Halogenated carborane molecular ferroelectric crystals with high-temperature phase transition

Abstract

Molecular ferroelectrics possess spontaneous polarization, finding applications in sensors, memory devices, capacitors, among others. However, most of these materials are hybrids of organic and inorganic compounds, and few single-component pure organic ferroelectrics have been reported to have high phase transition temperatures. In this study, employing a quasi-spherical approach, five o-carborane cage molecular ferroelectric derivatives, comprising two B(9)-mono-substituted o-carboranes, B(9)–Br-o-carborane (1), and B(9)–Cl-o-carborane (2), along with three B(9,12)-di-substituted o-carboranes, B(9,12)–Br, Cl-o-carborane (3), B(9,12)–Br₂-o-carborane (4), and B(9,12)–Cl₂-o-carborane (5) were obtained by halogenation. All five compounds crystallize in the ferroelectric polar point group (2, m, mm2) and exhibit above-room-temperature ferroelectricity which are confirmed by the ferroelectric hysteresis loops and domain switching. Furthermore, the maximum transition temperature of the dibromosubstituted o-carborane molecule 4 reaches 437 K, which is 160.5 K higher compared to the precursor o-carborane. This finding offers a novel avenue for investigating organic single molecular ferroelectrics with high-temperature phase transition.

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Introduction

Ferroelectric materials have garnered significant attention due to their spontaneous polarization and the ability to switch polarization direction with an external electric field, which enables a multitude of applications.^1–4 For instance, the nonlinear characteristics of ferroelectric materials can be exploited to fabricate tunable capacitors. Ferroelectric thin film materials can be utilized for memory storage, enhancing the erasing capability of the memory by altering the direction of applied voltage.^5,6 Thus far, research on ferroelectric materials has primarily focused on inorganic salts or organic–inorganic hybrid perovskite materials. However, both materials contain metallic elements, limiting their applications in biological and health engineering. Over the past decade, researchers have increasingly directed their efforts toward addressing these challenges through the exploration of pure organic molecular ferroelectric crystals.

It is evident that organic ferroelectrics offer distinct advantages, including notable structural adaptability that serves as a platform for the design of high-temperature ferroelectric materials, and minimal metallic toxicity, making them more suitable for applications related to the human body.^7–11 However, the majority of organic molecular ferroelectrics exhibit relatively low ferroelectric phase transition temperatures (T_c). In order to propel the manipulation of ferroelectricity and multifunctionality forward, there exists an urgent imperative to pioneer and create groundbreaking high-T_c organic molecular ferroelectrics. However, proposing an effective, widely applicable, and simple method to enhance the transition temperature presents significant challenges.

Ferroelectric materials discovered in the past century include simple organic salts such as Rochelle salt¹² and tri-glycine sulfate (TGS),¹³ as well as some multicomponent ferroelectric molecules like DabcoHReO₄,¹⁴ [C(NH₂)₃]₄Br₂SO₄,¹⁵ and [3.2.1-dabco]BF₄.¹⁶ However, the reports on single-component organic ferroelectric crystals are not many and the T_c of these compounds are not particularly high, thus the exploration of high T_c single-molecule organic ferroelectric crystals is currently receiving widespread attention.

In molecular design strategies, specific chemical groups can be introduced through precise molecular modifications to increase the phase transition temperatures. The strategies widely applied mainly include isotope effects, and fluorine doping.^17–19 The substitution of hydrogen/halogen has emerged as a more prevalent and effective guiding principle for the chemical design and optimization of molecular ferroelectrics.^20–27 While the hydrogen/halogen substitution effect has been demonstrated to be highly effective for typical ordered-disordered multicomponent molecular ferroelectrics, its role in single-component organic crystals remains unclear, particularly those with displacement ferroelectric mechanisms. Furthermore, most reported organic ferroelectric crystals belong to certain types of oxygen- and nitrogen-containing organic molecules. Therefore, the design of novel single-component organic ferroelectric crystals with high T_c could significantly contribute to the development of strategies and mechanisms aimed at enhancing T_c.

Carboranes have been recognized as a fundamental material for applications in medicinal chemistry,^28–33 metal complexes,^34–37 supramolecular assemblies, macrocycles,³⁸ and luminescent materials.^39–42 In recent years, active investigation has expanded its application scope into materials science, batteries, catalysis, and other fields.^28,43–51 However, there have been scarce reports on carboranes serving as ferroelectric materials. The unique cage-like structure of the o-carborane endows it with high molecular symmetry. At room temperature, o-carborane adopts a centrosymmetric space group,⁵² whereas ferroelectric crystals must crystallize in one of the ten polar point groups (1, 2, m, mm2, 4, 4mm, 3, 3m, 6, and 6mm). Hence, it is preliminarily inferred that o-carborane itself is not a high T_c ferroelectric material.

