New Molecular Ferroelectrics Accompanied by Ultrahigh Second

Apr 25, 2016 - Ordered Matter Science Research Center, College of Chemistry and Chemical Engineering, Southeast University, Nanjing 211189, People's R...
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New Molecular Ferroelectrics Accompanied by Ultrahigh SecondHarmonic Generation Chuang Liu,† Kaige Gao,† Zepeng Cui,† Linsong Gao,† Da-Wei Fu,‡ Hong-Ling Cai,*,† and X. S. Wu*,† †

Collaborative Innovation Center of Advanced Microstructures, Lab of Solid State Microstructures, School of Physics, Nanjing University, Nanjing 210093, People’s Republic of China ‡ Ordered Matter Science Research Center, College of Chemistry and Chemical Engineering, Southeast University, Nanjing 211189, People’s Republic of China S Supporting Information *

ABSTRACT: Second-harmonic generation (SHG) is one of the outstanding properties for practical applications. However, the great majority of molecular ferroelectric materials have very low nonlinear optical coefficients, attenuating their attractive performance. Here we synthesized (4-amino-2-bromopyridinium)(4-amino-2-bromopyridine)tetrafluoroborate (1), whose second-order nonlinear optical coefficient reaches up to 2.56 pm V−1, 2.67 times of that of KDP, and (4-amino-2-bromopyridinium)tetrafluoroborate (2), possessing a more incredible large second-order nonlinear optical coefficient as high as 10.24 pm V−1, 10.67 times that of KDP. The compound 1 undergoes two reversible phase transitions at around T1 = 244.1 K and T2 = 154.6 K, caused by dramatic changes of the protonated cations and order−disorder of anions, which was disclosed by differential scanning calorimetry, heat capacity, dielectric anomalies, SHG, and singlecrystal X-ray diffraction analysis. The pyroelectric measurements reveal that compound 1 is a Rochelle salt type ferroelectric, which has a large spontaneous polarization of about 3 μC/cm2.

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research the most promising alternatives, molecule ferroelectrics. Ferroelectric crystals, a kind of noncentrosymmetric (NCS) crystal belonging to the 10 polar point groups (C1, C2, C3, C4, C6, Cs, C2v, C3v, C4v, and C6v), simultaneously show many functionally relevant outstanding performances, such as piezoelectricity, pyroelectricity, and second-harmonic generation (SHG). There are lots of NCS materials with exceedingly excellent functional properties, such as lithium borate (LBO),28 barium borate (BBO),29 potassium fluoroboratoberyllate (KBBF),30 and deuterated potassium phosphate (KDP).31 During the assemblies and syntheses of new molecule-based ferroelectric crystals, we scheme out two molecular ferroelectrics with a very large SHG effect: (Habpy)(abpy)BF4 (abpy = 4-amino-2-bromopyridine) (1) (Figure 1a) and (Habpy)BF4 (2) (Figure 1b), which have extraordinary large second-order NLO coefficients reaching up to 2.56and 10.24 pm V−1, respectively, distinguished from other molecule ferroelectric materials, such as (pyrrolidinium)MnCl3 (0.58 pm V−1),16 4-(cyanomethyl)anilinium perchlorate (0.6 pm V−1),26 and 4-methoxyanilinium tetrafluoroborate 18-crown-6 (1.2 pm V−1).27 Herein, we describe the syntheses, crystal structure, and excellent properties by means of differential

olecular ferroelectrics, a sort of multifunctional electroactive materials, carrying outstanding characteristics of light weight, good processability, mechanical flexibility, and structural tenability in contrast to inorganic ferroelectrics, have widespread applications recently in the emerging fields of sensors devices, ferroelectric random access memory, and nonlinear optical (NLO) devices on account of high dielectric constant and electro-optic effects.1−6 Originally, the association between organic molecules and ferroelectricity was discovered in sodium potassium tartrate tetrahydrate, the so-called Rochelle salt, which contains organic tartrate ions, by J. Valasek7 in 1921. Very soon, the discovery and design of ferroelectrics were on the heels of another, such as KH2PO4 (KDP),8 BaTiO3,9 LiNbO3, and so on. New approaches for organic ferroelectrics have achieved considerable progress, in view of the contemporary ferroelectric mechanisms that consist of charge-transfer complexes of electron donors and acceptors, proton donor−acceptor compounds with intermolecular hydrogen bonds,10−13 metal−organic frameworks analogous to perovskite,14−17 rotator−stators on the strength of crown ether,18−20 and so forth.4,5,21−23 Many significative organic ferroelectrics were discovered, for instance, diisopropylammonium bromide,24,25 4-(cyanomethyl)anilinium perchlorate,26 4methoxyanilinium tetrafluoroborate 18-crown-6,27 and so forth. In consideration of the shortcoming of inorganic ferroelectrics and the bright prospect of organic ferroelectrics, there are a lot of opportunities and challenges for physicists and chemists to © XXXX American Chemical Society

