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PAPER View Article OnlineView Journal | View Issue
CrystEngCommThis journal is © The Royal Society of Chemistry 2014
a Laboratory of Coordination Chemistry, Shenyang University of Chemical
Technology, Shenyang, 100142, PR China. E-mail: [email protected] LumiLab, Department of Solid State Sciences, Ghent University, Ghent, 9000,
Belgium. E-mail: [email protected]
† Electronic supplementary information (ESI) available. CCDC 957777–957783.For ESI and crystallographic data in CIF or other electronic format see DOI:10.1039/c3ce42197j
Cite this: CrystEngComm, 2014, 16,
1777
Received 30th October 2013,Accepted 6th December 2013
DOI: 10.1039/c3ce42197j
www.rsc.org/crystengcomm
A family of 3D lanthanide–organic frameworksconstructed from parallelogram secondarybuilding units: synthesis, structures and properties†
Ya-guang Sun,*a Jian Li,a Ke-long Li,a Zhen-he Xu,a Fu Ding,a Bao-yi Ren,a
Shu-ju Wang,b Li-xin You,a Gang Xionga and Philippe F. Smet*b
A family of lanthanide–organic frameworks, {Ln2(pyip)(ox)2.5(H2O)5}n (Ln = La (1), Pr (2), Sm (3), Eu (4), Gd
(5), Tb (6), Dy (7)) has been synthesized based on mixed bridging ligands, H2pyip and oxalate anion
(H2pyip = 5-(4-pyridyl)-isophthalic acid, H2ox = oxalic acid) and characterized by elemental analysis,
Fourier transform infrared spectroscopy, steady state and time-resolved luminescence spectroscopy,
thermogravimetric analysis, powder X-ray diffraction and single-crystal X-ray diffraction. The single-crystal
X-ray diffraction analysis results show that complexes 1–7 are isostructural. They crystallize in the same
P21/c space group and exhibit a (3,4)-connected {42·6·82·10}{82·10} topology, being a 2-nodal net
topology. The luminescence properties of 3, 4, 6, and 7 were investigated and related to the structural
properties. The magnetic studies reveal that strong ferromagnetic coupling exists in compound 6.
Introduction
Considerable experimental and theoretical efforts have beenfocused on the design and synthesis of metal–organic frame-works (MOFs), mainly because of their intriguing structures,1
interesting physical and chemical properties2 and potentialapplications in photoluminescence,3 magnetism,4 catalysis5
and gas adsorption.6 Many types ofMOFs have been constructedwith varying combinations of metal ions and organic linkers.7
In contrast with the prolific production of MOFs based ontransition metal ions, the design and synthesis of multi-dimensional lanthanide frameworks is currently a challengingtask due to the typically unspecific coordination properties oflanthanide ions.8 However the MOFs containing lanthanideions usually provide new materials with special properties,9
recently leading to considerable research efforts into theseMOFs. The electronic transitions within the well-shielded 4fn
configuration of the lanthanide ions yield emission linescovering the visible and near to mid-infrared parts of the opti-cal spectrum. The disadvantage of most lanthanide ions,namely, the difficult excitation due to low absorption strength,can be overcome by sensitization via appropriately chosen
ligands.10 Several specific luminescence-based applications oflanthanide containing MOFs have been proposed, such asthermometry, small molecule sensing, as pH sensors and labelsin multiplexed bioanalytical assays and as encryption tags.11
For MOF construction, the selection of ligands is veryimportant. Among the ligands, one should select multi-functional ligands which have versatile coordination modesand potential hydrogen-bonding donors and acceptors.12 Asan example, 5-(4-pyridyl)-isophthalic acid (H2pyip) containstwo carboxylate groups, which can provide more coordinationsites to form polymers. In addition, the pyridyl can comple-ment the carboxylate ligands to construct high-connectednets.13 Furthermore, the introduction of an auxiliary ligandwould greatly facilitate the formation of various lanthanidemixed-ligand polymers. Here we choose the “shorter” and“more simple” organic ligand, oxalate, as the auxiliary ligand.This strategy is based on the following considerations. Firstly,we aim at bridging the adjacent Ln ions, because the oxalatecan play an important role in forming a chain.14 Secondly,it can improve the solubility of the rare earth oxide. Finally,the coordination with two types of ligands for the lanthanideionmay reduce the number of water molecules in the coordina-tion sphere of the lanthanide(III) ion. This reduces the vibra-tional quenching, hence increasing the luminescent intensity.15
On the basis of the above considerations, we choose the5-(4-pyridyl)-isophthalic acid (H2pyip) as the primary ligand andintroduce the oxalate anion as the auxiliary ligand. The reactionwith the lanthanide oxide then leads to Ln2(pyip)(ox)2.5(H2O)5(Ln = La (1), Pr (2), Sm (3), Eu (4), Gd (5), Tb (6), Dy (7)).
