Directional Intermolecular Interactions for Precise Molecular Design of

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Directional Intermolecular Interactions for Precise Molecular Design of a High-T Multiaxial Molecular Ferroelectric c

Chen-Kai Yang, Wang-Nan Chen, Yan-Ting Ding, Jing Wang, Yin Rao, Wei-Qiang Liao, Yongfa Xie, Wennan Zou, and Ren-Gen Xiong J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 07 Jan 2019 Downloaded from http://pubs.acs.org on January 7, 2019

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Journal of the American Chemical Society

Directional Intermolecular Interactions for Precise Molecular Design of a High-Tc Multiaxial Molecular Ferroelectric Chen-Kai Yang,† Wang-Nan Chen,† Yan-Ting Ding,† Jing Wang,† Yin Rao,† Wei-Qiang Liao,† Yongfa Xie,*,§Wennan Zou,*,‡ and Ren-Gen Xiong*,†,# †

Ordered Matter Science Research Center, Nanchang University, Nanchang 330031, People’s Republic of China Institute for Advanced Study, Nanchang University, Nanchang 330031, People’s Republic of China § College of Chemistry, Nanchang University, Nanchang, 330031, People’s Republic of China # Jiangsu Key Laboratory for Science and Applications of Molecular Ferroelectrics, Southeast University, Nanjing 211189, People’s Republic of China Supporting Information

‡

ABSTRACT: Quasi-spherical molecules have recently been developed as promising building blocks for constructing highperformance molecular ferroelectrics. However, although the modification of spherical molecules into quasi-spherical ones can efficiently lower the crystal symmetry, it is still a challenge to precisely arouse a low-symmetric polar crystal structure. Here, by introducing directional hydrogen-bonding interactions in the molecular modification, we successfully reduced the cubic centrosymmetric Pm3̅m space group of [quinuclidinium]ClO4 at room temperature to the orthorhombic polar Pna21 space group of [3oxo-quinuclidinium]ClO4. Different from the substituent groups of –OH, –CH3, and =CH2, the adding of =O group with H-acceptor to [quinuclidinium]+ forms directionally N–H···O=C hydrogen-bonded chains, which plays a critical role in the generation of polar structure in [3-oxo-quinuclidinium]ClO4. Systematic characterization indicates that [3-oxo-quinuclidinium]ClO4 is an excellent molecular ferroelectric with a high Curie temperature of 457 K, a large saturate polarization of 6.7 μC/cm2, and a multiaxial feature of 6 equivalent ferroelectric axes. This work demonstrates that the strategy of combining quasi-spherical molecule building blocks with directional intermolecular interactions provides an efficient route to precisely design new eminent molecular ferroelectrics.

INTRODUCTION Ferroelectrics have become a workhorse in numerous technologies, finding applications in data storage, capacitors, sensors, microactuators, and nonlinear optical devices.1 During the century-long history, extensive efforts have been directed toward the design and synthesis of new ferroelectric families, including complex oxides, molecular crystals, polymers, and liquid crystals.2 Ferroelectric oxides always stand at the forefront of materials science,3 whereas it is only in recent years that the investigation of molecular ferroelectrics shows signs of a revival.4 In contrast to the former with inherent structural rigidity, molecular ferroelectrics naturally lend themselves to the crafting of structure and function using the strategies of crystal engineering.5 Such a broader design flexibility offers a cornucopia of research opportunities to facilitate the development of highly organized molecular systems in future low-cost flexible devices and wearable devices.6 Nevertheless, effective ways of controlling the crystal structure and of engineering desired ferroelectric properties from designed building blocks are still a largely uncharted territory. Ferroelectricity, which means the spontaneous polarization can be switched by applying an external electric field, is closely related to the crystal structures.7 Only the ten polar point groups: 1 (C1), 2 (C2), m (C1h), mm2 (C2v), 4 (C4), 4mm (C4v), 3

