Temperature-dependent fluorescence anisotropies of a nonpolar solute 9-phenylanthracene (9-PA) have been measured in 1-alkyl-3-methylimidazolium-based ionic liquids with anions such as bis(trifluoromethylsulfonyl)imide ([Tf2N–]), tris(pentafluoroethyl)trifluorophosphate ([FAP–]), tetrafluoroborate ([BF4(–)]), and hexafluorophosphate ([PF6(–)]) to find out if the organized structure of the ionic liquid has a bearing on solute rotation. Analysis of the experimental data using the Stokes–Einstein–Debye hydrodynamic theory indicates that there is no significant variation in the solute–solvent coupling constants (Cobs) with an increase in the length of the alkyl chain on the imidazolium cation for the ionic liquids with [Tf2N–] and [FAP–] anions. However, in the case of ionic liquids with [BF4(–)] and [PF6(–)] anions, the rotation of 9-PA for a given viscosity at constant temperature becomes progressively faster and C(obs) decreases by a factor of 2.4 from ethyl to octyl derivatives. Quasihydrodynamic theories of Gierer–Wirtz and Dote–Kivelson–Schwartz could not account for the significant decrease in the C(obs) values. The observed behavior has been rationalized in terms of the organized structure of the ionic liquids having [BF4(–)] and [PF6(–)] anions, which results as a consequence of the high charge-to-size ratio of these anions compared to [Tf2N–] and [FAP–].

In an attempt to find out whether the length of the alkyl chain on the imidazolium cation has a bearing on solute rotation, temperature-dependent fluorescence anisotropies of three structurally similar solutes have been measured in a series of 1-alkyl-3-methylimidazolium (alkyl = methyl, ethyl, propyl, butyl, and hexyl) bis(trifluoromethylsulfonyl)imides. Solute-solvent coupling constants obtained from the experimentally measured reorientation times with the aid of Stokes-Einstein-Debye hydrodynamic theory indicate that there is no influence of the length of the alkyl chain on the rotation of nonpolar, anionic, and cationic solutes 9-phenylanthracene (9-PA), fluorescein (FL), and rhodamine 110 (R110), respectively. It has also been noticed that the rotational diffusion of 9-PA is closer to the predictions of slip hydrodynamics, whereas the rotation of negatively charged FL and positively charged R110 is almost identical and follows stick hydrodynamics in these ionic liquids. Despite having similar shape and size, ionic solutes rotate slower by a factor of 3-4 compared to the nonpolar solute. Interplay of specific and electrostatic interactions between FL and the imidazolium cation of the ionic liquids, and between R110 and the bis(trifluoromethylsulfonyl)imide anion, appear to be responsible for the observed behavior. These results are an indication that the length of the alkyl chain on the imidazolium cation does not alter their physical properties in a manner that has an effect on solute rotation.

To understand the influence of organized structure of the ionic liquids on the rotational diffusion of a hydrogen bond donating ionic solute, reorientation times (τr) of rhodamine 110 (R110) have been measured in 1-alkyl-3-methylimidazolium ([Rmim+]) based ionic liquids with anions tetrafluoroborate ([BF4-]) and hexafluorophosphate ([PF6-]). The viscosity (η) was varied by changing the temperature (T) and also the alkyl chain length on the imidazolium cation (ethyl, butyl, hexyl, and octyl). It has been noticed that τr versus η/T plots contain two slopes corresponding to lower and higher values of η/T for ionic liquids with [BF4-] as well as [PF6-] anions. For lower values of η/T (<0.2 and <0.3 mPa s K(-1), respectively, for [Rmim+][BF4-] and [Rmim+][PF6-]), rotational diffusion of R110 follows Stokes-Einstein-Debye hydrodynamic theory with stick boundary condition due to specific interactions between the solute and the anions of the ionic liquids. In contrast, at higher η/T, the rotational diffusion of the solute is faster than the stick predictions and this trend could not be explained by the quasihydrodynamic theories of Gierer-Wirtz and Dote-Kivelson-Schwartz as well. Diminishing hydrogen bonding interactions between the solute and the anions, which transpire as a consequence of the organized structure of the ionic liquids, are responsible for the observed behavior.

Rotational diffusion of a nonpolar solute 9-phenylanthracene (9-PA) and a cationic solute rhodamine 110 (R110) has been examined in a series of 1-alkyl-3-methylimidazolium (alkyl = octyl, decyl, dodecyl, tetradecyl, hexadecyl, and octadecyl) bis(trifluoromethylsulfonyl)imides to understand the influence of alkyl chain length on solute rotation. In this study, reorientation times (τr) have been measured as a function of viscosity (η) by varying the temperature (T) of the solvents. These results have been analyzed using the Stokes-Einstein-Debye (SED) hydrodynamic theory along with the ones obtained for the same solutes in 1-alkyl-3-methylimidazolium (alkyl = methyl, ethyl, propyl, butyl, and hexyl) bis(trifluoromethylsulfonyl)imides (Gangamallaiah and Dutt, J. Phys. Chem. B 2012, 116, 12819-12825). It has been noticed that the data for 9-PA and R110 follows the relation τr = A(η/T)(n) with A being the ratio of hydrodynamic volume of the solute to the Boltzmann constant and n = 1 as envisaged by the SED theory. However, upon increasing the alkyl chain length from methyl to octadecyl significant deviations from the SED theory have been observed especially from the octyl derivative onward. From methyl to octadecyl derivatives, the value of A decreases by a factor of 3 for both the solutes and n by a factor of 1.4 and 1.6 for 9-PA and R110, respectively. These observations have been rationalized by taking into consideration the organized structure of the ionic liquids, whose influence appears to be pronounced when the number of carbon atoms in the alkyl chain attached to the imidazolium cation exceeds eight.

To understand how the organized structure of the ionic liquids influences the location and mobility of nondipolar solutes, rotational diffusion of 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 1-alkyl-3-methylimidazolium (alkyl = ethyl, butyl, hexyl, and octyl) tetrafluoroborates. Both the solutes are structurally similar-the sole difference being the two NCH3 groups of DMDPP are replaced by two NH groups in DPP. The rotational diffusion of DPP is found to be significantly slower than DMDPP due to specific interactions between the NH groups of the solute and the anion of the ionic liquid. It has been observed that for a given viscosity and temperature, the rotational diffusion of DMDPP becomes progressively faster with an increase in the length of the alkyl chain on the imidazolium cation. DMDPP resides in the nonpolar domains of these ionic liquids whose sizes increase with an increase in the length of the alkyl chain, and as a result it experiences microviscosity that is lower than the bulk viscosity. However, an increase in the length of the alkyl chain has no apparent effect on the rotational diffusion of DPP because specific interactions with tetrafluoroborate necessitate the solute to be located in the vicinity of the anion. The results of this work exemplify that despite having similar size and shape, the rotational diffusion of DMDPP and DPP is quite contrasting as their sites of solubilization and the nature of interactions with the surroundings are vastly different owing to subtle variations in their chemical structures.