Unusual Long-Range Ordering Incommensurate Structural

Publication Date (Web): October 16, 2017. Copyright ... Here, we present that successive ordering of dynamic dipoles in an organic molecular ferroelec...
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Cite This: J. Am. Chem. Soc. 2017, 139, 15900-15906

Unusual Long-Range Ordering Incommensurate Structural Modulations in an Organic Molecular Ferroelectric Zhihua Sun,†,§ Jian Li,‡,§ Chengmin Ji,† Junliang Sun,*,‡ Maochun Hong,† and Junhua Luo*,† †

State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China ‡ College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China S Supporting Information *

ABSTRACT: The incommensurate (IC) behaviors of ferroelectrics have been widely investigated in inorganic oxides as an exciting branch for aperiodic materials, whereas it still remains a great challenge to achieve such intriguing effects in organic systems. Here, we present that successive ordering of dynamic dipoles in an organic molecular ferroelectric, N-isopropylbenzylaminium trichloroacetate (1), enables unusual incommensurately modulated structures between its paraelectric phase and ferroelectric phase. In particular, 1 exhibits three distinct IC states coupling with a long-range ordering modulation. That is, the incommensurately modulated lattice is ∼7 times as large as its periodic prototype, and the IC structure is well solved using a (3 + 1)D superspace group with the modulated wavevector q = (0, 0, 0.1589). To the best of our knowledge, 1 is the first organic ferroelectric showing such a long-range ordering IC structural modulation. In addition, structural analyses reveal that slowing down dynamic motions of anionic moieties accounts for its modulation behaviors, which also results in dramatic reorientation of dipolar moments and concrete ferroelectric polarization of 1 (∼0.65 μC/cm2). The combination of unique IC structural modulations and ferroelectricity makes 1 a potential candidate for the assembly of an artificially modulated lattice, which will allow for a deep understanding of the underlying chemistry and physics of aperiodic materials.

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INTRODUCTION Ferroelectric materials, which show switchable spontaneous polarization (Ps), have been widely used as the basic elements for electric−optical devices, nonlinear optical switches, and sensors.1−4 Generally, the emergence of ferroelectricity is inseparable from phase transitions, changing from a highsymmetry paraelectric phase (PEP) to a low-symmetry ferroelectric phase (FEP).5 An exceptional case, however, is the incommensurate (IC) lattice of the periodic distortions, which has been an important topic in condensed matter science.6 For instance, the long-range IC charge fluctuation was found to exert a significant influence on the superconductivity of (Y,Nd)Ba2Cu3O6+x.7 Particularly, because of the irrational vectors, the incommensurately modulated ferroelectrics exhibit a three-dimensional long-range order but lack universal translation of the lattice periodic symmetry.8 In terms of this unique concept, IC ferroelectric materials have been proven to be promising candidates for the design of artificially modulated lattices.9 Scientists have achieved the modulated structures in a few ferroelectric oxides, such as Ba 1−x Ca x Nb 2 O 6 , 10 Bi2Mn4/3Ni2/3O6,11 and YMnO3.12 More recently, a relaxor ferroelectric oxide of PbBiNb5O15 was reported to show quite strong IC structural modulation.13 Despite some IC ferroelectric oxides, most inorganic materials require high-temperature syntheses and even contain environmentally poisonous metals (e.g., lead). This becomes a big hindrance for further © 2017 American Chemical Society

development of optoelectronic devices based on IC ferroelectrics. Alternatively, molecular ferroelectrics have emerged as competing candidates, showing behaviors comparable to those of BaTiO3, including large Ps and high Curie temperature (Tc).14 Because of the structural diversity and flexibility, organic ferroelectrics have been developed as the key materials in masslight, printable, and bendable electronic devices. However, reports on the organic IC ferroelectrics are quite sparce,15 owing to the lack of knowledge regarding control of the motions of dipole moments between periodic lattices. In this context, it is a great opportunity to design the incommensurately modulated structures in the pure organic ferroelectric systems. Structurally, the appearance of IC modulation shows up in the system that contains the periodic lattices with different ranges but similar magnitudes.16 For ferroelectrics, the driving source to IC-modulated structures is the competition between ferroelectric lattice and paraelectric prototype, which is closely related to the positional freedom of molecular dipoles. From the viewpoint of phase transition kinetics, this requires that atomic disordering or reorientation undergoes a successive deviation from the prototypic lattices (i.e., FEP or PEP); that is, the IC structure originates from the distortion of one periodic Received: August 22, 2017 Published: October 16, 2017 15900

