Femtosecond Solvation Dynamics in a Neat Ionic Liquid and Ionic Liquid Microemulsion: Excitation Wavelength Dependence

Small Molecule Sorting: A Fluorescence Study of Microemulsions

The application of microemulsions to a wide range of industries relies on their ability to solubilize small molecules with vastly different structures. Herein, we use multiple fluorescence techniques to probe ionic (rhodamine 6g, r6g), polar (coumarin 153, c153), nonpolar (diphenylanthracene, DPA), and amphiphilic (laurdan) small molecules in a nonionic, bicontinuous microemulsion of varying hydration. All fluorophores investigated were found to associate with the surfactant region despite their different structures and properties. The hydration of the surfactant layer was found to increase linearly with water addition, but while this initially increases the fluidity of the surfactant layer, fluorescence anisotropy of c153 and r6g indicates a stiffening of the surfactant at water content >60%. This stiffening of the surfactant layer at higher water content correlates with a morphological change in the microemulsion from a bicontinuous structure to droplets. In contrast, the nonpolar DPA shows a change in partitioning as hydration changes, increasing its association with the oil domain. Overall, these studies elucidate not only the capability of these microemulsions to host a range of small molecules in the surfactant layer with tunable position but also the ability to probe the driving force of bulk structural changes in these heterogeneous fluids.

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.

Structural Transitions at the Water/Oil Interface by Ionic-Liquid-like Surfactant, 1-Butyl-3-methylimidazolium Dioctyl Sulfosuccinate: Measurements and Mechanism

Reverse micelle (RM) aggregates have a wide range of applications in various areas of science and technology. A continuous demand exists to replace interfacial surfactant molecules with various nonconventional amphiphiles. Ionic liquid (IL)-like surfactants (IL-surf's) constitute a class of such molecules that are being researched extensively. Here, we have formulated several water/IL-surf/oil microemulsions by optimizing the core droplet size with varying oil phases. The best composition of water/[BMIM][AOT]/IPM ([BMIM][AOT]: 1-butyl-3-methylimidazolium dioctyl sulfosuccinate; IPM: isopropyl myristate) was then analyzed in detail through experimental and computer simulations. Our results from DLS measurements suggest a structural transition from spherical aggregates in the parent water/[Na][AOT]/IPM solution to cylindrical droplets in the IL-surf-based system. The Raman and ATR-FTIR spectral analysis suggest a variation in the microstructure of the water/oil interface due to the differential interaction of the counterions with AOT headgroups and water. Molecular dynamics simulation results provided the direct image of the interface showing a structured versus uneven water/oil interface in [Na][AOT] versus [BMIM][AOT] RMs, where the larger [BMIM] cations weakly bind with the AOT headgroups due to their low charge density. Finally, an application of this IL-surf-based formulation was tested by carrying out a Heck cross-coupling reaction that showed significantly higher yield under milder reaction conditions.

Subpicosecond Hot Hole Transfer in a Graphene Quantum Dot Composite with High Efficiency

Extraction of hot carriers is of prime importance because of its potential to overcome the energy loss that limits the efficiency of an optoelectronic device. Employing a femtosecond upconversion setup, herein we report a few picoseconds carrier cooling time of colloidal graphene quantum dots (GQDs) is at least an order of magnitude slower compared to that in its bulk form. A slower carrier cooling time of GQDs compared to that of the other semiconductor quantum dots and their bulk materials is indeed a coveted property of GQDs that would allow one easy harvesting of high energy species employing a suitable molecular system as shown in this study. A subpicosecond hot hole transfer time scale has been achieved in a GQD-molecular system composite with high transfer efficiency. Our finding suggests a dramatic enhancement of the efficiency of GQD based optoelectronic devices can possibly be a reality.

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.

