Two new zinc(II) coordination polymers based on asymmetric tetracarboxylic acid for fluorescent sensing

Accepted Manuscript Research paper Two new zinc(II) coordination polymers based on asymmetric tetracarboxylic acid for fluorescent sensing Jing Sun, Ping Zhang, Hui Qi, Jia Jia, Xiaodong Chen, Shubo Jing, Li Wang, Yong Fan PII: DOI: Reference:

S0020-1693(17)31111-8 https://doi.org/10.1016/j.ica.2017.09.038 ICA 17897

To appear in:

Inorganica Chimica Acta

Received Date: Revised Date: Accepted Date:

17 July 2017 13 September 2017 14 September 2017

Please cite this article as: J. Sun, P. Zhang, H. Qi, J. Jia, X. Chen, S. Jing, L. Wang, Y. Fan, Two new zinc(II) coordination polymers based on asymmetric tetracarboxylic acid for fluorescent sensing, Inorganica Chimica Acta (2017), doi: https://doi.org/10.1016/j.ica.2017.09.038

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Two new zinc(II) coordination polymers based on asymmetric tetracarboxylic acid for fluorescent sensing Jing Sun,a Ping Zhang,a Hui Qi,b Jia Jia,c Xiaodong Chen,a Shubo Jing,a Li Wang* and Yong Fan* a

College of Chemistry, Jilin University, Changchun 130012, Jilin, P. R. China.

e-mail: [email protected], [email protected]. b

The Second Hospital of Jilin University, Changchun 130041, P.R. China

c

College of Chemistry, Baicheng Normal University, Baicheng 137000, Jilin, P. R.

China. Abstract Two

new

luminescent

[Zn(H2bptc)(2,2′-bipy)(H2O)]·3H2O

coordination

zinc (1)

polymers

(CPs),

and [Zn2(bptc)(H2O)]·(4,4′-bipy)0.5 (2)

(H4bptc = 2,4,4′,6-biphenyl tetracarboxylic acid, 2,2′-bipy = 2,2′-bipyridyl, 4,4′-bipy = 4,4′-bipyridyl), have been solvothermally synthesized using zinc nitrate and asymmetric 2,4,4′,6-H4bptc ligand in the presence of auxiliary ligands. It is noted that H4bptc ligand exhibits two different coordination modes in these two CPs, constructing disparate architectures by bridging the different building units. CP 1 shows an infinite zigzag ribbon constructed from ZnO2(H2O)(2,2′-bipy) units and bridging H2bptc2- ligands. CP 2 displays a complicated 3D framework with 1D oval channels that built up from binuclear [Zn2(COO)3] second building units (SBUs) and bptc4- ligands. Moreover, they both present excellent sensing of metal ions and nitro explosives, especially for Fe3+ ion and 2,4,6-trinitrophenol (TNP). The KSV values of Fe3+ ion and TNP are 2.581 × 10 4 and 1.26 × 105 L·mol-1 for 1, 2.826 × 104 and 3.422 1

× 10 5 L·mol-1 for 2, respectively. The fluorescence quenching mechanism of 1 and 2 caused by TNP is ascribed to electron transfer and resonance energy transfer, while caused by Fe3+ may due to resonance energy transfer.

Keywords: Zinc CPs; asymmetric tetracarboxyllic acid; fluorescent sensing

2

1. Introduction As new porous solid materials, coordination polymers (CPs) built by the assembly of metal containing units and proper organic linking groups have received tremendous attention in the last two decades [1]. The tunable nature of the framework and the functionality through rational design endows this new material with promising potential applications in the fields of gas separation and storage [2], luminescence [3], heterogeneous catalysis [4], chemical sensing [5], proton conducting [6], drug delivery [7] and biomedical imaging [8]. Among the large number of reported CPs, luminescent CPs are of especially interest because the collaborative functionalities of permanent porosity and luminescent property have enabled them distinguish from traditional inorganic or organic luminescent materials [9]. There is an increasing trend in the exploration of functional luminescent CPs, especially for their potential applications in fluorescent sensors [10]. So far, a wide range of luminescent CPs for sensing cations [11], anions [12], small molecules [13], vapors, and explosives [14] have been reported. As well known, for the luminescent CP-based sensors, the most common detection method is a change in their fluorescence intensity, that is, analyte molecules quench the excited states of metal ions or fluorescent organic linkers, thereby turning off or reducing the luminescent intensity of the parent CPs [15]. Moreover, the luminescent properties of CPs are sensitive to and dependent on their structural characteristics, coordination environment of metal ions, nature of the pore surfaces, and their interactions with guest species through π-π interactions and hydrogen 3

