Control of spin-spin exchange interactions in polynitroxides through inclusion within γ-cyclodextrin

Supramolecular interactions of polynitroxides with γ-cyclodextrin (γ-CD) were used to control intramolecular electron spin-spin exchange couplings in these stable radicals. In the absence of γ-CD, the binitroxide T2 and trinitroxide T3 showed strong spin-spin coupling as evidenced by the high exchange coupling constant of J = 120 MHz for T2 in its EPR spectra. Host/guest complexes are formed upon addition of γ-CD to the polynitroxides where the nitroxide moiety interacts with the γ-CD cavity, which causes suppression of spin exchange. By variation of the host/guest ratio, the nature of a trinitroxide spin system can be tuned from a three-spin to a two-spin to a one-spin system. In addition to the host/guest ratio, the spin exchange coupling can also be tuned by temperature, which shifts the equilibrium between CD-encapsulated nitroxides and free nitroxides.