Energy transduction through FRET in self-assembled soft nanostructures based on surfactants/polymers: current scenario and prospects

The self-assembled systems of surfactants/polymers, which are capable of supporting energy funneling between fluorophores, have recently gained significant attraction. Surfactant and polymeric micelles form nanoscale structures spanning a radius of 2-10 nm are generally suitable for the transduction of energy among fluorophores. These systems have shown great potential in Förster resonance energy transfer (FRET) due to their unique characteristics of being aqueous based, tendency to remain self-assembled, spontaneous formation, tunable nature, and responsiveness to different external stimuli. This review presents current developments in the field of energy transfer, particularly the multi-step FRET processes in the self-assembled nanostructures of surfactants/polymers. The part one of this review presents a background and brief overview of soft systems and discusses certain aspects of the self-assemblies of surfactants/polymers and their co-solubilization property to bring fluorophores to close proximity to transduce energy. The second part of this review deals with single-step and multi-step FRET in the self-assemblies of surfactants/polymers and links FRET systems with advanced smart technologies including multicolor formation, data encryption, and artificial antenna systems. This review also discusses the diverse examples in the literature to present the emerging applications of FRET. Finally, the prospects regarding further improvement of FRET in self-assembled soft systems are outlined.

Anomalous Spectral Modulation of 4-Aminophthalimide inside Acetonitrile/AOT/ n-Heptane Microemulsion: New Insights on Reverse Micelle to Bicontinuous Microemulsion Transition

The behavior of acetonitrile/sodium 1,4-bis(2-ethylhexyl)sulfosuccinate (AOT)/ n-heptane microemulsion, whether it remains as reverse micelle (RM) or bicontinuous microemulsion (BMC), has been controversial and even termed as a "problem system". Herein, we investigate the microemulsion using spectral and dynamical responses of a hydrophilic solvatochromic fluorophore 4-aminophthalimide (4-AP) at different w_s values (=[acetonitrile]/[AOT]). Interestingly, we found that emission parameters of 4-AP within the microemulsion vary differently at low and high w_s regimes. The quantum yield (ϕ_f) and lifetime (τ_f) of 4-AP first increase up to w_s = ∼1 and, thereafter, decrease upon a further increase in the w_s values. The emission maximum of 4-AP significantly shifts to a higher wavelength from 445 nm at w_s = 0 to 475 nm at w_s = 8. Interestingly, unlike aqueous RMs, the emission maximum at w_s = 1 matches with the emission maximum in neat acetonitrile and the emission maximum shifts to even longer wavelength at a higher w_s. Steady-state anisotropy also shows a break around w_s = 1; anisotropy decreases very sharply from w_s = 0 to 1 and, thereafter, remains nearly constant. Solvation dynamics becomes progressively faster with an increase in the acetonitrile content only in the low w_s regimes but remains almost independent of w_s after w_s > 2. All of the results collectively indicate that the morphology of the microemulsion may change at an intermediate w_s (∼1); below this, the system behaves like reverse micelles, and above this, the system may remain as BMC. The conjecture was further supported by dynamic light scattering measurements, where we observed a gradual increment of the average size at the low acetonitrile content (up to w_s = 1) but, thereafter, the size distribution becomes multimodal and sizes cannot be estimated correctly.

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.

Solvation, rotational relaxation and fluorescence correlation spectroscopic study on ionic liquid-in-oil microemulsions containing triple-chain surface active ionic liquids (SAILs)

In this article, solvation dynamics and rotational relaxation approaches have been applied to explore the microheterogeneity of surface active ionic liquid (SAIL) containing microemulsions, i.e. [P₁₃][Tf₂N] or [N₃₁₁₁][Tf₂N]/[BHD][AOT]/[IPM].

Enhanced fluorescence of [[5′-(4-hydroxyphenyl)[2,2′-bithiophen]-5-yl]methylene]-propanedinitrile (NIAD-4): solvation induced micro-viscosity enhancement

The solvation induced micro-viscosity enhancement effects on the fluorescence of a novel amyloid fibril marker (NIAD-4) were investigated in different alcoholic and aprotic solvents.

Understanding Structure-Property Correlation in Monocationic and Dicationic Ionic Liquids through Combined Fluorescence and Pulsed-Field Gradient (PFG) and Relaxation NMR Experiments

