Designing Chiral, Pro-Polar Structures for Observing Ferroelectricity

Communication pubs.acs.org/crystal

Designing Chiral, Pro-Polar Structures for Observing Ferroelectricity: Molecular Analogue of KNO3 Hemant M. Mande and Prasanna S. Ghalsasi* Department of Chemistry, Faculty of Science, The Maharaja Sayajirao University of Baroda, Vadodara, India S Supporting Information *

ABSTRACT: Observation and creation of a pro-chiral center is a prerequisite in chemical synthesis for generating chiral structures. On the other hand, temporary and reversible generation of chiral or polar structures using physical forces such as electricity is a need of present technology for observing nonlinear behavior. Thus, one can synthesize a pro-polar structure, akin to a pro-chiral center using chemical interactions, for facilitating physical forces in generating chiral/polar structures, a novel way for designing molecular materials. At room temperature, the structure of KNO3 and anilinium nitrate (AnHNO3) is centrosymmetric, although the latter showed molecular properties in the form of distinct helical NH···O hydrogen bonding to generate a dipole moment, which in turn gets canceled due to the formation of antiparallel double helical chains. We could separate these helical chains of NH···O hydrogen bonding, by chemical design, synthesis of analogues molecules (para-ethyl anilinium nitrate), and crystallization through a gel as well slow evaporation technique, to obtain a pro-polar and chiral structure with molecular ferroelectric behavior. Thus, creation of a pro-polar structure, presented here by modifying the cation in KNO3 to observe room temperature ferroelectric behavior, will help in designing novel molecular ferroelectric materials.

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structure (space group R3m). The phase III is obtained intermittently after cooling centrosymmetric phase I (C3 or D6 symmetric, above 483 K) from the stable phase II (space group Pnma) at RT.14 These reversible structural changes have been correlated to order−disorder transitions, originating from (a) changes in planarity of nitrate anion possibly due to its origin in asymmetric vibrations15,16 and (b) formation a of unequal distances between adjacent K+ and NO3− along the c-axis17 3.941 Å and 5.051 Å (Figure S2a, Supporting Information). To observe and modulate these changes for generating a noncentrosymmetric structure near room temperature in KNO3, many modifications were reported in the literature, starting from thin film formation to dispersion in solid matrix (such as polymers) to nanoparticles.18−24 Although all these methods helped in lowering the ferroelectric transition temperature, it was mainly controlling macroscopic or morphological arrangement. Here we will discuss a soft or molecular approach for “tuning” a similar structural change. This concept can be correlated to the approach used by Tokura et. al for generating various organic ferroelectrics.25 A close look of the KNO3 structure suggests K+ in a stator role surrounded by hexagonally arranged movable nitrate anions. Stator creates a non-centrosymmetric structure, PhaseIII, by the movements of nitrate anions with the help of an

ransforming a structure or framework into a polar and/or chiral space group with minimal physical force or chemical interaction is a challenging task. Generation of noncentrosymmetric (NCS) or acentric crystal class structure, i.e., crystal class lacking a center of inversion, with physical force imparts technologically important properties pertaining to nonlinear behavior.1−4 Here physical forces in terms of electricity and/or temperature manipulate reversible movement of ions and hence generate a chiral or polar structure by displacive and/or order−disorder transition. BaTiO3 (ionic structure, displasive), purely organic (charge transfer, order− disorder and/or displasive), and organic polymers (entanglement of polymers) after discovery in K-tartrate remain wellknown examples of ferroelectric materials, observed due to accidental discovery.5−9 Interestingly, a chemical synthesis of chiral molecules is carried out as formation of a key intermediate “pro-chiral center” has been a routine practice for over 100 years in asymmetric syntheses.10 One can extrapolate this concept for technology, use of physical forces, for generating polarity by using pro-polar compounds. Here we show for the first time chiral and pro-polar structures, synthesized and designed by molecular chemistry/chemical synthesis, assist physical forces to display ferroelectric behavior. Broadly, this strategy of generating pro-polar structures will help in constructing novel molecular materials. KNO3 shows ferroelectric behavior in its re-entrant phase (phase-III) and is a topic of basic research from undergraduate experiment11 to future technology.12,13 The ferroelectric behavior is due to generation of a non-centrosymmetric © 2015 American Chemical Society

