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2D lanthanide coordination polymers constructed from semirigid ligand 4-(pyridin-3-yloxy)-phthalic acid: Synthesis, structure and luminescence
Accepted Manuscript 2D Lanthanide Coordination polymers constructed from semirigid ligand 4(pyridin-3-yloxy)-phthalic acid: synthesis, structure and luminescence Juan Chai, Ping Zhang, Xiangxiang Shi, Jianing Xu, Yong Fan, Li Wang PII: DOI: Reference:
S0277-5387(19)30054-3 https://doi.org/10.1016/j.poly.2019.01.039 POLY 13712
To appear in:
Polyhedron
Received Date: Revised Date: Accepted Date:
14 September 2018 21 December 2018 14 January 2019
Please cite this article as: J. Chai, P. Zhang, X. Shi, J. Xu, Y. Fan, L. Wang, 2D Lanthanide Coordination polymers constructed from semirigid ligand 4-(pyridin-3-yloxy)-phthalic acid: synthesis, structure and luminescence, Polyhedron (2019), doi: https://doi.org/10.1016/j.poly.2019.01.039
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2D Lanthanide Coordination polymers constructed from semirigid ligand 4-(pyridin-3-yloxy)-phthalic acid: synthesis, structure and luminescence Juan Chai, Ping Zhang, Xiangxiang Shi, Jianing Xu, Yong Fan*, Li Wang* College of Chemistry, Jilin University, Changchun, 130012, Jilin, P. R. China. E-mail: [email protected]; [email protected].
Abstract Three new compounds (Ln-CPs) [Ln(PPDA)(C2O4)0.5(H2O)2]n∙nH2O (Ln = Tb (1), Eu (2) and Gd (3)) were synthesized based on semirigid ligand 4-(pyridin-3-yloxy)-phthalic acid (H2PPDA) under solvothermal conditions. Ln-CPs 1-3 exhibit isomorphic 2D layered network with rhombic windows. Notably, oxalic acid in the resulting framework is formed by in situ reaction under solvothermal and acidic conditions. Solid state luminescent studies indicate that 1 and 2 show characteristic green and red emissions of the corresponding Ln 3+ ions, respectively, while 3 exhibits blue emission arising from PPDA 2- ligand. Then, by adjusting the co-doping ratio of different Ln 3+ ions into the well-defined host framework, a series of codoped Ln-CPs Eu3+ -doped 1 and Eu3+, Tb3+ -doped 3 are synthesized, showing tunable luminescence emissions including white-light emitting. Keywords: Lanthanide coordination polymer; Solvothermal; Luminescence; Color-tunable; White-light emission.
1. Introduction 1
Recently, full-colour luminescent materials, in particular those with white-light emission, have received consideration attention due to their wide applications in color-tunable phosphors, full-color displays, optical memory devices, low-cost back-lighting, and backlights [1-3]. It is well known that one common approach to realize multi-color and white-light emissions is to incorporate and control the intensity of three fundamental red, green and blue (RGB) light emitting components in the bulk materials [4-5]. Coordination polymers (CPs) built from lanthanide ions and organic ligands offer a unique platform for the development of luminescence devices and optical displays due to their high photoluminescence efficiency and narrow band emission ability [6-7]. Considering the unity of the Ln-CPs in the structure, the emission color output can be controlled precisely by co-doping the different Ln3+ ion into the same framework. As well known, the multi-color and white-light luminescence can be achieved by co-doping Eu3+, Tb3+ ions and a certain blue light emission source (usually organic ligands) as RGB light emitting components into the frameworks of isostructural Ln-CPs [8]. The luminescence of these doped Ln-CPs can be effectively tuned by varying the stoichiometric ratio of Ln3+ ions and the excitation wavelengths [9]. For example, Zhang et al. synthesized a series of codoped Ln-CPs EuxTb1-xL by changing the ratio of Eu3+ and Tb3+, which exhibited the full-color of the luminescence and white light emission [10]. Du et al. synthesized a highly luminescent chameleon by varying the stoichiometric ratio of Eu3+, Tb3+ and Gd3+ ions in a Ln-CP [11]. Although luminescent Ln-CPs have been designed for white-light emission materials, bioimaging and molecular sensing materials, excitation-modulated luminescence materials based on Ln-CPs are still in their infancy owing to the difficulties associated with incorporating different luminescent components into a well-defined framework [12-13]. 2
In order to gain high luminescence, besides the metal ions, an accurate choice of the ligand is crucial for the success of the rational design of Ln-CPs. Ligands used in the construction of Ln-CPs should be able to efficiently transfer the energy absorbed by chromophores to the lanthanide ions (antenna effect) [14-15]. In addition, the ligand should comprise multiple binding sites to saturate the Ln3+ coordination sites. Among various organic ligands, polycarboxylate ligands, as good candidates for the construction of multifunctional CPs, have aroused a good deal of interest owing to their various coordination modes and strong coordination ability. Semirigid ligand 4-(pyridin-3-yloxy)-phthalic acid (H2PPDA) as a multi-carboxylate ligand can exhibit a great variety of coordination modes [16]. Recent reports comfirmed that semirigid 4-(pyridin-3-yloxy)-phthalic acid (H2PPDA) as bridges to connect Ln metal ions into highly stable frameworks, in which H2PPDA was able to act as antenna chromophore to efficiently absorb and transfer energy to the lanthanide ions. For example, Hu et al. synthesized a serial of Ln-CPs by mixed ligands of H2PPDA and terephthalic acid (H2bdc). They exhibited 2D layered structures and served as excellent selective fluorescent sensing materials for nitrobenzene derivatives-based explosives, Fe3+ ion and Cr2O72− ion [17]. In this work, based on multi-carboxylate ligand H2PPDA, three new compounds [Ln(PPDA) (C2O4)0.5(H2O)2]n∙nH2O (Ln = Tb (1), Eu (2) and Gd (3)) was synthesized under solvothermal condition, featuring a 2D layered structure with rhombic window. It is noted that the oxalic acid anions in the framework are in situ generated by the ligand (H 2PPDA) under solvothermal and acidic conditions. 1 and 2 show characteristic green and red emissions of the corresponding Ln 3+ ions, respectively, 3 exhibits blue emission arising from the PPDA2- ligand. In terms of 3
1-3 showing characteristic green, red and blue emissions, respectively, we try to prepare two series of new doped Ln-CPs Eu3+ -doped 1 and Eu3+, Tb3+ -doped 3 with careful adjustment of the relative concentrations of the lanthanide ions and excitation wavelengths. As a result, Eu3+ -doped 1 have been prepared and the emission colors can be tuned from green to red by varying the molar ratios of Eu 3+ ion from 3% to 12% in 1, meanwhile, Eu3+, Tb3+ -doped 3, Eu0.03Tb0.10Gd0.87, emits white light with quantum yield of 38% upon excitation at 370 nm.
