Eu3Tb) - NSFC

Article pubs.acs.org/crystal

Structural and Photophysical Studies on Geometric (Er2Yb2/Yb2Er2) and Configurational (EuTb3/Eu3Tb) Isomers of Heterotetranuclear Lanthanide(III) Complexes Hai-Bing Xu,*,†,‡ Jian-Guo Deng,‡ Li-Yi Zhang,† and Zhong-Ning Chen*,† †

State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China ‡ New Materials R&D Center, Institute of Chemical Materials, China Academy of Engineering Physics, Chengdu, Sichuan 621900, China S Supporting Information *

ABSTRACT: Heterotetranuclear geometrical (Er2Yb2/Yb2Er2) and configuational (EuTb3/Eu3Tb) isomeric lanthanide(III) complexes have been synthesized and characterized by spectroscopy as well as X-ray crystallography. The geometric Er2Yb2/ Yb2Er2 isomers exhibit dual emissions from both erbium(III) and ytterbium(III) ions. For the EuTb3/Eu3Tb configurational isomers, the TbIII subunits transfer energy to the EuIII centers in the EuTb3 complex, whereas the TbIII ion in the TbEu3 complex serves mainly as a structural stabilizer.

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INTRODUCTION Because of the forbidden Laporte rule for the lanthanide(III) ions, introduction of a sensitizer with a high extinction coefficient as light-harvesting antenna is usually necessary for achieving highly efficient lanthanide luminescence.1−3 So far, antenna chromophores have included organic ligands,4,5 organometallic chromophores,5−7 and lanthanide subunits.8−10 The multifunctional luminescent lanthanide materials with the first two classes of sensitizers have been extensively investigated in the literature and have found many applications in immunoassays,11 dual-imaging agents,12 DNA13 and metal-ion14 recognition, singlet oxygen15 and fluoride4,16 luminescence probes, electronic luminescence materials,17 and single-molecule magnets.18 Heterometallic complexes containing two or more different lanthanide ions are particularly attractive because of their potential to achieve dual emissions and up-conversion, which are useful in the dual detection of analytes combined with magnetic resonance imaging (MRI) or X-ray techniques for luminescence imaging in cells,19−21 multiple signal detection,22,23 barcoded materials,24 and bioimaging nanoprobes,25 among others. Nevertheless, because of the similar coordination preferences together with a smooth and monotonic contraction from La to Lu (from 1.216 to 1.032 Å), it is very challenging to control the synthesis of such heterometallic lanthanide complexes.8 Although there are few reports on such f−f′ mixed-metal arrays (excluding statistical crystallization9), the synthetic routes are quite complicated,8 and none of the heterometallic lanthanide(III) arrays have been characterized unambiguously by X-ray crystallography.8 Additionally, studies on lanthanide © 2012 American Chemical Society

emissions (including near-infrared and visible emissions) sensitized by another lanthanide subunit have been less explored in heterometallic lanthanide(III) complexes.8a Recently, we established a feasible synthetic approach to access hybrid lanthanide materials using potential bridging ligands. For instance, 8-hydroxyquinoline (Hq) and its derivatives display various coordination modes including chelating (I), bridging with μ-phenol (II), and chelating-bridging with μ- (III) and μ3-phenoxo modes (IV) (Scheme 1). Reactions of bis(2-methylScheme 1. Coordination Modes of q−

8-hydroxyquinolinato)aluminum(III) [ Al(Mq)2] or/and tetrameric bis(8-hydroxyquinolinato) zinc(II) [(Znq2)4] with the Ln(β-diketonate) subunit afforded Al3Ln24 or Zn2Ln26 complexes. These p−f or d−f heteronuclear complexes exhibit typical whitelight emission because of the complementary light-emitting colors from both the residual luminescence of Al(Mq)2 or Znq2 and the sensitized emission of the Eu(III) subunit. Received: October 23, 2012 Revised: November 15, 2012 Published: December 6, 2012 849