Carboranes exhibit high structural rigidity and stability, ensuring that ferroelectric crystals maintain their structural integrity even at high T_c. Furthermore, by regulating the introduction of various types and sizes of functional groups at the vertices, large dipole moments can be induced. The presence of these high dipole moments enhances crystal polarization, and the strong dipole interactions that result may lead to higher phase transition temperatures. This approach has enabled the synthesis of carborane ferroelectric crystals with high T_c. Therefore, the potential to design a series of high-T_c carborane-based ferroelectric materials is of great significance. Researchers can leverage these advantages to develop materials with higher transition temperatures, greater stability, and enhanced performance, particularly in high-temperature or harsh environments.

Drawing upon the theory of quasi-spherical structures and the concept of chemical design aimed at reducing crystal symmetry, extensive modification and manipulation of the symmetry of o-carborane derivatives were conducted. In order to diminish the symmetry and polarity of o-carborane, we initially conceived introducing highly electronegative functional groups at both carbon and boron vertices, and notably, halogen substituents caught our attention. While similar to C–H bonds in having low chemical polarity and high bond dissociation energy, B–H bonds are also generally unreactive. However, the symmetrical modulation at the boron vertex appears to be more effective than at the carbon vertex. Nevertheless, the large number of B–H bonds with similar chemical environments presents a significant challenge for achieving selective functionalization.^53–55 Recently, our group reported the first discovery of carborane ferroelectric crystals, B(9)-OH/SH-o-carborane.⁵⁶ Yet, investigating the effects of other substituent positions on ferroelectric properties and T_c is of significant importance. Therefore, we tried to functionalize the positions B(9) and B(12), which are furthest from the carbon atom and exhibit the highest electron cloud density.

In this study, we introduced five halogen-substituted derivatives of o-carborane (Scheme 1). This includes mono-substituted B(9)–Br/Cl-o-carborane and di-substituted B(9,12)–Br, Cl/Br₂/Cl₂-o-carborane, with the highest phase transition temperature reaching 437 K, a remarkable increase of 160.5 K compared to that of o-carborane, surpassing that of most single-molecule organic ferroelectric crystals. Furthermore, this provides a novel avenue for the further design of high-temperature phase-transition carborane ferroelectric crystals.

Scheme 1 The mono- and di-substituted o-carborane cage derivatives 1–5.

Results and discussion

Compounds 1–5 were synthesized by reacting o-carborane with halogenation reagents (trichloroisocyanuric acid, N-bromosuccinimide) in hexafluoroisopropanol solvent.^57,58 The purity of them was confirmed using infrared and nuclear magnetic resonance spectroscopy. In infrared spectra, the prominent absorption peaks at approximately 2600 cm⁻¹ are ascribed to the stretching vibration of B–H bonds, while the absorption bands around 3060 cm⁻¹ are designated for C–H stretching vibrations. Boron cage absorption modes (δ B–B) are discerned at 1200, 1130, 1000, 830, 710, and 620 cm⁻¹ as shown in Fig. S1.† The ¹H, ¹H{¹¹B} ¹¹B, ¹¹B{¹H}, ¹³C NMR spectra of compounds 1–5 with peak assignments elucidated are provided in the ESI.† Thermal analysis revealed that compounds 1–5 exhibit good thermal stability, with the mono-substituted and the di-substituted derivates decomposition commencing at temperatures around 470 K and 520 K (Fig. S2†).

The single crystal X-ray diffraction measurements at room temperature indicated that compounds 1–5 crystallize in the polar ferroelectric point groups (2, m, mm2),^59,60 in which compounds 1 and 2 crystallize in the space groups P2₁ and Cc, respectively, and compounds 3–5 crystallize in the space group Pna2₁. The crystal parameters and refined information of compounds 1–5 are listed in Tables S1–S7,† and the corresponding crystal structures are shown in Fig. S3–S5.† The structures of mono-substituted compounds 1–2 exhibit similarities, as do the structures of di-substituted compounds 3–5. Consequently, we focus our detailed discussion on compounds 1 and 3 below.

