Rational Design of Three Two-Fold Interpenetrated MetalâOrganic

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Rational Design of There Two-Fold Interpenetrated MOFs: Luminescent Zn/Cd-MOFs for Detection of 2,4,6Trinitrophenol and Nitrofurazone in the Aqueous Phase Zhi-Wei Zhai, Shuang-Hua Yang, Man Cao, Lin-Ke Li, Chenxia Du, and Shuang-Quan Zang Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b01335 • Publication Date (Web): 04 Oct 2018 Downloaded from http://pubs.acs.org on October 4, 2018

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Crystal Growth & Design

Rational Design of There Two-Fold Interpenetrated MOFs: Luminescent Zn/Cd-MOFs for Detection of 2,4,6-Trinitrophenol and Nitrofurazone in the Aqueous Phase Zhi-Wei Zhai,†,‡ Shuang-Hua Yang,‡ Man Cao,† Lin-Ke Li,† Chen-Xia Du,*,† Shuang-Quan Zang*,† †

College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou, 450001, P. R. China ‡

School of Environmental Engineering and Chemistry, Luoyang Institute of Science and Technology, Luoyang 471023, P. R. China

ABSTRACT:

Through

dual-ligand

strategy,

{[Zn2(Py2TTz)2(BDC)2]·2(DMF)·0.5(H2O)}n

(1),

{[Co2(Py2TTz)2(BDC)2]·2(DMF)·0.5(H2O)}n pyridyl)thiazolo[5,4-d]thiazole,

BDC

=

(3)

three

mixed

ligands

MOFs,

namely,

{[Cd2(Py2TTz)2(BDC)2]·2(DMF)}n (where

Py2TTz

1,4-benzenedicarboxylate,

and

= DMF

(2),

2,5-Bis(4=

N,N-

dimethylformamide), were synthesized under solvothermal conditions. The single-crystal X-ray diffraction analyses reveal that three MOFs possess similar two-fold interpenetrated 3D framework structures with pcu topology. The fluorescence properties of compounds 1 and 2 were investigated systematically. The results show that compounds 1 and 2 display good fluorescent properties, which can be efficiently quenched by trace amount of nitroaromatics 2,4,6-trinitrophenol (TNP) and antibiotics nitrofurazone (NZF) in water media. The large Ksv value and small LOD demonstrate that compounds1 and 2 can serve as good fluorescent sensors for TNP and NFZ detection in aqueous system. Density functional theory calculations, spectral overlap experiments, coupled with luminescence decay experiments, confirm that the luminescence quenching mechanism involves dynamic and static quenching mechanism and is dominated by the photo-induced electron transfer (PET) process and the Förster resonance energy transfer (FRET) process simultaneously.

ACS Paragon Plus Environment

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■ INTRODUCTION With ever-increasing concern for environmental safety and public health, it is becoming increasingly imperative to detect and remove pollutions from wastewater. As one kind of the major pollutants, nitroaromatics, are widely used as chemical intermediates and explosive materials. Among the nitroaromatics commonly used, 2,4,6-trinitrophenol (TNP) has the highest explosive power and has been identified as a toxic pollutant. Its release into the environment causing the contamination of water and soil, resulting in unpleasant effects on human health.1-9 Similarly, antibiotics are extensively used even overused so as to treat bacterial infections in humans, animals and aquacultures, leading to high levels of antibiotic residues, which are hazardous to human and wildlife.10-15 Moreover, either nitroaromatics or antibiotics are highly toxic and difficult to degrade, which results in the long-term harm to environment.16-20 Therefore, high sensitively and selectively detecting and removing nitroaromatics and antibiotics is an urgent issue. Currently, detection methods mainly depend on precision apparatus such as mass spectrometry (MS),21 ion mobility spectrometry (IMS),22 liquid chromatography-tandem mass spectrometry (LC-MS),23 etc. All these methods are complex, inconvenient, and expensive. In contrast to instrumental methods, luminescent sensing has been regarded as a particularly promising approach because of its fast response, high sensitivity, energy saving, low cost, easy operation, and so on.24-29 As important sensing materials for various analytes, luminescent MOFs have attracted great concern and made progress since Li et al. reported a luminescent Zn-MOF and pioneered the luminescent MOFs sensors for detection of nitroaromatics in 2009.30 The numerous luminescent MOFs have been synthesized as sensing materials for detecting metal ions (Cu2+, Hg2+, Pb2+, Fe3+, Al3+, etc.),31-36 inorganic anions (Cr2O72-, CrO42-, AsO43-, etc.),37-40 nitroaromatics (2,4,6trinitrophenol, 2,4,6-trinitrotoluene, etc.),41-42 other small organic molecules,43-50 and so forth. However, most of them are usually conducted in organic solvent instead of water, which is not desirable for practical application. Consequently, it is very crucial to prepare water stable luminescent MOFs as a base for developing the detection in water phase. Among various strategies to explore novel luminescent MOFs, constructing framework containing luminescent ligands and d10 electron configurational metal ions is simple and effective one. Given that the thiazolo[5,4-d]thiazole moiety with rigid conjugated plane has good fluorescence property. It also possesses some appealing features towards applications in organic electronics.51-56 We


