Orientational ordering of guest induced structural phase transition coupled with switchable dielectric properties in a host–guest crystal: bis(thiourea) thiazolium chloride

Abstract

A new thiourea-based supramolecular compound (TA)[(TU)₂Cl] (1, TA = thiazolium, TU = thiourea) has been successfully synthesized and systematically characterized, including differential scanning calorimetry (DSC), variable-temperature structural analysis and dielectric measurements. 1 undergoes a structural phase transition at 213.7 K (T_c), accompanied by distinct switchable dielectric anomalies. The TA cation is disordered over four equivalent positions above the T_c and becomes less-disordered over the two equivalent positions below the T_c. The structural relative disorder-order transition of the TA cation contributes to the switching of the dielectric constant between high- and low-dielectric states. Supramolecular inclusion compounds designed with dynamic components have been proved to be a good strategy for searching for new phase transition and switchable dielectric materials.

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Introduction

Solid-to-solid phase transition materials have received extensive attention because of their wide applications in sensing, data communication, signal processing, thermal energy storage, etc.^1–5 Notably, some physical properties of the materials, such as switchable dielectric constants, can be switched between different states before and after the phase transitions, which make them excellent in switches and sensors.^6–11 Many attempts have been made to prepare this kind of dielectric switches.^12,13 From these great works, the theme involves finding an effective way to design molecules with dynamic components that can experience rotational and static states under external stimuli like temperature, which is responsible for implementing structural phase transition. In particular, structurally organized and functionally integrated supramolecular architectures may be good candidates of switchable dielectric materials.^14–16 For example, Ye and Zhang et al. reported two new supramolecular crystals, where the motion-active 2,6-diisopropylanilinium cations play key roles in determining the ferroelectric phase transitions.^17,18 Moreover, supramolecular strategies based on the association with polymolecular organization such as host–guest inclusion compounds, tend to be an effective motivator in the promotion of the emerging field of the switchable dielectric phase transition materials.^19,20 The host–guest supramolecular inclusion clathrates, (HPy)[(TU)₂X] (HPy = pyridinium, TU = thiourea, X = halogenide ion or nitrate) salts, have been reported with structural phase transitions and ferroelectricity.²¹ In these compounds, the cation guest occupies the space enclosed by TU molecules and X ions, making themselves a large freedom of thermal motions in the negative host channels. Such cationic disordered/rotational and ordered/static states caused by the variation of temperature are key elements in designing supramolecular phase transition and switchable dielectric materials. Therefore, it is very conductive to investigate and get a better understanding of the dynamics of cation guests and design functional materials with prominent physical properties in the system of host–guest inclusion compounds.

Inspired by the pioneering work and in the process of exploring new switchable dielectric materials, we here report a thiourea-based supramolecular compound (TA)[(TU)₂Cl] (1, TA = thiazolium), which shows a structural phase transition with remarkable dynamic changes of the TA cation, accompanied by distinct dielectric switching behaviour. Systematic characterizations including differential scanning calorimetry measurements, single crystal X-ray diffraction structural analysis, and dielectric measurements reveal that the prominent dielectric anomalies is attributed to the cationic order–disorder transition. This finding affords an effective strategy to prepare supramolecular compounds with new functional properties.

Experimental

Synthesis

All reagents and solvents in the syntheses are confirmed pure and commercially obtained from Aladdin and used without further purification. The target product (TA)[(TU)₂Cl] (TU = thiourea, TA = thiazolium) was synthesized through the reaction of thiazole (99%), thiourea (99%) and concentrated hydrochloric acid in a solution of ethanol. Moderate HCl (1.0 g, 10 mmol) was added into the ethanol solution containing thiazole (0.869 g, 10 mmol). After stirring for 10 minutes, the solution was added dropwise into the boiling ethanol solution containing thiourea (1.522 g, 20 mmol). Colorless need-like crystals were collected after cooling. The crude needle crystals were purified by recrystallization from ethanol. Yield: 67%. Mp: 396 K. ¹H-NMR (D₂O, 300 MHz) δ: 8.01 (s, 1H, C2–H), 8.14 (s, 1H, C3–H), 9.78 (s, 1H, C1–H) (Fig. S1, ESI†). ¹³C-NMR (D₂O, 300 MHz) δ: 124.7 (C3), 134.3 (C2), 157.6 (C1), 181.7 (C4) (Fig. S2, ESI†). Selected IR peaks (KBr, cm⁻¹): 3340 (b), 3128 (s), 3040 (s), 1590 (s), 618 (m) (Fig. S3, ESI†). Anal. calcd for C₅H₁₂ClN₅S₃: C, 21.93; H, 4.42; N, 25.58, S, 35.13. Found: C, 21.64; H, 4.83; N, 25.27, S, 35.49 (Table S1, ESI†). Phase purity of the crystals was proved by powder X-ray diffraction pattern (Fig. S4, ESI†). TGA measurement shows that the sample is stable below about 350 K (Fig. S5, ESI†). The variable-temperature PXRD measurements also demonstrate the process of phase transition (Fig. S6, ESI†).

