Structural design and functional characterization of dicationic pyrazolium salts as organic ionic plastic crystals

Abstract

A series of dicationic pyrazolium hexafluorophosphate (PF₆⁻) salts connected by a butylene bridge were synthesized with side linear alkyl chains ranging from C₁ to C₁₂. Thermal analysis revealed that most compounds exhibit one or more solid–solid phase transitions (T_ss), and several compounds displayed clear characteristics of organic ionic plastic crystals (OIPCs), with entropy of fusion (ΔS_f) values below 40 J mol⁻¹ K⁻¹. Among them, 1,4-bis[N-(N′-octylpyrazolium)]butane PF₆⁻ exhibited the softest plastic crystal morphology, as confirmed by polarized optical microscopy (POM) and wide-angle X-ray scattering (WAXS), and achieved a high ionic conductivity of 1.31 × 10⁻³ S cm⁻¹ at 70 °C upon incorporation of 30 mol% lithium bis(trifluoromethanesulfonyl)imide (LiTf₂N). These findings demonstrate that the structural tunability of dicationic pyrazolium salts plays a key role in modulating solid-state ionic conductivity, providing insights into the design of next-generation electrochemical materials.

---

Introduction

Pyrazolium-based ionic compounds have emerged as promising alternatives to conventional imidazolium salts for various electrochemical applications.^1–5 A cation pyrazolium features a five-membered aromatic ring with nitrogen atoms at the 1- and 2-positions and is a structural isomer of imidazolium cation. Notably, pyrazolium lacks an acidic proton, which enhances its alkaline stability compared to imidazolium salts with the acidic proton at C2-position.^6,7 In addition, the acidic proton at the C2 position of the imidazolium cation can form hydrogen bonds with anions, promoting stronger ionic pairing compared to the pyrazolium cation and thereby affecting ionic conductivity. The 1-methyl-2-butylpyrazolium dicyanamide compound exhibits a viscosity (∼30 cP) similar to its imidazolium analogue, 1-methyl-3-butylimidazolium dicyanamide, but shows lower ionic conductivity (8.5 vs. 11.0 mS cm⁻¹). In imidazolium salts, the acidic C2 proton engages in strong, directional hydrogen bonding with anions, a feature that significantly affects ion association, while charge delocalization within the imidazolium ring contributes to maintaining relatively high ionic mobility, resulting in a lower activation energy for ionic conductivity (∼11 kJ mol⁻¹). By contrast, pyrazolium cations do not contain this acidic site and instead promote weaker, non-directional interactions through their alkyl groups, which enhance ion aggregation and increase the activation energy for ionic conductivity (12.1 kJ mol⁻¹).⁸ Accordingly, the ionic conductivity of pyrazolium-based salts can vary significantly depending on the nature of the alkyl substituents.

Pyrazolium-based cations have been reported in combination with various anions to form ionic liquids (ILs)^9–13 or organic ionic plastic crystals (OIPCs).^13–16 Both ILs and OIPCs commonly exhibit high thermal stability, wide electrochemical windows, and intrinsic flame retardancy.^17–21 In particular, OIPCs are defined as solids exhibiting one or more solid–solid phase transitions (T_ss) and a small melting entropy (ΔS_f) below 40 J mol⁻¹ K⁻¹,²² and have recently attracted attention as solid-state electrolytes due to their high ionic conductivity through soft crystalline phases at specific temperatures.^23–25

Structurally related cyclic quaternary ammonium species such as imidazolium, pyridinium, and pyrrolidinium have been widely studied in their symmetric dicationic forms, many of which exhibit typical OIPC characteristics.^26–30 Notably, dicationic salts combining the hexafluorophosphate (PF₆⁻) anions with symmetric cationic frameworks such as 1,2-bis[N-(N′-decylimidazolium)]ethane PF₆⁻ (T_ss = 62, 104 °C, ΔS_f = 11 J mol⁻¹ K⁻¹)²⁶ and 1,3-bis[N,N′-(4-propylpyridinium)]propane PF₆⁻ (T_ss = 107 °C, ΔS_f = 23 J mol⁻¹ K⁻¹)²⁸ have been reported to exhibit OIPC behaviors. However, most reported dicationic pyrazolium salts have focused on tuning the length of the alkylene bridging chain, and their properties have predominantly been studied as ionic liquids states.^31–33 Furthermore, compared to analogous imidazolium-based dicationic salts, dicationic pyrazolium compounds are reported in very limited species, and examples incorporating the PF₆⁻ anions are particularly rare.

In this study, symmetric dicationic pyrazolium PF₆⁻ salts were synthesized by connecting two pyrazolium units via a C4 butylene bridge and varying the side alkyl chain length. As a result, most of the synthesized compounds exhibited one or more solid–solid phase transitions, and some were confirmed to obtain OIPC characteristics. In particular, 1,4-bis[N-(N′-octylpyrazolium)]butane 2PF₆⁻ was found to exhibit temperature-dependent phase transitions and soft crystalline phases at specific temperatures, as confirmed by polarized optical microscopy (POM) and wide-angle X-ray scattering (WAXS) analysis. The solid-state material obtained by incorporating 30 mol% Li salt into 1,4-bis[N-(N′-octylpyrazolium)]butane PF₆⁻ exhibited a relatively high ionic conductivity of 1.31 × 10⁻³ S cm⁻¹ at 70 °C. Therefore, the structure–property relationships of the synthesized dicationic pyrazolium PF₆⁻ salts were comprehensively investigated for thermal properties and ionic conductivities, along with some unique features analyzed via POM and WAXS.

Results and discussion

Synthesis and NMR analysis

Dicationic pyrazolium PF₆⁻ salts were synthesized via a three-step procedure, as illustrated in Scheme 1. 1-Alkylpyrazoles were obtained via alkylation of pyrazole with 1-bromoalkane in the presence of NaOH. Menshutkin reaction of the 1-alkylpyrazoles (2.2 equiv.) with 1,4-dibromobutane (1.0 equiv.) afforded the corresponding dicationic pyrazolium bromide salts. Anion exchange with KPF₆ in deionized water subsequently yielded the target dicationic pyrazolium PF₆⁻ salts. The absence of residual halides was confirmed by the AgNO₃ test.^34,35 The final products, 1,4-bis[N-(N′-alkylpyrazolium)]butane PF₆⁻ salts, were denoted as m-PF₆ according to the alkyl chain length (R = C_mH_2m+1).

Scheme 1 Synthesis of 1,4-bis[N-(N′-alkylpyrazolium)butane 2PF₆⁻ (m-PF6) salts.