Received: March 17, 2016 Accepted: April 25, 2016

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Figure 1. Molecular structure of compounds 1 (a) and 2 (b).

scanning calorimetry (DSC), specific heat capacity (HC), single-crystal X-ray diffraction (XRD) analysis, Raman spectra, anisotropic dielectric anomalies, SHG, pyroelectric measurement, and ferroelectric hysteresis loops. To ascertain the structure and purity of compounds 1 and 2, we quantitatively measured the mass fractions of carbon, hydrogen, and nitrogen in compounds 1 and 2 by means of CHN elemental analysis. The results are 27.65, 2.48, and 12.88% for compound 1 and 22.94, 2.29, and 10.73% for compound 2, which are coincident with the theoretical values (27.7, 2.54, and 12.91% for compound 1 and 23.0, 2.3, and 10.74% for compound 2). These two single crystals are very stable without any deliquescence and decomposition in the temperature range of 100−400 K up to 5 months. As necessary evidence, calorimetric measurements play an advanced role in demonstrating phase transition behaviors that are involved in absorbing or releasing of heat. The temperature-dependent DSC curve (Figure 2a) shows two endothermic peaks at T2 = 154.6 K and T1 = 244.1 K in the heating process and two exothermic peaks at around T1 = 239.1 K and T2 = 145.5 K in the cooling process, which discloses two couples of reversible phase transitions. For convenience, the phases above T1, between T1 and T2, and below T2 are defined as hightemperature phase (HTP), intermediate-temperature phase (ITP), and low-temperature phase (LTP), respectively. The entropy change (ΔS) at around T1 and T2 estimated from the DSC are about 1.78 and 0.656 J mol−1 K−1. In consideration of the Boltzmann equation, ΔS = R ln N, where R is the universal gas constant and N represents the ratio of independent positions with equal probability in the crystal phases above and below the phase transition temperature, the values of N(T1) and N(T2) are calculated to be about 1.24 and 1.08, manifesting the disorder−order feature of the first phase transition and displacive mechanism of the second phase transition. HC measurements are also performed in the temperature range between 130 and 270 K (Figure 2a), which show two obvious thermal anomalies at T1 and T2, consistent exactly with the DSC measurements. Nevertheless, there is not any thermal anomalies in compound 2 in the investigating temperature range (Figure S1), indicating that no phase transitions take place.

Figure 2. (a) The temperature dependence of DSC and Cp of compound 1 in the heating and cooling runs, revealing the phase transitions at 244 and 155 K. (b) The temperature dependence of the real part (ε′) of the complex dielectric constant of compound 1 in cooling and heating processes. The inset is zoomed to show the phase transition at T2. (c) The comparison of real parts of dielectric constants measured along different axes at a frequency of 5 kHz.

In general, accompanied by the structural phase transition induced by the pressure, temperature, electric field, and shock wave, prominent anomalies in dielectric behaviors will be engendered, which is associated with dipolar motions, such as molecular disorder or rotations, and is significant evidence of a ferroelectric phase transition. In good accordance with thermal anomalies, these are indeed observed in the reversible heating− cooling cycles (Figure 2b) in compound 1. The temperaturedependent permittivity ε′ variation (the real part (ε′) of the complex dielectric constant ε = ε′ − iε″, where ε″ is the imaginary part) shows a sharp peak at T1 and step-like anomaly at T2 (inset of Figure 2b) at various frequencies, which correlates well with DSC results. For the first sharp peak at T1, as the main characteristic of a ferroelectric transition, the dielectric peak value at a frequency of 500 Hz is up to 2075, which is about 1 order of magnitude larger than that at room temperature (about 196). Nevertheless, the anomaly becomes step-like at T2, with a double aggrandizement from LTP to ITP, which is a unique characteristic of improper ferroelectrics at the 1757

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Figure 3. Projects of compound 1 in (a) the HTP (293 K) along the common b axis, (b) the ITP (223 K) along the common b axis, and (c) the LTP (103 K) along the [101] direction, showing the similarities of the crystal structures and the differences of orientation states of the cations and the order−disorder of anions. (d) Projects of the compound 2 along the common b axis.

