Temperature-triggered order–disorder phase transition in molecular-ionic material N-butyldiethanolammonium picrate monohydrate

Abstract

A new molecular-ionic phase transition material, N-butyldiethanolammonium picrate monohydrate (BEAPM), which exhibits reversible switchable dielectric performances, has been successfully assembled. This compound undergoes a first-order solid-state phase transition at 160 K (T_c), which is confirmed by the thermal analyses including differential scanning calorimetry (DSC), specific heat (C_p) and dielectric measurements. Variable-temperature single crystal X-ray diffraction reveals that the origin of its phase transition is ascribed to the order–disorder transformation of oxygen atoms of the poly-nitro aromatic system, i.e. the picrate anions. Interestingly, the dielectric constants display clear temperature-dependant anomalies with the temperature approaching to T_c. The evident step-like anomalies of dielectric constants demonstrate two distinct states below and above the T_c, respectively. This result signifies that BEAPM could be conceived as a potential switchable dielectric material. These findings make us believe that BEAPM might be a potential solid–solid dielectric phase transition material.

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1. Introduction

Solid–solid phase transition materials (SSPTMs) have attracted much attention, not only for the significant role they play in designing and exploring multi-functional materials, including ferroelectrics, nonlinear optical switches, tunable and switchable dielectric materials,^1,2 but also their great potential for applications, such as data storage and signal processing.^3–6 Some properties of SSPTMs can be switched between at least two states, which have been utilized in switches, memory devices and sensors.^7,8 For instance, under the external stimulus, dielectric responses of these materials will be switched between different states. Therefore, constructing such functional solid–solid phase transition compound is greatly important and until now various designing strategies have been developed to induce such kind of phase transition materials. Among them, introduction of the disordered moieties into molecular crystals is one of the most effective strategies, since the order–disorder transformation plays an important role in structural phase transitions.^9–11

Compounds with excellent ferroelectricity and large dielectric constants have been assembled, in which the order–disorder structural transformations of cations dominate phase transitions.^12–14 For example, Zhang et al. has synthesized molecular salts, using the disordered cation, which easily experience order–disorder transformations associated with tunable and switchable dielectric properties.¹⁵ Typical molecular motions resulted from the orderings of the cation, have been found to cause the structural phase transitions, which display excellent dielectric responses.¹⁶ As described above, most phase transitions originate from the order–disorder transformation of suitable moieties in the cation. However, the anionic counterpart also displays this phenomenon in some compounds. For example, Sun et al. has reported a molecular order–disorder phase transition material showing exceptional switchable dielectric properties,¹⁷ in which the dynamic highly disordered anion trifluoromethanesulfonate becomes more ordered at room temperature.¹⁸ Similarly, such orderings of the anion, have also been shown in the simple molecular nonlinear optical switch, induced by structural phase transition.¹⁹ The anionic entities either synergistically¹⁸ or solely, can induce the phase transition in a material, where the oxygen atoms of anion like oxalate²⁰ and aromatic poly-nitro system like picrate²¹ reorient themselves. As a result, their disordering becomes frozen by lowering the temperature below T_c. This classifiable strategy, using the moieties that contain oxygen atoms with possible order–disorder transformation, is quite promising. The disordered aromatic poly-nitro system fulfills this probability to some extent, as the oxygen atoms of picrate reorient between two different states at high temperature and turn ordered while decreasing down the temperature.²¹ These findings inspire us to intervene such kind of organic anions containing oxygen atoms to explore a new approach for the phase transition materials. Therefore, picric acid is chosen, where the oxygen atoms are dynamic and sensitive to the temperature change and thus might be assembled into smart SSPTMs with the order–disorder systems.

As a continuous study of order–disorder systems in SSPTMs,^22–26 the disordered picrate anion has been incorporated to build a new solid-to-solid phase transition compound, N-butyldiethanolammonium picrate monohydrate (BEAPM), which undergoes a reversible phase transition at 160 K. The phase transition has been confirmed by various methods, including the variable temperature single-crystal structure analyses, differential scanning calorimetry (DSC), specific heat (C_p) and the dielectric measurements. As demonstrated by the single crystal structure, order–disordered transformation of the oxygen atoms of the nitro group in picrate anion is the origin of this phase transition. Also important to mention that no compound is reported with phase transition in which N-butyldiethanolamine has been incorporated. Therefore, to the best of our knowledge, this is the first concrete example, where N-butyldiethanolamine is combined with organic acid inducing an order–disorder transformation. The order–disorder of anion in BEAPM is triggered by temperature, which leads to the switchable dielectric properties between two states. This finding might provide the possible route to design new SSPTMs, where the disordered anion can be utilized with suitable flexible counterpart that might be helpful to discover materials with various properties. The combination of such potential anions along with the flexible cations is utilized in a compound; therefore the possible resulting synergetic effect might enhance the dielectric response in such a system.

