Yu-Jie Zhong

Ke-Qing Zhao
email: kqzhao@sicnu.edu.cn

Bi-Qin Wang

Ping Hu

Hirosato Monobe
email: monobe-hirosato@aist.go.jp

Benoît Heinrich

Bertrand Donnio
email: bdonnio@ipcms.unistra.fr

2-Phenylbenzothiophene-based Liquid Crystalline Semiconductors

Keywords: organic semiconductor, time-of-flight photocurrent technique, 2-phenylbenzothiophene, smectic mesophase. 1

Highlight: 1. New mesomorphous phenylbenzothiophene derivatives were synthesized in two steps from a cheap and abundant precursor; 2. They display photoluminescence in mid-to-high quantum yields; 3. They exhibit smectic mesophase; 4. Charge transports measured by time-of-flight technique are in the 10 -3 cm 2 V -1 s -1 range.

Introduction

The benzo [b]thiophene core exhibits a wide range of biological and pharmacological activities thanks to structural similarities with many natural and synthetic molecules in drug design. [1] For instance, the 2-phenylbenzo[b]thiophene motif is the main parent structure of cancer drugs like raloxifene and arzoxifene, [2] and it is also used in the treatment of Alzheimer's disease. [3] Benzothiophene and its derivatives have now became indispensable building blocks for chemistry and for the development of the drug industry as it has been found to possess many more biological activities. [1] Therefore, as important pharmaceutical intermediate, it is produced in large scale, and thus is abundant, cheap and widely commercially available. However, despite this economical advantage, its natural rod-like shape, easy synthetic derivatization and appealing electronic structure, 2phenylbenzo[b]thiophene has not yet been considered in the design of liquid crystalline materials with potentially interesting semiconducting behaviour.

Thiophene-containing liquid crystalline (LC) semiconductors (Figure 1) perform a high degree of πdelocalization and optical tunability due to the combination of their intermolecular well-ordered morphology and unique electronic structure, which is an essential requirement for applications involving optoelectronic and photonic devices. [4][5][6][7][8][9] In addition, such materials usually show good charge carrier mobility, which depends strongly on the degree of ordering of the self-organized mesophases and on the orientation of the LC molecules, indicating the promising prospects of thiophene-based organic semiconductors for realizing cheap, flexible and printed electronics applications. [10][11][12][13][14] Among the promising thiophene-based candidates for OFETs are the benzothienobenzothiophene (BTBT) derivatives (Figure 1), [15][16][17][18] which present good solubility in organic solvents as well as forming well-ordered crystalline thin films, resulting in high carrier mobility. [19][20][21][22] Recently, our group has reported BTBT based semiconductors containing disk and calamitic LC molecules, which exhibit bipolar carrier mobility in the range of 10 -3 cm 2 V -1 s -1 in the columnar mesophase, and carrier mobility of hole approaching 0.07 cm 2 V -1 s -1 in the smectic phase, as measured by TOF technique. [23,24] Today, a large range of new thiophene-containing semiconductors of various shapes and structures are being investigated and their semiconducting performances evaluated in ordered mesophases (Figure 1). Charge mobilities higher than 10 cm 2 V -1 s - 1 , [25][26][27][28] have been achieved with various thiophene-based LC materials, even in solution process, [29,30] proving that they are expected to be promising materials to display good OTFTs device performances.

However, some economic constraints and environmental drawbacks need to be tackled and considered in this active field of research. First, from a synthetic view point, the chemistry of such organic semiconducting molecules is usually not straightforward, and often requires metal-catalysed coupling methods for connecting the different active parts through polluting, expensive, timeconsuming and sometimes low-yields synthetic procedures. [31] Thus, the easy access to an abundant precursor resource is always beneficial and cost-and time-saving in materials science, especially to carry out systematic studies on structure-property relationships. Secondly, the other fundamental prerequisite that drives the good functioning of the organic devices, and which has been extensively investigated in LC materials, is the ability to prepare high quality organic thin crystalline films (defectfree), with good molecular orientation, either as-deposited or after thermal annealing of the liquid crystal phase, leading to effective charge carrier transport in organic field-effect transistors (OFETs). [32][33][34][35][36][37][38] Thus, since they combine features of both crystalline and amorphous materials, the molecular design of novel LC semiconductors, supported by time-saving and efficient synthetic strategies, is highly desirable (self-organization, fluidity, and self-healing abilities, high control of the molecular orientation, one-or two-dimensional charge transport properties) for the discovery of new materials and the modulation of their physical properties. Yet, few works of LC materials consisting of a very simple benzothiophene system have been described in patents and literature, though it has already been proved versatile in the design of various mesogens. [39][40][41] Figure 1. Representative thiophene-based LC semiconductors reported in the literature.

In this work, a series of 2-phenylbenzo[b]thiophene (nPBT) mesogenic derivatives was synthesized in two easy steps and in good yields from abundant available starting material, i.e. 6-methoxy-2-(4methoxyphenyl)benzo[b]thiophene, which already possesses a natural rod-like and -conjugated core, prerequisite of LC phase formation and semiconducting behaviour. Further stepwise chemical oxidation of the benzothiophene moiety resulted in two additional sets of functional derivatives, namely the corresponding sulfoxide homologues (nPBTO) and one sulfone derivative (10PBTO2), which were also found to be mesomorphous. The liquid crystalline properties, photo-physical behaviour and charge transport properties were influenced greatly by adjusting the length of terminal alkyl chain and by the degree of oxidation of the benzothiophene moiety. All nPBT showed a SmA phase in addition to a low-temperature, high-ordered smectic phase, while only a SmA phase was observed for the oxidized derivatives nPBTO and 10PBTO2. The photo-physical properties, highly depending on the oxidation degree, displayed strong UV/Vis absorption and intense yellowish green luminescence, with high quantum yields, in the range of 38 ~ 62%. The charge transport carrier of three PBT compounds was also precisely measured in various mesophases as a function of chain-length by photocurrent TOF technique, and found to exhibit hole mobility rate of about 4.2  10 -3 cm 2 V -1 s -1 , in the low temperature ordered mesophase.

Results and discussion

Synthesis and characterization

The synthesis of the benzo[b]thiophene derivatives was straightforward, and is shown in Scheme 1. The dihydroxy compound 2 was prepared by demethylation of the readily available precursory 1 with pyridine hydrochloride at 190°C, and obtained as a white solid in almost quantitative yield (87%). Then, Williamson etherification of 2 with n-alkylbromide at both ends gave the desired nPBT derivatives (white crystalline solids) in 80-92% yields. The synthesis of the nPBTO derivatives was based on the general procedure described in literature, consisting of the oxidation of nPBT by addition of trifluoroacetic acid and a 30% aqueous solution of H2O2 at 0°C. [42] The sulfoxide compounds were obtained as yellow-green powders in yields ranging between 50 and 73%. The sulfone derivative 10PBTO2 could be directly obtained from 10PBT in 70% yield, by simply increasing the amount of trifluoroacetic acid and extending the reaction time. It is worth noting that as the alkyl chain-length was increased, the solubility of nPBT compounds deteriorated in chloroform, whereas compounds nPBTO and 10PBTO2 showed almost the same good solubility. The structures of all synthesized compounds were characterized by means of 1 H and 13

Mesmorphic properties

The mesomorphic behaviour of all compounds was investigated by polarized optical microscopy (POM, Figure S1) and differential scanning calorimetry (DSC, Figure S2, Table S1). From POM observations, all the phenylbenzothiophenes and the oxidized derivatives are liquid crystalline and solely show smectic phases consistent with their calamitic shape. The nPBT compounds exhibit all a high temperature smectic A (SmA), which was unequivocally recognized by the formation of smooth, focal conic, typical fan-shaped textures, as observed for all of them on slow cooling from the isotropic liquid (Figures 2 andS1). For two short homologues of this series, an additional phase was also detected by POM and DSC (Figures S1 andS2, Table S1), monotropic for 5PBT and enantiotropic for 6PBT, assigned as SmB by POM. On further cooling, another less fluid, higher-ordered smectic-like phase was detected, prior to crystallization, by an abrupt textural change with the transformation of the fans and focal conics to a spherulitic texture (Figures 2 andS2), which is reminiscent of a higher molecular re-organization within the layers. [43] Both nPBTO and 10PBTO2 showed a single SmA phase (Figure 2) recognized by the characteristic fan-shaped optical textures. All the transition temperatures were confirmed by DSC (Figure S2).