2

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expect that the assembly of the fluorescent ligand 2,5-bis(4-pyridyl)thiazolo[5,4-d]thiazole (Py2TTz), with d10 metal ions may render the MOFs with distinctive luminescent properties. Herein, through dual-ligand strategy, a rigid coplanar ligand Py2TTz was successfully introduced

into

three

mixed

{[Zn2(Py2TTz)2(BDC)2]·2(DMF)·0.5(H2O)}n {[Co2(Py2TTz)2(BDC)2]·2(DMF)·0.5(H2O)}n pyridyl)thiazolo[5,4-d]thiazole,

BDC

=

(1), (3)

ligands

MOFs,

namely,

{[Cd2(Py2TTz)2(BDC)2]·2(DMF)}n (where

Py2TTz

1,4-benzenedicarboxylate,

and

= DMF

(2),

2,5-Bis(4=

N,N-

dimethylformamide). Three MOFs possess similar 3D pillar-layered framework structures consisting of 2D sheet-like structure (constructed by ligand BDC2- and binuclear secondary building unit M2(CO2)4, where M = Zn, Cd, Co) and Py2TTz pillar. Three compounds are twofold interpenetrated frameworks with pcu topology. Considering the strong π-conjugated effect of ligand Py2TTz and the d10 electron configuration of Zn(II)/Cd(II) ions, the fluorescence properties of compounds 1 and 2 were investigated. Interestingly, compounds 1 and 2 not only demonstrate selectively detecting 2,4,6-trinitrophenol (TNP) for nitroaromatics in aqueous media, but also display selectively sensing nitrofurazone (NZF) for antibiotics in water solution. Furthermore, the sensing mechanism was systematically investigated. ■ EXPERIMENTAL SECTION Materials and methods. All the chemicals for experiment are of reagent grade and used as received without further purification. 2,5-Bis(4-pyridyl)thiazolo[5,4-d]thiazole (Py2TTz) was prepared according to reported procedure but slightly modified.51,57 Infrared spectra (IR) were recorded in the range of 400-4000 cm-1 on a Nexus 870 FTIR spectrometer with KBr pellets. Elemental analyses (EA) were conducted with a Perkin-Elmer 240 elemental analyzer. Powder X-ray diffraction (PXRD) patterns of the samples were recorded in the 2θ = 5-50 ° range on a Bruker D8 Advance diffractometer with Cu Kα radiation (λ = 1.5418 Å) at room temperature. Thermogravimetry analyses (TGA) were performed on a TA Q50 thermogravimeter in the temperature range 30-800 °C with a heating rate of 10 °C·min-1 under Ar atmosphere. UVvisible absorption spectra were obtained on a TU-1900 spectrophotometer. Luminescence spectra were recorded using a HORIBA FluoroLog-3 fluorescence spectrophotometer. HOMO and LUMO energies of selected analytes and Py2TTz were calculated via density functional theory (DFT) using the Gaussian 09 program at B3LYP/6-31G* level.


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Synthesis of compound {[Zn2(Py2TTz)2(BDC)2]·2(DMF)·0.5(H2O)}n (1). A mixture of Zn(NO3)2·6H2O (11 mg, 0.037 mmol), Py2TTz(7.5 mg, 0.025 mmol) and H2BDC (6 mg, 0.036 mmol) in DMF (2 mL) and H2O (0.5 mL) was sonicated for 30 minutes. Then, the reaction mixture was transferred to a PTFE-lined stainless-steel vessel and placed in the oven, which was heated at 100 °C for 2 days. The colorless prism-shaped crystals were generated after cooling to room temperature. The crystals were filtered off, washed with DMF, and dried in air (14.2 mg, 93% yield based on Py2TTz ligand). Elemental analysis (%) calcd for C50H39N10O10.5S4Zn2: C 49.76, H 3.26, N 11.61, S 10.63; found: C 50.33, H 3.15, N 11.23, S 10.81. Main infrared spectral data (KBr, cm-1): 3415 (s), 3056 (w), 2923(w), 1672 (m), 1614 (s), 1560(m), 1398 (s), 1213 (m), 1016 (s), 827 (m), 746 (m), 705 (m), 617 (s), 507 (s). (IR spectrum is shown in Figure S1). Synthesis

of

compound

{[Cd2(Py2TTz)2(BDC)2]·2(DMF)}n

(2).

