Tunable Emission and Selective Luminescence Sensing in a Series of

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Tunable Emission and Selective Luminescence Sensing in a Series of Lanthanide Metal-Organic Frameworks with Uncoordinated Lewis Basic Triazolyl Sites Zhen-Jing Li, Xiu-Yuan Li, Yang-Tian Yan, Lei Hou, Wen-Yan Zhang, and Yao-Yu Wang Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b01453 • Publication Date (Web): 04 Jan 2018 Downloaded from http://pubs.acs.org on January 5, 2018

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Crystal Growth & Design

Tunable Emission and Selective Luminescence Sensing in a Series of Lanthanide Metal-Organic Frameworks with Uncoordinated Lewis Basic Triazolyl Sites Zhen-Jing Li, Xiu-Yuan Li, Yang-Tian Yan, Lei Hou, Wen-Yan Zhang and Yao-Yu Wang*

Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of the Ministry of Education, Shaanxi Key Laboratory of Physico-Inorganic Chemistry, College of Chemistry & Materials Science, Northwest University, Xi’an 710127, P. R. China. *To whom correspondence should be addressed. E-mail: [email protected]. (Yao-Yu Wang).

ABSTRACT:

Four

isostructural

lanthanide

metal-organic

frameworks

(Ln-MOFs)

{[Ln(L)1.5(H2O)]·4H2O}n (1-Ln) (Ln = Sm, Eu, Gd, and Tb ) have been successfully synthesized under solvothermal conditions with 2-(1H-1,2,4-triazol-1-yl) terephthalic acid (H2L) and Ln(NO3)3·nH2O (n = 0, 6). 1-Ln shows a binodal (3,8)-connected three-dimensional framework possessing a 1D pore channel decorated with uncoordinated Lewis basic triazolyl sites. The 1-Eu and 1-Tb exhibit bright red and green emissions with absolute quantum yields of 9.1% for 1-Eu and 53.3% for 1-Tb. The luminescence explorations demonstrated that 1-Tb exhibits high quenching efficiency and low detection limit for sensing Fe3+ and nitrobenzene. Meanwhile, the fluorescence intensity of the quenched 1-Tb samples was resumed after washing with ethanol, which showing a highly selective and recyclable luminescence sensing for Fe3+ and nitrobenzene. Importantly, by doping different concentrations of Eu3+ and Tb3+ ions, a series of dichromatic doped 1-EuxTb1-x MOFs were fabricated, showing an unusual fluent change of the emissions color from green, yellow, orange, orange-red, and red.

INTRODUCTION Lanthanide metal-organic frameworks (Ln-MOFs) have attracted tremendous interest for superior properties and gigantic prospective application in numerous domain, such as luminescence,1, molecular magnetism,3 gas storage/separation,4,

5

2

proton conductivity,6 and catalysis.7 It is well

known that trivalent lanthanide ions are promising candidates for constructing unique luminescent ACS Paragon Plus Environment

Crystal Growth & Design 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

materials owing to their high luminescence quantum yield, narrow and intense band emission, giant Stokes shift, and long luminescence lifetimes.8-11 Ln-MOFs bearing active recognized sites are generally considered as great luminescent probes and sensors for the recognition and sensing of cations, anions, small molecules, vapors, pH values and even temperature.12-15 In many reports, some Ln-MOFs have been made for the detection of Ba2+, Al3+, Pb2+, Hg2+, F- and so forth.16-20 Besides, Fe3+ plays a major role in biological and environmental systems due to its vital cell functions like haemoglobin formation. Haemoglobin excess or deficiency is so detrimental that it may cause damage to nucleic acids and proteins. In addition, chemical sensors for the detection of nitroaromatics explosive-like substances are of high importance concerning homeland security, environmental, and humanitarian implications. Among various nitroaromatics, nitrobenzene as a simple nitro-containing compound is the basic and the simplest constituent of explosives. Likewise, nitrobenzene is also a highly toxic and notorious environmental pollutant that can give rise to serious health problems. Therefore, it is an extremely urgent issue to explore an effective method for recyclable probe for the sensitive detection of nitrobenzene and Fe3+ by taking environmental and health issues into consideration.21-24 Additionally, combining different lanthanide ions into isostructural Ln-MOFs to construct mixed Ln-MOFs is an effective method to generate dual-emission under one excitation wavelength. Any desired color emission can be acquired by modifying the molar ratio of the starting reactants.25-28 As we all know, Laporte forbidden f-f electronic transitions of Ln3+ give rise to low molar absorption coefficients, which causes Ln3+ to have very weak emissions when directly excited.29, 30 Therefore, the selection of the organic ligand incorporating strong absorbing chromophores has a remarkable effect on constructing luminescent Ln-MOFs.31 The strategy of utilizing aromatic carboxylate ligands as chromophores has been widely applied to build Ln-MOFs with unique structure and excellent luminescent properties because the Ln3+ usually reveal high coordination number and oxophilicity.32-34 Nitrogen-containing linkers such as imidazolate, pyrazolate, triazolate and tetrazolate, often yield MOFs having the more stable structure and extra active sites due to the stronger metal-ligand bond and Lewis basic nitrogen-rich units.35-38 Along this line, a bifunctional organic ligand 2-(1H-1,2,4-triazol-1-yl) terephthalic acid (H2L), which contain both aromatic carboxylate and triazole moieties, has been designed for the following merit: on the one hand, the aromatic carboxylate and triazole ligands are two most selective candidates in building MOFs due to ACS Paragon Plus Environment

