Counterion effects on the mesomorphic and electrochemical properties of guanidinium salts

Abstract

Ionic liquid crystals (ILCs) combine the ion mobility of ionic liquids with the order and self-assembly of thermotropic mesophases. To understand the role of the anion in ILCs, wedge-shaped arylguanidinium salts with tetradecyloxy side chains were chosen as benchmark systems and their liquid crystalline self-assembly in the bulk phase as well as their electrochemical behavior in solution were studied depending on the anion. Differential scanning calorimetry (DSC), polarizing optical microscopy (POM) and X-ray diffraction (WAXS, SAXS) experiments revealed that for spherical anions, the phase width of the hexagonal columnar mesophase increased with the anion size, while for non-spherical anions, the trends were less clear cut. Depending on the anion, the ILCs showed different stability towards electrochemical oxidation and reduction with the most stable being the PF₆ based compound. Cyclic voltammetry (CV) and density functional theory (DFT) calculations suggest a possible contribution of the guanidinium cation to the oxidation processes.

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Introduction

Among the large class of thermotropic liquid crystals, which form bulk mesophases (i.e. liquid crystalline phases) of orientationally ordered lamellar, columnar or cubic geometries upon change of the temperature, ionic liquid crystals (ILCs) constitute an important subclass.^1–10 From a molecular perspective, ILCs carry cationic (or less often anionic) headgroups, non-polar side chains with or without mesogenic subunits and inorganic (or organic) counter ions, resulting in strong Coulomb interaction and nano segregation into mesophases showing anisotropic physical properties on the one hand and on the other hand, ionic liquids (ILs) are organic, low melting salts, often with adjustable polarity, fluidity, viscosity and a wide electrochemical window.^1–10 These unique properties make ILCs promising candidates for a variety of applications, in particular as electrolytes in dye sensitized solar cells (DSSCs),^11–14 batteries, fuel cells and capacitor materials.^15–17 Despite the growing interest in electrochemical applications, most electric and ionic conductivity as well as cyclic voltammetry studies have been carried out with imidazolium ILCs.¹ Guanidinium ILCs, on the other hand, provide various opportunities to tailor their liquid crystalline self-assembly regarding mesophase geometry and temperature range^18–23 to obtain rare mesophases such as tilted lamellar smectic C phase (SmC)^24,25 and de Vries (i.e. minimal layer contraction at the transition from tilted SmC to non-tilted SmA phase),^24,25 or photoresponsive behavior.²⁶ ILs based on guanidinium cations and trifluorosulfonimide anions have been previously investigated regarding their electrochemical properties^27–29 as additives in electrolytes for DSSCs,³⁰ as well as guanidinium-functionalized anion exchange polymer electrolytes.³¹ In addition, the electrochemical properties of guanidines and the respective guanidinium salts were studied with respect to their applications as redox active ligands of metal complexes.^32–35 Moreover, aminocyclopropenium ions have been introduced as deltic guanidinium ions, i.e. macrosteres of the guanidinium ions,^36,37 which were successfully employed as redox-active organic salt for Na-ion batteries,³⁸ persistent radical cations,³⁹ polyelectrolytes,⁴⁰ polymerized ionic liquids for ion transport⁴¹ and materials for electrophotocatalysis.⁴² ILCs are expected to have certain advantages over ILs in battery and solar cell applications, such as 1D, 2D or 3D ordered charge transport depending on the mesophase type.^43,44 Thus, a basic understanding of both mesomorphic properties, e.g. phase transition temperatures and phase width as well as geometry and packing behavior of the liquid crystalline self-assembly and electrochemical properties of the respective ILCs is required, as the electrochemical stability towards oxidation and reduction directly influences the useable electrochemical window in an application.^45,46 Indeed, many ILs and ILCs have been investigated for their electrochemical stability windows.^47–53 However, electrochemical studies of guanidinium-based compounds are quite rare and there are no reports on the electrochemical properties of guanidinium-based ILCs.^{27,29,54–63}

On the other hand, counterions also play an important role in both the mesomorphic and electrochemical properties^{1,2,53,65–71} of ILCs as well as hydrophilicity, viscosity, and electrochemical window.^27,72–74

Thus, in order to understand their liquid crystalline self-assembly in the bulk phase and their electrochemical behavior in solution, we investigated a series of wedge-shaped 3,4,5-trisalkoxy-phenyl-guanidinium salts Gua(14)X with different counterions X = Cl, Br, I, BF₄, PF₆, OMs, SCN, N(CN)₂, NO₃, NTf₂, OTf, OTs, N₃, CN, OCN, OAc. The 3,4,5-trisalkoxy-phenyl-guanidinium mesogen Gua(14) was chosen as a cationic moiety because it is a very robust mesogen, forming a columnar mesophase in the presence of the chloride anion and undulated lamellar SmA phases for bulky divalent Mo cluster anions⁶⁴ (Fig. 1). Our current study revealed that the electrochemical stability window of these guanidinium ILCs in solution was strongly affected by the counterion. In the bulk mesophase, the counterion had a pronounced influence on the stability and temperature range, whereas the packing geometry was little affected. The details are discussed below.