2. Experimental

2.1 Synthesis

All reagents and solvents were obtained commercially and used without further purification. N-Butyldiethanolammonium picrate monohydrate (BEAPM) was synthesized by evaporating the aqueous solution, containing N-butyldiethanolamine base and picric acid in stoichiometric amount of 1 : 1 molar ratio. The reaction mixture was stirred and heated for 5 minutes to homogenize the components thoroughly. As the solution became cool, considerable size yellow crystalline product was obtained. The deuterated sample of BEAPM was obtained by dissolving both the components in D₂O and kept for three days at 50 °C. The resultant product was recrystallized twice from D₂O.

2.2 Thermal measurements

Differential scanning calorimetry (DSC) and specific heat (C_p) measurements were recorded on NETZCSCH DSC 200 F3 instrument by heating and cooling rate of 10 K min⁻¹ in the temperature range from 105–225 K. These measurements were accomplished under nitrogen atmosphere in aluminium crucibles. Experiments of specific heat (C_p) were performed by using a comparison method, where sapphire standard was used as the reference. Firstly we run the baseline on the desired temperature range and then sapphire disks for calibration were used as standard prior to the measurement. Then the sample was measured and compared with the sapphire recorded formerly.

2.3 Dielectric measurements

For dielectric experiments, well-ground powder samples connected by silver-conducting paste on electrodes, were used for measuring the complex dielectric permittivities, ε = ε′ − iε′′. The dielectric constants were measured using a TH2828A impedance analyzer at different frequencies of 1 MHz, 100 kHz, 10 kHz and 5 kHz with the measuring AC voltage fixed at 1 V.

2.4 Powder X-ray diffraction (PXRD) measurement and IR spectrum

MiniFlex II Powder X-Ray Diffractometer was used to record room temperature PXRD as a confirmation of the phase purity. The recorded pattern of PXRD of BEAPM matches well with the simulated result from the single-crystal structure at room temperature (Fig. S1†). For IR spectrum VERTEX 70 Infrared Spectrometer was used (Fig. S2†), where the peak at approximately 1620–1560 cm⁻¹ is assigned to stretching vibration absorption of the ammonium ion (–NH⁺), revealing the protonation of N-butylammonium cation in BEAPM. The broad peak near 3375 cm⁻¹ is the result of hydrogen bonding between the ions and water molecule.

2.5 Single crystal structure determination

Variable-temperature X-ray single-crystal diffraction data of BEAPM were collected using Super Nova CCD diffractometer with graphite monochromated Cu-Kα radiation (λ = 1.54184 Å) at low (100 K) and high (185 K) temperatures, respectively. For data collection, cell refinement and data reduction, Crystal Clear software package (Rigaku) was utilized. Crystal structures were solved by the direct methods and refined by the full-matrix least-squares method based on F² using the SHELXLTL software package.³⁰ All non-hydrogen atoms were refined anisotropically. The positions of hydrogen atoms were generated geometrically which were located at the cation (n-butyldiethanolamine), and the hydrogen atoms in benzene ring were determined from the Fourier electron density map. Crystallographic data and details of data collection and refinement are listed in Table S1. CCDC 1425998 and 1425999 for BEAPM contains the supplementary crystallographic data for this paper.†

3. Results and discussion

3.1 Thermal analysis

DSC measurement is an efficient and useful tool to confirm the existence of a reversible temperature-triggered phase transition.^27,28 Preliminary DSC measurement is carried out in BEAPM, depicting (Fig. 1a) a couple of exothermic (135 K) and endothermic (160 K) peaks along with a large heat hysteresis of ∼25 K, which indicate the discontinues first order one. In addition to DSC, the specific heat capacity (C_p) curve further confirmed the presence of phase transition (see Fig. 1b). The curve in C_p gives a sharp peak at 160 K, which is consistent with the DSC measurement. The enthalpy (ΔH) associated with the endothermic peak was calculated as 5.35 J g⁻¹, and thus the corresponding total entropy change (ΔS) was estimated to be 13.97 J mol⁻¹ K⁻¹ using the equation of ΔS = ΔH/T_c. According to the Boltzmann equation, ΔS = R ln N, in which R is the gas constant and N is the number of geometrical distinguishable orientations, the value of N comes out to be 1.68, suggesting that BEAP adopts two possible disordered states over two sites.²⁸

Fig. 1 (a) The reversible DSC curves (b) temperature dependence of C_p of BEAP.

