Tuning radical interactions in trisradical tricationic complexes by varying host-cavity sizes

Although host–guest pairing interactions between bisradical dicationic cyclobis(paraquat-p-phenylene) (BB(2()˙(+))) and the bipyridinium radical cation (BIPY˙(+)) have been studied extensively, host molecules other than BB(2()˙(+)) are few and far between. Herein, four bisradical dicationic cyclophanes with tunable cavity sizes are investigated as new bisradical dicationic hosts for accommodating the methyl viologen radical cation (MV˙(+)) to form trisradical tricationic complexes. The structure–property relationships between cavity sizes and binding affinities have been established by comprehensive solution and solid-state characterizations as well as DFT calculations. The association constants of the four new trisradical tricationic complexes are found to range between 7400 and 170 000 M(–1), with the strongest one being 4.3 times higher than that for [MV⊂BB](3()˙(+)). The facile accessibility and tunable stability of these new trisradical tricationic complexes make them attractive redox-controlled recognition motifs for further use in supramolecular chemistry and mechanostereochemistry.