A redox-active ionic liquid manifesting charge-transfer interaction between a viologen and carbazole and its effect on the viscosity, ionic conductivity, and redox process of the viologen

Abstract

Redox-active ionic liquids (RAILs) are gaining attention as a material that can create a wide range of functions. We herein propose a charge-transfer (CT) RAIL by mixing two RAILs, specifically a carbazole-based ionic liquid ([CzC₄ImC₁][TFSI]) as a donor and a viologen-based ionic liquid ([C₄VC₇][TFSI]₂) as an acceptor. We investigated the effect of CT interaction on the physicochemical properties of the CT ionic liquid (CT-IL) using the results of temperature-dependent measurements of UV-vis absorption, viscosity, and ionic conductivity as well as cyclic voltammograms. We employed the Walden analysis and the Grunberg–Nissan model to elucidate the effect of the CT interaction on the viscosity and ionic conductivity. The CT interaction reduces the viscosity by reducing the electrostatic attraction between the dicationic viologen and TFSI anion. It also reduces the ionic conductivity by the CT association of the dicationic viologen and carbazole. The electrochemically reversible responses of the viologens in [C₄VC₇][TFSI]₂ and CT-IL are consistent with the Nernstian and the interacting two-redox site models. Notably, the transport and electrochemical properties are modulated by CT interaction, leading to unique features that are not present in individual component ILs. The inclusion of CT interaction in RAILs thus provides a powerful means to expand the scope of functionalized ionic liquids.

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1. Introduction

Charge transfer (CT) interaction between a donor and acceptor has been of great interest because it can determine the material structures, electric conduction, and photoconduction in solid state.¹ In addition, it can give rise to unique dynamical physicochemical properties of Stoddart's supramolecules,^2,3 intermediate complex of chemical reaction,⁴ and photochemically active species^5–8 in solution phase. CT interaction never emerges in its individual component species but appears as a synergetic effect. The CT interaction depends on the combination of a donor and acceptor, and one can exploit such combinations in ionic liquids to produce new photoactivity and to modulate their physicochemical properties.

A myriad of ionic liquids (ILs) with various physicochemical properties have been synthesized. Because ILs are designer solvents⁹ that can be readily created by combining organic cations and anions, one can straightforwardly tune their viscosity, ionic conductivity, and melting point, as well as the solubility of substrate molecules. This leads to some notable advantages over typical organic solvents. For example, ILs are often preferred due to the higher solubility of solutes such as cellulose¹⁰ and CO₂.^11,12 Therefore, they are frequently applied to extraction and separation chemistry.¹³ ILs also provide interesting functionalities rarely found in organic molecular solvents,¹⁴ leading to unique IL-based liquid materials. In addition, organic cations can be modified to further transform ILs into functionalized ionic liquids (FILs) or task-specific ionic liquids.¹⁵

Extensive developments of FILs have led to functional liquid materials that are not merely a solvent or medium. The high concentrations in FILs (around 1–2 M) lead to strong interactions between functional groups and ions; these interactions can play dominant roles in new IL-based functional materials. For example, magnetic ILs consisting of metal complexes have been reported.^16–18 Hisamitsu et al. reported photo-functionalized ionic liquids with photon-up-conversion characteristics,^19,20 with concentrated chromophores being a critical design in their FILs. Murray et al. reported conductive redox-active ILs (RAILs) that comprise metal complexes and viologens.^21–23 In addition, Saielli et al. reported the thermal properties of various viologen-based ILs and ionic liquid crystals, which are correlated with their structure.^24–26 These findings demonstrate the potential of viologens to be a potent basic component for developing and creating FILs.

Several applications of RAILs in electrochemical reactions and devices have also been reported; they are used as components in supercapacitors,^27,28 redox flow batteries,²⁹ and electrochromic devices,^30–32 and as electron mediators for photochemical catalysts.³³ Additional examples have been given in recent reviews.^34,35 They highlight the extended utility of RAILs as a class of FILs. We note that, in most of the cases, functionalities of FILs originate from individual functional groups and ions.

Let us now consider the use of CT interaction in ILs to create a new class of FILs. Kato et al. first reported an ion pair formation between an imidazolium and iodide in a neat imidazolium-based IL ([BMIM][I]), resulting from CT interaction between them.³⁶ They indicate that the Madelung potential of [BMIM][I] determines the stabilization of the CT complex. Ogura et al. reported CT interaction between pyridinium as an acceptor and iodide as a donor in neat pyridinium-based IL ([Epy][I]) at 100 °C.³⁷ They found that the CT complex in the neat [Epy][I] shows a different photo-absorption band from the diluted solution in dichloromethane, and they pointed out the importance of CT interaction between cation and anion in the neat [Epy][I]. Aster and Vauthey studied the CT interaction between anions of ILs and electron accepting additives in a photo-induced electron transfer reaction between the species.³⁸ They concluded that the ILs play an active role in such photochemical systems rather than being an inert medium. With the incorporation of CT interactions into ILs, we may create a new functionality that is not possessed by individual donors and acceptors, and thereby modulate the physicochemical properties of the materials. In this study, we aim to exploit the functionality of viologens to design FILs based on CT interaction. In this regard, CT interactions between donors and viologens have been applied for assembling molecules,^2,39 photo-induced electron transfer reactions,^5–8,40,41 alkali metal sensing,⁴² and thermochromism.⁴³ These studies demonstrated the potential of incorporating CT interactions into viologen-based ILs to pave a new path in IL research. To the best of our knowledge, research into RAILs with externally introduced CT interaction has not been attempted.

