Crystal Structure and Photoluminescent Properties of a Unique Trinuclear Zn(II)Complex Based on 2-Substituted-8-Hydroxyquinoline

LIU Ji,SONG Kai,YUAN Guozan

(School of Chemistry and Chemical Engineering,Anhui University of Technology,Ma'anshan 243002,China)

Crystal Structure and Photoluminescent Properties of a Unique Trinuclear Zn(II)Complex Based on 2-Substituted-8-Hydroxyquinoline

LIU Ji,SONG Kai,YUAN Guozan

(School of Chemistry and Chemical Engineering,Anhui University of Technology,Ma'anshan 243002,China)

Using solvothermal method,a novel trinuclear complex,namely[Zn3ClL5](1),was fabricated by assembly of ZnCl2salt with an 8-hydroxyquinolinate ligand HL.The complex was characterized by elemental analysis,IR spectroscopy,powder and singlecrystal X-ray diffraction.X-ray structural analysis shows that 1 exhibits a trimeric core structure,which is bridged by five 8-hydroxyquinolinate-based ligands.The luminescent properties(fluorescent emission,lifetime,and quantum yield)of 1 were investigated by photoluminescence(PL),the results show that 1 emits yellow luminescence in the solid state.

8-hydroxyquinoline;trinuclear complex;crystal structure;photoluminescent properties

Organic light-emitting diodes(OLEDs)are being an active field of research because of their potential applications,including flat-panel displays and solid-state lightings[1-3].Since the first report on the use of aluminum tris-8-hydroxyquinoline(AlQ3)as an emissive material in vacuum-deposited LEDs,organic chelate metal complexes based on 8-hydroxyqunoline derivatives have attracted a lot of attentions[4-5].8-Hydroxyquinoline can bridge the gaps between the neutral 2,2′-bipyridine(bipy)and the dianionic catecholate because it possesses one pyridine donor of bipy and one phenolate unit of catecholate[6].Notably,recent reports have suggested that the electrontransporting mobility of zinc complexes with 8-hydroxyquinoline goes beyond that of aluminum complexes.Thus,zinc complexes may be potential candidates to enhance the electron-transporting properties for OLEDs[7-9].In this way,our group has been demonstrated that five multinuclear Zn(II)complexes with a 8-hydroxyquinolinate ligand exhibit a varying degree of red shifts and enhanced fluorescence intensities compared with free ligand[10].

In order to obtain the tunable emissions of 8-hydroxyquinoline metal chelates,one of the alternative meth-ods is to introduce some functional groups into the quinoline ring[11].The thiophene derivatives are a very important class of organic materials due to their biological,electronic,magnetic,and optical properties[12].In fact,Umbach et al.revealed that the improvement of electroluminescent performance of OLEDs might be achieved by changing the chain length of(end-capped)oligothiophenes[13].On the other hand,the high polarizability of sulfur atom in thiophene ring may lead to a stabilization of the conjugated chain and excellent charge transport property, which is one of the most crucial assets for applications in organic and molecular electronics[14].

According to our previous reports,multinuclear 8-hydroxyquinolinate-based complexes can be synthesized from 2-substituted 8-hydroxyquinolinate ligands under solvothermal conditions[15].In this context,we report the self-assembly of a novel trinuclear complex by reacting ligand(E)-2-[2-(3-thienyl)ethenyl]-8-quinolinol (HL)with zinc chloride.The complex was characterized by IR spectroscopy,powder and single-crystal X-ray diffraction.In addition,fluorescent properties(emission wavelength,lifetime and quantum yield)of the complex have also been investigated in the solid state.

1 Experimental Section

1.1 Materials and Characterization Methods

All of the chemicals are commercial available,and used without further purification.HL ligand was synthesized according to our previous report[15].Elemental analyses of C,H and N were performed with an EA1110 CHNS-0 CE elemental analyzer.The IR(KBr pellet)spectrum was recorded(400-4000 cm-1region)on a Nicolet Magna 750 FT-IR spectrometer.Powder X-ray diffraction(PXRD)data were collected on a DMAX2500 diffractometer using Cu Kα radiation.The calculated PXRD patterns were produced using the SHELXTL-XPOW program and single crystal reflection data.All fluorescence measurements were carried out on a LS 50B Luminescence Spectrometer(Perkin Elmer,Inc.,USA).The room-temperature(RT)lifetime measurements were determined on a FLS920 time-resolved and steady-state fluorescence spectrometer(Edinburgh Instruments).

