Reversible Phase Transition Triggered by OrderâDisorder

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Reversible Phase Transition Triggered by Order-Disorder Transformation of Carboxyl Oxygen Atoms Coupled with Distinct Reorientations in [HN (C4H9)3] (fumrate)0.5•(fumaric acid)0.5 Muhammad Adnan Asghar, Zhihua Sun, Tariq Khan, Chengmin Ji, Shuquan Zhang, Sijie Liu, Lina Li, Sangen Zhao, and Junhua Luo Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.5b01452 • Publication Date (Web): 31 Dec 2015 Downloaded from http://pubs.acs.org on January 1, 2016

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Reversible Phase Transition Triggered by Order-Disorder Transformation of Carboxyl Oxygen Atoms Coupled with Distinct Reorientations in [HN (C4H9)3] (fumrate)0.5·(fumaric acid)0.5 Muhammad Adnan Asghar, †‡ Zhihua Sun, †* Tariq Khan, †‡ Chengmin Ji, † Shuquan Zhang, † Sijie Liu, †‡ Lina Li, † Sangen Zhao†* and Junhua Luo†*

†

Key Laboratory of Optoelectronic Materials Chemistry and Physics, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, China ‡ University of the Chinese Academy of Sciences, Beijing 100039, China ABSTRACT: A new reversible molecular phase transition material, [HN(C4H9)3] (fumrate)0.5·(fumaric acid)0.5 (1) has been successfully synthesized. Differential scanning calorimetry (DSC) measurement shows a pair of reversible peaks at 181.9 K and 178 K on heating and cooling modes, respectively. The large thermal hysteresis of ~3.9 K discloses its reversible first-order structural phase transition. Specific heat capacity and dielectric constant measurements around Tc further confirm its phase transition behaviors. The detailed structural analyses of 1 at variable temperatures reveal that its structural phase transition is mainly accomplished by the order-disorder transformations of carboxyl oxygen atoms and distinct reorientations among cations and anion-acid infinite sheets together with proton-dynamic motion. All these results open a new way to construct potential phase transition materials through the selection of flexible aliphatic cations with simple carboxyl based anions.

■ INTRODUCTION Reversible phase transition materials, of which the optoelectronic responses can be systematically converted by application of external pressure, heat or light, have caught wide interests due to their potential applications in NLO switches, ferroelectrics, switchable dielectrics, phase shifters, sensing, signal processing and data storage, etc. 16 Engineering of solid-to-solid temperature-dependent reversible phase change materials are not only significant for the searching of technologically potential materials, but also helpful for the investigation of structure-property relationship. 7-9 Recently, breakthroughs in this field have been made by the discovery of a correlation between the order-disorder transformation of moieties and switchable dielectric/ferroelectric properties. 10-14 For example, Xiong et al. explored an amphidynamic crystal of [(CH3)2NH2]2[KCo(CN)6] exhibiting tunable and switchable dielectric constants, where the order-disorder transformation of dimethylammonium cation affords the driving force for its reversible structural changes. 15 In addition, some haloacetic acids, such as trichloroacetic acid, trifluoroacetic acid, dichloroacetic acid and difluoroacetic acid, also play an imperative role during the phase transitions of molecular crystals by the orderings of halogen atoms. 16, 17 For instance, dichloroacetate-anion-based salt of potassium hydrogen bis(dichloroacetate)-18-crown-6 undergoes a dielectric phase transition triggered by the ordering of unique pendulum-like motions of dichloroacetate anion. 18 Moreover, structural phase transitions of several classic ferroelectrics are ascribed to the proton transfer from the hydroxyl group, along with the disordering or displacement of other parts. 19-21 Take the first known ferroelectric of Rochelle salt as an example, its phase changes are primarily induced by the proton transfer of hydrogen bonds between water and L-tartrate molecules, together with the molecular displacements around potassium atoms. 22

