8-Hydroxyquinolinate-Based Metal–Organic Frameworks: Synthesis

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

8‑Hydroxyquinolinate-Based Metal−Organic Frameworks: Synthesis, Tunable Luminescent Properties, and Highly Sensitive Detection of Small Molecules and Metal Ions Xiangxiang Zhao, Shilin Wang, Liyan Zhang, Suya Liu, and Guozan Yuan* School of Chemistry and Chemical Engineering, Anhui University of Technology, Maanshan 243032, P. R. China

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S Supporting Information *

ABSTRACT: Five new metal−organic frameworks, [Zn2L2]· 2DMF·2MeOH (1), [Zn2L2(py)2] (2), [Cd2L2]·Diox·MeOH· 6H2O (3), [Mn2L2]·2DMF·2MeOH (4), and [Co2L2]·2DMF· 4H2O (5), were assembled by using a novel 8-hydroxyquinolinate derivative H2L with different metal ions. Complex 1 features a 3D porous network consisting of meso-helical chains (P + M) built from metal−ligand coordination bonds. The adjacent dinuclear ZnII building blocks in 2 are connected together to generate a 2D grid network. In complex 3, each binuclear motif is bound to four ZnII ions to produce a 2D layer structure that stacks into a 3D porous structure. The framework of complex 4 is isostructural to 5, featuring a 21 helical chain built from [M2L2] units (M = Mn or Co). The adjacent meso-helices associated in parallel are interconnected by the phenolate μ2-O atoms of H2L to give rise to a 2D network. Distinct solid-state luminescence properties of 1−3 were observed, arising from their different metal nodes and frameworks. In particular, complex 1 exhibited excellent stability in both common organic solvents and H2O, thus facilitating its utility as a chemical sensor. Remarkably, luminescent 1 showed highly sensitive detection for nitroaromatic molecules in methanol and Fe3+ ion in H2O even in the presence of other interfering metal cations.

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INTRODUCTION Luminescent metal−organic frameworks (LMOFs) have aroused broad scientific interest over the past 2 decades because of the key virtues derived from their crystalline nature, facile syntheses, tunable structures and porosities, and nanoscale processability.1 Owing to the structural diversity of LMOFs, not only can their organic and inorganic components act to generate luminescence, but also encapsulated guest molecules/ions can be luminescence-responsive.2−4 In this way, LMOFs possess collaborative multifunctionalities compared with other organic and inorganic luminescent materials and have been widely explored for various potential applications including nonlinear optics, biomedical imaging, sensors, and light-emitting and display devices.5−8 Despite the tremendous progress achieved in this field, only a few LMOFbased sensors reported so far have been employed in an aqueous environment because of their poor water-resistance.9 Therefore, the fabrication of LMOF-based sensors integrated with tunable luminescence properties and water stability remains a challenge for chemists. With rapid development of the social activities of human beings and industry, toxic and hazardous chemical species have directly threatened the environment and health of human beings.10 It is therefore highly demanded to develop effective sensors and detection methods for monitoring these chemicals.11 Nitroaromatic compounds (NACs), as one type © XXXX American Chemical Society

of the important chemical intermediates, are commonly used in a myriad ways for the chemical production of aniline, pesticides, dyes, and explosives.12 Sensitive and efficient detection of NACs has drawn tremendous attention for a variety of reasons, including homeland security, human health, and environmental problems. In recent years, it has been shown that luminescent MOFs are a promising class of sensors to detect nitroaromatic explosives.13 On the other hand, FeIII ion is an indispensable element in living organisms and has a vital part in biological processes related to proteins, enzymes, transcriptional events, and hemoglobin formation.14 Both deficiency and overload of Fe3+ may lead to various severe diseases, for example, iron-deficiency anemia, skin ailments, and hematological manifestations.15 Thus, sensing the Fe3+ ion with high selectivity and sensitivity is critical in life systems. Various methods have been developed for Fe3+ detection, including atomic absorption spectroscopy, spectrophotometry, and voltammetry.16 However, these methods are not always convenient and available when Fe3+ is interfered by other metal ions. Therefore, developing a simple and sensitive method for the detection of Fe3+ is imperative. 8-Hydroxyquinoline and its derivatives have been well established as versatile ligands to construct functional metal− Received: October 23, 2018

