Reversible Thermal Dielectric Switch Triggered by Blooming-Flower

Aug 2, 2018 - Reversible Thermal Dielectric Switch Triggered by Blooming-Flower Structural Phase Transition in Ionic Crystal without Metal. Yu-Wei Zha...
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Reversible Thermal Dielectric Switch Triggered by Blooming-Flower Structural Phase Transition in Ionic Crystal without Metal Yu-Wei Zhang, Ping-Ping Shi, Wan-Ying Zhang, Qiong Ye,* and Da-Wei Fu* Ordered Matter Science Research Center, Jiangsu Key Laboratory for Science and Applications of Molecular Ferroelectrics, Southeast University, Nanjing 211189, China

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S Supporting Information *

ABSTRACT: Due to having excellent properties of sensitive switchable physical and/or chemical response, simple preparation, and environmentally friendly processing, bistable switches (electric switching between “on” and “off” bistable states) have gradually developed into an ideal class of highly smart materials. However, most of them contain metals, especially heavy metals, which are highly toxic to the environment, and metal-free switch materials are rarely reported. Based on this issue, we successfully designed and synthesized organic ion crystals and realized thermal dielectric switching characteristics. Differential scanning calorimetry and dielectric measurements show that the large-size crystal (FTEDA)(BF4)2 (1) can be regarded as an sensitive dielectric bistable switching between high (switch on) and low (switch off) dielectric states. Variable-temperature single crystal structure reveals one-half of the BF4− anions in the crystal undergoes order− disorder transition around 200 K, similar to the transition between flower buds and blooming flowers. This flower-style transition of BF(1)4−/BF(0.5)8− triggered the rapid switching performance; those properties establish the basis of their applications in excellent temperature-responsive electrical switches, especially lightweight devices.

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properties of crystallization.30,31 Experimentally, the method of template pattern, namely, introducing the order−disorder motion or liable to twisted section, is widely employed. Further, the use of predesigned inorganic anions and organic cations as the specific basic building units has proven successful for the construction of crystals.20,32 The 1,4-diazoniabicyclo[2.2.2]octane (dabco) has to be mentioned, which undergoes order−disorder transition under external stimulation because of its high symmetry and structural flexibility. Among them, well-known examples include the dabco monosalts, namely, (Hdabco)A, where A = ClO4−, BF4−, ReO4−, Br−, and I−, exhibiting striking thermodynamic properties.33,34 Incomprehensibly, the reports focusing on reversible structural phase transition about dabco derivatives have been scarce. Some compounds, containing BF4− anion which is prone to cause order−disorder phase transition, are synthesized and systematically characterized. Intriguing examples, i.e., (Hdabco)(BF4)35,36 and [(H2dabco)(H2O)](BF4)2,37 which exhibit a range of solid−solid phase transitions due to the dynamics or motions of the anions. All of these findings reveal that BF4− anion can be used as a substrate in the search for stimuli-responsive materials. Herein, a novel organic ionic crystal (F-TEDA)(BF4)2 (1, F-TEDA = 1chloromethyl-4-fluoro-1,4-diazoniabicyclo[2.2.2]octane) aroused our interest because of its similar structural characteristic, whose interesting structure has been preliminarily disclosed by Banks et al.38 However, the study on the

INTRODUCTION Switchable materials which hold multiple alternative states so that they can be widely used as switch of electrical signal, including optoelectronic devices, sensors, actuators, capacitors, and data storage, have received extraordinary research attention due to not only the strictly fundamental features they pose, but also in electrical and electronic industries in which they are basic elements.1−15 Considered as stimuliresponsive or solid-state materials, those materials can switch their physical/chemical properties and respond to environmental stimuli such as temperature, light, pressure, electric and magnetic fields, radiation, and so on.16−21 On a microscopic level, a reorientational order−disorder transformation of the moiety in a molecular system plays a vital role to design molecule-based switchable bistable materials. When the changes are associated with a molecular rotor, dielectricity can transit between low- and high-dielectric states, i.e., an occurrence of a switchable dielectric constant is due to the reorientation transformation of the components between static (frozen) and dynamic (motional) phases.22−25 The design and synthesis of inorganic−organic hybrid compounds get attention due to their attractive structural diversities and prospective applications as functional materials.26−29 However, predicting the switching of reversibly highand low-state as well as similar properties in molecular-based amphidynamic materials is not smooth at the present stage because of numerous unknowns on the complicated interplay among intermolecular interactions during crystallization. Fortunately, crystal engineering affords a powerful tool to partly manipulate the relationship between structures and © XXXX American Chemical Society

Received: May 16, 2018

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DOI: 10.1021/acs.inorgchem.8b01306 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 1. (a) The as-grown crystal of compound 1 with large size of 13 × 3 × 3 mm3. (b) DSC curves of 1 measured in the temperature range of 120−280 K.

