Thermal-Induced Dielectric Switching with 40K ... - ACS Publications

Apr 12, 2018 - Yang-Hui LuoChen ChenChang HeYing-Yu ZhuDan-Li HongXiao-Tong ... Yang-Hui LuoJing-Wen WangWen WangXiao-Tong HeDan-Li ...
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Thermal-Induced Dielectric Switching with 40K Wide Hysteresis Loop Near Room Temperature Yang-Hui Luo, Chen Chen, Dan-Li Hong, Xiao-Tong He, Jing-Wen Wang, and Bai-Wang Sun J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b00597 • Publication Date (Web): 12 Apr 2018 Downloaded from http://pubs.acs.org on April 12, 2018

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Thermal-Induced Dielectric Switching with 40K Wide Hysteresis Loop Near Room Temperature

Yang-Hui Luo, * Chen Chen, Dan-Li Hong, Xiao-Tong He, Jing-Wen Wang, and Bai-Wang Sun*

School of Chemistry and Chemical Engineering, Southeast University, Nanjing, 211189, PR. China. E-mail: [email protected], [email protected].

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Abstract A thermal-induced dielectric switching has been realized in two ion-pair crystal [C2H6N5]+—[H2PO4]- (1, C2H6N5 = 3, 5-diamino-1, 2, 4-triazolinium) through single-crystal-to-single-crystal phase transition (SCSC-PT). Upon cooling from room temperature, the one-dimensional cation stripes that composed of [C2H5N5]+ cations have rotated sharply around the c axis with 90 degrees, accompanied by transition of crystal stacking from loose un-paralleled (dynamic state) to compression paralleled (static state), and reorientation of dipoles on [C2H5N5]+ cation, which thus resulted in high-dielectric state (HDS) to low-dielectric state (LDS) transformation. While on the warming run, the reverse process was rather sluggish, resulting a reversible dielectric switching with ultra-large (about 40K wide) hysteresis loop near room temperature. It is thought that the large-sized polar cation stripes play a predominant influence on the switching properties of 1.

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Materials showing switchable physical and/or chemical properties continue to draw attention of scientists with respect to not only the doubtlessly interesting fundamental aspects, but also the potential applications.1,2 Usually, switchable properties are highly sensitive to such external stimuli as light, irradiation, temperature, press, guest molecules, electric/magnetic fields, etc.3-6 As have been expected, the bulk physical/chemical properties will be turned between at least two distinct states triggered by multiple stimuli, that is bi-stability. Which has the potential to be developed into sensors, displays, switching devices, information storages, signal processing and memory devices.7-12 Among these switchable properties, magnetization and polarity have occupied a special position. Switchable magnetization refers to the spin-arrangement of respective sites and the coupling interactions between them, that is the so-called “spin crossover” (SCO) which refers to the spin-state transition between a high-spin (HS) and a low-spin (LS) state.13,14 Whereas polarity is related to the redistribution of positive and negative charge centers, and this redistribution will resulted in dielectric switching between high- and low-dielectric states (HDS and LDS) that can be viewed as an electrical counterpart to SCO.15,16 As a consequence, the switchable dielectric compounds have emerged as a promising candidate to act as effective stimuli-responsive materials.

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For practical application, a molecular switch must be required to show a remarkable bi-stability.17-20 More ideally, it should operate under ambient conditions with a wide hysteresis loop at around room temperature (RT).21 From the microscopic viewpoint of molecule-based crystalline structures, the bi-stability is often manifests a reversible structural phase-transition (PT) at molecular level. One thing deserves noting that: in most cases, the degree of thermal hysteresis can be significantly affected by the cooperativity during structural PT, that is the motion resistance of atomic /molecular within the tightly stacked crystalline environment.22 On the one hand, the dielectric switching is originated from the reorientation of polar components (atoms、ions or molecules) between static (frozen) and dynamic (motional) states.23,24 On the other hand, the reorientation of large-sized polar components will be more sluggish than the small-sized ones, which thus can promote the generation of unusual thermal hysteresis loop.25 Hence, the assemble of large-sized polar components into switchable dielectric compounds would make a significant difference. Zhang et, al.26-28 have reported a series of metal-cyanide based perovskite-type metal-organic frameworks, which have shown distinct dielectric constant switching triggered by guest molecules but with negligible thermal hysteresis loop; Wen et, al.29 has prepared two bulky ion-pair compounds which have shown above-room temperature dielectric constant switching with about 10K wide hysteresis loop. More recently, Tang et, al.30

