White-light Emitting Materials and Highly Sensitive Detection of Fe3+

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White-light Emitting Materials and Highly Sensitive Detection of Fe3+ and Polychlorinated Benzenes Based on Ln-MOFs Jing-Jing Huang, Jie-Hui Yu, Fu-Quan Bai, and Ji-Qing Xu Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00773 • Publication Date (Web): 07 Aug 2018 Downloaded from http://pubs.acs.org on August 10, 2018

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Crystal Growth & Design

White-light Emitting Materials and Highly Sensitive Detection of Fe3+ and Polychlorinated Benzenes Based on Ln-MOFs Jing-Jing Huanga, Jie-Hui Yu*,a, Fu-Quan Bai*,b, Ji-Qing Xua a College

of Chemistry, State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin University, Changchun,

Jilin, 130012, China b

Institute of Theoretical Chemistry, Laboratory of Theoretical and Computational Chemistry, Jilin University, Changchun,

Jilin, 130023, China Email: [email protected]; [email protected]

Abstract We employed a rigid bridging-type tetracarboxylic acid molecule, namely 5,5'-(pyridine-3,5-diyl)diisophthalic acid (H4L; containing two isophthalic

acid

moieties

and

one

pyridine

spacer),

to

construct

three

new

isomorphic

Ln-MOF

materials

[(CH3)2NH2]2[Ln2(L)2(H2O)2]⋅2DMF⋅2H2O (Ln3+ = Tb3+ 1, Eu3+ 2, Gd3+ 3; DMF = N,N-dimethylformamide). Since the pyridyl N atom does not coordinate to Ln3+, the larger pores are observed in the 3-D networks of 1-3. Based on their photoluminescence properties, the white-light emitting materials 4-7 with longer fluorescence lifetime (ms grade) and higher quantum yield (e.g. 48.29% for 5) are fabricated. We also find that the title Ln-MOF materials not only can selectively sense polychlorinated benzenes, but also can highly sensitively detect the Fe3+ ion (Ksv = 7.58 × 104 M-1). This should be associated with these three structural factors in 1-3: the larger

π-conjugated structure of L4-; the larger porous structures in 1-3; the existence of uncoordinated N atom on L4-. The test paper experiments reveal that 1 can be made into the test paper (a naked eye probe) to quickly detect the analytes.

Introduction Considerable attention has been paid to the design and construction of novel polycarboxylate-based lanthanide metal-organic framework (Ln-MOF) materials due to their structural diversity,1-5 and the potential applications in the manufacture of white-light emitting materials,6-9 and the sensing on environmental pollutants as harmful metal ions10-14 and toxic organic molecules.15-19 The polycarboxylate ligands not only control the structures of the as-synthesized Ln-MOF materials, but also affect their functional properties. Since the forbidden 4f-4f transition prevents the direct excitation of the Ln3+ luminescence, the selected polycarboxylate ligand is first able to sensitized the Ln3+ luminescence through a energy transfer from the π-conjugated organic chromophore to the Ln3+ center, namely the so called “antenna effect”.20-21 The most important is that the more energy is transferred, thus the fabricated white-light material may possess the better luminescence property as the higher quantum yield.22-24 Based on this, the polycarboxylic acidic molecule with a larger π-conjugated structure may be an ideal candidate for the ligand precursor,25-27 and the emission efficiency of the obtained white-light material may be significantly enhanced. On the other hand, the sensing of coordination polymer materials on organic molecules and metal ions is generally based on a luminescence quenching mechanism.28-32 Selectivity and sensitivity are determined by quenching efficiency of the analyte on luminescence intensity of the material. The selected polycarboxylate ligand should be able to effectively improve this quenching efficiency value. Therefore, the selected ligand should possess the following several structural features: (i) forming a porous network with the Ln3+ ion. The luminescence quchening should originate from the interaction between electronic donor and electronic acceptor. Here the polycarboxylate ligand serves as an electronic donor, since it has the rich electron. While the analyte acts as an electronic acceptor due to its electron deficient nature. The material with a porous structure has a larger surface, thus the introduced trace analyte can easily access the electronic donor; (ii) the exposed active site;33-36 Via this exposed active site, the analyte can be closely adsorbed on the surface of the electronic donor; (iii) the bigger π-conjugated rings. The molecule with the bigger π-conjugated rings can rapidly release a signal, once contacting the trace analyte. Based on the statement above, we design a rigid tetracarboxylic acid, namely 5,5'-(pyridine-3,5-diyl)diisophthalic acid (H4L). It is composed of two isophthalic acid moieties and one pyridine spacer, as shown in Scheme 1. First of all, it has a larger π-conjucted structure. Then all of the donor atoms (N, O) are located on the meta-positions, which is helpful to the formation of porous material. The most important is the existence of the N atom on H4L. If only the small pores are formed, the analyte can not enter the pores. However, due to 1

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the existence of this N atom, the H4L molecules can link the Ln3+ centers to form a network with the larger pores, based on the Hard-Soft-Acid-Base (HSAB) theory. In this theory, Ln3+ as a typical soft acid does not interact with the N atom (a typical hard base). Since this N atom is uncoordinated, it can serve as an active site, directly interacting with the introduced analyte. In this article, we employed this H4L molecule to construct three new isomorphic Ln-MOFs [(CH3)2NH2]2[Ln2(L)2(H2O)2]⋅2DMF⋅2H2O (Ln3+ = Tb3+ 1, Eu3+ 2, Gd3+ 3; DMF = N,N-dimethylformamide). Based on their photoluminescence properties, four white-light emitting materials 4-7 were obtained. The sensing ability of 1 on the polychlorinated benzenes and the metal ions are also investigated.