bonding [16]. Therefore, the deliberate selection of organic linkers is very important to ensure the construction of luminescent sensing CPs. In this respect, the organic linkers with conjugated π electrons might be the good candidate for constructing luminescent CPs [17]. To our knowledge, highly symmetric bipehenyltetracarboxylic acids, such as biphenyl-2,2′,5,5′-tetracarboxylic acid (2,2′,5,5′-H4bptc) [18], biphenyl-3,3′,4,4′-tetracarboxylic

acid

(3,3′,4,4′-H4bptc)

[19]

and

biphenyl-3,3′,5,5′-tetracarboxylic acid (3,3′,5,5′-H4bptc) [20] have been widely used to synthesize luminescent CPs. However, the investigation of asymmetric bipehenyltetracarboxylate, 2,4,4′,6-biphenyl tetracarboxylic acid (2,4,4′,6-H4bptc), remains rarely explored, so far only one Zn(II) CP based on 2,4,4′,6-H4bptc has been reported [21]. Compared with highly symmetric bipehenyltetracarboxylates, 2,4,4′,6-H4bptc with four asymmetric carboxylic groups has following interesting characters: (1) Due to the asymmetric distributions of four carboxylic groups, four carboxylic groups can be partlly or completely deprotonated, which can form versatile coordination modes to construct novel CPs. (2) Two phenyl rings can rotate around the C-C bond, which can result in the dihedral angles of carboxylic groups on them being flexible and adaptable to meet the requirements for constructing the architectures of CPs. Therefore, 2,4,4′,6-H4bptc can be a promising building block to prepare novel luminescent CPs. In this work, we designed and synthesized two new luminescent CPs, [Zn(H2bptc)(2,2′-bipy)(H2O)]·3H2O

(1)

and [Zn2(bptc)(H2O)]·(4,4′-bipy)0.5 (2)

(2,2′-bipy = 2,2′-bipyridyl, 4,4′-bipy = 4,4′-bipyridyl) based on asymmetric 4

2,4,4′,6-H4bptc ligand in the presence of different N-donor auxiliary ligands. 1 shows an infinite zigzag ribbon, while 2 possesses a complicated three-dimensional (3D) framework. These new CPs display strong solid-state luminescence emission at room temperature. Remarkably, they exhibit good luminescent sensing for metal ions and nitroaromatic explosives, especially for Fe3+ ion and 2,4,6-trinitrophenol (TNP) . 2. Experimental 2.1 Chemicals and materials 2,4,4′,6-H4bptc was prepared by the literature methods [22]. All commercially available reagents and starting materials were of reagent-grade quality and used without further purification. Fourier transform infrared (FTIR) spectra were recorded in the range 400-4000 cm-1 on a Nicolet Impact 410 spectrometer. Powder X-ray diffraction patterns (PXRD) were performed using a SHIMADZU XRD-6000 diffractometer with Cu-Kα radiation (λ = 1.5418Å), with the step size and the count time of 0.02° and 4 s, respectively. Elemental analyses (C, H and N) were performed using an Elementar Vario EL cube CHNOS Elemental Analyzer. Thermogravimetric (TG) analysis was carried out using a PerkinElmer TGA7 instrument, with a heating rate of 10 °C min-1 under air atmosphere. The UV-visible absorption spectra were recorded on a Shimadzu UV-1601PC spectrophotometer. The luminescence measurement was performed on an Edinburgh Instrument FLS 920 spectrophotometer. X-ray photoelectron spectroscopy (XPS) was carried out on an ESCALAB 250 X-ray electron spectrometer using Al Κα radiation. 2.2 Synthesis of [Zn(H2bptc)(2,2′-bipy)(H2O)]·3H2O (1) . 5

