CrystEngComm PAPER. Introduction

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1 PAPER View Article Online View Journal View Issue Cite this:, 2013, 15, 4489 Received 28th January 2013, Accepted 27th March 2013 DOI: /c3ce40184g 1-D helical chain, 2-D layered network and 3-D porous lanthanide organic frameworks based on multiple coordination sites of benzimidazole-5,6-dicarboxylic acid: synthesis, crystal structure, photoluminescence and thermal stability3 Ping Wang, a Rui-Qing Fan,* a Yu-Lin Yang,* a Xin-Rong Liu, a Peng Xiao, a Xin-Yu Li, a Wuliji Hasi a and Wen-Wu Cao b One-dimensional to three-dimensional lanthanide coordination polymers 1 8 based on benzimidazole- 5,6-dicarboxylic acid (H 3 BIDC) have been synthesized under hydrothermal conditions at different ph values, generally formulated as {[Pr(HBIDC)(ox) 0.5 (H 2 O)]?H 2 O} n (1), [Yb(HBIDC)(ox) 0.5 (H 2 O) 2 ] n (2), and [Ln(HBIDC)(ox) 0.5 (H 2 O) 3 ] n [Ln = Ho (3), and Tb (4)] and {[Ln(H 2 BIDC)(HBIDC)(H 2 O) 3 ]?3H 2 O} n [Ln = Tb (5), Sm (6), Dy (7), and Gd (8), H 2 ox = 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 {4 12?6 3 } in coordination polymer 1. Coordination polymer 2 exhibits a 4-connected 4 4 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 2 1 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 CH 2 Cl 2 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 ( ms at298k, ms at 77 K in the solid-state, ms inch 2 Cl 2 at 77 K) than coordination polymer 5 ( ms at 298 K, ms at 77 K in the solid-state, ms inch 2 Cl 2 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 ( cm 21 ) and the lowest triplet state energy level ( cm 21 )ofh 3 BIDC 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 H 3 BIDC ligand to lanthanide ions follows the sequence of Tb 3+,Dy 3+ >Sm 3+. Finally, thermal behaviors of all coordination polymers were studied by thermogravimetric analysis, which exhibited high thermal stability. Introduction a Department of Chemistry, Harbin Institute of Technology, Harbin , P. R. China. fanruiqing@hit.edu.cn; ylyang@hit.edu.cn; Fax: b Materials Research Institute, The Pennsylvania State University, University Park, PA 16802, USA 3 Electronic supplementary information (ESI) available. CCDC For ESI and crystallographic data in CIF or other electronic format see DOI: /c3ce40184g In recent years, novel metal organic frameworks (MOF) based on crystal engineering have attracted extensive attention not only due to their intriguing topologies and diverse structures but also owing to their interesting physical and chemical properties, such as photoluminescence, magnetism, gas storage, ion exchange, and catalysis, etc. 1 4 Among present contributions, metal ions are interlinked by organic bridging ligands containing functional groups such as imidazole and/or This journal is ß The Royal Society of Chemistry 2013, 2013, 15,

2 carboxylate groups to form infinite network structures, such as one-dimensional (1-D) chains 5 and ladders, 6 two-dimensional (2-D) grids, 7 three-dimensional (3-D) microporous networks, 8 interpenetrated mode, 9 and helical networks. 10 Recently, we have synthesized a number of metal organic coordination polymers and studied the luminescent properties of these complexes. 4 However, lanthanides, with high and variable coordination numbers (6 CN 12) 11 and flexible coordination environments, showed very limited success in the design of predetermined molecular architectures, therefore, rational design and synthesis lanthanide coordination polymer crystal materials with luminescent properties are still facing a great challenge. 2d,10b The coordination polymer, especially with luminescence in the range of nm from lanthanide ions such as Nd(III), Er(III), and Yb(III), are particular attractive because of their potential application in various optical and medical devices. 12,13 Lanthanides have intense affinity to oxygen atoms, so ligands containing oxygen atoms (such as oxalic acid) can be used as stable bridging ligands in the synthetic process of lanthanide complexes. 14,15 In the carboxylate family, N-heterocyclic multicarboxylic acids have been widely used to construct MOF for their potential application. 16 So far, to our knowledge, the solid state luminescence spectra of lanthanide coordination polymers have been researched at room temperature, but their luminescence properties in solutions or at 77 K have been rarely investigated. The investigation of luminescence emissions of Sm 3+ and Dy 3+ ions have been often studied based in the visible region, the emission of which were limited in the near infrared region Herein, we have chosen a multifunctional ligand, benzimidazole-5,6-dicarboxylic acid (H 3 BIDC) as the main ligand, which is a derivative of 4,5-imidazoledicarboxylic acid, and exhibits several interesting characteristics: (I) multifunctional coordination sites containing imidazole and carboxylate groups which provide a high likelihood for construction of different dimensional coordination polymers. (II) H 3 BIDC can be partially or completely deprotonated to generate H 2 BIDC 2, HBIDC 22, and BIDC 32 by controlling the ph carefully. (III) H 3 BIDC usually adopts difference coordination motifs, such as terminal monodentate chelating to one metal center, bridging bidentate in a syn syn, syn anti, and anti anti configuration to two metal centers, and bridging tridentate to two metal centers. (IV) The p-conjugated system in the benzimidazole ring is a good medium for transferring energy. H 3 BIDC possesses the capability to chelate and bridge metal centers in various coordination modes through the nitrogen atoms of the benzimidazole ring and carboxylate oxygen atoms, and allows us to explore further by adding another auxiliary organic ligand oxalic acid (H 2 ox) to the reaction mixture. To the best of our knowledge, cases of lanthanide coordination polymers linked by H 3 BIDC have been presented and the lanthanide coordination polymers based on benzimidazole-5,6-dicarboxylic acid are summarized in Table 1. In this work, we report the synthesis, structures, thermal properties and luminescent properties of eight lanthanide coordination polymers containing 1-D to 3-D structures formulated as {[Pr(HBIDC)(ox) 0.5 (H 2 O)]?H 2 O} n (1), [Yb(HBIDC)(ox) 0.5 (H 2 O) 2 ] n (2), [Ln(HBIDC)(ox) 0.5 (H 2 O) 3 ] n [Ln = Ho (3), and Tb (4)] and {[Ln(H 2 BIDC)(HBIDC)(H 2 O) 3 ]?3H 2 O} n [Ln = Tb (5), Sm (6), Dy (7), and Gd (8)]. The luminescence properties in the visible region and fluorescence lifetimes of coordination polymers 4 7 in the solid state at room and liquid nitrogen temperature and dispersed in CH 2 Cl 2 as suspensions at 77 K were discussed. Particularly, NIR emission spectra of Yb (2), Sm (6), and Dy (7) coordination polymers in solid state at room temperature were measured. Finally, thermal behaviors of all 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 [(Eu 2 (Hbidc) 2 (ox) 2?