Compound 1 crystallizes in the monoclinic P2₁ space group at room temperature, with cell parameters of a = 6.8936(14) Å, b = 24.047(5) Å, c = 7.1162(15) Å, and α = γ = 90°, β = 118.610(3)°, as detailed in Table S2.† The single-crystal structure of compound 1 is vividly displayed in Fig. 1a. Additionally, Fig. 1b illustrates the stacking diagram of compound 1 containing molecules of bromo-carborane, in which each halogen atom engages in intermolecular hydrogen bonding interactions with the hydrogen atoms positioned on the C and B vertices of adjacent carborane cages. The H-bonds link along the c-direction infinitely extending with the bond lengths of C–H⋯Br⋯H–B are measured as approximately 2.794 Å and 2.699 Å, forming a short contact.

Fig. 1 The single-crystal structure and intermolecular interactions of crystals 1 (a and b) and 3 (c and d).

Compound 3 crystallizes in the orthorhombic crystal system, with unit cell parameters of a = 12.9028(15) Å, b = 7.3660(8) Å, c = 11.6524(13) Å, and α = γ = β = 90°. The crystal structure of 3 is depicted in Fig. 1c. As shown in Fig. 1d, the chlorine atom of the di-substituted o-carborane forms intermolecular hydrogen bond to the C–H group on an adjacent borane molecule, with a bond length of approximately 2.594 Å. And, the stacking diagram of compound 3 demonstrates infinite growth facilitated by Cl⋯H–C hydrogen bonding interactions to form a chain structure.

Differential scanning calorimetry (DSC) analyses were performed to examine the thermal properties of 1–5. The DSC analysis diagrams for 9-monohalogenated o-carboranes and 9,12-dihalogenated o-carboranes are presented in Fig. 2a and b, respectively. The DSC measurement results presented in Fig. 2a reveal distinct thermal behaviors for 1 and 2 during the heating process, with endothermic peaks observed at 367 K and 353 K, respectively. These peaks indicate that both samples undergo high-temperature phase transitions. Two thermal hysteresis of 30 K and 16 K are observed, corresponding to the phase transitions occurring at 367 K (T_c) and 353 K (T_c), respectively. These transitions are identified as first-order phase transitions. The fitting of the Boltzmann equation ΔS = R ln N (where R represents the gas constant and N value signifies the ratio of geometrically distinguishable orientations) yields N values of 10.55 and 9.11 for 1 and 2, respectively. This outcome underscores the order–disorder characteristic of these phase transitions. Notably, 1 exhibits a marginally elevated transition temperature compared to its chlorinated counterpart. This discrepancy arises from the greater mass of a bromine atom relative to a chlorine atom. Consequently, the structural transformation of compound 1 necessitates a higher energy input than that of 2.

Fig. 2 (a and b) DSC curves of 1–5 upon heating and cooling. (c and d) Temperature-dependent real part (ε′) of dielectric constant at different frequencies measured for 1 and 3. (e and f) Temperature-dependent SHG intensity of 1 and 3. The inset shows the SHG intensity at 293 K.

Moreover, in our pursuit of enhancing the phase transition temperatures of molecules, we have achieved success in incorporating dual halogen atoms at vertices 9 and 12 of the o-carborane framework. This strategic modification has yielded di-halogenated carboranes with phase transition temperatures approximately 70 K higher than those of their mono-halogenated counterparts. Fig. 2b illustrates the DSC analysis for 9,12-dihalogenated o-carboranes, revealing distinct thermal signatures associated with their phase transitions. And, the hierarchy of phase transition temperatures aligns as follows: 4 > 3 > 5, thereby corroborating the aforementioned principle. By comparing with the phase transition temperature 276.5 K of o-carborane,⁵² the introduction of halogen substituents maximally increases the temperature by up to ΔT = 160.5 K.

As shown in Table S8,† we have summarized the reports on pure organic single-molecule compounds and their phase transition temperatures from recent years. The phase transition temperatures are comparable to those of most organic alcohols, amines, and silicon-based compounds. Compounds 1–5 are novel single-molecule organic ferroelectric materials, among which compound 4 exhibits a phase transition temperature as high as 437 K. This finding offers new referenced value for the exploration of high-temperature phase transition materials in the future.