2. Experimental 2.1. Materials and Measurements Unless otherwise noted, all other chemicals used in this work were of reagent grade. They were commercially available and used as purchased without further purification. Powder X-ray diffraction (XRD) data were obtained using SHIMADZU XRD-6100 diffractometer with Cu Kα radiation (λ = 1.5418 Å) in the 2θ range of 4-40°, with the step size and the count time of 0.06° and 6 s, respectively. The elemental analyses (C, H and N) were performed on a Vario EL cube elemental analyzer. FT-IR spectra were recorded on a Nicolet Impact 410 spectrometer between 400 and 4000 cm -1 using the KBr pellet method. Thermogravimetric analyses (TGA) were carried out on a Perkin-Elmer TGA 7 thermogravimetric analyzer with a heating rate of 10 °C min-1 from room temperature to 800 °C under air atmosphere. Photoluminescence (PL) spectra, fluorescence lifetimes and quantum yield were measured with an Edinburgh Instruments FLS920 spectrophotometer. Elemental analyses for Tb, Gd, and Eu were obtained using a PLASMA-SPEC(I) ICP atomic emission spectrometer. 4
2.2 Synthesis of 1-3 A mixture of Ln(NO3)3∙6H2O (1 ml, 0.08 mmol∙ml-1) and H2PPDA (0.0260 g, 0.1 mmol) were dispersed in C 2H5OH (3 ml), HNO3 (0.3 ml, 6 mmol∙ml-1) and H2O (3 ml) in a 23 ml Teflon-lined autoclave and then heated under autogenous pressure at 120 °C for two days, then cooled to room temperature under ambient conditions. Yellow sheet crystals were recovered by filtration, washed with distilled water, and dried in air. Yield: 75% for 1, 70% for 2, 80% for 3 (based on Ln(NO3)3∙6H2O). Elemental analysis (%) for 1: Anal. Calc.: C, 30.29; H, 1.17; N, 2.72. Found: C, 30.94; H, 1.27; N, 2.85. For 2: Anal. Calc.: C, 30.65; H, 1.18. N, 2.75; Found: C, 30.40; H, 1.95; N, 2.59. For 3: Anal. Calc.: C, 30.35; H, 1.17; N, 2.72. Found: C, 30.75; H, 1.88; N, 2.85. Selected IR (KBr pellet, cm-1) for 1 (4000-400 cm-1): 3390 (s), 3223 (m), 2930 (w), 1557 (s), 1411 (s), 1037 (w), 858 (m), 777 (m), 722 (w), 580(w), 520(w). (Fig. S1). The isostructural Ln-CPs Eu3+ -doped 1 and Eu3+, Tb3+ -doped 3, can be easily synthesized by using mixed lanthanide salts of Eu(NO 3)3∙6H2O, Tb(NO3)3∙6H2O and Gd(NO3)3∙6H2O following the same procedure applied to 1. The corresponding ratios in the starting codopants were listed in Table 1. Analyses of the relative molar concentration of the individual lanthanide elements are consistent with the corresponding ratios in the starting codopants (Table 1). Table 1 Doped molar ratios and elemental analyses (ICP) for Eu 3+ -doped 1, and Eu3+, Tb3+ -doped 3. Doped molar ratio
Wt% Tb calcd
Wt% Eu calcd 5
Wt% Gd calcd
of Tb : Eu : Gd 97%:3%:0% 95%:5%:0% 93%:7%:0% 88%:12%:0% 10%:3%:87%
(found) 96.06 (96.42) 95.32 (95.64) 93.57 (93.22) 87.69 (87.52) 10.01 (9.74)
(found) 2.96 (3.26) 4.79 (5.12) 6.71 (6.53) 11.53 (11.23) 2.91 (2.72)
(found) 0 0 0 0 87.08 (87.42)
2.3. X-ray crystallography Crystal data of 1 were obtained using a Rigaku R-AXIS RAPID diffractometer equipped with graphite-monochromated Mo-Kα (λ = 0.71073 Å) radiation in the ω scanning mode at room temperature. All structures were solved by direct methods and refined by full-matrix least-squares fitting on F2 by the SHELXTL-97 crystallographic software package [18]. All non-hydrogen atoms were refined anisotropically. All the hydrogen atoms were placed in fixed, calculated positions using a riding model. Selected crystallographic data and refinement parameters of 1 are listed in Table S2, while the selected bond lengths and angles are presented in Table S3. Unfortunately, the single crystals of 2 and 3 cannot be used for single-crystal X-ray diffraction analysis because of their poor qualities. The powder X-ray diffraction analyses show that they have the same structures as 1. Their formulas were determined based on the elemental and thermogravimetric analyses data. CCDC-1817644 contains the supplementary crystallographic data for this paper.
3. Results and Discussion 3.1. Crystal structures of 1
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Fig. 1. (a) Coordination environment of Tb3+ ions in 1. (b) Distorted tricapped trigonal prism TbO9 in 1. The single-crystal X-ray analysis of 1 revealed that it crystallizes in the monoclinic space group P-1. The asymmetric unit consists of one Tb 3+ ion, one PPDA2- ligand, half of oxalic acid anion and two coordinated water molecules. The coordination environment of Tb3+, shown in Fig. 1a, is nine coordinate with nine oxygen atoms, five of which are from one PPDA2− ligand, four oxygen atoms are from oxalic acid anion and coordinated water molecules, respectively, resulting in a slightly distorted tricapped trigonal prism geometry (Fig. 1b). The Tb-O bond lengths varies from 2.3747 to 2.6303 Å, the bond angles of O-Tb-O are in the range of 64.62 to 150.53 °, which are well matched to those observed in similar Tb-CPs [19].
