基于铕取代多金属氧簇的手性发光液晶材料

袁 鸿 张 静 魏学红 方慧敏 袁世芳 吴立新

(1山西大学应用化学研究所，太原 030006；2吉林大学，超分子结构与材料国家重点实验室，长春 130012；3山西大学化学化工学院，太原 030006)

基于铕取代多金属氧簇的手性发光液晶材料

袁 鸿1,3张 静1,*魏学红3,*方慧敏3袁世芳1吴立新2,*

(1山西大学应用化学研究所，太原 030006；2吉林大学，超分子结构与材料国家重点实验室，长春 130012；3山西大学化学化工学院，太原 030006)

将铕取代的多金属氧簇引入手性液晶体系是构筑多功能手性发光软材料的有力工具。圆二色谱表明手性两亲分子可以通过静电相互作用诱导非手性多金属氧簇显示出超分子手性。差示扫描量热法、偏光显微镜和变温X射线衍射证实这种有机无机杂化的多金属氧簇复合物具有热致液晶性质。复合物的薄膜显示出本征发光，并且我们可以通过温度调控复合物的发光性质。用手性介晶阳离子静电包覆多金属氧簇是构筑多金属氧簇基手性发光液晶材料的有效方法。

多金属氧簇；光学活性；液晶；发光；超分子自组装；表面活性剂

Key Words:Polyoxometalate;Optical activity;Liquid crystal;Luminescence;Supramolecular selfassembly;Surfactant

1 Introduction

Intense research has focused on the construction of functional chiral liquid crystal materials through supramolecular self-assembly1－4.Introduction of luminescent units into chiral liquid crystals has been proved to be an effective method to fabricate multifunctional soft materials5－9.Chiral luminescent liquid crystals (CLLCs)which integrate chiral,luminescence and self-orgnization properties of liquid crystals into single molecules supply advantages that reach beyond the sum of the individual properties.The design and development of CLLCs is particularly appealing owing to their potential applications in circularly polarized luminescence, asymmetric catalysis,chiral sensing,and chiroptical switch10－14. Polyoxometalates(PMs)have been developed into a class of fascinating building blocks in the exploitation of functional materials owing to their versatile properties15－22.We have recently demonstrated that the electrostatic encapsulation of achiral PMs with chiral amphiphiles could lead to optically active mesomorphic hybrid materials23.Chiral liquid crystals provide an asymmetric microenvironment for the organization of PMs,the properties of which could be modified and new functional materials could be fabricated.One of our persistent interests is concentrated on constructing PM-based CLLCs.The europium-substituted PMs exhibit intense red emission and are expected to be excellent luminescent building components24,25.It is promising that incorporation of europium-substituted PMs into chiral amphiphiles could allow to add luminescence to the anisotropy-related properties of chiral liquid crystals and afford novel multifunctional materials.

Herein we describe the first example of the CLLC based on surfactant-encapsulated PM complex,CS-Eu(Fig.1).We believe that the strategy applied herein is favorable for the design of novel nanohybrid-based chiral luminescent liquid crystalline materials.

Fig.1 Structural illustration of CS-Eu

2 Experimental

2.1 Materials

11-Bromoundecanoic acid(98%)and cholesterol(99%)from Aladdin were empolyed as received.4-Dimethylaminopyridine (DMAP,98%),N,N′-dicyclohexylcarbodiimide(DCC,99%)and N,N-dimethyldodecylamine(97%)were supplied from Sigma without further purification.Silica gel(100－200 mesh)was used for the purification of column chromatography.The polyoxometalate K13[Eu(SiW11O39)2]·20H2O was synthesized according to the published procedures26.The chiral surfactant(CS)containing cholesterol group and the resulted complex CS-Eu were prepared by referencing the procedures reported previously23.