Fig. 1 Preliminary work on luminescent clustomesogens [Gua(n)]₂[Mo₆Q₈X₆]⁶⁴ derived from guanidinium ILCs Gua(n)Cl and our new studies on one of these ILCs (Gua(14)X) with different anions X.

Results and discussion

Synthesis and NMR studies of guanidinium ILCs

The guanidinium salts Gua(14)X were prepared via salt metathesis from the known tristetradecyloxyphenylguanidinium chloride Gua(14)Cl⁶⁴ according to the procedure proposed by Butschies¹⁸ (Scheme 1). After recrystallization, the resulting guanidinium salts Gua(14)X were isolated in 79–91% yield.

Scheme 1 Synthesis of Gua(14)X.

Upon exchange of the anion, a characteristic upfield shift of the guanidinium N–H signal was observed, as exemplified for Gua(14)Br (Fig. 2b) and Gua(14)N(CN)₂ (Fig. 2c) as compared to Gua(14)Cl (Fig. 2a). A stacked plot showing the NMR spectra of the whole series Gua(14)X is shown in Fig. S1 (ESI†). The chemical shift δ(N–H) decreased with increasing anion size (Fig. S2, ESI†). These observations are in good agreement with previously reported NMR studies of guanidinium ILCs.^18,75 We surmised that the observed counterion dependent chemical shift of the N–H signal in the ¹H NMR spectrum might be caused by ion pairing in solution, as was independently reported by Aidas⁷⁶ for imidazolium and by Saielli⁷⁷ for pyridinium ionic liquids. Our concentration-dependent NMR studies of Gua(14)Cl in CDCl₃ revealed that the guanidinium N–H signal shifted only very little with decreasing concentration. In addition, a broadening and doubling of the N–H peak were observed (Fig. S3, ESI†). These results suggest that the ion pairs in Gua(14)X are generated by a combination of Coulomb interaction and H-bonding,^78,79 while the peak broadening is probably due to the formation of H-bonded aggregates resulting in rotameric equilibria (for a detailed analysis of such guanidinium rotamers, see ref. 18 and 75). The importance of H-bonding between the anion and cation was also pointed out by Donnio for azatriphenylene ILCs.⁷⁰ In addition, we performed NMR experiments in different solvents, which revealed that the δ(N–H) shift decreased with increasing polarity of solvents (Fig. S4, ESI†), presumably due to the weakening of the contact ion pairs and formation of solvent-separated ion pairs.

Fig. 2 Sections of the ¹H NMR spectra (400 MHz, r.t., CDCl₃) of guanidinium salts (a) Gua(14)PF₆ (blue), (b) Gua(14)N(CN)₂ (green) and (c) Gua(14)Cl (red) showing the anion-dependent shift of the guanidinium N–H signal.

It should be noted that any attempts to prepare guanidinium ILCs Gua(14)X with counterions of weak acids, i.e. X = N₃, OAc, CN, OCN failed (Scheme 1). The resulting reaction products from the salt metathesis did not show the characteristic HN=C signal for the guanidinium moiety in the ¹H NMR spectra (for details, see Fig. S1, ESI†). Presumably, for guanidinium salts with anions of weak acids, the acid–base equilibrium is shifted towards the neutral guanidine base and HX.⁸⁰ Indeed, the free guanidine Gua(14) could be isolated and characterized by NMR (for details, see ESI†).⁸¹ In the bulk phase, such equilibration would lead to evaporation of the acids HN₃, HOAc, HCN, and HOCN during repetitive heating/cooling cycles in DSC, POM or XRD analyses. Thus, further experiments with these counterions were abandoned.

Mesomorphic properties of guanidinium ILCs

Investigation by differential scanning calorimetry (DSC) and polarizing optical microscopy (POM) revealed enantiotropic mesomorphism for guanidinium salts Gua(14)X with counterions X = Cl, Br, I, BF₄, PF₆, OMs, SCN, N(CN)₂, NO₃, while guanidinium salts with X = NTf₂, OTf, OTs were non-mesomorphic (Table 1) (all DSC curves are shown in Fig. S7 and S8, ESI†).

