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Two Zn(II)-organic frameworks based on “V”-shaped terpyridine ligand and dicarboxylic ligands: Fascinating architectures and efficient luminescent aqueous-phase dual-responsive detection
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Two Zn(II)-organic frameworks based on “V”-shaped terpyridine ligand and dicarboxylic ligands: Fascinating architectures and efficient luminescent aqueous-phase dual-responsive detection Jinfang Zhang *, Qingxia Qiu , Qian Xiang , Simeng Ren , Chi Zhang ** International Joint Research Center for Photoresponsive Molecules and Materials, School of Chemical and Material Engineering, Jiangnan University, Wuxi, 214122, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Zn(II)-organic frameworks Structural elucidation Luminescence dual-responsive sensing
A “V”-shaped terpyridine ligand 3,3’:50 ,300 -terpyridine (L) was firstly applied to construct metal-organic frameworks. Two Zn-MOFs, [Zn2(L) (BDC)2(H2O)]n (1) and {[Zn(L) (ISO)]⋅H2O}n (2) (H2BDC ¼ terephthalic acid, H2ISO ¼ isophthalic acid) had been successfully synthesized by L and auxiliary dicarboxylic ligands. Structurally, 1 features an unprecedented (3,3,8)-connected framework constructed from tridentate L units, mononuclear Zn units, binuclear [Zn2(CO)2] units and BDC2 bridges. While 2 shows a rare (3,5)-connected architecture assembled from tridentate L units, mononuclear Zn units and ISO2 bridges. The different structures and topological diversities between 1 and 2 can be attributed to different twist angles between pyridine rings in L and distinct bridging angles from auxiliary ligands. Moreover, luminescent sensing experiments exhibit that 1 can act as a promising dual-responsive sensor for detecting TNP and Fe3þ in aqueous phase with high efficiency. The luminescence test paper was successfully developed to detect TNP for the practical applications. In addition, the emission quenching mechanisms of 1 towards TNP and Fe3þ were elucidated.
1. Introduction Metal-organic frameworks (MOFs) have attracted tremendous attention not only for their aesthetically fascinating structures and topological diversities [1], but also for their wide applications in gas storage [2,3], biomedicine delivery [4,5], chemical sensors [6–8] and so on. The deliberate design of organic ligand is beneficial to modulate the architectures and properties of MOFs. In comparison with ordinary organic ligands, V-shaped semi-rigid terpyridine ligands have several unique advantages: (1) the semi-rigid V-shaped ligands obtain multiple coordination sites and the free rotation of pyridyls on both sides, which can yield new MOFs with diverse structures and topological features [9,10]; (2) they contain semi-rigid π-conjugated skeletons, which offer advantages to produce strong luminescent characters. Therefore, it is of great significance to design V-shaped semi-rigid terpyridine ligands as the organic building units for constructing MOFs. Nitro-containing compounds (NCs), such as Nitromethane (NM), 2,4dinitrotoluene (2,4-DNT), 2,6-dinitrotoluene (2,6-DNT), nitrobenzene (NB) and 2,4,6-trinitrophenol (TNP) are the principal components of
high explosives and have potential threat to human health and global security. Particularly, TNP, a yellow crystalline solid commonly known as picric acid, is considered to be a powerful explosive and has an effect on human health even in a trace quantity [11,12]. Moreover, TNP and its biologically transformed products have been identified as highly toxic species to biota and may lead to chronic diseases such as sycosis and cancer [13]. On the other hand, Fe3þ is one of the most essential elements in the human body and biological tissues [14]. An excess of Fe3þ is harmful to human health. For this reason, sensing and detecting of Fe3þ also play a significant role in life science, environmental science, etc [15,16]. Thus, the development of highly efficient sensors for detecting TNP and Fe3þ is urgently needed. Among various detection methods, MOF-based luminescence detection has received considerable attention because of its quick response, high sensitivity and high selectivity etc [17–22]. Therefore, the exploitation of dual-responsive LMOF-based sensors for detecting Fe3þ and TNP is of great significance. Based on above considerations, a semi-rigid V-shaped ligand 3,3’:50 ,300 -terpyridine (L) was firstly developed to construct MOFs.
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (J. Zhang), [email protected] (C. Zhang). https://doi.org/10.1016/j.jssc.2020.121849 Received 10 October 2020; Received in revised form 4 November 2020; Accepted 4 November 2020 Available online xxxx 0022-4596/© 2020 Elsevier Inc. All rights reserved.
Please cite this article as: J. Zhang et al., Two Zn(II)-organic frameworks based on “V”-shaped terpyridine ligand and dicarboxylic ligands: Fascinating architectures and efficient luminescent aqueous-phase dual-responsive detection, Journal of Solid State Chemistry, https://doi.org/10.1016/ j.jssc.2020.121849
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[Zn2(L) (BDC)2(H2O)]n (1) and {[Zn(L) (ISO)]⋅H2O}n (2) (H2BDC ¼ terephthalic acid, H2ISO ¼ isophthalic acid) had been successfully synthesized (Scheme 1). The different twist angles of L and linear/angular coordination modes of H2BDC/H2ISO have resulted in an unprecedented (3,3,8)-connected framework for 1 and a rare (3,5)-connected architecture for 2. Moreover, 1 displays good water stability, and highly efficient sensing responses to TNP and Fe3þ. 2. Experimental section
mmol), H2ISO (16.6 mg, 0.1 mmol), 4 mL acetonitrile and 2 mL distilled water was added into a 25 mL Teflon-lined stainless steel vessel and stirred for 30 min in air, and heated at 120 C for 3 days. After cooling to room temperature at a rate of 2 C/h, colorless crystals were obtained by filtration and washed with distilled water (yield: 42.0% based on Zn). IR (KBr pellet, cm1): 3076 (s), 1620 (vs), 1557 (m), 1366 (s), 1162 (s), 1032 (m), 811 (m), 755 (w), 699 (m), 655 (m). Anal. (%) calcd for C23H17N3O5Zn: C, 57.46; H, 3.56; N, 8.74. Found: C, 57.50; H, 3.60; N, 8.70.
2.1. Materials and general methods
2.4. Crystal structure determination
All reagents and solvents were purchased commercially and used without further purification. L was synthesized under laboratory conditions according to the literature [23]. Uv–visible absorption spectra were recorded on a TU-1901 double-beam Uv–vis spectrophotometer. Elemental analysis for C, H and N were performed on a VARIOEL 3 microanalyzer. FT-IR spectra as KBr pellet was obtained by Nicolet Impact 470 FTIR. Thermogravimetric analyses (TGA) were performed with a TGA/1100SF thermal analyzer with a heating rate of 10 C min1 under a N2 atmosphere at a flow rate of 30 cm3 min1. Powder X-ray diffraction (PXRD) measurements were carried out on Bruker D8 ADVANCE X-ray diffractometer with Cu Kα1 radiation. The simulated PXRD patterns were calculated using Mercury 3.3.
Single crystal X-ray diffraction data for 1 and 2 were collected on a Bruker D8 VENTURE CMOS X-ray diffractometer with a graphite monochromated CuKα radiation (λ ¼ 1.54178 Å). Cell parameters were refined on all observed reflections using the program APEX3. The structures of 1 and 2 were solved by direct methods and refined by full-matrix leastsquares on F2 using the SHELX-2015 program package [24,25]. All non-hydrogen atoms were refined with anisotropic displacement parameters. Single-crystal diffraction experiment and refinement data for 1, 2 are listed in Table 1. The chosen bond lengths and angles are reported in Table S1. CCDC numbers are 2,013,661 and 2,013,662 for 1 and 2. 2.5. Luminescence sensing measurements
2.2. Synthesis of [Zn2(L) (BDC)2(H2O)]n (1)
Well-ground powder of sample 1 (1 mg) was suspended in 4 mL water to get emulsions. After ultrasonic treatment for 30 min, a well-dispersed and stable suspension of 1 was obtained. Then it was used for luminescence measurements and investigated at room temperature. For metalion sensing experiments, the aqueous solutions of 2 mM M(NO3)x (x ¼ 1–3) (Mxþ ¼ Naþ, Kþ, Ca2þ, Ba2þ, Mn2þ, Mg2þ, Cr3þ, Cu2þ, Co2þ, Hg2þ, Pb2þ, Fe3þ) were prepared for luminescence measurements. And the water solutions of 1 mM NCs, including NM, NB, 2,4-DNT, 2,6-DNT, TNP were also prepared for luminescence measurements.