Received: August 26, 2015 Revised: November 21, 2015 Published: December 14, 2015 3

DOI: 10.1021/acs.cgd.5b01236 Cryst. Growth Des. 2016, 16, 3−7

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external physical force of 6 kV/cm from the cencentrosymmetric structure. This external physical force is of use only in the temperature range of 401−381 K. Therefore, substituting stator K+ by organic cation, one can introduce molecular forces for assisting external physical forces for easy and reversible centrosymmetric to non-centrosymmetric structural change. The driving force for these molecular forces can be engineered by employing pro-polar or chiral structures. With this intention, here we synthesized nitrate salts of anilinium (1), para-methyl anilinium (2), and para-ethyl anilinium (3). Two different pathways were employed to form nitrate salt: (a) direct use of HNO3 acid as a nitrate source (Scheme 1, Supporting Information), here sugar-like cubical crystals were obtained by a slow evaporation technique; (b) Al(NO3)3 as an nitrate source, a green syntheses. Here thin hairlike crystals were obtained from a gel mediated method (Scheme 2, Supporting Information).26 Single crystal X-ray diffraction reveals both these pathways for aniline (a1 and b1), para-methyl aniline (a2 and b2), and para-ethyl aniline (a3 and b3) showed a systematic relative change in the position of the cation with respect to the nitrate anion. Figure 1 shows the nitrate anion maintains

Figure 2. NH···O hydrogen bond assisted helical chains between organic cation and the nitrate anion along the c-axis in (a) a1: double helical chain; (b) a2: helical chains in antiparallel direction; and (c) a3M and a3P: homochiral helical chains.

or para-position, steric interaction, for removing centrosymmetric structural arrangement. To our expectations, although para-methyl anilinium nitrate (a2) shows unequal distance between neighboring cation−anion, (along the c-axis 3.380 and 3.454 Å) but crystallized in nonchiral space group P21/n (Figure 1b). This is because the generated dipole by NH···O hydrogen bonding is getting nullified by the presence of antiparallel directions of helical chains. That means, NH···O hydrogen bonded interwoven double helical chains in a1 get separated in a2 but in the antiparallel direction, as shown in Figure 2a,b. By further increasing steric interactions, a case of para-ethyl anilinium nitrate (a3) salt shows cations with not only unequal distance between three neighboring nitrate ions, 3.432, 3.470, 3.554 Å but also they lie slightly above and below the nitrate anion plane, Figure 1c. This resulted in the formation of single helical homochiral NH···O hydrogen bonding (as shown in Figure 2c) and hence crystallization in chiral space group P212121. The structures of a1, a2 and a3, show two distinct changes in neighboring nitrate−nitrate anions (a) angle between planes having nitrate anion (along the ab-axis) increases from 82.59 to 88.64 Å; (b) distance along the a-axis (parallel) decreases from 1.130 to 0.713 Å, respectively. Solid-state CD measurements for the bulk a3 crystals are CD silent, contradicting its crystallization in chiral space group. Therefore, we carried out CD spectra of a single/one crystal. To our expectation, we observed positive CD activity (Figure S4, Supporting Information). This prompted manually separation of crystals, a set of more than 20, and separate individual crystals study using single crystal X-ray diffraction and CD spectroscopy. The final experimental result was conglomerate crystallization, both the isomers in a 1:1 ratio. A single crystal X-ray study showed homochiral helical chains due to NH···O hydrogen bonding, assisting resolution of crystals as right handed (a3P) or left handed (a3M) helical chain isomers (Figure 2c).28 To extend this finding, nitrate salts of aniline derivatives with para-isoproyl and para-n-propyl were studied. para-Isopropyl anilinium nitrate (a4) crystallized into the Pbca space group, while para-n-propyl anilinium nitrate (a5) crystallized into the P21/c space group. Compound a4 formed a continuous interwoven double helical NH···O hydrogen bonding similar to a1. On the other hand, a5 showed a single helical chain of NH···O hydrogen bonding but in the opposite direction similar to a2.