cation, one unprotonated 4-amino-2-bromopyridine, and one tetrafluoroborate anion, while compound 2 (Table S2) consists of one protonated 4-amino-2-bromopyridinium cation and one tetrafluoroborate anion. The families of pyridinium salts are well-known to undergo two phase transitions in the temperature range from 100 to 400 K (Table S3). It is worthy of note that these pyridinium salts32−34 possess three phases with a couple of centrosymmetric structures, while compound 1

second phase transition. Improper ferroelectrics are different from the proper ferroelectrics, which is attributed to their switchable dipole moment among the phase transition temperature. According to the Landau theory of phase transition, the order parameter of the phase transition of an improper ferroelectric is determined by another physical quantity instead of spontaneous polarization, which is a secondary effect in improper ferroelectric transitions. The dielectric constants in the ITP decrease sharply with increasing frequency from 500 Hz to 1 MHz. However, the dielectric constants in the HTP and LTP make a slight difference at variational frequencies. The permittivity along different crystallographic axes shows significant anisotropy (Figure 2c). The permittivity along the c axis, as mentioned above, shows remarkable anomaly at the transition temperature of T1. However, the permittivities along the other two directions become much smaller. The anomaly of the permittivity along the b axis is almost negligible. These outstanding dielectric properties, including ultrahigh dielectric constant and significant anisotropy, which could be tuned in three dielectric states and switched by the stepwise phase transitions, make compound 1 a promising dielectric for multifunctional electric devices. As mentioned, compound 1 reveals two reversible phase transitions. It facilitates us to determine a series of single-crystal structures at variable temperatures of 293, 223, and 103 K, corresponding to the three phases (Figure 3a−c), in order to confirm the phase transitions and understand the origin of the ferroelectricity. It is very strange that the crystal symmetry of compound 1 at these three temperatures belongs to a space group of Cc, which is a monoclinic crystal system. Nevertheless, compound 2 belongs to a space group of Pca21, an orthorhombic crystal symmetry (Figure 3d), without any phase transitions in the investigated temperature range. Crystal structural determination reveals that compound 1 (Table S1) is composed of one protonated 4-amino-2-bromopyridinium

244K

155K

= Cc =⇒ = Cc , follows the sequence of NCS space groups, Cc =⇒ and compound 2 reveals Pca21 NCS space groups without any phase transitions within the investigative temperature scope. For convenience, the protonated 4-amino-2-bromopyridinium cations are denoted as P, while those unprotonated are represented as U in Figure 3. The crystal structure of compound 1 (Figure 3a) consists of one 4-amino-2bromopyridinium cation and a disordered BF4− as the counterpart anion as well as a 4-amino-2-bromopyridine at room temperature. At a temperature of 223 K (Figure 3b), the ITP of the compound remains a polar space group, in which the tetrahedral BF4− anions are still disordered accompanied by some displacive deformation. The geometry details reveal that the bond angle of F1−B−F2 transforms from 64.5(4) to 71.6(5)° with an increment of 7.1(1)°, that of F3−B−F4 transforms from 35.1(3) to 30.5(4)° with a variation of −4.5(9)°, that of F5−B−F6 decreases from 86.6(5) to 69.0(6)° with a decrement of 17.5(9)°, and that of F7−B−F8 increases from 21.4(3) to 24.0(4)° with a variation of 2.6(1)° from HTP to ITP, respectively. In addition, the change of the dynamical state of the cations also makes contribution to the reversible phase transition around T1. Raman spectra (Figure S2) of compound 1 show striking differences between HTP and ITP, which confirms the mechanism and dynamics of the phase transitions. The differences are not only ascribed to the symmetric deformation and asymmetric stretching of BF4− anions but also to the torsion mode, out-of-plane bending, and in-plane bending of the cations.35,36 At a temperature of 1758