(H 2 O) 3 ] P1 2-D 21 2 [Tb 2 (Hbidc) 2 (ox)(h 2 O) 2 ]?4H 2 O P1 2-D 21 3 [Er 2 (Hbidc) 2 (ox)] P1 3-D 21 4 [Eu(C 9 H 4 N 2 O 4 )(C 9 H 5 N 2 O 4 )(H 2 O) 3 ]?2H 2 O} n P1 1-D 43 5 [Eu 2 (C 9 H 5 N 2 O 4 ) 2 (SO 4 ) 2 (H 2 O) 6 ]?6H 2 O} n P1 1-D 44 6 [Ln 2 (Hbidc) 2 (SO 4 )(H 2 O) 3 ] n (Ln = La, Pr, Sm, Gd) P1 2-D 7a 7 [Ln 4 (Hbidc) 4 (SO 4 ) 2 ] n?2nh 2 O (Ln = Eu, Tb, Dy, Er) P2 1 /c 3-D 7a 8 {[Ln 3 (bidc) 4 (phen) 2 (NO 3 )]?2H 2 O} n (Ln = Gd, Eu, Tb) P D 45 9 {[Tb(L)(HL)(H 2 O)]?H 2 O} n P2 1 /n 2-D 16d 10 {[Ln 2 L 2 (HL) 2 (H 2 O) 2 ]} n (Ln = Ho, Er, Lu) P2 1 /c 2-D 16d 11 [Ln 2 L 3 (H 2 O)] [Ln = Eu, Tb] P2 1 /c 3-D [Pr(L)(HL)H 2 O]?H 2 O P2 1 /n 2-D {[Er(Hbmdc)(bmdc)(H 2 O) 3 ]?3H 2 O} n (1) P2 1 /c 1-D [Er 2 (Hbmdc) 2 (bmdc) 2 (H 2 O) 8 ]?8H 2 O(2) P1 0-D [Ln(bidc)(Ac)?H 2 O] n (Ln = Tb (1), Dy (2)) P2 1 /c 2-D {[Ln(Hbidc)(ox) 1/2 (H 2 O)]?H 2 O} n [Ln = Pr, Nb, Sm] P1 3-D [Ln(Hbidc)(ox) 1/2 ] n [Ln = Eu, Gd] P1 3-D [Dy(H 2 bidc)(hbidc)(h 2 O) 8 ]?8H 2 O P1 0-D {[Dy(Hbidc)(H 2 O) 2 (Htzac)]?3H 2 O} n P1 1-D [Dy(C 2 O 4 ) 0.5 (Hbidc)(H 2 O) 3 ] n P1 2-D {[Dy 2 (Hbidc) 2 (H 2 O)(SO 4 )]?H 2 O} n Pc 3-D [Pr(C 9 H 4 N 2 O 4 )(C 2 H 3 O 2 )(H 2 O)] n P1 2-D , 2013, 15, This journal is ß The Royal Society of Chemistry 2013

3 Experimental section Materials and methods All reagents were commercially available and used without further purification. Infrared spectra were obtained from KBr pellets using a Nicolet Avatar-360 Infrared spectrometer in the cm 21 region. Powder X-ray diffraction (PXRD) patterns were recorded in the 2h range of 10 40u using Cu Ka radiation by Shimadzu XRD-6000 X-ray Diffractometer. Elemental analyses were performed on a Perkin-Elmer 240c element analyzer. Inductively coupled plasma (ICP) analysis was performed on a Perkin-Elmer Model Optima 3300 DV ICP spectrometer. Luminescence spectra and fluorescence lifetimes were measured with by an Edinburgh FLSP920 combined steady state fluorescence and phosphorescence lifetime spectrometer. The thermal analysis was performed on a ZRY- 2P thermogravimetric analyzer from 30 uc to 700 uc with heating rate of 10 uc min 21 under a flow of air. Synthesis of {[Pr(HBIDC)(ox) 0.5 (H 2 O)]?H 2 O} n (1) A mixture of Pr(NO 3 ) 3?5H 2 O (41.7 mg, 0.1 mmol), H 3 BIDC (20.6 mg, 0.1 mmol) and C 2 H 2 O 4?2H 2 O (12.6 mg, 0.1 mmol) was dissolved in distilled water (8.0 ml), adjusted to ph # 7.8 with an aqueous solution of piperazine while stirring, continued to stir 20 min, then sealed in a 20 ml Teflon-lined stainless steel autoclave and heated at 120 uc for 5 days. After the mixture was cooled slowly to room temperature, green block crystals (1) were obtained (yield, 89%, based on Pr). Elemental analysis for 1:C 10 H 8 N 2 O 8 Pr (M r : ). 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, cm 21 ): 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 (H 2 O) 2 ] n (2) and [Ln(HBIDC)(ox) 0.5 (H 2 O) 3 ] n [Ln = Ho (3), and Tb (4)] The methods used for the syntheses of coordination polymers 2 4 are similar. Ln(NO 3 ) 3?5H 2 O (Ln = Yb, Ho, Tb) was used instead of Pr(NO 3 ) 3?5H 2 O, the following steps are similar to that for coordination polymer 1. The mixture was dissolved in distilled water and adjusted to ph # 7.4 or 7.0 with an aqueous solution of piperazine while stirring. After the mixture was cooled slowly to room temperature, colorless block crystals (2) were obtained (yield, 88%, based on Yb), ivory block crystals (3) were obtained (yield, 81%, based on Ho), colorless block crystals (4) were obtained (yield, 83%, based on Tb). Elemental analysis for 2: C 10 H 8 N 2 O 8 Yb (M r : ). 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, cm 21 ): 3434 (s), 1630 (vs), 1400 (m), 1331 (s), 864 (m), 800 (m), 489 (m). Elemental analysis for 3: C 10 H 10 HoN 2 O 9 (M r : ). Calcd: C, 25.71; N, 6.00; H, 2.16%. Found: C, 25.92; N, 6.23; H, 2.34%. ICP analysis gave the following composition: Ho, 35.71% (calcd is 35.31%). IR (KBr, cm 21 ): 3445 (s), 1627 (vs), 1416 (s), 1324 (s), 1196 (m), 1091 (m), 813 (m), 502 (m). Elemental analysis for 4: C 10 H 10 N 2 O 9 Tb (M r : ). Calcd: C, 26.05; N, 6.07; H, 2.19%. Found: C, 26.31; N, 6.29; H, 2.33%. ICP analysis gave the following composition: Tb, 34.79% (calcd: 34.46%). IR (KBr, cm 21 ): 3445 (s), 1623 (vs), 1411 (s), 1319 (s), 1192 (m), 1089 (m), 809 (s), 497 (s). Synthesis of {[Tb(H 2 BIDC)(HBIDC)(H 2 O) 3 ]?3H 2 O} n (5) A mixture of Tb(NO 3 ) 3?5H 2 O (22.1 mg, 0.05 mmol) and H 3 BIDC (10.3 mg, 0.05 mmol) was dissolved in distilled water (5.0 ml), adjusted to ph # 4.5 with an aqueous solution of piperazine while stirring, continued to stir 20 min, then sealed in a 20 ml Teflon-lined stainless steel autoclave and heated at 120 uc for 5 days. After being slowly cooled to room temperature, colorless clubbed crystals were obtained (yield, 58%, based on H 3 BIDC). Elemental analysis for C 18 H 21 N 4 O 14 Tb (M r : ): 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 composition: Tb, 23.82% (calcd: 23.50%). IR (KBr, cm 21 ): 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(H 2 BIDC)(HBIDC)(H 2 O) 3 ]?3H 2 O} n [Ln = Sm (6), Dy (7), and Gd (8)] The methods used for the syntheses of coordination polymers 6 8 are similar. Ln(NO 3 ) 3?5H 2 O (0.05 mmol Ln = Sm, Dy, and Gd) was used instead of Tb(NO 3 ) 3?5H 2 O, the following steps are similar to that for coordination polymer 5. After the mixture was cooled slowly to room temperature, suitable crystals of 6 8 for single-crystal X-ray diffraction were obtained. For 6, light yellow clubbed crystals were obtained (yield, 53%, based on H 3 BIDC). Elemental analysis for C 18 H 21 N 4 O 14 Sm (M r : ). Calcd: C, 32.38; H, 3.17; N, 8.39%. Found: C, 32.76; H, 3.32; N, 8.73%. ICP analysis gave the following composition: Sm, 22.89% (calcd: 22.52%). IR (KBr, cm 21 ): 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 H 3 BIDC), elemental analysis for C 18 H 21 DyN 4 O 14 (M r : ): 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, cm 21 ): 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 H 3 BIDC), elemental analysis for C 18 H 21 GdN 4 O 14 (M r : ): calcd: C, 32.05; N, 8.30; H, 3.14%. Found: C, 32.37; N, 8.52; H, 3.38%. ICP analysis gave the following composition: Gd, 23.70% (calcd: 23.31%). IR (KBr, cm 21 ): 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 for coordination polymers 1 8 were collected on a Rigaku R-AXIS RAPID IP or a Siemens SMART 1000 CCD diffractometer equipped with graphite-monochromated Mo Ka radiation (l = Å). The crystal structures were resolved by direct method and refined by semi-empirical formula from equivalents and full-matrix least squares based on F 2 using the SHELXTL 5.1 software package. 20 All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were fixed at This journal is ß The Royal Society of Chemistry 2013, 2013, 15,

