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Construction of four low-dimensional NIR-luminescence-tunable Yb(III) complexes† Zhi-Peng Zheng,a Xiu-Xia Zhang,a Teng Li,a Jian Yang,a Lei-Ming Wei,a Li-Guo Zhang,a Xiao-Ming Lina and Yue-Peng Cai*a,b Four low-dimensional ytterbium(III)-organic compounds through hydrothermal reactions of quinoline2,3-dicarboxylic acid (2,3-H2qldc) and oxalic acid (H2ox) with Yb2O3, namely, [Yb(2,3-qldc)(ox)1/2(H2O)3·(H2O)4]n (1), [Yb(2,3-qldc)(ox)1/2(H2O)2·(H2O)2]n (2), [Yb(2,3-Hqldc)(ox)(H2O)2·(H2O)]n (3) and [Yb(2,3-Hqldc)(ox)(H2O)·(H2O)2]n (4), were first synthesized and characterized by elemental analysis (EA), infrared spectroscopy (IR), thermogravimetric analysis (TG), and single-crystal X-ray diffraction. When the reactant ratio of 2,3-H2qldc : H2ox : Yb2O3 is 2 : 1 : 1, 1-D chain-like complex 1 with three coordinated water molecules around the Yb(III) ion was obtained in mixed solvents of H2O and CH3OH (v : v = 10 : 1) at 70 °C, and with the increase of temperature to 100 °C, the same reactants gave 2-D 63 topological layerlike complex 2 with two coordinated water molecules in the coordination sphere of the Yb(III) ion. However, when the reactant ratio was changed to 1 : 1 : 1, two 2-D 63 topological layer-like complexes 3 (70 °C) and 4 (100 °C) were obtained at different temperatures, in which the coordination water molecules
Received 30th May 2014, Accepted 24th July 2014 DOI: 10.1039/c4dt01601g www.rsc.org/dalton
in 3 and 4 are two and one, respectively. Obviously, these results reveal that the reaction temperature and reactant ratios play critical roles in the structural direction of these low-dimensional compounds. Interestingly, with the gradual loss of coordination water molecules to the Yb(III) ion, the near infrared (NIR) emission of four Yb(III)-based compounds 1–4 can be gradually strengthened with increasing order of 1 < 3 < 2 < 4, indicating that these ytterbium(III) complexes have tunable near infrared luminescence.
Introduction As a consequence of low toxicity and efficient light transmission of biological tissues in part of the NIR spectral range (0.9 to 1.5 µm approximately),1 ytterbium(III) coordination compounds can be used as near infrared (NIR) luminescent materials applied in biomedicine,2 electroluminescence3 and laser systems,4 in which the central Yb(III) ions are coordinated to organic ligands with exceedingly delocalized π systems acting as antennas to effectively harvest and transfer energy, known as the antenna effect.1b,c,5 On the other hand, compared with that of visible-lightemitting lanthanide (Sm3+, Tb3+, Eu3+ etc.) coordination
a School of Chemistry and Environment, South China Normal University, Guangzhou Key Laboratory of Materials for Energy Conversion and Storage, Guangzhou 510006, PR China. E-mail: [email protected]; Fax: +86-020-39310; Tel: +86-020-39310383 b State Key Laboratory of Structure Chemistry, Fujian, Fuzhou 350002, PR China † Electronic supplementary information (ESI) available. CCDC 1003835, 1003836, 1003837 and 1003838 for 1–4. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4dt01601g
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compounds, the NIR luminescence of lanthanide (Nd3+, Yb3+, Er3+) coordination compounds is less explored, which is due to the low energy of the NIR luminescence and its susceptibility of being quenched by non-radiative energy transfer from Ln(III) excited states to high-energy O–H, N–H or C–H oscillators,6 making it one of the challenges in constructing NIR luminescent lanthanide complexes. Therefore, several efforts2,7, including taking advantage of macro cryptand-like ligands to protect central lanthanide ions and fluoridation of C–H bonds, have been made to alleviate non-radiative decays by sheltering the Ln3+ excited levels from high non-radiative transition probability by O–H and C–H oscillators from solvent molecules such as water, methanol and ethanol. Moreover, using an auxiliary ligand to displace solvent molecules is another effective way to assuage NIR luminescence quenching from coordinated solvent molecules. For example, oxalate (ox−) has no high-energy vibrational groups and high tendency to chelate lanthanide ions; accordingly, it is a good candidate for a bridging ligand in assembling lanthanide compounds.8 In the present contribution, apart from 2,3-quinoline dicarboxylic acid (2,3-H2qldc) with a large π-conjugating system as the main ligand, we choose oxalic acid (H2ox) as the auxiliary
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Scheme 1 Assembly of compounds 1–4 related to the reaction temperature and the reactant ratio.
ligand, reacting with Yb2O3 under hydrothermal conditions, to construct ytterbium(III) compounds with good near-infrared emission. By changing the reaction temperature and the reactant ratio, four ytterbium compounds containing different coordination water molecules, namely, [Yb(2,3-qldc)(ox)1/2(H2O)3·(H2O)4]n (1), [Yb(2,3-qldc)(ox)1/2(H2O)2·(H2O)2]n (2), [Yb(2,3-Hqldc)(ox)(H2O)2·(H2O)]n (3), and [Yb(2,3-Hqldc)(ox)(H2O)·(H2O)2]n (4) were constructed as indicated in Scheme 1. Their structures have been determined by single-crystal X-ray diffraction analyses and further characterized by infrared spectra (IR), elemental analyses (EA), and powder X-ray diffraction (PXRD). A further study reveals that the four complexes in the solid state have similar NIR-emission spectra with the increasing order of 1 < 3 < 2 < 4 in the emission intensity, consistent with the decreasing order of coordination water molecules in four complexes 1–4. To the best of our knowledge, compounds 1–4 represent the first example of systematic investigation into coordination chemistry and YbIII-based near-infrared emission in the system of quinoline-2,3-dicarboxylic acid and oxalic acid.
