Six Ln (III) Coordination Polymers with a Semirigid Tetracarboxylic

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Six Ln(III) Coordination Polymers with a Semirigid Tetracarboxylic Acid Ligand: Bifunctional Luminescence Sensing, NIR-Luminescent Emission and Magnetic Properties Ke-Xia Shang, Jing Sun, Dong-Cheng Hu, Xiao-Qiang Yao, Li-Hua Zhi, Chang-Dai Si, and Jia-Cheng Liu Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b01565 • Publication Date (Web): 01 Mar 2018 Downloaded from http://pubs.acs.org on March 3, 2018

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Crystal Growth & Design

Six Ln (III) Coordination Polymers with a Semirigid Tetracarboxylic Acid Ligand: Bifunctional Luminescence Sensing,

NIR-Luminescent

Emission

and

Magnetic

Properties Ke-Xia Shang,†,‡ Jing-Sun,† Dong-Cheng Hu,*,† Xiao-Qiang Yao,† Li-Hua Zhi,† Chang-Dai Si,§ and Jia-Cheng Liu*,† †

College of Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou 730070,

People's Republic of China ‡

School of Medicine, Hexi University, Zhangye 734000, People's Republic of China

§

College of Chemical Engineering and Technology, Tianshui Normal University, Tianshui 741001,

People's Republic of China

* Supporting Information ABSTRACT: Two series of lanthanide metal organic frameworks (Ln–MOFs) [Ln(HL)(H2O)3]·H2O and [Ln(HL)(H2O)2] (Ln = Tb, 1; Er, 4; Tm, 5; Yb, 6 and Dy, 2; Ho, 3) have been successfully assembled by Ln3+ ions and asymmetric polycarboxylate ligand 3-(3′,5′-dicarboxyl-phenoxy) phthalic acid (H4 L). The crystal structures, photoluminescence and magnetic properties of these compounds have been investigated. Complexes 1, 4, 5 and 6 are isostructural and show 2D layer frameworks constructed by hydrogen bonding interactions between the coordination water molecules and the O atoms of the carboxyl group. Complexes 2 and 3 show 3D frameworks with two different interlayer channels decorated by ligands and lanthanide metals. The emission spectra show that complex 1 displays an intense green light emission and can selectively and sensitively detect for Fe3+ ion and nitromethane. In addition, Near Infra–Red (NIR) luminescence and magnetic susceptibilities measurments further shows that these compounds are promising functional materials.

INTRODUCTION Over the last two decades, the research on metal–organic frameworks (MOFs) have attracted great attention not only because of their interesting structural features but also due to their extraordinary physical properties. 1–13 Compared with transition metals, the subcategory of lanthanide metal–organic frameworks (Ln–MOFs) deserves special attention, due to the internal nature of their unique electron structure such as long luminescence lifetime, large Stocks shifts, and high purity of colors which arise from f-f transitions via an ‘antenna effect’ 14–16. Meanwhile, Ln–MOFs can generate strong luminescent signals with visible emission colors and can be developed as a new type of promising chemical sensors. Therefore, fluorescent sensors based on luminescent Ln–MOFs have been widely investigated for the

