Hydrogen-Bonded Displacive-Type Ferroelastic Phase Transition in

Article pubs.acs.org/crystal

Hydrogen-Bonded Displacive-Type Ferroelastic Phase Transition in a New Entangled Supramolecular Compound Yuanyuan 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, Chinese Academy 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 the removal and incorporation of guest molecules, but rarely generate phase transitions by themselves. In this study, we report a new entangled hydrogen-bonded supramolecular compound, [(n-C4H9)2NH2]2H2C4O4·H4C4O4 (1, H4C4O4 = fumaric acid), which undergoes a reversible ferroelastic phase transition with the Aizu notation of 2/mF1̅. Differential scanning calorimetry and specific heat measurements 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 phase transition. Variable-temperature single-crystal X-ray diffraction analyses reveal that the cooperative displacements of hydrogen bonds induce the structural phase transition, which arise from the twisting motions of the fumaric acid molecules. Simultaneously, two types of independent hydrogen bonding layers in the entanglement 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 entangled framework.

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transfers, or proton disordering.16−25 The hydrogen bond, an important interaction for binding different molecules, could be utilized to construct a supramolecular assembly of hydrogen donor (D) and acceptor (A) molecules using its highly directional nature. Nevertheless, hydrogen bonds are much weaker than covalent bonds binding atoms into molecules; thus, the hydrogen-bonded aggregates could readily undergo transformations to shear the structural network and lower the symmetry of the crystal structure. As is typically found in the case of a KH2PO4 (KDP) crystal, which is a representative ferroelectric, a site-to-site proton transfer over very short O− H···O hydrogen bonds causes the ferroelectric phase transition.19−24 More recently, Horiuchi et al. have proposed an intermolecular proton migration in the hydrogen bonds responsible for thermally induced phase transition in the cocrystal of 2,5-dihydroxy-p-benzoquinones with pyridine derivatives, formed by short hydrogen bonds (O···N distance 2.61−2.70 Å, N+···O− distance 2.54−2.55 Å).25 Namely, the

INTRODUCTION Ferroelastic materials have been key to the design and exploitation of multiferroic materials, which enable the manipulation of magnetic ordering and/or polarization ordering by an external stress through switching of the ferroelastic state, with a significant amount of applications such as piezoelectric sensors and mechanical switches.1−6 When shear stresses are applied to a ferroelastic material, in which a phase transition often happens between the paraelastic and ferroelastic phases, the material shows a highly nonlinear strain−stress curve called a hysteresis. As it is rather a difficult experimental undertaking to measure ferroelastic hysteresis with any acceptable degree of accuracy, it has become customary to term a material “ferroelastic” on condition that a phase transition which may generate ferroelasticity occurs (or may occur).5,6 Theoretically, such a phase transition should belong to the 94 species ferroelastic phase transitions defined by Aizu.7 Various approaches have been obtained to induce the phase transitions.8−15 Among them, the progress associated with transformations of the hydrogen-bonded aggregates is one of the most promising strategies, which derive from the breaking and formation of alternative hydrogen bonds, proton © 2014 American Chemical Society

Received: October 15, 2014 Revised: November 13, 2014 Published: November 14, 2014 457

dx.doi.org/10.1021/cg501529f | Cryst. Growth Des. 2015, 15, 457−464

Crystal Growth & Design

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

process exchanges two hydrogen-bond tautomers O−H···N and N+−H···O−, accompanied by rearrangement of the geometries of both π molecules, giving rise to a large spontaneous polarization and dielectric constant. As the characteristic dibasic acid (D), the fumaric acid can easily provide protons to form a variety of hydrogen-bonded networks in the compounds. However, up to now, reports on the fumarate compounds associated with phase transition have been very scarce. Herein, we introduce a new hydrogen-bonded compound [(n-C4H9)2NH2]2H2C4O4·H4C4O4, with a dibutylamine molecule as the base (A) and fumaric acid as the acid (D), which undergoes a reversible ferroelastic phase transition at 228.8 K (Tc). The origin of the structural phase transition is ascribed not only to the proton dynamics in the hydrogen bonds but also to the cooperative displacements of hydrogen bonds between two phases. In the present new compound, it is interesting to be found that two types of independent hydrogen bonding layers along different planes, formed by N−H···O and O−H···O hydrogen bonds, completely interpenetrated to establish an entangled supramolecular network, through the elongation of hydrogen bonds that lead to the intergrowth of one framework into the other. In the entanglement, the cooperative displacements of hydrogen bonds have been discovered to yield two crystallographically nonequivalent frameworks in the low temperature phase, resulting in the symmetry breaking. Another fascinating feature with respect to the entangled framework is the excellent flexibility,26−28 where chemically noninterconnected frameworks can show dynamic movement by the dislocation of their mutual positions, also easily inducing the structural phase transition. This feature is quite promising for finding new paradigms with various structure−property relationships, and for the design of new ferroic materials.