Table 1 Phase transition temperatures T in °C (−enthalpies ΔH in kJ mol⁻¹, if available) of guanidinium ILCs Gua(14)X^ab

X	Cr3			Cr2			Cr1			Colh			I
a Values were obtained from DSC with cooling/heating rates of 5 K min^−1. b The following phases were observed: Cr1, Cr2, Cr3 crystalline, Colh hexagonal columnar, I isotropic. ● phase observed,—phase not observed.
Cl	—			—			●	34.44	(−34.74)	●	86.96	(−55.09)	●	2nd H
	—			—			●	25.19	(32.09)	●	89.28	(1.18)	●	2nd C
Br	—			●	34.44	(−31.94)	●	85.63	(−0.45)	●	93.11	(−1.03)	●	2nd H
	—			—			●	25.22	(31.84)	●	91.43	(1.09)	●	2nd C
I	—			—			●	33.89	(−33.90)	●	100.37	(−0.99)	●	2nd H
	—			—			●	24.79	(32.86)	●	99.12	(0.99)	●	2nd C
BF4	—			—			●	36.16	(−33.8)	●	112.30	(−1.0)	●	2nd H
	—			—			●	27.85	(31.5)	●	112.04	(1.1)	●	2nd C
PF6	—			●	34.16	(−32.96)	●	42.82	(−9.89)	●	106.18	(−1.02)	●	2nd H
	—			—			●	25.76	(33.80)	●	105.92	(1.03)	●	2nd C
OMs	—			—			●	35.01	(−37.15)	●	82.30	(−1.06)	●	2nd H
	—			—			●	25.62	(35.3)	●	82.60	(1.02)	●	2nd C
SCN	●	35.40	(−10.63)	●	38.02	(−16.37)	●	55.00	(−54.73)	●	76.59	(−0.85)	●	2nd H
	—			—			●	30.15	(40.0)	●	75.02	(0.84)	●	2nd C
N(CN)2	—			—			●	45.24	(−27.11)	●	70.59	(−22.87)	●	2nd H
	—			—			●	40.14	(51.61)	●	70.33	(0.84)	●	2nd C
NO3	●	11.13	(−10.47)	●	33.94	(−1.80)	●	57.17	(−12.03)	●	104.13	(−1.11)	●	2nd H
	●	19.91	(8.61)	●	29.13	(1.94)	●	46.29	(11.95)	●	103.98	(1.02)	●	2nd C

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The data in Fig. 3 suggest that there are four types of guanidinium salts Gua(14)X depending on the size and topology of the anion. For example, guanidinium salts with spherical anions X = Cl, Br, I, BF₄, PF₆ exhibited broad mesophases with up to 84 K phase widths and clearing temperatures up to 112 °C. The anion size had only little effect on the phase behavior. For example, although the anion diameter of PF₆ is 1.5 times larger than Br, both guanidinium salts Gua(14)Br and Gua(14)PF₆ showed similar melting and clearing transition. The second type of guanidinium salts Gua(14)X carrying anions derived from Brønsted acid sulfonates were mostly non-mesomorphic (X = OTf, OTs) except for the mesylate (X = OMs), which displayed a broad mesophase between 26 °C and 83 °C. The third type contained linear anions, such as X = SCN, OCN, N₃. But only the synthesis of Gua(14)SCN led to the desired product, and it exhibited mesomorphic properties.

Fig. 3 Mesophase widths [K] of guanidinium ILCs Gua(14)X with different anions.

The fourth type consisted of bent or C₃-symmetrical anions such as N(CN)₂ or NO₃. Both guanidinium salts Gua(14)N(CN)₂ and Gua(14)NO₃ displayed mesophases, however the temperature range for nitrate was larger as compared to dicyanamide, presumably due to the improved delocalization and net charge compensation in the latter case. The following phases were observed: hexagonal columnar Col_h, crystalline Cryst. Assignment of phase types was based on the XRD experiments. For details, see Fig. 5 and Table 2 and the corresponding text.

Table 2 XRD results of guanidinium ILCs Gua(14)X^ab

Gua(14)X	Anion X (θ/Å)	Mesophase (T/°C)	Reflexes/Å exp. (calc.)	Miller indices	Lattice parameters/Å
a Anion diameters (except OMs, N(CN)2) were taken from ref. 84. b Anion diameters of mesylate and dicyanamide were estimated from Chem3D ball & stick models.
Gua(14)Cl	Cl	Colh at 65 °C	33.24	(10)	a = 38.38
	3.62	P6mm	19.20 (19.19)	(11)
			16.62 (16.62)	(20)
			4.65	Halo
Gua(14)Br	Br	Colh at 65 °C	33.23	(10)	a = 38.38
	3.92	P6mm	19.18 (19.19)	(11)
			16.60 (16.62)	(20)
			4.56	Halo
Gua(14)I	I	Colh at 65 °C	29.86	(10)	a = 34.48
	4.40	P6mm	4.52	Halo
Gua(14)BF4	BF4	Colh at 65 °C	34.06	(10)	a = 39.33
	4.64	P6mm	19.77 (19.67)	(11)
			17.07 (17.03)	(20)
			4.12	Halo
Gua(14)PF6	PF6	Colh at 65 °C	33.18	(10)	a = 38.32
	5.80	P6mm	19.16 (19.67)	(11)
			16.60 (17.03)	(20)
			4.59	Halo
Gua(14)OMs	Oms	Colh at 65 °C	33.85	(10)	a = 39.09
	3.77	P6mm	19.52 (19.54)	(11)
			16.93 (16.93)	(20)
			4.53	Halo
Gua(14)SCN	SCN	Colh at 65 °C	32.88	(10)	a = 37.97
	4.26	P6mm	18.96 (18.98)	(11)
			16.47 (16.44)	(20)
			4.53	Halo
Gua(14)N(CN)2	N(NC)2	Colh at 65 °C	34.77	(10)	a = 40.15
	4.51	P6mm	20.08 (20.07)	(11)
			17.39 (17.38)	(20)
			4.52	Halo
Gua(14)NO3	NO3	Colh at 65 °C	34.01	(10)	a = 39.27
	4.00	P6mm	19.70 (19.64)	(11)
			17.01 (17.01)	(20)
			4.59	Halo