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selective detection of environmental contaminants due to their ability to provide a simple, selective, and visible detection method. In the past few years, the potential application of lanthanide complexes in magnetic materials has also received much attention. Owing to the strong spin−orbit coupling, the magnetic structure of the lanthanides is more complex than that of the transition metal system. 12–13 Since Fe3+ plays a significant role in environmental and ecological system, particularly exists in the structures of numerous enzymes, proteins, and transcriptional events.22–23 Either its deficiency or excess from the normal permissible limit can result in the physical illness like anemia, mental decline, arthritis, diabetes, cancer and etc.24–26 Therefore, the selective detection of Fe3+ ions is very important for human health. In addition, with fast-growing activities from human society and industry, a great amount of hazardous metal ions and toxic organic small molecules are released into the environment for human survival and caused many serious adverse effects on people’s health.27–28 Thus, it is extremely important and urgent to develop a new method to detect and sense small pollutants and metal cations through the naked eyes.54 In recent years, various Ln–MOF materials have been developed for sensing and detection of various metal ions and small molecules.29–31 Moreover, considerable efforts have been made on the detection of Ag+, Mg2+, Fe3+ ions and some small molecules based on Ln-MOFs.32–34, 54 Nevertheless, few works have been devoted to the exploitation of fluorescent probes that are sensitive to Fe3+ ions and nitromethane simultaneously.18 In order to construct functional Ln–MOF materials, the choice of organic ligand is vital in the assembly of structures. Semirigid V–shaped multicarboxylate ligands may be good candidates for following reasons: (a) compared to rigid carboxylate ligands, semirigid polycarboxylate ligands have a more flexible coordination mode; (b) compared to flexible carboxylate ligands, semi-rigid polycarboxylate ligands can provide relatively robust and stable molecular frameworks; (c) variable coordination modes of semi-rigid multicarboxylate ligands provide the possibilities of space distortion and complexes with different structures. 11, 12, 35–37. Based on the above considerations, in this contribution a semirigid tetracarboxylic acid ligand [H4L= 3-(3′, 5′-dicarboxyl-phenoxy) phthalic acid] was selected as organic ligand. Two series of Ln-MOFs: [Ln(HL)(H2O)3]·H2O and [Ln(HL)(H2O)2] (1,Tb; 4,Er; 5,Tm; 6,Yb and 2,Dy; 3,Ho) have been successfully synthesized under hydrothermal conditions. Importantly, the luminescent sensing properties of 1 was investigated for sensing Fe3+ and nitromethane simultaneously with good selectivity and sensitivity. The characteristic emission of the corresponding HoIII, ErIII and YbIII ions also observed within the NIR range. The magnetic properties of 2 and 3 have also been investigated.

EXPERIMENTAL SECTION Materials and General Methods. The ligand 3-(3′, 5′-dicarboxylphenoxy) phthalic acid (H4L) was synthesized according to the literature of our previous work.

11

Other chemicals and solvents were

commercially available and used as received without further purification. Elemental analyses of C, H, and N were performed on a VxRio EL Instrument. FT–IR spectra were recorded from KBr pellets in the range of 4000–400 cm−1 on a Bio-Rad FTS-7 spectrometer. Powder X–ray diffraction (PXRD) patterns were obtained at 293 K. on a Philips PW 1710-BASED diffractometer. Thermogravimetric analysis (TGA) experiments were carried out on a PerkinElmer TG-7 analyzer heated from 25 to 800 °C at a heating rate of 20 °C/min under N2 atmosphere. The near–infrared (NIR) luminescence spectra and fluorescence quantum yield were carried out on FLS-920 steady-state transient


