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 BBR^4(+•) 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⁺/BBR^4(+•) states, and (2) the formally tetraradical form of the rotaxane, BBR^4(+•), exhibits a diamagnetic ground state, which, as a result of the slow shuttling motions within BBR^4(+•), has a long enough lifetime to be characterized by ¹H NMR spectroscopy.

Short abstract

A rotaxane was designed using a redox-active ring-in-ring recognition motif to create a nanoswitch that exhibits hysteresis for switching between states with strikingly different electronic features.

Introduction

Mechanically interlocked molecules¹⁻¹⁷ (MIMs) are of interest on account of their unique chemical,¹⁸⁻²² physical,²³⁻²⁷ and electronic properties.²⁸⁻³⁶ Their distinct characteristics have been employed in a variety of advanced nanomaterials, including nanoswitches,^{28−32,36−42} molecular muscles,⁴³⁻⁴⁸ molecular motors,⁴⁹⁻⁵¹ and nanoscopic stabilized radicals.^33,34,52,53 The features of these nanomaterials are realized, at least in part, by the control over the individual component’s relative motion,⁶ or lack thereof, that is a consequence of the mechanical link. Thus, it is of interest to develop even more precise methods for controlling the movement and spatial arrangement of the mechanically interlocked components, while paradoxically, the practical demands of possible applications⁵⁴ require that this control be achieved using minimal synthetic effort.

The use⁵⁵⁻⁵⁷ of radical-pairing interactions⁵⁸⁻⁶² to template the formation of catenanes and rotaxanes has been a significant recent development⁵⁵⁻⁵⁷ in the field of functional MIMs, not only offering a facile means to their synthesis but also imbuing the products with distinct electronic properties^33,34 and stimulus responsive motion.^32,33,48 This research began with the discovery^63,64 that the dicationic diradical state of cyclobis(paraquat-p-phenylene), CBPQT^2(+•), and the monocationic monoradical state of methyl viologen, MV^+•, form a strongly associated tricationic trisradical complex [CBPQT⊂MV]^3(+•). Variations of this host–guest complex have been used (Scheme 1) to template⁵⁵⁻⁵⁷ the formation of a variety of MIMs, such as the [2]rotaxane⁵⁷CBPQT-RV-[2]R⁶⁺ and the homo[2]catenane³⁴bis-CBPQT-[2]C^(7+)(1•). The restricted motion of the interlocked cyclophanes in bis-CBPQT-[2]C^(7+)(1•) results in this MIM exhibiting an air-stable radical state,³⁴ while reduction and oxidation of CBPQT-RV-[2]R⁶⁺ toggle between strong attraction and repulsion of its viologen components.⁵⁷ These features are promising for applications in molecular electronics^32,36/muscles^47,48/machines,^65,66 but there are currently limitations. For example, bis-CBPQT-[2]C^(7+)(1•) exhibits only one translational state,³⁴ while CBPQT-RV-[2]R⁶⁺ exhibits³³ very rapid redox-stimulated shuttling behavior. As a consequence, both MIMs have limited potential for hysteretic^28,30,32 electrochemical behavior that is desired for electronic applications. Additionally, rotaxanes based on the trisradical complex [CBPQT⊂MV]^3(+•) are inherently paramagnetic in their reduced states, a property which prevents characterization of these states by the easily accessible and well-developed techniques of NMR spectroscopy.⁶⁷

Scheme 1. Structural Formulas and Space-Filling Representations of Examples of Radical Host–Guest Complexes and Their Mechanically Interlocked Derivatives.

The radical ring-in-ring complex⁶⁸[MS⊂m-CBPQT]^4(+•)—formed between a small dicationic diradical cyclophane⁵⁹m-CBPQT^2(+•) and a dicationic diradical molecular square⁶⁹MS^2(+•)—offers a possible solution to the shortcomings of the first generation of radical MIMs. Although [MS⊂m-CBPQT]^4(+•) resembles (Scheme 1) an expanded version of [CBPQT⊂MV]^3(+•), the former was found⁶⁸ to exhibit much slower rates of association/dissociation. This second generation complex is also comparable to bis-CBPQT-[2]C^(7+)(1•) insofar as both are composed of two bis-viologen cyclophanes, but unlike the catenane, MIMs based on [MS⊂m-CBPQT]^4(+•) have the potential to display bistability. Lastly, [MS⊂m-CBPQT]^4(+•) is a ground-state singlet, suggesting that NMR spectroscopy might be used to characterize MIMs designed around this complex. Herein, we describe a ring-in-ring rotaxane—a simple but as yet uncommon^16,70,71 type of MIM—which incorporates a derivative of m-CBPQT^2(+•) in its dumbbell component as a recognition unit for MS^2(+•).

