Journal of Materials Chemistry C c6tc05574e

A Q1 Q2supra-molecular switchable dielectric materialwith non-linear optical properties

Tariq Khan, Muhammad Adnan Asghar, Zhihua Sun,*Aurang Zeb, Chengmin Ji and Junhua Luo*

A molecular PTM, 1-[C6H13NH][18-crown-6][ClO4]monohydrate (1), has been Q4synthesized, showingreversible switchable dielectric anomalies, which wasconfirmed by differential scanning calorimetry and specificheat measurements. 1 displays non-linear opticalproperties with a second harmonic generation responseof B0.8 times compared to that of potassium dihydrogenphosphate. Variable-temperature single crystal X-raydiffraction discloses that the origin of the phase transitionis ascribed to the order–disorder transformation of a per-chlorate anion, the methyl group of a cation and the tor-sional angular change in a crown molecule. This findingsuggests that 1 could be conceived as a potential solid-state switchable dielectric and non-linear optical material.

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A Q1 Q2supra-molecular switchable dielectric materialwith non-linear optical properties†

Tariq Khan,ab Muhammad Adnan Asghar,ab Zhihua Sun,*a Aurang Zeb,ab

Chengmin Jia and Junhua Luo*a

A novel molecular phase transition material (PTM), 1-methylpiperidinium perchlorate [18-crown-6]

monohydrate (1), has been synthesized, which exhibits reversible switchable dielectric anomalies near

room temperature. The phase transition in 1 is from noncentrosymmetric to noncentrosymmetric,

displaying non-linear optical (NLO) properties with a second harmonic generation response of B0.8 times

compared to that of potassium dihydrogen phosphate. Thermal analyses of 1, including differential

scanning calorimetry and specific heat measurements, confirm the first-order solid-state phase transition

at 260 K. The dielectric constants display temperature-dependent anomalies with the temperature

approaching the transition point (Tc), where the evident step-like anomalies demonstrate two distinct

states below and above the Tc value, respectively. Variable-temperature single crystal X-ray diffraction

discloses that the origin of its phase transition is ascribed to the order–disorder transformation of a

perchlorate anion, the methyl group of a cation and the torsional angular change in a crown molecule;

that is, all three components contribute to the emergence of this phase transition. This result suggests that

1 could be conceived as a potential switchable dielectric and NLO material. These findings make us believe

that 1 might be a potential solid-state switchable dielectric PTM and this strategy provides an effective

approach to design novel NLO switched materials.

Introduction

Phase transition materials (PTMs) with the coexistence ofphysical properties have gained extensive recognition owingto their large panel of potential applications in various fields oftechnology. On account of their wide-ranging properties suchas second-nonlinear optical (NLO), ferroelectric, piezoelectricand phase switching, the PTMs are used in data communica-tion, memory devices, switches and mechanical energytransfer, optoelectronics and photonics etc.1–3 For example, instructural PTMs, the dielectric response is reversibly switchedbetween two different states in response to an external stimulus,which can be incorporated into switches, sensors and memorydevices.3 Similarly, NLO materials are useful in emerging optical

signal processing and conspicuously promising for terahertzgeneration, and detection4 as well as in the development ofphotonic devices.5 Among them, the molecular materials are ofsubstantial interest due to their relatively high nonlinearities incontrast to their inorganic counterparts.6,7 Because the inorganicNLO materials are usually difficult to process, strong attention isneeded to build up new molecular NLO materials.8

As far as the switching of certain properties is concerned,molecular PTMs represent the state-of-the-art nowadays, andare comparatively designated as efficient and easily processablesuccessors to inorganic materials.9,10 From the mechanismperspective, the order–disorder transformation of flexible mole-cular components is conceived as an effective strategy fordesigning smart PTMs of significant interest. Previously, anumber of such potential materials, which show not onlydielectric switching but also intricate functional responses,have been investigated by researchers with immenseeffort.11,12 For instance, Xiong et al. have reported that DIBAB,a molecular material, exhibits ferroelectric properties with highspontaneous polarization and large dielectric response.13 Thetemperature-dependent order–disorder transformation of anorganic cation induces the reversible phase transition in thismolecular material.13 Likewise, the disordering of an anion is alsoknown to induce phase changes associated with certain properties;for instance, dielectric constants of [(DIPA)([18-crown-6)][ClO4] Q5can

