Substituted Azolium Disposition: Examining the Effects of Alkyl Placement on Thermal Properties

1 Center for Multidimensional Carbon Materials (CMCM), Institute for Basic Science (IBS), Ulsan 44919, Republic of Korea 2 Organisch-Chemisches Institut, Westfälische Wilhelms-Universität Münster, Corrensstraße 40, 48149 Münster, Germany 3 UNIST Central Research Facilities (UCRF) and School of Natural Science, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Republic of Korea 4 Department of Chemistry, UNIST, Ulsan 44919, Republic of Korea 5 Department of Energy Engineering, UNIST, Ulsan 44919, Republic of Korea


Introduction
Attachment of relatively long alkyl or fluoroalkyl chains to the nitrogen atoms of heterocyclic cations, such as imidazolium moieties, typically bestows thermotropic liquid-crystalline (LC) properties due to an enhancement in amphiphilicity [1][2][3][4].Ionic liquid crystals (ILCs) are of high interest because they hold potential for use in a variety of applications [5][6][7][8][9].For instance, it has already been demonstrated that ILCs may be used as non-volatile electrolytes in dye-sensitized solar cells [10][11][12], as organized reaction media [13], and as active components in electrochromic devices that do not require additional electrolytes [14][15][16].On a more fundamental level, a comprehensive knowledge of the structural parameters that determine the thermal properties of ILCs and the long-range supramolecular organization in their mesophases contributes to a better understanding of the short-range nanosegregation phenomena that occur in room-temperature ionic liquids (ILs) [17][18][19][20][21][22][23][24][25][26][27].Such structure-property relationships may also improve molecular designs of new ILCs and ILs, and facilitate tailoring for specific applications.
Although imidazolium moieties are among the most widely used cationic cores to obtain ILCs, the majority of studies have focused on N-substituted derivatives due to the ease of their synthesis [5][6][7][28][29][30].

Materials and Methods
The synthesis of compounds 1-n and 2-4 has been described elsewhere [50,51].Scheme 1. Left: overview of the 4,5-bis(n-alkyl)azolium salts that were investigated in this work.Right: previously reported imidazolium salts that feature two relatively long alkyl chains [38,54].

Materials and Methods
The synthesis of compounds 1-n and 2-4 has been described elsewhere [50,51].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 Crystals 2019, 9, 34 3 of 13 temperature stage.The latter was equipped with a precision XY positioner, and was coupled to an Instec LN 2 -SYS liquid nitrogen cooling system and an Instec mK2000 programmable temperature controller.Images were recorded by a QImaging Retiga 2000R CCD camera that was coupled to the microscope.The samples were pressed between an untreated glass slide and glass coverslip (0.13-0.17 mm thick, Duran) prior to analysis.
Differential scanning calorimetry (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.The measurements were performed using the Tzero TM Heat Flow T4P option.High-purity sapphire disks were used for the Tzero TM calibration and high-purity indium was used as a standard for temperature and enthalpy calibrations.DSC data analysis was performed with the Universal Analysis 2000 software (version 4.5A) from TA Instruments (New Castle, DE, USA).The abbreviations used to describe the thermal phase properties are explained in the main text and in the captions of the figures and tables.
Thermogravimetric analysis (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.High-purity nickel was used as a standard for temperature calibration (based on its Curie temperature).
Synchrotron-based X-ray scattering measurements 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 1D intensity profiles, which were obtained from azimuthal averaging of the 2D patterns of non-aligned samples (with dezingering applied to the data of two separate measurements), were used for phase type assignments.Small-to wide-angle X-ray scattering (SWAXS) patterns (for periodicities up to 67 Å) were recorded using 12.3984 keV X-ray radiation (wavelength λ = 1.00 Å) and a sample-to-detector distance (SDD) of ca.431 mm.Diffraction angles were calibrated using a lanthanum hexaboride (LaB 6 ) standard (NIST SRM 660c).Samples were contained in borosilicate glass (glass #50) capillaries with an outer diameter of 0.4 mm and a wall thickness of 10 µm and were irradiated for 10-30 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 heating stage to achieve temperature control.The samples were allowed to equilibrate at each temperature before starting a measurement.
Molecular models were created using the Chem3D Pro 15.1 software package (PerkinElmer Informatics, Inc., Waltham, MA, USA).

