Hydrogen-Bonded Displacive-Type Ferroelastic Phase Transition in aNew Entangled Supramolecular CompoundYuanyuan Tang,†,‡,§ Zhihua Sun,†,§,∥ Chengmin Ji,† Lina Li,†,§ Shuquan Zhang,†,§ Tianliang Chen,†,§

and Junhua Luo*,†,‡,§

†Key Laboratory of Optoelectronic Materials Chemistry and Physics, Fujian Institute of Research on the Structure of Matter, ChineseAcademy of Sciences, Fuzhou 350002, China‡College of Chemistry, Fuzhou University, Fuzhou 350116, China§State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences,Fuzhou 350002, China∥State Key Laboratory of Crystal Material, Shandong University, Jinan 250100, China

*S Supporting Information

ABSTRACT: The framework entanglements show structural transitions by theremoval and incorporation of guest molecules, but rarely generate phase transitionsby themselves. In this study, we report a new entangled hydrogen-bondedsupramolecular compound, [(n-C4H9)2NH2]2H2C4O4·H4C4O4 (1, H4C4O4 =fumaric acid), which undergoes a reversible ferroelastic phase transition with theAizu notation of 2/mF1. Differential scanning calorimetry and specific heatmeasurements confirm its typical second-order phase transition at around 228.8 K(Tc), while the results of the deuterated analogue (2) are different with those of 1,indicating that proton dynamic motions in hydrogen bonds contribute to the phasetransition. Variable-temperature single-crystal X-ray diffraction analyses reveal thatthe cooperative displacements of hydrogen bonds induce the structural phasetransition, which arise from the twisting motions of the fumaric acid molecules.Simultaneously, two types of independent hydrogen bonding layers in theentanglement are altered in response to the transformation of hydrogen bonds aggregates at the low temperature phase,causing the symmetry breaking. These findings will open up a new avenue for the design of ferroic materials with an entangledframework.

■ INTRODUCTION

Ferroelastic materials have been key to the design andexploitation of multiferroic materials, which enable themanipulation of magnetic ordering and/or polarization order-ing by an external stress through switching of the ferroelasticstate, with a significant amount of applications such aspiezoelectric sensors and mechanical switches.1−6 When shearstresses are applied to a ferroelastic material, in which a phasetransition often happens between the paraelastic andferroelastic phases, the material shows a highly nonlinearstrain−stress curve called a hysteresis. As it is rather a difficultexperimental undertaking to measure ferroelastic hysteresiswith any acceptable degree of accuracy, it has becomecustomary to term a material “ferroelastic” on condition thata phase transition which may generate ferroelasticity occurs (ormay occur).5,6 Theoretically, such a phase transition shouldbelong to the 94 species ferroelastic phase transitions definedby Aizu.7 Various approaches have been obtained to induce thephase transitions.8−15 Among them, the progress associatedwith transformations of the hydrogen-bonded aggregates is oneof the most promising strategies, which derive from thebreaking and formation of alternative hydrogen bonds, proton

transfers, or proton disordering.16−25 The hydrogen bond, animportant interaction for binding different molecules, could beutilized to construct a supramolecular assembly of hydrogendonor (D) and acceptor (A) molecules using its highlydirectional nature. Nevertheless, hydrogen bonds are muchweaker than covalent bonds binding atoms into molecules;thus, the hydrogen-bonded aggregates could readily undergotransformations to shear the structural network and lower thesymmetry of the crystal structure. As is typically found in thecase of a KH2PO4 (KDP) crystal, which is a representativeferroelectric, a site-to-site proton transfer over very short O−H···O hydrogen bonds causes the ferroelectric phasetransition.19−24 More recently, Horiuchi et al. have proposedan intermolecular proton migration in the hydrogen bondsresponsible for thermally induced phase transition in thecocrystal of 2,5-dihydroxy-p-benzoquinones with pyridinederivatives, formed by short hydrogen bonds (O···N distance2.61−2.70 Å, N+···O− distance 2.54−2.55 Å).25 Namely, the

