Solute Rotation and Solvation Dynamics in a Room-Temperature Ionic Liquid

Spectroscopic and Chemical Properties of Ionic Liquids: Computational Study

A brief account is given of highlights of our computational efforts - often in collaboration with experimental groups - to understand spectroscopic and chemical properties of ionic liquids (ILs). Molecular dynamics, including their inhomogeneous character, responsible for key spectral features observed in dielectric absorption, infra-red (IR) and fluorescence correlation spectroscopy (FCS) measurements are elucidated. Mechanisms of chemical processes involving imidazolium-based ILs are illustrated for CO2 capture and related reactions, transesterification of cellulose, and Au nanocluster-catalyzed Suzuki cross-coupling reaction with attention paid to differing roles of IL ions. A comparison with experiments is also made.

Dielectric relaxation and dielectric decrement in ionic acetamide deep eutectic solvents: Spectral decomposition and comparison with experiments

Frequency-dependent dielectric relaxation in three deep eutectic solvents (DESs), (acetamide+LiClO4/NO3/Br), was investigated in the temperature range, 329 ≤ T/K ≤ 358, via molecular dynamics simulations. Subsequently, decomposition of the real and the imaginary components of the simulated dielectric spectra was carried out to separate the rotational (dipole-dipole), translational (ion-ion), and ro-translational (dipole-ion) contributions. The dipolar contribution, as expected, was found to dominate all the frequency-dependent dielectric spectra over the entire frequency regime, while the other two components together made tiny contributions only. The translational (ion-ion) and the cross ro-translational contributions appeared in the THz regime in contrast to the viscosity-dependent dipolar relaxations that dominated the MHz-GHz frequency window. Our simulations predicted, in agreement with experiments, anion-dependent decrement of the static dielectric constant (ɛs ∼ 20 to 30) for acetamide (ɛs ∼ 66) in these ionic DESs. Simulated dipole-correlations (Kirkwood g factor) indicated significant orientational frustrations. The frustrated orientational structure was found to be associated with the anion-dependent damage of the acetamide H-bond network. Single dipole reorientation time distributions suggested slowed down acetamide rotations but did not indicate presence of any "rotationally frozen" molecule. The dielectric decrement is, therefore, largely static in origin. This provides a new insight into the ion dependence of the dielectric behavior of these ionic DESs. A good agreement between the simulated and the experimental timescales was also noticed.

Insight into the structure and transport properties of pyrrolidinium-based geminal dicationic-organic ionic crystals: inravelling the role of alkyl-chain length

The present study has been undertaken with an aim to design and develop safer and more efficient all solid-state electrolytes, so that the issues associated with the use of conventional room temperature ionic liquid-based electrolytes can be tackled. To fulfil this objective, a series of geminal di-cationic Organic Ionic Crystals (OICs), based on C₃-, C₆-, C₈- and C₉-alkylbridged bis-(methylpyrrolidinium)bromide are synthesized, and the structural features, thermal properties and phase behaviours of these as synthesized OICs have been investigated. Additionally, a number of electro-analytical techniques have been employed to assess their suitability as an efficient electrolyte composite (OIC:I₂:TBAI) for all solid-state dye sensitised solar cells (DSSCs). The structural analysis has revealed that along with excellent thermal stability and well-defined surface morphology, all thsese OICs exhibit a well-ordered three-dimensional network of cations and anions that can serve as a conducting channel for the diffusion of iodide ions. Electrochemical investigations have shown that OICs with an intermediate length of alkyl bridge (C₆- and C₈-alkyl bridged) show better electrolytic performance than those that are based on OICs with a relatively shorter (C₃-) or longer (C₉-) alkyl-bridge chain. A careful analysis of the above data has essentially demonstrated that the length of the alkyl bridge chain plays a significant role in determining the structural organisation, morphology and eventually the ionic conductivity of OICs. Overall, the comprehensive knowledge on OICs that has been extracted from the current study is expected to be helpful to explore further new types of OIC-based all solid-state electrolytes with improved electrolytic performance for targeted applications.

Choline chloride and ethylene glycol based deep eutectic solvent (DES) versus hydroxyl functionalized room temperature ionic liquids (RTILs): assessing the differences in microscopic behaviour between the DES and RTILs