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Experimental sectionMaterials and physical measurements
All chemicals were of reagent grade and used without furtherpurification. All syntheses were carried out in 23 mL Teflon-lined autoclaves under autogenous pressure. Water used inthe syntheses was distilled before use. The C, H, and N analyseswere carried out using a Vario MICRO E III elemental analyzer.IR spectra were recorded in the range 400–4000 cm−1 witha Perkin-Elmer Spectrum spectrometer using KBr pellets.Thermogravimetric analyses (TGA) were performed on a SDTQ600 instrument with a heating rate of 10 °C min−1. PowderX-ray diffraction (XRD) patterns were collected on a RigakuMini Flex II diffractometer using Cu Kα radiation (λ = 1.54056 Å)under ambient conditions. Solid-state photoluminescence spec-tra were measured at room temperature using an EdinburghFS920 fluorescence spectrometer. The instrument is equippedwith a Xe900 xenon arc lamp as the excitation light source.All measurements were taken at room temperature, unlessotherwise mentioned. Measurements at low temperature wereperformed using an Optistat CF cryostat (Oxford Instruments).Luminescence decay measurements were obtained using apulsed nitrogen laser (excitation wavelength of 337 nm, pulselength of 800 ps, repetition rate of 1 Hz) and a 1024-channelintensified CCD (Andor Technology) attached to a 0.5 m Ebertmonochromator. Variable-temperature magnetic susceptibili-ties were measured on a Quantum Design MPMS-7 SQUID mag-netometer. Diamagnetic corrections were made with Pascal'sconstants for all constituent atoms.
Syntheses of complexes [Ln2(pyip)(ox)2.5(H2O)5]n
A mixture of H2pyip (0.4 mmol), Ln2O3 (0.15 mmol, Ln = La,Pr, Sm, Eu, Gd, Tb, Dy), Na2C2O4 (0.3 mmol), water (10 mL)and two drops of HClO4 was sealed in a 23 mL Teflon reactorand was kept under autogenous pressure at 160 °C for 3 days,then cooled to room temperature over 2 days. Well shapedblock crystals were obtained for all compounds. Finally, thecrystals were washed with deionized water.
1. Yield 55.6% (based on La2O3). IR spectrum (KBr, cm−1):3438m, 3265m, 1620s, 1570m, 1537m, 1513m, 1449m,1422m, 1357m, 1306m, 1236w, 1175w, 1080w, 917w, 877w,844w, 781m, 729m, 656m, 609w, 462w. Elem. anal. calcd. for1 (%): C, 26.07; H, 2.07; N, 1.69. Found: C, 25.49; H, 2.01; N, 1.60.
2. Yield 60.3% (based on Pr6O11). IR spectrum (KBr, cm−1):3415w, 3057w, 1707s, 1631s, 1512w, 1445w, 1407w, 1359w,1269w, 1175s, 930w, 882w, 839m, 793w, 764m, 724m, 674s,656w, 608w, 568w. Elem. anal. calcd. for 2 (%): C, 25.95; H,2.06; N, 1.68. Found: C, 25.72; H, 2.01; N, 1.66.
3. Yield 65.9% (based on Sm2O3). IR spectrum (KBr, cm−1):3448m, 3080m, 1686s, 1635s, 1613s, 1547m, 1505m, 1455w,1421w, 1391m, 1314m, 1242w, 1222w, 1174w, 788m, 769w,732w, 673w, 656w, 609w, 578w, 493w. Elem. anal. calcd. for 3(%): C, 25.37; H, 2.01; N, 1.64. Found: C, 25.24; H, 1.99; N, 1.50.