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Fig. 2. (a) View of the 1D chain structure of 1 in the ac-plane. (b) View of the 2D layer structure of 1 in the ab-plane. (c) View of the 3D supramolecular structure of 1 constructing by the 2D layers in a …ABAB… fashion (hydrogen atoms are omitted for clarity). In the structure of 1, all carboxyl groups of the ligands are deprotonated and exhibit chelating/bridging and bidentate chelating coordination modes (Fig. S4). As shown in Fig. 2a, two crystallographically equivalent Tb atoms are bridged by two μ3-O(8) from PPDA2- to form a binuclear building block, which was further connected with each other through two carboxylic groups to form a 1D infinite chain along a direction. The adjacent 1D chains are connected by oxalic acids to form the 2D layer framework with the rhombic windows (Fig. 2b). Along c direction, adjacent 2D layers are packed parallel in a …ABAB… fashion to generate a 3D supramolecular structure via van der Waals forces, which are different from common molecular interactions, such as hydrogen bond and π-π interactions (Fig. 2c). The benzene ring and imidazole ring of adjacent layers are not parallel. The dihedral angle between the benzene ring and imidazole ring of adjacent layer is approximately 73.325°. 3.2. Characterization Powder X-ray diffraction patterns of 1-3 are consistent with the simulated ones based on the single-crystal structures, which indicates the phase-purity of as-synthesized product (Fig. S2). Thermogravimetric analyses (TGA) of 1-3 were carried out under air atmosphere with a heating rate of 10 °C min -1 from ambient temperature up to 800 °C (Fig. S3). Herein, only the thermal stability of 1 was taken 8
as a representative example for discussion. In the temperature range of 20-200 °C, 1 display a continuous weight loss (7.7%), which corresponds to the loss of the coordinated and free water molecules (cal. 10.5%). The second weight loss (58.6%) is attributed to the coordinated C 2O42- and PPDA2- ligand (cal. 57.0%). The remaining weight is attributed to the final product of metal oxide. 1-3 display similar IR spectra in the range of 4000-400 cm-1 based on their similar structural characteristics (Fig. S1). No strong absorption bands around 1700 cm-1 is observed in the IR spectra, which indicates that the ligand is completely deprotonated. The characteristic strong absorption bands in the usual region around 1411 cm-1 and 1557 cm-1, respectively, are attributed to the symmetric and asymmetric stretching vibrations of the carboxylate groups of PPDA2- anions, respectively. The broad band centered around 3390 cm-1 indicates the presence of free water molecules. 3.3. Photoluminescent properties 3.3.1. Visible luminescent properties of 1-3
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Fig. 3. Solid state emission spectra of 1 (a), 2 (b), 3 (c) and H2PPDA ligand (d) collected at room temperature. Luminescent properties of 1-3 and H2PPDA ligand were investigated at room temperature. As shown in Fig. 3d, the H2PPDA ligand displayed a broad fluorescence emission band centered at 445 nm upon excitation at 370 nm and the emission band arose from the intramolecular π→π* transition of the ligand. Under excitation at 320 nm, 1 emitted the characteristic green luminescence of Tb 3+ (Fig. 3a), with four characteristic bands at 489 (5D4→7F6), 544 (5D4→7F5), 589 (5D4→7F4), and 622 (5D4→7F3) nm. The 5D4→7F6 and 5D4→7F5 emissions result from an electric dipole transition and a magnetic dipole transition, respectively. Upon excitation at 330 nm, 2 emitted the characteristic red luminescence of the Eu 3+. The emission spectrum has four characteristic bands at 590 ( 5D0→7F1), 614 (5D0→7F2), 651 (5D0 10
→7F3) and 690-698 (5D0→7F4) nm (Fig. 3b), respectively. The strongest intensity of emission at 614 nm in the red region is attributed to the 5D0→7F2 transition, which is an electric dipole transition, the so-called hypersensitive transition. When excited at 345 nm, 3 presented a blue emission with a broad band centered at 404 nm (Fig. 3c), which exhibits much enhanced blue emission and a slight blue shift [20] compared with the free H2PPDA ligand. The increase of the luminescence intensity for 3 is attributed to the chelation of the ligand to the metal center, which reduces the nonradiative relaxation process. Furthermore, the emission band from the PPDA 2ligand around 404 nm is not detected, implying efficient energy transfer from the ligand to the central Ln 3+ ions in 1 and 2. At the same time, the luminescence decay curves of 1 and 2 were obtained at room temperature (Fig. S5). The decay curves of them were both well fitted to a double exponential function: I = I0 + A1exp (-t/τ1) + A2 exp (-t/τ2), where I and I0 are the luminescent intensities at time: t = t and t = 0, respectively, whereas τ 1 and τ2 are defined as the luminescent lifetimes: τ 1 = 0.931 ms and τ2 = 0.406 ms for 1, τ1 = 1.08 ms and τ2 =0.027 ms for 2, respectively. We found that the lifetime of Eu 3+ is much longer than that of Tb 3+. This indicates ligands can more efficiently transfer the energy absorbed to the Eu 3+ by antenna effect [21]. 3.3.2. Tunable luminescence
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Fig. 4. (a) Emission spectra of Eu3+ -doped 1 CPs with different Eu3+ molar ratios. (b) CIE chromaticity diagram for Eu3+ -doped 1 CPs when excited at 330 nm. Considering that the emissions of 1-3 consist of three base colors as well as isomorphism, theoretically, production of tunable luminescence and white light should be achievable if the rational combination of different lanthanide ions as dopants in carefully stoichiometric ratio incorporated into one isostructural CPs. As expected, with the increase of amount of Eu 3+ introduced into isomorphic Ln-CPs, the corresponding emissions increase gradually. As shown in Fig. 4, the Eu3+ -doped 1 Ln-CPs generate tunable colors from green to green-yellow, yellow, orange and red-orange and red, by adjusting the co-doping ratio of Eu3+ from 3% to 12%. The corresponding CIE chromaticity photographs of the tunable colors generated from the Eu3+-doped 1 CPs excited under 330 nm. The emission spectra of Tb3+ significantly decrease, while the emission spectra of Eu 3+ are largely enhanced with the increase of Eu3+ concentration. There is a phenomenon that the red component of the color emission keeps strong even for the lower Eu3+/Tb3+ ratios. This approves the emission of the Eu3+ ions in these doped CPs are further sensitized by the Tb 3+ ions in Eu3+-doped 1.