Received: October 15, 2014Revised: November 13, 2014Published: November 14, 2014

Article

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© 2014 American Chemical Society 457 dx.doi.org/10.1021/cg501529f | Cryst. Growth Des. 2015, 15, 457−464

process exchanges two hydrogen-bond tautomers O−H···Nand N+−H···O−, accompanied by rearrangement of thegeometries of both π molecules, giving rise to a largespontaneous polarization and dielectric constant. As thecharacteristic dibasic acid (D), the fumaric acid can easilyprovide protons to form a variety of hydrogen-bondednetworks in the compounds. However, up to now, reports onthe fumarate compounds associated with phase transition havebeen very scarce.Herein, we introduce a new hydrogen-bonded compound

[(n-C4H9)2NH2]2H2C4O4·H4C4O4, with a dibutylamine mole-cule as the base (A) and fumaric acid as the acid (D), whichundergoes a reversible ferroelastic phase transition at 228.8 K(Tc). The origin of the structural phase transition is ascribednot only to the proton dynamics in the hydrogen bonds butalso to the cooperative displacements of hydrogen bondsbetween two phases. In the present new compound, it isinteresting to be found that two types of independent hydrogenbonding layers along different planes, formed by N−H···O andO−H···O hydrogen bonds, completely interpenetrated toestablish an entangled supramolecular network, through theelongation of hydrogen bonds that lead to the intergrowth ofone framework into the other. In the entanglement, thecooperative displacements of hydrogen bonds have beendiscovered to yield two crystallographically nonequivalentframeworks in the low temperature phase, resulting in thesymmetry breaking. Another fascinating feature with respect tothe entangled framework is the excellent flexibility,26−28 wherechemically noninterconnected frameworks can show dynamicmovement by the dislocation of their mutual positions, alsoeasily inducing the structural phase transition. This feature isquite promising for finding new paradigms with variousstructure−property relationships, and for the design of newferroic materials.

■ EXPERIMENTAL SECTIONSynthesis. All chemical reagents were used without further

purification. Compound 1 was prepared through reaction ofdibutylamine and fumaric acid with a 1:1 molar ratio. Thedibutylamine (2.61 g, 0.02 mol) was added into fumaric acid (2.32g, 0.02 mol) in water (100 mL), and then the reaction mixture wasstirred for an additional 20 min at room temperature. Crystals of 1were obtained by slow evaporation of the synthesized solution at roomtemperature after several days (Figure S1, Supporting Information).The deuterated compound 2 was obtained by refluxing a D2O (99.9%)solution of 1 for 72 h at 50 °C. After removing the solvent by heating,the resultant polycrystals were twice recrystallized from D2O. Singlecrystals were grown by slow evaporation of a D2O solution of 2 atroom temperature in the vacuum conditions after several days. 1HNMR spectra of 1 and 2 were obtained on a 400 MHz and reported inparts per million (δ) relative to the response of the solvent (DMSO)or to tetramethylsilane (0.00 ppm) (Figure S3, SupportingInformation), from which the deuterated percentage of acidichydrogens in 2 can be estimated to be about 45.2%. In the IR spectraof 1 (Figure S4, Supporting Information), the peaks at approximately1687 and 1614 cm−1 are assigned to stretching vibration absorption ofthe carbonyl group (CO), which definitely reveals the existence offumaric acid in 1. The phase purity of 1 and 2 is verified by the powderXRD (PXRD) patterns, which match very well with the patternsimulated from the single-crystal structure at room temperature(Figure S5, Supporting Information).Single-Crystal Structure Determination. Variable-temperature

X-ray single-crystal diffraction data of 1 were collected using a SuperNova CCD diffractometer with the graphite monochromated Mo-Kαradiation (λ = 0.71073 Å) at low temperature (216 K) and hightemperature (250 K), respectively. The CrystalClear software package

