Intramolecular association of covalently linked viologen radicals

Publisher Website
Google Scholar
Abstract
A series of compounds has been synthesised having two identical viologen groups separated by organic bridges of varying degree of flexibility. Chemical or radiolytic reduction results in formation of the mono viologen π-radical cations, which undergo disproportionation to form the doubly reduced viologen. In this latter species both viologen units are reduced and there is a distinct stabilisation effect if the molecule can form an intramolecular cofacial dimer. The disproportionation constants have been determined by cyclic voltammetry and depend markedly upon the type of bridging group employed. Using pulse radiolysis it was shown that the mono radical could be stabilised by intramolecular association with the unreduced viologen. This is achieved by fairly slow conformational rearrangement in competition with bimolecular electron transfer to the preferred conformation. All the mono radicals disproportionate and the bimolecular rate constants have been measured. Although the viologens can store several electrons on one molecule, they do not reduce water to H2 in the absence of a catalyst.