Electron- and Photo-Induced Processes

Toward a universal method for true redox potentials

A novel method to determine redox potentials without electrolyte is presented. The method is based on a new ability to determine the dissociation constant, K°_d, for ion pairs formed between any radical anion and any inert electrolyte counterion. These dissociation constants can be used to determine relative shifts of redox potential as a function of electrolyte concentration, connecting referenced potentials determined with electrochemistry (with 0.1 M electrolyte) to electrolyte-free values. Pulse radiolysis created radical anions enabling determination of equilibrium constants for electron transfer between anions of donor and acceptor molecules as a function of electrolyte concentration in THF. The measurements determined “composite equilibrium constants”, K_eqC, which contain information about the dissociation constant for the electrolyte cations, X⁺, with the radical anions of both the donor, K°_d (D^–•,X⁺) and the acceptor, K° _d(A^–•,X⁺). Dissociation constants were obtained for a selection of radical anions with tetrabutylammonium (TBA⁺). The electrolyte was found to shift the reduction potentials of small molecules 1-methylpyrene and trans-stilbene by close to +130 mV whereas oligo-fluorenes and polyfluorenes experienced shifts of only (+25 ± 6) mV due to charge delocalization weakening the ion pair. These shifts for reduction of aromatic hydrocarbon molecules are smaller than shifts of +232 and +451 mV seen previously for benzophenone radical anion with TBA⁺ and Na⁺ respectively where the charge on the radical anion is localized largely on one C═O bond, thus forming a more tightly bound ion pair.

Phys. Chem. A., 2020, 124, 5487 DOI: 10.1021/acs.jpca.0c02948