Structures and Photophysical Properties of Homo- and Heteronuclear

Aug 16, 2010 - Eight new lanthanide(III) complexes, {Ln(hfac)3}2(μ-HMq)2 (Ln = Eu, 1; Tb, 2; Yb, 3; hfac− = hexafluoroacetylacetonate; HMq ...
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DOI: 10.1021/cg100778w

Structures and Photophysical Properties of Homo- and Heteronuclear Lanthanide(III) Complexes with Bridging 2-Methyl-8-hydroxylquinoline (HMq) in the μ-Phenol Mode

2010, Vol. 10 4101–4108

Hai-Bing Xu, Jia Li, Li-Yi Zhang, Xin Huang, Bing Li, 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 Received June 10, 2010; Revised Manuscript Received July 30, 2010

ABSTRACT: Eight new lanthanide(III) complexes, {Ln(hfac)3}2(μ-HMq)2 (Ln = Eu, 1; Tb, 2; Yb, 3; hfac- = hexafluoroacetylacetonate; HMq = 2-methyl-8-hydroxylquinoline), {Eu(hfac)2(H2O)-(OC6H4CN)}2(μ-HMq)2 (4, HOC6H4CN = 4-hydroxybenzonitrile), Er5(hfac)9(μ-HMq)2(μ4-CH3COO)2(μ3-CH3COO)(μ-CH3COO)(μ3-OH)2 (5), and Cr(Mq)(μ-HMq)(μ3-OH)(μ-OH)(μ-CH3COO)2{Ln(hfac)2}2 (Ln = Eu, 6; Tb, 7; Er, 8), with bridging HMq in the μ-phenol mode were synthesized and characterized by X-ray crystallography, except for 6. Each LnIII center in 1-4 is surrounded by eight O donors to form a distorted square-antiprism, except that two O binding sites from one hfac- in 1-3 are displaced by one H2O and one OC6H4CN- in 4. Complex 5 could be viewed as a combination of two {Er(hfac)2}2(μ-HMq) moieties linked to a central ErIII ion through bridging acetates and μ3-OH, in which the central ErIII ion is nine-coordinated to give a distorted capped square antiprism whereas the other four ErIII ions are eight-coordinated with approximate square antiprisms. CrLn2 heterotrinuclear complexes 6-8 are ligated by one μ3-OH-, one μ-OH-, one bridging HMq, one chelating-bridging Mq-, and two μ-CH3COO-. These complexes display typical linelike emission from the corresponding lanthanide(III) ions with a microsecond range of lifetime. Introducing some amount of fluoride to these lanthanide(III) complexes induces significant enhancement in intensities and lifetimes for both visible and near-infrared (NIR) luminescence from lanthanide centers because strong F 3 3 3 O-H interaction can effectively suppress the O-H vibrational radiationless process.

Introduction Lanthanide complexes have attracted a lot of attention due to their extensive applications.1 It is well-known that most lanthanide oxo/hydroxide clusters and their analogues could be stabilized by ligands such as alkoxides,2-4 phenoxides,5 diketonates,6 or the o-nitrophenolates.7 Alkoxide and aryloxide ligands OR- show extensive uses in both transition metal and lanthanide complexes due to their variable steric requirements and favorable capability to bridge two or more metal centers.8 Over the years, intense interest has also been generated in the alkoxide or phenoxide chemistry of yttrium and lanthanide elements, owing to a renewed interest in the preparation of ceramic materials.9 However, the related lanthanide(III) complexes with an 8-hydroxylquinoline derivative as a bridging ligand in the μ-phenol or protonated μ-phenoxide bonding mode have been less explored.10,11 Due to the weak absorption efficiency of f-f transitions in lanthanide(III) ions, the use of strongly absorbing organic or organometallic sensitizer for light harvesting is usually necessary to achieve highly efficient lanthanide(III) luminescence.12 Particularly, lanthanide(III) emission is readily quenched by the nonradiative O-H, N-H, and C-H oscillators, resulting in remarkably decreased luminescence intensities and shortened excited-state lifetimes from lanthanide(III) centers.13,14 To achieve long-lived lanthanide(III) luminescence with high efficiency, several feasible approaches have been reported to circumvent this limitation.13,14 First, the quantum yield of lanthanide emission can be maximized when lanthanide(III) ions are encapsulated with polydentate ligands, so as to *To whom correspondence should be addressed. E-mail: [email protected]. r 2010 American Chemical Society

prevent any solvent molecules and anions close to the lanthanide centers.15-18 Second, fluorinated ligands and deuterated solvents were frequently adopted to eliminate vibrational deactivation induced by O-H, N-H, and C-H oscillators in close proximity to the lanthanide(III) centers.19-23 Third, fluoride ions are used to bind directly to the lanthanide(III) ions, thereby replacing coordinated solvent molecules or weakly binding ligands.14 Furthermore, we have recently reported another significant means to improve lanthanide(III) luminescence by introducing a suitable amount of fluoride,12,24 thus resulting in formation of strong O-H 3 3 3 F and/or N-H 3 3 3 F interactions so as to suppress significantly nonradiative O-H or N-H oscillators. In this paper, 2-methyl-8-hydroxylquinoline (HMq) is utilized as a bridging ligand featuring a μ-phenol mode, inducing formation of HMq-linked dinuclear complexes 1-4 and pentanuclear complex 5. These complexes are good candidates to explore the correlation between the quenching effect due to the hydroxyl O-H oscillators and fluoride-enhanced lanthanide(III) luminescence. Since Al-Mq11 and Zn-Mq25 (Mq = 2-methyl-8-hydroxylquinoline) chromophores could be served as favorable light-harvesting antenna sensitizers to achieve sensitized lanthanide luminescence, we are interested in preparation of CrLn2 heterotrinuclear complexes [Cr(Mq)(HMq)](μ-OH)(μ3-OH)(μ-CH3COO)2{[Ln(hfac)2]2} (Ln = Eu, 6; Tb, 7; Er, 8; hfac- = hexafluoroacetylacetonate) using Cr-Mq as a metalloligand to incorporate with the lanthanide( β-diketonate) subunit. Sensitized lanthanide(III) luminescence is indeed achieved in these CrLn2 heterotrinuclear complexes, suggesting that energy transfer is most likely operating from the Cr-Mq chromophore to the lanthanide(III) center. Published on Web 08/16/2010

