Florescence Quenching within Lithium Salt-Added Ionic Liquid.

Abstract

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 ( xLiTf2N) 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, xLiTf2N, and the temperature. Pyrene lifetimes decrease with increasing temperature at a given xLiTf2N 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 ( KD) in the process. KD along with the estimated bimolecular quenching rate constant ( kq) within LiTf2N-added [emim][Tf2N] first increases with increasing LiTf2N until xLiTf2N ∼ 0.10, decreasing monotonically thereafter until xLiTf2N = 0.40. The decrease in KD and kq with increasing xLiTf2N is attributed to the exponentially increased dynamic viscosity with increasing xLiTf2N of the ([emim][Tf2N] + LiTf2N) system. The initial increase in KD and kq 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 xLiTf2N. The role of structural changes within the system beyond viscosity increase on the quenching process is amply highlighted.