With the increase of the alkyl chain-length, the clearing temperature of the nPBT terms is decreased (Figure 3). The range of the SmA phase however expands gradually to ca. 25°C (maximum SmA range for 10PBT). Meanwhile, the interval of the low-temperature phase (referred to as SmX, see below) decreased at the expense of the expansion of the crystalline phase (Cr). This highly ordered phase has completely disappeared in the longest homolog 14PBT. The thermal behaviour of the next series is simpler, with the rapid enlargement of the SmA range with the increase of the alkyl chain length to reach a 40°C range for the longer homologues (maximum SmA range for 10PBTO), due essentially to a smooth decrease of the clearing temperatures (for n ≥ 8) and an average melting between 110-110°C. The isotropization temperatures of nPBTO are higher than of their corresponding nPBT homologs (n ≥ 8), and likely dependent on the stronger dipolar interactions in the latter. [44,45] The reason for the appearance of the highly ordered phase (SmX) in PBT series may be attributed to the presence of the aromatic-rich -electron system and a strong permanent dipole on the sulfur atom in the thiophene ring, which endows molecules with high anisotropic polarizability and occurrence of spontaneous polarization. [46][47][48] The electron-withdrawing ability of the sulfur atom in sulfoxide nPBTO is stronger than in nPBT resulting in the increase of the isotropization temperature. Further oxidation is detrimental to mesophase stability, which, when compared to the two other decyl derivatives, 10PBT and 10PBTO, is reduced down to 10°C only for 10PBTO2, between ca. 110-120°C. The smectic nature of the mesophases of all benzo[b]thiophene derivatives was confirmed by smallangle X-ray scattering (Figures 4 andS3, Table S2). On cooling from the isotropic liquid, in the upper temperature phase, most compounds show a single, intense fundamental reflection (001) along a few higher order peaks (002, 003, …), in the ratio 1:2:3:…, characterizing the lamellar arrangement of the molecules, along a diffuse scattering at high Bragg angles, corresponding to the molten chains (Figure 4, hch). The SAXS patterns of the nPBTO terms, of lesser quality (less peaks and of lower intensity) than those of the nPBT series and that of 10PBTO2, but still are in agreement with the formation of a smectic phase. Combined with POM, the mesophase can be readily assigned to as SmA in all cases. As expected, the d-spacings in the SmA increase almost linearly with the alkyl chain length (Figure 4) for both series nPBT and nPBTO, respectively, whereas the molecular area, obtained from the ratio between the molecular volume (Vmol) and the smectic periodicity (Table S2), do not greatly vary with n, but logically increase upon oxidation of the thiophene ring along the sequence nPBT (A  27.5-30Å 2 )  nPBTO (A  32.5-35Å 2 )  nPBTO2 (A  34.5Å 2 ). Upon further cooling, the SAXS patterns of the PBT terms revealed numerous sharp and equidistant (00l) reflections over the whole angular range (up to 13 for 14PBT, Figure S3) confirming the resilience of the long-range lamellar layering, with very sharp interfaces, which persists even in the lower temperature crystalline phase (Cr). The scattering halo corresponding to the molten chains is no more detectable on the SAXS patterns, suggesting that the chains are in a crystalized or pseudo crystalized state, and thus that the phase is not a true liquid crystalline phase. The supramolecular organizations within the SmX (standing for crystalline phase with a highly welldefined smectic structure) and crystalline (Cr) phases appear very similar to each other (similar spacing) and close to the SmA phase.

Photophysical Properties -UV/Vis Absorption and Photluminescence properties

The impact of the oxidation of the thiophene ring on the photophysical properties was evaluated in dilute THF solutions (10 -5 molL -1 ), and in thin films for all benzo[b]thiophene derivatives, nPBT, nPBTO and 10PBTO2. The absorption spectra recorded for solutions of nPBT exhibit two main absorption maxima at 240 and 320 nm, while for those of nPBTO and 10PBTO2 showed three strong absorption maxima centred at 235, 305 and 375 nm, and at 252, 312 and 373 nm, respectively (Figures 5 andS4). Solutions of nPBT showed a maximum emission at 365 nm with two shoulder peaks at 350 and 382 nm, and a double maximum emission in thin film, at ca. 372 and 393 nm. Emission spectra for the solutions of nPBTO exhibited one broad emission maximum at 477 nm while centred at 480 nm in thin film, whereas 10PBTO2 showed a similar emission maximum slightly shifted at 459 nm in THF solution and at 476 nm in thin film. It is worth noting that nPBTO showed yellowgreen light emission in THF and thin film under the illumination of UV light of 365 nm (Figure S4). Quantum yields calculated for nPBT were around 50%, slightly lower for nPBTO, at around 40%; 10PBTO2 exhibited the highest quantum yield with a value of 62% (Tables 1 andS3). The length of the alkyl chains has almost no influence on the absorption and emission spectra which are fully superimposable.

Table 1. Photo-physical properties of the three decyl representative benzo[b]thiophene derivatives. Quantum yields (QY) were measured with solution concentration of 510 -5 in THF excited at 310 nm (10PBT), at 365 nm (10PBTO) and at 350 nm (10PBTO2). The data for the other compounds are in Table S3.

Mesogens λabs (nm)

ε × 10

Charge transport properties

Time-of-flight (TOF) technique was used to investigate the charge carrier mobility of the various liquid crystalline materials. For this study, both the hole and negative charge mobilities of three presentative benzothiophene derivatives, nPBT with n = 6, 8, 10, were studied as a function of chain-length. The transient photocurrent decay curves obtained at different temperatures from the isotropic down to the crystalline phase showed reasonable shapes of decay. The textures of the three samples sandwiched in ITO cells, filled by capillary attraction from the isotropic liquid phase, were observed by POM of at various temperatures (Figures S6, S8 and S10). All of compounds exhibited a typical fanshaped texture in the SmA/B phase(s) as well as a spherulitic texture in the SmX phase. When the temperature was decreased from the isotropic to the crystalline phase, the POM images showed spherulitic texture with the presence of grain boundary on the substrate, which indicated that the molecules still retained the layer structure of the SmX phase in the crystalline phase. The charge carrier mobility was performed by TOF technique for the representative samples in a sandwich type ITO cell. The transient photocurrent measurements were performed on cooling at a very slow rate from the isotropic liquid to crystalline phase, at different temperatures with an applied electric field strength of 20000 ~ 50000 V cm -1 . The double-logarithm plot of positive transport decay curves for 6PBT, 8PBT and 10PBT, respectively, showed in Figure 6, enable the accurate read out of the transit time of charge carriers at various electric field, thus proving that the transient photocurrent curves of three mesogens are non-dispersive. With the order degree of mesophase raised on cooling, the transit time of charge carriers decreased for all samples (Figures S5, S7 and S9). The positive charge carrier mobility of three mesogens is shown in the cooling process without electric field dependence (Figure 6), while electron mobility was not observed. The hole transport of 6PBT

shows mobilities in the order of 2  10 -4 cm 2 V -1 s -1 in the isotropic liquid. In the SmA/SmB phase (150 o C) the mobility rate increases to 5.5  10 -4 cm 2 V -1 s -1 , followed by a second sharp increase up to 4.2  10 -3 cm 2 V -1 s -1 at the transition to SmX phase (142 o C). Meanwhile, the corresponding hole mobility values exhibited a slight temperature dependence both in the isotropic and smectic phases, it decreases very smoothly with the temperature falling. The charge carrier mobility in crystalline phase was not observed owing to cracks at domain boundaries. Similarly to 6PBT, the positive charge mobilities of 8PBT and 10PBT display lower values in the SmA phase than in the higher ordered SmX phase. The charge mobility of holes shows a slight dependence on the alkyl chain length, decreasing slowly with increasing length. These results are similar to several literature reported, [49] both in discotic and calamitic molecules. In contrast, TOF measurements for the sulfoxide (PBTO) and sulfone (PBTO2) derivatives did not yield any transit photo current.

The presence of this more ordered, low-temperature mesophase in the phase sequence, thus results in a significant improvement of the charge transport properties thanks to the pre-organizing templating effect of the SmA mesophase above, favoured by slow cooling.