Intermolecular and interionic interactions in ILs have been investigated to understand the relationship between the structures and dynamic transport properties such as viscosity and ionic conductivity. In these species, electrostatic, van der Waals, hydrogen bonding, dipole–dipole, and π–π interactions all play important roles in the formation of microstructures, leading to unanticipated transport features and functionalities.^44,45 Understanding these strong and weak interactions will provide new insights into the transport properties and functionalities in ILs. In this regard, it is noteworthy that the effect of CT interaction in ILs on the physicochemical and electrochemical properties is presently unclear.

In this study, we present a “RAIL with CT interaction” (CT-IL) originating from two redox centers, a carbazole and viologen; it is based on an equimolar mixture of a viologen-based IL and a carbazole-based IL. As we shall see, in the CT-IL, the CT interaction between the carbazole and viologen modulates the physicochemical properties such as viscosity and ionic conductivity, as well as redox properties including redox reversibility of viologens. Our approach provides a convenient means for creating and tuning of CT-IL, with properties that are distinct from those of the components.

2. Experimental

Chemical structures of the carbazole-based RAIL ([CzC₄ImC₁][TFSI]) and the viologen-based RAIL ([C₄VC₇][TFSI]₂) in this study are shown in Scheme 1, with a photo of the combined CT-IL straightforwardly prepared from an equimolar mixture between them. Synthetic procedures of the RAILs, water content in the RAILs, and experimental setups are described in the ESI.† All measurements in this study were conducted in neat RAILs, namely without any solvent, unless otherwise mentioned. [C₄VC₇][TFSI]₂ remains in a supercooled liquid state for a few hours at room temperature after it was heated above the melting point (52 °C).⁴⁶ Although the melting points of [CzC₄ImC₁][TFSI] and CT-IL could not be determined by differential scanning calorimetry (DSC) because of the absence of any peak assignable to the melting point, their solidification was not observed at room temperature (see the DSC thermograms in Fig. S1†). Therefore, [C₄VC₇][TFSI]₂, [CzC₄ImC₁][TFSI], and CT-IL can be handled as room temperature ILs.³²

Scheme 1 Preparation of a redox-active ionic liquid (RAIL) with charge transfer (CT) interaction from carbazole-based and viologen-based RAILs.

3. Results and discussion

3.1 Optical properties

3.1.1 Absorption spectra. To investigate the CT characteristics of the RAILs, we recorded the absorption spectra of [CzC₄ImC₁][TFSI], [C₄VC₇][TFSI]₂, and the CT-IL in the neat system (Fig. 1(a)). The neat [CzC₄ImC₁][TFSI] and [C₄VC₇][TFSI]₂ have no absorption band over the visible region as well as their diluted acetonitrile solutions (Fig. S2†). For the neat CT-IL, a new broad absorption band at 427 nm was observed. The absorption band is attributable to the absorption by the CT complex between the carbazole and viologen.⁷ Specifically, our absorption spectrum is in good agreement with the CT absorption spectrum reported by Yonemura et al.⁵ for a diluted carbazole-tethered viologen, in terms of not only peak wavelength but also line width. Thus, the formation of additional species such as higher aggregates of the CT complex in CT-IL is unlikely. The CT absorption band of CT-IL at 427 nm monotonically decreased with increasing the temperature, holding the peak wavelength invariant. This further supports a simple equilibrium of 1 : 1 complex of carbazole and viologen, the same as in the diluted system. Taking these considerations into account, the temperature dependence can be explained by the association/dissociation equilibrium of the carbazole and viologen (Cz + V⁺⁺ ⇌ CzV⁺⁺).

Fig. 1 Absorption spectra of [CzC₄ImC₁][TFSI], [C₄VC₇][TFSI]₂, and CT-IL. (a) The neat RAILs (20 µm optical path length) at 298 K. (b) The temperature-dependence of the neat CT-IL. The inset plot is the absorbance of the CT absorption at 427 nm wavelength as a function of the temperature. (c) x mM [CzC₄ImC₁][TFSI] (x = 2, 5, 10, 20) in [C₄VC₇][TFSI]₂ at 297 K and (d) y mM [C₄VC₇][TFSI]₂ in [CzC₄ImC₁][TFSI] (y = 2, 5, 10, 20) at 297 K (0.1 cm optical path length).

As a first approximation, we do not consider carbazole–carbazole interaction and viologen–viologen interaction. The total concentration (c₀) of the carbazoles and viologens of CT-IL here was 0.92 M in the 1 : 1 (molar ratio) mixture (c₀ = [Cz]₀ = [V⁺⁺]₀) (see the ESI†). The concentration of the CT complex (c_CT) can be described by the thermodynamic equilibrium constant.

