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Synthesis and characterization of Tb3+ and Eu3+ metal-organic frameworks with TFBDC2− linkers
Optical Materials xxx (2017) 1e7
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Synthesis and characterization of Tb3þ and Eu3þ metal-organic frameworks with TFBDC2 linkers nas Skaudzius a, Aivaras Kareiva a Andrius Laurikenas a, *, Arturas Katelnikovas b, Ramu a b
Department of Inorganic Chemistry, Vilnius University, Naugarduko 24, LT-03225, Vilnius, Lithuania Department of Analytical and Environmental Chemistry, Vilnius University, Naugarduko 24, LT-03225, Vilnius, Lithuania
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 January 2017 Received in revised form 20 March 2017 Accepted 21 May 2017 Available online xxx
In this study, terbium Tb3þ and europium Eu3þ metal-organic frameworks (MOFs) based on 2,3,5,6tetrafluoro-1,4-benzenedicarboxylic acid (TFBDC) were synthesized by precipitation and diffusioncontrolled precipitation methods. The powders insoluble in aqueous media and polar solvents were obtained. The phase and chemical composition, microstructure and properties of Tb3þ and Eu3þ MOFs were evaluated and discussed. The synthesized terbium and europium metal-organic frameworks were characterized by thermal (TG/DSC) analysis, X-ray diffraction (XRD) analysis, infrared (FTIR) spectroscopy, scanning electron microscopy (SEM) and fluorescence spectroscopy (FLS). © 2017 Elsevier B.V. All rights reserved.
Keywords: Rare earth metals Terbium Europium Metal-organic framework 2,3,5,6-Tetrafluoro-1,4-benzenedicarboxylic acid
1. Introduction Lanthanide coordination chemistry is characterized by high coordination numbers and adjustable coordination geometries and structures, which are dependent on a variety of factors, such as ionic radius, reaction temperature, atmosphere, coordinating solvents and the nature of counter anions. Metal-organic frameworks (MOFs) present opportunity for tuning the luminescence behaviour because of the possibility to trap in the network pores molecules which can influence the lanthanide emission. Lanthanide metalorganic frameworks containing organic fluorinated ligands possessing luminescence are considered as promising substances for photochemical light emitting diodes (pcLEDs) and light conversion molecular devices (LCMD) [1e5]. The compounds with anions of aliphatic perfluorocarboxylic acids form one of the well-studied groups of luminescent lanthanide complexes with O-containing ligands [3e5]. According to the literature, the introduction of F atoms into the composition of an organic ligand enhances the luminescence intensity [5e10]. The authors [5e8] discussed that the photoluminescence intensity of the partially desolvated polymer is higher than that of the
* Corresponding author. E-mail address: [email protected] (A. Laurikenas).
desolvated Eu(III) polymer containing the coordinated ions of tetrafluoroterephthalic acid. According to the X-ray diffraction (XRD) analysis data, the structures of these compounds are coordination 2D polymers [8e10]. The first step in this study was to perform the effective synthesis of the lanthanide compounds with 2,3,5,6tetrafluoro-1,4-benzenedicarboxylic acid (TFBDC). Next, the aim of this study was the development of suitable and simple synthesis route for the new coordination compounds based on TFBDC2 ligands, especially the compounds containing the photoluminescence sensitizer 1.10-phenanthroline (Phen). Thus, the main purpose of the present work was the synthesis and characterization of new Tb(III) and Eu(III) coordination compounds based on TFBDC2 ligands and containing the PL sensitizer 1.10phenanthroline.
