Hydrogen-Bonded Switchable Dielectric Material Showing theBistability of Second-Order Nonlinear Optical PropertiesJing Zhang,‡,# Zhihua Sun,*,‡ Weichuan Zhang,‡ Chengmin Ji,‡ Sijie Liu,‡ Shiguo Han,‡

and Junhua Luo*,‡

‡State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences,Fuzhou, 350002, China#College of Chemistry and Chemical Engineering, Fuzhou University, Fuzhou, 350108, China

*S Supporting Information

ABSTRACT: Physical properties of phase transition materials are directly linked to the transformation of structural moieties.Here, we report a new hydrogen-bonded binary crystal of cyclohexylaminium chlorodifluoroacetate (1), consisting of two distincttypes of the anion−cationic structural moieties, which shows an order−disorder phase transition around 153 K (phase transitionpoint, Tc). Thermal analyses, dielectric, and second-order nonlinear optical (NLO) measurements solidly confirm the symmetrytransformation from the point group mmm to mm2. It is noteworthy that 1 exhibits a coexistence of switchable dielectricbehaviors and bistable NLO effects. In detail, it is NLO-active below Tc, while its quadratic NLO effects completely disappearabove Tc. Such a change reveals the bistable feature of its quadratic NLO properties. Further, variable-temperature structureanalyses reveal that the emergence of NLO effects below Tc is ascribed to partial ordering of anions and small-angle reorientationof cations. This mechanism is different from that of the precedent NLO-switching materials, which affords a potential avenue todesign new electric-ordered molecular compounds as multifunctional materials.

1. INTRODUCTION

Stimuli-responsive materials are those that are able to changephysical and/or chemical properties under external stimuli suchas temperature, pressure, light radiation, electric and magneticfields, etc.1 The bulk switchable performance between differentstates can be used in sensors, switches, memory devices, and soforth.2 This has been well illustrated by switchable dielectricmaterials, of which the dielectric constants can be reversiblyswitched between low-dielectric and high-dielectric states. Sucha transformation is often established through the reorientationsor molecular motions of polar components during structuralchanges. For instance, the ordered solid-state molecules usuallyexhibit smaller dielectric constants than that in the disorderedstate, owing to the freezing of molecular motional freedom.3

Meanwhile, it is noteworthy that the structural dynamicsdirectly involves collective quadratic nonlinear optical (NLO)properties. As one class of the most important photoelectricdevices, the NLO switches demonstrate similar bistablebehaviors analogous to switchable dielectric materials.3 A

recent advance discloses that both dielectric constants andNLO properties can be switched in a reversible mannerthrough structural phase transitions.3c Nevertheless, it stillremains challenging to design switchable dielectric materialswith bistable NLO effects, since the macroscopic arrangementof NLO-active moieties should be controlled into a non-centrosymmetric (NCS) manner.4

Using the conceptual scheme of structural phase transitions,organic salts and inorganic−organic hybrids have been recentlyfound to show prominent bistable NLO properties.5 Partic-ularly, crystals that undergo structural changes from acentrosymmetric (CS) phase to the NCS state displayoutstanding physical properties in the vicinity of phasetransition point (Tc). For instance, the hydrogen-bondedbinary compounds of [Hdabco]+[A]− (dabco = 1, 4-

Received: February 17, 2017Revised: April 10, 2017Published: April 20, 2017

Article

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© 2017 American Chemical Society 3250 DOI: 10.1021/acs.cgd.7b00240Cryst. Growth Des. 2017, 17, 3250−3256