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fluorescence spectrometer at room temperature. The emission/excitation spectra were carried out on F97pro fluorescence spectrometer at room temperature. Magnetic susceptibility data were obtained on microcrystalline samples, using a Quantum Design MPMS (SQUID)-VSM magnetometer in the range of 1.8-300 K. Synthesis. [Tb(HL)(H2O)3]·H2O (1). Tb (NO3)3·6H2O (0.2 mmol, 90.6 mg), H4 L (0.2 mmol, 69.3 mg), and H2O (8 mL) were placed in a 25 mL Teflon-lined stainless steel autoclave, and heated at 120 °C for 3 days, and then cooled to room temperature over 48 h. Colorless block crystals suitable for single crystal X–ray diffraction analysis were collected by filtration, washed several times with H2O, and dried in air at ambient temperature. Yield: 42 % (based on Tb). Elemental analysis (%):Found (calcd) for C16H15O13Tb (Mr = 574.20): C 33.47 (33.28); H 2.63 (2.67); IR (KBr, cm−1): 3399(w), 1636(s), 1544(s), 1465(w), 1402(s), 1311(m), 1236(m), 1113(w) ,996(m) ,924(w), 777(s), 694(m) and 519(w). (Figure S3). [Dy(HL)(H2O)2] (2). Dy(NO3)3·6H2O (0.2 mmol, 86.4 mg), H4L (0.2 mmol, 69.3 mg), H2O (8 mL) and hydrochloric acid solution (0.05mL, 6M) were placed in a 25 mL Teflon-lined stainless steel autoclave, and heated at 170 °C for 3 days, and then cooled to room temperature over 48 h. Pale yellow rod crystals suitable for single crystal X–ray diffraction analysis were collected by filtration, washed several times with H2O, and dried in air at ambient temperature. Yield: 36 % (based on Dy). Elemental analysis (%): Found (calcd) for C16H11O11Dy (Mr = 541.75): C 33.42 (35.47); H 2.02 (2.05); IR (KBr, cm−1): 3556(w), 3371(m),2954(w),1569(s), 1369(s), 995(m), 899(m), 773(s), 651(m) and 509(w). [Ho(HL)(H2O)2] (3). Orange rod-shaped crystals (32 %, based on Ho) were obtained after filtration by the same procedure as that for 2 except that Ho (NO3)3·6H2O (0.2 mmol, 88.2mg) was used instead of Dy (NO3)3·6H2O. Elemental analysis (%): Found (calcd) for C16H11O11Ho (Mr = 544.18): C 34.20 (35.31); H 2.63 (2.64); IR (KBr, cm−1): 3557(w), 3367(m), 2803(w), 1570(s), 1396(s), 996(m), 899(m), 774(s), 643(m) and 477(w). [Er(HL)(H2O)3]·H2O (4). Er (NO3)3·6H2O (0.2 mmol, 92.3 mg), H4 L (0.2 mmol, 69.3 mg), and H2O (8 mL) were placed in a 25 mL Teflon-lined stainless steel autoclave, and heated at 170 °C for 3 days, and then cooled to room temperature over 48 h. Pale pink block crystals suitable for single crystal X– ray diffraction analysis were collected by filtration, washed several times with H2O, and dried in air at ambient temperature. Yield: 41 % (based on Er). Elemental analysis (%):Found (calcd) for C16H15O13Er (Mr = 582.54): C 33.06 (32.99 ); H 2.69 (2.60); IR (KBr, cm−1): 3607(w), 3402(m), 1635(s), 1538(s), 1480(m), 1456(m), 1405(s), 1313(m) , 1238(s), 1109(m), 1001(s), 818(s), 768(s), 707(m) and 603(w). [Tm(HL)(H2O)3]·H2O (5). Pale yellow block crystals (27 %, based on Tm) were obtained after filtration by the same procedure as that for 4 except that Tm (NO3)3·6H2O (0.2 mmol, 92.1mg) was used instead of Er (NO3)3·6H2O. Elemental analysis (%): Found (calcd) for C16H15O13Tm (Mr = 584.21): C 32.96 (32.89); H 2.68 (2.59); IR (KBr, cm−1): 3409(w), 3076(m), 1635(s), 1550(s), 1481(m), 1454(m), 1402(s), 1290(m), 1236(s), 1110(m), 996(s), 866(m), 817(s), 765(s), 709(m), 611(m), 516(w) and 451(w). [Yb(HL)(H2O)3]·H2O (6). Yb (NO3)3·6H2O (0.2 mmol, 89.8 mg), H4L (0.2 mmol, 69.3 mg), and H2O (8 mL) were placed in a 25 mL Teflon-lined stainless steel autoclave, and heated at 80 °C for 3 days, and then cooled to room temperature over 48 h. Colorless block crystals suitable for single crystal X–



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ray diffraction analysis were collected by filtration, washed several times with H2O, and dried in air at ambient temperature. Yield: 35 % (based on Yb). Elemental analysis (%): Found (calcd) for C16H15O13Yb (Mr = 588.32): C 32.79 (32.66); H 2.52 (2.57); IR (KBr, cm−1): 3606(w), 3390(m), 1636(s), 1530(s), 1469(m), 1400(s), 1315(m), 1236(m), 999(s) ,922(w) ,775(s), 698(m) 509(w) and 443(w). Fluorescence Sensing Experiments. The grounded powder samples of 1 (40 mg) were immersed in 40 mL purified water and ultrasonicated for 30 min, and then standed over night to form a stable suspensions for fluorescence study. Under ultraviolet light, the suspension showed the visible green emission. The fluorescence emission of the suspension was measured and exhibited very similar luminescence characteristic to 1 in the solid state, which suggests 1 has good water stability. Equal volumes 1000µL different aqueous suspension containing 1×10−2 M of M(NO3)x and 30µL organic solvent molecules were added in the suspension of crystals for luminescence test. X-ray Crystallography. X-ray single-crystal diffraction data were collected on a Bruker Smart Apex CCD area detector diffractometer with graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å) at room temperature using ω-scan technique. The structures were solved by direct methods and refined by a full-matrix least-squares refinement on F2 with SHELXL-97 and SHELXS-97.38–39The hydrogen atoms except those of water molecules were generated geometrically and refined isotropically using the riding model. The relevant crystallographic data are shown in Table 1. Selected bond distances and angles are given in Table S1 in the Supporting Information. Hydrogen bonds of 1 is listed in Table S2. CCDC 1540464 (1), 1577171 (2), 1540465 (3), 1555832 (4), 1540462 (5) and 1540463 (6) include all supplementary crystallographic data of complexes 1-6.