2. Materials and methods Lanthanide salts, 2,3,5,6-tetrafluoro-1,4-benzenedicarboxylic acid (TFBDC), triethylamine, N,N-dimethylformamide and methanol were purchased from Aldrich and used directly without further purification. Compounds Eu2(H2O)4(TFBDC)3$DMF (1a), Tb2(H2O)4(TFBDC)3$DMF (2a), Eu2(Phen)2(TFBDC)3$2H2O (1b), Tb2(Phen)2(TFBDC)3$DMF (2b) were synthesized by precipitation, compounds Eu(TFBDC)(NO3)(DMF)2$DMF (1c) and
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Please cite this article in press as: A. Laurikenas, et al., Synthesis and characterization of Tb3þ and Eu3þ metal-organic frameworks with TFBDC2 linkers, Optical Materials (2017), http://dx.doi.org/10.1016/j.optmat.2017.05.037
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Tb(TFBDC)(NO3)(DMF)2$DMF (2c) by diffusion-controlled precipitation methods. 2.1. Eu2(H2O)4(TFBDC)3·DMF (1a) Concentrated NH4OH (5 mL) was added to the solution of Eu(NO3)3$6H2O (0.041 g, 0.012 mmol) in water (30 mL) with continuous stirring, After 24 h, the formed Eu(OH)3 precipitates were filtered on a glass filter and washed with hot and then cold distilled water to the neutral pH of the filtrate. TFBDC (0.044 g, 0.018 mmol) was added to the wet precipitate of Eu(OH)3. The mixture was placed into the vial and stirred until a solution was formed. Next, a N,N-dimethylformide/methanol (3:1v/v) mixture (20 mL) was added to the above solution and stirred for 2 h. Formed white precipitates were filtered on a dense paper filter, washed with absolute ethanol three times and then with hexane and dried in air. 2.2. Tb2(H2O)4(TFBDC)3·DMF (2a) 1a procedure using Tb(NO3)3$6H2O (0.054 g, 0.012 mmol) and TFBDC (0.044 g, 0.0186 mmol) yielded a white precipitate of 2a. 2.3. Eu2(Phen)2(TFBDC)3·2H2O (1b) A solution of 1.10-phenanthroline hydrate (0.018 g, 0.1 mmol) in ethanol (10 mL) was added with stirring to previously obtained Eu2(H2O)4(TFBDC)3$DMF solution, which resulted in the formation of a white precipitate. After 24 h, the precipitate was filtered on a dense paper filler, washed with absolute ethanol and then with hexane and dried in air.
2.5. Eu(TFBDC)(NO3)(DMF)2·DMF (1c) Eu(NO3)3$6H2O (0.675 g, 2 mmol) and TFBDC (0.2643 g, 1.1 mmol) were dissolved in 10 mL of a mixture of methanol and N,N-dimethylformamide (3:1 v/v). The solution was poured into 10 ml chromatographic vial, closed with a perforated cap and placed in a desiccator. In addition, a beaker with 10 mL of triethylamine dissolved in 10 mL of the same solvent mixture was also placed in the desiccator. After 45 days on vial walls the formed crystals were filtered, washed with methanol and hexane, after that dried in air. 2.6. Tb(TFBDC)(NO3)(DMF)2·DMF (2c) 1c procedure using 0.906 g (2 mmol) Tb(NO3)3$6H2O and 0.2643 g (1.1 mmol) TFBDC yielded a white precipitate of 2c. FTIR analysis of compounds was conducted using Bruker Alpha FTIR spectrometer with Platinum ATR single reflection diamond module. Thermal analyses were conducted from room temperature to 600 C under N2 atmosphere using Perkin Elmer STA6000 TGA/ DSC thermal analyser and Pyris software. The heating rate was 1 C/ min. XRD data were collected at room temperature on a Rigaku Miniflex II system with a graphite monochromator, using Cu Ka1 radiation (speed 4 /min). Excitation and emission measurements were acquired using Edinburgh Instruments FLS980 fluorescence spectrometer. Scanning electron microscope (SEM) Hitachi TM3000 was used to study the main morphological features of obtained crystals. 3. Results and discussion 3.1. Infrared (FTIR) spectroscopy
2.4. Tb2(Phen)2(TFBDC)3·DMF (2b) 1b procedure using 1.10-phenanthroline hydrate (0.018 g, 0.1 mmol) yielded white precipitate of 2b.
The FTIR spectra of Eu3þ (1a, 1b, 1c) metal-organic frameworks are shown in Fig. 1. Characterization of Eu3þ containing metal organic frameworks revealed that the FTIR spectra of 1a and 1b do
Fig. 1. FTIR spectra of TFBDC and synthesized Eu3þ (1a, 1b, 1c) metal-organic frameworks [2,5].