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It was confirmed that the emission of the Eu3+ ions in Eu3+ -doped 1 is further sensitized by the Tb3+ ions within the same frameworks. Tb3+-to-Eu3+ energy transfer behaviour also has been comfirmed by the luminescent decay test, in which the luminescence lifetimes of Tb3+ decrease, while the luminescence lifetimes of Eu3+ are enhanced with the increase of Eu 3+ concentration in in the same frameworks (Fig. S5).
Fig. 5. (a) The emission spectra of Eu3+, Tb3+ -doped 3 by changing the excitation wavelengths from 330 to 380 nm. (b) The CIE chromaticity diagram of the Eu3+, Tb3+ -doped 3 under excitation wavelengths from 330 to 380 nm. Notably, developing high performance white-light emitting compounds has attracted much attention due to their important applications in general lighting sources [22,23]. In this sense, we successfully designed and fabricated novel Eu3+, Tb3+ -doped 3, which is isomorphous to 1 and able to simultaneously show the characteristic emissions of the Eu 3+ and Tb3+ ions, as well as the broad blue emission of the ligand (Fig. 5a). As shown in Fig. 5b, the CIE chromaticity coordinate positions locate in the white light luminescence zone when the excitation wavelength increases from 330 nm to 380 nm. The CIE coordinates (0.290, 0.335),
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with the excitation wavelength of 370 nm, is very close to the targeted value (0.333, 0.333). The quantum yield of luminescence is 38% for Eu 0.03Tb0.10Gd0.87 sample.
4. Conclusions In summary, three new compounds [Ln(PPDA)(C2O4)0.5(H2O)2]n∙nH2O (Ln = Tb (1), Eu (2) and Gd (3)) have been synthesized under solvothermal conditions. They exhibit isomorphic 2D layered network, in which oxalic acid is generated by in situ reaction. Considering 1 and 2 show the characteristic emissions of the corresponding Ln3+ ions, 3 emits blue light, with careful stoichiometric tuning Eu 3+ ion from 3% to 12%, Eu3+ -doped 1 CPs display emission colors from red to green, meanwhile, Eu3+, Tb3+ -doped 3 has been designed and presented white emission upon excitation at 370 nm and their emission can be switched yellow to yellow-green, and white. Appendix A. Supplementary data CCDC <1817644> contains the supplementary crystallographic data for 1. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: [email protected].
Acknowledgments The authors gratefully acknowledge the financial support through the National Natural Science Foundation of China (Grant Nos. 21171065, 21201077，21603004 and 51775232). 14
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[14] X.Y. Ren, L.H. Lu, Chin. Chem. Lett. 26 (2015) 1439. [15] D.K. Singha, P. Majee, S.K. Mondal, P. Mahata, Eur. J. Inorg. Chem. 8 (2015) 1390. [16] L.L. Zhang, P. Gao, Y.H. Zhang, M. Hu, J. Coord. Chem. 68 (2015) 3932. [17] X.L. Zhang, Z.Y. Zhan, X.Y. Liang, C. Chen, X.L. Liu, Y.J. Jia, M. Hu, Dalton Trans. 47 (2018) 3272. [18] G. Sheldrick, University of Goettingen, Goettingen, Germany, 1997 [19] S.Y. Wang, L. Shan, Y. Fan, J. Jia, J.N. Xu, L. Wang, J. Solid State Chem. 245 (2017) 132. [20] T. Ding, X.X. Wang, M. Zhang, S.M. Ou, T.J. Hu, CrystEngComm 19 (2017) 3313. [21] J.J. Liu, W. Sun, Z.L. Liu, RSC Adv. 6 (2016) 25689. [22] W.F. Zhao, C. Zou, L.X. Shi, J.C. Yu, G.D. Qian, C.D. Wu, Dalton Trans. 41 (2012) 10091. [23] G.J. He, D. Guo, C. He, X.L. Zhang, X.W. Zhao, C.Y. Duan, Angew. Chem. Int. Ed. 48 (2009) 6132.
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Figure captions
Fig. 1. (a) Coordination environment of Tb3+ ions in 1. (b) Distorted tricapped trigonal prism TbO9 in 1. Fig. 2. (a) View of the 1D chain structure of 1 in the ac-plane. (b) View of the 2D layer structure of 1 in the ab-plane. (c) View of the 3D supramolecular structure of 1 constructing by the 2D layers in a …ABAB… fashion (hydrogen atoms are omitted for clarity). Fig. 3. Solid state emission spectra of 1 (a), 2 (b), 3 (c) and H2PPDA ligand (d) collected at room temperature. Fig. 4. (a) Emission spectra of Eu 3+ -doped 1 CPs with different Eu3+ molar ratios. (b) CIE chromaticity diagram for Eu 3+ -doped 1 CPs when excited at 330 nm. Fig. 5. (a) The CIE chromaticity diagram of the Eu3+, Tb3+ -doped 3 by changing the excitation wavelengths from 330 to 380 nm. (b) The CIE chromaticity diagram of the Eu3+, Tb3+ -doped 3 under excitation wavelengths from 330 to 380 nm.
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Table captions
Table 1 Doped molar ratios and elemental analyses (ICP) for Eu 3+ -doped 1, and Eu3+, Tb3+ -doped 3.
