Solid State Sciences 78 (2018) 7e15
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Syntheses, structures and properties of four organiceinorganic hybrid nicotinate-bridging rare-earth-containing phosphotungstates Peijun Gong a, Jingjing Pang a, Cuiping Zhai a, *, Junwei Zhao a, b, ** a
Henan Key Laboratory of Polyoxometalate Chemistry, Institute of Molecular and Crystal Engineering, College of Chemistry and Chemical Engineering, Henan University, Kaifeng, Henan 475004, China b State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 November 2017 Received in revised form 27 January 2018 Accepted 31 January 2018 Available online 5 February 2018
Four novel organiceinorganic hybrid nicotinate-bridging dimeric rare-earth (RE)-containing phosphotungstates [H2N(CH3)2]8[RE(H2O)(NA)(a-HPW11O39)]2$24H2O (RE ¼ HoIII for 1, ErIII for 2, TbIII for 3, DyIII for 4; HNA ¼ nicotinic acid) have been synthesized from the reaction of trivacant Keggin precursor Na9[aPW9O34]∙16H2O, RE(NO3)3$6H2O, HNA by employing dimethylamine hydrochloride as organic solubilizing agent in the conventional aqueous solution system, which have been further characterized by elemental analyses, IR spectra, thermogravimetric analyses and single-crystal X-ray diffraction. Structural analysis indicates that the hybrid dimeric {[RE(H2O)(NA)(a-HPW11O39)]2}8 polyoxoanion in 1e4 can be considered as two head-to-head mono-RE-containing Keggin [RE(H2O)(NA)(a-HPW11O39)]4 subunits bridged by two (h2,m-1,1)-nicotinate linkers, which stands for the first organiceinorganic hybrid REcontaining phosphotungstates functionalized by nicotinate ligands. What's more, the solid-state photoluminescence properties and lifetime decay behaviors of 1e4 have been measured at room temperature and their photoluminescence spectra display the characteristic emission bands of corresponding trivalent RE cations. © 2018 Elsevier Masson SAS. All rights reserved.
Keywords: Rare-earth substituted polyoxometalate Phosphotungstate Nicotinic acid Photoluminescence
1. Introduction Polyoxometalates (POMs), as a charming class of metal oxide clusters with explicit sizes and shapes, have attracted considerable attention in recent years due to their potential applications in catalysis, medicine, nanotechnology, magnetism, sensing and materials science [1e6]. On the other hand, bearing abundant energy levels of 4f configurations, relatively larger ionic radii and high coordination numbers with flexible coordination geometries, RE cations are endowed with unique and fascinating optical properties [7,8]. Moreover, on account of their excellent photostability and long luminescence lifetimes, RE-based luminescent materials have been receiving consistent attention because of a wide range of applications such as in biomarkers, lighting, photovoltaic devices
* Corresponding author. ** Corresponding author. Henan Key Laboratory of Polyoxometalate Chemistry, Institute of Molecular and Crystal Engineering, College of Chemistry and Chemical Engineering, Henan University, Kaifeng, Henan 475004, China. E-mail addresses:
[email protected] (C. Zhai),
[email protected] (J. Zhao). https://doi.org/10.1016/j.solidstatesciences.2018.01.011 1293-2558/© 2018 Elsevier Masson SAS. All rights reserved.