The structures of the synthesized m-PF₆ salts were confirmed by NMR analysis. Fig. 1 shows the ¹H NMR spectra of 1-PF₆ and 4-PF₆. For both compounds, the butylene bridge protons appeared at the same positions: δ 2.2 ppm (N⁺–CH₂CH̲₂) and 4.8 ppm (N⁺–CH̲₂). However, the pyrazolium ring protons of 1-PF₆ showed two peaks at δ 6.9 and 8.3 ppm, while those of 4-PF₆ appeared as three peaks at δ 6.9, 8.4, and 8.5 ppm. This difference is likely due to the influence of the longer side alkyl chain on the electronic environment of the pyrazolium ring. In comparison, 1,4-bis[N-(N′-ethylimidazolium)]butane PF₆⁻, which has a structure similar to 1-PF₆, exhibited upfield-shifted bridge protons at δ 2.05 and 4.41 ppm, and its imidazolium ring protons were observed at δ 7.66 (N⁺–CH), 7.68 (N⁺–CH), and 8.86 (N⁺–CH–N⁺) ppm, compared to those of the pyrazolium ring.³³ The pyrazolium ring protons appear further downfield than those of the C4 and C5 on imidazolium ring. This trend can be rationalized in terms of electronic effects: the 1,2-nitrogen arrangement in pyrazolium modifies the π-electron density across the ring, leading to a lower local electron density at the proton positions. As a result, the ring protons experience stronger de-shielding and resonate further downfield compared to the corresponding imidazolium protons. The bridge protons were also observed further downfield in pyrazolium than in imidazolium. These differences can be attributed not only to structural variations but also to the positional effect of the PF₆⁻ anion. This observation is consistent with previous reports suggesting that hydrogen bonding between the acidic C2 proton of imidazolium and the anion brings the anion closer to the imidazolium ring.⁸

Fig. 1 ¹H NMR spectra of 1-PF₆ (up) and 4-PF₆ (down) (500 MHz, 23 °C, acetone-d₆).

Thermal properties

The thermal properties of the synthesized 1,4-bis[N-(N′-alkylpyrazolium)]butane PF₆⁻ compounds are summarized in Table 1. The thermal stabilities of the m-PF₆ series were investigated by thermogravimetric analysis (TGA), based on the temperatures corresponding to 5% weight loss. Overall, the decomposition temperatures showed a decreasing trend with increasing the side chain length, which is consistent with previously reported thermal degradation behavior observed in dicationic bis-imidazolium PF₆⁻ systems.³³

Table 1 Thermal properties of the 1,4-bis(N-alkylpyrazolium)butane PF₆⁻ salt

Code (m-X)	DSC transitions^a (°C)		Melting point^d (°C)	TGA (°C) (5 wt% loss)
	T ss (∑(ΔSss),^b J mol^−1 K^−1)	T m (ΔSf, J mol^−1 K^−1)
a DSC temperatures were recorded on the second heating scan at a scan rate of 10 °C min^−1. b ∑(ΔSss) values were calculated from the summation of each solid–solid phase transition entropy. c DSC temperatures were recorded on the second heating scan at a scan rate of 1 °C min^−1. d The melting point measured at 1 °C min^−1 from room temperature was lower than the DSC Tm. e DSC temperature was recorded on the third heating scan at a scan rate of 10 °C min^−1 (see Fig. S30). f Appeared liquid-like, as amorphous regions predominated over crystalline domains.
1-PF6	—	274 (68.0)	273.7–274.6	321
2-PF6	—	214 (79.2)	211.2–213.0	315
3-PF6	—	186 (97.0)	179.7–181.1	299
4-PF6	150 (5.5)	165 (77.6)	154.2–155.8	304
5-PF6	—	148 (72.5)^e	145.2–146.9	308
6-PF6	144 (54.3)	154 (1.4)	144.9–146.6^f	288
7-PF6	—	138 (70)	137.2–138.6	297
8-PF6	−14, −7, 113^c, 131^c (53.7)	137 (5.1)	135.7–136.9	294
9-PF6	91, 128 (133)	135 (69.1)	131.1–132.3	276
10-PF6	−25, 44, 129 (185)	140 (34.2)	139.5–140.1	261
11-PF6	98, 133 (141)	150 (32.3)	143.7–144.5	277
12-PF6	11, 56, 130 (176.8)	153 (43.4)	150.9–152.1	283

---

To investigate the thermal behaviors of the m-PF₆ series, melting points were first observed visually, followed by differential scanning calorimetry (DSC) analysis. Fig. 2 shows the melting temperatures (T_m) of the m-PF₆ compounds as a function of side alkyl chain length. The synthesized salts exhibited a trend in which T_m decreased up to the n-heptyl (C₇) derivative and increased from the n-decyl (C₁₀) derivative onward. This trend is consistent with previous observations in bis-imidazolium salt systems,³⁶ where shorter alkyl chains result in entropy-dominated melting behaviors, while longer chains promote stronger van der Waals interactions between molecules. In addition, from n-butyl (C4) to n-heptyl (C7), an odd–even effect was observed, where compounds with even-numbered side chains exhibited higher melting points than those with odd-numbered chains. This behavior is also similar to the previously reported ionic salts, 1,4-bis(N-alkylpyrrolidinium)butane compounds.³⁰

Fig. 2 Melting points changes and relationship for side-chain length of the synthesized m-PF₆ (2nd heating scan, heating rate = 10 K min⁻¹)

Except for the five compounds (1-PF₆, 2-PF₆, 3-PF₆, 5-PF₆ and 7-PF₆), all synthesized dicationic pyrazolium PF₆⁻ salts exhibited one or more solid–solid phase transitions during DSC investigations. Both 10-PF₆ and 11-PF₆ displayed more than two solid–solid transitions, with low melting entropies (ΔS_f) of 34.2 and 32.3 J mol⁻¹ K⁻¹, respectively, satisfying the criteria for OIPCs as defined by MacFarlane.²² In the case of 6-PF₆, a phase transition was observed after the melting determined by visual inspection (Fig. S31). For 8-PF₆, the melting peak overlapped with solid–solid transitions in the DSC thermogram (Fig. S33); therefore, a more precise measurement was carried out at a slow scan rate of 1 °C min⁻¹ between 100 and 160 °C. As shown in Fig. 3, 8-PF₆ exhibited solid–solid transitions at 113 and 131 °C, with the melting point clearly separated at 136 °C. For 6-PF₆, the same two transitions were observed at 144 and 153 °C when measured at a scan rate of 1 °C min⁻¹ (Fig. S31). Therefore, its perfect liquid phase might be observed over 153 °C during the DSC experiment.

Fig. 3 DSC thermograms of 8-PF₆ (red) and 6-PF₆ (blue) (heating rate = 1 °C min⁻¹).

Crystalline phase morphologies observed by POM

The temperature-dependent crystalline phase behaviors of the bis-pyrazolium salts was investigated using polarized optical microscopy (POM). Fig. 4 displays the morphological evolution of 6-PF₆, 8-PF₆, and 11-PF₆ observed at different temperatures. At 30 °C, 6-PF₆ exhibited numerous radially grown spherulitic structures, which refer to a typical crystalline phase at that temperature. At 150 °C, above its solid–solid transition temperature (T_ss, 140 °C), the sample showed highly aligned, elongated domains corresponding to a soft crystalline phase, a typical feature of OIPCs.^27,28,30

Fig. 4 POM images of (a) and (b) 6-PF₆, (c)–(e) 8-PF₆, and (f)–(h) 11-PF₆ during cooling from their isotropic phases. (a) Spherulitic rigid crystalline phase observed at 30 °C, (b) plastic crystalline phase at 150 °C, (c) aligned rigid crystalline phase at 30 °C, (d) plate-like soft crystalline phase at 120 °C, (e) mixed phase of soft crystals and amorphous domains at 133 °C, (f) irregularly intergrown crystal domains at 70 °C, (g) aligned soft crystalline phase with undulating structures perpendicular to the growth direction at 120 °C, (h) well-aligned soft crystalline phase at 140 °C. All scale bars represent 100 µm.

