Introduction

The development of metal-organic frameworks (MOFs) has provided an excellent platform for rational design and synthesis of functional materials with desired properties1,2,3,4,5. Through careful design or select of organic ligand, functional MOFs with gas storage/separation, catalysis and luminescent sensors were successfully constructed and reported in the past decades6,7,8,9,10,11,12,13. In particular, construction of MOF-based luminescent sensor has been received much attention of chemists due to its potential application in detection of nitroaromatic compounds (NACs). Nitroaromatics (NACs), which are composed of a benzene ring functionalized with several nitro-groups, have become serious pollution sources due to their explosivity and high toxicity. Beside, the structural tailorability and chemical versatility provide a significant level of tenability to detection of NACs with MOFs14. In general, it is highly dependent on the organic ligand or metal ion/cluster for the fluorescent behaviour of a MOF material. Hence, a large number of luminescent MOFs based on organic ligands with chromophores or metal ions such as Cd2+, Zn2+, Ln3+, or the combination of these two parts have been widely synthesized and their application in fluorescent detection of NACs has been studied in recent years15,16,17,18,19,20,21. Surprisingly, study of fluorescent Cu(II) MOF on the application of sensing NACs remains unexplored, although porous Cu(II) MOFs with high gas uptake can be easily constructed by the assembly of carboxylate ligands with Cu(II) ions22,23,24,25. Furthermore, the detection sensitivity is somewhat low for the reported luminescent MOFs on the detection of NACs. Thus, construction of luminescent MOFs with high-sensitivity detection of NACs is still a great challenge to chemists.

On the other hand, organic ligands used for the construction of functional MOFs are limited in phenyl- or anthracene-based carboxylate or N-donor ligands26,27,28,29,30,31. However, the application of iptycene–based organic ligands in the assembly of porous MOFs is quite rare32. Actually, iptycene-based conjugated polymers were widely synthesized and studied, which exhibits excellent fluorescence sensing applications33. Moreover, the rigid iptycene groups can significantly prevent aggregation of the polymers to generate excellent properties34. Similar results should be achieved if the iptycene-based ligands are used to construct functional MOFs. Very recently, MOFs constructed from triptycene- and pentiptycene-based carboxylate ligands were reported by MacLachlan and coworkers35. However, porous MOFs based on pentiptycene-based ligands with gas uptake and fluorescent sensors remain unexplored to date.

Continuing our previous work by use of rigid tetracarboxylate ligands (Fig. 1a,b) to construct porous MOFs36,37,38,39,40, we further extended the central anthracene core to pentiptycene core to synthesize a new pentiptycene-based tetracarboxylate ligand, H4L (Fig. 1c). The solvothermal reaction between H4L and Cu(NO3)2∙3H2O resulted in the formation of a large amount of green crystals. Single-crystal X-ray diffraction, elemental analysis and TGA measurements reveal that the formula of the complex is [Cu3(L)1.5(H2O)3]∙3DEF∙20H2O (UPC-21). UPC-21 exhibits selective adsorption of CO2 and highly efficient detection of NACs. To the best of our knowledge, the sensitivity of UPC-21 on the detection of NACs keeps a record among the reported MOF-based fluorescent sensors.

Figure 1
figure 1

The tetracarboxylate ligands used in our previous works (a,b) and this work (c).

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Results and Discussion

Crystal structure of UPC-21

Single crystal X-ray diffraction analyses reveal that UPC-21 crystallizes in the monoclinic C2/c space group and is a 3D porous framework based on Cu2(COO)4 paddlewheel SBUs. There are three Cu2+ ions, three halves of L4− ligands and three coordinated water molecules in the asymmetric unit. Hence, the paddlewheel SBUs are connected by the backbone of L4− ligands to generate a 3D porous framework. If the tetracarboxylate ligand of L4− can be considered as a 4-connected planar linker and the paddlewheel SBU as a square planar node, the present framework is a NbO network with the short Schläfli vertex notation of the net being {6^4. 8^2}. Indeed, the framework contains spindle-shaped cages and each cage is connected with other eight similar cages. Thus, UPC-21 can also be considered as formed by the infinite connection of the spindle-shaped cages (Fig. 2). The total accessible volume in UPC-21 is 54.2% using the PLATON/VOID routine41, after the removal of the axial coordinated water molecules of the paddlewheel SBU.