Single crystal X-ray diffraction

Variable-temperature X-ray single-crystal diffraction data were collected at 273 and 133 K on a Rigaku Saturn 724⁺ CCD diffractometer equipped with Rigaku low-temperature gas spray cooler device with Mo-Kα (λ = 0.71075 Å) radiation from a graphite monochromator. Data processing including empirical absorption corrections was performed by using the Crystal clear software package (Rigaku, 2005). The structures were solved by direct methods and refined by the full-matrix method based on F² by means of the SHELXLTL software package. Non-H atoms were refined anisotropically by using all reflections with I > 2σ(I). All H atoms were generated geometrically and refined by using a “riding” model with U_iso = 1.2U_eq (C and N). Crystallographic data and structure refinement at 273 and 133 K are listed in Table 1.

Table 1 Crystal data and structure refinements for 1 at 273 K and 133 K

a R1 = ∑||Fo| − |Fc||/∑|Fo|.b wR2 = [∑w(Fo^2 − Fc^2)^2/∑w(Fo^2)^2]^1/2.c Maximum and minimum residual electron density.
T/K	273	133
Formula	C20H48Cl4N20S12	C5H12ClN5S3
Mw	1095.30	273.83
Crystal system	Orthorhombic	Orthorhombic
Space group	Cmcm (no. 63)	Pmcn (no. 62)
a/Å	14.430(9)	14.311(10)
b/Å	10.330(9)	10.228(7)
c/Å	8.417(7)	8.393(6)
α/°	90	90
β/°	90	90
γ/°	90	90
V/Å^3	1255(2)	1229(2)
Z	1	4
ρcalcd/g cm^−3	1.450	1.480
μ/mm^−1	0.777	0.793
Refs collected/unique	4412/794	8314/1461
R1^a, wR2^b (I > 2σ(I))	0.0644, 0.1861	0.1198, 0.3049
GOF	1.128	1.224
Δρ^c/e Å^−3	1.003, −0.889	1.095, −0.764

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Thermal measurements

Thermogravimetric analysis was performed on a DSC/DTA-TG STA 449 F3 instrument at the rate of 10 K min⁻¹ under nitrogen atmosphere in the temperature range of 300–920 K. DSC measurements upon heating and cooling scans were performed on a TA Instruments SDT-Q10 with the crystals having a heating/cooling rate of 10 K min⁻¹ in the temperature range of 180–245 K under nitrogen at atmospheric pressure in aluminum crucible.

Dielectric measurements

Polycrystalline samples were pressed into pellets with silver conduction paste deposited on the surfaces as the electrodes. Dielectric measurements of the sample were performed on a TongHui 2828 impedance analyzer at frequencies of 100 Hz to 1000 kHz with an applied electric field of 0.5 V in the temperature range of 100–300 K.

Results and discussion

Phase transition of 1

It is well-known that DSC measurements can be used as an effective thermodynamic approach to detect a phase transition stimulated by temperature variation. As shown in Fig. 1, DSC curves measured in the range of 180–245 K exhibit an exothermic peak at 210 K and an endothermic peak at 213.7 K in a cooling–heating run, respectively. This suggests that 1 undergoes a reversible phase transition at T_c = 213.7 K with a small thermal hysteresis of 3.7 K. The broad and round-like peaks illustrate the continuous character of the transition, manifesting that the phase transition is of second-order type. Furthermore, the corresponding average enthalpy and entropy changes are estimated to be 116 J mol⁻¹ and 0.54 J mol⁻¹ K⁻¹ for the phase transition, respectively.