4. Yield 59.9% (based on Eu2O3). IR spectrum (KBr, cm−1):3423m, 3056w, 1702s, 1652m, 1630m, 1560w, 1512w, 1445w,
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1407w, 1315w, 1269w, 1174s, 929w, 913w, 881w, 838m, 793w,763m, 722m, 674m, 656m, 607w, 568m. Elem. anal. calcd. for4 (%): C, 25.28; H, 2.00; N, 1.64. Found: C, 25.20; H, 1.85;N, 1.58.
5. Yield 73.0% (based on Gd2O3). IR spectrum (KBr, cm−1):3424m, 3059m, 1701m, 1632s, 1617s, 1573m, 1512w, 1444m,1421m, 1359m, 1315w, 1296w, 1175m, 930w, 915w, 878w,839w, 827w, 789w, 764m, 723w, 675m, 656w, 606w, 569w,496w. Elem. anal. calcd. for 5 (%): C, 24.97; H, 1.98; N, 1.62.Found: C, 24.83; H, 1.88; N, 1.57.
6. Yield 57.2% (based on Tb4O7). IR spectrum (KBr, cm−1):3446m, 3085m, 1689s, 1616s, 1551m, 1506m, 1458m, 1423m,1394m, 1316m, 1243w, 1223w, 1176w, 935w, 879w, 789s,770m, 734w, 651w, 579w, 492w. Elem. anal. calcd. for 6 (%):C, 24.87; H, 1.97; N, 1.61. Found: C, 24.81; H, 1.91; N, 1.53.
7. Yield 66.7% (based on Dy2O3). IR spectrum (KBr, cm−1):3439m, 3084m, 1701s, 1633s, 1552m, 1512m, 1458m, 1422m,1394m, 1316m, 1175m, 930w, 880w, 838w, 789m, 764m,732w, 656w, 610w, 568w, 492w. Elem. anal. calcd. for 7 (%):C, 24.67; H, 1.96; N, 1.60. Found: C, 24.74; H, 1.91; N, 1.53.
X-ray diffraction studies
All data collections were carried out at 293 K on a BrukerSMART Apex CCD diffractometer with Mo-Kαmonochromatedradiation (λ = 0.71073 Å) using the ω–2θ scan technique. Anempirical absorption correction was applied. The structureswere solved by direct methods and refined by full-matrix least-squares against F2 using the SHELXTL crystallographic soft-ware package.16 Anisotropic thermal parameters were assignedto all non-hydrogen atoms. Hydrogen atoms were placed in cal-culated positions and refined using a riding mode. Analyticalexpressions of neutral atom scattering factors were employed,and anomalous dispersion corrections were incorporated. Forthe crystal data of compound 1, the large residue electron den-sities are around the La ions with distances of about 1 Å; thesecan be series termination errors. Full crystallographic data,selected bond lengths and angles for 1–7 are listed in Tables 1, 2and S1 (see ESI†), respectively.
Results and discussionStructural analysis of [Ln2(pyip)(ox)2.5(H2O)5]n
Single-crystal X-ray analysis revealed the seven complexes tobe isostructural, therefore only the structure of 4 is discussedin detail. Fig. 1 shows the coordination environment of Eu(III)ions in 4. The asymmetric unit of 4 contains two Eu(III) ions,one H2pyip ligand, two and a half ox ligands and five coordi-nating water molecules. The coordination geometry of thetwo Eu(III) ions in complex 4 is different. Eu1(III) is surroundedby nine oxygen atoms and exhibits a bicapped trigonalprismatic geometry, with the Eu1(III) atom in the center. Sixoxygen atoms (O7, O8, O11, O12,O13, O14) originate fromthree different ox ligands, while three oxygen atoms are fromcoordinated water molecules (O17, O18, O19). O13, O17,O18, O19, O7 and O11 form a trigonal prism. O8 and O12act as the capping atoms. Eu2(III) is also nine-coordinated
This journal is © The Royal Society of Chemistry 2014
Table 1 Crystal data and structure refinement for 1–4
Compound 1 2 3 4
Formula C18H18 La2NO19 C18H18 Pr2NO19 C18H18 Sm2NO19 C18H18Eu2NO19