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at about 155 K in the cooling process, revealing a first-order phase transition. In the warming process, it shows a reversible phase transition with very small thermal hysteresis. The transition temperatures observed from SHG are consistent with those observed from DSC and dielectric properties. It confirms that there exist exactly two structural phase transitions, though the variations observed from XRD are small within a same space group. In addition, the SHG activity and performance of compound 1 are still very well without any attenuation after 10 heating−cooling circulations, manifesting compound 1’s unexceptionable switching reversibility. The difference of SHG properties between compounds 1 and 2 reveals that the variation of polarization is one of the crucial factors affecting the SHG intensity. The spontaneous polarization of ferroelectrics can be determined from pyroelectric behavior because ferroelectrics are definitely pyroelectric materials. The temperature dependence of spontaneous polarization (Ps) can be obtained by integrating the pyroelectric current i in the circuit between two polar surfaces in a heating process after poling with a +300 or −300 V voltage (Figure 5a). Ps can be calculated by i Ps = ∫ AR dT , where A is the area of the sample and R is the heating or cooling rate. After poling with a voltage of +300 V (−300 V), two inverse pyroelectric current peaks with values of −2.75 nA (2.77 nA) at T2 and +3.23 nA (−3.21 nA) at T1 are detected, indicating the screen charge releasing and accumulating processes. Usually, crystal symmetry increases with increasing temperature. The integral polarizations under a +300 V (−300 V) poling voltage are about 0.85 μC cm−2 (−0.92 μC cm−2) in the LTP and 3.29 μC cm−2 (−3.13 μC cm−2) in the ITP, respectively, assuming that of HTP to be zero. Moreover, two step-like anomalies instead of the gradual enhancement indicate that the two phase transitions should be of first-order, coincident with the results of the DSC, dielectric measurements, and SHG in compound 1. The temperaturedependent spontaneous polarization behavior is very similar to that of BaTiO3, which has a step-like increase during ferroelectric to ferroelectric transitions (space group R3m → Amm2 → P4mm) and then gradually decreases to zero during the ferroelectric to paraelectric transition (P4mm → Pm3m). The value of the temperature-dependent spontaneous polarization behaves likes the second-order nonlinear optic susceptibility χ (2) (Figure 4), consistent with Landau phenomenological theory χ(2) = 6ε0βPs, where β is almost independent of the temperature. Nevertheless, no temperature-dependent pyroelectric current can be detected in compound 2 by means of the abovementioned method, which indicates that the polarization of compound 2 has no variation because it has no phase transition. The Chynoweth technique can measure the pyroelectric current eliminating the effects of charge injection, in which samples are heated or cooled periodically by turning on or off a pulse laser. Taking this technique into application, we can observe the pyroelectric current (Figure 5b) and the integrated spontaneous polarization (Figure 5c) in a period of 100 s, with poling a 300 V voltage to the sample before the measurement. The pyroelectric coefficients of compound 2 are calculated to be about 191.5 μC m−2 K−1, smaller than that of TGS (550 μC m−2 K−1) but larger than that of PVDF (27 μC m−2 K−1). First, when the laser is turned off, the temperature of the sample decreases quickly and then remains a constant value, which induces the pyroelectric current to decrease abruptly and then

103 K (Figure 3c), although the space group is still Cc, an apparent structural change occurs. Half of the disordered BF4− anions in the ITP become ordered, which gives rise to the single orientation of these anions, generating the remnant polarization due to symmetry breaking. Furthermore, it is unambiguously disclosed that some 4-amino-2-bromopyridinium cations (P) are deprotonated accompanied by the protonated process of the corresponding 4-amino-2-bromopyridine (U), that is, the protonic migration phenomenon, which weakens the spontaneous polarization of ITP. The actual ferroelectricity of compound 1 may show both order−disorder of BF4− and moving protonic displacive characteristics, which are not mutually exclusive. However, no structure transition is observed in compound 2, and it includes only one protonaccepting 4-amino-2-bromopyridinium cation and one protondonating BF4− (Figure 1b). What is more, the spontaneous polarization is similar to that of compound 1, originating from the behavior of protonated cations in the c direction, depicted in Figure 3d. SHG is very sensitive to symmetry breaking, especially to those caused by hydrogen bonds that cannot be revealed by XRD. The numerical values of the NLO coefficients for SHG are determined by comparison with a KDP reference. As expected, compounds 1 and 2 are both SHG-active at room temperature because they belong to noncentrosymmetric space groups of Cc and Pca21, respectively. They have very large SHG intensity (Figure 4); for example, the SHG intensity of 1 is

Figure 4. Variable-temperature second-order NLO coefficients of the powder samples of compounds 1 and 2.

