3D Rare earth porous coordination frameworks with formamide generated in situ syntheses: Crystal structure and down- and up-conversion luminescence Xue Ma a , Jing Tian a,b , Hong-Y. Yang a , Kai Zhao a , Xia Li a,n a Department of Chemistry, Capital Normal University, Beijing 100048, People’s Republic of China b Experiment and Teaching Resource Management Centre, Yibin University, Yibin, 644000, People’s Republic of China article info Article history: Received 9 January 2013 Received in revised form 7 February 2013 Accepted 11 February 2013 Available online 27 February 2013 Keywords: Rare earth complex Formate Crystal structure Fluorescence abstract The reaction of RE(NO) 3 6H 2 O and formamide yielded the coordination polymers, [RE(HCOO) 4 ] [NH 2 CHNH 2 ] þ (RE ¼Y 1, Eu 2, Gd 3, Tb 4, Dy 5, Er 6, and Yb 7). They possess 3D porous frameworks with the 1D rhombic channels occupied by [NH 2 CHNH 2 ] þ cations. Complexes 2 and 4 display the characteristic down-conversion emissions corresponding to 5 D 0 - 7 F J (J ¼1–4) transitions of Eu(III) ion and 5 D 4 - 7 F J (J ¼6 3) transitions of Tb(III) ion, respectively. Longer lifetime values of 2.128 70.002 ms ( 5 D 0 ) for 2 and 2.132 70.002 ms ( 5 D 4 ) for 4 have been observed. The up-conversion spectra of the Y:Yb,Er and Gd:Yb,Er codoped complexes exhibit three emission bands around 410 ( 4 H 9/2 - 4 I 15/2 , blue), 518–570 ( 4 S 3/2 , 2 H 11/2 - 4 I 15/2 , green), and 655 nm ( 4 F 9/2 - 4 I 15/2 , red). & 2013 Elsevier Inc. All rights reserved. 1. Introduction Rare earth complexes with distinct luminescent and magnetic properties are currently of interest because of their potential applications in the optics, magnetism, and biology [1–8]. There is great interest in the study of the luminescence properties of rare earth complexes. Down-conversion luminescence have been exten- sively studied such as Eu(III)- and Tb(III)-complexes [2–8], while up- conversion luminescence complexes are less studied [9–11], which is mainly because the existence of multiphonon relaxation could decrease the efficiency of the up-conversion process. Metal–organic frameworks (MOFs) have attracted interest owing to their interesting structural architectures and various potential applications in gas adsorption and separation, catalysis, ion exchange, magnetism and luminescence, etc. [12–19]. The architectures of MOFs are derived from metal, metal cluster, or metal oxide building blocks acting as nodes and organic ligands acting as linkers. The properties of ligands such as various coordination modes, variable lengths, and relative orientation of donor atoms play a fundamental role in determining the structure of MOFs. Carboxylate ligands have been extensively employed for the synthesis of MOFs due to their versatile coordination con- formations and strong coordination ability [15–19]. As the smal- lest carboxylate, the formate ion (HCOO ) has been found to display multiple bridging modes like syn–syn, anti–anti, syn–anti. Formate is a three-atom short linker which can mediate a relatively stronger magnetic interaction and avoid interpenetra- tion to form frameworks with a cavity. Moreover, the formate anion without high-energy C–H and N–H vibrational groups as ligand is beneficial to the increasing intensity of fluorescence. A variety of complexes containing formate as the bridging ligand has been prepared [20–26]. We have noticed that the reactant formamide undergoes hydrolysis to formate after completion of the reaction and several works have been reported [20–26]. So, the coordination polymers [RE(HCOO) 4 ] [NH 2 CHNH 2 ] þ (RE ¼ Y 1, Eu 2, Gd 3, Tb 4, Dy 5, Er 6, and Yb 7) were obtained by reaction between RE(NO) 3 6H 2 O and formamide in solvothermal condi- tions. Formate anions present in the structure were generated upon hydrolysis of formamide by the lattice water present in the starting metal salts. The down-conversion luminescence of com- plexes 2 and 4 and the up-conversion luminescence of the Y:Yb,Er and Gd:Yb,Er doped complexes were investigated. 