Switchable Dielectric Phase Transition Induced by Ordering of

Article pubs.acs.org/crystal

Switchable Dielectric Phase Transition Induced by Ordering of Twisting Motion in 1,4-Diazabicyclo[2.2.2]octane Chlorodifluoroacetate Xiaojun Shi,†,∥ Junhua Luo,*,†,‡ Zhihua Sun,†,‡ Shigeng Li,†,∥ Chengmin Ji,† Lina Li,†,∥ Liang Han,†,∥ Shuquan Zhang,† Daqiang Yuan,‡ and Maochun Hong†,‡ †

Key Laboratory of Optoelectronic Materials Chemistry and Physics, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, China ‡ State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, China ∥ Graduate School of the Chinese Academy of Sciences, Beijing 100039, China S Supporting Information *

ABSTRACT: A novel molecular electric ordered compound [(Hdabco)+ClF2CCOO−] (complex 1, dabco = 1,4-diazabicyclo[2.2.2]octane) has been discovered as a switchable dielectric material, in which the N−H···O hydrogen bonds connect the anions and cations together to form a dimer structure. It undergoes a reversible second order solid state phase transition at 165 K (Tc), which was confirmed by thermal analyses including differential scanning calorimetry (DSC) and specific heat (Cp) and dielectric measurements. Variable temperature single crystal X-ray diffraction analyses revealed its order−disorder transformation of twisting motion characteristic behaviors for the phase transition. Owing to the ordering of twisting motions of the chlorodifluoroacetate anions, the structure of 1 demonstrates a phase transition from high crystallographic symmetry with space group of Pnma to the low state with space group of P21/c; that is, an Aizu notion of mmmF2/m occurs. Emphatically, the dielectric constant of 1 displays a distinctive step-like anomaly; namely, it possesses a high dielectric state in the room temperature phase (RTP) and a low state in the low temperature phase (LTP). Such distinctive dielectric performances disclose that 1 might be considered as a potential switchable dielectric material. We believe that all the findings would afford a useful strategy in exploring new electric ordering materials.

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INTRODUCTION Molecular switchable dielectrics, as attractive functional electric materials with the dielectric constants undergoing abrupt change between the high and low dielectric states, have attracted more and more attention, due to their unique physical and chemical properties with potential applications in data communication, signal processing, sensing, phase shifters and varactors, etc.1 A lot of progress has been achieved in designing solid state dielectrics;2 however the main challenge is still how to get two or more obviously different dielectric states for dielectric materials. As one of the most promising strategies to assemble such functional materials, the engineering of molecular complexes which undergo structural transformations usually effectively acquires the typically temperature-dependent dielectric states.3 Namely, designing such complexes could afford the potential possibilities to explore novel switchable dielectric materials. For example, the order−disorder behaviors of the guest molecule in a perovskite-type cage compound leads to striking dielectric anomalies during the phase transition.4 Meanwhile, structural transformation induced by the molecular motions during the reversible phase transition could also be used to assemble switchable molecular dielectrics.5 Since crystalline molecular assemblies binary of 1,4-diazabicyclo[2.2.2]octane (dabco) which undergo reversible structure transitions © 2013 American Chemical Society

have demonstrated specific responses to external electric or temperature stimuli, they have provided a lot of room for us to explore switching dielectrics from the viewpoint of structural engineering; for example, the crystals of [Hdabco]+[ClO4]− and [Hdabco]+[BF4]− display structural change from space group Pm21n to P4/mmm, which was first reported by Katrusiak and Szafrański.6 More recently, a series of metal−organic frameworks assembled by dabco which undergo the structure transitions have been synthesized by Xiong et al.7 Moreover, the cocrystal compound dabco·p-nitrophenol which exhibits structural transformation induced by the motions of dabco was reported by our group.8 All these findings reveal that the dabco component would afford the potential to design and construct new functional electric ordering materials because of its striking features as donor−acceptor of proton or order−disorder structural transformation. Encouraged by the pioneering works, herein we present a new switchable molecular dielectric [(Hdabco)+ClF2CCOO−] (1) which exhibits a reversible second order phase transition at 165 K. Received: January 25, 2013 Revised: March 14, 2013 Published: March 22, 2013 2081

dx.doi.org/10.1021/cg400146b | Cryst. Growth Des. 2013, 13, 2081−2086

Crystal Growth & Design

Article

that 1 could be regarded as a potential switchable dielectric material.

