Download pdf - CrystEngComm - Pennsylvania State University ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c3ce40184g CrystEngComm PAPER This journal is The Royal

Cite this: CrystEngComm, 2013, 15,4489

1-D helical chain, 2-D layered network and 3-D porouslanthanide–organic frameworks based on multiplecoordination sites of benzimidazole-5,6-dicarboxylicacid: synthesis, crystal structure, photoluminescenceand thermal stability3

Received 28th January 2013,Accepted 27th March 2013

DOI: 10.1039/c3ce40184g

www.rsc.org/crystengcomm

Ping Wang,a Rui-Qing Fan,*a Yu-Lin Yang,*a Xin-Rong Liu,a Peng Xiao,a Xin-Yu Li,a

Wuliji Hasia and Wen-Wu Caob

One-dimensional to three-dimensional lanthanide coordination polymers 1–8 based on benzimidazole-

5,6-dicarboxylic acid (H3BIDC) have been synthesized under hydrothermal conditions at different pH

values, generally formulated as {[Pr(HBIDC)(ox)0.5(H2O)]?H2O}n (1), [Yb(HBIDC)(ox)0.5(H2O)2]n (2), and

[Ln(HBIDC)(ox)0.5(H2O)3]n [Ln = Ho (3), and Tb (4)] and {[Ln(H2BIDC)(HBIDC)(H2O)3]?3H2O}n [Ln = Tb (5), Sm

(6), Dy (7), and Gd (8), H2ox = oxalic acid]. All coordination polymers have been characterized by elemental

analysis, infrared spectra and single-crystal X-ray diffraction. The structural diversity, luminescence and

thermal properties of all coordination polymers have been investigated. Coordination polymers 1–8

exhibit four different structural types: topological analysis has given the 3-D pcu network, with the point

symbol of {412?63} in coordination polymer 1. Coordination polymer 2 exhibits a 4-connected 44 topology,

and coordination polymers 3–4 appear as 2-D (6,3)-connected hcb network topology. The 1-D helical

infinite chain of coordination polymers 5–8 around the crystallographic 21 axis spread along the b axis

direction, with different 1-D helical infinite chains forming 3-D supramolecular framework via hydrogen

bonds and p–p stacking interactions. The coordination polymers 4 and 5 could be triggered to have

intense characteristic lanthanide-centered green luminescence under UV excitation in the solid state at

room and liquid nitrogen temperature, or dispersed in CH2Cl2 at 77 K. In coordination polymers 4 and 5,

the oxalic acid introduced into coordination polymer 4 as a second ligand further sensitized the trivalent

terbium ion, and resulted in longer fluorescence lifetimes of coordination polymer 4 (1058.58 ms at 298 K,

679.42 ms at 77 K in the solid-state, 867.82 ms in CH2Cl2 at 77 K) than coordination polymer 5 (595.06 ms at

298 K, 583.19 ms at 77 K in the solid-state, 584.38 ms in CH2Cl2 at 77 K). In coordination polymers 6 and 7,

we not only measured emission spectra in the visible region, but also detected the infrequent NIR emission

spectra in the near infrared region of samarium and dysprosium ions. The singlet excited state (30 303

cm21) and the lowest triplet state energy level (24 390 cm21) of H3BIDC ligand were calculated based on

the UV-vis absorbance edges of ligand and the phosphorescence spectrum of Gd(III) coordination polymer

(8) at 77 K, showing that the effective extent of energy transfer from H3BIDC ligand to lanthanide ions

follows the sequence of Tb3+, Dy3+ > Sm3+. Finally, thermal behaviors of all coordination polymers were

studied by thermogravimetric analysis, which exhibited high thermal stability.

Introduction

In recent years, novel metal–organic frameworks (MOF) basedon crystal engineering have attracted extensive attention notonly due to their intriguing topologies and diverse structuresbut also owing to their interesting physical and chemicalproperties, such as photoluminescence, magnetism, gasstorage, ion exchange, and catalysis, etc.1–4 Among presentcontributions, metal ions are interlinked by organic bridgingligands containing functional groups such as imidazole and/or

aDepartment of Chemistry, Harbin Institute of Technology, Harbin 150001, P. R.

China. E-mail: [email protected]; [email protected]; Fax: +86-451-86418270bMaterials Research Institute, The Pennsylvania State University, University Park, PA

16802, USA

3 Electronic supplementary information (ESI) available. CCDC 902074–902081.For ESI and crystallographic data in CIF or other electronic format see DOI:10.1039/c3ce40184g

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carboxylate groups to form infinite network structures, such asone-dimensional (1-D) chains5 and ladders,6 two-dimensional(2-D) grids,7 three-dimensional (3-D) microporous networks,8

interpenetrated mode,9 and helical networks.10 Recently, wehave synthesized a number of metal–organic coordinationpolymers and studied the luminescent properties of thesecomplexes.4 However, lanthanides, with high and variablecoordination numbers (6 ¡ CN ¡ 12)11 and flexiblecoordination environments, showed very limited success inthe design of predetermined molecular architectures, there-fore, rational design and synthesis lanthanide coordinationpolymer crystal materials with luminescent properties are stillfacing a great challenge.2d,10b The coordination polymer,especially with luminescence in the range of 900–1700 nmfrom lanthanide ions such as Nd(III), Er(III), and Yb(III), areparticular attractive because of their potential application invarious optical and medical devices.12,13 Lanthanides haveintense affinity to oxygen atoms, so ligands containing oxygenatoms (such as oxalic acid) can be used as stable bridgingligands in the synthetic process of lanthanide complexes.14,15

In the carboxylate family, N-heterocyclic multicarboxylic acidshave been widely used to construct MOF for their potentialapplication.16

So far, to our knowledge, the solid state luminescencespectra of lanthanide coordination polymers have beenresearched at room temperature, but their luminescenceproperties in solutions or at 77 K have been rarely investigated.The investigation of luminescence emissions of Sm3+ and Dy3+

ions have been often studied based in the visible region, theemission of which were limited in the near infrared region.17–19

Herein, we have chosen a multifunctional ligand, benzimida-zole-5,6-dicarboxylic acid (H3BIDC) as the main ligand, which isa derivative of 4,5-imidazoledicarboxylic acid, and exhibitsseveral interesting characteristics: (I) multifunctional coordina-

tion sites containing imidazole and carboxylate groups whichprovide a high likelihood for construction of different dimen-sional coordination polymers. (II) H3BIDC can be partially orcompletely deprotonated to generate H2BIDC2, HBIDC22, andBIDC32 by controlling the pH carefully. (III) H3BIDC usuallyadopts difference coordination motifs, such as terminalmonodentate chelating to one metal center, bridging bidentatein a syn–syn, syn–anti, and anti–anti configuration to two metalcenters, and bridging tridentate to two metal centers. (IV) Thep-conjugated system in the benzimidazole ring is a goodmedium for transferring energy. H3BIDC possesses the cap-ability to chelate and bridge metal centers in various coordina-tion modes through the nitrogen atoms of the benzimidazolering and carboxylate oxygen atoms, and allows us to explorefurther by adding another auxiliary organic ligand–oxalic acid(H2ox) to the reaction mixture. To the best of our knowledge,cases of lanthanide coordination polymers linked by H3BIDChave been presented and the lanthanide coordination polymersbased on benzimidazole-5,6-dicarboxylic acid are summarizedin Table 1. In this work, we report the synthesis, structures,thermal properties and luminescent properties of eightlanthanide coordination polymers containing 1-D to 3-Dstructures formulated as {[Pr(HBIDC)(ox)0.5(H2O)]?H2O}n (1),[Yb(HBIDC)(ox)0.5(H2O)2]n (2), [Ln(HBIDC)(ox)0.5(H2O)3]n [Ln =Ho (3), and Tb (4)] and {[Ln(H2BIDC)(HBIDC)(H2O)3]?3H2O}n

[Ln = Tb (5), Sm (6), Dy (7), and Gd (8)]. The luminescenceproperties in the visible region and fluorescence lifetimes ofcoordination polymers 4–7 in the solid state at room and liquidnitrogen temperature and dispersed in CH2Cl2 as suspensionsat 77 K were discussed. Particularly, NIR emission spectra of Yb(2), Sm (6), and Dy (7) coordination polymers in solid state atroom temperature were measured. Finally, thermal behaviors ofall coordination polymers were also presented.