Zn(NO3)2·6H2O (0.0223 g, 0.075 mmol), H4bptc (0.0165 g, 0.05 mmol), 2,2′-bipy (0.0078 g, 0.05 mmol), deionized water (6 ml) and NaOH (0.0008 g, 0.02 mmol) were added to a 15 ml Telfon-lined stainless steel autoclave, and the solution was heated at 160 °C for 96 h, then cooled down to room temperature at a rate of 0.2 °C·min-1 . The resulting yellow crystals were obtained after washed with deionized water several times and dried in air. The yield of 1 was 72% based on Zn. Anal. Calcd for C26H22N2O12Zn: C, 50.34%; H, 3.55%; N, 4.52%. Found: C 50.56%; H, 3.61%; N, 4.56%. IR (KBr, cm-1): 3420(s), 1702(m), 1607(s), 1379(m), 1314(w), 1188(s), 1130(w), 1048(w), 814(w), 770(w), 704(w), 565(w). 2.3 Synthesis of [Zn2(bptc)(H2O)]·(4,4′-bipy)0.5 (2) . Zn(NO3)2·6H2O (0.0297 g, 0.1 mmol), H4bptc (0.0165 g, 0.05 mmol), 4,4′-bipy (0.0078 g, 0.05 mmol), deionized water (4 ml), ethanol (4 ml) and HNO3 (20 µl, 0.32 mmol) were added to a 15 ml Telfon-lined stainless steel autoclave, and the solution was heated at 160 °C for 96 h. Then, the reaction mixture was slowly cooled to room temperature. Colorless crystals of 2 were collected from the final reaction system by filtration, washed several times with H2O, and dried in air at ambient temperature. (Yield: 67% based on Zn ). Anal. Calcd for C21H12 NO9Zn2: C, 45.57%; H, 2.17%; N, 2.53%. Found: C 45.36%; H, 2.24%; N, 2.56%. IR (KBr, cm-1): 3420(s), 1604(s), 1366(s), 774(w), 705(w), 520(w). 2.4 X-ray crystallography The crystallographic data for 1 were collected on a Bruker D8 Quest diffractometer with graphite-monochromated Mo-Kα (λ = 0.71073 Å) radiation and 6

the crystallographic data for 2 were collected on a Rigaku R-AXIS RAPID diffractometer at room temperature. All structures were solved by direct methods and refined by the full matrix least-squares method against F2 values using the SHELXTL crystallographic software package [23]. All non-hydrogen atoms were located from the Fourier maps and refined anisotropically. The hydrogen atoms of the ligands were generated geometrically. The distribution of peaks in the channels of 1 was chemically featureless to refine using conventional discrete-atom models. To resolve this issue, the contribution of the electron density by the remaining water molecule was removed by the SQUEEZE routine in PLATON [24]. The final formula was derived from crystallographic data combined with elemental and thermogravimetric analyses data. Selected crystallographic data and refinement parameters of 1 and 2 are listed in Table S1, and selected bond lengths and angles data are presented in Table S2. Topology information for 2 was obtained using TOPOS 4.0 [25]. CCDC-1554404 and 1554406 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge at www.ccdc.cam.ac.uk [or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; Fax: (internat.) +44 1223/336 033; E-mail: [email protected]]. 2.5 Fluorescent sensing The fluorescence properties of 1 and 2 in the solid state and various solvent suspensions were examined at room temperature. To examine the potential of them for sensing metal ions and nitro explosives, the powders of 1 or 2 (5 mg) were immersed in water for metal ions and ethanol for nitroaromatic explosives (4 ml), respectively, 7

then the mixture was treated by the ultrasonication for 30 min and aged for 24 h to obtain a stable suspension. In a typical fluorescence quenching experiment, the suspensions of 1 and 2 were excited at λex = 350 nm and λex = 330 nm, respectively. 3. Results and discussion 3.1 Characterization The phase purities of 1 and 2 were confirmed by PXRD measurements and each PXRD pattern of the as-synthesized sample is very consistent with the simulated one (Fig. S1). The differences in intensity may be owing to the preferred orientation of the powder samples. The thermal stabilities of them were studied by using thermogravimetric analysis (TGA). For 1, two weight loss steps can be found in the TG curve (Fig. S2a). The first weight loss (11.1%) between 75 °C and 110 °C, corresponds to the loss of free and coordination water molecules (calcd 11.6%). The second weight loss can be attributed to the elimination of coordinated 2,2′-bipy and H2bptc2- ligands. For 2, the gradual weight loss of 4.5% from 130 to 180 °C corresponds to the loss of one coordination H2O molecule (Fig. S2b). The sharp weight loss above 400 oC corresponds to the decomposition of the framework. 3.2 Structural descriptions [Zn(H2bptc)(2,2′-bipy)(H2O)]·3H2O

(1).