about 2.67 times of that of KDP; particularly, the SHG intensity of 2 (Figure S4) is about 10.67 times of that of KDP and 1.3 times that of potassium titanyl phosphate (KTP),37 which is a very notable NLO crystal with a large second-order nonlinear coefficient and low cost. Compound 2 keeps this high SHG intensity in the whole measurement temperature range of 130− 300 K due to the fact that no phase transitions occur, as determined from XRD and DSC, which is important and useful for a potential NLO crystal candidate for frequency doubling, tunable microwave devices, and optical parametric oscillators. Although the space group of compound 1 is Cc, a NCS one, in the whole investigative temperature range from 130 to 300 K determined from XRD, very apparent phase transition evidence can be observed from SHG measurement (Figure 4). The second-order NLO coefficient (χ(2)) of compounds 1 and 2 can be calculated by comparing with that of KDP (0.96 pm V−1). The χ(2) of compound 1 increases sharply from 2.56 to 6.24 pm V−1 at about 244 K and then decreases sharply to 2.05 pm V−1 1759

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Figure 6. Ferroelectric hysteresis loops of compound 1 at different temperatures with a frequency of 50 Hz in the [101] direction.

PZT (20−80 kV cm−1),38 and PVDF (500 kV cm−1).5 The Ps is evaluated to be about 1.38 μC cm−2, with remanent polarization (Pr) reaching 0.35 μC cm−2. Compared with these recently reported small-molecule organic ferroelectrics, the Ps of compound 1 is evidently larger than that of Rochelle salt (0.2 μC cm−2),7 (2,6-diisopropylanilinium)([18]crown6)]BF4 (0.3 μC cm−2),20 and [4-NH2C5H4NH][SbCl4] (0.35 μC cm−2)39 and comparable to that of methoxyanilinium perrhenate 18-crown-6 (1.2 μC cm−2),27 [H-55DMBP][Hia] (1.2 μC cm−2),40 and (C5H9NH3) (CdCl3) (1.7 μC cm−2)15 while significantly smaller than that of (3-pyrrolinium) (CdCl3) (5.1 μC cm−2),41 (pyrrolidinium)MnBr3 (6 μC cm−2),17 and diisopropylammonium bromide (23 μC cm−2).24 In summary, this work has successfully demonstrated that (Habpy) (abpy)BF4 (1) and (Habpy)BF4 (2) are two aboveroom-temperature molecular ferroelectrics with extraordinary second-order NLO coefficient, reaching up to 2.56 and 10.24 pm V−1, respectively. Compound 1 undergoes two reversible phase transitions at around T1 = 244 K and T2 = 155 K, disclosed by DSC, HC, dielectric anomalies, SHG, pyroelectric measurement, single-crystal XRD analysis, and hysteresis loops. The SHG intensity of 2 is even larger than that of KTP, which suggests that compound 2 is a potential NLO crystal candidate for tunable visible laser sources, microwave devices, and optical parametric oscillators.

Figure 5. (a) The temperature-dependent pyroelectric currents of compound 1 polarized by opposite electric voltages (±400 V) and the temperature dependence of polarization determined by integration of pyroelectric currents. (b) Pyroelectric current density of compound 2 modulated by a pulse laser at room temperature. (c) The integral pyroelectric current density of compound 2.

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decay to zero in about 24 s. In the next moment, the laser is turned on, and the inverse result is measured. Furthermore, it is non-negligible that the current pulse was obtained after many circulations without any attenuation at room temperature. To further verify the ferroelectricity, the basic characterization of spontaneous polarization was confirmed by the measurements of polarization−electric field (P−E) hysteresis loops. P−E hysteresis loops for various temperatures were obtained at a frequency of 50 Hz in the [101] direction in the ITP (Figure 6). It reveals that the coercive fields (Ec) of compound 1 increase slightly, while the spontaneous polarizations decrease slightly with decreasing temperature. The moving protons only within the hydrogen bond would generally contribute to the minimizing steric difficulties for ferroelectricity. Hence, the coercive field of compound 1 is on the order of 1.8 kV cm−1, lower than that of most the smallmolecule organic ferroelectrics and the metal−organic ferroelectrics, and is 3 orders of magnitude lower than that of polymer ferroelectrics, for instance, (pyrrolidinium)MnCl3 (2.3 kV cm−1),16 diisopropylammonium bromide (5 kV cm−1),24 (2,6-diisopropylanilinium)([18]crown-6)]BF4 (12 kV cm−1),20