(Rigaku) was applied for data collection, cell refinement, and datareduction.29 Crystal structures were solved by the direct methods andrefined by the full-matrix least-squares method based on F2 using theSHELXLTL software package.30 All non-hydrogen atoms were refinedanisotropically. The positions of hydrogen atoms that were located atthe carbons were generated geometrically, and the hydrogen atoms incarboxyl groups were determined from the Fourier electron densitymap. Besides, as shown in Table S1 (Supporting Information), the C−O bond lengths and the O−C−O bond angles demonstrate that thehydrogen atoms are closely shared with the carboxylic acids moiety inthe same fumaric acid. Some atoms (especially carbon atoms in theterminal at the cations and oxygen atoms) also have a very slightswing. We try to solve the disorder, and yet the swings are too slight tosplit. Crystallographic data and details of data collection andrefinement at 216 and 250 K are listed in Table 1.

DSC and Specific Heat Measurement. The DSC and specificheat measurements were carried out on a NETZCSCH DSC 200 F3instrument by heating and cooling at a rate of 10 K/min in thetemperature range from 110 to 270 K. These measurements wereperformed under a nitrogen atmosphere in aluminum crucibles.

Dielectric and Second Harmonic Generation (SHG) Measure-ments. In the dielectric experiments, the single-crystal plates of 1 withsilver pasted as the electrodes were used for measuring the complexdielectric permittivities, ε = ε′ − iε″. Its dielectric constants weremeasured using a TH2828A impedance analyzer at the respectivefrequencies of 5, 10, 100, and 1000 kHz with the measuring ACvoltage fixed at 1 V. Powder SHG measurements were carried out bythe Kurtz−Perry method. The measurements were performed using aQ-switched Nd:YAG laser at 1064 nm with an input pulse of 350 mV.

■ RESULTS AND DISCUSSIONThermal Properties. The structural phase transition of 1

accompanied by thermodynamic anomalies was confirmed byDSC and specific heat measurements in the temperature rangeof 200−250 K (Figure 1). In the heating and cooling modes ofthe DSC curves, a pair of broad peaks are recorded with theendothermic peak at 228.8 K (Tc) on heating and anexothermic one at 224.4 K upon cooling (Figure 1a). Inorder to certify the second-order phase transition, DSC

Table 1. Crystal Data and Structure Refinement of 1 at 216and 250 K

sum formula C24H46N2O8 C24H46N2O8

formula weight 490.63 490.63temperature (K) 250(2) 216(2)crystal system monoclinic triclinicspace group C2/c P1a/Å 16.6386(9) 9.0851(5)b/Å 19.8615(10) 12.8719(4)c/Å 9.0962(5) 12.9349(6)α/deg 90 100.258(3)β/deg 92.422(4) 90.572(4)γ/deg 90 93.606(4)volume (Å3) 3003.3(3) 1485.17(12)Z 8 2Dcalcd, g cm−3 1.085 1.097μ, cm−1 0.80 0.81F(000) 1072.0 536.0completeness (%) 99.7 99.4goodness-of-fit on F2 1.050 1.055Tmin/Tmax 0.967/0.980 0.967/0.976R1 (on Fo

2, I > 2σ(I))a 0.0656 0.0780wR2 (on Fo

2, I > 2σ(I))a 0.1876 0.2472aαR1 = ∑∥Fo| − |Fc∥/∑|Fo|, wR2 = [∑(|Fo|

2 − |Fc|2)/∑|Fo|

2]1/2.

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measurements under different cooling/heating rates wereperformed (Figure S6, Supporting Information). The limitingthermal hysteresis (0.1 K) estimated from the scansextrapolated to a scanning rate of 0 K·min−1 is close to 0 K,which is a characteristic of a second-order transition.31−34

Specific heat capacity measurement further confirms thepresence of phase transition and shows a typical anomaly ataround 229 K, corresponding well to the DSC results (Figure

1c). It is notable that the λ shape of the anomaly peak mostlikely resembles the features of a second-order phase transition,like that of triglycine sulfate (TGS).35 The entropy change ΔSis estimated to be 0.877 J/mol·K from the heat capacity data.Given that Boltzmann’s equation ΔS = R ln N, in which R is thegas constant and N is the ratio of the numbers of possibleconfigurations, N = 1.11 is obtained. The N value, which iscloser to 1, suggests that the phase transition is not an order−

Figure 1. DSC curves of (a) 1 and (b) 2. The temperature dependence of specific heat capacity of (c) 1 and (d) 2.

