Publication 536

Contribution from the Department of Chemistry, Box 870336, University of Alabama, Tuscaloosa, Alabama 35487, and the Laboratoire d'Electrochimie Moléculaire, Unité mixte de Recherche Université-CNRS No. 7591, Université de Paris 7 -Denis Diderot, Case Courier 7107, 2 Place Jussieu, 75251 Paris Cedex 05, France

Examination of cyclic voltammetric responses reveals that inversion of the standard potentials of the first and second electron transfers occurs in the oxidation of -carotene and 15,15'-didehydro--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 systems, and more generally in symmetrical molecules containing conjugated long chains, are investigated by quantum chemical calculations. Two main interconnected 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 of the di-ion providing additional stabilization. In contrast, the charge in the ion radical is delocalized over the whole molecular framework, thus disfavoring its stabilization by interaction with the solvent. The combination of the two solvation effects allows potential inversion to occur as opposed 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 electron-accepting terminal groups for reductions (as the two carbonyl groups in canthaxanthin) and of hole-accepting terminal groups for oxidations (as in -carotene).