A 3D metal–organic framework [NH2(CH3)2][Cd6(L)4(DMF)6(HCOO)](DMF = N,N-dimethylformamide) (1) has been synthesized using a tripodal ligand H3L (2,4,6-tris[1-(3-carboxylphenoxy)ylmethyl]mesitylene). The obtained complex exhibits a 3D framework containing hexanuclear {Cd6} building units formed by two trinuclear {Cd3} clusters that are connected via HCOO anions. For complex 1, the participation of the fluorescent ligand H3L not only gives rise to a strong photoluminescence emission as expected, but more interestingly, that ligand originated characteristic band could be quenched selectively by nitrobenzene with a low detection limit, showing its potential as a highly sensitive and selective sensor for nitrobenzene. Based on an electron transfer quenching mechanism, the fluorescence sensing ability of 1 is also applicable for other electron-deficient nitroaromatic compounds with high selectivity and sensitivity, i.e., 1,4-dinitrobenzene, 1,3-dinitrobenzene, 2,4-dinitrotoluene, and 4-nitrotoluene, suggesting 1 a promising fluorescence sensor for detecting and recognizing the same kind of chemicals.

Metal–organic frameworks (MOFs) have aroused great attention due to their wide range of applications, including gas separation and storage, sensing and recognition, catalysis, etc.1–3 Very recently, a class of MOFs featuring intriguing luminescent properties has demonstrated great potential in photocatalysis, bio-imaging, chemical sensing, and nonlinear optics, etc.4 Among these applications, MOF chemical sensors based on fluorescence quenching are of great promise and also a simple procedure according to the donor-acceptor electron-transfer mechanism.5–7 For instance, Ghosh et al. recently reported a luminescent MOF highlighted with a highly selective fluorescence quenching toward 2,4,6-trinitrophenol.8(a) In contrast, the MOF-based luminescent sensors have certain advantages over conventional fluorophores. That is, when serving as recognition/binding sites, their specific structures and functionalities can help to enhance the host–guest interactions between MOF frameworks and target analytes.9 Additionally, the fine tuning of the electronic and porous properties of MOFs can be realized by rational design and deliberate selection of appropriate organic linkers and metal nodes. Moreover, the immobilization of organic struts in MOFs results in strong emissions due to reduced non-radiative relaxation.9 However, due to the porosity of MOF materials, a pre-activation manipulation has to be carried out before sensing to provide more void space because these MOF-based sensors often exhibit poor performance when their pores are occupied by solvents or other guest molecules.8 Therefore, the convenient and simple materials are desiderative for practical application.

As we know, nitrobenzene is a highly volatile and explosive organic solvent. And it is also the basic constituent of nitroaromatic compounds. So how to quickly and quantitatively detect nitroaromatic compounds has shown a great attraction for scientists. At present, trained canines and many kinds of modern instruments (such as, Ion mobility spectroscopy, X-ray dispersion, and Raman spectroscopy) are currently being employed for the sensitive and selective sensing of nitrobenzene,10(a) but usually suffering from the disadvantages like high operational cost and portability issues during in-field use,10(b) which greatly restrict their widespread application. In the past few years, several MOFs with fluorescence quenching ability have been designed and used for recognition and detection of electron-deficient nitroaromatic molecules.11 Besides, MOF-based photoluminescence chemosensors show great promise because of their excellent sensitivity, short response time, reusability, and operability.11 Especially, the development of nitrobenzene sensors under the gas phase condition is more significant when taking the safety and environmental into consideration.12 

In this work, a three dimensional (3D) luminescent MOF [NH2(CH3)2][Cd6(L)4(DMF)6(HCOO)] (1) has been successfully synthesized by using the fluorescent ligand H3L. As expected, the as-synthesized 1 exhibits a strong fluorescence that could be further enhanced or quenched selectively by different analytes. Among them, complex 1 shows a good potential as a luminescent sensor material for nitrobenzene, and it is very impressive for its quick response and high sensitivity toward a trace amount of nitrobenzene in either solution or vapor state. In addition, 1 can also detect trace amounts of other nitrobenzene derivatives, including 4-nitrotoluene, 1,4-dinitrobenzene, 1,3-dinitrobenzene, 2,4-dinitrotoluene, etc.