A

DOI: 10.1021/acs.inorgchem.8b03001 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry organic materials. 1 7 For example, aluminum tris(hydroxyquniolate) has been applied in luminescent devices.18 Our previous reports revealed that 8-hydroxyquinolinate ligands can facilitate the formation of multinuclear metal building units with enhanced rigidity and coplanarity, which further assemble into 3D frameworks with hydrophobic channels.19 The above two features will be beneficial for the water stability of the resulting MOFs. Inspired by this, we envision that water-resistant LMOFs bearing good sensitivity and selectivity toward NACs and metal cations can be fabricated using rationally designed ligands and suitable nodes. Herein one 8-hydroxyquinolinate ligand with a carboxylate group was synthesized, characterized, and employed in the assembly of five MOFs, [Zn2L2]·2DMF· 2MeOH (1; DMF = N,N-dimethylformamide and MeOH = methanol), [Zn2L2(py)2] (2; py =pyridine), [Cd2L2]·Diox· MeOH·6H2O (3; Diox = 1,4-dioxane), [Mn2L2]·2DMF· 2MeOH (4), and [Co2L2]·2DMF·4H2O (5), based on the following reasons: (i) the lowest unoccupied molecular orbitals of 8-hydroxyquinoline are mainly located on the pyridyl ring, implying that the introduction of a carboxylphenyl group can tune the band gap of target complexes, thus modulating the luminescence properties;20 (ii) the carboxylphenyl group in H2L can further enhance the rigidity, π-electron density, and band gap of the ligand, facilitating the formation of robust porous frameworks; (iii) strong metal−ligand bonds can be achieved by the bidentate NO coordination sites of H2L. 1−5 exhibit a variety of 2D and 3D supramolecular frameworks (Scheme 1). Remarkably, complex 1 is capable of not only

Synthesis of (E)-2-[2-(4-Carboxy)ethenyl]-8-hydroxyquinoline (H2L). 4-Formylbenzoic acid (3.0 g, 20 mmol) and 8hydroxyquinaldine (3.18 g, 20 mmol) were added to an oblique two-necked flask containing acetic anhydride (15 mL). Then the reaction was heated at 120 °C for 72 h. Upon cooling, the product was added into ice H2O (300 mL) and kept stirring for ∼12 h. The brown solid was filtered and rinsed three times by H2O. (E)-2-[2-(4Carboxy)ethenyl]-8-acetoxyquinoline (5.4 g, 81%) was obtained as a brown solid. The above-obtained product was dissolved in a mixture of H2O (55 mL) and pyridine (175 mL). Then the reaction mixture was heated to 100 °C and kept there overnight. After cooling, H2O (500 mL) was poured into the flask. The resultant yellow solid was filtered, rinsed by H2O, and dried to obtain the target product H2L with a yield of 92%. 1H NMR (DMSO-d6, 400 MHz) δ: 13.00 (s, 1H), 9.63 (s, 1H), 8.32 (d, J = 8.4 Hz, 1H), 8.21 (d, J = 16 Hz, 1H), 8.01 (d, J = 8.4 Hz, 2H), 7.81 (t, J = 8.4 Hz, 3H), 7.61 (d, J = 16 Hz, 1H), 7.35−7.41 (m, 2H), 7.11 (dd, J = 1.6, 1.6 Hz, 1 H). 13C NMR (DMSO-d6, 400 MHz): δ 167.5, 153.5, 153.3, 150.0, 141.2, 138.6, 137.1, 136.6, 133.6, 130.8, 130.4, 128.3, 127.8, 127.6, 124.4, 121.7, 118.0, 111.8. ESI-MS: m/z 290.1 ([M + 1]+). Synthesis of Complexes 1−5. A mixture of MX2 (M = Zn2+, Co2+, and Mn2+; X = Cl−; 0.1 mmol), H2L (0.05 mmol), N,Ndimethylformamide (DMF) or 1,4-dioxane (Diox) (5 mL), MeOH [or ethanol (EtOH)] (2.5 mL), and H2O (0.5 mL) was heated at 80 °C for 12 h in a capped vial. Crystals of 1−5 qualified for singlecrystal X-ray diffraction (SCXRD) were harvested after slow cooling to RT. They were rinsed by ether and dried in air. Yield: 1, 18.8 mg, 82%; 2, 15.1 mg, 70%; 3, 19.6 mg, 75%; 4, 18.6 mg, 83%; 5, 18.3 mg, 80%. Complex 1. 1 was synthesized via a solvothermal reaction of ZnCl2 and H2L according to the above-mentioned method. Elem anal. Calcd for C44H44N4O10Zn2: C, 57.47; H, 4.82; N, 6.09. Found: C, 57.03; H, 4.98; N, 5.85. FTIR (KBr pellets): 3432(m), 2933(w), 1653(s), 1598(s), 1552(m), 15018(m), 1445(m), 1381(s), 1338(m), 1276(m), 1102(m), 964(w), 839(w), 757(w), 679(w), 595(w), 425(w). Complex 2. Introducing pyridine (0.25 mL) into the above reaction system, yellow crystals 2 were obtained. Elem anal. Calcd for C46H32N4O6Zn2: C, 63.69; H, 3.72; N, 6.46. Found: C, 63.25; H, 3.96; N, 6.01. FTIR (KBr pellets): 3426(m), 3056(w), 2923(w), 1602(s), 1552(m), 1499(w), 1445(s), 1375(s), 1342(s), 1275(m), 1215(w), 1176(w), 1099(m), 1043(w), 1012(w), 878(w), 834(m), 774(w), 741(m), 695(m), 525(w), 425(w). Complex 3. When the same synthetic procedure as that of 1 was adopted but Diox was replaced with DMF, 3 was separated as red block crystals. Elem anal. Calcd for C41H46N2O15Cd2: C, 47.73; H, 4.49; N, 2.72. Found: C, 47.27; H, 4.63; N, 2.45. FTIR (KBr pellets): 3415(s), 2974(m), 2926(m), 1590(m), 1545(m), 1439(s), 1395(s), 1333(m), 1273(m), 1174(w), 1096(m), 1048(m), 967(w), 879(w), 834(w), 748(w), 671(m), 478(w). Complexes 4 and 5. 4 and 5 was prepared by solvothermal reactions of MnCl2 or CoCl2 with H2L based on the aforementioned method. Compound 4. Elem anal. Calcd for C44H44N4O10Mn2: C, 62.63; H, 5.26; N, 6.64. Found: C, 62.24; H, 5.76; N, 6.37. FTIR (KBr pellets): 3429(s), 2971(w), 2929(w), 1653(s), 1587(m), 1543(m), 1501(w), 1444(s), 1394(s), 1334(m), 1276(w), 1101(m), 1056(w), 962(w), 838(w), 759(w), 676(m), 588(w), 480(w), 420(w). Compound 5. Elem anal. Calcd for C42H44N4O12Co2: C, 55.15; H, 4.85; N, 6.13. Found: C, 55.08; H, 6.12; N, 5.96. FTIR (KBr pellets): 3432(m), 2931(w), 1653(s), 1588(m), 1543(m), 1501(m), 1385(s), 1333(m), 1272(m), 1110(m), 965(w), 838(w), 765(w), 682(w), 586(w), 421(w). X-ray Crystallography. SCXRD data of complexes 1−5 were recorded on a Bruker APEX area-detector X-ray diffractometer with Cu Kα radiation (λ = 1.54178 Å) or Mo Kα radiation (λ = 0.71073 Å). The empirical absorption correction was performed by the SADABS program.21a The structures were solved by direct methods and refined by full-matrix least squares on F2.21b For complex 3, the SQUEEZE option was used to delete the Diox and H2O molecules because they could not be satisfactorily modeled.21c Crystallographic