Fw 830.15 834.15 853.03 856.25Temp (K) 293(2) 293(2) 293(2) 293(2)Crystal system Monoclinic Monoclinic Monoclinic MonoclinicSpace group P21/c P21/c P21/c P21/ca (Å) 6.8098(14) 6.7147(13) 6.7129(13) 6.6740(13)b (Å) 31.143(6) 30.942(6) 30.855(6) 30.729(6)c (Å) 13.070(4) 12.909(4) 12.888(4) 12.805(4)α (°) 90 90 90 90β (°) 120.24(2) 120.14(2) 120.73(2) 120.67(2)γ (°) 90 90 90 90V (Å)3 2394.7(10) 2319.4(10) 2294.6(10) 2258.8(9)Z 4 4 4 4λ (Å) 0.71073 0.71073 0.71073 0.71073ρc (g cm−3) 2.303 2.389 2.469 2.518μ/mm−1 3.617 4.252 5.170 5.606F(000) 1596 1612 1636 1644Reflections collected/unique
18 027/4131[R(int) =0.1499]
22 648/5336[R(int) =0.0914]
15 760/4006 [R(int) =0.0784]
17 613/3966 [R(int) =0.0384]
GOF on F2 1.082 1.105 1.183 1.121R1 [I > 2σ(I)] 0.1078 0.0405 0.0588 0.0280wR2 [I > 2σ(I)] 0.2906 0.0942 0.1404 0.0672
Table 2 Crystal data and structure refinement for 5–7
Compound 5 6 7
Formula C18H18Gd2NO19 C18H18Tb2NO19 C18H18Dy2NO19
Fw 866.83 870.17 877.33Temp (K) 293(2) 293(2) 293(2)Crystal system Monoclinic Monoclinic MonoclinicSpace group P21/c P21/c P21/ca (Å) 6.6750(13) 6.6660(13) 6.6610(13)b (Å) 30.726(6) 30.662(6) 30.580(6)c (Å) 12.841(4) 12.798(4) 12.763(3)α (°) 90 90 90β (°) 120.81(2) 120.79(2) 121.00(2)γ (°) 90 90 90V (Å)3 2262.0(9) 2247.1(9) 2228.4(8)Z 4 4 4λ (Å) 0.71073 0.71073 0.71073ρc (g cm−3) 2.545 2.572 2.615μ/mm−1 5.916 6.347 6.759F(000) 1652 1660 1668Reflections collected/unique 21 727/5184 [R(int) = 0.0796] 22 910/5168 [R(int) = 0.1125] 16 230/5030 [R(int) = 0.0777]GOF on F2 1.179 1.105 1.100R1 [I > 2σ(I)] 0.0605 0.0565 0.0522wR2 [I > 2σ(I)] 0.1450 0.1277 0.1408
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but the coordination geometry around it can be described asa distorted monocapped tetragonal antiprism, with theEu2(III) in the center. The vertices of the tetragonal antiprismare all occupied by the oxygen atoms. Four oxygen atomsbelong to ox ligands (O5, O6, O9, O10), three oxygen atomsoriginate from two different H2pyip ligands (O1A, O3, O4)and two oxygen atoms are from two coordinated water mole-cules (O15, O16), as shown in Fig. 1. O1A, O16, O3, O10 arealmost in a plane; O6, O5, O4, O9 are almost in a plane, too.The two planes form a tetragonal antiprism. O15 acts as thecapping atom. The Eu–O bond lengths are in the range of2.310(4)–2.531(4) Å, the O–Eu–O bond angles are in the range
This journal is © The Royal Society of Chemistry 2014
of 52.17(12)–151.97(14)° (Table S1†) which are similar tothose for other Ln carboxylate complexes described inliterature.17
Two adjacent Eu1(III) are bridged by ox ligands to form aone-dimensional chain along the c axis as shown in Fig. 2(a),represented by the green chain. Eu1(III) and Eu2(III) are alsobridged by the oxygen atoms (O5, O6, O7, O8), leading to ashort ox chain represented by the pink chain (Fig. 2(a)). Thetwo kinds of ox chains connect together to construct a paral-lelogram secondary building unit. The secondary buildingunits further link to each other by ox ligands to form a euro-pium oxalate layer structure in the bc plane. Notably, the rare
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Fig. 1 The coordination environments of Eu(III) ions in 4. Hydrogen atoms are omitted for clarity.
