Three Novel Zn-Based Coordination Polymers: Synthesis, Structure, and Eﬀective Detection of Al3+ − and S2 Ions

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Article Three Novel Zn-Based Coordination Polymers: Synthesis, Structure, and Eﬀective Detection of Al3+ − and S2 Ions

1,2, 1,2, 1,2 1,2 1,2 1,2 Yuna Wang †, Xiaofeng Zhang †, Yanru Zhao , Suoshu Zhang , Shifen Li , Lei Jia , Lin Du 1,2,* and Qihua Zhao 1,2,* 1 School of Chemical Science and Technology Pharmacy, Yunnan University, Kunming 650091, China; [email protected] (Y.W.); [email protected] (X.Z.); [email protected] (Y.Z.); [email protected] (S.Z.); [email protected] (S.L.); [email protected] (L.J.) 2 Key Laboratory of Medicinal Chemistry for Natural Resource Education Ministry, Yunnan University, Kunming 650091, China * Correspondence: [email protected] (L.D.); [email protected] (Q.Z.); Tel.: +86-871-6503-5640 (L.D.) These authors contributed equally to this work. † Received: 27 December 2019; Accepted: 12 January 2020; Published: 17 January 2020

Abstract: Three novel Zn-based coordination polymers (CPs), [Zn(MIPA)]n (1), {[Zn(MIPA)(4,40- bipy) (H O)] 1.5H O}n (2), and {[Zn(MIPA)(bpe)] H O}n (3) (MIPA = 4-methoxyisophthalic acid, 0.5 2 · 2 · 2 4,40-bipy = 4,40-bipyridine, bpe = (E)-1,2-di(pyridine-4-yl)ethane), were constructed by ligand 4-methoxyisophthalic acid under solvothermal conditions. Compound 1 features a beaded 2D-layer architecture, while compound 2 presents a 2-fold interpenetrating structure with a uninodal three-connected hcb topology. Compound 3 has a 3-fold interpenetrated four-connected dmp topology. Photoluminescence investigations of compound 2 were explored in detail, by which ions 3+ 2 were detected, and it was observed to have the highest quenching efficiency toward Al and S − ions. 3+ 2 The possible fluorescence quenching mechanisms of 2 toward Al and S − ions were also explored. To the best of our knowledge, this is the first potential dual-responsive luminescent probe based on 3+ 2 a Zn(II) coordination polymer for detecting Al and S − ions via a luminescence quenching effect in ethanol.

Keywords: coordination polymers; 4-methoxyisophthalic acid; ion detection; ethanol

1. Introduction As interest in environmental and health concepts grows, the effects of various ions on the biological environment and human health are popular focus points. Aluminum and sulfur ions play an important part in our daily lives and are not able to be substituted due to their peculiarly superior properties, such as the light weight and corrosion resistance of the metal aluminum, and the participation of sulfur ions in physiological processes [1,2]. However, if the aluminum ion or sulfur ion content in the body is high, it will affect human health. The concentration of aluminum ions accumulated in the human body can result in metabolic disorders. Abnormal production of sulfur ions is associated with many diseases, including loss of consciousness and respiratory paralysis [3,4]. In addition, sulfur ions are important pollutants in the environment and are widely found in various polluted water sources. Therefore, it is very important to urgently establish a rapid and effective analytical method for the determination of aluminum and sulfur ions or to find a substance that has a specific and sensitive ability to recognize them. Coordination polymers (CPs) have aroused much interest, owing to their potential applications in catalysis [5–7], adsorption [8,9], separation [10], electrochemistry [11–13], and as chemical

Molecules 2020, 25, 382; doi:10.3390/molecules25020382 www.mdpi.com/journal/molecules Molecules 2020, 25, 382 2 of 14 Molecules 2020, 25, x FOR PEER REVIEW 2 of 14 sensors[14–17]. [14In– 17recent]. In years, recent coordination years, coordination polymers polymers (CPs) used (CPs) as used fluorescent as fluorescent sensors sensors have havebeen 3+ 3+ 2− 2 beensuccessfully successfully applied applied in the in determination the determination of various of various analytes analytes such such as metal as metal ions ions (Fe (Fe, Cr2,O Cr4 2,O etc.)4 −, etc.)[18–20], [18– small20], small molecules molecules (acetylacetone, (acetylacetone, water, water, etc.) etc.) [21,22], [21,22 and], and explosives explosives (TNT, (TNT, PA, PA, etc.) etc.) [23–25]. [23–25]. Coordination polymerspolymers have have become become one one of theof the hotspots hotspots of material of material chemistry chemistry as new as fluorescent new fluorescent probes becauseprobes because of their of high their sensitivity high sensitivity and selectivity, and selectivity, ease of operation, ease of operation, convenience, convenience, and fast responseand fast timeresponse [26– 29time]. There[26–29]. has There been evidencehas been thatevidence Zn-based that coordinationZn-based coordination polymers presentpolymers prominent present superiority,prominent superiority, especially inespecially the field in of the fluorescence field of fluo sensingrescence [ 30sensing,31]. He[30,31]. et al. He designed et al. designed a Zn-based a Zn- 3+ 3+3+ 3+ 2 2− 22− multi-responsivebased multi-responsive luminescent luminescent probe prob withe awith discriminatory a discriminatory ability ability for Fe for ,Fe Al, Al, SiF, SiF6 −6, Cr, Cr2O2O77 −,, nitrofurantoin (NFT), and nitrofurazonenitrofurazone (NFZ) viavia thethe changechange ofof luminescenceluminescence intensityintensity [[32].32]. The Zn(II) ion, as aa dd1010 metal ion, can greatly enhance the rigidity and fluorescentfluorescent yield of coordinationcoordination polymers [[33,34].33,34]. Furthermore, Zn-based CPs are able to displaydisplay thethe ligand-centeredligand-centered characteristiccharacteristic emission. AsAs such, such, Zn-based Zn-based CPs CPs used used as fluorescent as fluorescent sensors sensors usually usually change thechange fluorescence the fluorescence properties byproperties means of by the means incorporation, of the incorporation, functionalization, functionalization, and modification and modification of the ligands of [ 35the,36 ligands]. Therefore, [35,36]. it isTherefore, fascinating it is and fascinating significant and to significant design and to synthesize design and luminescent synthesize materialsluminescent using materials Zn(II) ionsusing and Zn(II) the controlions and of the ligands. control of ligands. Herein, wewe reported reported three three novel Zn-basednovel Zn-based coordination coordination polymers: [Zn(MIPA)] polymers:n (1[Zn(MIPA)]), {[Zn(MIPA)(4,4n (1),0- bipy){[Zn(MIPA)(4,4’-bipy)(H O)] 1.5H O}0.5n (H(2),2O)]·1.5H and {[Zn(MIPA)(bpe)]2O}n (2), andH O}{[Zn(MIPA)(bpe)]·Hn (3) (MIPA = 4-methoxyisophthalic2O}n (3) (MIPA acid, = bpe4- 0.5 2 · 2 · 2 =methoxyisophthalic(E)-1,2-di(pyridine-4-yl)ethane), acid, bpe = which(E)-1,2-di(pyridine-4-yl)ethane), were constructed by introducing which anwere N-donor constructed ligand andby controllingintroducing solvents. an N-donor The solid ligand fluorescence and controlling properties solv of compoundsents. The solid1–3 were fluorescence investigated, properties and results of 3+ 2 clearlycompounds indicated 1–3 thatwere compound investigated,2 has and agood results fluorescence clearly indicated performance that compound in Al and 2 S has− detection, a good withfluorescence high selectivity performance and sensitivity. in Al3+ and As S2 far− detection, as we know, with this high is theselectivity first Zn-based and sensitivity. coordination As far polymer as we 3+ 2 whichknow, canthis be is usedthe first as a dualZn-based functional coordination fluorescent polymer material which to detect can Albe usedand as S −aions dual by functional means of luminescencefluorescent material quenching to detect in ethanol. Al3+ and S2− ions by means of luminescence quenching in ethanol.