A

mixture

of

Cd(NO3)2·4H2O (8 mg, 0.026 mmol), Py2TTz(7.5 mg, 0.025 mmol) and H2BDC (4.5 mg, 0.027 mmol) in DMF (2 mL) and H2O (0.5 mL) was sonicated for 30 minutes. Then, the reaction mixture was transferred to a PTFE-lined stainless-steel vessel and placed in the oven. Subsequently, the temperature was kept at 100 °C for 2 days before cooling to room temperature. The pale yellow needle-shaped crystals were filtered off, washed with DMF, and dried in air (13.8 mg, 85% yield based on Py2TTz ligand). Elemental analysis (%) calcd for C50H38N10O10S4Cd2: C 46.48, H 2.96, N 9.93, S 10.84; found: C 47.69, H 2.90, N 10.41, S 10.14. Main infrared spectral data (KBr, cm-1): 3415 (s), 3060 (w), 2921(w), 1673 (m), 1618 (m), 1581(m), 1384 (m), 1213 (w), 1027 (m), 835 (m), 752 (m), 703 (m), 620 (m), 511 (m). (IR spectrum is shown in Figure S1). Synthesis of compound {[Co2(Py2TTz)2(BDC)2]·2(DMF)·0.5(H2O)}n (3). A mixture of Co(NO3)2·6H2O (8 mg, 0.027 mmol), Py2TTz(7.5 mg, 0.025 mmol) and H2BDC (5 mg, 0.03 mmol) in DMF (2 mL) and H2O (0.5 mL) was sonicated for 30 minutes. Then, the reaction mixture was transferred to a PTFE-lined stainless-steel vessel and placed in the oven, which was heated at 100 °C for 2 days. The light orange needle-shaped crystals were generated after cooling to room temperature. The crystals were filtered off, washed with DMF, and dried in air (13.5 mg, 95% yield based on Py2TTz). Elemental analysis (%) calcd for C50H39N10O10.5S4Co2: C 50.29, H 3.29, N 11.73, S 10.74; found: C 49.09, H 3.10, N 11.80, S 10.99. Main infrared spectral data


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(KBr, cm-1): 3415 (s), 3060 (w), 2921(w), 1673 (m), 1618 (s), 1581(m), 1384 (m), 1213 (w), 1027 (m), 835 (m), 752 (m), 703 (m), 620 (m), 511 (m). (IR spectrum is shown in Figure S1). X-ray Crystallography. Single-crystal X-ray diffraction measurements of compounds 1 and 3 were performed on a Rigaku XtaLAB Pro diffractometer with Mo-Kα radiation (λ = 0.71073 Å) at 150 K and 100 K, respectively, while the diffractions of compound 2 were collected with Cu-Kα radiation (λ = 1.54178 Å) at 120 K. Crystallographic data collection and reduction were performed using the program CrysAlisPro.58 The structures were solved with direct methods (SHELX),59 and refined by full-matrix least squares on F2 using OLEX2,60 which utilizes the SHELXL-2014 module.61 The hydrogen atoms were placed geometrically. All nonhydrogen atoms were refined anisotropically. The relevant crystallographic data are summarized in Table S1. Selected bond lengths and angles are listed in Tables S2-S4. CCDC numbers of compounds 1-3 are 1861683, 1861684, and 1861686, respectively. Luminescence Measurements. All luminescence measurements were performed at room temperature. The luminescence emissions of samples in different solvents were recorded with various emulsions. The homogeneous emulsions were prepared as follow: the finely ground sample powders (1 mg) were introduced into different solvents (3 mL) and then subjected to ultrasonication for 30 min so as to disperse completely. Luminescence quenching experiments were recorded by gradually adding the analytes into the aqueous emulsion of the samples. The concentration of analytes employed in the quenching experiments is 1×10-3 M. ■ RESULTS AND DISCUSSION Structure Description. The single-crystal X-ray diffraction analyses reveal that compounds 1 and 3 have similar structures and connections, while compound 2 possesses different space structure and coordination mode, although they were obtained under the same experimental condition, which illustrates that the final structures of MOFs can be influenced by the nature of metal ions. Compounds 1 and 3 are isostructural. Herein, take compound 1 for example, their structures are described in detail. Compound 1 crystallizes in the orthorhombic crystal system with Iba2 space group. The asymmetric unit consists of two Zn(II) ions, two Py2TTz ligands, two BDC2- coligands, two disordered free DMF molecules, and half a free water molecule. Each Zn(II) center exhibits a six-coordinated distorted octahedron geometry (ZnO4N2) by four oxygen atoms from three BDC2- ligands in the equatorial positions and two nitrogen atoms from two