Figure 1. Generation of chiral structure in an achiral molecular salt of nitrate anion with (a) anilinium (b) para-methyl anilinium, and (c) para-ethyl anilinium cation. Nitrate anions form a hexagonal array (green) along the c-axis and are stacked alternate to the cationic center (marked in black circles). Dipole formation or chirality induction is observed due to changes in the cations’ relative position triggered by molecular interactions. (Carbon and hydrogen atoms are omitted for clarity.)

hexagonal array and helps in generating chirality in the structure by distinct changes in the position of the cation. An investigation of these structural changes is presented here with the help of temperature-dependent Fourier transform infrared, circular dichroism spectroscopy, and polarization electric field loop measurements. Anilinium nitrate (AnHNO3-a1) crystallizes in the orthorhombic system with centrosymmetric space group Pbca.27 The structure shows nitrate anion and anilinium cation equidistant (3.400 Å) from each other forming a 2D lattice, a case very similar to KNO3 at room temperature, as shown in Figure 1a. Nitrate anion forms NH···O hydrogen bonding with three neighboring anilinium cations with two oxygen atoms, while the third oxygen atom remains free. Each anilinium cation forms hydrogen bonding with three neighboring different nitrate anions. Here, NH···O hydrogen bonding interactions show a molecular signature by forming (i) a layer parallel to the acplane, with two anilinium cations at the center between adjacent layers of nitrate anions (in the same plane); (ii) a continuous interwoven double helical-like structure along the caxis, as shown in Figure 2a. The position of the anilinium cation in Figure 2a can be poked by introducing a substituent at the 44

DOI: 10.1021/acs.cgd.5b01236 Cryst. Growth Des. 2016, 16, 3−7

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Two significant differences in the structure of a3 compared to a1 and a2 are (a) establishment of nonplanarity in NO3− anion: the nitrogen atom lies above the three oxygen atom plane by 0.009 Å; (b) shift in the position of para-ethylAnH+ cation: crystal packing of nitrate anion remains hexagonal but the paraethylAnH+ cation lies above and below the planes nitrate anion (Figure 1), while in a1, a cation is present at the center of the hexagon-like structure, and in a2 it is slightly off-centered (Figure 1). Thus, one can systematically generate chirality in the achiral molecule by changing the position of the cationic center in the structure without disturbing the hexagonal arrangement of nitrate anions, as shown pictorially in Figure 3, which will be discussed in detail in subsequent sections.

Figure 5. PE Loop measurement of (a) b1 and (b) b3.

polarization (Ps) (obtained by extrapolation of the linear parts of the loops to zero field) remained constant irrespective of the frequency, while Pr increases as the frequency decreases. At 298 K and 50 Hz, we obtained Pr = 1.71 μC/cm2 and coercive field (the intercept of the loop with the field axis) Ec = 1.94 kV/cm for b1. Pr and Ec values for b3 are 2.09 μC cm−2 and 3.37 kV/ cm. These values are close to 3.5 μC cm−2 and 6 kV/cm for phase-III of KNO3.25,29 Observation of high-temperature paraelectric phase (molecular or structural dipoles behave linearly with applied electric field) is considered as a signature of ferroelectric paraelectric transition. But in the present case due to an increase in leakage current with increasing temperature, a general phenomenon in molecular ferroelectrics, we could not observe the presence of the paraelectric phase using a P-E study. However, it was expected clearly in the case of b3 because (a) DSC measurements showed reversible phase transition temperature at 349.35 K (Figure S6 and Table S7, Supporting Information), and (b) high temperature single crystal X-ray data suggested a phase transition with symmetry breaking, or conversion into a nonpolar space group (Figure S7 and Table S8, Supporting Information). P-E hysteresis at room temperature after heating to 340 K showed complete reversible behavior, in accordance with observed reversible data of DSC and single crystal X-ray diffraction (Figure S9, Supporting Information). That means we designed a molecular analogue of KNO3 which showed room temperature ferroelectric behavior. As expected, both b1 and b3 showed ferroelastic30 domain structures under an optical polarizing microscope at room temperature. The reversible decreases and in some places totally diminishing of domains with an increase in temperature marked formation of the paraelastic phase (Figure S10, Supporting Information).31 Interestingly, in the case of a3 the unit cell volume of the crystal decreases by 48.10% during transformation from noncentrosymmetric space group P212121 (19) to centrosymmetric (paraelectric) Pbca (61) [whose subgroups includes Pca21 (No. 29), P212121 (No. 19), and P21/c (No. 14)] with half in symmetry elements from 8 (E, C2, 2C2′ i, 3σh) to 4 (E, C2,2C2′) a clear-cut case of second order transition (Scheme S3, Supporting Information) in accordance with the Curie symmetry principle32,33 for ferroelectric and ferroelastic compounds. From a microscopic point of view, the loss of the n-glide mirror can result in the loss of inversion center i. The 2-fold C2 should remain unchanged, leading to the space group P212121. This prompted us to look close at the single crystal data along the polar axis for the crystals grown using slow evaporation (a-type) and the gel mediated technique (btype). The anilinium cations in a1 and b1 are elongated almost perpendicularly along the a-axis in an antiparallel fashion, but they are slightly off-centered by 0.3 Å in b1 generating a polar