DOI: 10.1021/jacs.7b08950 J. Am. Chem. Soc. 2017, 139, 15900−15906

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SHG intensities were compared with that of KH2PO4 (i.e., KDP), for which the SHG coefficient χ(2) is ∼0.39 pm/V.19 Single-Crystal X-ray Crystallography. Data of single-crystal diffraction were collected on an Agilent Technologies SuperNova dual wavelength CCD diffractometer with Mo Kα radiation (λ = 0.71073 Å) at different temperatures (100, 140, 150, 190, 240, and 290 K). Data reduction and multiscan absorption correction were performed by the CrysAlisPro software.20 Crystal structures except for incommensurately modulated structures were solved with direct method and refined by the SHELX software.21 The non-hydrogen atoms were refined in the anisotropic modes, with positions of H atoms being generated geometrically. Moreover, the crystal structures at IC phase (ICP) were determined with the charge-flipping algorithm using the program SUPERFLIP22 and refined against F2 by the program JANA2006.23 VESTA was used as a visualization of Fourier electron density.24 Crystal data and structural information for 1 are listed in Tables S1−S3, and CCDC 999204−999208 contain the crystallographic data for this paper.

lattice. Thus, a delicate balance must be achieved between the dynamic branch and static remaining (not strictly). The former affords a driving force to break through the energy barrier of phase transitions, while the later provides cooperation to preserve its preliminary crystallographic feature. As shown in the IC ferroelectric of NaNO2, the ordered orientation of NO2− groups inside the lattice leads to its sinusoidally modulated structure between 163 and 166 °C.17 Nevertheless, it is challenging to achieve such intriguing effects in organic ferroelectrics due to their more complicated structures. Here, we designed a new molecular ferroelectric that shows diverse incommensurately modulated structures, N-isopropylbenzylaminium trichloroacetate (1), by introducing the constrained disordered moiety into a H-bonding system. In addition to notable ferroelectric properties, 1 displays three distinct IC states between its PEP and FEP. Particularly, an unusual long-range ordered IC periodicity of ∼7 times as large as its prototype lattice is established from the diffuse satellite reflections. At 150 K, the satellite reflections have to be indexed with four integers as H = ha* + kb* + lc* + mq with a modulated vector of q = (0, 0, 0.1589) and m ≠ 0. The dynamic anions account for such incommensurately modulated structures as well as the long-range ferroelectric order.18 To the best of our knowledge, 1 is the first molecular ferroelectric with such diverse IC structural modulations. The combined ferroelectricity and IC properties make 1 a potential candidate for the artificially modulated lattices, which also allows for a deep understanding of the underlying concepts of chemistry and physics in aperiodic materials.

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RESULTS AND DISCUSSION It is well-known that most ferroelectric materials are inseparable from the symmetry breaking phase transitions, which result from quite subtle structure changes. For 1, the preliminary DSC and Cp−T measurements clearly disclose that it undergoes successive phase transitions (Figure 1). As shown in the DSC

