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DOI: 10.1039/D5QI01256B (Research Article) Inorg. Chem. Front., 2025, Advance Article

The role of anionic design in modulating material properties: from SHG switching to ferroelastic switching†

Jia-Zi She, Yun-Hui Yu, Yan-Ran Weng, Dan Lu, Yong Yu, Ying Zhou, Hui-Peng Lv and Yong Ai*
Ordered Matter Science Research Center, Nanchang University, 330031, P. R. China. E-mail: yong.ai@ncu.edu.cn

Received 6th June 2025 , Accepted 7th July 2025

First published on 10th July 2025

Abstract

Crystallographic engineering and chemical synthesis modification play crucial roles in modulating material properties and designing multifunctional materials, such as those with second-harmonic generation (SHG) switching and ferroelastic behaviors. Herein, we present the role of anion-based design in crystalline materials through anion H/F substitution. Specifically, the material [NMe4][CH3SO3] exhibits a high phase transition temperature (Tc = 462 K) and excellent SHG switching performance. The introduction of polyfluorinated substitutions results in a new organic ferroelastic material, [NMe4][CF3SO3], which exhibits ferroelastic phase transition at 309 K. The transition from the non-centrosymmetric [NMe4][CH3SO3] to the centrosymmetric [NMe4][CF3SO3] leads to a shift in material properties, transitioning from SHG switching to ferroelastic switching. Our findings underscore the importance of anion-based design in the development of advanced multifunctional phase-transition materials.

Introduction

Phase transition materials undergo alterations in their physical properties with temperature variations, exhibiting multiple functional characteristics, such as switchable second-order nonlinear optical (NLO), dielectric, ferroelastic, ferroelectric, etc.1–10 Especially in second-harmonic generation (SHG) switching and ferroelastic switching materials, phase transitions play a crucial role. SHG switching materials driven by symmetry-breaking structural phase transitions have garnered considerable attention due to their unique combination of tunable optical properties and robust switching performance.11–14 The mechanism relies on reversible transitions between centrosymmetric and non-centrosymmetric states, where symmetry changes during phase transitions induce sharp modulation of SHG responses, enabling reproducible ‘on–off’ switching.15,16 Notably, organic molecular phase transition materials exhibit superior chemical tunability and dynamically adjustable nonlinear optical properties during phase transitions, resulting in excellent SHG switching performance.17–20 During phase transitions, the material's crystal structure undergoes significant changes, leading to symmetry breaking that can induce ferroelastic behavior.21,22 However, designing ferroelastic phase transitions necessitates adherence to one of the 94 ferroelastic phase transition types outlined in Aizu's rule.23

The low phase transition temperatures have significantly hindered the application and development of phase transition materials. Crystallographic engineering, particularly through fluorine substitution, plays a pivotal role in enhancing phase transition temperatures and modulating material properties.24 The pronounced electronegativity difference between hydrogen and fluorine atoms induces substantial variations in molecular polarizability within H/F-substituted systems, potentially leading to significant alterations in macroscopic physical properties.25 For instance, monofluorination leads to significant Tc enhancement in those hybrid molecular materials, such as (fluoropyrrolidinium)MnCl3,26 [(CH3)3PCH2F][Cd(SCN)3],27 N,N-dimethyl-3-fluoropyrrolidinium CdCl3,28 (4FHQ)2RbEu(NO3)629 and so on.30–33

Furthermore, multiple fluorine substitutions on the cations demonstrate remarkable efficacy in modifying the transition temperature, polarity, photochromism, and crystal structures, as evidenced by those hybrid compounds, such as [4,4-DFPD]2PbI4,34 [4,4-DFHHA]2PbI4,35 [3,3-Difluorocyclobutylammonium]2 CuCl4,36 [DFCBA]2CrCl4.37 Besides, multiple fluorine substitutions can also induce the weaker intermolecular interactions within amantadine-pentafluorobenzoic acid and lead to a ferroelastic phase transition.38 In addition to modulating material properties through H/F substitution in the cationic part of molecular crystals, H/F substitution in the anionic component can also be used. Fu et al. reported the flexible organic ferroelectric material PEA-TFMS using an anionic fluorination strategy, showing the feasibility of inducing plastic ferroelectricity.39 Sugata Ray achieved multiferroicity in fluorinated inorganic oxides, Ca2Mn2O5−xFy, by engineering anionic networks through fluorination and vacancy ordering.24

