Structure of a Luminescent MOF-2 Derivative with a Core of Zn(II)-Terephthalate-Isoquinoline and Its Application in Sensing of Xylenes

: A new blue photoluminescent 2D metal–organic framework, 1 , with formula {[Zn 2 ( µ 2 -BDC) 2 ( i Q) 2 ]} ∞ has been synthesized in a high yield under solvothermal conditions by reacting Zn(II) ions with 1,4-benzenedicarboxylic acid (H 2 BDC) and isoquinoline ( i Q) in DMF. Compound 1 was thoroughly characterized by single-crystal X-ray di ﬀ raction, solid-state cross-polarization magic-angle spinning 13 C NMR, X-ray powder di ﬀ raction, scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDS), and thermoanalysis. The crystal structure of 1 showed interpenetrated 2D frameworks consisting of dinuclear paddle-wheel cores Zn 2 ; moreover, this material possessed thermostability up to 310 ◦ C. The CPMAS 13 C-NMR spectrum of 1 is consistent with the symmetry of the crystal structure. Luminescence studies showed that 1 strongly enhances its ﬂuorescence emission in the presence of xylene isomers with a pronounced selectivity to p-xylene.


Introduction
There is a growing interest in the development of photoluminescent metal-organic materials (MOMs) [1][2][3][4], where the color emission upon excitation is a significant feature of these materials that has been recently used in several applications such as emitting devices/materials [5,6], optical sensing of vapor phase analytes [3,7], detection of ions [8,9], bioanalytes [10], explosives [11,12], and detection of volatile organic solvents (VOCs) [8,13,14]. Most known MOMs-based chemosensors for organic solvents act in a "turn-off" response where the fluorescence intensity of the compound is quenched upon addition of the analyte [7,15,16]. The photoluminescent properties of MOMs are basically dependent on the used metal center, its coordination environment, the presence of aromatic ligands, as well as the supramolecular interactions such as π-π stacking in the final arrangement [1]. From a synthetic point view, the combination of divalent d 10 transition metal ions with rigid aromatic dicarboxylic acids and auxiliary N-donor ligands such as pyridines [17], imidazole groups [18], and isoquinline rings [19] can be used to obtain crystalline metal-organic structures with adjustable emission properties. In this context, the planar symmetric 1,4-benzenedicarboxylate (BDC) anion is a very common bridging

Physical Measurements
All chemicals are commercially available (H 2 BDC, 98%; isoquinoline, 98%; and Zn(NO 3 ) 2 ·4(H 2 O), 98% from Sigma-Aldrich (St. Louis, Missouri, United States)) and were used as received. The FT-IR spectrum was recorded in the range of 4000-600 cm -1 by using the standard Pike ATR cell on a Bruker Tensor 27 FT-IR spectrophotometer (Bruker Optik GmbH, Ettlingen, Germany). Elemental analysis for C, H, and N were carried out by standard methods using a Vario Micro-Cube analyzer.
Thermogravimetric analyses were performed using TA Instruments equipment, under a dinitrogen atmosphere, at a heating rate of 10 • C min -1 , and from 25 to 475 • C.
The CPMAS 13 C NMR experiment was recorded with a Bruker Advance II 300 spectrometer (Bruker BioSpin GmbH, Rheinstetten, Germany) (operating at 75 MHz for 13 C) on a 4 mm rotor double resonance. The CPMAS experiment was recorded with a contact time of 2 ms and a delay of 5 s at a 7 kHz spinning rate at ambient temperature.
The shape and size of the crystals of compound 1 were examined with scanning electron microscopy (SEM) using a JSM-6510LV microscope from JEOL (JEOL, Ltd, Akishima, Tokyo, Japan) equipped with a Bruker QUANTAX 200 energy-dispersive X-ray spectrometer (EDS) (Bruker Nano GmbH, Adlershof, Berlin, Germany) for elemental characterization. The crystals were let to dry at room-temperature conditions and fixed on Al stubs with carbon tape and finally coated with a thin layer of gold using a Denton IV sputtering chamber before SEM imaging acquisition.
Luminescence spectra in suspensions of 1 were recorded on an Agilent Cary Eclipse equipped with a crystal holder and a thermostated holder for quartz cuvettes. In all sensing studies, as-synthesized single-crystalline samples of 1 were used. Suspensions of 1 were prepared by stirring for 20 min at 25 • C in spectrophotometric-grade solvents with a final concentration of 25 µM. For the titration experiment of xylenes, a stock suspension of 1 (1 mg mL −1 ) in acetonitrile was prepared by ultrasonic treatment by 20 min at r.t., and subsequently, 45 µL of this suspension was diluted with 2500 uL of acetonitrile, to give a final concentration of 25 µM, and the emission spectra (λ ex = 330) of the suspension were recorded after adding aliquots of xylenes.