For 8-PF₆, a highly birefringent and well-defined rigid crystalline phase was observed at 30 °C, with distinct domain boundaries. When heated to 120 °C, above its first T_ss (113 °C), the morphology changed into stacked fan- or plate-like domains, indicating the formation of a relatively soft crystalline phase. At 133 °C, beyond the second T_ss (131 °C), a very soft crystalline morphology consisting of weak interference colors and well-aligned flexible domains coexisting with amorphous regions were observed.

11-PF₆ initially showed disordered and entangled crystalline domains (Fig. 4f), which transformed into highly oriented soft crystalline structures with undulating features perpendicular to the growth direction. While the morphologies observed in Fig. 4g and h appear similar, the vertically contracting domains present in Fig. 4g disappeared in Fig. 4h, resulting in more uniformly aligned soft crystalline textures. These thermally induced phase transformations are similar to the previously reported phase behaviors of dicationic OIPC materials.^27,28,30

Temperature-dependent structural evolution by WAXS

The solid–solid phase transition features of OIPCs can be also observed by X-ray analysis. Fig. 5 presents the 1D wide-angle X-ray scattering (WAXS) patterns of 8-PF₆ and 11-PF₆ at various temperatures. In Fig. 5a, the WAXS profile of 8-PF₆ at 25 °C shows multiple sharp diffraction peaks, particularly within the 2θ range of 12.7–24.1°, which reflect well-ordered packing within unit cells and repetitive long-range order across crystalline domains. These features indicate a rigid crystalline phase with high crystallinity. At 120 °C, peaks at 2θ = 4.2°, 12.7°, and 13.5° disappear, and those between 15–30° become broadened or show reduced intensity. Notably, the disappearance of the 4.3° peak and the emergence of a new peak at 5.6° suggest a structural rearrangement to a newly ordered but softer crystalline phase.³⁷ These changes imply partial loss of crystallinity and the formation of new crystalline domains. At 135 °C, the appearance of broad peaks in the 2θ = 0–10° region suggests the presence of limited crystalline domains or short-range ionic ordering, while the broadening and weakening of peaks above 10° indicate a transition to a plastic crystalline phase. These WAXS observations are consistent with the POM results discussed earlier.

Fig. 5 1D wide-angle X-ray scattering spectra of (a) 8-PF₆ and (b) 11-PF₆ at different temperatures.

As shown in Fig. 5b, the 1D WAXS pattern of 11-PF₆ at 25 °C also exhibits multiple sharp peaks, indicative of a highly ordered rigid crystalline phase. At 120 °C, most peaks in the 2θ = 15–25° range remain, but their intensities decrease and become broadened, similar to the behavior observed for 8-PF₆, suggesting a transition to a soft crystalline phase. At 140 °C, the overall pattern remains similar to that at 120 °C, although subtle changes in the 10–20° range suggest further structural relaxation. These results support the POM observations (Fig. 4g and h), indicating that 11-PF₆ transforms into an even softer crystalline phase at 140 °C compared to 120 °C.

For 6-PF₆, sharp diffraction peaks were observed at 130 °C, indicative of a rigid crystalline structure. At 150 °C, the peak at 2θ = 3.8° remains, but those in the 5–30° range show a significant decrease in intensity and pronounced broadening, suggesting a transition into a plastic crystalline phase (Fig. S38).

Ionic conductivity and phase transition behavior

The ionic conductivities of the synthesized m-PF₆ were measured by electrochemical impedance spectroscopy (EIS) after annealing each sample at the target temperature for 10 minutes. Fig. 6 shows the temperature-dependent ionic conductivities of 8-PF₆ and 11-PF₆. Across the entire temperature range, 8-PF₆ exhibited higher ionic conductivity than 11-PF₆, which is attributed to the softer crystalline phase of 8-PF₆ observed in the POM analysis.

Fig. 6 Temperature-dependent ionic conductivities of 8-PF₆, 11-PF₆ and their solid electrolytes containing 30 mol% LiTf₂N, measured by electrochemical impedance spectroscopy.

For both samples, changes in the slope of the conductivity curves were observed at the solid–solid phase transition temperatures (Fig. S39 and S40), indicating a change in ion transport mechanism upon phase transition. This behavior is consistent with previous reports on OIPCs, where conductivity is strongly influenced by phase transitions.^28–30

Notably, the solid-state electrolyte prepared by incorporating 30 mol% lithium bis(trifluoromethanesulfonyl)imide (LiTf₂N) into 8-PF₆ exhibited higher ionic conductivity than the corresponding 11-PF₆ based system under identical conditions. This enhancement is ascribed to the more flexible crystalline framework of 8-PF₆, which facilitates more efficient Li⁺ ion transport pathways. Specifically, the 8-PF₆ + 30 mol% LiTf₂N sample showed a high ionic conductivity of 1.31 × 10⁻³ S cm⁻¹ at 70 °C. This enhancement in ionic conductivity is consistent with previously reported OIPC-Li salt composite systems, where Li⁺–anion interactions were found to induce lattice disorder and facilitate ion transport.^38,39

Conclusion

In this study, a series of symmetric dicationic bis-pyrazolium PF₆⁻ salts (m-PF₆) with varying side chain lengths were synthesized, and their thermal properties, phase transition behaviors, and ionic conductivity were systematically investigated. Differential scanning calorimetry (DSC) revealed that most of the compounds exhibited one or more solid–solid phase transitions (T_ss). Notably, 6-PF₆ showed a single solid–solid phase transition with the lowest melting entropy (ΔS_f = 1.4 J mol⁻¹ K⁻¹) among the series. Compounds 8-PF₆, 10-PF₆ and 11-PF₆ exhibited two or more solid–solid phase transitions, with ΔS_f values below 40 J mol⁻¹ K⁻¹, indicating their classification as organic ionic plastic crystals (OIPCs).

Polarized optical microscopy (POM) and wide-angle X-ray scattering (WAXS) analyses confirmed that 8-PF₆ undergoes a transition into a soft crystalline phase upon solid–solid phase transition, characterized by enhanced domain alignment and coexistence with amorphous regions. Compared to 11-PF₆, 8-PF₆ displayed a more flexible crystalline framework, which directly influenced its ionic transport properties.

Electrochemical impedance spectroscopy (EIS) measurements showed that pristine 8-PF₆ exhibited an ionic conductivity of 1.47 × 10⁻⁶ S cm⁻¹ at 70 °C, which is approximately ten times higher than that of 11-PF₆ (1.22 × 10⁻⁷ S cm⁻¹). Furthermore, incorporation of 30 mol% LiTf₂N significantly enhanced conductivity in both systems. The solid mixture of 8-PF₆ + 30 mol% LiTf₂N exhibited a conductivity of 1.31 × 10⁻³ S cm⁻¹ at 70 °C, which is more than 18 times higher than the corresponding 11-PF₆ based composite (7.26 × 10⁻⁵ S cm⁻¹). These results suggest that the soft crystalline nature of OIPCs plays a critical role in facilitating efficient ion transport in solid-state systems.