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have demonstrated an approximately 15K wide hysteresis loop of dielectric relaxation at around 228K, in an succinimide lithium(I) hybrids. However, none of these above-mentioned results meet the requirements for practical application. Control of the polar components assemble and thus the dielectric switching operate at around room temperature with ultra-large hysteresis loop remains a main challenge for the fabricating of functioning dielectric devices. Here in this context, we report a crystalline two ion-pair compound composed of 3, 5-diamino-1, 2, 4-triazolinium cation (1+) and phosphate ion (1-), that is crystal [C2H6N5]+—[H2PO4]- (1). Upon cooling and heating, 1 shown reversible dielectric switching between distinct HDS and LDS with an approximately 40K wide thermal hysteresis loop near the room temperature. Interestingly, this dielectric switching was accompanied by a SCSC-PT, which was originated from the reorientation of the large-sized polar 3, 5-diamino-1, 2, 4-triazolinium cation stripes. Where, the polar cation stripes undergo a 90 degrees rotation around the c axis, accompanied by a dynamic change between the paralleled and un-paralleled stacking models that switched by temperature. More importantly, this SCSC-PT has induced exchange of the positive and negative charge centers on each [C2H6N5]+ cations, which primary responsible for the dielectric switching properties of 1. Crystal 1 was precipitated from an aqueous solution of 3, 5-diamino-1, 2, 4-triazole and phosphate (Supporting Information) with large white block habit.

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Firstly, we solved its structure at room temperature (293 K), it has shown orthorhombic space group Fdd2 with a = 13.481(3) Å, b = 36.938(8) Å, c =

Figure 1. Crystal structure of compound 1 under different temperature. (a) The formation of 1D ladder-like 3, 5-diamino-1, 2, 4-triazolinium cations stripe and (b) the connecting motif of 3D framework; The height of “steps” in the stripe undergo a decrease from HDS (c) to LDS (d); The dihedral angle and plane-plane distances between adjacent stripes for HDS (e) and LDS (f) have been presented.

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6.0738(12) Å (Supporting Information, Tables S1). In the crystal, each cation of 3, 5-diamino-1, 2, 4-triazolinium connects to two other one through N-HON hydrogen bonding contacts (NON distance of 3.036 Å) along the crystallography c axis, and four different phosphate ions through N-HOO hydrogen bonding contacts (NOO distances 2.766-2.988 Å) along the crystallography b axis (Figure S1a and Table S2). While for each phosphate ion, its connects to three other ions and four 3, 5-diamino-1, 2, 4-triazolinium cations, through O-HOO (OOO distances of 2.535 and 2.585 Å) and N-HOO (NOO distances of 2.754-2.988 Å) hydrogen bonding contacts, along the crystallography a and b axis, respectively (Figure S1b and Table S2). Interestingly, the adjacent 3, 5-diamino-1, 2, 4-triazolinium cations were connected

into

crystallography

one-dimensional

c

direction

(1D)

(Figure

ladder-like 1a).

Where,

stripe the

along height

the and

centroids-centroids distances of the adjacent “steps” within one stripe were found to be 0.590 (Figure 1c) and 6.074 Å (Figure S2a), respectively. These 1D ladder-like stripes were further stacked along both the a and b direction, supported by the two-dimensional (2-D) phosphate ion ([H2PO4]-) arrays, into three-dimensional (3D) framework (Figure 1b). It is notable that, in the 2-D arrays, the [H2PO4]- ions were formed a dimer unit through double O-HOO (OOO distances of 2.585 Å) hydrogen bonding interactions, and each dimer unit was further connected to other fours through single O-HOO (OOO

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distances of 2.535 Å) hydrogen bonding interactions (Figure S3a). For the 3-D framework, the cation stripes stacked un-paralleled along the crystallography a direction in OABABO fashion and fixed in the both sides of the 2-D arrays, with the dihedral angle and centroids-centroids distances between adjacent stripes of 0.76° (Figure 1e) and 3.700 Å (Figure S2b), respectively. Thermal analysis revealed that crystal 1 can maintained to 200 °C upon heating (Figure S4).