With the aim of understanding the differences in the behavior of deep eutectic solvents (DESs) and room temperature ionic liquids (RTILs) in terms of their structure, dynamics, and intra- and intermolecular interactions, three different ILs and one DES having similar functionalities (hydroxyl) have been investigated by using both ensembled average and single-molecule spectroscopic techniques. Specifically, for this purpose, a choline chloride based DES (ethaline) and three hydroxyl functionalized ILs (1-(2-hydroxyethyl)-3-imidazolium bis(trifluoromethanesulfonyl)imide ([OHEMIM][NTF₂]), N-(2-hydroxyl ethyl)-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide ([OHEMPy][NTf₂]), and N-(2-hydroxyethyl)-N,N-dimethylpropan-1-aminium bis(trifluoromethanesulfonyl)imide ([OHC3CH][NTf₂])) are employed and investigated by EPR, time-resolved fluorescence, NMR and FCS studies. Estimation of polarity through EPR spectroscopy has revealed that the hydroxyl ILs employed in these studies are hyper-polar (close to water) in nature, whereas the polarity of the DES is found to be close to those of aliphatic polyhydroxy-alcohols. Interestingly, both time-resolved fluorescence anisotropy and FCS studies on these systems have suggested that the hydroxyl ILs are more dynamically heterogeneous than the DES. More interestingly, PFG-NMR measurements have indicated that the fluid structure of ethaline is relatively more associated as compared to those of the ILs despite the fact that all the cations have the same hydroxyl functionalities. All these investigations have essentially demonstrated that, despite having similar functionalities, both the DES and hydroxyl ILs employed in the present study exhibit microscopic behaviours that are significantly different from each other, indicating the interplay of various intermolecular interactions within the constituent species in governing the behaviours of these solvent systems.

Assessing rotation and solvation dynamics in ethaline deep eutectic solvent and its solutions with methanol

Steady-state and time-resolved fluorescence were used to investigate the solvation of coumarin 153 (C153) and coumarin 343 (C343) in methanol + ethaline binary solutions, a deep eutectic solvent composed of a 1:2 molar ratio choline chloride + ethylene glycol. In addition, time-resolved anisotropy decays were used to determine the solute's rotational reorientation time as a function of viscosity. Measurements were made in solutions covering the entire range of mole fraction. Viscosity measurements were used to characterize the bulk solvent properties, and as expected, addition of methanol resulted in an decreased viscosity, showing an exponential decrease with mole fraction, up to ∼50-fold at x_MeOH = 1.0. Probe rotational reorientation times were found to be biexponential at x_MeOH < 0.3 for C153 and x_MeOH < 0.5 for C343 and monoexponential at richer methanol content. In proportion to viscosity, C153 and C343 average rotation times decreased ∼30-fold from x_MeOH = 0 to 0.9 and showed a power law dependence of ∼η^0.85. Rotation times approached the stick boundary limit on dilution with methanol. Time-resolved Stokes shifts quantified the solvation dynamics and were nearly single exponential for C153 but were clearly biexponential for C343. Solvation times also tracked with viscosity according to a power law dependence, with exponents of 0.3 and 0.4 for C153 and C343, respectively. The dilution effect of methanol was not linear in proportion to the viscosity change and alone cannot account for the change in solvation. Dilution also showed a different correlation to solvation than did temperature variations to govern the viscosity change.

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.

Solvation dynamics: improved reproduction of the time-dependent Stokes shift with polarizable empirical force field chromophore models

The inclusion of explicit polarization in molecular dynamics simulation has gained increasing interest during the last several years. An understudied area is the role of polarizability in computer simulations of solvation dynamics around chromophores, particularly for the large solutes used in experimental studies. In this work, we present fully polarizable ground and excited state force fields for the common fluorophores N-methyl-6-oxyquinolium betaine and Coumarin 153. While analyzing the solvation responses in water, methanol, and the highly viscous ionic liquid 1-ethyl-3-methylimidazolium trifluoromethanesulfonate we found that the inclusion of solute polarizability considerably increases the agreement of the obtained Stokes shift relaxation functions with experimental data. Solute polarizability slows down the inertial solvation response in the femtosecond time regime and enables the chromophore to adapt its dipole moment to the environment. Furthermore, the developed chromophore force field reproduces the solute dipole moments in both the electronic ground and excited state in environments ranging from gas phase to highly polar media correctly. Based on these studies it is anticipated that polarizable models of chromophores will lead to an improved understanding of the relationship of their environment to their spectroscopic properties.

Polarizability in ionic liquid simulations causes hidden breakdown of linear response theory

The validity of linear response theory (LRT) in computer simulations of solvation dynamics, i.e. the time-dependent Stokes shift, has been debated widely during the last decades. Since the use of LRT is computationally less expensive than the calculation of the true nonequilibrium response, it is often invoked for large systems exhibiting a particularly slow solvation response, e.g. ionic liquids. In the case of ionic liquids, LRT does not only need to capture the correct overall dynamics of the system, but also the contributions and timescales of the respective cation and anion movement. We show by large scale computer simulations that the contribution of the permanent dipoles to the solvation response obeys LRT to some extent, whereas the induced contributions in polarizable simulations lead to a failure of LRT for the respective ion contributions.