2. Experimental 2.1. Materials and physical measurement All of the starting materials were commercially available reagents for analytical grade and were used without further purification. Lanthanide nitrates were produced from lanthanide oxides and nitric acid. Elemental analyses (C, H and N) were Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/jssc Journal of Solid State Chemistry 0022-4596/$ - see front matter & 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jssc.2013.02.015 n Corresponding author. Fax: þ86 10 6890 2320. E-mail address: [email protected] (X. Li). Journal of Solid State Chemistry 201 (2013) 172–177
Transcript
Journal of Solid State Chemistry 201 (2013) 172–177
Contents lists available at SciVerse ScienceDirect
Journal of Solid State Chemistry
0022-45
http://d
n Corr
E-m
journal homepage: www.elsevier.com/locate/jssc
3D Rare earth porous coordination frameworks with formamide generatedin situ syntheses: Crystal structure and down- andup-conversion luminescence
Xue Ma a, Jing Tian a,b, Hong-Y. Yang a, Kai Zhao a, Xia Li a,n
a Department of Chemistry, Capital Normal University, Beijing 100048, People’s Republic of Chinab Experiment and Teaching Resource Management Centre, Yibin University, Yibin, 644000, People’s Republic of China
a r t i c l e i n f o
Article history:
Received 9 January 2013
Received in revised form
7 February 2013
Accepted 11 February 2013Available online 27 February 2013
The reaction of RE(NO)3 �6H2O and formamide yielded the coordination polymers, [RE(HCOO)4]�
[NH2CHNH2]þ (RE¼Y 1, Eu 2, Gd 3, Tb 4, Dy 5, Er 6, and Yb 7). They possess 3D porous frameworks
with the 1D rhombic channels occupied by [NH2CHNH2]þ cations. Complexes 2 and 4 display the
characteristic down-conversion emissions corresponding to 5D0-7FJ (J¼1–4) transitions of Eu(III) ion
and 5D4-7FJ (J¼6�3) transitions of Tb(III) ion, respectively. Longer lifetime values of 2.12870.002 ms
(5D0) for 2 and 2.13270.002 ms (5D4) for 4 have been observed. The up-conversion spectra of the
Y:Yb,Er and Gd:Yb,Er codoped complexes exhibit three emission bands around 410 (4H9/2-4I15/2, blue),
518–570 (4S3/2, 2H11/2-4I15/2, green), and 655 nm (4F9/2-
4I15/2, red).
& 2013 Elsevier Inc. All rights reserved.
1. Introduction
Rare earth complexes with distinct luminescent and magneticproperties are currently of interest because of their potentialapplications in the optics, magnetism, and biology [1–8]. There isgreat interest in the study of the luminescence properties of rareearth complexes. Down-conversion luminescence have been exten-sively studied such as Eu(III)- and Tb(III)-complexes [2–8], while up-conversion luminescence complexes are less studied [9–11], whichis mainly because the existence of multiphonon relaxation coulddecrease the efficiency of the up-conversion process.
Metal–organic frameworks (MOFs) have attracted interestowing to their interesting structural architectures and variouspotential applications in gas adsorption and separation, catalysis,ion exchange, magnetism and luminescence, etc. [12–19]. Thearchitectures of MOFs are derived from metal, metal cluster, ormetal oxide building blocks acting as nodes and organic ligandsacting as linkers. The properties of ligands such as variouscoordination modes, variable lengths, and relative orientation ofdonor atoms play a fundamental role in determining the structureof MOFs. Carboxylate ligands have been extensively employed forthe synthesis of MOFs due to their versatile coordination con-formations and strong coordination ability [15–19]. As the smal-lest carboxylate, the formate ion (HCOO�) has been found to
ll rights reserved.