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Under the POM, all mesomorphic guanidinium salts displayed fan-shaped textures, indicating the presence of a columnar mesophase in analogy to the known chloride salt Gua(14)Cl.⁶⁴ Typical examples are shown in Fig. 4 for the bromides, Gua(14)Br (Fig. 4a) and dicyanamide Gua(14)N(CN)₂ (Fig. 4b). The POM textures of all guanidinium salts are summarized in Fig. S5 and S6 (ESI†).

Fig. 4 POM textures of (a) Gua(14)Br at 98 °C and (b) Gua(14)N(CN)₂ at 71 °C upon cooling from the isotropic phase (100× magnification, heating/cooling rate 5 K min⁻¹).

The mesophase geometry of the liquid crystalline guanidinium salts Gua(14)X was assigned by X-ray diffraction (XRD) using small- and wide-angle scattering (SAXS, WAXS), respectively. For example, bromide Gua(14)Br showed three sharp reflections at 33.24 Å, 19.20 Å and 16.62 Å respectively, which were assigned to the (10) (11) and (20) reflections with reciprocal d-spacings of 1 : 1/√3 : 1/2 of a hexagonal columnar (Col_h) mesophase with space P6mm and a lattice parameter a = 38.38 Å (Fig. 5a). In the wide-angle section, a broad halo around 4.6 Å was visible due to the liquid-like disorder of the alkyl side chains (Fig. 5b).

Fig. 5 (a) Small angle scattering (SAXS) and (b) wide angle scattering (WAXS) profile of Gua(14)Br at 65 °C upon cooling with the corresponding diffraction patterns (inset). (c) SAXS and (d) WAXS profiles of Gua(14)PF₆ at 65 °C upon cooling with the corresponding diffraction patterns (inset).

In agreement with the DSC results, Gua(14)PF₆ exhibited a similar diffractogram and lattice parameter (a = 38.32 Å) (Fig. 5c and d) despite a much larger size of the PF₆ anion. Overall, when comparing the lattice parameters of the various guanidinium salts Gua(14)X shown in Table 2, no obvious trends were visible. This is quite surprising, as one would expect that the counterion contributes to a significant extent to the size of the pizza-slice packing (Fig. 6), which we propose for these wedge-shaped ILCs in agreement with the previous work.^{64,69,75,82,83} Details of the XRD data are summarized in Fig. S9–S11 (ESI†). It should be noted that in several reported cases, anion variation does not change the mesophase geometry or space group, i.e. for wedge-shaped mesogens with 3 alkoxy side chains, the space filling of the cation strongly favors the hexagonal columnar (P6mm) geometry.^69,75,83 However, for ILCs with bulky or non-spherical anions (e.g. with long side chains), mesophases with other space groups can be formed.⁷⁰

Fig. 6 (a) Possible packing model for the wedge-shaped guanidinium ILCs Gua(14)X with schematic representations in pizza slice-like form and (b) the hexagonal columnar (Col_h) mesophase: cationic head group (blue); anion (orange); periphery with chains (purple).

The thermal stabilities of the mesomorphic compounds were determined through TGA measurements. The detailed results can be found in Fig. S14 and Table S9 (ESI†). Overall, the compounds showed decomposition temperatures between 240 °C (Gua(14)NO₃) and 305 °C (Gua(14)N(CN)₂). However, for most compounds, decomposition started around 280 °C, i.e. far above the clearing temperatures.

Electrochemical studies of guanidinium ILCs

The redox stability of the mesomorphic guanidinium salts Gua(14)X was investigated by cyclic voltammetry (CV). The electrochemical stability window ΔEW is defined as the potential window where no significant oxidative or reductive processes of the sample occur, and consequently no noteworthy current can be observed when applying the CV potential.^50,85 Therefore, ΔEW is described by eqn (1) with E_an and E_cat being the respective anodic and cathodic potential limits of each individual compound.^86,87

ΔEW = E_an − E_cat (1)

One of the obstacles for measuring ΔEW for the guanidinium salts Gua(14)X was their low solubility in solvents that are commonly used in CV measurements. Consequently, several factors had to be considered for the actual measurements: (I) use of the same solvent for all compounds (better comparison), (II) a reasonable solvent stability window in the cell set-up, (III) solubility of the internal standard (ferrocene) and conducting salt in the solvent for the measurement of a solvent stability window. Based on these factors, only dichloromethane (DCM) was suitable as a solvent for the CV measurements. The combination of DCM and ferrocene as well as Bu₄NPF₆ as conducting salt showed a stability window in the aforementioned cell set-up between +1.2 and −2.0 V.