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Py2TTz ligands at the axial positions (Figure 1a). The Zn-O and Zn-N distances are in the ranges of 2.013(2)-2.460(3) Å and 2.142(3)-2.189(3) Å, respectively (Table S2). In the BDC2- ligand, the two carboxylate groups display two different coordination modes: one adopting a bidentate chelating mode to bind one Zn(II) center and another adopting a bidentate bridging mode to link two Zn(II) centers. Four carboxylate groups from four BDC2- ligands bridge two crystallographically independent Zn(II) ions to form a binuclear secondary building unit (SBU) Zn2(CO2)4. The binuclear Zn(II) SBUs are linked by BDC2- ligands to yield a 2D wave-like sheet structure (Figure 1b). Further, the adjacent 2D layers are joined together by Py2TTz ligands acting as pillars to form a 3D pillar-layered framework (Figure 1c). If the binuclear Zn(II) SBU is regarded as a 6-connected node and the organic ligands as spacers, the whole architecture can be simplified as a common pcu topology with a point symbol of {412.63} (Figure 1f). Interestingly, compound 1 reveals two-fold interpenetrated networks but still exhibits significant channels when viewed from either a or c axes (Figure 1d-1e). The free void volume calculated by the PLATON program is approximately 24.8 % of the total crystal volume after removal of guest water molecules and DMF molecules. In compound 3, each Co(II) center exhibits also a six-coordinated distorted octahedron geometry CoO4N2) (Figure S2a). The Co-O and Co-N distances are in the ranges of 1.988(4)-2.232(4) Å and 2.134(3)-2.160(3) Å, respectively (Table S4). Two Co(II) centers are bridged by four carboxylate groups to yield a binuclear Co(II) SBU, which is further connected by Py2TTz to generate a 3D framework (Figure S2b). Similar to compound 1, compound 3 consists of two-fold interpenetrated networks and its void volume is approximately 24 % of the total crystal volume by PLATON calculations.


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Figure 1. a) The coordination environments around Zn(II) ions; hydrogen atoms and lattice solvent molecules are omitted for clarity. Symmetry codes: #1. 1/2 - x, 1/2 - y, 1/2 + z; #2 1 - x, + y, - 1/2 + z; #3 + x, -1 + y , + z. b) 2D sheet showing the connectivity of BDC2- ligand with Zn2(CO2)4 SBUs. c) View of a single network of two-fold interpenetrated 3D pillar-layered framework of compound 1. d) View of two-fold interpenetrated 3D framework of compound 1 along c axis. e) View of two-fold interpenetrated 3D framework of compound 1 along a axis. f) Topology network of compound 1.


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Compound 2 crystallizes in the monoclinic crystal system with P21/c space group. The asymmetric unit consists of two Cd(II) ions, two Py2TTz ligands, two BDC2- coligands, and two free DMF molecules. Each Cd(II) center is six-coordinated by four oxygen atoms from three BDC2- ligands and two nitrogen atoms from two Py2TTz ligands (Figure 2a). The Cd-O and CdN distances are in the ranges of 2.244(4)-2.411(4) Å and 2.296(5)-2.350(5) Å, respectively (Table S3). Two Cd(II) centers are bridged by four carboxylate groups from four BDC2- ligands to form a binuclear SBU Cd2(CO2)4. Unlike compound 1, the binuclear Cd(II) SBUs are connected by BDC2- ligands to yield a planar 2D layer structure (Figure 2b), which is further extended to a 3D pillar-layered framework by Py2TTz ligand (Figure 2c). If the binuclear Cd(II) SBU is regarded as a 6-connected node and the organic ligands as spacers, the whole architecture can be simplified as a common pcu topology with a point symbol of {412.63} (Figure 2d). Similarly, compound 2 exhibits a 2-fold interpenetration of architecture. PLATON calculations reveal that the void volume is approximately 25 % per unit cell after removal of guest DMF molecules.


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Figure 2. a) The coordination environment of compound 2; hydrogen atoms and lattice solvent molecules are omitted for clarity. Symmetry codes: #1 1 - x, 1 - y, 1 - z; #2 2 - x, 1 - y, 1 - z; #3 x, 0.5 - y, - 0.5 + z; #4 1 - x, 0.5 + y, 1.5 - z; #5 - 1 + x, y, z; #6 - x, - y, 1 - z; #7 - x, - 0.5 + y, 1.5 - z; #8 1 - x, - y, 1 - z. b) 2D sheet showing the connectivity of BDC2- ligand with Cd2(CO2)4 SBUs. c) View of a single network of two-fold interpenetrated 3D pillar-layered framework of compound 2. d) Two-fold interpenetrated 3D pillar-layered framework of compound 2.

Powder X-ray Diffraction. To confirm the phase purity of bulk samples, PXRD experiments of compounds 1-3 were conducted. As shown in Figures S3-S5, experimental PXRD patterns of three as-synthesized samples are consistent with the simulated ones from their single crystal data, which indicate the phase purity and the effective synthetic method of three samples. Furthermore, stabilities of compounds 1 and 2 in water were investigated so as for aqueous phase