In this work, we explore the effects of H/F substitution on the anionic components to modulate the phase transition and crystalline symmetry. Herein, we choose quasi-spherical tetramethylammonium molecules as cation, which is prone to order–disorder phase transitions,40–42 successfully constructed two multifunctional materials, namely [NMe4][CH3SO3] (CH-NMe4) and [NMe4][CF3SO3] (CF-NMe4). As shown in Scheme 1, the organic compound CH-NMe4 exhibits high Tc of 462 K and SHG switching behaviours. CH-NMe4 crystallizes in the non-centrosymmetric Pca21 space group at 293 K, demonstrating significant nonlinear optical switching with a large on–off ratio and high stability. In contrast, the H/F substituted compound CF-NMe4 crystallizes in the centrosymmetric P21/m space group at 260 K. Reversible ferroelastic domain patterns confirm its ferroelastic phase transition. This work highlights the synergy between crystallographic engineering and H/F substitution in tailoring material functionalities, offering valuable insights for advancing nonlinear optical materials and ferroelastics.


image file: d5qi01256b-s1.tif
Scheme 1 The design strategy for SHG switching CH-NMe4 and ferroelastic switching CF-NMe4.

Results and discussion

Materials

Crystals of CH-NMe4 and CF-NMe4 are grown in ethanol solution, slowly evaporated by the solvent at a constant temperature of 30 °C, and colourless, clear crystals for single crystal measurement were obtained. Thermogravimetric analysis (TGA) confirms exceptional thermal stability, with decomposition onsets occurring at 634 K for CH-NMe4 and 705 K for CF-NMe4 (Fig. S1, ESI†), indicating their robust thermal stability.

Single crystal structures

At 293 K, the CH-NMe4 crystallized in the orthorhombic Pca21 space group with cell parameter of a = 11.994(2) Å, b = 6.1359 (13) Å, c = 11.923(3) Å, α = β = γ = 90°. The non-centrosymmetric crystalline structure of CH-NMe4 demonstrated intrinsic piezoelectric behavior. Quasi-static measurements revealed a d33 coefficient of 11 pC N−1, confirming its piezoelectric responsiveness (Fig. S2, ESI†). The CF-NMe4 crystallized in the monoclinic P21/m space group at 260 K. The parameters are a = 10.3722(3) Å, b = 8.6225(2) Å, c = 11.6086(3) Å, α = γ = 90°, β = 103.824° for CF-NMe4. The detailed cell parameters of CH-NMe4 and CF-NMe4 are listed in Table S1, ESI.† Subsequently, we performed powder X-ray diffraction (PXRD) to verify the phase purity. The experimental patterns show excellent agreement with simulated profiles (Fig. S3, ESI†), with all observed peaks aligning precisely with simulated reflections, confirming the phase purity of both CH-NMe4 and CF-NMe4. As depicted in Fig. 1a, the asymmetric unit of the CH-NMe4 consists of one NMe4+ cation and one CH3SO3− anion. The CH3SO3− parameters for CH-NMe4 are comparable to those found in the ordered phases of other known compounds with methanesulfonic acid,43 with S–O bond lengths ranging from 1.446 to 1.458 Å. The CH3SO3− anion is involved in four non-classical hydrogen bonds C–H⋯O. The strongest hydrogen bonds in the structure exist between the O atom on the methanesulfonic acid and the N atom on the tetramethylamine, which are at the closest distance: H⋯A = 2.40 Å, D⋯A = 3.349 Å, D–H–A = 170° (Fig. 1b, Fig. S4a and Table S2, ESI†). Ions of opposite charges are alternately arranged along c-axis, forming a two-dimensional structure perpendicular to the b-axis under the weak interaction force. For CF-NMe4, the asymmetric unit of the crystal structure consists of two NMe4+ cations and two CH3SO3− anions, as depicted in Fig. 1c. There exists C–H⋯O hydrogen-bonding interactions in the structure, forming infinite linear hydrogen-bonded chains along the b-axis, as shown in Fig. 1d and Fig. S4b, ESI.† The non-classical hydrogen bonds of the CH-NMe4 and CF-NMe4 crystals are summarized in Tables S2 and S3, ESI.† At 260 K, CF-NMe4 exhibits six types of C–H⋯O hydrogen bonds, with bond lengths ranging from 3.373 Å to 3.442 Å, and corresponding bond angles ranging from 148° to 153°. Unlike most studies in which the introduction of monofluoro substitution brings about polarity,26,28,33 the multiple fluorine substitution on the anions induces a notable reduction in crystal symmetry, leading to the formation of a centrosymmetric structure in CF-NMe4.
image file: d5qi01256b-f1.tif
Fig. 1 The asymmetric unit of (a) CH-NMe4 at 293 K and (c) CF-NMe4 at 260 K. Packing structure of (b) CH-NMe4 at 293 K and (d) CF-NMe4 at 260 K. The dash lines denote the C–H⋯O hydrogen bond.