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Table 1 Doped molar ratios and elemental analyses (ICP) for Eu 3+ -doped 1, and Eu3+, Tb3+ -doped 3. Doped molar ratio of Tb : Eu : Gd 97%:3%:0% 95%:5%:0% 93%:7%:0% 88%:12%:0% 10%:3%:87%
Wt% Tb calcd (found)
Wt% Eu calcd Wt% Gd (found) calcd (found) 2.96 (3.26) 0 4.79 (5.12) 0 6.71 (6.53) 0 11.53 (11.23) 0 2.91 (2.72) 87.08 (87.42)
96.06 (96.42) 95.32 (95.64) 93.57 (93.22) 87.69 (87.52) 10.01 (9.74)
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Graphical abstract synopsis
Tunable luminescence from green to red was successfully achieved by co-doping Eu into isostructural Tb framework. White-light emission was successfully realized by co-doping Eu/Tb into analogic Gd framework.
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S0277-5387(19)30054-3 https://doi.org/10.1016/j.poly.2019.01.039 POLY 13712
To appear in:
Polyhedron
Received Date: Revised Date: Accepted Date:
14 September 2018 21 December 2018 14 January 2019
Please cite this article as: J. Chai, P. Zhang, X. Shi, J. Xu, Y. Fan, L. Wang, 2D Lanthanide Coordination polymers constructed from semirigid ligand 4-(pyridin-3-yloxy)-phthalic acid: synthesis, structure and luminescence, Polyhedron (2019), doi: https://doi.org/10.1016/j.poly.2019.01.039
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
2D Lanthanide Coordination polymers constructed from semirigid ligand 4-(pyridin-3-yloxy)-phthalic acid: synthesis, structure and luminescence Juan Chai, Ping Zhang, Xiangxiang Shi, Jianing Xu, Yong Fan*, Li Wang* College of Chemistry, Jilin University, Changchun, 130012, Jilin, P. R. China. E-mail: [email protected]; [email protected].
Abstract Three new compounds (Ln-CPs) [Ln(PPDA)(C2O4)0.5(H2O)2]n∙nH2O (Ln = Tb (1), Eu (2) and Gd (3)) were synthesized based on semirigid ligand 4-(pyridin-3-yloxy)-phthalic acid (H2PPDA) under solvothermal conditions. Ln-CPs 1-3 exhibit isomorphic 2D layered network with rhombic windows. Notably, oxalic acid in the resulting framework is formed by in situ reaction under solvothermal and acidic conditions. Solid state luminescent studies indicate that 1 and 2 show characteristic green and red emissions of the corresponding Ln 3+ ions, respectively, while 3 exhibits blue emission arising from PPDA 2- ligand. Then, by adjusting the co-doping ratio of different Ln 3+ ions into the well-defined host framework, a series of codoped Ln-CPs Eu3+ -doped 1 and Eu3+, Tb3+ -doped 3 are synthesized, showing tunable luminescence emissions including white-light emitting. Keywords: Lanthanide coordination polymer; Solvothermal; Luminescence; Color-tunable; White-light emission.
1. Introduction 1
Recently, full-colour luminescent materials, in particular those with white-light emission, have received consideration attention due to their wide applications in color-tunable phosphors, full-color displays, optical memory devices, low-cost back-lighting, and backlights [1-3]. It is well known that one common approach to realize multi-color and white-light emissions is to incorporate and control the intensity of three fundamental red, green and blue (RGB) light emitting components in the bulk materials [4-5]. Coordination polymers (CPs) built from lanthanide ions and organic ligands offer a unique platform for the development of luminescence devices and optical displays due to their high photoluminescence efficiency and narrow band emission ability [6-7]. Considering the unity of the Ln-CPs in the structure, the emission color output can be controlled precisely by co-doping the different Ln3+ ion into the same framework. As well known, the multi-color and white-light luminescence can be achieved by co-doping Eu3+, Tb3+ ions and a certain blue light emission source (usually organic ligands) as RGB light emitting components into the frameworks of isostructural Ln-CPs [8]. The luminescence of these doped Ln-CPs can be effectively tuned by varying the stoichiometric ratio of Ln3+ ions and the excitation wavelengths [9]. For example, Zhang et al. synthesized a series of codoped Ln-CPs EuxTb1-xL by changing the ratio of Eu3+ and Tb3+, which exhibited the full-color of the luminescence and white light emission [10]. Du et al. synthesized a highly luminescent chameleon by varying the stoichiometric ratio of Eu3+, Tb3+ and Gd3+ ions in a Ln-CP [11]. Although luminescent Ln-CPs have been designed for white-light emission materials, bioimaging and molecular sensing materials, excitation-modulated luminescence materials based on Ln-CPs are still in their infancy owing to the difficulties associated with incorporating different luminescent components into a well-defined framework [12-13]. 2
In order to gain high luminescence, besides the metal ions, an accurate choice of the ligand is crucial for the success of the rational design of Ln-CPs. Ligands used in the construction of Ln-CPs should be able to efficiently transfer the energy absorbed by chromophores to the lanthanide ions (antenna effect) [14-15]. In addition, the ligand should comprise multiple binding sites to saturate the Ln3+ coordination sites. Among various organic ligands, polycarboxylate ligands, as good candidates for the construction of multifunctional CPs, have aroused a good deal of interest owing to their various coordination modes and strong coordination ability. Semirigid ligand 4-(pyridin-3-yloxy)-phthalic acid (H2PPDA) as a multi-carboxylate ligand can exhibit a great variety of coordination modes [16]. Recent reports comfirmed that semirigid 4-(pyridin-3-yloxy)-phthalic acid (H2PPDA) as bridges to connect Ln metal ions into highly stable frameworks, in which H2PPDA was able to act as antenna chromophore to efficiently absorb and transfer energy to the lanthanide ions. For example, Hu et al. synthesized a serial of Ln-CPs by mixed ligands of H2PPDA and terephthalic acid (H2bdc). They exhibited 2D layered structures and served as excellent selective fluorescent sensing materials for nitrobenzene derivatives-based explosives, Fe3+ ion and Cr2O72− ion [17]. In this work, based on multi-carboxylate ligand H2PPDA, three new compounds [Ln(PPDA) (C2O4)0.5(H2O)2]n∙nH2O (Ln = Tb (1), Eu (2) and Gd (3)) was synthesized under solvothermal condition, featuring a 2D layered structure with rhombic window. It is noted that the oxalic acid anions in the framework are in situ generated by the ligand (H 2PPDA) under solvothermal and acidic conditions. 1 and 2 show characteristic green and red emissions of the corresponding Ln 3+ ions, respectively, 3 exhibits blue emission arising from the PPDA2- ligand. In terms of 3
1-3 showing characteristic green, red and blue emissions, respectively, we try to prepare two series of new doped Ln-CPs Eu3+ -doped 1 and Eu3+, Tb3+ -doped 3 with careful adjustment of the relative concentrations of the lanthanide ions and excitation wavelengths. As a result, Eu3+ -doped 1 have been prepared and the emission colors can be tuned from green to red by varying the molar ratios of Eu 3+ ion from 3% to 12% in 1, meanwhile, Eu3+, Tb3+ -doped 3, Eu0.03Tb0.10Gd0.87, emits white light with quantum yield of 38% upon excitation at 370 nm.