H4L Scheme 1 Molecular structure of H4L

Experimental Section Materials and physical measurement All of the materials and solvents were purchased commercially and used without further purification. IR (Infrared) spectrum was collected on a Perkin Elmer Spectrum 1 spectrophotometer in 4000-400 cm-1 area with a powder sample on a KBr plate. Powder XRD (X-ray diffraction) patterns were recorded on a Rigaku/max-2550 diffractometer using Cu-Kα radiation (λ = 1.5418Å). CHN analysis was carried out on a Pekin-Elmer 2400LS II elemental analyzer. TG (Thermogravimetric) behavior was investigated on a Pekin-Elmer TGA-7 instrument with a heating rate of 10 oC min-1 in air. Fluorescence spectra were recorded on a LS 55 florescence/phosphorescence spectrophotometer at room temperature. Commission International de l’Eclairage (CIE) color coordinates were calculated on the basis of international CIE standards. Fluorescence lifetime and quantum yield were measured on an Edinburgh Instrument FLS920 steady-state transient fluorescence spectrometer at room temperature. The integral sphere method is used to integrate the spectrum, and the quantum yield is the ratio of the photons emitted to photons absorbed.

Synthesis of coordination polymers [(CH3)2NH2]2[Tb2(L)2(H2O)2]·2DMF·2H2O 1. The yellow columnar crystals of 1 were obtained from an easy solvothermal reaction of Tb(NO3)3·6H2O (23 mg, 0.05 mmol) with H4L (20 mg, 0.05 mmol) in a 6 mL DMF-H2O (1:1) solution (pH = 4 adjusted by dilute HNO3) at 160 oC for 3 days. Yield: ca. 65% based on Tb(III). Anal. Calcd for C52H56N6O22Tb2 1: C, 43.53; H, 3.93; N, 5.86. Found: C, 43.02; H, 3.62; N, 5.06%. IR (cm-1): 3422 (w), 1625 (s), 1560 (s), 1442 (s), 1372 (s), 1108 (w), 1022 (m), 936 (w), 893 (m), 785 (m), 731 (m), 662 (m).

[(CH3)2NH2]2[Eu2(L)2(H2O)2]·2DMF·2H2O 2. The yellow columnar crystals of 2 were obtained from an easy reaction to that of 1 except Eu(NO3)3·6H2O (22 mg, 0.05 mmol) in place of Tb(NO3)3·6H2O. Yield: ca. 62% based on Eu(III). Anal. Calcd for C52H56N6O22Eu2 2: C, 43.95; H, 3.97; N, 5.91. Found: C, 43.81; H, 3.65; N, 5.20%. IR (cm-1): 3425 (w), 1631 (s), 1560 (s), 1441 (s), 1370 (s), 1113 (w), 1027 (m), 932 (w), 889 (m), 785 (m), 733 (m), 655 (m).

[(CH3)2NH2]2[Gd2(L)2(H2O)2]·2DMF·2H2O 3. The yellow columnar crystals of 3 were obtained from an easy reaction to that of 1 except Gd(NO3)3·6H2O (23 mg, 0.05 mmol) in place of Tb(NO3)3·6H2O. Yield: ca. 68% based on Gd(III). Anal. Calcd for C52H56N6O22Gd2 3: C, 43.63; H, 3.94; N, 5.87. Found: C, 42.98; H, 3.60; N, 5.06%. IR (cm-1): 3425 (w), 1631 (s), 1554 (s), 1441 (s), 1372 (s), 1113 (w), 1018 (m), 936 (w), 889 (m), 785 (m), 733 (m), 655 (m). 2


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X-ray crystallography The crystallographic data of 1-3 reveal that they are isomorphic with each other. But the crystal data for 3 does not pass the cif-checking examination due to the existence of a level A alert. Table 1 gives the crystal data of 1 and 2. The data was collected with Mo-Kα radiation (λ = 0.71073 Å) on a Rigaku R-AXIS RAPID IP diffractometer. With the SHELXTL program, the structure was solved using direct method.37 The non-hydrogen atoms were assigned anisotropic displacement parameters in the refinement. The hydrogen atoms on the benzene rings were treated using a riding model. The hydrogen atoms on the coordinated water molecules were not located. The structures were then refined on F2 using SHELXL-97.37 Based on the difference Fourier map, the lattice water molecules, DMF, and the [(CH3)2NH2]+ cations can be found. But they suffer from the severe disorder, and can not be modeled properly. So the diffuse electron densities resulting from these molecules are removed by PLATON/SQUEEZE to produce a set of solvent-free diffraction intensities.38 The guest molecules were indentified by the elemental analysis, TG analysis and the charge balance principle. This method has been extensively employed in the related reports. The CCDC numbers are 1561361 for 1 and 1831589 for 2, respectively.