2. Results

2.1. Crystal Structure of [Zn(MIPA)]nn (1) Single crystalcrystal X-ray X-ray analysis analysis showed showed that that compound compound1 crystallizes 1 crystallizes in the in orthorhombic the orthorhombic Pbca spacePbca group,space group, presenting presenting a 2D multi-metallic a 2D multi-metallic linear clusterlinear cluster structure. structure. The asymmetrical The asymmetrical unit consists unit consists of one Zn(II)of one ionZn(II) and ion one and MIPA one ligand. MIPA Eachligand. hexacoordinated Each hexacoordinated Zn(II) center Zn(II) with center a ZnO with6 binding a ZnO6 setbinding displays set adisplays distorted a octahedraldistorted octahedral geometry, createdgeometry, by fivecreated carboxylic by five O carboxylic atoms from O fouratoms diff fromerent four MIPA different ligands (Zn–O:MIPA ligands 1.958(4), (Zn–O: 1.997(4), 1.958(4), 2.008(4), 1.997(4), 2.016(4) 2.008(4), Å) and 2. one016(4) methoxy Å) and O one atom methoxy (Zn–O: O 2.482(4) atom (Zn–O: Å) (Figure 2.482(4)1a). 5 TheÅ) (Figure MIPA ligand 1a). The is completely MIPA ligand deprotonated is completely with the deprotonated coordination with mode the of µ coordination4-κ O0,O00,O000 mode,O0000 :Oof00000 μ4-. (Schemeκ5O’,O’’,O1,’’’ mode,O’’’’:O I).’’’’’ Two. (Scheme carboxyl 1, mode groups I). from Two di carboxylfferent MIPA groups ligands fromare different used to MIPA bridge ligands the neighboring are used Zn(II) ions into 1D multi-metallic (Zn–O)n chains (Zn Zn distance is 3.519 Å), and the neighboring to bridge the neighboring Zn(II) ions into 1D multi-metallic··· (Zn–O)n chains (Zn···Zn distance is 3.519 1DÅ), multi-metallicand the neighboring chains are1D furthermulti-metallic connected chains into are a beaded further 2D-layer connected structure into a bybeaded MIPA 2D-layer ligands (Figure1b,c). The 2D-layer structure is further assembled into a 3D supermolecule frame by π π structure by MIPA ligands (Figure 1b,c). The 2D-layer structure is further assembled into a ···3D interactionsupermolecule between frame layers by π··· (Figureπ interaction1d). between layers (Figure 1d).

Scheme 1. Coordination modes of an MIPA (4-methoxyisophthalic(4-methoxyisophthalic acid)acid) ligand.ligand.

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Figure 1. (a) Coordination environment of compound 1, [Zn(MIPA)]n.(b) 1D metal chain of 1. The a, b Figure 1. (a) Coordination environment of compound 1, [Zn(MIPA)]n. (b) 1D metal chain of 1. The a, and c represent a axis, b axis and c axis, respectively. (c) The 2D layers of 1.(d) The π π interaction b and c represent a axis, b axis and c axis, respectively. (c) The 2D layers of 1. (d) The π······π interaction between layers. All hydrogen atoms and guest molecules are omitted for clarity. Symmetry codes: between layers. All hydrogen atoms and guest molecules are omitted for clarity. Symmetry codes: (1) (1) x, 0.5 y, 0.5 + z, (2) 0.5 + x, y, 0.5 z. x, 0.5 − y, −0.5 −+ z, (2) 0.5 + x, y, 0.5 − z. − 2.2. Crystal Structure of {[Zn(MIPA)(4,4 -bipy) (H O)] 1.5H O}n (2) 0 0.5 2 · 2 2.2. Crystal Structure of {[Zn(MIPA)(4,4′-bipy)0.5(H2O)]·1.5H2O}n (2) Single crystal X-ray analysis revealed that 2 crystallizes in the monoclinic C2/c space group. The asymmetricSingle crystal unit isX-ray composed analysis of revealed one Zn(II) that ion, 2 crystallizes one MIPA, halfin the of monoclinic a 4,40-bipy C linker,2/c space one group. coordinated Thewater asymmetric molecule, unit and one-and-a-halfis composed of free one water Zn(II) molecules. ion, one MIPA, The Zn1 half unit of isa four-ligated4,4′-bipy linker, with one distorted coordinatedtetrahedral ZnOwater3N molecule, geometry and by two one-and-a-half carboxylic O free atoms water (Zn–O: molecules. 1.991(3), The 1.948(3) Zn1 Å)unit from is four- two MIPA ligatedligands, with one distorted coordinated tetrahedral water molecule ZnO3N (Zn–O:geometry 2.055(3) by two Å), carboxylic and one N O atom atoms (Zn–N: (Zn–O: 2.024(4) 1.991(3), Å) from 1.948(3)a 4,40-bipy Å) from linker two (Figure MIPA2 liga). Thends, fully one deprotonatedcoordinated water MIPA molecu ligandsle (Zn–O: connect 2.055(3) the neighboring Å), and one Zn (II) Nions atom into (Zn–N: a 1D W-type2.024(4) chain.Å) from As a shown 4,4′-bipy in Schemelinker (Figure1, mode 2). II, The all thefully carboxylic deprotonated groups MIPA of MIPA 2 ligandsligands connect present the a syn-syn- neighboringµ2-κ O,O Zn 0(II)coordination ions into a mode.1D W-type The 1Dchain. chains As shown are arranged in Scheme in a parallel1, 2 modemode II, by all the the 4,4 0carboxylic-bipy linkers, groups which of resultsMIPA inligands a zigzag present 2D hexagone-pattern a syn-syn-μ2-κ O,O fold′ coordination (Figure3a,b). The mode.terminally-coordinated The 1D chains are water arranged molecules in a parallel that are mode suspended by the between4,4′-bipy the linkers, two layers which prevent results itsin a further zigzagextension 2D hexagone-pattern to a high-dimensional fold (Figure network. 3a,b). Two The of terminally-coordinated the folds are interlocked water with molecules each other that to give a are2-fold suspended interpenetrating between structurethe two (Figurelayers prevent3c). To better its further observe extension the structure, to a high-dimensional topology analysis was network.carried out.Two The of the Zn(II) folds ions are can interlocked be considered with aseach three-connected other to give nodes,a 2-fold while interpenetrating all the MIPA and structurebpe ligands (Figure can be3c). viewed To better as linkers. observe Therefore, the structure, the structure topology of analysis compound was2 carriedcan be simpliﬁedout. The as a Zn(II)uninodal ions three-connectedcan be consideredhcb astopology three-connected with the nodes, Schläﬂi while symbol all the of6 MIPA3 (Figure and3d). bpe ligands can be viewed as linkers. Therefore, the structure of compound 2 can be simplified as a uninodal three-connected hcb topology with the Schläfli symbol of 63 (Figure 3d).