Figure 2
figure 2

Crystal structure of UPC-21.

(a) the square-planar linker of L4− ligand in UPC-21. (b) The square-planar node of the paddlewheel SBU. (c) The NbO topological net. (d) Schematic representation of the linkage among the spindle-shaped cages.

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Gas sorption properties

In order to confirm the permanent porosities of UPC-21, various gas adsorption isotherms of UPC-21 were measured under various temperature. Before the measurement, the freshly prepared sample of UPC-21 was extracted by soxhlete extraction with acetone for 36 hours, then activated at 80 °C to generate solvent-free sample of UPC-21. As shown in Fig. 3, desolvated UPC-21 displays a typical Type-I adsorption isotherm with the Brunauer-Emmett-Teller (BET) surface area and Langmuir surface area being 1117.0 and 1253.6 m2g−1, respectively. Low-pressure H2 and CO2 uptakes of desolvated sample of UPC-21 were also determined using volumetric gas adsorption measurements, which exhibit the classical reversible Type-I isotherms. The type of adsorption suggests typically microporous having been retained after the removal of guest molecules. The PXRD pattern is consistent with that of the pristine sample, which also indicates that the structure of UPC-21 is relatively stable after removing the guest molecules. Under the conditions of 77 K and 1 bar, the desolvated UPC-21 has a maximum H2 uptake of 154 cm3 g−1. The H2 isosteric heat of adsorption (Qst) for UPC-21 was calculated by fitting the H2 adsorption isotherms at 77 K and 87 K to a Virial-type expression. At zero coverage, the Qst has the estimated value of 7.4 kJ mol−1, which is comparable to other MOF-5 series42,43. CO2 measurement for the desolvated UPC-21 indicates that the maximum CO2 uptake is 86.7 cm3 g−1 under 273 K and 1 bar and the corresponding Qst of 33.6 kJ mol−1 was calculated by fitting the CO2 adsorption isotherms at 273 K and 295 K to a Virial-type expression. The Qst value is much higher than other reported MOF materials44, indicating that the framework of UPC-21 possesses high affinity to CO2 molecules.

Figure 3
figure 3

Gas uptakes for UPC-21.

(a) The N2 sorption isotherms for UPC-21 at 77 K. (b) The H2 adsorption capacity for UPC-21 at 77 K and 87 K. (c) The CO2 adsorption capacity for UPC-21 at 273 K and 295 K. (d) The Qst of UPC-21 for H2. (e) The Qst of UPC-21 for CO2

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Selective adsorption

To further verify the selectivity potential to CO2 over N2 and CH4, the adsorption isotherms of CO2, CH4 and N2 were measured at 273 K (Fig. 4) and 295 K (Figure S5). As shown in Fig. 4, the maximum CO2, CH4 and N2 uptakes of 86.7, 25.4 and 7.8 cm3 g−1, respectively, were found at 273 K and 1 atm. To further evaluate the feasibility of the separations, ideal adsorbed solution theory (IAST) was performed on the basis of the experimentally recorded adsorption isotherms of UPC-21. According to the calculation results over the 10:90 and 50:50 CO2/CH4 or CO2/N2 mixed gas, the CO2/CH4 selectivities are 12.7–7.4 and 10.9–7.5 and the CO2/N2 selectivities are 70.5–34.3 and 52.2–47.4 for 10:90 and 50:50 mixtures, respectively. These results further indicate that UPC-21 exhibits highly selective adsorption of CO2 over CH4 and N2 at 273 K45.

Figure 4
figure 4

Selective gas adsorption for UPC-21.