Experimental Physical measurements All materials were of reagent grade obtained from commercial sources and used without further purification, and solvents were dried by standard procedures. Elemental analyses for C, H, N were performed on a Perkin-Elmer 240C analytical instrument. IR spectra were recorded on a Nicolet FT-IR-170SX spectrophotometer in KBr pellets. Thermogravimetric analyses were performed on a Perkin-Elmer TGA7 analyzer with a heating rate of 10 °C min−1 under a flowing nitrogen atmosphere. The luminescent spectra for the solid state were recorded at room temperature on a Edinburgh-FLS-920 with a xenon arc lamp as the light source. In the measurements of emission and excitation spectra the pass width is 5.0 nm. The X-ray powder diffraction (PXRD) patterns were recorded with a Siemens D5005 diffractometer with Cu-Kα (λ = 1.5418 Å) radiation. Preparation of complexes 1–4. A mixture of Yb2O3 (39 mg, 0.1 mmol), ligand 2,3-H2qldc (44 mg, 0.2 mmol), oxalic acid (13 mg, 0.1 mmol for 1, 2 and 26 mg, 0.2 mmol for 3, 4), H2O
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(10 mL), and CH3OH (0.2 mL) was heated from room temperature to 70 °C for 80 h in a 25 mL Teflon lined stainless-steel autoclave and then cooled to room temperature at a rate of 10 °C h−1 to obtain the colourless crystals for compounds 1 and 3 suitable for a single crystal diffraction test. By contrast, when the reaction temperature rose up to 100 °C, the two abovementioned reactions gave colourless crystal compounds 2 and 4, respectively. 1. Yield 57% (based on Yb), colorless crystals. Elemental analysis calcd (%) for C12H19NO13Yb: C, 25.79; H, 3.40; N, 2.51. Found: C, 25.81; H, 3.30; N, 2.53. FT-IR (KBr, cm−1): 3457(vs), 2364(s), 2072(m), 1663(s), 1400(s), 1123(s), 996(w), 794(w), 638(w), 534(w). 2. Yield 49% (based on Yb), colorless crystals. Elemental analysis calcd (%) for C12H13NO10Yb: C, 28.56; H, 2.58; N, 2.78. Found: C, 28.78; H, 2.65; N, 2.80. FT-IR (KBr, cm−1): 3446(br, vs), 2360(w), 2065(w), 1635(s), 1400(s), 1136(s), 999(m), 785(w), 669(w), 617(w), 538(w), 488(w). 3. Yield 48% (based on the Yb), colorless crystals. Elemental analysis calcd (%) for C13H12NO11Yb: C, 29.36; H, 2.26; N, 2.64. Found: C, 29.41; H, 2.29; N, 2.66. FT-IR (KBr, cm−1): 3477(br, vs), 1730(s), 1687(s), 1618(vs), 1465(s), 1386(m), 1386(s), 1309(w), 1143(w), 1049(w), 876(m), 800(m), 619(m), 481(w), 455(w). 4. Yield 51% (based on the Yb), colorless crystals. Elemental analysis calcd (%) for C26H26N2O23Yb2: C, 28.87; H, 2.41; N, 2.59. Found: C, 28.89; H, 2.47; N, 2.63. FT-IR (KBr, cm−1): 3460 (br, vs), 3330(s), 1620(vs), 1543(vs), 1463(vs), 1407(m), 1321(m), 1047(m), 873(m), 808(s), 771(m), 748(w), 682(w), 592(w), 442(w).
X-ray data collection and structure refinement Data collections were performed at 298 K on a Bruker Smart Apex II diffractometer with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) for 1–4. Absorption corrections were applied using the multiscan program SADABS.9 Structural solutions and full-matrix least-squares refinements based on F2 were performed with the SHELXS-9710 and SHELXL-9711 program packages, respectively. All the non-hydrogen atoms were refined anisotropically. The hydrogen atoms were placed at calculated positions and included in the refinement in the riding model approximation. The organic hydrogen atoms were generated geometrically (C–H = 0.93 or 0.96 Å), and the water hydrogen atoms were located from difference maps and refined with isotropic temperature factors. The carbon atoms C4–C10 and the nitrogen atom N1 of the quinoline ring in compound 2 are located in two positions with 0.5 occupancy of each position. Details of the crystal parameters, data collections, and refinements for complexes 1–4 are summarized in Table S1.† Selected bond lengths and angles for four complexes are shown in Table S2,† hydrogen-bonding data of these four complexes are listed in Table S3 (ESI†). Further details are provided in the ESI.† Crystallographic data for the structures reported in this paper have been deposited with the Cambridge Crystallographic Data Centre. CCDC 1003835, 1003836, 1003837 and 1003838 are for four new compounds 1–4, respectively.
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Results and discussion
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IR spectroscopic and thermogravimetric analyses Through hydrothermal syntheses, four new compounds 1–4 were synthesized and characterized by EA, IR and X-ray singlecrystal/powder diffraction (see Fig. S1, ESI†). The IR spectra of 1–4 are similar, and the strong and broad absorption bands in the range of 3100–3600 cm−1 in four compounds may be attributed to the characteristic peaks of ν(O–H) from coordinated water molecules or ν(N–H) stretching vibrations in compound 4. The strong vibrations appearing around 1635, 1590, and 1450 cm−1 correspond to the asymmetric and symmetric stretching vibrations of the carboxyl group, respectively.12 The absence of strong bands ranging from 1690 to 1730 cm−1 for complexes 1, 2 and 4 indicates that the ligands 2,3-H2qldc and H2ox in these complexes are deprotonated, compared with those of free ligands. However, strong peaks in the wavelength range from 1730 to 1687 cm−1 for complex 3 show the existence of the free carboxyl group from the ligand 2,3-H2qldc, further confirmed by X-ray single crystal diffraction. Thermal stability of these complexes was investigated by the TGA technique (see Fig. S2, ESI†). Complexes 1–4 were all examined by TGA, among which the first step of weight loss (12.17% for 1, 7.65% for 2, 3.02% for 3, 6.53% for 4) at around 100 °C can be assigned to removal of lattice water molecules, and subsequent weight loss (10.57% for 1, 7.12% for 2, 7.11% for 3, 3.85% for 4) at about 160 °C correspond to the loss of coordinated water molecules, while abrupt weight loss in the range of 300 to 400 °C could be owing to decomposition of 2,3-qldc2− and ox2− to generate the final product of Yb2O3. Structural analysis and discussion [Yb(2,3-qldc)(ox)1/2 (H2O)3·(H2O)4]n (1). Single-crystal X-ray diffraction analyses reveal that 1 crystallizes in the triclinic ˉ space group and features a 1-D chain-like structure system, P1 constructed from lanthanide core [Yb(2,3-qldc)(H2O)3]22+ units and ox2− bridges. Its asymmetrical unit comprises of one ytterbium ion, one independent deprotonated 2,3-qldc2− ligand, one-half ox2− ligand, three coordinated as well as three and a half lattice water molecules. As shown in Fig. 1, each Yb3+ is
Fig. 1 The coordination environments of the YbIII ion in compound 1 containing partial atomic labels, and the lattice water molecules are omitted for clarify. Symmetry code: (a) 2 − x, 1 − y, 1 − z, (b) 1 − x, 2 − y, 1 − z.
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Scheme 2
Fig. 2
Four coordination modes of 2,3-qldc2− in complexes 1–4.
1-D chain in compound 1 along the a-axis.
octa-coordinated in a distorted bicapped trigonal prismatic geometry with three oxygen atoms (O1, O2, O3a) from two 2,3qldc2− ligands, two oxygen atoms (O5b, O6) from one ox2− ligand and three oxygen atoms (O7, O8, O9) from three coordinated water molecules, respectively. The Yb–O distances are in the range of 2.284(9)–2.411(9) Å. In the present case, the ligand 2,3-H2qldc was completely deprotonated and each 2,3qldc2− ligand via a μ2–η1:η1:η1:η1 coordination manner (mode I in Scheme 2) links two Yb(III) centers to generate a 14-membered ring, which was further, by means of chelation-bridging coordination mode μ2–κ4-O of ox2−, connected to form a 1-D chain-like structure as depicted in Fig. 2. Moreover, through inter-chain π⋯π stacking interactions between two quinoline rings from two adjacent chains with the separation of 3.521–3.985 Å, a 3-D supramolecular framework containing the 1-D channel with a size of 8.8 × 13 Å2 is assembled, in which water molecules are trapped and the framework is stabilized via hydrogen bonding O–H⋯O(N) interactions (Fig. S3 and Table S3†). [Yb(2,3-qldc)(ox)1/2(H2O)2·(H2O)2]n (2). When heated up to 100 °C, with the reactant ratio of H2qldc : H2ox : Yb2O3 still being 2 : 1 : 1, all the four oxygen atoms from two carboxyl groups of the fully deprotonated 2,3-qldc2− were coordinated to Yb3+ ions, showing the coordination mode II in Scheme 2, and complex 2 was obtained. The single crystal diffraction anaˉ lysis shows that complex 2 crystalizes in the triclinic system, P1 space group and presents a 2-D layer structure constructed from lanthanide chains [Yb(2,3-qldc)(H2O)2]2n2n+ (Fig. S4†) and ox2− pillars. As shown in Fig. 3, the Yb3+ ion is
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Fig. 3 The coordination environments of the YbIII ion in compound 2 containing partial atomic labels, and the lattice water molecules are omitted for clarify. Symmetry code: (a) x, 1 − y, 0.5 + z; (b) x, −1 + y, z.