Table 1. Crystal Data and Structure Refinement Results for Complexes 1−6 comple

1

2

3

4

5

6

153(2)

153(2)

296(2)

296(2)

296(2)

C16H11O11Dy

C16H11O11Ho

C16H15O13Er

C16H15O13Tm

C16H15O13Yb

574.20

541.75

544.18

582.54

584.21

588.32

triclinic

monoclinic

monoclinic

triclinic

triclinic

triclinic

P -1

P21/c

P21/c

P -1

P -1

P -1

2

4

4

2

2

2

a (Å)

7.744(3)

12.4515(7)

12.438(3)

7.7018(17)

7.716(5)

7.7133(7)

b (Å)

10.924(4)

14.7347(8)

14.689(4)

10.829(2)

10.828(6)

10.8484(10)

c (Å)

10.991(4)

9.1023(5)

9.089(2)

10.945(2)

10.940(6)

10.8976(10)

x temp (K) empirical

296(2) C16H15O13Tb

formula formula weight crystal system Space group

Z

α (deg)

85.835(10)

90

86.053(5)

85.851(7)

85.9380(10)

β (deg)

81.313(10)

9.1023(5)

109.266(3)

81.401(5)

81.023(8)

81.0450(10)

γ (deg)

78.067(12)

90

90

78.001(5)

78.223(8)

78.2030(10)

898.4(6)

1577.24(15)

1567.6(7)

882.2(3)

883.0(9)

881.02(14)

3

V (Å )

90


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ρcalcd

2.123

2.282

2.306

2.193

2.197

2.218

µ (mm−1)

4.011

4.805

5.116

4.832

5.100

5.384

GOF

1.022

1.141

1.266

1.071

1.139

1.107

R1 (I > 2σ(I))a

0.0709

0.0194

0.0229

0.0560

0.0366

0.0498

wR2(I>2σ(I))a

0.1833

0.0585

0.0722

0.1822

0.1296

0.1480

R1 (all data)b

0.0745

0.0202

0.0265

0.0566

0.0417

0.0538

0.1875

0.0590

0.0942

0.1834

0.1574

0.1662

3

(g/cm )

wR2(all data)

b

a

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

RESULTS AND DISCUSSION Complexes 1, 4, 5 and 6 were synthesized via hydrothermal conditions with mixed Ln(NO3)3·6H2O and H4L without adding hydrochloric acid, while hydrochloric acid was added in the synthesis procedures of complexes 2 and 3 under identical conditions. It’s difficult to obtain good single crystals of 2 and 3 without hydrochloric acid and only get the powder. That is to say, compared to complexes 1, 4, 5 and 6, the addition of hydrochloric acid is more favorable for the formation of 2 and 3. As a result, compound 2 and 3 have different structures with compounds 1, 4, 5 and 6. Description of Crystal Structures. Herein, only structure of complex 1 is described in detail on account of complexes 1, 4, 5 and 6 are isostructural frameworks. Single crystal X–ray analysis reveals that compond 1 crystallizes in the triclinic crystal system with P-1 space group. As shown in Figure 1a, each asymmetric unit of 1 contains one Tb3+ metal center, one HL3− ligand, three coordinated and one solvated water molecule. Each terbium ion is coordinated with nine oxygen atoms in a distorted capped square antiprism configuration: six oxygen atoms from three bidentate carboxyl groups of three different HL3− ligands, and three oxygen atoms from three different coordinated water molecules. The carboxylate groups of H4L ligand shows a bidentate chelating bonding mode (Figure S1). As to HL3− in 1 the dihedral angle is 120.08° of two phenyl rings, leaving a carboxyl group protonated and uncoordinated. TbO9 Secondary Building Units (SBU) are interconnected with each other through 2and 3-carboxyl groups to afford a 1D chain along the c direction (Figure1b). The adjacent chains were further extended by the hydrogen-bond interaction between O(11)–H(11B)...O(4) and O(12)– H(12C)...O(2) to give a 2D layered structure (Figure 1c, d).