Table 1 Crystal data of 1 and 2. 1

2

Formula

C52H56N6O22Tb2

C52H56N6O22Eu2

M

1434.91

1420.99

T(K)

293(2)

293(2)

Crystal system

triclinic

triclinic

Space group

P-1

P-1

a (Å)

9.7337(19)

9.7329(3)

b (Å)

15.767(3)

15.7974(5)

c (Å)

17.977(4)

17.7693(6)

α (o)

65.66(3)

114.295(10)

( o)

89.99(3)

91.08(10)

γ (o)

84.53(3)

95.362(10)

V (Å3)

2500.1(9)

2474.31(14)

Z

2

2

Dc (g cm-3)

1.542

1.539

µ (mm-1)

2.874

2.580

Reflections collected

19449

53008

Unique reflections

8567

12246

Rint

0.0448

0.0279

Gof

1.071

1.093

R1, I > 2σ(I)

0.0672

0.0581

wR2, all data

0.1964

0.1788

β

Results and discussion Synthetic analysis At 160 oC, the simple reactions between Ln3+ and H4L in DMF and H2O (pH = 4) created 1-3. In the reactions, DMF plays a key role: (i) controlling the formation of the larger pores. H4L has a potential to form a framework with the larger pores; (ii) stabilizing the skeleton structure. Without DMF in the pores, the 3-D skeletons for 1-3 are unstable; (iii) balancing the systematic charge through forming the [(CH3)2NH2]+ cations. In the reactions, the temperature also plays an important role. It directly affects the quality and the yield for the crystals. The lower temperature is unbeneficial to the decarbonylation of DMF, whereas the higher temperature is unfavorable to the 3



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encapsulation of DMF in the pores. In the reactions, pH for the reactive system is also critical. Whether pH ≥ 5 or pH ≤ 3, no target products were obtained. The weakly acidic environment maybe influences the protonation of (CH3)2NH (forming [(CH3)2NH2]+), while the strong acidic environment maybe affects the deprotonation of H4L.

Structural description

Fig. 1 Asymmetric unit (a), coordination environment around Tb1 (b), coordination mode of L4- (c), 2D layer (d), topology of 2D layer (e), projection plot of 3D network in bc plane (f), and topology of 3D network (g) in 1 (a: x, y+1, z; b: x-1, y+1, z; c: -x, -y+1, -z+1).

The crystal data indicate that 1 is a L4--extended 3-D Tb3+ coordination polymer. It crystallizes in the space group P-1, and the asymmetric unit is composed of two Tb3+ ions (Tb1, Tb2), two L molecules (L1, L2), and two coordinated water molecules (Ow1, Ow2) (see Fig. 1a). Since two Tb3+ ions possess the same coordination environments, and two L molecules adopt the same coordination modes, here only Tb1 and L1 as the representatives will be described. As shown in Fig. 1b, Tb1 with a distorted tricapped triangular prismatic geometry (Fig. S3) is coordinated by eight carboxylato O atoms (O2, O3c, O4c, O5b, O6b, O7a, O8a, O10), and one water molecule (Ow1). The Tb1-O distances span a wide range from 2.295(8) Å to 2.556(9) Å. L1 exhibits a η1:η1:η1:η1:η1:η1:η1:η1:μ5 coordination mode. Three carboxylate groups adopt a chelating mode, whereas the fourth one adopts a bidentate bridging mode (see Fig. 1c). Bridged by L, 1 self-assembles into a 3-D porous network. In order to better understand the structure of this 3-D porous network, the 5'-position carboxylate group on L is temporarily ignored, producing a new ligand L'. As displayed in Fig. 1d, the L' molecules connect the Tb3+ centers into a 2-D layer network, which is based on a dinuclear Tb2(COO)24+ unit. With L′ as the bridges, each dinuclear unit interacts with neighboring six dinuclear units. According to the topological viewpoint, this dinuclear unit can be regarded as a 6-connected metal node, while L' can be viewed as a 3-connected ligand node. So this 2-D layer network can be simplified into a (3,6)-connected net with a PbI2 topology (see Fig. 1e). Via the chelating role of the 5'-position carboxylate group, the adjacent 2-D layers mentioned above interlock to form the title 3-D network of 1. Fig. 1f is the projection of this 3-D network in the bc plane, in which two types of pores A and B are found. Since the pyridyl N atom is uncoordinated, pore A has a larger window (ca. 15 × 10 Å2). The window size for pore B is ca. 8.5 × 7.5 Å2. The guest molecules as DMF, H2O and [(CH3)2NH2]+ should occupy the space of these pores. Based on the elemental analysis and the TG analysis as well as the charge 4


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balance principle, the category and the number for these guest molecules can be confirmed.39-41 Fig. 1g shows the topological structure of the 3-D porous network of 1, which is a (4,8)-connected net.

The title three compounds 1-3 are new, and have been never reported. They are the first examples of H4L-based rear-earth metal coordination polymers. To date, only two H4L-based Cu(II) complexes have been reported.45-46 As expected, just the carboxylato O atoms coordinate to the metal centers. The pyridyl N atom is not involved in the coordination. Just because the pyridyl N atom does not coordinate to the Ln3+ centers, the larger pores appear in the 3-D network of 1. The most important is that the exposed N atoms in the framework may act as the potential functional sites for the fluorescence response of the heavy metal ions. In the structural description section, the 5'-position carboxyl group on L is ignored. If the 3-position or 5-position carboxyl group on L is neglected, the remaining three carboxyl groups also connect the Tb3+ centers to form a 2-D layer with a PbI2 topology (Fig. S4). In the projections of the (010) and (001) directions, the pores are also observed. The widow sizes are 14.5 × 10.5 Å2, 15 × 10.5 Å2 (see Fig. S5a) and 12.5 × 10.5 Å2 (see Fig. S5b), respectively. Based on the PLATON program, the potential solvent accessible void volume in the 3-D network of 1 is 773.4 Å3.