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detection. As shown in Figures S3-S4, PXRD patterns of compounds 1 and 2 immersed in water 48 h are slightly different from ones of their as-synthesized samples, but return to their origins after immersion in DMF for 24 h. These results can be explained by the breathing behavior.62-63 For the rigid interpenetrated frameworks, subtle differences of guest content and composition will lead to different structures, and this transformation is reversible.64-67 Therefore, the slight reversible changes in PXRD patterns of compounds 1 and 2 are caused not by collapse of the framework, but by solvent effect. That is to say, the frameworks of compounds 1 and 2 remain constant in water solution and are stable enough for aqueous phase detection. Thermogravimetric Analyses. To evaluate their thermal stabilities, thermogravimetric analyses (TGA) of compounds 1-3 were examined. As shown in Figure S6, three compounds have high thermal stability. Compound 1 undergoes a gradual weight loss of 12.92% until 317 °C due to the loss of water and DMF molecules in the crystal lattice (calculated value is 12.86%), and then shows a steep mass loss assigned to the decomposition of the framework. Compound 2 experiences a weight loss of 11.22% in the range of 120-280 °C, which can be attributed to the liberation of the DMF molecules in the pores of the framework (calculated value is 11.31%). The collapse of frameworks commences about 350 °C. Compound 3 exhibits two main steps of weight loss: the first weight loss (roughly 13.21%) is observed between 90-235 °C resulting from the loss of solvents (calculated value is 12.99%), the second one is contributed to the decomposition of the framework starting from 390 °C. Luminescent Properties. Considering the strong π-conjugated effect of the rigid coplanar Py2TTz ligand and d10 electron configuration of central metal ions, compounds 1 and 2 are likely to exhibit good fluorescent properties. Thus, the fluorescent properties of free ligand and compounds 1 and 2 were investigated. As shown in Figure S7, the free ligand Py2TTz exhibits two emission peaks at 440 nm and 451 nm upon excitation at 413 nm in the solid state, which can be assigned to π*-π transition.68-69 Upon excitation at 414 nm for compound 1 and 417 nm for compound 2, the emission peaks of compounds 1 and 2 in powder samples appear at 453 nm and 518 nm, respectively. Compared with the free ligand Py2TTz, the emission peak of compound 1 is slightly red shifted, which can be attributed to metal perturbed emission of ligand. However, compound 2 displays a large red shift of 67 nm, which may be assigned to the ligandto-metal charge transfer (LMCT).70-71 Additionally, the fluorescent intensities of two compounds


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are much stronger than that of the free ligand, suggesting that the luminescent efficiencies of these compounds may be affected by the metal centers and coordination environments together. As described above, compounds 1 and 2 possess good fluorescent properties in the solid state and also have good stabilities in water. Then, can compounds 1 and 2 be used for aqueous phase detection? It has been reported that different solvents may affect fluorescence property of MOFs sample.71-74 To investigate the feasibility of aqueous phase detection, the fluorescent properties in different solvents were examined with solvent emulsions of compounds 1 and 2. The finely ground sample powders (1 mg) were introduced into different solvents (3 mL) and then subjected to ultrasonication for 30 min so as to disperse completely. The solvents used in the tests are water, EtOH, acetonitrile, DMF, 1,4-dioxane, ethyl acetate (EA), and methylbenzene (MB). As shown in Figure S8a and Figure S8b, it is obvious that all solvent emulsions of compounds 1 and 2 are solvent-dependent, both compounds show appropriate fluorescent emissions when they are dispersed in different solvents. For compound 1 (Figure S8a), the emission peaks for organic solvent emulsions are all slightly blue shifted in comparison with the emission peak (453 nm) in the solid state, whereas it happens to red shift in water. For compound 2 (Figure S8b), the emission peaks of all solvent emulsions display different extent blue shifts compared with its solid state emission peak (518 nm). This phenomenon may be largely ascribed to the interactions between the guest solvent molecules and the host frameworks.75 The good water stabilities together with good luminescent properties of compounds 1 and 2 prompt us to explore their fluorescent sensing properties in water. Detection of Nitroaromatics. The fluorescent spectra of compounds 1 and 2 were conducted with addition of various nitroaromatics aqueous solution. The volume of aqueous emulsions of compounds 1 and 2 employed in the quenching experiments is 2 mL. The concentration of nitroaromatics aqueous solution is 1×10-3 M. The nitroaromatics employed to test consist of 2nitrophenol (2-NP), 4-nitrophenol (4-NP), 2,4,6-trinitrophenol (TNP), 2-nitrotoluene (2-NT), 2,4-dinitrotoluene

(2,4-DNT),

2,6-dinitrotoluene

(2,6-DNT),

nitrobenzene

(NB),

1,3-

dinitrobenzene (1,3-DNB), and so on (Figure S9). As shown in Figure 3a, different nitro compounds exhibit different degree of luminescence quenching towards the emission intensity of compound 1. In particular, TNP exhibits distinctly greater quenching degree than other nitro compounds under the same test concentration, which implies that compound 1 can selectively detect TNP. To further explore the sensitivity, titration experiments were performed with