dx.doi.org/10.1021/cg3015546 | Cryst. Growth Des. 2013, 13, 849−857

Crystal Growth & Design

Article

ethanol to give 1410 mg (84%) of NH4L as an off-white solid, which was used without further purification. mp: 270−275 °C. ESI-MS (DMSO): m/z 320 [M−NH4]−. Tris(4-nitrophenyl-2,2′-bipyridine-6-carboxylato) Lanthanide Complexes (3a and 4a). An aqueous (5 mL) solution of Ln(OTf)3 (0.1 mmol) was added dropwise to an aqueous (80 mL) solution of NH4L (101 mg, 0.3 mmol) with stirring. A precipitate immediately formed with stirring at room temperature for 3 h. The precipitate was filtered, washed with water and ethanol, and then dried. 3a (EuL3). Anal. Calcd for C51H30EuN9O12·3H2O: C, 52.50; H, 3.11; N, 10.80. Found: C, 52.18; H, 3.03; N, 10.69. IR (KBr, cm−1): 1653 s (CO). Yield: 98%. 4a (TbL3). Anal. Calcd for C51H30TbN9O12·3H2O: C, 52.19; H, 3.09; N, 10.74. Found: C, 52.65; H, 2.97; N, 10.42. IR (KBr, cm−1): 1653 s (CO). Yield: 95%. EuTb3 (3) and Eu3Tb (4) Complexes. 3a or 4a and 3 equiv of Ln(hfac)2(CH3COO)(H2O)2 (Ln = Tb, 3; Eu, 4) were added to dichloromethane with stirring, giving a clear solution. After being filtered, the concentrated dichloromethane solutions were layered with n-hexane to afford the products as pale yellow crystals. 3 (EuTb3). Yield: 78%. Anal. Calcd for C87H45EuF36N9O30Tb3·3H2O·0.5C6H14: C, 34.80; H, 1.88; N, 4.06. Found: C, 34.56; H, 1.85; N, 3.93. ICP analysis showed that the molar ratio of Tb3+ to Eu3+ was 9.06:2.74, corresponding to 22.5% Eu3+ of the total metal composition. ESIMS (CH3OH−CH2Cl2, m/z): ESI-MS (CH3OH−CH2Cl2, m/z): 1455 ([M − 2CH3COO− + H2O]2+), 1463 ([M − 2CH3COO− + 2H2O]2+), 1473 ([M − 2CH3COO− + 3H2O]2+). IR (KBr, cm−1): 1651 s (CO). 4 (TbEu3). Yield: 75%. Anal. Calcd for C87H45TbF36N9O30Eu3·C6H14·CH2Cl2: C, 35.66; H, 1.94; N, 3.98. Found: C, 35.87; H, 1.86; N, 3.98. ICP analysis showed that the molar ratio of Tb3+ to Eu3+ was 3.07:9.92, corresponding to 75.5% Eu3+ of the total metal composition. ESI-MS (CH3OH−CH2Cl2, m/z): 2692 ([M − L + H2O]+). IR (KBr, cm−1): 1651 s (CO). GdTb3. This compound was prepared by the same procedure as used for 3 but with GdL3 and 3 equiv of Tb(hfac)2(CH3COO)(H2O)2 for reaction. Yield: 70%. Anal. Calcd for C87H45GdF36N9O30Tb3·3H2O: C, 34.06; H, 1.68; N, 4.11. Found: C, 34.03; H, 1.65; N, 4.13. R (KBr, cm−1): 1653 s (CO). LaEu3. This compound was prepared by the same procedure as used for 3 but with LaL3 and 3 equiv of Eu(hfac)2(CH3COO)(H2O)2. Yield: 76%. Anal. Calcd for C87H45LaF36N9O30Eu3·3H2O: C, 34.50; H, 1.70; N, 4.16. Found: C, 34.53; H, 1.75; N, 4.17. IR (KBr, cm−1): 1655 s (CO). La3Eu. This compound was prepared by the same procedure as used for 4 but with EuL3·H2O and 3 equiv of La(hfac)2(CH3COO)(H2O)2. Yield: 78%. Anal. Calcd for C87H45EuF36N9O30La3·3H2O: C, 34.80; H, 1.71; N, 4.20. Found: C, 34.76; H, 1.77; N, 4.15. IR (KBr, cm−1): 1655 s (CO). Physical Measurements. Elemental analyses (C, H, N) were carried out on a Perkin-Elmer model 240C elemental analyzer. ICP data were measured on a Jobin Yvon Ultima2 inductively coupled plasma OES spectrometer. Electrospray ion mass spectroscopy (ESIMS) was performed on a Finnigan LCQ mass spectrometer using dichloromethane and methanol mixture as the mobile phase. UV−vis absorption spectra were measured on a Perkin-Elmer Lambda 25 UV− vis spectrophotometer. Infrared (IR) spectra were recorded on a Magna750 FT-IR spectrophotometer with KBr pellets. Emission and excitation spectra in the UV−vis region were recorded on a PerkinElmer LS 55 luminescence spectrometer with a red-sensitive photomultiplier-type R928 detector. Emission lifetimes in the solid state and in degassed solutions were determined on an Edinburgh Analytical Instrument (F900 fluorescence spectrometer) using an LED laser at 397-nm excitation, and the resulting emission was detected by a thermoelectrically cooled Hamamatsu R3809 photomultiplier tube. The instrument response function at the excitation wavelength was deconvolved from the luminescence decay. Near-infrared (NIR) emission spectra were measured on an Edinburgh FLS920 fluorescence spectrometer equipped with a Hamamatsu R5509-72 supercooled photomultiplier tube at 193 K and a TM300 emission monochromator with an NIR grating blazed at 1000 nm. The NIR emission spectra

Taking advantage of the various coordination modes of 8-hydroxyquinolinato (Scheme 1) and acetate (Scheme S1, Supporting Information), we have been able to establish a convenient and feasible synthetic route to access geometric and configurational f−f′ mixed metal isomers. Herein, we describe the two f−f′ heterometallic geometric isomers {Erq2(hfac)}2[(μ3OH)]2[Ybq(hfac)]2 (Er2Yb2, 1) and {Ybq2(hfac)}2[(μ3-OH)]2[Erq(hfac)]2 (Yb2Er2, 2), together with the two configurational isomers {Eu(CH3COO)3}{Tb(L)(hfac)2}3 (EuTb3, 3) and {Tb(CH3COO)3}{Eu(L)(hfac)2}3 (TbEu3, 4), where q− = 8-hydroxyquinolinato, hfac− = hexafluoroacetylacetonate, and L− = 4-nitrophenyl-2,2′-bipyridine-6-carboxylato. As revealed by photophysical studies, Er2Yb2/Yb2Er2 geometric isomers exhibit typical dual emissions from both erbium and ytterbium ions. For LnLn′3 configurational isomers, the TbIII subunits transfer energy to the EuIII center in the EuTb3 complex, whereas the TbIII ion in the TbEu3 array acts mainly as a structural stabilizer.

