An ionic liquid is an ionic substance in which its constituent ions are in motion. Consequently, it is not possible to understand the nature of an ionic liquid without understanding how its constituent ions move. However, gaining a thorough understanding the nature of ion motion in ionic liquids remains a fundamental challenge.

 Single molecule fluorescence (SMF) confocal spectroscopy offers a powerful, but as yet largely underutilized tool to measure the diffusion of probe ions in an ionic liquid under a variety of conditions.

 To enable SMF measurements, it was necessary to develop the synthesis of ionic liquids with very low background fluorescence. The SMF derived diffusivity of the cationic probe Rhodamine 123 in dilute (ca. 100 pM) solutions in [C₄C₁im][NTf₂] solutions was compared to diffusivity of the ionic liquid cations derived from Pulse Gradient Stimulated Echo NMR measurements. The SMF derived diffusivity of the cationic probe Rhodamine 123 was measured under various applied potentials and throughout the system’s relaxation back to its unperturbed state. This relaxation was found to be markedly slower than in salt solutions in molecular solvents.