Single two-electron transfers vs successive one-electron transfers in polyconjugated systems illustrated by the electrochemical oxidation and reduction of carotenoids

Examination of cyclic voltammetric responses reveals that inversion of the standard potentials of the first and second electron transfers occurs in th e oxidation of alpha -carotene and 15,15'-didehydro-beta -carotene (but not in their reduction) as well as in the reduction of canthaxanthin (but not in its oxidation). The factors that control potential inversion in these sy stems, and more generally in symmetrical molecules containing conjugated lo ng chains, are investigated by quantum chemical calculations. Two main inte rconnected effects emerge. One is the localization of the charges in the di -ion toward the ends of the molecule at a large distance from one another, thus minimizing Coulombic repulsion. The same effect favors the solvation o f the di-ion providing additional stabilization. In contrast, the charge in the ion radical is delocalized over the whole molecular framework, thus di sfavoring its stabilization by interaction with the solvent. The combinatio n of the two solvation effects allows potential inversion to occur as oppos ed to the case where the two electrophores are linked by a saturated bridge where potential inversion cannot occur. Localization of the charges in the di-ion, and thus potential inversion, is favored by the presence of electr on-accepting terminal groups for reductions las the two carbonyl groups in canthaxanthin) and of hole-accepting terminal groups for oxidations (as in beta -carotene).