The molar absorption coefficient (ε_CT) of the CT complex between carbazoles and viologens was reported to be around 400 ± 70 M⁻¹ cm⁻¹ at 420 nm.⁷ However, the absorption coefficient is invalid in our CT-IL because if we use it as an approximated value, the concentration of the CT complex is estimated to be over 1.0 M at 298 K with the Lambert–Beer relation (Abs_CT = ε_CTLc_CT) despite c₀ = 0.92 M. Usually, those parameters can be determined from the concentration dependence of the donor and acceptor in the same solvent. However, CT-IL here consists of only the donor ([CzC₄ImC₁][TFSI]) and acceptor ([C₄VC₇][TFSI]₂) without solvent. That is, the donor and the acceptor themselves play the role of a medium as well. Thus, the concentration and the fraction changes give rise to the polarity changes of the solvent (medium). To approximate ε_CT and K_CT of the CT-IL, we prepared two sets of diluted solutions, (1) x mM [CzC₄ImC₁][TFSI] diluted by [C₄VC₇][TFSI]₂ (x = 2, 5, 10, 20) and (2) y mM [C₄VC₇][TFSI]₂ diluted by [CzC₄ImC₁][TFSI] (y = 2, 5, 10, 20), and measured the absorption spectra. Note that two sets of solutions satisfy the following concentration relationship: [CzC₄ImC₁] (x mM, solute) ≪ [C₄VC₇] (1.8 M, solvent) in system (1) and [C₄VC₇] (y mM, solute) ≪ [CzC₄ImC₁] (2.7 M, solvent) in system (2). Besides, the polarity of [C₄VC₇][TFSI]₂ and [CzC₄ImC₁][TFSI] could be close to that for the CT-IL. Fig. 1(c) and (d) show absorption spectra of the CT complex in systems (1) and (2). The absorbances were proportional to the concentration of the diluted species. In these cases, i.e. with a large excess of C₄VC₇ or CzC₄ImC₁, the absorbance of the CT complex can be described as follows.

where [Cz]₀ and [V]₀ are the starting concentration of the carbazole and viologen. The concentration of the CT complex can be approximated to be that for the diluted [Cz]₀ or [V]₀ when K_CT⁻¹[V]₀⁻¹ ≪ 1 namely K_CT ≫ 0.56 M⁻¹ for eqn (2) (system (1)) and K_CT⁻¹[Cz]₀⁻¹ ≪ 1 namely K_CT ≫ 0.37 M⁻¹ for eqn (3) (system (2)). Under the assumptions of K_CT⁻¹[V]₀⁻¹ ≪ 1 and K_CT⁻¹[Cz]₀⁻¹ ≪ 1, the molar absorption coefficients of the CT complex were determined from the slopes of the obtained straight lines in Fig. 1(c) and (d) to be 780 and 980 M⁻¹ cm⁻¹. Those values greater than the previously reported value⁷ are possibly due to the polarity difference of the medium. Furthermore, the peak wavelength of the CT absorption in CT-IL (427 nm) locates between the wavelengths in [C₄VC₇][TFSI]₂ (414 nm) and [CzC₄ImC₁][TFSI] (436 nm). The polarity of the CT-IL, i.e., with an equimolar mixture of [CzC₄ImC₁][TFSI] and [C₄VC₇][TFSI]₂, can be in between the polarities of [CzC₄ImC₁][TFSI] and [C₄VC₇][TFSI]₂. The concentration of the CT complex (c_CT) in CT-IL is 0.48 M at 298 K with an assumption of ε_CT to be 880 M⁻¹ cm⁻¹ in the CT-IL, which is taken as an average of the absorption coefficients in Fig. 1(c) and (d). Around 50 mol% carbazole and viologen in the CT-IL achieve the CT complexation. Its thermodynamic parameters are ΔG = −2.2 kJ mol⁻¹, ΔH = −13.9 kJ mol⁻¹, and ΔS = +39.4 J mol⁻¹ K⁻¹ (see Fig. S3 and Table S1†). The theoretical evaluation using density functional theory (DFT) showed ΔG = −3.6 kJ mol⁻¹, ΔH = −49.8 kJ mol⁻¹ and ΔS = +155 J mol⁻¹ K⁻¹ at 298 K with a model compound of CzV⁺⁺ in an IL medium (see the ESI†). The theoretical evaluation provided reasonable ΔG for CT-IL, although the assumption of the isolated CT complex surrounded by the continuous IL dielectric medium was far different from the real system.

3.1.2 Fluorescence quenching. Emissions of 10 µM [CzC₄ImC₁][TFSI] in acetonitrile and neat [CzC₄ImC₁][TFSI] were observed at around 360 nm when excited at 293 nm and 430 nm when excited at 328 nm (Fig. S4†). The emission of 10 µM [CzC₄ImC₁][TFSI] in acetonitrile is attributed to the monomolecular fluorescence of carbazole. The emission of neat [CzC₄ImC₁][TFSI] was located at longer wavelengths with a vibration structure. The origin of the emission in neat [CzC₄ImC₁][TFSI] is not clear but it is beyond our focus in this study. On the other hand, we observed no emission in CT-IL system. This can be attributed primarily to the fast fluorescence quenching by photo-induced electron transfer from the excited carbazole to the viologen.^5,47 As can be seen above, neat CT-IL shows rather different optical properties from individual RAILs and the diluted CT-IL in acetonitrile.

