How is the Charge of Reactants Affects the Reaction Dynamics in Ionic Liquids?

It is known that in ionic liquid solvents, ionic solutes often get "caged" in ionic grids, so they diffuse slower than neutral solutes. Will that affect the dynamics of reactions that occur in ionic liquids, when an ionic reactant is involved? To answer this question, we studied the dynamics of a photo-induced electron transfer reaction in ionic liquids. We used neutral electron acceptors, and cationic/neutral/anionic donors, and measure the reaction dynamics.

 

The reaction kinetics are evaluated by measuring the fluorescence quenching of an excited electron acceptor (fluorophore) in the presence of electron donor (quencher).  We choose different donors and acceptors with a variety of redox potentials, which enables us to examine reactions that span a 2.0 eV range in driving force.

The solvents are ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethyl-sulfonyl)amide (Im2,1+/NTf2−, viscosity = 32 cP at 25 degree C) and trihexyl(tetradecyl)phosphoniumbis(trifluoromethylsulfonyl)amide (P14,6,6,6+/NTf2−, viscosity = 346 cP), as well as conventional solvent acetonitrile (viscosity =  0.343 cP).

If we plot the observed quenching rate constants kq vs. the reaction free energy, the following Rehm-Weller plot is obtained. Data are plotted in solid dots and circles if the donors are anionic or neutral, respectively. When the reaction free energy is "enough negative", kq stops increasing when the free energy continues to be lower. At this point we say the "diffusion limit" of the reaction is reached. In acetonitrile, the diffusion limited rate constant is independent of the charge of electron donors. But in both ionic liquids, the circles are above the solid dots, indicating a larger diffusion-limited rate constant when the donor is neutral.

Therefore, we conclude that in ionic liquid solvents, reactions are faster if the reactants are neutral. The "intrinsic" electron transfer rate constants k0, which describe the dynamics of charge transfer between proximate acceptor-donor pairs, can be obtained by diffusion-reaction models. The intrinsic electron transfer k0, in which the effect of diffusion has been stripped off, is not dependent on the charge of the donors. 

Please read our papers for more details if you are interested in this work:

 

Boning Wu, Mark Maroncelli and Edward W. Castner, Jr., “Photoinduced Bimolecular Electron Transfer in Ionic Liquids.”, J. Am. Chem. Soc., 2017, 139, 14568–14585, doi: 10.1021/jacs.7b07611.

Boning Wu, Min Liang, Mark Maroncelli and Edward W. Castner, Jr., “Photo-Induced Bimolecular Electron Transfer from Cyano-Anions in Ionic Liquids.”, J. Phys. Chem. B, 2015, 119, 14790–14799, doi: 10.1021/acs.jpcb.5b09216.

The Rhem-Weller plots does not include quenching data when donors are cations. Please read the following paper for more details.

Boning Wu, Min Liang, Nicole Zmich, Jasmine Hatcher, Sharon I. Lall-Ramnarine, James F. Wishart, Mark Maroncelli and Edward W. Castner, Jr., "Photo-Induced Bimolecular Electron Transfer in Ionic Liquids: Cationic Electron Donors", J. Phys. Chem. B, 2018, 122, 2379 – 2388, doi: 10.1021/acs.jpcb.7b12542.

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