4 calculated positions and refined by using a riding mode except water molecules. The CCDC contain the crystallographic data 1 8 for this paper. Crystal structure data and details of the data collection and the structure refinement are listed in Table 2, selected bond lengths, bond angles and hydrogen bonding data of coordination polymers 1 8 are listed in Tables S1 S3 (ESI3). Results and discussion Syntheses and characterization Eight lanthanide coordination polymers, {[Pr(HBIDC)(ox) 0.5 (H 2 O)]?H 2 O} n (1), [Yb(HBIDC)(ox) 0.5 (H 2 O) 2 ] n (2), [Ln(HBIDC)(ox) 0.5 (H 2 O) 3 ] n [Ln = Ho (3), and Tb (4)] and {[Ln(H 2 BIDC)(HBIDC)(H 2 O) 3 ]?3H 2 O} n [Ln = Tb (5), Sm (6), Dy (7), and Gd (8)], have been synthesized under hydrothermal conditions. The crystals of coordination polymers 1 8 were synthesized by reacting H 3 BIDC with the corresponding lanthanide nitrates at 120 uc for 5 days under hydrothermal conditions at different ph values. The reaction route of 1 8 is shown in Scheme 1. Coordination polymer 1 displayed a 3-D MOF structure, 2 4 displayed different 2-D structures and 5 8 displayed 1-D structures, and these results can be easily understood considering the following factors. First, the reaction ph value is one of main influencing factors in the synthesis of coordination polymers 1 8. H 3 BIDC is partially deprotonated to generate H 2 BIDC 2, and HBIDC 22, a low ph value will restrain the deprotonation of H 3 BIDC, and result in difficulty in coordinating to the metal centers. Higher ph value in the reaction system can enhance the coordination competence of H 3 BIDC ligand in the crystal structure. In the synthesis of coordination polymers 1 8, ph values for coordination polymers 1 4 (at ca. 7 8) are higher than those for coordination polymers 5 8 (at ca. 4.5), which results in forming the 3-D and 2-D structure of coordination polymers 1 4. Therefore, higher ph value is one reason for the formation of 2-D and 3-D structures rather than 1-D structures, which is consistent with the experimental results. Second, ox 22 ligands enhance the coordination interaction between metal centers and ligands, and weaken the coordination competition of water, so high dimensional coordination polymers 1 4 (3-D and 2-D) were obtained. Moreover, comparison of different dimensional structure of coordination polymers 1 4, caused by different lanthanide ionic radius, suggests that the different metal sources play an important role in the structural assemblies of corresponding coordination polymers. Coordination polymers 1 8 were characterized by IR spectra, elementary analysis and single-crystal X-ray diffraction. The results of single-crystal X-ray diffraction analyses indicate that coordination polymer 1 shows a 3-D pcu network, with the point symbol of {4 12?6 3 }, coordination polymer 2 has 4-connected 4 4 topology, coordination polymers 3 4 appear as 2-D (6,3)-connected hcb networks, and coordination polymers 5 8 show a 1-D helical infinite chain. Coordination polymers 1 4 possess identical ligands H 3 BIDC and H 2 ox, and their IR spectra are similar. The asymmetric and symmetric stretching vibrations of carboxyl groups in coordination polymers 1 4 appeared at ca cm 21 and ca cm 21 in the IR spectra, respectively, which are red shifted compared with the stretching vibration of carboxyl group in free oxalic acid (ca cm 21 ) and H 3 BIDC (ca cm 21 ). That is because the electron cloud density for carbon oxygen bonds in coordination polymers 1 4 is in the range of that for CLO and C O. The stretching vibration and the plane rocking vibration of C C appeared at ca cm 21 and ca. 800 cm 21, respectively, which are blue shifted compared with the free ligands. This phenomenon may due to the increased rigidity of C C caused by the coordination between the metal centers and ligands. Simultaneously, the absorbed energy of stretching vibration and the plane rocking vibration correspondingly increased. Coordination polymers 5 8 possess the identical ligand H 3 BIDC, coordination polymers 5 8 are isomorphous, and the IR spectra of coordination polymers 5 8 are similar. The asymmetric and symmetric stretching vibrations of carboxyl group of H 3 BIDC ligand in coordination polymers 5 8 appeared at ca cm 21 and ca cm 21 in the IR spectra, respectively, which are red shifted compared with the stretching vibration of carboxyl group in free H 3 BIDC (ca cm 21 ). That is because the electron cloud density for carbon oxygen bonds in coordination polymers 5 8 is in the range of that for CLO and C O. The stretching vibration and the plane rocking vibrations of C C appeared at ca cm 21 and ca. 790 cm 21, respectively, which are blue shifted compared with the free ligands. This phenomenon may due to the increased rigidity of C C caused by the coordination between the metal centers and ligands. Simultaneously, the absorbed energy of stretching vibration and the plane rocking vibration correspondingly increased. Description of crystal structure The X-ray structural analysis of coordination polymer 1 reveals that it contains one Pr 3+ cation, one HBIDC 22 anion, half ox 22 anion, one coordination water molecule, and one free water molecule with all in general positions in the asymmetric unit (Fig. 1a). Center metal Pr1 is nine-coordinated: each Pr(III) is coordinated by two oxygen atoms from one chelating ox 22 ligand (O5, O6A), and five oxygen atoms from three crystallographically independent HBIDC 22 ligands (O1, O2A, O3A, O3B, O4A), one