3.2 Viscosity and ionic conductivity

3.2.1 Arrhenius and Vogel–Fulcher–Tammann analysis. Temperature dependences of the viscosity and the ionic conductivity of [CzC₄ImC₁][TFSI], [C₄VC₇][TFSI]₂, and CT-IL are shown in Fig. 2(a) and (b). The viscosity increases in the order of CT-IL < [C₄VC₇][TFSI]₂ < [CzC₄ImC₁][TFSI] at temperatures between 293 K and 333 K, while the ionic conductivity shows a different order of [CzC₄ImC₁][TFSI] < CT-IL < [C₄VC₇][TFSI]₂ from the viscosity. The viscosity of CT-IL is the lowest among the three RAILs, while the conductivity of CT-IL is not the highest among them. To rationalize the trends, the activation energy was evaluated using the Arrhenius equations (eqn (4)), and the approximated interpolation equations (eqn (5)) were expressed by the Vogel–Fulcher–Tammann (VFT) formalism:

Fig. 2 Temperature-dependent transport properties of [CzC₄ImC₁][TFSI], [C₄VC₇][TFSI]₂, and CT-IL. (a) Viscosity, (b) conductivity, (c) Walden product of RAILs, and (d) interaction parameters W₁₂ of CT-IL evaluated using the Grunberg–Nissan model with neat [CzC₄ImC₁][TFSI] and [C₄VC₇][TFSI]₂. Solid lines in (a)–(c) are represented by fitted VTF lines. At the Walden product (c), the conductivities were divided by the formal concentrations (c_F) as constant values in the whole temperature range, 2.7 M for [CzC₄ImC₁][TFSI] (2.7 M CzC₄Im⁺C₁ and 2.7 M TFSI⁻), 1.8 M for [C₄VC₇][TFSI]₂ (1.8 M C₄V⁺⁺C₇ and 3.6 M TFSI⁻) and 0.92 M for CT-IL (0.92 M CzC₄Im⁺C₁, 0.92 M C₄V⁺⁺C₇, and 2.7 M TFSI⁻).

The best fit parameter-sets are listed in Table 1. The Arrhenius plot of the viscosities and ionic conductivities (Fig. S6†) shows a little deviation from straight lines. The coefficient of determination (R²) for the viscosity of CT-IL is 0.9914, and that for the ionic conductivity of [CzC₄ImC₁][TFSI] is 0.9761 on the basis of the Arrhenius equation. Taking these into consideration, we can see that these RAILs obey VFT formalism resulting in better R² values over 0.999.

Table 1 Fitting results of the viscosity and ionic conductivity data

RAILs	Arrhenius parameter set			Vogel–Fulcher–Tammann parameter set
	η = η0 exp(Eηa/RT)			η = η0 exp(DηT0/(T − T0))
	σ = σ0 exp(−Eσa/RT)			σ = σ0 exp(−DσT0/(T − T0))
	E ηa/kJ mol^−1	ln(η0/Pa s)	R ^2	D η T 0/K	T 0/K	ln(η0/Pa s)	R ^2
	E σa/kJ mol^−1	ln(σ0/S cm^−1)		D σ T 0/K		ln(σ0/S cm^−1)
a Viscosity data and the VTF parameters were taken from ref. 46.
[C4VC7][TFSI]2^a	63.1	−23.58	0.9941	1147	193.9	−9.053	0.9997
	64.2	16.21	0.9917	659.0	219.8	−1.303	0.9999
[CzC4ImC1][TFSI]	87.0	−32.03	0.9952	1413	194.7	−10.66	0.9999
	109	31.42	0.9761	774.5	230.3	−1.005	0.9996
CT-IL	62.8	−24.44	0.9914	880.8	200.6	−8.217	0.9995
	77.0	20.36	0.9896	1019	203.8	0.1992	0.9997

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The activation energy (E_ηa) for the viscosity follows the order CT-IL ≈ [C₄VC₇][TFSI]₂ < [CzC₄ImC₁][TFSI]. In comparison, activation energy (E_σa) for the conductivity shows a different order [C₄VC₇][TFSI]₂ < CT-IL < [CzC₄ImC₁][TFSI]. This suggests that the activation processes of viscosity and conductivity are different. This tentative conclusion is also supported by the difference in the trends for D_ηT₀ and D_σT₀ in VTF formalism, as interpreted from the activation energy, i.e., CT-IL < [C₄VC₇][TFSI]₂ < [CzC₄ImC₁][TFSI] for the viscosity and [C₄VC₇][TFSI]₂ < [CzC₄ImC₁][TFSI] < CT-IL for the conductivity. The activation energy (E_ηa) of the viscosity of CT-IL is reduced by mixing the two RAILs. To examine the characteristics of the ionic transportation, we use the Walden analysis and the Grunberg–Nissan^48–51 analysis below.

3.2.2 Walden analysis. For the investigation of transportation parameters of ILs, the Walden relation (ησ = constant) has often been used, even though it originally holds in a diluted ionic solution. The Walden product, ησ or ησ/c, remains constant, independent of the temperature in the absence of inter-ionic interactions. Fig. 2(c) shows the temperature dependence of the Walden product of the RAILs obtained by using the formal concentration (c_F), while the full Walden plot is shown in Fig. S7.† The density of ILs and thus c_F of the ions slightly decreased with increasing temperature because of volumetric expansion. However, the changes are negligibly small.^5,31 Therefore, we assumed the formal concentrations are temperature-independent constants for the Walden analysis.

In comparison, the Walden product of CT-IL increased with increasing temperature, while that of [CzC₄ImC₁][TFSI] remains nearly constant with a slight increase, and that of [C₄VC₇][TFSI]₂ slightly decreases. To put these results into perspective, the Walden product of a conventional IL [BMIM][BF₄] remains almost constant.⁵² In CT-IL, the ionic association and the CT complex formation can be deeply involved in the transport properties. Therefore, the temperature dependent Walden product of CT-IL is attributable to both equilibria: association and dissociation of (1) ionic components (between the viologen, imidazolium, and TFSI), and (2) CT complex (between the carbazole and viologen).