Results
We examined the thermal properties of 4,5-bis(n-alkyl)azolium salts 1-n and 2-4 using TGA, DSC, and polarized-light optical microscopy (POM).Key results are summarized in Table 1.Figures showing the TGA and DSC thermograms can be found in the Supplementary Material (Figures S1-S7).Cr → Iso 9 68 7  38.1 ~206 1 Abbreviations: Cr = crystalline phase; g = glass; SmA = smectic A phase; Iso = isotropic liquid phase.HR1 = first heating run; HR2 = second heating run. 2 Onset temperatures obtained by DSC during the second heating run (unless indicated otherwise) at a rate of 10 • C•min −1 and under an atmosphere of N 2 (50 mL•min −1 ).A small hole was pierced into the lid of the DSC sample pans.The glass transition temperature was defined by the inflection point of the signal recorded in the respective DSC thermogram. 3Enthalpy change. 4Temperature at which 1% weight loss was measured by TGA (neglecting initial small weight losses attributed to the release of H 2 O).n.d.= not determined. 5During the second heating run, melting was preceded by an exothermic recrystallization event with a peak temperature of 22 • C (∆H = −0.4kJ•mol −1 ) (Figure S1). 6This transition involved two, partially resolved transitions between 37 • C and 68 • C (Figure S2).The peak temperatures of the two signals and the total enthalpy change are listed.Examination by POM revealed that the sample became plastic after the first transition upon heating. 7Peak temperature. 8The transition to the LC mesophase upon heating was preceded by multiple transitions (Figure S4).The peak temperature of the last endothermic signal just before the temperature range of the SmA phase is given.Examination by POM revealed that the sample gradually became plastic above ~50 • C upon heating.The enthalpy change associated with the transition from the SmA phase to the solid state at 40 • C upon cooling was measured to be 12.2 kJ•mol −1 . 9Melting was preceded by solid-to-solid transitions (Figures S5 and S6).
The slightly higher thermal stability measured for 2 as compared to salt 1-15 can be ascribed to the replacement of the acidic H(2) proton of the imidazolium with a methyl group.The result is in accordance with the generally observed higher thermal stabilities of 2-methylsubstituted 1,3-dialkylimidazolium ILs relative to their unsubstituted counterparts [60].The 3,4,5-trialkylsubstituted thiazolium salt 3 showed a relatively low thermal stability, with weight losses starting to occur at about 135 • C under the conditions employed for the TGA measurements.The most thermally stable salt among the compounds that were studied was 4 (T 1% ≈ 206 • C), despite the fact that it contains nucleophilic Cl − anions as well as amino groups.
DSC and POM investigations of compounds 1-n and 2-4 led to the conclusion that imidazoliums 1-11, 1-15, and 2 are thermotropic ILCs but imidazolium 1-7, thiazolium 3, and guanidinium 4 are not.To the best of our knowledge, 2 is the first example of a LC pentasubstituted imidazolium salt.Salt 1-7, which was obtained as a partially crystallized compound, melted to an isotropic liquid around room temperature.In contrast, solid samples of 1-11, 1-15, and 2, which are more amphiphilic than 1-7, melted to birefringent mesophases which produced "oily streaks" when viewed by POM (Figure 1).Homeotropic domains were observed as well.Further heating facilitated transitions to isotropic liquid states, for which, in the case of 1-15 and 2, only weak signals were observed in the DSC thermograms (Figure 2 and Figures S2-S4).Upon cooling, ill-shaped "bâtonnets" were seen by POM upon entering the LC states and gradually transformed into focal-conic-like textures with further cooling.In the case of 1-11, the sample spontaneously aligned homeotropically when cooled from the isotropic liquid state and the LC mesophase vitrified around −23 • C; glass transitions continued to be seen during subsequent heating/cooling cycles (Figure 2a).The non-mesomorphic salts 3 and 4 melted directly to isotropic liquids without passing through intermediate LC phases.The melting point data recorded for all 4,5-bis(n-pentadecyl)azolium salts (1-15 and 2-4) are similar and situated in the range of 68 to 76 • C. −23 °C; glass transitions continued to be seen during subsequent heating/cooling cycles (Figure 2a).The non-mesomorphic salts 3 and 4 melted directly to isotropic liquids without passing through intermediate LC phases.The melting point data recorded for all 4,5-bis(n-pentadecyl)azolium salts (1-15 and 2-4) are similar and situated in the range of 68 to 76 °C.Synchrotron-based small-to wide-angle X-ray scattering (SWAXS) measurements of the LC mesophases adopted by 1-15 and 2 afforded patterns that were characterized by two sharp reflections in the small-angle region, in addition to a diffuse wide-angle scattering signal centered at 4.4-4.6Å (Figure 3, Table 2 and Figure S8).The latter corresponds to the lateral short-range order of the molten alkyl chains (c.f.h ch ) and the ionic headgroups (c.f.h ion ), respectively, which were not resolved in the experimental data.The reciprocal d-spacings of the sharp small-angle reflections were related by a 1:2 ratio and the signals can be indexed as the (001) and (002) reflections that originate from the formation of layers.Collectively, the POM and SWAXS data indicate that the LC mesophases adopted by salts 1-15 and 2 are SmA phases.A slightly larger layer thickness was found for 2 as compared with 1-15 at similar temperatures (see Table 2), which can be ascribed to the protruding 2-methyl substituents in the former.Based on the POM observations, the mesophase adopted by 1-11 is also a SmA phase.SWAXS patterns that were recorded for 3 at different temperatures revealed that the sample adopted a lamellar structure in the solid state before melting to an isotropic liquid at about 73 • C (Figure S9).originate from the formation of layers.Collectively, the POM and SWAXS data indicate that the LC mesophases adopted by salts 1-15 and 2 are SmA phases.A slightly larger layer thickness was found for 2 as compared with 1-15 at similar temperatures (see Table 2), which can be ascribed to the protruding 2-methyl substituents in the former.Based on the POM observations, the mesophase adopted by 1-11 is also a SmA phase.SWAXS patterns that were recorded for 3 at different temperatures revealed that the sample adopted a lamellar structure in the solid state before melting to an isotropic liquid at about 73 °C (Figure S9).Table 2. Summary of synchrotron-based SWAXS data recorded for the LC mesophases adopted by 1-15 and 2, including calculated structural parameters and corresponding assignments.values refer to the measured and calculated diffraction spacings, respectively.d calcd.= <d 001 > = [Σ l d 00l l]/N 00l , in which N 00l = the number of (00l) reflections. 2I is the intensity of each reflection: VS = very strong, W = weak, sh = sharp reflection, and br = broad reflection. 3hkl are the Miller indices of the reflections.h 1 indicates the center position of the diffuse wide-angle signal that originates from the lateral short-range order of the ionic moieties (c.f.h ion ) and the molten alkyl chains (c.f.h ch ). 4 V mol is the molecular volume, which was estimated as V mol (T) = (M cation /0.6022)f + V iodide , in which M cation is the molecular mass of the cations (in g•mol −1 ), f is a temperature-correcting factor (f = 0.9813 + 7.474 × 10 −4 T with T in • C) and V iodide is the partial volume of the iodide anions as determined from reference salts [61].A M is the cross-sectional area that is occupied by molecular assemblies along the sequence of smectic layers and was calculated as A M (T) = 2V mol (T)/d(T) [38,61].The cross-sectional area of one fully stretched aliphatic chain, σ ch [62], is listed for comparison.
From the structural parameters that were obtained from the SWAXS measurements as well as the temperature-dependent molecular volumes calculated for 1-15 and 2, it can be concluded that the SmA phases are characterized by alternating, nanosegregated ionic and aliphatic sublayers, with a head-to-head arrangement of the ionic headgroups in the former, and partially interdigitated and folded alkyl chains in the latter [63,64].The values calculated for the molecular cross-sectional areas, A M (see Table 2), are comparable to those previously reported for the SmA phases adopted by Crystals 2019, 9, 34 8 of 13 2-aryl-1,3-dimethylimidazolium iodide salts having two alkyl chains per cation (6-n (n = 6, 10, 14)) [38].The supramolecular arrangements found in all of these SmA phases are similar.We note that 1-15 has been reported to spontaneously form thermodynamically stable vesicles in buffered aqueous media without the addition of other lipids [52].As such, the structure of its thermotropic LC mesophase may resemble the local structure of the vesicle bilayer membranes.It was also found that 1-7 does not form bilayer vesicles under similar conditions [52].Herein we report that it does not form a thermotropic LC mesophase either.