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EXPERIMENTAL PROCEDURES

Material and Reagents. All manipulations were performed under a dry argon atmosphere using Schlenk techniques and a vacuum-line system. The solvents were dried, distilled, and degassed prior to use, except those for spectroscopic measurements, which were of spectroscopic grade. Lanthanide oxide, 8-hydroxyquinoline (Hq), 4-nitrobenzaldehyde (alfa), sodium pyruvate, and hexafluoroacetylacetone (Hhfac) were obtained from commercial sources. Ln(hfac)3(H2O)2, Ln(hfac)2(CH3COO)(H2O)2 (Ln = Nd, Eu, Tb, Yb), and Lnq3·H2O were prepared by the literature procedures.16,26,28c [{Ln(q)2(hfac)}2(μ3-OH)2{Ln′(q)(hfac)}2] (Ln = Er, Yb; Ln′ = Yb, Er). Lnq3·H2O (Ln = Er, 1; Yb, 2) and equimolar Ln′(hfac)3(H2O)2 (Ln′ = Yb, 1; Er, 2) were added to dichloromethane with stirring for 30 min, giving a clear solution. After filtration, the concentrated dichloromethane solutions were layered with n-hexane to afford the products as yellow crystals. 1 (Er2Yb2). Yield: 78%. Anal. Calcd for C74H42Er2F24N6O16Yb2·H2O: C, 36.64; H, 1.83; N, 3.46. Found: C, 36.91; H, 1.78; N, 3.18. ICP analysis showed that the molar ratio of Yb3+ to Er3+ was 7.85:7.29, corresponding to 47.3% Er3+ of the total metal composition. ESI-MS (CH3OH−CH2Cl2, m/z): 995 ([M − 2hfac]2+). IR (KBr, cm−1): 2855 m, 2929 m (H−O), 1655 s (CO). 2 (Yb2Er2). Yield: 75%. Anal. Calcd for C74H42Yb2F24N6O16Er2·H2O: C, 36.64; H, 1.83; N, 3.46. Found: C, 36.71; H, 1.84; N, 3.38. ICP analysis showed that the molar ratio of Yb3+ to Er3+ was 7.09: 8.03, corresponding to 52.3% Er3+ of the total metal composition. ESI-MS (CH3OH−CH2Cl2, m/z): 995 ([M − 2hfac]2+). IR (KBr, cm−1): 2854 m, 2930 m (H−O), 1653 s (CO). Gd2Yb2. This compound was prepared by the same procedure as used for 1 but with Gdq3·H2O and equimolar Yb(hfac)3(H2O)2. Yield: 72%. Anal. Calcd for C74H42Gd2F24N6O16Yb2·H2O: C, 36.95; H, 1.84; N, 3.49. Found: C, 37.01; H, 1.89; N, 3.58. IR (KBr, cm−1): 2855 m, 2929 m (H−O), 1655 s (CO). Yb2Gd2. This compound was prepared by the same procedure as used for 2 but with Ybq3·H2O and equimolar Gd(hfac)3(H2O)2. Yield: 70%. Anal. Calcd for C74H42Yb2F24N6O16Gd2·H2O: C, 36.95; H, 1.84; N, 3.49. Found: C, 36.99; H, 1.85; N, 3.44. IR (KBr, cm−1): 2855 m, 2929 m (H−O), 1655 s (CO). Ammonium 4-Nitrophenyl-2,2′-bipyridine-6-carboxylate (NH4L). To a 500 mL round-bottom flask were added an ethanol (200 mL) solution of 4-nitrobenzaldehyde (2.42 g, 16 mmol) and an aqueous (30 mL) solution of sodium pyruvate (2.64 g, 24 mmol). The mixture was neutralized with HCl with stirring at 298 K for two days, giving (E)-4-(4-nitrophenyl)-2-oxobut-3-enoic acid as a pale yellow precipitation. (E)-4-(4-Nitrophenyl)-2-oxobut-3-enoic acid (1.11g, 5 mmol), 2-pyridacylpyridinium iodide (1.63g, 5 mmol), and ammonium acetate (8.1 g, 105 mmol) were then added to 100 mL of methanol. The solution was heated under reflux for 1 day to produce an off-white precipitate. After being allowed to cool to room temperature, the precipitate was filtered and washed with water and 850



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Table 1. Crystallographic Data of 1−4 empirical formula formula weight space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z ρcalc (g/cm−3) μ (mm−1) radiation (λ, Å) temp (K) R1(Fo)a wR2(Fo2)b GOF a

1·H2O

2·H2O

3·3H2O·6C6H14

4·3C6H14

C74H44Er2F24N6O17Yb2 2425.76 P2(1)/n 13.1738(8) 24.5677(11) 13.2235(7) 90 100.718(3) 90 4205.1(4) 2 1.925 4.299 0.71073 293(2) 0.0379 0.1020 1.059

C74H44Yb2F24N6O17Er2 2425.76 P2(1)/n 12.969(4) 24.709(3) 13.227(3) 90 101.432(10) 90 4154.6(16) 2 1.937 4.351 0.71073 293(2) 0.0562 0.1514 1.065

C123H135EuF36N9O33Tb3 3580.12 R3̅ 24.8037(6) 24.8037(6) 42.7922(19) 90 90 120 22799.6(13) 6 1.564 1.900 0.71073 293(2) 0.0598 0.1717 1.089

C105H87Eu3F36N9O30Tb 3253.64 R3̅ 24.8541(7) 24.8541(7) 42.446(4) 90 90 120 22707(2) 6 1.428 1.793 0.71073 293(2) 0.0515 0.1382 1.061

R1 = ∑||Fo| − |Fc||/∑|Fo|. bwR2 = [∑w(Fo2 − Fc2)2/∑w(Fo2)2]1/2.