A mixture of Zn(NO3)2⋅4H2O (59.4 mg, 0.2 mmol), L (11.7 mg, 0.05 mmol), H2BDC (16.6 mg, 0.1 mmol) and 6 mL distilled water was added into a 25 mL Teflon-lined stainless steel vessel and stirred for 30 min in air, and heated at 140 C for 3 days. After cooling to room temperature at a rate of 2 C/h, yellow crystals were obtained by filtration and washed with distilled water (yield: 55.2% based on Zn). IR (KBr pellet, cm1): 3064 (s), 2542 (vs), 1678 (m), 1575 (s), 1375 (s), 1358 (m), 1283 (m), 742 (w), 706 (m), 558 (m). Anal. (%) calcd for C31H21N3O9Zn2: C, 52.42; H, 2.98; N, 5.92. Found: C, 52.60; H, 2.70; N, 5.90.
3. Results and discussion 3.1. Structure description of [Zn2(L) (BDC)2(H2O)]n (1)
2.3. Synthesis of {[Zn(L) (ISO)]⋅H2O}n (2) A mixture of Zn(NO3)2⋅4H2O (29.7 mg, 0.1 mmol), L (11.7 mg, 0.05
Single-crystal X-ray diffraction shows that 1 crystallizes in the
Scheme 1. The formation of 1 and 2. 2
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Table 1 Summary of crystal data and structure refinement parameters for 1 and 2. Compound
1
2
Molecular formula Formula weight Temperature (K) Wavelength (Å) Crystal system Space group a (Å) b (Å) c (Å) α ( ) β ( ) γ ( ) V (Å3) Z ρcalc/Mg⋅m3 μ/mm1 F (000) Reflections collected Unique reflections Rint No. parameters GOF R1 [I > 2σ(I)] wR2 [I > 2σ(I)] Δρmax/Δρmin (e Å3)
C31H21N3O9Zn2 710.25 273 (2) 1.54178 Triclinic P-1 9.7941 (3) 10.1493 (3) 15.4226 (5) 90.0060 (10) 97.8630 (10) 112.8540 (10) 1397.04 (8) 2 1.688 2.668 720 46,689 4927 0.0492 433 1.127 0.0363 0.1027 1.111/-0.541
C23H17N3O5Zn 480.79 273 (2) 1.54178 Orthorhombic Pna21 17.9865 (4) 11.5895 (3) 9.5998 (2) 90 90 90 2001.12 (8) 4 1.596 2.080 984 7982 3093 0.0243 512 1.195 0.0278 0.0674 0.376/-0.427
triclinic P-1 space group and exhibits an unprecedented (3,3,8)-connected topology constructed from tridentate L units, mononuclear Zn units, binuclear [Zn2(CO)2] units and BDC2 bridges. The asymmetric unit is composed of two Zn2þ, one L ligand, one whole and two halves of BDC2 ligands, one terminally coordinated water molecule (Fig. S1a). In the structure of 1, BDC2 ligands connect with two crystallographically independent Zn2þ and adopt three kinds of coordination modes (Mode A: μ4-η2η2, Mode B: μ2-η1η1η1η1, Mode C: μ2-η1η1η1, Fig. S2). Zn1 is encircled by four carboxylate O atoms from three BDC2 (Mode A and C), two N atoms from two L ligands (Fig. 1a). Two Zn1 centers are bridged by two carboxylic O atoms from two independent BDC2 in Mode A to constitute a binuclear [Zn2(CO)2] unit. Unlike Zn1 showing an octahedral coordinated geometry, Zn2 center is located in a distorted trigonal bipyramidal {ZnO4N} geometry, fabricated by one N atom from L unit, three O atoms from two carboxyl groups (Mode B and C) and one O atom from coordinated H2O (Fig. 1b). The binuclear [Zn2(CO)2] and mononuclear Zn2þ building units constitute a 2D layer via the carboxyl groups (Fig. 2a), then these adjacent layers are bridged by L units to give rise to a 3D coordination framework (Fig. 2b). L coordinates with one Zn2 and two Zn1 in a tridentate coordination mode (Fig. 1b). The twist angles between the central pyridyl ring (N1C6–C10) of L and the pyridyl rings (N3C11–C15) and (N2C1–C5) are 48.77 and 10.35 , respectively (Fig. S3a). Each L unit bridges two binuclear [Zn2(CO)2] units and one mononuclear Zn2 unit by its three pyridyl N atoms therefore in a 3-connected mode (Fig. 1b); each mononuclear Zn2 unit connects another Zn2, one binuclear [Zn2(CO)2] unit and one L unit, and is also in a 3-connected manner (Fig. 1b); while each binuclear [Zn2(CO)2] unit links another two binuclear clusters, two mononuclear units and four L units in a 8-connected mode (Fig. 1a). Topologically, L units and mononuclear Zn2 units can be regarded as 3-connected nodes, whereas binuclear [Zn2(CO)2] units act as 8-connected nodes (Scheme 2). Therefore, 1 can be simplified as a (3,3,8)-connected topology with a Schl€afli symbol of (4⋅82)2 (46⋅612⋅88⋅102) (Scheme 2). TOPOS analysis shows that it is a new topology [26].
Fig. 1. View of the coordination and connection environments of Zn1 (a) and Zn2 (b) centers (all H atoms are omitted for clarity, Zn light green, C grey, O pink, and N blue), Symmetry code: a: x, 1-y, 1-z; b: 1-x, 1-y, 1-z; c: 1 þ x, y, z; d: 1þx, y, z; e: 1-x, 1-y, 1-z; f: 2 þ x, 1 þ y, z; g: 2-x, 2-y, 1-z; h: 1 þ x, 1 þ y, z; i: 1-x, 2-y, 1-z; j: 1-x, 3-y, -z. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
mononuclear Zn units, and ISO2 bridges. The asymmetric unit is composed of one Zn2þ, one L ligand, one ISO2 ligand and one free water molecule (Fig. S1b). In the structure of 2, Zn center is arranged in a sixcoordinated fashion, displaying a slightly distorted octahedral geometry. It is encircled by three O atoms from two ISO2 ligands, and three N atoms from three L units (Fig. 3a). Each L unit links three Zn centers by its three pyridyl N atoms in a 3-connected manner. Zn center connects another two Zn centers by two ISO2 bridges in μ2-η1η1η1 mode (Fig. 3b) and three L units through pyridyl N atoms in a 5-connected manner (Fig. 3a). The twist angles between the central pyridyl ring (C6–C10, N1) of L and the pyridyl rings (C1–C5, N3), (C11–C15, N2) are 43.12 and 49.85 , respectively (Fig. S3b). The structural study shows that each L unit links to three Zn centers and each Zn center connects three L units to afford a 3D (3,3)-connected framework (Scheme 3); the complexity of the (3,3)-connected framework is further increased by the incorporation of ISO2 bridges to overall forming a 3D (3,5)-connected framework (Scheme 3). In view of the topology, L units can be seen as 3-connected nodes and Zn centers act as 5-connected nodes. Therefore, 2 can be simplified as a binodal (3,5)connected 3D framework with the Schl€afli symbol of (3∙92) (32∙94∙104) (Scheme 4). To the best of our knowledge, MOFs possessing (3,5)-connected net are very rare [27,28]. Comparing the structures of 1 and 2, L shows the same 3-connected manners, while different twist angles (Fig. S3). The twist angles (43.12 and 49.85 ) of L in 2 are larger than those (10.35 and 48.77 ) in
3.2. Structure description of {[Zn(L) (ISO)]⋅H2O}n (2) Single-crystal X-ray diffraction shows that 2 crystallizes in the orthorhombic crystal system of Pna21 space group and exhibits a rare (3,5)-connected 3D framework assembled from tridentate L units, 3
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experimental patterns of them exactly match to those simulated from the single crystal data, confirming their high phase purities. Furthermore, the stability of 1 in H2O was also investigated. After immersing 1 in H2O for 7 days, the PXRD remained unchanged, which indicates that 1 possesses good water stability (Fig. S5). The thermal stabilities of 1 and 2 were investigated by TGA, 1 can maintain the framework unchanged up to 240 C, then collapses (Fig. S6a). While 2 loses guest water molecules from 75 to 175 C, and can maintain its framework up to 300 C (Fig. S6b). 3.4. Luminescence properties The solid-state luminescence properties of 1, 2 and L were monitored at ambient temperature. Excited at 311 nm, the emission peak of L is 371 nm, while the emissions of 1 and 2 are 388 nm and 383 nm which are both slightly red-shifted relative to free L ligand (Fig. S7). The central metal of 1 and 2 was Zn2þ, and the outermost electron arrangement was a stable d10 configuration, which was not easily oxidized and reduced in the framework. These emission bands were substantially neither MLCT nor LMCT. L ligand contains conjugated pyridine rings, so the emissions of 1 and 2 can be attributed to π→π* or π→n electron transition centered on the ligand [29]. The higher emission intensities of 1 and 2 compared with free L could be tentatively assigned to the influence of coordination interactions between Zn2þ and L [30]. The twist angles (10.35 and 48.77 ) of L in 1 are smaller than those (43.12 and 49.85 ) in 2 (Fig. S3), which results in higher coplanarity of L in 1 than that in 2, and leads to more efficient π–π stacking interactions for 1 than that for 2. As a result, the emission intensity of 1 is stronger than that of 2. The water suspension of 1 exhibits the strongest emission at 437 nm under the excitation at 320 nm (Fig. S8). The luminescence emission of 1 in water is red-shifted (49 nm) in comparison with that of 1 in the solid state, which may be due to the guest-dependent interaction between the framework and guest molecules [31]. The excellent water stability of 1 and strong emission in water suspension prompted us to examine its ability of sensing TNP and Fe3þ in aqueous medium. Fig. 2. The 2D layer (a) views from different directions. the 3D packing diagram (b).