Table 1 Summary of lanthanide–organic frameworks based on benzimidazole-5,6-dicarboxylic acid

Empirical formula Space group Dimension References

1 [(Eu2(Hbidc)2(ox)2?(H2O)3] P1 2-D 212 [Tb2(Hbidc)2(ox)(H2O)2]?4H2O P1 2-D 213 [Er2(Hbidc)2(ox)] P1 3-D 214 [Eu(C9H4N2O4)(C9H5N2O4)(H2O)3]?2H2O}n P1 1-D 435 [Eu2(C9H5N2O4)2(SO4)2(H2O)6]?6H2O}n P1 1-D 446 [Ln2(Hbidc)2(SO4)(H2O)3]n (Ln = La, Pr, Sm, Gd) P1 2-D 7a7 [Ln4(Hbidc)4(SO4)2]n?2nH2O (Ln = Eu, Tb, Dy, Er) P21/c 3-D 7a8 {[Ln3(bidc)4(phen)2(NO3)]?2H2O}n (Ln = Gd, Eu, Tb) P212121 1-D 459 {[Tb(L)(HL)(H2O)]?H2O}n P21/n 2-D 16d10 {[Ln2L2(HL)2(H2O)2]}n (Ln = Ho, Er, Lu) P21/c 2-D 16d11 [Ln2L3(H2O)] [Ln = Eu, Tb] P21/c 3-D 4612 [Pr(L)(HL)H2O]?H2O P21/n 2-D 4613 {[Er(Hbmdc)(bmdc)(H2O)3]?3H2O}n (1) P21/c 1-D 2414 [Er2(Hbmdc)2(bmdc)2(H2O)8]?8H2O (2) P1 0-D 2415 [Ln(bidc)(Ac)?H2O]n (Ln = Tb (1), Dy (2)) P21/c 2-D 4716 {[Ln(Hbidc)(ox)1/2(H2O)]?H2O}n [Ln = Pr, Nb, Sm] P1 3-D 4817 [Ln(Hbidc)(ox)1/2]n [Ln = Eu, Gd] P1 3-D 4818 [Dy(H2bidc)(Hbidc)(H2O)8]?8H2O P1 0-D 2319 {[Dy(Hbidc)(H2O)2(Htzac)]?3H2O}n P1 1-D 2320 [Dy(C2O4)0.5(Hbidc)(H2O)3]n P1 2-D 2321 {[Dy2(Hbidc)2(H2O)(SO4)]?H2O}n Pc 3-D 2322 [Pr(C9H4N2O4)(C2H3O2)(H2O)]n P1 2-D 49

4490 | CrystEngComm, 2013, 15, 4489–4506 This journal is � The Royal Society of Chemistry 2013

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Experimental section

Materials and methods

All reagents were commercially available and used withoutfurther purification. Infrared spectra were obtained from KBrpellets using a Nicolet Avatar-360 Infrared spectrometer in the4000–400 cm21 region. Powder X-ray diffraction (PXRD)patterns were recorded in the 2h range of 10–40u using CuKa radiation by Shimadzu XRD-6000 X-ray Diffractometer.Elemental analyses were performed on a Perkin-Elmer 240celement analyzer. Inductively coupled plasma (ICP) analysiswas performed on a Perkin-Elmer Model Optima 3300 DV ICPspectrometer. Luminescence spectra and fluorescence life-times were measured with by an Edinburgh FLSP920 com-bined steady state fluorescence and phosphorescence lifetimespectrometer. The thermal analysis was performed on a ZRY-2P thermogravimetric analyzer from 30 uC to 700 uC withheating rate of 10 uC min21 under a flow of air.

Synthesis of {[Pr(HBIDC)(ox)0.5(H2O)]?H2O}n (1)

A mixture of Pr(NO3)3?5H2O (41.7 mg, 0.1 mmol), H3BIDC (20.6mg, 0.1 mmol) and C2H2O4?2H2O (12.6 mg, 0.1 mmol) wasdissolved in distilled water (8.0 mL), adjusted to pH # 7.8 withan aqueous solution of piperazine while stirring, continued tostir 20 min, then sealed in a 20 mL Teflon-lined stainless steelautoclave and heated at 120 uC for 5 days. After the mixturewas cooled slowly to room temperature, green block crystals (1)were obtained (yield, 89%, based on Pr). Elemental analysis for1: C10H8N2O8Pr (Mr: 425.09). Calcd: C, 28.26; N, 6.60; H, 1.90%.ICP analysis gave the following composition: Pr, 33.39%(calcd: 33.15%). Found: C, 27.98; N, 6.37; H, 2.19%. IR (KBr,cm21): 3419 (s), 1636 (vs), 1399 (s), 1315 (s), 1134 (w), 960 (w),803 (m), 628 (m), 502 (m).

Synthesis of [Yb(HBIDC)(ox)0.5(H2O)2]n (2) and[Ln(HBIDC)(ox)0.5(H2O)3]n [Ln = Ho (3), and Tb (4)]

The methods used for the syntheses of coordination polymers2–4 are similar. Ln(NO3)3?5H2O (Ln = Yb, Ho, Tb) was usedinstead of Pr(NO3)3?5H2O, the following steps are similar tothat for coordination polymer 1. The mixture was dissolved indistilled water and adjusted to pH # 7.4 or 7.0 with anaqueous solution of piperazine while stirring. After themixture was cooled slowly to room temperature, colorlessblock crystals (2) were obtained (yield, 88%, based on Yb),ivory block crystals (3) were obtained (yield, 81%, based onHo), colorless block crystals (4) were obtained (yield, 83%,based on Tb). Elemental analysis for 2: C10H8N2O8Yb (Mr:457.22). Calcd: C, 26.27; N, 6.13; H, 1.77%. Found: 26.01; N,5.97; H, 1.79%. ICP analysis gave the following composition:Yb 37.99% (calcd: 37.85%). IR (KBr, cm21): 3434 (s), 1630 (vs),1400 (m), 1331 (s), 864 (m), 800 (m), 489 (m). Elementalanalysis for 3: C10H10HoN2O9 (Mr: 467.13). Calcd: C, 25.71; N,6.00; H, 2.16%. Found: C, 25.92; N, 6.23; H, 2.34%. ICPanalysis gave the following composition: Ho, 35.71% (calcd is35.31%). IR (KBr, cm21): 3445 (s), 1627 (vs), 1416 (s), 1324 (s),1196 (m), 1091 (m), 813 (m), 502 (m). Elemental analysis for 4:C10H10N2O9Tb (Mr: 461.13). Calcd: C, 26.05; N, 6.07; H, 2.19%.Found: C, 26.31; N, 6.29; H, 2.33%. ICP analysis gave thefollowing composition: Tb, 34.79% (calcd: 34.46%). IR (KBr,

cm21): 3445 (s), 1623 (vs), 1411 (s), 1319 (s), 1192 (m), 1089(m), 809 (s), 497 (s).

Synthesis of {[Tb(H2BIDC)(HBIDC)(H2O)3]?3H2O}n (5)

A mixture of Tb(NO3)3?5H2O (22.1 mg, 0.05 mmol) and H3BIDC(10.3 mg, 0.05 mmol) was dissolved in distilled water (5.0 mL),adjusted to pH # 4.5 with an aqueous solution of piperazinewhile stirring, continued to stir 20 min, then sealed in a 20 mLTeflon-lined stainless steel autoclave and heated at 120 uC for5 days. After being slowly cooled to room temperature,colorless clubbed crystals were obtained (yield, 58%, basedon H3BIDC). Elemental analysis for C18H21N4O14Tb (Mr:676.31): calcd: C, 31.97; N, 8.28; H, 3.13%. Found: C, 32.22;N, 8.46; H, 3.31%. ICP analysis gave the following composi-tion: Tb, 23.82% (calcd: 23.50%). IR (KBr, cm21): 3419 (s), 3137(s), 1564 (vs), 1527 (vs), 1474 (vs), 1428 (vs), 1366 (vs), 1276 (m),1274 (m), 1039 (w), 876 (m), 806 (m), 786 (m), 694 (m), 646 (m),619 (m).