2. Experimental 2.1. Materials and Measurements Unless otherwise noted, all other chemicals used in this work were of reagent grade. They were commercially available and used as purchased without further purification. Powder X-ray diffraction (XRD) data were obtained using SHIMADZU XRD-6100 diffractometer with Cu Kα radiation (λ = 1.5418 Å) in the 2θ range of 4-40°, with the step size and the count time of 0.06° and 6 s, respectively. The elemental analyses (C, H and N) were performed on a Vario EL cube elemental analyzer. FT-IR spectra were recorded on a Nicolet Impact 410 spectrometer between 400 and 4000 cm -1 using the KBr pellet method. Thermogravimetric analyses (TGA) were carried out on a Perkin-Elmer TGA 7 thermogravimetric analyzer with a heating rate of 10 °C min-1 from room temperature to 800 °C under air atmosphere. Photoluminescence (PL) spectra, fluorescence lifetimes and quantum yield were measured with an Edinburgh Instruments FLS920 spectrophotometer. Elemental analyses for Tb, Gd, and Eu were obtained using a PLASMA-SPEC(I) ICP atomic emission spectrometer. 4
2.2 Synthesis of 1-3 A mixture of Ln(NO3)3∙6H2O (1 ml, 0.08 mmol∙ml-1) and H2PPDA (0.0260 g, 0.1 mmol) were dispersed in C 2H5OH (3 ml), HNO3 (0.3 ml, 6 mmol∙ml-1) and H2O (3 ml) in a 23 ml Teflon-lined autoclave and then heated under autogenous pressure at 120 °C for two days, then cooled to room temperature under ambient conditions. Yellow sheet crystals were recovered by filtration, washed with distilled water, and dried in air. Yield: 75% for 1, 70% for 2, 80% for 3 (based on Ln(NO3)3∙6H2O). Elemental analysis (%) for 1: Anal. Calc.: C, 30.29; H, 1.17; N, 2.72. Found: C, 30.94; H, 1.27; N, 2.85. For 2: Anal. Calc.: C, 30.65; H, 1.18. N, 2.75; Found: C, 30.40; H, 1.95; N, 2.59. For 3: Anal. Calc.: C, 30.35; H, 1.17; N, 2.72. Found: C, 30.75; H, 1.88; N, 2.85. Selected IR (KBr pellet, cm-1) for 1 (4000-400 cm-1): 3390 (s), 3223 (m), 2930 (w), 1557 (s), 1411 (s), 1037 (w), 858 (m), 777 (m), 722 (w), 580(w), 520(w). (Fig. S1). The isostructural Ln-CPs Eu3+ -doped 1 and Eu3+, Tb3+ -doped 3, can be easily synthesized by using mixed lanthanide salts of Eu(NO 3)3∙6H2O, Tb(NO3)3∙6H2O and Gd(NO3)3∙6H2O following the same procedure applied to 1. The corresponding ratios in the starting codopants were listed in Table 1. Analyses of the relative molar concentration of the individual lanthanide elements are consistent with the corresponding ratios in the starting codopants (Table 1). Table 1 Doped molar ratios and elemental analyses (ICP) for Eu 3+ -doped 1, and Eu3+, Tb3+ -doped 3. Doped molar ratio
Wt% Tb calcd
Wt% Eu calcd 5
Wt% Gd calcd
of Tb : Eu : Gd 97%:3%:0% 95%:5%:0% 93%:7%:0% 88%:12%:0% 10%:3%:87%
(found) 96.06 (96.42) 95.32 (95.64) 93.57 (93.22) 87.69 (87.52) 10.01 (9.74)
(found) 2.96 (3.26) 4.79 (5.12) 6.71 (6.53) 11.53 (11.23) 2.91 (2.72)
(found) 0 0 0 0 87.08 (87.42)
2.3. X-ray crystallography Crystal data of 1 were obtained using a Rigaku R-AXIS RAPID diffractometer equipped with graphite-monochromated Mo-Kα (λ = 0.71073 Å) radiation in the ω scanning mode at room temperature. All structures were solved by direct methods and refined by full-matrix least-squares fitting on F2 by the SHELXTL-97 crystallographic software package [18]. All non-hydrogen atoms were refined anisotropically. All the hydrogen atoms were placed in fixed, calculated positions using a riding model. Selected crystallographic data and refinement parameters of 1 are listed in Table S2, while the selected bond lengths and angles are presented in Table S3. Unfortunately, the single crystals of 2 and 3 cannot be used for single-crystal X-ray diffraction analysis because of their poor qualities. The powder X-ray diffraction analyses show that they have the same structures as 1. Their formulas were determined based on the elemental and thermogravimetric analyses data. CCDC-1817644 contains the supplementary crystallographic data for this paper.