Experimental

Materials

Pyrazole (>98%), 1-methylpyrazole (>98%), 1,4-dibromobutane (98%) and potassium hexafluorophosphate (KPF₆) were purchased from Tokyo Chemical Industry (TCI). All other chemicals and solvents were purchased from Dukan Chemical (Korea) and used as received.

Instrumentation

¹H and ¹³C NMR spectra were obtained on Varian VNMRS 500 MHz spectrometer. DSC results were obtained on a TA Instrument DSC250 differential scanning calorimeter with a scan rate of 10 K min⁻¹ under N₂. TGA results were obtained on a TA Instrument SDT Q600 Simultaneous TGA/DSC with a heating rate of 10 K min⁻¹ under N₂. Melting points were observed on EZ-melt-automated melting point system MPA120 apparatus with a 1 K min⁻¹ heating rate. Polarized optical microscope (POM) images were taken by POM (LV100ND POL/DS, Nikon). Images were recorded using a charge-coupled device (CCD) camera (DS-Ri2, Nikon). The temperature was controlled with the heating stage (Linkam LTS420) and a temperature controller (Linkam T95-HS). Ionic conductivity was measured using a BioLogic SP-200 electrochemical workstation with an EL-CELL chamber. Samples were annealed for 10 min at the target temperature before measurement. Impedance spectra were collected over the frequency range of 1 MHz to 0.1 Hz. WAXS measurements were performed using a Xenocs Xeuss 3.0 system equipped with an 8 keV Cu Kα source (beam size: 50 µm, divergence ≤0.4 mrad). Scattering profiles were collected at selected temperatures using a Linkam temperature-controlled stage (−20 to 150 °C). Kapton film was used for q-value calibration. All data were collected as 1D azimuthally integrated patterns.

General synthetic procedures for 1-alkylpyrazole

A solution of pyrazole (1.20 equiv.), 1-bromoalkane (1.00 equiv.) and 50% aqueous NaOH (1.00 equiv.) in tetrahydrofuran (THF) was refluxed overnight. After cooling to room temperature, the solvent was removed under reduced pressure using a rotary evaporator. The residue was extracted with n-hexane/water three times. The combined organic layers were washed with water and then dried over anhydrous MgSO₄. Column chromatography through a short silica gel column with n-hexane as the eluent afforded a clear liquid.

1,4-Bis[N-(N′-methylpyrazolium)butane PF₆⁻ (1-PF₆). 1-Methylpyrazole (4.00 g, 0.0487 mol) and 1,4-dibromobutane (4.78 g, 0.221 mol) were dissolved in acetonitrile (30 mL) and refluxed for 48 hours under a nitrogen atmosphere. Upon completion of the reaction, the solvent was removed under reduced pressure using a rotary evaporator. The resulting residue was dissolved in methanol and precipitated sequentially with diethyl ether and acetone to yield a pure white solid (6.99 g, 85%). The synthesized bromide salt (2.00 g, 0.00526 mol) was dissolved in deionized water, and an aqueous solution of KPF₆ (2.13 g, 0.00116 mol in DI water) was added dropwise with stirring. The mixture was stirred at room temperature for 6–8 hours. The resulting precipitate was filtered and washed with deionized water twice. Drying in a vacuum oven gave pure 1-PF₆ as a white crystalline solid (2.46 g, 92%). ¹H NMR (500 MHz, 23 °C, acetone-d₆): δ 2.3 (m, 4H), 4.4 (s, 6H), 4.8 (m, 4H), 6.9 (t, J = 3, 2H), 8.4 (d, 4H). ¹³C NMR (250 MHz, 23 °C, acetone-d₆): δ 27, 38, 51, 139, 140.

1-Ethylpyrazole. The general 1-alkylpyrazole procedure was followed using pyrazole (4.00 g, 0.0588 mol), 50% aqueous NaOH (3.92 g, 0.0490 mol) and 1-bromoethane (5.34 g, 0.0490 mol). A clear liquid (2.12 g, 45%) was obtained. ¹H NMR (500 MHz, 23 °C, CDCl₃): δ 1.46 (t, J = 7, 3H), 4.15 (m, 2H), 6.22 (t, J = 2, 1H), 7.36 (d, J = 2, 1H), 7.48 (d, J = 2, 1H).

1,4-Bis[N-(N′-ethylpyrazolium)butane PF₆⁻ (2-PF₆). The same procedure as for 1-PF₆ was followed. 1-Ethylpyrazole (2.00 g, 0.0208 mol) and 1,4-dibromobutane (2.04 g, 0.00946 mol) in acetonitrile (20 mL) were reacted to afford the crystalline white bromide salt (2.68 g, 69%). The bromide salt (1.00 g, 0.00245 mol) and KPF₆ (0.992 g, 0.00539 mol) were used to give 2-PF₆ (1.05 g, 80%). ¹H NMR (500 MHz, 23 °C, acetone-d₆): δ 1.6 (t, J = 7, 6H), 2.2 (m, 4H), 4.6 (q, J = 7, 4H), 4.7 (m, 4H), 7.0 (t, J = 3, 2H), 8.4 (d, J = 3, 2H), 8.5 (d, J = 3, 2H). ¹³C NMR (250 MHz, 23 °C, acetone-d₆): δ 15, 27, 47, 51, 109, 138, 139.

1-Propylpyzaole. The general 1-alkylpyrazole procedure was followed using pyrazole (3.00 g, 0.0441 mol), 50% aqueous NaOH (2.94 g, 0.0367 mol) and 1-bromopropane (4.51 g, 0.0367 mol). A clear liquid (2.27 g, 56%) was obtained. ¹H NMR (500 MHz, 23 °C, CDCl₃): δ 0.90 (t, J = 7, 3H), 1.87 (m, 2H), 4.09 (t, J = 7, 2H), 6.22 (t, J = 2, 1H), 7.36 (d, J = 2, 1H), 7.48 (d, J = 2, 1H).

1,4-Bis[N-(N′-propylpyrazolium)butane PF₆⁻ (3-PF₆). The same procedure as for 1-PF₆ was followed. 1-Propylpyrazole (2.00 g, 0.0182 mol) and 1,4-dibromobutane (1.78 g, 0.00825 mol) in acetonitrile (20 mL) were reacted to afford the crystalline white bromide salt (2.77 g, 77%). The bromide salt (1.00 g, 0.00229 mol) and KPF₆ (0.928 g, 0.00504 mol) were used to give 3-PF₆ (1.15 g, 89%). ¹H NMR (500 MHz, 23 °C, acetone-d₆): δ 1.0 (t, J = 7, 6H), 2.0 (m, 4H), 2.2 (m, 4H), 4.6 (t, J = 7, 4H), 4.8 (m, 4H), 7.0 (t, J = 3, 2H), 8.4 (d, J = 3, 2H), 8.5 (d, J = 3, 2H). ¹³C NMR (250 MHz, 23 °C, acetone-d₆): δ 12, 24, 27, 51, 53, 109, 139.

1-Butylpyrazole. The general 1-alkylpyrazole procedure was followed using pyrazole (3.00 g, 0.0441 mol), 50% aqueous NaOH (2.94 g, 0.0367 mol) and 1-bromobutane (5.03 g, 0.0367 mol). A clear liquid (4.01 g, 88%) was obtained. ¹H NMR (500 MHz, 23 °C, CDCl₃): δ 0.93 (t, J = 7, 3H), 1.31 (m, 2H), 1.84 (m, 2H), 4.12 (t, J = 7, 2H), 6.23 (t, J = 2, 1H), 7.36 (d, J = 2, 1H), 7.49 (d, J = 2, 1H).