Synthesis of {[Ln(H2BIDC)(HBIDC)(H2O)3]?3H2O}n [Ln = Sm(6), Dy (7), and Gd (8)]

The methods used for the syntheses of coordination polymers6–8 are similar. Ln(NO3)3?5H2O (0.05 mmol Ln = Sm, Dy, andGd) was used instead of Tb(NO3)3?5H2O, the following stepsare similar to that for coordination polymer 5. After themixture was cooled slowly to room temperature, suitablecrystals of 6–8 for single-crystal X-ray diffraction wereobtained. For 6, light yellow clubbed crystals were obtained(yield, 53%, based on H3BIDC). Elemental analysis forC18H21N4O14Sm (Mr: 667.74). Calcd: C, 32.38; H, 3.17; N,8.39%. Found: C, 32.76; H, 3.32; N, 8.73%. ICP analysis gavethe following composition: Sm, 22.89% (calcd: 22.52%). IR(KBr, cm21): 3418 (s), 3131 (s), 1563 (vs), 1472 (s), 1419 (vs),1360 (s), 1280 (m), 1262 (m), 957 (w), 874 (m), 791 (m), 759 (m),685 (w), 613 (m). For 7 (yield, 59%, based on H3BIDC),elemental analysis for C18H21DyN4O14 (Mr: 679.89): calcd: C,31.80; N, 8.24; H, 3.18%. Found: C, 31.98; N, 8.51; H, 3.41%.ICP analysis gave the following composition: Dy, 23.79%(calcd: 23.90%). IR (KBr, cm21): 3411 (s), 3137 (s), 1565 (vs),1527 (vs), 1474 (vs), 1429 (vs), 1366 (vs), 1272 (m), 1271 (m),1039 (w), 878 (m), 812 (m), 786 (m), 694 (m), 646 (m), 619 (m).For 8 (yield, 60%, based on H3BIDC), elemental analysis forC18H21GdN4O14 (Mr: 674.64): calcd: C, 32.05; N, 8.30; H, 3.14%.Found: C, 32.37; N, 8.52; H, 3.38%. ICP analysis gave thefollowing composition: Gd, 23.70% (calcd: 23.31%). IR (KBr,cm21): 3418 (s), 3137 (s), 1564 (vs), 1527 (vs), 1474 (vs), 1428(vs), 1370 (vs), 1286 (m), 1040 (w), 948 (w), 876 (m), 807 (m),786 (m), 694 (m), 619 (m).

X-ray crystal structure determination

The X-ray diffraction data taken at room temperature forcoordination polymers 1–8 were collected on a Rigaku R-AXISRAPID IP or a Siemens SMART 1000 CCD diffractometerequipped with graphite-monochromated Mo Ka radiation (l =0. 71073 Å). The crystal structures were resolved by directmethod and refined by semi-empirical formula from equiva-lents and full-matrix least squares based on F2 using theSHELXTL 5.1 software package.20 All non-hydrogen atomswere refined anisotropically. Hydrogen atoms were fixed at


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calculated positions and refined by using a riding mode exceptwater molecules. The CCDC 902074–902081 contain thecrystallographic data 1–8 for this paper. Crystal structure dataand details of the data collection and the structure refinementare listed in Table 2, selected bond lengths, bond angles andhydrogen bonding data of coordination polymers 1–8 arelisted in Tables S1–S3 (ESI3).

Results and discussion

Syntheses and characterization

Eight lanthanide coordination polymers,{[Pr(HBIDC)(ox)0.5(H2O)]?H2O}n (1), [Yb(HBIDC)(ox)0.5(H2O)2]n

(2), [Ln(HBIDC)(ox)0.5(H2O)3]n [Ln = Ho (3), and Tb (4)] and{[Ln(H2BIDC)(HBIDC)(H2O)3]?3H2O}n [Ln = Tb (5), Sm (6), Dy(7), and Gd (8)], have been synthesized under hydrothermalconditions. The crystals of coordination polymers 1–8 weresynthesized by reacting H3BIDC with the correspondinglanthanide nitrates at 120 uC for 5 days under hydrothermalconditions at different pH values. The reaction route of 1–8 isshown in Scheme 1. Coordination polymer 1 displayed a 3-DMOF structure, 2–4 displayed different 2-D structures and 5–8displayed 1-D structures, and these results can be easilyunderstood considering the following factors. First, thereaction pH value is one of main influencing factors in thesynthesis of coordination polymers 1–8. H3BIDC is partiallydeprotonated to generate H2BIDC2, and HBIDC22, a low pHvalue will restrain the deprotonation of H3BIDC, and result indifficulty in coordinating to the metal centers. Higher pHvalue in the reaction system can enhance the coordinationcompetence of H3BIDC ligand in the crystal structure. In thesynthesis of coordination polymers 1–8, pH values forcoordination polymers 1–4 (at ca. 7–8) are higher than thosefor coordination polymers 5–8 (at ca. 4.5), which results informing the 3-D and 2-D structure of coordination polymers 1–4. Therefore, higher pH value is one reason for the formationof 2-D and 3-D structures rather than 1-D structures, which isconsistent with the experimental results. Second, ox22 ligandsenhance the coordination interaction between metal centersand ligands, and weaken the coordination competition ofwater, so high dimensional coordination polymers 1–4 (3-Dand 2-D) were obtained. Moreover, comparison of differentdimensional structure of coordination polymers 1–4, causedby different lanthanide ionic radius, suggests that the differentmetal sources play an important role in the structuralassemblies of corresponding coordination polymers.

Coordination polymers 1–8 were characterized by IR spectra,elementary analysis and single-crystal X-ray diffraction. Theresults of single-crystal X-ray diffraction analyses indicate thatcoordination polymer 1 shows a 3-D pcu network, with thepoint symbol of {412?63}, coordination polymer 2 has 4-con-nected 44 topology, coordination polymers 3–4 appear as 2-D(6,3)-connected hcb networks, and coordination polymers 5–8show a 1-D helical infinite chain.

Coordination polymers 1–4 possess identical ligandsH3BIDC and H2ox, and their IR spectra are similar. Theasymmetric and symmetric stretching vibrations of carboxyl

groups in coordination polymers 1–4 appeared at ca. 1630cm21 and ca. 1400 cm21 in the IR spectra, respectively, whichare red shifted compared with the stretching vibration ofcarboxyl group in free oxalic acid (ca. 1700 cm21) and H3BIDC(ca. 1717 cm21). That is because the electron cloud density forcarbon–oxygen bonds in coordination polymers 1–4 is in therange of that for CLO and C–O. The stretching vibration andthe plane rocking vibration of C–C appeared at ca. 1320 cm21

and ca. 800 cm21, respectively, which are blue shiftedcompared with the free ligands. This phenomenon may dueto the increased rigidity of C–C caused by the coordinationbetween the metal centers and ligands. Simultaneously, theabsorbed energy of stretching vibration and the plane rockingvibration correspondingly increased.

Coordination polymers 5–8 possess the identical ligandH3BIDC, coordination polymers 5–8 are isomorphous, and theIR spectra of coordination polymers 5–8 are similar. Theasymmetric and symmetric stretching vibrations of carboxylgroup of H3BIDC ligand in coordination polymers 5–8appeared at ca. 1560 cm21 and ca. 1474 cm21 in the IRspectra, respectively, which are red shifted compared with thestretching vibration of carboxyl group in free H3BIDC (ca. 1717cm21). That is because the electron cloud density for carbon–oxygen bonds in coordination polymers 5–8 is in the range ofthat for CLO and C–O. The stretching vibration and the planerocking vibrations of C–C appeared at ca. 1370 cm21 and ca.790 cm21, respectively, which are blue shifted compared withthe free ligands. This phenomenon may due to the increasedrigidity of C–C caused by the coordination between the metalcenters and ligands. Simultaneously, the absorbed energy ofstretching vibration and the plane rocking vibration corre-spondingly increased.