2.2 Preparation of CS-Eu

The synthesis of the novel complex,CS-Eu,is accomplished by encapsulating the Eu-substituted PM K13Eu(SiW11O39)2· 20H2O with CS by referencing the literature27,28.A solution of CS dissolved in chloroform was added dropwise into an aqueous solution of PM with stirring at room temperature.The initial molar ratio of CS to PM was controlled at 10:1.After 3 h of stirring at 45 °C, the organic phase was separated and washed with deionized water three times.Then the complex CS-Eu was obtained by evaporating the solvent to dryness.Finally,the product was dried in a vacuum desiccator until the weight remained constant.The chemical formula of CS-Eu is suggested to be[CS]12KEu(SiW11O39)2, identified by infrared(IR)spectroscopy(Fig.S1,Supporting Information),elemental analysis and thermal gravimetric analysis (Fig.S2,Supporting Information).

CS-Eu.IR(KBr,ν/cm－1):3450,3033,2925,2852,1735,1674, 1467,1378,1249,1172,1012,946,894,777,723(Table S1, Supporting Information).Elemental analysis:Anal.Calcd(%)for CS-Eu(C624H1152N12O102KSi2W22Eu):C,50.82;H,7.87;N,1.14. Found(%):C,50.33;H,7.79;N,1.20;corresponding to a chemical formula:[CS]12KEu(SiW11O39)2.Thermogravimetric analysis(TGA)displays a mass residue of 33.4%at 800 °C,which is in agreement with the calculated value of 35.8%from the given formula[CS]12KEu(SiW11O39)2.

2.3Measurements

IR spectra were recorded on a Germany Bruker Optics VERTEX 80v Fourier transform infrared spectrometer,equipped with a DTGS detector in pressed KBr pellets.A resolution of 4 cm－1was chosen,and 32 scans were signal-averaged.Elemental analysis(C, H,N)was performed on a Flash EA1112 from ThermoQuest Italia SPA.TGA was conducted using a Q500 thermal analyzer in flowing air with a heating rate of 10 °C·min－1in the temperature range from 25 to 800 °C.The UV/Vis spectra were taken by a Varian CARY 50 Probe spectrometer.Circular dichroism(CD) spectra were carried out on a Bio-Logic MOS-450 spectropolarimeter with a step size of 1 nm and at a speed of 5 s·nm－1.The optical textures of the mesophases were studied with a ZeissAxioskop 40 polarizing microscope equipped with a Linkam THMSE 600 hot stage,a central processor,and a DF1 cooling system.Differential scanning calorimetric(DSC)measurements were performed on a Netzsch DSC 204 using a 5 °C · min－1scanning rate.All the samples were sealed in aluminum capsules in air,and the atmosphere of holder was sustained under dry nitrogen.For variable-temperature X-ray diffraction(XRD)experiments,a Germany Bruker AXS D8 ADVANCE X-ray diffractometer using Cu Kαradiation at a wavelength of 0.154 nm with a mri Physikalische Geräte GmbH TC-Basic temperature chamber was employed.Luminescence measurements were carried out on a F-4600 FL spectrophotometer and a xenon lamp was used as the excitation source.

Fig.2 (a)CD and(b)UV/Vis spectra of CS-Eu,(c)CD and(d)UV/Vis spectra of CS casting film on quartz substrate

3 Results and discussion

3.1 Optical activities of CS-Eu

To obtain insights into the chiroptical activity of CS-Eu,circular dichroism(CD)characterization was performed.Apparently,the peripheral chiral surfactants are effective promoters to induce CSEu to display chiral signals.It is believed that the cholesterol moiety with(S)configuration tends to give a negative sign for the first Cotton effect29,30.CS-Eu is actually the case,which exhibited a negative Cotton effect at 212 nm and a positive Cotton effect at 338 nm(Fig.2(a)).More importantly,by comparing the CD spectra of CS-Eu and CS(Fig.2(b)),we observed a positive CD signal at 262 nm assigned to the induced circular dichroism(ICD) of PM in CS-Eu definitively,demonstrating that encapsulation by chiral surfactants is a feasible strategy for developing PM-based materials with optical activities.The characteristic band at 262 nm is derived from O → W ligand to metal charge transfer(LMCT) band31as observed in the UV/Vis spectrum(Fig.2(c)),the ICD should be attributed to be supramolecular chirality32－35which originates from the chirality transfer from the chiral organic moieties to the LMCT band through electrostatic interactions.The optical activity of CS-Eu is very stable,indicated by the observation of no apparent changes in CD spectrum over several months,which is of practical significance in the potential applications of the chiral structure.