However, searching for new phase transition materials composed of carboxyl and hydroxyl-based anions remains quite interesting, because simple acids containing carboxyl groups are easy to form hydrogen bonds which play a vital role for structural changes. To the best of our knowledge, phase transitions induced by order-disorder transformation of carboxyl oxygen atoms are comparatively scarce. 23 Whereas, it should be emphasized that sometime only order-disordered transformation of carboxyl-based anions is not adequate to stimulate structural phase transitions. 24,25 The introduction of other molecular motions, such as twisting and/or reorientations, would enhance the possibilities to assemble phase transition materials. In this context, we propose a potential strategy to design new phase transition compounds, that is, combining the carboxyl-based acids with the branched/flexible aliphatic amines. Particularly, the aliphatic amines with long and flexible alkyl chains have more opportunities to trigger molecular twisting and/or reorientations. Here, the combination of flexible tri-pod-like tri-n-butyl amine with fumaric acid yields the fruitful results. As the continuing work of our group to explore phase transition materials, 26-32 we report that a new molecular crystal of [HN(C4H9)3] (fumrate)0.5·(fumaric acid)0.5 (1), undergoes a reversible first-order phase transition at ~182 K, which is confirmed by the differential scanning calorimetry (DSC), specific heat capacity (Cp) and dielectric constant measurements. Variable-temperature single-crystal structure analyses reveal order-disorder transformations of carboxyl oxygen atoms stimulate its phase transition, as well as the distinct reorientations between cations and the anion-acid chains parallel with proton-dynamic motion. 12 This finding affords a potential pathway to assemble phase transition compounds by combining carboxyl-based acids with branched/flexible aliphatic amines.

■ EXPERIMENTAL SECTION Synthesis. All chemical reagents were purchased from Sigma Aldrich with high purity and used without further


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purification. Colourless polycrystalline material of 1 was harvested through slow evaporation method by dissolving tri-n-butylamine and fumaric acid in ethanol with a molar ratio of 2:1 at ambient conditions. Repeated recrystallization of 1 yields needle-like single crystals after several days. The phase purity of 1 was verified by comparing the room-temperature experimental and simulated powder Xray diffraction patterns (Figure S1). The stoichiometry of 1 was determined by X-ray diffraction and elemental analyses. Calcd. for C16 H31 N O4; C: 63.76, H: 10.39, N: 4.65. Found C: 63.74, H: 10.38, N: 4.66. Thermal measurements. Specific heat capacity and differential scanning calorimetry (DSC) measurements of 1 were recorded on a Netzsch DSC 200 F3 instrument with a heatingcooling rate of 5 K min-1 under the nitrogen atmosphere in the temperature range of 150-220 K using aluminum crucibles. Dielectric measurements. The complex dielectric permittivity (ε = ε′ - iε′′) of 1 was recorded on TH2828 A impedance analyzer at the frequencies of 500 Hz, 10 kHz, 100 kHz and 1 MHz in the temperature range of 150-200 K with the measuring AC voltage fixed at 1 V. Single-crystal Structure determination. Variable Singlecrystal X-ray diffraction were performed on Super Nova diffractometer loaded with Cu-Kα radiation (λ = 1.54184 Å). A colorless crystal of approximate dimensions 0.33 × 0.29 × 0.25 mm was used in data collections at 160 and 260 K. The SHELXS97 was used to solve the crystal structures by direct method and refined by full-matrix method on F2 using SHELXLTL software package. 33 All hydrogen atoms were generated geometrically and all non-hydrogen atoms were refined anisotropically. Crystallographic data and details of data collection and refinement are listed in Table S1. Crystallographic data and details of data collection and refinement are listed in Table S1. CCDC 1064095-1064096 for 1 contains the supplementary crystallographic data for this paper, which can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

■ RESULTS AND DISCUSSION Thermal properties of 1. DSC is reliable thermodynamic technique to detect the reversible phase transition nature of a compound with respect to temperature change. Here, the polycrystalline samples of 1 were subjected to DSC measurements in an aluminium container under nitrogen conditions. Upon cooling and heating, an exothermic anomaly at 178 K and an endothermic anomaly at 181.9 K were observed, respectively (phase transition point, Tc = 181.9 K, Figure 1). The large thermal hysteresis of ~3.9 K during cooling and heating suggests its first-order phase transition behaviour. In addition, an entropy change (∆S) is calculated with a value of 3.27 J·mol-1·K-1at about 182 K from the Cp measurements. From the Boltzmann equation ∆S = RlnN, where R is the gas constant and N is the ratio of the numbers of respective geometrically distinguishable orientations, N is calculated as 1.48, which signifies an order–disorder transformation of 1. 34, 35

Figure 1. The DSC and Cp curves of 1.

