Ternary Complex Formation by Cucurbit[7]uril Leads to Large Shifts in the Reduction Potentials of Suitable Viologens

This work addresses the effects of supramolecular complexation by cucurbit[7]uril (CB[7]) on the cell potential of aqueous batteries based on ferrocene and 4,4′-bipyridinum (viologen) electroactive materials. It was previously known that the oxidation potential of cationic ferrocene derivatives is shifted to more positive values by the formation of highly stable 1 : 1 binary complexes with CB[7]. Here, we found that the formation of 1 : 2 ternary complexes between suitably designed viologens and CB[7] leads to a pronounced shift to more negative values in the first one-electron reduction potential of the viologen. Combining these two effects, we predict that the corresponding aqueous redox flow battery (RFB) -containing cationic ferrocene in the catholyte and viologen in the anolyte- will exhibit a potential of 1.04 V in the absence of CB[7] and 1.30 V in the presence of suitable concentrations of CB[7]. This substantial potential increase might be improved in future research, as the supramolecular complexation effect seems to be rather general.