Figure 2. (a) Asymmetric unit of 1 at 250 K. (b) Asymmetric unit of 1 at 216 K.

Figure 3. Temperature dependence of (a) cell parameter changes for three axis lengths and (b) cell volume and three angles in the range from 200to 270 K for 1.

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disorder mechanism but rather a displacive mechanism, wellconsistent with the crystal structure analyses below.36−38 Toinvestigate whether proton dynamic motion in the compoundoccurs, the deuteration effect was determined by DSC andspecific heat measurements. In comparison, the deuteratedanalogue 2 also shows a heat anomaly, appearing atapproximately 216.7 K upon cooling and 221.9 K uponheating, with a distinct change of 7 K, indicating that the protondynamic motions in the hydrogen bond are responsible for thestructural phase transition (Figure 1b,d).39−41

Structure Discussion. In order to understand the phasetransition more clearly, single-crystal X-ray structure determi-nations of 1 were performed at 216 K (low-temperature, LT)and 250 K (high-temperature, HT), respectively. The crystalstructure of the ionic [(n-C4H9)2NH2]2H2C4O4·H4C4O4 crystalis primarily established by the electrostatic cation−anion andhydrogen-bonding interactions. At the HT phase, 1 crystallizesin the monoclinic space group C2/c, and cell parameters are a =16.6386(9) Å, b = 19.8615(10) Å, c = 9.0962(5) Å, α = 90°, β= 92.422(4)°, γ = 90°, V = 3003.3(3) Å3, and Z = 8. Theasymmetric unit contains one-half of di-ionic fumaric acid, one-half of un-ionized fumaric acid, and one dibutylammoniumcation (DBA = dibutylammonium) (Figure 2a). The structureof 1 in the LT phase is triclinic with a space group of P1 , andcell parameters are a = 9.0851(5) Å, b = 12.8719(4) Å, c =12.9349(6) Å, α = 100.258(3)°, β = 90.572(4)°, γ =93.606(4)°, V = 1485.17(12) Å3, and Z = 2. The cellparameters of 1 show a great change between the two phases, inwhich the cell volume is halved at the LT phase in comparisonto that at the HT phase. The components of its asymmetric cellunit are doubled that of the HT phase, in good agreement withthe gradually decreasing structural symmetry triggered by thephase transition during the cooling process (Figure 2b).Figure 3 exhibits the unit cell parameters of 1 as a function of

temperature on cooling from 270 to 200 K, in which theanisotropic lattice parameter variations around Tc are clearlyevident, confirming the structural changes during the phasetransition. The relationship between the two temperature cellsis that aHT corresponds to cLT, bHT corresponds to bLT, and cHT

corresponds to aLT. The lattice constant aLT shows nodiscernible anomalies compared with cHT, but bLT and cLT

expand by about 53.9% and 27.9% with an inflection point inthe vicinity of Tc, respectively. At the same time, the monoclinicsymmetry is reduced to triclinic, which is characterized by thechanges of α angle and γ angle, from 90° in the HT phase toabout 100.2° and 93.5°, respectively, in the LT phase.Moreover, the great change occurred in cell volume, whichpresents an approximately 2-fold decrease in cooling process.All cell parameters, with the exception of the a-axis length,show abrupt changes around Tc, which is a sign of structuralphase transition.From the viewpoint of symmetry breaking, the structure of 1