were corrected using a calibration curve supplied with the instrument. The emission quantum yields (Φ) of 3 and 4 in degassed dichloromethane solutions at room temperature were calculated as Φs = Φr(Br/Bs)(ns/nr)2(Ds/Dr) using [Ru(bpy)3]Cl2 in acetonitrile as the standard (Φem = 0.062), where the subscripts r and s denote reference standard and the sample solution, respectively, and n, D, and Φ are the refractive index of the solvent, the integrated intensity, and the luminescence quantum yield, respectively. The quantity B was calculated by B = 1 − 10−AL, where A is the absorbance at the excitation wavelength and L is the optical path length.10 All solutions used for the determination of emission lifetimes and quantum yields were prepared under a vacuum in a 10 cm3 round-bottom flask equipped with a sidearm 1-cm fluorescence cuvette and sealed from the atmosphere by a quick-release Teflon stopper. Solutions used for luminescence determination were prepared after rigorous removal of oxygen by three successive freeze−pump−thaw cycles. Crystal Structural Determination. Single crystals of 1·H2O, 2·H2O, 3·3H2O·6C6H14, and 4·3C6H14 suitable for X-ray diffraction were grown by layering n-hexane onto the dichloromethane solutions. Crystals coated with epoxy resin or sealed in capillaries with mother liquors were measured on a Siemens SMART CCD diffractometer by the ω-scan technique at room temperature using graphite-monochromated Mo Kα radiation (λ = 0.71073 Å). Lp and absorption corrections were applied in the reflection reduction process. The structures were solved by direct methods, and the heavy atoms were located from E-map. The remaining non-hydrogen atoms were determined from the successive difference Fourier syntheses. The non-hydrogen atoms were refined anisotropically except for the F atoms, and the hydrogen atoms were generated geometrically with isotropic thermal parameters. The structures were refined on F2 by full-matrix least-squares methods using the SHELXTL-97 program package. For 1−4, the refinements were carried out by fixing the C−F distances (1.32 ± 0.01 Å) with the occupancy factor of each Fn and F′n being 0.5. Crystallographic data of 1−4 are summarized in Table 1. The unit cell contains 36 hexane and 18 hydrate molecules for 3 and 18 hexane molecules for 4, which were treated as a diffuse contribution to the overall scattering without specific atom positions by SQUEEZE/PLATON.

dichloromethane solutions were layered with n-hexane to afford geometric isomers of heterotetranuclear lanthanide(III) complexes as yellow crystals. ICP data indicated that the molar ratios of ErIII to YbIII in 1 and 2 are about 1:1.08 and 1.13:1, respectively. ESI-MS (CH3OH−CH2Cl2) studies revealed the presence of positive ion fragments 995 ([M − 2hfac]2+) (Figures S1 and S2, Supporting Information). On one hand, as 8-hydroxyquinolinato (q−) in the precursor Lnq3·H2O exhibits chelating coordination mode (I). The unsaturated coordinated q− has potential bonding ability, inducing a further binding to Ln′(hfac)3(H2O)2. On the other hand, as Lnq3·H2O was isolated from a basic aqueous solution with pH ≈ 9 whereas Ln′(hfac)3(H2O)2 was isolated from an acidic solution with pH ≈ 6, a neutralization reaction might occur upon mixing Lnq3·H2O with Ln′(hfac)3(H2O)2 in dichloromethane solutions. These are likely the two reasons that make the reaction occur, so that μ3-OH bridged Er2Yb2/Yb2Er2 isomers were successfully isolated. Precursors LnL3 were synthesized by the reaction of NH4L with Ln(OTf)3 in aqueous solutions. The EuTb3/Eu3Tb configurational isomers were prepared by the reaction of LnL3 with 3 equiv of Ln′(hfac)2(CH3COO)(H2O)2 in dichloromethane solutions until the solution became clear. After filtration, crystallization by layering n-hexane onto the concentrated dichloromethane solutions afforded the configurational heterotetranuclear lanthanide isomers as pale yellow crystals. As the carboxylates in the precursor LnL3 exhibit monodentate coordination, this implies that the COO− group has further coordination (Scheme S1, Supporting Information) capability to link Ln′(hfac)2(CH3COO)(H2O)2 so as to isolate LnLn′3 heterotetranuclear complexes. ICP data indicated that the molar ratios of EuIII to TbIII in 3 and 4 are 1:3.30 and 3.23:1, respectively. The complexes were all characterized by elemental and ICP analyses, IR spectroscopy, ESI-MS (Figures S3 and S4, Supporting Information), and X-ray crystallography. The crystallographic data of 1·H2O (CCDC 808800), 2·H2 O (CCDC 808803), 3·3H2O·6C6H14 (CCDC 808801), and 4·3C6H14 (CCDC 808802) are summarized in Table 1. Crystal Structures. Selected bond lengths and angles of 1− 4 are presented in Table 2. As 1 and 2 are geometric isomers,

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RESULTS AND DISCUSSION Syntheses. The synthetic routes of heterometallic lanthanide isomers are shown in Scheme 2. The geometric isomers 1 and 2 were prepared by mixing equimolar Lnq3·H2O and Ln′(hfac)3(H2O)2 in dichloromethane solutions with stirring until the solutions became clear. After filtration, the concentrated 851



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Scheme 2. Synthetic Routes to Lanthanide Isomers 1−4

Table 2. Selected Bond Distances (Å) and Angles (deg) for Compounds 1·H2O, 2·H2O, 3·3H2O·6C6H14, and 4·3C6H14 1·H2O Er1−O1 Er1−O2 Er1−O3 Er1−O4(H) Er1−N1 Er1−N2 Yb1−O1′ Yb1−O2′ Yb1−O3 Yb1−O4(H) Yb1−N3 Er1−Yb1 Er1−Yb1′ Er1−O1−Yb1′ Er1−O2−Yb1′ Er1−O3−Yb1 Er1−O4(H)−Yb1 Er1−O4(H)−Yb1′ Yb1−O4(H)−Yb1′

2·H2O 2.334(4) 2.293(4) 2.348(4) 2.284(4) 2.504(5) 2.464(6) 2.335(4) 2.367(4) 2.324(4) 2.300(3) 2.484(5) 3.7782(4) 3.4587(4) 95.60(14) 95.83(15) 107.94(16) 111.04(15) 96.76(14) 108.09(14)