1,4-Bis[N-(N′-butylpyrazolium)butane PF₆⁻ (4-PF₆). The same procedure as for 1-PF₆ was followed. 1-Butylpyrazole (2.00 g, 0.0161 mol) and 1,4-dibromobutane (1.58 g, 0.00732 mol) in acetonitrile (20 mL) were reacted to afford the crystalline white bromide salt (3.12 g, 92%). The bromide salt (1.00 g, 0.00215 mol) and KPF₆ (0.872 g, 0.00474 mol) were used to give 4-PF₆ (1.08 g, 84%). ¹H NMR (500 MHz, 23 °C, acetone-d₆): δ 1.0 (t, J = 7, 6H), 1.4 (m, 4H) 2.0 (m, 4H), 2.2 (m, 4H), 4.6 (t, J = 7, 4H), 4.8 (m, 4H), 7.0 (t, J = 3, 2H), 8.4 (d, J = 3, 2H), 8.5 (d, J = 3, 2H). ¹³C NMR (250 MHz, 23 °C, acetone-d₆): δ 15, 21, 27, 32, 51, 52, 110, 139.

1-Pentylpyrazole. The general 1-alkylpyrazole procedure was followed using pyrazole (3.00 g, 0.0441 mol), 50% aqueous NaOH (2.94 g, 0.0367 mol) and 1-bromopentane (5.55 g, 0.0367 mol). A clear liquid (4.72 g, 93%) was obtained. ¹H NMR (500 MHz, 23 °C, CDCl₃): δ 0.87 (t, J = 7, 3H), 1.25–1.31 (m, 4H), 1.85 (m, 2H), 4.12 (t, J = 7, 2H), 6.23 (t, J = 2, 1H), 7.36 (d, J = 2, 1H), 7.49 (d, J = 2, 1H).

1,4-Bis[N-(N′-pentylpyrazolium)butane PF₆⁻ (5-PF₆). The resulting 1-pentylpyrazole (2.00 g, 0.0145 mol) and 1,4-dibromobutane (1.42 g, 0.00658 mol) were dissolved in acetonitrile (20 mL) and refluxed for 48 hours under a nitrogen atmosphere. Upon completion of the reaction, the solvent was removed under reduced pressure using a rotary evaporator. The resulting residue was dissolved in methanol and reprecipitated from acetone three times to yield a pure white solid (3.01 g, 93%). The synthesized bromide salt (1.00 g, 0.00203 mol) was dissolved in deionized water, and an aqueous solution of KPF₆ (0.822 g, 0.00447 mol in DI water) was added dropwise with stirring. The mixture was stirred at room temperature for 6–8 hours. The resulting precipitate was filtered and washed with deionized water twice. Drying in a vacuum oven gave pure 5-PF₆ as a white crystalline solid (1.18 g, 93%). ¹H NMR (500 MHz, 23 °C, acetone-d₆): δ 0.9 (t, J = 7, 6H), 1.4 (m, 8H) 2.0 (m, 4H), 2.2 (m, 4H), 4.6 (t, J = 7, 4H), 4.8 (m, 4H), 7.0 (t, J = 3, 2H), 8.4 (d, J = 3, 2H), 8.5 (d, J = 3, 2H). ¹³C NMR (250 MHz, 23 °C, acetone-d₆): δ 15, 24, 27, 29, 30.3, 30.7, 51. 52, 109, 139.

1-Hexylpyrazole. The general 1-alkylpyrazole procedure was followed using pyrazole (3.00 g, 0.0441 mol), 50% aqueous NaOH (2.94 g, 0.0367 mol) and 1-bromohexane (6.06 g, 0.0367 mol). A clear liquid (4.88 g, 87%) was obtained. ¹H NMR (500 MHz, 23 °C, CDCl₃): δ 0.87 (t, J = 7, 3H), 1.25–1.31 (m, 6H), 1.85 (m, 2H), 4.12 (t, J = 7, 2H), 6.23 (t, J = 2, 1H), 7.36 (d, J = 2, 1H), 7.49 (d, J = 2, 1H).

1,4-Bis[N-(N′-hexylpyrazolium)butane PF₆⁻ (6-PF₆). The same procedure as for 5-PF₆ was followed. 1-Hexylpyrazole (2.00 g, 0.0131 mol) and 1,4-dibromobutane (1.29 g, 0.00597 mol) in acetonitrile (20 mL) were reacted to afford the crystalline white bromide salt (2.98 g, 96%). The bromide salt (1.00 g, 0.00192 mol) and KPF₆ (0.778 g, 0.00423 mol) were used to give 6-PF₆ (1.09 g, 87%). ¹H NMR (500 MHz, 23 °C, acetone-d₆): δ 0.9 (t, J = 7, 6H), 1.3 (m, 8H), 1.4 (m, 4H), 2.0 (m, 4H), 2.2 (m, 4H), 4.6 (t, J = 7, 4H), 4.8 (m, 4H), 7.0 (t, J = 3, 2H), 8.4 (d, J = 3, 2H), 8.5 (d, J = 3, 2H). ¹³C NMR (250 MHz, 23 °C, acetone-d₆): δ 15, 24, 27.3, 27.4, 33 51, 52, 109, 139.

1-Heptylpyrazole. The general 1-alkylpyrazole procedure was followed using pyrazole (3.00 g, 0.0441 mol), 50% aqueous NaOH (2.94 g, 0.0367 mol) and 1-bromohepthane (6.58 g, 0.0367 mol). A clear liquid (5.82 g, 95%) was obtained. ¹H NMR (500 MHz, 23 °C, CDCl₃): δ 0.87 (t, J = 7, 3H), 1.25–1.31 (m, 8H), 1.85 (m, 2H), 4.12 (t, J = 7, 2H), 6.23 (t, J = 2, 1H), 7.36 (d, J = 2, 1H), 7.49 (d, J = 2, 1H).

1,4-Bis[N-(N′-heptylpyrazolium)butane PF₆⁻ (7-PF₆). The resulting 1-heptylpyrazole (2.00 g, 0.0111 mol) and 1,4-dibromobutane (1.18 g, 0.00547 mol) were dissolved in acetonitrile (20 mL) and refluxed for 48 hours under a nitrogen atmosphere. Upon completion of the reaction, the solvent was removed under reduced pressure using a rotary evaporator. The resulting residue was dissolved in methanol and reprecipitated from THF two times to yield a pure white solid (2.01 g, 67%). The synthesized bromide salt (1.00 g, 0.00182 mol) was dissolved in deionized water, and an aqueous solution of KPF₆ (0.738 g, 0.00401 mol) was added dropwise with stirring. The mixture was stirred at room temperature for 6–8 hours. The resulting precipitate was filtered and washed with deionized water twice. Drying in a vacuum oven gave pure 7-PF₆ as a white crystalline solid (1.13 g, 91%). ¹H NMR (500 MHz, 23 °C, acetone-d₆): δ 0.9 (t, J = 7, 6H), 1.3 (m, 8H), 1.4 (m, 8H), 2.0 (m, 4H), 2.2 (m, 4H), 4.6 (t, J = 7, 4H), 4.8 (m, 4H), 7.0 (t, J = 3, 2H), 8.4 (d, J = 3, 2H), 8.5 (d, J = 3, 2H). ¹³C NMR (250 MHz, 23 °C, acetone-d₆): δ 15, 24, 27.4, 27.6, 30.3, 30.7, 33, 51, 52, 109, 139.