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incremental addition of TNP solution to emulsion of compound 1. As shown in Figure 3b, the emission intensities of compound 1 gradually reduce with increasing amount of TNP solution. When the addition of TNP solution reaches to 300 µL, namely the concentration of TNP up to 130 µM, the emission intensity of compound 1 is quenched by 91%. However, upon addition of 300 µL 4-NP and 2-NP, the emission intensity of compound 1 are quenched only by 50% and 35% respectively. The quenching efficiencies of the remaining nitroaromatics towards compound 1 are not obvious. The fluorescence quenching efficiency of TNP was analyzed using the SternVolmer (SV) equation I0/I = 1 + Ksv[C], where Ksv is the quenching constant (M-1), [C] is the molar concentration of the analyte, I0 and I are the emission intensities before and after the addition of analyte, respectively.76 It is shown that the SV plot for TNP is nearly linear in the low concentration range but subsequently deviates from linearity and bends upward at higher concentrations (Figure 4), which may be ascribed to self-absorption or an energy-transfer process.77 The quenching constant Ksv of compound 1 for TNP is calculated to be 3.257 × 104 M1

based on the linear part. Limit of detection (LOD) is calculated by using equation LOD = 3σ/k,

where σ is defined as the standard deviation of a blank sample and k represents the slope of the linear calibration plot.76 LOD of compound 1 for TNP is calculated to be 0.93 µM. Similarly, compound 2 also was employed to sense nitro compounds (Figures S10-S11). Similar to compound 1, compound 2 can also selectively detect TNP. With increscent amount of TNP solution, the luminescent intensities of compound 2 decrease gradually. The emission of compound 2 is quenched by 96% when the addition of TNP is 300 µL. However, under the same concentration, quenching efficiencies of 4-NP and 2-NP for compound 2 are only 55% and 40%, respectively. Quenching efficiencies of the remaining nitroaromatics towards compound 2 are not obvious. Further, LOD and Ksv of compound 2 for TNP are estimated to be 0.90 µM and 4.063 × 104 M-1, respectively. Interestingly, the observed LOD and Ksv value for compounds 1 and 2 are comparable to the good results reported for aqueous phase detection of TNP (Table S5). The large Ksv value and small LOD demonstrate that compounds 1 and 2 can serve as good fluorescent sensors for TNP detection in aqueous system.


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Figure 3. a) Quenching efficiencies towards compound 1 upon addition of 300 µL different nitroaromatics. b) Change of the emission intensities towards compound 1 upon incremental addition of TNP.

Detection of Antibiotics. The sensing results of compounds 1 and 2 towards nitroaromatics examine

inspire

detection

us

of

to

further

antibiotics

in

aqueous solution. A series of antibiotics (Figure

S12),

namely

nitrofurazone

(NZF), nitrofurantoin (NFT), furazolidone (FZD), ronidazole (RDZ), metronidazole (MDZ), ornidazole (ODZ), dimetridazole (DTZ),

sulfamethazine

sulfadiazine (CAP),

(SDZ),

thiamphenicol

(SMZ),

chloramphenicol (THI)

,

were

selected (the concentrations of antibiotics

Figure 4. Stern-Volmer plot of compound 1 for sensing of TNP in aqueous solution, the inset is linear fitting part.

and the volume of water emulsions of compounds 1 and 2 are the same as in the detection of nitroaromatics). Similar to nitroaromatics, antibiotics containing nitro group can be responded by compounds 1 and 2. As shown in Figure S13a, NZF exhibits a drastic quenching effect towards the luminescence of compound 1; nevertheless, the remaining antibiotics display a relatively


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small influence. The result indicates that compound 1 may act as a potential sensor for detecting NZF. Moreover, to further explore the sensitivity, the luminescence titration spectra were investigated. As shown in Figure S13b, with addition of NZF up to 300 µL, the quenching efficiency of compound 1 can reach above 87%. However, under the same concentration, quenching efficiencies of NFT and FZD for compound 1 are only 48% and 30%, respectively. Quenching efficiencies of the remaining antibiotics towards compound 1 are not obvious. The SV plot for NZF displays almost linear shape in the low concentration range and slightly upper bends at the high concentration level (Figure S14). Based on the linear part of the SV plot, the Ksv value of compound 1 for NZF is calculated to be 1.726 × 104 M-1. In the light of data of blank experiments and titration experiments, LOD of compound 1 for NZF is computed to be 0.91 µM. The quenching efficiencies of compound 2 toward the above-mentioned eleven antibiotics were also investigated (Figure 5). Obviously, the fluorescence of compound 2 also can be selectively quenched by NZF. The emission intensity of compound 2 is quenched nearly 93% when the quantity of NZF reaches 300 µL. However, under the same concentration, quenching efficiencies of NFT and FZD for compound 2 are only 51% and 31%, respectively. Moreover, quenching efficiencies of the remaining antibiotics towards compound 2 are not obvious. Similarly, the SV plot of NZF is also linear at low concentration range and bent upward at higher concentration range (Figure 6). Based on the blank experiments and titration experiments, LOD and Ksv of compound 2 for NZF are estimated to be 0.85 µM and 4.538 × 104 M-1, respectively. Interestingly, the observed LOD and Ksv value for compounds 1 and 2 are comparable to the values reported for aqueous phase detection of NZF (Table S6). The large Ksv value and small LOD indicate that compound 2 may serve as a potential fluorescent sensor for NZF detection in the aqueous system.


