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Tetra(4-imidazoylphenyl)ethylene based metal-organic frameworks for highly selective detection of TNP and Fe3+
Journal Pre-proof Tetra(4-imidazoylphenyl)ethylene based metal-organic frameworks for highly 3+ selective detection of TNP and Fe Mengni Peng, Kun Huang, Xianglin Li, Defang Han, Qi Qiu, Linhai Jing, Dabin Qin PII:
S0022-4596(19)30498-0
DOI:
https://doi.org/10.1016/j.jssc.2019.120993
Reference:
YJSSC 120993
To appear in:
Journal of Solid State Chemistry
Received Date: 26 July 2019 Revised Date:
30 September 2019
Accepted Date: 7 October 2019
Please cite this article as: M. Peng, K. Huang, X. Li, D. Han, Q. Qiu, L. Jing, D. Qin, Tetra(4imidazoylphenyl)ethylene based metal-organic frameworks for highly selective detection of TNP and 3+ Fe , Journal of Solid State Chemistry (2019), doi: https://doi.org/10.1016/j.jssc.2019.120993. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Inc.
Tetra(4-imidazoylphenyl)ethylene based metal-organic frameworks for highly selective detection of TNP and Fe3+ Mengni Peng, Kun Huang*, Xianglin Li, Defang Han, Qi Qiu, Linhai Jing, Dabin Qin* Key Laboratory of Chemical Synthesis and Pollution Control of Sichuan Province, School of Chemistry and Chemical Engineering, China West Normal University, Nanchong 637002, P. R. China *Corresponding authors. Email: [email protected]; [email protected]. Tel. and Fax: +86-817-2568081.
1
Abstract:
Two
new
3D
metal-organic
frameworks
(MOFs),
namely
{[Cd4(L)2(oba)4]·4H2O}n (1) and {[Co4(L)2(BPDC)4·4H2O]·5MeCN·5H2O}n (2), have been fabricated by reactions of tetra(4-imidazoylphenyl)ethylene (TIPE) (L) with cadmium
nitrate
and
4,4’-oxybisbenzoic
acid
(H2oba),
cobalt
nitrate
and
4,4’-dicarboxylbenzophenone (H2BPDC), respectively, under hydrothermal condition. X-ray crystallographic analysis indicates 1 is a 4-fold interpenetrating 3D porous structure, displaying a (4, 4)-connected 4-c afw network with the point symbol of {{3.75}{3.7.7.7.7(2).7(2)}}. 2 shows a 3D→3D 4(2+2)-fold interpenetrating 2-nodal (4, 5)-connected 4,4-c 4T67 network with the Schälfli symbol of {{4.64.8}2{42.62.82}}. Significantly, fluorescence measurements reveal that 1 and 2 have good selectivity and sensitivity toward picric acid (TNP) through photo-induced electron transfer mechanism. Besides, 1 can also be employed as a highly selective sensor for Fe3+, making it a promising bifunctional sensor. Keywords: Metal-organic frameworks; Tetraphenylethylene; Fluorescence; TNP; Ferric ion 1. Introduction MOFs are generally constructed through organic ligands and metal ions or clusters, which have attracted tremendous interests over the past decades because their fantastic architectures and luminescent properties [1-3] provide diverse applications in the fields of gas storage/separation [4], catalysis [5], magnetism [6], sensors [7], drug delivery [8, 9] and so on. As we know, some tetra-imidazole derived ligands showed great potential in architecting MOFs with fantastic structures and interesting topologies [10, 11]. In recent years tetraphenylethylene (TPE) skeleton based luminescent materials draw extensive attention in molecular probe design due to their unique aggregation-induced emission (AIE) nature [12-14]. Thus, the introduction of TPE backbone to MOF structure may contribute to develop MOF based luminescent materials. The strategy of utilizing mixed-ligand for preparing MOFs with diverse structure and functions recently attracted much attention because the combination of two types of ligands such as Lewis acid and base may facilitate the charge balance, weak interaction, etc., which favor fully utilizing the coordination atoms or group as well as producing fantastic topologies [15, 16]. As stated above, it is highly desirable to produce MOFs with attractive structure by involving
2
TPE derived tetra-imidazole and dicarboxylic acid. Nitroaromatic compounds (NACs) are generally considered as highly toxic pollutants and dangerous explosives, which pose a huge threat to human security and gradually cause environmental concerns [17]. Among them, TNP are commonly found pollutant in groundwater and soil, which may lead to severe health problems as well as fearfully environmental pollution [18]. Unfortunately, current methods for detection TNP are suffering from high-cost, complicated operation, low selectivity and sensitivity, etc. [19-21]. On the other hand, ferric ion (Fe3+), the most abundant transition metal ions in biological system, is of essential to maintain human health with a normal levels because it plays critical roles in homoeostatic processes of biological systems [22, 23]. However, the abnormal levels of Fe3+ can led to several diseases including Alzheimer's disease and Parkinson's disease. Meanwhile, the excessive distribution of Fe3+ is also considered to be environmental pollution, which is harmful to public health [24-26]. Up to date, a great many fluorescent sensors have been designed for TNP or Fe3+ with high selectivity and sensitivity [27-39], however, it is still challenging to detect TNP and Fe3+ by using a same sensor probably due to the distinct recognizing modes. As mentioned above, it is highly expected and significant to explore suitable, low-cost and simple tools for selectively and sensitively monitoring of TNP as well as Fe3+ in environmental or biological systems. For the reasons mentioned above, we herein designed and synthesized two new MOFs {[Cd4(L)2(oba)4]·4H2O}n (1) and {[Co4(L)2(BPDC)4·4H2O]·5MeCN·5H2O}n (2) by utilizing mixed-ligand strategy (Scheme 1). Among them, 1 can be developed as bifunctional sensor with high selectivity and sensitivity towards TNP and Fe3+, while 2 has a specific fluorescence quenching response towards TNP. Though there are a number of MOF based sensors that can specifically respond to TNP and Fe3+ respectively (Scheme S1), so far as we know, limited number of MOF based sensors can be utilized as bifunctional sensors for selective detection of TNP and Fe3+ [28, 29] (Table 1). By contrast, MOFs 1 exhibits larger values of Ksv, lower/comparable detection limit towards Fe3+/TNP respectively when compared to that of bifunctional sensors for TNP and Fe3+ as listed in Table 1. It is anticipated this work would shed light on the development of MOF based bifunctional sensors.
3
Scheme 1 Structures of tetra(4-imidazoylphenyl)ethylene (L), dicarboxylic acids H2oba and H2BPDC. Table 1 Some MOF based sensors for TNP (NACs) and Fe3+.