Table 1. Crystal Data and Structure Refinement for Compound 1 at 100 K and 293 K moiety formula

C6H13N2,C2ClF2O2

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C6H13N2, C2ClF2O2

formula weight 242.65 242.65 temperature (K) 100(2) 293(2) crystal system monoclinic orthorhombic space group P21/c Pnma a/Å 9.116(5) 6.123(5) b/Å 18.023(10) 9.135(7) c/Å 21.227(10) 19.405(16) β/deg 115.33(2) 90 volume (Å3) 3152(3) 1085.4(15) Z 12 4 F(000) 1512 504 goodness-of-fit on F2 1.178 1.109 reflns collected/unique 26358/7192 6061/949 R(int) 0.0723 0.0271 Tmin/Tmax 0.4504/1.0000 0.5795/1.0000 R1 (on Fo2, I > 2σ(I)) 0.0816 0.0888 wR2 (on Fo2, I > 2σ(I)) 0.2530 0.2684 αR1 = Σ∥Fo| − |Fc∥/Σ|Fo|, wR2 = [Σ(|Fo|2 − |Fc|2)/Σ|Fo|2]1/2

EXPERIMENTAL SECTION

Synthesis. All chemical reagents were used without further purification. Compound 1 was synthesized by dissolving dabco (1.12 g, 0.01 mol) and chlorodifluoroacetic acid (CDFA) (1.30 g, 0.01 mol) in methanol (10 mL) at room temperature. Crystals of 1 were obtained by slow evaporation of the mixture solution in the refrigerator (about 10 °C) within several days. In the IR spectra of 1 (Figure S1, Supporting Information), the peak at approximately 1689 cm−1 is ascribed to stretching vibration absorption of the carbonyl group (CO), which indicates the existence of CDFA in 1. The powder XRD (PXRD) pattern verified the purity of 1 (Figure S2). Single-Crystal X-ray Crystallography. X-ray diffraction data of 1 were collected using a Rigaku Saturn 007 diffractometer with Mo−Kα radiation (λ = 0.71073 Å) at low temperatures (100 K), and using Rigaku Saturn Mercury CCD diffractometer with Mo−Kα radiation (λ = 0.71073 Å) at room temperatures (293 K). Empirical absorption correction was applied when the data processing was performed using the Crystalclear software package (Rigaku, 2005). The structures were solved by direct methods and refined with a full-matrix least-squares technique (SHELX-97). Non-H atoms were refined anisotropically using all reflections with I > 2σ(I). All H atoms were added geometrically and refined using “riding” model with Uiso = 1.2Ueq (C and N). The packing views and the asymmetric units were drawn with DIAMOND (Brandenburg and Putz, 2004) Visual Crystal Structure Information System Software. Crystallographic data and structure refinement of the LT phase and RT phase are listed in Table 1. Dielectric Constant Measurement. Because of the failure in obtaining large crystals, the powder-pressed pellet pasted with silver

Variable temperature structural determination, thermal analyses including differential scanning calorimetry (DSC) and specific heat (Cp), and dielectric measurements have been used to investigate its phase transition. It is found that the ordering of twist motions of chlorodifluoroacetic anions in 1 mainly accounts for its switchable dielectric phase transition. All the results reveal

Figure 1. The structure packing diagrams of 1 (a) at 100 K and (b) at 293 K. Oscillation photographs at the reflection of the same crystal orientation at (c) 293 K and (d) 100 K. 2082



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conducting glue of 1 was used for dielectric measurement. The temperature dependence of the dielectric constant was measured in the temperature range of 113−293 K at the frequency range 5 kHz to 1 MHz by using the automatic impedance TongHui 2828 analyzer. DSC and Specific Heat Measurement. The DSC measurement was performed on NETZCSCH DSC 200 F3 instrument by heating and cooling rate of 2 K/min in the temperature range of 123−193 K. These measurements were carried out under nitrogen atmosphere in aluminum crucibles. The specific heat measurement was performed on a PPMS-9T instrument in the temperature range of 123−193 K.