Fig. 1 Temperature dependence of DSC curves of 1 obtained in a cooling–heating cycle.

Crystal structures of 1

The TU molecules and the chloride anions linking through the hydrogen bonds constitute the basic host channels (Fig. 2a). The rhombic networks provide confined space for the cation guests to reside within. The perspective view of 1 brings to light its typical supramolecular inclusion structure (Fig. 2b). The depth arrangement here depicts the periodic nature and the inclusive thiazolium cations of the TU–Cl cavity.

Fig. 2 (a) Space-filling representations of the TU–Cl host channels of bis(thiourea) thiazolium chloride(1); (b) perspective view of the supramolecular inclusion compound 1 at HTP. Thiazolium (TA) cation resides in the channels formed by the thiourea molecules and chloride anions. Hydrogen atoms were omitted for clarity.

Variable-temperature X-ray single-crystal diffraction data were recorded at 273 K and 133 K, respectively. It appears that 1 crystallizes in the centrosymmetric Cmcm space group in the high-temperature phase (HTP) and Pmcn in the low-temperature phase (LTP), which are in accordance with those of the reported bis(thiourea) thiazolium bromide.^22b Whereas, the cell parameters of a, b and the volume are a little smaller than those of bis(thiourea) thiazolium bromide, which may be due to the smaller size of the chloride atom in comparison with the bromide atom. In addition, 1 maintains an isostructural structure with the HPy and the bis(thiourea) imidazolium chloride salts with a slight difference in the cell parameters.²³ In general, the clathrate behaviour of the host network that acted against the cations with different sizes suggests structural compatibility between the host and guest in meeting the fundamental requirements to allow the synthesize of more related materials through similar cationic inclusion structures.

The structure of 1 is organised via hydrogen-bonding interactions (Table S2, ESI†). As illustrated in Fig. 3a and b, there are N–H⋯S and N–H⋯Cl hydrogen bonds among the TU molecules and the chloride anions, which contribute to the TU–Cl host structure. Additionally, the hydrogen bonds are of variable strength under the external temperature stimulus.

Fig. 3 Hydrogen bonds between the TU molecules and chloride ions in the host structures are shown in (a) HTP and (b) LTP. Hydrogen bonds between the guest TA cations and the host TU molecules are shown in (c) HTP and (d) LTP. Unit of the distances shown as digitals is Å. Hydrogen bonds are presented as rosy dotted lines.

The donor–acceptor distances of the N–H⋯S and N–H⋯Cl hydrogen bonds of the TU–Cl channels are 3.396(4) Å and 3.307(3) Å in the HTP and 3.369(3)–3.372(3) Å and 3.259(2)–3.292(2) Å in the LTP, respectively. These hydrogen bonds are comparable with those previously reported in other isostructural compounds.^21–23 Apart from these hydrogen bonds, there are hydrogen-bonding interactions between the TU–Cl networks and the TA cations as well. The TA cation situated in the hydrogen-bonded cavity of TU–Cl undergoes the transition from disordered to less-disordered states with the decreasing of the temperature. It can be seen from Fig. 3c that the TA cations in the HTP are highly disordered with undistinguishable characterization of the hydrogen bonding. Whereas the hydrogen bonds for those of the LTP structure are clearly demonstrated in Fig. 3d with N–H⋯S bond distances 3.320(8) Å owing to its relative ordered state.

The most concerned dynamic modality of the TA cation is refined as exceedingly disorder over four equivalent positions and denoted as four cycles with four specific colors at 273 K (Fig. 4). The four cycles mainly orient in two directions with every two cycles occupying the same carbon atoms, such as the green and the purple cycles sharing the C3 atom in the HTP. The TA cation occupies a special Wyckoff position with a 2₁2/m symmetry site. The symmetry elements in the HTP adopt the same kind of orientations as those in bis(thiourea) imidazolium chloride compound, which has one mirror plane, one 2-fold axis, one 2-fold screw axis and an inversion center. In 1, the mirror plane is perpendicular to the a-axis, the 2-fold axis lies along the a-axis, and the 2-fold screw axis goes radially through the TA cation, which makes a center of symmetry come into being at the intersection point. Note that the TA cation withstands a dynamic hopping/rotation disorder in the HTP. When upon cooling in the LTP, the symmetry breaking takes place on the 2-fold axis along the a-axis, which vanishes after the phase transition and results in the disappearance of the inversion center (Fig. 4). As a result, the TA cation is disordered over two identified equivalent positions, which is related by the mirror plane and the 2-fold screw axis with an equivalent occupancy of 0.5 of each cycle. This two-fold disorder of the TA cation may be a static disorder that has been forced to occur to satisfy symmetry conditions, and is further verified by the following dielectric measurements. Additionally, neither the four-fold TA cycles in the HTP nor the two-fold TA cycles in the LTP are located in the same plane. The angles between the adjacent non-parallel TA planes are 15.09° and 15.95° in the HTP and LTP, respectively.