Scheme 1. Synthetic Routes of H2L Ligand and Complexes 1−5

sensing NACs through luminescence quenching but also behaving as a highly selective and sensitive luminescent sensor to detect Fe3+ in water (H2O) because of its high water stability.

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EXPERIMENTAL SECTION

Materials and Measurements. All of the chemicals for syntheses are commercially available and were used as received. Fourier transform infrared (FTIR; KBr pellets) spectra were performed on a Nicolet Magna 750 FTIR spectrometer. 1H and 13C NMR spectra were recorded on a Mercury plus 400 spectrometer. Powder X-ray diffraction (PXRD) data were conducted on a DMAX2500 diffractometer with Cu Kα radiation. Thermogravimetric analysis (TGA) was measured with a STA449C integration thermal analyzer. Fluorescence spectra and room temperature (RT) lifetimes were obtained on a LS 50B luminescence spectrometer, and a FLS920 time-resolved and steady-state fluorescence spectrometer (Edinburgh Instruments), respectively. UV−vis absorption spectra were recorded on a TU-1810 UV−vis spectrophotometer. Electrospray ionization mass spectrometry (ESI-MS) spectra were obtained with a Finnigan LCQ mass spectrometer using dichloromethane (DCM)/methanol (MeOH) as the mobile phase. B