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Figure 5. a) Quenching efficiencies towards compound 2 upon addition of 300 µL different antibiotics. b) Change of the emission intensities towards compound 2 upon incremental addition of NZF.

Fluorescence quenching mechanism. To

investigate

the

mechanism

of

fluorescence quenching by nitroaromatics and

antibiotics,

PXRD

spectra

of

compounds 1 and 2 before and after sensing experiments (immersed in analytes aqueous solution 48 h) were measured. As shown in Figures S15-S16, PXRD spectra of

two

compounds

after

sensing

experiments have slight difference from the ones before sensing experiments, but return to their origins after immersion in

Figure 6. Stern-Volmer plot of compound 2 for sensing of NZF in aqueous solution, the inset is linear fitting part.

DMF. It means that the structures of compounds 1 and 2 are highly robust under the experimental conditions and remained unchanged during the sensing experiments, which rules out the possibility of collapse of the framework leading to the luminescence quenching. Considering the strong π-conjugated effect of ligand Py2TTz and the nature of electron deficiency of analytes, the photo-induced electron transfer (PET) mechanism is proposed. Generally, the excited electrons lying in the higher LUMO of the fluorescent ligand are transferred to the lower LUMO of the electron-deficient analytes, resulting in fluorescence quenching. The lower the LUMO energy of analytes, the more easily the electrons are transferred to the acceptor.78 To investigate LUMO energy levels of analytes, the frontier molecular orbitals of above-mentioned analytes and ligand are calculated by density


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functional theory (DFT), as shown in Figure 7 and Table S7. It is obvious that TNP has the lowest LUMO energy level among tested nitroaromatics, which explains its highest quenching efficiency. However, the order of quenching efficiency is not fully in accordance with the LUMO energies of nitroaromatics, which indicates that the PET process is not the only mechanism for fluorescence quenching. Additionally, the nonlinear behavior observed from SV plot of TNP indicates that Förster resonance energy transfer (FRET) process also contributes to luminescence quenching.79 It is known that FRET process can occur if the absorbance spectrum of the analyte has an effective overlap with the emission spectrum of the MOF, the larger overlap between the absorbance band of the analyte and the emission band of the MOF, the greater the probability of energy transfer from the MOF to the analyte and hence more efficient luminescence quenching. To assess the spectral overlap between the absorption spectra of the analytes and the emission spectrum of the MOF, UV-vis absorption spectra of nitroaromatics, antibiotics in aqueous solution were measured. As shown in Figures S17, the absorption spectra of TNP, 4-NP, 2-NP have relative larger overlaps with the emission spectra of compounds 1 and 2, which demonstrate the efficient FRET process from the luminescent compounds 1 and 2 to the analytes. Meanwhile, it explains why 4-NP and 2-NP have some extent of quenching towards compounds 1 and 2 even though their LUMO energy values are higher than Py2TTz. For antibiotics, the order of quenching efficiency is also not fully in accordance with the LUMO energies of antibiotics, especially NZF displays the strongest quenching efficiency but its LUMO energy is not the lowest. On the other hand, the absorption spectra of NZF, NFT, FZD have relative larger overlaps with the emission spectra of compounds 1 and 2 (Figures S18), which indicate the probability of FRET process and are consistent with their quenching efficiency. Especially the spectral overlap between the absorption spectra of NZF and the emission spectrum of compounds 1 and 2 is the largest among tested antibiotics, which explains the highest quenching efficiency of NZF. Additionally, the nonlinear behavior observed from SV plot of NZF indicates that FRET process also contributes to luminescence quenching.79 Therefore, the above-mentioned results clearly support that luminescence quenching is dominated by the PET process and the FRET process simultaneously. Moreover, the nonlinear behavior of SV plot also implies the coexistence of dynamic and static quenching mechanism.80 Hence the SV equation can be defined as I0/I = (1 + KD[C])(1 + KS[C]), where KS and KD are static and dynamic quenching constants, respectively. At lower


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concentration, KSKD[C]2 can be ignored. Furtherly, the SV equation can be simplified as I0/I = 1 + (KD + KS)[C]. Meanwhile, KD can be obtained according to equation τ0/τ = 1 + KD[C], where τ0 and τ are lifetimes with and without addition of analyte, respectively. Accordingly, KS also can be obtained through equation KSV = KD + KS. The lifetimes of compounds 1 and 2 were measured at various concentration of analytes, so as to confirm which quenching mechanism (dynamic or static) is predominant. Based on the luminescence decay experiments and the aforesaid equations, KD and KS of compound 1 are calculated to be 0.287 × 104 M-1 and 2.97 × 104 M-1 for TNP, 0.39 × 104 M-1 and 1.336 × 104 M-1 for NZF, respectively. KD and KS of compound 2 are 0.125 × 104 M-1 and 3.938 × 104 M-1 for TNP, 0.12 × 104 M-1 and 4.418 × 104 M-1 for NZF, respectively (Figures S19 and S20). Larger KS indicate that static quenching is predominant process. All in all, the luminescence quenching mechanism of compounds 1 and 2 is complicated, which involves dynamic and static quenching mechanism and is dominated by the PET process and the FRET process simultaneously.

