Solvent and Rotational Relaxation of Coumarin 153 in a Protic Ionic Liquid Dimethylethanolammonium Formate

Solute dynamics of a hydrophobic molecule in a menthol-thymol based type-V deep eutectic solvent: effect of composition of the components

Type-V deep eutectic solvents (DESs) are a newly emerging unique class of solvents obtained by physical mixing and heating of non-ionic components. These solvents show deviation from the thermodynamic ideality. Compared to type-I to IV DESs, type-V DESs are less explored and their physical chemistry is in its nascent stage. In this work, we have chosen a type-V DES based on menthol-thymol (MT) for our working media. Solvent and rotational dynamics were studied with varying temperature using a well-known solvatochromic probe, Coumarin 153 (C153). We prepared the MT-based DES using a reported procedure at three molar ratios: 1 : 1 (M1T1), 1 : 1.5 (M1T1.5), and 2 : 1 (M2T1) of menthol (M) and thymol (T). Time-resolved emission spectra (TRES) were constructed with varying temperature. Utilizing TRES, the decay of the solvent correlation function (C(t)) was plotted. We have correlated the solvent relaxation time in these DESs as a function of viscosity. The time-resolved anisotropy decays were also collected to perceive the rotational relaxation dynamics of C153 as a function of temperature. The decay of solvent relaxation was found to be bi-exponential, and the average solvation time (〈τs〉) in M2T1 was found to be longer than those of M1T1.5 and M1T1. The rotational reorientation times (〈τrot〉) also follow the same trend. We have analysed the rotational dynamics of C153 in type-V DESs employing the Stokes-Einstein-Debye (SED) hydrodynamic model. The rotational dynamics in DESs demonstrate a good correlation with the SED model with a little deviation. In MT-based DESs, the solute's rotational relaxation times approach hydrodynamic stick boundary condition at low viscosity (or at high temperatures) for all molar compositions. Using the Arrhenius-type equations, we have correlated the activation energies for the rotational motion of C153, along with the viscous flow and non-radiative pathways for all the DESs.

Deep Eutectic Solvents: A Review of Fundamentals and Applications

Deep eutectic solvents (DESs) are an emerging class of mixtures characterized by significant depressions in melting points compared to those of the neat constituent components. These materials are promising for applications as inexpensive "designer" solvents exhibiting a host of tunable physicochemical properties. A detailed review of the current literature reveals the lack of predictive understanding of the microscopic mechanisms that govern the structure-property relationships in this class of solvents. Complex hydrogen bonding is postulated as the root cause of their melting point depressions and physicochemical properties; to understand these hydrogen bonded networks, it is imperative to study these systems as dynamic entities using both simulations and experiments. This review emphasizes recent research efforts in order to elucidate the next steps needed to develop a fundamental framework needed for a deeper understanding of DESs. It covers recent developments in DES research, frames outstanding scientific questions, and identifies promising research thrusts aligned with the advancement of the field toward predictive models and fundamental understanding of these solvents.

Solvation properties of protic ionic liquids and molecular solvents

Ionic liquids (ILs) are highly tailorable solvents with many potential applications. Knowledge about their solvation properties is highly beneficial in the utilization of ILs for specific tasks, though for many ILs this is currently unknown.

Solvation properties of protic ionic liquid–molecular solvent mixtures

In this study, we have investigated the solvation properties of binary mixtures of PILs with molecular solvents. The selected binary solvent systems are the PILs ethylammonium nitrate (EAN) and propylammonium nitrate (PAN) combined with either water, methanol, acetonitrile or DMSO.

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

With an aim to understand the behavior in terms of the intermolecular interactions, structure and dynamics of dicationic and monocationic ionic liquids (ILs), two imidazolium-based dicationic ionic liquids (DILs), 1,8-bis-(3-methylimidazolium-1-yl)octane bis-(trifluoromethylsulfonyl)amide ([C₈(mim)₂][NTf₂]₂), 1,9-bis-(3-methylimidazolium-1-yl)nonane bis-(trifluoromethylsulfonyl)amide ([C₉(mim)₂][NTf₂]₂), and one monocationic ionic liquid (MIL), 1-butyl-3-methyl imidazolium bis(trifluoromethylsulfonyl)amide ([C₄(mim)][NTf₂]), have been investigated through combined fluorescence, electron paramagnetic resonance (EPR), NMR and fluorescence correlation spectroscopy (FCS). The DILs were synthesized by following a standard synthetic protocol and subsequently characterized by different analytical techniques. Steady state absorption, emission and EPR spectroscopic data reveal that DILs are less polar compared to MIL. The polarities of the DILs and MIL were found to be close to those of acetonitrile and short chain alcohols, respectively. The excitation wavelength dependent emission data reveals that DILs are more micro-heterogeneous in nature than MIL. The rotational diffusion of two organic solutes, perylene and 8-methoxypyrene-1,3,6-sulfonate (MPTS), were examined in the DILs and MIL. The rotational diffusion data for perylene and MPTS were analyzed in light of the Stokes-Einstein-Debye (SED) hydrodynamic theory. The rotation of perylene in the DILs was observed to be relatively faster to that in the MIL, and it goes beyond the limit predicted by the SED theory. In order to explain the rotational motion of perylene in DILs, the data was analyzed further by invoking quasi-hydrodynamic theory. The observed rotational behavior of perylene has been explained by considering the fact that perylene is located in the nonpolar region of ILs, and larger solvent molecules (DILs) induce a lower friction to the rotating solute. Interestingly, unlike perylene, rotations of MPTS in both of the ILs were observed to be much hindered indicating a relatively stronger MPTS-IL interaction than perylene-IL interaction. More interestingly, rotation of MPTS was observed to be faster in the DILs than that in the MIL despite the fact that DILs are more viscous than MILs. Relatively faster rotation of MPTS in DILs has been explained by resorting to NMR and FCS studies. The outcomes of the NMR and FCS studies revealed that DILs in the experimental condition exist in their folded form and because of this structural restriction of DILs it becomes difficult for the bulky MPTS to make stronger hydrogen bonding interactions with DILs, which eventually makes the rotation of MPTS in DILs faster. Essentially, the outcomes of all of these studies have demonstrated that the behavior of DILs is quite different to that of the usual MILs.