Results and Discussion

Synthesis of a Ring-In-Ring Rotaxane BBR·8PF₆

In order to synthesize a rotaxane utilizing the host–guest complex [MS⊂m-CBPQT]^4(+•) as a template, it was, first of all, necessary to prepare a derivative of the m-CBPQT^2(+•) guest that would be amenable to further functionalization. Since copper-catalyzed alkyne–azide cycloaddition (CuAAC) has recently been shown⁵⁷ to be useful for synthesizing radically templated MIMs, a bis-azide substituted cyclophane m-N₃-CBPQT·4PF₆ was targeted.

The synthesis (Scheme 2) of m-N₃-CBPQT·4PF₆ was accomplished in four steps, starting from the known⁷² 3,5-bis(hydroxymethyl)aniline (1). This starting material can be obtained commercially, or by employing a published procedure⁷² involving the reduction of inexpensive dimethyl 5-aminoisophthalate. The intermediate compounds 2, 3, and 4, were obtained in good yields and, moreover, could be purified by crystallization or by washing with an appropriate solvent. The target cyclophane, m-N₃-CBPQT·4PF₆, was also obtained pure without the need for chromatography. A mixed Br^–/PF₆^– salt of the product precipitated from the reaction mixture in nearly pure form; additional crops were obtained by concentrating the supernatant. Pure m-N₃-CBPQT·4PF₆ was obtained in 41% yield after washing the precipitate sparingly with MeCN, followed by salt metathesis with aqueous NH₄PF₆. This relatively easy synthesis is notable since, at the outset, it seemed possible that incorporating a cyclophane component into the dumbbell of a rotaxane would introduce a considerable synthetic burden at an early stage.

Scheme 2. Synthesis of m-N₃-CBPQT·4PF₆.

Scheme 3 depicts the use of m-N₃-CBPQT·4PF₆ in the synthesis of the Box-in-Box-Rotaxane·8PF₆ (BBR·8PF₆), a name that recognizes the common use⁷³ of the term “box” to refer to rigid viologen-based cyclophanes. Copper powder was used as a reducing agent in order to generate the host–guest complex [MS⊂m-N₃-CBPQT]^4(+•) from a 1:1 ratio mixture of m-N₃-CBPQT·4PF₆ and MS·4PF₆ in MeCN. As reported previously in the synthesis⁵⁷ of CBPQT-RV-[2]R·6PF₆, this reduction generates (MeCN)₄Cu⁺ as a byproduct, which mediates the cycloadditions between [MS⊂m-N₃-CBPQT]^4(+•) and a bulky alkyne. The tetracationic tetraradical rotaxane BBR^4(+•), which is formed initially, is converted into its octacationic state, BBR⁸⁺, upon oxidation with air. Examination of the crude product by ¹H NMR spectroscopy indicated that 70–80% of m-N₃-CBPQT·4PF₆ was converted to the rotaxane, while the remainder was converted to the m-Box-Dumbbell⁴⁺ (m-BDB⁴⁺) component that was not encircled by the molecular square. The rotaxane was isolated in 54% yield after a series of solvent washes, salt metathesis steps, crystallization of the tetracationic tetraradical form, and reoxidation with air. The purification of BBR·8PF₆ is, thus, more complicated than that for m-N₃-CBPQT·4PF₆, but the individual steps are operationally simple and the need for chromatography is once again avoided.

Scheme 3. Synthesis of the Box-In-Box Rotaxane BBR·8PF₆.

Spectroscopic Characterization of BBR⁸⁺ and BBR^4(+•)

The mechanically interlocked nature of the rotaxane was confirmed by ESI-HRMS, DOSY NMR spectroscopy, and UV–vis–NIR dilution measurements of its reduced BBR^4(+•) state. See the synthetic details and Figures S3 and S10. The ¹H NMR spectrum (Figure 1) also reveals the mechanically interlocked nature of BBR⁸⁺. All the resonances of the dumbbell component of BBR⁸⁺ exhibit anisochronicity because the molecular square is confined, at least on the NMR exchange time scale, to one end of the dumbbell. The asymmetry is observed most clearly in (i) the −CH₂O– proton resonances and (ii) the ortho Ar–H signals of the phenolic group. These signals are shifted upfield considerably for the side of the dumbbell that is encircled by the molecular square, indicating that this electron deficient cyclophane resides over the electron rich ether unit.

Figure 1.

¹H NMR spectrum of a CD₃CN solution of BBR·8PF₆. The spectrum is abridged in the region of the residual CD₂HCN and HOD resonances, while all of the signals for the BBR⁸⁺ cation are displayed. Selected resonances are labeled in order to illustrate the coconstitutional asymmetry of the rotaxane. See Figure S1 for full ¹H NMR spectroscopic assignments.