nitrogen atom (N1A) of another crystallographically independent HBIDC 22 ligand, and the remaining coordination site is occupied by oxygen atom of one coordinated water molecule (O7), displaying a tricapped trigonal prismatic arrangement, as shown in Fig. 1b. The structure is different from that of erbium coordination polymer which has been reported in the literature, 21 this is due to the presence of coordinated water molecules. The Pr1 O bond lengths vary from 2.395(4) to 2.639(4) Å, the Pr1 N bond length is 2.601(5) Å. The O Pr1 O bond angles range from 63.92(1)u to (1)u, the O Pr1 N bond angles range from 71.01(2)u to (2)u. In the coordination polymer 1, the HBIDC 22 ligand displays the m 4 -g 1 O :g 1 O :g 2 O :g 1 O :g 1 N mode to connect four Pr(III) ions (Scheme 2A), and the 6-carboxylate group adopts an anti anti mode to link two metal ions. Pr1 and Pr1A are linked together to form a binuclear unit [Pr 2 (m 2 - O) 2 ] by two m 2 -O bridges. The distance between two praseody- 4492, 2013, 15, This journal is ß The Royal Society of Chemistry 2013

5 Table 2 Crystal data and structure refinements for coordination polymers 1 8 Identification code Empirical formula C10H8N2O8Pr C10H8N2O8Yb C10H10HoN2O9 C10H10N2O9Tb C18H21N4O14Tb C18H21N4O14Sm C18H21DyN4O14 C18H21GdN4O14 Formula weight Crystal system Triclinic Triclinic Triclinic Triclinic Monoclinic Monoclinic Monoclinic Monoclinic Space group P1 P1 P1 P1 P21/c P21/c P21/c P21/c Unit cell dimensions a/å 7.501(7) 6.733(2) 6.482(2) 6.506(1) (2) (2) (8) (2) b/å 9.435(9) 7.450(3) 9.848(3) 9.834(1) 8.957(2) 9.006(2) 8.962(6) 8.982(2) c/å 9.706(9) (4) (4) (1) (5) (5) (2) (5) a (u) (1) 87.68(4) (3) 99.88(1) b (u) 93.26(1) 77.44(4) (3) (1) 94.70(3) 94.80(3) 94.77(1) 94.84(1) c (u) 99.50(1) 80.43(3) (3) (1) Z D calcd /Mg m m/mm F(000) h range (u) 2.39 to to to to to to to to Limiting indices 28 h 8, 211 k = 11, 211 l h = 7, 28 k 8, 212 l h = 7, 211 k 11, 212 l h = 7, 29 k 11, 211 l h = 14, 211 k 10, 229 l h = 13, 29 k 10, 228 l h = 13, 210 k 11, 229 l h = 14, 211 k 11, 229 l 29 Data/restraints/parameters 2052/0/ /0/ /0/ /0/ /0/ /0/ /0/ /0/334 GOF on F Final R indices [I. 2sigma(I)] R1 a wr2 b R indices (all data) R wr a R1 = g F o 2 F c /g F o. b wr 2 ={g[w(f o 2 2 Fc 2 ) 2 ]/g[w(fo 2 ) 2 ]} 1/2. This journal is ß The Royal Society of Chemistry 2013, 2013, 15,

6 Scheme 1 Reaction routes of coordination polymers 1 8. mium atoms is Å. The binuclear units are linked by HBIDC 22 ligands to form a one-dimensional trapezoid-like chain (Fig. 1c), the trapezoid-like chains are connected by m 2 - Fig. 1 (a) The metal coordination environment in 1 with labeling scheme and 50% thermal ellipsoids (hydrogen atoms and free water are omitted for clarity). Symmetry codes: O2A and O3A 1 2 x, 22 y, 12 z; O4A and O3B 21 +x, y, z; O5A and O6A 2x,12 y, 2z; N1A 1 2 x,22 y, 2z. (b) Polyhedral representation of the coordination sphere of the Pr 3+ centre in 1. (Hydrogen atoms, and BIDC 32 ligands are omitted for clarity.) (c) The illustration of a 1-D trapezoid-like chain in 1. (d) The illustration of 2-D layer structure in 1 (water molecules and hydrogen atoms are omitted for clarity). (e) Three-dimensional framework in 1. (f) Schematic topological view of the 3-D structure of {4 12?6 3 } topology in 1. (The water molecules and hydrogen atoms in the framework are omitted for clarity. Color code: [Pr 2 (m 2 -O) 2 ], pink ball; HBIDC 22, blue line; ox 22 turquoise line.) Scheme 2 The coordination modes of H 3 BIDC ligand in the coordination polymers 1 8 (A for 1, B for 2 4, C and D for 5 8). O1 of HBIDC 22 ligands to construct a two-dimensional network (Fig. 1d). Furthermore, the two adjacent 2-D coordination networks are further connected to adjacent praseodymium cations by chelate carboxylate group of the ox 22 ligands to generate the 3-D network, as illustrated in Fig. 1e. The free water molecules are inserted into the channels. Topological analysis has been applied for better understanding the connectivity of 3-D framework in coordination polymer 1. If coordinated water molecules and lattice water molecules are ignored, the binuclear unit [Pr 2 (m 2 -O) 2 ] as a secondary building unit is connected to six adjacent praseodymium dimers by four HBIDC 22 anions and two ox 22 anions. It can be viewed as a 6-connected octahedral node, the HBIDC 22 and ox 22 ligands act as linear bridges between the binuclear unit nodes. Such connectivity repeats infinitely, resulting in a 3-D pcu network, with the point symbol of {4 12?6 3 }. The topology is shown in Fig. 1f and the pore size is Å (diagonal-to-diagonal distances). The effective free volume of the channels without these guest molecules is estimated to be 50.1 Å 3 by PLATON software, 22 almost 8.4% of the per unit cell volume of Å 3. [Yb(HBIDC)(ox) 0.5 (H 2 O) 2 ] n (2). Single-crystal X-ray diffraction analysis shows that coordination polymer 2 includes one crystallographically unique Yb(III) ion, one HBIDC 22 ligand, half ox 22 ligand, and two coordinated water molecules in the independent symmetry unit (Fig. 2a). The Yb(III) atom has eight-coordinated bi-capped triprismatic coordination geometry (Fig. 2b) with three oxygen atoms (O1, O3A, and O4A) deriving from three HBIDC 22 ligands, two oxygen atoms (O5A, O6) from the ox 22 anion, and three oxygen atoms from three water molecules (O7, O7A, and O8). The O(cap) Yb1 O(cap) bond angle is (1)u for O3A Yb1 O5A, the O(cap) Yb1 O(prism) bond angles range from 69.64(1)u to (1)u, the O(prism) Yb1 O(prism) bond angles range from 71.22(1)u to (1)u. The Yb1 O(cap) bond lengths are 2.292(3) Å for Yb1 O3A, 2.349(3) Å for Yb1 O5A, the Yb1 O(prism) bond lengths vary from 2.258(3) to 2.486(3) Å. In the coordination 4494, 2013, 15, This journal is ß The Royal Society of Chemistry 2013