Overall, for most compounds Gua(14)X, the oxidation end of ΔEW was determined by the cation. All compounds showed a distinctive oxidation peak around +0.9 V, which can therefore be attributed to the cation effect as only the cation remains unchanged (Fig. 7). The peak itself was slightly shifted depending on the anion but always appeared around +0.9 V (Table 3). It must be pointed out that a direct comparison with the literature data is not trivial as many of the guanidinium IL examples found in the literature are structurally only slightly similar to the guanidinium salts Gua(14)X investigated in this study. Most guanidinium derivatives for which CV data can be found in the literature are either part of a piperidinium structure or contain six alkyl chains of various lengths. The pure guanidinium trifluoromethane sulfonate investigated by Zhu et al. showed an anodic stability of at least +0.8 V (vs. pseudo Pt).⁵⁴ Several guanidinium ILs containing three different anions OTf, NTf₂, and N(CN)₂ studied by Gnahm et al.⁵⁵ exhibited maximum anodic potential limits of +0.6 V to +1.0 V when in contact with an Au surface and compared against. an Ag pseudo-reference electrode.

Fig. 7 (a) Cyclic voltammograms of all guanidinium salts Gua(14)X. Cyclic voltammograms of Gua(14) cations with (b) halide and pseudo-halide anions, (c) fluorine-containing anions, and (d) additional anions.

Table 3 Electrochemical properties of all guanidinium salts Gua(14)X studied

Compound	E an [V vs. Fc/Fc^+]		E cat [V vs. Fc/Fc^+]	ΔEW [V]peak	ΔEW [V]cut-off
	Peak maximum	Cut-off
a Could not be determined. b Not referenced to Fc/Fc^+. c 1st CV cycle, as not reproducible in multiple cycles.
Gua(14)Cl	0.91	0.47	−1.03	1.94	1.50
Gua(14)Br	0.45; 0.85	0.17	−1.10	1.95	1.27
Gua(14)I	0.27; 0.61	0.08
Gua(14)BF4	0.87	0.55	−1.11	1.98	1.66
Gua(14)PF6	0.97	0.66	−1.79	2.76	2.45
Gua(14)OMs	0.98	0.44	−1.82	2.80	2.26
Gua(14)SCN	0.96	0.31	−1.83	2.79	2.14
Gua(14)N(CN)2	0.96	0.79	−1.74	2.70	2.53 ^c
Gua(14)NO3		0.66	−1.52		2.18

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Similar compounds were also investigated by Berger et al.⁵⁶ Their guanidinium ILs showed anodic limits vs. Pt between +1 V and +1.4 V for different alkyl chain lengths and +1.5 V to +1.7 V for piperidinium-based ILs.⁵⁶ More guanidinium stability windows were described in the dissertation of Arkhipova,⁶³ ranging from +0.6 V to +1.6 V depending on the alkyl chains and anions, which were then further used in the investigation as possible electrolytes for lithium ion batteries in combination with lithium salts.^57,63 Other functionalized guanidinium ILs showed significantly higher anodic limits.^28,29,58 Aromatic complexes with guanidino-functionalized aromatic ligands which usually contain more than one guanidine-function also showed an oxidation process around +1 V.^59–62 An overview of the literature guanidinium compounds and their cyclic voltammetry data for better comparison can be found in Table S8 (ESI†).

Some of the guanidinium salts Gua(14)X (X = Cl, Br, OMs, SCN, I, BF₄) investigated in this study showed an influence of the anion upon the oxidation peak/cut-off potential or a limiting influence on the oxidative side of ΔEW, which will be discussed below. However, no such negative effect was observed for Gua(14)N(CN)₂, Gua(14)NO₃ and Gua(14)PF₆, when analyzing the cut-off current (Table 3). Many of the anions represented in this study (BF₄, OMs and PF₆) have been used in previous electrochemical studies of ILs, showing that there are numerous instances of oxidative stability for substances containing these anions up to at least +2 V.^47,88–90 As a result, the peak around +1 V most likely originates in the cation. This observation is in agreement with a previous report by Himmel and co-workers,⁶² indicating that guanidino-functionalized aromatic compounds can act as electron donors. Our hypothesis was also supported by DFT calculations (see below).