3. Results and Discussion 3.1. Crystal structures of 1
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Fig. 1. (a) Coordination environment of Tb3+ ions in 1. (b) Distorted tricapped trigonal prism TbO9 in 1. The single-crystal X-ray analysis of 1 revealed that it crystallizes in the monoclinic space group P-1. The asymmetric unit consists of one Tb 3+ ion, one PPDA2- ligand, half of oxalic acid anion and two coordinated water molecules. The coordination environment of Tb3+, shown in Fig. 1a, is nine coordinate with nine oxygen atoms, five of which are from one PPDA2− ligand, four oxygen atoms are from oxalic acid anion and coordinated water molecules, respectively, resulting in a slightly distorted tricapped trigonal prism geometry (Fig. 1b). The Tb-O bond lengths varies from 2.3747 to 2.6303 Å, the bond angles of O-Tb-O are in the range of 64.62 to 150.53 °, which are well matched to those observed in similar Tb-CPs [19].
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Fig. 2. (a) View of the 1D chain structure of 1 in the ac-plane. (b) View of the 2D layer structure of 1 in the ab-plane. (c) View of the 3D supramolecular structure of 1 constructing by the 2D layers in a …ABAB… fashion (hydrogen atoms are omitted for clarity). In the structure of 1, all carboxyl groups of the ligands are deprotonated and exhibit chelating/bridging and bidentate chelating coordination modes (Fig. S4). As shown in Fig. 2a, two crystallographically equivalent Tb atoms are bridged by two μ3-O(8) from PPDA2- to form a binuclear building block, which was further connected with each other through two carboxylic groups to form a 1D infinite chain along a direction. The adjacent 1D chains are connected by oxalic acids to form the 2D layer framework with the rhombic windows (Fig. 2b). Along c direction, adjacent 2D layers are packed parallel in a …ABAB… fashion to generate a 3D supramolecular structure via van der Waals forces, which are different from common molecular interactions, such as hydrogen bond and π-π interactions (Fig. 2c). The benzene ring and imidazole ring of adjacent layers are not parallel. The dihedral angle between the benzene ring and imidazole ring of adjacent layer is approximately 73.325°. 3.2. Characterization Powder X-ray diffraction patterns of 1-3 are consistent with the simulated ones based on the single-crystal structures, which indicates the phase-purity of as-synthesized product (Fig. S2). Thermogravimetric analyses (TGA) of 1-3 were carried out under air atmosphere with a heating rate of 10 °C min -1 from ambient temperature up to 800 °C (Fig. S3). Herein, only the thermal stability of 1 was taken 8
as a representative example for discussion. In the temperature range of 20-200 °C, 1 display a continuous weight loss (7.7%), which corresponds to the loss of the coordinated and free water molecules (cal. 10.5%). The second weight loss (58.6%) is attributed to the coordinated C 2O42- and PPDA2- ligand (cal. 57.0%). The remaining weight is attributed to the final product of metal oxide. 1-3 display similar IR spectra in the range of 4000-400 cm-1 based on their similar structural characteristics (Fig. S1). No strong absorption bands around 1700 cm-1 is observed in the IR spectra, which indicates that the ligand is completely deprotonated. The characteristic strong absorption bands in the usual region around 1411 cm-1 and 1557 cm-1, respectively, are attributed to the symmetric and asymmetric stretching vibrations of the carboxylate groups of PPDA2- anions, respectively. The broad band centered around 3390 cm-1 indicates the presence of free water molecules. 3.3. Photoluminescent properties 3.3.1. Visible luminescent properties of 1-3
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Fig. 3. Solid state emission spectra of 1 (a), 2 (b), 3 (c) and H2PPDA ligand (d) collected at room temperature. Luminescent properties of 1-3 and H2PPDA ligand were investigated at room temperature. As shown in Fig. 3d, the H2PPDA ligand displayed a broad fluorescence emission band centered at 445 nm upon excitation at 370 nm and the emission band arose from the intramolecular π→π* transition of the ligand. Under excitation at 320 nm, 1 emitted the characteristic green luminescence of Tb 3+ (Fig. 3a), with four characteristic bands at 489 (5D4→7F6), 544 (5D4→7F5), 589 (5D4→7F4), and 622 (5D4→7F3) nm. The 5D4→7F6 and 5D4→7F5 emissions result from an electric dipole transition and a magnetic dipole transition, respectively. Upon excitation at 330 nm, 2 emitted the characteristic red luminescence of the Eu 3+. The emission spectrum has four characteristic bands at 590 ( 5D0→7F1), 614 (5D0→7F2), 651 (5D0 10
→7F3) and 690-698 (5D0→7F4) nm (Fig. 3b), respectively. The strongest intensity of emission at 614 nm in the red region is attributed to the 5D0→7F2 transition, which is an electric dipole transition, the so-called hypersensitive transition. When excited at 345 nm, 3 presented a blue emission with a broad band centered at 404 nm (Fig. 3c), which exhibits much enhanced blue emission and a slight blue shift [20] compared with the free H2PPDA ligand. The increase of the luminescence intensity for 3 is attributed to the chelation of the ligand to the metal center, which reduces the nonradiative relaxation process. Furthermore, the emission band from the PPDA 2ligand around 404 nm is not detected, implying efficient energy transfer from the ligand to the central Ln 3+ ions in 1 and 2. At the same time, the luminescence decay curves of 1 and 2 were obtained at room temperature (Fig. S5). The decay curves of them were both well fitted to a double exponential function: I = I0 + A1exp (-t/τ1) + A2 exp (-t/τ2), where I and I0 are the luminescent intensities at time: t = t and t = 0, respectively, whereas τ 1 and τ2 are defined as the luminescent lifetimes: τ 1 = 0.931 ms and τ2 = 0.406 ms for 1, τ1 = 1.08 ms and τ2 =0.027 ms for 2, respectively. We found that the lifetime of Eu 3+ is much longer than that of Tb 3+. This indicates ligands can more efficiently transfer the energy absorbed to the Eu 3+ by antenna effect [21]. 3.3.2. Tunable luminescence
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Fig. 4. (a) Emission spectra of Eu3+ -doped 1 CPs with different Eu3+ molar ratios. (b) CIE chromaticity diagram for Eu3+ -doped 1 CPs when excited at 330 nm. Considering that the emissions of 1-3 consist of three base colors as well as isomorphism, theoretically, production of tunable luminescence and white light should be achievable if the rational combination of different lanthanide ions as dopants in carefully stoichiometric ratio incorporated into one isostructural CPs. As expected, with the increase of amount of Eu 3+ introduced into isomorphic Ln-CPs, the corresponding emissions increase gradually. As shown in Fig. 4, the Eu3+ -doped 1 Ln-CPs generate tunable colors from green to green-yellow, yellow, orange and red-orange and red, by adjusting the co-doping ratio of Eu3+ from 3% to 12%. The corresponding CIE chromaticity photographs of the tunable colors generated from the Eu3+-doped 1 CPs excited under 330 nm. The emission spectra of Tb3+ significantly decrease, while the emission spectra of Eu 3+ are largely enhanced with the increase of Eu3+ concentration. There is a phenomenon that the red component of the color emission keeps strong even for the lower Eu3+/Tb3+ ratios. This approves the emission of the Eu3+ ions in these doped CPs are further sensitized by the Tb 3+ ions in Eu3+-doped 1.
