Pentiptycene-Based Luminescent Cu (II) MOF Exhibiting Selective Gas Adsorption and Unprecedentedly High-Sensitivity Detection of Nitroaromatic Compounds (NACs)

The assembly of a fluorescent pentiptycene-based ligand with copper ion resulted in the formation of a 3D porous metal-organic framework (UPC-21) based on well-known paddlewheel SBUs. UPC-21 exhibits selective adsorption of CO2 over CH4 and N2 at 273 K and 295 K, C2H2 over CH4 at 273 K. The most significant performance of UPC-21 is its highly efficient detection of NACs such as 4-NP, 1,4-DNB, NB, and 1,3-DNB with the calculated quenching constants, Ksv, being 3.097 × 106, 1.406 × 106, 4.420 × 105, and 1.498 × 105 M−1 for 4-NP, 1,4-DNB, NB, 1,3-DNB, respectively, which keeps a record on the fluorescence detection of NACs. This is the first porous Cu(II) MOF that exhibits fluorescent detection of NACs with high sensitivities.

coworkers 35 . 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 MOFs [36][37][38][39][40] , we further extended the central anthracene core to pentiptycene core to synthesize a new pentiptycene-based tetracarboxylate ligand, H 4 L (Fig. 1c). The solvothermal reaction between H 4 L and Cu(NO 3 ) 2 •3H 2 O 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 [Cu 3 (L) 1

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 Cu 2 (COO) 4 paddlewheel SBUs. There are three Cu 2+ ions, three halves of L 4− ligands, and three coordinated water molecules in the asymmetric unit. Hence, the paddlewheel SBUs are connected by the backbone of L 4− ligands to generate a 3D porous framework. If the tetracarboxylate ligand of L 4− 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 routine 41 , after the removal of the axial coordinated water molecules of the paddlewheel SBU.
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 m 2 g −1 , respectively. Low-pressure H 2 and CO 2 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 H 2 uptake of 154 cm 3 g −1 . The H 2 isosteric heat of adsorption (Q st ) for UPC-21 was calculated by fitting the H 2 adsorption isotherms at 77 K and 87 K to a Virial-type expression. At zero coverage, the Q st has the estimated value of 7.4 kJ mol −1 , which is comparable to other MOF-5 series 42,43 . CO 2 measurement for the desolvated UPC-21 indicates that the maximum CO 2 uptake is 86.7 cm 3 g −1 under 273 K and 1 bar, and the corresponding Q st of 33.6 kJ mol −1 was calculated by fitting the CO 2 adsorption isotherms at 273 K and 295 K to a Virial-type expression. The Q st value is much higher than other reported MOF materials 44 , indicating that the framework of UPC-21 possesses high affinity to CO 2 molecules. Selective adsorption. To further verify the selectivity potential to CO 2 over N 2 and CH 4 , the adsorption isotherms of CO 2 , CH 4 , and N 2 were measured at 273 K ( Fig. 4) and 295 K ( Figure S5). As shown in Fig. 4, the maximum CO 2 , CH 4 , and N 2 uptakes of 86.7, 25.4, and 7.8 cm 3 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 CO 2 /CH 4 or CO 2 /N 2 mixed gas, the CO 2 /CH 4 selectivities are 12.7-7.4 and 10.9-7.5, and the CO 2 /N 2 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 CO 2 over CH 4 and N 2 at 273 K 45 .
The separation of CH 4 /C 2 H 2 is also significant in the industrial process. Therefore, the gas separation task becomes increasingly challenge in recent years [46][47][48] . As shown in Fig. 5(a), the maximum C 2 H 2 uptake is 72.6 cm 3 g −1 at 273K and 1 atm, compared to CH 4 uptake of 25.4 cm 3 g −1 . The selectivity for C 2 H 2 /CH 4 mixture was calculated using the ideal adsorbed solution theory (IAST) as well. Figure 5 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 H 4 L 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 metal 49 . 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.
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 L 4− ligands after the analytes diffuse into the channels of the framework. In contract, when 1,4-DNB was added gradually to H 4 L 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 OHgroup. 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.    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 Zn 2+ and Cd 2+ etc. and H 4 L 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 H 4 L 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 N 2 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. 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 PLATON 2 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.