7 Fig. 2 (a) The metal coordination environment in complex 2 with labeling scheme and 50% thermal ellipsoids (hydrogen atoms are omitted for clarity). Symmetry codes: O3A 1 + x, y, z; O4A 21 2 x, 2y, 2z; O5A and O6A 21 2 x, 12 y, 2z; O7A 2x, 2y, 2z. (b) Polyhedral representation of the coordination sphere of the Yb 3+ centre, with display bi-capped trigonal prismatic arrangement in the complex 2. (Hydrogen atoms, and BIDC 32 ligands are omitted for clarity.) (c) The illustration of a 1-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-D supramolecular structure of coordination polymer 2 (hydrogen bonding turquoise dashed line). polymer 2, HBIDC 22 ligand displays the m 3 -g O 1 :g O 1 :g O 1 mode to connect three Yb(III) ions (Scheme 2B), and the 6-carboxylate group adopts a syn syn mode to link two metal ions, with the uncoordinated nitrogen atoms acting as the hydrogen-bonding acceptor. The oxygen atoms O7 and O7A of two water molecules as bridging oxygen m 2 -OH 2 to link two adjacent metal ytterbium ions form dimeric unit [Yb 2 (CO 2 ) 2 (m 2 -OH 2 ) 2 ], adjacent dimeric units [Yb 2 (CO 2 ) 2 (m 2 -OH 2 ) 2 ] are connected to form one-dimensional chain by HBIDC 22 ligands (see Fig. 2c). It was also found that within the one-dimensional chain, the separation of Yb1 Yb1 (bridged by bridging oxygen m 2 -OH 2 )is 3.851(1) Å. Then, these chains are further connected into a spectacular 2-D grid by the chelate carboxylate group of the ox 22 ligands, the separation of Yb1 Yb1 (chelated by ox 22 )is 6.101(2) Å, the structure of which differs from coordination polymer 2, as illustrated in Fig. 2d. A 3-D supramolecular structure is obtained by weak p p stacking interactions (p p stacking interactions of Å) and hydrogen bonding interactions (see Fig. 2e), which are formed by the nitrogen atoms and carboxylic oxygen atoms of HBIDC 22, oxygen atoms of coordination water and the hydrogen atoms of coordination water and benzimidazole ring. The obtained structure is very similar to the complex based on H 3 BIDC, 21 except for a different coordination mode of water molecules in crystal lattice. In order to identify the connectivity in ligands and metals, the topology of the whole framework is investigated. If coordinated water molecules was ignored, and the dimeric unit [Yb 2 (CO 2 ) 2 (m 2 -OH 2 ) 2 ] considered as the node, and HBIDC 22 and ox 22 ligands as the linkers, the simplified topological representation of the coordination polymer 2 exhibits a 4-connected 4 4 topology, which is described in Fig. 3. [Tb(HBIDC)(ox) 0.5 (H 2 O) 3 ] n (4). Coordination polymers 3 and 4 are isomorphous, therefore, only the structure of coordination polymer 4 is described in detail. Single crystal X-ray diffraction analysis reveals that coordination polymer 4 crystallize in a triclinic system, P1 space group. As shown in Fig. 4, the asymmetric unit of 4 consists of one crystallographically unique Tb(III) ion, one HBIDC 22 ligand, half ox 22 ligand, and three coordinated water molecules. Each Tb(III) ion coordinates to three water oxygen atoms, three carboxylic oxygen atoms from three HBIDC 22 ligands and two carboxylic oxygen atoms from ox 22 ligand, forming a distorted This journal is ß The Royal Society of Chemistry 2013, 2013, 15,

8 Fig. 3 (a) The defined nodes of 4-connected dimeric unit [Yb 2 (CO 2 ) 2 (m 2 -OH 2 ) 2 ] in coordination polymer 2. (b) and (c) The defined linkers of 2-connected HBIDC 22 and ox 22 ligands in coordination polymer 2. (d) The 2-D net of 4 4 topology in its most symmetrical form distinguished by different colors. bi-capped triprismatic coordination geometry. The isomorphic dysprosium coordination polymer of compound 4 has been reported by Sun and co-workers. 23 The difference from reports in the literature of isomorphic dysprosium coordination polymer is that it forms a distorted square-antiprismatically coordination geometry. The Tb O bond lengths vary from 2.320(4) to 2.465(4) Å. All the Tb O distances are compatible with the values of previously published terbium compounds. 17 The O Tb O bond angles range from 66.70(1)u to (1)u.In coordination polymer 4, the 6-carboxylate group adopts a syn anti mode to link two metal ions of HBIDC 22 ligand, HBIDC 22 ligand displays the m 3 -g 1 O :g 1 O :g 1 O mode to connect three Tb(III) ions (Scheme 2B). Each H 3 BIDC ligand uses three carboxylate group oxygens linking three adjacent Tb(III) ions to form a 1-D trapezoid-like double chain structure along the crystallographic a axis (see Fig. 4c), with Tb Tb separations of 5.69 Å, 6.06 Å and 6.51 Å, and the dihedral angles between the benzimidazole ring and coordinating carboxylate group are 64.67(2) and 33.18(2)u. Two adjacent 1-D trapezoid-like double chains form a 2-D layer structure (Fig. 4d) through the ox 22 ligands. A 3-D supramolecular structure is obtained by p p stacking interactions (p p stacking interactions of Å) and hydrogen bonding interactions (see Fig. 4e), which are formed by nitrogen atoms and carboxylic oxygen atoms of HBIDC 22 and ox 22 ligands and the hydrogen atoms of coordination water and benzimidazole ring. If coordinated water molecules are ignored, the metal terbium center can be viewed as a 3-connected node connecting adjacent three terbium centers by HBIDC 22 and ox 22 ligands, HBIDC 22 and ox 22 ligands act as linear bridges between the metal terbium center nodes. Such connectivity repeats infinitely, resulting in a 2-D (6,3)-connected hcb network, the pore size of the 6-member ring is Å (Fig. 4f). Coordination polymers 1 4 have the same triclinic system and P1 space group, but they exhibit different kinds of crystal structures. The structural differences between 1 4 are due to two reasons: one is the different coordination motifs of HBIDC 22 ligands: in the coordination polymer 1, HBIDC 22 ligand displays the anti anti mode and forms m 4 - g 1 O :g 1 O :g 2 O :g 1 N mode (Scheme 2A); in the coordination polymers 2 4, HBIDC 22 ligand displays the m 3 -g 1 O :g 1 1 O :g O mode (Scheme 2B), and the 6-carboxylate group adopts a syn syn mode for coordination polymer 2 and syn anti mode for coordination polymers 3 and 4. The other reason is the incremental coordination water of rare earth ions. {[Sm(H 2 BIDC)(HBIDC)(H 2 O) 3 ]?3H 2 O} n (6). Coordination polymers 5 8 are isomorphous, therefore, only the structure of coordination polymer 6 is described in detail. Single crystal X-ray diffraction analysis reveals that coordination polymer 6 crystallizes in the monoclinic system, P2 1 /c space group. The coordination polymer 6 has 37 non-hydrogen atoms in the asymmetric unit, which contains one Sm 3+ cation, different deprotonation anions H 2 BIDC 2 and HBIDC 22 of two H 3 BIDC ligands, three coordination water molecules, and three free water molecules with all in general positions (Fig. 5a). Center metal Sm1 is nine-coordinated: six chelated carboxyl oxygen atoms (O1, O2, O3, O4, O5, and O6) from two HBIDC 22 ligands and one H 2 BIDC 2 ligand and three oxygen atoms (O9, O10, and O11) from three water molecules, and displays a slightly distorted tricapped trigonal prismatic arrangement (Fig. 5b). The Sm O distances range from 1.908(9) to 2.392(1) Å, which are within the range of reported complexes , 2013, 15, This journal is ß The Royal Society of Chemistry 2013