Dielectric testing was conducted on materials 1 and 3 to analyze the dielectric constants at different temperatures and frequencies (100 Hz, 1 kHz, 10 kHz, 100 Hz, and 1 MHz). Simultaneously, changes in dielectric constants before and after phase transitions during both heating and cooling cycles are recorded. As illustrated in Fig. 2c and d, significant dielectric anomalies are observed near the respective phase transition points of compounds 1 and 3, occurring at approximately 367 K and 437 K, consistent with the findings from DSC. Dielectric changes observed around the T_c of compounds 1 and 3 are approximately 5-fold and 3-fold, respectively, with the dielectric constants reaching as high as 500 and 1500 F m⁻¹. These notable variations and high values of dielectric constants indicate the occurrence of ferroelectric phase transitions. Moreover, the dielectric constants of compounds 2, 4, and 5, also demonstrated nonlinear relationships with temperature and frequency, corresponding to their respective phase transitions, Fig. S6 and S7.†

Second harmonic generation (SHG) is commonly employed to investigate crystal symmetry alterations in crystal structures 1–5 during the transition process. The changes in SHG intensity with temperature for compounds 1 and 3 are presented in Fig. 2e and f, respectively, revealing distinct nonlinear trends. The inset plots in Fig. 2e and f demonstrate that the SHG signal intensities of compounds 1 and 3 reach up to 0.45 and 0.25, respectively, prior to their respective phase transition temperatures of 367 K and 437 K. Upon approaching the phase transition temperature, the signal intensity experiences a sudden drop to zero, remaining constant thereafter. Similarly, upon cooling from elevated temperatures to room temperature, the SHG signal intensity reverts to its initial value, completing one cycle, which is consistent with the respective findings from DSC and dielectric measurements discussed earlier.

In addition, cyclic tests are conducted and the SHG signal intensity of the material remained unchanged despite five consecutive cycles of heating and cooling. Similar results are obtained for compounds 2, 4, and 5, which matched well with their respective experiments. Both the SHG conversion plots and multiple cycling plots for all compounds are presented in Fig. S8 and S9.†

To further investigate the structural phase transition of large ΔT, the temperature-variable powder X-ray diffraction (PXRD) measurements were conducted around the T_c of compounds 1 and 3 as shown in Fig. 3a and b. The temperature-variable PXRD results of other compounds 2, 4 and 5 are shown in Fig. S10.† Upon heating to T_c (367 K), the peaks in the 12–20° range of compound 1 merged, confirming the occurrence of the phase transition. The XRD patterns at high temperature of 373 K were analyzed to simulate the single-crystal cell parameters, revealing that 1 adopts a monoclinic crystal system with space group P2₁/c after T_c. In contrast, 3 exhibits a shift in diffraction peaks between 10–20° around T_c, and the simulation of powder diffraction peaks at 433 K reveals orthorhombic crystal system with space group Pnna.

Fig. 3 Temperature-dependent PXRD results of 1 (a) and 3 (b).

The simulation results of both compounds are consistent with the cell parameters at high temperature, corresponding respectively to the 2/mF2 and mmmFmm2 ferroelectric phase transitions of the 88-type.⁶¹ The simulated cell parameters and single crystal X-ray diffraction pattern of five compounds at high temperatures are presented in Fig. S11 and S12.†

Gaussian software calculations reveal a dipole moment magnitude of approximately 4.26 D for o-carborane, directed from the molecular center towards the midpoint between two carbon atoms. Upon introduction of halogen groups at the farthest boron vertices, 9 and 12, significant changes in dipole moment direction and magnitude are observed. When halogen atoms with high electronegativity are introduced into the molecule, electrons are drawn towards these atoms, causing the center of positive charge in the molecule to shift towards B9. Meanwhile, the center of negative charge remains near the carbon atom. This results in an increased separation of positive and negative charges. Consequently, the dipole moments of 1 and 3 increase, orienting from B9 toward the carbon atom, with values of 6.03 D and 7.71 D in Fig. 4, respectively. The dipole moment magnitudes of both compounds increase by 1.77 D and 3.45 D, respectively, compared to the unsubstituted carborane, indicating the changes in polarization intensity and direction may confer ferroelectricity to halogenated carboranes. The results of other three compounds are speculated in Fig. S13.†

Fig. 4 The dipole moments calculated for o-carborane and compounds 1 and 3.

To further investigate the ferroelectric properties of halogenated carborane thin films, we conducted in-depth studies on one compound each from two mono-substituted and three di-substituted carboranes, namely 1 and 3, respectively. Thin film samples are prepared via drop-casting, followed by the investigation of their ferroelectric behavior using piezoresponse force microscopy (PFM), which encompassed the investigation of domain structure and polarization reversal.