The proton resonances of BBR⁸⁺ exhibit only slight broadening of ≤2 Hz at half-height in ¹H NMR spectra that were collected at 75 °C compared with those recorded at 25 °C in CD₃CN. See Figures S11 and S12. Significant broadening, let alone coalescence, was not observed at 75 °C, even for the pair of potentially exchangeable resonances that exhibited the smallest separation (ca. 6.5 Hz at 500 MHz) from each other. From this latter observation, it can be inferred⁷⁴ that the molecular square translates across the cyclophane portion of the dumbbell at a rate, k_ex, that is ≪29 s^–1 at 75 °C. Furthermore, it can be inferred that the degenerate coconformations of BBR⁸⁺ have lifetimes of >1 s at 25 °C. In contrast, the ¹H NMR spectrum of CBPQT-RV-R·6PF₆ in CD₃CN exhibits significant broadening/coalescence for several of the signals at 25 °C. Although the rates^33,57 of shuttling of rings along dumbbells in hexacationic rotaxanes of this type are slower in CD₃COCD₃ solutions, fast shuttling is observed^33,57 for these rotaxanes, even in this solvent, at elevated temperatures. The differences in the shuttling rates observed for hexacationic rotaxanes and BBR⁸⁺ can be attributed to the greater charge repulsion present in the latter rotaxane, as well as to the comparatively tight fit⁶⁸ between the cyclophane components of BBR⁸⁺.

The conversion of BBR⁸⁺ to BBR^4(+•) was probed (Figure 2a) by UV–vis–NIR spectroscopic monitoring of the sequential addition of 1–4 equiv of the 1 e^– reductant⁷⁵ Cp₂Co to a solution of BBR·8PF₄ in MeCN. The resulting spectra exhibit linear increases with each equivalent that is added, indicating that BBR⁸⁺ is converted portionwise directly to BBR^4(+•). Intermediate oxidation states must, therefore, not be thermodynamically accessible to any significant extent. The spectrum obtained following the addition of 4 equiv of Cp₂Co corresponds to full conversion of BBR⁸⁺ to BBR^4(+•), which exhibits an intense NIR band (λ_max = 950 nm) that is very similar to that¹⁶ (λ_max = 941 nm) observed for the host–guest complex [MS⊂m-CBPQT]^4(+•). This comparison indicates that this radical ring-in-ring complex provides an accurate coconformational model for the associated cyclophane components of BBR^4(+•).

Figure 2.

Titrations of solutions of BBR·8PF₆ with 1 equiv at a time of Cp₂Co. (a) UV–vis–NIR spectra of a 0.050 mM solution of BBR·8PF₆ in MeCN after the addition of 0–4 equiv of Cp₂Co. (b) Partial ¹H NMR spectra displaying the ^tBu resonances of a 1 mM solution of BBR·8PF₆ in CD₃CN, after the addition of 0–4 equiv of Cp₂Co.

¹H NMR spectroscopy was also employed to monitor the titration of BBR·8PF₆ with Cp₂Co in CD₃CN. The two ^tBu proton resonances of BBR⁸⁺ are observed (Figure 2b) to undergo stepwise decreases in intensities upon sequential addition of Cp₂Co, while the single ^tBu resonance of BBR^4(+•) grows in strength concomitantly. Other ¹H resonances of BBR^4(+•) are also observed (Figure 3a) in the spectrum after the addition of 4 equiv of the reductant. The signals for both BBR^4(+•) and BBR⁸⁺ are observed in the ¹H NMR spectra collected during intermediate stages of the titration. See Figures S24–S26. Selective broadening of some resonances of each oxidation state are, however, evident at the midpoint of the titration. Since this broadening does not affect all of the signals, it can be concluded that the two oxidation states do not rapidly undergo degenerate interconversion—a 4-electron based process—on the ¹H NMR time scale. Instead, there may be some degree of single-electron transfer to provide BBR^(7+)(1•) and BBR^(5+)(3•) transiently, a process which would affect primarily the NMR signals arising from protons that are on or near the cyclophane components.

Figure 3.