Determination of guest molecules On the basis of the charge balance principle and the crystal data, the molecular formula for 1 is preliminarily defined as [(CH3)2NH2]2[Tb2(L)2(H2O)2]·guest. The cation [(CH3)2NH2]+ comes from the in situ decomposition and protonation for DMF.42-44 In order to identify the guest molecules, the TG analysis was performed (see Fig. S2). In the temperature range of 20-110 oC, a minor weight-loss is observed, suggesting there are the lattice H2O molecules in 1. In the temperature range of 110-420 oC, ca. 12.7% weight loss is observed, much higher than the weight loss of the coordinated water molecules (Calcd: 2.5%), indicating that DMF should be present in 1. Therefore, the molecular formula for 1 can be further written as [(CH3)2NH2]2[Tb2(L)2(H2O)2]·xDMF·yH2O. Ca. 2.6% weight loss in the first step suggests there are two lattice H2O molecules in asymmetric unit for 1 (Calcd: 2.5%). Ca.12.1% weight loss for the second step implies that there exist two DMF in the asymmetric unit of 1 (Calcd: 12.7%). The residue content of ca. 26.5% is comparable with that of the calculated (25.5%), implying the inferred formula composition for 1 is accurate. This result is further verified by the CHN analysis of the product.

Photoluminescence investigations

5



Fig. 2 Solid-state photoluminescence spectra of 1 (a), 2 (b), 3 (c) and H4L (d).

In order to reveal the photoluminescence properties of 1-3, and better understand the photoluminescence mechanism, the photoluminescence behaviors of 1, 2, 3 and H4L were investigated. As shown in Fig. 2, upon excitation, 1 and 2 exhibit the characteristic transitions of the Ln3+ ions. For the emission of 1 (λex = 330 nm), the hypersensitive peak at 544 nm, corresponding to the 5D4→7F5 transition of the Tb3+ ion, determines the green-light emission of 1. The other three weak peaks at 490 nm, 584 nm and 620 nm, should be attributed to the transitions of 5D4→7F6, 5D4→7F4, and 5D4→7F3, respectively. For the emission of 2 (λex = 330 nm), the four peaks at 590 nm, 613 nm, 651 nm, and 699 nm should be assigned to the 5D0→7FJ (J = 1-4) transitions of the Eu3+ ion. The red light of 2 is dominated by the emission at 613 nm. 3 shows a blue light emission with the maximum at 433 nm (λex = 373 nm). The free ligand H4L exhibits a similar emission (λem = 412 nm) when excited at 333 nm. So the emission peak of 3 at 433 nm should be attribute to the internal ligand π* → π charge transfer. The energy of the lowest excited states of Gd3+ 6P7/2 is too high to accept the energy of the ligand, so the characteristics emission peak of 4f-4f transition at 311 nm for Gd3+ is not observable. Based on the results above, we can know that (i) H4L can sensitize Tb3+ and Eu3+ to emit the characteristic green or red light; (ii) although Gd3+ is not sensitized by L4- , that is not important yet. Here a blue-light emitting Ln-MOF material is required, while the Gd3+ complex just gives a blue-light emission (characteristic emission for Gd3+ appearing at 311 nm). It belongs to a ligand-emitted material.

Fig. 3 Solid-state emission spectra of 4 excited from 310 to 380 nm (a); CIE chromaticity diagram for 4 excited at 350 nm.

The white-light emitting materials have attracted much interest because of their potential applications in solid-state lighting, full-color displays, and backlights.47-49 The title three compounds emit RGB light, and they are isomorphic. Therefore, it will be possible to construct the white-light emitting materials by doping Eu3+/Tb3+/Gd3+ into the identical framework. Moreover, a rational collocation of emission intensities for the RGB light is necessary. By varying the excitation wavelength, the emission intensities of RGB light can be appropriately adjusted. After a careful screening, compound 4 with a composition of [(CH3)2NH2]2[Eu0.016Tb0.078Gd1.906(L)2(H2O)2]·2DMF·2H2O was obtained. Fig. 3 records the emission spectra of 4 upon excitation from 310 nm to 380 nm. When excited at 350 nm, the corresponding CIE chromaticity coordinates (0.317, 0.326) are very close to those of the pure white light (0.333, 0.333), suggesting that 4 is a white-light emitting material. The quantum yield for 4 is 43.41%. The Powder XRD analysis proves that 4 is isomorphic with 1-3 (Fig. S1b). Compound 4 demonstrates that the mixture of Tb3+/Eu3+/Gd3+ can afford a white-light emitting material, in which L4- around Gd3+ provides the blue-light source, while the energy of L4- around Tb3+/Eu3+ is almost transferred. Actually, only a mixture of Tb3+/Eu3+ can also afford a white-light emitting material, as revealed by the relevant documents.50-51 Here the excitation wavelength plays a key role. When excited at some wavelengths, only part of energy transfers from the ligand to the excited state of the Ln3+ center. The remaining energy for the ligand can still ensure itself to emit the blue-light upon excitation. Based on this, we achieved three tunable white-light emitting materials [(CH3)2NH2]2[TbxEu2-x(L)2(H2O)2]·2DMF·2H2O (x = 0.6 for 5; x = 0.8 for 6; x = 1.8 for 7) by accurately controlling the 6


