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.

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Introduction

Mechanically interlocked molecules¹ (MIMs), e.g., bistable [2]catenanes and [2]rotaxanes,² have attracted a copious amount of interest in the area of molecular nanotechnology on account of their ability to serve as active components in molecular electronic devices (MEDs)³ and nanoelectromechanical systems (NEMs),⁴ in addition to mechanized nanoparticles (MNPs)⁵ for drug delivery. A bistable [2]rotaxane consists of two components—namely, a dumbbell containing two recognition sites and a macrocycle which binds preferentially to one of the recognition units in its ground state and then can be switched reversibly between the recognition sites by the application of external stimuli of a chemical,⁶ electrochemical⁷ or photochemical⁸ nature. In a bistable [2]rotaxane or catenane, consisting of 1,5-dioxynaphthalene (DNP) and tetrathiafulvalene (TTF) as electron-rich recognition units and cyclobis(paraquat-p-phenylene) (CBPQT⁴⁺) as the electron-deficient cyclophane, the CBPQT⁴⁺ ring prefers to reside on the TTF unit rather than on the DNP unit in its ground state co-conformation (GSCC) on account of a 1.4 kcal mol⁻¹ difference in their binding affinities.⁹ Once the TTF unit is oxidized—chemically or electrochemically—to either its radical cation or to its TTF²⁺ dication, the CBPQT⁴⁺ ring moves on to the DNP unit as a consequence of Coulombic repulsion between the TTF²⁺ dication and the CBPQT⁴⁺ ring. Upon reduction of the TTF²⁺ dication, neutral TTF is regenerated, while the CBPQT⁴⁺ ring remains encircled around the DNP unit. This state, which slowly relaxes back to the GSCC, is defined as the metastable state co-conformation (MSCC). Increasing the life-time of the MSCC is crucial for the development of non-volatile flash memory devices. Inserting steric and/or electrostatic barriers¹⁰ between the two recognition sites has been shown^11–13 to increase the life-time of the MSCC. Recently, it has been demonstrated that the rate of shuttling of the rings between two identical stations in a degenerate [2]rotaxane can be decreased dramatically by inserting functionalized azobenzenes,¹¹ tosylimines¹² or bulky alkyl groups.¹³

A considerable amount of research effort has been expended with the focus of developing molecular switches based on the donor–acceptor [2]rotaxane system in which the electron-rich recognition motif serves as the dumbbell for the electron-deficient ring component. There are, however, few examples¹⁴ of switchable “reverse” donor–acceptor [2]rotaxanes in which electron-deficient recognition motifs serve as the dumbbell component for an electron-rich ring. The Sanders group^6a,15 has recently demonstrated a template-directed synthesis¹⁶ of a neutral [2]catenane and a bistable [2]rotaxane, wherein pyromellitic diimide (PmI) and naphthalene diimide (NpI) serve as the two electron-deficient recognition motifs and BDNP38C10 serves as an electron-rich ring. The switching was stimulated both chemically, using Li⁺ ions, and electrochemically. Examples of electrochemical switching of reverse donor–acceptor [2]rotaxanes have been reported somewhat infrequently in the literature.¹⁷ We have recently demonstrated¹⁸ the synthesis of a reverse donor–acceptor bistable [2]catenane (Fig. 1a) containing NpI and 4,4′-bipyridinium (BIPY²⁺) units as two electron-deficient stations, BDNP38C10 as the ring component and di-tertbutyl functionalized tetraarylmethane, incorporated between the two stations, as a “speed bump” for circumrotation of the ring. Although this system was not electrochemically switchable on account of the size of the speed bump, it encouraged us to design and synthesize a bistable [2]rotaxane incorporating a speed bump somewhat smaller in size. Herein, we report the synthesis and full characterization of the reverse donor–acceptor bistable [2]rotaxaneR·2PF₆ (Fig. 1b)—containing NpI and BIPY²⁺ as the two electron-deficient stations, BDNP38C10 as the ring component, and the centrally located diethyl-fuctionalized tetraarylmethane unit as a speed bump—which can be switched electrochemically. Reduction of the NpI unit to its NpI²⁻ dianion resulted in the passage of the BDNP38C10 ring over the speed bump onto the neutral BIPY⁰ station as a consequence of the repulsive interaction between the dianion and the electron-rich ring.

Fig. 1 Structural formulas of (a) a [2]catenane, (b) the [2]rotaxaneR·2PF₆ and (c) the dumbbell 12·2PF₆ and their graphical representations.