Figure 1. (a) Coordination environment of Tb3+ cations in 1. (b) The [TbO9] SBU units are interconnected to 1D chain along the c direction. (c) The structure extended by the hydrogen-bond between adjacent chains. (d) The two-dimensional layered structure.

For 2 and 3, they are also isostructural frameworks. Here, only structure of 2 is described in detail. Single crystal X–ray analysis reveals that compound 2 crystallizes in the monoclinic crystal system with P21/c space group. As shown in Figure 2a, each asymmetric unit of 2 contains one Dy3+ metal center, one HL3− ligand and two coordinated water molecule. Each dysprosium ion is coordinated with nine oxygen atoms in a distorted capped square antiprism configuration: four oxygen atoms from two bidentate carboxyl groups of two different HL3− ligands, three oxygen atoms from three monodentate carboxyl groups of three HL3− ligands and two oxygen atoms from two different coordinated water molecules. In 2, the carboxylate groups of HL3- ligand adopt three distinct bonding modes: tridentate, monodentate coordination and bridging chelating to coordinate with the center Dy3+ ions (FigureS2). As to HL3− in 2, the dihedral angle is 112.10° of two phenyl rings, leaving a carboxyl group protonated and all coordinated with Dy3+ ions. Two adjacent dysprosium atoms are interconnected with each other by two different O7C, O7D atoms from two HL3- ligands to generate a dinuclear secondary building unit (SBU) with a Dy···Dy separation of 4.1122(1)Å. These SBUs are further bridged through O1, O2 atoms of 3'-carboxyl groups from another two HL3- ligands into a 1D chain along the c direction (Figure 2b). The HL3- ligands connect the adjacent layers through the carboxyl oxygen atom to form a two-dimensional layered structure along b direction (Figure 2c) and then further connected by the HL3ligands to construct a 3D framework view along c direction. There are two different channels in the frame structure of 2 (Figure 2d).

Figure 2. (a) Coordination environment of Dy3+ cations in 2. (b) 1D chain constructed by Dy2 SBUs along c direction. (c) The 2D network connected by the HL3- ligands along c direction. (d) Three-dimensional framework of 2.

Powder X-ray Diffraction and Thermal Analyses. X–ray powder diffraction (PXRD) patterns for compounds 1−6 are consistent with that simulated from single crystal data, showing a high phase purity of the synthesized samples (Figure S4). The thermal stabilities of 1–6 were also tested using


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thermogravimetric analysis (TGA) (Figure S5). The TGA curves of the samples show that componds 1, 4, 5 and 6 have similar weight loss processes. The weight loss of 11.60% (calcd 12.54%) for 1, 13.90% (calcd 12.36%) for 4, 12.60% (calcd 12.32%) for 5 and 15.00% (calcd 12.24%) for 6 can be regarded as the loss of three coordinated water and one guest water between 92 and 300 °C. The major framework of them can stay stable up to about 420 °C and then they begin to collapse with the loss of organic ligands. For 2 and 3, the first major weight loss of 15.50% (calcd 15.04%) and 14.92% (calcd 15.11%) from 29 to 144°C, corresponding to two coordinated water per formula unit, the major framework can keep stable to 350°C and then they begin to collapse with the loss of organic ligands. The TGA curves of the samples show that compounds 2 and 3 have similar weight loss processes. Photoluminescent Properties. Luminescent spectra for all complexes 1, 3, 4 and 6 were conducted (Figure 3 and Figure 4). The emission spectrum of 1 reveals four peaks at 488, 542, 583 and 618nm corresponding to the 5D4→7FJ (J = 6–3) transitions of Tb3+ ion, respectively. The emission spectra indicate that complex 1 displays an intense green light emission at 542 nm under excitation at 335 nm. Complex 1 also shows a single-exponential function luminescent decay with the lifetime of 883.35 µs (Figure S6 and Table S3), and the absolute quantum yields was found to be 29.39% at room temperature, which is comparative to the analogous complexes in the previously reported Tb-MOFs.15– 16,28

However, no characteristic emission peak of the corresponding Dy3+ ion was observed in complex