Figure 3. Schematic drawing showing systematic generation of chirality in achiral molecule. Nitrate anion present at the vertex of triangles and red-colored (+) show the position of the cationic center.

Unusual crystal growth from Al(NO3)3 directed gel oozes hair-like crystal morphology for AnHNO3-b1, a work reported previously by our group.26 Both AnHNO3-b1 and AnHNO3-a1, show similar crystal packing and the same orthorhombic Pbca space group, Z = 8 (Figure S5 and Table S2, Supporting Information). Similarly, two different morphologies (a3 and b3) for para-ethyl anilinium nitrate (Figure 4) were observed with a space group (orthorhombic, P212121).

Figure 4. Morphology of (a) a3 and (b) b3 crystals; molecular view of compound a3 with thermal ellipsoids is drawn at the 50% probability in the center.

Single crystal X-ray data on these hair-like crystals reveals cations are not equidistance from three neighboring nitrate anions in b1 (3.396, 3.424, 3.434 Å) and b3 (3.411, 3.420, 3.524 Å). Cation in b1 and b3 is NH···O hydrogen bonded to three different nitrate anions with average hydrogen bond lengths of 2.850 and 2.904 Å, respectively. But, more importantly, the nitrogen atom in the nitrate anion remained above the plane formed by three oxygen atoms, by 0.030 and 0.046 Å in b1 and b3. Ferroelectric behavior is one of the best way to observe reversible generation of noncentrosymmetric structure due to alignment of dipoles in individual molecules with the help of an electric field, physical force. Figure 5 shows polarization versus electric field (P-E) hysteresis loops at various frequencies for b1 and b3, where one can study the extent of induced polarization. We have not observed “ferroelectric” behavior for a1, a2, and a3. Figure 5 shows nonzero remnant polarizations (Pr) at zero field and clear hysteresis loop at room temperature. The saturation 5