3.2.3 Grunberg–Nissan analysis. As an alternative method of analysis, we evaluated the viscosity of binary mixtures using the Grunberg–Nissan model.^48–51where η₁, η₂, and η_mix are the viscosities of compounds 1, 2, and their mixture, respectively; x₁ and x₂ are the molar fractions, and W₁₂ is the interaction parameter between compounds 1 and 2. If the interaction parameter W₁₂ is negligibly small, the viscosity of the mixture is a fraction-weighted logarithmic average, that is, it signifies a non-associated liquid. When W₁₂ is non-zero positive/negative, the viscosity of the mixture should be greater/smaller than those of the individual compounds. The experimentally obtainable interaction parameters for viscosity, and those for ionic conductivity, in the analogy to the viscosity, can be evaluated using the following expression.

Fig. 2(d) shows the temperature dependence of Δln η and Δln σ⁻¹ for CT-IL, obtained using the continuous data interpolated with the VTF formula of the viscosity and conductivity as well as the fitted parameters in Table 1. The interaction parameters were negative, and the magnitudes of both viscosity and conductivity decreased monotonically with increasing the temperature. The curvatures of Δln η and Δln σ⁻¹ are different, where Δln σ⁻¹ shows more convex behavior.

In the CT-IL system, two kinds of equilibria should be considered: (1) electrostatic cation–anion pair association and dissociation and (2) formation of the CT complex. In ionic liquids, electrostatic attraction between anions and cations dominates the attractive interaction that leads to the association of cations and anions (Fig. 3(a) and (b)). The associations can be examined in terms of ionicity by the impedance technique and nuclear magnetic resonance.⁵³ The decreased viscosity in CT-IL upon mixing [CzC₄ImC₁][TFSI] and [C₄VC₇][TFSI]₂ can be attributed to decreasing electrostatic attraction because of the formation of the CT complex between the carbazole and the viologen (Fig. 3(c)). Our DFT calculation supports the decrease of the interionic attraction. We employed a fragment molecular orbital (FMO) calculation to evaluate the CT interaction which contributes to the charge donation from the carbazole to the viologen. We found that the carbazole donates 0.163 electrons to the viologen resulting in decreasing its cationic charge, i.e. the formal charge can be depicted as Cz^+0.163V^+1.837. Besides, the attraction enthalpy difference between the viologen and TFSI is 63.4 kJ mol⁻¹ with carbazole and 65.9 kJ mol⁻¹ without carbazole (geometries of the contact ionic pairs are shown in Fig. S17†). Thus, the CT attraction can weaken the electrostatic attraction between the viologen and TFSI. Therefore, we may expect that decreased viscosity would lead to increasing ionic conductivity. However, the ionic conductivity of CT-IL was in fact smaller than that of [C₄VC₇][TFSI]₂.

Fig. 3 Illustration of attractions between the cations (imidazolium and viologen) and the anion (TFSI).

To explain the smaller ionic conductivity of CT-IL than [C₄VC₇][TFSI]₂, we note that the actual concentration of free ionic species is important for ionic conductivity. Formation of the CT complex would lead to decreasing concentration of the free dicationic viologens and suppressing the movement and migration of the viologens and imidazoliums linked with carbazole. Because increasing temperature leads to dissociation of both the ions and the CT complex, this results in a steeper temperature dependence on Δln σ⁻¹ than on Δln η.

The viscosity and ionic conductivity of those RAILs can be affected by the temperature-dependent microstructural and dynamic heterogeneities of the ILs.^44,45 For our [CzC₄ImC₁][TFSI] and [C₄VC₇][TFSI]₂ systems, however, no data suggest the presence of the such phase-separated structures in these RAILs.

3.3 Redox properties

3.3.1 Brief overview of the redox of the carbazole and viologen in CT-IL. CVs of the neat CT-IL at 333 K and 50 mV s⁻¹ are shown in Fig. 4. A reversible redox wave of the viologen was observed around −0.6 V (region A). Anodic currents originating from the oxidation of carbazoles were found around +0.7 to +1.5 V (region B). Although no anodic current was found around +0.5 V during the first positive scan, a redox pair in that region appeared in the second and subsequent scans resulting from the redox reaction of electro-polymerized carbazole produced by the oxidation of monomeric carbazole in potential region B.^54,55 The similar responses of carbazole and poly-carbazole were likewise seen in neat [CzC₄ImC₁][TFSI] (Fig. S8†). However, the redox responses of the carbazole and the electropolymerized carbazole were complicated and the analysis was difficult. Therefore, we focused our attention on the redox response of the viologen.

Fig. 4 Overview of the electrochemical reaction of neat CT-IL. Multiple CV scans of neat CT-IL with a Au microelectrode at 333 K and 50 mV s⁻¹ in the viologen region and the carbazole region (black for the 1st, red for the 2nd, and blue for the 3rd scan). The capital alphabets denote the potential regions: (A) redox of the viologen; (B) redox of carbazole and the electro-polymerization; (C) redox of the electro-polymerized carbazole (poly(CzC₄ImC₁)).