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The strong absorption of X-rays by the lanthanide containing MOFs was also observed. Therefore, the identification of reflections in the XRD patterns was problematic. Again, XRD analysis of europium compounds revealed similar results. 3.4. Fluorescence spectroscopy (FLS)
Fig. 2. The schematic view of TFBDC.
not contain absorption band attributable to the carbon-oxygen v(C¼O) stretching at 1692 cm1, which is visible in TFBDC sample. The absorption bands observed in the FTIR spectra of 1a, 1b and 1c at 1575 and ~1511 cm1 correspond to the ring stretching in aromatic ring of TFBDC, and the bands visible at ~1311 and 1317 cm1 are due to the presence of the CeF functional groups in TFBDC (Fig. 2). Also C-H sp3 bend of N,N-dimethylformamide is visible at 1386 cm1. The sharp absorption bands at 840 and 843 cm1 correspond to the oop bending vibrations of psubstituted arene. FTIR assignments of synthesized Eu3þ MOFs are shown in Table 1. FTIR analysis of synthesized terbium compounds revealed very similar results. 3.2. Thermal analysis (DTG/TG/DSC) As shown in Fig. 3, synthesized Tb3þ metal-organic frameworks 2a and 2c have similar thermal stability and are stable up to 170 C [11]. In 2a case first weight loss from 20 to 220 C corresponds to the loss of 4 H2O and 1 DMF molecules. In the temperature range of 275e400 C and 415e600 C MOFs decompose possibly to the products of Ln3þF3 and Ln3þOF [11,12]. In 2b case, similar decomposition processes occur, also two molecules of 1.10phenanthroline starts to decompose at 320 C. In 2c case, weight loss at 375 C reveals the decomposition of nitrate anion to NO2. Terbium oxynitrate TbONO3 forms at this stage [13]. Comparing obtained TG and DSC data of synthesized europium metal-organic frameworks it was observed that the results of thermal analysis of 1a, 1b and 1c are similar to the discussed above for the synthesized terbium metal-organic frameworks. 3.3. X-ray diffraction (XRD) analysis The XRD patterns of Tb3þ (2a, 2b, 2c) metal-organic frameworks are presented in Fig. 4. A high background observed in the XRD patterns of Tb3þ (2a, 2b, 2c) compounds indicates the formation of amorphous phases [14].
The excitation spectra of Eu3þ MOFs (1a, 1b, 1c) presented in Fig. 5 consist of a broad band in the range of 300e350 nm and several sets of lines in the range of 350e600 nm. The broad excitation band can be attributed to the charge transfer transition from TFBDC2 ligands to Eu3þ ions. The excitation spectra of Tb3þ MOFs (2a, 2b, 2c) depicted in Fig. 5 consist of a broad band in the range of 300e362 nm and also several sets of lines in the range of 362e490 nm. The broad excitation band can be attributed to the charge transfer transition from TFBDC2 ligands to Tb3þ ions. The emission lines of synthesized Eu3þ metal-organic frameworks (lex 300 nm) are assigned to 5D0/7F0, 5D0/7F1, 5D0/7F2, 5 D0/7F3 and 5D0/7F4 transitions for those peaks located at 579, 593, 613, 618 and 699 nm, respectively (Fig. 6). It can be observed that the emission spectrum of (1b) is dominating by a very intense 5D0/7F2 transition at 613 nm. It is known that the 5D0/7F2 transition is an electric dipole transition and is very sensitive to the local symmetry of europium ions. For Tb3þ metal-organic frameworks (lex 300 nm), the emission lines shown in Fig. 6 B are assigned to 5D4/7F6, 5D4/7F5, 5D4/7F4, 5D4/7F3, 5 D4/7F2, 5D4/7F0 transitions for those peaks located at 488, 542, 582, 620,646 and 679 nm, respectively. Very intense 5D4/7F5 transition at 542 nm is observed for the emission spectrum of (2a). In the case of Tb3þ metal-organic frameworks, the lower emission intensity (2 times) is observed for the 2b compound, which could be explained by the presence of the coordinated water molecules. Less intensive emission bands for the 1c might be resulted from coordinated DMF molecules, which exhibit fluorescence quenching properties [3e5]. The presence of coordinated Phen molecules in 1b resulted in a significant (by ~1.8 times) increase in the intensity of the bands in the emission spectrum of 1b compared to the spectrum of 1a. This is due to the sensitizing ability of Phen molecules [8,10]. It should be mentioned that the position of bands and intensities of emission spectra of 2a and 2c compounds are very similar. 3.5. Scanning electron microscopy (SEM) SEM analysis of Ln2(H2O)4(TFBDC)3$DMF (Tb3þ, Eu3þ) (Fig. 7) revealed that compounds exhibit nonhomogenous surface with an average crystallite size of 1.5 mm for Eu3þ and 2.7 mm for Tb3þ. Besides, the Ln2(Phen)2(TFBDC)3$2H2O exhibits better crystallinity with formation of sharp needle-like particles for the Eu3þ and rectangular plate-like agglomerates for the Tb3þ. The average crystal size of 19 mm for Eu3þ and 16 mm for Tb3þ were determined. The SEM analysis of (TFBDC)(NO3)(DMF)2$DMF revealed the formation of neat rectangular plate-like crystals of similar size (9.5 mm for Eu3þ and 10.6 mm for Tb3þ). Such microstructure and porosity of amorphous metal-organic frameworks are presumably suitable for
Table 1 FTIR assignments of synthesized Eu3þ MOFs [2,5]. Band wavelength, cm1 Assignment
840,843
1311
oop bending of p-substituted CeF antisymm. arene stretch.