1.2 X-ray Crystallography

Single-crystal XRD data for compound 1 was all collected on a Bruker APEX area-detector X-ray diffractometer with MoKaradiation(λ=0.710 73 ?).The empirical absorption correction was applied by using the SADABS program(G.M.Sheldrick,SADABS,program for empirical absorption correction of area detector data;University of G?ttingen,G?ttingen,Germany,1996).The structure was solved using direct method,and refined by full-matrix least-squares on F2(G.M.Sheldrick,SHELXTL97,program for crystal structure refinement,University of G?ttingen,Germany,1997).

1.3 Synthesis of Complex 1

[Zn3ClL5](1).A mixture of ZnCl2(13.5 mg,0.1 mmol),HL(12.5 mg,0.05 mmol),H2O(0.2 mL),DMF (0.5 mL),and MeOH(2.0 mL)in a capped vial was heated at 80℃for one day.Yellow block like crystals of 1, which were suitable for single-crystal X-ray diffraction,were collected after washed with ether and dried in air.Light yellow crystals of 1 were obtained with 82%yield based on HL ligand.Yield:11.7 mg(81%).Elemental analysis data and IR of 1:Anal(%)calcd for[Zn3ClL5],C75H50ClN5O5S5Zn3:C,62.38;H,3.47;N,4.85.Found: C,62.25;H,3.61;N,4.79.FTIR(KBr pellet):3 432.82(w),3 091.03(w),3 045.98(w),1 625.73(m),1 598.87(m), 1 553.87(m),1 500.67(w),1 445.88(s),1 412.13(m),1 376.34(m),1 336.32(s),1 275.56(m),1 157.42(w), 1 102.02(s),960.28(m),865.99(w),830.37(m),802.49(w),774.18(m),745.89(s),695.32(w),617.67(w), 566.42(w),511.33(s),470.47(w).

2 Results and Discussion

2.1 Synthesis Considerations

As outlined in Scheme 1,single crystals of[Zn3ClL5](1)were readily obtained in good yield by heating Zn-Cl2and HL in a mixture of DMF and MeOH.The formulation was supported by elemental analysis,IR and singlecrystal X-ray diffraction.Moreover,when ethanol or 2-butanol was used as a solvent instead of methanol,the same products were obtained.

2.2 Structural Description

Single-crystal X-ray diffraction analysis reveals that complex 1 crystallizes in the monoclinic space group P21/c with Z=4.The asymmetric unit of 1 consists of three crystallographically independent Zn(II)atoms,five ligands L,and one coordinated chlorine atom.As shown in Fig.1,the Zn1 atom is five-coordinated by two nitrogen and three phenol oxygen atoms from three ligands L forming a distorted trigonal bipyramidal geometry.The coordination environment of the Zn2 atom is a distorted octahedron geometry,in which Zn2 is equatorially bonded to NO3 donors of three ligands.The two axial sites on the metal are occupied by a pyridine nitrogen atom and one phenol oxygen atom.The Zn3 atom is four-coordinated by one nitrogen atom,two phenol oxygen atoms and a chlorine atom displaying a distorted tetrahedron.The three Zn(II)cations in 1 are bridged by phenolato oxygen atoms of four ligands with the Zn…Zn distances of 3.272 4(2)and 3.274 0(2)?.The bond lengths and angels around Zn(II)are 2.115 6-2.184 3 ? for Zn-N, 1.943 4-2.558 8 ? for Zn-O,2.185 4 ? for Zn-Cl, 75.70°-167.29°for O-Zn-O,78.86°-154.93° for O-Zn-N,and 103.06°-112.84°for N-Zn-N, respectively.

There are numerous weak C–H···O intramolecular hydrogen bonds in compound 1 between the phenolato oxygen and the C-H group of quinoline ring or ethenyl(C…O=3.105-3.363 ?,C-H…O=123°-160°),which play a vital role in the construction of trimeric Zn(II)units(Fig.2).As shown in Fig.3,the trimeric Zn(II)units are linked into a supramolecular chain along the b axis through C-H…π(between the C-H groups of the thiophene rings and the quinolone rings of the neighboring units,3.61(3)?)intermolecular non-covalent interactions.Furthermore,owing to abundance π…π intermolecular interactions betweenthiophene rings of adjacent ligands(interligand distance of ca.3.684(12)?)(Fig.4),the parallel 1D chain were expanded into a 2D network in the bc plane.