Single-crystal

X-ray

diffraction

analysis reveals that 1 crystallizes in the monoclinic system C2/c space group, and there are one crystallographically independent Zn2+ ion, one H2bptc2- ligand, one 2,2’-bipy molecule, one coordinated water molecule and three lattice water molecules 8

in the asymmetric unit. As show in Fig. 1a, the central Zn2+ ion is coordinated by three oxygen atoms from the carboxylate moiety of two different H2bptc2- anions, one oxygen atom of the coordinated water molecule and two nitrogen atoms from one 2,2’-bipy ligand, showing a distorted octahedral geometry. The Zn-O bond distances range from 2.0101(18) to 2.318(2) Å and Zn-N distances are 2.084(2) and 2.123(2) Å, the O-Zn-O angles vary from 58.21(7)o to 153.30(7)o, the O-Zn-N angles vary from 92.63(10)o to 169.85(9)o, all of which are comparable to those reported in the related Zn(II) compounds[26]. The 2,2’-bipy ligand is in a classical coordination mode with its two nitrogen atoms chelating to one metal center. The bite angle of N1-Zn1-N2 is 77.89(10)o, slightly larger than the corresponding value in the literatures [27]. Notably, four carboxyl groups of H4bptc ligand are incompletely deprotonated, in agreement with the IR spectra (Fig. S1c), in which the strong absorption peaks in range of 1690-1730 cm-1 are observed. As shown in Fig. 1b, each H2bptc2- ligand adopts monodente and bidentate chelating coordination modes to link two zinc centers with the dihedral angle of two benzene rings is 52.68o.

9

Fig. 1 (a) Coordination environment of the Zn(II) atoms in 1 (hydrogen atoms and lattice water molecules are omitted for clarity). (b) View of the coordination modes of H2bptc2- ligand in 1. (c) The 1D zigzag chain of 1 built from ZnO2(H2O)(2,2′-bipy) units and bridging H2bptc2- ligands. (d) The π-π stacking interactions of 1 .

In the structure of 1, the adjacent ZnO2(H2O)(2,2′-bipy) units are linked by two bridging H2bptc2- ligands to give a zigzag ribbon (Fig. 1c). The zigzag ribbon and its enantiomer alternately stack along the a axis and the distances of the plane-to-plane between pyridyl rings of 2,2’-bipy ligands in the adjoining chains are 3.5 Å, which suggests the contiguous ribbons are packed through the π-π stacking interactions (Fig. 1d). [Zn2(bptc)(H2O)]·(4,4′-bipy)0.5 (2). MOF 2 crystallizes in the tetragonal system and I41/a space group. The asymmetric unit of 2 is composed of two crystallographically independent Zn2+ ions, one unique bptc4- ligand and one 10

coordinated water molecule. Zn1 is four-coordinated by four oxygen atoms from four different bptc4- ligands, displaying a distorted tetrahedral geometry. Zn2 is five-coordinated by four oxygen atoms from four different bptc4- ligands and one oxygen atom from the coordinated water molecule to form a slightly distorted trigonal bipyramidal geometry (Fig. 2a). The Zn-O bond distances range from 1.919(2)-2.138 (3) Å. The O-Zn-O angles vary from 81.58(13) to 178.7(13) Å , all of which are in good agreement with those usually described in previous studies for zinc-oxygen [28]. The four carboxylate groups of bptc4- ligand are completely deprotonated, which can be confirmed by the IR spectra (Fig. S1d) and each carboxylate group adopts µ2-η1:η1 coordination fashion to link two zinc centers (Fig. 2b). Interestingly, 4,4’-bipy ligand is not coordinated with any zinc centers, only as guest molecule appears in the pores. As shown in Fig. 2c, two crystallographically distinct zinc atoms, Zn1 and Zn2, are linked by three bismonodentate carboxylate groups to produce a binuclear [Zn2(COO)3] second building unit (SBU). Each binuclear SBU is surrounded by five individual bptc4- ligands (Fig. 2d). Finally, the bptc4- ligands link the binuclear SBUs to result in a complicated 3D framework as illustrated in Fig. 3a. From the topological point of view, the bptc4- ligand and the binuclear [Zn2(COO)3] SBU can be defined as a 5-connected node, respectively. Thus, the overall structure of 2 can be reduced to a novel 3D (5, 5)-connected network with the extend point symbol of {4 4·63·83} (Fig. 3b).