Figure 7. a) HOMO and LUMO energies for the selected nitroaromatics and Py2TTz. b) HOMO and LUMO energies for the selected antibiotics and Py2TTz

■ CONCLUSION In summary, utilizing dual-ligand strategy, three mixed ligands MOFs have been successfully synthesized under solvothermal conditions. Their structures have been characterized by single-


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crystal X-ray diffraction analysis. Three MOFs possess similar two-fold interpenetrated and 3D pillar-layered framework structures with pcu topology. Meanwhile, good fluorescent properties and aqueous stabilities endow compounds 1 and 2 with potentialities for detecting in water. The results show that compounds 1 and 2 can be employed as good luminescent sensors for selectively detecting nitroaromatics TNP and antibiotics NZF in water phase. Furthermore, the mechanism of fluorescence sensing was systematically investigated. Density functional theory calculations, coupled with the spectral overlap experiments, confirm that PET process and FRET process jointly dominate the luminescence quenching of compounds 1 and 2 towards the analytes. Luminescence decay experiments indicate the quenching mechanism involves dynamic and static quenching simultaneously. On this basis, using Py2TTz and aromatic dicarboxylic or tricarboxylic acid, we will continue to construct other mixed ligands luminescent MOFs, explore their applications in the detection of environmental pollutants, reveal the relationships between their fluorescence properties and their structures. The relevant studies are underway in our laboratory. ■ ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI:10.1021/acs.cgd.xxxxxxx. Crystallographic data, TGA, PXRD patterns, IR spectra for complexes 1-3; solid-state emission spectra for liand and compounds 1 and 2; Stern-Volmer plots, spectral overlap plots for compounds 1 and 2; HOMO and LUMO energies for ligand and analytes. Accession Codes CCDC 1861683, 1861684, and 1861686 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033. ■ AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected].


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*E-mail: [email protected]. ORCID Shuang-Quan Zang: 0000-0002-6728-0559 Notes The authors declare no competing financial interest. ■ ACKNOWLEDGMENTS The authors are grateful for thefinancial aid from the National Natural Science Foundation of China (No. 21371154, 21371153), the Program for Innovative Research Team (in Science and Technology) in Universities of Henan Province (No. 19IRTSTHN022). ■ REFERENCES (1) Lustig, W. P.; Mukherjee, S.; Rudd, N. D.; Desai, A. V.; Li, J.; Ghosh, S. K. Metal-organic frameworks: functional luminescent and photonic materials for sensing applications. Chem. Soc. Rev. 2017, 46, 3242-3285. (2) Xiao, J. D.; Qiu, L. G.; Ke, F.; Yuan, Y. P.; Xu, G. S.; Wang, Y. M.; Jiang, X. Rapid synthesis of nanoscale terbium-based metal-organic frameworks by a combined ultrasoundvapour phase diffusion method for highly selective sensing of picric acid. J. Mater. Chem. A 2013, 1, 8745-8752. (3) Hu, Z. C.; Deibert, B. J.; Li, J. Luminescent metal-organic frameworks for chemical sensing and explosive detection. Chem. Soc. Rev., 2014, 43, 5815-5840. (4) Mantha, R.; Taylor, K. E.; Biswas, N.; Bewtra, J. K. A Continuous System for Fe0 Reduction of Nitrobenzene in Synthetic Wastewater. Environ. Sci. Technol. 2001, 35, 32313236. (5) Douvali, A.; Tsipis, A. C.; Eliseeva, S. V.; Petoud, S.; Papaefstathiou, G. S.; Malliakas, C. D.; Papadas, I.; Armatas, G. S.; Margiolaki, I.; Kanatzidis, M. G.; Lazarides, T.; Manos, M. J. Turn-On Luminescence Sensing and Real-Time Detection of Traces of Water in Organic Solvents by a Flexible Metal-Organic Framework. Angew. Chem., Int. Ed. 2015, 54, 1651-1656. (6) He, G.; Peng, H. N.; Liu, T. H.; Yang, M. N.; Zhang, Y.; Fang, Y. A novel picric acid film sensor via combination of the surface enrichment effect of chitosan films and the aggregationinduced emission effect of siloles. J. Mater. Chem. 2009, 19, 7347-7353.


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For Table of Contents Use Only

Through dual-ligand strategy, three mixed ligands MOFs (compounds 1-3) have been successfully synthesized through solvothermal method. They possess similar two-fold interpenetrated 3D framework structures. Interestingly, compounds 1 and 2 display good fluorescent properties, which can be efficiently quenched by trace amount of nitroaromatics 2,4,6-trinitrophenol (TNP) and antibiotics nitrofurazone (NZF) in water media.


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Rational Design of Three Two-Fold Interpenetrated MetalâOrganic

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