Figure 2. Views of the molecular structures of compound 1 in the (a) LTP and (c) HTP. The two-site disordered BF4− anions are distinguished by different bond colors. Comparison of the change of anionic structures in the (b) LTP and (d) HTP. All hydrogen atoms are omitted for clarity.

also used to confirm the purity of the crystalline sample of 1 (Figure S2). Single-Crystal X-ray Crystallography. A Rigaku Saturn 724+ diffractometer equipped with a graphite monochromator low-temperature gas spray cooler device by using Mo Kα (λ = 0.71073 Å) radiation was used to collect single-crystal diffraction data of 1 at 293 K (high temperature phase, HTP) and 123 K (low-temperature phase, LTP), respectively. Data processing was handled by CrystalClear software package, and the structures were solved by direct methods and successive Fourier synthesis and then refined by full-matrix least-squares refinements on F2 using the SHELXLTL-97 software package. H atoms bonded to C atoms were placed in calculated positions, with C−H = 0.93 Å and Uiso(H) = 1.2Ueq(C). H atoms bonded to C atoms were refined freely with isotropic displacement parameters. Crystal data collection and structure refinement details are summarized in Table S1. General Measurements. Elemental analysis of C, H, and N was measured on a Vario MICRO analyzer. Infrared spectroscopy was carried out by a Nicolet 5700 spectrometer. A variation-temperature powder X-ray diffraction patterns were

dielectricity and concerning practical applications of 1 was not included. A series of experiments have proved that 1 undergoes a phase transition between low-temperature phase (LTP) and high-temperature phase (HTP) in a wide temperature range owing to the disordering of one-half of the BF4− anions. Undoubtedly, the compound has extended the family of the dielectric switch.

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EXPERIMENTAL SECTION Synthesis. All the analytical grade chemicals were used as received without further purification. Compound 1 was synthesized by the combination of 1-chloromethyl-4-fluoro1,4-diazoniabicyclo[2.2.2]octane chloride (1.765 g, 5 mmol) and sodium tetrafluoroborate (1.10 g, 10 mmol)38 in H2O (15 mL). The colorless block crystal (Figure 1a) of 1 was acquired by slow evaporation of the aqueous solution at room temperature (yield 85%, based on Cl). In the IR spectra of 1 (Figure S1), the peaks at approximately ∼1100−1020 and 540 cm−1 are ascribed to stretching vibration absorption of the BF4− anions. Elemental analysis calcd (%) for 1: C 23.73, H 3.98, N 7.91; found: C 23.69, H 4.02, N 7.90. The PXRD was B

DOI: 10.1021/acs.inorgchem.8b01306 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 3. Packing diagrams of 1 at LTP and HTP, showing the similarities of lattice framework at different temperature and differences in states of motion about BF4− anions. The ordered BF4− anions form the blue tetrahedrons, and the disordered BF4− anions form the red twisted tetrahedrons. H atoms were omitted for clarity.

Figure 4. Symmetry breaking in 1 from HTP (Pnma) to LTP (P21/c).

and N is possible arrangement, it is found that N = 2.3, showing that an order−disorder phase transition has happened. Crystal Structure of 1. To determine the microscopic mechanism of phase transition, the structural details of compound 1 are revealed by the determination of variabletemperature (VT) crystal structures at 293 K (HTP) and 123 K (LTP), respectively. VT single-crystal structure analysis was introduced for 1 to explain the change of the microstructure, revealing structural changes in different phases. At LTP, the basic structure unit consists of one F-TEDA cation, two BF4− anions which both are ordered (Figure 2a, amplify one of two identical BF4− anions analogized to the unblooming bud for clear analysis of the structure in Figure 2b). Compound 1 crystallizes in the monoclinic space group P21/c (no. 14) with cell parameters of a = 7.593(16) Å, b = 13.83(3) Å, c = 12.64(3) Å. In the LTP, B atoms also existed as twisted and tetrahedrally coordinated by four F atoms uniformly with B−F distance from 1.338 to 1.437 Å. In the process of increasing temperature, a half of two BF4− anions achieved transition from order to disorder, so that asymmetric cell unit of 1 is changed obviously (Figure 2c, the disordered BF4− anion analogized to the blooming flower in Figure 2d.). At HTP, 1 crystallizes in the orthorhombic space