Experimental section

General methods and materials

Starting materials and reagents were purchased from Aldrich or Fisher and used as received. Compound 6,⁶9·2PF₆¹⁸ and NpI were prepared following procedures reported in the literature. All reactions were performed under an atmosphere of nitrogen and in dry solvents, unless otherwise noted. Analytical thin-layer chromatography (TLC) was performed on aluminum sheets, precoated with silica gel 60-F254 (Merck 5554). Flash chromatography was carried out using silica gel 60 (Silicycle) as the stationary phase. HPLC purification was performed on a preparative RP-HPLC instrument, using a C18 column. ¹H and ¹³C NMR spectra were recorded on either a Bruker Avance 500 MHz, or a Bruker Avance 600 MHz spectrometer. Chemical shifts are reported in ppm relative to the signals corresponding to the residual non-deuterated solvents (CDCl₃: δ 7.26 ppm, CD₃CN: δ 1.94 ppm). High resolution electrospray ionization (HR ESI) mass spectra were measured on Agilent 6210 LC-TOF with Agilent 1200 HPLC introduction. High-resolution matrix-assisted laser desorption/ionization (MALDI) mass spectra were measured on a Bruker Autoflex III mass spectrometer. Electrochemical experiments were carried out at room temperature in argon-purged DMF solutions, with a Gamry Reference 600 potentiostat interfaced to a PC. Cyclic voltammetry experiments were performed using a glassy carbon working electrode (0.071 cm², Cypress Systems). Its surface was polished routinely with 0.05 μm alumina-water slurry on a felt surface immediately before use. The counter electrode was a Pt coil and the reference electrode was a standard Ag/AgCl electrode. The concentration of the sample and supporting electrolyte (TBAPF₆) were 1 × 10⁻³ mol L⁻¹ and 0.1 mol L⁻¹, respectively.

Compound R·2PF₆

Compound 11 (97 mg, 0.068 mmol) and BDNP38C10 (48 mg, 0.074 mmol) were dissolved in CHCl₃ (3 mL) in a 10 mL flask and stirred at 25 °C for 24 h. The solvent was removed under vacuum and the crude product was purified by column chromatography [SiO₂:CHCl₃] to obtain pseudorotaxaneR (69 mg). TBTA (18 mg) and Cu(MeCN)₄PF₆ (12 mg) were added to the solution of R in CHCl₃ (3 mL), 10·2PF₆ (80 mg, 0.034 mmol) dissolved in DMF (1 mL). The solution was stirred at 25 °C for 24 h. The solvent was evaporated off under vacuum and the solid product was purified by RP-HPLC (H₂O–MeCN–0.1% TFA/80–100% in 40 min λ = 254 nm). The purple fraction was collected and concentrated. The resulting solid was dissolved in a minimal amount of MeCN and added to a saturated solution of NH₄PF₆ in H₂O. The precipitate was collected and washed with H₂O several times and dried under vacuum to afford the rotaxaneR·2PF₆ as a reddish-white solid (27 mg, 23%). ¹H NMR (CD₃CN, 600 MHz, 298 K): δ = 8.75 (d, J = 7.2 Hz, 4H), 8.54 (s, 4H), 8.54 (s, 4H), 8.18 (s, J = 7.2 Hz, 4H), 8.12 (s, 4H), 7.78 (s, 1H), 7.77 (s, 1H), 7.53 (s, 1H), 7.52 (s, 1H), 7.29–6.58 (m, 116), 6.37 (d, J = 7.5 Hz, 2H), 6.07 (d, J = 7.5 Hz, 2H), 5.03 (t, J = 7.5 Hz, 2H), 4.91 (q, J = 7.5 Hz, 2H), 4.59–3.68 (m, 40H), 3.48 (t, J = 7.5 Hz, 2H), 3.37 (t, J = 7.5 Hz, 2H), 2.5 (q, J = 7.2 Hz, 4H), 2.40–1.25 (m, 48H), 1.1 (t, J = 7.5 Hz, 6H) ppm. ¹³C NMR (CD₃CN, 125 MHz, 298 K): δ = 163.9, 163.8, 157.9, 157.7, 154.1, 153.9, 149.4, 149.3, 146.8, 145.6, 145.5, 145.5, 142.8, 142.6, 142.2, 140.6, 140.5, 140.4, 140.3, 132.6, 132.5, 131.5, 131.1, 127.8, 127.7, 126.6, 126.2, 126.1, 125.4, 125.3, 124.9, 124.7, 123.9, 114.6, 114.3, 114.2, 114.1, 114.0, 106.4, 104.5, 72.0, 71.9, 71.8, 70.7, 70.4, 69.0, 68.6, 68.3, 68.3, 63.9, 63.8, 63.7, 62.8, 62.0, 50.7, 50.6, 41.1, 41.0, 37.6, 36.0, 34.9, 32.6, 31.5, 30.5, 30.3, 30.1, 30.0, 29.9, 29.8, 29.5, 29.2, 28.7, 28.6, 28.4, 27.7, 27.6, 26.3, 25.6, 24.8, 23.7, 23.6, 23.3, 15.9, 15.8, 14.3. ESI-HRMS calcd for m/z = 1470.8275 [M − 2PF₆]²⁺, found m/z = 1470.8265.