1510
1575
1386
1665
1692
ring stretching in aromatic ring
ring stretching in aromatic ring
C-H sp3 bend in DMF molecule
C¼O stretch
C¼O stretch
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Fig. 3. TG, DTG and DSC curves of synthesized Tb3þ metal-organic frameworks.
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Fig. 4. XRD patterns of TFBDC and synthesized Tb3þ (2a, 2b, 2c) metal-organic frameworks.
Fig. 5. Excitation spectra of Eu3þ (1a, 1b, 1c) (A) and Tb3þ (2a, 2b, 2c) (B) metal-organic frameworks.
the drug delivery [15,16]. 4. Conclusions In this study, the lanthanide elements (Ln: Tb3þ, Eu3þ) and 2,3,5,6-tetrafluoro-1,4-benzenedicarboxylic acid (TFBDC) based metal-organic frameworks (MOFs) Ln2(H2O)4(TFBDC)3$DMF, Ln2(Phen)2(TFBDC)3$2H2O and Ln(TFBDC)(NO3)(DMF)2$DMF were synthesized by precipitation and diffusion-controlled precipitation methods. XRD analysis data revealed the formation of amorphous MOFs. It was demonstrated, that thermal decomposition of lanthanide element MOFs occurred in the temperature ranges of 275e400 C and 415e600 C to the products of Ln3þF3 and Ln3þOF.
The most intensive emission was observed for Eu2(Phen)2(TFBDC)3$2H2O at 613 nm and for Tb2(H2O)4(TFBDC)3$DMF at 542 nm (lex 300 nm). The SEM micrographs revealed the slightly different surface morphology of the synthesized MOFs. The surface of Ln2(H2O)4(TFBDC)3$DMF was nonhomogenous with an average crystallite size of ~1.5 mm for Eu3þ and ~2.7 mm for Tb3þ. The Ln2(Phen)2(TFBDC)3$2H2O exhibited better crystallinity with formation of sharp needle-like particles for the Eu3þ (~19 mm) and rectangular plate-like agglomerates for the Tb3þ (~16 mm). The Ln(TFBDC)(NO3)(DMF)2$DMF were composed of a neat rectangular plate-like crystals with similar size (~9.5e10.6 mm).
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Fig. 6. Emission spectra of synthesized Eu3þ (1a, 1b, 1c) (A) and Tb3þ (2a, 2b, 2c) (B) metal-organic frameworks.
Fig. 7. SEM images of synthesized Eu3þ (1a, 1b, 1c) and Tb3þ (2a, 2b, 2c) compounds.
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Acknowledgements This research was funded by a grant (No. S-LZ-17-6) from the Research Council of Lithuania. References [1] Huijie Zhang, Ruiqing Fan, Wei Chen, Xubin Zheng, Kai Li, Ping Wang, Yulin Yang, Two new dysprosiumeorganic frameworks containing rigid dicarboxylate ligands: synthesis and effect of solvents on the luminescent properties, J. Luminescence 143 (2013) 611e618, http://dx.doi.org/10.1016/ j.jlumin.2013.06.001. [2] Weiqiang Fan, Jing Feng, Shuyan Song, Yongqian Lei, Song Dang, Hongjie Zhang, Synthesis and luminescent properties of organiceinorganic hybrid macroporous materials doped with lanthanide (Eu/Tb) complexes, Opt. Mater. 33 (3) (2011) 582e585, http://dx.doi.org/10.1016/ j.optmat.2010.09.034. [3] G.F. de S a, O.L. Malta, C. de Mello Doneg a, A.M. Simas, R.L. Longo, P.A. SantaCruz, E.F. da Silva, Spectroscopic properties and design of highly luminescent lanthanide coordination complexes, Coord. Chem. Rev. 196 (1) (2000) 165e195, http://dx.doi.org/10.1016/s0010-8545(99)00054-5. [4] M.A. Katkova, Alexey G. Vitukhnovsky, Mikhail N. Bochkarev, Coordination compounds of rare-earth metals with organic ligands for electroluminescent diodes, Russ. Chem. Rev. 74 (12) (2005) 1089e1109, http://dx.doi.org/ 10.1070/rc2005v074n12abeh002481. ska, Anja-Verena Mudring, [5] Christiane Seidel, Chantal Lorbeer, Joanna Cybin Uwe Ruschewitz, Lanthanide coordination polymers with tetrafluoroterephthalate as a bridging ligand: thermal and optical properties, Inorg. Chem. 51 (8) (2012) 4679e4688, http://dx.doi.org/10.1021/ic202655d. [6] J.C.G. Bünzli, A.S. Chauvin, H.K. Kim, E. Deiters, S.V. Eliseeva, Lanthanide luminescence efficiency in eight- and nine-coordinate complexes:Role of the radiative lifetime, Coord. Chem. Rev. 254 (2010) 2623e2633, http:// dx.doi.org/10.1016/j.ccr.2010.04.002. [7] T. Wei Duan, B. Yan, Lanthanide ions (Eu3þ, Tb3þ, Sm3þ, Dy3þ) activated ZnO embedded zinc 2,5-pyridinedicarboxylic metaleorganic frameworks for luminescence application, J. Mater. Chem. C 3 (2015) 2823e2830, http://