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Experimental Procedures Materials and Reagents. All manipulations were performed under 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 were of spectroscopic grade. 2-Methyl-8-hydroxylquinoline (HMq), hexafluoroacetylacetone (Hhfac), and 4-hydroxybenzonitrile (4-HOC6H4CN) were commercially available. Ln(hfac)2(CH3COO)(H2O)2 (Ln = Eu, Tb, Er). To an aqueous solution of lanthanide acetates (pH = 5-7) was dropwise added 2.2 equiv of Hhfac to produce a precipitate. Upon stirring at room temperature for 3 h, the precipitate was filtered, and the solid was washed with water and dried under vacuum to afford the quantitative products without further purification. Eu(hfac)2(CH3COO)(H2O)2. Anal. Calcd: C, 21.80; H, 1.37; Eu, 22.98. Found: C, 21.88; H, 1.38; Eu, 22.99. IR (KBr, cm-1): 1652s (hfac); 1557 (CH3COO). ESI-MS (CH3OH): m/z (%) 662 (M þ Hþ). Tb(hfac)2(CH3COO)(H2O)2. Anal. Calcd: C, 21.57; H, 1.36; Tb, 23.79. Found: C, 21.56; H, 1.33; Tb, 24.06. IR (KBr, cm-1): 1652s (hfac); 1557 (CH3COO). ESI-MS (CH3OH): m/z (%) 669 (M þ Hþ). Er(hfac)2(CH3COO)(H2O)2. Anal. Calcd: C, 21.31; H, 1.34; Er, 24.73. Found: C, 20.99; H, 1.22; Er, 24.99. IR (KBr, cm-1): 1652s (hfac); 1567 (CH3COO). ESI-MS (CH3OH): m/z (%) 641 (M 2(H2O) þ Hþ). {Ln(hfac)3}2(μ-HMq)2 (Ln = Eu, 1; Tb, 2; Yb, 3). 2-Methyl-8hydroxylquinoline (318 mg, 2 mmol), CrCl3 3 6H2O (266 mg, 1 mmol), and 4-hydroxybenzonitrile (238 mg, 2 mmol) were added to absolute ethanol (30 mL) with stirring under reflux for 6 h. Upon cooling down, the mixture was neutralized with triethylamine and water (100 mL) was added to the solution to produce a precipitate. After filtration, the solid was washed thoroughly with water, ethanol, and diethyl ether, and dried under vacuum for 1 day to afford a pale green product. Then 50 mg of the pale green product reacted with 85 mg of Ln(hfac)3(H2O)226,27 (Ln = Eu, Tb, and Yb) in 30 mL of dichloromethane solution with sonication for 15 min. After filtration, the concentrated dichloromethane solutions were layered with n-hexane to afford green crystals. 1 (Ln = Eu). Yield: 43%. Anal. Calcd for C50H24Eu2F36N2O14: C, 32.21; H, 1.30; N, 1.50. Found: C, 32.19; H, 1.22; N, 1.42. IR (KBr, cm-1): 1654s (CdO), 2922w (OH), 3427w (OH). 2 (Ln = Tb). Yield: 45%. Anal. Calcd for C50H24Tb2F36N2O14: C, 31.97; H, 1.29; N, 1.49. Found: C, 31.94; H, 1.19; N, 1.43. IR (KBr, cm-1): 1654s (CdO), 2928w (OH), 3432w (OH). 3 (Ln = Yb). Yield: 45%. Anal. Calcd for C50H24Yb2F36N2O14: C, 31.50; H, 1.27; N, 1.47. Found: C, 31.46; H, 1.15; N, 1.41. IR (KBr, cm-1): 1654s (CdO), 2929w (OH), 3430w (OH). {Eu(hfac)2(H2O)(OC6H4CN)}2(μ-HMq)2 (4). 2-Methyl-8-hydroxylquinoline (318 mg, 2 mmol), aluminum isopropoxide (204 mg, 1 mmol), and 4-hydroxybenzonitrile (238 mg, 2 mmol) were added to absolute ethanol (60 mL) with stirring at 85 °C for 6 h, producing Al(Mq)2(OC6H4CN-4) as a precipitate. Al(Mq)2(OC6H4CN-4) (0.3 mmol, 139 mg) and Eu(hfac)3(H2O)2 (0.2 mmol, 162 mg) were added to dichloromethane (30 mL) with stirring for 1 h. The concentrated solution was precipitated by addition of Et2O to give the main product [Al3(Mq)4(HMq)(μ3-OH)2(μ-OH)3{Eu(hfac)3}2].11 The filtrate was rotary evaporated, giving complex 4 as a yellow product. Crystallization of 4 by layering n-hexane onto a dichloromethane solution afforded yellow crystals. Yield: 18%. Anal. Calcd for C54H34Eu2F24N4O14: C, 37.65; H, 1.99; N, 3.25. Found: C, 37.24; H, 2.05; N, 3.12. IR (KBr, cm-1): 1641s (CdO), 2364m (CtN), 2920s (OH), 3420s (OH). Er5(hfac)9(μ-HMq)2(μ4-CH3COO)2(μ3-CH3COO)(μ-CH3COO)(μ3-OH)2 (5). 2-Methyl-8-hydroxylquinoline (318 mg, 2 mmol), CrCl3 3 6H2O (266 mg, 1 mmol), and 4-hydroxybenzonitrile (238 mg, 2 mmol) were added to absolute ethanol (30 mL) with stirring under reflux for 6 h. Upon cooling down, the mixture was neutralized with triethylamine, and water (100 mL) was added to produce a precipitate. After filtration, the precipitate was washed thoroughly with water, ethanol, and diethyl ether, and dried under vacuum for 1 day to afford a pale green product. Then 50 mg of the pale green product reacted with 350 mg of Er(hfac)2(CH3COO)(H2O)2 in 30 mL of dichloromethane solution with sonication for 15 min. After filtration, the concentrated dichloromethane solutions were layered with n-hexane to afford pale yellow crystals. Yield: 47%. ESI-MS (CH3OH-CH2Cl2): m/z 3287