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Table 1. Crystal Data and Structure Refinement of 1 at 216 and 250 K sum formula formula weight temperature (K) crystal system space group a/Å b/Å c/Å α/deg β/deg γ/deg volume (Å3) Z Dcalcd, g cm−3 μ, cm−1 F(000) completeness (%) goodness-of-fit on F2 Tmin/Tmax R1 (on Fo2, I > 2σ(I))a wR2 (on Fo2, I > 2σ(I))a

EXPERIMENTAL SECTION

Synthesis. All chemical reagents were used without further purification. Compound 1 was prepared through reaction of dibutylamine and fumaric acid with a 1:1 molar ratio. The dibutylamine (2.61 g, 0.02 mol) was added into fumaric acid (2.32 g, 0.02 mol) in water (100 mL), and then the reaction mixture was stirred for an additional 20 min at room temperature. Crystals of 1 were obtained by slow evaporation of the synthesized solution at room temperature 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. Single crystals were grown by slow evaporation of a D2O solution of 2 at room temperature in the vacuum conditions after several days. 1H NMR spectra of 1 and 2 were obtained on a 400 MHz and reported in parts per million (δ) relative to the response of the solvent (DMSO) or to tetramethylsilane (0.00 ppm) (Figure S3, Supporting Information), from which the deuterated percentage of acidic hydrogens in 2 can be estimated to be about 45.2%. In the IR spectra of 1 (Figure S4, Supporting Information), the peaks at approximately 1687 and 1614 cm−1 are assigned to stretching vibration absorption of the carbonyl group (CO), which definitely reveals the existence of fumaric acid in 1. The phase purity of 1 and 2 is verified by the powder XRD (PXRD) patterns, which match very well with the pattern simulated 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 Super Nova CCD diffractometer with the graphite monochromated Mo-Kα radiation (λ = 0.71073 Å) at low temperature (216 K) and high temperature (250 K), respectively. The CrystalClear software package

a

C24H46N2O8 490.63 250(2) monoclinic C2/c 16.6386(9) 19.8615(10) 9.0962(5) 90 92.422(4) 90 3003.3(3) 8 1.085 0.80 1072.0 99.7 1.050 0.967/0.980 0.0656 0.1876

C24H46N2O8 490.63 216(2) triclinic P1̅ 9.0851(5) 12.8719(4) 12.9349(6) 100.258(3) 90.572(4) 93.606(4) 1485.17(12) 2 1.097 0.81 536.0 99.4 1.055 0.967/0.976 0.0780 0.2472

αR1 = ∑∥Fo| − |Fc∥/∑|Fo|, wR2 = [∑(|Fo|2 − |Fc|2)/∑|Fo|2]1/2.

DSC and Specific Heat Measurement. The DSC and specific heat measurements were carried out on a NETZCSCH DSC 200 F3 instrument by heating and cooling at a rate of 10 K/min in the temperature range from 110 to 270 K. These measurements were performed under a nitrogen atmosphere in aluminum crucibles. Dielectric and Second Harmonic Generation (SHG) Measurements. In the dielectric experiments, the single-crystal plates of 1 with silver pasted as the electrodes were used for measuring the complex dielectric permittivities, ε = ε′ − iε″. Its dielectric constants were measured using a TH2828A impedance analyzer at the respective frequencies of 5, 10, 100, and 1000 kHz with the measuring AC voltage fixed at 1 V. Powder SHG measurements were carried out by the Kurtz−Perry method. The measurements were performed using a Q-switched Nd:YAG laser at 1064 nm with an input pulse of 350 mV.

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RESULTS AND DISCUSSION Thermal Properties. The structural phase transition of 1 accompanied by thermodynamic anomalies was confirmed by DSC and specific heat measurements in the temperature range of 200−250 K (Figure 1). In the heating and cooling modes of the DSC curves, a pair of broad peaks are recorded with the endothermic peak at 228.8 K (Tc) on heating and an exothermic one at 224.4 K upon cooling (Figure 1a). In order to certify the second-order phase transition, DSC 458



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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 200 to 270 K for 1.