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Please cite this article in press as: A. Laurikenas, et al., Synthesis and characterization of Tb3þ and Eu3þ metal-organic frameworks with TFBDC2 linkers, Optical Materials (2017), http://dx.doi.org/10.1016/j.optmat.2017.05.037
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Synthesis and characterization of Tb3þ and Eu3þ metal-organic frameworks with TFBDC2 linkers nas Skaudzius a, Aivaras Kareiva a Andrius Laurikenas a, *, Arturas Katelnikovas b, Ramu a b
Department of Inorganic Chemistry, Vilnius University, Naugarduko 24, LT-03225, Vilnius, Lithuania Department of Analytical and Environmental Chemistry, Vilnius University, Naugarduko 24, LT-03225, Vilnius, Lithuania
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 January 2017 Received in revised form 20 March 2017 Accepted 21 May 2017 Available online xxx
In this study, terbium Tb3þ and europium Eu3þ metal-organic frameworks (MOFs) based on 2,3,5,6tetrafluoro-1,4-benzenedicarboxylic acid (TFBDC) were synthesized by precipitation and diffusioncontrolled precipitation methods. The powders insoluble in aqueous media and polar solvents were obtained. The phase and chemical composition, microstructure and properties of Tb3þ and Eu3þ MOFs were evaluated and discussed. The synthesized terbium and europium metal-organic frameworks were characterized by thermal (TG/DSC) analysis, X-ray diffraction (XRD) analysis, infrared (FTIR) spectroscopy, scanning electron microscopy (SEM) and fluorescence spectroscopy (FLS). © 2017 Elsevier B.V. All rights reserved.
Keywords: Rare earth metals Terbium Europium Metal-organic framework 2,3,5,6-Tetrafluoro-1,4-benzenedicarboxylic acid
1. Introduction Lanthanide coordination chemistry is characterized by high coordination numbers and adjustable coordination geometries and structures, which are dependent on a variety of factors, such as ionic radius, reaction temperature, atmosphere, coordinating solvents and the nature of counter anions. Metal-organic frameworks (MOFs) present opportunity for tuning the luminescence behaviour because of the possibility to trap in the network pores molecules which can influence the lanthanide emission. Lanthanide metalorganic frameworks containing organic fluorinated ligands possessing luminescence are considered as promising substances for photochemical light emitting diodes (pcLEDs) and light conversion molecular devices (LCMD) [1e5]. The compounds with anions of aliphatic perfluorocarboxylic acids form one of the well-studied groups of luminescent lanthanide complexes with O-containing ligands [3e5]. According to the literature, the introduction of F atoms into the composition of an organic ligand enhances the luminescence intensity [5e10]. The authors [5e8] discussed that the photoluminescence intensity of the partially desolvated polymer is higher than that of the
* Corresponding author. E-mail address: [email protected] (A. Laurikenas).
desolvated Eu(III) polymer containing the coordinated ions of tetrafluoroterephthalic acid. According to the X-ray diffraction (XRD) analysis data, the structures of these compounds are coordination 2D polymers [8e10]. The first step in this study was to perform the effective synthesis of the lanthanide compounds with 2,3,5,6tetrafluoro-1,4-benzenedicarboxylic acid (TFBDC). Next, the aim of this study was the development of suitable and simple synthesis route for the new coordination compounds based on TFBDC2 ligands, especially the compounds containing the photoluminescence sensitizer 1.10-phenanthroline (Phen). Thus, the main purpose of the present work was the synthesis and characterization of new Tb(III) and Eu(III) coordination compounds based on TFBDC2 ligands and containing the PL sensitizer 1.10phenanthroline.