Xu et al. ([M - H]-). Anal. Calcd for C73H41Er5F54N2O30 3 C6H14: C, 28.12; H, 1.64; N, 0.83. Found: C, 28.03; H, 1.64; N, 0.85. IR (KBr, cm-1): 1665s (CdO), 2932w (OH), 3436w (OH). Cr(Mq)(μ-HMq)(μ 3 -OH)(μ-OH)(μ-CH 3 COO)2 {Ln(hfac)2 }2 (Ln = Eu, 6; Tb, 7; Er, 8). 2-Methyl-8-hydroxylquinoline (318 mg, 2 mmol), CrCl3 3 6H2O (266 mg, 1 mmol), and 4-hydroxybenzonitrile (238 mg, 2 mmol) were added to absolute ethanol (30 mL) with stirring under reflux for 6 h. Upon cooling to room temperature, the mixture was neutralized with triethylamine, and water (100 mL) was added to produce a precipitation which was taken by filtration. The solid was redissolved in ethanol. After it was filtered, the filtrate was rotary-evaporated to afford a blue product. Then 50 mg of the blue product reacted with 150 mg of Ln(hfac)2(CH3COO)(H2O)2 in 30 mL of dichloromethane with sonication for 15 min. After filtration, the concentrated dichloromethane solutions were layered with n-hexane to afford the products as blue crystals. 6 (Ln = Eu). Yield: 50%. Anal. Calcd for C44H29F24N2CrEu2O16 3 H2O: C, 31.62; H, 1.87; N, 1.68. Found: C, 31.54; H, 1.83; N, 1.66. IR (KBr, cm-1): 1661s (CdO), 2933w (OH), 3412w (OH). 7 (Ln = Tb). Yield: 55%. Anal. Calcd for C44H29F24N2CrTb2O16 3 H2O: C, 31.35; H, 1.85; N, 1.66. Found: C, 31.24; H, 1.85; N, 1.62. IR (KBr, cm-1): 1662s (CdO), 2942w (OH), 3420w (OH). 8 (Ln = Er). Yield: 60%. Anal. Calcd for C44H29F24N2CrEr2O16 3 H2O: C, 31.05; H, 1.84; N, 1.65. Found: C, 31.05; H, 1.74; N, 1.55. IR (KBr, cm-1): 1660s (CdO), 2932w (OH), 3422w (OH). Crystal Structural Determination. Single crystals of 1-4, 5 3 C6H14, 7 3 H2O 3 0.5CH2Cl2, and 8 3 H2O 3 0.5CH2Cl2 suitable for X-ray diffraction were grown by layering n-hexane onto the corresponding dichloromethane solutions, respectively. Data collection was performed on an SCXMini diffractometer for 1-4, a Rigaku Mercury CCD diffractometer for 5 3 C6H14, and a SATURN724 CCD diffractometer for 7 3 H2O 3 0.5CH2Cl2 and 8 3 H2O 3 0.5CH2Cl2 by an ω scan technique at room temperature using graphite-monochromated Mo KR (λ = 0.71073 A˚) radiation. The Lp corrections were carried out in the reflection reduction process. The structures were solved by a direct method. 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.28 The refinements were carried out by fixing the C-F distances (1.32 ( 0.01 A˚), with the occupancy factors of F and F0 being 0.50, respectively. Crystallographic data are summarized in Table 1. Selected bond distances (A˚) and angles (deg) for 1, 4, 5 3 C6H14, and 7 3 H2O 3 0.5CH2Cl2 are presented in Table 2. Physical Measurements. Elemental analyses (C, H, N) were carried out on a Perkin-Elmer model 240C automatic instrument. Electrospray mass spectra (ES-MS) were recorded on a Finnigan LCQ mass spectrometer using dichloromethane-methanol as mobile phase. UV-vis absorption spectra were measured on a Perkin-Elmer Lambda25 UV-vis spectrometer. Infrared 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 Perkin-Elmer LS 55 luminescence spectrometer with a red-sensitive photomultiplier type R928. The steady-state nearinfrared (NIR) emission spectra were measured on an Edinburgh FLS920 fluorescence spectrometer equipped with a Hamamatsu R5509-72 super cooled photomultiplier tube at 193 K and a TM300 emission monochromator with an NIR grating blazed at 1000 nm. Corrected spectra were obtained via a calibration curve supplied with the instrument. The emission lifetimes above 10 μs were obtained by using an Edinburgh Xe900 450 W pulse xenon lamp as the excitation light source. The emission lifetimes below 10 μs were determined using an LED laser at 397 nm excitation. The emission quantum yields of Eu and Tb complexes were measured in degassed dichloromethane solutions at 298 K and estimated relative to [Ru(bpy)3](PF6)2 in acetonitrile as the standard (Φem = 0.062) and calculated by Φs = Φr(Br/Bs)(ns/nr)2(Ds/Dr), where the subscripts s and r refer to the sample and reference standard solution, respectively, n is the refractive index of the solvents, d is the integrated intensity, and Φ is the luminescence quantum yield.29,30 The quantity B is calculated by B = 1 - 10-AL, where A is the