[43]

Conclusion

We have successfully synthesized a series of benzothiophene-based liquid crystalline semiconductors and further oxidized derivatives from an abundant commercially available pharmaceutical precursor intermediate. The facile grafting of linear alkyl chains was indeed sufficient to promote the formation of smectic mesophases, showing the strong mesogenic character of this molecular species. The length of the alkyl chain and the degree of oxidation of the benzothiophene moiety were found to greatly influence the phase transition temperatures and thermal behaviour. Furthermore, the optical properties, emitting blue or yellow light with high quantum yields in the range of 38-62%, also are dramatically affected by the oxidation of the sulfur atom of the thiophene moiety. The charge carrier mobility of compound 6PBT measured by the photocurrent TOF technique revealed highest mobility range (about 4.2  10 -3 cm 2 V -1 s -1 ) in the SmX phase, depending strongly on the order degree of the mesophase. Therefore, the utilization of this available pharmaceutical intermediate to design organic semiconducting and luminescent functional materials shows high promising applicative potentials, providing some additional structural molecular adjustments. For instance, PBT compounds have potential applications in the field of organic field-effect transistors (OFETs), as the TOF mobility results have been demonstrated, while PBTO and PBTO2 derivatives can be used in fluorescent and lightemitting applications, as they emit blue and blue to green light, with mid-to-high quantum yields. The design of new mesogens based on 2-phenylbenzo[b]thiophene maybe be strongly improved by straightforward molecular derivatization, for example, through the substitution of the aromatic core by electron-donotaing or accepting groups, or by the extension of the molecular anisotropy and consequently enhanced  conjugation to boost the electronic properties. Moreover, the inclusion of such moieties into larger oligomeric, dendritic and polymeric structures would further facilitate their processing and incorporation into corresponding field-effect transistors and fluorescent sensors.

Supporting information

The file contains experimental techniques, synthetic details, POM images, DSC traces, SAXS patterns and indexation, UV-Vis and luminescence spectra, TOF tables, 1 H and 13 C NMR spectra and HMRS spectra. Supplementary data to this article can be found online at https://doi. org/10.1016/j.dyepig.2019.107964.

  C NMR (Figures S11), elemental analysis, and high resolution mass spectroscopy (HRMS, Figures S12).Details on the synthesis are given in the Supplementary Information.

Scheme 1 .

 1 Scheme 1. Synthesis yields and nomenclature of the benzo[b]thiophene derivatives (nPBT) and corresponding oxidized homologues (nPBTO and 10PBTO2). Reagents and conditions: (i) pyridine hydrochloride, reflux, 1 h, 190 o C (87%). (ii) n-Bromoalkane, K2CO3, 2-butanone, 24 h, 100 o C, (80-92%). (iii) Trifluoroacetic acid, anhydrous CH2Cl2, 30% H2O2, 2-6 h, 0 o C (50-73%). (iv) Excess of trifluoroacetic acid, anhydrous CH2Cl2, 30% H2O2, 8 h, 0 o C (70%).

Figure 2 .

 2 Figure 2. Representative POM images, observed on cooling from the isotropic liquid: clockwise from top left: 10PBT at 120 o C in SmA; 10PBT at 114 o C in SmX; 10PBTO at 145 o C in SmA; (d) 10PBTO2 at 77 o C in SmA.

Figure 3 .

 3 Figure 3. Bar graph summarizing the thermal behaviour of nPBT, nPBTO and 10PBTO2 derivatives (2nd heating).

Figure 4 .

 4 Figure 4. Two representative SAXS patterns (6PBT, top, and 10PBTO2, middle). Variation of the lamellar periodicity, d, and molecular area, A, with the alkyl chain length, n, for nPBT, nPBTO and 10PBTO2 derivatives (bottom).

Figure 5 .

 5 Figure 5. (a) Normalized absorption (straight lines) and emission spectra (dotted lines) in micromolar THF solution obtained for 10PBT, 10PBTO and 10PBTO2 (chosen as representative compounds); (b) Normalized emission spectra of thin films of compounds 10PBT, 10PBTO and 10PBTO2. (c) Images of the micromolar THF solutions (top panel) and thin films (bottom panel) of nPBTO (n = 4-14) under UV light (365 nm).

Figure 6 .

 6 Figure 6. Representative charge carrier mobility measured by the TOF technique. (Left) Electric field dependency of positive photocurrent decay curves (normal plot; Inset double-logarithm plot) at 120 o C (6PBT, with cell thickness is 17.7 m), 100 o C (8PBT, with cell thickness is 17.7 m) and 100 o C (10PBT, with cell thickness is 18.2 m), from top to bottom. (Right) Temperature dependence of hole mobility (positive charge carrier transports), measured in the different phases on cooling for 6PBT (cell thickness is 17.7 m), 8PBT (cell thickness is 17.7 m) and 10PBT (cell thickness is 18.2 m).

4 (L mol -1 cm -1 ) λem (nm) solution λem (nm) film

						QY [%]
	10PBT	238	1.28	350	374	50.8
		321	1.95	366	393
				382
	10PBTO	244	1.83	478	483	41.9
		313	160
		379	1.07
	10PBTO2	252	2.00	459	476	62.6
		312	1.61
		373	1.44

Acknowledgements This research was financially supported by the National Natural Science Foundation of China (Fund numbers: 51773140, 21772135). KQZ and HM thank the support of NSFC-JSPS joint project (50811140156). BD and BH thank CNRS and University of Strasbourg for support.

Conflict of interest

The authors declare no conflict of interest.


Karel Goossens

Lena Rakers

Benoît Heinrich

Guillermo Ahumada

Takahiro Ichikawa

Bertrand Donnio

Joo Tae

Christopher W Shin

Frank Bielawski

Glorius

Bielawski

Frank Glorius

Anisotropic, Organic Ionic Plastic Crystal Mesophases from Persubstituted Imidazolium Pentacyanocyclopentadienide Salts

niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

A wide variety of organic cations have been used to obtain ionic liquids (ILs) as "designer" solvents [START_REF]Ionic Liquids in Synthesis[END_REF][START_REF] Endres | Air and Water Stable Ionic Liquids in Physical Chemistry[END_REF][START_REF] Hallett | Room-Temperature Ionic Liquids: Solvents for Synthesis and Catalysis[END_REF] as well as to synthesize ionic liquid crystals (ILCs) expected to show anisotropic physical properties.(4-7) Besides other applications, both classes of materials have attracted widespread interest for their potential use as nonvolatile electrolytes in various electrochemical devices, such as lithium-ion or sodium-ion batteries, fuel cells, and dye-sensitized solar cells. [START_REF] Galiński | Ionic Liquids As Electrolytes[END_REF][START_REF] Armand | Ionic-Liquid Materials for the Electrochemical Challenges of the Future[END_REF][START_REF] Macfarlane | Energy Applications of Ionic Liquids[END_REF][START_REF] Watanabe | Application of Ionic Liquids to Energy Storage and Conversion Materials and Devices[END_REF][START_REF] Mcewen | Electrochemical Properties of Imidazolium Salt Electrolytes for Electrochemical Capacitor Applications[END_REF][START_REF] Wu | Electrolytes in Dye-Sensitized Solar Cells[END_REF][START_REF] Yoshio | One-Dimensional Ion-Conductive Polymer Films: Alignment and Fixation of Ionic Channels Formed by Self-Organization of Polymerizable Columnar Liquid Crystals[END_REF][START_REF] Yamanaka | Dye-Sensitized TiO2 Solar Cells Using Imidazolium-Type Ionic Liquid Crystal Systems As Effective Electrolytes[END_REF] 1,3-Disubstituted imidazoliums are emblematic cations which are among the most frequently selected partners for incorporation into ILs and ILCs. [START_REF] Lee | Functionalized Imidazolium Salts for Task-Specific Ionic Liquids and Their Applications[END_REF][START_REF] Douce | Imidazolium-Based Liquid Crystals: A Modular Platform for Versatile New Materials With Finely Tuneable Properties and Behaviour[END_REF][START_REF] Scalfani | Analysis of the Frequency and Diversity of 1,3-Dialkylimidazolium Ionic Liquids Appearing in the Literature[END_REF][START_REF] Bara | Beyond 1,3-Difunctionalized Imidazolium Cations[END_REF] The nature of the imidazolium substituents as well as the type of the anion selected can strongly impact the physical properties of the corresponding salts, including their melting and clearing temperatures, decomposition temperature, viscosity, and ionic conductivity. For example, fluorinated anions are known to increase thermal stability and reduce viscosity. [START_REF] Bonhôte | An archetypical example is the bis(trifluoromethylsulfonyl)imide (i.e., [N(SO2CF3)2]or [NTf2] -) anion; see, for example[END_REF] Likewise, halogen-free, cyano-substituted anions, such as [N(CN)2] -and [C(CN)3] -, have been reported to yield ILs that feature low viscosities and high ionic conductivities. [START_REF] Macfarlane | Low Viscosity Ionic Liquids Based on Organic Salts of the Dicyanamide Anion[END_REF][START_REF] Macfarlane | Ionic Liquids Based on Imidazolium, Ammonium and Pyrrolidinium Salts of the Dicyanamide Anion[END_REF] Moreover, some of us recently reported a series of [N(CN)2] -salts that self-organized into liquid-crystalline (LC) mesophases. [START_REF] Park | Dicyanamide Salts That Adopt Smectic, Columnar, or Bicontinuous Cubic Liquid-Crystalline Mesophases[END_REF] Although LC phases are often observed upon the attachment of long alkyl or fluoroalkyl chains, or mesogenic groups, to cationic species, [START_REF] Goossens | Ionic Liquid Crystals: Versatile Materials[END_REF][START_REF] Mansueto | Ionic Liquid Crystals[END_REF] the use of relatively short substituents commonly results in nonmesomorphic ILs or, in some cases, salts that adopt organic ionic plastic crystal (OIPC) phases. OIPCs constitute an emerging field of materials science and are intensively investigated as "solid" electrolytes for use in applications that require high ionic conductivities. [START_REF] Pringle | Organic Ionic Plastic Crystals: Recent Advances[END_REF][START_REF] Pringle | Recent Progress in the Development and Use of Organic Ionic Plastic Crystal Electrolytes[END_REF][START_REF] Macfarlane | Ionic Liquids and Their Solid-State Analogues As Materials for Energy Generation and Storage[END_REF][START_REF] Rana | Toward Protic Ionic Liquid and Organic Ionic Plastic Crystal Electrolytes for Fuel Cells[END_REF][START_REF] Basile | Ionic Liquids and Organic Ionic Plastic Crystals: Advanced Electrolytes for Safer High Performance Sodium Energy Storage Technologies[END_REF][START_REF] Jin | Structure and Transport Properties of a Plastic Crystal Ion Conductor: Diethyl(Methyl)(Isobutyl)Phosphonium Hexafluorophosphate[END_REF][START_REF] Luo | 1,2,4-Triazolium Perfluorobutanesulfonate As an Archetypal Pure Protic Organic Ionic Plastic Crystal Electrolyte for All-Solid-State Fuel Cells[END_REF] The plasticity of OIPC mesophases and their ability to flow under mechanical stress stem from structural disorder due to rotational or reorientational short-range motions of the cationic and/or anionic species that are otherwise part of a long-range correlated lattice. [START_REF] Macfarlane | Pyrrolidinium Imides: A New Family of Molten Salts and Conductive Plastic Crystal Phases[END_REF] The short-range dynamic disorder leads to enhanced translational motion for individual ions as well as to lattice vacancies. [START_REF] Jin | Conformational Dynamics in an Organic Ionic Plastic Crystal[END_REF] Whereas N-alkyl-N-methylpyrrolidinium cations have been extensively used to obtain OIPC phases, examples of imidazolium-based plastic crystals are scarce. [START_REF] Lee | 1,2-Bis[N-(N′alkylimidazolium)]ethane salts: a new class of organic ionic plastic crystals[END_REF][START_REF] Kidd | Cation and Anion Transport in a Dicationic Imidazolium-Based Plastic Crystal Ion Conductor[END_REF][START_REF] Lee | Bis-Imidazolium Iodide Organic Ionic Plastic Crystals and Their Applications to Solid State Dye-Sensitized Solar Cells[END_REF][START_REF] Yoshizawa-Fujita | A New Class of Proton-Conducting Ionic Plastic Crystals Based on Organic Cations and Dihydrogen Phosphate[END_REF][START_REF] Luo | Imidazolium Methanesulfonate As a High Temperature Proton Conductor[END_REF][START_REF] Chong | Enhancing Solid-State Conductivity Through Acid or Base Doping of Protic Imidazolium and Imidazolinium Triflate Salts[END_REF][START_REF] Zhu | Structure and Ion Dynamics in Imidazolium-Based Protic Organic Ionic Plastic Crystals[END_REF] MacFarlane and co-workers attributed the lower tendency of imidazolium salts to adopt OIPC phases to charge delocalization which ultimately weakens interionic interactions. [START_REF] Macfarlane | Structural Studies of Ambient Temperature Plastic Crystal Ion Conductors[END_REF] Consequently, short-chain-substituted imidazolium salts often melt to a liquid state before a stable plastic crystal phase is formed.

In search of new ionic compounds and molecular architectures that adopt LC or OIPC phases, or that behave as ILs at reasonably low temperatures, we considered salts with 1,2,3,4,5-pentaalkyl-1Himidazol-3-ium ([(Cn)5im] + ) cations. Although the influence of installing alkyl or aryl substituents in the 2-positions of imidazoliums on the thermal characteristics of the corresponding ILs and ILCs has been explored, [START_REF] Fox | Flammability, Thermal Stability, and Phase Change Characteristics of Several Trialkylimidazolium Salts[END_REF][START_REF] Mukai | Self-Organization of Protonated 2-Heptadecylimidazole As an Effective Ion Conductive Matrix[END_REF][START_REF] Yoshio | Columnar Liquid-Crystalline Imidazolium Salts. Effects of Anions and Cations on Mesomorphic Properties and Ionic Conductivity[END_REF][START_REF] Kouwer | Synthesis and Mesomorphic Properties of Rigid-Core Ionic Liquid Crystals[END_REF][START_REF] Endo | Effects of Methylation at the 2 Position of the Cation Ring on Phase Behaviors and Conformational Structures of Imidazolium-Based Ionic Liquids[END_REF][START_REF] Goossens | T-Shaped Ionic Liquid Crystals Based on the Imidazolium Motif: Exploring Substitution of the C-2 Imidazolium Carbon Atom[END_REF][START_REF] Nestor | Methimazolium-Based Ionic Liquid Crystals: Emergence of Mesomorphic Properties Via a Sulfur Motif[END_REF] peralkylated derivatives (i.e., [(Cn)5im][X] with n ≥ 2 ) have not yet been reported in detail.(48,49) Persubstitution was envisioned to result in a protective hydrophobic shell around the cationic heterocycle and to facilitate long-range molecular ordering, particularly in combination with planar counterions. Recently, some of us prepared 4,5-dialkylsubstituted imidazoles with relatively long alkyl substituents by utilizing Radziszewski-type chemistry as a key step. The corresponding 1,3dimethylimidazolium salts were obtained upon methylation and applied as lipid analogues in model cell systems or used as precursors for N-heterocyclic carbenes for the stabilization of nanoparticles. [START_REF] Richter | Tailor-Made N-Heterocyclic Carbenes for Nanoparticle Stabilization[END_REF][START_REF] Wang | A Remarkably Simple Class of Imidazolium-Based Lipids and Their Biological Properties[END_REF][START_REF] Wang | Anti-Tumor Activity and Cytotoxicity in Vitro of Novel 4,5-Dialkylimidazolium Surfactants[END_REF][START_REF] Rühling | Influence of the Headgroup of Azolium-Based Lipids on Their Biophysical Properties and Cytotoxicity[END_REF][START_REF] Drücker | Imidazolium Salts Mimicking the Structure of Natural Lipids Exploit Remarkable Properties Forming Lamellar Phases and Giant Vesicles[END_REF] We also reported 1,2,3-trimethyl-4,5-bis(n-pentadecyl)imidazolium iodide to be the first example of a LC pentasubstituted imidazolium salt. [START_REF] Goossens | Substituted Azolium Disposition: Examining the Effects of Alkyl Placement on Thermal Properties[END_REF] We describe here how the previously reported synthetic methodology may be extended to obtain [(Cn)5im][X] salts that feature alkyl substituents larger than a methyl group (i.e., n-C7H15, n-C11H23 or n-C15H31). Derivatives with planar nitrate ([NO3] -) or "π-electronic" pentacyanocyclopentadienide ([Cp(CN)5] -) anions (Scheme 1) were subsequently prepared from the halide salts by anion metathesis. The thermal characteristics of the aforementioned compounds as well as those of permethylated homologues were also examined. During the course of these studies, it was discovered that [(C1)5im][Cp(CN)5] adopted an unprecedented, anisotropic OIPC mesophase and was found to exhibit high ionic conductivity values, especially after doping with a Na + salt. [Cp(CN)5] -was identified as a stable and compact mesogenic building block that maintains a "charge-segregated" stacking, even at elevated temperatures. The following discussions and implications of the findings will be focused accordingly.