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Figure 2. The coordination environment of Zn1. All hydrogen atoms and guest molecules are omitted FigureFigure 2.2. The coordination environmentenvironment ofof Zn1.Zn1. All hydrogenhydrogen atomsatoms andand guestguest moleculesmolecules areare omittedomitted for clarity. Symmetry code: (1) x, 1 − y, 0.5 + z. forfor clarity.clarity. SymmetrySymmetry code:code: (1)(1) xx,, 11 − y, 0.5 + zz.. −

FigureFigure 3. (a) The 2D2D layerlayer ofof compoundcompound2 2,, {[Zn(MIPA)(4,4 {[Zn(MIPA)(4,40-bipy)′-bipy)0.50.5(H2O)]·1.5HO)] 1.5H22O}O}nn. .The The a, a, b b and and c c Figure 3. (a) The 2D layer of compound 2, {[Zn(MIPA)(4,4′-bipy)0.5(H2O)]·1.5H· 2O}n. The a, b and c representrepresent aa axis,axis, bb axisaxis and and cc axis,axis, respectively. respectively. ( (bb)) The The corresponding corresponding node-and-linker node-and-linker diagram diagram of of the the represent a axis, b axis and c axis, respectively. (b) The corresponding node-and-linker diagram of the 2D2D layers layers in in 22..( (cc)) The The interpenetrating interpenetrating structure structure of of 22..( (dd) )The The corresponding corresponding node-and-linker node-and-linker diagram diagram 2D layers in 2. (c) The interpenetrating structure of 2. (d) The corresponding node-and-linker diagram ofof structures structures interspersed between 2D layers in 2. of structures interspersed between 2D layers in 2. 2.3. Crystal Structure of {[Zn(MIPA)(bpe)] H2O}n (3) 2.4. Crystal Structure of {[Zn(MIPA)(bpe)]·H· 2O}n (3) 2.4. Crystal Structure of {[Zn(MIPA)(bpe)]·H2O}n (3) CompoundCompound 33 crystallizescrystallizes in in the orthorhombic space space group Pnna,, presenting presenting a a 3-fold 3-fold Compound 3 crystallizes in the orthorhombic space group Pnna, presenting a 3-fold interpenetratinginterpenetrating 3D3D structure.structure. An An asymmetric asymmetric unit unit contains contains half half of a of Zn(II) a Zn(II) ion, halfion, ofhalf an MIPAof an ligand,MIPA ligand,halfinterpenetrating of ahalf bpe of linker, a bpe3Dand structure.linker, half ofand An a freehalf asymmetric waterof a free molecule. unitwater contains Themolecule. Zn(II) half The ionof a exhibitsZn(II) Zn(II) ion ion, distorted exhibits half of tetrahedral distortedan MIPA ZnOligand,N halfgeometry of a bpe as it linker, coordinates and withhalf of two a carboxylicfree water O molecule. atoms (Zn–O: The 1.934(3)Zn(II) ion Å) fromexhibits two distorted diﬀerent tetrahedral2 2 ZnO2N2 geometry as it coordinates with two carboxylic O atoms (Zn–O: 1.934(3) Å) from twoMIPAtetrahedral different ligands, ZnO MIPA and2N2 twoligands,geometry pyridyl and as Ntwoit atomscoordinate pyridyl (Zn–N: Ns withatom 2.059(4) twos (Zn–N: carboxylic Å) from2.059(4) diOﬀ atomÅ)erent froms bpe(Zn–O: different ligands 1.934(3) bpe (Figure ligandsÅ) from4a). (FigureMIPAtwo different ligands 4a). MIPA MIPA bind ligands the ligands, two bind adjacent and the two two Zn(II) pyridyl adjacent ions N resultingatomZn(II)s (Zn–N:ions in resulting the 2.059(4) formation in Å) the from of formation a 1Ddifferent chain of (Figurebpea 1D ligands chain4b). Here,(Figure each 4a). carboxylate MIPA ligands group bind adopts the two a syn-syn- adjacentµ Zn(II)-κ2O,O ionscoordination resulting in (Scheme the formation1, mode of II). a 1D The chain 1D (Figure 4b). Here, each carboxylate group adopts2 a syn-syn-0 μ2-κ2O,O′ coordination (Scheme 1, mode 2 II).(Figure The 4b).1D helicalHere, eachchain carboxylate further links group with adopts four ahelices syn-syn- inμ 2opposite-κ O,O′ coordination directions through (Scheme the 1, modefour II). The 1D helical chain further links with four helices in opposite directions through the four

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Molecules 2020, 25, x FOR PEER REVIEW 5 of 14 helical chain further links with four helices in opposite directions through the four coplanar bpe ligands fromcoplanar the viewbpe ligands of the b fromaxis. the Consequently, view of the itb isaxis. extended Consequently, into a 3D it porousis extended framework into a (Figure3D porous4c). Moreover,framework three (Figure identical 4c). Moreover, frameworks three interpenetrate identical frameworks with each other interpenetrate to stabilize thewith whole each structureother to andstabilize almost the block whole the structure pores. The and two-connected almost block th bridginge pores. of The the two-connected MIPA ligand and bridging the bpe of ligandthe MIPA can beligand viewed andas the a linearbpe ligand link. Thecan Zn(II)be viewed ions as can a belinear regarded link. The as four-connected Zn(II) ions can nodes. be regarded Therefore, as four- the overallconnected structure nodes. of Therefore,3 is simpliﬁed the overall as a uninodal structure four-connected of 3 is simplified 3-fold as interpenetratinga uninodal four-connected 3D structure 3- with a point symbol of (65 8) (Figure4d). fold interpenetrating 3D structure· with a point symbol of (65·8) (Figure 4d).