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Table 1. Crystallographic Data of 1-8 1

2

3

4

empirical formula

C25H12C25H12TbF18- C25H12YbF18- C54H34Eu2F24EuF18NO7 NO7 NO7 N4O14 formula weight 932.32 939.28 953.40 1722.77 P1 P1 P1 space group P1 a (A˚) 12.153(5) 12.237(5) 12.031(5) 10.901(3) b (A˚) 12.769(5) 12.880(5) 12.710(5) 13.091(3) c (A˚) 12.973(5) 13.110(5) 12.972(5) 13.635(2) R (deg) 105.396(2) 105.356(3) 105.535(3) 63.912(7) β (deg) 111.867(4) 111.992(12) 112.0000(10) 66.895(7) γ (deg) 106.23 106.188(5) 106.269(3) 81.286(9) 3 1632.4(10) 1674.3(12) 1603.2(11) 1606.8(6) V (A˚ ) Z 2 2 2 1 1.897 1.863 1.975 1.780 Fcalc (g/cm-3) -1 2.067 2.254 3.065 2.067 μ (mm ) radiation (λ, A˚) 0.71073 0.71073 0.71073 0.71073 temp (K) 293(2) 293(2) 293(2) 293(2) 0.0387 0.0827 0.0437 0.0439 R1 (Fo)a 2 b 0.1011 0.2425 0.1020 0.1152 wR2 (Fo ) GOF 1.044 1.123 1.142 1.115 P P P P a R1 = ||Fo| - |Fc||/ |Fo|. b wR2 = [ w(Fo2 - Fc2)2/ w(Fo2)2]1/2.

5 3 C6H14 C79H55Er5F54N2O30 3374.55 P21 15.016(4) 21.867(6) 18.240(5)

7 3 H2O 3 0.5CH2Cl2

8 3 H2O 3 0.5CH2Cl2

C44.5H32ClCrTb2F24N2O17 1683.53 C2/c 39.421(17) 13.229(5) 26.149(11)

C44.5H32ClCrEr2F24N2O17 1744.69 C2/c 39.39(2) 13.188(6) 26.076(14)

90.591(5)

115.568(6)

115.570(8)

5989(3) 2 1.871 3.615 0.71073 293(2) 0.0558 0.1477 1.070

12301(9) 8 1.818 2.584 0.71073 293(2) 0.0733 0.2190 1.055

12218(11) 8 1.897 3.079 0.71073 293(2) 0.0764 0.2076 1.041

Table 2. Selected Bond Distances (A˚) and Angles (deg) for Compounds 1, 4, 5 3 C6H14, and 7 3 0.5CH2Cl2 3 H2O 1 Eu1-Eu1A Eu1-O7 Eu1A-O7 C16-O7 C16-O7-Eu1 C16-O7-Eu1A Eu1-O7-Eu1A O7-Eu1-O7A

3.9292(11) 2.411(3) 2.370(3) 1.352(5) 120.1(2) 126.1(2) 110.56(10) 69.44(10)

4 Eu1-Eu2 Eu1-O9 Eu1-O10 Eu2-O9 Eu2-O10 O9-C12 O10-C22 C12-O9-Eu1 Eu2-O9-Eu1 C22-O10-Eu1 Eu2-O10-Eu1 O9-Eu1-O10 O9-Eu2-O10

3.9746(9) 2.413(11) 2.425(10) 2.383(12) 2.422(11) 1.387(17) 1.29(2) 124.0(9) 111.9(4) 121.1(10). 110.2(4) 68.7(3) 69.2(3)

absorbance at the excitation wavelength and L is the optical path length.

Results and Discussion Preparation. Synthetic routes to complexes 1-8 are shown in Scheme 1. The precursor complexes were synthesized by the reaction of 2-methyl-8-hydroxylquinoline (HMq), CrCl3 3 6H2O, and 4-hydroxybenzonitrile (4-HOC6H4CN) in absolute ethanol. The mixture was neutralized with triethylamine, and water was added to produce a precipitation. The precipitation contains two species, with the blue one being dissolved in ethanol whereas the pale green one is insoluble in ethanol. Green complexes 1-3, prepared by the reaction of the pale green precursor with Ln(hfac)3(H2O)2 in dichloromethane, are sensitive to air at room temperature. Stable yellow complex 5 was synthesized by the reaction of pale green precursor with Ln(hfac)2(CH3COO)(H2O)2 in dichloromethane. Complex 4 was a byproduct from the reaction of Al(Mq)2(OC6H4CN-4) with Eu(hfac)3(H2O)2 in dichloromethane, in which the main product is [Al 3 (Mq)4 (HMq)(μ3-OH)2(μ-OH)3{Eu(hfac)3}2].11 The CrLn2 heterotrinuclear clusters 6-8 were prepared by the reaction of Ln(hfac)2(CH3COO)(H2O)2 (Ln = Eu, Tb, Er) with the blue precursor in dichloromethane. Crystallization by layering