Er1−O1 Er1−O2 Er1−O3 Er1−O4(H) Yb1−N1 Yb1−N2 Yb1−O1 Yb1−O2 Yb1−O3 Yb1−O4(H) Er1− N3 Er1−Yb1 Er1−Yb1′ Yb1−O1−Er1′ Er1−O2−Yb1′ Er1−O3−Yb1 Yb1−O4(H)−Er1 Yb1−O4(H)−Er1′ Er1−O4(H)−Er1′

3·3H2O·6C6H14 2.368(7) 2.326(7) 2.315(7) 2.298(7) 2.461(9) 2.493(10) 2.296(7) 2.353(7) 2.344(8) 2.289(7) 2.483(9) 3.7755(11) 3.4545(7) 95.6(2) 95.2(3) 108.3(3) 110.8(3) 96.6(3) 108.4(3)

4·3C6H14

Tb1−O1 Tb1−O2′ Tb1−O7 Tb1−N1 Tb1−N2

2.484(4) 2.426(4) 2.357(4) 2.598(5) 2.529(4)

Eu1−O1 Eu1−O2 Eu1−O7 Eu1−N1 Eu1−N2

2.495(3) 2.435(4) 2.370(4) 2.613(4) 2.549(4)

Eu1−O1 Eu1−O7 Eu1−O8

2.467(3) 2.448(4) 2.432(4)

Tb1−O1 Tb1−O7 Tb1−O8

2.482(3) 2.459(4) 2.436(4)

Tb1−Eu1

4.0196(3)

Tb1−Eu1

4.038

Eu1−O1−Tb1 Tb1−O7−Eu1

108.56(13) 113.56(15)

Tb1−O1−Eu1 Eu1−O7−Tb1

108.42(12) 113.46(14)

of 8-hydroxyquinolinato, and two μ3-OH−, also giving a distorted square-antiprism. The Er−O3′ [2.315(7) Å] and Yb−O3 [2.344(8) Å] distances for μ-phenoxo of 8-hydroxyquinolinato are much longer than the Er−O4 [2.298(7) Å] and Yb−O4 [2.289(7) Å] distances for μ3-OH−. The Er′−O3−Yb bond angle [108.3(3)°] is larger than that of Er−O4−Yb [96.6(3)°]. The 8-hydroxyquinolinato exhibits a chelating-bridging mode as shown in Scheme 1 (mode III). The bridging μ3-OH caps ErYb2 atoms, with Er(Yb)−O distances of 2.284(4), 2.299(4), and 2.343(4) Å and Er(Yb)−O−Yb angles in the range of 96.7−110.1°. As 3 and 4 are configurational isomers, only the structure of 3 is described here. In 3 (Figure 2), the EuTb3 tetranuclear

only the structure of 1 is described here (Figure 1). In 1, the Er2Yb2 tetranuclear cluster can be viewed as a combination of two Er(q)2(hfac) units and two Yb(q)(hfac) components linked by two μ3-OH−. The Yb···Yb distance is 3.76 Å, and the Er···Er distance is 6.19 Å. The Er···Yb separations are 3.78 and 3.46 Å. As shown in Figure 1b, it is intriguing that the Er2Yb2 structure affords two incomplete cubanes that share a common plane built by Yb, O4 (μ3-OH−), Yb′, and O4′ (μ3-OH−). In 1, two ErIII ions exhibit eight-coordinated environments from one hfac−, two chelating 8-hydroxyquinolinato, one μ-phenoxo of 8-hydroxyquinolinato, and one μ3-OH−, forming a square antiprism geometry. The two YbIII ions are also eight-coordinated from one hfac−, one chelating 8-hydroxyquinolinato, two μ-phenoxos 852



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Figure 1. (a) Perspective view of the asymmetric unit of 1 with atomic labeling scheme. (b) View of 1 showing two incomplete cubanes that share a common plane. The F atoms on the trifluoromethyl are omitted for clarity.

cluster can be viewed as a fan structure composed of three Tb(hfac)2(L) fragments linked by the central EuIII ion through μ3-acetate coordination mode. The EuTb3 core is arranged into a distorted triangular pyramid with three TbIII ions in the basal plane and the EuIII ion at the apex. The three TbIII atoms are nine-coordinated with two hfac−, one chelating L−, one μ3-CH3COO−, and one μ3-COO− from another bridging L−, affording a distorted capped square antiprism. The apical EuIII atom is also nine-coordinated to oxygen atoms from three chelating CH3COO− groups and three μ3-COO− groups from the bridging L−, to give a distorted capped square antiprism. Although the EuIII atom in the precursor EuL3 is also nine-coordinated to three oxygen atoms and six nitrogen atoms from L−, it is likely that the better coordination capability of oxygen than nitrogen atoms to the lanthanide(III) ion induces the structural translation to occur. The μ3-CH3COO− adopts a chelating-bridging bonding mode so as to link three Tb(hfac)2(L) fragments with central EuIII ion. The bridging L− is also featured with a μ3-COO− mode to link EuIII and TbIII ions into a symmetric fashion with Tb···Tb and Eu···Tb distances across μ3-COO− of 6.522(10) and 4.016(9) Å, respectively. As the geometric isomers (1 and 2) are monoclinic with the space group of P2(1)/n (Table 1), the asymmetric unit of the complex contains only two lanthanide ions with Ln′Ln array (Figure 1a). On one hand, the lanthanide-β-diketonate complex is less stable than the lanthanide-quinolinolate complex, as it is

Figure 2. (a) Perspective view of the asymmetric unit of 3 with atomic labeling scheme. (b) View of 3 showing a fan structure composed of three Tb(hfac)2(L) fragments linked by the central EuIII ion through μ3-acetate. The F atoms on the trifluoromethyl are omitted for clarity.