3.3.2 Redox reversibility of the viologen: the Nernstian and the interacting two-redox site models. CVs of the viologen in region A in Fig. 4 produced peaked diffusion-limited waveforms due to semi-infinite planar diffusion to the electrode surface. For analytical evaluation of electrochemical reversibility of the viologen, we investigated CVs of the viologens at some concentrations with slow scan rates at which the CVs show a sigmoidal waveform. Reducing the scan rate brought the CVs to a steady state without current and shape changes. Thus, the CVs can be regarded as electrochemically reversible responses. Fig. 5 shows steady-state CVs showing sigmoidal waveforms at 333 K for four systems: neat [C₄VC₇][TFSI]₂, 0.84 M [C₄VC₇][TFSI]₂ in [BMIM][TFSI], 10 mM [C₄VC₇][TFSI]₂ in [BMIM][TFSI], and CT-IL. Although hysteresis was found due to a contribution of the planar diffusion (see the ESI†), all samples showed sigmoidal waveforms, whose forward and reverse scan curves were superimposable by shifting the potential axis less than 19 mV. That is, the systems again showed electrochemically reversible CVs whose line shapes did not vary at lower scan rates. However, the slope of the CVs at E = E_1/2 was dependent on the system. The three systems, neat [C₄VC₇][TFSI]₂, 0.84 M [C₄VC₇][TFSI]₂ in [BMIM][TFSI] and CT-IL showed a steeper slope than 10 mM [C₄VC₇][TFSI]₂ diluted in [BMIM][TFSI].

Fig. 5 Steady-state CVs of [C₄VC₇][TFSI]₂ at various concentrations at 333 K with a Au-UME using a slow scan rate within the reduction region of viologens. (a) Neat [C₄VC₇][TFSI]₂ at 0.1 mV s⁻¹, (b) 0.84 M [C₄VC₇][TFSI]₂ in [BMIM][TFSI] (1 : 2 molar ratio mixture of [C₄VC₇][TFSI]₂ and [BMIM][TFSI]) at 0.5 mV s⁻¹, (c) 10 mM [C₄VC₇][TFSI]₂ in [BMIM][TFSI] at 1 mV s⁻¹, and (d) CT-IL at 0.5 mV s⁻¹. The currents were normalized by the steady-state limiting current. Nernstian parameters related to the CVs are listed in Table 2.

In principle, when the electrochemical reaction is described by a simple electrochemical equilibrium (Ox + e⁻ ⇄ Red) of a redox couple and the redox species are not interacting with each other, the Nernst equation is established and can provide the concentration ratio of the redox species on the electrode surface [Ox]_surf/[Red]_surf. In that situation, a steady-state voltammogram can be described by the ideal Nernst response eqn (9) under the assumption of equal diffusion coefficients D_Ox = D_Red with n_app = 1.

where f = F/RT, i_ss is the steady-state limiting current whose sign is defined so that the cathodic current goes negative, E_1/2 is the half-wave potential and n_app is an apparent number of electrons representing the redox equilibrium. Here, n_app can take a non-integer value to appropriately describe the redox characteristics of CVs. In fact, the diluted system of 10 mM [C₄VC₇][TFSI]₂ in [BMIM][TFSI] produced a Nernstian CV with n_app = 1.00. However, the other systems with higher concentrations of the viologens indeed produced Nernstian CVs with greater n_app than unity, i.e. the slope of the CV at E = E_1/2 is steeper than that with n_app = 1 (see Table 2). It implies that the reduced viologens are interacting attractively. The attractive interaction brings about a situation where the reduction potential of an Ox species close to a Red species shifts positively when the Ox species accepts an electron from the electrode to be the Red species. The schematic representation of the attractive reaction can be proposed as follows.

V²⁺ + e⁻(eld) ⇄ V⁺˙ (step 1)

V⁺˙ + V²⁺(neighbor) ⇄ V⁺˙⋯V²⁺ (step 2)

V⁺˙⋯V²⁺ + e⁻(eld) ⇄ V⁺˙⋯V⁺˙ (step 3)

Table 2 Analyzed parameters of steady state current in the sigmoidal CVs of RAILs consisting of [C₄VC₇][TFSI]₂ at 333 K (Fig. 5)

RAILs	Concentration of C4VC7/M	i ss/nA	D app/cm^2 s^−1	n app	ΔE = E1 − E2/mV	Viscosity (η)/mPa s	Conductivity (σ)/S cm^−1
a Forward scan. b Reverse scan. c Pure [BMIM][TFSI] from ref. 58 using the VFT equation. d Pure [BMIM][TFSI] from ref. 59 using the VFT equation.
(a) Neat [C4VC7][TFSI]2	1.8	−8.38	2.4 × 10^−8	1.60^a	−55.6^a	456	8.0 × 10^−4
				1.66^b	−64.6^b
(b) Mixture of [C4VC7][TFSI]2 and [BMIM][TFSI] (1 : 2 molar ratio)	0.84	−13.0	8.0 × 10^−8	1.43^a	−20.0^a	N/A	N/A
				1.39^b	−15.6^b
(c) Diluted [C4VC7][TFSI]2 in [BMIM][TFSI]	0.010	−0.466	2.4 × 10^−7	1.00	+39.9	15.4^c	1.1 × 10^−2^d
(d) CT-IL	0.92	−1.85	1.0 × 10^−8	1.22^a	+13.2^a	214	4.5 × 10^−4
				1.23^b	+7.6^b