3.5. Luminescence sensing of TNP in H2O Luminescent titration experiments are performed with gradual dropping NCs into the H2O suspensions of 1. As illustrated in Figs. 4a, 5, and S9, the emission intensity of 1 can be weakened by all NCs, while quenching efficiencies for NCs are distinctly different. When the volume of NCs reached to 260 μL, quenching efficiencies of TNP, 2,4-DNT, 2,6DNT, NB, NM are 84%, 29%, 18%, 16%, 13%, respectively (Fig. 5). PXRD patterns keep with each other between original sample and that after sensing TNP (Fig. S5), indicating that the framework of 1 is still high integrity after TNP detection in its H2O suspension. In order to understand the luminescence quenching process, luminescence quenching efficiency can be quantitatively analyzed using the
1 (Fig. S3). The auxiliary ligands in 1 and 2 act as linear linkers with distinct bridging angles. The bridging angles of BDC2 and ISO2 are about 180 and 118 , respectively (Scheme 1). The framework differences between 1 and 2 can be attributed to the different twist angles of L and different bridging angles of auxiliary ligands. 3.3. PXRD and TGA The phase purity of 1 and 2 were confirmed using experimental powder X-ray diffraction (PXRD). As shown in Fig. S4, the individual
Scheme 2. The topological formation of 1. Left: the corresponding topological nodes for the building units. Right: the topology of 1. Color code: L blue, Zn2 units light green, [Zn2(CO)2] units purple. 4
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Fig. 3. Coordination environments (a) of Zn2þ, ISO2 and L in 2.1D chain (b) formed by L ligands and 1D helix chain formed by ISO2 viewed approximately along a axis.
Scheme 3. Left: the 3D (3,3)-connected framework constructed by Zn centers and L units. (only L units and Zn centers were considered); right: the overall 3D (3,5)connected packing diagram of 2 approximately along the b axis (Zn light green, N blue, O pink, C grey).
Scheme 4. The topological formation of 2. Left: the corresponding topological nodes for the building units. Right: the topology of 2. Color code: L pink, Zn blue.
Stern-Volmer (SV) equation: I0/I ¼ 1þKsv [M] [32], where I0 and I represent the maximum luminescence intensity of 1 before and after addition of relevant analytes, Ksv is the quenching constant (M1), and [M] is the concentration of analyte (mM). The S–V plot for TNP is nearly linear at low concentration, and the Ksv value of 1 is found to be 6.17 104 M1 (Fig. 4b), which is obviously higher than those for 2,4-DNT (4.69 103 M1), 2,6-DNT (3.51 103 M1), NB (2.96 103 M1) and NM (1.9 103 M1), respectively (Fig. S10). The obtained Ksv value is comparable to or even better than those for the most previous LMOF-based TNP sensors (Table 2). The emission intensity of 1 decreased sharply in about 30s when the addition solution amount of TNP (1 mM) was 260 μL, and slightly changed in the following 5 min (Fig. S11), indicating the fast response of 1 for sensing TNP. Moreover, the detection limit (LOD) of 1, calculated with 3σ/slope [33,34] (σ: standard), are 6.17 104 mM, 1.52 103 mM, 2.12 103mM, 2.73 103 mM, and 3.61 103 mM for TNP, 2,4-DNT, 2,6-DNT, NB and
NM, respectively (Figs. 4c and S12). These results demonstrate that 1 possesses high sensitivity for sensing TNP in aqueous medium. In order to assess the selectivity of 1 for TNP in the presence of other NCs, a competitive luminescence quenching test was conducted. Initially, the luminescence intensity of 1 in water was recorded, then two equal portions of an aqueous NM solution (total 40 μL, 1 mM) were added into the above suspension, and the slight luminescence quenching was recorded. However, a significant luminescence quenching was observed when the same quantity of the TNP aqueous solution was added to the above solution (Fig. 6). The trend repeated even in subsequent two cycles in which NM and TNP were added. The similar results were observed in competitive tests with other NCs. It is reasonable to conclude that 1 shows highly selectivity for TNP over other NCs (Fig. 6). In an attempt to verify the recyclability of 1, fluorescence quenching efficiencies were recorded (Fig. S13). The suspension with 1 and TNP was centrifuged and washed several times with water, then dried at
5
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Fig. 5. Quenching efficiencies of 1 for different NCs (1 mM, 260 μL).
Table 2 The quenching constant of reported LMOFs for sensing TNP in H2O suspension. Molecular Formula
Quenching Constant (Ksv/ M1)
Ref.
[Zn2(L) (BDC)2(H2O)]n
6.17 104 3.20 10
This work [35]
1.36 104 2.10 104
[36] [37]
1.5 104 2.90 104 2.68 104 3.14 104 3.11 104 1.63 104
[37] [38] [39] [40] [41] [42]
[(CH3)2NH2]3 [Zn4Na(BPTC)3] ∙4CH3–OH∙2DMF Zn2(H2L)2(Bpy)2(H2O)3⋅H2O [Eu3 (bpydb)3(HCOO) (μ3-OH)2(H2O)]⋅(x solvent) [Eu3 (bpydb)3(HCOO) (μ3-OH)2(H)] Zr6O4(OH)4(L)6 [Cd (5-BrIP) (TIB)]n [Zn (μ2-1H-ade) (μ2-SO4)]n [Zn2(L)2 (azp)] [Cd2 (2-abpt) (Hbtca) (H2btca)0.5(H2O)]n
4
coated papers at ambient environment. Then the H2O solutions of NCs were dropped onto these pristine papers. It was noticed that the paper in contact with TNP showed significant luminescence quenching, while those dipped in other NCs had negligible fluorescence quenching responses (Fig. S14). This demonstrates the potential of 1 for more practical sensing applications. 3.6. Luminescence sensing of Fe3þ in H2O The sensing experiments of 1 toward metal ions were carried out. After immersion in 2 mM solutions of various metal ions, 1 displays markedly different luminescence intensity. As the addition of Fe3þ, the luminescence intensity of 1 is obviously quenched (Fig. 7). However, a slight luminescence decline was observed for Naþ, Kþ, Ca2þ, Ba2þ, Mn2þ, Mg2þ, Cr3þ, Cu2þ, Co2þ, Hg2þ, Pb2þ. The sensing sensitivity of 1 towards Fe3þ was studied by monitoring the luminescence intensity of 1 with gradual addition of Fe3þ (2 mM) (Fig. 8a). The luminescence of suspension was significantly quenched (90%) when the concentration is 1.61 104 M. Quantitatively, the quenching efficiency can be estimated using S–V equation at low concentrations, and the Ksv was calculated to be 1.99 104 M1 (Fig. 8b), which is superior to those in ever reported works (Table 3). As shown in Fig. S10, the emission intensity gives a drastic decrease within 30s when the solution amount of Fe3þ was 220 μL, and subsequently changes a little, which suggests its fast response for Fe3þ sensing. In addition, the LOD was found to be 1.55 103 mM (Fig. 8c). These results show that 1 exhibits high detection sensitivity towards Fe3þ. To explore the selective detection ability of 1 for Fe3þ, the
Fig. 4. Emission spectra (a) of 1 dispersed in H2O upon gradual addition of TNP solution (1 mM) (λex ¼ 320 nm). The plot (b) of relative emission intensity of 1 in H2O suspension (inset: s-v plot of I0/I-1 vs the concentration of TNP (0–0.002 mM); the detection limit (c) for TNP of 1 in H2O suspension.
ambient temperature and the fluorescence respond was recorded. After five cycles, the luminescence intensities of 1 remain nearly unchanged, indicating that 1 displays high recyclability and stability in TNP detecting. PXRD pattern in H2O also confirms its strong stability (Fig. S5). The results suggest that 1 can be used as a promising practical TNP sensor. For practical application, a luminescence test paper was prepared to realize intuitive and portable detection of TNP. In detail, the pristine papers were immersed in H2O suspension of 1 and dried to be MOF6
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Fig. 6. Competitive luminescence quenching of 1 upon addition of other NCs followed by TNP.