and drug carriers [9,10]. POMs can be reliably utilized as fundamental building blocks to combine with RE ions for designing and preparing novel RE-containing POM-based materials (RCPBMs) because of their coordination ability of numerous accessible oxygen atoms, various compositions and manifold properties [11e14]. Therefore, in the field of POM chemistry, continuous exploration and discovery on RCPBMs has become an important and challengeable research topic. In the past two decades, overwhelming efforts have been contributed to synthesize novel RCPBMs with intriguing structures and properties. For instance, in 2001, Francesconi et al. utilized the reaction of lacunary Keggin-type [PW9O34]9 precursor with EuIII or YIII ions to prepare two tetraREIII incorporated tetrameric heteropolyoxometalates [(PRE2W10O38)4(W3O14)]30 (RE ¼ EuIII, YIII) and studied their fluorescence properties [15]. Subsequently, Mialane et al. reported a family of inorganic 1-D and 2-D silicotungstates based on building units {REn(a-SiW11O39)} (RE ¼ NdIII, EuIII, GdIII, YbIII), whose solid-state structures are strongly dependent on the nature of used RE cations [16]. In 2009, Niu's group separated the first monovacant Keggin phosphotungstate dimers [{(a-PW11O39H)RE(H2O)3}2]6 (RE ¼ NdIII, GdIII) constituted by two [a-PW11O39]7 polyoxoanions
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and two RE cations [17]. In 2014, Reinoso and co-workers isolated the Naþ-directing dimeric RCPBMs [RE4(H2O)6(b-GeW10O38)2]12 (RE ¼ GdIII, TbIII, DyIII, HoIII, ErIII, TmIII, YbIII, LuIII) and the Csþdirecting tetrameric RCPBMs [{RE4(H2O)5(GeW10O38)2}2]24 (RE ¼ HoIII, ErIII, TmIII, YbIII, LuIII) [18]. Above all, the majority of the previously reported examples are concentrated on the purely inorganic RCPBMs. However, the relevant investigations on organiceinorganic hybrid RCPBMs are relatively limited. In 2003, Kortz et al. succeeded in obtaining the first Dawson-type acetateconnecting mono-RE-substituted phosphotungstate [{La(CH3COO)(H2O)2(a2-P2W17O61)}2]16 utilizing the reaction of K12[aH2P2W12O48]$24H2O and LaCl3$7H2O in sodium acetate buffer [19]. In 2006, Mialane and co-workers discovered a novel tetrameric [{Yb(P2W17O61)}4(C2O4)3(H2O)4]34 complex constructed from four [Yb(H2O)4(P2W17O61)]7 units linked by three oxalate ligands [20]. In 2014, our group also synthesized peculiar oxalate-bridging lacunary Lindqvist-type RE-substituted isopolyoxotungstates [RE2(C2O4)(H2O)4(OH)W4O16]10 and {[RE(C2O4)W5O18]4}20 2 III III III III (RE ¼ Eu , Ho , Er , Tb ) through a one-step reaction procedure [21]. Whereafter, Niu's group isolated a family of tartrate-bridging RE-containing arsenotungstates [RE2(C4H4O6)(C4H2O6)(AsW9O33)]18 (RE ¼ HoIII, ErIII, TmIII, YbIII, 2 LuIII, YIII) [22]. It is conspicuous that the above RCPBMs are based on lacunary POM units, the reason of which is that lacunary POM units have more well-defined vacant sites and higher negative charges compared to plenary POM units, which might be beneficial to the assembly of nucleophilic lacunary POM units with RE ions to generate various RCPBMs [23,24]. Notably, the above-mentioned researches on organiceinorganic hybrid RCPBMs are mainly concentrated on the aliphatic acid ligands, such as acetate, tartrate and oxalate. However, to the best of our knowledge, there are only sporadic reports on RE-containing lacunary Keggin-type phosphotungstates with pyridine carboxylic acid ligands [25e27], which not only hints at the great difficulty and challenge in preparing these novel structures but also provides us an excellent opportunity to develop this domain. In this background, recently, we have launched the exploration on the reaction of the precursor Na9[a-PW9O34]∙16H2O, RE(NO3)3$6H2O, nicotinic acid (HNA) and dimethylamine hydrochloride to prepare organiceinorganic hybrid RE-containing Keggin-type phosphotungstates with pyridine carboxylic acid ligands by the aqueous solution method. As we expected, four organiceinorganic hybrid nicotinate-bridging REcontaining phosphotungstates [H2N(CH3)2]8[RE(H2O)(NA)(aHPW11O39)]2$24H2O (RE ¼ HoIII for 1, ErIII for 2, TbIII for 3, DyIII for 4) have been obtained, which represent the first nicotinate-bridging dimeric RE-containing phosphotungstates. Furthermore, the luminescence properties and the lifetime decay behaviors of solid-state 1e4 have been studied at room temperature and their luminescence spectra exhibit the characteristic emission bands of corresponding trivalent RE cations. 