2, this may be due to its poor ligand-induced sensitization effect.12 Meanwhile, NIR luminescent spectra of complex 3 excited at 350 nm reveals three bands at 984, 1477 and 1180 nm, respectively, which can be attributed to the 5F5→5I7, 5F5→5I6 and 5I6→5I8 transitions of Ho3+ ion (Figure 4a). Complex 4 exhibits a typical long wavelength emission of Er3+ ion around 1547nm (Figure 4b) under excitation at 350 nm, which originates from the 4I13/2→ 4I15/2 transition. The Yb3+ ions emission signal was observed with an apparent maximum at 993 nm for complex 6 under excitation at 350 nm, and it is attributed to the 2F5/2→2F7/2 transition (Figure 4c). The emission peak is split into two components (979 and 993 nm), which can be attributed to the splitting of the emitting levels as a consequence of ligand field effects.40–41 As one of the carboxyl groups in H4 L ligand is uncoordinated, we investigate their sensing ability for metal cations and small organic molecules. The results of fluorescence sensing measurements show that 1 has a good selectively sensing for iron ion and nitromethane.

Figure 3 PL excitation (a) and emissions (b) spectra of 1 in solid state at room temperature.



Figure 4 NIR emission spectra of (a) 3 (b) 4 and (c) 6 at room temperature excited at 350 nm.

To examine the potential of 1 in sensing metal ions, equal volumes of different aqueous solutions containing 1×10−2 M of M(NO3)x (M = K+, Ca2+, Cu2+, Fe3+, Pb2+, Ag+, Mg2+, Cr3+, Zn2+, Co2+, Ni2+, Cd2+) were added in the suspension of crystals. Obviously, only Fe3+ ions show an excellent quenching effect on the luminescence of complex 1 (Figure 5a). Furthermore, the process of detection is accompanied by visible changes in color. As we can see from Figure 5b, under the irradiation of 365nm, the green solution significantly changed a lot after the addition of iron ions, which makes its simple to distinguish by the naked eye. Figure 5c shows the photo-luminescence intensity decreased to 1.69 % for 1.

Figure 5. (a) Photoluminescence spectra of 1 introduced into various cations. (b) Photographs showing color changes after adding metal ions under 365 nm ultraviolet light. (c) Luminescence intensities after introduced into various cations (5D4→7F5 transition intensity of 1).

To further understand the fluorescence response of complex 1 toward Fe3+ ions, the fluorescence spectra upon the addition of 1×10−2M Fe3+ to 1 suspension of aqueous solutions were collected (Figure 6a). The Stern−Volmer equation can be used to rationalize the quenching effect: I0/I =1+Ksv[M], where


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I0 is the original fluorescence intensity and I is the fluorescence intensity upon adding Fe3+, [M] is the molar concentration (mol/L) of Fe3+, and Ksv is the Stern−Volmer constant. As expected, the reduction of fluorescence intensity is related to the concentration of metal ions. The Stern−Volmer curves for Fe3+ ion are nearly linear at low concentrations (R2 = 0.9966) (Figure 6b). However, with the increase of concentration, the curves deviate from the linearity which can be explained by an energy-transfer process or self-absorption.42–46 A Ksv value of 2063 M−1 is calculated directly from the experimental database at low concentrations (insets of Figure 6b). The experimental results show complex 1 has relatively high Ksv value with Fe3+, which are comparable to those of known Ln−MOFs (Table S4).18,43,47–48 The low detection limit and the high quenching constant for Fe3+ make clear that complex 1 is an excellent sensor for the sensitive and selective detection of Fe3+.

Figure 6. (a) Photoluminescence spectra and (b) SV curve for 1 by gradual addition of 10−2 M Fe3+ ions in aqueous solution. The inset demonstrate the quenching linearity relationship at low concentrations of Fe3+ ion.

As a kind of novel sensing material, luminescent Ln–MOFs have been broadly used in the detection of small organic molecules in the environment via the markedly changes of luminescent signals. When excited at 335 nm, the characteristic bands of Tb3+ ions in aqueous solutions is obversed indicating that an efficient energy transfer from the sensitizer to the Tb3+. The finely grinded samples (3mg) of 1 was dispersed in aqueous solution, and then 30 µL of each of the different organic solvents were added to the suspension of crystal samples. Several common solvents, such as methanol (MeOH), ethanol (EtOH), acetonitrile (CH3CN), dichloromethane (CH2Cl2), trichloromethane (CHCl3), N,N-dimethylacetamide (DMA), N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), acetone, nitromethane (NM) and H2O, were selected to investigate. As shown in Figure 7, the suspensions of compound 1 still display the characteristic emission peaks of Tb3+ ions in various solvents, while the emission intensities of the mixture are largely dependent on the types of solvents. Delightedly, the fluorescence intensities of complex 1 almost fully quenched with the addition of nitromethane, which reveals an intense quenching effect (Figure 7a and Figure S7). The quenching efficiency reached 94 % for 1 (Figure 7b).