recorded by a Rigaku SmartLab X-ray diffraction instrument. A NETZSCH DSC 200F3 with a heating/cooling rate of 5 K· min−1 under nitrogen at atmospheric pressure was used to receive the value of differential scanning calorimetry (DSC). Dielectric constant curves were obtained on powdered samples on a Tonghui TH2828A impedance analyzer in the frequency range from 5 kHz to 1 MHz with an applied electric field of 1.0 V.

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RESULTS AND DISCUSSION

Phase Transitions of 1. The changes of temperature as the most efficient approach in external stimulus can achieve the switching of the physical/chemical properties.24,39,40 Differential scanning calorimetry (DSC) is an effective measurement to conclude the existence of a reversible phase transition due to thermal stimuli.41 The phase transition of 1 was first determined by the DSC measurement. In the whole measured temperature range of 120−280 K, the compound shows reversible endo- and exothermic peaks (Figure 1b). For 1, a pair of peaks appears below the room temperature centered at 200 K, corresponding to phase transition in the temperature range. According to the peak type and thermal hysteresis, it is probably first-order phase transition. On the basis of Boltzmann equation, ΔS = R ln(N), where R is gas constant C

DOI: 10.1021/acs.inorgchem.8b01306 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 5. (a) Dielectric constant of 1 measured in the pulverulent state of 5 kHz, 10 kHz, 100 kHz, and 1 MHz upon cooling, in which there is the dielectric diagram above room temperature as illustration. Temperature-dependent dielectric constant of the crystal samples along (b) a-axis, (c) baxis, and (d) c-axis at different frequencies upon cooling. (e) The recoverable switching of dielectric effect of the pressed powder pellet. (f) Schematic of the testing device.

Dielectric Transitions of 1. It is known that the dielectric response at variable temperature indicates thermally activated molecular rotations and structural changes.43 Specifically, as a physical property of the material, dielectric constant ε (ε = ε′ − i ε″, where ε′ is the real part and the ε″ imaginary part) could act as a signal to judge the existence of structural phase transitions sensitively due to the changes of polarization.44,45 The relational spectra of temperature and dielectric constant (the real part and imaginary part because the value of real part is sufficient to settle the interrelated problem, the spectrum of imaginary part is not present here) were acquired on crystal samples and powder-pressed plate-shaped particles to verify its switchable characteristic. Actually, the order−disorder transition of BF4− anion in crystal lattices would arouse tunable dielectric constant between the low- and high-dielectric states. The temperature-dependent dielectric constant of 1 is shown in Figure 5. For 1, the ε′ curves measured on powder sample show the dielectric switches whose position are in low temperature (Figure 5a). In the process of heating−cooling cycle, exothermic and endothermic peaks appear around 198 and 205 K at 1 MHz, respectively, which agrees well with the DSC results and corresponds to the order−disorder structural phase transitions of the crystals. In addition, the dielectric

group Pnma (no. 62) with a = 12.600(6) Å, b = 7.660(6) Å, and c = 14.112(6) Å, which is consistent with the structural information reported by Banks et al.38 The basic unit consists of three discrete components, i.e., one F-TEDA cation and two different twisted tetrahedral BF4− anions. In the HTP, one of BF4− anions is in the state of disorder in which the F atoms are split into two positions with site occupancies of 0.25 and 0.25 (Figure 2c). Conversely, the F atoms in the other BF4− anion remain in a relatively static state with site occupancies of 0.5 and 0.5 in the HTP. Meanwhile, the BF4− anions consisted of a boron atom and four adjacent F atoms, which exhibit distorted tetrahedron with the B−F distance of 1.27−1.48 Å (Table S2). As result, the phase transition in 1is mainly attributed to the change of thermodynamic behavior of the BF4− anions. In the crystalline packing unit of complex 1, there are no classic hydrogen bonds or other intermolecular forces to exist in the molecular structure of 1, leading to the 0D single molecule (Figure 3). Meanwhile, during the process of phase transition, the species of the symmetric elements vary from (E, C2, σh, i) in the LTP to (E, C2, C’2, C”2, i, σh, σV, σ′V) in the HTP, as shown in Figure 4, which is coincident with the Curie symmetry principle.42 D