2. Experimental 2.1. Materials and physical methods The trivacant precursor Na9[a-PW9O34]∙16H2O was prepared according to the literature [28] and identified by IR spectra. All other chemical reagents were purchased and used without further purification. Elemental analyses (C, H and N) were performed on a PerkineElmer 2400-II CHN S/O analyzer. IR spectra were recorded from a solid sample pelletized with KBr pellets on a Nicolet 170 SXFT-IR spectrometer in the range of 4000e400 cm1. Thermogravimetric (TG) analyses were carried out on a Mettler-Toledo TGA/ SDTA 851e instrument in the temperature region of 25e800 C with a heating rate of 10 C∙min1 under the flowing nitrogen
atmosphere. Luminescence and lifetime spectra were obtained on a FLS 980 Edinburgh Analytical Instrument apparatus equipped with a 450 W xenon lamp and a mF900 high energy microsecond flash lamp as the excitation sources. 2.2. Syntheses of 1e4 2.2.1. Synthesis of 1 Na9[a-PW9O34]∙16H2O (1.000 g, 0.367 mmol) was dissolved in distilled water (15 mL) under stirring and the pH of the solution was adjusted to 3.0 using HCl (2 mol∙L1). After the solution was stirred for about 30 min, (CH3)2NH$HCl (0.700 g, 8.59 mmol), Ho(NO3)3$6H2O (0.190 g, 0.414 mmol) and HNA (0.115 g, 0.934 mmol) were successively added. After the pH value of the mixture solution was adjusted to 3.0 by NaOH (2 mol∙L1) and stirred for another 15 min. The final solution was kept in a 90 C water bath for 40 min and cooled to room temperature and filtered. Slow evaporation of the filtrate at room temperature led to light yellow bar-shaped crystals of 1 after one week. Yield: 32% based on Na9[a-PW9O34]∙16H2O. Anal. calcd (%) for 1: C, 4.97; H, 1.88; N, 2.07. Found (%): C, 5.16; H, 1.98; N, 2.21. 2.2.2. Synthesis of 2 The synthesis process of 2 is similar to 1 except that Er(NO3)3$6H2O (0.190 g, 0.412 mmol) was used instead of Ho(NO3)3$6H2O. After one week, light pink stick crystals of 2 were obtained in about 31% yield based on Na9[a-PW9O34]∙16H2O. Anal. calcd (%) for 2: C, 4.97; H, 1.88; N, 2.07. Found (%): C, 5.18; H, 2.03; N, 2.17. 2.2.3. Synthesis of 3 The synthesis process of 3 is similar to 1 except that Tb(NO3)3$6H2O (0.190 g, 0.419 mmol) was used instead of Ho(NO3)3$6H2O. After one week, colorless bar-shaped crystals of 3 were obtained in about 31% yield based on Na9[a-PW9O34]∙16H2O. Anal. calcd (%) for 3: C, 4.98; H, 1.88; N, 2.07. Found (%): C, 5.18; H, 1.99; N, 2.20. 2.2.4. Synthesis of 4 The synthesis process of 4 is similar to 1 except that Dy(NO3)3$6H2O (0.190 g, 0.416 mmol) was used instead of Ho(NO3)3$6H2O. After one week, colorless stick crystals of 4 were obtained in about 31% yield based on Na9[a-PW9O34]∙16H2O. Anal. calcd (%) for 4: C, 4.97; H, 1.88; N, 2.07. Found (%): C, 5.15; H, 2.01; N, 2.17. 2.3. X-ray crystallography Single-crystal X-ray diffraction intensity data for 1e4 were recorded on a Bruker APEX-II CCD diffractometer using graphite monochromatized Mo Ka radiation (l ¼ 0.71073 Å) at 296(2) K. Both structures were solved by direct methods and refined on F2 by full-matrix least-squares techniques using SHELXLe97 program package [29]. Routine Lorentz polarization and empirical absorption corrections were applied. All non-hydrogen atoms were refined anisotropically. The positions of hydrogen atoms attached to carbon atoms were geometrically placed. All hydrogen atoms were refined isotropically as a riding model using the default SHELXTL parameters. No hydrogen atoms associated with water molecules were located from the difference Fourier map. In the structure refinements of 1e4, four [H2N(CH3)2]þ ions and fourteen lattice water molecules were found from the Fourier maps. However, there are still solvent accessible voids in the check cif reports of crystal structures, suggesting that some counter cations and water molecules should exist in the structures that cannot be found