Description of crystal structure

The X-ray structural analysis of coordination polymer 1 revealsthat it contains one Pr3+ cation, one HBIDC22 anion, half ox22

anion, one coordination water molecule, and one free watermolecule with all in general positions in the asymmetric unit(Fig. 1a). Center metal Pr1 is nine-coordinated: each Pr(III) iscoordinated by two oxygen atoms from one chelating ox22

ligand (O5, O6A), and five oxygen atoms from three crystal-lographically independent HBIDC22 ligands (O1, O2A, O3A,O3B, O4A), one nitrogen atom (N1A) of another crystal-lographically independent HBIDC22 ligand, and the remain-ing coordination site is occupied by oxygen atom of onecoordinated water molecule (O7), displaying a tricappedtrigonal prismatic arrangement, as shown in Fig. 1b. Thestructure is different from that of erbium coordinationpolymer which has been reported in the literature,21 this isdue to the presence of coordinated water molecules. The Pr1–O bond lengths vary from 2.395(4) to 2.639(4) Å, the Pr1–Nbond length is 2.601(5) Å. The O–Pr1–O bond angles rangefrom 63.92(1)u to 148.74(1)u, the O–Pr1–N bond angles rangefrom 71.01(2)u to 141.49(2)u. In the coordination polymer 1,the HBIDC22 ligand displays the m4-gO

1:gO1:gO

2:gO1:gN

1 modeto connect four Pr(III) ions (Scheme 2A), and the 6-carboxylategroup adopts an anti–anti mode to link two metal ions. Pr1and Pr1A are linked together to form a binuclear unit [Pr2(m2-O)2] by two m2-O bridges. The distance between two praseody-


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Tab

le2

Cry

stal

dat

aan

dst

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refin

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tsfo

rco

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inat

ion

po

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1–8

Iden

tifi

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12

34

56

78

Em

piri

cal

form

ula

C1

0H

8N

2O

8Pr

C1

0H

8N

2O

8Y

bC

10H

10H

oN2O

9C

10H

10N

2O

9T

bC

18H

21N

4O

14T

bC

18H

21N

4O

14Sm

C1

8H

21D

yN4O

14

C1

8H

21G

dN

4O

14

Form

ula

wei

ght

425.

0945

7.22

467.

1346

1.13

676.

3166

7.74

679.

8967

4.64

Cry

stal

syst

emT

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inic

Tri

clin

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ricl

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Tri

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onoc

lin

icM

onoc

lin

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lin

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lin

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ace

grou

pP1

P1P1

P1P2

1/c

P21/c

P21/c

P21/c

Un

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501(

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733(

2)6.

482(

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506(

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11.2

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)11

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435(

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mium atoms is 4.202 Å. The binuclear units are linked byHBIDC22 ligands to form a one-dimensional trapezoid-likechain (Fig. 1c), the trapezoid-like chains are connected by m2-

O1 of HBIDC22 ligands to construct a two-dimensionalnetwork (Fig. 1d). Furthermore, the two adjacent 2-D coordi-nation networks are further connected to adjacent praseody-mium cations by chelate carboxylate group of the ox22 ligandsto generate the 3-D network, as illustrated in Fig. 1e. The freewater molecules are inserted into the channels.

Topological analysis has been applied for better under-standing the connectivity of 3-D framework in coordinationpolymer 1. If coordinated water molecules and lattice watermolecules are ignored, the binuclear unit [Pr2(m2-O)2] as asecondary building unit is connected to six adjacent praseo-dymium dimers by four HBIDC22 anions and two ox22 anions.It can be viewed as a 6-connected octahedral node, theHBIDC22 and ox22 ligands act as linear bridges between thebinuclear unit nodes. Such connectivity repeats infinitely,resulting in a 3-D pcu network, with the point symbol of{412?63}. The topology is shown in Fig. 1f and the pore size is17.27 6 9.44 Å (diagonal-to-diagonal distances). The effectivefree volume of the channels without these guest molecules isestimated to be 50.1 Å3 by PLATON software,22 almost 8.4% ofthe per unit cell volume of 595.5 Å3.

[Yb(HBIDC)(ox)0.5(H2O)2]n (2). Single-crystal X-ray diffrac-tion analysis shows that coordination polymer 2 includes onecrystallographically unique Yb(III) ion, one HBIDC22 ligand,half ox22 ligand, and two coordinated water molecules in theindependent symmetry unit (Fig. 2a). The Yb(III) atom haseight-coordinated bi-capped triprismatic coordination geome-try (Fig. 2b) with three oxygen atoms (O1, O3A, and O4A)deriving from three HBIDC22 ligands, two oxygen atoms (O5A,O6) from the ox22 anion, and three oxygen atoms from threewater molecules (O7, O7A, and O8). The O(cap)–Yb1–O(cap)bond angle is 125.51(1)u for O3A–Yb1–O5A, the O(cap)–Yb1–O(prism) bond angles range from 69.64(1)u to 142.62(1)u, theO(prism)–Yb1–O(prism) bond angles range from 71.22(1)u to144.90(1)u. The Yb1–O(cap) bond lengths are 2.292(3) Å forYb1–O3A, 2.349(3) Å for Yb1–O5A, the Yb1–O(prism) bondlengths vary from 2.258(3) to 2.486(3) Å. In the coordination

Scheme 1 Reaction routes of coordination polymers 1–8.

Fig. 1 (a) The metal coordination environment in 1 with labeling scheme and50% thermal ellipsoids (hydrogen atoms and free water are omitted for clarity).Symmetry codes: O2A and O3A 1 2 x, 2 2 y, 1 2 z; O4A and O3B 21 + x, y, z;O5A and O6A 2x, 1 2 y, 2z; N1A 1 2 x, 2 2 y, 2z. (b) Polyhedral representationof the coordination sphere of the Pr3+ centre in 1. (Hydrogen atoms, and BIDC32

ligands are omitted for clarity.) (c) The illustration of a 1-D trapezoid-like chain in1. (d) The illustration of 2-D layer structure in 1 (water molecules and hydrogenatoms are omitted for clarity). (e) Three-dimensional framework in 1. (f)Schematic topological view of the 3-D structure of {412?63} topology in 1. (Thewater molecules and hydrogen atoms in the framework are omitted for clarity.Color code: [Pr2(m2-O)2], pink ball; HBIDC22, blue line; ox22 turquoise line.)

Scheme 2 The coordination modes of H3BIDC ligand in the coordinationpolymers 1–8 (A for 1, B for 2–4, C and D for 5–8).


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polymer 2, HBIDC22 ligand displays the m3-gO1:gO

1:gO1 mode

to connect three Yb(III) ions (Scheme 2B), and the 6-carboxylategroup adopts a syn–syn mode to link two metal ions, with theuncoordinated nitrogen atoms acting as the hydrogen-bond-ing acceptor. The oxygen atoms O7 and O7A of two watermolecules as bridging oxygen m2-OH2 to link two adjacentmetal ytterbium ions form dimeric unit [Yb2(CO2)2(m2-OH2)2],adjacent dimeric units [Yb2(CO2)2(m2-OH2)2] are connected toform one-dimensional chain by HBIDC22 ligands (see Fig. 2c).It was also found that within the one-dimensional chain, theseparation of Yb1…Yb1 (bridged by bridging oxygen m2-OH2) is3.851(1) Å. Then, these chains are further connected into aspectacular 2-D grid by the chelate carboxylate group of theox22 ligands, the separation of Yb1…Yb1 (chelated by ox22) is6.101(2) Å, the structure of which differs from coordinationpolymer 2, as illustrated in Fig. 2d. A 3-D supramolecularstructure is obtained by weak p–p stacking interactions (p–pstacking interactions of 3.488 Å) and hydrogen bondinginteractions (see Fig. 2e), which are formed by the nitrogenatoms and carboxylic oxygen atoms of HBIDC22, oxygen atomsof coordination water and the hydrogen atoms of coordinationwater and benzimidazole ring. The obtained structure is very

similar to the complex based on H3BIDC,21 except for adifferent coordination mode of water molecules in crystallattice.