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Step 3 is the dimerization reaction of the reduced viologen as discussed below. If ΔG in step 3 is negatively large and the reactions from step 1 to step 3 are regarded as a sequential redox process with two redox sites, we can approximately describe the reaction as a two-consecutive one-electron transfer process. In that case, the reduction potential difference ΔE = E₁ − E₂, the first and second reduction potentials E₁ and E₂, can negatively increase with increasing the attractive interaction which means the system prefers the dimerized viologens (V⁺˙⋯V⁺˙). The two steps of the reduction reaction eventually cannot be distinguished from each other but seen to be coalescent apparently resulting in an indistinguishable two-electron reduction reaction. Under the assumption that diffusion coefficients of all species involved in the reaction are the same, the steady-state voltammogram can be described as follows.^56,57

where E₁ and E₂ are the macroscopic redox potentials for the two redox steps. Here we defined an interaction parameter, ΔE = E₁ − E₂, which reflects an inter-site interaction in the redox reaction as described above. If there is no inter-site interaction in the system, i.e. redox potentials of all steps are equal (ε₁ = ε₂ = ε₃ = ε₄ in ref. 56), it turns out that ΔE = (2 ln 2)RT/F = +39.8 mV at 333 K and n_app becomes unity. If the inter-site interaction is repulsive/attractive, i.e. one reduced site discourages/encourages the reduction reaction on another site, then ΔE > +39.8 mV for repulsive and ΔE < +39.8 mV for attractive interaction.

The results from analyzing the CVs in Fig. 5 are listed in Table 2. The diluted solution of 10 mM [C₄VC₇][TFSI]₂ exhibited a typical Nernstian CV with n_app = 1 and ΔE = +39.8 mV, which indicates the absence of interaction between viologens. Other viologen systems exhibit ΔE < +39.8 mV and n_app > 1, which suggest attractive interaction between viologens. A possible attraction of viologens was the dimerization between reduced species (V⁺˙ + V⁺˙ ⇌ V⁺˙⋯V⁺˙). In relation to such dimerization, the dimerization constants of viologens (K_D = [V⁺˙⋯V⁺˙]/[V⁺˙]²) in water and organic solvent systems are typically 10³ to 10⁵ M⁻¹ (ref. 60–62) with the corresponding Gibbs energy changes being −18.8 to −29.1 kJ mol⁻¹.⁶¹ In our previous electroreflectance spectroscopy experiments, we have also observed, in the steady-state CV, intramolecular dimerization of a diluted bis-viologen in water as a result of attractive interaction.⁵⁷ In addition, we have also reported electrochromic devices based on viologen-based RAILs, with dimerizations being observed as a result of highly concentrated viologens.^31,33

The concentrated viologens in this study reached around 1–2 M, and we therefore deem dimerization as induced by reduction possible. The order of −ΔE and n_app values is (c) 10 mM [C₄VC₇][TFSI]₂ (no attraction) < (d) CT-IL < (b) 0.84 M [C₄VC₇][TFSI]₂ < (a) neat [C₄VC₇][TFSI]₂. Neat [C₄VC₇][TFSI]₂ (a) exhibited the greatest n_app and smallest ΔE because of its highest viologen concentration. Interestingly, the concentration of C₄VC₇ in (d) CT-IL (0.92 M) is similar to that for (b) 0.84 M [C₄VC₇][TFSI]₂, while the n_app and ΔE values were quite different. This can be explained by the CT complexation between the viologen and carbazole, which weakened the inter-viologen dimerization. Three such mechanisms were plausible. One mechanism is that CT complexation weakens the attraction of free V⁺⁺ species to V⁺˙; the other mechanism is that the reduction potential of the CT complex (CzV⁺⁺) might be more negative than that for free V⁺⁺, which then results in the increase of ΔE greater than the value in 0.84 M [C₄VC₇][TFSI]₂.

The diffusion coefficients can be evaluated by the steady state limiting current (i_ss) with the Saito equation (i_ss = 4nFrcD) for microelectrodes.⁶³ The migration effect in the absence of the supporting electrolyte system should not be ignored but the contribution to the limiting current in our system could be small.⁶⁴ Therefore, we directly used the observed limiting current to evaluate the diffusion coefficient without any migration correction. The diffusion coefficients of the viologen (D_V) in [C₄VC₇][TFSI]₂ and CT-IL at 60 °C are, respectively, 2.4 × 10⁻⁸ and 1.0 × 10⁻⁸ cm² s⁻¹ in the CVs of Fig. 5. The diffusion coefficients of the concentrated system can be explained by the Dahms–Ruff model (D_V = D_phys + D_ex)^65,66 as the sum of physical diffusion and electron hopping processes. Although we cannot clarify the details of the process in the redox reaction at this stage, we need to mention the relation between the diffusion coefficient in the CVs and the transport properties such as viscosity and ionic conductivity. Interpretation of these diffusion coefficients should be presently made for two limiting cases.