octa-coordinated with four O atoms (O1, O2, O3a, O4b) from three different 2,3-qldc2− ligands, two oxygen atoms (O5, O6c) from the chelation-bridging ox2− ligand, and two remaining oxygen atoms (O7, O8) from two coordinated water molecules, exhibiting the coordination geometry of a distorted bicapped trigonal prism. The Yb–O distances are in the range of 2.255(1)–2.397(1) Å. Meanwhile, complex 2 could be structurally viewed as the temperature-driven transformation in solution from complex 1 by the removal of one coordinated water molecule and then coordinated to the Yb3+ ion (in the adjacent chain) of the uncoordinated oxygen atom (O4) from the carboxyl group of the ligand 2,3-qldc2−, with structural change from the 1-D chain in 1 to the 2-D 63 topological layer in 2, in which the Yb3+ ion is viewed as the node and the ligands 2,3qldc2− and ox2− as linkers (Fig. 4). Moreover, inter-layer π⋯π stacking interaction with the shortest distance of 3.365 Å between the centers of two quinoline rings from two neighbour layers, links the adjacent pillar-chained layers to form a 3-D supramolecular network containing the 1-D channel with a size of 3.8 × 6.5 Å2, in which water molecules are residing and the network is stabilized via hydrogen bonding interactions (Fig. S5 and Table S3†). [Yb(2,3-Hqldc)(ox)(H2O)2·(H2O)]n (3). When the reactant ratio of H2qldc : ox : Yb2O3 (1 : 1 : 1) is applied, compound 3 is obtained at reaction temperature of 70 °C. 3 crystallizes in the ˉ space group and presents a 2-D layer structriclinic system, P1 ture, in which every Yb3+ ion is octa-coordinated to two μ1oxygen atoms (O3, O4a) of the carboxyl group from two partially deprotonated 2,3-Hqdc−, four oxygen atoms (O5, O6, O7, O8) from two ox2−, the remaining two oxygen atoms (O9, O10) from two coordinated water molecules, exhibiting a distorted bicapped trigonal prismatic coordination geometry (Fig. 5). The Yb–O distances fall in the range of 2.296(6) to 2.401(6) Å. Because one of the two carboxyl groups in the ligand 2,3H2qldc was not deprotonated, the resulting ligand 2,3-Hqldc− adopts the μ2–η1:η1 coordination fashion (mode III in Scheme 2), together with the μ2–κ4 ox2− bridging ligand, the 2-D pillar-chained layer is assembled as shown in Fig. 6a and S6,† which can be simplified as Fig. 6b, exhibiting a 3-connected 63 topology. Similar to compound 2, a 3-D supramolecular framework containing the 1-D channel with a size of
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Fig. 4
2-D sheet with 63 topology extended in the ab plane in 2.
Fig. 5 The coordination environments of the YbIII ion in compound 3 containing partial atomic labels, and the lattice water molecules are omitted for clarify. Symmetry code: (a) 1 − x, 2 − y, 1 − z, (b) −x, 1 − y, 1 − z, (c) −x, 2 − y, 1 − z.
3.5 × 6.7 Å2 may be assembled via inter-layer π⋯π stacking interaction with the shortest distance of 3.328 Å between the centers of two quinoline rings from two neighbour layers, in which water molecules are residing and stabilized via hydrogen bonding interactions (Fig. S7 and Table S3†). [Yb(2,3-Hqldc)(ox)(H2O)·(H2O)2]n (4). Compound 4 was obtained at a temperature of 100 °C with a reactant molar ratio
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Fig. 8 2-D pillar-chained layer with 63 topology extended in the bc plane in 4.
Fig. 6
2-D layer with 63 topology extended in the ab plane in 3.
Fig. 7 The coordination environments of the YbIII ions in compound 4, and the lattice water molecules are omitted for clarify. Symmetry code: (a) 1 − x, 2 − y, 1 − z; (b) −x, 1 − y, 1 − z; (c) −x, 2 − x, 1 − z.
of 2,3-H2qldc : H2ox : Yb2O3 (1 : 1 : 1). Like the above three comˉ space pounds 1–3, 4 also crystallizes in the triclinic system, P1 3+ group. Its asymmetric unit contains one Yb ion, two half ox2−, one 2,3-Hqldc−, one coordinated water molecule as well as two and half lattice water molecules as indicated in Fig. 7, in which the Yb3+ ion is octa-coordinated to three oxygen atoms (O1, O3, O4a) from two ligands of 2,3-Hqldc−, four oxygen atoms (O5, O6b, O7, O8c) from two ox2− and the remaining
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one oxygen atom (O9) from one coordinated water molecule. The Yb–O distances are in the range of 2.276(6) and 2.417(6) Å, consistent with those in compounds 1–3 (Table S2†). Unlike 1–3, the nitrogen atom of the ligand 2,3-H2qldc in compound 4 was protonated to generate a partially deprotonated 2,3Hqldc− ligand and shows μ3–η1:η1:η1 coordination mode (mode IV in Scheme 2). Based on the above coordination modes of 2,3-Hqldc− and ox2−, these YbIII ions are connected to the 2-D pillar-chained layer with 63 topology like complexes 2–3 as displayed in Fig. 8 and S8.† In 4, a similar 3-D supramolecular framework containing a 1-D channel with a size of 3.3 × 6.6 Å2 assembled by inter-layer π⋯π stacking interaction with the shortest distance of 3.357 Å between the centers of two quinoline rings from two neighbour layers is shown in Fig. S9.† Meanwhile, the existence of hydrogen bonds O(N)–H⋯O between water molecules trapped in 1-D channels and the nitrogen atom from the quinoline ring may contribute to the stability of the 3-D supramolecular framework structure (Table S3†). Effect of temperature and the reactant ratio on molecular assemblies In this article, four crystal structures of the Yb(III), 2,3-qldc2− and ox2− system have been described and discussed, which have been prepared with a reactant ratio of 2,3-H2qldc : H2ox : Yb2O3 as 2 : 1 : 1 or 1 : 1 : 1 at the reaction temperature of 70 °C or 100 °C. From the above description and discussion, it can be seen that different reaction temperatures/reactant ratios have a great influence on the assembly of these metal–organic complexes, especially on the coordination behavior of the
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carboxyl group of 2,3-H2qldc, and the number of coordinated water molecules and subsequently the dimensionality of the final products. The overview presented in Scheme 1 shows that via the hydrothermal synthesis method at low reaction temperature (∼70 °C), only three or two carboxyl oxygen atoms of 2,3-H2qldc were coordinated to Yb3+ (Scheme 2, modes I/III), leaving three/two water molecules occupying the other coordination positions besides chelating coordinated ox2−, generating a 1-D chain with three coordinated water molecules per Yb(III) ion in 1 and a 2-D layer with two coordinated water molecules per Yb(III) ion in 3. However, at high reaction temperature (∼100 °C), coordination ability of carboxyl oxygen atoms of 2,3H2qldc has been further strengthened, and four or three carboxyl oxygen atoms of 2,3-H2qldc can be coordinated to the Yb3+ ion (Scheme 2, modes II/IV), and there are only two/one coordinated water molecules in the coordination sphere of the YbIII ion besides chelating coordinated ox2−, therefore two resulting complexes 2 and 4 have the same 2-D layer-like networks with 63 topologies, but with two or one coordinated water molecules per YbIII ion in 2 and 4, respectively (Scheme 1). As indicated above, obviously directing coordination of these quinoline-involved ligands to Yb3+ centers is the synergy of various factors, but the reaction temperature and the reactant ratio indeed play an important role in directing coordination modes of the ligand 2,3-H2qldc, subsequently leading to different assemblies of metal–organic complexes in the present reaction system and under the reaction conditions employed (Scheme 1), and further influencing their fluorescent properties. NIR luminescent properties The NIR luminescent spectra (Fig. 9) reveal that the largest excitation peak exists at 339 nm and all complexes 1–4 exhibit a NIR emission at around 980 nm, which can be assigned to the 2F5/2→2F7/2 transition of the Yb(III) ion. Observation of Ybbased NIR luminescence for all complexes 1–4 supports that 2,3-qldc serves as an energy-acceptable “antenna” for the central Yb3+ ions, absorbing energy from UV radiation and
Fig. 9
NIR excitation and emission spectra of complexes 1–4.