Thermal Characteristics and Structural Elucidation of Crystal and Plastic Crystal Phases

The thermal characteristics of the synthesized pentaalkylimidazolium salts were examined using thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and polarized-light optical microscopy (POM). The key results are summarized in Table 1 (see also Figures S8-S23). The decomposition temperatures of [(C1)5im][X] salts were measured to be significantly higher than those of the homologues with longer alkyl chains and decreased in the following order: [Cp(CN)5] -> [NO3] -> I -. None of the amphiphilic salts 5-X-n (X -= Br -, [NO3] -, and [Cp(CN)5] -; n = 7, 11, and 15) were found to adopt LC mesophases. The compounds melted directly to isotropic liquids without passing through intermediate LC phases and, in the case of [Cp(CN)5] -salts, were found to be viscous liquids at ambient temperature. Likewise, variable temperature, small-to wide-angle X-ray scattering (SWAXS) measurements on powder samples of 5-X-n (X -= Br -, [NO3] -; n = 11 and 15) revealed that the pristine samples were solids of low crystallinity and that cooling from the melts yielded even lower crystallinities because of slow crystallization kinetics (Figures S36-S39). We refer to Section S8 in the Supporting Information for a comparison with the thermal characteristics of long-chainsubstituted 1-alkyl-3-methylimidazolium ([Cnmim] + ) and 1,3-dialkylimidazolium ([(Cn)2im] + ) salts and for an in-depth discussion of the effect of symmetric pentasubstitution on the mesomorphic properties of imidazolium salts. The salts 5-I-1 and 5-NO3-1 also did not exhibit mesophases based on POM and DSC observations. Significantly different results were obtained upon analysis of 5-Cp(CN)5-1. DSC measurements revealed two pronounced thermal events during heating and cooling of the salt (Figure S16) apart from a small enthalpy change due to a crystal-to-crystal transition (Cr1 → Cr2, see Table S2). POM studies revealed that 5-Cp(CN)5-1 melts to a plastic, birefringent, enantiotropic mesophase at ∼234 °C before clearing to an isotropic liquid at ∼240 °C. The observed textures indicated that a columnar type of mesophase formed with a pronounced tendency toward homeotropic alignment (Figures 1 andS1-S5). a See Table S2 for additional data and information. b Abbreviations: Cr = crystalline phase; g = glass; S, S1, and S2 = solid phase(s) (not or only partially crystalline); Mhex = plastic crystal mesophase with 2D hexagonal symmetry; and Iso = isotropic liquid phase. c Onset temperatures obtained by DSC during the second heating run at a rate of 10 °C min -1 and under an atmosphere of N2. A small hole was pierced into the lid of the DSC sample pans. d Enthalpy change. e Entropy change. f Temperature at which 1% weight loss was measured by TGA at a rate of 5 °C min -1 and under an atmosphere of N2 (neglecting initial small weight losses attributed to the release of H2O). g Peak temperature. h An accurate value forthe entropy of fusion, ΔSfus, could not be obtained because of the overlap between transitions. S5). The single-crystal XRD results for the Cr1 phase of 5-Cp(CN)5-1 were used to derive the orthorhombic unit cell parameters of its Cr2 phase from the SWAXS powder pattern that was recorded at 200 °C (Figure S33 and Table S7). The cross-sectional area Scol of one column of stacked [Cp(CN)5] -anions along the a-axis surrounded by [(C1)5im] + cations in the Cr2 phase was calculated as (b × c)/4 = 126.6 Å 2 . The thickness of one columnar slice was estimated as hmol = Vmol/Scol = a/4 = 3.94 Å (where Vmol is the molecular volume). The results were then compared to the geometric parameters of the thermotropic mesophase that were derived from synchrotron-based SWAXS measurements at 240 °C.

In addition to several broad scattering signals, the SWAXS pattern contained four sharp reflections characterized by an inverse d-spacing ratio of 1:√3:2:√7 (Figure 3 and Table S8). The signals were indexed as the [START_REF] Macfarlane | Energy Applications of Ionic Liquids[END_REF], [START_REF] Watanabe | Application of Ionic Liquids to Energy Storage and Conversion Materials and Devices[END_REF], [START_REF] Bonhôte | An archetypical example is the bis(trifluoromethylsulfonyl)imide (i.e., [N(SO2CF3)2]or [NTf2] -) anion; see, for example[END_REF], and (21) reflections of a two-dimensional (2D) hexagonal lattice with a lattice parameter ahex ≈ 12.31 Å; hence, the columnar mesophase was designated as Mhex. The 2D lattice area Scol corresponded to (√3/2)ahex 2 ≈ 131.2 Å 2 and hmol to Vmol/Scol = 4.1(±0.2) Å (Vmol was estimated to be 536(±20) Å 3 from reference dilatometric data).

Figure 3. Synchrotron-based SWAXS pattern that was recorded for the Mhex mesophase of 5-Cp(CN)5-1 at 240 °C (the X-ray wavelength used was 1.00 Å). See also Table S8 in the Supporting Information.

The geometric parameters described above indicated that the local molecular arrangements in the crystal phases and the mesophase are similar, with the differences attributable to thermal expansion (see also Section S6.2 in the Supporting Information). In the mesophase, the [Cp(CN)5] -anions are πstacked into columns, albeit in an irregular manner as evidenced by the broad hπ scattering signal centered at ∼3.6 Å (Figure 3). Heating above the melting point also reduces the correlation between the respective, tilt-induced (c.f., hmol > hπ) orientations of the [Cp(CN)5] -stacks and the relative positions of the surrounding [(C1)5im] + cations. This process is accompanied by the onset of rotation of the [Cp(CN)5] -stacks. The disorder causes the formation of columns with an average cylindrical shape and facilitates merging of the peripheral cations to form a continuum (Figure 4). The broad scattering signal hion, centered at ∼5.9 Å (Figure 3), arises from the average, close-packed distances between the [(C1)5im] + cations that have lost long-range positional order. The resulting time-and space-averaged hexagonal symmetry and organization, as observed by SWAXS, is similar to that of hexagonal columnar LC mesophases (Colhex) adopted by classical discotic mesogens that consist of planar aromatic cores with peripheral alkyl chains, in which case the molten alkyl chains form a continuum. [START_REF] Wöhrle | Discotic Liquid Crystals[END_REF] Despite the structural similarity to Colhex phases, the mesophase adopted by 5-Cp(CN)5-1 is not a liquid crystal phase that involves molten chains but rather an ionic plastic crystal (OIPC) phase for which the mesomorphism originates from dynamic disorder. Indeed, 5-Cp(CN)5-1 solely consists of cations and anions that lack flexible chains and have a defined conformation because of their rigid structure (except for the ability of the methyl groups in the [(C1)5im] + cations to rotate about their C-C or C-N bond axes( 61)). In OIPC phases, the individual cations and/or anions typically rotate around the lattice points of a three-dimensional (3D) lattice, leading to an isotropic or weakly anisotropic arrangement of quasi-globular objects. In contrast, the OIPC phase of 5-Cp(CN)5-1 features a 2D, and thus intrinsically anisotropic, lattice resulting from the rotation of entire ionic assemblies around a columnar axis. The molecular arrangements in the crystalline solid phases, featuring approximately cylindrical assemblies of stacked anions "wrapped" by cations, prefigure the aforementioned motions in the OIPC phase. We note that 5-Cp(CN) Although the structure of the Mhex OIPC phase is, to the best of our knowledge, unprecedented, comparison to nonionic systems may be valuable. For instance, the so-called "rotator phases" that are adopted by long-chain n-alkanes in certain temperature ranges in which they are still mainly in their "stiff", all-trans conformations also consist of units that rotate about parallel axes, which, in those cases, correspond to the chain backbones. [START_REF] Sirota | Rotator Phases of the Normal Alkanes -An X-Ray-Scattering Study[END_REF] In the columnar mesophases that were reported by Ros and co-workers, the rotating units are columns of π-stacked indene-and pseudoazulene-based aromatic cores that are arranged in a 2D hexagonal sublattice. [START_REF] Barberá | Breaking the Mold of Discotic Liquid Crystals[END_REF][START_REF] Basurto | Indene and Pseudoazulene Discotic Liquid Crystals: A Synthetic and Structural Study[END_REF] The molecules did not contain peripheral chains but featured polarizable atoms (halogens, S) and/or polar groups (-C≡N, >C═O) that facilitated stacking in the solid state, which was proposed to be a prerequisite for the formation of soft columnar mesophases, and assisted in decorrelating the neighboring columnar stacks at higher temperatures to form the plastic phases. [START_REF] Barberá | Breaking the Mold of Discotic Liquid Crystals[END_REF][START_REF] Basurto | Indene and Pseudoazulene Discotic Liquid Crystals: A Synthetic and Structural Study[END_REF] Compound 5-Cp(CN)5-1 also exhibits solid-state molecular stacking, albeit only with regard to the planar anions. In contrast, the nonmesomorphic iodide salt 5-I-1 lacks a columnar stacking motif in the crystalline solid state (Figures 5 andS25-S27).( 72) Collectively, these results identify [Cp(CN)5] -as a stable and compact mesogenic building block as it appears to maintain a so-called "charge-segregated" stacking, even at elevated temperatures.( 73 Finally, the thermodynamic parameters of the aforementioned phase transitions were evaluated. The entropy of fusion, ΔSfus, that was measured for 5-Cp(CN)5-1 (23 J K -1 mol -1 , see Table 1) is significantly lower than the upper limit of 40 J K -1 mol -1 for OIPCs that was proposed by MacFarlane and co-workers. [START_REF] Macfarlane | Pyrrolidinium Imides: A New Family of Molten Salts and Conductive Plastic Crystal Phases[END_REF][START_REF] Macfarlane | Structural Studies of Ambient Temperature Plastic Crystal Ion Conductors[END_REF] The measured ΔSfus value even approaches Timmermans' original criterion of ΔSfus < 20 J K -1 mol -1 for plastic crystal behavior.( 76