EXPERIMENTAL SECTION Synthesis. Being of reagent grade, all reagents and solvents were used without further purification. Compounds 1 and 2 were obtained by slow evaporation of a clear ethanol solution containing a corresponding molar ratio of 4-amino-2bromopyridine and tetrafluoroboric acid. Compounds 1 and 2 were prepared by dissolving the 4-amino-2-bromopyridine into tetrafluoroboric acid solution at a mole ratio of 1:1 and 1:2 at room temperature. The transparent block-shaped crystals of 1 and the colorless needle-like crystals of 2 were achieved (Figure S3). Measurement Section. The elemental analysis was recorded on a Heraeus CHN-0-Rapid elemental analyzer. Single-crystal XRD experiments were carried out using a Rigaku Saturn 924 diffractometer with Mo−Kα radiation (λ = 0.71073 Å) at various temperatures. The structures were solved by direct methods and refined by the full-matrix method based on F2 using the SHELXLTL software package. All nonhydrogen atoms were refined anisotropically, and the positions of all hydrogen atoms were generated geometrically. DSC and HC 1760

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(7) Valasek, J. Piezo-electric and allied phenomena in Rochelle salt. Phys. Rev. 1921, 17, 475−481. (8) Busch, G.; Scherrer, P. Eine neue seignette-elektrische Substanz. Naturwissenschaften 1935, 23, 737−737. (9) Cross, L.; Newnham, R. History of ferroelectrics. Ceramics and Civilization; The American Ceramic Society, Inc., 1987; Vol. 3, pp 289−305. (10) Horiuchi, S.; Tokunaga, Y.; Giovannetti, G.; Picozzi, S.; Itoh, H.; Shimano, R.; Kumai, R.; Tokura, Y. Above-room-temperature ferroelectricity in a single-component molecular crystal. Nature 2010, 463, 789−792. (11) Horiuchi, S.; Kumai, R.; Tokunaga, Y.; Tokura, Y. Proton dynamics and room-temperature ferroelectricity in anilate salts with a proton sponge. J. Am. Chem. Soc. 2008, 130, 13382−13391. (12) Gao, K.; Gu, M.; Qiu, X.; Ying, X.; Ye, H.-Y.; Zhang, Y.; Sun, J.; Meng, X.; Zhang, F.; Wu, D.; et al. Above-room-temperature molecular ferroelectric and fast switchable dielectric of diisopropylammonium perchlorate. J. Mater. Chem. C 2014, 2, 9957−9963. (13) Sun, Z.; Chen, T.; Luo, J.; Hong, M. Bis (imidazolium) LTartrate: A Hydrogen-Bonded Displacive-Type Molecular Ferroelectric Material. Angew. Chem., Int. Ed. 2012, 51, 3871−3876. (14) Liao, W.-Q.; Zhang, Y.; Hu, C.-L.; Mao, J.-G.; Ye, H.-Y.; Li, P.F.; Huang, S. D.; Xiong, R.-G. A lead-halide perovskite molecular ferroelectric semiconductor. Nat. Commun. 2015, 6, 7338. (15) Zhang, Y.; Ye, H.-Y.; Zhang, W.; Xiong, R.-G. Roomtemperature ABX 3-typed molecular ferroelectric:[C5H9−NH3][CdCl3]. Inorg. Chem. Front. 2014, 1, 118−123. (16) Zhang, Y.; Liao, W.-Q.; Fu, D.-W.; Ye, H.-Y.; Chen, Z.-N.; Xiong, R.-G. Highly Efficient Red-Light Emission in An OrganicInorganic Hybrid Ferroelectric: (Pyrrolidinium)MnCl3. J. Am. Chem. Soc. 2015, 137, 4928−4931. (17) Zhang, Y.; Liao, W.-Q.; Fu, D.-W.; Ye, H.-Y.; Liu, C.-M.; Chen, Z.-N.; Xiong, R.-G. The First Organic-Inorganic Hybrid Luminescent Multiferroic: (Pyrrolidinium)MnBr3. Adv. Mater. 2015, 27, 3942. (18) Ye, H. Y.; Zhang, Y.; Fu, D. W.; Xiong, R. G. A Displacive-Type Metal Crown Ether Ferroelectric Compound: Ca(NO3)2 (15-crown5). Angew. Chem., Int. Ed. 2014, 53, 6724−6729. (19) Fu, D.-W.; Zhang, W.; Cai, H.-L.; Zhang, Y.; Ge, J.-Z.; Xiong, R.-G.; Huang, S. D. Supramolecular bola-like ferroelectric: 4methoxyanilinium tetrafluoroborate-18-crown-6. J. Am. Chem. Soc. 2011, 133, 12780−12786. (20) Ye, H.-Y.; Li, S.-H.; Zhang, Y.; Zhou, L.; Deng, F.; Xiong, R.-G. Solid State Molecular Dynamic Investigation of An Inclusion Ferroelectric:[(2, 6-Diisopropylanilinium)([18] crown-6)] BF4. J. Am. Chem. Soc. 2014, 136, 10033−10040. (21) Sun, Z.; Luo, J.; Zhang, S.; Ji, C.; Zhou, L.; Li, S.; Deng, F.; Hong, M. Solid-State Reversible Quadratic Nonlinear Optical Molecular Switch with an Exceptionally Large Contrast. Adv. Mater. 2013, 25, 4159−4163. (22) Sun, Z.; Tang, Y.; Zhang, S.; Ji, C.; Chen, T.; Luo, J. Ultrahigh Pyroelectric Figures of Merit Associated with Distinct Bistable Dielectric Phase Transition in a New Molecular Compound: Di-nButylaminium Trifluoroacetate. Adv. Mater. 2015, 27, 4795−4801. (23) Sun, Z.; Chen, T.; Liu, X.; Hong, M.; Luo, J. Plastic Transition to Switch Nonlinear Optical Properties Showing the Record High Contrast in a Single-Component Molecular Crystal. J. Am. Chem. Soc. 2015, 137, 15660−15663. (24) Fu, D.-W.; Cai, H.-L.; Liu, Y.; Ye, Q.; Zhang, W.; Zhang, Y.; Chen, X.-Y.; Giovannetti, G.; Capone, M.; Li, J.; et al. Diisopropylammonium bromide is a high-temperature molecular ferroelectric crystal. Science 2013, 339, 425−428. (25) Gao, K.; Liu, C.; Cui, Z.; Zhu, J.; Cai, H.-L.; Wu, X. Roomtemperature growth of ferroelectric diisopropylammonium bromide with 12-crown-4 addition. CrystEngComm 2015, 17, 2429−2432. (26) Cai, H.-L.; Zhang, W.; Ge, J.-Z.; Zhang, Y.; Awaga, K.; Nakamura, T.; Xiong, R.-G. 4-(Cyanomethyl) anilinium perchlorate: a new displacive-type molecular ferroelectric. Phys. Rev. Lett. 2011, 107, 147601.