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Cite this: DOI: 10.1039/c6tc05574e

a State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the

Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian, 350002,

P. R. China. E-mail: sunzhihua@fjirsm.ac.cn, jhluo@fjirsm.ac.cn;

Fax: +86 0591-63173126; Tel: +86 0591-63173126b Graduate School of the University of Chinese Academy of Sciences, Beijing,

100039, P. R. China

† Electronic supplementary information (ESI) available: CIF files, crystal data andstructural refinement, PXRD, symmetry diagram, SHG diagram, packing views ofcrystal structures, thermal ellipsoid maps. CCDC 1501909 and 1501910.Q3 For ESIand crystallographic data in CIF or other electronic format see DOI: 10.1039/c6tc05574e

Received 23rd December 2016,Accepted 13th February 2017

DOI: 10.1039/c6tc05574e

rsc.li/materials-c

This journal is �c The Royal Society of Chemistry 2017 J. Mater. Chem. C, 2017, 00, 1�6 | 1

Journal ofMaterials Chemistry C

PAPER

be switched between two different states along with NLO proper-ties, which are triggered by the order–disorder transition of aperchlorate anion and a crown-ether molecule.14

Primarily, this performance can be achieved in molecularsystems containing donor and acceptor groups, which conse-quentially provide efficient intramolecular charge transfer andform a highly polar system. More importantly, the structuremust be non-centrosymmetric (NCS) along with an appropriatepacking arrangement. To explore such PTMs, we have designedassemblies by utilizing their molecular entities,15 the flexible1-methylpiperidine was incorporated along with a flexible[18]-crown-6 molecule and various acids. It would be intriguingand effective to manufacture PTMs induced by the synergeticeffect of disordering of an anion and a cation coupled with theconformational transformation of a crown-ether. Through thisstrategy the desired and potential candidate can be achievedbecause of the flexible amine and its polar nature. Therefore,along with dielectric switching, properties related to theacentric nature of the title compound such as second harmonicgeneration (SHG) can be generated.

Herein, we report a molecular PTM, [(C5H10)(CH3)NH]-[18-crown-6][ClO4] monohydrate (1), which exhibits switchabledielectric anomalies near room-temperature. Its phase transi-tion at B260 K (Tc, upon heating) is confirmed by thermalanalyses, i.e., differential scanning calorimetry (DSC) and heatcapacity (Cp–T) measurements. Single crystal structural studiesreveal that the stern order–disorder transformation of an anionand a cation together with the conformational change inneutral crown-ether accounts for the phase transition in 1.Upon lowering the temperature below Tc, the dynamics of bothcounter ions are frozen and the conformation of crown-etheris converted to a relaxed staggered conformer. InterestinglyQ6 ,1 is SHG-active and its SHG intensities remain almostunchanged (B0.8 � KDP, KH2PO4) below and above Tc.Variable-temperature single crystal analysis discloses that 1 crys-tallizes in two different polar space groups at a low-temperaturephase (LTP, below Tc) and a high-temperature phase (HTP, aboveTc). Therefore, the phase transition in 1 is from NCS to NCS,which is quite rare. The dielectric constants of 1 exhibittemperature-switched changes between two different states, i.e.,the high- and low-state. As described, the structural analysesilluminate the origin of the phase transition, where both thecounter ions and the neutral crown-ether synergistically contri-bute to the structural phase transition. The cation also forms asupra-molecular moiety with the crown-ethers, which are inter-locked by a water molecule. To the best of our knowledge,although there are a large number of PTMs with CS-to-NCSstructural changes, 1 is an unusual example showing theNCS-to-NCS change with SHG signals at both phases. Further-more, the synergistic contribution of all three components in thetemperature-triggered transformation is rare. Hence, 1 should bethe first concrete example, in which 1-methylpiperidine combineswith other components to trigger the NCS-to-NCS phase transi-tion, along with the NLO properties. These observations suggestthat our findings may offer a new potential pathway to explorenew functional PTMs through thisQ7 strategy (Scheme 1).

ExperimentalSynthesis of 1

All reagents and solvents were obtained commercially and usedwithout further purification. 1-Methylpiperidinium perchlorate18-crown-6 monohydrate (1) was synthesized by evaporating theaqueous solution containing 1-methylpiperidine base, perchloricacid and 18-crown-6 in a stoichiometric amount of 1 : 1 : 1 molarratio. The reaction mixture was stirred for 10 minutes to homo-genize the components thoroughly. After 1 week, a colourlesscrystalline product of considerable size was obtained. A MiniFlexII Powder X-ray Diffractometer was used to record room tem-perature PXRD as a confirmation of the phase purity in 1 (Fig. S1,ESI†). The recorded pattern matches well with the simulatedresult from the single-crystal structure at room temperature.