Synthesis of {[Zn
Initially, in a 4 mL vial, 32.6 mg of Zn(NO 3 ) 2 ·4H 2 O (0.10 mmol) and 15.0 mg of BDC (0.09 mmol) were dissolved with 3 mL of DMF. After two and a half hours, 24 µL (0.20 mmol) of isoquinoline was added using a high-precision micropipette. The solution was stirred for 15 min at 75 • C and was then placed in a Parr bomb and heated at 115 • C for 16 h. After cooling the mixture, colorless crystals were obtained, which were filtered out and washed with DMF. Yield: 82%, based on the metal precursor.
Elemental analysis (%), C 34  Luminescence spectra in suspensions of 1 were recorded on an Agilent Cary Eclipse equipped with a crystal holder and a thermostated holder for quartz cuvettes. In all sensing studies, assynthesized single-crystalline samples of 1 were used. Suspensions of 1 were prepared by stirring for 20 min at 25 °C in spectrophotometric-grade solvents with a final concentration of 25 µ M. For the titration experiment of xylenes, a stock suspension of 1 (1 mg mL −1 ) in acetonitrile was prepared by ultrasonic treatment by 20 min at r.t., and subsequently, 45 µ L of this suspension was diluted with 2500 uL of acetonitrile, to give a final concentration of 25 µ M, and the emission spectra (λex = 330) of the suspension were recorded after adding aliquots of xylenes.

Synthesis of {[Zn2(μ2-BDC)2(iQ)2]}∞, 1
Initially, in a 4 mL vial, 32.6 mg of Zn(NO3)2·4H2O (0.10 mmol) and 15.0 mg of BDC (0.09 mmol) were dissolved with 3 mL of DMF. After two and a half hours, 24 μL (0.20 mmol) of isoquinoline was added using a high-precision micropipette. The solution was stirred for 15 min at 75 °C and was then placed in a Parr bomb and heated at 115 °C for 16 h. After cooling the mixture, colorless crystals were obtained, which were filtered out and washed with DMF. Yield: 82%, based on the metal precursor.

Crystallographic Investigations
The relevant details of the crystal, data collection, and structural refinement can be found in Table 1. Crystallographic data for 1 were collected on a Bruker APEX II CCD diffractometer at 296 K using Mo-Kα radiation (k = 0.71073 Å ), from an Incoatec Iµ S source and Helios optic monochromator [37]. Suitable crystals were coated with hydrocarbon oil (Parabar), and the specimen was collected with a glass fiber and fixed with glue. The structure was solved using intrinsic phasing (SHELXT) [38] and refined by full-matrix least-squares on F2 using the shelXle GUI [39]. The hydrogen atoms of the C-H bonds were placed in idealized positions and their position was refined with Uiso linked to the parent atom with distance restraints. Crystallographic data for 1 have been deposited at the Cambridge Crystallographic Data Center (CCDC) with the number 1994765.

Crystal Structure of {[Zn2(μ2-BDC)2(iQ)2]}∞, 1
The X-ray crystallographic analysis shows that 1 crystalizes in the monoclinic space group P 2/n, and the asymmetric unit consists of one Zn(II) ion, one BDC dianion, and one isoquinoline molecule ( Figure 1). Each Zn(II) ion is five-coordinated in a slightly distorted square pyramidal geometry, coordinated by four carboxyl oxygen atoms of BDC and the nitrogen atom from the isoquinoline molecule. The basal plane is defined by O1, O2, O3, and O4 from four BDC ligands. The apical position is occupied by the nitrogen atom from the coordinated isoquinoline molecule. Toward the apical ligating atom, the metal ion is deviated from the corresponding basal plane by 0.378 Å . The basal planes are found to be very little tetrahedrally distorted with a τ value of 0.012 [τ = (159.01-158.29)/60 = 0.012] [40]. The Zn-O bond lengths vary from 2.003(3) to 2.065(4) Å , and the O-Zn-O bond angles range from 88.55 (17) to 158.99(17) ( Table 2); these values are representative of different coordination geometries of Zn(II) ions in coordination polymers with dicarboxylate-bridging ligands [41][42][43].