EXPERIMENTAL SECTION

Synthesis. Raw materials of 1 were synthesized by the reaction of equivalent mole ratio of trichloroacetic acid and N-isopropylbenzylamine. Block crystals were grown from the aqueous solutions by the temperature lowering method (in Figure S1), and its bulk purity was verified by powder X-ray diffraction (PXRD, Figure S2) and elemental analysis (C, H, and N). Anal. Calcd for C12H16Cl3NO2 (%): C, 46.10; H, 5.16; N, 4.48. Found (%): C, 46.05; H, 5.12; N, 4.43. Measurement Methods. The PXRD patterns were collected using the MiniFlex II X-ray diffractometer. The differential scanning calorimetry (DSC) experiment was carried out on a NETZSCH DSC 200 F3 in the temperature range of 100−300 K with a heating/cooling rate of ∼10 K/min, and the specific heat (Cp) studies were performed on the physical property measurement system (model 6000, Quantum Design USA). In electric experiments, single crystals covered by the silver conduction paste on the surfaces were used as electrodes. The complex dielectric permittivities (ε = ε′ − iε″, where ε′ and ε″ are the real part and imaginary part, respectively) were measured by the twoprobe AC impedance method, using an impedance analyzer (TH2828A) under an applied electric field of 0.5 V. Pyroelectric currents were measured along the b-axis of single crystals by using a high resistance meter/electrometer (Keithley 6517B), in which the released currents were recorded as a function of temperature. Thus, temperature-dependent polarization can be obtained by integrating the pyroelectric currents with respect to time. Polarization hysteresis loops (P−E loops) were measured with applied electric field parallel to its baxis direction, using the Sawyer−Tower circuit method (Radiant Precision Premier II). Polycrystalline samples of 1 with particle sizes of 72−100 μm were adopted to measure its temperature-dependent second harmonic generation (SHG) effects. The fundamental light was generated from a Nd:YAG laser (λ = 1064 nm, the pulse duration is 5 ns, and peak power is ∼1.6 MW), and the SHG signals were collected using a fluorescence spectrometer (FLS 920, Edinburgh Instruments), equipped with a variable-temperature system (DE202, 120-190 K).

Figure 1. DSC and the Cp−T traces of 1.

traces, two pairs of exothermal peaks are observed at ∼150.3/ 146.1 and ∼159.3/155.6 K in the heating/cooling runs. In addition, the Cp−T curve also displays two thermal anomalies at 149.9 and 159.1 K upon heating, which coincide with DSC results. For convenience, the phase transition temperatures at 146.1 and 155.6 K (in the cooling run) were labeled as Tc and Ti, respectively. To deeply understand the origin of phase transitions, we determined crystal structures of 1 at different temperatures (Table S1, Supporting Information). Structure analyses reveal that 1 crystallizes in the orthorhombic space group of Pbca (the point group of mmm) above Ti but transforms to Pb21a (the point group of mm2) below Tc. From the viewpoint of symmetry breaking, an Aizu notation of mmmFmm2 confirms that the phase transition of 1 belongs to the ferroelectric type.25 Between Tc and Ti, some satellite reflections around the main reflections with strong intensities are clearly observed from the single-crystal X-ray diffraction patterns, reminiscent of the possible IC structure modulations (as discussed below). In this context, we denote the phase above Ti as PEP, the phase between Ti and Tc as incommensurate phase (ICP), and the phase below Tc as FEP (as shown in Figure 1). 15901

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prototype of Pbca changes to a polar space group Pb21a, reminiscent of possible ferroelectricity in 1. What makes 1 more intersting is the emergence of ICP between Tc and Ti. As shown in Figure 3, at 100 and 240 K, the

At PEP (T = 240 K), the basic unit of 1 is composed of the H-bonding dimers, in which anions and cations are linked by notable N−H···O hydrogen bonds with the donor−acceptor distances of 2.794 and 3.185 Å (in Figure 2 and Table S3).

Figure 3. (0hk) Plane diffraction patterns for 1 at 100 K (a,d), 150 K (b,e), and 240 K (c,f). All the main reflections at 150 K possess the same forbidden reflection law and diffraction intensities with those at PEP.