display multiple bridging modes like syn–syn, anti–anti, syn–anti.Formate is a three-atom short linker which can mediate arelatively stronger magnetic interaction and avoid interpenetra-tion to form frameworks with a cavity. Moreover, the formateanion without high-energy C–H and N–H vibrational groups asligand is beneficial to the increasing intensity of fluorescence. Avariety of complexes containing formate as the bridging ligandhas been prepared [20–26]. We have noticed that the reactantformamide undergoes hydrolysis to formate after completion ofthe reaction and several works have been reported [20–26]. So,the coordination polymers [RE(HCOO)4]�[NH2CHNH2]þ (RE¼Y 1,Eu 2, Gd 3, Tb 4, Dy 5, Er 6, and Yb 7) were obtained by reactionbetween RE(NO)3 �6H2O and formamide in solvothermal condi-tions. Formate anions present in the structure were generatedupon hydrolysis of formamide by the lattice water present in thestarting metal salts. The down-conversion luminescence of com-plexes 2 and 4 and the up-conversion luminescence of the Y:Yb,Erand Gd:Yb,Er doped complexes were investigated.
2. Experimental
2.1. Materials and physical measurement
All of the starting materials were commercially availablereagents for analytical grade and were used without furtherpurification. Lanthanide nitrates were produced from lanthanideoxides and nitric acid. Elemental analyses (C, H and N) were
X. Ma et al. / Journal of Solid State Chemistry 201 (2013) 172–177 173
performed on an elementer Vario EL elemental analyzer. Infrared(IR) spectra were measured on a Bruker Tensor37 spectrophot-ometer using KBr pellets. X-ray diffraction carried out on aPANaytical X’Pert PRO MPD diffractometer for CuKa radiation(l¼1.5406 A), with a scan speed of 21 min�1 and a step size of0.021 in 2y. The simulated PXRD patterns were obtained from thesingle-crystal X-ray diffraction data. The experimental PXRDpatterns match well with the calculated ones obtained from thesingle-crystal structures, confirming the phase purity of the bulksamples. Fluorescence spectra were recorded on an FL4500fluorescence spectrophotometer (Japan Hitachi company) at roomtemperature. The lifetimes were measured at room temperatureon FLS920 Steady State & Time-resolved Fluorescence Spectro-meter (Edinburgh Instrument). The emission quantum yieldswere measured at room temperature using a Quantum YieldMeasurement System Fluorologs-3 (HORIBA company) with a450 W Xe lamp coupled to a monochromator for wavelengthdiscrimination, an integrating sphere as sample chamber, and ananalyzer R928P for signal detection. Up-conversion spectra wereobtained with a Hitachi F4500 fluorescence spectrophotometerwith an external 980 nm excitation source (Beijing Hi-TechOptoelectronic Co., China) instead of the xenon source in thespectrophotometer and with a fiber-optic accessory. Thermogra-vimetric analyses (TGA) were carried out using a shimadzu DTG-60AH thermal analyzer (Japan) under air from room temperatureto 800 1C with a heating rate of 10 1C/min. CD measurements onthe complexes indicate that the products are probably racemicmixture.
2.2. Preparation of [RE(HCOO)4]�[NH2CHNH2]þ (RE¼Y 1, Eu 2,
Gd 3, Tb 4, Dy 5, Er 6, and Yb 7)
0.2 mmol RE(NO)3 �6H2O (RE¼Y 1, Eu 2, Gd 3, Tb 4, Dy 5, Er 6,and Yb 7) and 5 ml formamide were transferred into a Teflonlined autoclave and then heated at 120 1C for 3d, followed by
Table 1Crystallographic data and structure refinement of the complexes 1–7.