(a) ¹H NMR spectrum of a CD₃CN solution of BBR^4(+•) that was prepared in situ by the addition of 4 equiv of Cp₂Co to a solution of BBR·8PF₆. The spectrum is abridged in the region of the residual CD₂HCN and HOD signals, and the height of the ^tBu resonance is truncated. See Figure S4 for the full spectrum. (b) Partial ¹H NMR spectra, collected over a temperature range of −20 to −57 °C, of a solution of BBR^4(+•) in CD₃COCD₃ that was prepared in situ by stirring a solution of BBR·8PF₆ over Zn dust. Although the resonances arising from the BBR^4(+•) cation are best resolved in the spectrum collected at −57 °C, signal H is obscured by the H₂O/HOD signal at this temperature. In the interest of preserving space, the ^tBu resonance of BBR^4(+•) is not displayed. See Figure S14 for the full ¹H NMR spectrum recorded at −57 °C.

The ¹H NMR spectrum of BBR^4(+•) exhibits several well-resolved signals, including those for the four outward facing protons of the m-phenylene linker, which are located very close to the viologen radical cations. The ability to observe sharp signals from these protons can be attributed to the strong radical-pairing interactions between the two cyclophanes, a phenomenon which provides a diamagnetic ground state. In contrast, the closest observable protons in the ¹H NMR spectrum (Figure S6) of the free dicationic diradical dumbbell, m-DBD^2(+•), are the −CH₂O– methylene protons, a full seven bonds away from the m-phenylene positions that can be observed in the spectrum of BBR^4(+•). Other ¹H NMR resonances arising from the cyclophane components of BBR^4(+•) could not be observed individually in the spectra collected at 25 °C, but broad signals, such as that observed in the inset of Figure 3a, can be attributed to these groups. These broad signals are most well-resolved when 3–4 equiv of Cp₂Co has been added, or when Zn dust was used as the reducing agent, but disappear upon the addition of even a small excess (4.25 equiv) of Cp₂Co. See Figures S26 and S27. Further characterization of lower (<4+) oxidation states of this rotaxane is described below in the section “Lower Oxidation States of BBR^x(+•)”.

¹H NMR spectra of BBR^4(+•) were collected at reduced temperatures in CD₃COCD₃ in the hopes of resolving the broad resonances observed in the spectra collected at 25 °C. These resonances sharpened and resolved into individual signals in the spectra collected at temperatures between +25 and −57 °C. Resonances for all the possible protons of BBR^4(+•) can be observed (Figure 3b) clearly at −50 °C. At −57 °C, the proton resonances of the cyclophane units are even better resolved, although one of them becomes obscured by the H₂O/HOD residual signal. The resonances of BBR^4(+•) integrate appropriately in the spectrum collected at −57 °C, and J-coupling (J = 5.4 Hz) can even be discerned for the two viologen Ar–H doublets that do not overlap with other signals. See Figure S15 for a depiction of the integrated spectrum.

The four aromatic resonances of the viologen units can be assigned based on the shielding of the protons that is expected by considering their proximity to other aromatic units. The innermost viologen protons of the smaller cyclophane are, thus, assigned to the most upfield signal since these protons are surrounded on both sides by other viologen units. The 2-positions of the pyridinium groups, in contrast, give rise to ¹H NMR signals with normal pyridinum chemical shifts. These protons are not in the proximity of other viologen units because of the nearly perpendicular geometry that is enforced between these units on the smaller cyclophane compared with the larger one. The assumptions of this analysis were validated by ¹H NMR spectroscopic studies of the cyclophane⁵⁹o-CBPQT^2(+•), which also exhibits diamagnetic NMR behavior. See Figure S8. The o-phenylene linkers of this small cyclophane enforce an eclipsed, rather than perpendicular, alignment of the viologen units, such that the 2- and 3-positions on the pyridinum groups are shielded to a similar extent by the adjacent viologen unit. This geometric arrangement is reflected in the ¹H NMR chemical shifts of o-CBPQT^2(+•); the viologen units give rise to more tightly clustered ¹H NMR chemical shifts than those observed in the spectra of BBR^4(+•).

It is remarkable that well-resolved ¹H NMR spectra could be obtained for BBR^4(+•) and o-CBPQT^2(+•). The latter species was first reported⁵⁹ ca. 30 years ago, but it was not investigated by ¹H NMR spectroscopy even though EPR spectroscopy indicated that it is diamagnetic. To our knowledge,^67,76−80 well-resolved ¹H NMR spectra have never been observed for any other diamagnetic viologen radical dimers, which in most cases can be attributed to rapid equilibration between the dimers and a significant concentration of the paramagnetic monomers. It is conceivable that similar processes—equilibration between BBR^4(+•) and a metastable coconformation (MSCC), BBR^4(+•)_D, in which the two cyclophane components are dissociated from each other—might be responsible for the broadening of the cyclophane resonances in the ¹H NMR spectrum of BBR^4(+•) at 25 °C. This possibility can, however, be discounted based on variable scan-rate cyclic voltammetry described in the next section.