Fig. 7. Quenching efficiency of 1 in aqueous solutions of different ions.
interference experiment was conducted. Firstly, the luminescence intensity of 1 in water suspension was recorded. Then the Fe3þ aqueous solution (220 μL, 2 mM) with equal concentrations different interference metal ions was added into the suspension, and the changes of emission intensities were recorded. As shown in Fig. 9 and Fig. S15, obvious decrease can be observed with addition of Fe3þ upon the existence of listed ions, which indicates its high selective performance. Moreover, 1 can be recovered by centrifugation and deionized water washing, and the original emission intensity and quenching ability have negligible change after 5 cycles (Fig. S16). PXRD pattern demonstrates that 1 is still stable after recycling (Fig. S5). 3.7. Mechanism of luminescence detection To better understand the mechanism for highly efficient sensing of TNP and Fe3þ of 1, a series of experiments had been conducted. The absorption of TNP has the greatest overlap with the excitation and emission of 1, while a small amount of overlap was observed for other NCs (Fig. S17). This implies an energy absorption competition between 1 and TNP as well as a resonance energy transfer from 1 to the nonemissive analyte TNP [47–49]. Moreover, the conduction band of MOFs usually lie at higher energy than the LUMOs of electron deficient NCs, which facilitates electron transfer from the conduction band of MOFs to the LUMOs of NCs upon excitation, and thus results in luminescence quenching [50]. TNP possesses the lowest LUMO energy level
Fig. 8. (a) Emissions of 1 upon incremental addition of 2 mM Fe3þ solution; (b) The plot of relative emission intensity of 1 in H2O suspension (inset: s-v plot of I0/I-1 vs the concentration of Fe3þ (0–0.004 mM); (c) LOD of 1 in H2O suspension.
among the analyzed NCs [51], thus electron transfer most easily occurs from 1 to TNP. Therefore, the highly efficient luminescence quenching of 1 for TNP can be attributed to energy absorption competition, resonance energy transfer and photo-induced electron transfer. The reason of highly efficient luminescence quenching of 1 by Fe3þ 7
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Declaration of competing interest
Table 3 The quenching constants of the reported MOFs for detecting Fe3þ. Molecular Formula
Quenching Constant (Ksv/ M1)
Ref.
[Zn2(L) (BDC)2(H2O)]n
1.99 104
This work [43] [44] [45] [46]
[Zn2(L)2 (TPA)]∙2H2O {[Tb(Cmdcp) (H2O)3]2(NO3)2⋅5H2O}n [Ag(CIP)] [Tb3 (TCA)2 (DMA)0.5(OH)3(H2O)0.5]⋅ 3H2O
6.40 5.53 7.10 2.69
3
10 103 103 103
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Fig. 9. Selective detection of 1 for Fe3þ in the presence of other interference metal ions.
would be explained from the following several ways. Firstly, PXRD pattern indicates that 1 can maintain its structural integrity after the introduction of Fe3þ. This reveals that the luminescence quenching is not caused by the collapse of framework (Fig. S5). Secondly, the Uv–vis absorption spectra of metal ions in aqueous solution were measured, the absorption spectrum of Fe3þ has an observable peak in the range of 260–398 nm, which has a large overlap with the excitation spectrum of 1 (Fig. S18). This means that there is an energy absorption competition between 1 and Fe3þ, resulting in highly efficient quenching in luminescence intensity [51]. 4. Conclusions In summary, MOFs were successfully constructed by a “V”-shaped terpyridine-ligand 3,3’:50 ,300 -terpyridine (L) for the first time. Different twist angles of L and distinct bridging angles of auxiliary ligands give rise to different structures for 1 and 2.1 shows an unprecedented (3,3,8)connected 3D framework, while 2 exhibits a rare (3,5)-connected architecture. Moreover, 1 can serve as an aqueous-phase dual-responsive luminescence sensor, and shows highly efficient detection of TNP and Fe3þ. The test paper provides a simple and reliable detection application for TNP. The mechanisms of 1 for detecting TNP and Fe3þ were elucidated. Further studies on the synthesis of “V”-shaped terpyridine-ligandbased MOFs and their luminescence sensing properties are currently underway. CRediT authorship contribution statement Jinfang Zhang: Supervision, Writing - review & editing. Qingxia Qiu: Investigation, Conceptualization, Writing - original draft. Qian Xiang: Investigation. Simeng Ren: Validation. Chi Zhang: Supervision.
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Contents lists available at ScienceDirect
Journal of Solid State Chemistry journal homepage: www.elsevier.com/locate/jssc
Two Zn(II)-organic frameworks based on “V”-shaped terpyridine ligand and dicarboxylic ligands: Fascinating architectures and efficient luminescent aqueous-phase dual-responsive detection Jinfang Zhang *, Qingxia Qiu , Qian Xiang , Simeng Ren , Chi Zhang ** International Joint Research Center for Photoresponsive Molecules and Materials, School of Chemical and Material Engineering, Jiangnan University, Wuxi, 214122, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Zn(II)-organic frameworks Structural elucidation Luminescence dual-responsive sensing
A “V”-shaped terpyridine ligand 3,3’:50 ,300 -terpyridine (L) was firstly applied to construct metal-organic frameworks. Two Zn-MOFs, [Zn2(L) (BDC)2(H2O)]n (1) and {[Zn(L) (ISO)]⋅H2O}n (2) (H2BDC ¼ terephthalic acid, H2ISO ¼ isophthalic acid) had been successfully synthesized by L and auxiliary dicarboxylic ligands. Structurally, 1 features an unprecedented (3,3,8)-connected framework constructed from tridentate L units, mononuclear Zn units, binuclear [Zn2(CO)2] units and BDC2 bridges. While 2 shows a rare (3,5)-connected architecture assembled from tridentate L units, mononuclear Zn units and ISO2 bridges. The different structures and topological diversities between 1 and 2 can be attributed to different twist angles between pyridine rings in L and distinct bridging angles from auxiliary ligands. Moreover, luminescent sensing experiments exhibit that 1 can act as a promising dual-responsive sensor for detecting TNP and Fe3þ in aqueous phase with high efficiency. The luminescence test paper was successfully developed to detect TNP for the practical applications. In addition, the emission quenching mechanisms of 1 towards TNP and Fe3þ were elucidated.
1. Introduction Metal-organic frameworks (MOFs) have attracted tremendous attention not only for their aesthetically fascinating structures and topological diversities [1], but also for their wide applications in gas storage [2,3], biomedicine delivery [4,5], chemical sensors [6–8] and so on. The deliberate design of organic ligand is beneficial to modulate the architectures and properties of MOFs. In comparison with ordinary organic ligands, V-shaped semi-rigid terpyridine ligands have several unique advantages: (1) the semi-rigid V-shaped ligands obtain multiple coordination sites and the free rotation of pyridyls on both sides, which can yield new MOFs with diverse structures and topological features [9,10]; (2) they contain semi-rigid π-conjugated skeletons, which offer advantages to produce strong luminescent characters. Therefore, it is of great significance to design V-shaped semi-rigid terpyridine ligands as the organic building units for constructing MOFs. Nitro-containing compounds (NCs), such as Nitromethane (NM), 2,4dinitrotoluene (2,4-DNT), 2,6-dinitrotoluene (2,6-DNT), nitrobenzene (NB) and 2,4,6-trinitrophenol (TNP) are the principal components of
high explosives and have potential threat to human health and global security. Particularly, TNP, a yellow crystalline solid commonly known as picric acid, is considered to be a powerful explosive and has an effect on human health even in a trace quantity [11,12]. Moreover, TNP and its biologically transformed products have been identified as highly toxic species to biota and may lead to chronic diseases such as sycosis and cancer [13]. On the other hand, Fe3þ is one of the most essential elements in the human body and biological tissues [14]. An excess of Fe3þ is harmful to human health. For this reason, sensing and detecting of Fe3þ also play a significant role in life science, environmental science, etc [15,16]. Thus, the development of highly efficient sensors for detecting TNP and Fe3þ is urgently needed. Among various detection methods, MOF-based luminescence detection has received considerable attention because of its quick response, high sensitivity and high selectivity etc [17–22]. Therefore, the exploitation of dual-responsive LMOF-based sensors for detecting Fe3þ and TNP is of great significance. Based on above considerations, a semi-rigid V-shaped ligand 3,3’:50 ,300 -terpyridine (L) was firstly developed to construct MOFs.
