NMR Spectroscopic Study of Orientational Order in Imidazolium-Based Ionic Liquid Crystals

Abstract

We report on molecular and local orientational order of a series of imidazolium-based ionic liquid crystals exhibiting layered smectic A mesophase. Materials constituting of 1-dodecyl-3-methylimidazolium cation, and different counter-ions, were investigated. We apply two-dimensional ¹³C-¹H dipolar NMR spectroscopy to quantify orientational order of C-H bonds of the organic cation. The experimental data supported the structural model of the interdigitated chains aligned with the smectic layer normal. Molecular order parameter S was found to increase in the anion sequence BF₄⁻ < I⁻ < Br⁻ < Cl⁻. This trend correlates well with ionic radius, negative charge delocalization, and hydrogen-bonding properties of the anions.

1. Introduction

Ionic liquids (ILs) are considered today as key materials in a broad range of emerging technological applications in the areas of electrochemistry, synthesis, catalysis and separation, and lubrication. ILs are commonly built up of bulky organic cations and smaller anions of organic or inorganic origin. The size difference contributes in decreasing melting point by hindering crystallization of the material. Ionic liquid crystals (ILCs) are ionic liquids capable of self-assembly into thermodynamically stable liquid-crystalline phases [1,2,3]. They thus can be defined as anisotropic liquids consisting entirely of ions as mesogenic units. ILCs exhibit the typical properties of ionic liquids such as ionic conductivity, as well as nano-scale structures with partial molecular ordering. Generally, mesophase formation is predetermined by a pronounced anisotropy of interparticle interactions, which in the case of molecular liquid crystals (LCs) with prevailing van der Waals interaction is a consequence of molecular anisotropic shape. In contrast, in ionic LCs with dominant electrostatic interparticle interactions, the essential factor contributing to mesophase stability is an anisotropic charge distribution.

ILCs typically form layered, columnar, and bicontinuous cubic phases while least-ordered nematic phases are rare. The properties of ILCs, including phase behaviour, are readily modified by selecting cations and anions [1]. Typically, ionic mesogenic materials are composed of positively charged heterocyclic cores linked to long n-alkyl chains and small inorganic or organic anions. Phase segregation of incompatible, charged (anions and cationic heads), and hydrophobic nonpolar moieties (aliphatic chains) induces the formation of layered smectic phases. Imidazolium-based ILCs have been most extensively investigated [4]. Charge delocalization in the imidazolium core reduces ionic interaction and thus promotes low transition temperatures. Imidazolium salts with a length of the alkyl chain of n ≥ 12 display interdigitated smectic A bilayer phases. Systematic trends in the thermodynamic phase properties depending on ionic/molecular structure have been revealed [1,2]. Mesophase temperature range and clearing temperature T_C (mesophase-to-isotropic phase transition temperature) increase with the chain length of the organic cation. Similarly, the mesophase temperature changes with the type of anions. For example, mesophase temperature range decreases in the anion order Cl⁻ > Br⁻ > I⁻ [1,5,6]. Larger size fluorinated anions with delocalized charge and lower capability to build hydrogen bonds, such as BF₄⁻ and PF₆⁻, lead to further decrease of melting and clearing temperatures [6,7,8]. Additionally, the phase diagram in hydroscopic ILCs is affected by water-anion hydrogen bonding [1,6,9].

Theoretical and experimental studies of orientational order in ILCs [1,10,11,12,13,14] revealed that, due to the contribution of the electrostatic interactions to the stabilization of layered structures, smectic phases in ILCs exhibit significantly lower value of the molecular orientational order parameter S as compared to that in its non-ionic counterparts. However, systematic studies of the orientational order parameter depending on ILCs structure and ionic composition are still lacking. In the present work, we report detailed investigation of the molecular and local bond orientational ordering in a series of ILCs with a fixed imidazolium-based cation and number of anions varying in structure, ionic radius, negative charge localization, and hydrogen bonding properties.