Another possible explanation for the selective broadening of the ¹H NMR signals of BBR^4(+•) is that rapid electron transfer between BBR^4(+•) and BBR⁸⁺ could transiently form BBR^(7+)(1•) and BBR^(5+)(3•), as is invoked (vide supra) to account for the broadening of signals of BBR⁸⁺ when a large concentration of BBR^4(+•) is present. This explanation is, however, inadequate when it comes to explaining the NMR spectroscopic data of BBR^4(+•) for two reasons: (1) Reduction of BBR⁸⁺ with an excess of Zn dust is expected to provide nearly complete and selective conversion to BBR^4(+•) (see Figure S35 for the relevant EPR spectrum), and (2) o-CBPQT^2(+•) consistently exhibits sharp ¹H NMR resonances at a much higher temperature (+25 °C) than does BBR^4(+•) (−57 °C). This latter observation is particularly significant because the rigid covalent linkers in o-CBPQT^2(+•) must greatly reduce the rearrangement energy needed for electron transfer between o-CBPQT^2(+•) and its higher oxidation states, such that the ¹H NMR spectrum of o-CBPQT^2(+•) should be more sensitive to electron transfer processes. Consistent with this assessment, ¹H NMR signals could not be observed for a 1:1 ratio mixture of o-CBPQT^2(+•) and o-CBPQT⁴⁺ in CD₃CN. See Figure S31.

The best explanation for the broadened ¹H NMR signals in the case of BBR^4(+•) is that there is a thermally accessible triplet electronic state. Previous DFT calculations⁶⁸ on [MS⊂m-CBPQT]^4(+•) revealed that its HOMO consists of a bonding combination of the viologen SOMO orbitals, while the LUMO corresponds to an antibonding combination. The lowest energy triplet state of BBR^4(+•) would involve the thermal excitation of an electron across this HOMO–LUMO gap, thus disrupting one of the radical-pairing interactions. The o-CBPQT^2(+•) cyclophane presumably has a larger HOMO–LUMO separation because the eclipsed arrangement of its viologen units provides greater orbital overlap.⁶² The relative HOMO–LUMO gaps for o-CBPQT^2(+•) and BBR^4(+•) were assessed by comparing their NIR absorption bands, which arise^60,61 from the photoinduced promotion of an electron from the HOMO to the LUMO level within radical-paired dimers. It should be noted that the energies of these photoexcitations do not provide a direct measure of the energies of thermally promoted singlet/triplet interconversion because the optical absorption is a singlet-to-singlet transition and is subject to the Franck–Condon principle. Since, however, these effects apply to both o-CBPQT^2(+•) and BBR^4(+•), it can be concluded that the HOMO–LUMO gap is larger in this smaller cyclophane (λ_max = 850 nm)⁵⁹ relative to the rotaxane (λ_max = 950 nm, Figure 2a), as expected based on the ¹H NMR spectra for each compound. Despite its influence on the ¹H NMR spectrum of BBR^4(+•), the triplet state of this rotaxane could not be observed by EPR spectroscopy (see Figure S35) even for samples that were warmed to 90 °C.

Electrochemical Characterization of BBR·8PF₆

Cyclic voltammetry was used to probe the interconversion between BBR⁸⁺, BBR^4(+•), and a neutral form of this rotaxane BBR⁰. These are the three oxidation states of the rotaxane that appear to be accessible based on the CV data presented in Figure 4a. The more negative BBR^4(+•)/BBR⁰ redox couple exhibits completely reversible behavior at scan rates between 50 and 500 mV/s, while, in contrast, the BBR⁸⁺/BBR^4(+•) redox couple displays a large peak separation (ΔE_p ≥ 130 mV) even at the lowest scan rates examined, i.e., 50 mV/s in Figure 4a and 10 mV/s in Figure S38. This large peak separation can be explained by a slow rate of dissociation for the two cyclophanes from the radical-paired state of BBR^4(+•), resulting in an overpotential for oxidation back to the BBR⁸⁺ state. Similar electrochemical behavior was observed⁶⁸ previously for the corresponding redox couple of a 1:1 molar ratio mixture of MS·4PF₆ and m-CBPQT·4PF₆, though, in contrast with BBR⁸⁺, reversible behavior was observed for the host–guest complex at sufficiently low scan rates (≤25 mV/s). Thus, the two cyclophane components of BBR^4(+•) come apart even more slowly than the two cyclophane components of [MS⊂m-CBPQT]^4(+•), a difference that might be attributed to steric factors associated with the dumbbell component of BBR^4(+•).

Figure 4.