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their strong coordination abilities and versatile coordinated modes;39, 40 on the other hand, the H2L with aromatic rings can be regarded as antennas or sensitizers to increase light absorption and transfer energy to the Ln3+ efficiently through “antenna effect”.41 In this paper, the reaction of Ln3+ and H2L under solvothermal condition afforded four isostructural 3D Ln-MOFs, {[Ln(L)1.5(H2O)]·4H2O}n (1-Ln, Ln = Sm, Eu, Gd, and Tb), possessing a 1D pore channel decorated by exposed Lewis basic triazolyl sites. In addition, a class of novel bimetallic Eu/Tb doped MOFs bearing tunable emission were obtained by changing the ratios of different Ln3+ ions in MOFs. Moreover, the luminescence studies indicate that 1-Tb displays the quenching effect on Fe3+ and nitrobenzene, which may be used as a chemical sensor in detecting these substances. Their crystal structures, topology, luminescent properties and mechanism have been studied in depth.

Scheme 1 2-(1H-1,2,4-triazol-1-yl) terephthalic acid (H2L).

EXPERIMENTAL SECTION Materials and General Methods. All chemical reagents were commercially available and used as received without further purification. The infrared (IR) spectra were obtained through a BRUKER EQUINOX-55 FT-IR spectro-photometer together with KBr discs from 4000 to 400 cm−1. The elemental analyses for C, H, and N were performed with a Perkin-Elmer 2400C elemental analyzer. The thermogravimetric analyses (TGA) were carried out using the NETZSCH TG 209 equipment from 30 to 800 °C with a heating rate of 10 °C·min-1 under a nitrogen atmosphere. Powder X-ray diffraction (PXRD) data were recorded on a Bruker D8 ADVANCE X-ray powder diffractometer (Cu Kα, λ = 1.5418 Å) with 2θ (5-50º), in which the X-ray tube was operated at 40 kV and 40 mA. The photoluminescence spectra for 1-Ln were performed on an Edinburgh FLS920 fluorescence spectrometer. Quantum efficiency was measured using the integrating sphere on a FluoroMax-4 ACS Paragon Plus Environment