diazabicyclo[2.2.2]octane; A = ClO4, BF4, and ReO4) showobvious NLO-switching activities due to the synergistic effectsof the individual components.6 In addition to dynamic multiplestates of the monoprotonated Hdabco+ cation, the ordering ofanionic components also makes an important contribution totheir NLO bistability.7 Owing to the appearance of strongelectric polarization, dielectric properties of such candidatesexhibit peak-like anomalies, instead of the low/high switchabledielectric states. In this context, phase transition materials withthe coexistence of bistable NLO activities and switchabledielectric orders have remained scarce. Encouraged by thepioneering work, we attempt to utilize the structural phasetransitions to switch dielectric and quadratic NLO properties insolid-state crystalline molecular materials. Here, a newhydrogen-bonded switchable dielectric of cyclohexylaminiumchlorodifluoroacetate (1) has been reported, which undergoesan order−disorder phase transition at Tc = 153 K. It isinteresting that 1 demonstrates both switchable dielectricresponses and bistable NLO activities, that is, the coexistence ofdielectric and NLO-switching properties. Further, structuralanalyses reveal that partial ordering of anionic moieties andsmall-angle reorientations of cationic parts contribute to itsphase transition, as well as the bistable NLO effects. It isbelieved that this finding provides a potential pathway to designnew electric-ordered multifunctional materials.

2. EXPERIMENTAL SECTIONCompound 1 was synthesized by the reaction of cyclohexylamine andchlorodifluoroacetic acid with a molar ratio of 1:1 at roomtemperature. Transparent block crystals were obtained by thetemperature-cooling method from the aqueous solutions with thecooling rate of 0.5°/day (Figure 1). Powder X-ray diffraction (PXRD)

of 1 was measured on a Rigaku Ultima-IV X-ray diffractometer. Thediffraction patterns were collected in the 2θ range of 5−45° with a stepsize of 0.02°, which match fairly well with the calculated results basedon its room-temperature structure. This result confirms the bulk purityof the grown crystals (Figure S1).

DSC and Cp measurements were performed using a NETZSCHDSC 200 F3 instrument in the temperature range between 100 and250 K. The crystalline samples were placed in the aluminum crucibleswith different heating and cooling rates under the nitrogenatmosphere. Dielectric measurements of powder and crystal samples,deposited with silver conducting glue, were performed using aTonghui TH2828A analyzer in the temperature range of 120−250 K(Figure S5). Temperature dependence of the real part (ε′) of thecomplex dielectric constant (ε = ε′ − iε″ where ε″ is the imaginarypart) was recorded with the frequency range of 100 kHz to 1 MHz.The UV/vis diffuse reflection spectrum of 1 was collected in ShimadzuUV−2600 spectrometer in the range of 800−200 nm at roomtemperature, and the absorption edge near the UV side of 1 ismeasured to be ∼260 nm (Figure S6). Thermogravimetric analysis(TGA) was performed on a Mettler Toledo TGA/SDTA 851eanalyzer (Figure S7). Measurements of variable-temperature NLOproperties were carried out on FLS 920, Edinburgh Instruments, in thetemperature range of 100−250 K, using an unexpanded laser beamwith low divergence (pulsed by Nd:YAG laser at a wavelength of 1064nm). The laser is Vibrant 355 II, OPOTEK. The potassiumdihydrogen phosphate (KDP) is used as the reference standard.

Single-crystal X-ray diffractions were performed on a SuperNovadiffractometer with Cu-Kα radiation (λ = 1.54184 Å). Crystals withapproximate dimensions of 0.29 × 0.36 × 0.52 mm3 were used for datacollections at different temperatures. The crystal data were processedby the Crystalclear software package (Rigaku, 2005). The variable-temperature crystal structures were solved by the direct method andrefined by the full-matrix least-squares refinements on F2 using theSHELXLTL software package. Non-hydrogen atoms were refinedanisotropically based on all the reflections with I > 2σ(I), andhydrogen atoms were generated by geometrical considerations.Crystallographic data and structure refinements of 1 are listed inTables S1 and S2.