Crystallographic Investigations
The relevant details of the crystal, data collection, and structural refinement can be found in Table 1. Crystallographic data for 1 were collected on a Bruker APEX II CCD diffractometer at 296 K using Mo-K α radiation (k = 0.71073 Å), from an Incoatec IµS source and Helios optic monochromator [37]. Suitable crystals were coated with hydrocarbon oil (Parabar), and the specimen was collected with a glass fiber and fixed with glue. The structure was solved using intrinsic phasing (SHELXT) [38] and refined by full-matrix least-squares on F2 using the shelXle GUI [39]. The hydrogen atoms of the C-H bonds were placed in idealized positions and their position was refined with U iso linked to the parent atom with distance restraints. Crystallographic data for 1 have been deposited at the Cambridge Crystallographic Data Center (CCDC) with the number 1994765.

Results and Discussion
The X-ray crystallographic analysis shows that 1 crystalizes in the monoclinic space group P 2/n, and the asymmetric unit consists of one Zn(II) ion, one BDC dianion, and one isoquinoline molecule ( Figure 1). Each Zn(II) ion is five-coordinated in a slightly distorted square pyramidal geometry, coordinated by four carboxyl oxygen atoms of BDC and the nitrogen atom from the isoquinoline molecule. The basal plane is defined by O1, O2, O3, and O4 from four BDC ligands. The apical position is occupied by the nitrogen atom from the coordinated isoquinoline molecule. Toward the apical ligating atom, the metal ion is deviated from the corresponding basal plane by 0.378 Å. The basal planes are found to be very little tetrahedrally distorted with a τ value of 0.012 [τ = (159.01 − 158.29)/60 = 0.012] [40]. The Zn-O bond lengths vary from 2.003(3) to 2.065(4) Å, and the O-Zn-O bond angles range from 88.55 (17) to 158.99(17) ( Table 2); these values are representative of different coordination geometries of Zn(II) ions in coordination polymers with dicarboxylate-bridging ligands [41][42][43].   In 1, each BDC ligand bridges four Zn(II) ions, with the two carboxylate moieties assuming a tetrakis(monodentate) coordination mode. As a consequence, pairs of Zn(II) ions are bridged by four carboxylates to form dinuclear secondary building units (SBUs) (Figure 2), which generate a 2D coordination network parallel to the ab plane, enclosing virtually a square grid with a size of 10.945 × 10.915 Å 2 . In the SBU, the Zn···Zn distance of 2.9740(11) Å is in the characteristic range of BDC-bridged dinuclear Zn(II) complexes [20,42,44]. Furthermore, a 3D supramolecular network is formed by interpenetrated layers with π-π stacking interactions between the BDC and the secondary ring in the isoquinoline (Figure 3), with a centroid-centroid displacement angle of 7.62 • and a distance equal to 3.47(8) Å. Although the presence of the isoquinoline ligand directly attached to the SBU could Crystals 2020, 10, 344 5 of 13 contribute to avoiding interpenetration in a similar approach as in the isoreticular series of MOF-2 derivates [45], this phenomenon does not occur in 1, as the layers along the crystallographic a axis are stabilized by the mentioned π-π interactions.

