Rotational and Translational Dynamics of Rhodamine 6G in a Pyrrolidinium Ionic Liquid: A Combined Time-Resolved Fluorescence Anisotropy Decay and NMR Study

NMR spectroscopy and time-resolved fluorescence anisotropy decay (TRFAD) are two of the most commonly used methods to study solute-solvent interactions. However, only a few studies have been reported to date using a combined NMR and TRFAD approach to systematically investigate the overall picture of diffusional and rotational dynamics of both the solute and solvent In this paper, we combined NMR and TRFAD to probe fluorescent rhodamine dye in a pyrrolidinium:based room temperature ionic liquid (RTIL), an emergent environmentally friendly solvent type used in several energy-related applications. A specific interaction of the R6G cation and [Tf2N] anion was identified, resulting in near stick boundary condition rotation of R6G in this RTIL. The diffusional rates of the R6G solute and [C(4)mpyr][Tf2N] solvent derived from H-1 NMR suggest the rates are proportional to their corresponding hydrodynamic radii. The H-1 and C-13 NMR studies of self rotational dynamics of [C(4)mpyr] [Tf2N] showed that the self rotational correlation time of [C(4)mpyr] is 47 +/- 2 ps at 300 K. At the same temperature, we find that the correlation time for N-CH3 rotation in [C(4)mpyr](+) is 77 +/- 2 ps, comparable to overall molecular reorientation. This slow motion is attributed to properties of the cation structure.