DOI: 10.1021/acs.inorgchem.8b03001 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry details are shown in Tables S1 and S2, and Tables S3−S7 summarize the selected bond distances and angles. The CCDC numbers of 1−5 are 1872664, 1872665, 1872668, 1872671, and 1872672, respectively. Sensing of Small Organic Molecules. Ground broken samples of complex 1 (2 mg) were soaked in different solvents [methenol, ethenol, N,N′-dimethylacetamide (DMA), nitrobenzene (NB), acetone, DMF, tetrahydrofuran (THF), acetone, acetonitrile (CH3CN), o-nitrotoluene (2-NT), and m-nitrotoluene (3-NT)] with an equal volume of 2 mL and treated by ultrasound for 30 min, and then the homogeneous suspensions were immediately used for luminescence measurement. Detailed detection for aromatic compounds was performed by gradually adding a 0.1 M methenol solution of NB, 2-NT, 3-NT, toluene, o-xylene, o-dimethoxybenzene, and ethylbenzene into a cuvette containing 1 (2 mg) immersed in MeOH (2 mL). Sensing of Metal Ions. The lapping as-synthesized materials 1 were then used for sensing tests. For each test, the ground sample (2 mg) was dispersed in different H2O solutions (3 mL) containing 1 × 10−3 mol·L−1 M(NO3)x (M = Al3+, Mn2+, K+, Co2+, Ni2+, Cu2+, Cd2+, Cr3+, and Fe3+), FeSO4, or Ln(NO3)3 (Ln3+ = Ho3+, Yb3+, Gd3+, Er3+, Tb3+, Pr3+, Ce3+, Dy3+, Nd3+, and Sm3+) and then treated by ultrasound for 30 min for luminescence detection. Detailed detection for Fe3+ was measured with increasing aqueous solutions of Fe3+ from 1 × 10−5 to 5 × 10−3 mol·L−1. The antiinterference experiments were carried out to verify the emission of Fe3+ in the presence of various other metal ions with an equal concentration in H2O of 1 × 10−3 mol· L−1. Upon excitation at 350 nm at RT, the luminescence spectra of the obtained suspensions were detected from 350 to 800 nm.

structed from four O atoms of two L ligands and a DMF molecule, while the apical vertices are occupied by one O atom and one N atom from two ligands. Each L ligand uses one bidentate NO coordination pocket to connect two Zn centers and one bidentate carboxylate group to bind another Zn from the adjacent binuclear building unit, with Zn−N and Zn−O bond lengths of 2.195(4) and 1.973(4)−2.065(3) Å, respectively. In the ab plane, the ZnII centers are linked by the terminal carboxylate groups and chelating NO donors of ligands to assemble into a 2D grid structure. It should be noted that mesohelical chains (P + M) built from binuclear ZnII units and L ligands are observed within the 2D network. These mesohelical chains are thus generated around the crystallographic 2(1) axis with a pitch of 9.850(4) Å, which is the same as the length of the b axis (Figure 2a,b). The adjacent 2D network further assembles into a 3D supramolecular framework via C−

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RESULTS AND DISCUSSION Synthetic Chemistry. The 8-hydroxyquinolinate ligand H2L involving one carboxylate group was synthesized by the condensation of 8-hydroxyquinaldine with 4-formylbenzoic acid followed by pyridine-catalyzed hydrolysis in 75% overall yield. H2L was characterized by 1H and 13C NMR spectroscopy (Figure S1) as well as ESI-MS (Figure S2). Heating a mixture of H2L and metal(II) salts (1:2 molar ratio) in DMF, MeOH, Diox, and H2O for 12 h at 80 °C afforded single crystals of 1 and 3−5 in good yield (Scheme 1). Pleasingly, upon the addition of pyridine in an appropriate amount to the reaction mixture, complex 2 was synthesized within the temperature range 70−100 °C. The five complexes were formulated based on SCXRD, elemental analysis, TGA, and IR spectroscopy. Crystal Structures. Single-crystal X-ray analysis on 1 revealed that it crystallized in space group P21/n of a monoclinic system. The asymmetric unit contains one crystallographically independent ZnII ion coordinated by one L ligand and one DMF as well as one guest MeOH (Figure S4). As depicted in Figure 1, the basic structural motif is a binuclear Zn unit, in which two Zn centers are linked by two μ2-O atoms from the phenolates on two L ligands [Zn−Zn distance: 3.257(1) Å]. The Zn atoms adopt a distorted squarebipyramidal geometry, where the equatorial plane is con-

Figure 2. View of the 2D network consisting of meso-helical chains (P + M) along the b axis (a) that are built from the metal−ligand coordination bonds (b) and the 3D supramolecular framework of 1 (c).