spectrophotometer. Inductively coupled plasma (ICP) spectroscopy was performed on an Agilent 725 ICP-OES spectrometer. UV-Vis spectroscopic studies were measured on a Hitachi U-3310 spectrometer. An Axis Ultra spectrometer was selected to measure X-ray photoelectron spectroscopy (XPS). Synthesis of {[Sm(L)1.5(H2O)]·4H2O}n (1-Sm). A mixture of Sm(NO3)3·6H2O (0.0444 g, 0.10 mmol), bpe (0.0091 g, 0.05 mmol) and H2L (0.0117 g, 0.05 mmol) in the mixed solution of H2O (2 mL), HNO3 (1:1, 1 drop) and DMA (2 mL) was placed into a glass vial (15 mL) and heated at 95 °C for 72 h, and then cooled to room temperature at a rate of 5 °C h-1. The yellow crystals of 1-Sm were obtained in 71% yield. Anal Calcd for C30H27N9O18Sm2 (%): C 32.69, H 2.47, N 11.44; found: C 32.48, H 2.51, N 11.56. IR (KBr, cm-1; Figure S6): 3489(s), 3393(w), 3134(m), 1621(s), 1517(s), 1451(s), 1400(s), 1289(s), 1200(s), 1141(m), 1111(s), 1045(s), 971(m), 890(m), 838(m), 772(s), 654(s), 565(s), 513(m). Synthesis of {[Eu(L)1.5(H2O)]·4H2O}n (1-Eu). A mixture of Eu(NO3)3·6H2O (0.0446 g, 0.10 mmol) and H2L (0.0117 g, 0.05 mmol) in the mixed solution of H2O (2 mL) and DMA (2 mL) was placed into a glass vial (15 mL) and heated at 95 °C for 72 h, and then cooled to room temperature at a rate of 5 °C h-1. The colorless crystals of 1-Eu were obtained in 75% yield. Anal Calcd for C30H27N9O18Eu2 (%): C 32.59, H 2.46, N 11.40; found: C 32.45, H 2.65, N 11.39. IR (KBr, cm-1; Figure S6): 3401(s), 3142(s), 1614(s), 1547(s), 1517(s), 1451(s), 1407(s), 1274(m), 1200(m), 1141(m), 1111(m), 1045(m), 971(w), 846(w), 779(m), 654(m), 566(w), 513(w). Synthesis of {[Gd(L)1.5(H2O)]·4H2O}n (1-Gd). The procedure was the same as that for 1-Sm, except that Sm(NO3)3·6H2O was replaced by Gd(NO3)3·6H2O (0.0451 g, 0.10 mmol). The yellow crystals of 1-Gd were obtained. Yield: ca. 78%. Anal Calcd for C30H27N9O18Gd2 (%): C 32.28, H 2.44, N 11.29; found: C 32.46, H 2.60, N 11.36. IR (KBr, cm-1; Figure S6): 3489(s), 3393(w), 3134(m), 1621(s), 1517(s), 1451(s), 1400(s), 1289(s), 1200(s), 1141(m), 1111(s), 1045(s), 971(m), 890(m), 838(m), 772(s), 654(s), 565(s), 513(m). Synthesis of {[Tb(L)1.5(H2O)]·4H2O}n (1-Tb). The procedure was the same as that for 1-Eu, except that Eu(NO3)3·6H2O was replaced by Tb(NO3)3 (0.0345 g, 0.10 mmol). The colorless crystals of 1-Tb were obtained. Yield: ca. 76%. Anal Calcd for C30H27N9O18Tb2 (%): C 32.19, H 2.43, N 11.26; found: C 32.10, H 2.59, N 11.34. IR (KBr, cm-1; Figure S6): 3408(s), 3134(s), 1614(s), 1547(s), 1517(s),

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1451(s), 1400(s), 1274(m), 1200(m), 1141(m), 1111(m), 1045(m), 971(w), 846(m), 779(m), 654(m), 566(w), 513(w). Synthesis of 1-EuxTb1-x. The procedure was the same as that for 1-Tb, as a typical doping experiment, the total molar amounts of Eu3+ and Tb3+ were kept the same as that for 1-Tb, and the doped Eu3+ molar amounts equal to 10-90% with respect to the total molar amounts. X-ray Data Collection and Structure Determination. The single-crystal X-ray diffraction measurements were carried out on a Bruker SMART APEXII CCD diffractometer to get the crystal data at 296(2) K using ω rotation scans with widths of 0.3º and Mo Kα radiation (λ = 0.71073 Å). The structures were solved by the direct methods and refined by full-matrix least-squares refinements based on F2 with the Olex2 program.42 Anisotropic thermal parameters were applied to non-hydrogen atoms, and all hydrogen atoms from the organic ligands were calculated and added at ideal positions. The disordered solvent molecules in structures cannot be well identified, so the SQUEEZE routine of PLATON program was adopted in structural refinement.43 The final formulae of 1-Ln were determined by single-crystal structures, elemental analysis results and TGA. Crystal data and structure refinements for 1-Ln and selected bond distances and angles are listed in Tables 1 and S1, respectively. Table 1 Crystal data and Structure Refinement for 1-Ln. Complex

1-Sm

1-Eu

1-Gd

1-Tb

formula

C30H19N9O14Sm2

C30H19N9O14Eu2

C30H19N9O14Gd2

C30H19N9O14Tb2

Mr

1030.24

1033.46

1044.04

1047.38

crystal system

triclinic

triclinic

triclinic

triclinic

space group

P1

P1

P1

P1

a (Å)

9.904(5)

9.9121(18)

9.8900(12)

9.8971(15)

b (Å)

11.002(5)

11.094(2)

11.0753(14)

11.0720(17)

c (Å)

11.591(6)

11.510(3)

11.5069(14)

11.4978(18)

α (deg)

113.002(7)

107.809(4)

111.912(2)

111.771(2)

β (deg)

102.412(8)

107.996(4)

103.593(2)

103.662(2)

γ (deg)

108.326(9)

109.169(3)

109.103(2)

109.103(2)

1017.8(9)

1013.54

1009.1(2)

1009.7(3)

1

1

1

1

ρ (g cm )

1.681

1.693

1.718

1.723

F (000)

498

500

502

504

0.0314

0.0227

0.0174

0.0162

1.089

1.103

1.164

1.169

3

V (Å ) Z −3

Rint 2

GOF on F



R1ɑ [I > 2σ(I)] b

wR2 (all data) ɑ

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0.0508

0.0403

0.0420

0.0422

0.1449

0.1191

0.1327

0.1958

R1 =∑(|F0| - |Fc|) / ∑|F0|.bwR2 = [Σw(F02 - Fc2)2 / Σw(F02)2]1/2.