The guanidinium ILCs Gua(14)X containing halide anions displayed typical redox behavior associated with these halides. The most complex cyclic voltammogram was obtained for Gua(14)I. Multiple signals between +0.2 and +0.6 V (Fig. S12a, ESI†) could be attributed to the electrooxidation of the iodide anion, with the first oxidation peak corresponding to the oxidation of iodide to molecular iodine followed by the formation of the triiodide anion. The second oxidation peak was then assigned to the dissociation of said triiodide.^91–93

The CV curves for the other guanidinium ILCs Gua(14)X with halide and pseudo halide counterions can be found in Fig. 7b. Gua(14)Br showed distinct peaks around +0.5 and +0.8 V with an additional shoulder at +0.7 V. These signals might be caused by the same mechanisms observed in Gua(14)I – namely the stepwise electrooxidation of the bromide anion.⁹⁴ Compared to the iodide anion, the potentials observed for bromide were shifted to higher values and the second oxidation steps might coincide with the oxidation of the cation; this could explain the prominent shoulder of the second peak. In contrast, the electrooxidation of the chloride anion was not visible in the CV data, as the potentials for the chloride oxidation steps seemed to be higher than for the oxidation of the guanidinium cation; this is consistent with the reports from the literature.⁹⁵

Gua(14)SCN was another guanidinium salt for which an influence of the anion on the oxidation limit was observed. The oxidation peak was slightly shifted towards lower potentials and was also very broad. This behavior can be justified by the pseudo-halide nature of the SCN anion. This anion exhibits quasi-halide electrochemical behavior, forming an (SCN)₂ dimer upon oxidation.^48,96 This process might again overlap with the oxidation of the cation as seen for Gua(14)Br, resulting in a broad peak and a comparatively low cut-off potential.

No real peak was visible in the voltammogram of Gua(14)NO₃ (Fig. 7d), as this compound only showed a broad oxidation signal. Even though this compound possessed one of the widest ΔEW_cut-offs, the oxidation current measured in the three scans showed decreasing signal intensities with each scan (Fig. S12c, ESI†), indicating a depletion of ions and slow diffusion close to and/or deactivation of the electrode surface by reaction products. Therefore, reproducible CV scans could not be obtained and no reliable statement on the oxidative stability of this compound could be made.

A similar observation was made for Gua(14)N(CN)₂ (Fig. 7d). This compound showed one of the widest ΔEW_cut-offs and highest E_an,cut-off for the 1st CV cycle. As ILs containing this anion were already shown to have oxidative stability up to +2 V, the oxidative process in this CV measurement might be attributed to the cation.^50,97 However, even in the 1st cycle, the experimental current was very low. Furthermore, the 2nd and 3rd cycles showed almost no current response when scanned in the same potential range. This non-reproducibility has already been observed for other guanidinium compounds containing the same anion.⁵⁵ These observations suggest that at least in part, the anion is also involved in the processes leading to the observed signals. Examples include, but are not limited to, the formation of the neutral dimer (NC)₂N–N(CN)₂.⁹⁷

The most stable compounds towards reduction can be seen in Fig. 8. Here, Gua(14)SCN, Gua(14)PF₆ and Gua(14)OMs showed the highest reductive stability. This high stability towards reduction has already been reported for compounds containing the OMs and PF₆ anion.^{47,89,98–100}

Fig. 8 (a) Cyclic voltammograms of selected guanidinium salts Gua(14)X scanned to lower potentials, (b) magnified view of cyclic voltammograms between −1.0 and −2.0 V.

When looking closely at the cyclic voltammograms of most compounds (except for Gua(14)PF₆, Gua(14)N(CN)₂ and Gua(14)NO₃), a small broad reduction peak or deviation from the current baseline between −0.5 and −1.0 V was observed. This process seems to be more or less pronounced depending on the different guanidinium salts Gua(14)X (Fig. 7a). We suggest two possible causes for this observation, both of which are supported by the results of more detailed investigations: (I) presence of traces of water in the compounds, (II) delayed reduction of some oxidation products.

Fig. 9a and b show the voltammograms of DCM + ferrocene, as well as Gua(14)OMs with and without added water. Clearly, the presence of water influences the reduction side of the voltammogram between −0.5 and −1.0 V. There was no signal in the data obtained from pure DCM, and only a small signal was visible in the data obtained from pure Gua(14)OMs. The same signal increased upon the addition of water. This suggested that the process between −0.5 and −1.0 V detected for the different guanidinium salts Gua(14)X could be associated with the reduction of water. A limiting influence of water on the electrochemical stability window of ILs is well known.^50,89

Fig. 9 (a) Cyclic voltammogram of DCM with and without water, (b) cyclic voltammogram of Gua(14)OMs with and without water, (c) cyclic voltammogram of Gua(14)Br with and without Fc and scanned to different potentials (curve without Fc normalized to the curve measured with Fc; original curves can be found in Fig. S12, ESI†), (d) cyclic voltammogram of Gua(14)Cl without Fc scanned to different end potentials.

However, as already mentioned above, this additional signal could also be caused by the re-reduction of oxidation products. This can be illustrated as follows: Fig. 9c and d display comparisons of different measurements of Gua(14)Br and Gua(14)Cl. For Gua(14)Br, the comparison between measurements with and without ferrocene showed that in the measurement without ferrocene, which was (normalized to the measurement with ferrocene) only scanned to a potential of +0.8 V, no reduction peak was visible (Fig. 9c). The same result could be observed for Gua(14)Cl, where a sample without ferrocene was scanned to +0.8 V and +1.2 V (not referenced to Fc/Fc⁺, Fig. 9d). The measurement up to +1.2 V showed a small peak at −0.4 V, whereas in the measurement stopped at a lower potential of +0.8 V, such a peak was absent, confirming the above hypothesis. However, as previously shown, the influence of traces of water could not be entirely ruled out. It is therefore not possible to exclusively assign this peak to either of the processes mentioned above, as they both are still reasonable possibilities at this stage.