Orientational ordering in LCs is most directly studied by nuclear magnetic resonance (NMR) spectroscopic technique [15,16,17]. Molecular and local bond molecular order parameters are estimated via measurement of the anisotropic spin couplings, such as dipolar and quadrupolar interactions and chemical shift anisotropy. A large anisotropy of diamagnetic susceptibility of mesogenic molecules/ions often leads to macroscopic molecular alignment with respect to direction of the magnetic field applied during NMR measurement. In this case, NMR spectra are obtained with high resolution, and site-specific anisotropic spin couplings are most straightforwardly accessed. In the present study, we apply two-dimensional (2D) ¹³C-¹H dipolar NMR spectroscopy to quantify orientational order of C-H bonds of a long-chain imidazolium-based cation in an ionic smectic phase. Molecular order parameter S is also estimated. The observed trends of the measured bond and molecular order parameter values depending on counter-ion are discussed.

2. Materials and Methods

Four samples of ionic mesogenic materials with the same cation C₁₂mim (1-dodecyl-3-methylimidazolium) and different anions, Cl⁻, Br⁻, I⁻, and BF₄⁻, were obtained from commercial sources: C₁₂mimCl, C₁₂mimBr, and C₁₂mimBF₄ were purchased from ABCR GmbH, Karlsruhe, Germany, and C₁₂mimI was received from Sigma-Aldrich Sweden AB, Stockholm, Sweden. All samples were tested for water content by recording the ¹H NMR spectra in the isotropic phase. As received, the C₁₂mimBr sample exhibited non-negligible water content of 1.3 wt % (23 mol %). The sample was dehydrated by equilibrating for 7 days in a desiccator with P₂O₅ powder. The water contents in the samples used for NMR measurements are indicated in Table S1 in the Supplementary Materials.

NMR measurements (in smectic phase were performed in samples aligned in the magnetic field on the NMR spectrometer with the phase director distributed in the plane perpendicular to the magnetic field vector. Experiments were performed using the Bruker 500 Avance III spectrometer (Bruker BioSpin GmbH, Rheinstetten, Germany) at Larmor frequencies of 500.1 and 125.7 for ¹H and ¹³C, respectively. Dipolar ¹H-¹³C spectra were recorded using proton detected/encoded local field (PDLF) NMR spectroscopy [18,19] (Figure S1 in Supplementary Materials). Other details of NMR experiment were described in a previous publication [13] and also included in the Supplementary Materials.

Radio frequency (rf)-induced sample heating effects and hardware performance limited the rf power level in our experiments. Applied heteronuclear and homonuclear decoupling powers were not sufficient to achieve high resolution and sensitivity simultaneously for all carbon sites in the molecules. Hence ¹H and ¹³C offset frequencies were set to the values corresponding to optimal signal resolution in the alkyl chain region.

3. Results and Discussion

3.1. Dipolar Spectra

Representative carbon-13 cross-polarization spectrum of the C₁₂mimBF₄ in the smectic phase is shown in Figure 1. Spectra of other samples (displayed in Supplementary Materials, Figure S2) exhibited similar structure and peak positions. The observed chemical shifts are given by the component ${\delta }_{\perp }^{LC}$ of the chemical shift tensor [13,16]. Unambiguous assignment of the spectral lines was obtained from the dipolar ¹³C-¹³C double-quantum correlation 2D spectra and was further confirmed by dipolar PDLF spectra with dipolar splittings and carbon chemical shifts correlated in orthogonal directions. Due to the high spectral resolution in the static samples, ¹³C-¹H dipolar couplings for all in-equivalent directly bound ¹³C and ¹H pairs could be directly measured in a 2D PDLF experiment. Representative dipolar cross-sections from PDLF spectrum of C₁₂mimBF₄ are displayed in Figure 2.