In order to identify the connectivity in ligands and metals,the topology of the whole framework is investigated. Ifcoordinated water molecules was ignored, and the dimericunit [Yb2(CO2)2(m2-OH2)2] considered as the node, andHBIDC22 and ox22 ligands as the linkers, the simplifiedtopological representation of the coordination polymer 2exhibits a 4-connected 44 topology, which is described inFig. 3.

[Tb(HBIDC)(ox)0.5(H2O)3]n (4). Coordination polymers 3 and4 are isomorphous, therefore, only the structure of coordina-tion polymer 4 is described in detail. Single crystal X-raydiffraction analysis reveals that coordination polymer 4crystallize in a triclinic system, P1 space group. As shown inFig. 4, the asymmetric unit of 4 consists of one crystal-lographically unique Tb(III) ion, one HBIDC22 ligand, halfox22 ligand, and three coordinated water molecules. EachTb(III) ion coordinates to three water oxygen atoms, threecarboxylic oxygen atoms from three HBIDC22 ligands and twocarboxylic oxygen atoms from ox22 ligand, forming a distorted

Fig. 2 (a) The metal coordination environment in complex 2 with labeling scheme and 50% thermal ellipsoids (hydrogen atoms are omitted for clarity). Symmetrycodes: O3A 1 + x, y, z; O4A 21 2 x, 2y, 2z; O5A and O6A 21 2 x, 1 2 y, 2z; O7A 2x, 2y, 2z. (b) Polyhedral representation of the coordination sphere of the Yb3+

centre, with display bi-capped trigonal prismatic arrangement in the complex 2. (Hydrogen atoms, and BIDC32 ligands are omitted for clarity.) (c) The illustration of a1-D chain in coordination polymer 2. (d) The illustration of 2-D layer structure in coordination polymer 2 (hydrogen atoms are omitted for clarity). (e) A 3-Dsupramolecular structure of coordination polymer 2 (hydrogen bonding turquoise dashed line).


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bi-capped triprismatic coordination geometry. The isomorphicdysprosium coordination polymer of compound 4 has beenreported by Sun and co-workers.23 The difference from reportsin the literature of isomorphic dysprosium coordinationpolymer is that it forms a distorted square-antiprismaticallycoordination geometry. The Tb–O bond lengths vary from2.320(4) to 2.465(4) Å. All the Tb–O distances are compatiblewith the values of previously published terbium compounds.17

The O–Tb–O bond angles range from 66.70(1)u to 149.22(1)u. Incoordination polymer 4, the 6-carboxylate group adopts a syn–anti mode to link two metal ions of HBIDC22 ligand, HBIDC22

ligand displays the m3-gO1:gO

1:gO1 mode to connect three

Tb(III) ions (Scheme 2B). Each H3BIDC ligand uses threecarboxylate group oxygens linking three adjacent Tb(III) ions toform a 1-D trapezoid-like double chain structure along thecrystallographic a axis (see Fig. 4c), with Tb…Tb separations of5.69 Å, 6.06 Å and 6.51 Å, and the dihedral angles between thebenzimidazole ring and coordinating carboxylate group are64.67(2) and 33.18(2)u. Two adjacent 1-D trapezoid-like doublechains form a 2-D layer structure (Fig. 4d) through the ox22

ligands. A 3-D supramolecular structure is obtained by p–pstacking interactions (p–p stacking interactions of 3.236 Å) andhydrogen bonding interactions (see Fig. 4e), which are formedby nitrogen atoms and carboxylic oxygen atoms of HBIDC22

and ox22 ligands and the hydrogen atoms of coordinationwater and benzimidazole ring.

If coordinated water molecules are ignored, the metalterbium center can be viewed as a 3-connected nodeconnecting adjacent three terbium centers by HBIDC22 andox22 ligands, HBIDC22 and ox22 ligands act as linear bridgesbetween the metal terbium center nodes. Such connectivityrepeats infinitely, resulting in a 2-D (6,3)-connected hcb

network, the pore size of the 6-member ring is 8.90 615.13 Å (Fig. 4f).

Coordination polymers 1–4 have the same triclinic systemand P1 space group, but they exhibit different kinds of crystalstructures. The structural differences between 1–4 are due totwo reasons: one is the different coordination motifs ofHBIDC22 ligands: in the coordination polymer 1, HBIDC22

ligand displays the anti–anti mode and forms m4-gO

1:gO1:gO

2:gN1 mode (Scheme 2A); in the coordination

polymers 2–4, HBIDC22 ligand displays the m3-gO1:gO

1:gO1

mode (Scheme 2B), and the 6-carboxylate group adopts a syn–syn mode for coordination polymer 2 and syn–anti mode forcoordination polymers 3 and 4. The other reason is theincremental coordination water of rare earth ions.

{[Sm(H2BIDC)(HBIDC)(H2O)3]?3H2O}n (6). Coordinationpolymers 5–8 are isomorphous, therefore, only the structureof coordination polymer 6 is described in detail. Single crystalX-ray diffraction analysis reveals that coordination polymer 6crystallizes in the monoclinic system, P21/c space group. Thecoordination polymer 6 has 37 non-hydrogen atoms in theasymmetric unit, which contains one Sm3+ cation, differentdeprotonation anions H2BIDC2 and HBIDC22 of two H3BIDCligands, three coordination water molecules, and three freewater molecules with all in general positions (Fig. 5a). Centermetal Sm1 is nine-coordinated: six chelated carboxyl oxygenatoms (O1, O2, O3, O4, O5, and O6) from two HBIDC22 ligandsand one H2BIDC2 ligand and three oxygen atoms (O9, O10,and O11) from three water molecules, and displays a slightlydistorted tricapped trigonal prismatic arrangement (Fig. 5b).The Sm–O distances range from 1.908(9) to 2.392(1) Å, whichare within the range of reported complexes.17

Fig. 3 (a) The defined nodes of 4-connected dimeric unit [Yb2(CO2)2(m2-OH2)2] in coordination polymer 2. (b) and (c) The defined linkers of 2-connected HBIDC22 andox22 ligands in coordination polymer 2. (d) The 2-D net of 44 topology in its most symmetrical form distinguished by different colors.


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In coordination polymer 6, H3BIDC ligands show two typesof coordination modes (Scheme 2C and 2D): the first one ismono-chelate coordination mode m1-gO

2, with one freecarboxyl oxygen atom uncoordinated to any metal ions(Scheme 2C). The second one adopts bis-chelate manner m2-gO

2:gO2 (Scheme 2D). The adjacent two samarium ions are

connected through chelating carboxyl oxygen atoms accordingto Scheme 2D, giving rise to an infinite one-dimensional right-handed helix and left-handed helix chains (see Fig. 5c). The1-D helical infinite chain around the crystallographic 21 axisspreads along the b axis direction, the pitch of the helix iscalculated to be 9.01 Å containing two Sm(III) ions per turn.The 2-D layer structure is formed by hydrogen bond interac-tions between two same helical chains, different 2-D layerforming a 3-D supramolecular framework alternately via p–pstacking interactions (see Fig. 5d) (p–p stacking interactions of3.793 Å). The isomorphism of coordination polymer 6 hasbeen reported by Liu and co-workers.24 Although we set 120 uCas reaction temperature rather than room temperature, it wasfound that the structure of 6 is similar to the structure

reported by Liu, basing on single-crystal X-ray diffraction data,which indicates that the reaction temperature has no or only aminor effect on the structure of coordination polymer 6.

In order to confirm the phase purity of the bulk materials,powder X-ray diffraction (PXRD) experiments were carried outon complexes 1–8 (see ESI,3 Fig. S5). The PXRD patternsindicated that the patterns are entirely consistent with thesimulated PXRD pattern generated based on the structuresdetermined from the single-crystal data of coordinationpolymers 1–3 and 5. The similar PXRD pattern (see ESI,3 Fig.S5) of coordination polymers 3–8 indicated that the complexesare isomorphous and proved the purity of coordinationpolymers 3–4 and 5–8.