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (J. Zhang), [email protected] (C. Zhang). https://doi.org/10.1016/j.jssc.2020.121849 Received 10 October 2020; Received in revised form 4 November 2020; Accepted 4 November 2020 Available online xxxx 0022-4596/© 2020 Elsevier Inc. All rights reserved.
Please cite this article as: J. Zhang et al., Two Zn(II)-organic frameworks based on “V”-shaped terpyridine ligand and dicarboxylic ligands: Fascinating architectures and efficient luminescent aqueous-phase dual-responsive detection, Journal of Solid State Chemistry, https://doi.org/10.1016/ j.jssc.2020.121849
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[Zn2(L) (BDC)2(H2O)]n (1) and {[Zn(L) (ISO)]⋅H2O}n (2) (H2BDC ¼ terephthalic acid, H2ISO ¼ isophthalic acid) had been successfully synthesized (Scheme 1). The different twist angles of L and linear/angular coordination modes of H2BDC/H2ISO have resulted in an unprecedented (3,3,8)-connected framework for 1 and a rare (3,5)-connected architecture for 2. Moreover, 1 displays good water stability, and highly efficient sensing responses to TNP and Fe3þ. 2. Experimental section
mmol), H2ISO (16.6 mg, 0.1 mmol), 4 mL acetonitrile and 2 mL distilled water was added into a 25 mL Teflon-lined stainless steel vessel and stirred for 30 min in air, and heated at 120 C for 3 days. After cooling to room temperature at a rate of 2 C/h, colorless crystals were obtained by filtration and washed with distilled water (yield: 42.0% based on Zn). IR (KBr pellet, cm1): 3076 (s), 1620 (vs), 1557 (m), 1366 (s), 1162 (s), 1032 (m), 811 (m), 755 (w), 699 (m), 655 (m). Anal. (%) calcd for C23H17N3O5Zn: C, 57.46; H, 3.56; N, 8.74. Found: C, 57.50; H, 3.60; N, 8.70.
2.1. Materials and general methods
2.4. Crystal structure determination
All reagents and solvents were purchased commercially and used without further purification. L was synthesized under laboratory conditions according to the literature [23]. Uv–visible absorption spectra were recorded on a TU-1901 double-beam Uv–vis spectrophotometer. Elemental analysis for C, H and N were performed on a VARIOEL 3 microanalyzer. FT-IR spectra as KBr pellet was obtained by Nicolet Impact 470 FTIR. Thermogravimetric analyses (TGA) were performed with a TGA/1100SF thermal analyzer with a heating rate of 10 C min1 under a N2 atmosphere at a flow rate of 30 cm3 min1. Powder X-ray diffraction (PXRD) measurements were carried out on Bruker D8 ADVANCE X-ray diffractometer with Cu Kα1 radiation. The simulated PXRD patterns were calculated using Mercury 3.3.
Single crystal X-ray diffraction data for 1 and 2 were collected on a Bruker D8 VENTURE CMOS X-ray diffractometer with a graphite monochromated CuKα radiation (λ ¼ 1.54178 Å). Cell parameters were refined on all observed reflections using the program APEX3. The structures of 1 and 2 were solved by direct methods and refined by full-matrix leastsquares on F2 using the SHELX-2015 program package [24,25]. All non-hydrogen atoms were refined with anisotropic displacement parameters. Single-crystal diffraction experiment and refinement data for 1, 2 are listed in Table 1. The chosen bond lengths and angles are reported in Table S1. CCDC numbers are 2,013,661 and 2,013,662 for 1 and 2. 2.5. Luminescence sensing measurements
2.2. Synthesis of [Zn2(L) (BDC)2(H2O)]n (1)
Well-ground powder of sample 1 (1 mg) was suspended in 4 mL water to get emulsions. After ultrasonic treatment for 30 min, a well-dispersed and stable suspension of 1 was obtained. Then it was used for luminescence measurements and investigated at room temperature. For metalion sensing experiments, the aqueous solutions of 2 mM M(NO3)x (x ¼ 1–3) (Mxþ ¼ Naþ, Kþ, Ca2þ, Ba2þ, Mn2þ, Mg2þ, Cr3þ, Cu2þ, Co2þ, Hg2þ, Pb2þ, Fe3þ) were prepared for luminescence measurements. And the water solutions of 1 mM NCs, including NM, NB, 2,4-DNT, 2,6-DNT, TNP were also prepared for luminescence measurements.
A mixture of Zn(NO3)2⋅4H2O (59.4 mg, 0.2 mmol), L (11.7 mg, 0.05 mmol), H2BDC (16.6 mg, 0.1 mmol) and 6 mL distilled water was added into a 25 mL Teflon-lined stainless steel vessel and stirred for 30 min in air, and heated at 140 C for 3 days. After cooling to room temperature at a rate of 2 C/h, yellow crystals were obtained by filtration and washed with distilled water (yield: 55.2% based on Zn). IR (KBr pellet, cm1): 3064 (s), 2542 (vs), 1678 (m), 1575 (s), 1375 (s), 1358 (m), 1283 (m), 742 (w), 706 (m), 558 (m). Anal. (%) calcd for C31H21N3O9Zn2: C, 52.42; H, 2.98; N, 5.92. Found: C, 52.60; H, 2.70; N, 5.90.
3. Results and discussion 3.1. Structure description of [Zn2(L) (BDC)2(H2O)]n (1)
2.3. Synthesis of {[Zn(L) (ISO)]⋅H2O}n (2) A mixture of Zn(NO3)2⋅4H2O (29.7 mg, 0.1 mmol), L (11.7 mg, 0.05
Single-crystal X-ray diffraction shows that 1 crystallizes in the
Scheme 1. The formation of 1 and 2. 2
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Table 1 Summary of crystal data and structure refinement parameters for 1 and 2. Compound
1
2
Molecular formula Formula weight Temperature (K) Wavelength (Å) Crystal system Space group a (Å) b (Å) c (Å) α ( ) β ( ) γ ( ) V (Å3) Z ρcalc/Mg⋅m3 μ/mm1 F (000) Reflections collected Unique reflections Rint No. parameters GOF R1 [I > 2σ(I)] wR2 [I > 2σ(I)] Δρmax/Δρmin (e Å3)
C31H21N3O9Zn2 710.25 273 (2) 1.54178 Triclinic P-1 9.7941 (3) 10.1493 (3) 15.4226 (5) 90.0060 (10) 97.8630 (10) 112.8540 (10) 1397.04 (8) 2 1.688 2.668 720 46,689 4927 0.0492 433 1.127 0.0363 0.1027 1.111/-0.541
C23H17N3O5Zn 480.79 273 (2) 1.54178 Orthorhombic Pna21 17.9865 (4) 11.5895 (3) 9.5998 (2) 90 90 90 2001.12 (8) 4 1.596 2.080 984 7982 3093 0.0243 512 1.195 0.0278 0.0674 0.376/-0.427
triclinic P-1 space group and exhibits an unprecedented (3,3,8)-connected topology constructed from tridentate L units, mononuclear Zn units, binuclear [Zn2(CO)2] units and BDC2 bridges. The asymmetric unit is composed of two Zn2þ, one L ligand, one whole and two halves of BDC2 ligands, one terminally coordinated water molecule (Fig. S1a). In the structure of 1, BDC2 ligands connect with two crystallographically independent Zn2þ and adopt three kinds of coordination modes (Mode A: μ4-η2η2, Mode B: μ2-η1η1η1η1, Mode C: μ2-η1η1η1, Fig. S2). Zn1 is encircled by four carboxylate O atoms from three BDC2 (Mode A and C), two N atoms from two L ligands (Fig. 1a). Two Zn1 centers are bridged by two carboxylic O atoms from two independent BDC2 in Mode A to constitute a binuclear [Zn2(CO)2] unit. Unlike Zn1 showing an octahedral coordinated geometry, Zn2 center is located in a distorted trigonal bipyramidal {ZnO4N} geometry, fabricated by one N atom from L unit, three O atoms from two carboxyl groups (Mode B and C) and one O atom from coordinated H2O (Fig. 1b). The binuclear [Zn2(CO)2] and mononuclear Zn2þ building units constitute a 2D layer via the carboxyl groups (Fig. 2a), then these adjacent layers are bridged by L units to give rise to a 3D coordination framework (Fig. 2b). L coordinates with one Zn2 and two Zn1 in a tridentate coordination mode (Fig. 1b). The twist angles between the central pyridyl ring (N1C6–C10) of L and the pyridyl rings (N3C11–C15) and (N2C1–C5) are 48.77 and 10.35 , respectively (Fig. S3a). Each L unit bridges two binuclear [Zn2(CO)2] units and one mononuclear Zn2 unit by its three pyridyl N atoms therefore in a 3-connected mode (Fig. 1b); each mononuclear Zn2 unit connects another Zn2, one binuclear [Zn2(CO)2] unit and one L unit, and is also in a 3-connected manner (Fig. 1b); while each binuclear [Zn2(CO)2] unit links another two binuclear clusters, two mononuclear units and four L units in a 8-connected mode (Fig. 1a). Topologically, L units and mononuclear Zn2 units can be regarded as 3-connected nodes, whereas binuclear [Zn2(CO)2] units act as 8-connected nodes (Scheme 2). Therefore, 1 can be simplified as a (3,3,8)-connected topology with a Schl€afli symbol of (4⋅82)2 (46⋅612⋅88⋅102) (Scheme 2). TOPOS analysis shows that it is a new topology [26].