(C3), 3m (C3v), 6 (C6) and 6mm (C6v), allow ferroelectric behaviors.8 Specifically, molecules must be assembled in particular ways to make sure that their dipole moments do not cancel each other out in order to achieve targeted polar crystal structures. Choice of the molecular building blocks as well the precise control over the assembly of them is therefore the handle that permits crystal design of molecular ferroelectrics. Together with scientists around the world, we have aimed at this challenging goal. As a result, a series of molecular design strategies have been developed and continuously improved while getting deeper insight into the structure-property relationship. Earlier studies revealed that, spherical molecules, such as 1,4-diazabicyclo[2.2.2]octonium (dabco), quinuclidinium (Q), and tetramethylammonium, often allow for dynamic rotation or reorientation, the freezing of which would easily induce a structural or even ferroelectric phase transition.4c,9 Then we also developed a pathway to introduce a dipole moment to the organic cation, so that the lowering of molecular symmetry may trigger a low-symmetric polar crystal structure, while the quasi-spherical geometry still makes molecular reorientation easy to arouse ferroelectric ordering.10 That is how we designed the metal-free perovskite ferroelectric [Me-dabco]NH4I3 (Me-dabco is N-methyl-N′diazabicyclo[2.2.2] octonium) more recently.11

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Scheme 1. The design of molecular ferroelectric [3-O-Q]ClO4 by modifying molecular structure of non-ferroelectric [Q]ClO4. Disappointedly but admittedly, it seems that the above ideas roelectric axes. This work demonstrates that only taking into are not always going to work very well. For example, we synaccount the roles of both the molecule building blocks and thesized quinuclidinium perchlorate ([Q]ClO4) in the hope of intermolecular interactions can fully enable the deliberate deobtaining another new high-temperature molecular ferroelecsign of ferroelectricity. We believe it will be illuminating in tric like the reported [Q]ReO4 and [Q]IO4,4c,9f but it adopts a the field of molecular ferroelectrics and greatly widen the scope of this chemistry. centrosymmetric cubic space group Pm3̅m at room temperature. And, the attachment of a hydroxyl group to the high RESULTS AND DISCUSSION symmetric [Q]+ cation failed to change the crystal structure symmetry, as the 3-hydroxyl-quinuclidinium ([3-OH-Q]+) Quinuclidine is a spherical molecule analogous to 1,4cations remain severe disorder in [3-OH-Q]ClO4. Although diazabicyclo[2.2.2]octane. By using quinuclidine as a building replacing –OH by another flexible –CH3 group lowers the unit, we obtained two high Curie temperature ferroelectrics, crystal symmetry to the orthorhombic Pnma space group and [Q]IO4 and [Q]ReO4, possessing the polar Pmn21 space group restrains the motion of cations in [3-methylat room temperature.4c,9f However, [Q]ClO4 has a centrosymquinuclidinium]ClO4 ([3-CH3-Q]ClO4), the centrosymmetric metric structure with cubic Pm3̅m space group at 298 K, in nature has not been changed. Even further attempt of a rigid which the quinuclidinium cation and perchlorate anion are =CH2 group can’t address such situation. [3-methylenehighly disordered (Figure S1 and Table S1). The centrosymquinuclidinium]ClO4 ([3-CH2-Q]ClO4) also has the Pnma metric structure excludes the possibility to have above-room+ symmetry, although the [3-CH2-Q] becomes ordered. At this temperature ferroelectricity. Although it shows a phase transipoint, every door seems to be close to us, is there any avenue tion at 289 K, the structure below 289 K in the lowof endowing ferroelectricity? The answer is exploiting intertemperature phase (LTP) is still a centrosymmetric one (Figmolecular interactions. The shapes, sizes, and symmetries of ure S1). Apparently, [Q]ClO4 is not a ferroelectric. molecules are important in the close packing principle, whereIn order to induce crystallization in a polar space group for as intermolecular interactions cause deviations from close ferroelectric, we then made a subtle molecular modification packing and hence affect the crystal structures and properties for quinuclidine by adding a substituent group to the third at a more subtle level.12 A long-standing issue at the core of position to change the crystal symmetry (Scheme 1). We first crystal engineering is the balance between close-packing efintroduce the flexible –OH group. Unfortunately, [3-OH13 fects and intermolecular interactions. Towards the desired Q]ClO4 still adopts the cubic Pm3̅m space group at 298 K and crystal packing and fine-tuning properties, one can employ at 93 K below the phase transition temperature of 210 K (Fignumerous intermolecular interactions to assemble molecular ure S2 and Table S1), isostructural to [Q]ClO4. The quasicrystals, among which hydrogen bonding with pronounced spherical [3-OH-Q]+ cation also shows freely motion in the directionality and relatively high strength is undoubtedly the isotropically spherical disorder state. When introducing the most effective and widely used tool for crystal design.14 Hereflexible –CH + 3 group, the crystal symmetry of [3-CH3-Q]ClO4 in we equipped a =O group to the [Q] cation, and expected reduced to the orthorhombic centrosymmetric space group that the formation of the typical and well-known N–H···O=C Pnma at 298 K (Figure S3 and Table S1). The motion of the hydrogen bond once found in amino acids, peptides and procation is obviously restricted, but the [3-CH3-Q]+ cation as teins would aid the self-assembly process. The results didn’t well as perchlorate anion are still orientationally disordered. let us down, through trial from [Q]+ to [3-OH-Q]+, [3-CH3-Q]+, [3-CH3-Q]ClO4 undergoes a symmetry-breaking phase transi+ + [3-CH2-Q] , and [3-oxo-quinuclidinium] ([3-O-Q] ) compotion from space group Pnma to P21/n, which is not among the nent, we finally harvest an excellent molecular ferroelectric, 88 species of ferroelectric phase transition. 15 We then select a [3-O-Q]ClO4, belonging to the polar orthorhombic space rigid =CH2 substituent group. The motion of the cation is furgroup Pna21 and having a high Curie temperature (Tc) of 457 ther frozen in [3-CH2-Q]ClO4, which also crystalizes in an K, a large saturate spontaneous polarization (PS) of 6.7 orthorhombic Pnma space group at 298 K (Figure S4 and 2 μC/cm , and a multiaxial characteristic with 6 equivalent fer-