2. Experimental 2.1. Materials and measurements All commercially available chemicals and regents were used without further purification. Ligand tetra(4-imidazoylphenyl)ethylene (L) was synthesized according to the literature [12]. IR spectra were measured on a Nicolet 6700 FT-IR spectrometer (KBr disk, 400-4000 cm-1). NMR spectra were recorded on an advance 400 Bruker (1H 400 MHz and
13
C 100 MHz). Elemental analyses were investigated by a Perkin-Elmer 240
elemental analyzer. Metal content was performed on ICP-MS (thermo ICP6300). Thermogravimetric analysis (TGA) was performed on a Netzsch STA 449 F3 with a heating rate of 10 °C·min-1 in a N2 atmosphere. Melting points were obtained on an X-4 micro-melting point apparatus. Powder X-ray diffraction (PXRD) investigations were carried out on a Dmax/Ultima IV diffractometer (Rigaku Co., Japan). 2.2. Crystal structural determination Diffraction data for 1 and 2 were obtained from a Bruker APEX-II CCD
4
diffractometer equipped with a graphite monochromated Mo-Kα radiation (λ = 0.71073 Å). Crystal structures were solved by direct methods and refined on F2 full-matrix least-squares procedures using SHELXL-97 program [40]. The terminal oxygen atoms of BPDC in 2 are disordered with higher thermal values probably due to the poor quality of the crystal, which were split into two sites (O11A and O11B) with different occupancies (0.48 and 0.52). All non-hydrogen atoms were refined anisotropically and hydrogen atoms were introduced at calculated positions and refined with a riding model. The topological analyses were conducted by TOPOS program [41]. The CCDC numbers of 1 and 2 were assigned to be 1898814 and 1898815, respectively. Corresponding crystallographic data for 1 and 2 were provided in Table 2 and Table S1. Table 2. Crystallographic data for compounds 1 and 2. Compound Empirical formula Formula weight Crystal system Space group a/Å b/Å c/Å α/deg β/deg γ/deg V/Å3 T/K Z Dcalc/g cm−3 µ/mm−1 F(000) Reflections collected Unique reflections GOF R1, I > σ(I) (all) a wR2, I > σ(I) (all) b a
1 C132H88N16O24Cd4 2731.83 Triclinic P1 10.106(3) 23.323(5) 26.521(8) 90.103(5) 98.342(6) 90.546(6) 6185(3) 100 2 1.467 6.140 2752.0 29085 19969 1.128 0.1015(13777) 0.3275
R1 = Σ||Fo| − |Fc||/|Σ|Fo|, b wR2 = [Σw(Fo2 − Fc2)2/Σw(Fo2)2]1/2.
5
2 C146H121N21O29Co4 2869.37 Monoclinic P21/n 12.3635(3) 55.7700(11) 19.2732(4) 90 92.4270(10) 90 13277.2(5) 100 4 1.435 4.557 5936.0 131448 20408 1.057 0.0543 (17755) 0.1445
2.3. Synthesis of {[Cd4(L)2(obba)4]·4H2O}n (1): To a Telfon-lined stainless autoclave was added Cd(NO3)2·4H2O (35.9 mg, 0.12 mmol), 4,4'-oxybis(benzoic acid) (H2oba, 25.8 mg, 0.10 mmol) and L (30 mg, 0.05 mmol) in mixture of MeCN (6 mL) and H2O (6 mL), which were heated to 160 °C for 96 h. The resulting mixture was cooled to room temperature with the appearance of large amount of yellow crystals. The crystals were collected by filtration, washed with CH2Cl2 and dried in air. Yield: 50%. Anal. Calcd. for C132H88Cd4N16O28, C, 56.66; H, 3.17; N, 8.01. Found: C, 56.73; H, 3.25; N, 7.99. IR (KBr pellets, cm−1): 3428 (w), 3134 (w), 1597 (s), 1519 (vs), 1396 (vs), 1306 (m), 1238 (s), 1160 (m), 1118 (w), 1059 (m), 1014 (w), 963 (m), 931 (w), 876 (w), 842 (w), 807 (w), 779 (m), 659 (m), 537 (w). 2.4. Synthesis of {[Co4(L)2(BPDC)4·4H2O]·5MeCN·5H2O}n (2): Co(NO3)2·6H2O (29.1 mg, 0.01 mmol), 4,4'-benzophenonedicarboxylic acid (H2BPDC, 27.0 mg, 0.10 mmol), and L (15.2 mg, 0.026 mmol) were dissolved in CH3CN-H2O (v/v = 6 mL/6 mL) and placed in a Telfon-lined stainless autoclave, which were heated to 160 °C for 96 h. After cooling to ambient temperature, a large number of violet crystals was observed and collected by filtration, washed with CH2Cl2 and dried in air. Yield: 55%. Anal. Calcd. for C146H121Co4N21O29, C, 61.09; H, 4.25; N, 10.25. Found: C, 60.05; H, 4.31; N, 10.28. IR (KBr pellets, cm−1): 3409 (w), 3130 (w), 2924 (w), 1656 (w), 1594 (s), 1522 (vs), 1386 (s), 1304 (m), 1270 (m), 1122 (w), 1061 (m), 963 (w), 932 (w), 837 (m), 810 (m), 733 (s), 657 (w), 540 (w). 2.5. Luminescence sensing The luminescent behaviors of 1 and 2 were measured by dispersing their ground powders (0.5 mg) respectively in acetonitrile (3 mL) with 5 min ultrasonication to obtain suspensions.
Nitroaromatic
compounds
including
2,4,6-trinitrophenol
(TNP),
2,4-dinitrophenylhydrazine (2,4-DNPH), 4-nitrobenzoic acid (4-NBA), 2-nitrophenol (2-NP),
3-nitrophenol
(3-NP),
4-nitrophenol
(4-NP),
2-nitroaniline
(2-NA),
nitrobenzene(NB), 2, 4-dinitroaniline (DNA), 4-nitrochlorobenzene (PNCB) and 4-nitroaniline (4-NA) were set at concentration of 10 mM. Stock solutions of various metal ions (10 mM) were prepared in distilled water. Test samples were prepared by adding appropriate volume (v% < 2%) of the stock solutions (NACs or metal ions) into
6
suspension of MOF. All data were obtained 3 min after the addition of guest ions at room temperature. 3. Results and discussion 3.1. Structural description and characterization {[Cd4(L)2(obba)4]·4H2O}n (1) The Single crystal X-ray analysis reveals that 1 crystallizes in the triclinic P 1 space group and the asymmetric unit contains one CdII ions, two oba2- anions and one L ligand. As shown in Fig. 1a, CdII presents six-coordinated octahedral geometry involving four O atoms and two N atoms from two oba2- anions and two L ligands, respectively. The lengths of Cd-O and Cd-N are in the range of 2.255(10)-2.485(12) Å and 2.226(11)-2.314(10) Å, respectively, agreeing with that in literature [42]. The ligand L links with four CdII ions to form two types of cavities with sizes of 30.9968(147) × 22.0013(152) Å2 and 31.0546(157) × 21.9910(167) Å2 (Fig. 1b). Besides, the adjacent Cd atoms (Cd1/Cd2 and Cd3/Cd4) are bridged by H2oba to form two types of smaller cavities with sizes of 13.9796(39) × 11.2568 Å2 and 14.0814(39) × 11.1286(141) Å2, which leads to the formation of a 2D layered network along a axis. Then an infinite 3D self-penetrating framework is further generated via bridging Cd centers of the adjacent neighboring layers in µ 2-obba2- mode (Fig. 1c). Thus, 1 presents an unusual structure with 3D 4-fold interpenetrating network, which can be simplified as a (4, 4)-connected 4-c afw net with the point symbol of {{3.75}{3.7.7.7.7(2).7(2)} } (Fig. 1d).
7
Fig. 1 a) Asymmetric unit of 1, hydrogen atoms were omitted for clarity. b) 2D channels of 1. c) 3D framework of 1. d) Topological architecture of 3D→3D interpenetrating networks of 1.