9 Fig. 4 (a) The metal coordination environment in 4 with labeling scheme and 50% thermal ellipsoids (hydrogen atoms are omitted for clarity). Symmetry codes: O3A 2x, 12 y, 12 z; O4A 1 2 x, 12 y, 12 z; O5A and O6A 1 2 x, 12 y, 22 z. (b) Polyhedral representation of the coordination sphere of the Tb 3+ centre, with display slightly distorted bi-capped trigonal prismatic arrangement in 4 (hydrogen atoms and BIDC 32 ligands are omitted for clarity.) (c) The illustration of a 1-D trapezoid-like double 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-D supramolecular 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 water molecules and hydrogen atoms in the framework are omitted for clarity. Color code: Tb, pink ball; HBIDC 22, blue line; ox 22 turquoise line). In coordination polymer 6, H 3 BIDC ligands show two types of coordination modes (Scheme 2C and 2D): the first one is mono-chelate coordination mode m 1 -g O 2, with one free carboxyl oxygen atom uncoordinated to any metal ions (Scheme 2C). The second one adopts bis-chelate manner m 2 - g O 2 :g O 2 (Scheme 2D). The adjacent two samarium ions are connected through chelating carboxyl oxygen atoms according to Scheme 2D, giving rise to an infinite one-dimensional righthanded helix and left-handed helix chains (see Fig. 5c). The 1-D helical infinite chain around the crystallographic 2 1 axis spreads along the b axis direction, the pitch of the helix is calculated to be 9.01 Å containing two Sm(III) ions per turn. The 2-D layer structure is formed by hydrogen bond interactions between two same helical chains, different 2-D layer forming a 3-D supramolecular framework alternately via p p stacking interactions (see Fig. 5d) (p p stacking interactions of Å). The isomorphism of coordination polymer 6 has been reported by Liu and co-workers. 24 Although we set 120 uc as reaction temperature rather than room temperature, it was found 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 a minor 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 out on complexes 1 8 (see ESI,3 Fig. S5). The PXRD patterns indicated that the patterns are entirely consistent with the simulated PXRD pattern generated based on the structures determined from the single-crystal data of coordination polymers 1 3 and 5. The similar PXRD pattern (see ESI,3 Fig. S5) of coordination polymers 3 8 indicated that the complexes are isomorphous and proved the purity of coordination polymers 3 4 and 5 8. Luminescence properties and lifetimes of coordination polymers Lanthanide ions are well-known to give rise to luminescence and are used as optical active centers in many of the phosphors used for lighting, scintillating, and plasma display panel applications. 25 The characteristic luminescence of This journal is ß The Royal Society of Chemistry 2013, 2013, 15,

10 Fig. 5 (a) The metal coordination environment in coordination polymer 6 with labeling scheme and 50% thermal ellipsoids (free water molecules and hydrogen atoms are omitted for clarity). Symmetry codes: O3A and O4A 2x, y, 0.52 z. (b) Polyhedral representation of the coordination sphere of the Sm 3+ centre, with display slightly distorted tricapped trigonal prismatic arrangement in the complex 6 (hydrogen atoms, and BIDC 32 ligands are omitted for clarity). (c) Right-handed helix (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 omitted for clarity). 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 light radiation to lanthanide ions. The luminescence and NIR emission spectra of coordination polymers 1 8 were measured, the characteristic NIR emission bands for corresponding lanthanide in coordination polymers 1 and 3 were not observed. For the other coordination polymers 4 8, the luminescence spectra were measured in the solid-state at room temperature and liquid nitrogen temperature, and dispersed in CH 2 Cl 2 solvents. In particular, in coordination polymers 6 and 7, not only were measured characteristic transition of corresponding rare earth ions in the visible region, but also determined were infrequent NIR emission spectra of samarium and dysprosium ions in the near infrared region, which is a rarely described phenomenon For the emission spectra of coordination polymer 2 (Fig. 6), the Yb 3+ ion emits in the range of nm, with a sharp peak around 995 nm assigned to the 2 F 5/2 A 2 F 7/2 transition of the Yb 3+ ion broader vibronic components at longer wavelength. 26 Similar splitting has been reported in previous literature reports. 27 This may be the splitting of energy levels of Yb 3+ ion as a consequence of ligand field effects. 27a The Yb 3+ ion plays an important role in laser emission because of its very simple f f energy level structure. 28 The solid-state emission spectra of terbium coordination polymers 4 and 5 at room and liquid nitrogen temperature, and dispersed in CH 2 Cl 2 solvent are shown in Fig. 7. Upon excitation by UV light, we have observed characteristic emission in the visible light region from the Tb 3+ ions in the coordination polymers 4 and 5 at l max = ca. 490, 544, 584, 622, and 650 nm, which showed characteristic transitions of the Tb 3+ ion. These originated from the characteristic 5 D 4 A 7 F J transition of a sensitized terbium emission, where J =6,5,4,3, and 2, respectively, and the relative intensity of the sharp-line band is 5 D 4 A 7 F 5 transition. 29 The luminescence measure- 4498, 2013, 15, This journal is ß The Royal Society of Chemistry 2013