Results and discussion

Synthesis

The synthesis of R·2PF₆ is outlined in Scheme 1. The reaction of commercially available methyl 4-methoxybenzoate with 4-ethylbromobenzene in the presence of magnesium turnings in dry THF afforded (80%) compound 1, which was reacted subsequently with phenol in presence of conc. HCl to yield compound 2 in 65% yield. De-O-methylation was achieved using BBr₃ in CH₂Cl₂ at RT, and subsequent alkylation employing 5-bromopentanol was carried out in dry MeCN in the presence of K₂CO₃ together with a catalytic amount of 18-crown-6 to yield compound 4, which was then reacted with tosyl chloride and Et₃N in the presence of a catalytic amount of DMAP to afford the monotosylate derivative 5. Reaction of compound 5 and 6⁶ with commercially available 1,4,5,8-naphthalenetetracarboxydiimide under Mitsunobu reaction conditions yielded the monotosylate derivative, which was subsequently converted into compound 11 by reacting the intermediate formed in the reaction in situ with NaN₃ in dry DMF at 80 °C.

Scheme 1 Synthesis of [2]rotaxaneR·2PF₆ and its dumbbell component 12·2PF₆.

Similarly, the tosylation of compound 6, using tosyl chloride and Et₃N in the presence of a catalytic amount of DMAP, afforded the tosyl derivative, which was converted into compound 8 by reacting it with NaN₃ in dry DMF at 80 °C. Subsequently, compound 8 was reacted with the dialkyne-functionalized viologen 9·2PF₆¹⁸ under Cu(I)-catalyzed azide–alkynecycloaddition (CuAAc) reaction conditions¹⁹ to afford 10·2PF₆. Reaction of 10·2PF₆ with compound 11 in the presence of BDNP38C10²⁰ using the so-called “threading-followed-by-stoppering” approach^19c,21 yielded an inseparable mixture of both the [2]- and [3]rotaxanes, and so we decided to form the pseudorotaxane R prior to making the [2]rotaxaneR·2PF₆. The pseudorotaxane R was obtained by mixing compound 11 with BDNP38C10 in CHCl₃ at room temperature in 70% yield. Finally, the reaction of R with 10·2PF₆ using CuAAc conditions afforded the [2]rotaxaneR·2PF₆ in 23% yield. The dumbbell 12·2PF₆ was obtained simply by reacting 11 with 10·2PF₆ under the same CuAAc conditions.

Characterization by ¹H NMR spectroscopy

All compounds have been fully characterized by ¹H and ¹³C NMR spectroscopies and by mass spectrometry. The ¹H NMR spectrum of the [2]rotaxaneR·2PF₆ is recorded (Fig. 2) in CD₃CN at 298 K.²² There are two sets of BDNP38C10 protons corresponding to the two translational isomers of R·2PF₆—namely, NpI ⊂ BDNP38C10 and BIPY²⁺ ⊂ BDNP38C10. The set of signals observed at higher frequencies, that is, δ = 6.40 (d), 6.85 (t), 7.12 (d) ppm (Fig. 2) were assigned, in turn, to the H_2/6, H_3/7, H_4/8protons of BDNP38C10 encircling the BIPY²⁺ station on account of the lower shielding effect between these two units. In contrast, the set of signals observed at lower frequencies, that is, δ = 6.10 (d), 6.60 (t) and 7.05 (d) ppm were assigned to the aromatic protons of BDNP38C10 encircling the NpI unit, i.e., NpI ⊂ BDNP38C10. Similarly, two sets of H_α and H_βprotons, corresponding to the encircled BIPY²⁺ and free BIPY²⁺ units, as well as another set of protons corresponding to the encircled and free form of the NpI station were observed. The relatively shielded signals at ∼ 8.50 (d) and 6.95 (d) ppm were assigned, respectively, to the H_α and H_βprotons of the encircled BIPY²⁺ and signals at δ = ∼ 8.78 (d) and 8.20 (d) ppm were assigned, respectively, to the H_α and H_βprotons of the free BIPY²⁺ unit. Likewise, the signals at δ = ∼ 8.10 (s) and 8.50 (s) ppm correspond to the encircled and free NpI stations, respectively. The assignment of these resonances was assisted by ¹H–¹H correlation spectroscopy (see ESI†).