Er1-Er2 Er1-Er5 Er2-Er5 Er3-Er4 Er3-Er5 Er4-Er5 Er1-O29 Er2-O29 Er5-O29 Er3-O30 Er4-O30 Er5-O30 Er1-O29-Er2 Er1-O29-Er5 Er2-O29-Er5 Er3-O30-Er4 Er3-O30-Er5 Er4-O30-Er5

5 3 C6H14 3.5795(10) 3.8221(11) 3.8252(9) 3.5599(10) 3.8796(10) 3.8896(9) 2.359(7) 2.351(8) 2.338(7) 2.356(8) 2.360(8) 2.290(7) 98.9(3) 108.9(3) 109.3(3) 98.0(3) 113.2(3) 113.5(3)

7 3 0.5CH2Cl2 3 H2O Cr-Tb1 Cr-Tb2 Tb1-Tb2 Cr-O13 Cr-O14 Cr-O15 Tb1-O13 Tb1- O15 Tb1-O16 Tb2-O14 Tb2-O15 Tb2-O16 Cr-O13-Tb1 Cr-O14-Tb2 Cr-O15-Tb1 Cr-O15-Tb2 Tb1-O15-Tb2 Tb1-O16-Tb2

3.3724(17) 3.4402(17) 3.8689(14) 1.987(6) 2.006(6) 1.966(5) 2.404(5) 2.378(5) 2.354(6) 2.433(6) 2.387(5) 2.359(6) 99.9(2) 101.2(2) 101.4(2) 104.0(2) 108.58(19) 110.4(2)

n-hexane onto the corresponding dichloromethane solutions afforded stable CrLn2 complexes as bluish-green crystals. The infrared spectra of these complexes show characteristic ν(OH) bands at 2920 and 3440 cm-1 and ν(CdO) bands at ca. 1665 cm-1. The structures of 1-5, 7, and 8 were determined by X-ray crystallography. Crystal Structures. As complexes, {Ln(hfac)3}2(μ-HMq)2 (Ln = Eu, 1; Tb, 2; Yb, 3) are isomorphous; only the crystal structure of 1 is described here. As depicted in Figure 1, the EuIII ions are coordinated by eight oxygen atoms with two from the μ-phenol of HMq and six from hfac-, forming a square antiprism geometry. The dinuclear lanthanide(III) centers are bridged by HMq through μ-phenol in a symmetric fashion, with the Eu 3 3 3 Eu distance being 3.93 A˚. The crystal structure of {Eu(hfac)2(H2O)(OC6H4CN)}2(μ-HMq)2 (4) is similar to that of 1 (Figure 2) except that two of the eight oxygen atoms around the EuIII ion are from one H2O and one phenolate of deprotonated 4-HOC6H4CN to give a distorted square-antiprism. The bridging HMq is featured with a μ-phenol mode to link two EuIII ions in a symmetric fashion, with the Eu 3 3 3 Eu distance being 3.9746(9) A˚. Due to the steric requirement, the two Eu(hfac)2 and coordination H2O, together with the deprotonated OC6H4CN4 units, are oriented at opposite directions.

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Scheme 1. Synthetic Routes to Complexes 1-3, 5, and 6-8

Figure 1. ORTEP drawing of 1 (30% thermal ellipsoid) with atomlabeling scheme. The H atoms and the F atoms on the trifluoromethyl are omitted for clarity.

Pentanuclear complex 5 3 C6H14 (Figure 3) can be viewed as two {Er(hfac)2}2(μ-HMq)(μ-CH3COO) fragments linked to a central ErIII ion through two μ3-OH- and two μ4CH3COO-. The five ErIII atoms are arranged into a distorted square pyramid with four of them (Er1-Er4) in the basal plane and another one (Er5) at the apex. The four ErIII atoms in the basal plane are coplanar and coordinated by eight oxygen donors to give a distorted square-antiprism, respectively. The apical ErIII atom is nine-coordinated to oxygen atoms from one hfac-, two μ4-CH3COO-, two μ3OH- ions, and one μ3-CH3COO- to give a distorted capped

Figure 2. ORTEP drawing of 4 (30% thermal ellipsoid) with atomlabeling scheme. The H atoms and the F atoms on the trifluoromethyl are omitted for clarity.

square antiprism. The bridging acetates adopt three types of bonding modes, including μ- (monatomic bridging), μ3(monatomic and triatomic bridging), and μ4-CH3COO(bridging chelating). The bridging HMq is featured with a μ-phenol mode similar to that in 1-4. Er1, Er2, and Er5 are capped by one μ3-OH- (O29), and Er3, Er4, and Er5 by the other μ3-OH- (O30) with the Er-O lengths being 2.290(7)2.360(8) A˚ and the Er-O-Er angles being 98.0(3)-113.5(3)°. The Er 3 3 3 Er distances across μ 3-OH - are 3.5599(10)3.8896(9) A˚ .

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Figure 4. ORTEP drawing of 7 (30% thermal ellipsoid) with an atom-labeling scheme. The H atoms and the F atoms on the trifluoromethyl are omitted for clarity.