11

Fig. 2 (a) Coordination environment of the Zn(II) atoms in 2 (hydrogen atoms are omitted for clarity). (b) View of the coordination modes of bptc4- ligand in 2. (c) Zn2(COO)3 binuclear SBU in 2. (d) Binuclear SBU is surrounded by five bptc4ligands in 2.

Fig. 3 (a) View of the 3D framework with 1D oval channels of 2 constructed from binuclear [Zn2(COO)3] SBUs and bptc4- ligands. (c) Schematic representation of the (5, 5)-connected {47·6 3}{44·6 3·83} topology.

12

3.3 Luminescence and sensing properties Upon excitation at 350 nm, 1 gives one characteristic emission band at 420 nm (Fig. S3b), which can be ascribed to the ligand-centred electronic transition. For 2, when excited at 330 nm, it exhibits characteristic emission peak at 415 nm (Fig. S3c), which can also be ascribed to the ligand-centred electronic transition [29]. Considering the excellent fluorescence properties of 1 and 2, we explored their potential sensing abilities. In order to identify the potential of them toward sensing of metal ions, the fluorescence spectra of 1 and 2 dispersed in deionized water solutions of M(NO3)n (M = K+, Na+, Ag+, Ni2+, Cd2+, Cu2+, Al3+, Cr3+ and Fe3+) with the concentration of 1.0 × 10 -2 mol·L-1 were firstly studied. As shown in Fig. 4, the metal ions have varying degrees of quenching effects on the luminescence intensities of 1 and 2, especially in case of Fe3+. These results demonstrate that 1 and 2 have high selectivity for the detection and recognition of Fe3+ ions compared to other metal ions. The detailed fluorescence quenching efficiency of 1 and 2 for different metal ions is shown in Fig. S4.

Fig. 4 Fluorescence intensities of 1 (a) and 2 (b) in 10-2 mol·L-1 aqueous solution of M(NO3)x (M = Na+, K+, Ag+, Cu2+, Cd2+, Ni2+, Al3+, Cr3+, Fe3+).

To study the influence of Fe3+ concentration on the luminescence intensities of 1 13

and 2, we prepared the aqueous solution of Fe3+ ions in the concentration range of 1 × 10-5 – 2 × 10 -4 mol·L-1. Fig. 5 shows that the luminescence intensities of Fe3+@1 and Fe3+@2 gradually decrease with increasing concentration of Fe3+ ions. There is a good linear correlation between the quenching level (I0/I) and the concentration of Fe3+ in the range from 1 × 10 -5 to 2 × 10-4 mol·L-1 with the static quenching equation [30]: I0/I = 1+ KSV[M], where I0 and I represent, respectively, the fluorescence intensities of 1 or 2 in the absence and presence of Fe3+, [M] is the concentration of the quencher (Fe3+) and KSV, the slope, is the association constant of the ground state complex. The KSV values are 2.581 × 10 4 L·mol-1 for 1 and 2.826 × 104 L·mol-1 for 2, respectively, which can be matched by those in the same solution-based MOFs for sensing Fe3+ (Table S3). This indicates that these two new CPs can be the candidates to easily identify the existence of a small amount of Fe3+ ions [31].

Fig. 5 Left: Fluorescence intensities of 1 (a) and 2 (b) at different Fe3+ concentrations 14

in aqueous solution. Right: Linear relationships for 1 (c) and 2 (d) in aqueous solutions of different concentrations of Fe3+.

To determine whether 1 and 2 act as a highly selective luminescent sensor for Fe3+, their selectivity detection and anti-interference sensing abilities were further performed by the competing experiments. The detailed experiments were described as follows: 0.08 ml of Fe3+ ions (10 -2 mol·L-1) were slowly dropped into the 4 ml suspension (contained 5 mg 1 or 2) of other metal ions (10-2 mol·L-1) in aqueous solutions, respectively. The fluorescence intensities of the resultant suspensions were recorded in Fig. 6. Strikingly, the fluorescence quenching effect of other metal ions on 1 and 2 show significant changes after adding the Fe3+ ions, indicating the presence of other metal ions do not disturb the detection of Fe3+ ions.