The spectral splitting in the PDLF experiment is resulted from the contribution of direct dipolar coupling d_CH and indirect spin coupling J_CH

$$\Delta \nu =\left|k(2d_{CH}+J_{CH})\right|$$

(1)

where k = 0.418 ± 0.04 is the experimentally calibrated dipolar scaling factor of the applied homonuclear decoupling sequence BLEW48 [20]. The dipolar coupling constant d_CH in a mesophase is the orientational average of the rigid lattice coupling constant $b_{CH}=-({\mu }_0/8{\pi }^2)({\gamma }_H{\gamma }_C\hslash /r_{CH}^3)$ (r_CH is atomic distance and the ${\gamma }_H,{\gamma }_C$ gyromagnetic ratios). For a single C-H bond in aliphatic moieties the generally accepted b_CH value, corrected for vibration effects, is −21.5 kHz [21,22]. The orientational averaging of the local C-H vector in the director frame is described by the local bond order parameter

$$S_{CH}=\langle P_2(\cos {\theta }_{PN})\rangle ,$$

(2)

with $P_2(\cos {\theta }_{PN})=(3{\cos }^2{\theta }_{PN}-1)/2$, where ${\theta }_{PN}$ is the angle between the C-H vector (principal frame) and the phase director N. For the samples with the director perpendicular to the external magnetic field and assuming uniaxial symmetry of the molecular motion, one thus obtains [23]

$$d_{CH}=-\frac{1}{2}{b}_{CH}{S}_{CH}$$

(3)

The values of the isotropic coupling J_CH, the sign of which is known to be positive, were measured in the carbon-13 spectra in the isotropic phase. The sign of the coupling constant d_CH was possible to infer considering director alignment and C-H bond angle to the main molecular axis.

3.2. C-H Bond Orders

Bond order parameter profiles of all four ILC samples in the smectic A phase were measured at similar temperature difference ΔT = T_C − T with respect to clearing point T_C (Figure 3). The C-H bond order parameter (S_CH) values for the alkyl chains were negative because of the directions of the C-H bond on average perpendicular to the long molecular axis. The samples exhibited common trends in the variation of the S_CH values. With decreasing temperature, the magnitudes of S_CH parameters of all groups were consistently increasing. Besides, at a fixed temperature, S_CH values, being nearly constant for a few first chain segments, dropped gradually towards the chain terminal. Similar trends have been reported in the recent experimental study of the longer alkyl chain cation in the C₁₄mimNO₃ material [13], and also in a molecular dynamics (MD) study of C₁₆mimNO₃ homologue [14]. The S_CH profiles were analyzed by applying a model of the alkyl chain with predominant trans conformation of the groups in vicinity of the aromatic core and with the symmetry axis of this part parallel to a long molecular axis. This model was further supported by measuring (in some of the samples) the C-C order parameter (S_CC) for the alkyl carbons separated by two bonds: Since the vectors C-H and C_n-C_n+2 were perpendicular to and along the molecular axis, respectively, the experimentally obtained ratios $S_{CC}\approx -2S_{CH}$ confirmed the validity of the model assumption [13].

Comparing the S_CH data in different samples at the same relative temperatures ΔT, one finds that the alkyl chains became more ordered in the following anion sequence: BF₄⁻ < I⁻ < Br⁻ < Cl⁻. This suggests that the molecular order parameter S is also increasing in the same anion order as discussed in the next section.

3.3. Molecular Order S

Considering statistical independence of the conformational and molecular rotational dynamics and assuming the validity of the described above model of the chain dynamics, one can estimate the molecular order parameter S using the following equation for the chain segments in trans conformation

$$S_{CH}=\langle P_2(\cos {\theta }_{PM})\rangle S\approx -0.5S$$

(4)

where ${\theta }_{PM}\approx 90°$ is the angle between the C-H internuclear vector and long molecular axis [13,16]. Thus, we calculated the order parameter applying Equation (4) to chain carbons C1-C3 and taking the average value. The results for four samples are compiled in Figure 4, where S values are plotted against the reduced temperature T_red = T/T_C.

The values of the order parameters were found in the range of 0.22 to 0.35 depending on the temperature and anion type. These values are significantly lower when compared to typical S > 0.5 in conventional smectic A phases of non-ionic LCs [24,25]. Similarly low S values in smectic phase were found in an analogous ILC with longer alkyl chain imidazolium-based cation and NO₃ anion, C₁₄mimNO₃ [13]. Likewise, MD simulation predicted the same range of S values in the homologue sample C₁₆mimNO₃ [12]. A relatively low value of the parameter S in ILCs was explained by the contribution of electrostatic interactions to molecular segregation. In neutral LCs, a high molecular order is essential for mesophase stability since the molecular mutual alignment is resulted from weak van der Waals interactions of particles of anisotropic shape. In contrast, in ILCs the dominant contribution is electrostatic interaction of particles with anisotropic charge distribution while van der Waals interaction is less important. Hence stable layer structure can form with relatively low orientational order.