3.2 Liquid crystal properties of CS-Eu

Interestingly,the non-covalent attached amphiphiles at the periphery of PM act as mesogenic precursors trigger CS-Eu to exhibit liquid crystalline properties,demonstrably characterized by differential scanning calorimetry(DSC),polarized optical microscopy(POM),and temperature-dependent X-ray diffraction (XRD).DSC traces of CS-Eu(Fig.S3,Supporting Information) displayed glass transitions at 10 and 9 °C on the first cooling and second heating processes,respectively.Due to the low enthalpy of CS-Eu,no clear peak corresponding to the phase transition from mesophase to isotropic state was detected at high temperature. When cooling from isotropic liquids,a birefringent texture with strong fluidity was investigated at 156 °C(Fig.S4(a),Supporting Information),suggesting the transition from isotropic state to mesophase.Mesomorphic properties were recognized by the formation of grain textures(Fig.3)upon cooling to 30 °C.On heating process,the birefringence was observed up to 163 °C(Fig. S5(b))and disappeared at 168 °C(Fig.S5(c)),indicating theclearing point of CS-Eu.

We employed temperature-dependent XRD to elucidate the mesophase and stacking structure of CS-Eu.As displayed in Fig.4 (a),a less-ordered layered structure with a spacing of 4.21 nm at 30 °C was calculated from the four equidistant diffractions at 4.21, 2.21,1.50,and 1.10 nm,which are assigned to(001),(002),(003), and(004)reflections,respectively36－38.In addition,a diffused halo centered at 20°corresponding to a spacing of 0.45 nm was observed,typical of disordered conformation of the aliphatic moieties39.Combining the grain texture,the mesophase of CS-Eu is assigned as chiral smectic A phase.It is noticed that CS-Eu reveals a dependence of layer spacing on temperature(Table S2,Supporting Information);that is,the layer spacing decreases slowly with temperature increasing,which is closely related to the increasing of gauche conformation of the alkyl chains during heating.The upword shifts of the asymmetrical and symmetrical CH2stretching vibrations in temperature-dependent IR spectra (Fig.4(b))clearly illustrate the increasing of gauche conformation of the alkyl chains.The increasing of gauche conformation will result in the shortened molecular length of alkyl chains in CS-Eu, which triggers the decreasing of the layer spacing.It is worth noting that CS-Eu is a room temperature ionic liquid crystal over a wide temperature range,which is meaningful for the potential applications.

Fig.3 Grain texture of CS-Eu at 30 °C on the cooling run

Fig.4 (a)Temperature-dependent XRD patterns of CS-Eu on heating runs,and(b)temperature dependence of the positions of IR bands of antisymmetrical and symmetrical CH2stretching mode of CS-Eu

Based on the XRD results,the possible aggregation structure of CS-Eu in mesophase could be speculated.The layer distance in the mesophase is much smaller than the simulated packing(ca 7.56 nm)of CS-Eu in sandwich structure,acquired by combining the short-axis diameter of PM(1.00 nm)and the length of two organic surfactants(3.28 nm,calculated by MM2 force field method). Taking this into account,it is reasonable to assume the presence of a conformational distortion or a partially interdigitation of the aliphatic moieties,as schematically illustrated in Fig.5.

3.3 Luminescent properties of CS-Eu

Apparently,from the red luminescence of CS-Eu under ultraviolet excitation(Fig.6(a)),we see that the luminescence of PM is well retained in the complex.The spectrum of CS-Eu(Fig.6(b)) displays the characteristic5D0－7Fj(j=0,1,2,3,4)transitions of Eu3+when excited at 254 nm,for instance,5D0－7F0,581 nm;5D0－7F1,594 nm;5D0－7F2,616 and 621 nm;5D0－7F3,653 nm;5D0－7F4,695 and 703 nm.The symmetrically forbidden transition5D0－7F0at 581 nm is clearly detected as a single peak, indicating the low symmetry and the existence of one local site symmetry for the chemical environment of the Eu3+ion in CS-Eu21.