Fig. 1. View of the coordination and connection environments of Zn1 (a) and Zn2 (b) centers (all H atoms are omitted for clarity, Zn light green, C grey, O pink, and N blue), Symmetry code: a: x, 1-y, 1-z; b: 1-x, 1-y, 1-z; c: 1 þ x, y, z; d: 1þx, y, z; e: 1-x, 1-y, 1-z; f: 2 þ x, 1 þ y, z; g: 2-x, 2-y, 1-z; h: 1 þ x, 1 þ y, z; i: 1-x, 2-y, 1-z; j: 1-x, 3-y, -z. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
mononuclear Zn units, and ISO2 bridges. The asymmetric unit is composed of one Zn2þ, one L ligand, one ISO2 ligand and one free water molecule (Fig. S1b). In the structure of 2, Zn center is arranged in a sixcoordinated fashion, displaying a slightly distorted octahedral geometry. It is encircled by three O atoms from two ISO2 ligands, and three N atoms from three L units (Fig. 3a). Each L unit links three Zn centers by its three pyridyl N atoms in a 3-connected manner. Zn center connects another two Zn centers by two ISO2 bridges in μ2-η1η1η1 mode (Fig. 3b) and three L units through pyridyl N atoms in a 5-connected manner (Fig. 3a). The twist angles between the central pyridyl ring (C6–C10, N1) of L and the pyridyl rings (C1–C5, N3), (C11–C15, N2) are 43.12 and 49.85 , respectively (Fig. S3b). The structural study shows that each L unit links to three Zn centers and each Zn center connects three L units to afford a 3D (3,3)-connected framework (Scheme 3); the complexity of the (3,3)-connected framework is further increased by the incorporation of ISO2 bridges to overall forming a 3D (3,5)-connected framework (Scheme 3). In view of the topology, L units can be seen as 3-connected nodes and Zn centers act as 5-connected nodes. Therefore, 2 can be simplified as a binodal (3,5)connected 3D framework with the Schl€afli symbol of (3∙92) (32∙94∙104) (Scheme 4). To the best of our knowledge, MOFs possessing (3,5)-connected net are very rare [27,28]. Comparing the structures of 1 and 2, L shows the same 3-connected manners, while different twist angles (Fig. S3). The twist angles (43.12 and 49.85 ) of L in 2 are larger than those (10.35 and 48.77 ) in
3.2. Structure description of {[Zn(L) (ISO)]⋅H2O}n (2) Single-crystal X-ray diffraction shows that 2 crystallizes in the orthorhombic crystal system of Pna21 space group and exhibits a rare (3,5)-connected 3D framework assembled from tridentate L units, 3
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experimental patterns of them exactly match to those simulated from the single crystal data, confirming their high phase purities. Furthermore, the stability of 1 in H2O was also investigated. After immersing 1 in H2O for 7 days, the PXRD remained unchanged, which indicates that 1 possesses good water stability (Fig. S5). The thermal stabilities of 1 and 2 were investigated by TGA, 1 can maintain the framework unchanged up to 240 C, then collapses (Fig. S6a). While 2 loses guest water molecules from 75 to 175 C, and can maintain its framework up to 300 C (Fig. S6b). 3.4. Luminescence properties The solid-state luminescence properties of 1, 2 and L were monitored at ambient temperature. Excited at 311 nm, the emission peak of L is 371 nm, while the emissions of 1 and 2 are 388 nm and 383 nm which are both slightly red-shifted relative to free L ligand (Fig. S7). The central metal of 1 and 2 was Zn2þ, and the outermost electron arrangement was a stable d10 configuration, which was not easily oxidized and reduced in the framework. These emission bands were substantially neither MLCT nor LMCT. L ligand contains conjugated pyridine rings, so the emissions of 1 and 2 can be attributed to π→π* or π→n electron transition centered on the ligand [29]. The higher emission intensities of 1 and 2 compared with free L could be tentatively assigned to the influence of coordination interactions between Zn2þ and L [30]. The twist angles (10.35 and 48.77 ) of L in 1 are smaller than those (43.12 and 49.85 ) in 2 (Fig. S3), which results in higher coplanarity of L in 1 than that in 2, and leads to more efficient π–π stacking interactions for 1 than that for 2. As a result, the emission intensity of 1 is stronger than that of 2. The water suspension of 1 exhibits the strongest emission at 437 nm under the excitation at 320 nm (Fig. S8). The luminescence emission of 1 in water is red-shifted (49 nm) in comparison with that of 1 in the solid state, which may be due to the guest-dependent interaction between the framework and guest molecules [31]. The excellent water stability of 1 and strong emission in water suspension prompted us to examine its ability of sensing TNP and Fe3þ in aqueous medium. Fig. 2. The 2D layer (a) views from different directions. the 3D packing diagram (b).
3.5. Luminescence sensing of TNP in H2O Luminescent titration experiments are performed with gradual dropping NCs into the H2O suspensions of 1. As illustrated in Figs. 4a, 5, and S9, the emission intensity of 1 can be weakened by all NCs, while quenching efficiencies for NCs are distinctly different. When the volume of NCs reached to 260 μL, quenching efficiencies of TNP, 2,4-DNT, 2,6DNT, NB, NM are 84%, 29%, 18%, 16%, 13%, respectively (Fig. 5). PXRD patterns keep with each other between original sample and that after sensing TNP (Fig. S5), indicating that the framework of 1 is still high integrity after TNP detection in its H2O suspension. In order to understand the luminescence quenching process, luminescence quenching efficiency can be quantitatively analyzed using the
1 (Fig. S3). The auxiliary ligands in 1 and 2 act as linear linkers with distinct bridging angles. The bridging angles of BDC2 and ISO2 are about 180 and 118 , respectively (Scheme 1). The framework differences between 1 and 2 can be attributed to the different twist angles of L and different bridging angles of auxiliary ligands. 3.3. PXRD and TGA The phase purity of 1 and 2 were confirmed using experimental powder X-ray diffraction (PXRD). As shown in Fig. S4, the individual
Scheme 2. The topological formation of 1. Left: the corresponding topological nodes for the building units. Right: the topology of 1. Color code: L blue, Zn2 units light green, [Zn2(CO)2] units purple. 4
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Fig. 3. Coordination environments (a) of Zn2þ, ISO2 and L in 2.1D chain (b) formed by L ligands and 1D helix chain formed by ISO2 viewed approximately along a axis.