PXRD, Thermal Stability, IR-Correlation, and Solid-State Structure-CPMAS 13 C NMR Correlation
The purity of the crystalline as-synthesized bulk sample of 1 was examined by powder X-ray diffraction (PXRD) measurements at room temperature ( Figure 4). The PXRD patterns of the synthesized sample is consistent with those simulated from single-crystal X-ray data, indicating high phase purity; thus, we used this sample for further solvent sensing studies. The thermal stability of 1 was tested Crystals 2020, 10, 344 6 of 13 by thermogravimetric analysis (TGA) under a dinitrogen atmosphere ( Figure 5A). Due to the lack of guest solvent molecules, compound 1 presents high stability up to 310 • C. The TGA curve of 1 shows a weight loss of 36.20% from 312 to 411 • C, corresponding to the removal of two coordinated iQ molecules (calcd 35.88%). The differential scanning calorimetry curve (DSC) shows a single endotherm peak at 375 • C with ∆H = 307 J/g, which can be attributed to the release of the aromatic N-donor ligand ( Figure 5B) [19].       The IR spectrum shows the common bands expected for coordinated carboxylate ligands to atoms Zn(II) along with the corresponding bands to isoquinoline. Two bands of asymmetric and symmetric stretches for the -CO 2 − groups at 1586 and 1410 cm -1 and the difference (∆ν = 176 cm −1 ) can be assigned to the tetrakis(monodentate) coordination mode of the BDC 2− ligand, which is in agreement with the crystal structure [46]. The solid-state CPMAS 13 C NMR spectrum of 1 ( Figure 6) shows three expected signals (red rectangles) for the BDC ligand at 129 and 136 ppm for aromatic carbon atoms (C3 and C4) and the ipso-carbon (C2), respectively, whereas the peak at 171 ppm can be ascribed to the carboxylate (C1) group. All of those assignments are in agreement with the 13 [48]. The blue rectangles denote the methine signals from the isoquinoline ligand, and, as can be seen, some of those peaks are superimposed along with the BDC signals, especially the 13 CH atoms C15, C17, C18, and C19, which are in the range of 127-129 ppm. The two downfield signals at 154 and 141 ppm can be ascribed to methine groups C11 and C13, respectively. In general, the 13 C CPMAS spectrum of 1 is consistent with its crystalline structure.    Figure 7 shows SEM micrographs of crystalline compound 1. Micrometric bars are observed with a well-defined plane facing along the transversal direction. The inset in 7A displays the edge of a bar with the typical cleavage for monoclinic structures (inset 7B), which confirms the crystal system obtained with X-ray diffraction studies. Even though different crystal sizes are shown, the morphology is homogenous, and this could be associated with the presence of only one phase based in the monoclinic unit-cell. EDS elemental mapping of compound 1 is shown in Figure 8. EDS analysis confirms the presence of C, O, and N as part of the chemical composition of the sample; furthermore, it can be observed that the signal of Zn atoms was detected all over the crystals, which can be related to a homogeneous distribution of the Zn in compound 1.

SEM-EDS Analysis
Crystals 2020, 10, x FOR PEER REVIEW 9 of 13 A similar "turn-on" response has been reported for some MOF-based sensors, and this has been ascribed to an aggregation-enhanced emission [14].
In order to estimate the sensitivity of 1 to p-xylene, a fluorescence titration experiment was carried out by addition of increasing amounts of this solvent to an acetonitrile suspension of 1. The addition of p-xylene (0-40 equiv.) induced a turn-on fluorescence response ( Figure 10). For the concentration range of [p-xylene] <200 μM, the emission shows a linear dependence, as shown in Figure 10 (inset). The detection limit (LOD) of p-xylene for 1, defined as LOD = 3σ/s, where σ is the standard deviation of the blank signals and s is the slope of the calibration curve with the linear fit (inset, Fig. 10), is 10 ± 0.3 μM.
Finally, in order to explore the chemical stability of 1 and potential guest introduction inside the layers, we compared PXRD patterns and FTIR spectra of the as-synthesized compound 1 and a sample of this MOF treated with p-xylene by stirring (30 min) at r.t. In general, the patterns and IR spectra are very similar to each other (Figure 11), before and after treatment, indicating that its crystal structure remains intact in the presence of p-xylene. This chemical stability can be favored by very limited access to solvents. In this sense, the enhancement of fluorescence can be mainly attributed to the solvation phenomenon.