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Fig. 10 Diagram of quenching effect on Yb3+ by vibrational energy transfer from an excited Yb3+ ion to proximate OH oscillators (ISC = intersystem crossing, ET = energy transfer).
transferring to the 2F5/2 level of central Yb3+ ions. With radiative transition of Yb3+ from 2F5/2 to 2F7/2, NIR luminescence at 980 nm of complexes 1–4 was emitted.1b,c,5 The relative intensity of the NIR emission follows the order of 4 > 2 > 3 > 1 under the same experimental conditions. This observation supports that the ligand 2,3-qldc2− may effectively transfer energy to the central Yb3+ ions, while the different intensity could be attributed to the stepwise elimination of coordinated water molecules in compounds 1–4. Upon receiving energy from the triplet excited state of the ligand 2,3-qldc2−, the nonradiative energy transfer from the emitting Yb3+ ion to the oscillating O–H group from the coordinated water molecules is very efficient since the energy gap of the radiative (f–f ) transition for the Yb3+ (2F5/2→2F7/2: ∼10 200 cm−1) matches well with the energy of the third vibrational quanta of O–H groups which were placed in the first coordination sphere with a short distance of about 3.0 pm from emitting Yb3+ to O–H groups (Fig. 10). Therefore via removing coordinated water molecules, energy loss of the emitting Yb3+ ions was reduced and the YbIII-based NIR luminescence was hence enhanced. Besides, for complexes 2 and 3, with the same number of coordinated water molecules per Yb3+ ion, the slight difference in their NIR luminescent intensity is possibly ascribed to the number of 2,3-qldc2− coordinated per Yb3+ ion (three for 2 and two for 3), which act as the main antenna harvesting energy for Yb3+ ions. Additionally, the visible emission spectra of complexes 1–4 and the ligand 2,3-H2qldc were also investigated, which revealed that complexes 1–4 exhibit ligand-centered emission at around 425 nm as denoted in Fig. 11, enhanced and redshifted from the emission band of the ligand at 415 nm, indicating that the energy transfer from the ligand to the Yb3+ ions was inefficient. Visible ligand-centered luminescence of complexes 1–4 could be ascribed to enhanced rigidity of the ligand and less energy loss through vibrational motion upon coordination of Yb3+.13 Emission intensity of complexes 1–4 are similar, revealing that coordinated water molecules have no or
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Notes and references
Fig. 11 Visible emission spectra of complexes 1–4 and the ligand 2,3-H2qldc at room temperature.
little impact on the visible emission properties of the four complexes 1–4.
Conclusions In summary, we have successfully assembled four low-dimensional ytterbium(III)-organic polymers from the reaction of 2,3H2qldc and H2ox with Yb2O3 under hydrothermal synthesis conditions, and firstly explore the lanthanide coordination chemistry of 2,3-H2qldc. The results show that the different assemblies of four YbIII-based complexes 1–4 with different structures and dimensions mainly derived from the change in the reaction temperature and the reactant ratio in the present reaction system. Further investigation into their NIR luminescence revealed that four complexes exhibit typical ytterbium(III) luminescence at 980 nm with a decreasing order of 4 > 2 > 3 > 1 in intensity, closely related to the number of coordination water molecules per Yb3+ ion in complexes 1–4. The relationship between structures and properties may provide a useful strategy to tune the NIR fluorescent properties of lanthanideorganic compounds, which could be exploited as NIR-luminescent-tunable materials.
Acknowledgements This work was supported by the National Natural Science Foundation of China (grant no. 91122008 and 21071056), the Research Fund for the Doctoral Program of Higher Education of China (20124407110007), higher school science and technology innovation key project of Guangdong Province (cxzd1113), and the Foundation for High-level Talents in Higher Education of Guangdong, China (C10301).