Figure 1. View of the binuclear ZnII building unit in complex 1. C

DOI: 10.1021/acs.inorgchem.8b03001 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

H···O hydrogen bonds between the C−H groups of the quinoline rings and carboxylate O atoms (Figure S11). Solvothermal reactions of CdI2 and the 8-hydroxyquinoline ligand H2L in Diox gave block-shaped crystals. As revealed by SCXRD analysis, complex 3 crystallizes in the Pbca space group of an orthorhombic system [a = 20.783(3) Å, b = 9.964(2) Å, and c = 39.876(4) Å at 170 K]. Two crystallographically independent Cd ions exist in 3. As shown in Figure 4a, two Cd1 atoms were bridged by two

H···N hydrogen bonds [C−H···N: 3.518(8) Å] between the C−H groups of the quinoline rings and the N atoms of the coordinated DMF molecules and C−H···O hydrogen bonds [C−H···O: 3.674 (8) Å] between the C−H groups of the quinoline rings and the O atoms of the carboxylate groups (Figure S7). The 3D framework possesses one type of 1D channel extending along the c axis (Figure 2c). The large square channel has the aperture dimensions of 5.0 × 4.2 Å2, with the MeOH and coordinated DMF molecules protruding into the channels. Complex 2 crystallizes in the triclinic space group P1̅ and contains two crystallographically independent ZnII ions, two L ligands, and two coordinated pyridine molecules in the asymmetric unit (Figure S8). As portrayed in Figure 3a,

Figure 3. (a) View of the coordination environment of two ZnII centers in 2. (b) View of the 2D supramolecular structure of 2.

complex 2 is constructed from two similar binuclear ZnII units with Zn−Zn separations of 3.2611(3) and 3.3037(2) Å, respectively. In the (Zn1)2L2 unit, the Zn1 centers are sixcoordinated to four O atoms of three L ligands, one quinoline N atom, and one coordinated pyridine N atom, generating a distorted octahedral geometry. The basal (Zn1)2O2 plane is almost coplanar with the quinoline rings of the binuclear (Zn1)2L2 motif (dihedral angle: 3.34°). The aromatic rings on the L ligand rotate around the central CC bond with a dihedral angle of 52.99°. The two ZnII centers in (Zn2)2L2 adopt a rectangular pyramidal geometry in which the equatorial plane is defined with three O atoms and one N atom from three L ligands, and the apical vertices are filled with a N atom of pyridine. The change of the coordination geometry accompanies adaptive conformation variations of the skeleton of L. The dihedral angle of L in (Zn2)2L2 decreases from 52.99 to 12.63°. Each binuclear Zn2L2 unit is connected with four neighboring Zn2L2 units by the bidentate NO donors and the carboxylate groups of L ligands, giving rise to a 2D grid network in the ab plane (Figure 3b), which further packs into a 3D supramolecular framework (Figure S10) via numerous intermolecular π···π interactions between quinoline rings of vicinal units (ca. 3.98 Å for the face-to-face distance) and C−

Figure 4. (a) Coordination environment of the Cd2+ center in 3. (b) 4-connected mode of binuclear CdII units with each other. (c) View of the 3D structure of complex 3.

phenolate O atoms to build a binuclear unit in which the two Cd centers are separated by 3.6785(3) Å. The Cd1 center environment can be described as a distorted squarebipyramidal geometry, with the CdII being coordinated in the equatorial plane to one N atom and three O atoms. The remaining coordination sphere of each Cd1 ion is further completed by one O atom and one N atom of two different quinoline ligands. The Cd2 centers is six-coordinated by two coordinated H2O ligands and four O atoms from two carboxylate units of two ligands, resulting in an octahedral coordination geometry. Each dimeric CdII building block is connected with four neighboring dimeric units (Figure 4b) via the coordination of four terminal carboxylate groups from four L ligands. As a result, each Cd2 metal center is thus linked by two 4connected (Cd1)2 cores, and each binuclear assembly is linked to four Cd2 ions to yield a 2D network (Figure S14). Notably, the C26−H26A···π- and π···π-stacking interactions between D

DOI: 10.1021/acs.inorgchem.8b03001 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry the quinoline rings of adjacent binuclear units are critical for the formation of a 2D layer network (Figure S15). The layered structure extends into a 3D open framework featuring a 1D channel along the b axis (with the largest dimension of 11.7 × 4.2 Å2) via other noncovalent interactions (Figure 4c). PLATON calculations illustrate that 27.2% void space accessible to solvent molecules (2248.4 Å3 per unit cell) is contained in complex 3. Single-crystal X-ray structure analyses showed that complex 5 is isostructural to 4, and complex 4 is therefore selected to elucidate the structural features. A SCXRD study showed that a 2D network structure is fabricated from a binuclear Mn2L4· 2DMF unit in 4. In the binuclear building unit, both of the two identical Mn centers lie in a distorted octahedral coordination sphere, being coordinated by one quinoline N atom and two phenolate μ2-O atoms of two different ligands L, two carboxylate O atoms, and one DMF (Figure 5a). The related