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Journal of the American Chemical Society Table S1). The cation becomes an ordered one, forming a hydrogen-bonded dimer with the perchlorate anion though N– H···O=Cl hydrogen bonds, where the H acceptor is O atom from the perchlorate anion. The centrosymmetric Pnma space group maintains at 93 K, which indicates that [3-CH2-Q]ClO4 is still a non-ferroelectric. It is found that in molecular crystals, intermolecular interactions including hydrogen bonds and halogen bonds also affect the crystal symmetry. 9c,10a We thus further replace the =CH2 by another rigid substituent group =O, in which the O atom can act as a H acceptor. As a result, the hydrogen-bonding interactions change significantly in [3-OQ]ClO4, which successfully crystalizes in the orthorhombic polar space group Pna21 at room temperature (Table S1). This space group belongs to the mm2 point group, among the ten point groups for ferroelectrics. The crystal structure of [3-O-Q]ClO4 at 298 K is comprised of two basic components, one [3-O-Q]+ cation and one perchlorate anion (Figure 1a). Although occupying general symmetry positions, both of them show some degree of disorder. The atomic coordinate of the O atom of [3-O-Q]+ cation was split into two parts while the atomic coordinates of all the four O atoms of perchlorate anion needed to be split to get a better refinement result. One [3-O-Q]+ cation connects with another one through N–H···O=C hydrogen bonds with donoracceptor distance of 3.045 Å, forming one-dimensional hydrogen-bonded chains along the a-axis in a head-to-tail arrangement (Figure 1a). For ionic crystals, this kind of directional hydrogen-bonding arrangement is favorable to induce crystallization in polar space groups, as found in many ionic ferroelectrics such as [Hdabco]ClO4,9c [Hdabco]BF4,9c and (R)-3hydroxlyquinuclidinium chloride.16 Additionally, the [3-O-Q]+ cation also forms N–H···O=Cl hydrogen bonds with the perchlorate anion, which have a directional arrangement as well (Figure 1a). In the packing view along the [011] direction, each [3-O-Q]+ cation sits at the center of eight perchlorate anions, showing a pseudo-cubic CsCl-type packing structure (Figure 1b).

Figure 1. (a) Crystal structure of [3-O-Q]ClO4 at 298 K, showing head-to-tail type one-dimensional hydrogen-bonded chains along the a-axis. Packing views of [3-O-Q]ClO4 at (b) 298 K and (c) 473 K.