X-ray crystal structural analysis confirms that complex 1 crystallizes in the trigonal space group R-3c and exhibits a 3D framework which contains hexanuclear {Cd6} building units formed by two trinuclear {Cd3} clusters connected by HCOO anions. Figure S1 illustrates that a total of three crystallographically independent Cd2+ centers are involved in 1, referred as Cd1, Cd2, and Cd3, respectively. Exhibiting a similar distorted octahedral geometry, both Cd1 and Cd2 are six-coordinated. In detail, Cd1 atom is surrounded by six carboxylate oxygen atoms from six different L3− ligands, while Cd2 atom is linked to four carboxylate oxygen atoms and two DMF molecules. Differently, Cd3 atom shows a pentagonal bipyramid geometry and is coordinated by five carboxylate oxygen atoms, one DMF molecule, and one HCOO anion. Interestingly, each Cd1 atom is bridged to two neighboring Cd2 and Cd3 atoms by three –O–C–O– bridges and three μ2-Ocarboxyl atoms to afford a trinuclear {Cd3} cluster with a Cd1⋅⋅⋅ Cd2 and Cd1⋅⋅⋅ Cd3 distance of 3.657 and 3.479 Å, respectively. It is worth noting that HCOO was generated from the decomposition of DMF solvent, playing a dominant role in the construction of the hexanuclear structure. In 1, the lengths of all Cd–O bonds vary between 2.185 and 2.509 Å.13 

Two types of L3− ligands are identified in complex 1 according to their different coordination modes (Figs. 1(a) and 1(b)). The carboxylic groups in mode I adopt μ211 and μ2-η2:η1 bridging coordination modes to link three pairs of {Cd1–Cd2} dinuclear units. Additionally, the three {Cd1–Cd2} dinuclear units arrange in an analogous isosceles triangular mode with outer Cd2⋅⋅⋅ Cd2 distances 17.280, 17.980, and 20.107 Å, which may be ascribed to the peripheral phenyl moieties of mode I ligand being perpendicular to the central benzene ring (Fig. S2). Interestingly, six {Cd1–Cd2} dinuclear cores are connected by six L3− ligands to induce a {Cd12L6} octahedral cage (Fig. 1(c)) with the internal diameter of 10 Å (Fig. S3(a)). While six of the eight faces are occupied by L3−, the other two still remain “open.” The distance between two diagonal Cd2+ atoms is 26.733 Å for this octahedral chamber. Due to the two open faces of the octahedral cage, the rounded window is skillfully formed (Fig. 1(d)). On the other hand, each mode II ligand connects three different octahedral cages by bridging three pairs of Cd1 and Cd3 atoms (Fig. S4(a)). Meanwhile, three octahedral cages are bridged by two parallel mode II ligands through four trinuclear {Cd3} clusters (Fig. 2(a) and S4(b)). Furthermore, every two octahedral cages are connected together by four mode II ligands (Fig. 2(b)). In this way, each octahedral cage wonderfully links 18 mode II ligands (Fig. 2(c)). Finally, by sharing the hexanuclear {Cd6} units, the octahedral cages are packed by the mode II ligands to give a 3D framework (Fig. 2(d)). This framework possesses 1D channels along the c direction (Fig. S3(b)) with dimensions of ca. 11 × 11Å2 (without considering van der Waals radii), which are occupied by dimethylamine cations.

FIG. 1.

(a) Coordination mode of L3− ligand (I). (b) Coordination mode of L3− ligand (II). (c) The octahedral cavities in 1 (the coordinated DMF molecules were omitted). (d) Three linear hexanuclear {Cd6} clusters bridged by six L3− ligands.

FIG. 1.

(a) Coordination mode of L3− ligand (I). (b) Coordination mode of L3− ligand (II). (c) The octahedral cavities in 1 (the coordinated DMF molecules were omitted). (d) Three linear hexanuclear {Cd6} clusters bridged by six L3− ligands.

Close modal
FIG. 2.