Figure 2. Perspective views of 1 at (a) PEP and (b) FEP. At PEP, its asymmetric unit consists of one H-bonding dimer, and the anion is highly disordered. In comparison, two H-bonding dimers with slightly distinct steric configurations are included, and all the anions become totally ordered at FEP.

main reflections are identified by the (hkl) indices and conform to the universal translation of long-range lattice periodic symmetry. In contrast, quite strong satellite reflections are clearly observed around the main diffraction peaks at 150 K (Figure 3e). Viewed from the reciprocal lattice of diffraction patterns (Figure 3b), the linear distibution of satellite reflections reveals the first-order modulation along its c*-axis, whereas no modulated vectors can be observed along a*- and b*-axes. Therefore, these diffraction patterns can be indexed with four parameters as H = ha* + kb* + lc* + mq with q = (0, 0, 0.1589) and m ≠ 0. As the q vector is not an irrational value, this unique feature confirms that the crystal structure of 1 is to be incommensurately modulated.16 Especially, all the reflection conditions follow the rule of (h0lm: h + m = 2n; 0klm: l = 2n; hk00: k = 2n), which suggests its modulated lattice should be Pbca(00g)0s0, similar to the paraelectric phase of Pbca. This centrosymmetric feature agrees well with the absence of pyroelectric and quadratic nonlinear optical properties at 150 K. At 148 K, the main diffractions transform to an acentric basic cell, showing an average superstructure of Pb21a(00g)s00. However, it fails to obtain the accurate modulated structure model by refining the crystal data, and we could only obtain an average structure from the main diffractions (Figure S6, Supporting Information). As the temperature further decreases to 146 K, the intensities of satellite reflections become much weaker, and the main diffractions still adopt an acentric basic cell, close to its ferroelectric phase of Pb21a at 100 K (Figure S6d). Hence, it is proposed that 1 shows an unusual multilevel modulation of IC structures, including centrosymmetric ICP (150 K), intermediate ICP (148 K), and acentric ICP (146 K). This result discloses that the modulated structures in 1 are gradually inhibited. In detail, the stable ICP at 150 K adopts a centrosymmetric supergroup of Pbca(00g)0s0, whereas the unstable IC states at 146 and 148 K exhibit polar characteristics. In this regard, we can conclude that the polarization should totally disappear at Ti. As far as we are aware, 1 should be the first example of organic molecular ferroelectrics showing such

Interestingly, the trichloromethyl moiety adopts a distorted ditetrahedron geometry. This positional disordering is also confirmed by their relatively elongated thermal ellipsoids (Figure S4). In contrast, the atoms of organic cations are solved at the exclusive positions without any disordering, which behave as the static part. As a result, the sterically chemical configuration of 1 could be vividly described as a rotator−stator assembly via intermolecular N−H···O hydrogen bonds, as shown in Figure 2. It is known that molecular motions in the rotator−stator systems have succeeded in the assembly of ferroelectrics, such as 4-methoxyanilinium tetrafluoroborate 18crown-6,26 in which motional behavior of rotators gives rise to the ferroelectric properties. It should be expected that dynamics of the rotator in 1 will also enable the potential ferroelectric responses. As temperature decreases below Tc, the ordering of dynamic anions results in the breakdown of its prototypic symmetry. At FEP (T = 100 K), the asymmetric unit of 1 is doubled compared to that of PEP, containing two H-bonding dimers with slightly distinct steric configuration (named A and B, Figure 2). This is solidly identified from the differences of donor−acceptor distances for N−H···O H-bonds (i.e., 2.777/ 3.210 Å for the dimer A and 2.856/3.059 Å for B, in Tables S2 and S3). Another distinguishable change is that the anionic components become ordered, with all the Cl atoms located at definite positions. The trichloromethyl group shows an ideal tetrahedron symmetry with almost equivalent C−Cl bond lengths (∼1.76 Å) and Cl−C−Cl bond angles (∼107°). The cationic configuration displays quite small alterations; that is, the isopropylaminium group deviates farther away from the benzene ring plane and thus compacts the space for N−H···O hydrogen bonds. This will enhance energy barriers to activate the possible dynamic motions of rotators, favoring an ordered arrangement of molecular dipoles.27 Hence, the frozen ordering of such dynamic components affords a driving force to its symmetry breaking (Figure S5); that is, the paraelectric 15902