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proportion of Eu3+/Tb3+. Powder XRD analysis demonstrates that the doped complexes 5-7 are isostructural to 1-3 (Fig. S1b). The solid-state emission spectra at the different wavelengths and the CIE chromaticity diagrams for 5-7 are shown in Fig. 4. Upon excitation at 350 nm for 5, the corresponding CIE chromaticity coordinates are (0.329, 0.322). Upon excitation at 340 for 6, the corresponding CIE chromaticity coordinates are (0.333, 0.330). Whereas upon excitation at 350 for 7, the corresponding CIE chromaticity coordinates are (0.325, 0.331). These coordinates all appear near the coordinates of pure white light (0.333, 0.333), implying that 5-7 are the expected white-light emitting materials. Note that 5 has a higher quantum yield (48.29%). The quantum yields for 6-7 are 35.03% for 6 and 38.56% for 7, respectively. The fluorescence lifetimes for 4-7 were also measured (Fig. S6). As listed in Table S2, compounds 4-7 possess the longer ms-grade fluorescence lifetimes. Meanwhile, we also find that from 5 to 7, even though the Eu3+ proportion is largely decreased (Tb3+/Eu3+ = 0.3/0.7 in 5, 0.9/0.1 in 7), the lifetime of Eu3+ is improved (0.602 ms for 5, 0.936 ms for 7). On the contrary, the Tb3+ proportion is largely increased from 5 to 7, but the improvement of the Tb3+ lifetime is not too high (τ1 = 0.055 ms, τ2 = 0.361 ms for 5; τ1 = 0.175 ms, τ2 = 0.656 ms for 7), compared with the change of the Eu3+ lifetime. This means that the energy transfer from Tb3+ to Eu3+ should occur. This situation has been observed in the reported document.52 Based on the photoluminescence properties of 1-3, the white-light emitting materials can be fabricated. Whether the mixture of Tb3+/Eu3+/Gd3+, or the mixture of Tb3+/Eu3+ can both afford the white-light emitting materials. The excitation wavelength not only influences the transferred energy from the ligand to the Ln3+ center, letting the emissions of ligand and Ln3+ co-exist, but also modulates the emission intensities of the RGB light. The as-synthesized white-light emitting materials 4-7 possess the longer fluorescence lifetimes, and the higher quantum yields. This is due to the existence of the bigger π-conjugated rings of L4-.

7



Fig. 4 Solid-state emission spectra for 5 (a), 6 (c) and 7 (e) excited from 310 to 380 nm; CIE chromaticity diagrams for 5 (b), 6 (d) and 7 (f).

Sensing of organic small molecules

Fig. 5 Emission spectra of 1 dispersed in different organic solvents (a), emission spectra of 1 with gradually increased 1,2,4-TCB (Inserted plot showing a fluorescence change after adding 1,2,4-TCB) (b), a plot showing a change of luminescence intensity of 1 with gradually added 1,2,4-TCB (c), and Stern-Volmer plot of Io/I vs concentration of 1,2,4-TCB (Inserted plot being Stern-Volmer plot at lower concentrations) (d).

Polychlorinated benzenes as the solvents and the raw materials have been widely used in industry for synthesizing the dyes and the phenols. As the pesticides and the fungicides,53-54 they have also been used in agriculture. They are likely to enter the human body via the food chain, threatening the human wealth.55-56 Meanwhile, they are likely to contaminate the water and the soil. 57 Therefore, finding an effective detecting method has become the focus of research. In order to reveal the sensing ability of 1 on polychlorinated benzenes, the fluorescence behaviors of 1 dispersed in the different organic solvents were measured. The selected organic solvents include DMF, ethanol, methanol, trichloromethane, aniline, xylene, toluene, o-aminophenol, chlorobenzene (CB), 1,2-dichlorobenzene (1,2-DCB) and 1,2,4-trichlorobenzene (1,2,4-TCB). Before the experiments, some suspensions are made as follows: a ground powder sample for 1 (3 mg) is dispersed in a solvent (3 mL), and then treated through ultrasonication for half an hour. As shown in Fig. 5a, three situations are observed: (i) dispersed in ethanol, DMF, methanol, trichloromethane, aniline, xylene, toluene or o-aminophenol, the emission intensities of suspension-state 1 are basically compare with each other, and show the maxima; (ii) dispersed in CB or 1,2- DCB, the emission intensities of the suspension-state 1 are largely decreased, but not quenched; (iii) dispersed in 1,2,4-TCB, the emission for the suspension-state 1 are almost completely quenched. These suggest that (i) ethanol, DMF, methanol, trichloromethane, aniline, xylene, 8


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toluene and o-aminophenol are the enhancers while ethanol can be selected as dispersible solvent; (ii) 1 as a sensor cannot detect CB and 1,2-DCB, but can selectively detect 1,2,4-TCB. 1 is stable in these organic solvents, which is characterized by the related XRD analysis (see Fig. S1c). Aiming at investigating the quenching efficiency for 1 on 1,2,4-TCB, the powder 1 was first dispersed into 3 mL ethanol to generate a suspension. Then 1,2,4-TCB was gradually added into this suspension. The corresponding photoluminescence intensities of the suspensions were recorded. As shown in Figs. 5b and 5c, with the increase of 1,2,4-TCB concentration, the emission intensity of the suspension gradually decreases. When the 1,2,4-TCB concentration is 0.6 mM, the fluorescence intensity of 1 decreases by 28%, while when the 1,2,4-TCB concentration is 100 mM, the fluorescence intensity of 1 reduces by 92%. Fig. 5d gives the Stern-Volmer plot of relative fluorescence intensity (Io/I) vs 1,2,4-TCB concentration, where Io and I represent the fluorescence intensity of 1 before and after adding 1,2,4-TCB. At the low concentration field, a good linear proportion (R2 = 0.996) is obtained. The calculated Ksv value is 0.47 × 103 M-1. When the 1,2,4-TCB concentration is larger than 20 mM, the curve is non-linear. This indicates that the fluorescence quenching process caused by 1,2,4-TCB is accompanied by both static and dynamic quenching.58-59