Crystal structural analysis. A comprehensive structural analysis is imperative for understanding the microscopic mechanism of the reversible phase transitions. For 1, variable-temperature single-crystal X-ray diffraction analyses were performed at 260 K (above Tc, high-temperature phase, HTP) and 160 K (below Tc, low-temperature phase, LTP), respectively. Compound 1 crystallizes in the triclinic class with centrosymmetric space group of P 1 at both phases. In the HTP, the cell parameters are a = 8.425 Å, b = 9.71 Å, c = 12.537 Å, α = 100.236°, β = 109.249°, γ = 95.15° and V = 940.75 Å3; while in the LTP, its cell parameters are a = 8.188 Å, b = 9.887 Å, c = 11.973 Å, α = 100.63°, β = 108.452°, γ = 94.113°, V = 894.86 Å3. As shown in Figure. 2, the unit cell parameters of 1 were recorded as a function of temperature from 240 K to 100 K. The cell constants show considerable change at about 182 K, suggesting its phase change behaviors, which agree fairly well with the results of DSC and dielectric measurements. Crystal structures at HTP and LTP. The asymmetric unit of 1 is composed of one protonated tri-n-butyl ammonium cation, half part of deprotonated fumarate anion and half molecule of fumaric acid at HTP and LTP (Figures 3 and 4). The packing structures of 1 in both phases are characterized by an extensive hydrogen bonding arrangement. In the ellipsoidal structure at HTP, the observed thermal values of oxygen atoms are found to be higher than that of the neighbouring carbon atoms, which would reveal the disrodering feature of oxygen atoms (Figure S4-b). For example, the terminal oxygen atoms of fumarate groups acquire higher thermal ellipsoidal state, which will be more suitable if splitted into two occupied sites as O2A and O2B with the occupancies of 0.5 and 0.5, respectively. In contrast, the flexible cations are found to be in the normal ordered state, although the terminal carbon atoms of three flexible arms of cations exhibit relatively large thermal vibrations. The detailed Ueq values of 1 are given in the Table S2.



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Figure 3. Asymmetric unit (a), hydrogen bonding (b) and packing diagram (c) of 1 at HTP. In the structure of HTP, the fumarate anions and fumaric acid molecules are connected with each other infinitely through relatively weak O–H···O- hydrogen bonding interactions between carboxylate moieties and adjacent acid molecules (distance between donor and acceptor is 2.498 Å, Figure 3b). The carboxyl atoms of acids act as donors to the adjacent carboxylate groups in intra anion-acid hydrogen bonds, which enhance the establishment of infinite sheets (as shown in Figures 3c and 4c). Moreover, each fumarate anion not only connects with two different fumaric acids, but also interlocks with two tri-n-butyl ammonium cations through N−H···O hydrogen-bonding interactions, facilitated by the protonated nitrogen atoms of cations and carboxyl oxygen atoms of anions. This type interlocking of cations creates the distinct layers above and below the anion-acid infinite sheets, respectively.

Figure 4. Asymmetric unit (a), hydrogen bonding (b) and packing diagram (b) of 1 at LTP. Structure difference between HTP and LTP. It is noteworthy that the disordered oxygen atoms of fumarate anions do not participate in the construction of H-bonding interactions at both phases. It means that the disordered oxygen atoms might possess a higher dynamic freedom in the crystal lattice, thus affording possibilities for the orderdisorder transformation at HTP. With the temperature decreasing from HTP to LTP, the disordered motions of carboxylate oxygen atoms become frozen, well corresponding to its more ordered low-temperature phase (as shown in Figure 5).

Figure 5. The order-disorder transformation of fumarate anions in 1 at HTP (left) and LTP (right).

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Such a fascinating order-disorder transformation can also be deduced from the tilting of angles between acid and anionic moieties. As discussed previously, the onedimensional infinite H-bonding sheets are formed between the anions and acid molecules at both HTP and LTP. However, these sheets display distinct reorientations at two states, as shown in Figure 6. The observed torsion angle among C13-O1-O3-C15 is calculated as 163.65° at HTP, which deviates abruptly to 157.0° at LTP. Besides, the flexible protonated tri-n-butyl ammonium groups show different geometrical configurations for both phases. As shown in Figure 7, three n-butyl parts of the cations (named as C1-C2-C3-C4, C5-C6-C7-C8 and C9-C10C11-C12) bear the torsion angles of 178.44°, 179.65° and -68.59° at HTP, while 179.0°, 178.58° and -67.65° at LTP, respectively. These obvious reorientations at both phases unveil the involvement of cations during phase change process. These detailed structural comparisons of 1 disclose that the order-disorder transformation of carboxyl oxygen atoms would afford the driving force for this first-order phase transition, together with the relative reorientations between anionic-acid sheets and the flexible cationic n-butyl parts. The deuterated analogue 2 was synthesized to investigate the proton dynamic motions during phase-transition. It is obvious that the deutrated compound exhibits obvious change of DSC shapes, compared to that of compound 1 in DSC measurements (Figure S3). The DSC peaks became broader and displaced from the original position to 183.9 K on heating and 175.3 K on cooling. However, the quite small change of phase transition point (~2 K) further confirms that order-disorder transformation of fumarate anions still dominates its structure changes, although the proton dynamic motions in the O–H···O hydrogen bond may also involve the phase-transition.