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Journal of the American Chemical Society rich and distinct IC structural modulations, which suggests that 1 has a strong tendency to be modulated. Moreover, we adopt the (3 + 1)-dimensional superspace group directly with the charge-flipping algorithm to solve its exact IC structure. The details of its superspace group determination are presented in Supporting Information. The refined structure model of ICP has a small residual electron density (the largest peak is ∼0.37 e/Å3), along with very low R values for the main diffractions and first-order satellite reflections (I > 3σ). Figure 4 depicts the approximate

Figure 4. Superlattice approximants of 1 at 150 K. (a,b) 1a × 1b × 7c modulated approximant along its a- and c-axes. (c,d) Reasonable IC superstructure with 1a × 1b × 7c modulated approximant along its aand c-axes, which cutoff the atoms (Cl1b, Cl2b, Cl3b and Cl4b, Cl5b, Cl6b) to where occupancies are below 0.5. The diameter of Cl atoms was enlarged in order to clearly show the modulation function of occupancy.

Figure 5. Dielectric properties of 1 measured on the single crystals along the b-axis direction. (a) Temperature dependence of the dielectric constant (ε′). Inset: Reciprocal of dielectric constants versus temperature, which obeys Curie−Weiss law in the vicinity of Tc. (b) Comparison of dielectric constants for 1 and NaNO2, of which like anomalies confirm their IC behaviors.

superstructures of 1 at 150 K, of which the asymmetric unit is greatly expanded in comparison to that of PEP and FEP. In its superstructure, the modulated cell length of c-axis is almost ∼7 times as large as that of its periodic prototype, whereas the a- and b-axes remain unchanged. This feature suggests that the IC structural modulation of 1 is well developed with long-range ordered modulations, coincident with the observation of satellite reflections. Structurally, this behavior is closely related to the positional and occupational modulations induced by the disordering of trichloroacetate anions. As shown in Figure 4a, highly disordered anions were initially modeled according to all the reflections. A more reasonable mode was finally determined by cutting chlorine atoms with the occupancies below 0.5 (in Figure 4b). Obviously, the large molecular freedom of trichloroacetate groups affords abundant possibilities to be relaxed in an energetically favorable state, which might promote its long-range ordering modulations. Similar molecular motions have been observed in other molecular IC ferroelectrics, such as trichloroacetamide.28 Based on this study, it is deduced that the unique structural modulations of 1 are driven and dominated by the occupational and positional disordering of anions. In detail, for this rotator−stator assembly, anionic moieties are highly disordered at PEP, acting as dynamic rotor-like units. At ICP, the rotors still preserve disordered feature but have an ordered occupancy, leading to the incommensurately modulated structures of 1 (see eqs 9−11, Supporting Information). Upon further cooling below Tc, thermal motions of dynamic parts are totally frozen into an ordered state, corresponding to its FEP. Microscopically, the frozen ordering of dynamic molecular motions during phase transitions would induce the notable dielectric response and ferroelectric polarization. As shown in Figure 5, temperature-dependent dielectric constants of 1

display sharp anomalies around Ti and Tc, coinciding well with its PEP−ICP−FEP phase transitions. In the vicinity of the maximum peak position at Tc, the ε′−T traces display sharp changes and clear frequency-dependent responses. The ε′ values are much larger than those in both higher and lower temperature regions, which obey the Curie−Weiss law of 1/ε′ = (T − T0)/C. The fitting curve affords the Curie constant C of ∼148 K (f = 1 MHz, inset in Figure 5a), falling around those of other molecular ferroelectrics, such as [NH2CH2COOH]2HNO3, RbHSO4,29 and bis(imidazolium) 30 L-tartrate. In particular, it is noteworthy that the shoulder-like dielectric anomalies are also observed around Ti, suggestive of its ICP-to-PEP phase transition. As shown in NaNO2 (inset in Figure 5b), this shoulder-like dielectric anomaly has evidenced its IC properties.17 For 1, the emergence of successive dielectric changes agrees fairly well with its thermal and structural analyses, which also confirms 1 is a ferroelectric with IC behaviors. In addition to remarkable dielectric anomalies, ferroelectric materials display switchable spontaneous polarizations induced by long-range ordering of molecular dipoles.32 Here, we performed the measurement of Ps versus electric field (E) on the crystals of 1. At 160 K (above Ti), the dependence of polarization to the applied field is linear, which reveals the paraelectric properties of 1 (Figure S8). As temperature decreases in the range of Ti and Tc, it fails to record clear hysteresis loops in this narrow temperature range. However, upon cooling below Tc, the traces become broader with a curved tail and grow into the typical ferroelectric loops (at ∼145 K), as illustrated in Figure 5. Such nonlinear P−E loops are the characteristic behaviors for ferroelectrics. The better hysteresis loops can be obtained by further decreasing the 15903