were measured on a NETZSCH DSC 200F3 in the temperature range of 120−350 K with a heating or cooling rate of 10 K/min under a nitrogen atmosphere. Otherwise, Raman spectra were collected using a Horiba Jobin Yvon HR800 spectrometer device by means of a 633 nm laser line from an air-cooled Ar ion laser. As for SHG experiments, an unexpanded laser beam with low divergence (pulsed Nd:YAG at a wavelength of 1064 nm, 5 ns pulse duration, 1.6 MW peak power, 10 Hz repetition rate) was used. The numerical values of the NLO coefficients for SHG have been determined by comparison with a KDP reference. The crystals were cut into the form of thin plates perpendicular to the crystal axes with an area of about 4 mm2 and thickness of about 0.8 mm and then deposited with silver conduction as electrodes before the measurements of anisotropic dielectric anomalies, pyroelectrics, and P−E hysteresis loops. Complex dielectric permittivities were measured with a Tonghui TH2828A LCR meter. To obtain the temperature dependence of spontaneous polarization, it was necessary to integrate the pyroelectric current measured using an electrometer (Keithley 6517B) with a constant heating rate. Moreover, by utilizing the Chynoweth technique at room temperature, the dynamic pyroelectric current was obtained by changing the temperature of the sample periodically using a pulsed laser with a power of 100 mW and wavelength of 1.47 mm modulated at a low frequency of 0.1 Hz. With regard to P−E hysteresis loops, we recorded them on a Precision Premier II (Radiant Technologies, Inc.).

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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/acs.jpclett.6b00618. Tables S1−S3 and Figures S1−S4: crystallographic information files for compound 1 at temperatures of 103, 223, and 293 K and compound 2 at a temperature of 293 K (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (H.-L.C.). *E-mail: [email protected] (X.S.W.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundations of China (11574138, 21427801, U1332205, 11274153), the Top-Notch Young Talents Program of China, the Project 973 (2014CB848800), and Dengfeng Project B of Nanjing University.

