A redox-active reverse donor–acceptor bistable [2]rotaxane

Abstract

The synthesis and the dynamic behavior of a bistable [2]rotaxane, based on a reverse donor–acceptor motif containing naphthalene diimide (NpI) and 4,4′-bipyridinium (BIPY²⁺) as two electron-deficient stations and bis-1,5-dioxynaphthalene[38]crown-10 (BDNP38C10) as the electron-rich ring, is described. A functionalized tetraarylmethane moiety has been incorporated between the two stations in order to control the free energy barrier for the shuttling of the BDNP38C10 on the dumbbell component. The bistable [2]rotaxane was synthesized using the so-called “threading-followed-by-stoppering” approach and characterized by NMR spectroscopy and mass spectrometry. Initially, the BDNP38C10 ring resides on the NpI station on account of the synthetic approach employed in the synthesis of the bistable [2]rotaxane. ¹H NMR spectroscopy was used to follow the equilibration process between the two translational isomers of the bistable [2]rotaxane—namely, NpI ⊂ BDNP38C10 and BIPY²⁺ ⊂ BDNP38C10. After 72 h, equilibrium was reached with a 3 : 2 ratio of the two translational isomers in favor of the NpI ⊂ BDNP38C10 co-conformation in CD₃CN. The rate of relaxation of the crown ether from NpI ⊂ BDNP38C10 back to BIPY²⁺ ⊂ BDNP38C10 was associated with a rate constant of 2.2 ± 0.3 × 10⁻⁵ s⁻¹ (t_1/2 = 3.4 h), corresponding to a free energy of activation of 23.8 ± 0.1 kcal mol⁻¹. Cyclic voltammetry (CV) reveals that the BDNP38C10 ring can be enticed to pass over the speed bump onto the neutral BIPY⁰ unit upon the generation of the NpI²⁻ dianion, even although the neutral BIPY⁰ has presumably little or no affinity for the BDNP38C10 ring.