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Journal of the American Chemical Society temperature from its Tc. At 135 K, the rectangle-like hysteresis loop is recorded, like that of triglycine sulfate (Figure S9). The saturated polarization Ps is estimated to be ∼0.65 μC/cm2, and the remanent polarization (Pr) is ∼0.63 μC/cm2. This Ps figure is fairly consistent with the result simulated using Landau theory (Figure 6b) and slightly larger than that of some other

Figure 7. (a) Temperature dependence of Ps obtained by integrating pyroelectric currents in the positive (green) and negative (brown) poling voltage. (b) Temperature-dependent SHG effects for 1. Inset: SHG intensities vs the wavelength at different temperatures.

be another solid indicator for the ferroelectric phase transition in 1.35 It is clear that positional and occupational disorder of dynamic moieties dominate long-range ordered IC modulations of 1, as well as its ferroelectricity. To understand the origin of intrinsic polarization, structure analyses for collective alignment of molecular dipole moments are illustrated in Figure 8. Above Tc, the disordered anions adopt a mirror symmetry, and thus the dipole moment is canceled out (i.e., μs = 0), corresponding to its PEP. However, such IC structural modulations are strongly dependent on thermal motions of dynamic parts, which become fully ordered below Tc. All the anionic moieties

Figure 6. Ferroelectric properties of 1. (a) P−E hysteresis loops measured along the b-axis direction at different temperatures ( f = 30 Hz). (b) Experimental and theoretical P−E loops simulated by using the expression of Landau theory, i.e., 1 1 G = 2 α0(T − Tc)Ps2 + 4 βPs4 − EP .31

ferroelectrics, such as Rochelle salt (∼0.2 μC/cm2), guanidinium aluminum sulfate hexahydrate (∼0.35 μC/cm2), trichloroacetamide (∼0.2 μC/cm2), and ammonium sulfate (∼0.25 μC/cm2).33 The related coercive electric field (Ec ∼ 4.8 kV/ cm) is also comparable with that of other molecular ferroelectrics (Table S4). Ferroelectric materials are a subgroup of pyroelectrics, of which the polarization is sensitive to temperature and pyroelectric current can be generated under the external thermal stimuli.34 Temperature-dependent polarization and pyroelectric current of 1 are depicted in Figure 7a. Upon heating, the significant current peak was produced in the vicinity of Tc, of which the direction can be easily reversed by altering the signs of external electric field (inset in Figure 7a). Temperature-dependent polarizations obtained by integrating pyroelectric current shows that the Ps value for 1 is ∼0.65 μC/ cm2 at 135 K, which coincides with the experimental result measured by P−E hysteresis loops. The direction reversal of pyroelectric current and polarization confirms ferroelectric properties for 1. Moreover, the SHG effect of 1 is also strongly dependent on its structures; that is, SHG signals emerge at FEP with the χ(2) value of 0.52 pm/V but fully disappear at PEP and ICP (in Figure 7b). The tendency of SHG effects versus temperature is consistent with that of Ps, following the Landau theory equation of χ(2) = 6ε0βPs, where ε0 and β are temperature-independent constants (the formula derivation is shown in Supporting Information). This result is considered to

Figure 8. Diagram for the generation of molecular dipole moments (μs) and Ps in 1. (a) Origin of μs. (b) Generation of Ps along its b-axis, as indicated by the red arrows. 15904

