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

Abstract

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·8PF6 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(+•) results in strong association of the two cyclophanes driven by two radical-pairing interactions. In these respects, BBR·8PF6 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·8PF6 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(+•) states, and (2) the formally tetraradical form of the rotaxane, BBR4(+•), exhibits a diamagnetic ground state, which, as a result of the slow shuttling motions within BBR4(+•), has a long enough lifetime to be characterized by 1H NMR spectroscopy.

Molecular Russian dolls

“…This observation is in line with the solid-state superstructure (Fig. 6 , see below) of the complex [ CBPQT ⊂ 1 ] 4(+•) , which shows that the BIPY +• units in the radical dimers (BIPY +• ) 2 are nearly perpendicular to each other, torsion angle 84°, such that the meta CHs of the BIPY +• units in 1 2(+•) and CBPQT 2(+•) are strongly shielded 45 , while the ortho CHs are not shielded.…”

“…At room temperature, however, only a single broad peak near 7.7 ppm was observed in the spectrum (Fig. 4 ) of a 1:1 mixture of 1 2(+•) and CBPQT 2(+•) in CD 3 CN, presumably because of a small thermal population of a paramagnetic state at room temperature 45 , 49 . Upon cooling to –20 °C, the signals of the complex are individually resolved, yet remain broad until further cooling to –40 °C, at which point all of the resonances of the host–guest complex are displayed (Fig.…”

“…Since the viologen diradical dicationic dimer (BIPY +• ) 2 and the tetraradical tetracationic complex [ m -CBPQT ⊂ SqBox] 4(+•) are reported 44 , 49 to be diamagnetic, based on EPR measurements, the tetraradical tetracationic host–guest complex [ CBPQT ⊂ 1 ] 4(+•) was also expected to be diamagnetic. Thus, we sought to characterize the host–guest complex by 1 H NMR spectroscopy, as we have previously succeeded 45 in doing for the tetraradical tetracationic state of a rotaxane, based on the [ m -CBPQT ⊂ SqBox] 4(+•) recognition motif. At room temperature, however, only a single broad peak near 7.7 ppm was observed in the spectrum (Fig.…”

“…Recently, we have expanded 44 the radical recognition motif to a tetraradical tetracationic host–guest complex in which a square-shaped diradical cyclophane [cyclobis(paraquat-4,4′-biphenylene)] 2(+•) ( SqBox 2(+•) ) encapsulates a cyclobis(paraquat- m -phenylene) ( m -CBPQT 2(+•) ) guest. Recognition between these two diradical dicationic cyclophanes is highly selective, while CBPQT 2(+•) and SqBox 2(+•) exhibit 44 , 45 no binding affinity as a consequence of their mismatched sizes. The m -CBPQT 2(+•) cyclophane is, however, unable to encapsulate any guests on account of the relatively small spacing distance between two BIPY +• units, thus precluding its use in the formation of more sophisticated hierarchical superstructures.…”

Electrochemically switchable rotaxanes: recent strides in new directions

“…The Stoddart group has recently investigated shuttling rates in a redox-switchable ring-in-ring rotaxane, where the axle itself incorporates a redox-active macrocycle. 61 Electrochemically induced shuttling was found to be signicantly slower compared to their previous systems. 62 The slow motion results in an electrochemical hysteresis for switching as shown by CV and digital simulations.…”