2. Materials and methods Lanthanide salts, 2,3,5,6-tetrafluoro-1,4-benzenedicarboxylic acid (TFBDC), triethylamine, N,N-dimethylformamide and methanol were purchased from Aldrich and used directly without further purification. Compounds Eu2(H2O)4(TFBDC)3$DMF (1a), Tb2(H2O)4(TFBDC)3$DMF (2a), Eu2(Phen)2(TFBDC)3$2H2O (1b), Tb2(Phen)2(TFBDC)3$DMF (2b) were synthesized by precipitation, compounds Eu(TFBDC)(NO3)(DMF)2$DMF (1c) and
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Please cite this article in press as: A. Laurikenas, et al., Synthesis and characterization of Tb3þ and Eu3þ metal-organic frameworks with TFBDC2 linkers, Optical Materials (2017), http://dx.doi.org/10.1016/j.optmat.2017.05.037
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Tb(TFBDC)(NO3)(DMF)2$DMF (2c) by diffusion-controlled precipitation methods. 2.1. Eu2(H2O)4(TFBDC)3·DMF (1a) Concentrated NH4OH (5 mL) was added to the solution of Eu(NO3)3$6H2O (0.041 g, 0.012 mmol) in water (30 mL) with continuous stirring, After 24 h, the formed Eu(OH)3 precipitates were filtered on a glass filter and washed with hot and then cold distilled water to the neutral pH of the filtrate. TFBDC (0.044 g, 0.018 mmol) was added to the wet precipitate of Eu(OH)3. The mixture was placed into the vial and stirred until a solution was formed. Next, a N,N-dimethylformide/methanol (3:1v/v) mixture (20 mL) was added to the above solution and stirred for 2 h. Formed white precipitates were filtered on a dense paper filter, washed with absolute ethanol three times and then with hexane and dried in air. 2.2. Tb2(H2O)4(TFBDC)3·DMF (2a) 1a procedure using Tb(NO3)3$6H2O (0.054 g, 0.012 mmol) and TFBDC (0.044 g, 0.0186 mmol) yielded a white precipitate of 2a. 2.3. Eu2(Phen)2(TFBDC)3·2H2O (1b) A solution of 1.10-phenanthroline hydrate (0.018 g, 0.1 mmol) in ethanol (10 mL) was added with stirring to previously obtained Eu2(H2O)4(TFBDC)3$DMF solution, which resulted in the formation of a white precipitate. After 24 h, the precipitate was filtered on a dense paper filler, washed with absolute ethanol and then with hexane and dried in air.
2.5. Eu(TFBDC)(NO3)(DMF)2·DMF (1c) Eu(NO3)3$6H2O (0.675 g, 2 mmol) and TFBDC (0.2643 g, 1.1 mmol) were dissolved in 10 mL of a mixture of methanol and N,N-dimethylformamide (3:1 v/v). The solution was poured into 10 ml chromatographic vial, closed with a perforated cap and placed in a desiccator. In addition, a beaker with 10 mL of triethylamine dissolved in 10 mL of the same solvent mixture was also placed in the desiccator. After 45 days on vial walls the formed crystals were filtered, washed with methanol and hexane, after that dried in air. 2.6. Tb(TFBDC)(NO3)(DMF)2·DMF (2c) 1c procedure using 0.906 g (2 mmol) Tb(NO3)3$6H2O and 0.2643 g (1.1 mmol) TFBDC yielded a white precipitate of 2c. FTIR analysis of compounds was conducted using Bruker Alpha FTIR spectrometer with Platinum ATR single reflection diamond module. Thermal analyses were conducted from room temperature to 600 C under N2 atmosphere using Perkin Elmer STA6000 TGA/ DSC thermal analyser and Pyris software. The heating rate was 1 C/ min. XRD data were collected at room temperature on a Rigaku Miniflex II system with a graphite monochromator, using Cu Ka1 radiation (speed 4 /min). Excitation and emission measurements were acquired using Edinburgh Instruments FLS980 fluorescence spectrometer. Scanning electron microscope (SEM) Hitachi TM3000 was used to study the main morphological features of obtained crystals. 3. Results and discussion 3.1. Infrared (FTIR) spectroscopy
2.4. Tb2(Phen)2(TFBDC)3·DMF (2b) 1b procedure using 1.10-phenanthroline hydrate (0.018 g, 0.1 mmol) yielded white precipitate of 2b.