(a) Three octahedral cages bridged by two parallel mode II ligands connecting four trinuclear {Cd3} clusters. (b) The two octahedral cages connected together by four mode II ligands. (c) One octahedral cage linking 18 mode II ligands. (d) The 3D framework of 1.

FIG. 2.

(a) Three octahedral cages bridged by two parallel mode II ligands connecting four trinuclear {Cd3} clusters. (b) The two octahedral cages connected together by four mode II ligands. (c) One octahedral cage linking 18 mode II ligands. (d) The 3D framework of 1.

Close modal

Since complex 1 is constructed from d10 metal ion Cd2+ and conjugated organic linker L3−, it is possible to be a candidate for potential photoactive material,14,15 which was confirmed by the observation that complex 1 exhibited a strong emission band at 353 nm upon an excitation at 290 nm (Fig. S5 in the supplementary material).16 The luminescent property of 1 was also examined in liquid suspension, along with the stability test of 1 performed in water. Therein, we found that, the experimental X-ray power diffraction pattern of 1, which was measured on the crystalline sample immersed in water for 3 day, agrees well with the simulated one. That demonstrates the phase purity and the stability of 1 in water. But since complex 1 also shows the inferior dispersibility in water, the fluorescent analysis was carried out without water. Then, DMF was employed instead as the dispersion medium, since a good stability and dispersibility were found during the synthesis procedure of complex 1 in this solvent. Before spectrum recording, 3 mg powder sample of ground single-crystalline complex 1 was dipped into DMF, which was treated by ultrasonic dispersion for 30 min, and subsequently aged for 3 day, forming the stable suspension. In this case, the suspension of 1 shows the emission peaks at 346 nm (λex = 290 nm), as illustrated in Fig. S6. Moreover, 1 exhibited strong emissions in either solid state or suspension, suggesting a good potential as a fluorescence sensor applicable in liquid phase.

The selective fluorescence sensing performance of 1 for small molecules was also examined in DMF suspension. As shown in Fig. 3, it was observed that the ligand originated emission of 1 could be completely quenched upon addition of 10 μL liquid nitrobenzene and 10 mg solid nitroaromatic compounds (1,4-dinitrobenzene, 1,3-dinitrobenzene, 2,4-dinitrotoluene, and 4-nitrotoluene). For comparison, only a slight change in the emission of 1 was observed (intensity change < ± 15%) with the addition of other organics of the same amount, such as, alcohols (methanol, ethanol, tert-butanol, and n-propanol), chloroalkanes (ClCH2CH2Cl, CH2Cl2, CHCl3, and CCl4), nitriles (acetonitrile), amides (DMAC = N,N- dimethylacetamide), ethers (THF = tetrahydrofuran), and non-nitro aromatic complexes (benzene, ortho-xylene, toluene, chlorobenzene, and benzonitrile). These results suggest that 1 could be employed as a highly selective chemical sensor for nitroaromatic compounds.

FIG. 3.

Fluorescence spectra of 1 measured in DMF with the addition of 10 μL different liquid organic reagents and 10 mg solid nitroaromatic compounds.

FIG. 3.

Fluorescence spectra of 1 measured in DMF with the addition of 10 μL different liquid organic reagents and 10 mg solid nitroaromatic compounds.

Close modal

The sensing sensitivity of 1 in DMF suspension was further explored by monitoring the change of its emissive response in correspondence with a gradual increase of nitrobenzene contents of the emulsions. The fluorescence titration experimental results, shown in Fig. 4, reveal that a significant decrease in fluorescence signal (97%) can be observed, when the concentration of nitrobenzene increases to 300 ppm. This performance is beyond or very close to that of other reported MOF-based fluorescence sensors.17 From Fig. S7, we found that even an amount of 5 ppm nitrobenzene can be effectively recognized with a good linear correlation (R2 = 0.986). From the slope of the fitting line, the detection limit of 1 is then estimated to be ca. 0.2 ppm (calculated by using 3σ/k, k: slope, σ: standard), indicating that complex 1 has a higher detection sensitivity towards nitrobenzene. Furthermore, the anti-interference ability of 1 against organic solvents was also investigated by introducing a simplest nitro-compound, nitrobenzene, into the DMF suspension and evaluated using the formula I/I0. Here, I is the maximum fluorescence intensity of 1 in DMF with an addition of 10 μL organics, while I0 is that of 1 dispersed in pure DMF. The other organics (10 μL) have weak effect on the luminescence intensity of 1 (Fig. 5). On this basis, only 300 ppm extra nitrobenzene were added to all parallel tests, the fluorescence intensity at maximum emission is sharply quenched, suggesting that the presence of organic solvents induce almost no influence on the fluorescence sensing of 1 toward nitrobenzene.