The role of viscosity in various dynamical processes of different fluorophores in ionic liquid–cosolvent mixtures: a femtosecond fluorescence upconversion study

Literature reports provide ample evidence of the dynamical studies of various fluorophores in different room-temperature ionic liquid (RTIL)–cosolvent mixtures.

Polymeric Ionic Liquid-Based Fluorescent Amphiphilic Block Copolymer Micelle for Selective and Sensitive Detection of p-Phenylenediamine

A highly sensitive and selective detection of p-phenylenediamine (PPD) is achieved by a fluorescence sensor, which is constructed by encapsulating the hydrophobic fluorescent 1-pyrenecarboxaldehyde (Py-CHO) into a polymeric ionic liquid (PIL)-based amphiphilic block copolymer (BCP) micelle. The amine-aldehyde condensation reaction between PPD and Py-CHO leads to the fluorescence quenching of Py-CHO, giving rise to the basis for the quantitative detection of PPD. The core cavity of the micelle formed by the self-assembly of PIL provides an excellent hydrophobic environment for the accommodation of fluorescent Py-CHO, offering significant improved sensitivity and selectivity for PPD detection. The amount of PIL in fabricating the amphiphilic BCP micelle, the BCP-Py-CHO micelle concentration, and the detection pH condition are investigated to obtain the best performance of this sensor. The accurate detection of PPD is achieved in the range of 0.02-10 μmol L^-1 under optimal conditions, and the detection limit is 0.007 μmol L^-1 (3σ/ s). The developed sensor is successfully applied to the determination of PPD contents in hair dyes, spiked water, and urine samples.

Ionic liquid mediated micelle to vesicle transition of a cationic gemini surfactant: a spectroscopic investigation

In this contribution, we have examined a composition dependent self aggregated structural modification of a catanionic mixture of the surface active ionic liquid (IL) 1-butyl-3-methylimidazolium octyl sulphate and a cationic gemini surfactant (14-5-14) in aqueous medium. We have observed that the hydrodynamic diameter of the aggregates increases with increasing IL concentration and microscopic evidence (HRTEM, FESEM, and LCSM) shows the formation of vesicle like aggregates (Dh ≈ 200 nm) at XIL = 0.5. The steady state fluorescence anisotropy of the membrane binding probe DPH shows a micelle to vesicle transition at this composition. The viscosity of the solution shows a peak at XIL = 0.3, indicating the formation of a worm like micelle as an intermediate of the micelle to vesicle transition. The rotational dynamics shows a stiffer surfactant packing in the vesicles compared to the micelles, whereas, the solvation dynamics measurements indicate a higher abundance of bound type water in the vascular medium compared to that for the micelle. The formed vesicles also show stability towards temperature and biomolecules, which can be used for respective applications.

Solvation dynamics in polar solvents and imidazolium ionic liquids: failure of linear response approximations

This study presents the large scale computer simulations of two common fluorophores, N-methyl-6-oxyquinolinium betaine and coumarin 153, in five polar or ionic solvents. The validity of linear response approximations to calculate the time-dependent Stokes shift is evaluated in each system. In most studied systems linear response theory fails. In ionic liquids the magnitude of the overall response is largely overestimated, and linear response theory is not able to capture the individual contributions of cations and anions. In polar liquids, the timescales of solvation dynamics are often not correctly reproduced. These observations are complemented by a detailed analysis of Gaussian statistics including higher order correlation functions, variance of the energy gap distribution and its time evolution. The analysis of higher order correlation functions was found to be not suitable to predict a failure of linear response theory. Further analysis of radial distribution functions and hydrogen bonds in the ground and excited state, as well as the time evolution of the number of hydrogen bonds after solute excitation reveal an influence of solvent structure in some of the studied systems.

How do the hydrocarbon chain length and hydroxyl group position influence the solute dynamics in alcohol-based deep eutectic solvents?

Effect of the hydrocarbon chain length and hydroxyl group position of hydrogen bond donor on the microscopic solution structure and diffusion dynamics of solutes is studied in a series of choline chloride/alcohol based deep eutectic solvents using ensemble and single-molecule measurements.

Is Solute Rotation in an Ionic Liquid Influenced by the Addition of Glucose?