To evaluate the thermal stability of luminescence of CS-Eu in the mesophase,the temperature dependence of luminescence intensity was studied.As shown in Fig.6(c),with temperature increasing,the characteristic peaks of Eu3+show a slow decrease in luminescent intensity owing to the quenching from nonradiative transition40.The results imply that the photophysical properties of CS-Eu can be adjusted by accurately controlling the temperature, which is favorable for the realization of Eu-substituted PM-based luminescent devices.

Fig.5 Schematic drawing of packing model of CS-Eu in the mesophase at 30 °C

Fig.6 (a)Photographs of CS-Eu under daylight(top)and under the irradiation of 254 nm light(bottom),(b)emission spectrum of CS-Eu casting film on quartz substrate(λex=254 nm),and (c)variable-temperature emission spectra of CS-Eu

4 Conclusions

We report the first demonstration of a multifunctional chiral lumescinent liquid crystal material,which was achieved by encapsulation of Eu-substituted PM with chiral surfactantsviaelectrostatic interactions.Importantly,optical activities in achiral PM could be induced successfully by chiral transfer from the peripheral chiral organic amphiphiles through intermolecular interactions.The synergistic properties combining the chiral liquid crystals and the inherent luminescence of PM endow the hybrid multifunctional material with unfathomable potential in circularly polarized luminescence,asymmetric catalysis,and chiroptical switches.

Supporting Information:available free of chargeviathe internet at http://www.whxb.pku.edu.cn.

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Chiral Luminescent Liquid Crystal Material Based on Europium-Substituted Polyoxometalate

YUAN Hong1,3ZHANG Jing1,*WEI Xue-Hong3,*FANG Hui-Min3
YUAN Shi-Fang1WU Li-Xin2,*
(1Institute of Applied Chemistry,Shanxi University,Taiyuan 030006,P.R.China;2State Key Laboratory of Supramolecular Structure and Materials,Jilin University,Changchun 130012,P.R.China;3College of Chemistry and Chemical Engineering,Shanxi University,Taiyuan 030006,P.R.China)

The incorporation of europium-substituted polyoxometalate(PM)into chiral amphiphiles is attractive for the fabrication of multifunctional chiral luminescent liquid crystalline materials.Chiral amphiphiles acted as good promoters to trigger the achiral PM to show induced supramolecular chirality through electrostatic interactions,as illustrated by circular dichroism(CD)spectra.Differential scanning calorimetry(DSC),polarized optical microscopy(POM),and temperature-dependent X-ray diffraction(XRD)analysis confirmed that the organic/inorganic hybrid polyoxometalate complex exhibited thermotropic mesomorphic behaviors.In a cast film,the complex displayed intrinsic luminescence that could be adjusted by accurately controlling the temperature.The electrostatic encapsulation of PM with chiral mesomorphic promoters provides an effective method for constructing PM-based chiral luminescent liquid crystalline materials.

O644

10.3866/PKU.WHXB201611032

Received:September 23,2016;Revised:November 2,2016;Published online:November 3,2016.

*Corresponding authors.ZHANG Jing,Email:jingzhang@sxu.edu.cn;Tel:+86-351-7018390.WEI Xue-Hong,Email:xhwei@sxu.edu.cn;

Tel:+86-351-7018390.WU Li-Xin,Email:wulx@jlu.edu.cn;Tel:+86-431-85168481.

The project was supported by the National Key Basic Research Program of China(973)(2013CB834503),National Natural Science Foundation of China(21502107,21574057,21101101),Natural Science Foundation of Shanxi Province,China(2014021019-5),Scientific Research Start-up Funds of Shanxi University,China(020451801001),Scientific and Technological Innovation Programs of Higher Education Institutions in Shanxi Province, China(2016118),and National Training Programs of Innovation and Entrepreneurship for Undergraduates,China(201610108010)and Scientific Instrument Center of Shanxi University,China.

国家重点基础研究发展规划项目(973)(2013CB834503),国家自然科学基金(21502107,21574057,21101101),山西省自然科学基金(2014021019-5),山西大学引进人才建设项目(020451801001),山西省高校科技创新项目(2016118),国家级大学生创新创业训练项目(201610108010)和山西大学大型科学仪器中心资助© Editorial office of Acta Physico-Chimica Sinica