Figure 7. (a) Photoluminescence spectra of 1 suspension introduced into various pure solvents. (b) Luminescence intensities after introduced into various pure solvents. (5D4→7F5 transition intensity of 1).

The further investigation of the possible sensing mechanism of complex 1 for Fe3+ ions and NM has been carried out. The photoluminescent of Ln–MOFs chiefly arising from the “antenna effect” included the following three steps: the organic ligands around the lanthanide ions absorbed the light, then energy is transferred from organic ligands to lanthanide ions, the luminescence was produced from lanthanide ions eventually.49 The underlying mechanism of luminescence quenching by Fe3+ ion could be explained from the following aspects. First, the FT-IR spectrometer and PXRD patterns (Figure S8 and S9) of the products of 1 treated by H2O, Fe3+ and NM system reveal that the basic frameworks are unchanged, so the framework collapse could be eliminated. Second, the crystalline product of 1 is a neutral compound and it is not likely to capture Fe3+ by exchanging cation methods. Hence, the luminescent quenching should not be attributed to the cation exchange process. Third, the competitive energy absorption was another possible reason for the quenching phenomena. According to the literature, if the excitation spectrum of the fluorophore (donor) has a certain degree of overlap with the absorption band of the analyte (acceptor), there might be a competitive energy absorption progress between the HL3- ligand and the Fe3+ solution.50–51 As shown in Figure 3, the maximum excitation wavelength of 1 was observed at 335nm, which shows a partially spectral overlap to the absorption spectrum of Fe3+ aqueous solution in the range of about 335nm, but other M(NO3)X solutions show no absorption at that region exactly (Figure S10), as reported in the literature of our previous work.52 In addition, because the carboxyl group of HL3− ligand remains uncoordinated, we conjecture that the weak interaction between Fe3+ and the uncoordinated carboxyl group may lead to the energy migration and luminescence quenching.20,53 To confirm this speculation, X-ray photoelectron spectroscopy (XPS) measurement was carried out. The O1s peak from free carboxyl group oxygen atoms at 531.68 eV in 1 is shifted to 531.86 eV induced by addition of Fe3+ in 1, suggesting the weak interaction between Fe3+ cations and carboxyl group basic sites in 1@Fe3+ (Figure 8). In a word, energy absorption and weak interaction may have synergistic effect on the quenching of fluorescence intensity of 1. Although the quenching mechanism toward solvent molecules is still unclear, the luminescence response of nitromethane may be due to the effect of ligand-to-metal energy transformation (LMET), because the emission efficiency of LMET determine the luminescent intensity.54 Similarly, the range of excitation peaks of 1 shows an almost completely spectral overlap the absorption spectrum of nitromethane (Figure S11) as reported at 330 nm in the literature.55–56 The luminescence decrease of 1 after the addition of NM can be put down to the competition of absorption of the light energy and the electronic interaction between the NM and HL3- ligands. The NM screening the light adsorbed by HL3-


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ligands, which give rise to the attenuation of the energy transfer from HL3- ligands to lanthanide ions, the luminescent intensity of 1 was obviously weaken.19,28,31 In order to further evaluate the fluorescence quenching effect of these two analytes (Fe3+ and NM) towards 1, the solid-state fluorescence quenching study was performed on 1. As shown in Figure S12 and S13, the fluorescence intensity of 1 was greatly weakened and quenched about 87% and 96% after treated with Fe3+ for 30min and 12h whereas almost no change after nitromethane treatment. Meanwhile, the color of 1 after treated with Fe3+ solution changed to orange-red under UV light irradiation, which maybe due to the rapid coordination reaction between the uncoordinated carboxyl group in 1 and Fe3+. However, the color of 1 separated from nitromethane did not changed, this maybe due to the evaporation of the solvent molecule so that the fluorescence intensity of solid 1 remained substantially unchanged. This finding reveals the bifunctional sensing advantage of MOF-1 sensors.

Figure 8. (a) XPS spectra for complex 1 (blue) and 1@ Fe3+ (red); (b) O 1s XPS for complex 1 (blue) and 1@ Fe3+ (red).