FigureFigure 4.4. ((aa)) TheThe coordinationcoordination environmentenvironment ofof Zn1.Zn1. ((b)) TheThe left-handedleft-handed andand right-handedright-handed chainschains ofof compound 3, {[Zn(MIPA)(bpe)] H O}n. The a, b and c represent a axis, b axis and c axis, respectively. compound 3, {[Zn(MIPA)(bpe)]·H· 22O}n. The a, b and c represent a axis, b axis and c axis, respectively. ((cc)) TheThe 3D3D supramolecularsupramolecular architecturearchitecture ofof3 3.(. (dd)) TheThe four-connectedfour-connected netnet ofof 33.. AllAll hydrogenhydrogen atomsatoms andand guest molecules are omitted for clarity. Symmetry code: (1) x, y + 1/2, z + 1/2. guest molecules are omitted for clarity. Symmetry code: (1) x, −−y + 1/2, −−z + 1/2. 2.4. Powder Diffraction (PXRD) Analysis and Thermogravimetric Analysis (TGA) 2.5. Powder Diffraction (PXRD) Analysis and Thermogravimetric Analysis (TGA) For compounds 1–3, bulk phase purity was confirmed by the PXRD pattern of the simulated For compounds 1–3, bulk phase purity was confirmed by the PXRD pattern of the simulated and as-synthesized compounds. The results are shown in Figure S1—it is clearly observable that the and as-synthesized compounds. The results are shown in Figure S1—it is clearly observable that bulk phase is in good agreement with the simulated data, confirming all of the compounds are high the bulk phase is in good agreement with the simulated data, confirming all of the compounds in purity and can support further study. Thermogravimetric analysis (TGA) of compounds 1–3 was are high in purity and can support further study. Thermogravimetric analysis (TGA) of1 carried out under an N2 atmosphere increasing from 25 to 800 ◦C with a flow rate of 10 ◦C min− . The compounds 1−3 was carried out under an N2 atmosphere increasing from 25 to 800 °C ·with a TG curves are given in Figure5. For compound 1, no obvious weight loss before 400 C was observed, flow rate of 10 °C·min−1. The TG curves are given in Figure 5. For compound 1, no obvious◦ weight which showed good thermostability. After that, rapid decomposition of the structure occurred. For 2, loss before 400 °C was observed, which showed good thermostability. After that, rapid a weight loss of 10.4% was detected in the temperature range of 60 120 C, which corresponds to the decomposition of the structure occurred. For 2, a weight loss of− 10.4%◦ was detected in the release of both free and coordinated water molecules (calculated, 11.77%). Subsequently, no significant temperature range of 60−120 °C, which corresponds to the release of both free and coordinated weight loss was observed until the structure collapsed at 275 C. Compound 3 was stable until 320 C, water molecules (calculated, 11.77%). Subsequently, no significant◦ weight loss was observed ◦ followed by the decomposition of its backbone. until the structure collapsed at 275 °C. Compound 3 was stable until 320 °C, followed by the decomposition of its backbone.

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Figure 5. Thermogravimetric curves of the three compounds. Figure 5. Thermogravimetric curves of the three compounds. 2.6.2.5. Solid-State Solid-State Luminescence Luminescence 2.6. Solid-State Luminescence The solid-state luminescent emissions emissions of of compounds compounds 11,, 22,, 3,, and and free ligands were collected at The solid-state luminescent emissions of compounds 1, 2, 3, and free ligands were collected at room temperature. temperature. The The 4,4´-bpy 4,40-bpy and and bpe bpe ligands ligands displayed displayed luminescent luminescent emission emission at 362 at 362nm nmand and417 room temperature. The 4,4´-bpy and bpe ligands displayed luminescent emission at 362 nm and 417 nm,417 nm,respectively, respectively, while while the MIPA the MIPA ligand ligand was not was observed not observed to produce to produce luminescent luminescent emissionemission (Figures nm, respectively, while the MIPA ligand was not observed to produce luminescent emission (Figures S2(Figures and S3). S2 As and illustrated S3). As illustratedin Figure 6a, in Figureupon ph6a,otoexcitation upon photoexcitation at 323 nm, atweak 323 and nm, broad weak emission and broad of S2 and S3). As illustrated in Figure 6a, upon photoexcitation at 323 nm, weak and broad emission of 1emission is observed, of 1 is with observed, a maximum with a peak maximum of 344 peaknm (Figure of 344 nmS4). (Figure Compound S4). Compound 2 exhibits stronger2 exhibits emissions stronger 1 is observed, with a maximum peak of 344 nm (Figure S4). Compound 2 exhibits stronger emissions atemissions 436 nm, at with 436 nm,an excitation with an excitation peak of peak346 nm, of 346 while nm, compound while compound 3 is not3 isobserved not observed to produce to produce any at 436 nm, with an excitation peak of 346 nm, while compound 3 is not observed to produce any luminescentany luminescent emission emission (Figure (Figure S5). The S5). luminescence The luminescence of 1 and of 21 isand due2 isto dueligand-based to ligand-based emission, emission, which luminescent emission (Figure S5). The luminescence of 1 and 2 is due to ligand-based emission, which maywhich be may connected be connected with the with metal-to-ligand the metal-to-ligand charge charge transfer, transfer, ligand-to-metal ligand-to-metal charge charge transfer, transfer, or may be connected with the metal-to-ligand charge transfer, ligand-to-metal charge transfer, or intraligandor intraligand transitions transitions between between ligands ligands [37–40]. [37–40 ].The The photoluminescent photoluminescent emission emission of of CPs CPs is is closely intraligand transitions between ligands [37–40]. The photoluminescent emission of CPs is closely associated with the central metal coordination environment and the property ofof thethe ligand.ligand. After associated with the central metal coordination environment and the property of the ligand. After adding and tuning the N-donor link linkers,ers, the coordination coordination environment of metal ions and the structure structure adding and tuning the N-donor linkers, the coordination environment of metal ions and the structure of the coordinationcoordination polymerspolymers are are changed. changed. All All of of those those have have eﬀ ectseffects on on the the rigidity rigidity of the of structurethe structure and of the coordination polymers are changed. All of those have effects on the rigidity of the structure andfurther further inﬂuence influence the energythe energy transfer transfer and chargeand charge transfer transfer involved involved in luminescence. in luminescence. and further influence the energy transfer and charge transfer involved in luminescence.