Fig. 6 Competitive binding studies of the different metal ions on 1 (a) and 2 (b): The luminescence intensities of 1 and 2 upon the addition of different metal ions followed by Fe3+ ions. The green bars represent the fluorescence intensities of MOFs in different metal ions aqueous solution (4 ml, 10 -2 mol·L-1); the red bars represent the fluorescence intensities of MOFs in the mixed solutions of Fe3+ (0.08 ml, 10 -2 mol·L-1) 15

and other metal ions (4 ml, 10-2 mol·L-1 ). In addition, the fluorescence intensities of 1 and 2 were also measured as a function of immersion time in aqueous solution of 10 -2 mol·L-1. As shown in Fig. S6, there are obvious fluorescence quenching effect of pure 1 and 2 when they were treated with Fe3+ solution for 1 h, and the fluorescent emissions of them were totally quenched after the immersion of 36 h and 24 h, respectively. As well known, mostly mechanisms of MOFs selectively sensing Fe3+ can be attributed to the cations exchange between cation-type MOF and Fe3+ [32] or the interaction between functional sites of the MOF and Fe3+ [33]. To study the possible reasons for the luminescence sensing of 1 and 2 for Fe3+ ion , the PXRD of Fe3+@1 and Fe3+@2 were carried out (Fig. S7). The results show that the frameworks of 1 and 2 still remain intact. This indicates the luminescence quenching is not caused by collapse of the frameworks. Moreover, the UV-Vis spectra for all the metal ions were recorded as shown in Fig. S8. Only the Fe3+ ion shows the maximum spectral overlap with the emission of 1 and 2, which indicates that the energy transfer mechanism exists in the selective fluorescence quenching with Fe3+ [34]. Additionally, the typical feature peak of Fe3+ ion can be observed in the X-ray photoelectron spectroscopy (XPS) (Fig. S9), which further indicates Fe3+ may diffuse into the pores or attach to the surface of 1 and 2. The analogous mechanism has also been recently reported by Bogale et al [35]. In summary, the result of the experiments indicate that energy transfer quenching mechanisms can be responsible for the sensing of Fe3+ in aqueous solution. 16

The fluorescence behaviors of 1 and 2 in ethanol with different analytes were also investigated to examine their potential for the sensing of nitro-explosives. 1 and 2 were dispersed in the ethanol solutions of various nitro-explosives including 2,4,6-trinitrophenol

(TNP),

2,4-dinitrotoluene

(2,4-DNT),

2,6-dinitrotoluene

(2,6-DNT), 4-nitrotoluene (PNT), nitrobenzene (NB) and nitromethane (NM) with the concentration of 1.0 × 10 -2 mol·L-1 for 24 h. As shown in Fig. 7, all nitro analytes can lead to varying degree of luminescence quenching effects, especially in the case of TNP. The detailed fluorescence quenching efficiency of 1 and 2 for different nitro-explosives is shown in Fig. S10.

Fig. 7 Fluorescence intensities of 1 (a) and 2 (b) emerging in 10-2 mol·L-1 ethanol solution of different nitro compounds.

Then, the quenching effects of 1 and 2 were examined as a function of TNP concentration in the range of 1 × 10-6 – 1 × 10 -5 mol·L-1, respectively. It can be clearly seen from Fig. 8, the concentration of TNP has a remarkable influence on the fluorescence intensities of 1 and 2, and the fluorescence intensity versus TNP

17

concentration can be well fitted by the Stern–Volmer equation, (I0/I) = 1+KSV[M], where I0 and I are the fluorescence intensities of 1 or 2 suspension without and with addition of analytes, respectively, [M] is the molar concentration of the analyte, and KSV is the quenching constant. On the basis of experimental data in Fig. 8, the KSV values are calculated to be 1.26 × 105 L·mol-1 for 1 and 3.422 × 105 L·mol-1 for 2, respectively. These KSV values are obviously higher than those of in the same solution-based MOFs (Table S4), indicating 1 and 2 can be promising luminescent sensor for nitro explosives.

Fig. 8 Left: Fluorescence intensity of 1 (a) and 2 (b) in ethanol solutions containing TNP of different concentrations. Right: Linear relationships for compounds 1 (c) and 2 (d) in ethanol solutions of different concentrations of TNP.