FIG. 4.

Fluorescence titration of 1 in DMF upon additions of different concentrations of nitrobenzene (λex = 290 nm).

FIG. 4.

Fluorescence titration of 1 in DMF upon additions of different concentrations of nitrobenzene (λex = 290 nm).

Close modal
FIG. 5.

Histograms of fluorescence intensity ratio (I/I0) for 1 dispersed in DMF after adding different organic reagents (yellow) and subsequent addition of nitrobenzene (black).

FIG. 5.

Histograms of fluorescence intensity ratio (I/I0) for 1 dispersed in DMF after adding different organic reagents (yellow) and subsequent addition of nitrobenzene (black).

Close modal

Liquid nitrobenzene has a considerable high vapour pressure at room temperature,8(a) so that we carried out a solid-gas monitoring equipment of nitrobenzene vapour in real-time.18 The experiment was performed with the powder sample loaded quartz plate, which was placed diagonally in the fluorescence spectrometer. The emission intensity of 1 reduces immediately upon exposing to nitrobenzene vapour (Fig. S8). Here, the quenching efficiency (%) can be evaluated using (I0 − I)/I0 × 100%, in which I and I0 represent the maximum fluorescence intensity of 1 with and without exposure to the nitrobenzene vapour, respectively. The quenching efficiency is above 80% at the time of 660 s (Fig. S9). The investigation on the cyclicity of 1 was also performed by using a simple heat treatment at 50 C for 0.5 h. The results indicated that the sample could be easily reactivated under this treatment, and the performance (Fig. S9) and stability (Fig. S10) of the sensor did not exhibit remarkable change in three detection-activation cycles. In addition, the rapid and convenient sensing also suggests that 1 can be potentially applicable for continuous sensing of gas phase nitrobenzene.

Among the tested aromatics, 1 only responds to nitro substituted analytes. It might be because that the nitro group as a typical electron withdrawing substituent can easily enhance the oxidation of the aromatic ring. Note that, 1 lacks an accessible path for encapsulating the targeted analytes so that its sensing ability should not be attributed to the guest-induced quenching mechanism.8 On the other hand, we observed that the particles of 1 exhibit good dispersion in DMF solution, resulting in a larger effective surface for adsorbing the nitroaromatic compounds molecules. Moreover, the enlarged adsorption surface area would largely facilitate the potential electron transfer from complex 1 to nitroaromatic compounds upon excitations, making an electron transfer quenching mechanism quite plausible.19 Therefore, the selective fluorescence sensing ability of 1 to nitroaromatic compounds is not only related to the electron-rich nature of the conjugated framework structure and electron-deficient nitroaromatics, but also depends on the dispersion performance of MOF particles in solvents. It should be mentioned that the observed solvent-dependent fluorescence quenching behavior is in line with that of recently reported literatures.20 

In conclusion, a 3D MOF was constructed starting from a C3-symmetrical carboxylate ligand. In the presence of a trace of nitrobenzene, the luminescent emission of this MOF was completely quenched. This finding indicates the highly selective and sensitive fluorescence response of this complex and strongly suggests that it could serve as an efficient fluorescence sensor for detecting nitrobenzene in either solution or vapor state.

This work was supported by the 973 Program of China (Grant No. 2014CB845600) and the NSFC (Nos. 21421001, 21290171, and 21403116), and MOE Innovation Team (No. IRT13022) of China. We thank Dr. Tong-Liang Hu for helpful discussions.

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Supplementary Material