Figure 6. ((aa)) Fluorescence spectra spectra of of compounds 1–3 in the solidsolid state.state. ( (b)) Fluorescence spectra of compoundFigure 6. ( a2) inFluorescence different diﬀerent solvents. spectra of compounds 1–3 in the solid state. (b) Fluorescence spectra of compound 2 in different solvents. 2.6. Selectivity toward Al3+ Ions 2.7. Selectivity toward Al3+ Ions 3+ 2.7. SelectivityCompound toward2 was Al selectedIons for use in studying the luminescence responses to various metal cations, Compound 2 was selected for use in studying the luminescence responses to various metal owing to its stronger solid-state emission performance. Before the experiment, the ﬂuorescence spectra cations,Compound owing to 2 wasits stronger selected forsolid-state use in studying emission the performance. luminescence Before responses the experiment,to various metal the of compound 2 in various solvents were compared and studied (Figure6b). In view of environmental fluorescencecations, owing spectra to its of strongercompound solid-state 2 in various em issionsolvents performance. were compared Before and the studied experiment, (Figure 6b).the protection and experimental results, ethanol was used as the main solvent. Compound 2 exhibits Influorescence view of environmental spectra of compound protection 2 in and various experi solventsmental were results, compared ethanol and was studied used as (Figure the main 6b). stronger emissions at 422 nm with an excitation peak of 309 nm in ethanol (Figure7a). A sample solvent.In view Compoundof environmental 2 exhibits protection stronger and emissions experimental at 422 results,nm with ethanol an excitation was used peak as of the 309 main nm solvent. Compound 2 exhibits stronger emissions at 422 nm with an excitation peak of 309 nm

Molecules 2020, 25, 382 7 of 14 MoleculesMolecules 20202020,, 2525,, xx FORFOR PEERPEER REVIEWREVIEW 77 ofof 1414 inin ethanolethanol (Figure(Figure 7a).7a). AA samplesample ofof 22 (1(1 mg)mg) waswas groundground intointo powderpowder andand disperseddispersed inin ethanolethanol of 2 (1 mg) was ground into powder and dispersed in ethanol2+ + solution2+ 2+ (4 mL)2+ containing2+ 2+ diff2+erent solutionsolution (4(4 mL)mL) containingcontaining2+ different+different2+ nitratesnitrates2+ (cations:(cations:2+ 2+ MnMn2+2,,+ AgAg+,, Mg2Mg+ 2+,, 3CoCo+ 2+,, ZnZn2+,, CdCd2+,, PbPb2+,, HgHg2+,, nitrates3+ (cations: Mn , Ag , Mg , Co , Zn , Cd , Pb , Hg , Al ) with a concentration of [M] Al3+ ) with a concentration of [M] = 0.4 mM. The suspensions were sonicated for 20 min, and then =Al0.4) with mM. a The concentration suspensions of were[M] = sonicated 0.4 mM. The for 20susp min,ensions and thenwere thesonicated photoluminescence for 20 min, and experiment then the photoluminescence experiment was conducted. As shown in Figure 7b, with the addition of wasthe photoluminescence conducted. As shown experiment in Figure was7b, withconducted. the addition As shown of the in diFigurefferent 7b, metal with cations,the addition most of metal thethe differentdifferent metalmetal cations,cations, mostmost metalmetal cationscations havehave differentdifferent effectseffects onon thethe characteristiccharacteristic peakpeak atat 3+ cations have different effects on the characteristic3+ peak at 422 nm. Notably, with the addition of Al 422422 nm.nm. Notably,Notably, withwith thethe additionaddition ofof AlAl3+ ions,ions, thethe luminescenceluminescence intensityintensity atat 422422 nmnm3+ ions, the luminescence intensity at 422 nm substantially3+ decreased. The addition of 0.4 mM Al ions substantiallysubstantially decreased.decreased. TheThe additionaddition ofof 0.40.4 mMmM AlAl3+ ionsions cancan produceproduce aa 90%90% quenchingquenching effect.effect. can produce a 90% quenching effect. These results indicate that 23+is a powerful sensor for detecting These results indicate that 2 is a powerful sensor for detecting Al3+ ions. AlThese3+ ions. results indicate that 2 is a powerful sensor for detecting Al ions.

FigureFigure 7.7.7. ( a((a)a)) Fluorescence FluorescenceFluorescence spectra spectraspectra of theofof the ligandthe ligandligand in ethanol. inin ethanol.ethanol. (b) Luminescence ((bb)) LuminescenceLuminescence intensities intensitiesintensities of 2 in di ofofff erent 22 inin metaldifferentdifferent cations. metalmetal cations.cations. 3+ 3+ To betterbetter understandunderstand the the response response toward toward Al Alions,3+ ions, the the fluorescence fluorescence spectra spectra upon upon the additions the To better understand the response toward Al ions, the fluorescence spectra upon3+ the were collected. As depicted in Figure8a, with the increasing concentration of Al 3+ ions, the additionsadditions werewere collected.collected. AsAs depicteddepicted inin FigureFigure 8a,8a, withwith thethe increasingincreasing concentrationconcentration ofof AlAl3+ ions,ions, emission intensity at 422 nm gradually decreased and then disappeared for the suspensions. The thethe emissionemission intensityintensity atat 422422 nmnm graduallygradually decdecreasedreased andand thenthen disappeardisappeareded forfor thethe suspensions.suspensions. SternThe SternVolmer−Volmer equation equation can becan used be used to rationalize to rationalize the quenchingthe quenching effect: effect:I0/I =I0/1I =+ 1K +sv K[M],sv[M], where The Stern− −Volmer equation can be used to rationalize the quenching3+ effect: I0/I = 1 + Ksv[M], 3+ Iwhere0 and I0 areand the I are fluorescence the fluorescence intensities intensities before before and and after after adding adding Al Alions,3+ ions, respectively; respectively; [M] [M] is the where I0 and I are the fluorescence 3intensities+ before and after adding Al ions, respectively; [M] 3+ molaris the molar concentration(mol concentration(mol/L)/L) of Al of Alions;3+ ions; and andKsv Kissv is the the Stern Stern−VolmerVolmer constant. constant. As expected, expected, the is the molar concentration(mol/L)3+ of Al ions; and Ksv is the Stern−−Volmer constant. As expected,2 3+ 2 Sternthe SternVolmer−Volmer curve curve for the for Al the ionAl3+ is ion linearly is linearly proportional proportional in the in range the ofrange 0–133 of uM0–133 (R uM= 0.921), (R2 = and the Stern− −Volmer curve for the Al ion is3 linearly1 proportional in the range of 0–133 uM (R = 3 −1 the0.921), value and of theKsv valueis calculated of Ksv is ascalculated 9.61 10 asM 9.61− (Figure × 103 M8−b).1 (Figure 8b). 0.921), and the value of Ksv is calculated× as 9.61 × 10 M (Figure 8b).