Fig. 4 Transformation of the states of the TA cation during the phase transition of 1. The TA cycle is disordered over four and two equivalent cycles in the HTP and LTP, respectively. The 2-fold axis is marked by 2 and the mirror plane by m. The symbol in pink represents the 2-fold screw axis and the open circle denotes the inversion center. Hydrogen atoms were omitted for clarity. Symmetry code: (A) −x, 1 − y, −z; (B) 1.5 − x, y, z.

Dielectric properties of 1

The temperature-dependent dielectric measurement is usually a significant and valuable indicator of structural phase transition. The dielectric anomalies shown as the nonlinear variation of the dielectric constant usually result from the dipole moment changes triggered by the structural phase transitions.^24,25 The motion-active TA cation can arouse dipolar reorientation under the external electric field. The temperature and frequency dependence on the dielectric constant was measured in the range of 100 Hz to 1000 kHz from 100 to 300 K, respectively. As presented in Fig. 5, the real part of the dielectric constant (ε′) distinctly shows a step-like reduction in a wide temperature range (170–230 K), disclosing the continuous and slow process of the structural phase transition of 1. Above 230 K, the curve of ε′ manifests a plateau and maintains stable value (about 24) corresponding to the high-dielectric state. Upon cooling, apparent decrease in the value of ε′ with a step-like type is observed in the vicinity of T_c. When the temperature is further depressed, the value of ε′ drops to about 9 at 150 K, which corresponds to the low dielectric state. This relative small value can help us preclude the dipolar reorientation of the TA cation, suggesting that the TA cation is static in the low-dielectric state. The obviously transition between high- and low-dielectric states makes 1 a potential switchable dielectric material. In combination with the single-crystal X-ray diffraction analysis, the TA cation is disordered over two equivalent positions in the LTP and it can be deduced to be static disorder related by some kinds of symmetry elements (Fig. 4). Therefore, the dynamic order–disorder transition of TA cation contributes to the promising switchable dielectric behaviour.

Fig. 5 Temperature dependence of the real part (ε′) and imaginary part (inset) (ε′′) of dielectric constant of 1 measured in a cooling–heating run at 1000 kHz.

The imaginary part (ε′′) also exhibits a pair of obvious wide peaks in the cooling–heating run with a relative bigger value of about 4 at around T_c, which further supports a phase transition and coincides with the results obtained from the DSC measurements. Moreover, the frequency dependence of the real part (ε′) is displayed in Fig. S7 (ESI†). It can be seen that there is no discrepancy on the phase transition temperature with the variation of the frequency but the value of ε′ does grow with the reducing of the frequency, especially at the temperature region above the T_c. Although the dielectric relaxation behaviour is unnoticeable in the curve of ε′, the dipolar relaxation can be identified in the imaginary part with distinct dispersion at the lower frequencies (Fig. S8, ESI†).^22a,23

Conclusions

In conclusion, we have discovered and characterized a new thiourea-based supramolecular inclusion compound (TA)[(TU)₂Cl], which suffers from a reversible structural phase transition, accompanied by prominent switchable dielectric response. Systematic characterizations reveal that the phase transition is of order–disorder type. The origin of the phase transition is the dynamic order–disorder transition of the TA cation guests, which experience dynamic disorder in the HTP and static disorder in the LTP. Thus, this discovery of the switchable dielectric behaviour encourages us to seek more supramolecular inclusion compounds that will improve the development and design for the synthesis and testing of additional novel-switchable dielectric materials.

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Footnote

† Electronic supplementary information (ESI) available: IR spectrum, PXRD pattern, bond distances and angles, and hydrogen-bond geometry of the crystal structures. CCDC 1432879 at 273 K, and 1432880 at 133 K. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6ra23952h

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