Differences in the behavior of dicationic and monocationic ionic liquids as revealed by time resolved-fluorescence, NMR and fluorescence correlation spectroscopy

A Combined Experimental/Computational Study of Dicationic Ionic Liquids with Bromide and Tungstate Anions

A panel of dicationic ionic liquids (DILs) with different rigid xylyl (ortho, meta, para) spacers and different anions (bromide and tungstate) has been synthetised and characterised through different experimental and computational techniques. Differences and analogies between the systems are analysed using information derived from their DFT structures, semiempirical dynamics, thermal behaviour, and catalytic properties versus the well-known reaction of CO2 added to epichlorohydrin. A comparison between the proposed systems and some analogues that present non-rigid spacers shows the key effect displayed by structure rigidity on their characteristics. The results show an interesting correlation between structure, flexibility, properties, and catalytic activity.

Structural, energetic and dynamic investigation of poly(ethylene oxide) in imidazolium-based ionic liquids with different cationic structures

In this work, two imidazolium-based ionic liquids (ILs) with different cations including dications (DIL) and monocations (MIL) were blended with poly(ethylene oxide) (PEO). The influence of ILs' structure on the structural and dynamic properties of a PEO/IL system was investigated by molecular dynamics (MD) simulation and density functional theory (DFT) methods. The simulation results show that DIL exhibits weaker interaction with PEO than MIL due to a stronger IL aggregation effect. The intermolecular interaction also makes the PEO chain tend to organize around the imidazolium ring of ILs, which causes the conformational entropy loss. Compared with PEO/MIL, this phenomenon is more significant in PEO/DIL because of the double positive centers of the dication and a longer hydrogen bond lifetime. MD simulation also demonstrates that DIL could act as a "crosslinker" to promote the formation of a physical crosslinking network which has strong dependence on the concentration of IL. The competition between physical crosslinking and plasticizing effects induces non-monotonic variations of relaxation time in PEO/DIL, which is consistent with its unusual change of the glass transition temperature (Tg). Despite stronger hydrogen bonding interactions between PEO and MIL demonstrated by atom-in-molecules (AIM) and reduced density gradient (RDG) analysis, the segmental mobility is slower in PEO/DIL according to the MSD curve. These differences in multiple structural or energetic factors finally lead to different conductive mechanisms and hence obtain different ionic conductivities.

Physico-Chemical Properties of Magnetic Dicationic Ionic Liquids with Tetrahaloferrate Anions

A series of imidazolium-based symmetrical and asymmetrical dicationic ionic liquids (DcILs) with alkyl spacers of different length and with [FeCl3 Br]- as counter ion have been synthesized. The synthesized DcILs are characterized by using FTIR and Raman spectroscopy as well as mass spectrometry, along with single-crystal XRD analysis. Physicochemical properties such as solubility, thermal stability and magnetic susceptibility are also measured. These compounds show low melting points, good solubility in water and organic solvents, thermal stability, and paramagnetism. The products of molar susceptibility and temperature (χmol ⋅T) for the synthesized DcILs have been found between 4.05 to 4.79 emu mol-1  K Oe-1 and effective magnetic moment values have also been determined to be compared to that expected from the spin-only approximation.

Assessing the Suitability of a Dicationic Ionic Liquid as a Stabilizing Material for the Storage of DNA in Aqueous Medium

The present study has been undertaken with an objective to find out a suitable medium for the long-term stability and storage of the ct-DNA structure in aqueous solution. For this purpose, the potential of a pyrrolidinium-based dicationic ionic liquid (DIL) in stabilizing ct-DNA structure has been investigated by following the DNA-DIL interaction. Additionally, in order to understand the fundamental aspects regarding the DNA-DIL interaction in a comprehensive manner, studies are also done by employing structurally similar monocationic ionic liquids (MILs). The investigations have been carried out both at ensemble-average and single molecular level by using various spectroscopic techniques. The molecular docking study has also been performed to throw more light into the experimental observations. The combined steady-state and time-resolved fluorescence, fluorescence correlation spectroscopy, and circular dichroism measurements have demonstrated that DILs can effectively be used as better storage media for ct-DNA as compared to MILs. Investigations have also shown that the extra electrostatic interaction between the cationic head group of DIL and the phosphate backbone of DNA is primarily responsible for providing better stabilization to ct-DNA, retaining its native structure in aqueous medium. The outcomes of the present study are also expected to provide valuable insights in designing new polycationic IL systems that can be used in nucleic acid-based applications.