The CO2/CH4 (a) and CO2/N2 (b) sorption isotherms for UPC-21 at 273 K. The CO2/CH4 (c) and CO2/N2 (d) selectivities for UPC-21 at 273 K calculated by the IAST method for two CO2 concentration.

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The separation of CH4/C2H2 is also significant in the industrial process. Therefore, the gas separation task becomes increasingly challenge in recent years46,47,48. As shown in Fig. 5(a), the maximum C2H2 uptake is 72.6 cm3 g−1 at 273K and 1 atm, compared to CH4 uptake of 25.4 cm3 g−1. The selectivity for C2H2/CH4 mixture was calculated using the ideal adsorbed solution theory (IAST) as well. Figure 5(b) presents the IAST- derived selectivity of C2H2/CH4 for UPC-21. Notably, UPC-21 exhibits substantially high selectivity at very low pressure. The C2H2/CH4 selectivities are 293.2–91.5 and 128.0–43.7 for 10:90 and 50:50 when the pressure is lower than 0.1 bar.

Figure 5
figure 5

(a) The C2H2 sorption isotherms for UPC-21 at 273 K. (b) Selectivities for C2H2/CH4 at 273 K calculated by the IAST method.

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Fluorescent property and NACs sensing

Fluorescent detection of NACs has received much attention of chemists. In the past decade, a large number of fluorescent MOFs were synthesized and reported. Although these fluorescent MOFs exhibit excellent sensing of NACs, the detection sensitivity needs to be further improved.

Considering the excellent fluorescent property of pentiptycene-based ligand, the photoluminescence of UPC-21 in the solid state was studied at room temperature. As shown in Fig. 6a, UPC-21 exhibits a luminescent emission peak at 465 nm, upon excitation at 330 nm. The emission band of UPC-21 can be ascribed to the organic linker, because similar emission at 418 nm was observed for the free H4L ligand. However, it is noteworthy that the change of luminescence for the UPC-21with free ligand under the same conditions, which probably can be considered as arising from strong coordination interactions between the ligand and metal49.

Figure 6
figure 6

Emission spectra for H4L and UPC-21.

(a) The solid state emission spectra for free ligand H4L and UPC-21 at room temperature. (b) The emission spectra for H4L dissolved in DMSO and UPC-21 dispersed in DMSO at room temperature.

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To verify UPC-21 sensing ability to NACs, fluorescent titrations were carried out with the gradual addition of analytes in DMSO to UPC-21 dispersed in DMSO at room temperature. The simplest nitroaromatic compound, nitrobenzene (NB), was first chosen as the analyte. Hence, UPC-21 dispersed in DMSO was titrated with NB in DMSO at room temperature and the fluorescence change was monitored by PL spectroscopy. Interestingly, the fluorescent intensity increased slightly with the incremental addition of NB, but decreased significantly after further addition of NB at 3 ppm, indicating that the introduction of NB produced significant quenching of fluorescence intensity of UPC-21. The fluorescence intensity decreased to 89% at 10 ppm and no further quenching was observed after 10 ppm (Fig. 7a), indicating that UPC-21 can sense NB molecule with high sensitivity.

Figure 7
figure 7

The emission spectra of UPC-21 titrated with NB (a), 4-NP (b), 1,4-DNB (c) and the relationships between emission intensities and concentrations for NB, 4-NP and 1,4-DNB (d).

