Florescence Quenching within Lithium Salt-Added Ionic Liquid

Salt-added ionic liquid media have emerged as a versatile alternative to the conventional electrolytes in several applications. A lithium bis­(trifluoromethylsulfonyl)­imide (LiTf₂N)-added ionic liquid 1-ethyl-3-methylimidazolium bis­(trifluoromethylsulfonyl)­imide ([emim]­[Tf₂N]) system up to a LiTf₂N mole fraction (<i>x</i>_LiTf₂N) of 0.40 is investigated using a fluorophore–quencher pair of pyrene–nitromethane in the 298.15–358.15 K temperature range. Excited-state intensity decay of pyrene fits best to a single-exponential decay function irrespective of the concentration of nitromethane, <i>x</i>_LiTf₂N, and the temperature. Pyrene lifetimes decrease with increasing temperature at a given <i>x</i>_LiTf₂N with lifetime becoming more sensitive to temperature at higher LiTf₂N concentration. The pyrene–nitromethane fluorophore–quencher pair follows a simplistic Stern–Volmer formulation, indicating the quenching to be purely dynamic in nature affording dynamic quenching constants (<i>K</i>_D) in the process. <i>K</i>_D along with the estimated bimolecular quenching rate constant (<i>k</i>_q) within LiTf₂N-added [emim]­[Tf₂N] first increases with increasing LiTf₂N until <i>x</i>_LiTf₂N ∼ 0.10, decreasing monotonically thereafter until <i>x</i>_LiTf₂N = 0.40. The decrease in <i>K</i>_D and <i>k</i>_q with increasing <i>x</i>_LiTf₂N is attributed to the exponentially increased dynamic viscosity with increasing <i>x</i>_LiTf₂N of the ([emim]­[Tf₂N] + LiTf₂N) system. The initial increase in <i>K</i>_D and <i>k</i>_q is controlled by the structural changes within the system as LiTf₂N is added to [emim]­[Tf₂N]. It is proposed that the presence of [Li­(Tf₂N)₂]⁻ anionic clusters stabilizes the partial positive charge that develops on excited pyrene during the electron/charge transfer to nitromethane during the quenching process. While the Stokes–Einstein formulation is not followed by the ([emim]­[Tf₂N] + LiTf₂N) system in general, it is found to be obeyed at fixed <i>x</i>_LiTf₂N. The role of structural changes within the system beyond viscosity increase on the quenching process is amply highlighted