Fig. 2 The transformation from 2D layers to 3D frameworks. (a) The 2D layers structure of complex 4; (b) The 3D frameworks structure of complex 4.
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earth oxalate layer has the shape of a parallelogram, which isdifferent from the circular layers reported earlier.18 The dif-ference is owing to the existence of the H2pyip ligands. EachH2pyip ligand takes (κ1-κ1-μ1)-(κ1-μ1)-μ2 coordination modesto link two Eu2(III) atoms. One is in the plane formed by theeuropium oxalate layers, the other one is out-of-plane. Botheuropium oxalate layers connect together by H2pyip ligandsto generate a three-dimensional framework (Fig. 2(b)). Froma topological point of view, Eu1(III) can be defined as a4-connected node and Eu2(III) can be defined as a3-connected node, so the topology of 4 can be described as a{42·6·82·10}{82·10} topological network; it is a 2-nodal nettopology (Fig. 3).
PXRD and thermal analyses
Powder X-ray diffraction (PXRD) analysis of compounds 1–7was performed at room temperature (see Fig. S1–S7 in ESI†).
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The patterns for 1–7 are in good agreement with the calcu-lated patterns obtained from the single-crystal structures,indicating that the single-crystal structure is really represen-tative for the main constituent of the correspondingsamples.
Thermogravimetric (TG) analysis was performed in N2
atmosphere on polycrystalline samples of compounds 1–7,and the TG curves are shown in Fig. 4. Since the curves ofthe seven compounds are similar, only complex 6 will bediscussed in detail as a representative compound. The firstweight loss of complex 6 from 150 °C to 300 °C is about10.17%, corresponding to the loss of the coordinated watermolecules (calcd: 10.36%). Above 300 °C, the frameworkgradually began to break down. The weight loss could beattributed to the decomposition of the organic ligand, with aresidual weight of ca. 42.11%, slightly lower than the theoret-ical calculation yielding 43.01%.
This journal is © The Royal Society of Chemistry 2014
Fig. 3 The topology framework of 4. The red sites represent theEu1(III) and the blue sites represent Eu2(III).
Fig. 4 The TG curves of compounds 1–7.
Fig. 5 Emission and excitation spectra for 4, 6 and 7 at roomtemperature. Emission spectra (thin lines) were recorded uponexcitation at 335 nm. Excitation spectra (thick lines) were obtained bymonitoring the peak emission wavelength. The inset shows the 5D0–
7F1transition at 70 K for 4 upon excitation at 335 nm.
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Luminescent properties
The trivalent lanthanide (Ln3+) ions are well-known lumines-cent ions. This is due to the splitting of the 4f1−13 configura-tion into 2S+1LJ spectroscopic levels, caused by Coulombrepulsion and spin–orbit coupling. Each of these states isfurther split into 2J + 1 Stark sublevels by the crystal field.Except for Ce3+, their excitation and emission spectra are ingeneral characterized by narrow bands (typical width of a fewnm) originating from internal 4fn–4fn electronic transitionsbetween the 2S+1LJ energy levels. The emission and excitationspectra for 4, 6 and 7 are shown in Fig. 5. The luminescenceintensity for the Sm3+ transitions in 3 is relatively weak. 4 ischaracterized by the typical orange-to-red emission bandsfor Eu3+, with the main transitions at 592 nm, 615 nm and
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690 nm due to transitions from the 5D0 excited state to thelower 7F1,
7F2 and7F4 levels of the 7FJ ground state multiplet,
respectively. The intensity ratio 7F2/7F1 is indicative of the
local symmetry of the coordination sphere around the Eu3+
ions. For high symmetry, including inversion symmetry, theemission spectrum is dominated by the 7F1 transition, whilefor low symmetry the 7F2 transition is much more intense,due to the hypersensitivity of the 5D0–
7F2 transition.10 For 4,the emission intensity ratio 7F2/
7F1 is 3.4, which is compati-ble with the symmetry of the local environment as derivedfrom the crystallographic data. For Eu3+, the transitions from5D0 to 7FJ will lead to at most 2J + 1 emission lines,depending on the local symmetry. At room temperature, arather featureless band is obtained for the 5D0 to 7F1 transi-tion at 592 nm (with a full width at half maximum (FWHM)of 4 nm). At 70 K, the thermal broadening is limited, andseparate emission lines are clearly observed (inset of Fig. 5).Six emission lines (FWHM, ~0.3 nm) can be distinguished.For this transition with J = 1, a single Eu(III) site leads to nomore than three transitions. Consequently, the presence oftwo distinct crystallographic sites for Eu(III) is reflected in the2 × 3 emissive transitions.