transforms from the monoclinic crystal system with a highcentrosymmetric space group of C2/c and the point group ofC2h at 250 K to the triclinic crystal system with a lowcentrosymmetric space group of P1 and the point group of Ci at216 K. Namely, a symmetry breaking phenomenon occursduring the transition from the HT phase to the LT phase withan Aizu notation of 2/mF1 .42 Symmetric elements decrease byhalf from four (E, i, C2, σh) to two (E and i), in strictaccordance with Landau phase transition theory. The spatialsymmetric operations change shown in Figure 4 unambiguouslyindicates that the symmetric elements (2 and m) disappear in 1

in the LT phase, which might be aroused by slightdisplacements of the atoms. It is in good agreement with theCurie symmetry principle that the ferroelastic space group P1 isa subgroup of the paraelastic one C2/c, whose maximalnonisomorphic subgroups include Cc, C2, and P1 . It can beseen that the inversion center i remains in both phases, which isalso confirmed by the temperature-dependent second harmonicgeneration (SHG) (Figure S7, Supporting Information). Thereis almost no signal in the temperature range of 200−280 K,indicating that the structure of 1 might turn from acentrosymmetric structure to another centrosymmetric one,which is consistent with X-ray single-crystal structureanalyses.43−45

The structures of 1 in the HT and LT phases are bothconstructed by an extensive hydrogen bonding network withO−H···O and N−H···O hydrogen bonds, which play a key rolein the emergence of phase transition. In the packing structureof 1 at the HT phase, strong O−H···O hydrogen bondsbetween the terminal COOH of fumaric acid units and COO−

groups of the alternating fumarate anions form a one-dimensional zigzag infinite chain in the (110) plane, with anO···O distance of 2.496(3) Å being shorter than the typical O−H···O hydrogen-bonding distance (Figure 5a). Furthermore,the adjacent anionic hydrogen-bonding chains are interlinkedvia rich N−H···O hydrogen bonds provided by the DBAcations. The rows of alternating cations and anions connectedby N−H···O hydrogen bonds are elongated along the c-axis,building the two-dimensional hydrogen bonding layers alongthe (1 10) plane, and there are hydrogen bonding layers alongthe (110) plane crystallographically equivalent to them by thecrystal symmetries of the 2-fold screw or c glide plane (FigureS8, Supporting Information). At the same time, thecombination of N−H···O and O−H···O hydrogen bonds inthe same layer forms R8

6 (36) ring motifs, each of which consistsof two DBA cations and six fumaric acids, including fourdeprotonated fumarate anions and two fumaric acid molecules,resulting in the occurrence of big holes (Figure S9, SupportingInformation). Interestingly, two kinds of chemically non-interconnected hydrogen bonding layers along different planesare found completely interpenetrated through the holes toestablish a three-dimensional entangled supramolecular net-work (Figure 5b). The N−H···O and O−H···O hydrogenbonding interactions and the interpenetration are responsible

Figure 4. Transformation of space group of 1 from the HT paraelasticphase (C2/c, No. 15) to an LT ferroelastic phase (P1 , No. 2).

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for stabilizing the overall three-dimensional architecture andtopology of the hydrogen-bonded framework.With the temperature decreasing from the HT phase to the

LT phase, little deformation of fumaric acid molecules isobserved during the phase transition. That is, the slight twistingmotions of the fumaric acid molecules are confirmed that thetorsion angles of O13−C11−C10−C10 and O30−C28−C27−C27 being 11.038(11)° and 14.003(10)°, respectively, exhibit asmall difference from the corresponding torsion angle of O13−C11−C10−C10 (11.771(5)°) in the HT phase (Table 2). Thehydrogen bond geometries are approximate to those in the HTphase. However, it is notable that the distances between thedonor and acceptor atoms for the O−H···O and N−H···Ohydrogen bonds show obvious differences in two phases,resulting from the twisting motions of the fumaric acidmolecules (Table S2). Furthermore, the two-dimensionalhydrogen bonding layers along (001) and (010) planes,which are symmetry-equivalent in the HT phase, emerge withtwo different conformations labeled as LTa and LTb based onthe relative different distances of hydrogen bonds and the slightmolecular movement, as illustrated in Figures 6 and 7. Incomparison, the O···O distance of the O−H···O hydrogenbond in the LTb part (2.508(3) Å) is a little longer than that inthe HT phase, but it is shorter in the LTa part (2.476(3) Å)(Figure 6). It is interestingly found that, similar to thedifferences of O···O distances, the changes of N···O distancesfor N−H···O hydrogen bonds compared to those in the HTphase are absolutely opposite in the LTa and LTb parts. Forinstance, the N···O distance of the N5−H5A···O16 hydrogenbond in the HT phase is 3.129(3) Å, whereas thecorresponding values turn into 3.007(3) Å in the LTa partand 3.208(3) Å in the LTb part, the intensities of hydrogenbond interactions varying greatly. It is proposed that therepresentative displacements of these hydrogen bonds accom-