Ionic Conductivity Displayed by Compound 5-Cp(CN)5-1 and Its Mixtures with Na[Cp(CN)5]

Because the Mhex mesophase described above featured structural characteristics that were potentially conducive for anisotropic ion conduction, a series of ionic conductivity measurements were performed. The analyses were done upon cooling over a range of temperatures using the alternating current impedance method in conjunction with a sample cell equipped with comb-shaped gold electrodes. [START_REF] Yoshio | Layered Ionic Liquids: Anisotropic Ion Conduction in New Self-Organized Liquid-Crystalline Materials[END_REF][START_REF] Ichikawa | Induction of Thermotropic Bicontinuous Cubic Phases in Liquid-Crystalline Ammonium and Phosphonium Salts[END_REF] The pure salt 5-Cp(CN)5-1 was measured first and, as shown in Figure 6, an Arrhenius plot was constructed using the recorded data. The ionic conductivity in the Mhex phase (σ = 2.6 to 3.0 × 10 -5 S cm -1 ) was determined to be comparable to that in the isotropic liquid phase. This indicates that the formation of a long-range, anisotropic 2D lattice in the former enables a mechanism of effective ion conduction that compensates for the higher viscosity of the mesophase. Evaluation of the data yielded an activation energy, Ea, of 19 kJ mol -1 for ion conduction in the Mhex phase, which was approximately two times lower than the Ea value that was calculated for the isotropic liquid phase (∼42 kJ mol -1 ). The results indicate a relatively low energy barrier to ion mobility in the Mhex phase, in accordance with the finding that the constituent ions are not engaged in a long-range 3D lattice. respectively. The substantial increase in ionic conductivity upon addition of the Na + salt, by a factor of 8, indicates that conductivity values in the mS cm -1 range may be realized along with potential utility as anisotropic electrolytes in sodium-ion batteries.

Conclusions

We report the synthesis of a series of pentaalkylimidazolium salts as well as studies of their thermal characteristics and supramolecular organization. None of the amphiphilic compounds with relatively long alkyl chains and Br -, [NO3] -, or [Cp(CN)5] -counterions appear to be LC, although they are all lowmelting salts. The amphiphilic [Cp(CN)5] -salts are ILs below room temperature. In contrast, [(C1)5im][Cp(CN)5] was found to self-organize into an anisotropic, plastic crystal mesophase upon heating. The phase features rotating columnar stacks of [Cp(CN)5] -anions that are arranged in a 2D lattice and positioned within a "continuum" formed by molten [(C1)5im] + cations that play the role of disorderly components. This binary system represents an original type of OIPC phase, which holds potential for use in electrochemical applications that require anisotropic ion conduction. Indeed, preliminary data show that high ionic conductivities can be achieved in the OIPC phase, particularly after doping with a constituent Na + salt. In a broader perspective, the discovery represents a new paradigm in the field of OIPCs and establishes new design parameters for preparing highly ionconductive, anisotropic electrolytes with tunable thermal characteristics. The results pertain to various domains of materials science, ranging from ILs through ionic plastic crystals and liquid crystals to the self-assembly of matter.

Future efforts may explore a systematic structure-property relationship of the [Cp(CN)5] -salts in order to identify the molecular requirements for the emergence of the OIPC mesophase. Various structural variations of the imidazolium motif, including changes leading to symmetry reduction, should be synthetically accessible, such as modifications of (i) the number and positions of methyl groups on the imidazolium rings (i.e., [(H)x(C1)5-xim] + , where x = 2-5) or (ii) the nature and disposition of substituents on the imidazolium cores (i.e., [(R)x(C1)5-xim] + , where x = 2-3, and R = -C≡N, -Cl, ..., or [(R1)x(R2)5-xim] + , where x = 2-3). The former materials may find utility in OIPC applications that require anisotropic proton conductivity. The latter can be expected to exhibit tunable thermal phase characteristics and surface activities in a manner that depends on the packing complementarities between the [Cp(CN)5] -anions and the modified imidazolium cations.

Methods

General Procedures

Nuclear magnetic resonance (NMR) spectra were recorded at ambient temperature (22.0(±1.0) °C, unless stated otherwise) on a Bruker ARX-300 spectrometer (operating at 300 MHz for 1 H), a Bruker AV-300 spectrometer (operating at 300 MHz for 1 H), a Bruker AV-400 spectrometer (operating at 400 MHz for 1 H), or a Bruker AVANCE III HD spectrometer (operating at 400 MHz for 1 H). Fourier transform infrared (FT-IR) spectra were recorded in attenuated total reflectance (ATR) mode on an Agilent Cary 630 FT-IR spectrometer using a germanium crystal (Ge-ATR). Highresolution mass spectra (HRMS) were recorded in electrospray ionization (ESI) mode on a Bruker Daltonics MicroTof mass spectrometer, a Thermo Scientific LTQ Orbitrap XL mass spectrometer, or a Waters Xevo G2-XS QTof mass spectrometer. Elemental analyses (carbon, hydrogen, nitrogen, and sulfur) were performed using a Thermo Scientific Flash 2000 Organic Elemental Analyzer. Optical textures were observed using an Olympus BX53-P polarized-light optical microscope that was equipped with a rotatable graduated sample platform and an Instec HCS402 dual heater temperature stage. The latter was equipped with a precision XY positioner and coupled to an Instec LN2-SYS liquid nitrogen cooling system and an Instec mK2000 programmable temperature controller. Images were recorded with a QImaging Retiga 2000R CCD camera that was coupled to the microscope via a C mount. DSC data were recorded under nitrogen (50 mL min -1 ) on a TA Instruments DSC Q2000 module equipped with an RCS90 cooling system at a heating rate of 10 °C min -1 and a cooling rate of 5 °C min -

1

. The quantity of sample analyzed was typically 4-5 mg. A small hole was pierced into the lid of the aluminum sample pans. TGA data were recorded under nitrogen (60 mL min -1 ) on a TA Instruments TGA Q500 module at a heating rate of 5 °C min -1 and using a platinum sample pan. The quantity of sample analyzed was typically 5-8 mg. Simultaneous TGA-DSC measurements were performed using a TA Instruments SDT Q600 module at a heating rate of 10 °C min -1 and under nitrogen (100 mL min - 1

). An alumina sample pan was used. The quantity of sample analyzed was approximately 6.5 mg. To determine the crystal structures of compounds 5-I-1 and 5-Cp(CN)5-1, X-ray intensity data were collected at 123 K on a Rigaku XtaLAB P200 diffractometer equipped with a Pilatus 200K detector, using ω scans and Cu Kα radiation (wavelength λ = 1.54187 Å). See Section S6 in the Supporting Information for further details. Most of the SWAXS patterns were obtained with a transmission Guinier-like geometry. A linear focalized monochromatic Cu Kα1 beam (λ = 1.5405 Å) was obtained using a sealed tube generator (600 W) equipped with a bent quartz monochromator. In all cases, the pristine powder was filled in Lindemann capillaries of 1 mm diameter and 10 μm wall thickness or in homemade sealed cells of 1 mm path length and equipped with 11 μm-thick aluminum windows. The diffraction patterns were recorded with a curved Inel CPS120 counter gas-filled detector linked to a data acquisition computer (periodicities up to 90 Å) and on image plates scanned by Amersham Typhoon IP with 25 μm resolution (periodicities up to 120 Å). The sample temperature could be varied between 20 and 200 °C with a precision of ±0.01 °C, and exposure times varied from 1 to 5 h. Synchrotron-based X-ray scattering measurements of the mesophase adopted by compound 5-Cp(CN)5-1 were performed at the PLS-II 6D UNIST-PAL Beamline of the Pohang Accelerator Laboratory (PAL), Pohang, Republic of Korea. The X-rays coming from the bending magnet were monochromated using Si(111) double crystals and focused at the detector position by the combination of a second, sagittal-type monochromator crystal and a toroidal mirror system. The diffraction patterns were recorded by a Rayonix MX225-HS 2D CCD detector (225 × 225 mm 2 square active area, full resolution 5760 × 5760 pixels) with 2 × 2 binning. The peak positions in the one-dimensional intensity profiles, which were obtained from azimuthal averaging of the 2D patterns, were used for phase-type assignments. SWAXS patterns were recorded using 12.3984 keV X-ray radiation (wavelength λ = 1.00 Å) and a sample-to-detector distance of ca. 436 mm. Diffraction angles were calibrated using a lanthanum hexaboride (LaB6) standard (NIST SRM 660c). Samples were contained in a borosilicate glass (glass #50) capillary with an outer diameter of 0.8 mm and a wall thickness of 10 μm and were irradiated for 5-10 s per measurement, depending on the saturation level of the detector. The capillaries were inserted into a custom-made brass holder that was placed into a Linkam TS1500V temperature stage to achieve temperature control. The samples were allowed to equilibrate at each temperature before starting a measurement. Dynamic ionic conductivity data were recorded with a Solartron (Schlumberger) SI 1260 impedance/gain-phase analyzer at a cooling rate of 2 °C min -1 , using a frequency range of 100 Hz to 20 MHz and a voltage of 0.6 V. The temperature was controlled using a Mettler-Toledo HS82 dual heater temperature stage that was coupled to a Mettler-Toledo HS1 programmable temperature controller. A glass sample cell with comb-shaped gold electrodes with thicknesses of 0.8 μm and spacings of 300 μm was used for all the measurements. The cell constant was determined using an aqueous KCl solution (0.01 M). Mixtures of 5-Cp(CN)5-1 and Na[Cp(CN)5] were prepared by slowly evaporating acetonitrile solutions of the two components followed by heating the samples to their isotropic liquid states.