Fluorescence anisotropy measurements and molecular dynamics (MD) simulations have been performed to understand the specific interactions of two structurally similar nondipolar solutes, 2,5-dimethyl-1,4-dioxo-3,6-diphenylpyrrolo[3,4-c]pyrrole (DMDPP) and 1,4-dioxo-3,6-diphenylpyrrolo[3,4-c]pyrrole (DPP), with neat 1-butyl-3-methylimidazolium dicyanamide ([BMIM][N(CN)₂]) and also in the presence of glucose. It has been observed that the measured reorientation times of DMDPP in neat [BMIM][N(CN)₂] follow the predictions of the Stokes-Einstein-Debye hydrodynamic theory with slip boundary condition. Addition of glucose (0.075 and 0.15 mole fraction) has no bearing on the rotational diffusion of the solute apart from the viscosity related effects. In contrast, the reorientation times of DPP in neat [BMIM][N(CN)₂] obey stick boundary condition as the hydrogen bond donating solute experiences specific interactions with the dicyanamide anion. No influence of the additive can be noticed on the rotational diffusion of DPP at 0.075 mole fraction of glucose. However, at 0.15 mole fraction of glucose, the reorientation times of the solute at a given viscosity and temperature decrease by 15-40% compared to those obtained in the neat ionic liquid. MD simulations indicate that each DPP molecule hydrogen bonds with two dicyanamide anions in neat ionic liquid. The simulations also reveal that, at 0.15 mole fraction of glucose, the concentration of anions hydrogen bonded to glucose increases significantly; therefore, the percentage of solute molecules that can form hydrogen bonds with two dicyanamide anions decreases to 84, which leads to faster rotation of DPP.

Probing the microscopic structural organization of neat ionic liquids (ILs) and ionic liquid-based gels through resonance energy transfer (RET) studies

With the aim to understand the role of the ionic constituents of ionic liquids (ILs) in their structural organization, resonance energy transfer (RET) studies between ionic liquids (donor) and rhodamine 6G (acceptor) have been investigated.

Excitation-energy dependence of solvation dynamics in room-temperature ionic liquids

Influence of the excitation energy of a probe solute molecule on its solvation dynamics and emission spectrum in 1-ethyl-3-methylimidazolium hexafluorophosphate (EMI(+)PF6 (-)) is studied via molecular dynamics simulations using a coarse-grained model description. By exciting the probe at different energies, each with an extremely narrow distribution, ensuing solvent relaxation and its dynamic variance are monitored using the isoconfigurational ensemble method. Resulting Stokes shift function, S(t), indicates that long-time solvent relaxation becomes slower with the decreasing excitation energy and approaches the equilibrium correlation function, C(t), of solvent fluctuations. This suggests that the system excited at the red-edge of the spectrum observes linear response better than that at the blue-edge. A detailed analysis of nonequilibrium trajectories shows that the effect of initial configurations on variance of relaxation dynamics is mainly confined to short times; it reaches a maximum around 0.1 ≲ t ≲ 1 ps and diminishes as time further increases. The influence of the initial velocity distribution, on the other hand, tends to grow with time and dominates the long-time variations of dynamics. The emission spectrum shows the red-edge effect in accord with previous studies.

Formic acid as a hydrogen storage material – development of homogeneous catalysts for selective hydrogen release

Liquid energy: formic acid is an ideal candidate for catalytic release and storage of hydrogen.

New insight into probe-location dependent polarity and hydration at lipid/water interfaces: comparison between gel- and fluid-phases of lipid bilayers

Location dependent polarity and hydration probed by a new series of 4-aminophthalimide-based fluorescent molecules (4AP-Cn;n= 2–10, 12) show different behaviour at gel- and fluid-phase lipid/water interfaces.

Does addition of an electrolyte influence the rotational diffusion of nondipolar solutes in a protic ionic liquid?

Rotational diffusion of two structurally similar nondipolar solutes, 2,5-dimethyl-1,4-dioxo-3,6-diphenylpyrrolo[3,4-c]pyrrole (DMDPP) and 1,4-dioxo-3,6-diphenylpyrrolo[3,4-c]pyrrole (DPP), has been examined in ethylammonium nitrate-lithium nitrate (EAN-LiNO3) mixtures to understand the influence of added electrolyte on the local environment experienced by the solute molecules. The measured reorientation times of both DMDPP and DPP in EAN-LiNO3 mixtures fall within the broad limits set by the hydrodynamic slip and stick boundary conditions. The hydrogen bond accepting DMDPP and the hydrogen bond donating DPP experience specific interactions with the cation and anion of the ionic liquid, respectively. Addition of LiNO3 (0.1 and 0.2 mole fraction) to EAN induces only viscosity related effects on the rotational diffusion of the two nondipolar solutes. These observations suggest that the local environment experienced by DMDPP and DPP in EAN is not altered upon the addition of LiNO3. Our results are consistent with the structural details available in the literature for EAN-LiNO3 mixtures.

Observations of probe dependence of the solvation dynamics in ionic liquids

Solvation and rotational dynamics of 4-aminophthalimide (4AP) in four ionic liquids (ILs) are measured using a combination of fluorescence upconversion spectroscopy and time-correlated single photon counting.

Investigation of the influence of alkyl side chain length on the fluorescence response of C153 in a series of room temperature ionic liquids

Variation of average solvation time with the product of temperature averaged viscosity and the radius of the cation of different room temperature ionic liquids (RTILs) with varying cationic chain length.