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(5) (a) Shi, P.-P.; Tang, Y.-Y.; Li, P.-F.; Liao, W.-Q.; Wang, Z.-X.; Ye, Q.; Xiong, R.-G. Chem. Soc. Rev. 2016, 45, 3811. (b) Xu, W.-J.; Li, P.F.; Tang, Y.-Y.; Zhang, W.-X.; Xiong, R.-G.; Chen, X.-M. J. Am. Chem. Soc. 2017, 139, 6369. (6) (a) Anderson, P. W.; Brinkman, W. F.; Huse, D. A. Science 2005, 310, 1164. (b) Zhao, L.; Fernández-Díaz, M. T.; Tjeng, L. H.; Komarek, A. C. Sci. Adv. 2016, 2, No. e1600353. (7) Ghiringhelli, G.; Le Tacon, M.; Minola, M.; Blanco-Canosa, S.; Mazzoli, C.; Brookes, N. B.; De Luca, G. M.; Frano, A.; Hawthorn, D. G.; He, F.; Loew, T.; Sala, M. M.; Peets, D. C.; Salluzzo, M.; Schierle, E.; Sutarto, R.; Sawatzky, G. A.; Weschke, E.; Keimer, B.; Braicovich, L. Science 2012, 337, 821. (8) Blinc, R. Advanced Ferroelectricity; Oxford University Press Inc.: New York, 2001. (9) (a) Tsurumi, T.; Miyasou, T.; Ishibashi, Y.; Ohashi, N. Jpn. J. Appl. Phys. 1998, 37, 5104. (b) Kimura, T.; Goto, T.; Shintani, H.; Ishizaka, K.; Arima, T.; Tokura, Y. Nature 2003, 426, 55. (c) Cheong, S.-W.; Mostovoy, M. Nat. Mater. 2007, 6, 13. (10) Graetsch, H. A.; Pandey, C. S.; Schreuer, J.; Burianek, M.; Mühlberg, M. Acta Crystallogr., Sect. B: Struct. Sci., Cryst. Eng. Mater. 2014, 70, 743. (11) (a) Argyriou, D. N.; Aliouane, N.; Strempfer, J.; Zegkinoglou, I.; Bohnenbuck, B.; Habicht, K.; Zimmermann, M. v. Phys. Rev. B: Condens. Matter Mater. Phys. 2007, 75, 020101. (b) Ishiwata, S.; Tokunaga, Y.; Taguchi, Y.; Tokura, Y. J. Am. Chem. Soc. 2011, 133, 13818. (12) Claridge, J. B.; Hughes, H.; Bridges, C. A.; Allix, M.; Suchomel, M. R.; Niu, H.; Kuang, X.; Rosseinsky, M. J.; Bellido, N.; Grebille, D.; Perez, O.; Simon, C.; Pelloquin, D.; Blundell, S. J.; Lancaster, T.; Baker, P. J.; Pratt, F. L.; Halasyamani, P. S. J. Am. Chem. Soc. 2009, 131, 14000. (13) Lin, K.; Zhou, Z.; Liu, L.; Ma, H.; Chen, J.; Deng, J.; Sun, J.; You, L.; Kasai, H.; Kato, K.; Takata, M.; Xing, X. J. Am. Chem. Soc. 2015, 137, 13468. (14) (a) Fu, D.-W.; Cai, H.-L.; Liu, Y.; Ye, Q.; Zhang, W.; Zhang, Y.; Chen, X.-Y.; Giovannetti, G.; Capone, M.; Li, J.; Xiong, R.-G. Science 2013, 339, 425. (b) Horiuchi, S.; Tokunaga, Y.; Giovannetti, G.; Picozzi, S.; Itoh, H.; Shimano, R.; Kumai, R.; Tokura, Y. Nature 2010, 463, 789. (15) Kamishina, Y.; Akishige, Y.; Hashimoto, M. J. Phys. Soc. Jpn. 1991, 60, 2147. (16) Blinc, R.; Levanyuk, A. P. Incommensurate Phases in Dielectrics; North-Holland Publishing Co.: Amsterdam, 1986. (17) Yamada, Y.; Shibuya, I.; Hoshino, S. J. J. Phys. Soc. Jpn. 1963, 18, 1594. (18) Lang, S. B.; Das-Gupta, D. K. Handbook of Advanced Electronic and Photonic Materials and Devices; Nalwa, H. S., Ed.; Academic: San Diego, CA, 2001; Vol. 4. (19) Cai, H.-L.; Zhang, Y.; Fu, D.-W.; Zhang, W.; Liu, T.; Yoshikawa, H.; Awaga, K.; Xiong, R.-G. J. Am. Chem. Soc. 2012, 134, 18487. (20) CrysAlis Pro; Oxford Diffraction Ltd.: Yarnton, Oxfordshire, England, 2010. (21) Sheldrick, G. M. Acta Crystallogr. 2015, C71, 3. (22) Palatinus, L.; Chapuis, G. J. Appl. Crystallogr. 2007, 40, 786. (23) Petříček, V.; Dušek, M.; Palatinus, L. Z. Kristallogr. - Cryst. Mater. 2014, 229, 345. (24) Momma, K.; Izumi, F. J. Appl. Crystallogr. 2011, 44, 1272. (25) Aizu, K. J. Phys. Soc. Jpn. 1969, 27, 387. (26) Fu, D.-W.; Zhang, W.; Cai, H.-L.; Zhang, Y.; Ge, J.-Z.; Xiong, R.-G.; Huang, S. D. J. Am. Chem. Soc. 2011, 133, 12780. (27) Blinc, R.; Ź eks, B. Soft Modes in Ferroelectrics and Antiferroelectrics; North-Holland Publishing Co.: Amsterdam, 1974. (28) Saito, K.; Yamamura, Y.; Kikuchi, N.; Nakao, A.; Yasuzuka, S.; Akishige, Y.; Murakami, Y. CrystEngComm 2011, 13, 2693. (29) Zhang, W.; Xiong, R.-G. Chem. Rev. 2012, 112, 1163. (30) Sun, Z.; Chen, T.; Luo, J.; Hong, M. Angew. Chem., Int. Ed. 2012, 51, 3871. (31) Gerra, G.; Tagantsev, A. K.; Setter, N. Phys. Rev. Lett. 2005, 94, 107602.