{[Co4(L)2(BPDC)4·4H2O]·5MeCN·5H2O}n (2) Crystal data indicates 2 crystallizes in the monoclinic P21/n space group. The asymmetric unit consists of four CoII, two L ligands, four BPDC2- and four coordinated water molecules. All CoII are different six-coordinated model to form distorted octahedra (Fig. 2a). Co1 ions bind with two N atoms (N12, N9) from two L molecules and four O atoms from two µ 1-BPDC2- anions (O2, O10) and terminal coordinated water molecules (O2W, O3W). Co3 is linked by two bridging η2-BPDC2- and two N atoms (N14, N16) from two L ligands. Co4 and Co2 have the same coordination mode, in which CoII are coordinated by two N atoms (N6 and N4 for Co4; N1 and N8 for Co2) from two L ligands, four O atoms (O11, O12, O17 and O1W for Co4; O19, O20, O14 and O4W for Co2) from two BPDC2- anions and one coordinated water molecules, and then bridged by µ 2-η2: η1-BPDC2- anions. [43] CoII ions are further connected by L and H2BPDC to form big cavities with the sizes of 40.3841(11) × 25.8524(9) Å2 and 40.5671(12) × 26.6026(9) Å2 along a axis (Fig. 2b), which are further extended to a 3D 2-fold interpenetrating network. In addition, a scarce 3D→3D 4(2+2)-fold interpenetrating network are generated involving the interpenetration of the two identical frameworks, which belongs to Class IIa interpenetration defined by Blatov et al [44] (Fig. 2c). By considering ligand L as 4-connected nodes and CoII as
8
5-connected nodes, 2 can be simplified as a 2-nodal (4,5)-connected 4,4,-c 4T67 network with the point symbol of {{4.64.8}2{42.62.82}} (Fig. 2d).
Fig. 2 a) Partially labeled plot showing the coordination environment around Co2+ in 2. b) 2D structure of 2. c) 3D packing diagram. d) Topology of 2.
3.2. FT-IR, X-ray powder diffraction analysis and thermogravimetric analysis 1 and 2 were characterized by FT-IR with characteristic absorption peaks agreeing with their structures as provided in Fig. S1. PXRD patterns for 1 and 2 were recorded at 20 oC with scanning range of 2 - 30 o and 2 - 35 o, respectively. As shown in Fig. 3a and Fig. 3b, simulated PXRD patterns obtained from the single crystal structures agree well with the as-synthesized samples, indicating the pure phase of the prepared MOFs. Next, TG analyses of 1 and 2 were conducted within temperature scope of 30 - 600 °C (Fig. 3c). According to TG curves, two obvious weight loss processes of 1 and 2 can be clearly observed. On the one hand, from 30 - 120 oC, 1 loses four free water molecules with weight loss of 2.7% (calcd. 2.64%). A continual weight loss can be observed from 120 340 oC corresponding to the loss of 4, 4'-Oxybisbenzoic acid (found 27.51%, calcd. 27.90%) and the structural decomposition occurs beyond 340 oC. On the other hand,
9
compound 2 has weight loss of 7.79% (calcd. 7.77%) form 30 - 150 oC associated with one free water and five acetonitrile molecules. From temperature range of 150 - 340 oC, a weight loss of 5.28% (calcd. 5.02%) can be observed, probably associated with the losses of four free water molecules and four coordinated water molecules. Then the structure of 2 begins to collapse at around 340 °C. The results suggest the good thermal stabilities of 1 and 2.
Fig. 3 PXRD patterns of a) 1, b) 2 and c) TGA curves of 1 and 2.
3.3. Fluorescence properties The solid state fluorescence spectra of 1, 2 and L were examined at room temperature. As depicted in Fig. 4, an obviously single emission peak of L can be observed centered at 450 nm when excited by 350 nm wavelength, which is analogues to that reported in literatures [13, 14], mainly corresponding to π-π* electronic transitions of TPE fluorophore. However, 1 and 2 exhibit distinct fluorescence quenching as well as blue shifts in the maximum emission (1: 435 nm; 2: 425 nm) compared to free L, which can probably be ascribed to a ligand-to-metal charge transfer (LMCT) process or the coordinating effects of ligands [45-47]. Next, emission spectra of 1 and 2 were investigated by dispersing the ground powders of them respectively in different solvents such as methanol, DMF, dimethylacetamid (DMA), acetone, acetonitrile, H2O, toluene and xylene. As shown in Fig. S2, 1 and 2 show single broad emission peaks in all solvents and excellent emission intensity in acetone is presented. However, they give very weak emission in water. Considering the fluorescence intensity and sensitivity, acetone is selected as test medium for 1 and 2 in the following measurements.
10
Fig. 4 Solid-state emission spectra of 1, 2 and L, λex: 350 nm for L and 1, λex: 250 nm for 2.
3.4. Sensing behaviors towards TNP Firstly, the emission profiles of ground powders of 1 and 2 in acetone were performed to evaluate their sensing capability towards nitroaromatic compounds (NACs) including 2-NA, 3-NA, 4-NA, 2-NP, 3-NP, 4-NP, PNCB, NBA, TNP, NB, DNA and DNPH, which posed a great threat to public health for their high toxicity and explosion. As shown in Fig. 5a, the addition of TNP (10 µM) causes emission quenching efficiencies of 88.0 % and 91.3% for 1 and 2 respectively, while the equivalent amounts of other NACs elicit no obvious emission changes compared to that of free MOFs with quenching efficiencies below 26.0%, indicating the high selectivity of 1 and 2 towards TNP. To better understand the emission quenching behaviors, titration assays were conducted by gradual addition of TNP (0 - 10 µM) to the suspensions of MOFs. Obviously, 1 and 2 have significant decreases in emission intensity upon the addition of TNP at low concentration (0.67 µM), suggesting their high sensitivity to TNP. In addition, 10 µM of TNP can induce the fluorescence quenching efficiencies of no less than 88.0% for 1 and 2 (Fig. 6). Meanwhile, 1 and 2 exhibit fast response time toward TNP (within 3 min) (Fig. S3). From the titration data, Stern-Volmer (SV) curves are obtained according to the formula: I0/I = Ksv [TNP] + 1, where I0 and I refer to the emission intensity in the absence and presence of TNP, respectively; Ksv represents the quenching constant; [TNP] stands for molar concentration of TNP (insets of Fig. 6). The SV plots exhibit linear correlations at the very low concentrations of TNP, which show linear divergences and upwards bend with the additions of higher concentrations of TNP, probably indicating a
11
combined static and dynamic quenching mechanism or energy transfer between MOFs and TNP [48, 49]. Subsequently, Ksv are calculated to be 1.9×105 M-1 and 4.7×105 M-1 for 1 and 2 respectively according to titration data, which are larger than that of reported MOF with TPE backbone [50] (Fig. S4). The above facts indicate 1 and 2 have high selectivity toward TNP among various NACs. Besides, the recyclability of all samples was evaluated by washing TNP treated samples with ethanol for 3 times, which were then use for sensing TNP again (Fig. S5). The results exhibit the good recyclability of 1 and 2 for five cycles with fluorescence quenching beyond 87%.
Fig. 5 Quenching efficiencies of 1 and 2 in different NACs (10 µM).