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1 (a) M. H. V. Werts, R. H. Woudenberg, P. G. Emmerink, R. van Gassel, J. W. Hofstraat and J. W. Verhoeven, Angew. Chem., Int. Ed., 2000, 39, 4542–4544; (b) L. Armelaoa, S. Quicib, F. Barigelletti, G. Accorsic, G. Bottarod, M. Cavazzinib and E. Tondelloe, Coord. Chem. Rev., 2010, 254, 487–505; (c) P. Martín-Ramos, P. S. P. da Silva, V. Lavín, I. R. Martín, F. Lahoz, P. Chamorro-Posada, M. R. Silva and J. Martín-Gil, Dalton Trans., 2013, 42, 13516–13526. 2 (a) J.-C. G. Bünzli, Chem. Rev., 2010, 110, 2729–2755; (b) S. V. Eliseeva and J.-C. G. Bünzli, Chem. Soc. Rev., 2010, 39, 189–227. 3 Z. R. Hong, C. J. Liang, R. G. Li, D. Zhao, D. Fan and W. L. Li, Thin Solid Films, 2001, 391, 122–125. 4 J. W. Stouwdam, G. A. Hebbink, J. Huskens and F. C. J. M. Van Veggel, Chem. Mater., 2003, 15, 4604–4616. 5 (a) G. F. de Sa, O. L. Malta, C. de Mello Donega, A. M. Simas, R. L. Longo, P. A. Santa-Cruz and E. F. da Silva Jr., Coord. Chem. Rev., 2000, 196, 165–195; (b) N. Sabbatini, M. Guardigli and J.-M. Lehn, Coord. Chem. Rev., 1993, 123, 201–228; (c) R. E. Whan and G. A. Crosby, J. Mol. Spectrosc., 1962, 8, 315–327. 6 (a) A. Beeby, I. M. Clarkson, R. S. Dickins, S. Faulkner and D. Parker, J. Chem. Soc., Perkin Trans. 2, 1999, 493–503; (b) D. Parker, Coord. Chem. Rev., 2000, 205, 109–130. 7 (a) J. Zhang, C. M. Shade, D. A. Chengelis and S. Petoud, J. Am. Chem. Soc., 2007, 129, 14834–14835; (b) K. A. White, D. A. Chengelis, M. Zeller, S. J. Geib, J. Szakos, S. Petoud and N. L. Rosi, Chem. Commun., 2009, 30, 4506–4508; (c) X. J. Hong, M. F. Wang, H. G. Jin, Q. G. Zhan, Y. T. Liu, H. Y. Jia, X. Liu and Y. P. Cai, CrystEngComm, 2013, 15, 5606–5611. 8 (a) C. Y. Sun, X. J. Zheng, X. B. Chen, L. C. Li and L. P. Jin, Inorg. Chim. Acta, 2009, 362, 325–330; (b) F. Wang, J. Q. Su, H. M. Peng, H. Y. Jia, Q. G. Zhan, H. G. Jin, J. H. Chen and Y. P. Cai, Inorg. Chem. Commun., 2012, 21, 8–11. 9 G. M. Sheldrick, SADABS, Version 2.05, University of Göttingen, Göttingen, Germany. 10 G. M. Sheldrick, SHELXS-97, Program for X-ray Crystal Structure Determination, University of Göttingen, Göttingen, Germany, 1997. 11 G. M. Sheldrick, SHELXS-97, Program for X-ray Crystal Structure Refinement, University of Göttingen, Göttingen, Germany, 1997. 12 K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds, John Wiley and Sons, New York, 1997. 13 (a) G. Wu, X. F. Wang, T. Okamura, W. Y. Sun and N. Ueyama, Inorg. Chem., 2006, 45, 8523; (b) D. M. Ciurtin, N. G. Pschirer, M. D. Smith, U. H. F. Bunz and H. C. zur Loye, Chem. Mater., 2001, 13, 2743; (c) Y. B. Dong, P. Wang, R. Q. Huang and M. D. Smith, Inorg. Chem., 2004, 43, 4727.
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Construction of four low-dimensional NIR-luminescence-tunable Yb(III) complexes† Zhi-Peng Zheng,a Xiu-Xia Zhang,a Teng Li,a Jian Yang,a Lei-Ming Wei,a Li-Guo Zhang,a Xiao-Ming Lina and Yue-Peng Cai*a,b Four low-dimensional ytterbium(III)-organic compounds through hydrothermal reactions of quinoline2,3-dicarboxylic acid (2,3-H2qldc) and oxalic acid (H2ox) with Yb2O3, namely, [Yb(2,3-qldc)(ox)1/2(H2O)3·(H2O)4]n (1), [Yb(2,3-qldc)(ox)1/2(H2O)2·(H2O)2]n (2), [Yb(2,3-Hqldc)(ox)(H2O)2·(H2O)]n (3) and [Yb(2,3-Hqldc)(ox)(H2O)·(H2O)2]n (4), were first synthesized and characterized by elemental analysis (EA), infrared spectroscopy (IR), thermogravimetric analysis (TG), and single-crystal X-ray diffraction. When the reactant ratio of 2,3-H2qldc : H2ox : Yb2O3 is 2 : 1 : 1, 1-D chain-like complex 1 with three coordinated water molecules around the Yb(III) ion was obtained in mixed solvents of H2O and CH3OH (v : v = 10 : 1) at 70 °C, and with the increase of temperature to 100 °C, the same reactants gave 2-D 63 topological layerlike complex 2 with two coordinated water molecules in the coordination sphere of the Yb(III) ion. However, when the reactant ratio was changed to 1 : 1 : 1, two 2-D 63 topological layer-like complexes 3 (70 °C) and 4 (100 °C) were obtained at different temperatures, in which the coordination water molecules
Received 30th May 2014, Accepted 24th July 2014 DOI: 10.1039/c4dt01601g www.rsc.org/dalton
in 3 and 4 are two and one, respectively. Obviously, these results reveal that the reaction temperature and reactant ratios play critical roles in the structural direction of these low-dimensional compounds. Interestingly, with the gradual loss of coordination water molecules to the Yb(III) ion, the near infrared (NIR) emission of four Yb(III)-based compounds 1–4 can be gradually strengthened with increasing order of 1 < 3 < 2 < 4, indicating that these ytterbium(III) complexes have tunable near infrared luminescence.
Introduction As a consequence of low toxicity and efficient light transmission of biological tissues in part of the NIR spectral range (0.9 to 1.5 µm approximately),1 ytterbium(III) coordination compounds can be used as near infrared (NIR) luminescent materials applied in biomedicine,2 electroluminescence3 and laser systems,4 in which the central Yb(III) ions are coordinated to organic ligands with exceedingly delocalized π systems acting as antennas to effectively harvest and transfer energy, known as the antenna effect.1b,c,5 On the other hand, compared with that of visible-lightemitting lanthanide (Sm3+, Tb3+, Eu3+ etc.) coordination
a School of Chemistry and Environment, South China Normal University, Guangzhou Key Laboratory of Materials for Energy Conversion and Storage, Guangzhou 510006, PR China. E-mail: [email protected]; Fax: +86-020-39310; Tel: +86-020-39310383 b State Key Laboratory of Structure Chemistry, Fujian, Fuzhou 350002, PR China † Electronic supplementary information (ESI) available. CCDC 1003835, 1003836, 1003837 and 1003838 for 1–4. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4dt01601g
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compounds, the NIR luminescence of lanthanide (Nd3+, Yb3+, Er3+) coordination compounds is less explored, which is due to the low energy of the NIR luminescence and its susceptibility of being quenched by non-radiative energy transfer from Ln(III) excited states to high-energy O–H, N–H or C–H oscillators,6 making it one of the challenges in constructing NIR luminescent lanthanide complexes. Therefore, several efforts2,7, including taking advantage of macro cryptand-like ligands to protect central lanthanide ions and fluoridation of C–H bonds, have been made to alleviate non-radiative decays by sheltering the Ln3+ excited levels from high non-radiative transition probability by O–H and C–H oscillators from solvent molecules such as water, methanol and ethanol. Moreover, using an auxiliary ligand to displace solvent molecules is another effective way to assuage NIR luminescence quenching from coordinated solvent molecules. For example, oxalate (ox−) has no high-energy vibrational groups and high tendency to chelate lanthanide ions; accordingly, it is a good candidate for a bridging ligand in assembling lanthanide compounds.8 In the present contribution, apart from 2,3-quinoline dicarboxylic acid (2,3-H2qldc) with a large π-conjugating system as the main ligand, we choose oxalic acid (H2ox) as the auxiliary
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Scheme 1 Assembly of compounds 1–4 related to the reaction temperature and the reactant ratio.
ligand, reacting with Yb2O3 under hydrothermal conditions, to construct ytterbium(III) compounds with good near-infrared emission. By changing the reaction temperature and the reactant ratio, four ytterbium compounds containing different coordination water molecules, namely, [Yb(2,3-qldc)(ox)1/2(H2O)3·(H2O)4]n (1), [Yb(2,3-qldc)(ox)1/2(H2O)2·(H2O)2]n (2), [Yb(2,3-Hqldc)(ox)(H2O)2·(H2O)]n (3), and [Yb(2,3-Hqldc)(ox)(H2O)·(H2O)2]n (4) were constructed as indicated in Scheme 1. Their structures have been determined by single-crystal X-ray diffraction analyses and further characterized by infrared spectra (IR), elemental analyses (EA), and powder X-ray diffraction (PXRD). A further study reveals that the four complexes in the solid state have similar NIR-emission spectra with the increasing order of 1 < 3 < 2 < 4 in the emission intensity, consistent with the decreasing order of coordination water molecules in four complexes 1–4. To the best of our knowledge, compounds 1–4 represent the first example of systematic investigation into coordination chemistry and YbIII-based near-infrared emission in the system of quinoline-2,3-dicarboxylic acid and oxalic acid.