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RESULTS AND DISCUSSION Crystal Structures of 1. The phase transition of 1 was confirmed by the determination of variable temperature crystal structures at 293 K (the room temperature phase, RTP) and 100 K (the low temperature phase, LTP), respectively (Figure 1a,b). The crystal structure of 1 at room temperature (293 K) belongs to the orthorhombic crystal system with a centrosymmetric space group of Pnma and the point group D2h, corresponding to its RTP. When the temperature decreases to 100 K, the crystal structure of 1 becomes a monoclinic crystal system with the centrosymmetric space group P21/c and the point group C2h, corresponding to its LTP. During the cooling process, the symmetry breaking occurs with an Aizu notation of mmmF2/m.9 In order to further investigate the structural changes of 1, we compared its RTP structure with the LTP structure. As a result, the cell parameters of 1 change remarkably between the RTP and LTP (Table 1). What is more, the crystal structure of 1 consists of three CDFA anions and three dabco cations in the asymmetric unit at the LT phase, while only half CDFA anion and half dabco cation in the asymmetric unit were discovered at the RT phase. To more deeply observe the structure change of 1, the oscillation photographs at reflection of the same crystal orientation at 293 K (Figure 1c) and 100 K (Figure 1d) were taken, respectively. From Figure 1c,d, it is obvious that the reflections at 100 K are almost three times that at 293 K in the given range, which further confirmed that the unit cell volume of the LTP is almost three times that of RTP. The Bragg reflection of (h k l) = (0 −4 8) at 293 K transfers to the reflection of (h k l) = (4 0 −7) at 100 K, suggesting that the a-axis at RTP changes to the b-axis at LTP. The temperature-dependent Bragg reflection clearly revealed a structure transition. In the RT phase crystal structure of 1, it is interestingly found that the ClF2CCOO− anions are disordered as shown in Figure 2a, which may result in the formation of a higher symmetry (Pnma). Meanwhile, the atoms of Cl1, C1, and C2 in the discrete ClF2CCOO− anions combined with the atoms of N1, C5, C6, and N2 in the (Hdabco)+ cations lie on a common crystallographic mirror plane in the crystal of 1 (Figure 2a). The N−H···O hydrogen bonds between the protonated nitrogen atoms of the (Hbadco)+ cations and the oxygen atoms of the neighboring ClF2CCOO− anions can be found in 1, which results in the noteworthy H-bonded dimers as shown in Figure 2b. The H-bonded dimers, which are antiparallel to each other in the packing structure of 1, form a double zigzag chain-like structure parallel to b-axis through molecular interaction (Figure 2c). In the LT phase, the disordered ClF2CCOO− anions are frozen and become more ordered (Figure 3a). In the asymmetric unit, it is notable that the ordered ClF2CCOO− anions emerge with three different conformations labeled as LTPa, LTPb, and LTPc based on the relative different twisting and devitation of the corresponding ClF2CCOO− anions with the plane of N, C, C, and N atoms in the (Hdabco)+ part as shown in Figure 3a. In the LTPa part, the two carbon atoms of the ClF2CCOO− anions

Figure 2. (a) Molecular structure of 1 with the atom labeling at 293 K; (b) the hydrogen bonded dimer in 1 at 293 K; (c) the antiparallel dimers construct a zigzag-like double chain by molecular interaction.