1c). It is notable that the λ shape of the anomaly peak most likely resembles the features of a second-order phase transition, like that of triglycine sulfate (TGS).35 The entropy change ΔS is 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 the gas constant and N is the ratio of the numbers of possible configurations, N = 1.11 is obtained. The N value, which is closer to 1, suggests that the phase transition is not an order−

measurements under different cooling/heating rates were performed (Figure S6, Supporting Information). The limiting thermal hysteresis (0.1 K) estimated from the scans extrapolated 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 the presence of phase transition and shows a typical anomaly at around 229 K, corresponding well to the DSC results (Figure 459



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disorder mechanism but rather a displacive mechanism, well consistent with the crystal structure analyses below.36−38 To investigate whether proton dynamic motion in the compound occurs, the deuteration effect was determined by DSC and specific heat measurements. In comparison, the deuterated analogue 2 also shows a heat anomaly, appearing at approximately 216.7 K upon cooling and 221.9 K upon heating, with a distinct change of 7 K, indicating that the proton dynamic motions in the hydrogen bond are responsible for the structural phase transition (Figure 1b,d).39−41 Structure Discussion. In order to understand the phase transition more clearly, single-crystal X-ray structure determinations of 1 were performed at 216 K (low-temperature, LT) and 250 K (high-temperature, HT), respectively. The crystal structure of the ionic [(n-C4H9)2NH2]2H2C4O4·H4C4O4 crystal is primarily established by the electrostatic cation−anion and hydrogen-bonding interactions. At the HT phase, 1 crystallizes in 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. The asymmetric unit contains one-half of di-ionic fumaric acid, onehalf of un-ionized fumaric acid, and one dibutylammonium cation (DBA = dibutylammonium) (Figure 2a). The structure of 1 in the LT phase is triclinic with a space group of P1̅, and cell 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 cell parameters of 1 show a great change between the two phases, in which the cell volume is halved at the LT phase in comparison to that at the HT phase. The components of its asymmetric cell unit are doubled that of the HT phase, in good agreement with the gradually decreasing structural symmetry triggered by the phase 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 the anisotropic lattice parameter variations around Tc are clearly evident, confirming the structural changes during the phase transition. The relationship between the two temperature cells is that aHT corresponds to cLT, bHT corresponds to bLT, and cHT corresponds to aLT. The lattice constant aLT shows no discernible anomalies compared with cHT, but bLT and cLT expand by about 53.9% and 27.9% with an inflection point in the vicinity of Tc, respectively. At the same time, the monoclinic symmetry is reduced to triclinic, which is characterized by the changes of α angle and γ angle, from 90° in the HT phase to about 100.2° and 93.5°, respectively, in the LT phase. Moreover, the great change occurred in cell volume, which presents 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 structural phase transition. From the viewpoint of symmetry breaking, the structure of 1 transforms from the monoclinic crystal system with a high centrosymmetric space group of C2/c and the point group of C2h at 250 K to the triclinic crystal system with a low centrosymmetric space group of P1̅ and the point group of Ci at 216 K. Namely, a symmetry breaking phenomenon occurs during the transition from the HT phase to the LT phase with an Aizu notation of 2/mF1̅.42 Symmetric elements decrease by half from four (E, i, C2, σh) to two (E and i), in strict accordance with Landau phase transition theory. The spatial symmetric operations change shown in Figure 4 unambiguously indicates that the symmetric elements (2 and m) disappear in 1

Figure 4. Transformation of space group of 1 from the HT paraelastic phase (C2/c, No. 15) to an LT ferroelastic phase (P1̅, No. 2).