Scheme 3. Left: the 3D (3,3)-connected framework constructed by Zn centers and L units. (only L units and Zn centers were considered); right: the overall 3D (3,5)connected packing diagram of 2 approximately along the b axis (Zn light green, N blue, O pink, C grey).
Scheme 4. The topological formation of 2. Left: the corresponding topological nodes for the building units. Right: the topology of 2. Color code: L pink, Zn blue.
Stern-Volmer (SV) equation: I0/I ¼ 1þKsv [M] [32], where I0 and I represent the maximum luminescence intensity of 1 before and after addition of relevant analytes, Ksv is the quenching constant (M1), and [M] is the concentration of analyte (mM). The S–V plot for TNP is nearly linear at low concentration, and the Ksv value of 1 is found to be 6.17 104 M1 (Fig. 4b), which is obviously higher than those for 2,4-DNT (4.69 103 M1), 2,6-DNT (3.51 103 M1), NB (2.96 103 M1) and NM (1.9 103 M1), respectively (Fig. S10). The obtained Ksv value is comparable to or even better than those for the most previous LMOF-based TNP sensors (Table 2). The emission intensity of 1 decreased sharply in about 30s when the addition solution amount of TNP (1 mM) was 260 μL, and slightly changed in the following 5 min (Fig. S11), indicating the fast response of 1 for sensing TNP. Moreover, the detection limit (LOD) of 1, calculated with 3σ/slope [33,34] (σ: standard), are 6.17 104 mM, 1.52 103 mM, 2.12 103mM, 2.73 103 mM, and 3.61 103 mM for TNP, 2,4-DNT, 2,6-DNT, NB and
NM, respectively (Figs. 4c and S12). These results demonstrate that 1 possesses high sensitivity for sensing TNP in aqueous medium. In order to assess the selectivity of 1 for TNP in the presence of other NCs, a competitive luminescence quenching test was conducted. Initially, the luminescence intensity of 1 in water was recorded, then two equal portions of an aqueous NM solution (total 40 μL, 1 mM) were added into the above suspension, and the slight luminescence quenching was recorded. However, a significant luminescence quenching was observed when the same quantity of the TNP aqueous solution was added to the above solution (Fig. 6). The trend repeated even in subsequent two cycles in which NM and TNP were added. The similar results were observed in competitive tests with other NCs. It is reasonable to conclude that 1 shows highly selectivity for TNP over other NCs (Fig. 6). In an attempt to verify the recyclability of 1, fluorescence quenching efficiencies were recorded (Fig. S13). The suspension with 1 and TNP was centrifuged and washed several times with water, then dried at
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Fig. 5. Quenching efficiencies of 1 for different NCs (1 mM, 260 μL).
Table 2 The quenching constant of reported LMOFs for sensing TNP in H2O suspension. Molecular Formula
Quenching Constant (Ksv/ M1)
Ref.
[Zn2(L) (BDC)2(H2O)]n
6.17 104 3.20 10
This work [35]
1.36 104 2.10 104
[36] [37]
1.5 104 2.90 104 2.68 104 3.14 104 3.11 104 1.63 104
[37] [38] [39] [40] [41] [42]
[(CH3)2NH2]3 [Zn4Na(BPTC)3] ∙4CH3–OH∙2DMF Zn2(H2L)2(Bpy)2(H2O)3⋅H2O [Eu3 (bpydb)3(HCOO) (μ3-OH)2(H2O)]⋅(x solvent) [Eu3 (bpydb)3(HCOO) (μ3-OH)2(H)] Zr6O4(OH)4(L)6 [Cd (5-BrIP) (TIB)]n [Zn (μ2-1H-ade) (μ2-SO4)]n [Zn2(L)2 (azp)] [Cd2 (2-abpt) (Hbtca) (H2btca)0.5(H2O)]n
4
coated papers at ambient environment. Then the H2O solutions of NCs were dropped onto these pristine papers. It was noticed that the paper in contact with TNP showed significant luminescence quenching, while those dipped in other NCs had negligible fluorescence quenching responses (Fig. S14). This demonstrates the potential of 1 for more practical sensing applications. 3.6. Luminescence sensing of Fe3þ in H2O The sensing experiments of 1 toward metal ions were carried out. After immersion in 2 mM solutions of various metal ions, 1 displays markedly different luminescence intensity. As the addition of Fe3þ, the luminescence intensity of 1 is obviously quenched (Fig. 7). However, a slight luminescence decline was observed for Naþ, Kþ, Ca2þ, Ba2þ, Mn2þ, Mg2þ, Cr3þ, Cu2þ, Co2þ, Hg2þ, Pb2þ. The sensing sensitivity of 1 towards Fe3þ was studied by monitoring the luminescence intensity of 1 with gradual addition of Fe3þ (2 mM) (Fig. 8a). The luminescence of suspension was significantly quenched (90%) when the concentration is 1.61 104 M. Quantitatively, the quenching efficiency can be estimated using S–V equation at low concentrations, and the Ksv was calculated to be 1.99 104 M1 (Fig. 8b), which is superior to those in ever reported works (Table 3). As shown in Fig. S10, the emission intensity gives a drastic decrease within 30s when the solution amount of Fe3þ was 220 μL, and subsequently changes a little, which suggests its fast response for Fe3þ sensing. In addition, the LOD was found to be 1.55 103 mM (Fig. 8c). These results show that 1 exhibits high detection sensitivity towards Fe3þ. To explore the selective detection ability of 1 for Fe3þ, the
Fig. 4. Emission spectra (a) of 1 dispersed in H2O upon gradual addition of TNP solution (1 mM) (λex ¼ 320 nm). The plot (b) of relative emission intensity of 1 in H2O suspension (inset: s-v plot of I0/I-1 vs the concentration of TNP (0–0.002 mM); the detection limit (c) for TNP of 1 in H2O suspension.
ambient temperature and the fluorescence respond was recorded. After five cycles, the luminescence intensities of 1 remain nearly unchanged, indicating that 1 displays high recyclability and stability in TNP detecting. PXRD pattern in H2O also confirms its strong stability (Fig. S5). The results suggest that 1 can be used as a promising practical TNP sensor. For practical application, a luminescence test paper was prepared to realize intuitive and portable detection of TNP. In detail, the pristine papers were immersed in H2O suspension of 1 and dried to be MOF6
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Fig. 6. Competitive luminescence quenching of 1 upon addition of other NCs followed by TNP.
Fig. 7. Quenching efficiency of 1 in aqueous solutions of different ions.
interference experiment was conducted. Firstly, the luminescence intensity of 1 in water suspension was recorded. Then the Fe3þ aqueous solution (220 μL, 2 mM) with equal concentrations different interference metal ions was added into the suspension, and the changes of emission intensities were recorded. As shown in Fig. 9 and Fig. S15, obvious decrease can be observed with addition of Fe3þ upon the existence of listed ions, which indicates its high selective performance. Moreover, 1 can be recovered by centrifugation and deionized water washing, and the original emission intensity and quenching ability have negligible change after 5 cycles (Fig. S16). PXRD pattern demonstrates that 1 is still stable after recycling (Fig. S5). 3.7. Mechanism of luminescence detection To better understand the mechanism for highly efficient sensing of TNP and Fe3þ of 1, a series of experiments had been conducted. The absorption of TNP has the greatest overlap with the excitation and emission of 1, while a small amount of overlap was observed for other NCs (Fig. S17). This implies an energy absorption competition between 1 and TNP as well as a resonance energy transfer from 1 to the nonemissive analyte TNP [47–49]. Moreover, the conduction band of MOFs usually lie at higher energy than the LUMOs of electron deficient NCs, which facilitates electron transfer from the conduction band of MOFs to the LUMOs of NCs upon excitation, and thus results in luminescence quenching [50]. TNP possesses the lowest LUMO energy level
Fig. 8. (a) Emissions of 1 upon incremental addition of 2 mM Fe3þ solution; (b) The plot of relative emission intensity of 1 in H2O suspension (inset: s-v plot of I0/I-1 vs the concentration of Fe3þ (0–0.004 mM); (c) LOD of 1 in H2O suspension.
among the analyzed NCs [51], thus electron transfer most easily occurs from 1 to TNP. Therefore, the highly efficient luminescence quenching of 1 for TNP can be attributed to energy absorption competition, resonance energy transfer and photo-induced electron transfer. The reason of highly efficient luminescence quenching of 1 by Fe3þ 7
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Declaration of competing interest
Table 3 The quenching constants of the reported MOFs for detecting Fe3þ. Molecular Formula
Quenching Constant (Ksv/ M1)
Ref.