Fig. 6 Plots showing a change of luminescence intensity of 1 with gradually added polychlorinated benzene ( a: 1,2,3,4-TCB; b: 1,2,4,5-TCB; c: PCB; d: HCB).

9



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Fig. 7 Stern-Volmer plots of Io/I vs concentration of polychlorinated benzenes (a: 1,2,3,4-TCB; b: 1,2,4,5-TCB; c: PCB; d: HCB; concentration range: 0-0.15 mM).

Based on the investigations above, we have found that the sensing ability of 1 on CB, 1,2-DCB and 1,2,4-TCB are different. In order to reveal how the number of the substituted Cl atom on benzene ring affects the sensing capacity of 1, we further studied the detecting ability of 1 on the other polychlorinated benzenes, such as 1,2,3,4-tetrachlorobenzene (1,2,3,4-TCB), 1,2,4,5-tetrachlorobenzene (1,2,4,5-TCB), pentachlorobenzene (PCB), and hexachlorobenzene (HCB). We first prepared four suspensions of 1 (sample: 3 mg; solvent: 3 mL). Then four polychlorinated benzenes were gradually added into the different suspensions, respectively (Fig. S7). For the suspension containing HCB, trichloromethane is selected as the dispersing agent, while for the others, ethanol acts as the dispersing agent. Fig. 6 shows fluorescence intensity of 1 vs polychlorinated benzene concentration plot. When the polychlorinated benzenes concentration is 0.01 mM, the fluorescence intensities of 1 decrease to 94.3% for 1,2,3,4-TCB, 93.5% for 1,2,4,5-TCB, 88.5% for PCB and 81.9% for HCB, respectively. When the polychlorinated benzenes concentration is 10 mM, the fluorescence intensities of 1 reduce to 14.9% for 1,2,3,4-TCB, 14.7% for 1,2,4,5-TCB, 4.6% for PCB and 1.6% for HCB, respectively. The Stern-Volmer plots of 1 in different concentrations of polychlorinated benzenes are displayed in Fig. S8. At the low concentrations (0-0.15 mM), the fitting curves all show a good agreement with the linear equation (Fig. 7). The calculated Ksv values are 5.11 × 103 M-1, 5.21 × 103 M-1, 8.84 × 103 M-1 and 12.23 × 103 M-1, respectively. This suggests that (i) the number of the substituted Cl atom affects the sensing ability of 1; (ii) the more the number is, the higher the quenching efficiency is; (iii) the title compound 1 can highly selectively sense the different polychlorinated benzenes. The fluorescence quenching of 1 caused by polychlorinated benzenes should be related to the substituted Cl atom on benzene ring. Cl is a typical electron withdrawing atom. When 1 encounters polychlorinated benzenes, the intrinsic energy transferring path in 1 is altered. The energy from L4- is transferred to the polychlorinated benzenes, instead of being transferred to the Tb3+ center (Fig. S11). That is to say, the antenna effect is restrained by the analyte, resulting in the occurrence of the quenching phenomenon. The better sensing ability of 1 on polychlorinated benzenes should be associated with the following several factors: (i) the larger π-conjugated structure for L4-. The fluorescence quenching derives from the interaction between electron donator (MOF material) and electron acceptor (analyte). The material with the bigger π-conjugated rings can rapidly release a signal once contacting the analyte; (ii) porous structure for the MOF 10


Page 11 of 18 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


material. The porous formation effectively increases the surface of the material. The larger surface makes the analyte easily access the electron donator. During the investigations, the material is generally grinded, and dispersed in a solvent. That is also to polychlorinated benzenes the surface of the material; (iii) the electron accepting ability for the analyte. In fact, Cl is a slightly weak electron withdrawing atom. Only one or two Cl atoms on benzene ring do not cause the fluorescence quenching of the material. Three Cl atom on benzene ring possesses the enough ability to quenching the fluorescence of the material. In addition, the more the number of the Cl atom on benzene ring, the higher the quenching efficiency is, as revealed by the Ksv values. Based on above statement, the title compound 1 can selectively sensing the various polychlorinated benzenes. The unique deficiency is that (CH3)2NH2+ in the pore impedes the contact of the analyte with the material. The UV-Vis adsorption region for 1,2,4-TCB in CH3OH or C2H5OH is lower than 330 nm.15 With 330 nm as the excitation wavelength, the analyte 1,2,4-TCB actually has no adsorption. However, with the addition of 1,2,4-TCB, the fluorescence intensity of 1 is gradually reduced, implying that the added 1,2,4-TCB changes the “antenna effect”. The energy absorbed by the ligand is transferred to the analyte 1,2,4-TCB. That is to say, the energy of the LUMO (lowest unoccupied molecular orbital for the ligand in 1 is higher than that of 1,2,4-TCB. Also because the energies of LUMOs for polychlorinated benzenes are gradually reduced with the increase of the number for the substituted Cl atom on benzene ring,15 the energy of the ligand can transfer to the LUMO of each polychlorinated benzene analyte. This is further confirmed by the DFT (density functional theory) calculations. The DFT calculations reveal that the energy of the LUMO for the ligand in 1 is -0.93411 eV, higher than that of each polychlorinated benzene analyte, as shown in Fig. S12.