RESULTS AND DISCUSSION Crystal Structure of {[Ln(L)1.5(H2O)]·4H2O}n (1-Ln, Ln = Sm, Eu, Gd, and Tb). Single crystal X-ray analysis revealed that the four 1-Ln MOFs are isostructural with the same triclinic P1ത space group and show a dinuclear-based 3D networks. Herein, 1-Tb is selected as a representative example to describe the structures in detail. The asymmetric unit of 1-Tb contains one Tb3+ ion, one and a half fully deprotonated L2- ligands, one coordinated water molecule, and four lattice water molecules (Figure 1a). Each nine-coordinated Tb3+ ion is located in a distorted tricapped trigonal prismatic geometry (Figure S1), surrounded by seven carboxylate oxygen atoms from five L2- ligands, one nitrogen atom from one L2- ligand, and one oxygen atom from water molecule. L2- in 1-Tb is fully deprotonated and adopts two kinds of coordination modes. I-L2- (μ4-η1:η1:η1:η2:η1; Figure 2a) connects four Tb3+ ions through one N atom from the triazolyl group, two O atoms from a μ1-η1:η1 carboxylic group and other two O atoms from a μ2-η2:η1 carboxylic group, while II-L2(μ4-η1:η1:η1:η1; Figure 2b) links four Tb3+ ions through four O atoms from two μ2-η1:η1 carboxylate group. Meanwhile, the neighboring Tb3+ ions are bridged by four carboxylic groups, generating a dinuclear terbium cluster [Tb2(COO)6N2] (Figure S2), namely, an edge-sharing polyhedron (Figure 1b). In this structure, the neighboring dinuclear units are connected by I-L2-, forming a 2D layer (Figure 1c). Adjacent layers are further associated together through II-L2-, resulting in a 3D framework possessing a 1D pore channel along the a axes. (Figure 1d). Notably, this channel is decorated with exposed Lewis basic triazolyl sites, which can sense small organic molecules and metal ions as a potential active site.


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Figure 1. (a) Coordination environment of Tb3+ ions in 1-Tb (symmetry codes: #1 = - x - 1, - y, - z 1; #2 = - x - 1, - y, - z; #3 = x + 1, y, z; #5 = - x - 1, - y - 1, - z - 1; (b) A edge-sharing polyhedron view of [Tb2(COO)6N2]; (c) 2D layer structure; (d) 3D framework.

Figure 2. Coordination modes of L2- in 1-Tb. Topologically, I-L2- can be considered as a 3-connected node (Figure 3a), II-L2- serves as a linker (Figure 3b), and each dinuclear metal cluster is regarded as an 8-connected node (Figure 3c). Thus 1-Tb possesses a binodal (3,8)-connected net with the point symbol of (43)2(46·618·84) calculated by TOPOS (Figure 3d).44 Comparing with the reported (3,8)-connected nets, the topology of 1-Tb belongs to tfz-d type net.45-47



Figure 3. (a, b and c) Ball-and-stick and schematic representations of 3-connected node (red), linker (green) and 8-connected node (green), respectively. (d) Schematic representation of a binodal (3,8)-connected tfz-d net of 1-Tb.

The PXRD and TGA of 1-Ln. The experimental PXRD patterns of 1-Ln match well with those simulated, indicating the phase purities of samples (Figure S3). Meanwhile, PXRD patterns of all bimetallic doped samples were found very similar to the simulated pattern of 1-Tb, demonstrating that they are isostructural with 1-Tb (Figure S4). 1-Ln show similar thermal behavior and undergo three steps of weight loss (Figure S5). The first weight loss corresponds to four lattice water molecules (calcd. 6.53%, obsed. 6.31% for 1-Sm; calcd. 6.51%, obsed. 6.16% for 1-Eu; calcd. 6.45%, obsed. 5.92% for 1-Gd; calcd. 6.43%, obsed. 6.15% for 1-Tb) before 115 °C. The second weight loss is attributable to one coordinated water molecule (calcd. 1.63%, obsed. 1.38% for 1-Sm; calcd. 1.63%, obsed. 1.29% for 1-Eu; calcd. 1.61%, obsed. 1.51% for 1-Gd; calcd. 1.61%, obsed. 1.51% for 1-Tb) at 115-190 °C. The further heating leads to the structural collapse.