Hirshfeld surfaces analysis

The 2D fingerprint plots enable detailed analysis and highlight the interaction forces between specific atoms, allowing for separation based on the varying contribution levels of different interactions depicted in the Fig. 2. To further explore the influence of anion fluorination on molecular dynamics, Hirshfeld surfaces and 2D fingerprint plots (Fig. 2) were generated using CrystalExplorer software, leveraging their CIF files for detailed analysis. In Fig. 2a, six distinct red spots on the Hirshfeld surface of CH-NMe4 signify the presence of C–H⋯O hydrogen bonds. A comprehensive analysis of the full 2D fingerprint plot in Fig. S5, ESI† further elucidates the relative contributions of various hydrogen bond types. Detailed analysis of the fingerprint plot reveals that H⋯H interactions dominate, contributing 65.2% of the total Hirshfeld surface area, while H⋯O interactions account for 34.7%. Additionally, the H⋯S.
image file: d5qi01256b-f2.tif
Fig. 2 Visualization map of the distribution of the interactions and the two-dimensional fingerprint plots of (a) CH-NMe4, and (b) CF-NMe4, show contributions from Houtside–Oinside.

Interaction represents only 0.1% of the Hirshfeld surface. In Fig. 2b, the Hirshfeld surface of CF-NMe4 exhibits five prominent red spots, signifying the presence of C–H⋯O hydrogen bond interactions. Analyzing the fingerprint plot reveals that O⋯H interactions occupy 49.0% of the entire Hirshfeld surface area, while F⋯H interactions account for 42.4%. Further examination of the complete 2D fingerprint plot in Fig. S6, ESI† provides additional details about the contributions of various hydrogen bond types. Specifically, Fig. S6e, ESI† represents the F⋯F interaction, contributing 6.9% to the Hirshfeld surface, while Fig. S6f and S6g, ESI† depict the F⋯O/S⋯H interactions, accounting for 1.3% and 0.3%, respectively. Anionic fluorination markedly influences the spacing between H atoms on cations and O atoms on anions, thereby reshaping structural packing by introducing steric effects. Simultaneously, it reorganizes hydrogen-bonding network formation pathways through enhanced electrostatic interactions, highlighting the profound impact of fluorination on molecular arrangements.