3+3+ FigureFigure 8.8. ((aa)) LuminescenceLuminescence intensitiesintensitiesintensities ofofof 2 22 in inin di differentdifferentfferentconcentrations concentrationsconcentrations of ofof Al AlAl3+ ions.ions. ( (bb)) Linear Linear relationrelation 33++ betweenbetween AlAl3+ ionsions and and II//II00 − 1.1. between Al ions and I/I0 − −1. To further study the practicality of compound 2 in the waste solution, different metal cations were ToTo furtherfurther studystudy thethe practicalitypracticality ofof compoundcompound 22 inin thethe wastewaste solution,solution, differentdifferent metalmetal cationscations introducedwere introduced into the into suspension the suspension in the in same the same conditions. conditions. As shown As shown in Figure in Figure9, most 9, metalmost metal ions hadions were introduced into the suspension3+ in the same conditions. As shown in Figure 9, most metal ions little effect on the detection of Al ions. These3+ experimental results further indicate that compound 2 had little effect on the detection of Al3+ ions. These experimental results further indicate that had little effect on the detection of Al ions.3+ These experimental results further indicate that is a highly selective luminescent sensor for Al ion detection3+ in ethanol. compoundcompound 22 isis aa highlyhighly selectiveselective luminescentluminescent sensorsensor forfor AlAl3+ ionion detectiondetection inin ethanol.ethanol.

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Figure 9. Luminescence intensities of 2 responding to interfering ions in the course of detecting Al3+ Figure 9. Luminescence intensities of 2 responding to interfering ions in the course of detecting ions.Figure 9. Luminescence intensities of 2 responding to interfering ions in the course of detecting Al3+ 3+ Alions. ions. The PXRDs were recorded after the sensing of metal ions. As confirmed by PXRD patterns The PXRDs were recorded after the sensing of metal ions. As confirmed by PXRD patterns (FigureThe S6), PXRDs the structure were recorded of 2 remains after unchangedthe sensing throughoutof metal ions. the sensingAs confirmed process, by thus PXRD eliminating patterns (Figure S6), the structure of 2 remains unchanged throughout the sensing process, thus eliminating the the(Figure collapse S6), ofthe the structure crystal structure. of 2 remains The unchanged Uv-vis absorption throughout spectrum the sensing was gained process, at room thus temperature eliminating collapse of the crystal structure. The Uv-vis absorption spectrum was gained at room temperature (Figurethe collapse S7). ofThere the crystal exists structure.a spectrum The over Uv-vislap betweenabsorption the spectrum Uv-vis absorptionwas gained ofat roomAl3+ ions temperature and the (Figure S7). There exists a spectrum overlap between the Uv-vis absorption of Al3+ ions and the excitation(Figure S7). band There of compound exists a spectrum 2, which overconfirmslap between that there the is Uv-viscompetitive absorption energy ofabsorption Al3+ ions betweenand the excitation band of compound 2, which confirms that there is competitive energy absorption between Alexcitation3+ ions and band compound of compound 2. As 2 a, whichresult, confirmsthe phenomenon that there can is co bempetitive explained energy either absorption by the competitive between Al3+ ions and compound 2. As a result, the phenomenon can be explained either by the competitive energyAl3+ ions absorption and compound or the combined2. As a result, action the of phenomenon energy-transfer can and be explained competitive either energy by theabsorption. competitive energy absorption or the combined action of energy-transfer and competitive energy absorption. energy absorption or the combined action of energy-transfer and competitive energy absorption. 2.8. Selectivity toward S2− Ions 2.7. Selectivity toward S2 Ions 2.8. Selectivity toward S2−− Ions Given the sensing ability of anions by compound 2, its ability to sense anions was also examined Given the sensing ability of anions by compound 2, its ability to sense anions was also examined − − − − − 2− − 2− 2− by theGiven addition the sensingof different ability sodium of anions salts by (anions: compound I , Cl 2, ,NO its ability3 , F , Br to, senseSO4 , anionsCH23COO was, COalso3 examined, and S2 ) by the addition of different sodium salts (anions:− I−, Cl− −, NO− 3−,F−− , Br−2,− SO4 −, CH−3COO2− , CO3 2− , inby ethanol the2 addition solutions, of different respectively, sodium into salts the (anions: ethanol I ,suspensions Cl , NO3 , F of, Br compound, SO4 , CH 23.COO As shown, CO3 in, and Figure S ) and S −) in ethanol solutions, respectively, into the ethanol suspensions of compound 2. As shown 10,in ethanoldifferent solutions, anions respectively,have different into degrees the ethanol of in fluencesuspensions on the of compoundintensity of 2 . luminescence,As shown in Figure most in Figure 10, different anions have different degrees of influence on the intensity of luminescence, 2− 2− notably,10, different S ions.anions2 When have S different ions2 are degrees present of in in solutions,fluence on th thee luminescence intensity of intensityluminescence, exhibits most a most notably, S − ions. When S − ions are present in solutions, the luminescence intensity exhibits 2− 2− 2− significantnotably, S decline. ions. When The concentrationS ions are presentof 1.75 mMin solutions, of S 2 ions th ecan luminescence quench the intensity90% luminescence exhibits a a significant decline. The concentration of 1.75 mM of S − ions can quench the 90% luminescence intensity.significant It decline.is clear thatThe 2 concentration is an excellent of luminescent 1.75 mM ofsensor S2− ions for thecan sensitive quench andthe 90%selective luminescence detection intensity. It is clear that 2 is an excellent luminescent sensor for the sensitive and selective detection of ofintensity. S2− ions. It is clear that 2 is an excellent luminescent sensor for the sensitive and selective detection S2 ions. of− S2− ions.

FigureFigure 10. 10. LuminescenceLuminescence intensities intensities of of 22 inin different diﬀerent anions. Figure 10. Luminescence intensities of 2 in different anions.