Synthetic Procedures

The reactions described below were performed in flame-dried or oven-dried glassware under an atmosphere of argon, unless specified otherwise. The given reaction temperatures correspond to the temperatures of the preheated oil baths that surrounded the reaction vessels. Unless specified otherwise, all reagents were purchased from commercial sources (Sigma-Aldrich, Alfa Aesar, Acros Organics, TCI Europe, and TCI Japan) and used without further purification. Anhydrous tetrahydrofuran (THF) was obtained by distillation in a continuous still under an atmosphere of argon over Na/benzophenone as the drying agent. The other anhydrous solvents were purchased from Carl Roth or Sigma-Aldrich and stored over molecular sieves under an atmosphere of argon. Anhydrous triethylamine was purchased from Sigma-Aldrich, distilled, and stored under an atmosphere of argon. The solvents that were used for column chromatography were of technical grade and were flashdistilled prior to use. During the syntheses of the precursors and the halide salts, reaction progress was monitored by thin-layer chromatography (TLC) on precoated, aluminum-backed plates (Merck Kieselgel 60 F254). TLC spots were visualized by irradiation with a UV lamp and by immersion into a KMnO4 stain. Purification by flash column chromatography was conducted using silica gel (60 Å, 35-70 μm, Kieselgel, Acros) with a head pressure of argon (0.1-0.3 atm). -imidazole] = 0.3 M) in a flame-dried Schlenk pressure tube. The reaction mixture was stirred for 3 days at 80 °C under an atmosphere of argon. After cooling to r.t., the mixture was diluted with water. The resulting biphasic system was extracted with dichloromethane (3 × 20 mL per mmol). The combined organic layers were dried over anhydrous MgSO4. After filtration, the solvent was removed under reduced pressure. The residue was purified by flash column chromatography (eluent: dichloromethane/methanol 100:0 → 93:7 v/v). To increase the isolated yield of pure product, the column fractions that contained a mixture of 1,2,4,5-tetraalkyl-1H-imidazole and 1,2,3,4,5-pentaalkyl-1H-imidazol-3-ium bromide were concentrated and resubjected to the aforementioned reaction conditions. See Section S1.2.2 in the Supporting Information for further details and analytical data.

Synthesis of 5-NO3-1

The following steps were performed in the dark. A solution of compound 5-I-1 (3.86 mmol, 1.0279 g) in 10 mL of water was slowly added to a solution of silver nitrate (3.86 mmol, 0.6561 g) in 15 mL of water under stirring. The reaction mixture was stirred for 1.5 h at r.t. Insoluble products were filtered using a paper filter. The filtrate was concentrated under reduced pressure at 40 °C. The residue was redissolved in 20 mL of ethanol, and the solution was filtered using a PTFE syringe filter (0.1 μm pore size). After removing the solvent under reduced pressure at 40 °C, the residue was redissolved in 10 mL of dichloromethane and the solution was kept in a freezer overnight. Although visually there were no signs of precipitation, the solution was filtered using a PTFE syringe filter (0.1 μm pore size). Finally, the solution was concentrated and dried under reduced pressure at 60 °C to afford the desired product as a white powder. Small amounts of the product were dissolved in water and tested with NaBr or AgNO3; the lack of a precipitate indicated that AgNO3 and the iodide salt had been successfully removed. Yield: 95% (0.737 g ). Subsequently, 30 mL of dichloromethane was added and the mixture was washed with 10 mL of water. The organic layer was separated and concentrated under reduced pressure at 40 °C. The residue was further dried overnight under reduced pressure at 40 °C, after which it was redissolved in 2.5 mL of dichloromethane. The solution was kept in a freezer overnight. Although visually there were no signs of precipitation, the solution was filtered using a PTFE syringe filter (0.1 μm pore size). Finally, the solution was concentrated and dried under reduced pressure at 50 °C. See Section S1.2.4 in the Supporting Information for further details and analytical data.

Scheme 1 .

 1 Scheme 1. Examples of 1,2,3,4,5-Pentaalkyl-1H-imidazol-3-ium Salts with Halide (X -= Br -and I -), [NO3] -, or [Cp(CN)5] -Anions

Figure 1 .

 1 Figure 1. POM images of the Mhex phase of 5-Cp(CN)5-1 at 239 °C. (Bottom) (a) Picture taken immediately after pressing the sample with a needle. (b-d) Pictures showing the further evolution of the texture over the course of 40 s during which time no pressure was applied to the sample [the same sample area is shown in images (a-d)]. All images were recorded using crossed polarizers.

Figure 2 .

 2 Figure 2. (Top) Packing in the crystal structure of 5-Cp(CN)5-1, showing the columnar stacking of the [Cp(CN)5] -anions and the linkage of the molecules through nonclassical C-H•••N hydrogen bonds (blue dotted lines). (Bottom) View along the b-axis, showing the ring-to-ring interactions between [Cp(CN)5] -anions (red dotted lines). See also Section S6.2 in the Supporting Information.

Figure 4 .

 4 Figure 4. Schematic view of the supramolecular packing in the Mhex mesophase of 5-Cp(CN)5-1. The 2D unit cell parameter is indicated by ahex. The phase features rotating columnar stacks of [Cp(CN)5] -anions that are arranged in a hexagonal lattice and positioned within a continuum formed by molten [(C1)5im] + cations that play the role of disorderly components. In classical LC Colhex mesophases, such a continuum is composed of molten chains.

)Figure 5 .

 5 Figure 5. Packing in the crystal structure of 5-I-1, showing the absence of a columnar stacking motif. The molecules are linked through nonclassical C-H•••N (blue dotted lines) and C-H•••I (purple dotted lines) hydrogen bonds as well as hydrogen-to-ring ("C-H•••π") interactions (red dotted lines). See also Section S6.1 in the Supporting Information.

  ) Based on this criterion, the entropy data support the hypothesis that the Mhex phase of 5-Cp(CN)5-1 involves motions of the [(C1)5im] + cations as well as the [Cp(CN)5] -anions.(31,40) The relatively small ΔSfus values of compounds 5-Br-7 and 5-Cp(CN)5-11 can be ascribed to incomplete crystallization during the DSC measurements.

Figure 6 .

 6 Figure 6. Dynamic ionic conductivity data (Arrhenius plots) recorded for 5-Cp(CN)5-1 and for mixtures of 5-Cp(CN)5-1 and Na[Cp(CN)5] (molar ratios: 0.975:0.025 and 0.95:0.05). The thermal phase transitions are indicated by dashed lines.

4. 2 . 7 .