The FTIR spectra of Eu3þ (1a, 1b, 1c) metal-organic frameworks are shown in Fig. 1. Characterization of Eu3þ containing metal organic frameworks revealed that the FTIR spectra of 1a and 1b do
Fig. 1. FTIR spectra of TFBDC and synthesized Eu3þ (1a, 1b, 1c) metal-organic frameworks [2,5].
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The strong absorption of X-rays by the lanthanide containing MOFs was also observed. Therefore, the identification of reflections in the XRD patterns was problematic. Again, XRD analysis of europium compounds revealed similar results. 3.4. Fluorescence spectroscopy (FLS)
Fig. 2. The schematic view of TFBDC.
not contain absorption band attributable to the carbon-oxygen v(C¼O) stretching at 1692 cm1, which is visible in TFBDC sample. The absorption bands observed in the FTIR spectra of 1a, 1b and 1c at 1575 and ~1511 cm1 correspond to the ring stretching in aromatic ring of TFBDC, and the bands visible at ~1311 and 1317 cm1 are due to the presence of the CeF functional groups in TFBDC (Fig. 2). Also C-H sp3 bend of N,N-dimethylformamide is visible at 1386 cm1. The sharp absorption bands at 840 and 843 cm1 correspond to the oop bending vibrations of psubstituted arene. FTIR assignments of synthesized Eu3þ MOFs are shown in Table 1. FTIR analysis of synthesized terbium compounds revealed very similar results. 3.2. Thermal analysis (DTG/TG/DSC) As shown in Fig. 3, synthesized Tb3þ metal-organic frameworks 2a and 2c have similar thermal stability and are stable up to 170 C [11]. In 2a case first weight loss from 20 to 220 C corresponds to the loss of 4 H2O and 1 DMF molecules. In the temperature range of 275e400 C and 415e600 C MOFs decompose possibly to the products of Ln3þF3 and Ln3þOF [11,12]. In 2b case, similar decomposition processes occur, also two molecules of 1.10phenanthroline starts to decompose at 320 C. In 2c case, weight loss at 375 C reveals the decomposition of nitrate anion to NO2. Terbium oxynitrate TbONO3 forms at this stage [13]. Comparing obtained TG and DSC data of synthesized europium metal-organic frameworks it was observed that the results of thermal analysis of 1a, 1b and 1c are similar to the discussed above for the synthesized terbium metal-organic frameworks. 3.3. X-ray diffraction (XRD) analysis The XRD patterns of Tb3þ (2a, 2b, 2c) metal-organic frameworks are presented in Fig. 4. A high background observed in the XRD patterns of Tb3þ (2a, 2b, 2c) compounds indicates the formation of amorphous phases [14].
The excitation spectra of Eu3þ MOFs (1a, 1b, 1c) presented in Fig. 5 consist of a broad band in the range of 300e350 nm and several sets of lines in the range of 350e600 nm. The broad excitation band can be attributed to the charge transfer transition from TFBDC2 ligands to Eu3þ ions. The excitation spectra of Tb3þ MOFs (2a, 2b, 2c) depicted in Fig. 5 consist of a broad band in the range of 300e362 nm and also several sets of lines in the range of 362e490 nm. The broad excitation band can be attributed to the charge transfer transition from TFBDC2 ligands to Tb3þ ions. The emission lines of synthesized Eu3þ metal-organic frameworks (lex 300 nm) are assigned to 5D0/7F0, 5D0/7F1, 5D0/7F2, 5 D0/7F3 and 5D0/7F4 transitions for those peaks located at 579, 593, 613, 618 and 699 nm, respectively (Fig. 6). It can be observed that the emission spectrum of (1b) is dominating by a very intense 5D0/7F2 transition at 613 nm. It is known that the 5D0/7F2 transition is an electric dipole transition and is very sensitive to the local symmetry of europium ions. For Tb3þ metal-organic frameworks (lex 300 nm), the emission lines shown in Fig. 6 B are assigned to 5D4/7F6, 5D4/7F5, 5D4/7F4, 5D4/7F3, 5 D4/7F2, 5D4/7F0 transitions for those peaks located at 488, 542, 582, 620,646 and 679 nm, respectively. Very