The luminescence color of 6 is green, due to the dominat-ing Tb3+ emission at 543 nm (5D4–
7F5). In 7, the emission isrelatively weak, although two characteristic emission bandsfor Dy3+ at 475 nm and 570 nm are observed. They are clearlysuperposed onto a broad emission band covering the shortwavelength side of the visible spectrum, which is due to hostrelated emission. The excitation spectra (Fig. 5) are almostidentical below 350 nm for 4, 6 and 7, with a broad excitationband between 250 and 350 nm. This similarity points at a
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good host sensitization by efficient energy transfer to the lan-thanide ion. The observed superposed bands in the spectraare narrow excitation bands originating from direct 4fn–4fn
excitations within the lanthanide ions.Fig. 6 shows the luminescence decay profiles for 4, 6
and 7. All decay profiles can be fitted with two exponentials,the decay constants τi are 60 μs and 205 μs for 4, 35 μs and140 μs for 6 and 0.9 μs and 2.0 μs for 7. For all polymers,both decay components contribute almost equally to the totalemission intensity. The decay values for 4 and 6 are consider-ably shorter than what can be expected for Eu3+ and Tb3+ inthe absence of non-radiative decay paths, with decay con-stants in the ms range. This can be explained by the presenceof coordinating water molecules, leading to vibrationalquenching. It could not be verified whether both decaycomponents are related to the presence of two differentlanthanide sites (with different geometry and number ofcoordinating water molecules).
Magnetic properties
The variable-temperature magnetic susceptibilities of com-pounds 4, 6 and 7 were measured in the range of 1.8–300 Kunder an applied field of 1000 Oe. The χmT vs. T plots arepresented in Fig. 7. For 2, the χmT value of 2.49 cm3 K mol−1
at room temperature is much higher than the theoreticalvalue of 0 cm3 K mol−1 for two uncoupled EuIII ions (S = 3,L = 3, 7F0) but close to previously reported values.19a,19b
The high values of χmT mainly stem from the population ofthe first (7F1 for Eu3+) and even higher excited states.19a,20
The χmT value rapidly decreases to the minimum value of0.04 cm3 K mol−1 at 2 K, which basically originates fromthermal depopulation of the excited levels. The magneticsusceptibility can be fitted using eqn (1),19 in which only thespin–orbital coupling of Eu3+ ions is considered:
Eu TIP NkTx
AB
2
3
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Fig. 6 Luminescence decay profiles (open symbols) obtained at room tememission peaks at 615 nm, 540 nm and 570 nm were monitored, respectivdecaying lifetime components (dotted lines).
A = 24 + (13.5x − 1.5)e−x + (67.5x − 2.5)e−3x + (189x − 3.5)e−6x
+ (405x − 4.5)e−10x + (742.5x − 5.5)e−15x + (1228.5x − 6.5)e−21x
(1)
B = 1 + 3e−x + 5e−3x + 7e−6x + 9e−10x + 11e−15x + 13e−21x
x = λ/kT
In this function, λ, k and TIP represent the spin–orbit cou-pling parameter, the Boltzmann constant and temperatureindependent magnetism, respectively. The best fitting resultsare λ = 519 cm−1, TIP = −0.013, and the agreement factorR = 4.21 × 10−3 in the range 2–300 K. The obtained λ value islarger than the commonly reported values.19 It is larger thanthe value derived from the luminescence spectroscopy, withthe energy difference between the different components of the7F1 multiplet (Fig. 5) and 7F0 ranging from 290 to 450 cm−1.Similar behavior was also encountered by Thompson et al.19
For 6, the χmT value is 23.25 cm3 K mol−1 at 300 K, whichapproaches the theoretical value of 23.64 cm3 K mol−1 basedon two Tb3+ ions in the 7F6 ground state (g = 3/2). As the tem-perature decreases, the curve goes to the minimum value of18.57 cm3 K mol−1 in the form of a parabola. The χm
−1 vs. Tplot of 6 obeys the Curie–Weiss law 1/χm = (T − θ)/C, depictedin Fig. 8, with the Curie constant C = 23.40 cm3 K mol−1 andthe Weiss constant θ = −4.04 K. However, the trend of theχmT values and the negative value of θ cannot unambiguouslyjustify the existence of antiferromagnetic coupling betweentwo adjacent Tb3+ ions. At room temperature, all Stark sub-levels of the 2S+1LJ ground state are thermally populated. Asthe temperature decreases, depopulation of these sublevelsoccurs and consequently χmT decreases. Only judging on thenegative value of θ and the decreasing trend of values of χmT,it is impossible to confirm the existence of antiferromagnetic/ferromagnetic coupling between adjacent Ln3+ ions, exceptfor gadolinium(III) with f7 configuration, whose orbitalmomentum is completely quenched in the ground state.19a
For 7, the χmT value is 28.12 cm3 K mol−1 at 300 K, whichis close to the theoretical value of 23.64 cm3 K mol−1
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perature upon pulsed excitation at 337 nm for 4, 6 and 7. The mainely. The full black line is a biexponential fit based on two exponentially
Fig. 7 χMT vs. T curves for 4, 6 and 7. The red solid line stands for thebest fitting.
