigated the effects of ligand and guest solvent molecules on energy transfer process between Tb and Eu. Different from BTC ligand, the bridging ligand TATB with the triplet state locating between the excited states of Tb and Eu shows effective facilitation in energy transfer between them. In this case, polar guest solvent molecules also influence the energy transfer efficiency. Therefore, while designing Ln-MOFs materials as sensors with dual emitting in liquid sensing system, ligands with the triplet state of being much higher than the excited state of lanthanide ions could be preferably selected because it can effectively prevent ligand-assisted energy transfer process, weaken the interference of guest solvent molecules inevitably entrapped in MOF cavity on energy transfer of lanthanide ions, and promote the contribution of target analyte in controlling energy transfer efficiency between lanthanide ions. This work was supported by National Natural Science Foundation of China (No.21271135), a start-up fund (Q410900712) from Soochow University, the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), and Young Thousand Talented Program.
Notes and references
Fig 4. (a) PL spectra of 1 with different guest solvent molecules trapped in the cavities. (b) PL spectra of 2 with different guest solvent molecules trapped in the cavites. (c) The histogram of the characteristic peak intensity ratio in 1 and 2. (CHY = cyclohexane; DEE = diethyl ether; THF = tetrahydrofuran; DMF = N,NDimethylformamide.)
1. (a) L. E. Kreno, K. Leong, O. K. Farha, M. Allendorf, R. P. Van Duyne and J. T. Hupp, Chem. Rev., 2012, 112, 1105-1125; (b) L.-J. Xu, G.-T. Xu and Z.-N. Chen, Coord. Chem. Rev., 2014, 273-274, 47-62; (c) X. Zhou, H. Li, H. Xiao, L. Li, Q. Zhao, T. Yang, J. Zuo and W. Huang, Dalton Trans., 2013, 42, 57185723; (d) Y. Takashima, V. M. Martinez, S. Furukawa, M. Kondo, S. Shimomura, H. Uehara, M. Nakahama, K. Sugimoto and S. Kitagawa, Nat.Commun., 2011, 2, 168. (e) Y.-P. He, Y.-X. Tan and J. Zhang, J. Mater. Chem. C, 2014, 2, 4436. 2. D. Tian, Y. Li, R.-Y. Chen, Z. Chang, G.-Y. Wang and X.-H. Bu, J. Mater. Chem. A, 2014, 2, 1465-1470. 3. Y. Cui, Y. Yue, G. Qian and B. Chen, Chem. Rev., 2012, 112, 1126-1162. 4. (a) Y. Cui, B. Chen and G. Qian, Coord. Chem. Rev., 2014, 273274, 76-86; (b) Z. Dou, J. Yu, H. Xu, Y. Cui, Y. Yang and G. Qian, Micropor. Mesopor.Mater., 2013, 179, 198-204. 5. A. R. Ramya, S. Varughese and M. L. Reddy, Dalton Trans, 2014, 43, 10940-10946. 6. (a) F. Wang and X. Liu, Acc. Chem. Res., 2014, 47, 1378-1385. (b) H. B. Zhang, M. Liu, X. Lei, T. Wen and J. Zhang, ACS Appl. Mat. Interfaces, 2014, 6, 12594-12599. 7. L. V. Meyer, F. Schonfeld and K. Muller-Buschbaum, Chem. Commun., 2014, 50, 8093-8108. 8. Y. Cui, H. Xu, Y. Yue, Z. Guo, J. Yu, Z. Chen, J. Gao, Y. Yang, G. Qian and B. Chen, J. Am. Chem. Soc., 2012, 134, 3979-3982.