pany large distortions of the atomic coordinates, which may bethe driving force of the structural phase transition.It is noteworthy that the cooperative displacements of

hydrogen bonds, which could be deduced from the stretch andshrinkage of O−H···O and N−H···O hydrogen bonds in theLT phase, give rise to the collective molecular movements. Asshown in Figures 5a and 7, the distances of O12−O12a/O29−O29a or O12−O12b/O29−O29b in both phases are nearly

Figure 5. (a) Unit cell packing diagrams of 1 viewed along the (1 10) plane at 250 K. The dashed lines stand for the hydrogen bonds; see Table S2 inthe Supporting Information. The atoms suffixed with a, b, and c are crystallographically equivalent to those atoms with the same numbers. Carbon-bound H atoms in the cations are omitted for clarity. (b) The topological network of 1 shows that two kinds of chemically noninterconnectedhydrogen bonding layers completely interpenetrated.

Table 2. Torsion Angles of 1 at 216 and 250 K

216 K (deg) 250 K (deg)

O13C11C10C10 11.038(11) O13C11C10C10 11.771(5)O30C28C27C27 14.003(10)O16C14C15C15 7.348(11) O16C14C15C15 6.121(5)O33C32C31C31 7.424(11)

Figure 6. Diagrams of hydrogen-bonded moieties in 1 at 216 K withdonor−acceptor distances (Å). The atoms suffixed with b arecrystallographically equivalent to those atoms with the same numbers.Some atoms are omitted for clarity. The dashed lines stand for thehydrogen bonds.

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identical (about 12.9 and 9.1 Å, respectively), while thediagonal distances of O12−O12c and O12a−O12b convertfrom 15.626/16.030 Å (HT) to 15.281/16.215 Å (LTa) and15.732/15.881 Å (LTb), respectively. In addition, the angle ofO12b−O12c−O12a in the LTa part (i.e., γ angle, 93.606(4)°)is larger than that in the HT phase (91.555(0)°), but thecorresponding value in the LTb part is smaller (i.e., β angle,90.572(4)°). In other words, the parallelogram of O12−O12a−O12c−O12b in the HT phase becomes a squashed one in theLTa part and an approximate rectangle in the LTb part. Thesefacts suggest that the two adjacent hydrogen-bonded chainsundergo a mutual shift, which are accompanied by thecooperative displacements of hydrogen bonds. Consideringthe twisting motions of the fumaric acid molecules and thecollective molecular movements in the LT phase, it is clearlydisplayed that the conformations of hydrogen-bonded layersalong (001) and (010) planes are not crystallographicallyequivalent any more (Figure S10, Supporting Information).Such an interesting transformation has been found to yield twononequivalent frameworks in the entanglement, therebyinducing the symmetric operations of the 2-fold screw axisand c glide plane to disappear from the HT phase to the LTphase (Figure 8).