Thermal measurements

DSC and Cp measurements were recorded using a NETZSCHDSC 200 F3 instrument with heating and cooling rates of10 K min�1 in the temperature range from 210 to 290 K. Thesemeasurements were accomplished under a nitrogen atmospherein aluminium crucibles. Experiments of Cp were performed byusing a comparison method, where a sapphire standard was usedas the reference. Firstly we run the baseline on the desiredtemperature range and then sapphire disks for calibration wereused as a standard prior to the measurement. Then the samplewas measured and compared with the sapphire recorded formerly.

Dielectric measurements

For dielectric experiments, well-ground powder samples con-nected by silver-conducting paste on electrodes were used formeasuring the complex dielectric permittivities, e = e0 � ie00. Thedielectric constants were measured using a TH2828A impe-dance analyzer at different frequencies of 1 MHz and 100 kHzwith a measuring AC voltage fixed at 1 V.

Single crystal structure determination

Variable-temperature X-ray single-crystal diffraction data of 1were collected using a SuperNova CCD diffractometer withgraphite monochromated Cu-Ka radiation (l = 1.54184 Å) atlow (220 K) and high (285 K) temperatures, respectively. Fordata collection, cell refinement and data reduction, the Crystal-Clear software package (Rigaku) was utilized. Crystal structureswere solved by direct methods and refined by the full-matrixleast-squares method based on F2 using the SHELXLTL softwarepackage.16 All non-hydrogen atoms were refined anisotropically.

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Scheme 1 Schematic representation of the structural formula of 1.

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The positions of the hydrogen atoms were generated geometri-cally and were located at the cation (1-methylpiperidinium).Crystallographic data and details of data collection and refine-ment are listed in Table S1 (ESI†). CCDC 1501909 and 1501910for 1 contains the supplementary crystallographic data forthis paper.

Results and discussionThermal analysis

DSC measurement affords an effective and useful technique toconfirm the existence of a reversible phase transition triggeredby temperature.17 In the case of 1, its phase transition ismanifested from thermal peaks (Fig. 1a); a couple of endo-thermic (260 K) and exothermic (234 K) peaks with a large heathysteresis of B26 KQ8 represent the discontinuous feature. Inaddition to DSC, the Cp–T curve also confirms the presence ofphase transition in 1 (Fig. 1b). The curve in Cp–T gives a sharppeak at 260 K, which is consistent with the DSC measurement.The enthalpy (DH) associated with the endothermic peak wascalculated as 7.12 J g�1, and thus the corresponding totalentropy change (DS) was estimated to be 13.93 J mol�1 K�1

using the equation DS = DH/Tc. According to the Boltzmannequation, DS = R ln N, in which R is the gas constant and N isthe number of geometrically distinguishable orientations, thevalue of N comes out to be 4.55, suggesting that 1 undergoes anorder–disorder phase transition.18

Second harmonic generation (SHG) measurement

SHG measurement is an effective way to test the NCS natureand the phase transition in a compound. However, for switch-ing of SHG signals, the phase transition should be from CS toNCS.19 As 1 crystallizes in acentric space groups at the LTP andthe HTP, it is expected to be NLO active below and above Tc. Tomeasure the relative value of the SHG efficiencies of 1, KDPpowder was chosen as the reference sample. Because the NLOactivity depends on the dipole moment of a material, thevariation of SHG intensities between the high and low statesof the molecule can be accredited to a miniature difference indipole moments. Here, the powdered sample of 1 was subjectedto a pulsed Nd:YAG laser at a wavelength of 1064 nm. At roomtemperature, the measured SHG intensities of 1 are estimatedto be B0.8 times compared to that of KDP. It is interesting thatthe intensities of the SHG signals remain unchanged at boththe HTP and the LTP, as shown in Fig. 2a, which is consistentwith its crystallographic symmetry. That is, both the spacegroups of 1 are polar at the HTP and the LTP (Cmc21 andPna21, as discussed below). Such a variation of SHG effects isreminiscent of the NCS-to-NCS phase transition in 1, whichchanges from Cmc21 to Pna21. The symmetry diagram is givenin Fig. S2 (ESI†). Furthermore, the SHG intensities were alsomeasured using a powder sample with different particle sizes.The SHG signals show a gradual enhancement with increasingparticle size and finally attain the saturation value (Fig. S3,ESI†), indicating that 1 is phase matchable.