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9. X. Rao, T. Song, J. Gao, Y. Cui, Y. Yang, C. Wu, B. Chen and G. Qian, J. Am. Chem. Soc., 2013, 135, 15559-15564. 10. C. Zhan, S. Ou, C. Zou, M. Zhao and C. D. Wu, Anal. Chem., 2014, 86, 6648-6653. 11. Y.-M. Zhu, C.-H. Zeng, T.-S. Chu, H.-M. Wang, Y.-Y. Yang, Y.-X. Tong, C.-Y. Su and W.-T. Wong, J. Mater. Chem. A, 2013, 1, 11312. 12. N. T. Binh, D. M. Tien, L. T. K. Giang, H. T. Khuyen, N. T. Huong, T. T. Huong and T. D. Lam, Mater. Chem. Phys., 2014, 143, 946-951. 13. (a) C. K. Brozek, V. K. Michaelis, T.-C. Ong, L. Bellarosa, N. López, R. G. Griffin and M. Dincă, ACS Cent. Sci., 2015, 1, 252260; (b) J. A. Botas, G. Calleja, M. Sanchez-Sanchez and M. G. Orcajo, Langmuir, 2010, 26, 5300-5303. (c) H. Yang, F. Wang, Y. X. Tan, Y. Kang, T. H. Li and J. Zhang, Chem. Asian. J., 2012, 7, 1069-1073. 14. S.-T. Zheng, T. Wu, C. Chou, A. Fuhr, P. Feng, and X. Bu, J. Am. Chem. Soc. 2012, 134, 4517-4520. 15. S.-T. Zheng, J. T. Bu, Y. Li, T. Wu, F. Zuo, P. Feng and X. Bu, J. Am. Chem. Soc., 2010, 132, 17062-17064. 16. Z. Dou, J. Yu, Y. Cui, Y. Yang, Z. Wang, D. Yang and G. Qian, J. Am. Chem. Soc., 2014, 136, 5527-5530.
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Ligand with its triplet state locating between the excited state of the Tb and Eu ionsshows effective facilitation in the energy transfer process between them, and the energy transfer process in this case is also influenced by polarguest solvent molecules.
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This article can be cited before page numbers have been issued, to do this please use: W. Yan, L. Wang, K. Yangxiao, Z. Fu and T. Wu, Dalton Trans., 2016, DOI: 10.1039/C5DT04844C.
This is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication. Accepted Manuscripts are published online shortly after acceptance, before technical editing, formatting and proof reading. Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article. We will replace this Accepted Manuscript with the edited and formatted Advance Article as soon as it is available. You can find more information about Accepted Manuscripts in the Information for Authors. Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal’s standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains.
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Effects of Ligand and Guest Solvent Molecule on Luminescent Property of Tb:Eu-Codoped Indium-Based MOF
Published on 17 February 2016. Downloaded by Gazi Universitesi on 17/02/2016 12:00:58.
Received 00th January 20xx, Accepted 00th January 20xx
Wenbo Yan, Le Wang, Kete Yangxiao, Zhixing Fu and Tao Wu*
DOI: 10.1039/x0xx00000x www.rsc.org/
Luminescent lantahanid MOF materials are good sensors for analyzing some specific gas or volatile small molecules. However, some potential interference factors coming from material itself, such as bridging ligand or guest solvent molecules entrapped in the channel of MOF, were usually ignored during sensing process. Hereby, two Tb:Eu-codoped Indium-based MOFs with different briding ligands were obtained for exploring effects of ligand and guest solvent molecules on their luminescent properties. The current studies demonstated that ligand with the triplet state locating between the excited states of Tb and Eu and polar guest solvent molecules encapsulated in lanthanide MOF with such type of ligand as linker can interfere sensing process since they can substantially facilitate energy transfer between Tb and Eu.