3. RESULTS AND DISCUSSION

3.1. Thermal Properties. Differential scanning calorimeter(DSC) and specific heat (Cp) measurements are effective todetect thermally induced solid-state phase transitions, especiallyfor crystals involving the dynamically disordered molecules.When the compound undergoes a structural phase transitionaccompanied by thermal entropy change, a heat anomaly can beobserved during heating and cooling process. Here, thestructural phase transition of 1 is preliminarily determined bythermal measurements, including DSC and Cp analyses. Asshown in Figure 2a, the DSC curves in the heating and cooling

Figure 1. Bulk crystals of 1 with the large size of 6 × 4 × 4 mm3.

Figure 2. (a) DSC curves in the heating and cooling runs and (b) the temperature dependence of Cp for 1 upon heating.

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runs clearly reveal a couple of reversible peaks. The respectivetemperature peaks are calculated to be ∼151/153 K (the phasetransition point, Tc) upon cooling and heating modes. Thepeak-like thermal anomaly of DSC trace suggests thediscontinuous characteristics for the structural phase transitionin 1. Besides, with the heating/cooling rates reducing to 2 K/min, a thermal hysteresis of ∼1.8 K was still observed (FigureS2), which discloses the first-order feature of its phasetransition.8 This result corresponds well to the thermal peakof its Cp−T trace, as shown in Figure 2b. Moreover, the relevantenthalpy change (ΔS) in the heating mode is calculated to be∼3.37 J·mol−1·K−1 from the Cp−T curve. According to theBoltzmann equation of ΔS = R ln N, where R is the gasconstant and N denotes the ratio of numbers of respectivegeometrically distinguishable orientations for the disorderedsystem, the N value is estimated as ∼1.5, which suggests a morecomplex phase transition than a typical order−disordertransition undergoing the two-site reorientations.9

3.2. Crystal Structure Analyses. To further confirm thestructural phase transition of 1, we have determined its variable-temperature structures. For convenience, its single-crystalstructure above Tc is labeled as high-temperature phase(HTP), and the phase below Tc is denoted as low-temperaturephase (LTP). Variable-temperature X-ray single-crystal dif-fraction reveals that 1 crystallizes in the orthorhombic crystalsystem at HTP, with a CS space group of Pcmn and point groupD2h. At 170 K, the cell parameters are determined as a = 10.912,b = 17.184, c = 17.738 Å, Z = 12, and V = 3326.29 Å3 (TableS1). Figure 3 shows that the asymmetric unit of 1 contains twotypes of structural moieties, which display different crystallo-graphic orientations. Apparently, one set consists of theprotonated cyclohexylaminium cation and the disorderedchlorodifluoroacetic anion (labeled as Type-B, Figure 3a),while the other only contains a half of anion−cationic moietieswith a highly symmetric molecular configuration (labeled asType-A). On the basis of crystallographic symmetry, all thecomponents including type-A and type-B moieties exhibit theCS configurations with respect to the mirror planes (FigureS3). Particularly, the Type-B chlorodifluoroacetate anions in 1are highly disordered; that is, both the F and Cl atoms of thechlorodifluoromethyl groups locate at two possible equilibriumpositions. These atoms reside over two sets of different atomicsites as C1, Cl1, F1, F2 and C1B, Cl1B, F1B, F2B, respectively. Thissimilar disordering has been observed in several other

molecular compounds, such as 4-diazabicyclo[2.2.2]octanetrifluoroacetate and tri-n-butylammonium trichloroacetate,10

which affords the driving force to their order−disorder phasetransitions. Moreover, it is interesting that the intramolecularN−H···O H-bonds between N atoms of cyclohexylaminiumcations and O atoms of anionic moieties construct the H-bonded dimmers (Figure 3b). As far as we are aware, the H-bonded binary compounds have been reported to show strongferroelectric polarization induced by order−disorder structuralchanges, such as diisopropylammonium chloride and diisopro-pylammonium bromide salts.11 Hence, it is expected that 1might also exhibit promising electric polarization, which isindispensable for the generation of quadratic NLO activities inthe crystalline samples.At 100 K, crystal structure of 1 still belongs to the