The crystallographic insight reveals that compound 1 ispolar at the LTP and the polar direction is along the b-axis(Fig. 3). Unusually, at the HTP, 1 still crystallizes in the polarspace group and it displays SGH signals with the same NLOintensities as that of the LTP. The disordered methyl groupattached to the nitrogen atom of the cyclic cation is dynamic inthe cationic plan. In this way, the polarity of molecular moietiesof 1 is not greatly affected. Besides, the arrangement of polardipoles still generates bulk dipole moments, which makes 1SHG-active at the HTP. Thus, SHG effects of 1 remain stablealthough it undergoes a structural phase transition near roomtemperature. Our SHG measurements confirm the polar natureof 1 at both phases with the NCS-to-NCS phase transition.

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55Fig. 1 (a) The DSC curves of 1. (b) Temperature dependence of Cp

obtained from the polycrystalline material.Fig. 2 SHG intensity of 1 below and above Tc (a) SHG response at bothphases. (b) SHG response of 1 relative to KDP.

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Single crystal structure determination

To further confirm the phase transition in 1, X-ray diffractionanalyses were performed at 220 K (LTP) and 285 K (HTP),respectively. At the HTP, 1 crystallizes in the orthorhombiccrystal system and the space group of Cmc21, with the cellparameters a = 10.8029(4), b = 15.9579(5), c = 14.6015(3) Å, Z = 4and V = 2517.18(13) Å3 (Table S1, ESI†). At the LTP, 1 belongsto the same orthorhombic crystal system but transformsto a different space group, Pn21a, with cell parameters a =31.3826(3) Å, b = 14.50650(10) Å, c = 10.79330(10) Å, Z = 4, andV = 4913.67(7) Å3. The crystal packing diagram shows thatits crystallographic axes obey the following relationship:aHTP B cLTP, bHTP B aLTP and cHTP B bLTP. In detail, thelengths of the bLTP and cLTP axes are almost equivalent to thoseof cHTP and aHTP, while the aLTP axis becomes twice as long asthat of bHTP. As a result, the cell volume has increased andbecomes two-fold below Tc. Such an obvious change of cellparameters in the vicinity of Tc is evidence of the occurrence ofthe phase transition in 1, which is in good agreement with thethermal measurements.20

At the HTP, the asymmetric unit of 1 is composed of oneprotonated 1-methylpiperidinium cation, one deprotonatedperchlorate anion, one neutral [18]-crown-6 and a water mole-cule, as shown in Fig. 3b. The packing diagram reveals anobvious disorder in all the tetrahedral ClO4

� anions andcations (see Fig. S7, ESI†). The oxygen atoms in ClO4

� aresternly disordered and located over different sites as O1A, O2A,O3A, O4A and O1A0, O2A0, O3A0, O4A0, respectively. Meanwhile,the methyl group of the cation is highly disordered, andoccupies two equivalent positions as C1 and C10. This strikingcharacteristic disordering of 1 is further confirmed by thethermal ellipsoids of oxygen and carbon atoms at high tem-perature, which are fairly large compared to that of the neigh-bouring atoms. This result is not the real occupation, but

actually an average result of the atomic disordering (Fig. S5b,ESI†). The splitting of ellipsoidal states provides a more accep-table crystal structure of 1 at the HTP. Upon decreasing thetemperature below Tc, the disordering state of the oxygen andcarbon atoms is frozen, where the fully ordered structure at theLTP corresponds well to the more stabilized state, as shown inFig. S5 (ESI†). These elaborated structural analyses suggest that theorder–disorder transformation of both counter ions is attributed tothe emergence of this first-order phase transition in 1.

In addition to the order–disorder transformation in 1, anotherstriking conformational change has also been observed;21 that is,the torsional angle (+O–C–C–O) of the crown-ether moleculechanges from 0.001 (HTP) to 69.11 (LTP). This drastic changereveals the conversion of the unstable eclipsed conformer to thestable staggered form. The pictorial illustration in Fig. S6 (ESI†)displays the conformational change across the O–C–C–O atomsof crown-ether. Consequently, this result implies that theseconformational changes cause the crown-ether entity to inducean alteration in spatial arrangements, which might minimize thesteric strain along the ring.