Figure 3. (a) ORTEP drawing of 5 (30% thermal ellipsoid) with atom-labeling scheme. The H atoms and the F atoms on the trifluoromethyl are omitted for clarity. (b) Skeleton of pentanuclear erbium(III) complex showing the bridging modes of the μ4-acetate, μ3-OH-, and μ-phenol of HMq.

Since CrLn2 heterotrinuclear complexes 7 and 8 are isomorphous, only the crystal structure of 7 is described here. As depicted in Figure 4, the CrTb2 cluster can be viewed as a combination of one Cr-Mq and two Tb(hfac)2 units linked by one μ3-OH-, one μ-OH-, one μ-phenol from HMq, one μ-phenoxo from Mq, and two μ-CH3COO- (triatomic bridging). It is intriguing that the CrTb2 structure affords one incomplete cubane11 composed of Tb1, Cr1, Tb2, O13 (Mq), O14 (HMq), O15 (μ3-OH-), and O16 (μ-OH-) atoms with the angle Tb2-Cr1-Tb1 being 69.20(3)°. Both Tb1 and Tb2 atoms are bound to eight oxygen donor atoms to give a distorted square-antiprism. The CrIII ion adopts an octahedral coordination environment composed of O5N donors from one μ-phenoxo of Mq (chelating-bridging), one μ-phenol of HMq, one μ3-OH- and two μ-CH3COO-. The deprotonated Mq adopts a chelating-bridging mode, a chelating CrIII center, as well as a bridging TbIII center through a μ-phenoxo, with the Cr-O13-Tb1 angle being 99.9(2)°. The HMq exhibits a bridging μ-phenol mode, with the CrO14-Tb2 angle being 101.2(2)°. The Cr-O13 [1.987(6) A˚] and Tb1-O13 [2.404(5) A˚] distances for μ-phenoxo (Mq) are distinctly shorter than the corresponding Cr-O14 [2.006(6) A˚] and Tb2-O14 [2.433(6) A˚] distances for μ-phenol (HMq). The CrTb2 atoms are capped by a μ3-OH- (O15) with the distance from the O15 to CrTb2 plane being 2.2 A˚ and the angles Cr-O15-Tb1 = 101.4(2)°, Cr-O15-Tb2 = 104.0(2)°, and Tb1-O15-Tb2 = 108.58(19)°. The Cr1 3 3 3 Tb1 and Cr1 3 3 3 Tb2 separations are 3.3724(17) and 3.4402(17) A˚, respectively. The Tb1 3 3 3 Tb2 distance is 3.8689(14) A˚.

Photophysical Properties. The absorption and emission data of 1-8 are summarized in Table 3. The UV-vis spectra of HMq, 1, 4, 5, and 7 in dichloromethane solutions are depicted in Figure 5. The ligand HMq in dichloromethane solution displays an intense absorption band at ca. 245 nm together with a broad band centered at 310 nm, tailing to 350 nm.31 In the UV-vis spectra of 1, 4, 5, and 7, there are mainly two sets of absorption bands, one from the ligandcentered transition of HMq/Mq at ca. 245 or 257 nm, and the other at ca. 314 nm from the intraligand π-π* transition of hfac.26,32,33 In addition, a broad low-energy absorption trailing to 420 nm arises, probably from the [π f π*] (HMq/Mq) transitions.31 Ln(hfac)3(H2O)2 has almost no absorption above 350 nm, while 1-8 exhibit appreciable absorption bands in the region of 350-420 nm, arising most likely from the extended π f π* transitions in HMq or Mq bound to the lanthanide(III) ions. Excitation of the ligand-centered charge-transfer band in the UV region for 1-4 (Figure 6) gives rise to typical luminescence from the corresponding lanthanide(III) ions with a microsecond range of lifetimes in both solid states and dichloromethane solutions at ambient temperature (Table 3). Meanwhile, the emission bands due to the 1(π f π*) excited state of the ligand in the UV region are remarkably attenuated due to an energy transfer from the ligands to lanthanide centers (Figure 6). The longer Eu-based lifetime for 1 (τ = 318 μs) than that for 4 (τ = 223 μs) in the solid state is attributable to exclusion of the solvent molecules coordinated to the EuIII ions for 1.24 Upon excitation at 260 nm < λex < 400 nm, 5 (Figure S4 of the Supporting Information) and the CrLn2 complexes (Ln = Eu, 6; Tb, 7; Er, 8) (Figures 7 and 8) exhibit linelike emission bands characteristic of the corresponding lanthanide(III) ions. The emissive lifetimes of lanthanide(III) luminescence are in microsecond ranges in both solid states and dichloromethane solutions at room temperature (Table 3). The lifetimes in these complexes are much longer than the corresponding values in the precusors Ln(hfac)3(H2O)2 and Lnq3 (Hq = 8-hydroxylquinoline),34 attributed probably to full exclusion of solvent molecule coordination to the LnIII ions.12-14 As the precursor Ln(hfac)3(H2O)2 does

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Table 3. Absorption and Luminescence Data of 1-8 at 298 K a

complex

medium

1

powder CH2Cl2 1-Fa powder CH2Cl2 2-Fa powder CH2Cl2 3-Fa powder CH2Cl2 4-Fa CH2Cl2 5-Fa powder CH2Cl2 6-Fa powder CH2Cl2 7-Fa CH2Cl2 8-Fa