in the LT phase, which might be aroused by slight displacements of the atoms. It is in good agreement with the Curie symmetry principle that the ferroelastic space group P1̅ is a subgroup of the paraelastic one C2/c, whose maximal nonisomorphic subgroups include Cc, C2, and P1̅. It can be seen that the inversion center i remains in both phases, which is also confirmed by the temperature-dependent second harmonic generation (SHG) (Figure S7, Supporting Information). There is almost no signal in the temperature range of 200−280 K, indicating that the structure of 1 might turn from a centrosymmetric structure to another centrosymmetric one, which is consistent with X-ray single-crystal structure analyses.43−45 The structures of 1 in the HT and LT phases are both constructed by an extensive hydrogen bonding network with O−H···O and N−H···O hydrogen bonds, which play a key role in the emergence of phase transition. In the packing structure of 1 at the HT phase, strong O−H···O hydrogen bonds between the terminal COOH of fumaric acid units and COO− groups of the alternating fumarate anions form a onedimensional zigzag infinite chain in the (1̅10) plane, with an O···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 interlinked via rich N−H···O hydrogen bonds provided by the DBA cations. The rows of alternating cations and anions connected by N−H···O hydrogen bonds are elongated along the c-axis, building the two-dimensional hydrogen bonding layers along the (1̅10) plane, and there are hydrogen bonding layers along the (110) plane crystallographically equivalent to them by the crystal symmetries of the 2-fold screw or c glide plane (Figure S8, Supporting Information). At the same time, the combination of N−H···O and O−H···O hydrogen bonds in the same layer forms R68 (36) ring motifs, each of which consists of two DBA cations and six fumaric acids, including four deprotonated fumarate anions and two fumaric acid molecules, resulting in the occurrence of big holes (Figure S9, Supporting Information). Interestingly, two kinds of chemically noninterconnected hydrogen bonding layers along different planes are found completely interpenetrated through the holes to establish a three-dimensional entangled supramolecular network (Figure 5b). The N−H···O and O−H···O hydrogen bonding interactions and the interpenetration are responsible 460



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

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

(deg)

250 K

(deg)

O13C11C10C10 O30C28C27C27 O16C14C15C15 O33C32C31C31

11.038(11) 14.003(10) 7.348(11) 7.424(11)

O13C11C10C10

11.771(5)

O16C14C15C15

6.121(5)

for stabilizing the overall three-dimensional architecture and topology of the hydrogen-bonded framework. With the temperature decreasing from the HT phase to the LT phase, little deformation of fumaric acid molecules is observed during the phase transition. That is, the slight twisting motions of the fumaric acid molecules are confirmed that the torsion angles of O13−C11−C10−C10 and O30−C28−C27− C27 being 11.038(11)° and 14.003(10)°, respectively, exhibit a small difference from the corresponding torsion angle of O13− C11−C10−C10 (11.771(5)°) in the HT phase (Table 2). The hydrogen bond geometries are approximate to those in the HT phase. However, it is notable that the distances between the donor and acceptor atoms for the O−H···O and N−H···O hydrogen bonds show obvious differences in two phases, resulting from the twisting motions of the fumaric acid molecules (Table S2). Furthermore, the two-dimensional hydrogen bonding layers along (001) and (010) planes, which are symmetry-equivalent in the HT phase, emerge with two different conformations labeled as LTa and LTb based on the relative different distances of hydrogen bonds and the slight molecular movement, as illustrated in Figures 6 and 7. In comparison, the O···O distance of the O−H···O hydrogen bond in the LTb part (2.508(3) Å) is a little longer than that in the HT phase, but it is shorter in the LTa part (2.476(3) Å) (Figure 6). It is interestingly found that, similar to the differences of O···O distances, the changes of N···O distances for N−H···O hydrogen bonds compared to those in the HT phase are absolutely opposite in the LTa and LTb parts. For instance, the N···O distance of the N5−H5A···O16 hydrogen bond in the HT phase is 3.129(3) Å, whereas the corresponding values turn into 3.007(3) Å in the LTa part and 3.208(3) Å in the LTb part, the intensities of hydrogen bond interactions varying greatly. It is proposed that the representative displacements of these hydrogen bonds accom-

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

pany large distortions of the atomic coordinates, which may be the driving force of the structural phase transition. It is noteworthy that the cooperative displacements of hydrogen bonds, which could be deduced from the stretch and shrinkage of O−H···O and N−H···O hydrogen bonds in the LT phase, give rise to the collective molecular movements. As shown in Figures 5a and 7, the distances of O12−O12a/O29− O29a or O12−O12b/O29−O29b in both phases are nearly 461



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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 the hydrogen bonds. (b) A schematic for displacements of hydrogen-bonded anionic chains from HT phase to LT phase in 1. The dashed lines stand for diagonal lines, where the distances marked.

