Redox-induced partner radical formation and its dynamic balance with radical dimer in cucurbit[8]uril.

N-(4-Hydroxy-phenoxyethyl)-N'-ethyl-4,4'-bipyridium (1) can form a stable 1 : 1 inclusion complex with CB[8] in aqueous solution, in which the hydroxyphenol (HP) moiety is back-folded and inserted together with the viologen moiety into the cavity of CB[8]. When the ethyl viologen dication (EV(2+)) in 1 is reduced, chemically or electrochemically, an intramolecular partner radical (EV(+)*-HP)/CB[8] can be detected, meanwhile, a dynamic balance between the partner radical and the intermolecular radical dimer (EV(+)*-HP)(2)/CB[8] can be observed.