Photoinduced Bimolecular Electron Transfer in Ionic Liquids

Boning Wu, Mark Maroncelli, Ed Castner

Research output: Contribution to journalArticle

7 Citations (Scopus)

Abstract

The present work seeks to better understand the role of solute diffusion and solvation dynamics on bimolecular electron transfer in ionic liquids (ILs). Steady-state and time-resolved measurements of the reductive fluorescence quenching of five fluorophores ("F") by six quenchers ("Q" electron donors) are reported in acetonitrile and two ionic liquids, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide and trihexyl(tetradecyl)phosphonium bis(trifluoromethylsulfonyl)amide. Data were collected on 66 different F-Q-solvent systems, which span a 2.0 eV range in driving force and viscosities that vary 1000-fold, enabling stringent tests of bimolecular electron transfer models. A Stern-Volmer analysis yielded much larger diffusion-limited rates than simple kinetic theory predictions in the ILs and the absence of a Marcus turnover. Use of an approximate solution to the spherical diffusion-reaction equation enabled testing of several models for the reaction rate distance dependence. The Smoluchowski and Collins-Kimball models, which assume reaction at a single distance, are able to fit the data collected in acetonitrile solutions reasonably well, but not the data in the IL solvents. An extended sink model, incorporating a finite reaction zone, was able to fit all data satisfactorily with only three adjustable parameters. Diffusion coefficients extracted from these fits were much larger for the neutral versus anionic quenchers and close to predicted values. Molecular dynamics simulations and density-functional methods were then used to explore solvation structures and electronic couplings. The electronic coupling between contact F-Q pairs was found to vary strongly with the relative location and orientation of the reactants. Information from these simulations was used to constrain a model based on classical Marcus theory, which provided physically reasonable fits with only two adjustable parameters, but required systematic reduction of the driving forces in order to suppress a rate turnover at large driving force. The latter requirement indicates that reaction rates in ionic liquids are limited by some factor not properly accounted for in bimolecular electron transfer models based on a spherical diffusion-reaction approach. Small-amplitude motions within contact F-Q pairs, which gate the electronic coupling, are suggested to be the limiting dynamics.

Original languageEnglish
Pages (from-to)14568-14585
Number of pages18
JournalJournal of the American Chemical Society
Volume139
Issue number41
DOIs
Publication statusPublished - Oct 18 2017

Fingerprint

Ionic Liquids
Ionic liquids
Electrons
Amides
Solvation
Acetonitrile
Reaction rates
Molecular Dynamics Simulation
Kinetic theory
Fluorophores
Viscosity
Time measurement
Fluorescence
Molecular dynamics
Quenching
Computer simulation
Testing

ASJC Scopus subject areas

  • Catalysis
  • Chemistry(all)
  • Biochemistry
  • Colloid and Surface Chemistry

Cite this

Photoinduced Bimolecular Electron Transfer in Ionic Liquids. / Wu, Boning; Maroncelli, Mark; Castner, Ed.

In: Journal of the American Chemical Society, Vol. 139, No. 41, 18.10.2017, p. 14568-14585.

Research output: Contribution to journalArticle

Wu, Boning ; Maroncelli, Mark ; Castner, Ed. / Photoinduced Bimolecular Electron Transfer in Ionic Liquids. In: Journal of the American Chemical Society. 2017 ; Vol. 139, No. 41. pp. 14568-14585.
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