Luminescence properties and lifetimes of coordinationpolymers

Lanthanide ions are well-known to give rise to luminescenceand are used as optical active centers in many of thephosphors used for lighting, scintillating, and plasma displaypanel applications.25 The characteristic luminescence of

Fig. 4 (a) The metal coordination environment in 4 with labeling scheme and 50% thermal ellipsoids (hydrogen atoms are omitted for clarity). Symmetry codes: O3A2x, 1 2 y, 1 2 z; O4A 1 2 x, 1 2 y, 1 2 z; O5A and O6A 1 2 x, 1 2 y, 2 2 z. (b) Polyhedral representation of the coordination sphere of the Tb3+ centre, with displayslightly distorted bi-capped trigonal prismatic arrangement in 4 (hydrogen atoms and BIDC32 ligands are omitted for clarity.) (c) The illustration of a 1-D trapezoid-likedouble chain in 4. (d) The illustration of 2-D layer structure in coordination polymer 4 (water molecules and hydrogen atoms are omitted for clarity). (e) A 3-Dsupramolecular structure of 4 (hydrogen bonding is turquoise dashed line.) (f) The layered structure is parallel to the ac plane with the 6-member ring (the watermolecules and hydrogen atoms in the framework are omitted for clarity. Color code: Tb, pink ball; HBIDC22, blue line; ox22 turquoise line).


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trivalent lanthanide ions mainly arises from the f–f transition,which leads to sharp line emission spectra. After coordination,the organic ligand can transfer its absorbed energy from lightradiation to lanthanide ions. The luminescence and NIRemission spectra of coordination polymers 1–8 were mea-sured, the characteristic NIR emission bands for correspond-ing lanthanide in coordination polymers 1 and 3 were notobserved. For the other coordination polymers 4–8, theluminescence spectra were measured in the solid-state atroom temperature and liquid nitrogen temperature, anddispersed in CH2Cl2 solvents. In particular, in coordinationpolymers 6 and 7, not only were measured characteristictransition of corresponding rare earth ions in the visibleregion, but also determined were infrequent NIR emissionspectra of samarium and dysprosium ions in the near infraredregion, which is a rarely described phenomenon.17–19

For the emission spectra of coordination polymer 2 (Fig. 6),the Yb3+ ion emits in the range of 920–1100 nm, with a sharp

peak around 995 nm assigned to the 2F5/2 A 2F7/2 transition ofthe Yb3+ ion broader vibronic components at longer wave-length.26 Similar splitting has been reported in previousliterature reports.27 This may be the splitting of energy levelsof Yb3+ ion as a consequence of ligand field effects.27a The Yb3+

ion plays an important role in laser emission because of itsvery simple f–f energy level structure.28

The solid-state emission spectra of terbium coordinationpolymers 4 and 5 at room and liquid nitrogen temperature,and dispersed in CH2Cl2 solvent are shown in Fig. 7. Uponexcitation by UV light, we have observed characteristicemission in the visible light region from the Tb3+ ions in thecoordination polymers 4 and 5 at lmax = ca. 490, 544, 584, 622,and 650 nm, which showed characteristic transitions of theTb3+ ion. These originated from the characteristic 5D4 A 7FJ

transition of a sensitized terbium emission, where J = 6, 5, 4, 3,and 2, respectively, and the relative intensity of the sharp-lineband is 5D4 A 7F5 transition.29 The luminescence measure-

Fig. 5 (a) The metal coordination environment in coordination polymer 6 with labeling scheme and 50% thermal ellipsoids (free water molecules and hydrogenatoms are omitted for clarity). Symmetry codes: O3A and O4A 2x, 0.5 + y, 0.5 2 z. (b) Polyhedral representation of the coordination sphere of the Sm3+ centre, withdisplay slightly distorted tricapped trigonal prismatic arrangement in the complex 6 (hydrogen atoms, and BIDC32 ligands are omitted for clarity). (c) Right-handedhelix (R) and left-handed helix (L) along the b axis direction of coordination polymer 6. (d) The 3-D structure along b axis (water molecules and hydrogen are omittedfor clarity).


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ments of 0.1 mg of polymers 4 and 5 dispersed in 10 mLCH2Cl2also showed characteristic 5D4 A 7FJ (J = 6, 5, 4, 3, and2) transition emission of terbium. This indicates that theTb(III) luminescence can be efficiently sensitized by theH3BIDC and H2ox ligands via the ‘‘antenna effect’’.12 Underdifferent conditions, orange and red luminescence is relativelyweak in comparison to the dominant green emission, resultingin a yellow-green emission color of the terbium coordination

polymers 4 and 5 (see the color coordinates diagram in Fig. S1,ESI3). This indicates that the coordination polymers 4 and 5 issuitable for green luminescence material in solid state or inCH2Cl2 solvent at room and liquid nitrogen temperature,meanwhile the color of 4 and 5 are relatively pure. In addition,we also performed time-resolved measurements of terbiumcoordination polymers 4 and 5 by using the time-correlatedsingle photon counting (TCSPC) technique. The fluorescencedecay curves of coordination polymers 4 and 5 are shown inFig. 7. The decay curves are well fitted into a single indexfitting attenuation function: I0 = I + Aexp(2t/t1), I0 and I are theluminescent intensities when time t = t and t = 0, respectively,and t1 is defined as the luminescent lifetime.30 Luminescencelifetimes of coordination polymers 4 and 5 under differentconditions are shown in Table 3. For coordination polymers 4and 5, the solid-state luminescence lifetimes at roomtemperature are longer than those at liquid nitrogen tempera-ture. The fluorescence lifetimes of 4 are longer than 5 in solidstate or in CH2Cl2 solvent at room and liquid nitrogentemperature, which is observed in the fluorescence decaycurves. This phenomenon can be attributed to the followingreasons: firstly, there are more free water molecules incoordination polymer 5 than that in coordination polymer 4,which can increase the radiationless transition and result inthe decreased of fluorescence intensity. Secondly, oxalic acidfurther sensitized trivalent terbium ion of coordination

Fig. 6 NIR emission spectra of coordination polymer 2 in the solid-state at roomtemperature.

Fig. 7 The solid-state emission spectra and luminescence decay curves of 4 and 5.


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polymer 4, and result in the fluorescence lifetime ofcoordination polymer 4 being longer than coordinationpolymer 5. Finally, coordination polymer 4 has the longestlifetime on account of the 2-D condensed structure, whichsuggests that the lanthanide ions in 4 are well shielded fromnonradiative deactivations.

The solid-state emission spectra of samarium coordinationpolymer 6 at room and liquid nitrogen temperature, anddispersed in CH2Cl2 solvent are shown in Fig. 8. The solid-state emission spectra at room temperature consists of fourmain bands arising from 4G5/2 A 6HJ transitions: the 4G5/2 A6H5/2 transition at 562 nm, the 4G5/2 A 6H7/2 transition at 596nm, the 4G5/2 A 6H9/2 transition at 642 nm, and the 4G5/2 A6H11/2 transition at 704 nm. In the solid-state emissionspectrum at room temperature of H3BIDC, we have observedligand-centered p* A p transitions at 399 nm (ESI3). In theemission spectra of coordination polymer 6, we have observedligand-centered p* A p transitions at ca. 420 nm, which arered-shifted compared with free ligand H3BIDC.31 This red-shiftmay be attributed to the metal-disturbed ligand-centered p* Ap transitions.4e,32 The 4G5/2 A 6H11/2 transition at 704 nm isdisappearing, which can be attributed to the followingreasons: there are a considerable number of excitation ionswhich exist at the 4G5/2 energy level, are transferred to 6F9/2

energy level by cross-relaxation, after available excitation ionsare consumed with radiation and relaxation process, thefluorescence emission is reduced.33 Such a hypersensitivetransition can also be found in Sm3+ (4G5/2 A 6H9/2), fulfillingthe selection rule DJ = 2 (electric-dipole allowed). It is proposedto use the 4G5/2 A 6H5/2 transition of Sm3+ as a reference,because it has a predominant magnetic dipole character (DJ =0).34 The relative intensity of the 4G5/2 A 6HJ transition and p*A p transition changes the color from yellow-white at roomtemperature to pink-white and blue-white at liquid nitrogentemperature (see the color coordinates diagram in Fig. 8).Luminescence emission of Sm3+ ions have been ofteninvestigated in visible region, NIR emission are limited innear infrared region.17,19 Characteristic transition of samar-ium ion not only was measured in the visible region, but also