MOFs are generally regarded as 3D frameworks assembled by metal ions as nodes and organic ligands as linkers. They can be utilized as ideal materials for sensing small molecular analytes since the permanent porosity and large surface area provide proper environment for host-guest interactions, and tunable luminescent properties of most MOF materials make detection signal to be easily readout.1, 2 Generally, photoluminescence of MOF materials stems from the organic linkers or inorganic parts, or both.3 As to the inorganic parts, lanthanide ions with unique luminescent properties have attracted great attention. Doping more than one type of lanthanide ions into MOF materials is common method to modify and control luminescent properties of the resulting materials.4 Among them, terbium (Tb) and europium (Eu) are most commonly used ions since their characteristic peaks are located in visible range and luminescent intensities are comparable.5 By changing the doping ratio of dual lanthanide ions, luminescent properties can be accurately controlled and modified.6 Mixed-lanthanide MOF sensors show several advantages over single-lanthanide MOFs. The detection signal is easier to be read out and detection sensitivity is higher.7 Such type of MOF materials
State and Local Joint Engineering Laboratory for Novel Functional Polymeric Materials, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Jiangsu 215123, China. E-mail: [email protected] †Electronic Supplementary Information (ESI) available: Synthesis, XRD, and TGA data. For ESI and crystallographicdata in CIF or other electronic format see DOI:XXXXXXX
have already been widely applied in sensing. For instances, Chen’s group successfully realized the sensing of temperature by virtue of the europium-doped terbium-based MOF, Eu0.0069Tb0.9931-DMBDC (DMBDC = 2,5-dimethoxy-1,4-benzenedicarboxylate).8 Lately, they also obtained another Ln-MOF (Tb0.9Eu0.1PIA, PIA = 5-(pyridine-4yl)isophthalate) to optimize the detection performance.9 Wu’s group also applied a luminescent mixed-lanthanide-based MOF material, [LnTCM(H2O)2]·3DMF·H2O (TCM = 4,4’,4”-(((2,4,6-trimethylbenzene-1,3,5-triyl)-tris(methylene))-tris(oxy))tribenzoicacid), in sensing volatile organic small molecules with a unique readout. The luminescent intensity ratio of Tb:Eu changes with different substrates through energy transfer process between lanthanide ions.10 The sensing mechanism for the dual-lanthanide MOF sensors relies on the change of luminescent intensity of lanthanide ions, which is usually realized through energy transfer process between lanthanide ions tuned by analytes. However, the bridging ligand sometimes can also serve as energy transporter to facilitate the energy transfer between lanthanide ions, which will cause interferences in sensing some specific analytes. Therefore, ligand selection becomes very important in creating lanthanide-based MOFs as sensor. In addition, most of MOF-involved sensing processes are carried out in liquid environment, in which guest solvent molecules are inevitably entrapped in the channel or pore of MOFs.2 It is generally accepted that polar solvent molecule can affect the dipole moment of organic linkers, which could potentially affect energy transfer process. Unfortunately, the influence of ligand and solvent guest molecules as potential interference factors has always been ignored in some solvent-involved sensing systems.11 Herein, we purposely synthesized two Tb:Eu-codoped indiumbased MOFs with different bridging ligands, Ln-In-TATB (1) and Ln-In-BTC (2) (TATB = 4,4’,4”-s-triazine-2,4,6-triyltriibenzoate, BTC = 1,3,5-benzene tricarboxylic acid, and Ln = Tb3+ and Eu3+), and investigated effects of ligand and guest solvent molecules on their luminescent properties. The current data demonstrated that the TATB ligand with the triplet state locating between the excited state of Tb and Eu and polar guest solvent molecules encapsulated in lanthanide MOF with TATB ligand can interfere sensing process since both of them can substantially facilitate energy transfer between Tb and Eu.
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Fig 1. Structure of Ln-In-TATB (1). (a) The [In3Ln2(OH)3O]10+ SBU. (b) TATB ligand. (c) The arrangement of SUB and TATB ligand. (d) View from a direction. (e) View from c direction.