Cyclic voltammograms (CVs) of a 0.125 mM solution of BBR·8PF₆ in MeCN containing a 0.1 M concentration of [Bu₄N][PF₆] electrolyte and ferrocene as an internal redox standard. In all the CVs, the current is normalized relative to the square root of the scan rate. (a) Scan rates of 0.05, 0.1, 0.2, and 0.5 V/s with a potential window that includes all accessible oxidation states of the rotaxane. (b) Scan rates of 0.5, 1, 2.5, 5.0, and 10.0 V/s over a potential window that includes only the BBR⁸⁺ and BBR^4(+•) oxidation states of the rotaxane. (c) Scan rate of 50.0 V/s over a potential window that includes only the BBR⁸⁺ and BBR^4(+•) oxidation states of the rotaxane. The capacitive current at 50.0 V/s was measured independently and subtracted from this CV.

The more positive redox couple of BBR⁸⁺ was examined in more detail to determine if the slow kinetics of association observed⁶⁸ previously between m-CBPQT^2(+•) and MS^2(+•) would be preserved within the context of the rotaxane. As the scan rate was increased beyond 500 mV/s, the CVs of BBR·8PF₆ revealed the appearance of a new reoxidation wave at potentials that were negative relative to those of the wave that is assigned to the radical-paired state of this rotaxane. See Figure 4b. The appearance of this wave can be attributed to the oxidation of a metastable coconformation (MSCC) BBR^4(+•)_D in which the two rings are dissociated from each other. Such a wave would appear at higher scan rates if, after reduction of BBR⁸⁺ to BBR^4(+•)_D, the two cyclophane components came together too slowly to form the associated BBR^4(+•)_A ground state coconformation (GSCC). The wave corresponding to the BBR^4(+•)_A state disappears almost completely at a scan rate of 50 V/s, such that the more negative BBR^4(+•)_D wave is part of a nearly reversible redox couple. The E_1/2 value (−0.70 V) is similar to those observed⁶⁸ for the individual MS⁴⁺/MS^2(+•) and m-CBPQT⁴⁺/m-CBPQT^2(+•) redox processes and is consistent with the assignment of the wave to a reversible BBR⁸⁺/BBR^4(+•)_D couple.

The electrochemical mechanism in Scheme 4 was used to simulate the CVs of the BBR⁸⁺/BBR^4(+•) redox couple. It is assumed that, like the individual cyclophanes, this rotaxane undergoes two-electron redox processes. Furthermore, the rate constant, k₁, for the conversion of BBR^(6+)(2•)_D to BBR^(6+)(2•)_A can be assumed to be lower than that (k₂) for the conversion of BBR^4(+•)_D to BBR^4(+•)_A simply because the latter association process occurs with less charge repulsion. This consideration implies that the pathway associated with k₂ will be the primary route that leads to the formation of the BBR^4(+•)_A state. From the CV data, it is also evident that k_–1 > k₂ because the metastable coconformation BBR^(6+)(2•)_A of the 6+ oxidation state does not have a long enough lifetime to permit observation of a BBR^(6+)(2•)_A/BBR⁸⁺_A oxidation wave. When these considerations were taken into account, CVs simulated with a rate constant of k₂ = 10–25 s^–1 provided good agreement⁸¹ with the experimental data relating to the appearance and disappearance of the BBR^4(+•)_D/BBR⁸⁺_D and BBR^4(+•)_A/BBR^(6+)(2•)_A oxidation waves. See Figure S41 for simulated CVs obtained with k₂ = 15 s^–1. The rate constant, k_–2, must be orders of magnitude smaller since the BBR^4(+•)_D/BBR^4(+•)_A equilibrium lies far toward the associated state.

Scheme 4. Proposed Mechanisms of BBR⁸⁺/BBR^4(+•) Interconversion.

CV studies demonstrate that electrochemical hysteresis is observed during the switching between BBR⁸⁺ and BBR^4(+•). Thus, this rotaxane preserves the comparatively slow kinetics of association/dissociation observed for the ring-in-ring complex⁶⁸ upon which it is based. In contrast, related hexacationic rotaxanes, e.g., CBPQT-RV-[2]R⁶⁺, exhibit³³ 2–3 reversible redox couples associated with the electrochemical switching between the dissociated 6+ state and the associated trisradical 3+ state, even though the corresponding host–guest complexes can exhibit^63,64 some degree of electrochemical hysteresis involving their association/dissociation at elevated scan rates (1–10 V/s). The slow rate of switching between BBR^4(+•)_D and BBR^4(+•)_A is particularly notable, given the distinct electronic nature, i.e., open- versus closed-shell electron configurations, of these two states of this rotaxane. Owing to their odd electron count, the trisradical rotaxanes cannot exhibit this same distinction between their associated and dissociated states since all coconformations of CBPQT-RV-[2]R^3(+•) are paramagnetic.