At 473 K in the high-temperature phase, [3-O-Q]ClO4 adopts a real cubic centrosymmetric space group Pm3̅m (point group m3̅m) with the CsCl-type structure (Table S1 and Figure 1c). Both the [3-O-Q]+ cation and perchlorate anion lie on high-symmetry special cites, which results in a totally disordered state to satisfy the site symmetry. The quasi-spherical [3-O-Q]+ cation was refined as a totally spherical molecule in spite of its molecular geometry, meaning that the [3-O-Q]+ cation shows isotropic motion. It is noted that the hightemperature structure of [3-O-Q]ClO4 is isostructural to the room-temperature structure of [Q]ClO4. After the introduction of small rigid =O group to spherical [Q] + cation, [3-O-Q]+ cation still has the quasi-spherical geometry, which is able to become a totally spherical one at high temperature, as observed for other quasi-spherical cations such as [MeHdabco]2+,11 3-ammonioquinuclidinium,11 (R)-3hydroxlyquinuclidinium,16 and 3-ammoniopyrrolidinium.17 From the viewpoint of polarization, the direction of [3-OQ]+ dipole is nearly perpendicular to the polar c-axis at 298 K (Figure 1b), the ferroelectric polarization from [3-O-Q]+ dipole is thus negligible. Therefore, we can employ the point charge model to evaluate the ferroelectric polarization from ionic part by assuming the positive center is located at N atom and the negative center is Cl atom. From the atom coordinate, the distance between positive and negative centers is about 1.04 Å, which will lead to a polarization of 6.86 μC/cm2 along the c-axis. Thus, the ferroelectricity in [3-O-Q]ClO4 crystal mainly stem from charge misalign between [3-O-Q]+ cations and perchlorate anions. When an external electric field is applied, the molecular cations and anions will flip to their opposite but equivalent position through their equilibrium states to reverse the whole ferroelectric polarization. In this process, the transition state may be referred to the structural configuration in paraelectric phase but with lower degree of disorder because the paraelectric configuration can be thought as the superposition of all equivalent ferroelectric polarization states. On the other hand, in the room temperature structure of [3CH2-Q]ClO4, all the [3-CH2-Q]+ cations canceled with each other exactly (Figure S5), resulting into a centrosymmetric symmetry of space group Pnma, under which the ferroelectricity is not allowed. Compared with polar [3-O-Q]ClO4 crystal, the underlying mechanism for the centrosymmetric array in [3CH2-Q]ClO4 may lies to the molecular length extension due to the additional two H atoms from C=C parts, which favors antiparallel packing fashion sterically. To have a deep insight into the structural difference among [3-X-Q]ClO4 (X = H, OH, CH3, CH2, and O) series, electron densities of these crystals from X-ray diffraction data are modelled, providing us with a comprehensive characterization of chemical bonding in crystals. As shown in Figures 2, S6 and S7, the highly ionic bond nature can be seen from the electron density distribution (slice) picture, supporting the effectiveness of the abovementioned point charge model analysis. Particularly, weak intermolecular interactions, such as hydrogen bonding effect, are quite different between [3-OQ]ClO4 and [3-CH2-Q]ClO4 crystals. In the [3-O-Q]ClO4 crystal, the N–H···O=Cl interaction is homo-directional and further stabilized by additionally directional N–H···O=C interactions (Figure 1a). Such an arrangement is responsible for the polarization along polar the c-axis. During the polarization reversal process, the directional hydrogen bond should be broken in the initial process, but rebuilt immediately when the opposite polarization state generates. In other words, the hy-