Steady state, time-resolved fluorescence and NMR experiments are carried out to gain deeper insights into the structure-property correlation in structurally similar monocationic and dicationic room-temperature ionic liquids (RTILs). The excitation wavelength dependent fluorescence response of fluorophore in 1-methy-3-propyllimidazolium bis(trifluoromethylsulfonyl)amide [C3MIm][NTf2] is found to be different from that of 1,6-bis(3-methylimidazolium-1-yl)hexane bis(trifluoromethylsulfonyl)amide [C6(MIm)2][NTf2]2 and 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide [C6MIm][NTf2]. The outcomes of the present solvent dynamics study in [C3MIm][NTf2] when compared with those in [C6(MIm)2][NTf2]2 and in [C6MIm][NTf2] from our previous studies (Phys. Chem. Chem. Phys. 2014, 16, 12918-12928) indicate the involvement of dipolar rotation of imidazolium cation during solvation. To correlate the findings of solvation dynamics study with the dipolar rotation of the imidazolium ring, pulsed-field gradient (PFG)-NMR technique for translational diffusion coefficient measurement and (1)H as well as (19)F spin-lattice relaxation measurements are employed. NMR investigation reveals that an ultrafast component of solvation can be related to the dipolar rotation of imidazolium cation; hence, the role of dipolar rotation of cations in governing the dynamics of solvation in ILs cannot be ignored. Analysis of the rotational relaxation dynamics data by the Stokes-Einstein-Debye hydrodynamic theory unveils distinctive features of solute-solvent interaction in [C3MIm][NTf2] and [C6(MIm)2][NTf2]2.

Dynamics of solvent response in methanol-chloroform binary solvent mixture: a case of synergistic solvation

Steady-state absorption, emission, and femtosecond transient absorption spectroscopies were used to ascertain the static and dynamic nature of the solvent response of methanol-chloroform binary solvent mixtures of different stoichiometric ratios using 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran (DCM) as the probe molecule. The appearance of synergistic solvation behavior in the steady-state absorption measurements can be explained in terms of solvent-solvent interactions through an extended hydrogen-bonding network. The disappearance of such synergistic behavior in the excited state of the DCM dye was recently proposed by us to be due to the weak nature of the intermolecular interactions present in binary solvent mixtures (J. Phys. Chem. B 2012, 116, 1345). It was anticipated and subsequently confirmed by the dynamics of the solvent response that the disruption of the weak interactive solvent network is the main reason for the absence of the synergism in the excited state. As expected, we observed the slowest dynamics for the mixture with X(MeOH) = 0.45, with an average solvation time of 12.03 ps, which is much higher than the values for the pure bulk counterparts (⟨τ(s)⟩(Methanol) = 4.32 ps and ⟨τ(s)⟩(Chloroform) = 1.32 ps). The unprecedented slowing of solvation for DCM is probably due to the rigid interactive methanol-chloroform solvent system in the first solvation shell, followed by solvent rearrangements around the solute dipole. Overall interactions present within the methanol-chloroform binary solvent mixture furnish clear evidence of solvent association through weak hydrogen bonding.

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.

Direct observation of the growth and shrinkage of microtubules by single molecule Förster resonance energy transfer

Single molecule Förster resonance energy transfer (FRET) has been applied to monitor the growth and the shrinkage of the dynamic microtubules.

Amphiphile self-assemblies in supercritical CO2 and ionic liquids

Supercritical (sc) CO2 and ionic liquids (ILs) are very attractive green solvents with tunable properties. Using scCO2 and ILs as alternatives of conventional solvents (water and oil) for forming amphiphile self-assemblies has many advantages. For example, the properties and structures of the amphiphile self-assemblies in these solvents can be easily modulated by tuning the properties of solvents; scCO2 has excellent solvation power and mass-transfer characteristics; ILs can dissolve both organic and inorganic substances and their properties are designable to satisfy the requirements of various applications. Therefore, the amphiphile self-assemblies in scCO2 and ILs have attracted considerable attention in recent years. This review describes the advances of using scCO2 or/and ILs as amphiphile self-assembly media in the last decade. The amphiphile self-assemblies in scCO2 and ILs are first reviewed, followed by the discussion on combination of scCO2 and ILs in creating microemulsions or emulsions. Some future directions on the amphiphile self-assemblies in scCO2 and ILs are highlighted.

Combined phase behavior, dynamic light scattering, viscosity and spectroscopic investigations of a pyridinium-based ionic liquid-in-oil microemulsion

Proposed model, phase diagram and variation in size (d), viscosity (η) and fluorescence intensity (FI) for 1-butyl-4-methyl pyridinium tetrafluoroborate ([b4mpy][BF4])–(Tween 20 + n-pentanol)–n-heptane ionic liquid-in-oil-microemulsion system.

Fluorescence response of a dipolar organic solute in a dicationic ionic liquid (IL): is the behavior of dicationic IL different from that of usual monocationic IL?

The solvation dynamics of coumarin 153 are compared in di and in mono-cationic-based ionic liquids for the first time.