The order parameter S was increasing in the anion sequence BF₄⁻ < I⁻ < Br⁻ < Cl⁻ (Figure 4). The observed trend correlated well with changes in the physicochemical properties of the anions. For example, the anions ability to build extended hydrogen-bond network within ionic sublayer increased in the same order [2,6]. This property contributed to thermodynamic stability of the phase and hence to increasing the order parameter S. The anion size (Table S1) and negative charge delocalization increased in the opposite sequence [26,27]. For more bulky anions, the occupied area of the imidazolium group should match the increased negative charge delocalization and weaker hydrogen bonding. The alignment of the imidazolium core in the ionic sub-layer can affect the ordering and conformation of the chains [2,28]. In fact, in a recent ²H NMR and MD study, there had been suggested a dramatic change in the headgroup conformation in the C₁₂mim cation when bulky BF₄⁻ was replaced by compact Cl⁻ [29]. The observed change in the values of the order parameter also correlates with the variation in the interlayer spacing, which is increasing in the same order [6]. Thus, the increasing interlayer spacing contributes to more extended chains with higher ordering.

While the order parameter characterizes the re-orientational molecular dynamics, anisotropy of translational mobility in LCs is also directly affected by the orientational order [30,31]. Having obtained in the present work, the data on the order parameters in studied ILC series, we are now in the position to explain some previously found trends in translation diffusion results. Molecular self-diffusion in the smectic phase of two ionic mesogens, C₁₂mimCl and C₁₂mimBF₄, had been investigated by Cifelli et al. [32]. Translational diffusion coefficients D_⊥ and D_|| of the organic cation were measured in directions perpendicular and parallel to the phase director. At comparable temperature, cation diffusion was slower in C₁₂mimCl sample. Interestingly, a significantly higher anisotropy ratio D_⊥/D_|| was observed in C₁₂mimCl compared to that in the sample C₁₂mimBF₄. This is consistent with the larger values of the molecular order parameter in C₁₂mimCl (Figure 4). Moreover, larger diffusion anisotropy implies faster diffusion of the cations within the layer and more restricted mobility between the layers. This contributes to better defined layers of the otherwise highly dynamic structure in agreement with higher mesophase stability of the material with Cl⁻.

4. Conclusions

We have applied NMR dipolar spectroscopic techniques to obtain quantitative information on molecular and local bond orientational order parameters in smectic A phase of a series of ionic mesogens. Materials constituting of the same long alkyl-chain imidazolium cation, and varying counter-ions, were investigated. The experimental data were consistent with the structural model of the interdigitated chains aligned with the smectic layer normal. In agreement with previous experimental and MD studies [12,13], the molecular alignment was characterized by a low molecular order parameter S in the range of 0.20 to 0.35. Since the ionic interactions contribute to the layer stabilization by inducing a ‘charge-ordered’ nanoscale segregation of ions, thermodynamically stable smectic structure in ILCs can be assembled with lower orientational order compared to that in molecular LCs [1,10,11,12]. Remarkably, the values of the order parameter S exhibited a consistent increase in the anion sequence BF₄⁻ < I⁻ < Br⁻ < Cl⁻. This new finding was rationalized by relating the S values to the trends in physicochemical properties of the anions such as ionic radius, charge delocalization, and increasing ability to build hydrogen bonds. Higher order was also observed for the salts with increasing interlayer spacing in the smectic A phase. Higher molecular orientational order, which is associated with the phase stability, presents an advantage in designs of ionic materials for anisotropic ion transport in various technological applications. In future studies, it would be interesting to study orientational order in hydrated mesogenic salts, where hydrogen bonds to water molecules contribute further to the structure stabilization.