Effect of Lithium-Ion on the Structural Organization of Monocationic and Dicationic Ionic Liquids

In recent times, ionic liquid-based (ILs) electrolytic system has emerged as suitable alternative to the conventional organic solvent-based electrolytic system. However, since, anion of ILs is known to form aggregates in the presence of lithium-ions (Li⁺), and this can influence the transport properties of Li⁺ ion in a significant manner, it is, therefore, important to understand how lithium-ions influence the structure and dynamics of ILs. With this objective, in the present study, intermolecular interaction, structural organization, and dynamics of monocationic ILs (MILs) and dicationic IL (DIL) have been studied in the absence and presence of lithium salt. Specifically, for this purpose, two MILs, 1-methyl-3-propylimidazolium bis(trifluoromethylsulfonyl)imide ([C₃C₁im][NTf₂]), 1-hexyl-3-propylimidazolium bis(trifluoromethylsulfonyl)imide ([C₆C₁im][NTf₂]), and a DIL, 1,6-bis(3-methylimidazolium-1-yl)hexane bis(trifluoromethylsulfonyl)amide ([C₆(mim)₂][NTf₂]₂) have been chosen in such a way that either the alkyl chain of MILs becomes equal or half of the spacer chain length of DIL. To understand the effect of the addition of lithium-ion on the structural organization of MILs and DIL, steady-state absorption and fluorescence spectroscopies, time-resolved fluorescence anisotropy and nuclear magnetic resonance (NMR) techniques have been used. Structural organization in the apolar and polar domains of ILs has been probed by following the rotational diffusion of suitably chosen solute in the concerned media through time-resolved fluorescence anisotropy (TRFA) measurements. TRFA studies have revealed that with the addition of Li⁺ ion, coordination between the Li⁺ ions and anions of MILs and DILs takes place in the ionic region leading to a change in the structural organization of the apolar regions of the respective medium. In fact, upon adding lithium-ions, a reduction in the packing of alkyl chains has also been observed for the MILs. However, not much change in the structural organization of the apolar region of the DIL has been observed when Li⁺ ion is added to it. In the presence of Li⁺ ions, a similar trend in the change of structural organization of polar regions for both MILs and DIL has been observed. Further, measurements of the self-diffusion coefficient through NMR have also supported the observation that Li⁺ ion also perturbs the nanostructural organization of the MIL in a significant manner than that it does for the DIL. The behavior of DIL in the presence of Li⁺ ion, as revealed by the present study, has been rationalized by considering the folded arrangement of DIL in the fluid-structure. Essentially, all of these investigations have suggested that the addition of lithium-ion significantly alters the microscopic behavior of MILs in comparison to that of DIL. The outcome of this study is expected to be helpful in realizing the potentials of these media as electrolytes in battery applications.

Comparison between pyrrolidinium-based and imidazolium-based dicationic ionic liquids: intermolecular interaction, structural organization, and solute dynamics

With an aim to understand the difference in the behaviour of imidazolium and pyrrolidinium-based dicationic ionic liquids (DILs) in terms of the intermolecular interactions, microscopic-structure and dynamics, two DILs, the imidazolium-based 1,9-bis(3-methylimidazolium-1-yl)nonane bis(trifluoromethanesulfonyl)imide and the pyrrolidinium-based 1,9-bis(1-methylpyrrolidinium-1-yl)nonane bis(trifluoromethanesulfonyl)imide, have been synthesized and subsequently investigated by exploiting combined steady sate and time resolved fluorescence, electron paramagnetic resonance (EPR) and nuclear magnetic resonance (NMR) spectroscopic techniques. Data obtained for DILs have also been compared with their corresponding mono-cationic counterpart (MILs) to evaluate and understand the distinctive characteristics of the DILs in contrast with the corresponding MILs. Steady state emission and EPR data have revealed that the pyrrolidinium-based DIL is slightly less polar than the imidazolium-based DIL. Temperature-dependent fluorescence anisotropy decay of two probes, perylene and MPTS (8-methoxypyrene-1,3,6-trisulfonate), has been measured in DILs as well as in MILs. Solute-solvent coupling constants obtained from the experimentally measured rotational correlation times with the aid of Stokes-Einstein-Debye hydrodynamic theory have indicated appreciable differences in the dynamics of both the solutes on going from MILs to DILs. More interestingly, the outcome of the NMR study has suggested that the alkyl spacer chain in the imidazolium-based DIL exists in the folded form, but the pyrrolidinium-based DIL remains in the straight chain conformation. Inherently, the outcomes of all of these studies have depicted that the microscopic structural organisations in imidazolium and pyrrolidinium-based DILs are different from each other as well as from their respective mono-cationic counterparts.