Fig. 6 Changes in emission intensity of a) 1 and b) 2 with incremental addition of TNP (0 - 10 µM), insets: SV curves of 1 and 2.
12
In consideration of the well-known electron-deficient nature of TNP, the emission quenching phenomena of MOFs induced by TNP may be ascribed to the electron transfer from the electron donor ligands to TNP molecule. To further illustrate this possible quenching mechanism, theoretical computation was performed based on density functional theory with [B3LYP/6-31G (d, p)/level by using Gauss 09 software. As shown in Fig. 7, the lowest unoccupied molecular orbital (LUMO) energies of L (-2.0885 eV), H2OBA (-2.0523 eV); H2BPDC (-2.9500 eV) are higher than that of TNP (- 3.8877 eV), which favor the electron transfer from the first excited orbits of ligands to the LUMO orbit of TNP and thus cause emission quenching processes through photo-induced electron transfer. Furthermore, MOFs immersed in TNP were examined by Powder X-ray diffraction and the XRD patterns demonstrated the almost intactness of crystalline phase (Fig. S6, Fig. S7).
Fig.7 The optimal structure and the molecular orbital energy of TNP, H2OBA; H2BPDC and L.
3.5. Sensing of Fe3+ To develop bifunctional MOF based sensing materials, experiments were further implemented to evaluate the potential of 1 and 2 for selectively sensing metal ions, in which various metal ions were added into the dispersed suspensions of MOFs (ground powders). As shown in Fig. 8a, quenching efficiency of almost 100% can be observed upon the addition of Fe3+ (19.8 µM) to suspension of 1. However, the equivalent amounts
13
of other ions listed in Figure 8a induce insignificant quenching efficiencies of no more than 30%, revealing the high selectivity of MOF 1 towards Fe3+. To apply in more complicated systems, competing experiments were conducted with the respectively adding various metal ions to the suspension of 1, followed by treatment of equivalent amount of Fe3+. As can be seen from Fig. 8b, the presence of other metal ions has almost no significant effect on the selectivity of 1 to Fe3+, further indicating the high fluorescence selectivity of 1 to Fe3+. Nerveless, MOF 2 gives poor selectivity to Fe3+ among various metal ions (Fig. S8).
Fig. 8 a) Quenching efficiencies of 1 in different metal ions (19.8 µM), b) the effects of various metal ions towards sensing of Fe3+, λex: 350 nm
To further understand the sensing behavior of 1 towards Fe3+, titration assays were implemented through gradually increasing levels of Fe3+ in suspension of 1. As depicted in Fig. 9a, the free 1 exhibit a single emission peak centered at 440 nm and the addition of a low concentration of Fe3+ (0.33 µM) can trigger an obvious emission decrease, indicated the sensitivity of 1 to Fe3+. Then the emission intensity decreases with the increasing amount of Fe3+, which is almost completely quenched when the Fe3+ concentration is 19.80 µM. Meanwhile, we note the spectral response is fast and can be finished within 2 min (Fig. S9). Subsequently, the SV curve was obtained from the titrating experiment with linear correlations at the low concentrations of Fe3+, which shows divergences from linearity and begins to bend up with the additions of higher concentrations of Fe3+, suggesting a probable combination of static and dynamic quenching processes between Fe3+ and MOFs [51]. Accordingly, Ksv was estimated to be
14
1.6 × 105 M-1, which were analogous to that in literatures [27, 28, 50]. Moreover, the recycling experiments of 1 showed about 85% fluorescence quenching rate was obtained after three cycles, while four cycles produced a fluorescence quenching rate of less than 63% (Fig. S10).
Fig. 9 (a) Changes in emission intensity of 1 with incremental addition of Fe3+. (b) SV curve of 1 for Fe3+ (inset: Ksv for 1).
Additionally, the luminescent quenching mechanism of 1 towards Fe3+ was investigated. Firstly, PXRD pattern of 1 soaked in Fe3+ aqueous solution (10-3 M) for 24 hours was recorded, which was distinct with the pattern of free 1 (Fig. S6), indicating the probability of Fe3+ induced crystal structure destruction. Furthermore, the suspensions of 1 immersed in ferric ion (1.0 × 10-3 mol·L−1) for 24 h was centrifuged, filtrated and evaluated by ICP analyses. The results indicate that the content of Fe3+ ion was close to 1.0 × 10-3 mol·L−1, while Cd2+ was observed with the concentration of about 4.28 × 10-4 mol·L−1, further demonstrating the above presumption of framework destruction as well as providing the evidence for moderate recyclability of 1 [52]. Besides, the UV-Vis spectra of all metal ions in acetone were recorded, which showed only the absorption peak of Fe3+ and the excitation spectrum of 1 have a major overlap (350 nm) (Fig. S11, Fig. S12). The results confirm that Fe3+ can partially absorb the excited energy of 1, leading to the quenching behavior, which is similar to that mentioned in literatures [53]. Thus, a combination of competitively excited energy adsorption and framework destruction may contribute to the fluorescence quenching behavior. 4. Conclusion
15
In summary, we herein reported two novel 3D interpenetrating porous MOFs based on tetra(4-imidazoylphenyl)ethylene and dicarboxylic acid. Owing to the good luminescent properties, these new MOFs can be developed as fluorescent sensors. On the one hand, 1 and 2 show high selectivity towards TNP by fluorescence quenching via PET processes. On the other hand, 1 can also selectively and quickly respond to Fe3+ with a fluorescence quenching of nearly 100%, which makes it a bifunctional sensor for selectively sensing of Fe3+ and TNP. It is anticipated this work would shed light on the development of MOF based bifunctional fluorescent sensors for detection. Acknowledgment This work was financially supported by the National Natural Science Foundation of China (21671159), Meritocracy Research Funds of China West Normal University (17YC030), Doctoral Scientific Research Start-up Foundation of China West Normal University (18Q022). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/xxx Reference [1] W.P. Lustig, S. Mukherjee, N.D. Rudd, A.V. Desai, J. Li, S.K. Ghosh, Metal-organic frameworks: functional luminescent and photonic materials for sensing applications, Chem. Soc. Rev. 46 (2017) 3242-3285. https://doi.org/10.1039/C6CS00930A. [2] H. Furukawa, K.E. Cordova, M. O'Keeffe, O.M. Yaghi, The Chemistry and Applications of Metal-Organic Frameworks, Science 341 (2013) 1230444. http://dx.doi.org/10.1126/science. 1230444. [3] Y.M. Zhang, S. Yuan, G. Day, X. Wang, X.Y. Yang, H.C. Zhou, Luminescent sensors based on metal-organic frameworks, Coord. Chem. Rev. 354 (2018) 28-45. https://doi.org/10.1016/j.ccr.2017.06.007. [4] X. Duan, H.Z. Wang, Z.G. Ji, Y.J. Cui, Y. Yang, G.D. Qian, A novel metal-organic framework for high storage and separation of acetylene at room temperature, J. Solid State Chem. 241 (2016) 152-156. https://doi.org/10.1016/j.jssc.2016.06.015.
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23
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12403-12409.
Two new MOFs have been prepared with interpenetrating 3D porous structure; 1 can be developed as bifunctional fluorescence sensor for TNP and Fe3+ with high selectivity; 2 showed specific fluorescence quenching response towards TNP; All MOFs exhibited recyclable utilization.