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from the weak residual electron peaks. These counter cations and water molecules are highly disordered and attempts to locate and refine them were unsuccessful. On the basis of charge-balance considerations, elemental analyses and TG analyses, another four [H2N(CH3)2]þ ions, two protons and ten water molecules were directly added to each molecular formula. This phenomenon is very common in POM chemistry [30,31]. The crystallographic data and structure refinements of 1e4 are summarized in Table 1. 3. Results and discussion 3.1. Structure description of 1e4 1e4 were synthesized by reaction of trivacant Keggin precursor Na9[a-PW9O34]∙16H2O, RE(NO3)3$6H2O, HNA and dimethylamine hydrochloride in the conventional aqueous solution system. We replaced the trivacant Keggin precursor with the monolacunary Keggin precursor under the similar conditions, however, we could not obtain the expected compounds. In such a case, the transformation of {PW9O34}9/{PW11O39}7 is necessary to prepare target compounds. Single-crystal X-ray diffraction analysis indicates that organiceinorganic hybrids 1e4 are isomorphic and crystallize in the monoclinic space group P2(1)/n. All of them possess a particular dimeric structure established by two mono-RE substituted Keggin [RE(H2O)(a-HPW11O39)]4 subunits bridged by two nicotinate bridges. Thus, only the structure of 1 is discussed here. The symmetrical molecular unit of 1 contains a REenicotinate-decorated Keggin-type phosphotungstate {[Ho(H2O)(NA)(a-HPW11O39)]2}8 hybrid dimeric core (Fig. 1a), eight discrete [H2N(CH3)2]þ cations and twenty-four lattice water molecules. The bond valence sum (BVS) calculations [32] manifest that the oxidation states of P, W and Ho elements in 1 are þ5, þ6 and þ 3, respectively (Table S1). Considering the charge balance, two protons need to compensate the negative charges. In addition, to localize probable binding sites of two protons, BVS calculations on all the oxygen atoms of the POM skeleton have been implemented (Table S2). The BVS value of O32 is 1.65, which is lower than 2, suggesting that this oxygen atom from the [a-HPW11O39]6e fragment is the possible site for binding proton, therefore, the molecular formula of 1 can be written as [H2N(CH3)2]8[RE(H2O)(NA)(a- HPW11O39)]2$24H2O. In the hybrid dimeric {[Ho(H2O)(NA)(a-HPW11O39)]2}8 core, two Ho3þ cations and two nicotinate-
Fig. 1. (a) Combined polyhedral and ball-and-stick view of {[Ho(H2O)(NA)(aHPW11O39)]2}8 core. (b) Combined polyhedral and ball-and-stick view of [Ho(H2O)(NA)(a-HPW11O39)]4 half-units. (c) The [a-PW11O39]7 segment in 1. (d) The trivacant Keggin precursor [a-PW9O34]9. (e) The m2-O and m3-O atoms in the {Ho2(NA)2}4þ cluster. (f) The square antiprismatic coordination environment of the Ho13þ ion in 1. Atoms with “A” in their labels are symmetrically generated (A: 2ex, 1ey, ez).
Table 1 Summary of crystallographic data and structural refinements for 1e4. Data
1
2
3
4
Empirical formula
C28H126Ho2N10 O108P2W22 6767.89 Monoclinic P2(1)/n 12.8793(12) 19.9154(18) 25.047(2) 90.00 101.894(2) 90.00 6286.4(10) 2 3.575 21.415 15 h 15 23 k 23 29 l 19 1.021 0.0477, 0.1246 0.0675, 0.1333
C28H126Er2N10 O108P2W22 6772.55 Monoclinic P2(1)/n 12.896(2) 20.008(3) 25.213(4) 90.00 101.879(3) 90.00 6366.2(18) 2 3.533 21.222 15 h 15 23 k 22 29 l 29 1.040 0.0411, 0.0970 0.0642, 0.1048
C28H126Tb2N10 O108P2W22 6755.87 Monoclinic P2(1)/n 12.8924(7) 19.8987(10) 25.0972(13) 90.00 101.9650(10) 90.00 6298.6(6) 2 3.562 21.240 15 h 15 20 k 23 29 l 29 1.018 0.0455, 0.1221 0.0667, 0.1310
C28H126Dy2N10 O108P2W22 6763.03 Monoclinic P2(1)/n 12.8641(9) 19.9130(15) 25.1373(18) 90.00 101.7240(10) 90.00 6304.9(8) 2 3.562 21.283 15 h 15 22 k 23 26 l 29 1.023 0.0489, 0.1261 0.0680, 0.1348
Formula weight Crystal system Space group a (Å) b (Å) c (Å) a (deg) b (deg) g (deg) V (Å3) Z Dc (g/cm3) m (mm1) Limiting indices
Goodness-of-fit on F2 R1, wR2 [I > 2s(I)] R1, wR2 [all data]
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bridging ligands in the central belt are sandwiched by two monovacant Keggin [a-HPW11O39]6 fragments. Moreover, the hybrid dimeric {[Ho(H2O)(NA)(a-HPW11O39)]2}8 core can be considered as two head-to-head monovacant [a-HPW11O39]6 units bridged by two Ho3þ ions with coordinating to two nicotinate chelator as well as a fusion of two equivalent mono-RE substituted Keggin [Ho(H2O)(NA)(a-HPW11O39)]4 half-units (Fig. 1b) through two HoeOeHo bridges with the Ho∙∙∙Ho distance of 4.128 Å. In the monovacant [a-PW11O39]7 fragment (Fig. 1c), there exists a central tetrahedral {PO4} group surrounded by three corner-shared {W3O13} trimers and an edge-shared {W2O10} dimer and three {WO6} octahedra from each {W3O13} trimer are fused together in the edge-shared mode. The occurrence of [a-PW11O39]7 indicates the transformation of the starting material [a-PW9O34]9 / [aPW11O39]7 fragment by two additional {WO6} octahedra incorporating into the defect sites of [a-PW9O34]9 (Fig. 1d), which probably because the trivacant [a-PW9O34]9 is actually metastable in acidic aqueous solution and can