well-known that the hexafluoroacetylacetonato (hfac−) ion can be easily substituted with external donor atoms.29 On the other hand, the molar ratio of ErIII to YbIII ions established by ICP is about 1:1. All of these facts indicate that the lanthanide(III) ions with two chelating hydroxyquinolinato ligands are from the Lnq3·H2O precursors. Similarly, as configurational isomers 3 (EuTb3) and 4 (Eu3Tb) are hexagonal with the R3̅ space group, the asymmetric unit of the complex contains only two lanthanide ions with Ln1/3Ln′ for 3 or LnLn′1/3 for 4. On one hand, the same coordination environment of the three lanthanide(III) ions in the basal plane is different from that of the apex lanthanide(III) ion. On the other hand, the molar ratio of EuIII to TbIII ions established by ICP is about 1:3 for 3 and 3:1 for 4. All of these facts suggest that the three metal ions in the basal plane are the same and from the 853



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Additionally, as shown in Table 1 and Figure 3a, the geometric isomer’s crystal contains some organic solvents. If the organic solvent has some interactions with Eu3+, the corresponding signals are also shifted to low field. Interestingly, the signals of the ethanol (quartet) are coincident with those of hfac− in LaEu3 (Figure 3a, bottom), so as to combine the two kinds of signals together, inducing the 1H NMR profiles and the integral areas with apparent discrepancy with respect to expectations. In turn, the mixed signals of solvent and CH3COO− in LaEu3 could also be separated by the shift reagent Eu3+ in EuLa3 (Figure 3a, top), resulting the unexpected 1H NMR profiles and the integral areas. As both hfac− and 8-hydroxyquinolinato are chelated to the La3+ and Eu3+ ions, the difference in the 1H NMR spectra of La2Eu2 and Eu2La2 is the integral area of 8-hydroxyquinolinato chelating to the Eu3+ ions. As shown in Figure 3b, the 1H NMR spectra of hfac− are split into two bands (3.47 and 3.54 ppm) with similar integral areas, indicating that the same number hfac− groups are chelated to the La3+ and Eu3+ ions in La2Eu2 and Eu2La2. Meanwhile, there are two sets of 8-hydroxyquinolinato signals in both La2Eu2/Eu2La2 complexes. One is observed at ca. 6.93, 7.02, 7.97, 9.98, and 11.2 ppm, and the other appears at 7.11, 7.54, 8.72, 10.2, and 11.31 ppm, respectively. The integral area of the first set is about half that of the second group in complex La2Eu2, whereas that of the first group is twice that of the second in complex Eu2La2, implying that the second group corresponds to the proton signals of 8-hydroxyquinolinato chelated to the Eu3+ ions. Thus, 1H NMR studies further confirm that the structures are consistent with those revealed by X-ray crystallography. Photophysical Properties. The UV−vis absorption and luminescence data of 1−4 are summarized in Table 3. 1 and 2 (Figure S7, Supporting Information) display mainly two sets of absorption bands, one from the ligand-centered transition of q− at ca. 245 nm (for 1) or 257 nm (for 2) and the other at ca. 310 nm due to intraligand π−π* transition of hfac−.6,28,30 In addition, a broad low-energy absorption trailing to 420 nm arises probably from the [n → π*] (q−) transitions.27,31 In the electronic spectra of 3 and 4 (Figure S7, Supporting Information), the absorption band at ca. 285 nm is due to the intraligand transition of L−, and the one at ca. 305 nm is also from hfac−.6,28,30 Upon excitation at 250 nm < λex < 420 nm, 1 and 2 display typically dual luminescence at ca. 1000 nm from YbIII ion and ca. 1535 nm from ErIII ion in both the solid state and dichloromethane solution (Figure 4 and Figure S8, Supporting Information) at ambient temperature. As the precursor Ln(hfac)3(H2O)2 is nonemissive upon excitation beyond 350 nm,6,28,30 the lanthanide emission in 1 and 2 should be sensitized by energy transfer from 8-hydroxyquinolinato with near-visible irradiation at 350 nm < λex < 420 nm. Because YbIII ion can serve as a sensitizer of ErIII, substantial energy transfer is operating from the YbIII chromophore to the ErIII subunit,1 as the lifetime of residual YbIII emission is 1.26 μs in 1 and 1.55 μs in 2 at ambient temperature. The Yb → Er energy-transfer rates kET can thus be estimated from the equation6,7,28,30

precursor Tb/Eu(hfac)2(CH3COO)(H2O)2, whereas the apex ion is another lanthanide(III) ion from the EuL3/TbL3 precursor. 1 H NMR Spectrometry. To further confirm the structures, we resorted to 1H NMR spectroscopy to determine the relative numbers of La2Eu2 and Eu2La2 or Eu3La and La3Eu pairs. For the predictability of europium’s 1H NMR spectra, Eu3+ was used as a shift reagent. Paramagnetic Eu3+ together with diamagnetic La3+ would help to confirm the structures. The 1H NMR spectra of the two pairs of complexes are shown in Figure 3 and Figures S5 and S6 (Supporting Information).

Figure 3. Part of the 1H NMR spectra highlighting the proton signals of (a) hfac− and acetate for LaEu3/EuLa3 and (b) 8-hydroxyquinolinato and hfac− for La2Eu2/Eu2La2 in CDCl3 at 400 MHz.