Tetra(4-imidazoylphenyl)ethylene
based
metal-organic
framework
with
interpenetrating 3D porous structures have been developed as bifunctional sensor for highly selective detection of TNP and Fe3+.
Declaration of interests ☐ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☒The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
There are no interests to declare
S0022-4596(19)30498-0
DOI:
https://doi.org/10.1016/j.jssc.2019.120993
Reference:
YJSSC 120993
To appear in:
Journal of Solid State Chemistry
Received Date: 26 July 2019 Revised Date:
30 September 2019
Accepted Date: 7 October 2019
Please cite this article as: M. Peng, K. Huang, X. Li, D. Han, Q. Qiu, L. Jing, D. Qin, Tetra(4imidazoylphenyl)ethylene based metal-organic frameworks for highly selective detection of TNP and 3+ Fe , Journal of Solid State Chemistry (2019), doi: https://doi.org/10.1016/j.jssc.2019.120993. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Inc.
Tetra(4-imidazoylphenyl)ethylene based metal-organic frameworks for highly selective detection of TNP and Fe3+ Mengni Peng, Kun Huang*, Xianglin Li, Defang Han, Qi Qiu, Linhai Jing, Dabin Qin* Key Laboratory of Chemical Synthesis and Pollution Control of Sichuan Province, School of Chemistry and Chemical Engineering, China West Normal University, Nanchong 637002, P. R. China *Corresponding authors. Email: [email protected]; [email protected]. Tel. and Fax: +86-817-2568081.
1
Abstract:
Two
new
3D
metal-organic
frameworks
(MOFs),
namely
{[Cd4(L)2(oba)4]·4H2O}n (1) and {[Co4(L)2(BPDC)4·4H2O]·5MeCN·5H2O}n (2), have been fabricated by reactions of tetra(4-imidazoylphenyl)ethylene (TIPE) (L) with cadmium
nitrate
and
4,4’-oxybisbenzoic
acid
(H2oba),
cobalt
nitrate
and
4,4’-dicarboxylbenzophenone (H2BPDC), respectively, under hydrothermal condition. X-ray crystallographic analysis indicates 1 is a 4-fold interpenetrating 3D porous structure, displaying a (4, 4)-connected 4-c afw network with the point symbol of {{3.75}{3.7.7.7.7(2).7(2)}}. 2 shows a 3D→3D 4(2+2)-fold interpenetrating 2-nodal (4, 5)-connected 4,4-c 4T67 network with the Schälfli symbol of {{4.64.8}2{42.62.82}}. Significantly, fluorescence measurements reveal that 1 and 2 have good selectivity and sensitivity toward picric acid (TNP) through photo-induced electron transfer mechanism. Besides, 1 can also be employed as a highly selective sensor for Fe3+, making it a promising bifunctional sensor. Keywords: Metal-organic frameworks; Tetraphenylethylene; Fluorescence; TNP; Ferric ion 1. Introduction MOFs are generally constructed through organic ligands and metal ions or clusters, which have attracted tremendous interests over the past decades because their fantastic architectures and luminescent properties [1-3] provide diverse applications in the fields of gas storage/separation [4], catalysis [5], magnetism [6], sensors [7], drug delivery [8, 9] and so on. As we know, some tetra-imidazole derived ligands showed great potential in architecting MOFs with fantastic structures and interesting topologies [10, 11]. In recent years tetraphenylethylene (TPE) skeleton based luminescent materials draw extensive attention in molecular probe design due to their unique aggregation-induced emission (AIE) nature [12-14]. Thus, the introduction of TPE backbone to MOF structure may contribute to develop MOF based luminescent materials. The strategy of utilizing mixed-ligand for preparing MOFs with diverse structure and functions recently attracted much attention because the combination of two types of ligands such as Lewis acid and base may facilitate the charge balance, weak interaction, etc., which favor fully utilizing the coordination atoms or group as well as producing fantastic topologies [15, 16]. As stated above, it is highly desirable to produce MOFs with attractive structure by involving
2
TPE derived tetra-imidazole and dicarboxylic acid. Nitroaromatic compounds (NACs) are generally considered as highly toxic pollutants and dangerous explosives, which pose a huge threat to human security and gradually cause environmental concerns [17]. Among them, TNP are commonly found pollutant in groundwater and soil, which may lead to severe health problems as well as fearfully environmental pollution [18]. Unfortunately, current methods for detection TNP are suffering from high-cost, complicated operation, low selectivity and sensitivity, etc. [19-21]. On the other hand, ferric ion (Fe3+), the most abundant transition metal ions in biological system, is of essential to maintain human health with a normal levels because it plays critical roles in homoeostatic processes of biological systems [22, 23]. However, the abnormal levels of Fe3+ can led to several diseases including Alzheimer's disease and Parkinson's disease. Meanwhile, the excessive distribution of Fe3+ is also considered to be environmental pollution, which is harmful to public health [24-26]. Up to date, a great many fluorescent sensors have been designed for TNP or Fe3+ with high selectivity and sensitivity [27-39], however, it is still challenging to detect TNP and Fe3+ by using a same sensor probably due to the distinct recognizing modes. As mentioned above, it is highly expected and significant to explore suitable, low-cost and simple tools for selectively and sensitively monitoring of TNP as well as Fe3+ in environmental or biological systems. For the reasons mentioned above, we herein designed and synthesized two new MOFs {[Cd4(L)2(oba)4]·4H2O}n (1) and {[Co4(L)2(BPDC)4·4H2O]·5MeCN·5H2O}n (2) by utilizing mixed-ligand strategy (Scheme 1). Among them, 1 can be developed as bifunctional sensor with high selectivity and sensitivity towards TNP and Fe3+, while 2 has a specific fluorescence quenching response towards TNP. Though there are a number of MOF based sensors that can specifically respond to TNP and Fe3+ respectively (Scheme S1), so far as we know, limited number of MOF based sensors can be utilized as bifunctional sensors for selective detection of TNP and Fe3+ [28, 29] (Table 1). By contrast, MOFs 1 exhibits larger values of Ksv, lower/comparable detection limit towards Fe3+/TNP respectively when compared to that of bifunctional sensors for TNP and Fe3+ as listed in Table 1. It is anticipated this work would shed light on the development of MOF based bifunctional sensors.
3
Scheme 1 Structures of tetra(4-imidazoylphenyl)ethylene (L), dicarboxylic acids H2oba and H2BPDC. Table 1 Some MOF based sensors for TNP (NACs) and Fe3+.