easily transform into the monolacunary [a-PW11O39]7 [33]. This phenomenon has been encountered when Hill and co-workers examined the stability of [PW9O34]9 [34]. Such similar cases have been also experienced in our group recently [35,36]. Interestingly, although both carboxyl oxygen atoms (O40 and O41) of nicotinate ligand play a bridging role in the connection of two [Ho(H2O)(NA)(a-HPW11O39)]4 units, the coordination modes of the two oxygen atoms are entirely different from each other: one oxygen atom (O40) possesses the m2O pattern bonding to the Ho13þ ion and one carboxyl carbon atom while the other oxygen atom (O41) possesses the m3-O pattern bonding to Ho13þ, Ho1A3þ ions and one carboxyl carbon atom (Fig. 1e). Furthermore, it is a remarkable fact that crystallographically unique Ho13þ cation binds to four terminal oxygen atoms (O3, O6, O13, O28) from the tetradentate [a-PW11O39]7 fragment [HoeO: 2.267(10)e2.298(9) Å], one carboxyl oxygen atom (O41A) from the other [Ho(H2O)(NA)(a-HPW11O39)]4 hybrid core and two oxygen atoms (O40, O41) from a bidentate-bridging NA ligand [HoeO: 2.421(10)e2.597(11) Å] and one O1W atom from one water molecule [HoeO: 2.371(10) Å], achieving an eight-coordinate distorted square antiprismatic coordination geometry (Fig. 1f). In the coordination configuration of the Ho13þ cation, the O40, O41, O41A, O1W group and O3, O6, O13, O28 group constitute the upper and bottom surfaces of the square antiprism and their standard deviations from the ideal planes are 0.0927 Å and 0.0045 Å, respectively. The dihedral angle for the upper and bottom planes is 3.7. The distances of the Ho13þ cation and the upper and bottom planes are 1.5499 Å and 1.0598 Å, respectively. These structural data indicate that the square antiprismatic geometry is somewhat distorted. On further inspection, it can be found that each dimeric {[Ho(H2O)(NA)(a-HPW11O39)]2}8 units interacts with adjacent dimeric units to propagate the 1-D linear chain via the hydrogen bonding interactions between the N atoms of discrete [H2N(2)(CH3)2]þ cations and the surface O atoms of the [aPW11O39]7 fragments with NH∙∙∙O distances of 2.69(4)3.04(3) Å (Fig. 2a). Furthermore, we can also see the 1-D linear chains are aligned in a parallel mode on the ac plane (Fig. 2bec). More interestingly, it can be seen from the 3-D packing arrangement of 1 (Fig. 3a) that the 1-D linear chains exhibit an arrangement mode of eABABe in the stagger fashion along the b axis, which may be in order to reduce steric hindrance as much as possible and favor to the closest packing of {[Ho(H2O)(NA)(a-HPW11O39)]2}8 units. Besides, the packing mode of 1 along a axis can be simplified to the structure in Fig. 3b and the simplified {[Ho(H2O)(NA)(aHPW11O39)]2}8 unit displays an amusing chair configuration. In a word, those hydrogen bonds between proton donors provided by the hydrogen atoms of discrete [H2N(CH3)2]þ cations and proton
Fig. 2. (a) The view of the 1-D linear chain in 1. (b) The stacking view of 1 in the ac plane. (c) The simplified stacking view of 1 in the ac plane.
acceptors from the surface oxygen atoms of phosphotungstate fragments not only link dimeric {[Ho(H2O)(NA)(a-HPW11O39)]2}8 units into a 1-D structure but also may be the key factor in increasing the stability of structure. 3.2. IR spectra of 1e4 IR spectra of 1e4 have been obtained by utilizing KBr pellets on a Nicolet 170 SXFT-IR spectrometer (Fig. S1). It is obvious that the similar characteristic peaks for the skeletal vibrations between 400 and 1100 cm1 indicate that 1e4 contain the same polyoxoanion skeleton. It should be emphasized that the characteristic vibration patterns resulting from monovacant Keggin [a-PW11O39]7 fragments have been detected. The two characteristic peak at 1094e1090 cm1 and 1048e1046 cm1 for 1e4 can be assigned to the stretching vibration of nas(PeOa) in the central PO4 unit, and the resonances for the absorption bands at 888e885 cm1, 827e823 cm1, 799e797 cm1 and 756e754 cm1 for 1e4 are ascribed to the nas(WeOb,c) stretching vibrations [37]. As for the peak at 951e950 cm1 for 1e4 are ascribed to the nas(WeOt) stretching vibration. While in the high-frequency region (n > 1100 cm1), two characteristic vibration bands attributable to the n(NH2) and n(CH2) vibrations appear at 3161e3159 cm1 and 2798e2794 cm1 and three characteristic vibration bands at 1466e1465 cm1, 1355e1353 cm1 and 1018e1017 cm1 for 1e4
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Fig. 4. The TG curves of 1e4.