In the case of D_phys ≫ D_ex. The above-mentioned magnitude relation, D_V([C₄VC₇][TFSI]₂) > D_V(CT-IL), cannot be simply explained by the magnitude relation of the viscosities (456 and 214 mPa s) with the Stokes–Einstein equation (D_phys = k_BT/6πaη, a is the molecular radius) for the physical diffusion process. To explain the diffusion coefficients within the Stokes–Einstein framework, we can write the following relation a_V,CT-IL/a_V,C4VC7 = D_V,CT-ILη_CT-IL/D_V,C4VC7η_C4VC7 and we obtained a_V,CT-IL/a_V,C4VC7 = 5.1. This can be translated by association of C₄VC₇ with not only CzC₄ImC₁ but also TFSI anions in the CT-IL.
In the case of D_ex ≫ D_phys. The diffusion coefficient electron hopping can be described by the Ruff equation (D_ex = δ²k_exc/6).^66,67 Center-to-center distances between viologens in [C₄VC₇][TFSI]₂ and CT-IL were estimated using the equation δ = (N_Ac)^−1/3 (where N_A is the Avogadro constant and c is the molar concentration), respectively, to be 0.97 and 1.2 nm. Thus, the electron hopping rate constants k_B were, respectively, 8.5 and 4.6 ×10⁶ M⁻¹ s⁻¹. The electron hopping rate constant k_ex in viscous solution can be described by an atmosphere relaxation model^68,69 which is an ion-pairing process to hold the electroneutrality in the electron hopping process. The magnitude relation of the diffusion coefficients agrees with that of ionic conductivities not that of the viscosities.

The details of the relationship among the transportation properties require further experiments such as diffusion ordered spectroscopy (DOSY)-NMR. We will perform and report it elsewhere in the future. Although the detailed mechanism of the redox process is at present not clear, what is clear is that CT interaction can notably modulate the physicochemical properties of ILs.

4. Conclusions

We demonstrated CT interactions in an IL produced by equimolar mixing of a carbazole-based ([CzC₄ImC₁][TFSI]) and a viologen-based ([C₄VC₇][TFSI]₂) RAIL. The existence of a CT complex between the carbazole and viologen was confirmed by UV-vis absorption spectra, which neither [CzC₄ImC₁][TFSI] nor [C₄VC₇][TFSI]₂ exhibited. The decrease of absorption with increasing temperature suggested the CT complex dissociation with increasing temperature. Transport properties are modulated by the new interaction; we found the viscosity and ionic conductivity of CT-IL to be different from those of the individual RAILs. The viscosity of CT-IL was smaller than those of [CzC₄ImC₁][TFSI] and [C₄VC₇][TFSI]₂. Likewise, the ionic conductivity of CT-IL was also smaller than those of [CzC₄ImC₁][TFSI] and [C₄VC₇][TFSI]₂. CV with a microelectrode showed that the redox response of the viologen was also modulated by CT interaction with the carbazoles. The apparent number of electrons involved in the redox reaction was larger than unity in concentrated solutions of [C₄VC₇][TFSI]₂, indicating an attractive interaction between the redox species in accordance with a two-site redox model. In turn, this implies that dimerization of reduced viologens occurred much faster than the CV timescale. In comparison, CT-IL showed less attractive behavior than neat [C₄VC₇][TFSI]₂, because CT complexation between the viologens and carbazoles suppresses viologen dimerization. Overall, the introduction of CT interaction into RAILs changes a wide range of chemical and physical properties. Thus, it represents a useful design tool for expanding the scope of FILs; a deep understanding of the relationship between the structures and the functions would be beneficial in advancing such systems.

Author contributions

H. Tahara conceived and designed this study. S. Yonemori and S. Yamamoto synthesized the compounds. H. Tahara performed viscosity and spectroscopic measurements. H. Tahara, Y. Tanaka, and S. Yamamoto performed electrochemical measurements. H. Tahara and C. Bun performed the quantum chemical calculations. H. Tahara and T. Sagara analyzed the data. H. Tahara produced the manuscript. H. Murakami, C. Bun and T. Sagara checked and modified the manuscript.

Funding sources

This work was supported by a Japan Society for the Promotion of Science (JSPS) Grant-in-Aid for Young Scientists (B) no. 26810052 (H. T.), Grant-in-Aid for Scientific Research (C) no. 20K05649 (H. T.), and the Hattori Hokokai Foundation (H. T.).

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was the result of using research equipment (¹H NMR, elemental analysis, mass spectroscopy) shared in the MEXT Project for promoting public utilization of advanced research infrastructure (Program for supporting introduction of the new sharing system) Grant Number JPMXS0422500320.

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Footnote

† Electronic supplementary information (ESI) available: Experimental method; DSC thermograms of [C₄VC₇][TFSI]₂, [CzC₄ImC₁][TFSI], and CT-IL; absorption spectra of a diluted solution of the RAILs; equilibrium constant of CT complex formation in CT-IL; fluorescence spectra of [CzC₄ImC₁][TFSI] in neat and diluted solutions; water content and IR spectra of RAILs; Arrhenius plots of the viscosities and ionic conductivities; Walden plots of the RAILs; multiple CV cycles of neat [CzC₄ImC₁][TFSI] in potential region B; ionic conductivity apparatus; two-electrode cell for electrochemical measurements; fitting diagrams of experimental CVs by the simple Nernst model; fitting diagrams of experimental CVs by the two redox-site model; uncompensated resistance effects of CVs by DigiElch; uncompensated resistance effects of Nernst analysis by DigiElch; scan rate dependence of CVs in neat [C₄VC₇][TFSI]₂ and CT-IL by DigiElch; DFT models of the viologen and carbazole for evaluating the Gibbs energy of the CT complex formation. See DOI: 10.1039/d0sc06244h

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