drogen bond is generated to stabilize the ferroelectric polarization states, which should be destroyed during the ferroelectric reversal process. The energy required for hydrogen bond breaking is a part of energy barrier for the polarization inversion. However, in [3-CH2-Q]ClO4, the main intermolecular interaction is the N–H···O=Cl interaction, whose spatial arrangement is forced by the static molecular hindrance. Thus, the main structural difference between [3-O-Q]ClO4 and [3CH2-Q]ClO4 crystals originates from the spatial arrangement governed by weak intermolecular interactions and static molecular hindrance. In addition, the situation that lack of directional intermolecular interaction is also found in [3-CH3Q]ClO4 crystal (Figure S6), which adopts a centrosymmetric packing pattern with space group of P21/n even at 93 K. With regard to [3-OH-Q]ClO4 and [Q]ClO4 crystals, the highly disordered behavior of both cations and anions shows high symmetric electron density distribution (Figure S7), which is broadly similar to the high-temperature phase of [3-O-Q]ClO4. Differential scanning calorimetry (DSC) curves of [3-OQ]ClO4 clearly present a pair of large heat anomalies in the heating-cooling run with a hysteresis of approximately 35 K, indicating a reversible first-order phase transition at Tc = 457 K (Figure 3a). Such a high Tc is rarely seen in molecular ferroelectrics,5,8 which is even significantly greater than that of the inorganic ferroelectric BaTiO3 (393 K). The high Tc makes [3-O-Q]ClO4 be able to adapt high-temperature working condition. Entropy change (ΔS) in the heating process is about 18.2 Jmol-1K-1, which gives a N (the ratio of the numbers of respective geometrically distinguishable orientations) value of 9 according to the Boltzmann equation, ΔS = RlnN (where R is the gas constant). The large N value suggests an orderdisordered type phase transition, in which the relatively ordered [3-O-Q]+ cations and perchlorate anions in the roomtemperature phase become highly disordered in the hightemperature phase. This is consistent with the structural analysis. In general, a phase transition will accompany anomalous dielectric behavior. As expected, the real part (ε′) of the complex dielectric constant of [3-O-Q]ClO4 displays an obvious dielectric anomaly in both the heating and cooling process (Figure 3b). The tremendous change of ε′ value in the vicinity of Tc at 1 MHz for polycrystalline sample reveals a ferroelectric-to-paraelectric phase transition feature. The temperature-dependent second harmonic generation (SHG) intensity experiments also verify the first-order phase transition in [3-O-Q]ClO4. As shown in Figure 3c, in the heating process, the SHG intensity decreases very slowly below Tc and changes abruptly to zero at around Tc. The zero SHG intensity above Tc and strong SHG intensity below Tc manifests a centrosymmetric and non-centrosymmetric phase, respectively, corresponding to the phase transition from the centrosymmetric space group Pm3̅m in room-temperature phase to the polar one Pna21 in high-temperature phase. Therefore, [3O-Q]ClO4 clearly undergoes a paraelectric-to-ferroelectric phase transition belonging to the m3̅ mFmm2 type from the Aizu rule.15 The paraelectric point group m3̅m and ferroelectric point group mm2 have 48 and 4 symmetry elements, respectively, which reveals a multiaxial ferroelectric with 12 crystallographically equivalent polarization directions, corresponding to 6 equivalent ferroelectric axes.

Figure 2. Electron density distribution ((010) slice) on the y = 1/4 plane in (a) [3-O-Q]ClO4 and (b) [3-CH2-Q]ClO4.

Figure 3. Ferroelectric related properties of [3-O-Q]ClO4. (a) DSC curves in a heating-cooling run, showing a high Tc of 457 K. (b) Temperature dependent ε′ (the real part of complex dielectric permittivity). (c) SHG intensity as a function of temperature. (d) P−E hysteresis loops measured at 293 K by using the doublewave method.

We then directly detect the ferroelectricity of [3-O-Q]ClO4 by the measurement of polarization−electric field (P−E) hysteresis loops. [3-O-Q]ClO4 shows a typical ferroelectric J−E curve with two opposite peaks at 293 K (Figure 3d). By accumulating the current, we obtained a standard P−E hysteresis loop. The saturate polarization (Ps) is about 6.7 μC/cm2, which is among the largest one in molecular ferroelectrics. The Ps of [3-O-Q]ClO4 is much higher than those for other perchlorate based ferroelectrics including 4-(cyanomethyl)anilinium perchlorate (0.75 μC/cm2)18 and [Hdabco]ClO4 (4 μC/cm2)9e, and some classical molecular ferroelectrics such as Rochelle salt (0.25 μC/cm2), thiourea (3.2 μC/cm2), and triglycine sulfate (3.8 μC/cm2).5a The experimental Ps is comparable to the polarization obtained from point charge model.