According to the crystal structures of Er2Yb2/Yb2Er2, the differences between the pair La2Eu2/Eu2La2 are the numbers of 8-hydroxyquinolinato chelating to the Eu3+ ions. As for LaEu3 and EuLa3, the differences in chemical environments are the chelating ligands around Eu3+. For LaEu3, six hfac− groups are bound to the Eu3+ ions, whereas three acetates are cheated to the Eu3+ ion in EuLa3. Taking into account hfac− as the internal standard substance, we could find the shift tendency with Eu3+ as the shift reagent. Therefore, we highlight part of the 1H NMR spectra on hfac− and 8-hydroxyquinolinato in the pair of La2Eu2/Eu2La2 together with hfac− and acetate in LaEu3/EuLa3 to distinguish the chemical environment of Eu3+. As shown in Figure 3a, the chemical shift of acetate chelating to La3+ is about 2.07 ppm in complex LaEu3. Once the acetate is chelated to the shift reagent Eu3+ in the complex EuLa3, the corresponding signal is shifted to low field at around 2.51 ppm. In the same way, the signal of hfac− chelating to the La3+ ion at 3.58 ppm in EuLa3 is also low-field-shifted to 3.78 ppm upon hfac− binding to the shift reagent Eu3+ in LaEu3.

kET = 1/τ − 1/τ0

(1)

where τ is the lifetime of residual Yb-based emission in 1 and 2 species and τ0 is the lifetime in the reference Gd2Yb2 (1.94 ± 0.03 μs) and Yb2Gd2 (2.64 ± 0.05 μs) complexes, which lack the corresponding energy transfer. The energy-transfer rates (kET) can thus be calculated as kET = 1/τEr2Yb2 − 1/τGd2Yb2 = (2.8 ± 0.4) × 105 s−1 for 1 and kET = 1/τYb2Er2 − 1/τYb2Gd2 = (2.7 ± 0.4) × 105 s−1 854



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O−H···F interactions could suppress the vibrational nonradiative processes, so as to improve the lanthanide(III) luminescence from f−f′ transitions.4,16 As anticipated, titration of 1 and 2 with [Bu4N]F in aerated dichloromethane solutions indeed induced a marked enhancement of the lanthanide(III) luminescence. As shown in Figure 5, addition of 2 equiv of F− to the

Table 3. Absorption and Luminescence Data of 1−4 compound

medium

1 (Er2Yb2)

CH2Cl2

Gd2Yb2

CH2Cl2

2 (Yb2Er2)

CH2Cl2

Yb2Gd2

CH2Cl2

3 (EuTb3)

Solid

CH2Cl2

4 (TbEu3)

234 (190400), 256 (153500), 304 (55900), 371 (13900)

239 (119000), 257 (147700), 307 (51300), 371 (15500)

284 (109200), 303 (113600)

Solid

CH2Cl2

GdTb3

λabs/nm (ε/M−1 cm−1)

CH2Cl2

284 (100200), 303 (109500)

λem/nm (τem)a at 298 K 1000 (1.26 ± 0.05 μs), 1535(1.13 ± 0.07 μs) 1000 (1.94 ± 0.03 μs) 1000 (1.55 ± 0.07 μs), 1535(1.09 ± 0.13 μs) 1000 (2.64 ± 0.05 μs) 543 (144 ± 0.5 μs), 615 (0.98 ± 0.03 ms) 543 (19.6 ± 0.2 μs), 615 (1.22 ± 0.03 ms) 543 (0.66 ± 0.01 μs), 615 (0.87 ± 0.01 ms) 543 (−c), 615 (1.05 ± 0.05 ms) 543 (28.2 ± 0.8 μs)

Φem × 10−3 b 0.63d 0.14d

0.78d 0.13d

13b

Figure 5. Emission spectra of 1 (λex = 333 nm) with different molar ratios of [Bu4N]F in dichloromethane solution at 298 K. The molar ratios between 1 and [Bu4N]F were 1:0 (gray), 1:0.2 (black), 1:1 (blue), 1:2 (red), and 1:4 (cyan).

27b

dichloromethane solution of 1 caused about a 2-fold enhancement in the YbIII emissive intensity. However, when more than 2 equiv of F− was added, the lanthanide(III) emission gradually attenuated. This is because the lanthanide(III) ions were gradually ligated by excess fluoride so as to destroy the original structures.4,16 As the molar absorption coefficient at 980 nm for YbIII is much larger than that for ErIII ion, it is likely that the adjacent YbIII ions could significantly promote the up-conversion efficiency of ErIII ion. However, such phenomena were not observed in our cases upon laser irradiation at 980 nm with 460 mW. The reason might be that the energy from the excited states of 2H11/2 (525 nm) and 4S3/2 (545 nm) to the 4I9/2 level are slightly higher than that of the triplet excited state of the ligand (q−) (around 17100 cm−1, 585 nm), thus quenching the up-conversion emission completely. In fact, it is challenging to attain up-conversion emissions in a single molecule, as both the direct coordination of μ3-OH− to the lanthanide(III) ions and the presence of organics can rapidly quench the up-converted emission.4,16,32 Nonetheless, the work presented here does show the potential for the fabrication of such complex for up-conversion applications. Upon excitation at 250 nm < λex < 380 nm, 3 also typically displays dual luminescence at ca. 543 nm from TbIII ion and ca. 615 nm from EuIII ion with microsecond to millisecond range of lifetimes in both the solid state and dichloromethane solution at ambient temperature (Table 1 and Figure 8). Titration experiments were performed to examine whether there is a communication between Tb3+ and Eu3+ centers through photoinduced energy transfer. As shown in Figure 6, titration of Tb(hfac)2(CH3COO)(H2O)2 with EuL3 in dichloromethane solution induces significant attenuation of the TbIII emission. Conversely, titration of EuL3 with Tb(hfac)2(CH3COO)(H2O)2 (Figure S9, Supporting Information) in dichloromethane solution results in marked enhancement of the EuIII-based emission. Both titration experiments suggest that the emissive energy of

a Excitation wavelengths in lifetime measurements: 400 nm for 1 and 2, 340 nm for 3 and 4. bQuantum yields of 3 and 4 in degassed dichloromethane determined relative to that of Ru(bpy)3(PF6)2 (Φ = 0.062) in degassed acetonitrile. cUndetectable in our instrument. d Quantum yields of Yb and Er complexes estimated by the equation ΦLn = τobs/τ0, in which τobs is the observed emission lifetime and τ0 is the radiative or natural lifetime with τ0 = 2 ms for YbIII and 8.26 ms for ErIII.7 These values refer to the lanthanide-based emission process only and take no account for the efficiency of intersystem crossing and energy-transfer processes.