 27 General Method for the Synthesis of 5-Cp(CN)5-n (n = 7, 11, 15) Under stirring, a solution of the appropriate bromide salt 5-Br-n (1 equiv) in dry acetonitrile ([5-Br-n] = 0.03 M) was slowly added to a solution of sodium pentacyanocyclopentadienide(57) (1 equiv) in dry acetonitrile ([Na[Cp(CN)5]] = 0.07 M). The reaction mixture was stirred for 1 h at r.t. (in the case of 5-Cp(CN)5-7 and 5-Cp(CN)5-11) or at 65-70 °C (in the case of 5-Cp(CN)5-15

Table 1 . Phase Transition Temperatures and Thermal Data Recorded for the Pentaalkylimidazolium Salts 5-X-n a compound transition b T (°C) c ΔH (kJ mol -1 ) d ΔS (J K -1 mol -1 ) e T1% (°C)f 5-I-1

 1

		Cr → Iso	214 g	22.7	∼47	∼231
	5-Br-7	S → Iso a	46 g	1.8	∼6	∼188
	5-Br-11	S → Iso a	40, 54, 59 a	42.0a	h	∼183
	5-Br-15	S → Iso a	78, 81a	82.8 a	h	∼193
	5-NO3-1	Cr → Iso	154 g	13.2	∼31	∼267
	5-NO3-7	S1 + Iso → S2 a	-8 g	-12.3	∼-46	∼175
		S2 → S1 + Iso a	19 g	11.4	∼39
		S1 + Iso → Iso a	45	1.1	3
	5-NO3-11	S → Iso	52 g	33.1	∼102	∼185
	5-NO3-15	S → Iso	72 g	77.6	∼225	∼204
	5-Cp(CN)5-1	Cr → Mhex	234	16.4	32	∼369
		Mhex → Iso	240	11.9	23
	5-Cp(CN)5-7	g → Iso	14	-	-	∼192
	5-Cp(CN)5-11	S → Iso	-6	9.7	36	∼175
	5-Cp(CN)5-15	S → Iso	16	92.9	321	∼221

  4.2.1. General Method for the Synthesis of Imidazole Precursors 4-n(n = 7, 11, and 15) 1,2-Diketone 3n (1 equiv), aldehyde 1-n (1.2 equiv), and NH4OAc (2.4 equiv) were dissolved in dry ethanol ([1,2diketone] = 0.5 M) in a flame-dried Schlenk pressure tube, after which three drops of HOAc were added. The reaction mixture was stirred for 18 h at 110 °C under an atmosphere of argon. After cooling to room temperature (r.t.), the mixture was quenched with a saturated aqueous solution of NaHCO3 and extracted with dichloromethane (3 × 30 mL per mmol). The combined organic layers were dried over anhydrous MgSO4. After filtration, the solvent was removed under reduced pressure. The residue was purified by flash column chromatography (eluent: dichloromethane/methanol 100:0 → 93:7 v/v). See Section S1.2.1 in the Supporting Information for further details and analytical data. Compound 5-I-1 was synthesized by slowly adding iodomethane (80.32 mmol, 5.0 mL) to a stirred solution of 1,2,4,5-tetramethyl-1H-imidazole (40.27 mmol, 5.00 g) in 15 mL of dry acetonitrile in an oven-dried Schlenk pressure tube. The reaction mixture was covered from light and stirred for 18 h at 80 °C under an atmosphere of nitrogen. The solvent and excess of iodomethane were removed under reduced pressure. The crude product was washed with diethyl ether (3 × 40 mL) and recrystallized from acetone. Drying under reduced pressure at 50 °C afforded the desired product as a white powder.

	4.2.2. Synthesis of 5-I-1 Yield: 95% (10.16 g). δH (400 MHz, CD2Cl2): 2.24 (s, 6H, (imidazolium C-4/C-5)-CH3),
	2.74 (s, 3H, (imidazolium C-2)-CH3), 3.70 (s, 6H, N-CH3). δC (101 MHz, CDCl3): 9.1, 12.7, 33.2, 125.7,
	142.8. IR (Ge-ATR): see Section S1.2.7 in the Supporting Information. HRMS (ESI, CH2Cl2, m/z): calcd for
	[C8H15N2]

+ ([M -I -] + ), 139.1230; found, 139.1233. Calcd for C8H15IN2 (266.13): C, 36.11; H, 5.68; N, 10.53. Found: C, 36.00; H, 5.70; N, 10.86.

4.2.3. General Method for the Synthesis of 5-Br-n (n = 7, 11, 15)

The appropriate 2,4,5-trialkyl-1Himidazole 4-n (1 equiv), the appropriate 1-bromoalkane CnH2n+1Br (15 equiv), and K2CO3 (2 equiv) were dissolved in dry THF (

[2,4,5-trialkyl-1H

  ] + ), 139.1230; found, 139.1228. Calcd for C8H15N3O3•0.25H2O (205.73): C, 47.75; H, 7.51; N, 20.88 (we note that, upon standing open to air, the compound rapidly absorbed additional water). Found: C, 47.42; H, 7.62; N, 20.45.4.2.5. General Method for the Synthesis of 5-NO3-n(n = 7, 11, 15) The following steps were performed in the dark. A solution of the appropriate bromide salt 5-Br-n (1 equiv) in ethanol ([5-Br-n] = 0.04 M) was slowly added to a solution of silver nitrate (1.1 equiv) in ethanol ([AgNO3] = 0.1 M) under stirring. The reaction mixture was stirred for 15 h at r.t. Subsequently, 6 mL of dichloromethane was added. Insoluble products were filtered using a paper filter. The filtrate was concentrated under reduced pressure at 40 °C. The residue was redissolved in 20 mL of dichloromethane and washed two times with 5 mL of water. The organic layer was dried over anhydrous MgSO4. After filtration, the solvent of the filtrate was removed under reduced pressure. See Section S1.2.3 in the Supporting Information for further details and analytical data. Under stirring, a solution of compound 5-I-1 (0.353 mmol, 0.0940 g) in 3 mL of water was slowly added to a solution of sodium pentacyanocyclopentadienide(57) (0.316 mmol, 0.0674 g) in 2 mL of water. The reaction mixture was stirred for 1 h at r.t. The off-white precipitate formed was filtered, washed with copious amounts of water, and then dried under reduced pressure at 50 °C for 48 h. Yield: 77% (0.080 g). δH (400 MHz, DMSO-d6): 2.20 (s, 6H, (imidazolium C-4/C-5)-CH3), 2.56 (s, 3H, (imidazolium C-2)-CH3),[START_REF] Hallett | Room-Temperature Ionic Liquids: Solvents for Synthesis and Catalysis[END_REF].61 (s, 6H, N-CH3). δC (101 MHz, DMSO-d6): 8.0, 9.7, 31.6, 101.7 (-C≡N in [Cp(CN)5] -), 113.0 (Cring in [Cp(CN)5] -), 125.0, 142.6. IR (Ge-ATR): see Section S1.2.7. HRMS (ESI, DMSO, m/z): calcd for [C8H15N2] + ([M -[Cp(CN)5] -] + ), 139.1230; found, 139.1226. Calcd for C18H15N7 (329.36): C, 65.64; H, 4.59; N, 29.77. Found: C, 65.26; H, 4.54; N, 29.40.

	4.2.6. Synthesis of 5-Cp(CN)5-1

). δH (400 MHz, DMSO-d6): 2.21 (s, 6H, (imidazolium C-4/C-5)-CH3), 2.57 (s, 3H, (imidazolium C-2)-CH3), 3.61 (s, 6H, N-CH3). δC (101 MHz, DMSO-d6): 7.9, 9.6, 31.5, 124.9, 142.6. IR (Ge-ATR): see Section S1.2.7. HRMS (ESI, CH3OH, m/z): calcd for [C8H15N2] + ([M -[NO3] -

Acknowledgments K.G., G.A., and C.W.B. were supported in part by the Institute for Basic Science (IBS-R019-D1). C.W.B. also acknowledges the BK21 Plus Program funded by the Ministry of Education and the National Research Foundation of Korea. L.R. and F.G. gratefully acknowledge the Deutsche Forschungsgemeinschaft (SFB 858). B.H. and B.D. thank the CNRS and the University of Strasbourg for support. K.G. and T.I. thank the Institute of Global Innovation Research at Tokyo University of Agriculture and Technology for financial support. The authors are grateful to Geonhui Park for synthesizing 5-I-1 and for recording the high-resolution mass spectrometric data. The authors thank Seong-Hun Lee for his assistance with the synchrotron SWAXS measurements. The experiments at the PLS-II 6D UNIST-PAL Synchrotron Beamline were supported in part by the Korean Ministry of Science and ICT (MSIT), Pohang University of Science and Technology (POSTECH), and the UNIST Central Research Facilities (UCRF).

Supporting Information

The Supporting Information is available free of charge at https://pubs-acs-org.scd-rproxy.ustrasbg.fr/doi/10.1021/acs.chemmater.9b02338. Methods, experimental details, and analytical data; POM, DSC, TGA, TGA-DSC, single-crystal XRD, and SWAXS data; and supplementary notes (PDF)

CIF files and CheckCIF reports of the crystal structures of compounds 5-I-1 and 5-Cp(CN)5-1 (ZIP)

Author Contributions

K.G. and L.R. contributed equally to this work. All authors have given approval to the final version of the manuscript.

Notes

The authors declare no competing financial interest.