2.3 Fluorescent Properties in the Solid State

To prove that the crystal structure of complex 1 is truly representative of their bulk materials,powder X-ray diffraction(PXRD)experiment was carried out on the as-synthesized sample.As shown in Fig.5,the peak positions of the experimental and simulated PXRD pattern are in good agreement with each other, which confirm the phase purity.The difference in intensity of some diffraction peaks may be owing to the preferred orientation of the crystalline sample.

Over the past several decades,fluorescent complexes are of great interest owing to their potential or real applications in many fields,including photochemistry,chemical sensors,and light-emitting diodes (LEDs)[16].Therefore,it is important to investigate the luminescent properties of the compound in view of potential applications.The luminescent behaviors of compound 1 were studied in the solid state at room temperature.As shown in Fig.6,complex 1 displays intense photoluminescence with the emission maximum at 546 nm,upon excitation at 370 nm.Compared with free ligand HL(Em:468 and 540 nm)[15], the bright and extensive yellow emission of 1 predominantly originate from metal-to-ligand charge transfer transition.However,in comparison to the emission spectrum of the free ligand HL,the red shift of emission band of compound 1 may be mainly arisen from the following two reasons.The coordination of metal ions effectively reduces the loss of energy via vibration motions and increases the conformational rigidity of the ligand[17].Additionally,the coordination of the ligand with Zn(II)ions forms additional five-membered rings,which increases the conjugation of ligand,thus reducing the energy gap between the π and π*molecular orbital of the ligand[18].

To further understand the fluorescent properties of complex 1,its fluorescent lifetime and quantum yield were monitored in the solid state at room temperature(Fig.7).The average lifetime was determined by allowing αiand τito vary,and then convoluting Eq.(1)with the instrument response function.The data was successfully modeled using double exponentials,and the average lifetime was determined by Eq.(2).In both equationsτrepresents lifetime,t represents time,and α is a pre-exponential factor.The average lifetime of compound 1 is 6.97 ns.Absolute quantum yield of complex 1 is 4.73%.Here,the photophysical properties of 1 are comparable with those found in 8-hydroxyquinolinate-based multinuclear coordination compounds.

3 Conclusion

In summary,a novel trinuclear Zn(II)complex was prepared under solvothermal condition,using Zn(II)salt and one thiophene-based quinolinate ligand synthesized from the cheap commercial available 2-methyl-8-hydroxyquinoline.The photophysical measurement of complex 1 show that it emits yellow luminescence at 546 nm(λem,max),upon excitation at 370 nm,in the solid state.The fluorescence lifetime and quantum yield were 6.97 ns and 4.73%,respectively.With a precise knowledge of their single-crystal structures,the present research holds great promise in the development of unique multinuclear Zn(II)photoluminescent materials,and may contribute to the comprehension of structure-property relationships.

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責(zé)任編輯：丁吉海

新型八羥基喹啉鋅配合物的晶體結(jié)構(gòu)及熒光性質(zhì)研究

劉 計(jì)，宋 鍇，袁國(guó)贊

（安徽工業(yè)大學(xué)化學(xué)與化工學(xué)院，安徽馬鞍山243002）

采用溶劑熱法，8-羥基喹啉類(lèi)配體與ZnCl2組裝得到一種新型的三核配合物1，并用元素分析，紅外光譜，粉末和單晶X-射線衍射對(duì)配合物1進(jìn)行表征。從X-射線單晶衍射分析可知配合物1是通過(guò)五個(gè)配體橋聯(lián)而成的三核結(jié)構(gòu)。采用熒光光譜研究了配合物1的固態(tài)熒光性質(zhì)（熒光發(fā)射，熒光壽命和量子產(chǎn)率），結(jié)果表明配合物1在固體狀態(tài)下發(fā)射黃色熒光。

8-羥基喹啉；三核配合物；晶體結(jié)構(gòu)；熒光性質(zhì)

2015-05-27

國(guó)家自然科學(xué)基金項(xiàng)目(21201002)

劉計(jì)(1990-),男，安徽懷遠(yuǎn)人，碩士生，研究方向?yàn)楣δ苡袡C(jī)-無(wú)機(jī)雜化材料。

袁國(guó)贊(1980-)，男，安徽宣城人，博士，教授，主要從事功能有機(jī)-無(wú)機(jī)雜化材料的設(shè)計(jì)合成及性能研究。

1671-7872(2015)-04-0338-05

O614.24 Document code:A

10.3969/i.issn.1671-7872.2015.04.008