TGA of 1 shows a weight loss of 22.2% (calcd: 22.8%) between 25−280 °C, which accounts for the liberation of two coordinated DMF and two MeOH molecules. Complex 2 decomposes until about 200 °C (weight loss for 2: 18.5%). This weight loss fits nicely with the release of two coordinated pyridine molecules (calcd: 18.2%). For 3, the guest molecules could be removed at 25−125 °C. Regarding 4, the loss of MeOH and coordinated DMF molecules below 315 °C (calcd: 23.4%) fits well 23.7% weight loss. The release of four H2O and two coordinated DMF molecules (calcd: 23.8%) is responsible for the initial weight loss of 5 in 25−315 °C (24.2%). However, TGA revealed that complexes 1−5 are thermally stable up to 350 °C and decomposition starts from there. Luminescent Properties. Because of the various applications in sensing and detection, photochemistry, and optoelectronic display devices, luminescent MOFs with d10 metals have attracted wide attention.22 Therefore, the solidstate luminescent behavior of complexes 1−3 and the free ligand H2L was studied at ambient temperature. Upon excitation at 350 nm, the ligand displays an intense emission band at 497 nm, which could be ascribed to the π → π* and n → π* electron transitions of ligands.13 Upon excitation at 350 nm, complexes 1−3 show bright emission peaks at 559, 560, and 576 nm, respectively (Figure 6). Their emissions are

Figure 5. (a) Coordination environment of the Mn2+ ion in 4. (b) 2D network of 4 consisting of meso-helical chains (P + M) in the bc plane.

Figure 6. Solid-state emission spectra of the free ligand H2L and complexes 1−3.

bond lengths and angles of Mn−O and Mn−N are comparable with other Mn-based compounds. The ligand L employs a twisted conformation with a dihedral angle of 22.84° between the quinoline and phenyl rings. Each L ligand uses one bidentate NO donor site and one carboxylate group to connect to two Mn ions, and each Mn ion is surrounded by two L ligands to build a 21 helical chain running along the b direction (Figure S17). In the bc plane, the phenolate μ2-O atoms of the ligands interconnect adjacent meso-helices (P + M) associated in parallel to generate a 2D network (Figure 5b). Assisted by other noncovalent interactions, the 2D network aggregates into a 3D network (Figure S18). PXRD and Thermal Stabilities of 1−5. To check the structural integrity of the thus-prepared complexes 1−5, PXRD was carried out. The PXRD patterns of 1−5 were in good agreement with the simulated patterns from their crystal data (Figure S22). The thermal stabilities of 1−5 were conducted through TGA experiments from RT to 800 °C with a heating rate of 10 °C·min−1 under a N2 atmosphere (Figure S23).

probably assigned to energy transfer from the L ligand to metal ions. Compared with the free ligand H2L, it is noteworthy that obvious red shifts have been observed for 1 (62 nm), 2 (63 nm), and 3 (79 nm) in varying degrees. A plausible reason for this red shift lies in the increase of the π conjugacy of the ligand and the mobility of the electron transfer within the binuclear skeleton when H2L is coordinated to metal ions.19,23 To further understand their luminescent properties in the solid state, the emission decay lifetimes of three complexes, 1− 3, were monitored. Combining eq 1 with the instrument response function, the average lifetime was confirmed by allowing αi and τi to vary. A double-exponential decay function was successfully applied to model the luminescent decay curves (Figure S24), and the value of the average lifetime was evaluated based on eq 2. In both equations, t is the time, τ is the lifetime, and α is the preexponential factor.24a The emission decay lifetimes for free ligand H2L and complexes 1−3 are 3.01, 4.04, 3.73, and 3.57 ns, respectively. The luminescence lifetime of complexes 1−3 is longer than that of E