Experimental Physical measurements All materials were of reagent grade obtained from commercial sources and used without further purification, and solvents were dried by standard procedures. Elemental analyses for C, H, N were performed on a Perkin-Elmer 240C analytical instrument. IR spectra were recorded on a Nicolet FT-IR-170SX spectrophotometer in KBr pellets. Thermogravimetric analyses were performed on a Perkin-Elmer TGA7 analyzer with a heating rate of 10 °C min−1 under a flowing nitrogen atmosphere. The luminescent spectra for the solid state were recorded at room temperature on a Edinburgh-FLS-920 with a xenon arc lamp as the light source. In the measurements of emission and excitation spectra the pass width is 5.0 nm. The X-ray powder diffraction (PXRD) patterns were recorded with a Siemens D5005 diffractometer with Cu-Kα (λ = 1.5418 Å) radiation. Preparation of complexes 1–4. A mixture of Yb2O3 (39 mg, 0.1 mmol), ligand 2,3-H2qldc (44 mg, 0.2 mmol), oxalic acid (13 mg, 0.1 mmol for 1, 2 and 26 mg, 0.2 mmol for 3, 4), H2O
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(10 mL), and CH3OH (0.2 mL) was heated from room temperature to 70 °C for 80 h in a 25 mL Teflon lined stainless-steel autoclave and then cooled to room temperature at a rate of 10 °C h−1 to obtain the colourless crystals for compounds 1 and 3 suitable for a single crystal diffraction test. By contrast, when the reaction temperature rose up to 100 °C, the two abovementioned reactions gave colourless crystal compounds 2 and 4, respectively. 1. Yield 57% (based on Yb), colorless crystals. Elemental analysis calcd (%) for C12H19NO13Yb: C, 25.79; H, 3.40; N, 2.51. Found: C, 25.81; H, 3.30; N, 2.53. FT-IR (KBr, cm−1): 3457(vs), 2364(s), 2072(m), 1663(s), 1400(s), 1123(s), 996(w), 794(w), 638(w), 534(w). 2. Yield 49% (based on Yb), colorless crystals. Elemental analysis calcd (%) for C12H13NO10Yb: C, 28.56; H, 2.58; N, 2.78. Found: C, 28.78; H, 2.65; N, 2.80. FT-IR (KBr, cm−1): 3446(br, vs), 2360(w), 2065(w), 1635(s), 1400(s), 1136(s), 999(m), 785(w), 669(w), 617(w), 538(w), 488(w). 3. Yield 48% (based on the Yb), colorless crystals. Elemental analysis calcd (%) for C13H12NO11Yb: C, 29.36; H, 2.26; N, 2.64. Found: C, 29.41; H, 2.29; N, 2.66. FT-IR (KBr, cm−1): 3477(br, vs), 1730(s), 1687(s), 1618(vs), 1465(s), 1386(m), 1386(s), 1309(w), 1143(w), 1049(w), 876(m), 800(m), 619(m), 481(w), 455(w). 4. Yield 51% (based on the Yb), colorless crystals. Elemental analysis calcd (%) for C26H26N2O23Yb2: C, 28.87; H, 2.41; N, 2.59. Found: C, 28.89; H, 2.47; N, 2.63. FT-IR (KBr, cm−1): 3460 (br, vs), 3330(s), 1620(vs), 1543(vs), 1463(vs), 1407(m), 1321(m), 1047(m), 873(m), 808(s), 771(m), 748(w), 682(w), 592(w), 442(w).
X-ray data collection and structure refinement Data collections were performed at 298 K on a Bruker Smart Apex II diffractometer with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) for 1–4. Absorption corrections were applied using the multiscan program SADABS.9 Structural solutions and full-matrix least-squares refinements based on F2 were performed with the SHELXS-9710 and SHELXL-9711 program packages, respectively. All the non-hydrogen atoms were refined anisotropically. The hydrogen atoms were placed at calculated positions and included in the refinement in the riding model approximation. The organic hydrogen atoms were generated geometrically (C–H = 0.93 or 0.96 Å), and the water hydrogen atoms were located from difference maps and refined with isotropic temperature factors. The carbon atoms C4–C10 and the nitrogen atom N1 of the quinoline ring in compound 2 are located in two positions with 0.5 occupancy of each position. Details of the crystal parameters, data collections, and refinements for complexes 1–4 are summarized in Table S1.† Selected bond lengths and angles for four complexes are shown in Table S2,† hydrogen-bonding data of these four complexes are listed in Table S3 (ESI†). Further details are provided in the ESI.† Crystallographic data for the structures reported in this paper have been deposited with the Cambridge Crystallographic Data Centre. CCDC 1003835, 1003836, 1003837 and 1003838 are for four new compounds 1–4, respectively.
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Results and discussion
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IR spectroscopic and thermogravimetric analyses Through hydrothermal syntheses, four new compounds 1–4 were synthesized and characterized by EA, IR and X-ray singlecrystal/powder diffraction (see Fig. S1, ESI†). The IR spectra of 1–4 are similar, and the strong and broad absorption bands in the range of 3100–3600 cm−1 in four compounds may be attributed to the characteristic peaks of ν(O–H) from coordinated water molecules or ν(N–H) stretching vibrations in compound 4. The strong vibrations appearing around 1635, 1590, and 1450 cm−1 correspond to the asymmetric and symmetric stretching vibrations of the carboxyl group, respectively.12 The absence of strong bands ranging from 1690 to 1730 cm−1 for complexes 1, 2 and 4 indicates that the ligands 2,3-H2qldc and H2ox in these complexes are deprotonated, compared with those of free ligands. However, strong peaks in the wavelength range from 1730 to 1687 cm−1 for complex 3 show the existence of the free carboxyl group from the ligand 2,3-H2qldc, further confirmed by X-ray single crystal diffraction. Thermal stability of these complexes was investigated by the TGA technique (see Fig. S2, ESI†). Complexes 1–4 were all examined by TGA, among which the first step of weight loss (12.17% for 1, 7.65% for 2, 3.02% for 3, 6.53% for 4) at around 100 °C can be assigned to removal of lattice water molecules, and subsequent weight loss (10.57% for 1, 7.12% for 2, 7.11% for 3, 3.85% for 4) at about 160 °C correspond to the loss of coordinated water molecules, while abrupt weight loss in the range of 300 to 400 °C could be owing to decomposition of 2,3-qldc2− and ox2− to generate the final product of Yb2O3. Structural analysis and discussion [Yb(2,3-qldc)(ox)1/2 (H2O)3·(H2O)4]n (1). Single-crystal X-ray diffraction analyses reveal that 1 crystallizes in the triclinic ˉ space group and features a 1-D chain-like structure system, P1 constructed from lanthanide core [Yb(2,3-qldc)(H2O)3]22+ units and ox2− bridges. Its asymmetrical unit comprises of one ytterbium ion, one independent deprotonated 2,3-qldc2− ligand, one-half ox2− ligand, three coordinated as well as three and a half lattice water molecules. As shown in Fig. 1, each Yb3+ is
Fig. 1 The coordination environments of the YbIII ion in compound 1 containing partial atomic labels, and the lattice water molecules are omitted for clarify. Symmetry code: (a) 2 − x, 1 − y, 1 − z, (b) 1 − x, 2 − y, 1 − z.