Complex 1 2 3
Empirical formula C5H9N2YO8 C5H9N2EuO8 C5H9N2GdO8
Formula weight 314.04 377.1 382.39
Temperature/K 296(2) 296(2) 296(2)
Crystal system Orthorhombic Orthorhombic Orthorhombic
Final R indices [I42sigma(I)] R1¼0.0276, R1¼0.0195, R1¼0.0196,
wR2¼0.0698 wR2¼0.0441 wR2¼0.0466
R indices (all data) R1¼0.0306, R1¼0.0208, R1¼0.0203,
wR2¼0.0713 wR2¼0.0446 wR2¼0.0469
Largest diff.peak to hole
(e A�3)
0.452 to �0.442 0.521 to �0.867 0.750 to 1.912
cooling to room temperature naturally. Large single crystals inregular cubic shape were collected and washed with ethanolseveral times. Yield: about 60% (based on RE). For 1, Anal. Calcdfor [Y(HCOO)4]�[NH2CHNH2]þ (314.04)(%): C, 19.12; H, 2.89; N,8.92; Found (%): C, 19.23; H, 3.20; N, 8.54. Selected IR (KBr pellet,cm�1): 3335(br), 2850(m), 1725(s), 1606(vs), 1385(s), 1121(m),794(vs), 733(s), 534(m). For 2, Anal. Calcd for [Eu(HCOO)4]�
[NH2CHNH2]þ (377.10) (%): C, 15.93; H, 2.41; N, 7.43; Found(%): C, 15.81; H, 2.49; N, 7.19. Selected IR (KBr pellet, cm�1)::3337(br), 2843(m), 1724(vs), 1600(vs), 1385(s), 1119(m), 791(vs),729(s), 533(m). For 3, Anal. Calcd for [Gd(HCOO)4]�[NH2CHNH2]þ
(382.38) (%): C, 15.70; H, 2.37; N, 7.32; Found(%): C, 15.78; H, 2.37;N, 7.35. Selected IR (KBr pellet, cm�1): 3335(br), 2844(m),1724(vs), 1603(vs), 1385(s), 1120(m), 792(vs), 729(s), 533(m). For4, Anal. Calcd for [Tb(HCOO)4]�[NH2CHNH2]þ (384.06) (%): C,15.64; H, 2.36; N, 7.29; Found(%): C, 15.89; H, 2.27; N, 7.51.Selected IR (KBr pellet, cm�1):: 3334(br), 2847(m), 1725(s),1602(vs), 1385(vs), 1121(m), 792(vs), 730(s), 534(m). For 5, Anal.Calcd for [Dy(HCOO)4]�[NH2CHNH2]þ (387.63) (%): C, 15.45; H,2.32; N, 7.22; Found(%): C, 15.73; H, 2.59; N, 7.36. Selected IR (KBrpellet, cm�1): 3334(br), 2848(m), 1724(s), 1603(vs), 1385(vs),1121(m), 794(vs), 730(s), 534(m). For 6, Anal. Calcd for [Er(HCOO)4]�[NH2CHNH2]þ (392.39) (%): C, 15.21; H, 2.30; N, 7.14;Found (%): C, 15.44; H, 2.53; N, 7.40. Selected IR (KBr pellet, cm�1):3332(br), 2851(m), 1725(s), 1603(vs), 1384(s), 1122(m), 795(vs),733(s), 534(m). For 7, Anal. Calcd for [Yb(HCOO)4]�[NH2CHNH2]þ
(398.17) (%): C, 15.08; H, 2.28; N, 7.04; Found (%): C, 15.10; H, 2.46;N, 7.12. Selected IR (KBr pellet, cm�1): 3337(br), 2854(m), 1726 (s),1604(vs), 1384(s), 1123(m), 797(vs), 734(s), 534(m).
2.3. X-ray crystal structure determination
The X-ray single-crystal data collections for the seven com-plexes were performed on a Bruker Smart Apex II CCD diffract-ometer equipped with a graphite monochromated MoKa radiation
0.415 to �1.096 0.779 to �0.599 1.125 to �1.231 0.549 to �0.610
X. Ma et al. / Journal of Solid State Chemistry 201 (2013) 172–177174
(l¼0.71073 A) at 293(2) K. Semiempirical absorption correctionwas applied on the complexes using the SADABS program. Thestructures were solved by direct methods and refined by fullmatrix least squares method on F2 using the SHELXS 97 andSHELXL 97 programs [27,28]. All non-hydrogen atoms in thecomplexes were refined anisotropically. The hydrogen atoms weregenerated geometrically and treated by a mixture of independentand constrained refinement. Summary of the crystallographic dataand details of the structure refinements are listed in Table 1. Theselected bond lengths and bond angles of the complexes are listedin Table S1.