Fig 2. (a) Solid-state PL spectra of 1 with different content ratio of Tb:Eu. (b)
(Table S1). To be noticed, crystal qualities of 1 and 2 View wereArticle not Online good DOI: 10.1039/C5DT04844C enough to get the structure from the single-crystal X-ray diffraction data. However, fortunately, powder XRD data (Fig S1, Fig S2) confirmed that 1 and 2 are isostructural with CPM-19 and CPM-5 (Fig S3), respectively. The TGA data (Fig S4) also showed both compound 1 and 2 have good thermal stability. The location of lanthanide ions in crystal structure has been identified by EDS mapping measurements (Fig S5). As iso-structure with CPM-19, the building unit of compound 1 also consists of three 6-coordinated In3+, one 9-coordinated Ln3+ and one 10-coordianted Ln3+, which can be regarded as the combination of cubic-like M4(μ3-O)4and pyramid-like M4(μ4-O) secondary building unit (SBU) (Fig. 1a-1c). These SBUs then coordinate with nine TATB ligands to form a 3D framework (Fig. 1d and 1e). Solid-state luminescent properties of compound 1 and 2 were firstly studied. All the samples were dried before measurements under vacuum to remove guest solvent molecules entrapped in the cavity. Both 1 and 2 show typical luminescent properties of lanthanide ions, and the emission colour can be well controlled. By changing the raw content ratio of Tb:Eu in both 1 and 2, the emitting colour can change from red to green. From emission spectra with the excitation wavelength of 315nm (Fig. 2a), despite the content ratio of Tb:Eu in the raw materials rising from 1-fold to 9-fold, the emitting colour of 1 remains red, and it turns to yellow only when the ratio is greater than 19-fold (Fig. 2b). Up to 39-fold, compound 1 appears yellow-green emission. The EDS data show that the content ratio of Tb to Eu in the final crystalline products has a linear relationship with that in the starting materials (Fig. 2c). However, the intensity ratio of the characteristic peak of Tb (5D4→7F5 at 545 nm) to Eu (5D0→7F2 at 615 nm) is non-linear to the raw ratio (Fig. 2d). Obviously, there must exist energy transfer between Tb and Eu. In the meanwhile, the emitting colour of 2 gradually changes from red to yellow, then to green with the content ratio of Tb to Eu increasing (Fig. 3a and 3b). From the EDS data, the final real content ratio has a linear relationship with raw content ratio (Fig. 3c). The
Photograph of 1 with different content ratio of Tb:Eu under UV lamp. (c) Plot of the raw content ratio versus real content ratio of Tb:Eu in 1. (d) Plot of the characteristic peak intensity ratio of Tb:Eu versus the raw content ratio in 1.
Due to the various coordination models and high coordination numbers for lanthanide ions, it is hard to construct well designed LnMOFs.12 Fortunately, partial in-situ doping methods can well solve this problem. Partial doping method is a common way to construct isostructural MOF materials, especially Ln-based MOFs.13 Compound 1 was prepared by heating the mixture of Eu(NO3)3·6H2O, Tb(NO3)3·5H2O, In(NO3)3·4.5H2O and TATB in DMF and H2O solution at 120oC for 5 days. Synthetic method was similar with the preparation of CPM-19, only replacing H3BTB (1,3,5-tri(4-carboxyphenyl)benzene) with H3TATB. The guestaccessible volume is 72.8% of the unit cell and the pore size are 7.4, 7.6, and 9.4 Å.14 Compound 2 was prepared by in-situ doping Tb3+ and Eu3+ into previously reported compound CPM-5. The guestaccessible volume of compound 2 is 47.9% of the unit cell with the pore size in the range of 1.68-4.00 Å as reported.15 By changing the content ratio of Tb and Eu in the raw reactants, series of compound 1 and 2 with different content ratio of Tb:Eu have been obtained. The content of Tb:Eu is characterized by EDS and ICP measurements
Fig 3. (a) Solid-state PL spectra of 2 with different content ratio of Tb:Eu. (b) Photograph of 2 with different content ratio of Tb:Eu under UV lamp. (c) Plot of the raw content ratio versus real content ratio of Tb: Eu in 2. (d) Plot of the characteristic peak intensity ratio of Tb:Eu versus the raw content ratio in 2.