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Efficient sensing of NB prompted us to investigate the potential of UPC-21 towards sensing other NACs such as 4-nitrophenol (4-NP), 1,4-dinitrobenzene (1,4-DNB), 1,3-dinitrobenzene (1,3-DNB), 4-Nitroaniline(4-NA) and p-nitrotoluene (4-NT). As expected, UPC-21 exhibits efficient sensing of 4-NP with high sensitivity. When only 6 ppm of 4-NP was added to UPC-21 dispersed in DMSO, nearly 98% of the initial fluorescence intensity was quenched. As contrast, the maximum fluorescence intensity of UPC-21 was decreased by 91, 69, 16 and 15% upon the addition of 1,4-DNB, 1,3-DNB, 4-NT and 4-NA at 10 ppm, respectively (Fig. 7 and Figure S6). These results indicate that UPC-21 can efficiently sense NB, 4-NP, 1,4-DNB and 1,3-DNB, in which 4-NP and 1,4-DNB can be detected most effectively. In order to further compare the efficiency of the sensor, the fluorescence quenching efficiency was calculated by use of the Stern-Volmer (SV) equation, I0/I = 1 + Ksv[A], where I0 is the initial fluorescence intensity before the addition of analyte, I is the fluorescence intensity after adding the analyte as quencher, [A] is the molar concentration of analyte and Ksv is the quenching constant (M−1). In particular, the quenching constant, Ksv, can reflect the efficiency of the sensor. Based on the equation, if the I0/I vs [A] plot is linear, then Ksv can be estimated accurately At very low concentrations, the changes of the fluorescence intensity are very slight for 4-NP, NB, 1,3-DNB and 1,4-DNB. The Stern-Volmer plots are nearly linear at relatively high concentrations for all analytes. The calculated quenching constants, Ksv, are 3.097 × 106, 1.406 × 106, 4.420 × 105, 1.498 × 105 M−1 for 4-NP, 1,4-DNB, NB, 1,3-DNB, respectively, which indicate that the quenching efficiency for the NACs in DMSO is the following order: 4-NP > 1,4-DNB > NB > 1,3-DNB (Fig. 8). The linear range for NACs detection is 2.5–6 ppm. Recently, several fluorescent MOFs with sensing of 4-NP, 1,4-DNB, NB and 1,3-DNB were synthesized and reported. As shown in Table 1, UPC-21 exhibits the highest sensitivity among the reported results. Furthermore, the detection limit (D) was calculated based on the equation: D = 3σ/k,(σ: standard, k:slope). The calculated detection limits, D, are about 0.0775, 0.0896, 0.0949 and 0.123 ppm for 1,4-DNB, 4-NP, NB and 1,3-DNB, respectively, which further confirm the high sensitivity of UPC-21 on the fluorescent detection of 4-NP, 1,4-DNB, NB and 1,3-DNB.

Table 1 Comparison of sensitivity of selected MOF-based sensors for the detection of NACs.
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Figure 8
figure 8

The Stern–Volmer plots for UPC-21 with NB (a), 4-NP (b), 1,4-DNB (c) and 1,3-DNB (d) in the relatively high concentration region. The solid lines represent fit to the concentration-resolved data using the Stern–Volmer equation.

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To further explore if UPC-21 can sense other highly explosive NACs with three substituted groups, the fluorescent titrations for 2,4-dinitrotoluene (2,4-DNT) and trinitrophenol (TNP) were carried out. Surprisingly, UPC-21 exhibits highly efficient sensing of TNP but no sensing for 2,4-DNT was observed (Figures S15 and S13). The maximum fluorescence intensity of UPC-21 was decreased by 64% upon the addition of 6 ppm TNP. The calculated Ksv is about 5.209 × 105 M−1 (Figure S16), which is much higher than other reported results50,51,52,53,54,55.

It is known that the fluorescent detection of NACs by MOFs materials should derive from the truth that the fluorescence quenching phenomenon can occur as a result of the electron transfer from the framework of MOFs to the electron-deficient NACs molecules. The high-sensitivity detection of 4-NP and 1,4-DNB by UPC-21 may result from the formation of strong interactions (such as π–π stacking) between the analytes and the side benzene rings of L4− ligands after the analytes diffuse into the channels of the framework. In contract, when 1,4-DNB was added gradually to H4L dissolved in DMSO, there is no obvious changes for the fluorescence intensity (Figure S18), confirming that the analytes entered into the channels of UPC-21 to interact with the framework. Beside, It should be note that UPC-21 can sense highly explosive for 4-NP, which is possible owing to the presence of the OH– group. With the involving of highly acidic OH−group, strong interaction is occurred via electrostatic interactions, which lead to the quenching effect maintained over a long range.