Figure 2. Rotation of cation stripes around the c axis with variation of temperature (a and b); Insert: DSC curves of 1 as measured in cooling-heating cycles (c). Upon cooling to 100K, the space group of 1 is still Fdd2, however, the unit cell a and b have exchanged with a = 37.1049(10) Å, b = 13.3329(3) Å, c =

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6.07102(16) Å (Table S1), which can be attribute to the rotation and reorientation of the large-sized polar 3, 5-diamino-1, 2, 4-triazolinium cation stripes. To understand the microscopic rotation and reorientation process of the cation stripes, a detailed comparison between the crystal structures at 293 and 100K have been performed. Results shown that the most striking changes in the crystal structure were the rotation about 90 degrees of the cation stripes around the c axis, from perpendicular to the a axis to perpendicular to b axis (Figure 2a and b) and the strengthening of intermolecular hydrogen bonding contacts (Table S3). Note that, this rotation was accompanied by a reorientation of the individual cation in the stripes: e.g. for the reoriented stripes, the height and centroids-centroids distances of the adjacent “steps” within one stripe were shorter to be 0.550 (Figure 1d) and 6.071Å (Figure S2c), respectively. More importantly, after reorientation, the cation stripes stacked paralleled along the b direction, with plane-plane distances of 3.456 Å (Figure 1f) and the centroids-centroids distances between 3, 5-diamino-1, 2, 4-triazolinium cation in adjacent stripes was shorter to 3.667 Å (Figure S2d). This dramatic rotation of the cation stripes is responsible primary for the SCSC-PT and dielectric switching for crystal 1 as described below. While for the 2-D arrays, the OOO distances for the dimer unit have shown a slight increase to 2.587 Å, which can be attributed to the pulling effect provided by transformation of the 1-D polar 3, 5-diamino-1, 2, 4-triazolinium

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cation stripes from un-paralleled to paralleled stacking. This pulling effect was accompanied by a decrease in the OOO distances (2.529 Å) between adjacent dimer units as sacrifices (Figure S3b).

Figure 3. Comparison between the (a) Hirshfeld dnorm surfaces, (b) Crystal Voids, and (c) Electron Density of polar 3, 5-diamino-1, 2, 4-triazolinium cations at HDS (left) and LDS (right), respectively. 3D Hirshfeld surfaces and 2D fingerprint plots analysis

31,32

(Figure 3 and

Figure S5-S6) were performed to compare the polar 3, 5-diamino-1, 2, 4-triazolinium cations at different dielectric states. Shown in Figure 3 were the

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Hirshfeld dnorm surfaces, Crystal Voids, and Electron Density of the polar 3, 5-diamino-1, 2, 4-triazolinium cation at HDS and LDS respectively. Where, the proportion of the significant hydrogen bonding contacts (Figure 3a, represented by the large deep red circular depressions on the dnorm surfaces) has shown an increased with the decrease of temperature, from 58.6% contribution to the total Hirshfeld surfaces at 293K to 62.5% at 100K (Table S4). On the contrary, the crystal voids have undergone a decrease with temperature, from V = 345.32 Å3, A = 1178.21 Å2 at 293K to V = 319.54 Å3, A = 1132.30 Å2 at 100K (Figure 3b), demonstrating a compressed crystal stacking at lower temperature that similar with the results of crystal structure analysis, and this compressed crystal stacking can be viewed as a static (frozen) state. More importantly, the SCSC-PT has accompanied by an exchange of the positive and negative charge centers on the polar 3, 5-diamino-1, 2, 4-triazolinium cations (Figure 3c), that is the reorientation of dipoles, thus, a dielectric switching with large relaxation can be expected with 1. The whole process of the SCSC-PT of 1 from 293 to 100K was also demonstrated by DSC (differential scanning calorimetry) analysis (Figure 2c). During the cooling-warming cycles, a sharp exothermic peak occurred at approximately 213 K with enthalpy change of 0.67 Jg-1 was observed on the cooling DSC curves, corresponding to the rotation and change of the stacking model of the polar 3, 5-diamino-1, 2, 4-triazolinium cation stripes from

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un-paralleled to paralleled stacking. While on the warming procedure, a broad endothermic peak appears in temperature range 223-240K with enthalpy change of 0.46 Jg-1. These results indicated that: one the one hand, the SCSC-PT of 1 from HDS to LDS can occurred reversibly in the solid-state; on the other hand, the un-paralleled to paralleled stacking transformation was more sharply than its reverse process, which will certainly result in ultra-large thermal hysteresis loop. From the above single-crystal structure analysis, the polar 3, 5-diamino-1, 2, 4-triazolinium cation stripes in 1 undergo approximately 90 degrees rotation along the c axis, accompanied by an un-paralleled to paralleled stacking model transformation at around the transition point. More importantly, within this rotational geometry, the dipoles within the 3, 5-diamino-1, 2, 4-triazolinium cation have undergone reorientation, that is the violently oscillation of the dipole moments within [C2H6N5]+ cation,33 A phenomenon that usually accompanied by large dielectric constant and reversible dielectric switching with ultra-large thermal hysteresis loop, as described below. Dielectric switching properties of crystal 1 were measured in temperature range 330-120 K and frequency range 0.5-1000 kHz with the powdered crystalline samples on cooling-warming cycles (Figure 4). At the room temperature, the real part (ε’) value at 1000 kHz indicates the HDS, and this value has undergone an increase to reach a peak value of about 6.4 at 293 K