Lower Oxidation States of BBR^x(+•)

CV studies indicate that only three thermodynamically stable oxidation states—namely, BBR⁸⁺, BBR^4(+•), and BBR⁰—exist for this ring-in-ring rotaxane. These results are consistent with data from Cp₂Co titration experiments on the conversion of BBR⁸⁺ to BBR^4(+•). In contrast, similar studies on the conversion of BBR^4(+•) to BBR⁰ reveal that at least two intermediate oxidation states are accessible. This result was evident from changes in the UV–vis–NIR spectrum (Figure 5a) of a solution of BBR·8PF₆ in Me₂CO⁸² as 4–14 equiv of Cp₂Co were added. Nearly identical results are obtained (Figure S19) when employing MeCN as the solvent.

Figure 5.

(a) UV–vis–NIR spectra of a 0.33 mM (initial concentration) solution of BBR·8PF₆ in Me₂CO following the additions of 4, 5, 6, 7, 8, and 14 equiv of Cp₂Co. The inset depicts the difference between the spectra obtained after adding 8 and 14 equiv of Cp₂Co, with the latter spectrum scaled appropriately to account for different concentrations of the fully reduced viologen units in each sample. (b) Relative integrated EPR signal intensities of 0.25 mM solutions of BBR·8PF₆ in Me₂CO after the additions of 4, 5, 6, 7, 8, and 14 equiv of Cp₂Co. See Figure S34 for the full EPR spectra. (c) Comparison of the relative integrated EPR signal intensities (red circles) with the visible light absorption of these samples at 680 nm (squares) that was measured by UV–vis–NIR spectroscopy.

Upon the addition of 5 and 6 equiv of the reductant, the NIR absorption band of BBR^4(+•) decreases in step sizes of >30%, and this decrease is accompanied by a shift in wavelength from λ_max(4equiv) = 950 nm to λ_max(6equiv) = 926 nm. Both observations indicate that BBR⁰ is not the only product formed upon partial reduction of BBR^4(+•), particularly because BBR⁰ does not absorb in the NIR region. The blue shift of the NIR band implies that there is a significant quantity of an intermediate oxidation state of this rotaxane that also features radical-pairing interactions. This new radical-paired state is assigned (Scheme 5) to BBR^2(+•)_A, which features two of the viologen units in a paired radical state and two in their fully reduced state. Owing to the mixed valency of the viologen units in BBR^2(+•)_A, its electronic absorption spectrum is very similar to that of a 1:1 mixture of BBR^4(+•) and BBR⁰, such that the individual concentrations of these three states of this rotaxane could not be determined in the sample mixture.

Scheme 5. Electrochemically Hidden Oxidation States of BBR^x(+•).

In addition to changes in the NIR region, the addition of 6 equiv of Cp₂Co resulted in the observation of subtle features in the visible region of the spectrum that suggested that unpaired viologen radicals are present. These spectroscopic features became more evident after 7 and 8 equiv of Cp₂Co had been added, while the NIR band disappeared almost completely. Note that full reduction of BBR⁸⁺ to the neutral BBR⁰ state is not achieved upon the addition of 8 equiv of Cp₂Co because the Cp₂Co/Cp₂Co⁺ redox couple has a potential⁷⁵E_Cp2Co/Co+ = −1.3 V which is not sufficiently negative compared to that of the BBR^4(+•)/BBR⁰ couple (E_BBR4+/BBR0 = −1.24 V). This limitation was overcome by the addition of a large excess of Cp₂Co (14 equiv) to BBR⁸⁺, resulting in complete reduction to BBR⁰ as indicated by a UV–vis spectrum consistent with the presence of only fully reduced viologen cyclophanes.

The inset in Figure 5a displays a spectrum corresponding only to the radical viologen units of BBR^+•. This spectrum was obtained by subtraction of the BBR⁰ spectrum, with appropriate scaling, from that obtained after the addition of 8 equiv of Cp₂Co to the solution of BBR·PF₆. The correct scaling of the BBR⁰ spectrum was assessed using the assumption that, in the resulting viologen radical spectrum, the absorption maximum at 618 nm and minimum at 440 nm would have a similar ratio of intensities to those observed for other unpaired viologen radicals. From this scaling, it was determined that 85% of the viologen units were in the fully reduced state after the addition of 8 equiv of Cp₂Co, leaving 15% in the radical state. A slightly higher estimate (18%) of the radical concentration was determined by comparing the integrated EPR signal intensity arising from BBR^+• with that of a MV^+• standard. The spectrum of the BBR^+• viologen radical resembles that observed⁶⁸ previously for a 1:1 mixture of MS^2(+•) and m-CBPQT^2(+•) under conditions in which there is little association between the two cyclophanes. Thus, it can be concluded that two mixed-valence isomers of BBR^+• are present, which are distinguished by whether the unpaired electron resides on the smaller or larger cyclophane.