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Journal of the American Chemical Society To further investigate the ferroelectricity of [3-O-Q]ClO4, we have also carried out PFM measurements on its thin film, which is an effective characterization tool for addressing the statics and dynamics of ferroelectric domains with no need of any special treatment.19 By measuring the amplitude and phase parameters in a PFM image, the relative strength of piezoelectric coefficient and the polarization orientation of domain can be acquired, respectively. Figure 4 shows the vertical and lateral PFM phase and amplitude images for the thin film of [3-O-Q]ClO4 in the annealed state, which emerges generally in a reconstructed domain structure after heat treatment above Tc. It is clear that the phase patterns are significantly different in two components, supporting the large amount of domains with various polarization directions. Moreover, the amplitude patterns are consistent well with the corresponding phase patterns, where the domain walls appear as darker lines.

Figure 5. The domain switching for the thin film of [3-O-Q]ClO4. The panels in each row are arranged as the sequence: topography (top), vertical PFM amplitude (middle) and phase (bottom) images of the film surface. The images in each column are arranged as the sequence of (a) Initial state. (b) After the switching produced by applying with the tip bias of -60 V for 1.5 s.

Figure 4. Vertical and lateral PFM phase (a, c) and amplitude (b, d) images for the thin film of [3-O-Q]ClO4. (e, f) Vertical PFM phase and amplitude signals as functions of the tip voltage for the selected point, showing local PFM hysteresis and butterfly loops.

Domain switching dynamic is an important subject in the physics of ferroelectrics, which needs an understanding on the sub-micrometer scale before considering any practical applications. We then performed the local PFM spectroscopic measurements on the thin film of [3-O-Q]ClO4 for studying the polarization switching behavior. As shown in Figure 4e,f, the typical hysteresis and butterfly loops triggered by the applied voltages are solid evidence for the polarization switching of ferroelectric domains. Meanwhile, we have also conducted PFM tip poling experiments to clearly visualize the domain switching process, on the thin film of [3-O-Q]ClO4 with a selected area of 50 μm × 50 μm. For the as-grown state, the piezoresponse in most regions is almost uniform in both phase and amplitude images, indicating single-domain state of these area (Figure 5a). Subsequently, a tip bias of -60 V is applied to a selected position for 1.5 s. From Figure 5b, the bidomain pattern with two-tone color contrast and the domain walls with nearly null signals occurred in the respective phase and amplitude images, which further confirms the polarization switching of ferroelectric domain. The generated bidomain pattern is not predefined, which depends on the lateral movement of domain wall. These PFM results constitute an abundant proof for the presence of stable and switchable polarization in the thin film of [3-O-Q]ClO4.

CONCLUSION



In summary, we successfully designed a new molecular ferroelectric [3-O-Q]ClO4 by utilizing hydrogen-bonding interactions in the molecular modification of the non-ferroelectric [Q]ClO4. Different from the substituent groups of –OH, –CH3, and =CH2, the introduce of =O group with H acceptor to [Q] + results in the formation of N–H···O=C hydrogen bonds. The directionally head-to-tail type hydrogen bonded chains in [3O-Q]ClO4 facilitate the crystallization in a polar space group at room temperature. The quasi-spherical [3-O-Q]+ cation becomes a isotropically spherical one in the high-temperature phase, inducing a paraelectric-to-ferroelectric phase transition. [3-O-Q]ClO4 shows a high Curie temperature of 457 K, a large saturate polarization of 6.7 μC/cm2, and a multiaxial characteristic with 6 equivalent ferroelectric axes. These ferroelectric performances are outstanding among molecular ferroelectrics. This finding demonstrates that the combination of quasispherical molecule building blocks and directional intermolecular interactions is an efficacious design strategy for achieving superior molecular ferroelectrics.