3.2 Single crystal structure determination

X-ray diffraction analyses of BEAPM were performed at 100 K (low-temperature phase, LTP) and 185 K (high-temperature phase, HTP), respectively. These analyses further confirmed the phase transition in BEAPM as clear observable differences can be seen in the LTP and HTP structures. At both the LTP and HTP, BEAPM crystallizes in the same monoclinic crystal system, with centrosymmetric space group P2₁/c and cell parameters LTP a = 13.50480(2) Å, b = 19.1864(3) Å, c = 7.05673(10) Å, β = 91.400(2)°, Z = 4, V = 1827.91(5) ų (Table S1, ESI†). The cell parameters at HTP are: a = 13.5233(2) Å, b = 19.2674(2) Å, c = 7.10340(10) Å, β = 91.5850 (10)°, Z = 4, V = 1850.15(4) ų. In comparison of the above values, it is clear that the difference of cell parameters between the two phases is negligible and the LTP structure is identical to that of HTP. This is an isosymmetric phase transformation with the only noticeable order–disorder change in the anion.

At both the HTP and LTP, the asymmetric unit of BEAPM is composed of one protonated N-butyldiethanolammonium cation, one deprotonated picrate anion and one water molecule (Fig. 2). The information, regarding crystal structure of BEAPM at HTP, describes prominent disorderly behavior along the three oxygen atoms of both the nitro groups at ortho position in picrate anion. These atoms are sternly disordered and reside over two sites as O2A, O5A, O6A and O2B, O5B, O6B respectively, as is shown in the encircled area in Fig. 3b. The striking feature in these observations demonstrates the serious disorder characteristics of BEAPM in the HTP, which is further confirmed by the thermal ellipsoids of oxygen atoms at high temperature. Such apparent disordered characteristics of BEAPM at HTP can also be observed by the thermal ellipsoidal behavior of O2A, O5A O6A atoms, which are larger than those of other neighboring atoms. In fact, this result is not the actual occupation, but an average result of the atomic disordering (Fig. S4a†). The splitting of these ellipsoidal stats leads the crystal structure of BEAPM at HTP into a more fairish one with the occupancy factors of O2A/O2B, O5A/O5B and O6A/O6B are 0.15 : 0.85, 0.15 : 0.85 and 0.13 : 0.87 respectively, which are not in equivalent distribution. This thermal splitting of the nitro groups in picrate anion assures the possible flexible nature of the aromatic poly-nitro system. With the decrease in temperature below phase transition point, the disordering of the oxygen atoms is frozen into a fully ordered state, and the LTP structure corresponds to more stabilized state, as demonstrated by Fig. S3b.† If a horizontal plane is dawn through all carbon atoms in the benzene ring of anion, the disordered atoms can be clearly seen above and below the plane (as shown in Fig. 3b). From this detailed structural analyses, it is clear that the order–disorder transformation is the driving force to the occurrence of this first-order phase transition in the testing material.

Fig. 2 (a) Illustration of the asymmetric unit of BEAPM at 185 K (HTP) and (b) asymmetric unit at 100 K (LTP).

Fig. 3 Order–disorder transformation of the picrate anion during the phase transition of BEAP. (a) The encircled parts exhibit the disordered state. (b) The anion becomes ordered below phase transition point. Hydrogen atoms attached to ring are omitted for clarity.

The versatile water molecule is part of both the HTP and LTP structures and is seen as sandwiched between the anion and cation, which can be viewed in the encircled area in the packing diagram Fig. 4. It is connected to both the counter ions through the formation of hydrogen bonding interactions. The strong intermolecular attractive forces bind the water molecules to the protonated nitrogen atom of cation from one side and to the deprotonated oxygen atom of picrate anion from the other side.

Fig. 4 Crystal packing diagram of BEAPM with hydrogen bonding network viewing from c-axis both at (a) LTP and (b) HTP. The water molecule is encircled in blue color.

