Shuttling Rates, Electronic States, and Hysteresis in a Ring-in-Ring Rotaxane

The trisradical recognition motif between a 4,4'-bipyridinium radical cation and a cyclo-bis-4,4'-bipyridinium diradical dication has been employed previously in rotaxanes to control their nanomechanical and electronic properties. Herein, we describe the synthesis and characterization of a redox-active ring-in-ring [2] rotaxane BBR center dot 8PF(6) that employs a tetraradical variant of this recognition motif. A square-shaped bis-4,4'-bipyridinium cyclophane is mechanically interlocked around the dumbbell component of this rotaxane, and the dumbbell itself incorporates a smaller bis-4,4'-bipyridinium cyclophane into its covalently bonded structure. This small cyclophane serves as a significant impediment to the shuttling of the larger ring across the dumbbell component of BBR8+, whereas reduction to the tetraradical tetracationic state BBR4(+center dot) results in strong association of the two cyclophanes driven by two radical-pairing interactions. In these respects, BBR center dot 8PF(6) exhibits qualitatively similar behavior to its predecessors that interconvert between hexacationic and trisradical tricationic states. The rigid preorganization of two bipyridinium groups within the dumbbell of BBR center dot 8PF(6) confers, however, two distinct properties upon this rotaxane: (1) the rate of shuttling is reduced significantly relative to those of its predecessors, resulting in marked electrochemical hysteresis observed by cyclic voltammetry for switching between the BBR8+/ BBR4(+center dot) states, and (2) the formally tetraradical form of the rotaxane, BBR4(+center dot), exhibits a diamagnetic ground state, which, as a result of the slow shuttling motions within BBR4(+center dot), has a long enough lifetime to be characterized by H-1 NMR spectroscopy.