EXPERIMENTAL SECTION Materials. Quinuclidine, 3-hydroxyl-quinuclidine, and 3oxo-quinuclidinium chloride were commercially available. 3Methylene-quinuclidine and 3-methyl-quinuclidine were synthesized by the following procedure. 3-methylene-quinuclidine. Methyltriphenylphosphonium bromide(142.8 g, 0.4 mol) was added to a suspension of tBuOK (33.6 g, 0.3 mol) in anhydrous THF (500 mL) at room temperature under N2 gas. The mixture was stirred at room temperature for 1 hour, and then heated to 45 oC for 1 hour. The mixture was turned into yellow suspension. After cooling to room temperature, 3-oxo-quinuclidine (25.0 g, 0.2 mol) (3oxo-quinuclidine was neutralized by NaOH aqueous of its commercially available 3-oxo-quinuclidinium chloride salt) in anhydrous THF (100 mL) was added dropwise to the yellow mixture at room temperature. The mixture was heated to reflux for 4 hours. After cooling to room temperature, the mixture was filtered, then the filtrate was concentrated by vacuum evaporator. Et2O/petroleum (1/1, 300 mL) was added to the resultant residue. The obtained white suspension was filtered through celite and the filter cake was washed with petroleum (3 × 50 mL). The filtrate was concentrated and dissolved in 200 mL ethyl acetate and then extracted with 1.0 M HCl aqueous (3 × 150 mL). The combined water phases were washed with CH2Cl2 (3 × 200 mL), then adjusted to pH = 12 with NaOH, and extracted with ethyl acetate (3 × 300 mL). The combined organic phases were dried over Na2SO4, filtered, and concentrated to afford a pale yellow oil (15.0 g, yield: 61%) (without further purification for next step). 1H NMR (CDCl3, 400 MHz) δ 4.75 (m, 1H), 4.64 (m, 1H), 3.46 (s, 2H) 2.92-2.78 (m, 4H), 2.39-2.36 (m, 1H), 1.74-1.67 (m, 4H). 3-methyl-quinuclidine. 3-methylene-quinuclidine (8.0 g, 65.0 mmol) in 50 mL ethanol was added to a suspension of PtO2(0.45g, 2.0 mmol) in ethanol (300 mL) at room temperature under H2 gas (with a H2 gas balloon). The mixture was stirred at room temperature for 3 hours (TLC showed all of 3methylene-quinuclidine reacted completely). The mixture was filtered through celite and the filtrate was concentrated to obtain a colorless oil (7.4 g, yield: 91%). 1H NMR (CDCl3, 400 MHz) δ 3.06-2.99 (m, 1H), 2.82-2.69 (m, 4H), 2.26-2.20 (m, 1H) 1.73-1.44 (m, 5H), 1.35-1.28 (m, 1H), 0.96-0.95 (d, 3H, J = 6.96).

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Colourless crystals of [Q]ClO4, [3-OH-Q]ClO4, [3-CH3Q]ClO4 and [3-CH2-Q]ClO4, were obtained by slow evaporation of aqueous solutions containing equimolar amounts of perchloric acid and quinuclidine, 3-hydroxyl-quinuclidine, 3methyl-quinuclidine and 3-methylene-quinuclidine, respectively. Slowly evaporating the aqueous solution of equimolar sodium perchlorate and 3-oxo-quinuclidinium chloride obtained colourless crystals of [3-O-Q]ClO4. Powder X-ray diffraction (PXRD) (Figure S8) and infrared (IR) spectroscopy (Figure S9) results confirmed the phase purity of all the asgrown crystals. Measurements. The methods of single-crystal X-ray diffraction, DSC, dielectric, SHG, P−E hysteresis loop, PFM, PXRD, and IR experiments were described in detail previously.4,10 The as-grown crystals of [3-O-Q]ClO4 were too small for us to prepare the single crystal sample along the polarization direction for P−E hysteresis loop measurements. We thus used the thin film sample of [3-O-Q]ClO4, which was also used for the PFM measurement. The precursor solution was prepared by dissolving 15 mg [3-O-Q]ClO4 in 1 mL deionized water. Spreading 20 µL of the precursor solution on a clean indium-doped tin oxide (ITO) glass substrate resulted the formation of thin film crystals after the slow solvent evaporation. The experimental PXRD patterns of the thin film sample matching well with the simulated ones verifies its phase purity (Figure S10). The PXRD patterns also indicate that the thin film clearly shows the (002) polarization direction (Figure S10). For the P−E hysteresis loop measurement, we dropped the liquid GaIn eutectic on the thin film sample as the top electrode to form a ITO/[3-O-Q]ClO4 thin film/GaIn capacitor architecture.

ASSOCIATED CONTENT Supporting Information. Figures S1–S10, Table S1, and discussion. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *[email protected]; *[email protected]; *[email protected]

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (21427801, 21831004 and 91856114).

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