obviously deviate from the plane with large deviation values of 0.5032 Å for C10 and 0.2715 Å for C9, respectively (Figure 3a). However, the slightly small deviation value of C2 is 0.2518 Å and C1 is 0.2383 Å in LTPb part, while the largest deviation value of 0.4359 Å for C17 and 0.6059 Å for C18 occurs in the LTPc part. Meanwhile, the inclined angle between the C−C (C9−C10 and C17−C18) bond of ClF2CCOO− anions and the above defined plane in the (Hdabco)+ cation is 8.3° in the LTPa part and 3.7° in the LTPc part, respectively. However, the C1−C2 bond in the LTPb part is almost parallel to the plane. More interestingly, the Cl−C−C−N of ClF2CCOO− anions in these three parts (LTPa, LTPb, and LTPc) exhibits large twisting conformations with torsion angles of 61.5(8)°, 33.9(2)°, and 62.4(8)° respectively, while it displays no twisting with the corresponding value of 0° at 293 K, which agrees quite well with its above-mentioned coplanarity as shown in Figure 2a. In other words, it can be considered that the ClF2CCOO− anions undergo a ordering of twisting motions in LTP. Therefore, it is indicated that the ordering of twisting motion of the ClF2CCOO− anions result in a lower crystallographic symmetry (P21/c) for 1 in LTP. At the 2083



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Figure 3. (a) Molecular structure of 1 with the atom-labeling at 100 K; (b) hydrogen-bonded dimer of 1 at 100 K.

The thermal ellipsoids of the C atoms of ClF2CCOO− are relatively large in RTP, indicating an averaged effect of the disordered atoms. So the only one conformation of ClF2CCOO− ions in RTP should be the statistical result of its disordering motion around their equilibrium position, which revealed a twisting motion of the ClF2CCOO− anion. In LTP, it can be seen that there are three different twisting conformations of the ClF2CCOO− ions, which originate from the ordering of twisting motion of ClF2CCOO− ions with the temperature decreasing. Thermal Properties of 1. Generally, DSC measurement is the direct way to detect whether the compound displays a reversible phase transition triggered by the temperature. In order to confirm the phase transition characteristic of 1, its DSC is measured in the temperature range of 113−293 K, which indicates that 1 undergoes a phase transition at approximately Tc = 165 K. The curves of one pair of peaks on heating and cooling centered around 165 K and appear with narrow thermal hysteresis as

same time, the weak hydrogen bonds are sensitive to temperature change and may have a significant impact on its properties.10 For example, we found that the donor−acceptor distances (N···O) (see Table S1) are strikingly different between RTP and LTP, the N3···O3 distance (2.731(4) Å) is shorter than the N3···O4 distance (3.331(5) Å) in the N−H···O hydrogen bonds between the (Hbadco)+ cations and ClF2CCOO− anions of 1 in LTP (Table S1), while in RTP all the N···O distances (3.243(17) Å) are same, which also demonstrates the twisting motion of 1. The (Hbadco)+ and ClF2CCOO− ions are also linked through N−H···O hydrogen bonds to form the antiparallel H-bonded dimers (Figure 3b), which are further linked into zigzag double chains by molecular interaction as in RTP (Figure S5). Hitherto, we have a deeper insight into the structure change of 1. The crystal structural analysis between low temperature and room temperature revealed the order−disorder transformation of twisting motion of the ClF2CCOO− ions in structure of 1. 2084



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Figure 4. (a) The DSC curves of 1 and (b) temperature dependence of specific heat capacity of 1.

Figure 5. (a) Temperature-dependent dielectric constant of 1 at different frequencies. (b) Temperature dependence of the dielectric permittivity measured at 1 MHz on heating and cooling.

notable that the dielectric permittivity exhibits a very strong temperature dependence; namely, the real part of dielectric constant ε′ at 1 MHz remains at about 11−11.5 below Tc and displays a marked change up to about 15.1 (Figure 5a). That is to say, the dielectric permittivity increases progressively until 161 K upon heating, and then it reaches the maximal value with a sudden increment amount of 4 units near Tc. Subsequently, the permittivity exhibits a plateau with a slight decrease. Such dielectric behaviors with the change of temperature are very consistent with the characteristic for switchable molecular dielectric. The dielectric studies reveal no dielectric relaxation process in the vicinity of the phase transition temperature at different frequencies (Figure S4). These results indicate that the motion of the dipolar is extremely fast.14 On the other hand, the behaviors of the dielectric permittivity as a function of temperature show a very reversible switchable; namely, dielectric anomalies occur during the heating or cooling processes (Figure 5b). These features of the dielectric properties make 1 a potential tunable dielectric material. Origin of the Switchable Dielectric Phase Transition of 1. According to the above studies, it becomes more and more clear that [(Hdabco)+ClF2CCOO−] undergoes an order−disorder transformation. Above Tc, the ClF2CCOO− ions show disorder and elevate the macroscopic crystallographic symmetry of 1. Below Tc, the corresponding moieties become more ordered.