For further clarifying the origin of the phase transition, it isvery significant whether the order−disorder behavior isobserved or not through diffraction study. The thermalellipsoids maps of 1 in LT and HT phases are shown inFigure S11 (Supporting Information). Although the thermalellipsoids of O12, C1, and C9 in the HT phase are larger thanother atoms, there is no any special anomaly during the phasetransition, thus de-emphasizing the disordered character ofatoms. The thermal ellipsoids of all other atoms also have noanomalous behavior. The above-mentioned results account forthat the phase transition is not order−disorder, but displacive-type, corresponding well to the DSC results. Moreover, takinginto account the significant deuteration effect and the distinctrelative displacements of hydrogen bonds upon the transition in1, we postulate the phase transition mechanism as a hydrogen-boned displacive type. The current X-ray diffraction is difficultto reliably detect the sites of hydrogen atoms in crystals;therefore, the forthcoming neutron diffraction experiments aredesired to elucidate a minute mechanism of the phase transitionof 1.

Dielectric Behaviors. Generally, accompanying with thepresence of structural phase transition, dielectric, pyroelectric,and other physical properties often present sharp anomalies.However, there would be no or only weak dielectric anomaliesoccurring close to the ferroelastic phase transition point, for thereason that the order parameter becomes independent oftemperature.46,47 The temperature dependence of the real partof the complex relative dielectric permittivity (ε′) of 1 taken at5, 10, 100, and 1000 kHz is shown in Figure 9. As expected, nodiscernible dielectric anomaly was observed in the measuredfrequency range, probably because the structural changes aretoo gentle to yield the dielectric anomaly. Thus, given norecognizable dielectric anomaly and the symmetry breakingaccording to the crystallographic data of 1, it is clear that thephase transition should be ferroelastic instead of ferroelectric orantiferroelectric.

Figure 7. (a) Packing diagrams of hydrogen-bonded chains viewed down c axis and b axis, respectively, in 1 at 216 K. The dashed lines stand for thehydrogen bonds. (b) A schematic for displacements of hydrogen-bonded anionic chains from HT phase to LT phase in 1. The dashed lines stand fordiagonal lines, where the distances marked.

Figure 8. A schematic for the changes of the framework entanglementsfrom HT phase to LT phase in 1.

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■ CONCLUSIONSIn summary, we have presented a new entangled hydrogen-bonded supramolecular compound, [(n-C4H9)2NH2]2H2C4O4·H4C4O4, with a dibutylamine molecule as the base (A) andfumaric acid as the acid (D). It undergoes a reversible second-order ferroelastic phase transition at 228.8 K, confirmed by thecombined DSC, specific heat capacity, and variable-temperaturesingle-crystal structural analyses. Study of the deuteratedanalogue of 1 demonstrates that proton dynamic motions inhydrogen bonds contribute to the phase transition. The originof structural phase transition is also attributed to thecooperative displacements of hydrogen bonds, which resultfrom the twisting motions of the fumaric acid molecules. At thesame time, two kinds of independent hydrogen bonding layersin the entanglement are altered in response to the trans-formation of hydrogen bonds aggregates at the low temperaturephase, inducing the symmetry breaking. The present work willoffer a new avenue for the design of new ferroic materials withan entangled framework.

■ ASSOCIATED CONTENT*S Supporting InformationIR spectrum, XRD patterns, DSC curves at different scanningrates, SHG data, packing views of the crystal structures, thermalellipsoids maps, and tables with C−O bonds and hydrogen-bond geometries of 1. XRD patterns and DSC and Cp curves of2. CCDC reference numbers 1021230 (HT (250 K) phase)and 1021231 (LT (216 K) phase) for 1. This material isavailable free of charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: jhluo@fjirsm.ac.cn (J.L.).NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work was financially supported by the National NatureScience Foundation of China (21222102, 21373220, 51102231,21171166, and 21301171), the One Hundred Talents Programof the Chinese Academy of Sciences, the 973 Key Programs ofthe MOST (2010CB933501, 2011CB935904), and Key Project

of Fujian Province (2012H0045). Z.S. is thankful for thesupport from “Chunmiao Project” of Haixi Institute of ChineseAcademy of Sciences (CMZX-2013-002). We thank Lijian Wufor providing the 1H NMR measurements.

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Figure 9. Temperature-dependent dielectric constant of 1 at differentfrequencies.

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