[Zn2(L) (BDC)2(H2O)]n
1.99 104
This work [43] [44] [45] [46]
[Zn2(L)2 (TPA)]∙2H2O {[Tb(Cmdcp) (H2O)3]2(NO3)2⋅5H2O}n [Ag(CIP)] [Tb3 (TCA)2 (DMA)0.5(OH)3(H2O)0.5]⋅ 3H2O
6.40 5.53 7.10 2.69
3
10 103 103 103
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. Acknowledgments This work was supporteikd by the National Natural Science Foundation of China (21671082, 51432006), the 111 Project (B13025). Appendix A. Supplementary data Supplementary data to this article can be found online at https://do i.org/10.1016/j.jssc.2020.121849. References [1] L. Carlucci, G. Ciani, D.M. Proserpio, Coord. Chem. Rev. 246 (2003) 247–289. [2] Y.W. Li, H. Yan, T. L Hu, H.Y. Ma, D.C. Li, S.N. Wang, Q.X. Yao, J.M. Dou, J. Xu, X.H. Bu, Chem. Commun. 53 (2017) 2394–2397. [3] C.Y. Gao, H.R. Tian, J.L. Ai, J. Li, S. Dang, Y.Q. Lan, Z.M. Sun, Chem. Commun. 52 (2016) 11147–11150. [4] J. Jiang, J. Yu, A. Corma, Angew. Chem. Int. Ed. 49 (2010) 3120–3145. [5] Z.B. Ma, B. Moulton, Coord. Chem. Rev. 255 (2011) 1623–1641. [6] J.F. Zhang, H.C. Xia, S.M. Ren, W. Jia, C. Zhang, New J. Chem. 44 (2020) 5285–5292. [7] J.F. Zhang, S.M. Ren, H.C. Xia, W. Jia, C. Zhang, J. Mater. Chem. C. 8 (2020) 1427–1432. [8] J.F. Zhang, Y.H. Liu, J.Y. Feng, L.P. Gong, G. Humphrey Mark, C. Zhang, Inorg. Chem. 58 (2019) 9749–9755. [9] L.L. Liu, C.X. Yu, J.M. Du, S.M. Liu, J.S. Cao, L.F. Ma, Dalton Trans. 45 (2016) 12352–12361. [10] F.L. Xun, X.L. Ling, L. Liu, H.L. Song, S.W. Ng, B.T. Su, Dalton Trans. 42 (2013) 10292–10303. [11] X. Luo, X. Zhang, Y.L. Duan, X.L. Wang, J.M. Zhao, Dalton Trans. 46 (2017) 6303–6311. [12] K.W. Hofmann, H.J. Knackmuss, G. Heiss, Appl. Environ. Microbiol. 70 (2004) 2854–2860. [13] S.S. Nagarkar, A.V. Desai, S.K. Ghosh, CrystEngComm 18 (2016) 2994–3007. [14] B. Wang, Q. Yang, C. Guo, Y. Sun, L.H. Xie, J.R. Li, ACS Appl. Mater. Interfaces 9 (2017) 10286–10295. [15] Z. Xiang, C. Fang, S. Leng, D. Cao, J. Mater. Chem. 2 (2014) 7662–7665. [16] R. Joseph, J.P. Chinta, C.P. Rao, Inorg. Chem. 50 (2011) 7050–7058. [17] X. Wang, Y. La, A. Ding, Y. Liu, Y.X. Lei, Adv. Funct. Mater. 22 (2012) 3547–3555. [18] L. Xiao, B. Yan, RSC Adv. 6 (2016) 11570–11576. [19] C.T. Lin, W.J. Boettcher, M. Chou, C. Creutz, N. Sutin, J. Am. Chem. Soc. 98 (1976) 6536–6544. [20] R.M. Melavanki, R.A. Kusanur, M.V. Kulakarni, J.S. Kadadevarmath, J. Lumin. 128 (2008) 573–577. [21] G.Y. Wang, C. Song, D.M. Kong, W.J. Ruan, Z. Chang, Y. Li, J. Mater. Chem. 2 (2014) 2213–2220. [22] J.F. Zhang, L.P. Gong, J.Y. Feng, J.J. Wu, C. Zhang, New J. Chem. 41 (2017) 8107–8117. [23] T.H. Chuang, Y.C. Chen, S. Pola, J. Org. Chem. 75 (2010) 6625–6630. [24] G.M. Sheldrick, Acta Crystallogr. A: Found. Adv. 71 (2015) 3–8. [25] G.M. Sheldrick, Acta Crystallogr. C: Struct. Chem. 71 (2015) 3–8. [26] TOPOSPRO, accessed, https://topospro.com/consulting/determining/. (Accessed May 2020). [27] H.Y. He, F.N. Dai, D.F. Sun, Dalton Trans. 5 (2009) 763–766. [28] H. Wang, F.Y. Yi, S. Dang, W.G. Tian, Z.M. Sun, Cryst. Growth Des. 14 (2014) 147–156. [29] L.S. Meng, L. Zhao, G.L. Guo, X. Lin, New J. Chem. 44 (2020) 9025–9036. [30] M.D. Allendorf, C.A. Bauer, R.K. Bhakta, R.J.T. Houk, Chem. Soc. Rev. 38 (2009) 1330–1352. [31] K.C. Stylianous, R. Heck, S.Y. Chong, J. Bacsa, J.T.A. Jones, Y.Z. Khimyak, D. Bradshaw, M.J. Rosseinsky, J. Am. Chem. Soc. 132 (2010) 4119–4130. [32] P. Taya, B. Maiti, V. Kumar, P. De, S. Satapathi, Sensor. Actuator. B Chem. 225 (2018) 2628–2634. [33] A. Buragohain, M. Yousufuddin, M. Sarma, S. Biswas, Cryst. Growth Des. 16 (2016) 842–851. [34] C.S. Cao, H.C. Hu, H. Xu, W.Z. Qiao, B. Zhao, CrystEngComm 18 (2016) 4445–4451. [35] E.L. Zhou, P. Huang, C. Qin, K.Z. Shao, Z.M. Su, J. Mater. Chem. 3 (2015) 7224–7228. [36] Y.J. Deng, N.J. Chen, Q.Y. Li, X.J. Wu, X.L. Huang, Z.H. Lin, Y.G. Zhao, Cryst. Growth Des. 17 (2017) 3170–3177. [37] X.Z. Song, S.Y. Song, S.N. Zhao, Z.M. Hao, M. Zhu, X. Meng, L.L. Wu, H.J. Zhang, Adv. Funct. Mater. 24 (2014) 4034–4041. [38] S Sanjog Nagarkar, V Aamod Desai, S.K. Ghosh, Chem. Commun. 50 (2014) 8915–8918.
Fig. 9. Selective detection of 1 for Fe3þ in the presence of other interference metal ions.
would be explained from the following several ways. Firstly, PXRD pattern indicates that 1 can maintain its structural integrity after the introduction of Fe3þ. This reveals that the luminescence quenching is not caused by the collapse of framework (Fig. S5). Secondly, the Uv–vis absorption spectra of metal ions in aqueous solution were measured, the absorption spectrum of Fe3þ has an observable peak in the range of 260–398 nm, which has a large overlap with the excitation spectrum of 1 (Fig. S18). This means that there is an energy absorption competition between 1 and Fe3þ, resulting in highly efficient quenching in luminescence intensity [51]. 4. Conclusions In summary, MOFs were successfully constructed by a “V”-shaped terpyridine-ligand 3,3’:50 ,300 -terpyridine (L) for the first time. Different twist angles of L and distinct bridging angles of auxiliary ligands give rise to different structures for 1 and 2.1 shows an unprecedented (3,3,8)connected 3D framework, while 2 exhibits a rare (3,5)-connected architecture. Moreover, 1 can serve as an aqueous-phase dual-responsive luminescence sensor, and shows highly efficient detection of TNP and Fe3þ. The test paper provides a simple and reliable detection application for TNP. The mechanisms of 1 for detecting TNP and Fe3þ were elucidated. Further studies on the synthesis of “V”-shaped terpyridine-ligandbased MOFs and their luminescence sensing properties are currently underway. CRediT authorship contribution statement Jinfang Zhang: Supervision, Writing - review & editing. Qingxia Qiu: Investigation, Conceptualization, Writing - original draft. Qian Xiang: Investigation. Simeng Ren: Validation. Chi Zhang: Supervision.
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