18

Generally, the fluorescence quenching of MOFs for nitro-explosives can be attributed to the electron transfer from the conduction band (CB) of MOFs to the LUMOs of the electron-deficient nitro analytes [10a, 36]. In addition, if the fluorophore and analyte are close to each other and the absorption band of the analyte has an effective overlap with the emission band of the fluorophore, the resonance energy transfer can occur from fluorophore to non-emissive analyte, resulting in dramatically enhance fluorescence-quenching efficiency [34, 37]. The maximum quenching efficiency observed for TNP is in good agreement with the LUMO energies of the nitro analytes [38], suggesting the fluorescence quenching can be attributed to the electron transfer. Moreover, the absorption spectrum of TNP exhibits a large overlap with the emissions of 1 and 2, whereas almost no overlap is observed for other nitro aromatics as shown in Fig. S11. This clearly suggests that energy-transfer mechanism also contributes to the fluorescence quenching for TNP except for the electron transfer. 4. Conclusions In summary, two new luminescent Zn(II)-CPs have been solvothermally synthesized by using same asymmetric tetracaboxylic ligand, 2,4,4′,6-H4bptc. Although the same carboxylate ligand was used, the different structures were observed in both compounds, which could be ascribed to the presence of different auxiliary N-donor ligands. 1 exihibits an infinite zigzag ribbon constructed from ZnO2(H2O)(2,2′-bipy) units and bridging H2bptc2- ligands, which can further form 2D layered structure by the π-π stacking interactions between pyridyl rings of 2,2’-bipy 19

ligands. 2 possesses a complicated 3D (5, 5)-connected network constructed from binuclear [Zn2(COO)3] SBUs and bptc4- ligands. Fluorescence sensing studies show that 1 and 2 display excellent fluorescence sensing ability for metal ions and nitro explosives, especially for Fe3+ ion and TNP. It indicates that 1 and 2 may potentially be used as multi-responsive luminescence-based sensors for the efficient detection of toxic and harmful substances. Noticeably, although the same carboxylate ligand and metal center are used, 2 shows better luminescent sensing abilities for Fe3+ and TNP than that of 1. This could be ascribed to their different structures. The microporous structure of 2 furnish the adequate diffusion routes within its 3D framework to facilitate the host−guest interaction between 2 and the analytes, presenting higher sensitivity of sensing. This work demonstrates that the variation of symmerty of organic ligand plays a significant role in the perparation of CPs with distinct structures and properties. Further investigations on such an interesting asymmetic ligand will be applied to other metal centers in our future work. Acknowledgments We acknowledge financial support from the National Natural Science Foundation of China (no. 21171065, 21201077 and 21401005). References [1] (a) G. Ferey, Chem. Mater. 13 (2001) 3084; (b) G. Ferey, C. Serre, Chem. Soc. Rev. 38 (2009) 1380; (c) Y.Q. Huang, B. Ding, H.B. Song, B. Zhao, P. Ren, P. Cheng, H.G. Wang, D.Z. Liao, S.P. Yan, Chem. Commun. 47 (2006) 4906; (d) J. Lee, O.K. Farha, J. Roberts, K.A. Scheidt, S.T. Nguyen, J.T. Hupp, Chem. Soc. 20

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Graphical abstract

Two new zinc(II) coordination polymers based on asymmetric tetracarboxylic acid for fluorescent sensing Jing Sun,a Ping Zhang,a Hui Qi,b Jia Jia,c Xiaodong Chen,a Shubo Jing,a Li Wang* and Yong Fan* a

College of Chemistry, Jilin University, Changchun 130012, Jilin, P. R. China.

e-mail: [email protected], [email protected]. b

The Second Hospital of Jilin University, Changchun 130041, P.R. China

c

College of Chemistry, Baicheng Normal University, Baicheng 137000, Jilin, P. R.

China.

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Highlights ►Two luminescent Zn-CPs were synthesized by the asymmetric tetracarboxylic acid. ► 1 exhibits the infinite zigzag ribbon built by ZnO2N2 units and H2bptc2- ligands. ► 2 shows a complicated 3D framework with 1D oval channels. ►1 and 2 exhibit excellent fluorescent sensing for Fe3+ ions and TNP.

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