orthorhombic crystal system but crystallizes in a NCS spacegroup of Pca21 with the polar point group C2v. During thecooling process, its crystallographic symmetry transforms fromthe point group mmm to mm2;12 that is, the eight symmetricelements (E, C2, C2′, C2″, i, σh, σv, and σv′) in the HTP are

halved into four elements (E, C2, σv, and σv′) in the LTP(Figure 4).13 Figure 4 shows the symmetry elements of 1 in theLTP within one unit cell and in the HTP within two unit cells,respectively. The doubling of the a-axis after phase transitionleads to that the sequential symmetry operation of reflectionand then translation along the a-axis become that of glide.Accordingly, the location of the a-glide is at that of the mirror

Figure 3. (a) Asymmetry unit of 1 with atomic numbering scheme at HTP. There are two types of cation−anionic structural moieties with thedistinct crystallographic configurations, which are named as type-A and type-B, respectively. (b) Hydrogen-bonding interactions between thenitrogen and oxygen atoms in 1 at HTP.

Figure 4. Symmetry transformation of 1 during its phase transition.

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in the HTP. Therefore, the disappearance of two 2-fold screwaxes and the inversion center leads to the NCS space group ofPca21 at LTP.Upon cooling down below Tc, the splitting of diffraction

peaks is reminiscent of the possible lowering of crystallographicsymmetry. On data reduction, the space group of Pcmn wasinitially assumed with the Rint value of 0.09. However, the lowervalue (Rint = 0.0697) was obtained for the solution in the spacegroup of Pca21 (Table S1, Supporting Information). The cellparameters become a = 21.5734, b = 17.1630, c = 17.7854, Z =24, and V = 6585.3 Å3. From the structural viewpoint, theasymmetric unit of 1 contains six cyclohexylaminium cationsand stoichiometric chlorodifluoroacetate anions at LTP. Asshown in Figure 5a, the orientational conformations of organiccations closely resemble those in the high-temperaturestructure. The most explicit change is that the disordering ofchlorodifluoroacetate anions has been greatly reduced; that is,two-thirds of the anionic moieties become ordered. However, itis unexpected that other two anions remain still disordered (inFigure 5a). For instance, both the F and Cl atoms of thechlorodifluoromethyl group in one disordered anion locate attwo possible equilibrium positions. These atoms reside overtwo sets of different atomic sites as Cl3/Cl3B, F5/F5B, and F6/F6B, respectively. Meanwhile, the C9 of another one is split toC9 and C9B. The partial ordering of chlorodifluoroacetateanions differs from the typical order−disorder transition,coinciding well with DSC result. From the packing view, due

to the disorder of its structural moieties, the dipolar momentsare canceled to display CS packing at HTP (Figure 3b). Incontrast, the disordering of anionic groups has been greatlyinhibited at LTP, and the cationic dipoles exhibit thereorientional arrangement. Hence, molecular dipoles couldnot cancel each other and finally result in electric polarizationof 1.Above-mentioned studies reveal that 1 crystallizes in the

space group Pca21 at LTP, which is expected to show electricpolarization or ferroelectricity.12 However, to define a ferro-electric, some sufficient conditions or requirements should alsobe satisfied in this system, such as ferroelectric hysteresis loop,electric domain and dielectric anomaly, etc. Among them, thespontaneous polarization (P) and electric field (E) hysteresisloops usually behave as the direct evidence for ferroelectriccrystals. Here, we also tried to measure the P−E hysteresisloops on crystal samples using the Sawyer−Tower circuit.Unfortunately, it fails to switch the electric polarization evenunder large electric field of ∼50 kV/cm. Actually, it is knownthat switching of polarization becomes almost impossible forsome polar materials, because their coercive electric field is solarge that crystals would be destroyed when being switched.16

At present, it is quite difficult to measure the ferroelectric P−Ehysteresis loops for crystals of 1 with an acceptable degree ofaccuracy. A further study on the domain motions is needed foraccurate determination of ferroelectricity of 1 in future.

