Discovery of Unusually Stable Reduced Viologen via Synergistic Folding and Encapsulation

Abstract

The preparation and identification of an unusually stable viologen radical has been demonstrated. The intramolecular interactions of bis-viologen species such as 1,1”-[1,2-phenylenebis(methylene)]bis[1’-methyl-4,4’-bipyridinium] (1) and 1,1”-(1,3-propanediyl)bis[1’-methyl-4,4’-bipyridinium] (2) upon reduction and complexation by cucurbit[8]uril (CB[8]) result in the formation of a unique monoradical species. Cyclic voltammetric responses for 1 and 2 in the presence of CB[8] showed a splitting of the degenerate 2 e− redox wave (corresponding to the 4+/2+ transition of the bis-viologen) into two separate 1 e− redox waves indicating the stabilization of the 3+ form of the bis-viologens. Visible light absorbance showed that this species had a distinct electronic structure, imparting a blue color unlike the 2+ state of the bis-viologen or the 1+ state of partially reduced methylviologen. The persistence of the blue color over time, even in the presence of dissolved O2, illustrated a significant stabilization of these monoradical species in comparison to any other partially reduced viologen radical. Molecular orbital calculations indicated that the origin of this stability arises from the SOMO of the monoradical that features the electron density as buried between the two cofacial viologen units, hindering electronic coupling with oxidants such as O2 and thereby providing enhanced stability for energy storage and electrochromic applications.