shown in Figure 4a. These indicate that 1 displays a reversible second order phase transition.11 From Figure 4a, the ΔH is estimated to equal to 33.78 (2) J/mol. The corresponding changes in entropy enable us to calculate with ΔS = ΔH/Tc, where Tc is the solid-state phase transition temperature. So the ΔS is estimated to 0.205(2) J/mol·K. According to Boltzmann’s equation ΔS = R ln N, where N is the ratio of possible configurations and R is the gas constant, the N value is ca. 1.025. Moreover, the specific heat curve of 1 exhibits a round-like peak at about 165 K, confirming the presence of a phase transition as a typical second order one (Figure. 4b),12 like that of triglycine sulfate (TGS). The ΔS is estimated with a value of 0.194(1) J/K·mol from the specific heat curve, and the ΔH is ca. 32.01(1) J/mol and N = 1.024, which are consistent with the DSC measurements. Dielectric Behaviors of 1. It is well-known that the phase transition will accompany anomaly of physical properties near the structure phase transition point, such as the dielectric constant.13 Herein, the powder pressed pellet of 1 was applied to the dielectric measurements and measured in the temperature range of 113−293 K. As expected, 1 displays a clear dielectric anomaly around 165 K, corresponding well to the phase transition temperature determined by DSC. The dielectric constant has a step-like enhancement around Tc; that is, the dielectric permittivities are quite high in RTP, while the LTP dielectric permittivities become lower into a different magnitude. It is 2085



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Additionally, the twisting of the ClF2CCOO− in the molecule has been observed in LTP (Figures 2b, 3b, and S3). Therefore, the order−disorder transformation of twisting motions of ClF2CCOO− anions induces the phase transition and two different dielectric states of 1. The role of N−H···O hydrogen bonds as the main interaction between the ClF2CCOO− anions and the Hdabco+ cations in 1 should not be ignored. It is clearly shown that the distances between the donors and acceptors change with the temperature decreasing (Table S1); thus this long-range coupling interaction should be indispensable for the occurrence of the phase transition.