Fluorescence quenching of coumarin 153 by hydroxyl-functionalized room temperature ionic liquids

Steady-state absorption and fluorescence as well as time-resolved fluorescence of coumarin 151 (C151) and coumarin 153 (C153) were measured in hydroxyl-functionalized ionic liquids ([HOEmim][BF4] and [HOEmim][N(CN)2]) and in nonhydroxyl-functionalized ionic liquids ([Emim][BF4] and [Emim][N(CN)2]). Both the steady-state fluorescence and time-resolved fluorescence observations reveal that hydroxyl-functionalized ionic liquid quenches the fluorescence of C153 while the nonhydroxyl-functionalized ionic liquid does not. We also measured the time-resolved fluorescence anisotropy of C151 and C153 in both [HOEmim][BF4] and [Emim][BF4]. It is found that the ratio of the rotational relaxation lifetime of C153 in [HOEmim][BF4] with respect to that in [Emim][BF4] is about 15% larger than that of C151 in [HOEmim][BF4] with respect to that in [Emim][BF4], indicating extra interaction between C153 and [HOEmim][BF4] exists except the effect of the viscosity of ionic liquid.

Experimental and theoretical studies on compositions, structures, and IR and NMR spectra of functionalized protic ionic liquids

DMCEAP and DMEOAP are composed of neutral and ionized species in the liquid phase as investigated through experimental spectroscopy and DFT calculations.

Photophysics and Rotational Dynamics of a Hydrophilic Molecule in a Room Temperature Ionic Liquid

We have studied the photophysics and rotational diffusion of hydrophilic solute 7-(N, N'-diethylamino)coumarin-3-carboxylic acid (7-DCCA) in a room temperature ionic liquid methyltrioctylammonium bis(trifluoromethylsulfonyl) imide ([N1888 ][NTf2 ]). Comparison of activation energies of viscous flow and nonradiative decay shows that the photophysical properties of 7-DCCA are not guided by the bulk viscosity of the medium but are dependent on the specific solute solvent interaction and structural heterogeneity of the medium. The rotational relaxation behaviour of 7-DCCA in [N1888 ][NTf2] shows significant deviation from the Stokes Einstein Debye hydrodynamic model of rotational diffusion. This is indicative of the influence of specific solute solvent interaction on the rotational relaxation behaviour of 7-DCCA. Comparison of activation energy of rotational relaxation with activation energy of viscous flow clearly reinforces our assumption that the structural heterogeneity of the medium and specific solute solvent interaction plays a dominant role on the rotational diffusion instead of bulk viscosity.

The solvation dynamics and rotational relaxation of protonated meso-tetrakis(4-sulfonatophenyl)porphyrin in imidazolium-based ionic liquids measured with a streak camera

For the steady-state behavior of diprotonated tetrakis(4-sulfonatophenyl)porphyrin (H4TPPS2-) in five imidazolium ionic liquids (ILs), the absorption positions of H4TPPS2- primarily depend on the constituent ions of the ILs whereas the emission positions of H4TPPS2- strongly depend on the polarity of the ILs. The fluorescence spectra of H4TPPS2- with different excitation wavelengths show no red-edge effect in our system. The dynamics of H4TPPS2- in ILs is further studied with a streak camera, and the relaxation process of ILs occurs on two different time scales. The short lived component attributed to the local motion of the cations and the anions around the porphine core varies from 32 to 196 ps. The long lived one originated from the collective diffusive motions of the cations and anions varies from 460 to 1072 ps. The average solvation time depends on the viscosity of the ILs. The rotational relaxation times of H4TPPS2- decrease as the viscosity of the ILs decreases.