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Marcus Bell-Shaped Electron Transfer Kinetics Observed in an Arrhenius Plot.

Morteza M Waskasi1, Gerdenis Kodis1, Ana L Moore1

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The Marcus theory of electron transfer predicts a bell-shaped rate dependence. Experiments on a fullerene-porphyrin dyad show this rate law holds true with temperature changes, not just chemical modifications.

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Area of Science:

  • Physical Chemistry
  • Photochemistry
  • Electron Transfer

Background:

  • The Marcus theory describes electron transfer rates based on reaction free energy.
  • A key prediction is a bell-shaped (inverted parabola) rate dependence.
  • Traditionally, this is observed by altering molecular structures.

Purpose of the Study:

  • To experimentally verify the Marcus theory's prediction of a bell-shaped rate dependence.
  • To investigate if temperature variation, rather than chemical modification, can induce this phenomenon.
  • To provide a clean confirmation of the Marcus energy gap law.

Main Methods:

  • Studied a fullerene-porphyrin dyad system.
  • Investigated photoinduced electron transfer and subsequent charge recombination.
  • Analyzed the rate of charge recombination as a function of inverse temperature.

Main Results:

  • Observed a bell-shaped dependence of charge recombination rate on inverse temperature.
  • The rate increased upon cooling, then decreased at lower temperatures.
  • This non-Arrhenius behavior resulted from significant temperature variations in reorganization and reaction free energies.

Conclusions:

  • Demonstrated that temperature variation can induce the Marcus inverted parabola effect.
  • Provided strong experimental evidence for the Marcus energy gap law without chemical alteration.
  • Highlighted the role of temperature-dependent reorganization and reaction free energies in electron transfer kinetics.