Fig. 1. View of the structure of 1:(a) Coordination environment of Y(III) ion.
NH2CHNH2þ ions are omitted for clarity. Symmetry codes: A, 1�x, y, 1.5�z. (b) 3D
porous framework occupied by NH2CHNH2þ cations (bottom) and Y-O-C-O-Y
double-stranded helices (top).
3. Results and discussion
3.1. Synthesis
Hydro(solvo)thermal synthesis with high temperature andpressure that favors the generation of suitable crystals. Hydro(-solvo)thermal synthesis technique provides a powerful tool forthe construction of materials containing unique structures andspecial properties. In our work, using solvothermal reactionbetween RE(NO3)3 6H2O and formamide at 120 1C for 72 h yieldedblock crystals with yield of about 60% based on metal. Indeed, athigh temperature and pressure of the solvothermal condition, thereaction is suggested to be as follows (Scheme 1). The formamidecan be hydrolyzed to formate by the lattice water present in thestarting metal salts and further to form formamidine cationNH2CHNH2
þ under the high temperature and pressure [20–26].The formamidinium ions are not coordinated to the metal ion andare H-bonded through their N–H groups to formate. Formamideplayed an important role as the source of formate and as thesolvent for the crystals in the synthesis of the complexes.
3.2. Description of the crystal structures
The seven complexes [RE(HCOO)4]�[NH2CHNH2]þ (RE¼Y 1,Eu 2, Gd 3, Tb 4, Dy 5, Er 6, and Yb 7) are isomorphous andcrystallize in orthorhombic system with the chiral space groupC2221, herein only the structure of 1 will be discussed in detail.Complex 1 possesses a 3D NaCl-like framework. The asymmetricunit of 1 consists of one Y(III) ion, four HCOO ligands and one freeNH2CHNH2
þ ion (Fig. 1a). The structure of 1 is constructed from{Y(HCOO)}n chains through the HCOO ligands into an extended3D Y–O–C–O–Y framework. Each Y(III) ion is coordinated to eightO atoms of formate anions in square antiprism. The Y–O bonddistances range from 2.284(2) to 2.417(2) A. The formate ligand inanti–anti mode acts as m�1,2 bridge and coordinates to twodifferent Y(III) ions. Each Y(III) ion is connected to its eight nearestneighbors by eight bridging HCOO ligands to form Y–O–C–O–Ychain connectivities. A chain is linked to eight adjacent chains toprovide 3D framework with the rectangular cavities along the a
axis direction (Fig. 1b). The neighboring two sides of the rectan-gular channels are constructed by left-handed helix. Each turn ofthe helix contains two HCOO ligands and two Y(III) ions with apitch of 6.675 A. The approximate dimensions of the rectangularporous are 6.680�6.680 A based on the Y � � �Y distances,
Scheme 1. A possible way of hydrolysis of formamide to formate.
corresponding to 28.7% of the unit cell volume by PLATON [29],which are occupied by NH2CHNH2
þ cations. There are the hydrogenbonds N–H � � �O between the NH2CHNH2
þ cations and formateswith the distances N �O of 3.133(3) and 3.460(3) A.