Detection of metal ions

Fig. 8 Emission spectra of 1 dispersed in various nitrate salts aqueous solutions (10-3 M) (a), emission spectra of 1 with gradually increased Fe3+ (Inserted plot showing a fluorescence change after adding Fe3+) (b), a plot showing a change of luminescence intensity of 1 with gradually added Fe3+ (c), and Stern-Volmer plot of Io/I vs concentration of Fe3+ (d).

The sensing ability of 1 on the metal ions was investigated. We still adopted the method of measuring the photoluminescence behaviors of the suspensions for 1. Herein, 1 (3 mg) was dispersed in the different nitrate salt aqueous solutions (3 mL). As shown in Fig. 8a, the 11



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emission intensities for all suspensions exhibit the differences. When dispersed in the K+/Na+/Zn2+/Cd2+/ Pb2+ solutions, the emission intensities for 1 have hardly changed. When dispersed in the Ni2+/Co2+/Cu2+/Cr3+ solutions, the emission intensities for 1 show a reduction with a different degree. However, when dispersed in the Fe3+ solution, the emission for 1 is completely quenched, which means 1 can act as a chemosensor to sense selectively Fe3+ ion. Iron(III) is an essential trace element in biochemical process, such as oxygen transport and oxygen storage in the body.60-61 However, an excess of iron will catalyze the production of reactive oxygen species, which is harmful to the organisms.62 Therefore, it is imperative to develop an efficient and convenient chemical sensor to detect iron(III). The sensitivity test of 1 on the Fe3+ ion was further carried out. At this time, the photoluminescence behaviors of 1 dispersed in the Fe(NO3)3 aqueous solutions with the different concentrations were investigated. As shown in Figs. 8b and 8c, with the increase of the Fe3+ concentration, the emission intensity of 1 gradually reduces. When the Fe3+ concentration is 15 µM, the fluorescence intensity for 1 declines by ca. 50%, whereas the Fe3+ concentration comes up to 100 µM, the fluorescence intensity for 1 decreases by ca. 88.5%. Fig. 8d gives the fitting cure of the Stern-Volmer plot, which is well in agreement with the linear equation. The calculated Ksv value of 7.58 × 104 M-1

is

obviously

larger

than

those

of

some

known

MOF-based

sensors,

4,4',4'',4'''-(4,4'-(1,4-phenylene)bis(pyridine-6,4,2-triyl))-tetrabenzoate) (Ksv = 1.66 ×

such 104

as M-1),63

[Zr6O4(OH)8(H2O)4(L1)2]

(L1

=

[Eu(L2)(L3)0.5(NO3)]·H2O (L2 =

2,5-di(pyridin-4-yl)terephthalate acid, L3 = biphenyl-4,4'-dicarboxylate) (Ksv = 5.16 × 104 M-1),64 and [Eu2K2(L4)2(H2O)6]·5H2O (L4 = 4-(3',5'-dicarboxylphenoxy)phthalate) (Ksv = 4.3 × 104 M-1).65 The large Ksv value implies that the title compound 1 can highly sensitively sense the trace Fe3+ ion. The fluorescence quenching caused by Fe3+ should be associated with the electron deficient nature of Fe3+. Due to this reason, the introduced trace Fe3+ ion has changed the intrinsic energy transferring path, so the quenching phenomenon occurs. K+, Na+, Zn2+, Cd2+ and Pb2+ all have a saturated electron configuration, so the fluorescence intensities of 1 have no obvious change after adding these metal ions. The electron deficient degree for Ni2+, Co2+ and Cu2+ is less than that of Fe3+, so they cannot thoroughly quench the emission of 1. The electron deficient nature of Cr3+ is comparable with that of Fe3+. However, since the CrIII-Npyridyl bond is somewhat weak, part of Cr3+ still exists in the form of hydrated ion in the suspension. As a result, Cr3+ cannot completely quench the emission of 1 yet. The title compound 1 exhibits a high sensibility on Fe3+, which should be mainly due to the following three factors: (i) the larger π-conjugated structure for L4-; (ii) the existence of the uncoordinated N atom on L4-. Owing to the presence of this N atom, the mixed trace Fe3+ ion can be quickly fixed on the surface of 1 via the FeIII-N bond. The TG analysis for 1@Fe3+ shows that the residue weight of ca. 36% is greater than that of 1 (ca. 27%), suggesting that there exists the bonding interaction between Fe3+ and N. Before the TG analysis, the sample was fully washed; (iii) the porous structure of 1. It is noteworthy that the cation (CH3)2NH2+ in the channels does not hinder the touch of Fe3+ with the uncoordinated N atom, because it can be exchanged by the mixed Fe3+ ion.

Fluorescent test paper

Fig. 9 Optical images of test paper coated with 1 under UV-vis lamp: organic molecule was dropped on surface (a); metal ion was dropped 12


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on surface (b); 1,2,4-TCB was dropped on surface and then washed (c); Fe3+ was dropped on surface and then washed (d).