identical (about 12.9 and 9.1 Å, respectively), while the diagonal distances of O12−O12c and O12a−O12b convert from 15.626/16.030 Å (HT) to 15.281/16.215 Å (LTa) and 15.732/15.881 Å (LTb), respectively. In addition, the angle of O12b−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 the corresponding 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 the LTa part and an approximate rectangle in the LTb part. These facts suggest that the two adjacent hydrogen-bonded chains undergo a mutual shift, which are accompanied by the cooperative displacements of hydrogen bonds. Considering the twisting motions of the fumaric acid molecules and the collective molecular movements in the LT phase, it is clearly displayed that the conformations of hydrogen-bonded layers along (001) and (010) planes are not crystallographically equivalent any more (Figure S10, Supporting Information). Such an interesting transformation has been found to yield two nonequivalent frameworks in the entanglement, thereby inducing the symmetric operations of the 2-fold screw axis and c glide plane to disappear from the HT phase to the LT phase (Figure 8).

For further clarifying the origin of the phase transition, it is very significant whether the order−disorder behavior is observed or not through diffraction study. The thermal ellipsoids maps of 1 in LT and HT phases are shown in Figure S11 (Supporting Information). Although the thermal ellipsoids of O12, C1, and C9 in the HT phase are larger than other atoms, there is no any special anomaly during the phase transition, thus de-emphasizing the disordered character of atoms. The thermal ellipsoids of all other atoms also have no anomalous behavior. The above-mentioned results account for that the phase transition is not order−disorder, but displacivetype, corresponding well to the DSC results. Moreover, taking into account the significant deuteration effect and the distinct relative displacements of hydrogen bonds upon the transition in 1, we postulate the phase transition mechanism as a hydrogenboned displacive type. The current X-ray diffraction is difficult to reliably detect the sites of hydrogen atoms in crystals; therefore, the forthcoming neutron diffraction experiments are desired to elucidate a minute mechanism of the phase transition of 1. Dielectric Behaviors. Generally, accompanying with the presence of structural phase transition, dielectric, pyroelectric, and other physical properties often present sharp anomalies. However, there would be no or only weak dielectric anomalies occurring close to the ferroelastic phase transition point, for the reason that the order parameter becomes independent of temperature.46,47 The temperature dependence of the real part of the complex relative dielectric permittivity (ε′) of 1 taken at 5, 10, 100, and 1000 kHz is shown in Figure 9. As expected, no discernible dielectric anomaly was observed in the measured frequency range, probably because the structural changes are too gentle to yield the dielectric anomaly. Thus, given no recognizable dielectric anomaly and the symmetry breaking according to the crystallographic data of 1, it is clear that the phase transition should be ferroelastic instead of ferroelectric or antiferroelectric.

Figure 8. A schematic for the changes of the framework entanglements from HT phase to LT phase in 1. 462



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

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Figure 9. Temperature-dependent dielectric constant of 1 at different frequencies.

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CONCLUSIONS In summary, we have presented a new entangled hydrogenbonded supramolecular compound, [(n-C4H9)2NH2]2H2C4O4· H4C4O4, with a dibutylamine molecule as the base (A) and fumaric acid as the acid (D). It undergoes a reversible secondorder ferroelastic phase transition at 228.8 K, confirmed by the combined DSC, specific heat capacity, and variable-temperature single-crystal structural analyses. Study of the deuterated analogue of 1 demonstrates that proton dynamic motions in hydrogen bonds contribute to the phase transition. The origin of structural phase transition is also attributed to the cooperative displacements of hydrogen bonds, which result from the twisting motions of the fumaric acid molecules. At the same time, two kinds of independent hydrogen bonding layers in the entanglement are altered in response to the transformation of hydrogen bonds aggregates at the low temperature phase, inducing the symmetry breaking. The present work will offer a new avenue for the design of new ferroic materials with an entangled framework.

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ASSOCIATED CONTENT

* Supporting Information S

IR spectrum, XRD patterns, DSC curves at different scanning rates, SHG data, packing views of the crystal structures, thermal ellipsoids maps, and tables with C−O bonds and hydrogenbond geometries of 1. XRD patterns and DSC and Cp curves of 2. CCDC reference numbers 1021230 (HT (250 K) phase) and 1021231 (LT (216 K) phase) for 1. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (J.L.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Nature Science Foundation of China (21222102, 21373220, 51102231, 21171166, and 21301171), the One Hundred Talents Program of the Chinese Academy of Sciences, the 973 Key Programs of the MOST (2010CB933501, 2011CB935904), and Key Project 463



Article

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