water molecules, the dehydration of eight protons, eight dimethylamine molecules and two nicotinic acid ligands. With respect to 3 and 4, the first weight loss of 5.73% (calcd. 6.40%) for 3 and 6.20% (calcd. 6.39%) for 4 from 25 to 150 C are assigned to the loss of twenty-four lattice water molecules. From 150 to 800 C, the second weight loss of 9.75% (calcd. 9.64%) for 3 and 9.59% (calcd. 9.63%) for 4 involve the liberation of two coordinate water molecules, the dehydration of eight protons, eight dimethylamine molecules and two nicotinic acid ligands. Markedly, the observed experimental values are in approximate consistence with the theoretical values. Fig. 3. (a) The 3-D packing arrangement of 1 along the a axis. (b) The simplified 3-D packing of 1 along the a axis and the simplified chair configuration of {[Ho(H2O)(NA)(a-HPW11O39)]2}8 core.
can be respectively assigned to the d(NH2), d(CH2) and nas(CeN) vibrations, confirming the presence of dimethylamine components in 1e4 [38]. Besides, the occurrence of the vibration bands at 1423e1420 and 1578e1574 cm1 for 1e4 imply the presence of the carboxylate groups [39]. Finally, the broad band centered at 3452e3445 cm1 for 1e4 is attributed to the lattice or coordinate water molecules. These IR spectral results agree well with the crystallographic structural analyses.
3.3. TG properties of 1e4 To investigate the thermal stability of 1e4, their TG analyses were performed under the flowing dry nitrogen atmosphere with temperature ranging from 25 to 800 C. As shown in Fig. 4, the TG curves of 1e4 exhibit two-step weight loss process. In terms of 1 and 2, the first weight loss of 5.89% (calcd. 6.38%) for 1 and 6.67% (calcd. 6.79%) for 2 from 25 to 150 C are assigned to the removal of twenty-four lattice water molecules. With further heating, the second weight loss of 9.91% (calcd. 9.85%) for 1 and 9.72% (calcd. 10.54%) for 2 from 150 to 800 C are attributed to two coordinate
3.4. PL properties of 1e4 In the past several decades, due to the fact that RE cations possess the unique spectroscopic properties with long luminescent lifetimes, excellent photostability, well-defined absorption and emission bands with high quantum yields, startling advances for RE-based luminescence materials (REBLMS) are stimulated by their continuously expanding applications and the urgent requirements of telecommunications, light emitting diodes, NIR-emitting materials, electroluminescent devices, biomedical imaging and sensing [40,41]. Hence, PL properties and lifetime decay behaviors of the solid state samples of 1e4 have been investigated at room temperature. Under excitation at 452 nm, the emission spectrum of 1 exhibits two characteristic bands with maxima at 532 and 660 nm, which are respectively assigned to the transitions from 5F4/5S2 and 5 F5 levels to the 5I8 ground state of the Ho3þ cation (Fig. 5a). The excitation spectrum of 1 was also obtained by monitoring the emission wavelength of Ho3þ (5F5/5I8) at 660 nm (Fig. 5b). It can be observed that excitation spectrum of 1 is dominated by two bands at 419 and 452 nm, which are ascribed to the transitions from the ground state 5I8 to excited levels 5G5 and 5G6 of the Ho3þ cation [42]. In order to obtain the decay lifetime, the luminescence decay curve of 1 has been measured upon monitoring the excitation at 452 nm and the most intense emission at 660 nm (Fig. 5c), which is fitted successfully by a double exponential function as the equation
12
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Fig. 5. (a) The emission spectrum of 1 under excitation at 452 nm at room temperature. (b) The excitation spectrum of 1 obtained by monitoring the emission wavelength at 660 nm. (c) The luminescence decay curve of 1. (d) The emission spectrum of 2 under excitation at 390 nm at room temperature. (e) The excitation spectrum of 2 obtained by monitoring the emission wavelength at 556 nm. (f) The luminescence decay curve of 2. (g) The emission spectrum of 3 under excitation at 378 nm at room temperature. (h) The excitation spectrum of 3 obtained by monitoring the emission wavelength at 544 nm. (i) The luminescence decay curve of 3. (j) The emission spectrum of 4 under excitation at 388 nm at room temperature. (e) The excitation spectrum of 4 obtained by monitoring the emission wavelength at 574 nm. (f) The luminescence decay curve of 4.