Phase transitions

Differential Scanning Calorimetry (DSC) serves as a robust analytical tool for precisely identifying phase transitions triggered by external thermal stimuli. Then, DSC measurements were conducted to explore the phase transition behaviors of CH-NMe4 and CF-NMe4. During the heating and cooling cycle of CH-NMe4, which ranged from 425 K to 490 K, a pair of endothermic and exothermic peaks were observed at 462 K and 453 K, respectively, as depicted in Fig. 3a. This observation indicates a reversible phase transition. Notably, the significant temperature hysteresis effect during the heating and cooling processes is characteristic of first-order phase transitions.44 It is noteworthy that CH-NMe4 exhibits an exceptionally high phase transition temperature (462 K), which significantly exceeds that most of organic materials,45–54 underscoring its unique thermal stability. For CF-NMe4, within the temperature range of 250–330 K, during the heating/cooling cycle, the DSC curve exhibits a pair of distinct thermal anomaly peaks at 309 K/283 K (Fig. 3b). The corresponding entropy changes (ΔS) for the endothermic peaks are approximately 2.98 and 17.9 J mol−1 K−1 for CH-NMe4 and CF-NMe4, respectively. Upon fluorination of the methanesulfonic acid anions, the phase transition temperature changes from 462 K to 309 K.
image file: d5qi01256b-f3.tif
Fig. 3 The DSC curves for (a) CH-NMe4 and (b) CF-NMe4. The temperature-dependent real part (ε′) of the dielectric constant was measured at 1 MHz for (c) CH-NMe4 and (d) CF-NMe4.

Phase transitions are typically accompanied by dielectric anomalies. Temperature-dependent dielectric measurements were performed at 1 MHz during a heating–cooling thermal cycle. For CH-NMe4, a step-like dielectric anomaly is observed near 460 K (Fig. 3c), marked by a discontinuous increase in the real permittivity (ε′) from 10.8 (low dielectric state) to 22.7 (high dielectric state), representing a 2.1-fold enhancement. Similarly, CF-NMe4 demonstrates distinct dielectric behavior: during heating, ε′ gradually increases from 4.5 at 307 K to 7.8 at 317 K, followed by stabilization (Δε′ < 0.5) at elevated temperatures (Fig. 3d). The value of ε′ in the high dielectric state is about 1.73 times that in the low dielectric state. The permittivity-temperature profiles were also measured at 1, 5, 10, and 100 kHz (Fig. S7, ESI†). As shown in Fig. S7,† the dielectric constant decreased with increasing frequency. Furthermore, the Tc deduced from permittivity-temperature curves is frequency-independent. Both compounds display thermal hysteresis (>5 K) between heating/cooling cycles, consistent with first-order transition characteristics observed in DSC.

To elucidate the structural phase transition, we attempted to measure the high-temperature crystal structures of CH-NMe4 and CF-NMe4. However, weak diffraction and severe ionic disorder at elevated temperatures hindered the successful collection of high-temperature phase data. Variable-temperature PXRD measurements were conducted for further analysis. A marked change in peak positions and intensities was observed upon heating to the phase transition point (Fig. S8, ESI†), confirming the occurrence of phase transition behavior leading to symmetry breaking. Critical evidence emerges from the progressive simplification of diffraction patterns, where specific reflections vanish above the transition temperature alongside reduced peak multiplicity. Rietveld refinement of high-temperature PXRD data reveals distinct structural transformations: CH-NMe4 transitions from an orthorhombic mm2 system to a tetragonal system, while CF-NMe4 undergoes monoclinic 2/m to tetragonal symmetry elevation (Fig. S9 and S10, ESI†). Although CH-NMe4 exhibits a polar crystal structure that satisfies the symmetry requirements for ferroelectricity, the piezoresponse force microscopy analysis showed no evidence of domain switching or polarization hysteresis under various experimental conditions.