Fig. 8 χM−1 vs. T curves for 6 (□) and 7 (○). The red solid line stands
for the best fitting.
Fig. 9 Temperature dependence of the in-phase (χ′) and out-of-phase(χ′′) ac susceptibilities at different frequencies for 6 with zero Oe dcfield and an oscillation of 3 Oe.
Fig. 10 Temperature dependence of the in-phase (χ′) and out-of-phase (χ′′) ac susceptibilities at different frequencies for 7 with zero Oedc field and an oscillation of 3 Oe.
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calculated for two Dy3+ ions (6H15/2, g = 4/3). Upon cooling,the χmT value is almost constant to about 100 K. And then,χmT values rapidly increase to the maximum value of 43.0cm3 K mol−1 at 2 K. This trend suggests that ferromagneticcoupling exists between the Dy3+ ions bridged by the oxalateligand in 7. The fitting to the plot of χm
−1 vs. T (Fig. 8) usingthe Curie–Weiss law 1/χm = (T − θ)/C above 40 K yields theCurie constant C = 27.42 cm3 Kmol−1 and theWeiss constant θ =2.90 K. The positive θ value further confirms the existence offerromagnetic coupling in 7. A similar magnetic behavior hap-pened in the compound {KDy(C2O4)2(H2O)4}n, where two types ofsuperexchange pathway of σ and π electronic coupling, simulta-neously offered by the oxalate, may be the main reason forferromagnetic superexchange between theDy3+ centers.21
To understand the dynamics of magnetization, alternatingcurrent (ac) magnetic measurements of compound 6 and 7were carried out under a zero dc field and an oscillation of 3Oe in the temperature range between 2 and 20 K and plots of
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χ′′ and χ′ vs. T are given in Fig. 9 and 10. No frequency depen-dence was observed for compounds 6 and 7, which indicatesthe absence of slow relaxation of the magnetization. Thismay be attributed to fast quantum tunneling and/or the lackof low lying excited levels.
Conclusion
In summary, seven lanthanide–organic frameworks with par-allelogram secondary building units were successfullyconstructed from H2pyip and oxalate ligands with lanthanideoxide. Single-crystal X-ray diffraction shows that compounds1–7 are isostructural. The shape of the rare earth containingoxalate layer in these compounds is a parallelogram, which isdifferent from the circular layers reported earlier. The differ-ence is owing to the presence of the H2pyip ligands, whichshows the importance of the ligand choice. Characteristicexcitation and emission bands for Dy3+, Tb3+ and Eu3+ were
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observed in the respective compounds. The luminescencecharacterization of the Eu3+ containing framework showedthe presence of two distinct crystallographic sites, in accor-dance with the structural data. Ferromagnetic couplings wereobserved in compound 6, based on Dy3+.
Acknowledgements
This work was conducted in the framework of a projectsponsored by the Natural Science Foundation of China(no. 21071100), the Doctor Scientific Startup Foundation ofLiaoning Province (no. 20111046), and the DistinguishedProfessor Project of Liaoning province.
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