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REFERENCES

(1) Jona, F.; Shirane, G. Ferroelectric crystals; Pergamon, 1962; Vol. 1. (2) Lines, M. E.; Glass, A. M. Principles and applications of ferroelectrics and related materials; Oxford University Press, 1977. (3) Smolenskiĭ, G. Ferroelectrics and related materials; Taylor & Francis, 1984; Vol. 3. (4) Zhang, W.; Xiong, R.-G. Ferroelectric metal−organic frameworks. Chem. Rev. 2012, 112, 1163−1195. (5) Horiuchi, S.; Tokura, Y. Organic ferroelectrics. Nat. Mater. 2008, 7, 357−366. (6) Scott, J. F. Applications of modern ferroelectrics. Science 2007, 315, 954−959. 1761

DOI: 10.1021/acs.jpclett.6b00618 J. Phys. Chem. Lett. 2016, 7, 1756−1762

Letter

The Journal of Physical Chemistry Letters (27) Fu, D.-W.; Cai, H.-L.; Li, S.-H.; Ye, Q.; Zhou, L.; Zhang, W.; Zhang, Y.; Deng, F.; Xiong, R.-G. 4-Methoxyanilinium Perrhenate 18Crown-6: A New Ferroelectric with Order Originating in Swinglike Motion Slowing Down. Phys. Rev. Lett. 2013, DOI: 10.1103/ PhysRevLett.110.257601. (28) Chen, C.; Wu, Y.; Jiang, A.; Wu, B.; You, G.; Li, R.; Lin, S. New nonlinear-optical crystal: LiB3O5. J. Opt. Soc. Am. B 1989, 6, 616−621. (29) Chen, C.; Wu, B.; Jiang, A.; You, G. A new-type ultraviolet SHG crystal-β-BaB2O4. Sci. Sinca (Ser. B) 1985, 28, 235−243. (30) Wu, B.; Tang, D.; Ye, N.; Chen, C. Linear and nonlinear optical properties of the KBe2 BO3F2 (KBBF) crystal. Opt. Mater. 1996, 5, 105−109. (31) Eimerl, D. Electro-optic, linear, and nonlinear optical properties of KDP and its isomorphs. Ferroelectrics 1987, 72, 95−139. (32) Pająk, Z.; Czarnecki, P.; Wąsicki, J.; Nawrocik, W. Ferroelectric properties of pyridinium periodate. J. Chem. Phys. 1998, 109, 6420− 6423. (33) Wasicki, J.; Czarnecki, P.; Pajak, Z.; Nawrocik, W.; Szczepanski, W. Ferroelectric properties of pyridinium perrhenate. J. Chem. Phys. 1997, 107, 576−579. (34) Czarnecki, P.; Nawrocik, W.; Pajak, Z.; Wasicki, J. Ferroelectric properties of pyridinium tetrafluoroborate. Phys. Rev. B: Condens. Matter Mater. Phys. 1994, 49, 1511. (35) Kandasamy, M.; Velraj, G. Molecular structure and vibrational spectra of 3-and 4-amino-2-bromopyridine by density functional methods. Spectrochim. Acta, Part A 2012, 91, 206−216. (36) Heimer, N. E.; Del Sesto, R. E.; Meng, Z.; Wilkes, J. S.; Carper, W. R. Vibrational spectra of imidazolium tetrafluoroborate ionic liquids. J. Mol. Liq. 2006, 124, 84−95. (37) Bierlein, J. D.; Vanherzeele, H. Potassium titanyl phosphate: properties and new applications. J. Opt. Soc. Am. B 1989, 6, 622−633. (38) Foster, C.; Bai, G.-R.; Csencsits, R.; Vetrone, J.; Jammy, R.; Wills, L.; Carr, E.; Amano, J. Single-crystal Pb (ZrxTi1‑x)O3 thin films prepared by metal-organic chemical vapor deposition: Systematic compositional variation of electronic and optical properties. J. Appl. Phys. 1997, 81, 2349−2357. (39) Jakubas, R.; Ciunik, Z.; Bator, G. Ferroelectric properties of 4NH2C5H4NH SbCl4. Phys. Rev. B: Condens. Matter Mater. Phys. 2003, DOI: 10.1103/PhysRevB.67.024103. (40) Horiuchi, S.; Kumai, R.; Tokura, Y. A supramolecular ferroelectric realized by collective proton transfer. Angew. Chem., Int. Ed. 2007, 46, 3497−3501. (41) Ye, H. Y.; Zhang, Y.; Fu, D. W.; Xiong, R. G. An Above-RoomTemperature Ferroelectric Organo-Metal Halide Perovskite:(3-Pyrrolinium) (CdCl3). Angew. Chem. 2014, 126, 11424−11429.

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