There are several factors affecting the reduction of dicationic 4,4'-bipyridinium derivatives ("viologens") such: solvent, charge transfer interactions, substituents at the nitrogen atoms, counter anion, etc.
More focused, I want to know how this factor should be considered to understand the shift of the reduction potential of the viologens by cyclic voltammetry in solution?
Well, assuming you are measuring the reduction of just the dication salt alone, charge transfer interactions should not be a factor. Likewise, providing the electrochemical measurements are carried out in a good solvent with sufficient electrolyte, changes in the counterions should also have little effect. This of course assumes that both dication and counterions are sufficiently solvated and ion-pairing is minimized. Lastly, assuming the substituents at the nitrogen are typical alkyl groups, again there should be little effect here as well. However, slight shifts could be seen in changes of very short chains (i.e .Me vs. Et. vs. Pr) in which the electron-donating ability of the alkyl groups can help stabilize the positive charge and shift the reduction to slightly higher potentials. Shifts between alkyl and aryl N-functionalization can have much greater effects. Most likely the largest effect would result from the solvent and electrolyte choices, which can have a variety of effects on the measurements (extent of aggregation, extent of ion-pairing, potential junction potentials, etc) and thus the resulting potentials. Hope this helps and if additional information is needed, let me know.
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