Figure 4. Emission spectra of 1 (blue) and 2 (red) (λex = 400 nm) in the solid state at 298 K.

for 2, which are much lower than the rates of d → f energy transfer using an organometallic chromophore as a near-infraredemitting sensitizer of the lanthanide ions.6,28,30a By comparison of the rates of Yb → Er energy transfer between 1 and 2, it was found that the Yb → Er energy transfer in 1 is slightly more efficient than that in 2. Because of the direct linkage of μ3-OH− to the lanthanide(III) ions in 1 and 2, the nonradiative O−H vibration excitation could severely quench the lanthanide(III) luminescence.33 It has been demonstrated that using fluoride to form strong 855



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Supporting Information), which is attributed to the low-lying excited states of L− and its inability for transfer energy to the TbIII centers, suggest a fast thermally activated back energy-transfer from the EuIII to the TbIII subunit, because some energy levels of the TbIII ion are too close to the emissive level of the EuIII ion. As shown in Figure 8, there is almost unobservable emission ascribed to TbIII ion in the emission spectrum of 4. Thus, the TbIII ion in 4 behaves mainly as a structural stabilizer.

Figure 6. Titration of Tb(hfac)2(CH3COO)(H2O)2 with EuL3 in dichloromethane solution, showing significant attenuation of the TbIII-based emission (λex = 335 nm). The molar ratios between Tb(hfac)2(CH3COO)(H2O)2 and EuL3 were 3:0 (cyan), 3:0.6 (red), 3:1.2 (blue).

the TbIII subunit is transferred to the EuIII ion in 3, inducing sensitized EuIII-centered emission with the TbIII subunit as the antenna chromophore. The Tb → Eu energy-transfer rate kET was estimated by eq 1, where τ is the lifetime of residual Tb-based emission in 3 and τ0 (28.2 ± 0.8 μs) is the lifetime in the reference GdTb3 complex, which lacks energy transfer from the TbIII triplet states. The energy-transfer rate (kET) can thus be calculated as kET = 1/τEuTb3 − 1/τGdTb3 = (1.6 ± 0.15) × 104 s−1, which is much lower than the rates of d → f energy transfer using organomentallic chromophores as the sensitizers of EuIII luminescence.6a,30b Upon direct excitation of the terbium absorption band at 488 nm, however, the emission signal of europium(III) ion at 615 nm was not detected. This phenomenon does not mean that Tb → Eu energy transfer is not operating. It is likely that the EuL3 complex absorbs some of the incident light at the excitation wavelength, and less light is thus available to excite the Tb complex. To verify this opinion, titration experiments using the Gd-based complex were carried out. As shown in Figure 7, titration of EuL3 with Gd(hfac)2(CH3COO)(H2O)2

Figure 8. Emission spectra of 3 (red) and 4 (blue) (λex = 335 nm) in dichloromethane solution at 298 K.

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CONCLUSIONS A convenient and feasible synthetic approach has been established for the specific preparation of geometric and configurational heterometallic isomers containing different lanthanides by incorporating a lanthanide subunit with another lanthanide precursor. Because of the incomplete energy transfer, the geometric lanthanide isomers (1 and 2) exhibit dual emissive states arising from both lanthanide chromophores. For configurational lanthanide isomers, the TbIII subunits transfer energy to the EuIII centers in 3 (EuTb3), whereas the TbIII ion in 4 (Eu3Tb) acts mainly as a structural stabilizer. It is demonstrated that energy-transfer rates with one lanthanide subunit as the sensitizer for another typical lanthanide emission in f−f heteronuclear complexes are lower than those with d-block organometallic chromophores as antennas for sensitized lanthanide luminescence in d−f heteronuclear complexes. This synthetic strategy would open up a significant approach for the design of heterometallic complexes containing two different lanthanide(III) ions, thus providing new possibilities for choosing specific metals with attractive properties in developing higherperformance dual-color probes and up-conversion materials.

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ASSOCIATED CONTENT

S Supporting Information *

Figure 7. Titration of EuL3 with Gd(hfac)2(CH3COO)(H2O)2 in dichloromethane solutions, showing marked enhancement of the EuIIIbased emission (λex = 335 nm). The molar ratios of EuL3 to Gd(hfac)2(CH3COO)(H2O)2 were 1:0 (cyan), 1:1 (red), 1:2 (magenta), and 1: 3 (blue).

Additional experimental and spectroscopic data together with X-ray crystallographic files of compounds 1−4. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

in dichloromethane solution also showed marked enhancement of the EuIII-based emission. This suggests that the hfac− ligands also act as partial sensitizers for Eu-based emission in our case. In the same way, titration of TbL3 with Eu(hfac)2(CH3COO)(H2O)2 in dichloromethane solution induces trivial enhancement of the TbIII-based emission. The reverse titration procedure, however, leads to a slight attenuation of the EuIII-based emission and trivial enhancement of the TbIII-based emission (Figures S10 and S11, Supporting Information). These facts together with undetectable TbIII emission for the precursor TbL3 (Figure S10,

*E-mail: [email protected] (H.-B.X.), [email protected] (Z.-N.C.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge financial support from the NSFC (20901077, 20931006, U0934003, and 91122006), the NSF of Fujian Province (2011J01065), and CAEP (2012B0302039). 856



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