2 3 4 5 6 7 8

λabs/nm (ε/(dm3 mol-1 cm-1)) 245 (49500), 260 (61600), 300 (53100), 380 (2500) 248 (49400), 260 (61200), 300 (53100), 380 (2600) 247 (51500), 260 (68400), 300 (53700), 380 (2800) 245 (62200), 265 (38000), 302 (37100), 380 (2500) 250(89100), 257 (96900), 302 (64300), 378 (4400) 257 (55600), 303 (47500), 368 (4600) 254 (56400), 303 (50000), 370 (4700) 257 (54800), 303 (46100), 370 (4500)

λem/nm (τem/μs)b 613 (318) 613 (269) 613 (488) 543 (104) 543 (22.2) 543 (30.7) 978 (20.8) 978 (14.4) 978 (17.1) 613 (223) 613 (393) 613 (611) 1537 (-)e 1537 (-)e 613 (437) 613 (584) 613 (612) 543 (14) 543 (18) 543 (24) 1535 (-)e 1535 (-)e

Φem (%)c 1.9 2.5 0.72d 1.6

0.6 0.9

a This represents the samples in dichloromethane solution upon addition of 0.2 equiv of F- for 1-3, 1.5 equiv for 4, and 3 equiv for 6-8. b Excitation wavelength at 310 nm. c The quantum yield of complexes in degassed dichloromethane was determined relative to that of Ru(bpy)3(PF6)2 (Φ = 0.062) in degassed acetonitrile. d The quantum yields of Yb complexes in dichloromethane solutions are 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.12c These values refer to the lanthanide-based emission process only and take no account for the efficiency of intersystem crossing and energy transfer processes. e Nondetectable in our instrument.

Figure 5. UV-vis absorption spectra of HMq (black), 1 (green), 4 (cyan), 5 (blue), and 7 (red) in dichloromethane at ambient temperature.

Figure 7. Excitation and emission spectra of fluoride-induced lanthanide emission enhancement in dichloromethane for 8 upon addition of 3 equiv of [Bu4N]F at ambient temperature.

Figure 6. Fluoride-induced lanthanide(III) emission enhancement of 1 (green), 2 (red), and 3 (blue) in dichloromethane solutions upon addition of 0.2 equiv of [Bu4N]F at ambient temperature.

Figure 8. Fluoride-induced lanthanide(III) emission enhancement of 6 (green), 7 (red), and 8 (blue) in dichloromethane solution upon addition of 3 equiv of [Bu4N]F at ambient temperature.

not display absorption at >350 nm,26,32,33 the lanthanide(III) luminescence in CrLn2 complexes should be sensitized by

energy transfer from Cr-Mq based light-harvesting chromophores upon near-UV irradiation at 350 nm < λex < 400 nm

Article

(Figures 7 and 8). In view of the short intramolecular Cr 3 3 3 Ln distances (3.34 and 3.42 A˚), substantial Cr f Ln energy transfer is most likely operating from Cr-Mq chromophores to the lanthanide(III) center so that the lanthanide luminescence is indeed “lighted up”. As depicted in Figure 8, the emission spectra exhibit five distinctly resolved bands for the CrEu2 complex 6 at 535, 591, 611, 651, and 699 nm due to 5 D1 f 7F0, 5D0 f 7F1, 7F2, 7F3, and 7F4 transitions, four emission bands for the CrTb2 complex 7 at ca. 487, 543, 583, and 619 nm due to 5D4 f 7F6, 7F5, 7F4, and 7F3 transitions, and one for the CrEr2 complex 8 at ca. 1535 nm due to the 4 I13/2 f 4I15/2 transition. Additionally, these CrLn2 complexes in dichloromethane solutions also afford a weak ligandbased emission centered at ca. 375 nm due likely to incomplete energy transfer from the ligands to the lanthanide(III) centers. As indicated in Table 3, the quantum yield of 1 (Φem = 1.9%) is higher than that of 4 (Φem = 1.6%) in dichloromethane solution, due most likely to additional two coordination water in 4, which could quench the Eu-centered emission through an O-H nonradiative oscillator. Obviously, the quantum yields of heteronuclear complexes 6 (Φem = 0.6%) and 7 (Φem = 0.9%) are lower than those of homonuclear complexes of 1 (Φem = 1.9%) and 2 (Φem = 2.5%) and even lower than that of 4 (Φem = 1.6%). This originates likely from the mismatched energy level for CrMq f Ln energy transfer,1,12 causing lanthanide(III)-centered luminescence in relatively low efficiency. Furthermore, the quantum yield of CrEu2 complex 6 (Φem = 0.6%) is much lower than that of Al3Eu2 complex (Φem = 6.1%),11 Zn2Eu2 complex (Φem = 7.8%),25a or Zn2Eu (Φem = 1.3%)25b with bridging HMq or Mq. This suggests that energy transfer from the Cr-Mq chromophore is much less effective than that from the Al-Mq or Zn-Mq subunit to the lanthanide(III) center. Fluoride-Induced Luminescence Enhancement. In view of the direct linkage of μ-HMq to the lanthanide(III) ions in 1-3 and additional coordination water into the inner coordination sphere of the lanthanide(III) ions in 4, the emissive states from lanthanide(III) centers would be partially deactivated through nonradiative O-H vibration excitation so as to severely quench lanthanide(III) luminescence. Inspired by the idea that fluorides could replace coordination water molecules or form strong O-H 3 3 3 F interactions so as to suppress the O-H oscillators, thus minimizing the nonradiative deactivation processes,11,24 we attempted to achieve fluoride-enhanced lanthanide(III) luminescence by introducing a small amount of [Bu4N]F to these species. As anticipated, titration of 1-4 with [Bu4N]F in aerated dichloromethane solutions induced indeed significant enhancement of LnIII-centered luminescence intensities and lifetimes. Addition of 0.2 (1-3) or 1.5 (4) equiv of F- to dichloromethane solutions of 1-3 (Figure 6) or 4 (Figure S2 of the Supporting Information) caused 4-, 3-, 1.6-, or 7-fold enhancement in emission intensity, respectively, and the lifetimes were increased from 269 to 488 μs for 1, from 22 to 31 μs for 2, from 14 to 17 μs for 3, and from 393 to 611 μs for 4 (Figure S5 of the Supporting Information). The significant enhancement in both emission intensities and lifetimes in the presence of a small amount of F- suggests that 1-4 could serve as potential luminescence probes for F- detection. As there exist additional two coordinated water molecules except for two bridging HMq in 4, a greater amount of F- for the maximum luminescent enhancement for 4 (1.5 equiv) is needed than that for 1 (0.2 equiv), leading to indeed more