11 Fig. 6 NIR emission spectra of coordination polymer 2 in the solid-state at room temperature. ments of 0.1 mg of polymers 4 and 5 dispersed in 10 ml CH 2 Cl 2 also showed characteristic 5 D 4 A 7 F J (J =6,5,4,3,and 2) transition emission of terbium. This indicates that the Tb(III) luminescence can be efficiently sensitized by the H 3 BIDC and H 2 ox ligands via the antenna effect. 12 Under different conditions, orange and red luminescence is relatively weak in comparison to the dominant green emission, resulting in 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 is suitable for green luminescence material in solid state or in CH 2 Cl 2 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 terbium coordination polymers 4 and 5 by using the time-correlated single photon counting (TCSPC) technique. The fluorescence decay curves of coordination polymers 4 and 5 are shown in Fig. 7. The decay curves are well fitted into a single index fitting attenuation function: I 0 = I + Aexp(2t/t 1 ), I 0 and I are the luminescent intensities when time t = t and t = 0, respectively, and t 1 is defined as the luminescent lifetime. 30 Luminescence lifetimes of coordination polymers 4 and 5 under different conditions are shown in Table 3. For coordination polymers 4 and 5, the solid-state luminescence lifetimes at room temperature are longer than those at liquid nitrogen temperature. The fluorescence lifetimes of 4 are longer than 5 in solid state or in CH 2 Cl 2 solvent at room and liquid nitrogen temperature, which is observed in the fluorescence decay curves. This phenomenon can be attributed to the following reasons: firstly, there are more free water molecules in coordination polymer 5 than that in coordination polymer 4, which can increase the radiationless transition and result in the decreased of fluorescence intensity. Secondly, oxalic acid further sensitized trivalent terbium ion of coordination Fig. 7 The solid-state emission spectra and luminescence decay curves of 4 and 5. This journal is ß The Royal Society of Chemistry 2013, 2013, 15,

12 Table 3 Luminescence data for coordination polymers 1 8 a Coordination polymers l ex (nm) l em (nm) t (ms) Conditions , 1091 Solid state, 298 K , 544, 584, 621, Solid state, 298 K , 545, 584, 622, Solid state, 77 K , 544, 584, 622, CH 2 Cl 2,77K , 545, 585, 622, Solid state, 298 K , 544, 586, 622, Solid state, 77 K , 544, 584, 622, CH 2 Cl 2,77K , 562, 596, 642, Solid state, 298 K , 562, 597, Solid state, 77 K , 597, CH 2 Cl 2,77K , 919, 940, 980, 994, 1125, 1196, 1286, 1417 Solid state, 298 K , 574, 663, Solid state, 298 K , 575, 664, Solid state, 77 K , 480, 575, 664, CH 2 Cl 2,77K , 965, 1151, 1325, 1503 Solid state, 298 K Solid state, 77 K a Emission of coordination polymers 1 and 3 not determined. polymer 4, and result in the fluorescence lifetime of coordination polymer 4 being longer than coordination polymer 5. Finally, coordination polymer 4 has the longest lifetime on account of the 2-D condensed structure, which suggests that the lanthanide ions in 4 are well shielded from nonradiative deactivations. The solid-state emission spectra of samarium coordination polymer 6 at room and liquid nitrogen temperature, and dispersed in CH 2 Cl 2 solvent are shown in Fig. 8. The solidstate emission spectra at room temperature consists of four main bands arising from 4 G 5/2 A 6 H J transitions: the 4 G 5/2 A 6 H 5/2 transition at 562 nm, the 4 G 5/2 A 6 H 7/2 transition at 596 nm, the 4 G 5/2 A 6 H 9/2 transition at 642 nm, and the 4 G 5/2 A 6 H 11/2 transition at 704 nm. In the solid-state emission spectrum at room temperature of H 3 BIDC, we have observed ligand-centered p* A p transitions at 399 nm (ESI3). In the emission spectra of coordination polymer 6, we have observed ligand-centered p* A p transitions at ca. 420 nm, which are red-shifted compared with free ligand H 3 BIDC. 31 This red-shift may be attributed to the metal-disturbed ligand-centered p* A p transitions. 4e,32 The 4 G 5/2 A 6 H 11/2 transition at 704 nm is disappearing, which can be attributed to the following reasons: there are a considerable number of excitation ions which exist at the 4 G 5/2 energy level, are transferred to 6 F 9/2 energy level by cross-relaxation, after available excitation ions are consumed with radiation and relaxation process, the fluorescence emission is reduced. 33 Such a hypersensitive transition can also be found in Sm 3+ ( 4 G 5/2 A 6 H 9/2 ), fulfilling the selection rule DJ = 2 (electric-dipole allowed). It is proposed to use the 4 G 5/2 A 6 H 5/2 transition of Sm 3+ as a reference, because it has a predominant magnetic dipole character (DJ = 0). 34 The relative intensity of the 4 G 5/2 A 6 H J transition and p* A p transition changes the color from yellow-white at room temperature to pink-white and blue-white at liquid nitrogen temperature (see the color coordinates diagram in Fig. 8). Luminescence emission of Sm 3+ ions have been often investigated in visible region, NIR emission are limited in near infrared region. 17,19 Characteristic transition of samarium ion not only was measured in the visible region, but also in the near infrared region. The NIR emission spectra of coordination polymer 6 were measured in the solid state at 298 K. In the NIR emission spectra of coordination polymer 6, we have observed characteristic emission in the near-ir from the Sm 3+ ions upon excitation of UV light. In the coordination polymer 6, the NIR emission spectrum (Fig. 8) consisting of several bands at l = 828, 919, 940, 980, 994, 1125, 1196, 1286 and 1417 nm are clearly observed. Discernible peaks at 919 nm and 940 nm, 980 nm and 994 nm, 1125 nm and 1196 nm are suspected to be the Stark splitting of 6 F 5/2, 6 F 7/2 and 6 F 9/2, respectively. The other emissions are assigned to the f f transitions of 4 G 5/2 to 6 F 3/2 (828 nm), and 6 F 11/2 (1286 nm and 1417 nm), respectively. The emission bands of coordination polymer 6 are shifted relative to the bands of the reported theoretical values. 35 In addition to the steady-state emission, we also performed time-resolved measurements by using TCSPC technique. The fluorescence decay curve of coordination polymer 6 is shown in Fig. 8. Luminescence lifetimes of coordination polymer 6 at different conditions are shown in Table 3. The fluorescence lifetimes of coordination polymer 6 at 298 K are longer than at 77 K, which is observed in the fluorescence decay curves. This phenomenon can be attributed to the quenching of Sm 3+ cation at low temperature. The solid-state emission spectra of dysprosium coordination polymer 7 at room and liquid nitrogen temperature, and dispersed in CH 2 Cl 2 solvent are shown in Fig. 9. In addition, we have observed characteristic emission in the near-ir from the Dy 3+ ions in coordination polymer 7 (see Fig. 9), which is a rarely described phenomenon. 17,18 The emission spectrum of coordination polymer 7 was obtained under the excitation wavelength at ca. 310 nm. The room and liquid nitrogen temperature emission spectra of coordination polymer 7 shows four emission bands in the visible region, two strong bands at ca. 480 nm ( 4 F 9/2 A 6 H 15/2 ) and ca. 575 nm ( 4 F 9/2 A 6 H 13/2 ) and two weaker bands at ca. 664 nm ( 4 F 9/2 A 6 H 11/2 ) and at ca. 750 nm ( 4 F 9/2 A 6 H 9/2 + 6 F 11/2 ). 36 In the dysprosium coordination polymers characteristic 4 F 9/2 A 6 H 9/2 + 6 F 11/2 transition is rarely observed and described. 29 At room temperature, the blue emission arises from a magnetic dipole 4500, 2013, 15, This journal is ß The Royal Society of Chemistry 2013