As illustrated in Fig. 5, a relatively flat area of 20 μm × 20 μm region was selected on the surface of thin film of 1. By applying a tip voltage of +60 V at the center position of the selected area using a conductive PFM tip, a clear flipping of domain orientation is observed on the surface of the film. This observation suggests that under a sufficiently strong electric field, the orientation of ferroelectric domains on the film can be reoriented, thereby achieving ferroelectric domain switching. This phenomenon epitomizes one of the most pivotal characteristics of ferroelectric materials which is the microscopic reflection of polarization switching. Subsequently, when a tip voltage of −60 V, with the same magnitude but opposite direction, was applied to thin film 1, almost the entire area reversed back, forming a nested switchable domain pattern, as highlighted in the blue box in Fig. 5i, which was evident in both amplitude and phase images. Next, we selected individual points on the thin film of 1 to examine the polarization state. Utilizing the tip of the PFM conductive probe, we applied voltage to manipulate the polarization switch at the point. As depicted in Fig. 5j and k, characteristic butterfly-shaped amplitude hysteresis loops and phase difference of 180° are generated, offering compelling evidence for the ferroelectric properties of 1.

Fig. 5 (a–c) The corresponding topographic image of a specific region on the surface of the crystal 1. (d–f) PFM amplitude before and after the application of electric writing with a tip voltage of ±60 V. (g–i) Phase images before and after the aforementioned electric writing process. (j and k) Phase loop and amplitude loop of the thin film of 1 through PFM switching spectroscopy measurements.

Subsequently, the ferroelectricity of di-substituted compound 3 was tested by identifying a well-defined 10 μm × 10 μm region on the surface of the thin film of 3. Initially, at the zero bias, the distinct lines of contrast in the amplitude map are observed as shown in Fig. 6d. Upon applying a −60 V voltage within the blue box designated in the central region, no alteration in the surface morphology of the film is noted. However, significant changes are evident in the amplitude map (Fig. 6e), accompanied by a discernible shift in the polarization direction within the region subjected to the applied negative voltage, as evidenced by the phase map (Fig. 6h), aligning with the direction of the applied voltage. To assess the reversibility of polarization within this domain, we applied a reverse positive voltage of +60 V within a smaller box, resulting in the anticipated reversal of polarization direction within the ferroelectric domain, as depicted in Fig. 6i. Altogether, this bidirectional polarization switch confirms the ability of the crystal to alter its polarization direction in response to changes in the electric field direction, strongly corroborating the ferroelectric properties of 3. Subsequently, polarization testing was conducted at individual points, confirming the ferroelectric nature of 3, as illustrated in the amplitude butterfly map and phase hysteresis loop shown in Fig. 6j and k. The polarization testing results for the remaining compounds (2, 4, and 5) are depicted in Fig. S14,† illustrating their individual points of polarization.

Fig. 6 Polarization switching measurements for the thin film of 3. Topography (a–c) and the simultaneously acquired out-of-plane PFM amplitude (d–f) and phase (g–i) images in the initial state and after the application of electric writing with tip voltage of ±60 V. (j and k) PFM amplitude and phase hysteresis loops illustrating the switching behavior of the thin film 3.

Conclusions

Under the guidance of the ferroelectric quasi-spherical theory and related design strategies, we successfully synthesized and introduced five ferroelectric crystals based on cage-like carborane molecules, including two monosubstituted and three disubstituted halogenated o-carboranes. By introducing halogen atoms to modulate the dipole moment, we enhanced the ferroelectric properties and synthesized carborane ferroelectric crystals with high phase transition temperatures. Notably, compound 4 exhibits an exceptionally high phase transition temperature of 437 K, which is approximately 160.5 K higher than that of unsubstituted o-carborane. While unsubstituted o-carborane is not a ferroelectric material, the ferroelectric properties of halogenated carboranes have been strongly validated through PFM evaluation of ferroelectric domain switching. The mono-substituted caborane 1 and di-substituted carborane 3 are identified as 2/mF2 and mmmFmm2-type ferroelectrics, respectively. This discovery establishes a trend linking structural phase transitions with ferroelectric properties, providing valuable insights for tuning ferroelectric performance by modifying the substituents on the carborane cage.

Author contributions

G. W. J. prepared the samples, measured the ferroelectric properties, and wrote the manuscript. C. W. K., L. Y. T. determined the structures. W. Z. H. and C. H. provided suggestions for research. All the authors discussed and commented on the manuscript.

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (22071094 and 22075123), the Jiangxi Provincial Natural Science Foundation (20213BCJ22055 and 20224ACB203004) and Graduate Innovative Special Fund Projects of Jiangxi Province (YC2023-B020).

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Footnote

† Electronic supplementary information (ESI) available. CCDC 2294556 (1), 2294802 (2), 2299657 (3), 2294555 (4) and 2294648 (5). For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4qi02011a

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