Discussion
The observation of SmA mesophases for imidazolium salts 1-n (n = 11, 15) and 2 was not unexpected, since structurally related 1,3-bis(n-alkyl)imidazolium salts with long alkyl chains are known to adopt thermotropic SmA phases [54][55][56][57][58][59].However, inspection of the literature data, some of which are collected in Table 3, shows that the points of the 4,5-disubstituted ILCs are lower than those of 1,3-analogues.For example, the alkyl substituents of iodide salt 5-16 contain only one more methylene group than 1-15 or 2, yet the clearing point of the former is 59 • C higher and its SmA phase is stable over a temperature range of 80 • C [54].Even the homologue with n-dodecyl groups in the 1-and 3-positions, 5-12, exhibits a similar clearing point as 1-15 and 2 despite its lower amphiphilic character.Since 5-12 also has a lower melting point, its mesophase temperature range is about 49 ] − is about 1.5 times as large [65]) [54,57].Compound 1-11 also has a higher melting point and lower clearing point than 5-12 [56].As such, 1,3-disubstitution appears to induce thermotropic LC mesomorphism in imidazolium salts more effectively than the 4,5-disubstituted analogues.
the ionic headgroups in the ionic sublayers, although: (1) similar A M values were found for the SmA phases adopted by 2-aryl-1,3-dimethylimidazolium iodide salts having two alkyl chains per cation (see above) and those smectic phases were stable until 147-164 • C [38], and (2) LC mesophases were not detected for thiazolium salt 3 which features only one N-methyl group.The latter observation provides another example of how subtle structural and electronic changes in organic salts may have a considerable impact on their physical properties.Although LC thiazolium salts have not yet been reported to the best of our knowledge, the apparent absence of mesomorphic properties for compound 3 does not exclude the possibility that 3-alkylthiazolium salts with relatively long alkyl chains may show such characteristics.We tentatively ascribe the higher melting point of imidazoliums 1-11 and 1-15 as compared with 5-12 and 5-16, respectively, to a closer proximity of the alkyl chains in the former.Such arrangement may facilitate van der Waals interactions between the chains and increase the melting point.
As mentioned above, a LC phase was not observed for 4. The potential beneficial effect of the N-H sites in the cationic headgroup, which could participate in hydrogen bonds within ionic sublayers, may be counteracted by a mismatch in cross-sectional area between the ionic moieties and the molten, long alkyl chains, which is a prerequisite for the development of a smectic LC mesophase.The latter effect may explain the direct transition to an isotropic liquid and the lowest measured melting point among the series (1-15)-4.
We also note that 6-15 exhibits enantiotropic cubic and columnar LC mesophases while homologues with shorter alkyl chains adopt only SmA phases [38].The absence of non-smectic LC phases for the salts discussed herein underscores the importance of the "taper angles" and precise geometric shapes of the polar headgroups of amphiphilic mesogens for inducing columnar and bicontinuous cubic mesophases [68][69][70].

