Coulombic-enhanced hetero radical pairing interactions

Spin–spin interactions between two identical aromatic radicals have been studied extensively and utilized to establish supramolecular recognition. Here we report that spin-pairing interactions could also take place between two different π-electron radicals, namely a bipyridinium radical cation ( BPY +• ) and a naphthalene-1,8:4,5-bis(dicarboximide) radical anion ( NDI ─• ). The occurrence of this type of previously unreported hetero radical-pairing interactions is attributed to enhancement effect of Coulombic attraction between these two radicals bearing opposite charges. The Coulombic-enhanced hetero radical pairing interactions are employed to drive host–guest recognition, as well as the reversible switching of a bistable [2]rotaxane. Homo radical spin-pairing interactions between two identical aromatic radicals are common in supramolecular chemistry, but hetero interactions between two different aromatic radicals are seldom observed. Here, the authors find that hetero radical pairing between a radical cation and a radical anion, together with Coulombic attraction, can drive host-guest recognition, representing a new supramolecular recognition motif.