SHG switching

The temperature-dependent second-harmonic generation (SHG) serves as a sensitive probe for identifying structural transformations between centrosymmetric and non-centrosymmetric states.16,55 To further investigate the symmetry changes associated with the phase transition, we conducted SHG measurements on CH-NMe4. At room temperature, CH-NMe4 demonstrates a moderate SHG intensity, measuring 0.8 times that of KDP (Fig. 4a), which aligns with its non-centrosymmetric packing in the Pca21 space group. Temperature-dependent SHG measurements demonstrate a distinct bistable response, as illustrated in Fig. 4b. During heating, strong SHG signals persist below 460 K (high-NLO state) but abruptly vanish above 480 K as the material transitions to a centrosymmetric phase (low-NLOstate). Conversely, upon cooling, the SHG response of CH-NMe4 remains zero until 477 K before sharply recovering at 466 K with a 5.6 V signal, demonstrating a 9 K thermal hysteresis. The complete suppression of SHG intensity in the high-temperature phase confirms symmetry elevation to a centrosymmetric space group. Remarkably, CH-NMe4 retains over 95% of its initial SHG intensity across 10 thermal cycles (Fig. 4c), achieving an exceptional switching contrast ratio (Ion/Ioff ≈ 13) that outperforms many existing SHG-switching materials.55–58 This unique combination of robust bistability, high switching contrast, and exceptional durability establishes CH-NMe4 as a highly promising candidate for nonlinear optical switching devices that demand precise state control and long-term operational stability.59 In contrast, the fluorine-substituted analogue, CF-NMe4, crystallizes in a centrosymmetric space group and thus shows no SHG signal.
image file: d5qi01256b-f4.tif
Fig. 4 (a) SHG signals of CH-NMe4 and KDP at room temperature. (b) The temperature dependencies of SHG signals under heating and cooling run of CH-NMe4. (c) Cycles of switching high and low NLO states of SHG signals of CH-NMe4.

Ferroelastic switching

The evolution of ferroelastic domains is a sign of ferroelastic phase transitions, which we investigated through temperature-dependent polarized light microscopy. For CF-NMe4, the appearance of stripe-like patterns in polarized microscopy images (at 260 K, Fig. 5a) serves as a definitive indicator of ferroelastic domain formation, which is distinct from material morphology (Fig. 5f). These domains exhibit alternating light–dark contrast, originating from the coexistence of multiple spontaneous strain orientations during the paraelastic-to-ferroelastic transition. Upon gradual heating, the ferroelastic domain structure progressively diminishes and fully vanishes near 313 K (Fig. 5b and e), signaling the transition to a paraelastic phase. The domain patterns reversibly reappear with restored stripe-like domain walls upon cooling back to 260 K (Fig. 5c and d), demonstrating robust thermal cyclability and highlighting the material's structural reversibility. DSC and temperature-dependent PXRD analyses confirm a reversible phase transition near 309 K. The temperature-dependent domain switching dynamics, coupled with the hysteretic recovery of domain patterns, provide unequivocal evidence of ferroelasticity in CF-NMe4. However, no significant ferroelastic domain evolution was observed in CH-NMe4, leading us to conclude that it lacks ferroelasticity. This structural transformation profoundly influences the material's properties, highlighting the critical role of chemical modifications in tailoring functional behaviors.
image file: d5qi01256b-f5.tif
Fig. 5 The ferroelastic domain walls evolution of CF-NMe4. (a–e) evolutions of the ferroelastic domain structure for CF-NMe4 in the cooling-heating runs, natural optical microscope images for (f) morphology of CF-NMe4. Ferroelastic domain evolution is indicated by the red rectangular frame.

Conclusions

In conclusion, we reported an organic nonlinear optical switching material CH-NMe4, and a molecular ferroelastic CF-NMe4. Structural analyses reveal that the polyfluorinated substitution on the anion markedly alters intermolecular interactions and packing arrangements, inducing a transformation from a non-centrosymmetric (space group Pca21) to a centrosymmetric (space group P21/m) crystal structure at room temperature. Through anion modification, both symmetry breaking and lattice flexibility are simultaneously regulated, resulting in the disappearance of polarity and triggering ferroelasticity. This work realizes the functional switching of phase transition materials through anion fluorination, demonstrating an effective chemical synthesis strategy and providing ideas for the development of multifunctional materials.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the ESI.† Crystallographic data has been deposited at the Cambridge Crystallographic Data Centre (CCDC) as a supplementary publication no. 2449758 and 2449759.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 22271131 and 22401132).

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Footnote

† Electronic supplementary information (ESI) available: Fig. S1–S10 and Tables S1–S3. CCDC 2449758 and 2449759. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d5qi01256b
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