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The sensing sensitivity of 2 toward S2−2 ions was further examined. As depicted in Figure 11a, the The sensing sensitivity sensitivity of of 22 towardtoward S S2− ions− ions was was further further examined. examined. As Asdepicted depicted in Figure in Figure 11a, 11 thea, titration curves show that the emission intensity of 2 decreased with the addition of increasing thetitration titration curves curves show show that that the the emission emission intensity intensity of of2 2decreaseddecreased with with the the addition addition of increasing concentrations of S22− ions. The luminescence intensity is highly sensitive to S22− ions. The luminescence concentrations ofof SS2−− ions.ions. The The luminescence luminescence intens intensityity isis highlyhighly sensitivesensitive toto SS2−− ions.ions. The The luminescence luminescence intensity is observed to be linearly proportional to the concentration of S2− ions2 in the range of 0−0.333 intensity is is observed observed to to be be linearly linearly proportional proportional to the to theconcentration concentration of S2− of ions S − inions the range in the of range 0−0.333 of mM. (R2 = 0.969)2 (Figure 11b), indicating the coexistence of dynamic and static quenching processes. 0mM.0.333 (R2 mM.= 0.969) (R (Figure= 0.969) 11b), (Figure indicating 11b), indicating the coexiste thence coexistence of dynamic of and dynamic static quenching and static quenchingprocesses. The− value Ksv is estimated to be 2.14 × 103 M−1. 3 1 processes.The value K Thesv is value estimatedKsv is to estimated be 2.14 × to10 be3 M 2.14−1. 10 M− . ×

Figure 11. (a) Luminescence intensities of 2 in different concentrations of S22− ions. (b) Linear relation Figure 11. (a) Luminescence intensities ofof 22 inin didifferentﬀerent concentrationsconcentrations ofofS S2−− ions. ( b) Linear relation 22− 0 between SS2− ionsions and and II//II − 1.1. between S − ions and I/I00 − −1. To furtherfurther checkcheck thethe selectiveselective quenchingquenching behaviorbehavior ofof SS22− towardtoward 2,, its its anti-interference anti-interference capability To further check the selective quenching behavior of S2−− toward 2, its anti-interference capability waswas studied.studied. In the same conditions, some other anionsanions werewere disperseddispersed inin suspensions.suspensions. As shown in was studied. In the same conditions, some other anions were dispersed in suspensions. As shown in Figure 1212,, mostmost anionsanions hadhad somesome contributioncontribution toto luminescenceluminescence quenching,quenching, butbut thethe quenchingquenching eeffectﬀect Figure 12, most anions had some contribution to luminescence quenching, but the quenching effect grew withwith thethe additionaddition ofof SS22− ions.ions. This This further further proved proved that compound 2 has remarkable selectivity grew with the addition of S2− ions. This further proved that compound 2 has remarkable selectivity towardtoward SS22− ionsions in in ethanol. ethanol. toward S2− ions in ethanol.

Figure 12. LuminescenceLuminescence intensities intensities of of 2 2respondingresponding to to interfering interfering ions ions in inthe the course course of detecting of detecting S2− Figure 12. Luminescence intensities of 2 responding to interfering ions in the course of detecting S2− Sions.2 ions. ions.− 2 The PXRDPXRD patternpattern ofof compoundcompound 22 afterafter sensingsensing SS2−− was recordedrecorded (Figure(Figure S8).S8). TheThe experimentexperiment The PXRD pattern of compound 2 after sensing S2− was recorded (Figure S8). The experiment resultsresults revealedrevealed that that the the structure structure of compoundof compound2 remains 2 remains unchanged unchanged throughout throughout the sensing the process,sensing results revealed that the structure of compound 2 remains unchanged throughout the sensing demonstratingprocess, demonstrating that the phenomenon that the phenomenon is unrelated is unre to thelated collapse to the of collapse the frame. of Thethe frame. Uv-vis The absorption Uv-vis process, demonstrating2 that the phenomenon is unrelated to the collapse of the frame. The Uv-vis spectrumabsorption of spectrum S − ions of and S2− the ions excitation and the excitation band of compound band of compound2 have an 2 observablehave an observable spectral overlap,spectral absorption spectrum of S2− ions and the excitation band of compound 2 have an observable spectral thusoverlap, competitive thus competitive absorption absorption cannot be cannot ruled be out ru (Figureled out S7).(Figure As aS7). result, As a theresult, phenomenon the phenomenon can be overlap, thus competitive absorption cannot be ruled out (Figure S7). As a result, the phenomenon explainedcan be explained both by both competitive by competitive energy energy absorption absorp ortion the or combined the combined action action of energy-transfer of energy-transfer and can be explained both by competitive energy absorption or the combined action of energy-transfer competitiveand competitive absorption. absorption. and competitive absorption.

Molecules 2020, 25, 382 10 of 14

3. Experimental

3.1. Materials and General Methods All reagents and solvents were purchased commercially and used without further purification. Infrared (IR) spectra were obtained through an FTS-400 FT-IR spectrometer together with a KBr pellet 1 from 4000 to 400 cm− . Powder X-ray diffraction analyses were determined on a Bruker D8 Advance with Cu Kα radiation (λ = 1.54 Å). Elemental analyses of C, H, and N were performed in an Elementar Vario EL III analyzer. Thermogravimetric analyses (TGA) were carried out on a Metler-Toledo 1 simultaneous SDT thermal analyzer (Bangkok, Thailand) at a heating rate of 10 ◦C min− under a N2 1 atmosphere (N2 flow rate = 0.06 L min− ). The luminescence spectra were measured on a Lengguang (Shanghai, China) F98 fluorescence spectrophotometer at room temperature.

3.2. Synthesis of [Zn(MIPA)]n (1) MIPA (4.90 mg, 0.0250 mmol) and Zn(NO ) 3H O (29.7 mg, 0.100 mmol) were dissolved in a 3 2· 2 mixed solvent comprising DMF/H2O/CH3OH (l/6/8, v/v/v, 2 mL) by sonication, and then sealed in a 5 mL glass vial and kept at 100 ◦C for 2 days. After being cooled to room temperature over 24 h, colorless slice crystals of 1 were obtained (yield 82% based on MIPA). Elemental analysis calculated 1 (%) for C9H6O5Zn: C 41.65, H 2.31. Found: C 42.09, H 2.33. IR (KBr, cm− ): 3436 (vs), 1629 (s), 1537 (m), 1358 (s), 1338 (m), 1001 (w), 791 (w), 636 (w).

3.3. Synthesis of {[Zn(MIPA)(4,4 -bpy) (H O)] 1.5H O}n (2) 0 0.5 2 · 2 MIPA (4.90 mg, 0.0250 mmol), 4,4 -bpy (1.95 mg, 0.0125 mmol), and Zn(NO ) 3H O (29.7 mg, 0 3 2· 2 0.100 mmol) were dissolved in a mixed solvent comprising DMF/H2O (1/2, v/v, 2 mL) by sonication, and then sealed in a 5 mL glass vial and kept at 100 ◦C for 2 days. After being cooled to room temperature over 24 h, colorless block crystals of 2 were obtained (yield 67% based on MIPA). Elemental analysis calculated (%) for C14H15ZnNO7.5: C 43.9, H 3.92, N 3.66. Found: C 43.25, H 3.71, N 3.78. IR 1 (KBr, cm− ): 3431 (vs), 1619 (s), 1543 (m), 1381 (s), 1267 (m), 1015 (w), 779 (w), 644 (w).