2. Experimental 2.1. Materials and measurements All commercially available chemicals and regents were used without further purification. Ligand tetra(4-imidazoylphenyl)ethylene (L) was synthesized according to the literature [12]. IR spectra were measured on a Nicolet 6700 FT-IR spectrometer (KBr disk, 400-4000 cm-1). NMR spectra were recorded on an advance 400 Bruker (1H 400 MHz and
13
C 100 MHz). Elemental analyses were investigated by a Perkin-Elmer 240
elemental analyzer. Metal content was performed on ICP-MS (thermo ICP6300). Thermogravimetric analysis (TGA) was performed on a Netzsch STA 449 F3 with a heating rate of 10 °C·min-1 in a N2 atmosphere. Melting points were obtained on an X-4 micro-melting point apparatus. Powder X-ray diffraction (PXRD) investigations were carried out on a Dmax/Ultima IV diffractometer (Rigaku Co., Japan). 2.2. Crystal structural determination Diffraction data for 1 and 2 were obtained from a Bruker APEX-II CCD
4
diffractometer equipped with a graphite monochromated Mo-Kα radiation (λ = 0.71073 Å). Crystal structures were solved by direct methods and refined on F2 full-matrix least-squares procedures using SHELXL-97 program [40]. The terminal oxygen atoms of BPDC in 2 are disordered with higher thermal values probably due to the poor quality of the crystal, which were split into two sites (O11A and O11B) with different occupancies (0.48 and 0.52). All non-hydrogen atoms were refined anisotropically and hydrogen atoms were introduced at calculated positions and refined with a riding model. The topological analyses were conducted by TOPOS program [41]. The CCDC numbers of 1 and 2 were assigned to be 1898814 and 1898815, respectively. Corresponding crystallographic data for 1 and 2 were provided in Table 2 and Table S1. Table 2. Crystallographic data for compounds 1 and 2. Compound Empirical formula Formula weight Crystal system Space group a/Å b/Å c/Å α/deg β/deg γ/deg V/Å3 T/K Z Dcalc/g cm−3 µ/mm−1 F(000) Reflections collected Unique reflections GOF R1, I > σ(I) (all) a wR2, I > σ(I) (all) b a
1 C132H88N16O24Cd4 2731.83 Triclinic P1 10.106(3) 23.323(5) 26.521(8) 90.103(5) 98.342(6) 90.546(6) 6185(3) 100 2 1.467 6.140 2752.0 29085 19969 1.128 0.1015(13777) 0.3275
R1 = Σ||Fo| − |Fc||/|Σ|Fo|, b wR2 = [Σw(Fo2 − Fc2)2/Σw(Fo2)2]1/2.
5
2 C146H121N21O29Co4 2869.37 Monoclinic P21/n 12.3635(3) 55.7700(11) 19.2732(4) 90 92.4270(10) 90 13277.2(5) 100 4 1.435 4.557 5936.0 131448 20408 1.057 0.0543 (17755) 0.1445
2.3. Synthesis of {[Cd4(L)2(obba)4]·4H2O}n (1): To a Telfon-lined stainless autoclave was added Cd(NO3)2·4H2O (35.9 mg, 0.12 mmol), 4,4'-oxybis(benzoic acid) (H2oba, 25.8 mg, 0.10 mmol) and L (30 mg, 0.05 mmol) in mixture of MeCN (6 mL) and H2O (6 mL), which were heated to 160 °C for 96 h. The resulting mixture was cooled to room temperature with the appearance of large amount of yellow crystals. The crystals were collected by filtration, washed with CH2Cl2 and dried in air. Yield: 50%. Anal. Calcd. for C132H88Cd4N16O28, C, 56.66; H, 3.17; N, 8.01. Found: C, 56.73; H, 3.25; N, 7.99. IR (KBr pellets, cm−1): 3428 (w), 3134 (w), 1597 (s), 1519 (vs), 1396 (vs), 1306 (m), 1238 (s), 1160 (m), 1118 (w), 1059 (m), 1014 (w), 963 (m), 931 (w), 876 (w), 842 (w), 807 (w), 779 (m), 659 (m), 537 (w). 2.4. Synthesis of {[Co4(L)2(BPDC)4·4H2O]·5MeCN·5H2O}n (2): Co(NO3)2·6H2O (29.1 mg, 0.01 mmol), 4,4'-benzophenonedicarboxylic acid (H2BPDC, 27.0 mg, 0.10 mmol), and L (15.2 mg, 0.026 mmol) were dissolved in CH3CN-H2O (v/v = 6 mL/6 mL) and placed in a Telfon-lined stainless autoclave, which were heated to 160 °C for 96 h. After cooling to ambient temperature, a large number of violet crystals was observed and collected by filtration, washed with CH2Cl2 and dried in air. Yield: 55%. Anal. Calcd. for C146H121Co4N21O29, C, 61.09; H, 4.25; N, 10.25. Found: C, 60.05; H, 4.31; N, 10.28. IR (KBr pellets, cm−1): 3409 (w), 3130 (w), 2924 (w), 1656 (w), 1594 (s), 1522 (vs), 1386 (s), 1304 (m), 1270 (m), 1122 (w), 1061 (m), 963 (w), 932 (w), 837 (m), 810 (m), 733 (s), 657 (w), 540 (w). 2.5. Luminescence sensing The luminescent behaviors of 1 and 2 were measured by dispersing their ground powders (0.5 mg) respectively in acetonitrile (3 mL) with 5 min ultrasonication to obtain suspensions.
Nitroaromatic
compounds
including
2,4,6-trinitrophenol
(TNP),
2,4-dinitrophenylhydrazine (2,4-DNPH), 4-nitrobenzoic acid (4-NBA), 2-nitrophenol (2-NP),
3-nitrophenol
(3-NP),
4-nitrophenol
(4-NP),
2-nitroaniline
(2-NA),
nitrobenzene(NB), 2, 4-dinitroaniline (DNA), 4-nitrochlorobenzene (PNCB) and 4-nitroaniline (4-NA) were set at concentration of 10 mM. Stock solutions of various metal ions (10 mM) were prepared in distilled water. Test samples were prepared by adding appropriate volume (v% < 2%) of the stock solutions (NACs or metal ions) into
6
suspension of MOF. All data were obtained 3 min after the addition of guest ions at room temperature. 3. Results and discussion 3.1. Structural description and characterization {[Cd4(L)2(obba)4]·4H2O}n (1) The Single crystal X-ray analysis reveals that 1 crystallizes in the triclinic P 1 space group and the asymmetric unit contains one CdII ions, two oba2- anions and one L ligand. As shown in Fig. 1a, CdII presents six-coordinated octahedral geometry involving four O atoms and two N atoms from two oba2- anions and two L ligands, respectively. The lengths of Cd-O and Cd-N are in the range of 2.255(10)-2.485(12) Å and 2.226(11)-2.314(10) Å, respectively, agreeing with that in literature [42]. The ligand L links with four CdII ions to form two types of cavities with sizes of 30.9968(147) × 22.0013(152) Å2 and 31.0546(157) × 21.9910(167) Å2 (Fig. 1b). Besides, the adjacent Cd atoms (Cd1/Cd2 and Cd3/Cd4) are bridged by H2oba to form two types of smaller cavities with sizes of 13.9796(39) × 11.2568 Å2 and 14.0814(39) × 11.1286(141) Å2, which leads to the formation of a 2D layered network along a axis. Then an infinite 3D self-penetrating framework is further generated via bridging Cd centers of the adjacent neighboring layers in µ 2-obba2- mode (Fig. 1c). Thus, 1 presents an unusual structure with 3D 4-fold interpenetrating network, which can be simplified as a (4, 4)-connected 4-c afw net with the point symbol of {{3.75}{3.7.7.7.7(2).7(2)} } (Fig. 1d).
7
Fig. 1 a) Asymmetric unit of 1, hydrogen atoms were omitted for clarity. b) 2D channels of 1. c) 3D framework of 1. d) Topological architecture of 3D→3D interpenetrating networks of 1.