intense 5D4/7F5 transition at 542 nm is observed for the emission spectrum of (2a). In the case of Tb3þ metal-organic frameworks, the lower emission intensity (2 times) is observed for the 2b compound, which could be explained by the presence of the coordinated water molecules. Less intensive emission bands for the 1c might be resulted from coordinated DMF molecules, which exhibit fluorescence quenching properties [3e5]. The presence of coordinated Phen molecules in 1b resulted in a significant (by ~1.8 times) increase in the intensity of the bands in the emission spectrum of 1b compared to the spectrum of 1a. This is due to the sensitizing ability of Phen molecules [8,10]. It should be mentioned that the position of bands and intensities of emission spectra of 2a and 2c compounds are very similar. 3.5. Scanning electron microscopy (SEM) SEM analysis of Ln2(H2O)4(TFBDC)3$DMF (Tb3þ, Eu3þ) (Fig. 7) revealed that compounds exhibit nonhomogenous surface with an average crystallite size of 1.5 mm for Eu3þ and 2.7 mm for Tb3þ. Besides, the Ln2(Phen)2(TFBDC)3$2H2O exhibits better crystallinity with formation of sharp needle-like particles for the Eu3þ and rectangular plate-like agglomerates for the Tb3þ. The average crystal size of 19 mm for Eu3þ and 16 mm for Tb3þ were determined. The SEM analysis of (TFBDC)(NO3)(DMF)2$DMF revealed the formation of neat rectangular plate-like crystals of similar size (9.5 mm for Eu3þ and 10.6 mm for Tb3þ). Such microstructure and porosity of amorphous metal-organic frameworks are presumably suitable for
Table 1 FTIR assignments of synthesized Eu3þ MOFs [2,5]. Band wavelength, cm1 Assignment
840,843
1311
oop bending of p-substituted CeF antisymm. arene stretch.
1510
1575
1386
1665
1692
ring stretching in aromatic ring
ring stretching in aromatic ring
C-H sp3 bend in DMF molecule
C¼O stretch
C¼O stretch
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Fig. 3. TG, DTG and DSC curves of synthesized Tb3þ metal-organic frameworks.
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Fig. 4. XRD patterns of TFBDC and synthesized Tb3þ (2a, 2b, 2c) metal-organic frameworks.
Fig. 5. Excitation spectra of Eu3þ (1a, 1b, 1c) (A) and Tb3þ (2a, 2b, 2c) (B) metal-organic frameworks.
the drug delivery [15,16]. 4. Conclusions In this study, the lanthanide elements (Ln: Tb3þ, Eu3þ) and 2,3,5,6-tetrafluoro-1,4-benzenedicarboxylic acid (TFBDC) based metal-organic frameworks (MOFs) Ln2(H2O)4(TFBDC)3$DMF, Ln2(Phen)2(TFBDC)3$2H2O and Ln(TFBDC)(NO3)(DMF)2$DMF were synthesized by precipitation and diffusion-controlled precipitation methods. XRD analysis data revealed the formation of amorphous MOFs. It was demonstrated, that thermal decomposition of lanthanide element MOFs occurred in the temperature ranges of 275e400 C and 415e600 C to the products of Ln3þF3 and Ln3þOF.
The most intensive emission was observed for Eu2(Phen)2(TFBDC)3$2H2O at 613 nm and for Tb2(H2O)4(TFBDC)3$DMF at 542 nm (lex 300 nm). The SEM micrographs revealed the slightly different surface morphology of the synthesized MOFs. The surface of Ln2(H2O)4(TFBDC)3$DMF was nonhomogenous with an average crystallite size of ~1.5 mm for Eu3þ and ~2.7 mm for Tb3þ. The Ln2(Phen)2(TFBDC)3$2H2O exhibited better crystallinity with formation of sharp needle-like particles for the Eu3þ (~19 mm) and rectangular plate-like agglomerates for the Tb3þ (~16 mm). The Ln(TFBDC)(NO3)(DMF)2$DMF were composed of a neat rectangular plate-like crystals with similar size (~9.5e10.6 mm).
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Fig. 6. Emission spectra of synthesized Eu3þ (1a, 1b, 1c) (A) and Tb3þ (2a, 2b, 2c) (B) metal-organic frameworks.
Fig. 7. SEM images of synthesized Eu3þ (1a, 1b, 1c) and Tb3þ (2a, 2b, 2c) compounds.
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