Luminescence properties. The lanthanide MOFs usually show excellent luminescent properties. Hence, the solid-state luminescent properties of 1-Ln were examined at room temperature. The free H2L ligand displays the maximum emission peak at 455 nm (λex = 380 nm), corresponding to intraligand π* → π charge transfer (Figure S7). 1-Gd presents a broad emission centered at 515 nm (λex = 365 nm) (Figure 4a), which can also be assigned to the intraligand π* → π transition of L2- ligand. Compared with the emission spectra of ligand, the emission band of 1-Gd is 60 nm red-shifted (Figure S7), indicating that the ligand


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coordinated with the metal ion influence the HOMO and LUMO levels of the ligand and change the transition energy. The CIE color coordinate is (0.28, 0.45) for 1-Gd. The 1-Gd exhibits the single-exponential decay with the lifetime of 3.43 ns (Figure S8a). The doping bimetallic or trimetallic Ln3+ ions approach has been widely applied to white-light emitting, but only several single components of white light emitting have been reported to date.48-54 1-Sm excited at 368 nm displays a broad band of 420-480 nm stem from the π* → π transition of L2ligands, and narrow peaks at 563, 596, 644 and 704 nm corresponding to the characteristic transitions of 4G5/2→6HJ (J = 5/2, 7/2, 9/2, 11/2) of Sm3+ ions (Figure 4b). The CIE color coordinates are (0.32, 0.31) for 1-Sm, closely with the ideal coordinate for pure white light of (0.33, 0.33) and 1-Sm can be deemed to be a single component white light material. The 1-Sm lifetimes are measured to be 2.08 and 13.76 µs, exhibiting the double-exponential decay (Figure S8b). Under the excitation of 396 nm, 1-Eu emits the characteristic bright red color of Eu3+ with the CIE color coordinate of (0.66, 0.34), indicating it is a good candidate as a red fluorescent material. The emission bands at 591, 619, 651 and 695 nm are assigned to 5D0→7FJ (J = 1, 2, 3, 4) transitions (Figure 4c). The electric dipole transitions 5D0→7F2 dominates the red color emission light. When excited at 352 nm, the emission spectrum of 1-Tb displays narrow bands at 491, 547, 583 and 623 nm, which correspond to the characteristic transitions of 5D4→7FJ (J = 6, 5, 4, 3) (Figure 4d). Among these characteristic peaks of the Tb3+ ion, the green luminescence peak at 547 nm is the most significant, which makes 1-Tb becomes a promising green emitting phosphors with the CIE color coordinate of (0.32, 0.51). In the emission spectra of 1-Eu and 1-Tb, the ligand-centered emission is completely quenched, denoting L2- sensitizes Eu3+ or Tb3+ ions via the “antenna effect”. 1-Eu and 1-Tb display the single-exponential luminescent decay with the lifetimes of 0.57 and 1.05 ms (Figure S8c and S8d), respectively, and the corresponding absolute quantum yields were found to be 9.1% and 53.3%.


Crystal Growth & Design 1 2 3 4(c) 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 4. Solid state excitation spectra of 1-Gd (a), 1-Sm (b), 1-Eu (c), 1-Tb (d) at room temperature. The inset shows the corresponding luminescence picture under UV-light irradiation at 365 nm.

Energy Transfer. In order to elucidate the energy transfer process of the Ln-MOFs, the energy levels of the relevant electronic states of H2L have been measured. The 1ππ* of H2L was evaluated by referring to the wavelength of the UV-vis absorbance edge (300 nm ≈ 33333 cm-1) (Figure S9). The 3

ππ* of H2L was calculated by phosphorescence spectrum of 1-Gd in the solid state at 77 K (Figure

S10).55 As the lowest excited state of the Gd3+ 6P7/2 is too high to accept energy from the 3ππ* of ligand, the shortest-wavelength phosphorescent band (430 nm ≈ 23256 cm-1) of the 1-Gd is the 3ππ* of the ligand.56 The energy gap ∆E (1ππ* - 3ππ*) is 10077 cm-1, which far outweighs the Reinhoudt’s empirical rule of threshold value (5000 cm-1) for an efficient intersystem crossing process.57 So the H2L can be considered as a suitable sensitizer for luminescence of Ln3+ ions. The experimentally observed 3ππ* of H2L (23256 cm-1) is both above to 5D0 level of Eu3+ (17300 cm-1) and 5D4 level of Tb3+ (20500 cm-1), but the ∆E (3ππ* - 5D0) for 1-Eu and ∆E (3ππ* - 5D4) for


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1-Tb is 5956 cm-1 and 2756 cm-1, respectively. On the basis of Latva’s empirical rule, occurring an optimal ligand-to-metal energy transfer process requires that the ∆E (3ππ* - 5DJ) is 2500-4000 cm-1 for Eu3+ and 2500 - 4500 cm-1 for Tb3+, therefore the energy transfer process in 1-Tb is more effective than 1-Eu.58 In addition, the higher absolute quantum yield of 1-Tb (53.3%) relative to 1-Eu (9.1%) further confirms the result.