Cryst. 2003, 30, 507. (g) Stasyuk, I.; Czapla, Z.; Dacko, S.; Velychko, O. J. Phys.: Condens. Matter. 2004, 16, 1963. (3) (a) Akutagawa, T.; Takeda, S.; Hasegawa, T.; Nakamura, T. J. Am. Chem. Soc. 2004, 126, 291. (b) Jakubast, R.; Galewskit, Z.; Sobczyk, L.; Matuszewskit, J. J. Phys. C: Solid State Phys. 1985, 18, L857. (c) Horiuchi, S.; Tokura, Y. Nat. Mater. 2008, 7, 357. (4) Zhang, W.; Cai, Y.; Xiong, R. G.; Yoshikawa, H.; Awaga, K. Angew. Chem. 2010, 122, 6758. (5) Sun, Z. H.; Luo, J. H.; Chen, T. L.; Li, L. N.; Xiong, R. G.; Tong, M. L.; Hong, M. C. Adv. Funct. Mater. 2012, 22, 4855. (6) (a) Katrusiak, A.; Szafranśki, M. Phys. Rev. Lett. 1998, 82, 576. (b) Olejniczak, A.; Katrusiak, A.; Szafranski, M. Cryst.Growth Des. 2010, 10, 3537. (c) Katrusiak, A.; Szafranśki, M.; McIntyre, G. J. Phys. Rev. Lett. 2002, 89, 215507. (7) (a) Ye, H. Y.; Fu, D. W.; Zhang, Y.; Zhang, W.; Xiong, R. G.; Huang, S. D. J. Am. Chem. Soc. 2009, 131, 12544. (b) Zhang, W.; Ye, H. Y.; Cai, H. L.; Ge, J. Z.; Xiong, R. G.; Huang, S. D. J. Am. Chem. Soc. 2010, 132, 7300. (c) Zhang, Y.; Zhang, W.; Li, S. H.; Ye, Q.; Cai, H. L.; Deng, F.; Xiong, R. G.; Huang, S. D. J. Am. Chem. Soc. 2012, 134, 11044. (8) Sun, Z. H.; Wang, X. Q.; Luo, J. H.; Zhang, S. Q.; Yuan, D. Q.; Hong, M. C. J. Mater. Chem. C 2013, 1, 2561−2567. (9) Aizu, K. Phys. Rev. B 1970, 2, 754. (10) (a) Kozak, A.; Grottel, M.; Kozioz, A. E.; Pajak, Z. J. Phys. C: Solid State Phys. 1987, 5433. (b) Szafranński, M. J. Phys. Chem. B 2011, 115, 8755. (11) (a) Pająk, Z.; Czarnecki, P.; Szafrańska, B.; Małuszyńska, H.; Fojud, Z. J. Chem. Phys. 2006, 124, 144502. (b) Ye, H. Y.; Zhen, J.; Chen, G. F.; Xiong, R. G. CrystEngComm 2010, 12, 1705. (c) Fu, D. W.; Zhang, W.; Cai, H. L.; Zhang, Y.; Ge, J. Z.; Xiong, R. G.; Huang, S. D.; Nakamura, T. Angew. Chem., Int. Ed. 2011, 50, 11947. (12) (a) Fu, D. W.; Zhang, W.; Cai, H. L.; Zhang, Y.; Ge, J. Z.; Xiong, R. G.; Huang, S. D. J. Am. Chem. Soc. 2011, 133, 12780. (b) Cai, H. L.; Zhang, W.; Ge, J. Z.; Zhang, Y.; Awaga, K.; Nakamura, T.; Xiong, R. G. Phys. Rev. Lett. 2011, 107, 147601. (13) Zhang, Y.; Awaga, K.; Yoshikawab, H.; Xiong, R. G. J. Mater. Chem. 2012, 22, 9841. (14) (a) Jakubas, R.; Piecha, A.; Pietraszko, A.; Bator, G. Phys. Rev. B 2005, 72, 104107. (b) Kulicka1, B.; Kinzhybalo1, V.; Jakubas1, R.; Ciunik1, Z.; Baran, J.; Medycki, W. J. Phys.: Condens. Matter. 2006, 18, ́ 5087. (c) Szklarz, P.; Chański, M.; Slepokura, K.; Lis, T. Chem. Mater. 2011, 23, 1082.

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CONCLUSION We have presented a new reversible switchable dielectric complex formed by 1,4-diazabicyclo[2.2.2]octane and chlorodifluoroacetic acid, which undergoes a phase transition at about 165 K. Crystal structures of 1 obtained at 293 and 100 K reveal that it undergoes a reversible phase transition from the RT space group of Pnma to the LT space group of P21/c. The symmetry breaking occurs with an Aizu notion of mmmF2/m. The origin of the switchable dielectric phase transition was ascribed to the movements of the anions from the equilibrium position, which is induced by the ordering of the twisting motions of the chlorodifluoroacetate anions. In addition, the striking steplike dielectric anomalies indicated that 1 is a kind of switchable molecular dielectric. This successful example may open an effective way to design switchable dielectric phase transition materials.

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ASSOCIATED CONTENT

S Supporting Information *

Powdered XRD patterns, IR spectrum, packing views of the crystal structures, and other mode explanation. CCDC reference numbers 918107 (LT (100 K)) and 918108 (RT (293 K)). This information is available free of charge via the Internet http:// pubs.acs.org.

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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 This work was financially supported by the National Natural Science Foundation of China (21222102, 51102231 and 21171166), the One Hundred Talents Program of the Chinese Academy of Sciences, and the 973 Key Programs of the MOST (2010CB933501, 2011CB935904) and Key Project of Fujian Province (2012H0045). We gratefully thank Master Z. Z. Xue for his help on the crystal structure measurement at low temperature. Z.S. thanks the support from “Chunmiao” Project of Haixi Institute of Chinese Academy of Sciences (CMZX-2013-002).

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REFERENCES

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Switchable Dielectric Phase Transition Induced by Ordering of

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