Redox interaction of star-burst tetranuclear complex

Abstract

Abstract Electrode reactions of the star-burst tetranuclear complex, in which a redox center is connected symmetrically to three redox centers through π-conjugated bridges, begin either from the central redox site or from the outer sites. Which route the electrode reaction selects depends on the interaction energies between the oxidized site (O) and the reduced site (R), between O and O, and between R and R. On the assumption that the electrochemical potential is represented by a simple sum of the interaction energies, the molar fractions of eight redox isomers were expressed as a function of the electrode potential and two differences in the interaction energies u 1 and u 2 . The theory was developed for normal pulse voltammetry and cyclic voltammetry. It predicted the appearance of at most three voltammetric waves depending on u 1 and u 2 . The finding that four waves did not appear was ascribed to a minor contribution of the half-oxidized (reduced) isomers. A technique for evaluating the interaction energy was presented. In order to verify the theory, voltammograms of 2,2′-bis( N -methyl-2′'-benzimidazolyl)-4,4′-bipyridine(dmbbbpy) bridged tetraruthenium complex were recorded which showed one-electron and three-electron oxidation waves. From the peak difference, we obtained u 1 u 1 − u 2 = −8.2kj mol −1 . The observation that the inverse slope of the log plot was larger than the theoretical value indicated the presence of direct interaction between outer redox sites.