Tuning of Luminescent Color for Bimetallic Doped Eu/Tb-MOFs. 1-Eu and 1-Tb are isostructural, which provides possibility to synthesize heterometallic compounds. A series of Eu/Tb co-doped MOFs were obtained successfully by adjusting different doped molar ratios of Eu3+ and Tb3+, and the molar ratios of Eu3+ and Tb3+ ions were determined by means of ICP spectroscopy (Table S2). A fluent change of the emission color among green, yellow, orange, orange-red, and red can be achieved when irradiated at 352 nm (Figure 5a). As for the heterometallic compounds, the emission spectrum displays dual emissions of Eu3+ and Tb3+ ions. With the molar ratios increasing from 1:9 to 9:1, the emission intensity of the Tb3+ ion at 5D4 → 7F5 (547 nm) decreases gradually, corresponding to the increase of the Eu3+ ion at 5D0 → 7F2 (619 nm). As shown in Figure 5c, the resultant Eu/Tb co-doped MOFs present wide visible colors of emissions from green, yellow, orange, orange-red, and red under the UV 365 nm lamp, indicating which can be systematically modulated through adjusting different contents of Tb3+ and Eu3+. In Figure 5b, points a-i represent CIE color coordinates for Eu/Tb co-doped MOFs change from (0.38, 0.56) to (0.62, 0.33). As for the Eu/Tb co-doped MOFs, the energy transfer efficiency (E) between the donor and the acceptor can be calculated from the lifetime of donor luminescence by equation:

E = 1-τda/τd in which τda and τd are the excited-state lifetimes of a donor in the presence and absence of an acceptor, respectively.53 The values of E raise with the increase of Eu3+ molar amounts (Figure 5d).



Figure 5. (a) Solid state emission spectra of doped 1-EuxTb1-x with different molar ratios when excited at 352 nm.; (b) the CIE chromaticity diagram for co-doped 1-EuxTb1-x; (c) the luminescence picture of 1-EuxTb1-x under UV-light irradiation at 365 nm; (d) the energy transfer efficiency from Tb3+ to Eu3+ in 1-EuxTb1-x.

Sensing of Metal Ions. Owing to the excellent luminescent properties as well as the exposed Lewis basic triazolyl sites, 1-Tb has attracted our interest to study its application in sensing metal ions. The as-synthesized samples (3 mg) were ground into powder and immersed in the individual ethanol solutions of M(NO3)n (3 mL, 0.1 mol L-1, M = Na+, K+, Mg2+, Cd2+, Zn2+, Co2+, Cu2+, Al3+, Li+, Ag+, Ni2+, Fe3+ and Pb2+, respectively) for 24h at room temperature to form Mn+@1-Tb (metal ion incorporated 1-Tb) solutions, and then treated by ultrasonic agitation for 30 minutes to form uniform dispersion suspensions before the luminescence study. As shown in Figure 6a and 6b, the Mn+@1-Tb solutions exhibit markedly different luminescence intensities, it was found that Co2+, Al3+ and Cu2+ cations slightly decreased the luminescent intensity of 1-Tb at 547 nm, while other metal ions (K+, Mg2+, Cd2+, Na+, Pb2+, Ni2+, Zn2+, Li+, Ag+) enhanced the luminescence to different extents. The most striking phenomenon is that Fe3+ ions have a very significant quenching effect on the ACS Paragon Plus Environment

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luminescence of 1-Tb. The obvious change of the luminescent intensities affected by Fe3+ relative to other metal ions implies the potential of 1-Tb for recognizing and sensing Fe3+ ions. The PXRD patterns of 1-Tb treated by various M(NO3)n ethanol solutions were measured, and the patterns showed that the basic framework of 1-Tb still remained unchangeable (Figure S11), indicating that luminescent quenching caused by Fe3+ ions was not attributed to collapse of the framework. The result indicates that 1-Tb has highly selective detection and specific recognition of Fe3+ ions in ethanol solutions. To further explore 1-Tb as a luminescent probe for Fe3+ ions, as shown in Figure 6c, a series of titration experiments were performed, in which the luminescence intensities of the sample are measured by the addition of Fe3+ (0.1 mol L-1) ions ethanol solution into the suspension of 1-Tb (dispersing 3 mg of 1-Tb sample into 3 mL of ethanol). The luminescence intensities decrease as the concentration of Fe3+ ions increase, those of 1-Tb are almost completely quenched at the concentration of 15.61 mM for Fe3+. The relationship between I0/I and the concentration of Fe3+ ion does not match with the Stern-Volmer equation, indicating the coexistence of the dynamic and static quenching processes, which can be well fitted as the following formula: I0/I = 0.601 exp([Fe3+]/3.452) - 0.639 (I0 and I are the luminescent intensity of 1-Tb in the absence and presence of Fe3+, respectively; [Fe3+] = Fe3+ concentration (mM); R2 = 0.970) (Figure 6d), and a good linear correlation is observed for the plot of I0/I-1 vs [Fe3+] in the range of 0-0.58 mM (Figure S12a). The detection limit determined following the 3σ/Ksv (σ is the standard deviations for ten repeated luminescent measurements of blank solutions, Ksv represents quenching rate constant) can reach as low as 4.1 × 10−6 M.59 Comparable with the reported values in some Ln-MOFs,60,

