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B: Liquids; Chemical and Dynamical Processes in Solution
Fluorescence Quenching within Lithium Salt-Added Ionic Liquid Media Anu Kadyan, and Siddharth Pandey J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.8b02723 • Publication Date (Web): 20 Apr 2018 Downloaded from http://pubs.acs.org on April 20, 2018
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The Journal of Physical Chemistry
Fluorescence Quenching within Lithium Salt-Added Ionic Liquid Media
Anu Kadyan and Siddharth Pandey*
Department of Chemistry, Indian Institute of Technology Delhi, Hauz Khas, New Delhi – 110016, India. *
To whom correspondence should be addressed.
E-mail:
[email protected], Phone: +91-11-26596503, Fax: +91-11-26581102
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ABSTRACT:
Salt-added ionic liquid media have emerged as versatile alternative to the
conventional electrolytes in several applications. Lithium bis(trifluoromethylsulfonyl)imide (LiTf2N)-added ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([emim][Tf2N]) system up to LiTf2N mole fraction (ݔ୧మ ) of 0.40 is investigated using fluorophore-quencher pair of pyrene-nitromethane in 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, ݔ୧మ and the temperature. Pyrene lifetimes decrease with increasing temperature at a given ݔ୧మ with lifetime becoming more sensitive to temperature at higher LiTf2N concentration. The pyrene-nitromethane fluorophore-quencher pair follows simplistic Stern-Volmer formulation indicating the quenching to be purely dynamic in nature affording dynamic quenching constants (KD) in the process. KD along with estimated bimolecular quenching rate constant (kq) within LiTf2N-added [emim][Tf2N] first increase with increasing LiTf2N till ݔ୧మ ~ 0.10, decreasing monotonically thereafter till ݔ୧మ = 0.40. The decrease in KD and kq with increasing ݔ୧మ is attributed to the exponentially increased dynamic viscosity with increasing ݔ୧మ 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 stabilize 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 ([emim][Tf2N] + LiTf2N) system in general, it is found to be obeyed at fixed ݔ୧మ . Role of structural changes within the system beyond viscosity increase on quenching process is amply highlighted.
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Introduction For the past couple of decades or so, room temperature ionic liquids have been constantly proving to be promising and desirable solvent systems for various applications varying from a solvent for chemical reactions to an alternative electrolyte for lithium-ion batteries.1,2 The uniqueness of the ionic liquid as media of utmost curiosity stems from the fact that they are constituted entirely of ions which results in superior physicochemical properties, such as, nonflammability, negligible vapor pressure and broad electrochemical window. These properties and features present them as viable and better alternatives to the organic solvents that are currently being used as electrolytes in various applications in electrochemistry. Concerning the safety issues associated with the common volatile organic solvents, research on the application of ionic liquids in lithium batteries has found its inception in recent years.3,4 A significant increase in the viscosity and adverse effects on pertinent electrochemical properties, such as, hindered ionic mobility of the lithium ions, on addition of inorganic lithium salts to ionic liquids have been reported.5–9 Recently, some experimental10–12 along with molecular
dynamics
simulation11–13
studies
have
suggested
the
aggregation
of
bis(trifluoromethylsulfonyl)imide anions ([Tf2N]‒) around the small cations, such as, Li+ in the mixture resulting in higher viscosities and reduced mobilities. This effect was further studied using Raman spectroscopy and a considerable Li+ concentration-dependent shift in the vibrational mode of the [Tf2N]‒ anion from ߥ ∼740 cm‒1 to ߥ ∼747 cm‒1 was found in ionic liquid based-systems.10,11,14–16This is explained by the formation of [Li(Tf2N)2]‒ anionic aggregates where the Li+ ion may be four-coordinated through the O atoms of two bidentate Tf2N− ions.14 which is also confirmed by the trend in the dynamic viscosity with increasing concentration of lithium salt.17 As lithium ions transport plays a major role in battery operations, 3 ACS Paragon Plus Environment
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it becomes necessary to study its effect on the structure and dynamics of ionic liquids, especially the alkylimidazolium Tf2N ionic liquids due to their relatively low viscosities, good electrochemical stabilities and broad operating temperature ranges: properties that are of interest for battery applications.4,18 Thus, Li salt-added ionic liquid systems and solute behavior therein have garnered increased attention of the researchers in recent times. 11,14,19–24 In this paper, we report the results of our investigation of the excited-state fluorescence lifetime quenching of a common and popular fluorophore pyrene by a well-known quencher nitromethane within Li salt LiTf2N-added ionic liquid 1-ethyl-3-methylimidazolium Tf2N ([emim][Tf2N]) up to 0.40 LiTf2N mole fraction (ݔ୧మ ) in the temperature range (298.15 to 358.15) K. The selection of the ionic liquid and the Li salt having the same anion [Tf2N‒] may offer key insight not only to the solute behavior and diffusion within a potentially-attractive electrolyte system, but also afford information on changes in the structural features of the ionic media as the Li+ ions are added.
Experimental Materials. Pyrene [≥99.0% (GC), puriss for fluorescence] was obtained in highest purity from Sigma-Aldrich Co. and stored under dried conditions. Electrochemical grade (>99.0% by mass purity) ionic liquid [emim][Tf2N] was purchased from Covalent Associates, Inc. [emim][Tf2N] was stored under argon in an Auto Secador desiccator cabinet. Before use, [emim][Tf2N] was rigorously dried under vacuum for at least 72 hours. Karl-Fisher titrator was used next to assess water content of the ionic liquid prior to its use. Ionic liquid was dried till the water content became less than 100 ppm. LiTf2N with 99.95% purity was purchased from Sigma-Aldrich and stored in the Auto Secador desiccator cabinet. The water content of the 4 ACS Paragon Plus Environment
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[emim][Tf2N] and ([emim][Tf2N] + LiTf2N) samples was also assessed after data acquisition using Karl-Fisher titrator and was found to be