Pertaining to both the HTP and LTP, crystal packing structures of BEAPM are characterized by a network of extensive hydrogen bonding with O–H⋯O and N–H⋯O hydrogen bonds (Fig. 4). These are in the form of discrete units, of which each one consists of two anions, two cations and two water molecules. The water molecule binds itself to various oxygen atoms of picrate and cation through the intermolecular interaction. The observed picrate-cation intermolecular interactions (O–H⋯O) exist between the deprotonated oxygen atom of anion and hydroxyl groups of the cation. The entire hydrogen-bonding skeleton implies that the picrate-water and cation-water intermolecular interactions construct together an interlocked system where water molecule occupies central position, as can be seen in the packing diagram, viewing from c-axis. It is noticed that corresponding length of all the hydrogen bonding were found almost the same at LTP and HTP. A list of the detailed values of hydrogen bonding of BEAPM is given in Table S2 in ESI.† Deuterated sample of BEAPM does not give any deviation in value of the transition point in both heating and cooling modes in DSC measurements; therefore the role of hydrogen bonding in emergence of this phase transition is excluded, shown in Fig. S4.† Besides the hydrogen bonding simultaneously, the clear non-covalent π–π interactions (Cg–Cg = 3.351 Å at 100 K and 3.361 Å at 185 K, respectively) exists between adjacent parallel rings of the adjacent picrate anions. This π–π stacking links the anions, binding them together in the form of chains extending in the a-axis, Fig. S5.†

3.3 Dielectric studies of BEAPM

Measurement of temperature-dependent dielectric constant is a significant indicator of phase transition in a material, which discloses its degree of electric polarizability. For molecular crystals, dielectric transition between the low- and high-dielectric states could be established by the reorientation transformation.²⁹ The measurements of temperature-dependent dielectric permittivities of BEAPM were carried out on the powder pressed pellets, and Fig. 5 displays the real part of the complex dielectric permittivities (ε′) at different frequencies (1 MHz and 100 kHz) from 120 to 150 K. The obvious step-like dielectric anomaly was clearly recorded at 135 K in cooling mode, which corresponds well to thermal measurements. On cooling until 145 K, decrease in the dielectric constant progressively showing an obvious reduction with a slope around T_c. The permittivity exhibits a plateau with a slight decrease. The values of ε′, for instance, display the changes from 4.2 to 4.8 during its phase transition, of which the variation was sharp in the surrounding of T_c. The step-like observed dielectric anomaly is an indication of the occurrence order–disorder transformation in BEAPM.³¹ By decreasing the temperature below 135 K, the anion becomes ordered. The order–disorder phenomenon of the oxygen atoms in anionic moieties is assumed to be the main cause of dielectric response in BEAPM. Therefore, the disordering exerts predominant influences on the dynamics of the anion. From the discussion this can be concluded that the thermally-induced switching behaviour, as demonstrated by dielectric properties as a function of temperature, BEAPM might be a potential switchable dielectric material.¹⁵

Fig. 5 Representation of the dielectric constants of BEAPM carried out on the powder pressed pellets at different frequencies on cooling mode.

4. Conclusions

In this work, we present a new molecular-ionic material with switchable dielectric reversible phase transition induced by order disorder transformation at 160 K. The N-butyldiethanolammonium cation is incorporated for the first time in BEAPM. This is being confirmed by thermal analyses, temperature-dependent dielectric measurement and single-crystal structural analyses. The drive towards the origin of this discontinuous order disordered phase transition in BEAPM is exclusively attributed to the disordering of the nitro groups in picrate anion. In detail, on cooling down the temperature, the disordered atoms in the anion are frozen and become ordered, resulting in the transformation in BEAP. It is believed that this new finding offers a new route to design potential phase transition materials, which might be paramount in development of switchable devices and compels motivation for the continued studies of these materials.

Acknowledgements

I am thankful to the CAS-TWAS President program of the University of the Chinese Academy of Sciences and NSFC (21373220, 51402296, 21301172, 21525104, 91422301, 51402296 and 21571178), the 973 Key Program of MOST (2011CB935904), the NSF for Distinguished Young Scholars of Fujian Province (2014J06015) on their financial support for this work. Z. S. and S. Z. thanks the supports from "Chunmiao Projects" of Haixi Institute of Chinese Academy of Sciences (CMZX-2013-002 and CMZX-2014-003).

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Footnotes

† Electronic supplementary information (ESI) available: IR spectrum, PXRD patterns, packing diagram showing thermal ellipsoids maps of anion, deuterated sample DSC and tables of crystal data and hydrogen bonds of compound 1. CCDC 1425998 and 1425999. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6ra12178k

‡ These authors contribute equally.

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