3.3. Down-conversion luminescence
Complexes 2 and 4 display intense red and green lumines-cence, respectively, and their luminescent properties in the solidstate were investigated at room temperature. The emission spec-trum of 2 excited at 395 nm exhibits the characteristic narrowbands of Eu(III) ion (Fig. 2a). The emission band at 590 nm with ashoulder peak of 592 nm corresponds to the 5D0-
7F1 (magnetic-dipole) transition. The strong emission band at 615 nm with ashoulder peak of 617 nm is attributed to the 5D0-
7F2 (electric-dipole) transition. The intensity of the 5D0-
7F2 transition isgreater than that of the 5D0-
7F1 transition, which indicates thatthe coordination environment of the Eu(III) ion is devoid of aninversion center. The weak emission peak at 650 nm correspondsto 5D0-
7F3 transition. The emission band splits into two peaks at686 and 692 nm, corresponding to the 5D0-
7F4 transition. Theemission spectrum of 4 excited at 350 nm shows four major peaksat 491, 544, 585, and 621 nm with the shoulder peaks of 490, 548,580, and 618 nm, respectively, resulting from Tb(III) emission,which correspond to the 5D4-
7FJ (J¼6�3) transitions (Fig. 2b).The most intense emission at 544 nm corresponds to the 5D4-
7F5
transition. The luminescence lifetimes of 2 and 4 were monitoredby the more intense emission at 615 nm (5D0-
7F2) for 2 and544 nm (5D4-
7F5) for 4 (Fig. S1). The emission decay curve can bedescribed by monoexponential kinetics, which indicates the exis-tence of a single chemical environment around the Ln(III) ion intheir complexes. Longer lifetime values of 2.12870.002 ms (5D0)for 2 and 2.13270.002 ms (5D4) for 4 have been observed, whichare high compared with some reported data for luminescent Eu(III)and Tb(III) complexes [30,31]. Longer lifetime values of 2 and 4 isprobably related to the absence of solvent molecules in thecoordination sphere, since such molecules are essential vibrationaldeactivators of the excited states of Ln(III) ions. The quantum yields
Fig. 2. Emission spectra of complexes 2 (a) and 4 (b) in the solid state at room
temperature.
Fig. 3. Up-conversion emission for the Er(III) ion in the Gd:Yb,Er/Y:Yb,Er codoped
coordination polymers.
Fig. 4. Up-conversion mechanism of the Y:Er,Yb codoped coordination polymer
under 980 nm excitation at room temperature. The solid lines represent emission
process, while the dashed lines represent photon excitation or nonradiative
relaxation.
X. Ma et al. / Journal of Solid State Chemistry 201 (2013) 172–177 175
of luminescence for 2 and 4 are 17.35% and 23.72%, respectively.The quantum yields and lifetime values for 2 and 4 are promisingwhen compared with the reported some Eu/Tb complexes [30–33].
3.4. Up-conversion luminescence
The Gd:Yb,Er and Y:Yb,Er codoped complexes with Yb(III)12 mol% and Er(III) 3 mol% ions were prepared. The PXRD plot(Fig. S2) is clear that these codoped materials are isostructural tothe original complexes. The fluorescence properties of thecodoped complexes were studied under excitation of 980 nmlaser (Fig. 3). The basic mechanism for up-conversion in Y(III)doped systems such as NaYF4:Er,Yb is well-known [9–11,34,35].In Gd:Yb,Er codoped complex, the lowest excited level (6P7/2) ofGd(III) is situated in the ultraviolet region, which is far higherthan excited levels of Yb(III) and Er(III) involved in the up-conversion processes. Thus, energy transfer from Yb(III) and Er(III)to Gd(III) can be avoided. Up-conversion enhancement for theGd(III) doped system is essentially the same ones as in the Y(III)doped system.