DOI: 10.1021/acs.inorgchem.8b01306 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 6. Simulated application of the multistable molecular switches based on thermoelectric signal conversion to reach switch on/off rapidly. When the temperature is higher than phase-transition temperature (Tc), the machine switch is turned on.

constant of compound 1 is obtained at the range of 298−430 K above room temperature (the relevant curve is drawn in the illustration of Figure 1b), indicating that there is no abnormal dielectric in compound 1 at an appropriate range of temperature. At 1 MHz, the real part value (ε′) of compound 1 starts with 3.9 at 180 K and then grows rapidly with the increase of temperature in the low-dielectric states. Around the temperature of phase transition, the values of ε′ travel into the range of 3.9 and 4.6, corresponding to a dielectric switching behavior. It is suggested anisotropic measurements along different crystallographic axes (a-, b-, and c-axes) might be more appropriate for characterizing the properties of the compound 1. Generally speaking, the property of anisotropy is associated with differences in physical and chemical performances due to the inconsistent of periodicity and density of atoms in generating different crystallographic axes.46−49 It shows dielectric anisotropy with step-like anomalies and distinct dielectric permittivities along the a-, b-, and c-axes (Figures 5b−d). At the frequency of 1 MHz, the maxima of dielectric permittivity reach 9.0, 7.6, and 6.0 along the a-, b-, and c-axes, respectively. This performance of dielectric anisotropy can be explained by investigating that the dielectric constant (ε′) has the property of crystallographic axis dependence. As shown in Figures 5b−d, anomalies along the c-axis are stronger in contrast to other axes. Further research on reversibility of dielectric switching in compound 1 was also investigated on the powder-pressed plate-shaped particles in Figure 5e (the schematic of testing device is presented in Figure 5f). As shown, the switching period and the intensity of the dielectric signals remain virtually unchanged compared with the initial value accompanying temperature changing after seven cycles, which is further evidence of a reversible phase transition. Effectively, it is rare to explore dielectric switching cycles in the previous studies and the excellent switching characteristics displayed above also provide strong evidence that 1 can be a promising tunable switching material. What is universally known is that temperature-dependent molecular-based material could be switched reversibly at lowand high-dielectric states under thermal stimulation via the

thermoelectric coupling. As shown in Figure 6, the implementation of an “on” or “off” switchable dielectric signal is imitated on the temperature-dependent single device cell at high- and low-temperature. When temperature is higher than phase-transition temperature (Tc), the machine switch is turned on. If lower, the machine is off. Consequently, compound 1 could be a potential and excellent candidate for dielectric switches.

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CONCLUSION In summary, a metal-free hybrid 1-chloromethyl-4-fluoro-1,4diazoniabicyclo[2.2.2]octane bis(tetrafluoroborate) (1) was synthesized and characterized. The compound displays reversible structural phase transitions at around 200 K. The phase transition is characterized by variable-temperature structure, and analyses reveal that the order−disorder transition of one-half of the BF4− anions play an important role in the process. The emergence of compound 1 with switchable dielectricity between high- and low-dielectric states enriches the family of reversible structural phase transition materials, opening a promising way to construct electronic multifunctional materials and being used to obtain a deeper development of electronic bistable switches in the special systems of metal-free ionic crystal.

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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.inorgchem.8b01306. IR spectrum, PXRD patterns (Figures S1 and S2, respectively), crystallographic data and refinement parameters (Table S1), and bond lengths and angles (Tables S2) (PDF) Accession Codes

CCDC 1580795−1580796 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, by emailing [email protected], or by contacting The E

DOI: 10.1021/acs.inorgchem.8b01306 Inorg. Chem. XXXX, XXX, XXX−XXX

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Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Qiong Ye: 0000-0002-3532-5388 Da-Wei Fu: 0000-0003-4371-097X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Project 973 (Grant 2014CB848800), the National Natural Science Foundation of China (Grant 21673038, 21771037), Natural Science Foundation of Jiangsu Province (JSNSF) (Grant BK20170659), and the research fund of SEU.

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DOI: 10.1021/acs.inorgchem.8b01306 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.8b01306 Inorg. Chem. XXXX, XXX, XXX−XXX