Magnetic Properties. The temperature dependence of magnetic susceptibility of 2 and 3 were measured in the temperature range of 2 to 300 K with an applied magnetic field of 1000 Oe. The data obtained for complexes are represented in Figure 9. At room temperature χMT =14.39 cm

3

K mol

−1

(theoretical value of 14.17 cm3 K mol −1) for one isolated Dy3+ ion (6H15/2, S = 5/2, L= 5, g= 4/3) in 2. Upon cooling, χMT starts to slowly decrease to 50 K, and then sharply decreases to 9.22 cm3 K mol−1, which can likely be ascribed to the thermal depopulation of the Stark sublevels, and the possible antiferromagnetic intramolecular Dy-Dy interactions or dipole-dipole interactions between the molecules.57–59 Between 50 and 300 K, the inverse magnetic susceptibilities can be fitted to the Curie– Weiss law with Cm =14.66 cm3 mol−1 K and θ = -3.58 K (Figure 9a). The negative θ value further supports the existence of anti-ferromagnetic exchange interactions between the Dy3+ ions.

Figure 9. Magnetic properties of (a) 2 and (b) 3 in the form of χMT and χM -1 versus T plot. The red solid line represents the fitting results over the range of 50–300 K.



For 3, the measured χMT= 13.66 cm3 K mol−1 at 300 K (theoretical value of 14.07cm3 K mol 3+

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−1

) for

5

one isolated Ho ion ( I8, S= 2, L= 6, g= 5/4). Upon cooling, χMT starts to slowly decrease to 50 K, and then sharply decreases to 5.16 cm3 K mol−1, which can likely be ascribed to either antiferromagnetic interactions and/or depopulation of the Stark sublevels.60 Between 50 and 300K, the inverse magnetic susceptibilities can be fitted to the Curie–Weiss law with Cm= 14.31 cm3 mol−1 K and θ = -10.92 K (Figure 9b). The negative θ value further supports the existence of antiferromagnetic exchange interactions between the Ho3+ ions.

CONCLUSIONS Six novel lanthanide coordination polymers involving an asymmetric semi-rigid V–shaped multicarboxylate ligand with multiple coordination sites have been successfully synthesized and characterized. Luminescent property studies demonstrate that complex 1 exhibits characteristic emission bands of the Tb3+ ions. Complexes 3, 4 and 6 show characteristic Near Infra–Red (NIR) emission peaks of corresponding lanthanide ions in the near infrared region. Luminescent sensing experiment suggested that complex 1 can selectively and sensitively detect for Fe3+ ion and nitromethane, which suggest that Tb-MOF can be used as promising bifunctional luminescence sensor materials. Magnetic studies show antiferromagnetic exchange couplings between neighbouring Ln3+ ions also exist in 2 and 3. We expect that the work on the semirigid tetracarboxylic acids-based Ln– MOFs for detecting harmful metal ions and organic small molecules will facilitate the development of multifunctional PL materials for actual applications in the near future. ASSOCIATED CONTENT Supporting Information

The Supporting Information is available free of charge via the Internet at http://pubs.acs.org/.

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. [email protected] Notes

The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by grants from the Natural Science Foundation of China (Nos. 21761031, 21461023, 21761030 and 21361023) and Gansu Provincial Natural Science Foundation of China (No. 1606RJZA110).

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Table of Contents Use Only

Six Ln (III) Coordination Polymers with a Semirigid Tetracarboxylic Acid Ligand: Bifunctional Luminescence Sensing,

NIR-Luminescent

Emission

and

Magnetic

Properties Ke-Xia Shang,

†, ‡

Jing-Sun, † Dong-Cheng Hu, *, † Xiao-Qiang Yao, † Li-Hua

Zhi, † Chang-Dai Si, § and Jia-Cheng Liu*, †

Six LnIII coordination polymers with a semirigid tetracarboxylic acid have been successfully synthesized via hydrothermal reactions. The luminescent explorations suggested that complex 1 can be a luminescent sensors for sensing Fe3+ ion and nitromethane through fluorescent quenching at the same time. The possible sensing mechanism of complex 1 towards Fe3+ ions and NM have also been carried out in detail.



254x190mm (96 x 96 DPI)


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Six Ln (III) Coordination Polymers with a Semirigid Tetracarboxylic

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