1-Octylpyrazole. The general 1-alkylpyrazole procedure was followed using pyrazole (2.00 g, 0.0294 mol), 50% aqueous NaOH (1.96 g, 0.0245 mol) and 1-bromoocthane (4.73 g, 0.0245 mol). A clear liquid (4.12 g, 93%) was obtained. ¹H NMR (500 MHz, 23 °C, CDCl₃): δ 0.87 (t, J = 7, 3H), 1.25–1.31 (m, 10H), 1.85 (m, 2H), 4.12 (t, J = 7, 2H), 6.23 (t, J = 2, 1H), 7.36 (d, J = 2, 1H), 7.49 (d, J = 2, 1H).

1,4-Bis[N-(N′-octylpyrazolium)butane PF₆⁻ (8-PF₆). The same procedure as for 7-PF₆ was followed. 1-Octylpyrazole (2.00 g, 0.0103 mol) and 1,4-dibromobutane (1.09 g, 0.00504 mol) in acetonitrile (20 mL) were reacted to afford the crystalline white bromide salt (1.83 g, 63%). The bromide salt (1.00 g, 0.00174 mol) and KPF₆ (0.702 g, 0.00382 mol) were used to give 8-PF₆ (1.05 g, 86%). ¹H NMR (500 MHz, 23 °C, acetone-d₆): δ 0.9 (t, J = 7, 6H), 1.2 (m, 12H), 1.3 (m, 4H), 1.4 (m, 8H), 2.0 (m, 4H), 2.2 (m, 4H), 4.6 (t, J = 7, 4H), 4.8 (m, 4H), 7.0 (t, J = 3, 2H), 8.4 (d, J = 3, 2H), 8.5 (d, J = 3, 2H). ¹³C NMR (250 MHz, 23 °C, acetone-d₆): δ 15, 24, 27.4, 27.6, 30.5, 30.63, 30.67, 33, 51, 52, 109, 139.

1-Nonylpyrazole. The general 1-alkylpyrazole procedure was followed using pyrazole (2.00 g, 0.0294 mol), 50% aqueous NaOH (1.96 g, 0.0245 mol) and 1-bromononane (5.07 g, 0.0245 mol). A clear liquid (4.55 g, 96%) was obtained. ¹H NMR (500 MHz, 23 °C, CDCl₃): δ 0.87 (t, J = 7, 3H), 1.25–1.31 (m, 12H), 1.85 (m, 2H), 4.12 (t, J = 7, 2H), 6.23 (t, J = 2, 1H), 7.36 (d, J = 2, 1H), 7.49 (d, J = 2, 1H).

1,4-Bis[N-(N′-nonylpyrazolium)butane PF₆⁻ (9-PF₆). The same procedure as for 7-PF₆ was followed. 1-Nonylpyrazole (2.00 g, 0.0103 mol) and 1,4-dibromobutane (1.01 g, 0.00468 mol) in acetonitrile (20 mL) were reacted to afford the crystalline white bromide salt (1.71 g, 60%). The bromide salt (1.00 g, 0.00165 mol) and KPF₆ (0.670 g, 0.00364 mol) were used to give 9-PF₆ (1.08 g, 88%). ¹H NMR (500 MHz, 23 °C, acetone-d₆): δ 0.9 (t, J = 7, 6H), 1.2 (m, 16H), 1.3 (m, 4H), 1.4 (m, 8H), 2.0 (m, 4H), 2.2 (m, 4H), 4.6 (t, J = 7, 4H), 4.8 (m, 4H), 7.0 (t, J = 3, 2H), 8.4 (d, J = 3, 2H), 8.5 (d, J = 3, 2H). ¹³C NMR (250 MHz, 23 °C, acetone-d₆): δ 15, 24, 27.5, 27.7, 30.7, 30.8, 31, 33, 51, 52, 109, 139.

1-Decylpyrazole. The general 1-alkylpyrazole procedure was followed using pyrazole (2.00 g, 0.0294 mol), 50% aqueous NaOH (1.96 g, 0.0245 mol) and 1-bromodecane (5.42 g, 0.0245 mol). A clear liquid (4.82 g, 94%) was obtained. ¹H NMR (500 MHz, 23 °C, CDCl₃): δ 0.87 (t, J = 7, 3H), 1.25–1.31 (m, 14H), 1.85 (m, 2H), 4.12 (t, J = 7, 2H), 6.23 (t, J = 2, 1H), 7.36 (d, J = 2, 1H), 7.49 (d, J = 2, 1H).

1,4-Bis[N-(N′-decylpyrazolium)butane PF₆⁻ (10-PF₆). The same procedure as for 7-PF₆ was followed. 1-Decylpyrazole (2.00 g, 0.00960 mol) and 1,4-dibromobutane (0.942 g, 0.00436 mol) in acetonitrile (20 mL) were reacted to afford the crystalline white bromide salt (1.74 g, 63%). The bromide salt (1.00 g, 0.00158 mol) and KPF₆ (0.640 g, 0.00348 mol) were used to give 10-PF₆ (1.12 g, 93%). ¹H NMR (500 MHz, 23 °C, acetone-d₆): δ 0.9 (t, J = 7, 6H), 1.2 (m, 20H), 1.3 (m, 4H), 1.4 (m, 8H), 2.0 (m, 4H), 2.2 (m, 4H), 4.6 (t, J = 7, 4H), 4.8 (m, 4H), 7.0 (t, J = 3, 2H), 8.4 (d, J = 3, 2H), 8.5 (d, J = 3, 2H). ¹³C NMR (250 MHz, 23 °C, acetone-d₆): δ 15, 24, 27.2, 27.4, 30.40, 30.46, 30.6, 30.7, 30.8, 30.9, 33, 51, 52, 109, 139.

1-Undecylpyrazole. The general 1-alkylpyrazole procedure was followed using pyrazole (2.00 g, 0.0294 mol), 50% aqueous NaOH (1.96 g, 0.0245 mol) and 1-bromoundecane (5.76 g, 0.0245 mol). A clear liquid (4.98 g, 91%) was obtained. ¹H NMR (500 MHz, 23 °C, CDCl₃): δ 0.87 (t, J = 7, 3H), 1.25–1.31 (m, 16H), 1.85 (m, 2H), 4.12 (t, J = 7, 2H), 6.23 (t, J = 2, 1H), 7.36 (d, J = 2, 1H), 7.49 (d, J = 2, 1H).