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upon cooling. This short-range increase of ε’ value with decrease of temperature can be interpreted as the fight between random thermal motion of the polar 3, 5-diamino-1, 2, 4-triazolinium cation stripes and the orientational polarization that induced by electric field.34 Then, a rapid decrease of this ε’ value was observed to reach a platform with value of about 2.9 at 264K upon further cooling, indicating a transformation from the HDS to the LDS, that is dielectric switching. Note that, this dielectric switching can be attribute to the transitions of the polar cation stripes from un-paralleled-to-paralleled stacking in the crystal. Interestingly, a typical dielectric relaxation has occurred as expected, with the peak value of ε’ decreases from 48 to 6.4, and the peaking temperature moves from 288 to 293K, as the frequency increases from 0.5 kHz to 1000 kHz, demonstrating the slows down of dipolar reorientation process with the decrease of the temperature.35 It is thought that the motion of polar cation stripes in the crystal 1 is more slowly than 0.5 kHz because the SCSC-PT temperature (213K, determined by DSC measurements) is much lower than the peaking temperature (288K) of ε’ at 0.5 kHz.

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Figure 4. The real part of the dielectric constant of 1 dependent on temperature at 1 MHz (left) and comparison of thermal hysteresis loop under different frequency (right). While on the warming runs, the ε’ value has undergone a gradual increase in the temperature range 210-250K below the transition point of 278K, indicating a much slower transition from LDS to HDS than its reverse process. As a consquences, an approximate 40K wide dielectric hysteresis loop (ε’-T) has appeared, i.e., dielectric bi-stability, which can be attribute to the motion resistance that derived from the rotation and reorientation of the polar 3, 5-diamino-1,

2,

4-triazolinium

cation

stripes

during

the

reversible

thermally-induced SCSC-PT. It should be noted that, the width of the dielectric hysteresis loop was increased with the decrease of frequency, from 40K wide at 1000 kHz to 50K wide at 0.5 kHz. This kind of bi-stability in dielectric constants with tunable hysteresis loop is somewhat rare, which may have the potential to be integrated into molecular dielectric switching devices. Again, a

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typical dielectric relaxation has occurred for the warming runs as expected. Interestingly, the gap between the ε’ value for the HDS and LDS have shown significant frequency-dependency. Where, the value for the HDS was near 58 times larger than that for LDS at 0.5 kHz, while this multiple relationship was decreased to be 32, 10, 6.8, 2.4 and 2.2 times, for 1, 5, 10, 100 and 1000 kHz, respectively. Such results suggesting a promising dielectric switch with large frequency application range. In summary, a thermal-induced dielectric switching with ultra-large hysteresis loop near room temperature has been realized in an easy prepared two ion-pair crystal [C2H5N5]+—[H2PO4]- (1, C2H5N5 = 3, 5-diamino-1, 2, 4-triazolinium) through single-crystal-to-single-crystal phase transition. This dielectric bi-stability was originated from the 90 degrees rotation of the one-dimensional polar 3, 5-diamino-1, 2, 4-triazolinium cation stripes around the c axis and the following change of the intermolecular interactions around the [C2H5N5]+ cation, including hydrogen-bonding contacts, crystal stacking and the exchange of positive and negative charge centers. The large-sized polar cation stripes play a predominant influence on the switching properties of 1.

The authors declare no competing financial interest.

Acknowledgements 15

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This research was supported by the Natural Science Foundation of China (Grant No. 21701023), Natural Science Foundation of Jiangsu Province (Grant No. BK20170660), Fundamental Research Funds for the Central Universities (No.3207047407) and PAPD of Jiangsu Higher Education Institutions.

Supporting Information Materials and physical measurements, Crystal data and structure refinement details for 1-HDS and 1-LDS, Geometrical parameters for hydrogen bonds, additional crystal structures and TGA/DSC profiles of compound 1, Comparison between Curvedness, Fragment Patch, Shape index and 2-D fingerprint plots of 3, 5-diamino-1, 2, 4-triazolinium cation at HDS and LDS, Summary of the percentage contributions to the Hirshfeld surfaces of 3, 5-diamino-1, 2, 4-triazolinium cation at HDS and LDS.

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