Figure 1 .
Figure 1.Polarized-light optical microscopy (POM) images of the SmA phases of (a) 1-11 at 24 °C (upon cooling from the isotropic liquid state and after applying pressure to the sample) and (b) 2 at 77 °C (during the first heating run of a pristine sample).

Figure 1 .
Figure 1.Polarized-light optical microscopy (POM) images of the SmA phases of (a) 1-11 at 24 • C (upon cooling from the isotropic liquid state and after applying pressure to the sample) and (b) 2 at 77 • C (during the first heating run of a pristine sample).

Figure 2 .
Figure 2. Differential scanning calorimetry (DSC) data recorded for (a) 1-11 and (b) 2 at a heating rate of 10 °C•min −1 and a cooling rate of 5 °C min −1 under an atmosphere of N2.Endothermic peaks point upward.

Figure 2 .
Figure 2. Differential scanning calorimetry (DSC) data recorded for (a) 1-11 and (b) 2 at a heating rate of 10 • C•min −1 and a cooling rate of 5 • C min −1 under an atmosphere of N 2 .Endothermic peaks point upward.

Figure 3 .
Figure 3. Synchrotron-based SWAXS data that were recorded for the SmA phase of 1-15 at 75 °C upon cooling (the X-ray wavelength used was 1.00 Å).

Figure 3 .
Figure 3. Synchrotron-based SWAXS data that were recorded for the SmA phase of 1-15 at 75 • C upon cooling (the X-ray wavelength used was 1.00 Å).

Table 2 .
Summary of synchrotron-based SWAXS data recorded for the LC mesophases adopted by 1-15 and 2, including calculated structural parameters and corresponding assignments.Cpd.Type of LC Mesophase T (°C) dobs.(Å)