1,4-Bis[N-(N′-undecylpyrazolium)butane PF₆⁻ (11-PF₆). The same procedure as for 7-PF₆ was followed. 1-Undecylpyrazole (2.00 g, 0.00899 mol) and 1,4-dibromobutane (0.883 g, 0.00409 mol) in acetonitrile (20 mL) were reacted to afford the crystalline white bromide salt (1.55 g, 57%). The bromide salt (1.00 g, 0.00151 mol) and KPF₆ (0.613 g, 0.00333 mol) were used to give 11-PF₆ (1.05 g, 88%). ¹H NMR (500 MHz, 23 °C, acetone-d₆): δ 0.9 (t, J = 7, 6H), 1.2 (m, 24H), 1.3 (m, 4H), 1.4 (m, 8H), 2.0 (m, 4H), 2.2 (m, 4H), 4.6 (t, J = 7, 4H), 4.8 (m, 4H), 7.0 (t, J = 3, 2H), 8.4 (d, J = 3, 2H), 8.5 (d, J = 3, 2H). ¹³C NMR (250 MHz, 23 °C, acetone-d₆): δ 15, 24, 27.5, 27.7, 30.70, 30.74, 31.0, 31.1, 31.28, 31.29, 51, 52, 109, 139.

1-Dodecylpyrazole. The general 1-alkylpyrazole procedure was followed using pyrazole (2.00 g, 0.0294 mol), 50% aqueous NaOH (1.96 g, 0.0245 mol) and 1-bromododecane (6.10 g, 0.0245 mol). A clear liquid (5.21 g, 90%) was obtained. ¹H NMR (500 MHz, 23 °C, CDCl₃): δ 0.87 (t, J = 7, 3H), 1.25–1.31 (m, 18H), 1.85 (m, 2H), 4.12 (t, J = 7, 2H), 6.23 (t, J = 2, 1H), 7.36 (d, J = 2, 1H), 7.49 (d, J = 2, 1H).

1,4-Bis[N-(N′-dodecylpyrazolium)butane PF₆⁻ (12-PF₆). The same procedure as for 7-PF₆ was followed. 1-Dodecylpyrazole (2.00 g, 0.00846 mol) and 1,4-dibromobutane (0.830 g, 0.00385 mol) in acetonitrile (20 mL) were reacted to afford the crystalline white bromide salt (1.61 g, 61%). The bromide salt (1.00 g, 0.00145 mol) and KPF₆ (0.588 g, 0.00319 mol) were used to give 12-PF₆ (0.98 g, 82%). ¹H NMR (500 MHz, 23 °C, acetone-d₆): δ 0.9 (t, J = 7, 6H), 1.2 (m, 28H), 1.3 (m, 4H), 1.4 (m, 8H), 2.0 (m, 4H), 2.2 (m, 4H), 4.6 (t, J = 7, 4H), 4.8 (m, 4H), 7.0 (t, J = 3, 2H), 8.4 (d, J = 3, 2H), 8.5 (d, J = 3, 2H). ¹³C NMR (250 MHz, 23 °C, acetone-d₆): δ 15, 24, 27.4, 27.7, 30.64, 60.68, 30.9, 31.0, 31.1, 31.25, 31.27, 51, 52, 109, 139.

Author contributions

Jong Chan Shin performed the synthesis, WAXS and POM analyses, and contributed to data interpretation and manuscript preparation. Suyeon Kim carried out the synthesis and ionic conductivity measurements. Eunji Yun conducted thermal analyses and assisted in synthesis. Minjae Lee conceived and supervised the project, guided the experimental design, and served as the corresponding author. All authors contributed to the discussion of the results. The manuscript was drafted by Jong Chan Shin with input from all authors.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article are provided in the supplementary information (SI). Supplementary information: NMR spectra, TGA, DSC, WAXS, and ionic conductivity data. See DOI: https://doi.org/10.1039/d5qm00697j.

Acknowledgements

This work was supported by the National Research Foundation of Korea (NRF), funded by the Ministry of Science and ICT (RS-2024-00360494), the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korean government (MCEE) (RS-2022-KP002707, and Jeonbuk Regional Energy Cluster Training of Human Resources), and the Regional Innovation System & Education (RISE) initiative, funded by the Ministry of Education and administered by the Jeonbuk RISE Center (Project No. 2025-RISE-13-KSU).