in the near infrared region. The NIR emission spectra ofcoordination polymer 6 were measured in the solid state at 298K. In the NIR emission spectra of coordination polymer 6, wehave observed characteristic emission in the near-IR from theSm3+ ions upon excitation of UV light. In the coordinationpolymer 6, the NIR emission spectrum (Fig. 8) consisting ofseveral bands at l = 828, 919, 940, 980, 994, 1125, 1196, 1286and 1417 nm are clearly observed. Discernible peaks at 919 nmand 940 nm, 980 nm and 994 nm, 1125 nm and 1196 nm aresuspected to be the Stark splitting of 6F5/2, 6F7/2 and 6F9/2,respectively. The other emissions are assigned to the f–ftransitions of 4G5/2 to 6F3/2 (828 nm), and 6F11/2 (1286 nm and1417 nm), respectively. The emission bands of coordinationpolymer 6 are shifted relative to the bands of the reportedtheoretical values.35 In addition to the steady-state emission,we also performed time-resolved measurements by usingTCSPC technique. The fluorescence decay curve of coordina-tion polymer 6 is shown in Fig. 8. Luminescence lifetimes ofcoordination polymer 6 at different conditions are shown inTable 3. The fluorescence lifetimes of coordination polymer 6at 298 K are longer than at 77 K, which is observed in thefluorescence decay curves. This phenomenon can be attrib-uted to the quenching of Sm3+ cation at low temperature.

The solid-state emission spectra of dysprosium coordinationpolymer 7 at room and liquid nitrogen temperature, anddispersed in CH2Cl2 solvent are shown in Fig. 9. In addition,we have observed characteristic emission in the near-IR fromthe Dy3+ ions in coordination polymer 7 (see Fig. 9), which is ararely described phenomenon.17,18 The emission spectrum ofcoordination polymer 7 was obtained under the excitationwavelength at ca. 310 nm. The room and liquid nitrogentemperature emission spectra of coordination polymer 7shows four emission bands in the visible region, two strongbands at ca. 480 nm (4F9/2 A 6H15/2) and ca. 575 nm (4F9/2 A6H13/2) and two weaker bands at ca. 664 nm (4F9/2 A 6H11/2)and at ca. 750 nm (4F9/2 A 6H9/2 + 6F11/2).36 In the dysprosiumcoordination polymers characteristic 4F9/2 A 6H9/2 + 6F11/2

transition is rarely observed and described.29 At roomtemperature, the blue emission arises from a magnetic dipole

Table 3 Luminescence data for coordination polymers 1–8a

Coordination polymers lex (nm) lem (nm) t (ms) Conditions

2 298 995, 1091 — Solid state, 298 K4 286 491, 544, 584, 621, 650 1058.58 Solid state, 298 K

295 490, 545, 584, 622, 651 679.42 Solid state, 77 K302 490, 544, 584, 622, 651 867.82 CH2Cl2, 77 K

5 305 491, 545, 585, 622, 650 595.06 Solid state, 298 K305 490, 544, 586, 622, 650 583.19 Solid state, 77 K307 492, 544, 584, 622, 650 584.38 CH2Cl2, 77 K

6 307 433, 562, 596, 642, 704 14.22 Solid state, 298 K303 406, 562, 597, 648 6.11 Solid state, 77 K305 420, 597, 646 7.35 CH2Cl2, 77 K307 828, 919, 940, 980, 994, 1125, 1196, 1286, 1417 — Solid state, 298 K

7 313 481, 574, 663, 752 7.63 Solid state, 298 K305 480, 575, 664, 748 6.91 Solid state, 77 K308 409, 480, 575, 664, 753 60.39 CH2Cl2, 77 K313 835, 965, 1151, 1325, 1503 — Solid state, 298 K

8 316 410 1085 Solid state, 77 K

a Emission of coordination polymers 1 and 3 not determined.


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transition, whereas the yellow emission is due to a forcedelectric dipole. The latter one can be considered as ahypersensitive transition based on the selection rules. Sincethe coordination environment of the optically active ion is ableto influence the hypersensitive, electric-dipole-governed tran-sition, while the magnetic dipole transition remains insensi-tive to the crystal field, a different ratio of the blue to yellowemission can be achieved, thus changing the visible impres-sion of the emission color from blue to yellow. Thecontribution of the greenish-blue luminescence is relativelysmall in comparison to the dominant yellow emission,resulting in a yellow emission color of the dysprosiumcoordination polymer 7 (see the color coordinates diagramin Fig. 9). The emission spectra of coordination polymers 7dispersed in CH2Cl2 solvent at liquid nitrogen temperaturecontains characteristic transitions of Dy(III) cations at 481, 575,664 and 753 nm, we have observed ligand-centered p* A p

transitions at 409 nm, which are 10 nm red-shifted comparedwith free ligand H3BIDC.30 This red-shift may be attributed tothe metal-disturbed ligand-centered p* A p transitions.4e,32

There exits vibration coupling of the phonon with thecoordinated water molecules from solution, it is an effectiveway for rare earth ion emission state non-radiative deactiva-

tion, consequently causing quenching fluorescence. Thecollision quenching interaction between ligands and coordi-nated water molecules is reduced when the coordinationpolymer is dispersed in CH2Cl2 at liquid nitrogen temperature.The rate of non-radiative decay process was also reduced aswell as the quenching effect from oxygen on the luminescence.For these reasons, ligand-centered p* A p transitions wereobserved in the cryogenic emission spectrum from CH2Cl2,and led to an increase of Dy3+ other transitions intensity.37 At77 K, ligand-centered p* A p transition contributions increase,thus changing the visible impression of the emission colorfrom yellow to light blue-white (see the color coordinatesdiagram in Fig. 9). Luminescence emissions of Dy3+ ion havebeen often investigated in visible region, NIR emission ofwhich are limited in near infrared region.17,18 The character-istic transition of Dy3+ ion is not only measured in the visibleregion, but also in the near infrared region. The NIR emissionspectra of coordination polymer 7 were measured in the solidstate at 298 K. In the NIR emission spectra of coordinationpolymer 7, we have observed characteristic emission in thenear-IR from the Dy3+ ions upon excitation of UV light. TheNIR emission peak of Dy3+ coordination polymer is a singlesharp transition. In the coordination polymer 7, the emission

Fig. 8 The solid-state emission spectra and NIR emission spectra, color coordinate diagram of the corresponding emission and the luminescence decay curves ofcoordination polymer 6.


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spectrum (Fig. 9) consists of several bands at l = 835, 965,1151, 1325 and 1503 nm, which are attributed to the f–ftransitions 4F9/2 A 6H7/2 + 6F9/2, 4F9/2 A 6H5/2, 4F9/2 A 6F3/2,6F1/2 + 6H9/2 A 6H15/2 and 6F5/2 A 6H11/2, respectively. Inaddition to the steady-state emission, we also performedTCSPC technique. The fluorescence decay curve of coordina-tion polymer 7 is shown in Fig. 9. Luminescence lifetimes ofcoordination polymer 7 under different conditions are shownin Table 3. The fluorescence lifetime of coordination polymer 7dispersed in CH2Cl2 solvent is longer than that in the solidstate, which is observed in the fluorescence decay curves. Thisphenomenon can be attributed to the following reason: thereexists vibration coupling of the phonon with the coordinatedwater molecules from solution, for rare earth ion, it is aneffective way for emission state non-radiative deactivation,thus causing quenching fluorescence consequently. Thecollision quenching interaction between ligands and coordi-nated water molecules is reduced when the coordinationpolymer is dispersed in CH2Cl2 at liquid nitrogen temperature.The rate of non-radiative decay process is also reduced as wellas quenching effect from oxygen on the luminescence.37

In the fluorescence decay curves of coordination polymers4–7, fluorescence lifetimes of coordination polymers 4–7 insolid state are shorter than luminescence lifetimes in CH2Cl2,

which can be attributed to the following reasons: first, thecollision quenching interaction between ligands and coordi-nated water molecules is reduced when the coordinationpolymer is dispersed in CH2Cl2 at liquid nitrogen temperature.The rate of non-radiative decay process is also reduced as wellas the quenching effect from oxygen on the luminescence.Second, in relaxation process, the reduction of cross-relaxationbetween rare earth ions restrains useful number of excitedions in CH2Cl2, and results in the fluorescence lifetimes ofcoordination polymers 4–7 in CH2Cl2 being longer than insolid state.