At the LTP, the asymmetric unit of 1 is doubled relative to thatof the HTP, consisting of two protonated 1-methylpiperidiniumcations, two deprotonated perchlorate anions, two [18]-crown-6and two water molecules (Fig. 3a). Interestingly, at both the HTPand the LTP, the crystal structures of 1 are characterized byhydrogen bonding with O–H� � �O and N–H� � �O (Fig. S8, ESI†).Therefore, the crown-ether, cation and water molecule constructa supramolecular discrete unit. The water molecule links thecation and the crown molecule together through hydrogenbonds within the length range of 2.81 to 284 Å in both phases.The entire hydrogen-bonding skeleton shows that cation–waterand water–18-crown-6 interactions construct an interlockedsystem, in which the water molecule occupies a centralposition. However, there is no considerable change in thelength of hydrogen bonds between the LTP and the HTP, asshown in Fig. S8 (ESI†).

From the above analyses, it is clear that the phase transitionin 1 originates from the order–disorder transformation ofcations and anions. Apart from the disordering of the counter-ions, the neutral crown-ether molecule in 1 also undergoes aconformational change, contributing to the occurrence ofphase transition. As the temperature decreases, the disorderingof the counterions has been inhibited and crown-ether isrestricted to a staggered conformer. Consequently, the com-plete frozen ordering gives rise to the phase transition, as wellas the symmetry change in 1.

Dielectric studies of 1

The temperature-dependent dielectric constant is an impera-tive indicator of the presence of phase transition in a testingmaterial, revealing its degree of electric polarizability.22 Here,the temperature-dependent real part (e0) of the complex dielec-tric permittivities of 1 was measured on the powder-pressedpellets at different frequencies (1 MHz and 100 kHz) from170 to 315 K. As Fig. 4 displays, the obvious step-like dielectricanomaly was recorded at 234 K in cooling mode, which

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Fig. 3 Diagram of the asymmetric unit of 1 at (a) the LTP and (b) the HTP.Hydrogen atoms are omitted for clarity.

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corresponds well to thermal measurements. As temperaturedecreases, the dielectric constant decreases progressively anddisplays a clear reduction vertically in the vicinity of Tc. Thevalues of e0, for instance, display changes from 1.2 to 3.7 duringthe phase transition, where the variation was sharp in thesurrounding of Tc. The observed dielectric anomaly is anindication of the occurrence of an order–disorder transforma-tion in 1 coupled with the conformationalQ9 change.31 Uponcooling, the disordering of 1 is frozen below Tc and it acquiresan ordered state. The switching dielectric response in 1 isassumed to be induced by the synergetic effect of the order–disorder transformation of the counterions along with theconformation of the neutral crown-ether molecule. Therefore,these phenomena exert influences on the dynamics of the ionsand the crown-ether entity, respectively. From these findings itcan be concluded that the thermally-induced switching material,1, might be a potential switchable dielectric material as demon-strated by dielectric properties as a function of temperature.23

Conclusions

A molecular PTM, [(C5H10)(CH3)NH][18-crown-6][ClO4] mono-hydrate (1), has been assembled through aqueous solutionat ambient temperature and its phase transition was charac-terized by single crystal X-ray diffraction measurements,thermal analyses and dielectric experiments. Single crystalstructure analyses reveal that the space groups at both phasesare NCS and 1 undergoes a polar–polar phase transition,which is also confirmed by SHG measurements. The SHGintensities were B0.8 times compared to that of KDP. Thecation forms a supramolecular structure with crown-etherthrough binding of a water molecule. Mechanistically, thisphase transition originates from the synergistic order–disordertransformation of an anion and a cation, as well as theconformation change of the crown-ether molecule. Therefore,this mechanism involves three components; the counterionsand the neutral crown molecule through two different modesof induction of its phase transition, suggesting a new pathway

to design and explore such a novel molecular PTM with NLOproperties.

Acknowledgements

T. K. is thankful to the CAS-TWAS President program of theUniversity of Chinese Academy of Sciences. This work is sup-ported by NSFC (21622108, 21525104, 91422301, 21373220,51402296 and 51502290). Z. S. thanks the support from ‘‘Chun-miao Projects’’ of the Haixi Institute of Chinese Academy ofSciences (CMZX-2013-002), and the State Key Laboratory ofLuminescence and Applications (SKLA-2016-09).

Notes and references

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Fig. 4 Temperature-dependent dielectric constants of 1 measured atdifferent frequencies on cooling mode.

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6 | J. Mater. Chem. C, 2017, 00, 1�6 This journal is �c The Royal Society of Chemistry 2017

Paper Journal of Materials Chemistry C