Conclusions

In conclusion, a new 3D porous Cu (II) MOF (UPC-21) based on a fluorescent pentiptycene-based ligand was synthesized and characterized. UPC-21 shows selective adsorption of CO2 over CH4 and N2 at 273 K and 295 K, C2H2 over CH4 at 273 K, which possesses potential application on gas separation. Significantly, due to the existence of fluorescent ligand of H4L in the compound, UPC-21 exhibits fluorescence detection of 4-NP, 1,4-DNB, NB and 1,3-DNB with high sensitivities. Furthermore, the calculated quenching constants, Ksv, are 3.097 × 106, 1.406 × 106, 4.420 × 105, 1.498 × 105 M−1 for 4-NP, 1,4-DNB, NB, 1,3-DNB, respectively, which keeps a record on the sensitivity of fluorescence detection of NACs. To the best of our knowledge, this work represents the first fluorescent Cu (II) MOF that exhibits high-sensitivity detection of nitroaromatic compounds such as 4-NP, 1,4-DNB. Further study will focus on the synthesis of fluorescent MOFs with other transition metal such as Zn2+ and Cd2+ etc. and H4L ligand, as well as their application on the fluorescence detection of NACs.

Method

Materials and measurements

All the chemical reagents were obtained from commercial sources and used without further purification. The ligand H4L was synthesized in 60% yield by a Sonogashira coupling reaction between bis-6,13-(4-acetenyl)pentiptycene and Dimethyl 5-iodoisophthalate followed by hydrolysis with dilute HCl. (Supporting Information, SI). Thermo-gravimetric analysis (TGA) experiments were carried out on a Mettler Toledo TGA instrument with a heating rate 10 °C /min at the range of 25–800 °C under a N2 atmosphere. Elemental analyses (C, H, N) were performed on a CE instruments EA 1110 elemental analyzer. The powder XRD data were obtained on an X-Pert PRO MPD diffractometer with Cu-Kα radiation. Photoluminescence spectra were recorded with a Hitachi F-7000 Fluorescence Spectrophotometer. Gas-sorption isotherms were carried out on the surface area analyzer ASAP-2020.

Synthesis of UPC-21

Crystal of UPC-21 was synthesized by solvothermal reaction of Cu(NO3)2∙3H2O (5.0 mg, 0.021 mmol) and H4L (2.0 mg, 0.0025 mmol) in the mixed solvents of N,N-diethylformamide (DEF) and water. The reaction system was heated to 75 °C for 1000 min and cooled to room temperature slowly. Green block crystals of UPC-21 were obtained and dried in air at ambient temperature (yield: 25%, based on Cu). Elemental analysis calc. (%) for UPC-21: C, 54.58; H, 5.63; N, 1.99. Found: C, 53.13; H, 5.30; N, 2.02.

Single-crystal X-ray diffraction study

Single crystal structure analysis of UPC-21 was performed on Agilent Xcalibur Eos Gemini diffractometer with Enhance (Cu) X-ray Source Cu-Kα (λ = 1.54178 Å). An empirical absorption correction was used via the multi-scan method. The structure were solved by direct methods and refined by full-matrix least-squares on F2 using SHELXL-97 1. The structure was examined using the Addsym subroutine of PLATON2 to assure that no additional symmetry could be applied to the models. Crystal and refinement parameters are listed in Table S1. CCDC No. 1418675 for UPC-21. These data can be obtained free of charge (http://www.ccdc.cam.ac.uk/data_request/cif). Full experimental details and crystallographic analysis are given in the Supplementary Information.

Additional Information

How to cite this article: Zhang, M. et al. Pentiptycene-Based Luminescent Cu(II) MOF Exhibiting Selective Gas Adsorption and Unprecedentedly High-Sensitivity Detection of Nitroaromatic Compounds (NACs). Sci. Rep. 6, 20672; doi: 10.1038/srep20672 (2016).