Figure 5. (a) Asymmetric unit of 1 at LTP. The pink lines denote N−H···O hydrogen bonds. (b) Packing diagram of 1 viewed along the bLTP-axis.The ammonium groups displace away from its prototypical mirror, which leads to the breaking of crystallographic symmetry. The carbon-bondedhydrogen atoms are omitted for clarity.

Figure 6. Temperature dependence of dielectric properties of 1. (a) The real part (ε′) of complex permittivity measured at different frequencies; (b)the ε′ values obtained in heating and cooling modes at 500 kHz.

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To further study the origin of phase transition in 1, weperformed a comparison of its structures at HTP and LTP. It isclear that partial ordering of chlorodifluoroacetate anionsdominates its phase transition. However, the role of cationicmoieties should not be underemphasized, although the cationicmoieties look the same during the phase transition. First, itassembles the components into a binary hydrogen-bondedarchitecture through intermolecular N−H···O hydrogen bonds,as shown in Figure 5b. These H-bonding interactions mightexert delicate constraints on the thermally induced atomicvibrations of anionic moieties. Such cooperative intermolecularinteractions are indispensable for the occurrence of polarizationin crystalline samples, which is an essential result of the long-range coupling interactions. Further, the cationic parts display asmall-angle reorientation during the phase transition. Take theammonium groups as example: they deviate away from theinitial equilibrium positions and lead to the disappearance ofmirror plane (as shown in Figure 5b). Hence, it is supposedthat its phase transition of 1 is induced by the partial orderingof anions, along with the small-angle reorientation of cations.3.3. Dielectric Properties. Bulk physical properties

generally show abrupt changes in the vicinity of Tc andvariation magnitude relates to the characteristics of structuralchanges.14 Here, temperature dependence of the real part (ε′)of the complex dielectric permittivities of 1 was measured oncrystal samples at different frequencies, and the (1 01) crystalsurface was determined according to the simulated morphology(in Figure S4). As shown in Figure 6a, the ε′ values of 1 remainalmost stable below Tc, well corresponding to its low-dielectricstate. Upon heating, its dielectric constants exhibit the step-likechanges around Tc, and the values increase up to ∼7.5 in thehigh-dielectric state. Such a reversible transformation betweenthe low-dielectric and high-dielectric states, resembling that ofimproper ferroelectric materials of (NH4)2Cd2(SO4)3 andGd2(MoO4)3,

15 discloses that 1 should be a switchabledielectric material. Besides, a relatively narrow thermalhysteresis of ∼2 K is observed in the heating and coolingruns, which corresponds well to the DSC results (Figure 6b).As mentioned above, the order−disorder structural changes of1 dominates its phase transition from a CS state to the NCSone. In this regard, these dielectric behaviors coincide fairly wellwith the partial ordering of dynamic motions in 1 during itsphase transition.3.4. Bistable NLO Properties. The low-temperature

structure of 1 belongs to the NCS space group Pca21. Hence,

it is expected to display the second harmonic generation(SHG) effects at LTP. First, we measured the UV/vis diffusereflection spectrum of 1 in the range of 800−200 nm, and theresult indicates that the absorption edge of 1 is ∼260 nm(Figure S6). The optical absorption is quite low at 532 nm,which suggests that 1 should be suitable for NLO applicationusing a Nd:YAG laser (λ = 1064 nm). As shown in Figure 7a,the temperature-dependent SHG signals confirm the structurechange of 1. In detail, there is not any SHG signal above 153 K,which corresponds fairly well to its CS structure of Pcmn. Withthe temperature decreasing below Tc, 1 becomes NLO-activeand exhibits nonzero SHG signals. This step-like change ofSHG signals suggests that 1 becomes NCS in LTP, beingconsistent with the structure analysis (space group changesfrom Pcmn to Pca21). When temperature decreases far awayfrom Tc, the SHG intensities become saturated and wereestimated to be ∼0.3 times as large as that of KDP (at 120 K,Figure 7b). Hence, the quadratic NLO coefficients (χ(2)) of 1 iscalculated to be ∼0.12 pm V−1 (χ(2)KDP = 0.39 pm V−1) atLTP.16 This value is comparable with some reported NLOswitching materials , such as (cystaminium)BiI5,