Energy transfer processes studies

To elucidate the energy transfer processes of the lanthanidecoordination polymers, the energy levels of the relevantelectronic states of the ligand have been investigated. Thesinglet and triplet energy levels of the ligand were estimated byreferring to their wavelengths of UV-vis absorbance edges andthe lower wavelength of the 0–0 phosphorescence band ofgadolinium complex.38 According to Reinhoudt’s empiricalrule,39 the intersystem crossing process becomes effectivewhen DE (S1 2 T1) is at least 5000 cm21. It is necessary thatthere exists a suitable energy gap between the ligand-centeredtriplet state and the lanthanide ion emissive states. Latva’s

Fig. 9 The solid-state emission spectra and NIR emission spectra, color coordinate diagram of the corresponding emission and the luminescence decay curves ofcoordination polymer 7.


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empirical rule40 states that an optimal ligand-to-metal energytransfer process need 2500–4500 cm21 for Ln(III).

The singlet energy level (S1) of H3BIDC is 30 303 cm21 (330nm) (Fig. S2, ESI3). The lowest triplet energy level of H3BIDC is24 390 cm21 (410 nm) (see Fig. S3, ESI3), which was calculatedby the low-temperature (77 K) phosphorescence spectrum ofgadolinium coordination polymer 8. The energy gap DE (S1 2

T1) for the H3BIDC is 5913 cm21. Thus, the intersystemcrossing is effective in H3BIDC. In coordination polymers 4–8,the experimentally observed (T1) energy level of H3BIDC is6490 cm21 higher than the 4G5/2 level of Sm(III) (17 900 cm21),3890 cm21 higher than the 5D4 level of Tb(III) (20 500 cm21)and 3390 cm21 higher than the 4F9/2 level of Dy(III) (21 000cm21), respectively. This phenomenon indicates the transitionfrom the triplet energy level of H3BIDC ligand to terbium anddysprosium is effective.41 We think that the luminescenceefficiency of the Ln(III) ion can be sensitized depending on thematching between energy levels of Ln(III) ions and ligand(H3BIDC). A model for the indirect excitation mechanism isproposed based on above discussion, as shown in Scheme 3.The energy transfer from the H3BIDC ligand to lanthanideions follows the sequence of Tb3+, Dy3+ > Sm3+.

In coordination polymer 4, the singlet excited state andlowest triplet excited state of H2ox is 30 842 cm21 (324 nm)and 23 753 cm21 (421 nm), respectively (Fig. S4, ESI3). Theenergy gap is 7089 cm21, therefore, the intersystem crossingenergy of H2ox is effective. The experimentally observed T1

energy level of H2ox is 3253 cm21 higher than the 5D4 level ofTb(III), T1 energy level of H3BIDC is 3890 cm21 higher than the5D4 level of Tb(III) in coordination polymer 4. This phenom-

enon indicates the transition from the triplet energy level ofH3BIDC and H2ox to terbium is effective. Therefore, theabsorbent energy of H2ox as second ligand directly transferredto trivalent terbium ion,37 a model is proposed to represent theindirect excitation process (see Scheme 3). Firstly, the ligandsH3BIDC and H2ox absorb the energy and are excited from thesinglet S0 ground state to the singlet S1 excited state. Then theenergy in the S1 state is transferred to the T1 state of theligands via the intersystem-crossing, and followed by therelaxation from the upper 4f levels to the first excited states ofLn3+ ions, resulting in the emission of the sensitized Ln3+

ions.42,27b

Thermal analysis coordination polymers 1–8

The thermal stability of complexes in air was examined by theTGA techniques in the temperature range of 30–700 uC. TheTGA curves of the complexes 1–8 are shown in Fig. S7 (in ESI3),thermal decomposition data for coordination polymers 1–8 arelisted as Table S4 (in ESI3).

The thermal behaviors of 1–8 from 25 uC to 700 uC under air(Fig. S7, ESI3) display two weight loss steps. For coordinationpolymers 1–4, they possess identical ligands H3BIDC andH2ox, the TGA curves of coordination polymers 1–4 are similarand coordination polymer 4 is used as an example. The TGAcurve (Fig. S7, ESI3) shows that coordination polymer 4 has twoweight loss stages. Coordination polymer 4 loses weight from120 uC to 330 uC corresponding to the loss of two coordinationwater molecules (weight loss: observed 7.34%, calculated7.81%). Finally, from 330 uC to 600 uC, the loss correspondingto remnant coordination water molecule and all H3BIDC andH2ox ligands (observed 51.83%, calculated 52.52%). A white

Scheme 3 Schematic energy 4f level diagram showing the emissive levels of the Sm3+, Gd3+, Tb3+, Dy3+ and Yb3+ ions, and model for the main intramolecular energytransfer process.


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residue of Tb2O3 (observed 40.83%, calculated 39.67%)remained.

Coordination polymers 5–8 are isomorphous, therefore, thethermal behaviors of 5–8 from 25 uC to 700 uC under air (Fig.S7, ESI3) display two similar weight loss steps, and the TGAcurve of complex 8 is used as an example. The first of 13.37%of 8, from 110 uC to 300 uC, is assigned to the loss of three freewater molecules and two coordination water molecules (calcd13.49%). The second weight loss of 60.58% from 350 uC to 600uC corresponds to the decomposition of all H3BIDC ligandsand one coordination water molecule. The remaining weightof 26.05% corresponds to the percentage (calculated 26.73%)of the Gd and O components, indicating that the final productis Gd2O3.

Conclusion

1-D, 2-D and 3-D lanthanide coordination polymers 1–8 weresynthesized under hydrothermal conditions. Topologicalanalysis illustrates that a 3-D pcu network, with the pointsymbol of {412?63} is displayed in coordination polymer 1. Theresult of X-ray single-crystal diffraction indicates coordinationpolymer 2 exhibits a 4-connected 44 topology, coordinationpolymers 3–4 appear 2-D structures with (6,3)-connected hcbnetwork topology, and coordination polymers 5–8 are 1-Dhelical infinite chains. Luminescence studies in the visiblerange demonstrates that oxalic acid and H3BIDC ligands cansensitize and improve the lanthanide ions’ fluorescenceproperty. The fluorescence lifetime of coordination polymer4 is longer than that of coordination polymer 5, which iscaused by the radiationless transition of coordinated watermolecules and the concurrent sensitization of oxalic acid andH3BIDC ligands. The coordination polymers 4 and 5 aresuitable green luminescence materials for application indifferent conditions, and the color of 4 and 5 are relativelypure. The luminescence properties in NIR region of thesamarium and dysprosium coordination polymers show raref–f transitions 4G5/2 A 6F3/2, 6F5/2, 6F7/2, 6F9/2, 6F11/2 (6) and 4F9/2

A 6H7/2 + 6F9/2, 4F9/2 A 6H5/2, 4F9/2 A 6F3/2, 6F1/2 + 6H9/2 A 6H15/2,6F5/2 A 6H11/2 (7). Based upon the TGA analyses, the structuresare rather stable, and which is undoubtedly due to themultidentate ligands forming a rigid framework for 1–8.

Acknowledgements

This work was supported by the National Natural ScienceFoundation of China (Grant Nos. 21071035 and 21171044),National key Basic Research Program of China (973 Program,No. 2013CB632900), and the key Natural Science Foundationof the Heilongjiang Province, China (No. ZD201009).

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