DOI: 10.1021/acs.inorgchem.8b03001 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

luminescent intensity without and with the addition of analytes, respectively. The emission intensities were almost completely quenched with high quenching efficiencies (96.8% for NB, 98.8% for 2-NT, and 98.7% for 3-NT) when 30 μL of a NAC solution in MeOH was added. The luminescent quenching of complex 1 by NACs may have occurred by the effect of photoinduced electron transfer and resonance energy transfer (RET) between complex 1 and the analytes.25,26 To further investigate the potential property of 1 for the sensing of aromatic compounds, some other aromatic analytes with electron-donating groups (such as toluene, o-xylene, odimethoxybenzene, and ethylbenzene) were also explored for the sensing behavior. Unlike the above nitroaramatics, an enhanced luminescence intensity was observed for 1 with these electron-rich aromatics (Figure S27). The varying degrees of luminescent enhancements may arise from electron transfer from aromatic analytes to complex 1.13b Sensing Metal Ions. The potential sensing ability of complex 1 toward the sensing of metal ions was also evaluated in H2O. A suspension of 1 incorporated with metal ions was prepared by dispersing the crystalline sample (3 mg) into the 0.001 mol·L−1 M(NO3)x or FeSO4 (3 mL, M = Al3+, Mn2+, K+, Co2+, Ni2+, Cu2+, Cd2+, Cr3+, and Fe3+) aqueous solutions for fluorescence measurements. As shown in Figure 8a,b, the emission intensities of the different suspensions are directly dependent on the selected metal ions. Interestingly, Fe3+ drastically quenched the luminescence intensity of 1, while no significant effect was observed for other metal ions on the emission. To prove the high selectivity to Fe3+ over other common competing metal ions, antiinterference sensing tests were further carried out under the same conditions (Figure 8c). Pleasingly, the interfering metal ions had little effect on the luminescence of the emulsion of 1, even in the presence of Fe2+. To further study the influence of other trivalent metal ions, the sensing ability of 1 for different lanthanide cations was investigated in an aqueous solution. Upon the addition of different Ln3+ cations (Ln3+ = Ho3+, Yb3+, Gd3+, Er3+, Tb3+, Pr3+, Ce3+, Dy3+, Nd3+, and Sm3+) to the suspension of 1, all of them led to enhancement of the emission intensity of 1 with varying degrees (Figure S28). However, the luminescence quenching was almost not influenced by the interfering Ln3+ ions (Figure S29), thus demonstrating the highly specific quenching effect from the Fe3+ ion. To assess the stability of complex 1, we investigated recycled and reused 1 in the sensing of the Fe3+ ion. The recycled 1 was washed with H2O until the solution became colorless and recovered in quantitative yield. Strong luminescent intensity was observed after five cycles of sensing experiments (Figure S30). Meanwhile, PXRD patterns showed that the recovered 1 remained crystalline and structurally intact (Figure S31). All of the results confirmed that complex 1 could be a promising candidate to selectively detect the Fe3+ ion even in the presence of other metal ions. The concentration-dependent luminescent measurements for the Fe3+ ion were conducted to further assess the sensitivity of 1 for Fe3+. Obviously, when the Fe3+ concentration was increased from 1 × 10−5 to 5 × 10−3 mol·L−1, the emission intensity of 1 at 540 nm gradually decreased. From the relationship between the concentration of the Fe3+ ion and the quenching efficiency (Figures 8d and S32), it was found that the quenching efficiency could reach up to 99.8% with the addition of a Fe3+ ion of 5 × 10−3 mol·L−1. In addition, the linear correlation between the amount of Fe3+ ion and the

the free ligand H2L. This may arise from the high-dimensional supramolecular structures and increased conformational rigidity in complexes 1−3.24b Moreover, the luminescence lifetime for 1−3 is comparable with those reported for other H2L-derived complexes. The shorter luminescence lifetimes of complexes 2 and 3 may be caused by a cooperative contribution from a competitive nonradiative decay process in the two complexes, distinct metal nodes, and different supramolecular frameworks.19 n

I (t ) =

∑ αi exp( − t /τi) i=1

τavg =

α1τ12 + α2τ2 2 α1τ1 + α2τ2

(1)

(2)

Sensing Organic Small Molecules. Recently, it has been demonstrated that luminescent MOFs are very promising to sense small organic molecules because of their sensitivity, quick response, selectivity, operability, and reversibility.7 Complexes 2 and 3 are unstable when soaked in organic solvents or aqueous solutions. The stable crystalline nature and structural integrity combined with the excellent luminescence properties of 1 prompted us to explore its utilization such as in the sensing of small organic molecules. To evaluate the potential application of 1 to detect NACs, the photoluminescent properties of 1 dispersed in common solvents DMA, DMF, THF, CH3CN, MeOH, EtOH, DCM, Diox, and acetone were investigated. To do this, 1 (2 mg) was finely ground into a powder, then soaked in a variety of organic solvents (2 mL), and ultrasonicated for 0.5 h to form stable suspensions for luminescence measurement. As portrayed in Figures 7 and S25, the emission intensities of 1 are strongly regulated by the solvent molecules, in which acetone displays a significant quenching effect.