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Scheme 2
Fig. 2
Four coordination modes of 2,3-qldc2− in complexes 1–4.
1-D chain in compound 1 along the a-axis.
octa-coordinated in a distorted bicapped trigonal prismatic geometry with three oxygen atoms (O1, O2, O3a) from two 2,3qldc2− ligands, two oxygen atoms (O5b, O6) from one ox2− ligand and three oxygen atoms (O7, O8, O9) from three coordinated water molecules, respectively. The Yb–O distances are in the range of 2.284(9)–2.411(9) Å. In the present case, the ligand 2,3-H2qldc was completely deprotonated and each 2,3qldc2− ligand via a μ2–η1:η1:η1:η1 coordination manner (mode I in Scheme 2) links two Yb(III) centers to generate a 14-membered ring, which was further, by means of chelation-bridging coordination mode μ2–κ4-O of ox2−, connected to form a 1-D chain-like structure as depicted in Fig. 2. Moreover, through inter-chain π⋯π stacking interactions between two quinoline rings from two adjacent chains with the separation of 3.521–3.985 Å, a 3-D supramolecular framework containing the 1-D channel with a size of 8.8 × 13 Å2 is assembled, in which water molecules are trapped and the framework is stabilized via hydrogen bonding O–H⋯O(N) interactions (Fig. S3 and Table S3†). [Yb(2,3-qldc)(ox)1/2(H2O)2·(H2O)2]n (2). When heated up to 100 °C, with the reactant ratio of H2qldc : H2ox : Yb2O3 still being 2 : 1 : 1, all the four oxygen atoms from two carboxyl groups of the fully deprotonated 2,3-qldc2− were coordinated to Yb3+ ions, showing the coordination mode II in Scheme 2, and complex 2 was obtained. The single crystal diffraction anaˉ lysis shows that complex 2 crystalizes in the triclinic system, P1 space group and presents a 2-D layer structure constructed from lanthanide chains [Yb(2,3-qldc)(H2O)2]2n2n+ (Fig. S4†) and ox2− pillars. As shown in Fig. 3, the Yb3+ ion is
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Fig. 3 The coordination environments of the YbIII ion in compound 2 containing partial atomic labels, and the lattice water molecules are omitted for clarify. Symmetry code: (a) x, 1 − y, 0.5 + z; (b) x, −1 + y, z.
octa-coordinated with four O atoms (O1, O2, O3a, O4b) from three different 2,3-qldc2− ligands, two oxygen atoms (O5, O6c) from the chelation-bridging ox2− ligand, and two remaining oxygen atoms (O7, O8) from two coordinated water molecules, exhibiting the coordination geometry of a distorted bicapped trigonal prism. The Yb–O distances are in the range of 2.255(1)–2.397(1) Å. Meanwhile, complex 2 could be structurally viewed as the temperature-driven transformation in solution from complex 1 by the removal of one coordinated water molecule and then coordinated to the Yb3+ ion (in the adjacent chain) of the uncoordinated oxygen atom (O4) from the carboxyl group of the ligand 2,3-qldc2−, with structural change from the 1-D chain in 1 to the 2-D 63 topological layer in 2, in which the Yb3+ ion is viewed as the node and the ligands 2,3qldc2− and ox2− as linkers (Fig. 4). Moreover, inter-layer π⋯π stacking interaction with the shortest distance of 3.365 Å between the centers of two quinoline rings from two neighbour layers, links the adjacent pillar-chained layers to form a 3-D supramolecular network containing the 1-D channel with a size of 3.8 × 6.5 Å2, in which water molecules are residing and the network is stabilized via hydrogen bonding interactions (Fig. S5 and Table S3†). [Yb(2,3-Hqldc)(ox)(H2O)2·(H2O)]n (3). When the reactant ratio of H2qldc : ox : Yb2O3 (1 : 1 : 1) is applied, compound 3 is obtained at reaction temperature of 70 °C. 3 crystallizes in the ˉ space group and presents a 2-D layer structriclinic system, P1 ture, in which every Yb3+ ion is octa-coordinated to two μ1oxygen atoms (O3, O4a) of the carboxyl group from two partially deprotonated 2,3-Hqdc−, four oxygen atoms (O5, O6, O7, O8) from two ox2−, the remaining two oxygen atoms (O9, O10) from two coordinated water molecules, exhibiting a distorted bicapped trigonal prismatic coordination geometry (Fig. 5). The Yb–O distances fall in the range of 2.296(6) to 2.401(6) Å. Because one of the two carboxyl groups in the ligand 2,3H2qldc was not deprotonated, the resulting ligand 2,3-Hqldc− adopts the μ2–η1:η1 coordination fashion (mode III in Scheme 2), together with the μ2–κ4 ox2− bridging ligand, the 2-D pillar-chained layer is assembled as shown in Fig. 6a and S6,† which can be simplified as Fig. 6b, exhibiting a 3-connected 63 topology. Similar to compound 2, a 3-D supramolecular framework containing the 1-D channel with a size of
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Fig. 4
2-D sheet with 63 topology extended in the ab plane in 2.
Fig. 5 The coordination environments of the YbIII ion in compound 3 containing partial atomic labels, and the lattice water molecules are omitted for clarify. Symmetry code: (a) 1 − x, 2 − y, 1 − z, (b) −x, 1 − y, 1 − z, (c) −x, 2 − y, 1 − z.
3.5 × 6.7 Å2 may be assembled via inter-layer π⋯π stacking interaction with the shortest distance of 3.328 Å between the centers of two quinoline rings from two neighbour layers, in which water molecules are residing and stabilized via hydrogen bonding interactions (Fig. S7 and Table S3†). [Yb(2,3-Hqldc)(ox)(H2O)·(H2O)2]n (4). Compound 4 was obtained at a temperature of 100 °C with a reactant molar ratio
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Fig. 8 2-D pillar-chained layer with 63 topology extended in the bc plane in 4.
Fig. 6
2-D layer with 63 topology extended in the ab plane in 3.