Photoluminescent Properties
Currently, luminescent MOMs have been widely studied for sensing different types of small-molecule volatile organic compounds and nitroaromatic compounds [7]; however, the sensing of BTX aromatic solvents (benzene, toluene, xylene isomers) still remains largely unexplored. In order to investigate the potential chemosensing properties of 1, its photoluminescence spectra of liquid suspensions in several organic solvents including the BTX aromatics, with constant stirring, were analyzed. Figure 9A shows the family of the spectra (λ ex = 330 nm) of 1 (~25 µM). In general, a fluorescence enhancement was observed in the presence of aromatic compounds containing electron-donating groups. Among the studied solvents, p-xylene displays the highest intensity (λ em = 409 nm), followed by m-xylene and o-xylene (λ em = 407 nm). Suspensions in toluene and benzene showed modest emissions at 409 nm, which are significantly lower than that observed for p-xylene. By contrast, alcohols (CH 3 OH and EtOH) and aprotic solvents (acetone, CHCl 3 , CH 3 CN, DMF, and THF) practically do not present fluorescent emission. Considering that the acetonitrile suspension does not display emission and this solvent is highly miscible with BTX aromatics, we used an acetonitrile suspension medium of 1 to carry out a solvent sensing experiment due to its potential ability to turn-on fluorescence by addition of xylene isomers, toluene, and benzene. Due to the environmental, chemical, and industrial relevance of xylene isomers, which are anthropogenic polluting solvents, there is growing interest in the development of new luminescent MOF-based sensors in which the analytical responses are based on an increase in fluorescence intensity, rather than quenching, because this limits their practical applications. In this line, the literature features only a few examples for xylene isomer sensing [14,34,35].
Next, the solvent selectivity of an acetonitrile suspension of 1 (~25 µM) was examined by addition of 40 equiv. of BTX, and an emission intensity increase at 409 nm was recorded. Benzene and toluene gave low responses ( Figure 9B). Interestingly, 1 showed sensitivity to xylene isomers, especially p-xylene with a considerable increase in its blue emission (I F /I 0 = 27.2), as shown in Figure 9B. The differing "turn-on" response of the methyl-substituted aromatic VOCs versus the polar aprotic solvents demonstrates the potential of this material to act as a selective and sensitive sensor, and a Crystals 2020, 10, 344 9 of 13 potential π-π interaction may arise as a mechanism of interaction between 1 and BTX, which depends on the electro-donating ability of the substituents, as shown in Figure 9B. This fluorescent enhancement may be related to the different interactions between compound 1 and xylene isomers, which have different shapes and volumes; thus, p-xylene is linear and can easily present π-π interactions with the isoquinoline fragments groups. A PLATON program analysis shows that there is about 5% of the crystal volume accessible to solvents [49], which is very limited.   It is well-known that the fluorescent emission of MOFs is strongly dependent on their chemical environment such as solvation, which can change the relative contributions of charge transfers and intraligand luminescence [2,7]. The weak fluorescence or null of 1 in suspensions of polar solvents such as water, alcohols, and DMF is not unexpected, because these solvents are among the most efficient vibrational quenchers [7]. On the other hand, solvation with hydrophobic aromatic molecules can favor charge transfer processes, which results in an increase in fluorescence.
A similar "turn-on" response has been reported for some MOF-based sensors, and this has been ascribed to an aggregation-enhanced emission [14].
In order to estimate the sensitivity of 1 to p-xylene, a fluorescence titration experiment was carried out by addition of increasing amounts of this solvent to an acetonitrile suspension of 1. The addition of p-xylene (0-40 equiv.) induced a turn-on fluorescence response ( Figure 10). For the concentration range of [p-xylene] <200 µM, the emission shows a linear dependence, as shown in Figure 10 (inset). The detection limit (LOD) of p-xylene for 1, defined as LOD = 3σ/s, where σ is the standard deviation of the blank signals and s is the slope of the calibration curve with the linear fit (inset, Figure 10), is 10 ± 0.3 µM.
Finally, in order to explore the chemical stability of 1 and potential guest introduction inside the layers, we compared PXRD patterns and FTIR spectra of the as-synthesized compound 1 and a sample of this MOF treated with p-xylene by stirring (30 min) at r.t. In general, the patterns and IR spectra are very similar to each other (Figure 11), before and after treatment, indicating that its crystal structure remains intact in the presence of p-xylene. This chemical stability can be favored by very limited access to solvents. In this sense, the enhancement of fluorescence can be mainly attributed to the solvation phenomenon.

Conclusions
A new Zn(II)-based metal−organic framework, 1, has been designed and synthesized by solvothermal synthesis combining terephthalic acid and isoquinoline in DMF. X-ray crystallographic studies revealed interpenetrated 2D networks containing a dinuclear Zn 2 paddle-wheel core . Photoluminescence properties of 1 in suspensions of xylene isomers show that 1 possesses a strong blue emission with a maximum at 409 nm. Compound 1, suspended in acetonitrile, can be used as a luminescent material for the sensing of p-xylene in the micromolar concentration range based on an increase in its fluorescent emission.
The strong emission of 1 in combination with the straightforward synthesis and its high thermal stability up to 310 • C makes it a promising candidate for the design of more sophisticated blue-light-emitting materials with potential application in sensing of methyl-substituted aromatic VOCs.