In conclusion, the most significant limitation on ΔEW of the investigated compounds could be found for the anions, BF₄, Br, Cl, I, and SCN. In contrast, the highest ΔEW_cut-off that can reproducibly be measured was found for Gua(14)PF₆ and the smallest ΔEW_cut-off was found for Gua(14)Br and Gua(14)Cl (Table 3).

Theoretical studies of guanidinium salts

It has been postulated above that for some guanidinium salts Gua(14)X, the distinctive peak at around +0.9 V (vs. Fc/Fc⁺) originates in a cation oxidation process. To further support this hypothesis, DFT calculations were performed for Gua(14)Br, Gua(14)NO₃ and Gua(14)PF₆. They were chosen as representative examples of compounds influenced by the anion (Br), seemingly not influenced by the anion (PF₆) and inconclusive with regard to the redox process (NO₃), when comparing ΔEW_peak. As CV measurements are based on redox processes and therefore electron transfers, an investigation of the HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) energies for each anion in comparison to the respective cation energy is helpful for understanding the redox processes.^101,102

All calculations were performed using the Gaussian 16 program package with the B3LYP, M062X, and PBE functionals and basis sets as implemented therein,¹⁰³ and carried out for isolated ions, i.e. a single anion or cation.

First test calculations were performed on guanidinium cations Gua(n) with different chain lengths n with B3LYP/6-31G* in order to investigate the influence of the alkyl chains on the HOMO and LUMO energies of the cation (Table S3, ESI†). The original cation Gua(14) and simplified versions with methyl Gua(1), ethyl Gua(2), propyl Gua(3) and butyl Gua(4) chains were tested. All structures were fully optimized starting from a “straight” chain conformation and the optimized structures for Gua(2) and Gua(14) are shown in Fig. S13 (ESI†). The calculations indicated that Gua(2) seems to be reasonably close to Gua(14). Therefore, Gua(2) was used as a reference in the following investigations. Also Gua(1), Gua(3) and Gua(4) were investigated to estimate the uncertainty introduced by the approximate treatment of the alkyl chains. Gua(14) was not used for further calculations because geometry optimization particularly when including polarizable continuum models (PCMs)¹⁰⁴ for dichloromethane (DCM) was difficult to converge for the long alkyl chains. Note that entropy effects or different conformations, which could lead to the different anodic limits for different alkyl chain lengths found in the experimental investigations reported above, are not considered here. In the following discussion, we only refer to Gua(2). The full results can be found in the ESI† (Tables S4–S7).

As a next step, different basis sets, i.e. aug-cc-pVDZ and aug-cc-VTZ, were tested for the cations and the investigated anions, i.e. Br, NO₃ and PF₆, again using B3LYP with PCM for DCM and Grimme's D3 dispersion corrections.¹⁰⁵ These levels of theory are labeled as B3LYP/DZ and B3LYP/TZ in the following discussion. In order to test the influence of the used density functional, the same calculations were performed at the M062X-D3/aug-cc-pVDZ (label: M062X/DZ) and PBE-D3/aug-cc-pVDZ (label: PBE/DZ) level of theory. The respective ionization potentials relative to that of Gua(2) are given in eqn (2), i.e.,

IP^HOMO_X = −ε_HOMO,X + ε_HOMO,Gua(2) (2)

where ε_HOMO,X is the HOMO energy of the respective compound. The resulting values are given in Table 4 and Table S4 (ESI†). A negative value suggests that the compound would be oxidized more easily than Gua(2), while a positive value suggests the opposite. The results depended only to a very small extent on the basis set or the length of the alkyl chain (see Table S4, ESI†). However, the choice of the functional can lead to rather large differences up to 2 eV. We conclude from the IP^HOMO_X that only PF₆ has a higher IP than the cation, while the other anions have similar or slightly lower IPs.

Table 4 IP^HOMO_X, c.f.eqn (2), derived from the HOMO energies of the optimized anions X, IP^vert_X, c.f.eqn (3), derived from Δ-SCF calculations, and IP^adia_X, c.f.eqn (4), derived from Δ-SCF calculations. All values are given in eV and relative to the IP of Gua(2)

		B3LYP/DZ	B3LYP/TZ	M062X/DZ	PBE/DZ
IP^HOMOX	Br	−0.65	−0.64	−0.59	−0.75
	NO3	−0.56	−0.55	0.12	−1.25
	PF6	3.09	3.03	4.12	2.13
IP^vertX	Br	−0.46	−0.50	−0.87	−0.25
	NO3	−0.17	−0.18	−0.06	−0.45
	PF6	3.43	3.37	4.08	2.65
IP^adiaX	Br	0.23	0.20	−0.10	0.36
	NO3	0.46	0.45	0.63	0.11
	PF6	4.09	4.03	3.56	3.25