Inspired by the above results, we attempted to produce a macroscopic fluorescent test paper to detect 1,2,4-TCB and Fe3+. The fluorescent test papers were made by immersing the filter paper into the ethanol suspension of 1, then drying in the air. When 1,2,4-TCB and Fe3+ were dropped on the surface, the green fluorescence of the test paper was completely disappeared under the Ultraviolet lamp. Whereas no significant changes were observed for the other kinds of organic solvents and metal ions (see Fig. 9a and 9b). The reversibility of the test paper was also examined. The test paper used for detecting 1,2,4-TCB can be recycled. After five cycles, the green fluorescence for the test paper is still strong. For each cycle, the test paper has undergone a soaking, washing and drying process. But the test paper used for detecting Fe3+ cannot be recycled. Just after one cycle, the green fluorescence cannot be restored. This further confirms that there exists the strong interaction between Fe3+ and 1 (coordination Fe-N bond). Furthermore, we attempted to use the test paper to detect the trace Fe3+ ion in H2O. As shown in Fig. 10, when the Fe3+ concentration is 1 mg/L or 2 mg/L, the fluorescence green of the test paper has no obvious change under the UV lamp, compared with the blank situation. But when the Fe3+ concentration is 3 mg/L, the fluorescence green of the test paper nearly disappears. This means that the detection limit of the test paper coated with 1 on Fe3+ is 3 mg/L (ca. 5 × 10-5 mol/L). The interference experiments of the other metal ions as Cr3+, Co2+, Cu2+ or Ni2+ were also investigated. The results indicate that these ions hardly interfere with the detection of the test paper on Fe3+. When the Fe3+ concentration is 3 mg/L, the fluorescence green of the test paper still disappears (also see Fig. 10). Here the concentration of the interfering ion is equal to that of Fe3+.

Fig. 10 Optical images of test paper coated with 1 under UV-vis lamp: Fe3+ or mixed ions was/were dropped on surface.

Conclusion In summary, we reported the synthesis, structural characterization, and the sensing property of three new isomorphic L4--based Ln-MOF materials 1-3. Based on their photoluminescence properties, four white-light emitting materials 4-7 were fabricated. Synthetically, the temperature, solvent (DMF), and the pH value play a key role in the reactions. Structurally, since the pyridyl N atom does not interacts with the Ln3+ center, the channels with the larger windows are found in the 3-D networks of the title compounds. The photoluminescence analysis reveals that H4L can sensitize Tb3+ and Eu3+ to emit the characteristic green or red light. Although Gd3+ cannot be sensitized, the Gd3+ complex exhibits a blue-light emission. Whether the mixture of Tb3+/Eu3+/Gd3+, or the mixture of Tb3+/Eu3+ can both afford the white-light emitting materials. Here the role of excitation wavelength cannot be ignored. It not only can alter the transferred energy, letting the emissions of ligand and Ln3+ co-exist, but also can modulate the emission intensities of RGB light. 4-7 possess the longer fluorescence lifetimes and the higher quantum yields, which is due to the bigger π-conjugated rings of L4-. According to a fluorescence quenching mechanism, the title compound 1 has been found not only to be able to highly selectively sense the different polychlorinated benzenes, but also to be able to highly sensitively sense the Fe3+ ion. The fluorescence quenching of 1 should be related to the electron deficient nature for polychlorinated benzenes and Fe3+. Due to their electron deficient nature, the intrinsic energy transferring path is changed, and 13



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the antenna effect is prohibited, so the quenching phenomenon occurs. The high selection of 1 on the polychlorinated benzenes should be associated with their electron accepting ability. The more the number of the Cl atom is, the strong the electron accepting ability is. While the polychlorinated benzene with the strong electron accepting ability can quickly quenching the fluorescence emission of 1. The high sensibility of 1 on Fe3+ should be associated with the following several factors: the larger π-conjugated structure for L4-; the larger pore observed in the 3-D network of 1; the existence of the uncoordinated N atom on the spacer of L4-. The test-paper experiments point out that 1 can be made into the test paper to readily detect 1,2,4-TCB and Fe3+.

Acknowledgements The authors are grateful to the financial aid from the National Natural Science Foundation of China (Grant No. 21771076, 21271083).

Supporting Information Available Projection plots; Powder XRD patterns for 1-7; IR spectra and TG curves for 1-3; Decay curves and lifetimes for 4-7; Emission spectra of 1 with added polychlorinated benzenes, and corresponding Stern-Volmer plots; Fluorescence spectra and TG curves for 1 before and after interacting with Fe3+; HOMO and LUMO energies for ligand and polychloriznated benzenes; DFT calculation process; CIE chromaticity coordinates for 5-7; Crystallographic files in CIF format. This material is available free of charge via the Internet at http://pubs.acs.org.

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For Table of Contents Use Only

White-light Emitting Materials and Highly Sensitive Detection of Fe3+ and Polychlorinated Benzenes Based on Ln-MOFs Jing-Jing Huang, Jie-Hui Yu*, Fu-Quan Bai*, Ji-Qing Xu

Structural characterization of three isomorphic 5,5'-(pyridine-3,5-diyl)diisophthalate-based Ln-MOFs were reported. Based on their photoluminescence properties, four white-light emitting materials were fabricated. The sensing ability of Tb3+ compound on polychlorinated benzenes and Fe3+ were investigated.

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White-light Emitting Materials and Highly Sensitive Detection of Fe3+

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