I ¼ A1exp(-t/t1) þ A2exp(-t/t2), where I represents the luminescence intensity, A1 and A2 are the pre-exponential factors, t is the time, and t1 and t2 are the fast and slow components of the luminescence lifetimes. The affording lifetimes t1 and t2 are 0.85 ms (50.03%) and 9.16 ms (49.97%). Meanwhile, the average decay time (t*) can be calculated by the following equation t* ¼ (A1t21 þ A2t22)/(A1t1 þ A2t2) and the calculated t* for 1 is 5.00 ms. As depicted in Fig. 5d, the emission spectrum of 2 displays four typical emission peaks of the Er3þ ion at 534, 556, 676 and 698 nm under excitation at 390 nm ultraviolet light, which are ascribed to 2H11/ 4 4 4 2 S3/2 / 4I15/2, F9/2 / 4I15/2 and H9/2 / 4I11/2 2 / I15/2,
transitions, respectively. By monitoring the most intense emission 4 S3/2 / 4I15/2 transition at 556 nm, the excitation spectrum of 2 has been obtained (Fig. 5e). The excitation spectrum of 2 is dominated by a broad peak from 382 nm to 397 nm with a maximum at 387 nm, which is attributable to the 4I15/2 / 4G11/2 transition [43]. Similarly, the measured luminescence decay curve of 2 (Fig. 5f) was fitted successfully with the double exponential function and the affording lifetimes t1 and t2 are 0.98 ms (48.90%) and 9.29 ms (50.10%) with the average lifetime of 5.22 ms. The solid-state emission spectrum of 3 under excitation at 378 nm displays four obvious emission peaks at 488, 544, 585 and 623 nm (Fig. 5g),
P. Gong et al. / Solid State Sciences 78 (2018) 7e15
which respectively correspond to the 5D4 / 7F6, 5D4 / 7F5, 5D4 / 7 F4 and 5D4 / 7F3 transitions of the Tb3þ cation [44]. In addition, the excitation spectrum of 3 was also monitored under the excitation at 378 nm and the more intense emission at 544 nm (5D4/7F5) of the Tb3þ ion (Fig. 5h), in which the broad excitation band results from the 7F5/5D3 transition (378 nm) of the Tb3þ ion [45]. To further determine the lifetime, the luminescence decay curve of 3 has also been recorded (Fig. 5i), which can be well fitted to a single exponential function [I ¼ Aexp(et/t)], yielding the lifetime value (t) of 434.53 ms and the pre-exponential factor (A) of 2598.26. When 4 is excited at 388 nm, its PL emission spectrum (Fig. 5j) exhibits three characteristic emission bands, which are assigned to the fef transitions of the Dy3þ ion, namely, the magnetic dipole transition (4F9/2 / 6H15/2) at 478 nm, the electric dipole transition (4F9/2 / 6H13/2) at 574 nm, and a weak emission band (4F9/2 / 6H11/2) at 662 nm [46]. By monitoring the emission at 574 nm, the excitation spectrum of 4 was obtained (Fig. 5k) and includes a sharp peaks at 388 nm, which is assigned to the transitions of the Dy3þ ion from the ground 6H15/2 to the higher energy levels of 4I13/2. The decayetime curve of 4 was monitored under the most intense emission at 574 nm(Fig. 5l), which can be well fitted with a second-order exponential function. The fitting lifetimes are t1 ¼ 8.07 ms (56.18%) and t2 ¼ 55.37 ms (43.82%) and the average lifetime was calculated to be t* ¼ 28.8 ms. As we all know, in the UVvisible region, the intramolecular energy transfer of the oxygen-tometal (O / W) charge-transfer transitions can sensitize the emission of RE3þ ions in the reported RCPBMs [46,47]. Thus, in order to detect whether the intramolecular energy transfer between the Ho3þ ions and the phosphotungstate fragments in 1 occurs in the UVvisible region, time-resolve emission spectrum (TRES) measurements were carried out under excitation at 452 nm with the emission wavelength from 500 to 700 nm. The TRES shows two characteristic bands of Ho3þ ion, whose peak shape remains essentially unchanged apart from a steady decrease in intensity at successive decay times from 11 to 58.2 ms (Fig. 6). This phenomenon declares that there are no intermolecular O / W ligand-to-metal charge transfer sensitizing the luminescence of Ho3þ cations during the emission course of 1 [48]. A better understanding of the true color is significant in the applications of lighting and display devices [49]. In order to determine the specific color produced by 1e4, the CIE coordinate diagram was obtained (Fig. 7) and their chromaticity coordinate are (0.55696, 0.43075), (0.46762, 0.52596), (0.31558, 0.53665) and (0.37554, 0.44882) for 1e4, which suggest that 1e4 emit orange, yellow green, yellow green and green fluorescence, respectively. Lately, the near-infrared (NIR) RE emitters especially for Nd3þ,
13
Fig. 7. The CIE chromaticity diagram of the emissions of 1e4.