Figure 7. Luminescent intensities of 1 in different organic molecules upon excitation at 350 nm.

The solvent-dependent quenching behavior shown above is essential for sensing of NACs; therefore, detailed experiments were performed to examine it. Three NACs (NB, 2-NT, and 3NT) were selected as analytes to track the luminescence responses. As observed, the emission intensities of the suspensions of 1 in MeOH decreased gradually when increasing NAC concentrations (2−30 μL, dispersing in MeOH with a concentration of 0.1 M; Figure S26). The equation (I0 − I)/I0 × 100% was used to calculate the quenching efficiency (%), where I0 and I represent the F

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discussed above implies that complex 1 has great potential to detect the Fe3+ ion in high selectivity and sensitivity. Sensing Mechanism. We have also explored the possible mechanism in luminescent quenching processes of 1 toward the Fe3+ ion. On the basis of the PXRD pattern of the recovered sample, the crystal structure of complex 1 remains intact, suggesting that the structure change of 1 is not responsible for this quenching process. Moreover, the cationic exchange of ZnII centers in neutral 1 with the Fe3+ ion is also very difficult. To prove whether the RET exits or not in the above system, aqueous solutions of different metal ions [M(NO3)x, FeSO4, and Ln(NO3)3] were investigated by UV−vis spectroscopy. From the spectra, a broad absorption band from 210 to 500 nm is observed for Fe3+, which overlaps with the excitation spectrum of 1 (Figure S34), while no overlaps are found for other metal cations. Additionally, a batch of UV−vis absorption spectra of Fe3+ with various concentrations were measured in aqueous solutions (Figure S35). The absorption intensity of Fe3+ enhances linearly with the concentrations, which is in good agreement with the change of the quenching efficiency. These results illustrate that the quenching process may be caused by a competitive absorption mechanism, where resonance energy was transferred from 1 to Fe3+ ion, as reported in other literature reports.27

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CONCLUSION In summary, we have designed and prepared five novel L-based MOFs, 1−5, from one novel 2-substituted H2L ligand. In the solid state, five complexes display different frameworks built from similar dinuclear MII building blocks. The solid-state luminescent properties were explored for H2L and 1−3 at RT. The different frameworks and metal centers of 1−3 lead to the disparate emission spectra and fluorescence lifetimes. Importantly, complex 1 can remain crystalline and structurally intact in common organic solvents and H2O. The luminescence sensing studies demonstrate that 1 is capable of sensing nitroexplosive compounds and Fe3+ through luminescence quenching. It is worth noting that 1 not only exhibits excellent luminescence sensing behavior for Fe3+ ions in H2O but also is resistant to the influence from competing metal cations including Fe2+ and Ln3+. Overall, the present study provides an effective method to synthesize L-based MOFs for sensing small organic molecules and Fe3+ ion. Further research centered on the synthesis of MOF-based sensors with multifunctional applications and high sensitivity is ongoing in our group.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b03001. Experimental details and spectral data (PDF)

Figure 8. Luminescence spectra (a) and intensity (b) of 1 after treatment with different cations (1.0 × 10−3 M). (c) Luminescence intensities of 1 soaked in the M(NO3)x or FeSO4 (1 mM; red color) aqueous solutions and mixed-metal cations including the Fe3+ ion (1 mM; blue color) upon excitation at 350 nm. (d) Quenching efficiency of 1 with different concentrations of Fe(NO3)3.

Accession Codes

CCDC 1872664, 1872665, 1872668, 1872671, and 1872672 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam. ac.uk/data_request/cif, or by emailing data_request@ccdc. cam.ac.uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

quenching efficiency (R2 = 0.995) only occurs at low concentrations (0−0.1 mmol·L−1; Figure S33). The detection limit for the Fe3+ ion determined following the 3δ IUPAC criteria can reach as low as 0.1 mmol·L−1, which is comparable with the values of some reported Zn-MOFs.25 Therefore, that G

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Guozan Yuan: 0000-0003-0074-1274 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grants 21671002 and 21603001), Anhui Province Natural Science Funds for Distinguished Young Scholar (Grant 1808085J25), and Training Programs of Innovation and Entrepreneurship for Undergraduates (Grant 201810360028).

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DOI: 10.1021/acs.inorgchem.8b03001 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.8b03001 Inorg. Chem. XXXX, XXX, XXX−XXX