Fig. 7 The coordination environments of the YbIII ions in compound 4, and the lattice water molecules are omitted for clarify. Symmetry code: (a) 1 − x, 2 − y, 1 − z; (b) −x, 1 − y, 1 − z; (c) −x, 2 − x, 1 − z.
of 2,3-H2qldc : H2ox : Yb2O3 (1 : 1 : 1). Like the above three comˉ space pounds 1–3, 4 also crystallizes in the triclinic system, P1 3+ group. Its asymmetric unit contains one Yb ion, two half ox2−, one 2,3-Hqldc−, one coordinated water molecule as well as two and half lattice water molecules as indicated in Fig. 7, in which the Yb3+ ion is octa-coordinated to three oxygen atoms (O1, O3, O4a) from two ligands of 2,3-Hqldc−, four oxygen atoms (O5, O6b, O7, O8c) from two ox2− and the remaining
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one oxygen atom (O9) from one coordinated water molecule. The Yb–O distances are in the range of 2.276(6) and 2.417(6) Å, consistent with those in compounds 1–3 (Table S2†). Unlike 1–3, the nitrogen atom of the ligand 2,3-H2qldc in compound 4 was protonated to generate a partially deprotonated 2,3Hqldc− ligand and shows μ3–η1:η1:η1 coordination mode (mode IV in Scheme 2). Based on the above coordination modes of 2,3-Hqldc− and ox2−, these YbIII ions are connected to the 2-D pillar-chained layer with 63 topology like complexes 2–3 as displayed in Fig. 8 and S8.† In 4, a similar 3-D supramolecular framework containing a 1-D channel with a size of 3.3 × 6.6 Å2 assembled by inter-layer π⋯π stacking interaction with the shortest distance of 3.357 Å between the centers of two quinoline rings from two neighbour layers is shown in Fig. S9.† Meanwhile, the existence of hydrogen bonds O(N)–H⋯O between water molecules trapped in 1-D channels and the nitrogen atom from the quinoline ring may contribute to the stability of the 3-D supramolecular framework structure (Table S3†). Effect of temperature and the reactant ratio on molecular assemblies In this article, four crystal structures of the Yb(III), 2,3-qldc2− and ox2− system have been described and discussed, which have been prepared with a reactant ratio of 2,3-H2qldc : H2ox : Yb2O3 as 2 : 1 : 1 or 1 : 1 : 1 at the reaction temperature of 70 °C or 100 °C. From the above description and discussion, it can be seen that different reaction temperatures/reactant ratios have a great influence on the assembly of these metal–organic complexes, especially on the coordination behavior of the
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carboxyl group of 2,3-H2qldc, and the number of coordinated water molecules and subsequently the dimensionality of the final products. The overview presented in Scheme 1 shows that via the hydrothermal synthesis method at low reaction temperature (∼70 °C), only three or two carboxyl oxygen atoms of 2,3-H2qldc were coordinated to Yb3+ (Scheme 2, modes I/III), leaving three/two water molecules occupying the other coordination positions besides chelating coordinated ox2−, generating a 1-D chain with three coordinated water molecules per Yb(III) ion in 1 and a 2-D layer with two coordinated water molecules per Yb(III) ion in 3. However, at high reaction temperature (∼100 °C), coordination ability of carboxyl oxygen atoms of 2,3H2qldc has been further strengthened, and four or three carboxyl oxygen atoms of 2,3-H2qldc can be coordinated to the Yb3+ ion (Scheme 2, modes II/IV), and there are only two/one coordinated water molecules in the coordination sphere of the YbIII ion besides chelating coordinated ox2−, therefore two resulting complexes 2 and 4 have the same 2-D layer-like networks with 63 topologies, but with two or one coordinated water molecules per YbIII ion in 2 and 4, respectively (Scheme 1). As indicated above, obviously directing coordination of these quinoline-involved ligands to Yb3+ centers is the synergy of various factors, but the reaction temperature and the reactant ratio indeed play an important role in directing coordination modes of the ligand 2,3-H2qldc, subsequently leading to different assemblies of metal–organic complexes in the present reaction system and under the reaction conditions employed (Scheme 1), and further influencing their fluorescent properties. NIR luminescent properties The NIR luminescent spectra (Fig. 9) reveal that the largest excitation peak exists at 339 nm and all complexes 1–4 exhibit a NIR emission at around 980 nm, which can be assigned to the 2F5/2→2F7/2 transition of the Yb(III) ion. Observation of Ybbased NIR luminescence for all complexes 1–4 supports that 2,3-qldc serves as an energy-acceptable “antenna” for the central Yb3+ ions, absorbing energy from UV radiation and
Fig. 9
NIR excitation and emission spectra of complexes 1–4.
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Fig. 10 Diagram of quenching effect on Yb3+ by vibrational energy transfer from an excited Yb3+ ion to proximate OH oscillators (ISC = intersystem crossing, ET = energy transfer).
transferring to the 2F5/2 level of central Yb3+ ions. With radiative transition of Yb3+ from 2F5/2 to 2F7/2, NIR luminescence at 980 nm of complexes 1–4 was emitted.1b,c,5 The relative intensity of the NIR emission follows the order of 4 > 2 > 3 > 1 under the same experimental conditions. This observation supports that the ligand 2,3-qldc2− may effectively transfer energy to the central Yb3+ ions, while the different intensity could be attributed to the stepwise elimination of coordinated water molecules in compounds 1–4. Upon receiving energy from the triplet excited state of the ligand 2,3-qldc2−, the nonradiative energy transfer from the emitting Yb3+ ion to the oscillating O–H group from the coordinated water molecules is very efficient since the energy gap of the radiative (f–f ) transition for the Yb3+ (2F5/2→2F7/2: ∼10 200 cm−1) matches well with the energy of the third vibrational quanta of O–H groups which were placed in the first coordination sphere with a short distance of about 3.0 pm from emitting Yb3+ to O–H groups (Fig. 10). Therefore via removing coordinated water molecules, energy loss of the emitting Yb3+ ions was reduced and the YbIII-based NIR luminescence was hence enhanced. Besides, for complexes 2 and 3, with the same number of coordinated water molecules per Yb3+ ion, the slight difference in their NIR luminescent intensity is possibly ascribed to the number of 2,3-qldc2− coordinated per Yb3+ ion (three for 2 and two for 3), which act as the main antenna harvesting energy for Yb3+ ions. Additionally, the visible emission spectra of complexes 1–4 and the ligand 2,3-H2qldc were also investigated, which revealed that complexes 1–4 exhibit ligand-centered emission at around 425 nm as denoted in Fig. 11, enhanced and redshifted from the emission band of the ligand at 415 nm, indicating that the energy transfer from the ligand to the Yb3+ ions was inefficient. Visible ligand-centered luminescence of complexes 1–4 could be ascribed to enhanced rigidity of the ligand and less energy loss through vibrational motion upon coordination of Yb3+.13 Emission intensity of complexes 1–4 are similar, revealing that coordinated water molecules have no or
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Notes and references
Fig. 11 Visible emission spectra of complexes 1–4 and the ligand 2,3-H2qldc at room temperature.
little impact on the visible emission properties of the four complexes 1–4.
Conclusions In summary, we have successfully assembled four low-dimensional ytterbium(III)-organic polymers from the reaction of 2,3H2qldc and H2ox with Yb2O3 under hydrothermal synthesis conditions, and firstly explore the lanthanide coordination chemistry of 2,3-H2qldc. The results show that the different assemblies of four YbIII-based complexes 1–4 with different structures and dimensions mainly derived from the change in the reaction temperature and the reactant ratio in the present reaction system. Further investigation into their NIR luminescence revealed that four complexes exhibit typical ytterbium(III) luminescence at 980 nm with a decreasing order of 4 > 2 > 3 > 1 in intensity, closely related to the number of coordination water molecules per Yb3+ ion in complexes 1–4. The relationship between structures and properties may provide a useful strategy to tune the NIR fluorescent properties of lanthanideorganic compounds, which could be exploited as NIR-luminescent-tunable materials.
Acknowledgements This work was supported by the National Natural Science Foundation of China (grant no. 91122008 and 21071056), the Research Fund for the Doctoral Program of Higher Education of China (20124407110007), higher school science and technology innovation key project of Guangdong Province (cxzd1113), and the Foundation for High-level Talents in Higher Education of Guangdong, China (C10301).
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