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Figure 9. Fluoride-induced lanthanide emission enhancement in dichloromethane for 7 before (black) and after addition of 0.3 (red), 0.6 (green), 1 (blue), and 3 (cyan) equiv of [Bu4N]F at ambient temperature.

remarkably increased emission intensity (7-fold) in comparison with the case of 1 (4-fold). As there are the direct coordination of μ3-OH- and μ-OH- to the lanthanide(III) ions as well as the presence of μ-phenol from the HMq in close proximity to the lanthanide(III) centers in CrLn2 complexes 6-8, the emissive states from lanthanide(III) centers would also be deactivated through nonradiative O-H and C-H vibration excitation, thus quenching the corresponding lanthanide(III) luminescence. As expected, titration of 6-8 with [Bu4N]F in aerated dichloromethane solution induced indeed a significant enhancement of the LnIII-centered luminescence intensities and lifetimes. Addition of 3 equiv of F- to dichloromethane solutions of 6, 7, or 8 caused 5-, 12-, or 2-fold enhancement in emission intensity (Figure 8), respectively. The corresponding emissive lifetimes were increased from 584 to 612 μs for 6 and from 18 to 24 μs for 7 (Figure S5), although the ErIIIbased lifetime for 8 could not be measured with our spectrometer. As shown in Figure 9, the TbIII-based emission in CrTb2 complex 7 is progressively increased with gradual addition of 0.3, 0.6, 1, and 3 equiv of [Bu4N]F in dichloromethane solutions at ambient temperature. The remarkable fluoride-induced enhancement in emission intensities and lifetimes suggests that CrLn2 complexes could also serve as potential luminescence probes for F- detection. Nevertheless, when more than 0.2 equiv of F- for 1-3, 1.5 equiv for 4, or 3 equiv for 6-8 is added, the lanthanide(III) emission is gradually attenuated (Figures S1 and S2 of the Supporting Information). This is because the lanthanide(III) ions would gradually be ligated by excess fluoride so as to destroy the original structures. This variation process was monitored by UV-vis absorption and emission spectral titration. With successive addition of fluoride to the dichloromethane solution of 7 (Figure S3 of the Supporting Information), the absorption band at ca. 300 nm is attenuated so as to disappear entirely upon addition of 30 equiv of fluoride; meanwhile, a new absorption band at ca. 280 nm appears. Similarly, when excess fluoride is added to the dichloromethane of 7, the TbIII-centered emission is gradually attenuated so as to disappear entirely, whereas the CrMq-based emission is markedly enhanced with a red shift (Figure S6 of the Supporting Information) due probably to elimination of the process of CrMq f Ln energy transfer.11 This further suggests that the structure of 7 could be destroyed by excess fluoride. Likewise, the direct coordination of μ3-OH- and μ-OH from the HMq in close proximity to the ErIII centers in

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pentanuclear erbium(III) complex 5 would quench erbium(III) luminescence. Titration of 5 with [Bu4N]F in aerated dichloromethane solution promotes significantly the ErIII-centered luminescence intensity (Figure S4 of the Supporting Information). Addition of 3 equiv of F- to 5 in a dichloromethane solution leads to an ca. 2-fold enhancement of the erbium(III)centered emission intensity. Conclusions A new family of polynuclear lanthaninde(III) clusters with bridging HMq in μ-phenol mode have been isolated and their structures established by X-ray crystallography. Upon excitation at >350 nm, sensitized lanthanide luminescence is achieved due to energy transfer from the Mq or Cr-Mq chromophore to the lanthanide(III) centers. It has been demonstrated that significantly fluoride-enhanced emission intensities and lifetimes occur for both visible and NIR luminescent lanthanide(III) complexes due to effective suppression of the O-H nonradiative vibrational excitation from the μ-OH and μ-phenol of HMq bound directly to the lanthanide(III) ions by formation of strong O-H 3 3 3 F hydrogen bonding interactions. Acknowledgment. This work was financially supported by the NSFC (20901077), the 973 project (2007CB815304) from the MSTC, and the NSF of Fujian Province (2008I0027 and 2008F3117). Supporting Information Available: Selected bond distances (A˚) and angles (deg) for compounds 2, 3, and 8, UV-vis absorption and emission spectra of titration, lifetime decay curves, and X-ray crystallographic files in CIF format for the structure determination of compounds 1-4, 5 3 C6H6, 7 3 H2O 3 0.5CH2Cl2, and 8 3 H2O 3 0.5CH2Cl2. This material is available free of charge via the Internet at http://pubs.acs.org.

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