61

the low

detection limit implies the promise of 1-Tb for the sensitive and selective detection of Fe3+ ions. Meanwhile, multiple cycles of Fe3+ sensing experiments were performed and showed that the material could greatly regain its intensity after washing by ethanol several times (Figure S17a). The result reveals that 1-Tb could be employed as a fluorescence sensor for detecting Fe3+ with high sensitivity and recyclability. The mechanism of the luminescence quenching triggered by Fe3+ ions can be explained as an electron transfer process from the donor to the acceptor. The ligand can act as electrons donors, while Fe3+ ions adopt unsaturated electron configuration, which makes the ions retain empty orbits and become the ideal electrons acceptors. When Fe3+ ions diffused into the 1D pore channel of 1-Tb and formed contacts with the uncoordinated triazolate moiety, electrons of ligand to transfer from the ACS Paragon Plus Environment


donor to the acceptor, thus the “antenna effect” of ligand has been restrained to a certain degree, leading to luminescence quenching.12, 62 To check the results, XPS was used to investigate 1-Tb and Fe3+@1-Tb. The N 1s peak from triazole at 396.1 and 397.6 eV in 1-Tb is shifted to 397.4 and 399.0 eV after the addition of Fe3+, indicating the weak interaction between uncoordinated N atoms and Fe3+ in the Fe3+@1-Tb, and the peak of 404.6 eV appears in Fe3+@1-Tb (Figure S13). A peak at 404.6 eV in the N 1s peak for Fe3+@1-Tb is attributed to NO3- counterions.63 Besides, luminescence quenching induced by Fe3+ ions also could result from the competitive adsorption between Fe3+ ions and 1-Tb because the UV-Vis adsorption spectrum of Fe(NO3)3 in ethanol shows the moderate overlap with the excitation spectrum of 1-Tb in ethanol solution (Figure S14).

Figure 6. (a) Luminescence spectra of 1-Tb immersed in ethanol with various metal ions (λex = 352nm); (b) Relative luminescence intensities at 547 nm of the Mn+@1-Tb solutions; (c) Luminescence spectra of 1-Tb@ethanol suspensions with Fe3+ concentration varying from 0 to 15.61 mM (excited at 352 nm); (d) The plot of I0/I vs [Fe3+].


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Sensing of Small Organic Molecules. Organic solvents are commonly used in commercial production. However, they have adverse impacts on human health and the environment. As a kind of novel sensing material, luminescent Ln-MOFs have been widely used in the detection of small organic molecules in the environment via the notable changes of luminescent signals caused by those pollutants. On the basis of the above consideration, further fluorescence sensing experiment was carried out to explore the effect of guest molecules on compounds. The finely ground sample of 1-Tb (3 mg) was immersed in 3 mL of different organic solvents. Several common organic solvents used in the texts are methanol (MeOH), ethanol (EtOH), tetrahydrofuran (THF), N,N-dimethylformamide (DMF), acetone, N,N-dimethylacetamide (DMA), dimethyl sulfoxide (DMSO), N-methyl pyrrolidone (NMP), acetonitrile (MeCN), 1,4-dioxane, dichloromethane (CH2Cl2), toluene (C6H5CH3), benzene (C6H6), chlorobenzene (C6H5Cl), cyclohexane, and nitrobenzene (NB). As shown in Figure 7a and 7b, the luminescence intensities are greatly dependent on different organic pure solvents, particularly for NB, which appears a significant quenching effect. The PXRD patterns collected for each 1-Tb@solvent are similar to that of 1-Tb, showing that the framework of 1-Tb is intact in all the solvents (Figure S15) and that luminescent quenching caused by NB is not attributed to collapse of the framework. Furthermore, the luminescence intensities are measured by the addition of NB (0.1 mol/L) into the suspension of 1-Tb (dispersing 3 mg of 1-Tb sample into 3 mL of ethanol) to research the quenching effect of NB to 1-Tb. As shown in Figure 7c, with the NB concentration increasing from 0 to 7.777 mM, the luminescence intensity of 1-Tb@ethanol suspension decreases distinctly. The relationship between I0/I and the concentration of NB obeys equation of I0/I = 0.012 exp([NB]/0.609) + 2.162 along with the coexistence of the dynamic and static quenching processes (I0 and I are the luminescent intensity of 1-Tb in the absence and presence of NB, respectively; [NB] = NB concentration (mM); R2= 0.991) (Figure 7d). Meanwhile, a good linear correlation is observed for the plot of I0/I-1 vs [NB] in the range of 0-0.57 mM and the detection limit of 4.0 × 10−6 M was also obtained based on 3σ/Ksv (Figure S12b).59 Compared with the reported Ln-MOFs,59,