The Cp₂Co titration was also monitored by EPR spectroscopy in order to provide the data displayed in Figure 5b,c. The integrated EPR signal intensity increased in approximately equal steps after addition of the fifth and sixth equivalents of the reductant, suggesting that BBR^3(+•) is not a major component of the resulting mixture of BBR^x(+•) oxidation states. The addition of a seventh equivalent of Cp₂Co resulted in a slight increase in the EPR signal intensity, corresponding to the maximum value that was observed during these experiments. This result is consistent with the assignment of the EPR signal primarily to the BBR^+• state of this rotaxane, which is formed upon the addition of 7 electrons to the initial BBR⁸⁺ state. The change in the EPR signal intensity between adding the sixth and seventh equivalents was, however, relatively small. This observation indicates that there might be some contribution to the EPR signal from a dissociated BBR^2(+•)_D state, as illustrated in Scheme 5, although this possibility can be concluded with less certainty than the existence of the BBR^2(+•)_A and BBR^+• states of this rotaxane.

Further information was sought using ¹H NMR spectroscopy, but in contrast to the higher oxidation states, only a single set of resonances, corresponding to groups remote from the viologen cyclophanes, could be observed in each of the spectra obtained after adding >4 equiv of Cp₂Co to a solution of BBR⁸⁺. See Figures S28–S30. It appears that electrons exchange rapidly between the lower oxidation states of this rotaxane, resulting in the observation of ¹H NMR signals that are the average of multiple states, including some with paramagnetic character. Since some of these states involve association between the two rings, while others do not, it can be inferred that the rings associate and dissociate more rapidly in the lower BBR^x(+•) (x < 4) oxidation states of this rotaxane than in the higher ones.

Conclusions

A recently characterized⁶⁸ tetracationic tetraradical ring-in-ring complex has now been incorporated into the design of a new rotaxane, BBR⁸⁺, which features a cyclophane unit within its dumbbell component. This rotaxane is similar in its design to those of hexacationic rotaxanes based on the tricationic trisradical complex [CBPQT⊂MV]^3(+•). The properties of BBR⁸⁺ are, however, influenced to a great extent by the rigid preorganization of two redox-active viologen units within the cyclophane unit of its dumbbell. The tetracationic charge and physical size of this unit create a significant barrier to the movement of the larger, square-shaped cyclophane from one end of the rotaxane to the other in its BBR⁸⁺ oxidation state. Relatively slow kinetics of translational motion are also observed in the case of the reduced BBR^4(+•) state of the rotaxane: strong radical-pairing between the two cyclophanes causes slow dissociation of the rings from this favorable state, while charge repulsion and steric features create a significant barrier to the reverse process. Only in the lower oxidation states of the rotaxane, which feature reduced charge repulsion and weakened radical-pairing, does the larger cyclophane move rapidly along the dumbbell component.

The slow shuttling processes exhibited by BBR⁸⁺ and BBR^4(+•) result in easily observable hysteresis for switching between these states, whereas closely related tris-viologen rotaxanes exhibit reversible electrochemical behavior. Furthermore, the associated and dissociated states of BBR^4(+•) have strikingly different electronic properties. The transient BBR^4(+•)_D coconformation exhibits, for example, redox processes comparable to those of individual open-shell viologen cyclophanes, whereas radical-pairing interactions provide diamagnetic character to the BBR^4(+•)_A state while also stabilizing it against oxidation. The distinct electronic features of BBR^4(+•)_A are underscored by the ability to observe well-resolved ¹H NMR spectra for this form of the rotaxane.

The striking differences in the electronic properties of the associated and dissociated states of the ring-in-ring rotaxane—as well as marked hysteresis in switching between them—make this rotaxane design promising for the development of molecular electronic materials. More broadly, the ability to identify the reduced BBR^4(+•)_A state of this rotaxane by ¹H NMR spectroscopy represents a major step forward in the investigation of complex functional MIMs using radical-pairing interactions. This realization provides a looking glass for peering at the formally radical states of these MIMs, which, in turn, will enable much more detailed understanding of their complex structural features than is available using UV–vis–NIR and EPR spectroscopic methods.

Supporting Information Available

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscentsci.7b00535.

• Experimental details, including full synthetic procedures, routine characterization data, cyclic voltammograms, UV–vis–NIR spectra, EPR and ¹H NMR spectra, and details of data processing (PDF)

The authors declare no competing financial interest.