13 Fig. 8 The solid-state emission spectra and NIR emission spectra, color coordinate diagram of the corresponding emission and the luminescence decay curves of coordination polymer 6. transition, whereas the yellow emission is due to a forced electric dipole. The latter one can be considered as a hypersensitive transition based on the selection rules. Since the coordination environment of the optically active ion is able to influence the hypersensitive, electric-dipole-governed transition, while the magnetic dipole transition remains insensitive to the crystal field, a different ratio of the blue to yellow emission can be achieved, thus changing the visible impression of the emission color from blue to yellow. The contribution of the greenish-blue luminescence is relatively small in comparison to the dominant yellow emission, resulting in a yellow emission color of the dysprosium coordination polymer 7 (see the color coordinates diagram in Fig. 9). The emission spectra of coordination polymers 7 dispersed in CH 2 Cl 2 solvent at liquid nitrogen temperature contains 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 compared with free ligand H 3 BIDC. 30 This red-shift may be attributed to the metal-disturbed ligand-centered p* A p transitions. 4e,32 There exits vibration coupling of the phonon with the coordinated water molecules from solution, it is an effective way for rare earth ion emission state non-radiative deactivation, consequently causing quenching fluorescence. The collision quenching interaction between ligands and coordinated water molecules is reduced when the coordination polymer is dispersed in CH 2 Cl 2 at liquid nitrogen temperature. The rate of non-radiative decay process was also reduced as well as the quenching effect from oxygen on the luminescence. For these reasons, ligand-centered p* A p transitions were observed in the cryogenic emission spectrum from CH 2 Cl 2, and led to an increase of Dy 3+ other transitions intensity. 37 At 77 K, ligand-centered p* A p transition contributions increase, thus changing the visible impression of the emission color from yellow to light blue-white (see the color coordinates diagram in Fig. 9). Luminescence emissions of Dy 3+ ion have been often investigated in visible region, NIR emission of which are limited in near infrared region. 17,18 The characteristic transition of Dy 3+ ion is not only measured in the visible region, but also in the near infrared region. The NIR emission spectra of coordination polymer 7 were measured in the solid state at 298 K. In the NIR emission spectra of coordination polymer 7, we have observed characteristic emission in the near-ir from the Dy 3+ ions upon excitation of UV light. The NIR emission peak of Dy 3+ coordination polymer is a single sharp transition. In the coordination polymer 7, the emission This journal is ß The Royal Society of Chemistry 2013, 2013, 15,

14 Fig. 9 The solid-state emission spectra and NIR emission spectra, color coordinate diagram of the corresponding emission and the luminescence decay curves of coordination polymer 7. spectrum (Fig. 9) consists of several bands at l = 835, 965, 1151, 1325 and 1503 nm, which are attributed to the f f transitions 4 F 9/2 A 6 H 7/2 + 6 F 9/2, 4 F 9/2 A 6 H 5/2, 4 F 9/2 A 6 F 3/2, 6 F 1/2 + 6 H 9/2 A 6 H 15/2 and 6 F 5/2 A 6 H 11/2, respectively. In addition to the steady-state emission, we also performed TCSPC technique. The fluorescence decay curve of coordination polymer 7 is shown in Fig. 9. Luminescence lifetimes of coordination polymer 7 under different conditions are shown in Table 3. The fluorescence lifetime of coordination polymer 7 dispersed in CH 2 Cl 2 solvent is longer than that in the solid state, which is observed in the fluorescence decay curves. This phenomenon can be attributed to the following reason: there exists vibration coupling of the phonon with the coordinated water molecules from solution, for rare earth ion, it is an effective way for emission state non-radiative deactivation, thus causing quenching fluorescence consequently. The collision quenching interaction between ligands and coordinated water molecules is reduced when the coordination polymer is dispersed in CH 2 Cl 2 at liquid nitrogen temperature. The rate of non-radiative decay process is also reduced as well as quenching effect from oxygen on the luminescence. 37 In the fluorescence decay curves of coordination polymers 4 7, fluorescence lifetimes of coordination polymers 4 7 in solid state are shorter than luminescence lifetimes in CH 2 Cl 2, which can be attributed to the following reasons: first, the collision quenching interaction between ligands and coordinated water molecules is reduced when the coordination polymer is dispersed in CH 2 Cl 2 at liquid nitrogen temperature. The rate of non-radiative decay process is also reduced as well as the quenching effect from oxygen on the luminescence. Second, in relaxation process, the reduction of cross-relaxation between rare earth ions restrains useful number of excited ions in CH 2 Cl 2, and results in the fluorescence lifetimes of coordination polymers 4 7 in CH 2 Cl 2 being longer than in solid state. Energy transfer processes studies To elucidate the energy transfer processes of the lanthanide coordination polymers, the energy levels of the relevant electronic states of the ligand have been investigated. The singlet and triplet energy levels of the ligand were estimated by referring to their wavelengths of UV-vis absorbance edges and the lower wavelength of the 0 0 phosphorescence band of gadolinium complex. 38 According to Reinhoudt s empirical rule, 39 the intersystem crossing process becomes effective when DE (S 1 2 T 1 ) is at least 5000 cm 21. It is necessary that there exists a suitable energy gap between the ligand-centered triplet state and the lanthanide ion emissive states. Latva s 4502, 2013, 15, This journal is ß The Royal Society of Chemistry 2013