Expanding the Chemical Space of Benzimidazole Dicationic Ionic Liquids

Benzimidazole dicationic ionic liquids (BDILs) have not yet been widely explored in spite of their potential. Therefore, two structurally related families of BDILs, paired with either bromide or bistriflimide anions and bearing alkyl spacers ranging from C3 to C6, have been prepared. Their thermal properties have been studied by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC), while their electrical properties have been assessed by cyclic voltammetry (CV). TG analysis confirmed the higher stability of the bistriflimide BDILs over the bromide BDILs, with minor variation within the two families. Conversely, DSC and CV allowed for ascertaining the role played by the spacer length. In particular, the thermal behavior changed dramatically among the members of the bistriflimide family, and all three possible thermal behavior types of ILs were observed. Furthermore, cyclic voltammetry showed different electrochemical window (C3(C1BenzIm)2/2Tf2N < C4(C1BenzIm)2/2Tf2N, C5(C1BenzIm)2/2Tf2N < C6(C1BenzIm)2/2Tf2N) as well as a reduction peak potential, shape, and intensity as a function of the spacer length. The results obtained highlight the benefit of accessing a more structurally diverse pool of compounds offered by dicationic ILs when compared to the parent monocationic ILs. In particular, gains are to be found in the ease of fine-tuning their properties, which translates in facilitating further investigations toward BDILs as designer solvents and catalysts.

Ion conformation and orientational order in a dicationic ionic liquid crystal studied by solid-state nuclear magnetic resonance spectroscopy

Ionic liquids crystals belong to a special class of ionic liquids that exhibit thermotropic liquid-crystalline behavior. Recently, dicationic ionic liquid crystals have been reported with a cation containing two single-charged ions covalently linked by a spacer. In ionic liquid crystals, electrostatic and hydrogen bonding interactions in ionic sublayer and van der Waals interaction in hydrophobic domains are the main forces contributing to the mesophase stabilization and determining the molecular orientational order and conformation. How these properties in dicationic materials are compared to those in conventional monocationic analogs? We address this question using a combination of advanced NMR methods and DFT analysis. Dicationic salt 3,3′-(1,6-hexanediyl)bis(1-dodecylimidazolium)dibromide was studied. Local bond order parameters of flexible alkyl side chains, linker chain, and alignment of rigid polar groups were analyzed. The dynamic spacer effectively “decouples” the motion of two ionic moieties. Hence, local order and alignment in dicationic mesophase were similar to those in analogous single-chain monocationic salts. Bond order parameters in the side chains in the dicationic smectic phase were found consistently lower compared to double-chain monocationic analogs, suggesting decreasing contribution of van der Waals forces. Overall dication reorientation in the smectic phase was characterized by low values of orientational order parameter S. With increased interaction energy in the polar domain the layered structure is stabilized despite less ordered dications. The results emphasized the trends in the orientational order in ionic liquid crystals and contributed to a better understanding of interparticle interactions driving smectic assembly in this and analogous ionic mesogens.

Microstructural and Dynamical Heterogeneities in Ionic Liquids

Ionic liquids (ILs) are a special category of molten salts solely composed of ions with varied molecular symmetry and charge delocalization. The versatility in combining varied cation-anion moieties and in functionalizing ions with different atoms and molecular groups contributes to their peculiar interactions ranging from weak isotropic associations to strong, specific, and anisotropic forces. A delicate interplay among intra- and intermolecular interactions facilitates the formation of heterogeneous microstructures and liquid morphologies, which further contributes to their striking dynamical properties. Microstructural and dynamical heterogeneities of ILs lead to their multifaceted properties described by an inherent designer feature, which makes ILs important candidates for novel solvents, electrolytes, and functional materials in academia and industrial applications. Due to a massive number of combinations of ion pairs with ion species having distinct molecular structures and IL mixtures containing varied molecular solvents, a comprehensive understanding of their hierarchical structural and dynamical quantities is of great significance for a rational selection of ILs with appropriate properties and thereafter advancing their macroscopic functionalities in applications. In this review, we comprehensively trace recent advances in understanding delicate interplay of strong and weak interactions that underpin their complex phase behaviors with a particular emphasis on understanding heterogeneous microstructures and dynamics of ILs in bulk liquids, in mixtures with cosolvents, and in interfacial regions.