---

Finally, ΔSCF calculations were performed to obtain vertical and relaxed ionization potentials. For this purpose, structures of the ions with one electron removed, i.e. the neutral form of the anions and doubly positively charged cations (ions-e⁻) were optimized in addition to the structures of all anions and cations (ions). The following energy differences were calculated:

where, for instance, is the energy of the neutral form of the anions or doubly positively charged cations calculated at the respective optimized structures of the anions or the cations. −EA^vert_X corresponds to the vertical ionization potential at the optimized structure of the ion-e⁻ species. The respective values are given in Table 4 and Table S7 (ESI†).

As observed, the Δ-SCF results for IP^vert_X showed similar trends to the HOMO-derived IPs. For IP^adia_X, all anions (except Br for M062X/DZ) possessed a larger IP than the cations.

This indicated that nuclear reorganization is much more important for the cation than for the anions. This assumption was also supported by the values of −EA^vert_X, which were again larger for the anions than for the cations (Table S7, ESI†). We expect that the IP^vert_X values are the ones which fit best to the experiment because of the probably rather long-time scale needed for the nuclear reorganization in the Gua(X) cations, i.e. we think that the electron transfer process is reasonably close to a vertical transition.

Overall, as already stated above, only the PF₆ anion showed a higher IP^vert_X value than the cation, whereas Br and NO₃ showed smaller IP^vert_X (Table 4). These results were in good agreement with the observations from the CV measurements (Fig. 6), as Gua(14)Br clearly showed oxidative processes that precede those of Gua(14)PF₆. However, Gua(14)NO₃ did not show a distinct oxidation peak but rather a broad oxidation process. This might indicate an overlay between anion and cation processes. DFT calculations seem to support this interpretation because the results for IP^vert_X revealed that the value for the NO₃ anion is very close to that of the guanidinium cation Gua(2).

Conclusion

To get some insight into counterion effects on the solution redox properties and bulk phase liquid crystalline self-assembly of ILCs, a series of wedge-shaped 3,4,5-(tris(tetradecyloxy))-phenylguanidinium salts Gua(14)X with different anions were prepared. Unfavorable acid–base equilibria of salts with counterions derived from weak acids precluded access to Gua(14)X with X = N₃, OAc, CN, OCN. In contrast, Gua(14)X with X = Cl, Br, I, BF₄, PF₆, N(CN)₂, SCN, NO₃, OMs could be isolated and characterized. These guanidinium ILCs formed stable enantiotropic Col_h mesophases. For ILCs with spherical anions (X = Cl, Br, I, BF₄, PF₆), clearing temperatures and mesophase ranges increased with the increasing size of the anion, except that BF₄ displayed the broadest phase and highest clearing transition. ILCs with non-spherical counterions seem to pack less efficiently in the Col_h phase, thus resulting in smaller phase widths and lower clearing transition. This trend is most obvious for the dicyanamide anion. In this particular case, the relatively large diameter of the anion and the less efficient packing in the mesophase resulted in the largest packing parameter a.

Electrochemical investigations of Gua(14)X revealed a distinctive oxidation around +0.9 V caused by the guanidinium cation. A contribution of the guanidinium cation to the oxidation signals was further supported by DFT calculations. The broadest electrochemical stability window was found for X = PF₆, while the smallest electrochemical stability window was found for X = Br. Overall, Gua(14)PF₆ showed both the best electrochemical performance in solution, the highest mesophase stability (i.e. highest clearing temperature) and the broadest temperature range of the Col_h phase. Thus, this ILC seems to be the most promising candidate for further elaboration as an ordered electrolyte material. Future work must show whether this is indeed the case.

Author contributions

M. E. and E. W. synthesized and characterized the compounds and investigated the mesomorphic properties via POM and DSC. M. M. performed the XRD (WAXS, SAXS) studies. A. L. performed all electrochemical studies and wrote the electrochemical section of the manuscript. A. Z. wrote the manuscript and checked the data. T. K. performed the theoretical calculations and wrote the respective section. F. G. provided his expertise for the analysis of the XRD data. A. T. and S. L. coordinated and supervised the research and wrote the manuscript. All coauthors proof-read the manuscript. M. E., A. L. and M. M. contributed equally to this work.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

Generous financial support by the Deutsche Forschungsgemeinschaft (LA 907/17-2, GI243/8-2, TA571/13-1, 13-2, 16-2 and HBFG and shared instrumentation grant no. INST 41/897-1 FUGG for 700 MHz-NMR), the Ministerium für Wissenschaft, Forschung und Kunst des Landes Baden-Württemberg, the Carl-Schneider Stiftung Aalen (shared instrumentation grant), the Bundesministerium für Bildung und Forschung (shared instrumentation grant # 01RI05177) is gratefully acknowledged.

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4cp00356j

‡ These coauthors contributed equally to this work.

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