Viologen-cucurbituril host/guest chemistry - redox control of dimerization versus inclusion.

Two calix[4]arene systems, C23 4+ and C24 4+ - where 2 corresponds to the number of viologen units and 3-4 corresponds to the number of carbon atoms connecting the viologen units to the macrocyclic core - have been synthesized and led to the formation of [3]pseudorotaxanes when combined with either CB[7] or CB[8]. The [3]pseudorotaxanes spontaneously dissociate upon reduction of the bipyridinium units as the result of intramolecular dimerization of the two face-to-face viologen radical cations. CB[7] and CB[8]-based [2]pseudorotaxanes containing monomeric viologen guest model compounds, MC3 2+ and MC 4+ , do not undergo decomplexation and dimerization following electrochemical reduction of their bipyridinium units.