Evidence for ring spinning in rotaxanes induced with an alternate electric field

An alternating electric field is used to address the structure of two hydrogen bond-assembled rotaxanes and their interlocked molecular fragments by recording the electro-optic Kerr effect. Both rotaxanes consist of two nitrogen hydrogen bond acceptors 1 and a fumaramide unit 2 in the thread. The macrocycle is common. The experimentally determined Kerr constant as a function of the frequency of the applied AC field shows a resonance at 50Hz for both rotaxanes. This resonance is absent for the corresponding uninterlocked components or the solvent itself. A second resonance, around 80Hz was found for the fumaramide rotaxane but not for the nitrone one. The resonances have been shown to be field strength and temperature dependent, shifting to lower values with increasing the applied voltage and/or temperature. Nuclear Magnetic Resonance (NMR) spectrometry shows that the only possible process is a 180° circumrotation of the macrocycle around the thread. The rate of the macrocycle circumrotation measured by NMR is in the regime of that seen for the Kerr effect resonance. Theoretical simulations of the submolecular motion show that in 1 the macrocycle ring rotates smoothly with little further perturbations. In comparison 2 presents a more complicated picture where several processes are coupled.