Notes and references

1. T. He, Y. F. Wang and J. H. Zeng, Pyrazolium Based Electrolyte for Solid-State Dye-Sensitized Solar Cells with High Fill Factor and Open-Circuit Voltage, J. Mater. Chem. C, 2016, 4, 8235–8244.
2. M. Chai, Y. Jin, S. Fang, L. Yang, S.-I. Hirano and K. Tachibana, Low-viscosity ether-functionalized pyrazolium ionic liquids as new electrolytes for lithium battery, J. Power Sources, 2012, 216, 323–329.
3. S. Shen, S. Fang, L. Qu, D. Luo, L. Yang and S. Hirano, Low-viscosity ether-functionalized pyrazolium ionic liquids based on dicyanamide anions: properties and application as electrolytes for lithium metal batteries, RSC Adv., 2015, 114, 93888–93899.
4. N. Wang, W. Luo, H. Shen, H. Li, Z. Xu, Z. Yue, C. Shi, H. Ye and L. Miao, Crystal engineering regulation achieving inverse temperature symmetry breaking ferroelasticity in a cationic displacement type hybrid perovskite system, Chin. Chem. Lett., 2024, 35, 108696.
5. R. A. Abu Talip, W. Z. N. Yahya, N. M. Mohamed, Y. H. Zaki, K. Ramly, S. N. A. Zaine, M. A. Bustam and P. K. Singh, A performance enhancement of dye-sensitized solar cells using pyrazolium-based ionic liquids electrolytes vs imidazolium-based ionic liquids electrolytes, J. Mol. Liq., 2024, 398, 124214.
6. T. Jian, C. Wang, T. Wang, X. Wang, X. Wang, X. Li, Y. Ding and H. Wei, Imidazolium structural isomer pyrazolium: A better alkali-stable anion conductor for anion exchange membranes, J. Membr. Sci., 2022, 660, 120843.
7. Y. Kim, Y. Wang, A. France-Lanord, Y. Wang, Y.-C. Wu, S. Lin, Y. Li, J. C. Grossman and T. M. Swager, Ionic highways from covalent assembly in highly conducting and stable anion exchange membrane fuel cells, J. Am. Chem. Soc., 2019, 141, 18152–18159.
8. C. Chiappe, A. Sanzone, D. Mendola, F. Castiglione, A. Famulari, G. Raos and A. Mele, Pyrazolium-versus imidazolium-based ionic liquids: structure, dynamics and physicochemical properties, J. Phys. Chem. B, 2013, 117, 668–676.
9. T. Wang, D. Zheng, J. Zhang, B. Fan, Y. Ma, T. Ren, L. Wang and J. Zhang, Protic pyrazolium ionic liquids: An efficient catalyst for conversion of CO₂ in the absence of metal and solvent, ACS Sustainable Chem. Eng., 2018, 6, 2574–2582.
10. J. Zhang, X. Zhu, B. Fan, J. Guo, P. Ning, T. Ren, L. Wang and J. Zhang, Combination of experimental and theoretical methods to explore the amino-functionalized pyrazolium ionic liquids: An efficient single-component catalyst for chemical fixation of CO₂ under mild conditions, Mol. Catal., 2019, 466, 37–45.
11. J. D. Ndayambaje, I. Shabbir, Q. Zhao, L. Dong, Q. Su and W. Cheng, Controlling dual-positively charged pyrazolium ionic liquids for efficient catalytic conversion of CO₂ into carbonates under mild conditions, Catal. Sci. Technol., 2024, 14, 293–305.
12. D. Vasilyev, E. Shirzadi, A. V. Rudnev, P. Broekmann and P. J. Dyson, Pyrazolium ionic liquid co-catalysts for the electroreduction of CO₂, ACS Appl. Energy Mater., 2018, 1, 5124–5128.
13. Y. Abu-Lebdeh, A. Abouimrane, P.-J. Alarco and M. Armand, Ionic liquid and plastic crystalline phases of pyrazolium imide salts as electrolytes for rechargeable lithium-ion batteries, J. Power Sources, 2006, 154, 255–261.
14. I. Sánchez, J. A. Campo, J. V. Heras, M. R. Torres and M. Cano, Pyrazolium salts as a new class of ionic liquid crystals, J. Mater. Chem., 2012, 22, 13239–13251.
15. P.-J. Alarco, Y. Abu-Lebdeh, N. Ravet and M. Armand, Lithium conducting pyrazolium imides plastic crystals: a new solid state electrolyte matrix, Solid State Ionics, 2004, 172, 53–56.
16. P.-J. Alarco, Y. Abu-Lebdeh and M. Armand, Highly conductive, organic plastic crystals based on pyrazolium imides, Solid State Ionics, 2004, 175, 717–720.
17. Z. Lei, B. Chen, Y.-M. Koo and D. R. MacFarlane, Introduction: Ionic liquids, Chem. Rev., 2017, 117, 6633–6635.
18. Z. Xue, L. Qin, J. Jiang, T. Mu and G. Gao, Thermal, electrochemical and radiolytic stabilities of ionic liquids, Phys. Chem. Chem. Phys., 2018, 20, 8382–8402.
19. H. Zhu, D. R. MacFarlane, J. M. Pringle and M. Forsyth, Organic ionic plastic crystals as solid-state electrolytes, Trends Chem., 2019, 1, 126–140.
20. J. M. Pringle, P. C. Howlett, D. R. MacFarlane and M. Forsyth, Organic ionic plastic crystals: recent advances, J. Mater. Chem., 2010, 20, 2056–2062.
21. J. M. Pringle, Recent progress in the development and use of organic ionic plastic crystal electrolytes, Phys. Chem. Chem. Phys., 2013, 15, 1339–1351.
22. D. R. MacFarlane, P. Meakin, J. Sun, N. Amini and M. Forsyth, Pyrrolidinium Imides: A New Family of Molten Salts and Conductive Plastic Crystal Phases, J. Phys. Chem. B, 1999, 103, 4164–4170.
23. M. L. Thomas, K. Hatakeyama-Sato, S. Nanbu and M. Yoshizawa-Fujita, Organic ionic plastic crystals: flexible solid electrolytes for lithium secondary batteries, Energy Adv., 2023, 2, 748–764.
24. H. Ueda, F. Mizuno, M. Forsyth and P. C. Howlett, Tailoring Silicon Composite Anodes with Li⁺-Containing Organic Ionic Plastic Crystals for Solid-State Batteries, J. Electrochem. Soc., 2024, 171, 020556.
25. M. Salado, T. H. Smith, N. Sirigiri, F. Chen, L. A. O’Dell, J. M. Pringle and M. Forsyth, Ammonium-based plastic crystals as solid-state electrolytes for lithium and sodium batteries, JACS Au, 2025, 5, 1663–1676.
26. M. Lee, U. H. Choi, S. Wi, C. Slebodnick, R. H. Colby and H. W. Gibson, 1,2-Bis[N-(N′-alkylimidazolium)]ethane salts: a new class of organic ionic plastic crystals, J. Mater. Chem., 2011, 21, 12280–12287.
27. H. Chae, Y.-H. Lee, M. Yang, W.-J. Yoon, D. K. Yoon, K.-U. Jeong, Y. H. Song, U. H. Choi and M. Lee, Interesting phase behaviors and ion-conducting properties of dicationic N-alkylimidazolium tetrafluoroborate salts, RSC Adv., 2019, 9, 3972–3978.
28. H. Chae, A. R. Lee, M. Yoon, U. H. Choi, G. Park and M. Lee, Organic ionic crystals: solid-phase structure and thermal properties of dicationic α,ω-bis[N,N′-(4-alkylpyridinium)]alkane hexafluorophosphate salts, Asian J. Org. Chem., 2019, 8, 1718–1725.
29. J. C. Shin, S.-I. Lim, K.-U. Jeong, J. P. Shim and M. Lee, Dicationic pyridinium salts as new organic ionics: Changes in solid-state phases and thermal/electrochemical properties, J. Ind. Eng. Chem., 2022, 107, 418–427.
30. J. C. Shin, T. Y. kim, H. J. Kim, U. H. Choi, H. S. Park and M. Lee, New organic ionic plastic crystals based on pyrrolidinium dication for a solid-phase electrolyte, Bull. Korean Chem. Soc., 2023, 44, 310–321.
31. T. V. Goncharova, L. V. Zatonskaya and A. S. Potapov, Synthesis of di(imidazolium) and di(pyrazolium) salts as precursors for N-heterocyclic dicarbene complexes, Procedia Chem., 2014, 10, 485–489.
32. L. V. Zatonskaya, I. A. Schepetkin, T. V. Petrenko, V. D. Ogorodnikov, A. I. Khlebnikov and A. S. Potapov, Synthesis and cytotoxicity of bis(pyrazol-1-yl)- alkane derivatives with polymethylene linkers and related mono- and dipyrazolium salts, Chem. Heterocycl. Compd., 2016, 52, 807–814.
33. M. Lee, Z. Niu, C. Slebodnick and H. W. Gibson, Structure and Properties of N,N-Alkylene Bis(N′-Alkylimidazolium) Salts, J. Phys. Chem. B, 2010, 114, 7312–7319.
34. G. Svehla, Vogel's Qualitative Inorganic Analysis, 7th edn, Prentice Hall, Harlow, 1996, ch. 4, pp. 4.14–4.16.
35. J. C. Shin and M. Lee, Expanded Structural Design of Organic Ionic Plastic Crystals Based on Linear Tris-Pyrrolidinium Salt, Chem. – Eur. J., 2025, 31, e202500903.
36. Y. Zhang and E. J. Maginn, Molecular dynamics study of the effect of alkyl chain length on melting points of [C_nMIM][PF₆] ionic liquids, Phys. Chem. Chem. Phys., 2014, 16, 13489–13499.
37. N. Sirigiri, F. Chen, C. M. Forsyth, R. Yunis, L. O’Dell, J. M. Pringle and M. Forsyth, Factors controlling the physical properties of an organic ionic plastic crystal, Mater. Today Phys., 2022, 22, 100603.
38. S. Abeysooriya, M. Lee, L. A. O’Dell and J. M. Pringle, Plastic crystal-based electrolytes using novel dicationic salts, Phys. Chem. Chem. Phys., 2022, 24, 4899–4909.
39. S. Abeysooriya, M. Lee, S. H. Kim, L. A. O’Dell and J. M. Pringle, Development of New Plastic-Crystal Based Electrolytes using Pyrrolidinium- Bis(fluorosulfonyl)imide Dicationic Salts, ChemSusChem, 2023, 16, e202202249.

---