{[Co4(L)2(BPDC)4·4H2O]·5MeCN·5H2O}n (2) Crystal data indicates 2 crystallizes in the monoclinic P21/n space group. The asymmetric unit consists of four CoII, two L ligands, four BPDC2- and four coordinated water molecules. All CoII are different six-coordinated model to form distorted octahedra (Fig. 2a). Co1 ions bind with two N atoms (N12, N9) from two L molecules and four O atoms from two µ 1-BPDC2- anions (O2, O10) and terminal coordinated water molecules (O2W, O3W). Co3 is linked by two bridging η2-BPDC2- and two N atoms (N14, N16) from two L ligands. Co4 and Co2 have the same coordination mode, in which CoII are coordinated by two N atoms (N6 and N4 for Co4; N1 and N8 for Co2) from two L ligands, four O atoms (O11, O12, O17 and O1W for Co4; O19, O20, O14 and O4W for Co2) from two BPDC2- anions and one coordinated water molecules, and then bridged by µ 2-η2: η1-BPDC2- anions. [43] CoII ions are further connected by L and H2BPDC to form big cavities with the sizes of 40.3841(11) × 25.8524(9) Å2 and 40.5671(12) × 26.6026(9) Å2 along a axis (Fig. 2b), which are further extended to a 3D 2-fold interpenetrating network. In addition, a scarce 3D→3D 4(2+2)-fold interpenetrating network are generated involving the interpenetration of the two identical frameworks, which belongs to Class IIa interpenetration defined by Blatov et al [44] (Fig. 2c). By considering ligand L as 4-connected nodes and CoII as
8
5-connected nodes, 2 can be simplified as a 2-nodal (4,5)-connected 4,4,-c 4T67 network with the point symbol of {{4.64.8}2{42.62.82}} (Fig. 2d).
Fig. 2 a) Partially labeled plot showing the coordination environment around Co2+ in 2. b) 2D structure of 2. c) 3D packing diagram. d) Topology of 2.
3.2. FT-IR, X-ray powder diffraction analysis and thermogravimetric analysis 1 and 2 were characterized by FT-IR with characteristic absorption peaks agreeing with their structures as provided in Fig. S1. PXRD patterns for 1 and 2 were recorded at 20 oC with scanning range of 2 - 30 o and 2 - 35 o, respectively. As shown in Fig. 3a and Fig. 3b, simulated PXRD patterns obtained from the single crystal structures agree well with the as-synthesized samples, indicating the pure phase of the prepared MOFs. Next, TG analyses of 1 and 2 were conducted within temperature scope of 30 - 600 °C (Fig. 3c). According to TG curves, two obvious weight loss processes of 1 and 2 can be clearly observed. On the one hand, from 30 - 120 oC, 1 loses four free water molecules with weight loss of 2.7% (calcd. 2.64%). A continual weight loss can be observed from 120 340 oC corresponding to the loss of 4, 4'-Oxybisbenzoic acid (found 27.51%, calcd. 27.90%) and the structural decomposition occurs beyond 340 oC. On the other hand,
9
compound 2 has weight loss of 7.79% (calcd. 7.77%) form 30 - 150 oC associated with one free water and five acetonitrile molecules. From temperature range of 150 - 340 oC, a weight loss of 5.28% (calcd. 5.02%) can be observed, probably associated with the losses of four free water molecules and four coordinated water molecules. Then the structure of 2 begins to collapse at around 340 °C. The results suggest the good thermal stabilities of 1 and 2.
Fig. 3 PXRD patterns of a) 1, b) 2 and c) TGA curves of 1 and 2.
3.3. Fluorescence properties The solid state fluorescence spectra of 1, 2 and L were examined at room temperature. As depicted in Fig. 4, an obviously single emission peak of L can be observed centered at 450 nm when excited by 350 nm wavelength, which is analogues to that reported in literatures [13, 14], mainly corresponding to π-π* electronic transitions of TPE fluorophore. However, 1 and 2 exhibit distinct fluorescence quenching as well as blue shifts in the maximum emission (1: 435 nm; 2: 425 nm) compared to free L, which can probably be ascribed to a ligand-to-metal charge transfer (LMCT) process or the coordinating effects of ligands [45-47]. Next, emission spectra of 1 and 2 were investigated by dispersing the ground powders of them respectively in different solvents such as methanol, DMF, dimethylacetamid (DMA), acetone, acetonitrile, H2O, toluene and xylene. As shown in Fig. S2, 1 and 2 show single broad emission peaks in all solvents and excellent emission intensity in acetone is presented. However, they give very weak emission in water. Considering the fluorescence intensity and sensitivity, acetone is selected as test medium for 1 and 2 in the following measurements.
10
Fig. 4 Solid-state emission spectra of 1, 2 and L, λex: 350 nm for L and 1, λex: 250 nm for 2.
3.4. Sensing behaviors towards TNP Firstly, the emission profiles of ground powders of 1 and 2 in acetone were performed to evaluate their sensing capability towards nitroaromatic compounds (NACs) including 2-NA, 3-NA, 4-NA, 2-NP, 3-NP, 4-NP, PNCB, NBA, TNP, NB, DNA and DNPH, which posed a great threat to public health for their high toxicity and explosion. As shown in Fig. 5a, the addition of TNP (10 µM) causes emission quenching efficiencies of 88.0 % and 91.3% for 1 and 2 respectively, while the equivalent amounts of other NACs elicit no obvious emission changes compared to that of free MOFs with quenching efficiencies below 26.0%, indicating the high selectivity of 1 and 2 towards TNP. To better understand the emission quenching behaviors, titration assays were conducted by gradual addition of TNP (0 - 10 µM) to the suspensions of MOFs. Obviously, 1 and 2 have significant decreases in emission intensity upon the addition of TNP at low concentration (0.67 µM), suggesting their high sensitivity to TNP. In addition, 10 µM of TNP can induce the fluorescence quenching efficiencies of no less than 88.0% for 1 and 2 (Fig. 6). Meanwhile, 1 and 2 exhibit fast response time toward TNP (within 3 min) (Fig. S3). From the titration data, Stern-Volmer (SV) curves are obtained according to the formula: I0/I = Ksv [TNP] + 1, where I0 and I refer to the emission intensity in the absence and presence of TNP, respectively; Ksv represents the quenching constant; [TNP] stands for molar concentration of TNP (insets of Fig. 6). The SV plots exhibit linear correlations at the very low concentrations of TNP, which show linear divergences and upwards bend with the additions of higher concentrations of TNP, probably indicating a
11
combined static and dynamic quenching mechanism or energy transfer between MOFs and TNP [48, 49]. Subsequently, Ksv are calculated to be 1.9×105 M-1 and 4.7×105 M-1 for 1 and 2 respectively according to titration data, which are larger than that of reported MOF with TPE backbone [50] (Fig. S4). The above facts indicate 1 and 2 have high selectivity toward TNP among various NACs. Besides, the recyclability of all samples was evaluated by washing TNP treated samples with ethanol for 3 times, which were then use for sensing TNP again (Fig. S5). The results exhibit the good recyclability of 1 and 2 for five cycles with fluorescence quenching beyond 87%.
Fig. 5 Quenching efficiencies of 1 and 2 in different NACs (10 µM).
Fig. 6 Changes in emission intensity of a) 1 and b) 2 with incremental addition of TNP (0 - 10 µM), insets: SV curves of 1 and 2.