DOI: 10.1021/acs.cgd.5b01236 Cryst. Growth Des. 2016, 16, 3−7

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binary a-axis. Apart from this a1 differs from b1 in nitrate anions arrangements in two ways (a) the angle between planes having nitrate anion (along ab-axis) increases from 80.59 to 88.64 Å; (b) the distance between the nearest neighbor along the a-axis (parallel) decreases from 1.570 to 0.713 Å, respectively. These microscopic changes lead to non-centrosymmetric structure (Aizu notation). FT-IR spectra also showed lowering of symmetry of the NO3− ion from D3h to C2v or Cs (Scheme S4, Supporting Information). Similarly, structures of a3 and b3 differ, where the latter displayed room temperature ferroelectricity. Here, only in b3 one of the para-ethylAnH+ lies along the binary axis. Apart from this the angle between two diagonally arranged nitrate anions, along the a-axis, differs in b3 (88.20°) and a3 (88.64°). These small changes create microscopic polarity in b3 along the b-axis. Reversible transformation of the centrosymmetric structure to non-centrosymmetric structure with physical forces or in other words structural analysis of the paraelectric and ferroelectric phases of a compound is often difficult and remain challenging because of small displacements of the ions with the highest correlation with each other. Interestingly, in the present case a simple signature can be observed in FT-IR spectra. The NO3− anion has D3h symmetry and shows four distinct vibrational fundamental modes: (a) symmetric stretching (υ1) mode, around 1040 cm−1; (b) antisymmetric stretching (υ3) mode, around 1330 cm−1; (c) out-of-plane bending (υ2) mode, around 800 cm−1; and (d) in-plane bending (υ4) mode, around 730 cm−1. The presence of two υ2 modes in FT-IR spectra of a1 (Figure 4a, 303 K) matches with centrosymmetric structured KNO 3 −II (paraelectric phase, at 295 K) (Supporting Information, Figure S11c), while b1 showed a signature feature of non-centrosymmetric structured KNO3−III (ferroelectric phase). FT-IR spectra of b1 contain two bands at 821 and 834 cm−1, while a1 contains a single band at 825 cm−1. Splitting of υ2 mode into a doublet is suggested due to lowering of the symmetry of the NO3− ion from D3h to C2v or Cs, a requirement of KNO3 type ferroelectric material.34−36 The modes observed at 821−840 (w) cm−1 and 725 (st) cm−1 in a1/b1 have exactly opposite intensity in a3/b3. On the other hand, out-of-plane bending (υ2) modes of NO3− ion in both a3 and b3 showed splitting into two bands at 821 and 825 cm−1 at room temperature. After increasing the temperature (Figure S12c,d, Supporting Information), both of these modes get merged into a single band, with a difference in peak position for b3 and a3 at 825 and 821 cm−1, respectively. These small changes drive ferroelectric behavior observed in b3. Although Raman measurements, ideally suited in the case of NO3− anion, showed distinct changes at RT for these compounds, high temperature measurements remained inconclusive. In KNO3, the ferroelectric ordering is debated but predicted on the basis of rotations of hexagonally packed nitrate anion in a systematic manner to induce polarization. We expect a similar mechanism for ferroelectric behavior in anilinium nitrate salts. However, apart from this what we observed is the change in planarity of the NO3− ligand when the ethyl group is introduced at the para position in AnHNO3. Now, if one considers the more electronegativity of nitrogen in paraethylAnH+ over AnH, then H+ is attracted to the former more effectively. Now, is this motion of H+ responsible for changes in NO3− rotation and hence the observed ferroelectric behavior? If the answer is yes, then it is closely related to KDP type hydrogen bonded ferroelectric behavior. One needs to closely study the “driving force” for ferroelectric behavior in this

system more carefully to reveal the true answer. We are presently working on this issue. Now-a-days obtaining chiral structure from an achiral molecule is no longer a surprise. It is not understood, however, how homochiral packing of helices in crystal can be induced systematically. We observed this here by deciphering an interwoven double helical NH···O hydrogen bonded network to antiparallel single helical arrangement and finally into a homochiral helical network for the resulting chiral structure, as shown in Figure 3. This chiral molecule can form a pro-polar structure by crystallization from gel, a prerequisite for generating a polar structure with minimum physical force, room temperature ferroelectric behavior. In short, we designed a chiral and propolar analogue of KNO3 which showed true “molecular ferroelectric” behavior. The present work on designing a chiral and pro-polar structure, which can be easily tuned by physical forces for reversible transformation into non-centrosymmetric or polar structures, will open a novel approach in designing not only molecular ferroelectrics but in general molecular materials.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.5b01236. Complete synthetic procedure and characterization data, crystal structure data, crystal parameters, ORTEP diagrams, variable temperature single crystal data and FT-IR data (PDF) Accession Codes

CCDC 1009873−1009874, 1408305, 841664, 967489, and 967495−967496 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_ [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS P.S.G. and H.M. thank UGC-DAE consortium for Scientific Research, Kalpakkam Node (CRS-K-04/19) for funding this research programme. P.S.G. thanks Department of Science and Technology, New Delhi (SR/S1/IC-43/2009) for financial support. We also thank Mr. Dasari Ganga Prasad for preliminary work. Authors thank DST-PURSE Single Crystal X-ray Diffraction facility at the Faculty of Science, The Maharaja Sayajirao University of Baroda, Vadodara. The authors thank Dr. V. Raghavendra Reddy, UGC-DAE consortium for Scientific Research, Indore for PE loop measurements.

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REFERENCES

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DOI: 10.1021/acs.cgd.5b01236 Cryst. Growth Des. 2016, 16, 3−7