64-66

this

detection limit is significantly low, implying that 1-Tb is very promising in the sensitive and selective detection for NB. Additionally, the recyclable performance of 1-Tb was also investigated. The samples of 1-Tb were added into NB to completely form 1-Tb@NB, and then 1-Tb@NB was simply washed with ethanol ACS Paragon Plus Environment


for several times. The results show that the fluorescence intensity of 1-Tb can be partially resumed (Figure S17b). In order to understand the luminescent quenching mechanism of 1-Tb toward NB, its UV−vis spectrum was determined. As shown in Figure S16, the UV−vis adsorption spectrum of NB shows the little overlap with the excitation spectrum of 1-Tb in ethanol solution, but a moderate overlap with absorption spectrum of H2L, so one of the reasons of luminescent quenching is competitive adsorption between NB and H2L.67 Moreover, NB with an electron-withdrawing substituent -NO2 group and its LUMO locates at lower energies, so it can become an excellent electron acceptor,68 which might drive the electrons to transfer from the ligand to NB and weaken the “antenna effect”, causing the luminescence quenching.

Figure 7. (a) Luminescence spectra of 1-Tb with various organic pure solvents (λex = 352 nm); (b) Relative luminescence intensities at 547 nm of the 1-Tb@Solvent solutions; (c) Luminescence spectra of 1-Tb@ethanol suspensions with nitrobenzene concentration varying from 0 to 7.777 mM (excited at 352 nm); (d) The plot of I0/I vs [NB].


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CONCLUSION In summary, we have successfully synthesized four novel Ln-MOFs using a bifunctional 2-(1H-1,2,4-triazol-1-yl) terephthalic acid ligand with Ln3+ ions under solvothermal conditions. All complexes show similar 3D structure with a 1D pore channel decorated by uncoordinated Lewis basic triazolyl units. Their luminescence properties and mechanism have been studied. 1-Tb shows the quenching effect on nitrobenzene and Fe3+ ions, and the solid products can keep its original network and be repeated use for the sensing experiments, which may be used as a chemical sensor in sensing nitrobenzene and Fe3+. Moreover, in view of similar structures of 1-Ln, a series of bimetallic doped Eu/Tb-MOFs have been successfully fabricated by tuning different doped molar ratios of Eu3+ and Tb3+. The doped Eu/Tb-MOFs exhibit an uncommon fluent change of the emissions color from green, yellow, orange, orange-red, and red. This contribution may spark a broad spectrum of interest in the fabrication of interesting luminescence Ln-MOFs by using a bifunctional ligand which combines the merit of aromatic carboxylate and triazole group.

ASSOCIATED CONTENT Supporting Information. Additional figures, TGA, PXRD, and selected bond length/angle table. This material is available free of charge via the Internet at http://pubs.acs.org. Crystallographic information files for 1-Sm, 1-Eu, 1-Gd and 1-Tb (CCDC 1580394-1580397) AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. (Yao-Yu Wang) Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work is supported by NSF of China (21531007, 21371142, 51374174 and 21471124).

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For Table of Contents Use Only

Title: Tunable Emission and Selective Luminescence Sensing in a Series of Lanthanide Metal-Organic Frameworks with Uncoordinated Lewis Basic Triazolyl Sites

Authors: Zhen-Jing Li, Xiu-Yuan Li, Yang-Tian Yan, Lei Hou, Wen-Yan Zhang, Ping Liu and Yao-Yu Wang* A series of 3D Ln-MOFs, which possesses a 1D pore channel decorated by exposed Lewis basic triazolyl sites, have been successfully fabricated by using a bifunctional ligand containing aromatic carboxylate and triazole moieties. And their luminescent properties and mechanism have been investigated in depth.


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Tunable Emission and Selective Luminescence Sensing in a Series of

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