deviate away from the mirror symmetry at FEP, showing a twisted angle of 5.8° (Figure 8a). Thus, the reorientation of dipoles gives rise to the components along the b-axis. According to the point electric charge model (Figure S10), the permanent dipole moment and electric polarization for 1 are, respectively, estimated to be 3.65 × 10−29 C·m and ∼1.2 μC/cm2, in accordance with experimental pyroelectric and P−E hysteresis measurements. As a result, the interplays between unique IC structural modulations and bulk ferroelectricity make 1 a promising candidate for the future assembly of artificially modulated periodic structure. In summary, we have designed and characterized a new organic molecular ferroelectric, which shows unique IC structural modulations between its FEP and PEP. In particular, three distinct IC states coupling with a long-range structural modulation are observed, evidenced by an exact (3 + 1)D superspace group. Such a modulation suppresses the long-range ordered arrangement of electric dipoles at ICP; however, the freezing of active rotators leads to the reorientation of dipoles as well as its ferroelectric behaviors. The comprehensive understanding of structural modulation and ferroelectricity will be inspiring to study the underlying concepts of physics and chemistry in aperiodic materials.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b08950. Crystals, PXRD, and basic physical properties; details for the refinement of IC structure (PDF) X-ray data for crystal structures (CIF) CheckCIF/PLATON report (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*[email protected] *[email protected] ORCID

Junliang Sun: 0000-0003-4074-0962 Junhua Luo: 0000-0002-7673-7979 Author Contributions §

Z. Sun and J. Li contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by NSFC (21622108, 21525104, 21601188, 91422301, 21373220, 51402296, and 51502290), the NSF of Fujian Province (2015J05040), the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB20000000), the Youth Innovation Promotion of CAS (2014262 and 2015240), and State Key Laboratory of Luminescence and Applications (SKLA-2016-09).

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DOI: 10.1021/jacs.7b08950 J. Am. Chem. Soc. 2017, 139, 15900−15906