Sm3þ, Ho3þ, Er3þ and Yb3þ ions have attracted great interest due to their possible applications in photonics, telecommunication, biomedical and so on [50e52]. As a consequence, in this paper, NIR PL properties and lifetime decay behaviors of 1 and 2 have been also measured at ambient temperature. Upon excitation at 452 nm, the NIR emission spectrum of 1 shows a characteristic emission band at 977 nm (Fig. 8a), which can be attributed to the 5F5 / 5I7 transition of the Ho3þ ion [53]. In addition, by monitoring the emission at 997 nm, the excitation spectrum of 1 has been collected, which is dominated by a strong band at 452 nm corresponding to the 5I8 / 5 G6 transition of the Ho3þ ion (Fig. 8b). Moreover, the NIR luminescence lifetime curve of 1 was collected, which can be also well fitted to the double exponential function affording the rapid and slow decays as 0.72 ms (42.35%) and 9.14 ms (57.65%), respectively (Fig. 8c). The average lifetime is about 5.58 ms. Upon excitation at 380 nm, the characteristic emission band of 2 appears at 1537 nm (Fig. 8d) that is attributed to the 4I13/2 / 4I15/2 transition of the Er3þ ion [54]. The excitation spectrum of 2 was collected by monitoring the emission at 1537 nm and exhibits a broad band at 380 nm (Fig. 8e), which stems from the transition of 4I15/2 / 4G11/2. The lifetime decay behavior of 2 (Fig. 8f) also abides by the double exponential function affording the rapid and slow decays as 0.86 ms (37.22%) and 9.34 ms (62.78%). The average lifetime was calculated to be 6.18 ms.
4. Conclusion
Fig. 6. The time-resolve emission spectrum of 1.
In this paper, four organiceinorganic hybrid nicotinate-bridging dimeric RE-containing phosphotungstates [H2N(CH3)2]8[RE(H2O)(NA)(a-HPW11O39)]2$24H2O (RE ¼ HoIII for 1, ErIII for 2, TbIII for 3, DyIII for 4) have been made in aqueous solution and characterized by elemental analyses, IR spectra, TG analyses and single-crystal Xray diffraction. Furthermore, the visible and near-IR photoluminescence properties and the lifetime decay behaviors of 1e4 have been investigated and discussed in detail. In summary, this work not only enriches the structural diversity of organiceinorganic hybrid RCPBMs, but also extended the potential applications of RCPBMs in the territory of RE luminescent materials. These research fruits will drive us to persistently explore and
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P. Gong et al. / Solid State Sciences 78 (2018) 7e15
Fig. 8. The NIR emission spectrum of 1 under excitation at 452 nm. (b) The excitation spectrum of 1 obtained by monitoring the emission wavelength at 977 nm. (c) The NIR lifetime decay curve of 1. (d) The NIR emission spectrum of 2 under excitation at 380 nm. (e) The excitation spectrum of 2 obtained by monitoring the emission wavelength at 1537 nm. (f) The NIR lifetime decay curve of 2.
prepare much more fascinating RCPBMs by means of the appropriate choice of different lacunary POM precursors and various organic ligands. The ongoing work and unremitting efforts in this area are in progress in our lab. Acknowledgements This work was supported by the Natural Science Foundation of China (21571048, 21771052), the Innovation Scientists and Technicians Troop Construction Projects of Henan Province (174100510016), the Program for Science & Technology Innovation Talents in Universities of Henan Province (16HASTIT001), the Foundation of State Key Laboratory of Structural Chemistry (20160016), the 2014 Special Foundation for Scientific Research Project of Henan University (XXJC20140001) and the Students Innovative Pilot Plan of Henan University (2017) (2017N018). Appendix A. Supplementary data Supplementary data related to this article can be found at https://doi.org/10.1016/j.solidstatesciences.2018.01.011. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
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