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It was confirmed that the emission of the Eu3+ ions in Eu3+ -doped 1 is further sensitized by the Tb3+ ions within the same frameworks. Tb3+-to-Eu3+ energy transfer behaviour also has been comfirmed by the luminescent decay test, in which the luminescence lifetimes of Tb3+ decrease, while the luminescence lifetimes of Eu3+ are enhanced with the increase of Eu 3+ concentration in in the same frameworks (Fig. S5).
Fig. 5. (a) The emission spectra of Eu3+, Tb3+ -doped 3 by changing the excitation wavelengths from 330 to 380 nm. (b) The CIE chromaticity diagram of the Eu3+, Tb3+ -doped 3 under excitation wavelengths from 330 to 380 nm. Notably, developing high performance white-light emitting compounds has attracted much attention due to their important applications in general lighting sources [22,23]. In this sense, we successfully designed and fabricated novel Eu3+, Tb3+ -doped 3, which is isomorphous to 1 and able to simultaneously show the characteristic emissions of the Eu 3+ and Tb3+ ions, as well as the broad blue emission of the ligand (Fig. 5a). As shown in Fig. 5b, the CIE chromaticity coordinate positions locate in the white light luminescence zone when the excitation wavelength increases from 330 nm to 380 nm. The CIE coordinates (0.290, 0.335),
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with the excitation wavelength of 370 nm, is very close to the targeted value (0.333, 0.333). The quantum yield of luminescence is 38% for Eu 0.03Tb0.10Gd0.87 sample.
4. Conclusions In summary, three new compounds [Ln(PPDA)(C2O4)0.5(H2O)2]n∙nH2O (Ln = Tb (1), Eu (2) and Gd (3)) have been synthesized under solvothermal conditions. They exhibit isomorphic 2D layered network, in which oxalic acid is generated by in situ reaction. Considering 1 and 2 show the characteristic emissions of the corresponding Ln3+ ions, 3 emits blue light, with careful stoichiometric tuning Eu 3+ ion from 3% to 12%, Eu3+ -doped 1 CPs display emission colors from red to green, meanwhile, Eu3+, Tb3+ -doped 3 has been designed and presented white emission upon excitation at 370 nm and their emission can be switched yellow to yellow-green, and white. Appendix A. Supplementary data CCDC <1817644> contains the supplementary crystallographic data for 1. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: [email protected].
Acknowledgments The authors gratefully acknowledge the financial support through the National Natural Science Foundation of China (Grant Nos. 21171065, 21201077，21603004 and 51775232). 14
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Figure captions
Fig. 1. (a) Coordination environment of Tb3+ ions in 1. (b) Distorted tricapped trigonal prism TbO9 in 1. Fig. 2. (a) View of the 1D chain structure of 1 in the ac-plane. (b) View of the 2D layer structure of 1 in the ab-plane. (c) View of the 3D supramolecular structure of 1 constructing by the 2D layers in a …ABAB… fashion (hydrogen atoms are omitted for clarity). Fig. 3. Solid state emission spectra of 1 (a), 2 (b), 3 (c) and H2PPDA ligand (d) collected at room temperature. Fig. 4. (a) Emission spectra of Eu 3+ -doped 1 CPs with different Eu3+ molar ratios. (b) CIE chromaticity diagram for Eu 3+ -doped 1 CPs when excited at 330 nm. Fig. 5. (a) The CIE chromaticity diagram of the Eu3+, Tb3+ -doped 3 by changing the excitation wavelengths from 330 to 380 nm. (b) The CIE chromaticity diagram of the Eu3+, Tb3+ -doped 3 under excitation wavelengths from 330 to 380 nm.
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Table captions
Table 1 Doped molar ratios and elemental analyses (ICP) for Eu 3+ -doped 1, and Eu3+, Tb3+ -doped 3.
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Table 1 Doped molar ratios and elemental analyses (ICP) for Eu 3+ -doped 1, and Eu3+, Tb3+ -doped 3. Doped molar ratio of Tb : Eu : Gd 97%:3%:0% 95%:5%:0% 93%:7%:0% 88%:12%:0% 10%:3%:87%
Wt% Tb calcd (found)
Wt% Eu calcd Wt% Gd (found) calcd (found) 2.96 (3.26) 0 4.79 (5.12) 0 6.71 (6.53) 0 11.53 (11.23) 0 2.91 (2.72) 87.08 (87.42)
96.06 (96.42) 95.32 (95.64) 93.57 (93.22) 87.69 (87.52) 10.01 (9.74)
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Graphical abstract synopsis
Tunable luminescence from green to red was successfully achieved by co-doping Eu into isostructural Tb framework. White-light emission was successfully realized by co-doping Eu/Tb into analogic Gd framework.
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