5a

(pyrrolidinium)CdCl3,16c and bis(imidazolium hydrochlorate)

dihydrate 18-crown-6.5c Actually, this change of SHG effectsclearly discloses the feature of 1 as a potential NLO-switchingmaterial. In detail, the complete vanishing of SHG signals atHTP corresponds to the low-NLO state, while the emergenceof SHG effects at LTP satisfies the essential requirement forNLO switches (treated as the high-NLO state). The NLOswitching contrast of between high-NLO state and low-NLOstate is calculated as ∼12, which is comparable with somecrystalline materials,17 such as anil crystals, thin films ofruthenium complexes and metal−organic frameworks, etc.18

Such a result suggests that 1 may be a potential NLO-switchingmolecular material.

4. CONCLUSIONS

In summary, the present work has successfully reported a solid-state molecular material, which undergoes an order−disorderphase transition at ∼153 K. It is noteworthy that it exhibitsswitchable dielectric responses and controllable NLO effects,that is, the coexistence of dielectric- and NLO-switchingbehaviors. Structural analyses disclose that partial ordering ofanionic moieties and small-angle reorientation of cationspredominate the phase transition. This coexistence mechanism

Figure 7. (a) Temperature dependence of second-order NLO effects of 1. The SHG intensities become almost saturated far away from the Tc. (b)Oscilloscope traces of SHG signals of 1 and the referential KDP. The quadratic NLO coefficient of 1 is estimated to be ∼0.3 times as large as that ofKDP.

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for the emergence of second-order NLO effects is distinct fromthe precedent NLO-switching compounds. It is believed that, inthe search for new multifunctional materials, this finding affordsa potential pathway to construct new electric-ordered molecularmaterials.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acs.cgd.7b00240.

Figure S1. Experimental and calculated powder X-raydiffraction patterns of 1; Figure S2. DSC traces measuredwith the different heating/cooling rates; Figure S3.Packing diagram of 1 viewed along the a HTP-axis;Figure S4. Crystal morphology of 1 simulated from thesingle-crystal structure; Figure S5. Temperature depend-ence of dielectric constants of 1 measured on powdersample; Figure S6. UV-Vis diffuse reflection spectrum of1 with the absorption edge at 260 nm; Figure S7. TGcurves of 1; Table S1. Crystal data and structurerefinement for 1; Table S2. N−H···O Hydrogen bondsof 1 at HTP (PDF)

Accession CodesCCDC 1498369 and 1498384 contain the supplementarycrystallographic data for this paper. These data can be obtainedfree of charge via www.ccdc.cam.ac.uk/data_request/cif, or byemailing data_request@ccdc.cam.ac.uk, or by contacting TheCambridge Crystallographic Data Centre, 12 Union Road,Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

■ AUTHOR INFORMATIONCorresponding Authors*(J.L.) E-mail: jluo2009@gmail.com.*(Z.S.) E-mail: sunzhihua@fjirsm.ac.cn.ORCIDJunhua Luo: 0000-0002-3179-7652NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work was supported by NSFC (21622108, 21525104,91422301, 21373220, 51402296, and 51502290), the StrategicPriority Research Program of the Chinese Academy of Sciences(XDB20000000) and the Youth Innovation Promotion of CAS(2014262). Z.S. thanks the State Key Laboratory ofLuminescence and Applications (SKLA-2016-09).

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