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.

The rotational dynamics of 1-alkyl-3-methylimidazolium-based ionic liquids has been investigated by monitoring their inherent fluorescence with the intent to unravel the characteristics of the emitting species. For this purpose, temperature-dependent fluorescence anisotropies of 1-alkyl-3-methylimidazolium (alkyl = ethyl and hexyl) ionic liquids with anions such as tris(pentafluoroethyl)trifluorophosphate ([FAP]), bis(trifluoromethylsulfonyl)imide ([Tf2N]), tetrafluoroborate ([BF4]), and hexafluorophosphate ([PF6]) have been measured. It has been observed that the reorientation times (τr) of the ionic liquids with an ethyl chain scale linearly with viscosity and were found to be independent of the nature of the anion. The experimentally measured τr values are a factor of 3 longer than the ones calculated for 1-ethyl-3-methylimidazolium cation using the Stokes-Einstein-Debye (SED) hydrodynamic theory with stick boundary condition, which suggests that the emitting species is not the imidazolium moiety but some kind of associated species. The reorientation times of ionic liquids with a hexyl chain, in contrast, follow the trend τr([FAP]) > τr([Tf2N]) = τr([BF4]) > τr([PF6]) at a given viscosity (η) and temperature (T). The ability of the ionic liquids with longer alkyl chains to form the organized structure appears to be responsible for the observed behavior considering the fact that significant deviations from linearity have been noticed in the τr versus η/T plots for strongly associating anions [BF4] and [PF6], especially at ambient temperatures.

Fluorescence anisotropies 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), have been measured in 1-methyl-3-octylimidazolium hexafluorophosphate-dibenzyl ether ([MOIM][PF6]-DBE) mixtures to understand how the addition of a low viscous nondipolar solvent influences solute rotation. The data when analyzed with Stokes-Einstein-Debye hydrodynamic theory reveals that the measured reorientation times of DMDPP are closer to the predictions of slip boundary condition, whereas those of DPP follow stick hydrodynamics. This outcome arises due to specific interactions between DPP and the solvent medium. Nevertheless, the important result of this study is that the rotational diffusion of DMDPP becomes gradually slower with an increase in the mole fraction of DBE (xDBE) for a given viscosity and temperature. In contrast, such a trend is not noticed for the hydrogen-bond donating solute DPP. Instead, two sets of reorientation times have been obtained, one corresponding to xDBE = 0.0-0.2 and the other xDBE = 0.4-1.0. The results for DMDPP have been rationalized on the basis of the organized structure of [MOIM][PF6], which attains homogeneity at the microscopic level with an increase in xDBE. In case of DPP, however, the propensity of the solute to be in the neighborhood of DBE, as a consequence of its stronger hydrogen bond accepting ability compared to the ionic liquid, appears to be the reason for the observed behavior.

Rotational diffusion of two structurally similar nonpolar and charged solutes has been examined in mixtures of an ionic liquid and an organic solvent of comparable size and viscosity with an intent to find out whether the organized structure of the former influences solute rotation. To this effect, temperature-dependent fluorescence anisotropies of 9-phenylanthracene (9-PA) and rhodamine 110 (R110) have been measured in n-propylammonium nitrate (PAN), propylene glycol (PG), and also four different compositions of PAN-PG mixtures. Analysis of the data carried out with the aid of Stokes-Einstein-Debye (SED) hydrodynamic theory indicates that the reorientation times of 9-PA and R110 scale more or less linearly with the ratio of viscosity to temperature and are found to be independent of the mole fraction of PAN. In other words, apart from the viscosity and temperature, rotational diffusion of both the solutes is not affected by the composition of PAN-PG mixtures. It has also been observed that the reorientation times of R110 are significantly longer compared to those of 9-PA due to the specific interactions prevailing between the cationic solute and PAN-PG mixtures. However, the important finding of this work is that, even though PAN forms an organized structure, rotational diffusion of the solute molecules is similar in both the ionic liquid and the organic solvent. The disordered lamellar structure present in PAN probably does not offer compact organized domains unlike ionic liquids with long alkyl chains wherein solute rotation is influenced significantly.

Rotational diffusion of a nondipolar solute 2,5-dimethyl-1,4-dioxo-3,6-diphenylpyrrolo[3,4-c]pyrrole (DMDPP) and a charged solute rhodamine 110 (R110) has been investigated in 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([BMIM][Tf2N]) and 1-butyl-2,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide ([BMMIM][Tf2N]) to understand the influence of the C2 methylation on solute rotation. The measured reorientation times of the nondipolar solute DMDPP are similar in both the ionic liquids and follow Stokes-Einstein-Debye hydrodynamic theory with slip hydrodynamics. In contrast, rotational diffusion of the charged solute R110 in [BMIM][Tf2N] obeys stick hydrodynamics due to specific interactions with the anion of the ionic liquid. Nevertheless, the intriguing result of this study is that the reorientation times of R110 in [BMMIM][Tf2N] deviate significantly from the predictions of stick hydrodynamics, especially at ambient temperatures. The solute-solvent boundary condition parameter Cobs, which is defined as the ratio of the measured reorientation time to the one calculated using the SED theory with stick boundary condition, for R110 is lower by a factor of 2 in [BMMIM][Tf2N] compared to [BMIM][Tf2N] at 298 K. Upon increasing the temperature, Cobs gradually increases and eventually matches with that obtained in [BMIM][Tf2N] at 348 K. It has been well established that methylation of the C2 position in [BMMIM][Tf2N] switches off the main hydrogen-bonding interaction between the anion and the cation, but increases the Coulombic interactions. As a consequence of the enhanced interionic interactions between the cation and anion of the ionic liquid, specific interactions between R110 and [Tf2N] diminish leading to the faster rotation of the solute. However, such an influence is not apparent in case of DMDPP as it does not experience specific interactions with either the cation or the anion of these ionic liquids.