Ionic liquid lubricants: when chemistry meets tribology

Ionic liquids (ILs) have emerged as potential lubricants in 2001. Subsequently, there has been tremendous research interest in ILs from the tribology society since their discovery as novel synthetic lubricating materials. This also expands the research area of ILs. Consistent with the requirement of searching for alternative and eco-friendly lubricants, IL lubrication will experience further development in the coming years. Herein, we review the research progress of IL lubricants. Generally, the tribological properties of IL lubricants as lubricating oils, additives and thin films are reviewed in detail and their lubrication mechanisms discussed. Considering their actual applications, the flexible design of ILs allows the synthesis of task-specific and tribologically interesting ILs to overcome the drawbacks of the application of ILs, such as high cost, poor compatibility with traditional oils, thermal oxidization and corrosion. Nowadays, increasing research is focused on halogen-free ILs, green ILs, synthesis-free ILs and functional ILs. In addition to their macroscopic properties, the nanoscopic performance of ILs on a small scale and in small gaps is also important in revealing their tribological mechanisms. It has been shown that when sliding surfaces are compressed, in comparison with a less polar molecular lubricant, ion pairs resist "squeeze out" due to the strong interaction between the ions of ILs and oppositely charged surfaces, resulting in a film that remains in place at higher shear forces. Thus, the lubricity of ILs can be externally controlled in situ by applying electric potentials. In summary, ILs demonstrate sufficient design versatility as a type of model lubricant for meeting the requirements of mechanical engineering. Accordingly, their perspectives and future development are discussed in this review.

Understanding the Behavior of Monocationic and Dicationic Room-Temperature Ionic Liquids through Resonance Energy-Transfer Studies

The present work has been undertaken with an objective to understand the differences in the local structural organization of imidazolium-based monocationic ionic liquids (MILs) and dicationic ionic liquids (DILs) through resonance energy-transfer (RET) studies. In this study, a neat IL is used as a donor and a charged species rhodamine 6G (R6G) is used as an acceptor unit because of the fact that they satisfy the spectroscopic criteria that are needed for an RET event to take place. Additionally, R6G, being a charged species, is expected to facilitate the electrostatic interactions with the ILs which are also charged. Specifically, two imidazolium-based germinal DILs and their monocationic counterparts are used for the present investigations. Additionally, the studies are carried out in some selected MILs where the lengths of the alkyl side chains are kept unchanged for MILs and DILs. Interestingly, the present data reveal that the RET interaction is more favorable for DILs than for MILs, even though the DILs are relatively bulkier than their monocationic counterparts. More interestingly, the RET interaction is also found to be more favorable for DILs than for MILs, where the length of the alkyl group is kept fixed for MILs and DILs. The result of the present study delineates that the alkyl chain length on the cation is not the sole factor contributing to the RET outcomes for DILs and MILs but the local structure of DILs also contributes significantly to the same. The current investigation clearly indicates that DILs have a more compact local structure than that of MILs. Essentially, the current study highlights that a cost-effective, noninvasive technique such as RET is quite effective in capturing the differences in the nanostructural organization of MILs and DILs.

An insight into the intermolecular vibrational modes of dicationic ionic liquids through far-infrared spectroscopy and DFT calculations

Dicationic ionic liquids (DILs) are a subclass of the ionic liquid (IL) family and are characterized by two cationic head groups linked by means of a spacer. While DILs are increasingly attracting interest due to their peculiar physico-chemical properties, there is still a lack of understanding of their intermolecular interactions. Herein, we report our investigations on the intermolecular vibrational modes of two bromide DILs and of a bistriflimide DIL. The minimal possible neutral cluster of ions was studied as a simplified model of these systems and was optimized at the DFT level. Normal modes of two sandwich-like conformers were then calculated using the harmonic approximation with analytical computation of the second derivatives of molecular energy with respect to the atomic coordinates. The calculated spectra were compared to far-infrared experimental spectra and two groups of peaks over three, for the two bromide DILs, and three over five, for the Tf₂N⁻ DIL, were described by the proposed neutral cluster model. Therefore, this model represents a reliable and computationally affordable model for the exploration of the intermolecular interactions of this kind of system.

The minimal cluster of ions represents a reliable and computationally affordable model for the exploration of the intermolecular interactions of dicationic ionic liquids.