3.4. Synthesis of {[Zn(MIPA)(bpe)] H O}n (3) · 2 Compound 3 was obtained by the same procedure used for the preparation of 2, except that 4,40-bpy was replaced by bpe (1.95 mg, 0.0125 mmol). Colorless block crystals of 3 were obtained (yield 74% based on MIPA). Elemental analysis calculated (%) for C21H17ZnN2O6: C 54.9, H 3.70, N 6.10. 1 Found: C 55.05, H 3.66, N 6.36. IR (KBr, cm− ): 3502 (s), 1605 (vs), 1500 (w), 1383 (m), 1269 (w), 1021 (s), 779 (s), 680 (w), 551 (m).

3.5. Crystallographic Data Collection and Refinement A Bruker Smart APEX II CCD area-detector was utilized to obtain the crystal data of compounds 1–3 at 293 K using ω rotation scans with widths of 0.3◦ and graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å). Empirical absorption corrections were applied using the SADABS program. All structures were solved by the direct method and refined by the full-matrix least-squares method on F2 with SHELXL-14 software [40–43]. Non-hydrogen atoms were defined by the Fourier synthesis method. Positional and thermal parameters were refined by the full-matrix least-squares method (on F2) to convergence. Hydrogen atoms of the ligands were initially localized from different Fourier maps and subsequently placed at geometrically ideal positions using riding models with isotropic displacement parameters derived from their carrier atoms. The hydrogen atoms of water were added by the different Fourier maps and refined with suitable constraints. For compound 3, the positioning of the methoxy groups is disordered. The pertinent crystallographic details for compounds 1–3 are provided in Table1, while the selected bond lengths and angles are given in Tables S1–S3 in the SI. Crystallographic data are available at the Cambridge Crystallographic Data Center (see in AppendixA). Molecules 2020, 25, 382 11 of 14

Table 1. Crystallographic data and structural parameters of compounds 1–3.

Compound 1 2 3

Empirical formula C9H6O5Zn C14H15ZnNO7.5 C21H17ZnN2O6 Formula weight 259.51 382.64 458.75 Crystal system Orthorhombic Monoclinic Orthorhombic Space group Pbca C2/c Pnna a/Å 6.9761 (17) 18.346 (3) 7.996 (2) b/Å 14.443 (4) 12.2149 (16) 17.355 (5) c/Å 17.319 (4) 16.850 (3) 15.110 (4) α/◦ 90.00 90.00 90.00 β/◦ 90.00 121.911(2) 90.00 γ/◦ 90.00 90.00 90.00 Volume/Å3 1745.0(8) 3205.26 2096.8(10) Z 8 8 8 3 Dc/g cm− 1.976 1.586 1.453 1 µ/mm− 2.810 1.571 1.211 F(000) 1040.0 1568.0 940.0 Independent reﬂections 2090 3772 2543 Rint 0.1138 0.0405 0.0494 Goodness-of-ﬁt on F2 0.954 1.024 1.036 R1, wR2(I > = 2σ(I)) 0.0508, 0.1046 0.0450, 0.1255 0.0553, 0.1621 R1, wR2 (all data) 0.1393, 0.1339 0.0776, 0.1421 0.1039, 0.1863 ∆ρ /∆ρ (e Å 3) 1.43/ 0.74 0.5/ 0.65 0.42/ 0.5 max min − − − − 1 2 2 2 2 2 2 2 1/2 R = Σ ||Fo| |Fc||/Σ |Fo|; [b] wR (F ) = [Σ w(Fo Fc ) /Σ w(Fo ) ] . 1 − −

4. Conclusions

In conclusion, three coordination polymers, [Zn(MIPA)]n (1), {[Zn(MIPA)(4,40- bpy)(H O) ] H O}n (2), and {[Zn(MIPA)(bpe)] H O}n (3) (MIPA = 4-methoxyisophthalic acid, bpe = 2 0.5 · 2 · 2 (E)-1,2-di(pyridine-4-yl)ethene), have been successfully synthesized by adding auxiliary ligands and controlling solvent effects. All the structures of compounds 1–3 were characterized by infrared, PXRD, and TGA studies. The fluorescence properties of compounds 1–3 were explored, and it was found that 2 has strong fluorescence emission. Fluorescence titration of compound 2 exhibits a selective 3+ 2 and highly sensitive quenching efficiency toward Al and S − ions in ethanol, even in the presence of disturbing ions. Either the presence of competitive energy absorption or the combined action of energy-transfer and competitive energy absorption is the main reason for the detection of the Al3+ and 2 3+ S − ions in ethanol. Moreover, this work is meaningful since it proves the potential for detecting Al 2 and S − ions based on coordination polymers.

Supplementary Materials: The following are available online. Figure S1: Powder diffraction (PXRD) of three compounds, Figure S2: Powder X-ray patterns of Al3+@compound 2 and as-synthesized 2, Figure S3: The 2 3+ ultraviolet spectra of compound 2,S − and Al in ethanol solutions, Figure S4: Powder X-ray patterns of 2 S −@compound 2 and as-synthesized 2, Figure S5: Infrared spectra of compounds, Table S1: Selected bond lengths (Å) and angles (o) of compound 1, Table S2: Selected bond lengths (Å) and angles (o) of compound 2, Table S3: Selected bond lengths (Å) and angles (o) of compound 3. Author Contributions: Conceptualization: X.Z. and Y.Z.; methodology, X.Z.; software, S.L.; validation, X.Z., S.Z., and X.Z.; formal analysis, Y.W.; investigation, Q.Z.; resources, L.J.; data curation, Y.W.; writing—original draft preparation, L.D.; writing—review and editing, Y.W. Please refer to the CRediT taxonomy for the term explanations. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by the National Natural Science Foundation of China (Project 21561033). Conflicts of Interest: The authors declare no conflict of interest. Molecules 2020, 25, 382 12 of 14

Appendix A CCDC 1863388 (1), 1863389 (2), and 1863390 (3) contain the supplementary crystallographic data for the present paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Center via the internet at http://www.ccdc.cam.ac.uk/data_request/cif, or by e-mailing [email protected].

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Sample Availability: Samples of the compounds 1–3 are available from the authors.

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