The proposed up-conversion mechanism in the Y:Yb,Er codopedsystem is described in the energy diagram, as shown in Fig. 4. Fig. 3shows the up-conversion peaks at 410 nm, 518–570 nm, and
656 nm for Gd(III)/Y(III) codoped complexes. For the 518–570 nmgreen emission, the mechanism is clearly known, which can bedescribed as a two photon up-conversion excitation mechanism.The Yb(III) ion was excited by a 980 nm photon from the groundstate 2F7/2 level to the excited state 2F5/2. Then the energy istransferred from Yb(III) to Er(III) (4I11/2). The Er(III) ion populated atthe 4I11/2 level absorbs a second-photon to the 4F7/2 state. Theexcited electrons at the 4F7/2 level then decay to the lowly 2H11/2
and 4S3/2 levels mainly through nonradiative relaxations. The decayfrom the 2H11/2 and 4S3/2 states to the ground state 4I15/2 generatedthe emissions at 518–570 nm (green), which is the strongest peakin the system. While the 653 nm weak red emission is generatedby the 4F9/2-
4I15/2 transition. The 4F9/2 level can be reached by thenonradiative decay from the 4S3/2 level. The other possible route,owing to the nonradiative decay from the 4I11/2 to the 4I13/2 level,the 4I13/2 level via a second-photon leads to promotion of Er(III) tothe 4F9/2 level. Finally, Er(III) at 4F9/2 level relaxes to the groundstate 4I15/2 configuration and emits weak red luminescence.The weak blue emission peak at 410 nm can be assigned to 2H9/2-4I15/2 transition of Er(III). The Er(III) ion populated at the 2H9/2 levelby two possible three-photon routes. The Er(III) ions in the 4F9/2
ground state absorb the third photon and promote to 2H9/2 state.The second, some of the Er(III) ions in the 4S3/2 level are pumped to
X. Ma et al. / Journal of Solid State Chemistry 201 (2013) 172–177176
the 4G11/2 level by the third photon, followed to 2H9/2 level bynonradiative relaxations. The decay from the excited state 2H9/2 tothe ground state 4I15/2, the blue light emission is observed.
3.5. TGA and temperature dependent PXRD
We adopt TGA and PXRD techniques to investigate the frame-work stability. The complexes 1–7 exhibit similar thermal beha-vior (Fig. S3), herein only 7 will be discussed in detail. The TGAcurve (Fig. 5) shows that the free NH2CHNH2
þ cations can beremoved at around 252.4 1C (the observed weight loss of 13.73%,calcd 13.96%). Up to 514 1C, the complex was completelydegraded and the final residue weight is 51.05%, which corre-sponds to the Yb2O3 as final product (Calc. 49.60%). The PXRDpattern (Fig. 6) at 100 and 200 1C is very similar to the roomtemperature (25 1C) one. The PXRD pattern at 300 1C is differentfrom that at 200 1C due to the removal of NH2CHNH2
þ cations. ThePXRD pattern at 400 1C is clearly different. So, the PXRD patternsindicate that the 3D framework changed after removal of the freeNH2CHNH2
þ cations.
Fig. 5. The TGA curve of complex 7.
Fig. 6. PXRD patterns of complex 7 at different temperatures.
4. Conclusions
Solvothermal reactions of formamide with a series of rareearth(III) salts yielded 3D porous coordination frameworks,[RE(HCOO)4]�[NH2CHNH2]þ (RE¼Y 1, Eu 2, Gd 3, Tb 4, Dy 5,Er 6, and Yb 7). An outstanding feature of the structures is the 1Drhombic channels and the [NH2CHNH2]þ counterions residewithin these channels. The solvothermal reaction allows us tocontrol the content of bonded water to the rare earth center. The3D structural skeleton changed between 200 and 300 1C. Thecomplexes 2 and 4 show the characteristic strong down-emissionof the Eu(III) for 2 and Tb(III) for 4 and have longer luminescencelifetimes. A significant blue, green, and red emission at 410, 518–570, and 656 nm for Gd:Yb,Er and Y:Yb,Er codoped complexes bypumping at 980 nm are observed. The up-conversion fluorescenceof Gd:Yb,Er codoped complex is first reported
Acknowledgments
This work is supported by the National Natural ScienceFoundation of China (21071101).
Appendix A. Supporting information
Supplementary data associated with this article can be foundin the online version at http://dx.doi.org/10.1016/j.jssc.2013.02.015. CCDC 916940–916946 contain the supplementary crystal-lographic data for 1–7. These data can be obtained free of chargevia http://www.ccdc.cam.ac.uk/conts/retrieving.html, or fromthe Cambridge Crystallographic Data Centre, 12 Union Road,Cambridge CB2 1EZ, UK; fax: (þ44) 1223-336-033; ore-mail: [email protected].
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