Electrochemistry of viologen dications in cholate media and competition between the cholate assemblies and the cucurbit[7]uril host.

ABSTRACT

The cathodic voltammetric behavior of N,N'-dimethyl-4,4'-bipyridinium (methylviologen, MV(2+)), N,N'-dipropyl-4,4'-bipyridinium (propylviologen, PV(2+)), N,N'-dibutyl-4,4'-bipyridinium (butylviologen, BV(2+)), and N-heptyl-N'-ethyl-4,4'-bipyridinium (heptyl-ethylviologen, HEV(2+)) was investigated in aqueous solution containing variable concentrations of sodium cholate. In general, the presence of cholate was found to solubilize the more hydrophobic forms of the viologen probes. Among the three accessible viologen oxidation states (V(2+), V(+•), and V), the intermediate cation radical (V(+•)) was preferentially stabilized by the cholate aggregates regardless of the nature of the N-alkyl substituents. This stabilization leads to anodic shifts in the first half-wave potential (V(2+)/V(+•)) and cathodic shifts in the second half-wave potential (V(+•)/V) for viologen reduction. Both potential shifts were considerably more pronounced as the hydrophobic character of the viologen probe increased. The presence of the cucurbit[7]uril host in the solution leads to the formation of very stable inclusion complexes with the viologen probes, which tend to eliminate or substantially decrease the interactions between the viologens and the cholate micellar aggregates.