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intensity ratio of the characteristic peaks versus raw content ratio is also linear correlation (Fig. 3d). These results show that there is no obvious energy transfer between Tb and Eu in 2. In addition, for the compound 1 and 2 with the same ratio of Tb:Eu (9:1), the quantum yield of 1 (25%) is quite higher than that of 2 (1.67%), which also indicates the existence of effective energy transfer in 1. The excited triplet state of ligand is key consideration while determining its role in the energy transfer between lanthanide ions. The excited state of Tb3+ and Eu3+ is about 20400 cm-1(5D4) and 17250 cm-1 (5D0), respectively.5, 16 Big energy gap between the excited state of Tb and Eu prevents effective energy transfer between them. The energy level of triplet state of BTC (23200 cm-1) is higher than the excited state of Tb and Eu,16 therefor, BTC ligand can not serve as energy transporter to facilitate energy transfer between Tb and Eu, as observed in compound 1. While the triplet state of TATB (calculated 19500 cm-1, Fig S6) locates in the middle of excited state of Tb and Eu, there occurs energy transfer between them, and finally influences the luminescent intensity ratio of Tb:Eu, as observed in compound 2. The influence of guest solvent molecules on the luminescent properties of 1 and 2 was also studied. To eliminate interference of guest molecules originally embedded in the pore during synthesis, the raw crystalline compounds were grounded, and then evacuated under vacuum at 120oC for 12h, following by dispersing ground vacuumed sample in solvents with different polarity. For compound 1, emission intensity ratio of characteristic peak of Tb:Eu decreased with solvent polarity increasing (Fig 4a, Fig S7). However, for compound 2, intensity ratio almost remains unchanged (Fig 4b). The histogram of characteristic peak intensity ratio of Tb:Eu clearly
shows that emission property of 1 is sensitive to solvent polarity, View Article Online 10.1039/C5DT04844C while emission property of 2 has no responseDOI: to solvent polarity (Fig 4c). So, it is clear that polar guest solvent molecules have influence on luminescent properties of 1 in which there exists ligand-assisted energy transfer between Tb and Eu, and have no effect on luminescent properties of 2 in which no ligand-assisted energy transfer process gets involved. In summary, we purposely synthesized two Tb:Eu-codoped indium-based MOF sand investigated the effects of ligand and guest solvent molecules on energy transfer process between Tb and Eu. Different from BTC ligand, the bridging ligand TATB with the triplet state locating between the excited states of Tb and Eu shows effective facilitation in energy transfer between them. In this case, polar guest solvent molecules also influence the energy transfer efficiency. Therefore, while designing Ln-MOFs materials as sensors with dual emitting in liquid sensing system, ligands with the triplet state of being much higher than the excited state of lanthanide ions could be preferably selected because it can effectively prevent ligand-assisted energy transfer process, weaken the interference of guest solvent molecules inevitably entrapped in MOF cavity on energy transfer of lanthanide ions, and promote the contribution of target analyte in controlling energy transfer efficiency between lanthanide ions. This work was supported by National Natural Science Foundation of China (No.21271135), a start-up fund (Q410900712) from Soochow University, the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), and Young Thousand Talented Program.
Notes and references
Fig 4. (a) PL spectra of 1 with different guest solvent molecules trapped in the cavities. (b) PL spectra of 2 with different guest solvent molecules trapped in the cavites. (c) The histogram of the characteristic peak intensity ratio in 1 and 2. (CHY = cyclohexane; DEE = diethyl ether; THF = tetrahydrofuran; DMF = N,NDimethylformamide.)
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DOI: 10.1039/C5DT04844C
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Dalton Transactions Accepted Manuscript
Ligand with its triplet state locating between the excited state of the Tb and Eu ionsshows effective facilitation in the energy transfer process between them, and the energy transfer process in this case is also influenced by polarguest solvent molecules.
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