12
In consideration of the well-known electron-deficient nature of TNP, the emission quenching phenomena of MOFs induced by TNP may be ascribed to the electron transfer from the electron donor ligands to TNP molecule. To further illustrate this possible quenching mechanism, theoretical computation was performed based on density functional theory with [B3LYP/6-31G (d, p)/level by using Gauss 09 software. As shown in Fig. 7, the lowest unoccupied molecular orbital (LUMO) energies of L (-2.0885 eV), H2OBA (-2.0523 eV); H2BPDC (-2.9500 eV) are higher than that of TNP (- 3.8877 eV), which favor the electron transfer from the first excited orbits of ligands to the LUMO orbit of TNP and thus cause emission quenching processes through photo-induced electron transfer. Furthermore, MOFs immersed in TNP were examined by Powder X-ray diffraction and the XRD patterns demonstrated the almost intactness of crystalline phase (Fig. S6, Fig. S7).
Fig.7 The optimal structure and the molecular orbital energy of TNP, H2OBA; H2BPDC and L.
3.5. Sensing of Fe3+ To develop bifunctional MOF based sensing materials, experiments were further implemented to evaluate the potential of 1 and 2 for selectively sensing metal ions, in which various metal ions were added into the dispersed suspensions of MOFs (ground powders). As shown in Fig. 8a, quenching efficiency of almost 100% can be observed upon the addition of Fe3+ (19.8 µM) to suspension of 1. However, the equivalent amounts
13
of other ions listed in Figure 8a induce insignificant quenching efficiencies of no more than 30%, revealing the high selectivity of MOF 1 towards Fe3+. To apply in more complicated systems, competing experiments were conducted with the respectively adding various metal ions to the suspension of 1, followed by treatment of equivalent amount of Fe3+. As can be seen from Fig. 8b, the presence of other metal ions has almost no significant effect on the selectivity of 1 to Fe3+, further indicating the high fluorescence selectivity of 1 to Fe3+. Nerveless, MOF 2 gives poor selectivity to Fe3+ among various metal ions (Fig. S8).
Fig. 8 a) Quenching efficiencies of 1 in different metal ions (19.8 µM), b) the effects of various metal ions towards sensing of Fe3+, λex: 350 nm
To further understand the sensing behavior of 1 towards Fe3+, titration assays were implemented through gradually increasing levels of Fe3+ in suspension of 1. As depicted in Fig. 9a, the free 1 exhibit a single emission peak centered at 440 nm and the addition of a low concentration of Fe3+ (0.33 µM) can trigger an obvious emission decrease, indicated the sensitivity of 1 to Fe3+. Then the emission intensity decreases with the increasing amount of Fe3+, which is almost completely quenched when the Fe3+ concentration is 19.80 µM. Meanwhile, we note the spectral response is fast and can be finished within 2 min (Fig. S9). Subsequently, the SV curve was obtained from the titrating experiment with linear correlations at the low concentrations of Fe3+, which shows divergences from linearity and begins to bend up with the additions of higher concentrations of Fe3+, suggesting a probable combination of static and dynamic quenching processes between Fe3+ and MOFs [51]. Accordingly, Ksv was estimated to be
14
1.6 × 105 M-1, which were analogous to that in literatures [27, 28, 50]. Moreover, the recycling experiments of 1 showed about 85% fluorescence quenching rate was obtained after three cycles, while four cycles produced a fluorescence quenching rate of less than 63% (Fig. S10).
Fig. 9 (a) Changes in emission intensity of 1 with incremental addition of Fe3+. (b) SV curve of 1 for Fe3+ (inset: Ksv for 1).
Additionally, the luminescent quenching mechanism of 1 towards Fe3+ was investigated. Firstly, PXRD pattern of 1 soaked in Fe3+ aqueous solution (10-3 M) for 24 hours was recorded, which was distinct with the pattern of free 1 (Fig. S6), indicating the probability of Fe3+ induced crystal structure destruction. Furthermore, the suspensions of 1 immersed in ferric ion (1.0 × 10-3 mol·L−1) for 24 h was centrifuged, filtrated and evaluated by ICP analyses. The results indicate that the content of Fe3+ ion was close to 1.0 × 10-3 mol·L−1, while Cd2+ was observed with the concentration of about 4.28 × 10-4 mol·L−1, further demonstrating the above presumption of framework destruction as well as providing the evidence for moderate recyclability of 1 [52]. Besides, the UV-Vis spectra of all metal ions in acetone were recorded, which showed only the absorption peak of Fe3+ and the excitation spectrum of 1 have a major overlap (350 nm) (Fig. S11, Fig. S12). The results confirm that Fe3+ can partially absorb the excited energy of 1, leading to the quenching behavior, which is similar to that mentioned in literatures [53]. Thus, a combination of competitively excited energy adsorption and framework destruction may contribute to the fluorescence quenching behavior. 4. Conclusion
15
In summary, we herein reported two novel 3D interpenetrating porous MOFs based on tetra(4-imidazoylphenyl)ethylene and dicarboxylic acid. Owing to the good luminescent properties, these new MOFs can be developed as fluorescent sensors. On the one hand, 1 and 2 show high selectivity towards TNP by fluorescence quenching via PET processes. On the other hand, 1 can also selectively and quickly respond to Fe3+ with a fluorescence quenching of nearly 100%, which makes it a bifunctional sensor for selectively sensing of Fe3+ and TNP. It is anticipated this work would shed light on the development of MOF based bifunctional fluorescent sensors for detection. Acknowledgment This work was financially supported by the National Natural Science Foundation of China (21671159), Meritocracy Research Funds of China West Normal University (17YC030), Doctoral Scientific Research Start-up Foundation of China West Normal University (18Q022). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/xxx Reference [1] W.P. Lustig, S. Mukherjee, N.D. Rudd, A.V. Desai, J. Li, S.K. Ghosh, Metal-organic frameworks: functional luminescent and photonic materials for sensing applications, Chem. Soc. Rev. 46 (2017) 3242-3285. https://doi.org/10.1039/C6CS00930A. [2] H. Furukawa, K.E. Cordova, M. O'Keeffe, O.M. Yaghi, The Chemistry and Applications of Metal-Organic Frameworks, Science 341 (2013) 1230444. http://dx.doi.org/10.1126/science. 1230444. [3] Y.M. Zhang, S. Yuan, G. Day, X. Wang, X.Y. Yang, H.C. Zhou, Luminescent sensors based on metal-organic frameworks, Coord. Chem. Rev. 354 (2018) 28-45. https://doi.org/10.1016/j.ccr.2017.06.007. [4] X. Duan, H.Z. Wang, Z.G. Ji, Y.J. Cui, Y. Yang, G.D. Qian, A novel metal-organic framework for high storage and separation of acetylene at room temperature, J. Solid State Chem. 241 (2016) 152-156. https://doi.org/10.1016/j.jssc.2016.06.015.
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Two new MOFs have been prepared with interpenetrating 3D porous structure; 1 can be developed as bifunctional fluorescence sensor for TNP and Fe3+ with high selectivity; 2 showed specific fluorescence quenching response towards TNP; All MOFs exhibited recyclable utilization.
Tetra(4-imidazoylphenyl)ethylene
based
metal-organic
framework
with
interpenetrating 3D porous structures have been developed as bifunctional sensor for highly selective detection of TNP and Fe3+.
Declaration of interests ☐ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☒The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
There are no interests to declare