Figure 6. Reorientations in anion-acid sheet at HTP and LTP.

Figure 7. Reorientational studies of cations at (a) HTP and (b) LTP.


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Dielectric properties. The structural phase transitions not only lead to thermal entropy change, but also help the stimulation of anomaly during dielectric measurements. The powder-pressed pellet of 1 was subjected to temperature-dependent dielectric measurements at 500 Hz, 10 kHz, 100 kHz, and 1 MHz. As shown in Figure 8, the step-like anomalies are clearly observed at about 182 K upon heating, which reveals the occurrence of phase transition. In detail, the sample exhibits an initial dielectric constant value of approximately 2.3 at 160 K. Around Tc, a sharp variation is recorded and the dielectric constants jump to 6.1 at 182 K (f = 500 Hz). As far as we are aware, such temperature-dependent step-like dielectric anomalies around Tc are reliable with the characteristics of switchable dielectric materials. 36 Besides, the comparative dielectric relaxation near Tc shows no obvious responses, which enlightens the fact that dipolar motion is very fast during phase transition. 37,38 Furthermore, the step-like dielectric anomalies together with the variabletemperature crystallographic studies of 1 disclose that the phase transition is neither ferroelectric nor antiferroelectric.

sition at about 182 K, being confirmed by the step-like dielectric anomalies, DSC and Cp measurements. Variable-temperature structural analyses of 1 further disclose that the origin of phase transition is ascribed to the synergetic corporations between order-disorder transformations of carboxyl oxygen atoms and distinct torsions among cations and anion-acid infinite chain together with protondynamic motion. Thus, we believe that the present finding would afford an opportunity for the design of potential phase transition materials through selecting the flexible aliphatic amines with simple carboxyl based acids.

■ ASSOCIATED CONTENT ■ ACKNOWLEDGMENT This work was financially supported by the National Nature Science Foundation of China (21222102, 21373220, 51102231, 21171166, and 21301171), the One Hundred Talents Program of the Chinese Academy of Sciences, the 973 Key Programs of the MOST (2010CB933501, 2011CB935904), and Key Project of Fujian Province (2012H0045). Dr. Sun and Zhao is thankful for the support from “Chunmiao Project” of Haixi Institute of Chinese Academy of Sciences (CMZX-2013-002 and CMZX-2013-003). T. K is thankful to the CAS-TWAS President program of the University of the Chinese Academy of Sciences for financial support. Supporting Information Available. PXRD patterns, TG-DTA, Deuterated sample DSC, thermal ellipsoids diagrams, and tables of compound 1. This information is available free of charge via the Internet http://pubs.acs.org.

■ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]; Figure 8. Temperature-dependent dielectric constants of 1 measured at 500 Hz, 10 kHz, 100 kHz and 1000 KHz. It is noteworthy that dielectric constants of 1 also exhibit the frequency-dependent characteristics. Above Tc, the dielectric constants demonstrate an obvious decline with the frequency increasing, which change from ~8.1 (at 500 Hz) to 5.4 (at 1000 KHz). That is, the larger values of dielectric constants could be expected at even lower frequency. Such a frequency-dependent dielectric response mainly results from the orientational motions of the structural groups in 1, well coinciding with the above structural analyses of reorientations between cations and anion-acid infinite sheets, which are responsible for the presence of the motions of dipolar moments during the order–disorder phase transition.39, 40

■ CONCLUSIONS In conclusion, we have discovered a new phase transition compound of [HN(C4H9)3] (fumrate)0.5·(fumaric acid)0.5, which exhibits a reversible second-order phase tran-

[email protected]; [email protected]

Notes; ; The authors declare no competing financial interests.

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Table of contents Reversible Phase Transition Triggered by Order-Disorder Transformation of Carboxyl Oxygen Atoms Coupled with Distinct Reorientations in [HN (C4H9)3] (fumrate)0.5·(fumaric acid)0.5 Muhammad Adnan Asghar, †‡ Zhihua Sun, †* Tariq Khan, †‡ Chengmin Ji, † Shuquan Zhang, † Sijie Liu, †‡ Lina Li, † Sangen Zhao†* and Junhua Luo†*

[HN(C4H9)3]HC2O2·H2C2O2 (1), undergoes a reversible first-order phase transition around 182 K, triggered by the order-disorder transformation of carboxyl oxygen atoms and distinct torsions between cations and anion-acid infinite chain together with proton dynamic motion.

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Reversible Phase Transition Triggered by OrderâDisorder

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Reversible Phase Transition Triggered by OrderâDisorder