Fig. 2 Partial ¹H NMR of R·2PF₆ and assignment of resonances recorded in CD₃CN at 298 K. The subscripts “E” and “F” correspond to encircled and free, respectively.

The synthetic strategy we employed allowed us to obtain the free energy of activation for the shuttling of the BDNP38C10 ring over the speed bump. Since the higher percentage of the BDNP38C10 ring encircling the NpI station decreases gradually over time, this equilibration process was followed (integration) by ¹H NMR spectroscopy (Fig. 3) at 298 K in CD₃CN. At the beginning, ∼85% of the BDNP38C10 ring encircles the NpI station based on the relative integrations of the peaks observed at δ = 6.40 and 6.10 ppm, resonances which correspond to the H_2/6protons of the BDNP38C10 ring encircling BIPY²⁺ and NpI, respectively. The rate of equilibration between the two translational isomers—namely, NpI ⊂ BDNP38C10 and BIPY²⁺ ⊂ BDNP38C10—occurs in a relatively rapid manner at room temperature and then slows down with time in a manner consistent with a first-order decay process (see ESI†). The bistable [2]rotaxane was observed to have reached equilibrium after 72 h with a 3 : 2 ratio of the two translational isomers, where NpI ⊂ BDNP38C10 was found to be the major co-conformation. The slightly higher population of the NpI ⊂ BDNP38C10 co-conformation can be rationalized by a “side on” interaction¹⁸ between the BIPY²⁺ with the BDNP38C10 ring, while encircling the NpI unit as a consequence of the conformational flexibility inherent in the structure. The rate of relaxation of the BDNP38C10 ring from NpI ⊂ BDNP38C10 to BIPY²⁺ ⊂ BDNP38C10 is associated with a rate constant of 2.2 ± 0.3 × 10⁻⁵s⁻¹, giving rise to a ΔG^‡ value of 23.8 ± 0.1 kcal mol⁻¹ (see ESI†). It is important to note that replacing the tert-butyl groups¹⁸ with ethyl groups on the central tetraarylmethane speed bump core decreases the activation barrier from 28.5 (measured at 343 K) to 23.8 kcal mol⁻¹.

Fig. 3 Partial ¹H NMR of R·2PF₆ in CD₃CN at 298 K recorded at time (a) 0 h, (b) 6 h, (c) 11 h, and (d) 72 h. Annotations are shown in color. Blue line indicates the co-conformation in which the BIPY²⁺ is encircled by BDNP38C10. Purple line indicates the co-conformation in which the NpI is encircled by BDNP38C10.

Switching behavior of the bistable [2]rotaxane

Electrochemical experiments—namely, cyclic voltammetry (CV), differential pulse voltammetry (DPV) and spectroelectrochemistry (SEC)—were carried out in argon-purged DMF solutions (0.1 M TBAPF₆ as a supporting electrolyte) at 298 K. As a consequence of the fact that the barrier to shuttling of the BDNP38C10 is relatively large in MeCN as evidenced by ¹H NMR spectroscopy, we chose to investigate the electrochemical switching behaviour of R·2PF₆ in DMF,²² in which the ¹H NMR spectrum reveals the shuttling behavior is much faster (see ESI†). All the potential values were referenced against the Ag/AgCl electrode (SSCE). Our focus in these experiments was centered on (i) corroborating the results from ¹H NMR spectroscopy (in CD₃CN) which shows that the BDNP38C10 ring exists approximately in a 1 : 1 ratio distributed between the two co-conformational isomers encircling either the NpI or the BIPY²⁺ station in DMF, a situation which corresponds to the ground state of the [2]rotaxaneR·2PF₆ and (ii) whether it is possible to induce switching of the BDNP38C10 ring electrochemically.

Table 1 summarizes the results obtained from the CV experiments. Both the model compounds, NpI²³ and 9·2PF₆¹⁸ exhibit (Fig. 4) two monoelectronic and reversible reduction processes. The dumbbell 12·2PF₆, which incorporates both BIPY²⁺ and NpI redox-active recognition units, shows four reversible reduction processes at the same potentials as the model compounds, indicating that the redox behavior of each of the recognition units within the dumbbell component are independent of each other.

Table 1 Reduction processes^a [reduction peak potential (redox potential)] of the BIPY²⁺ and NpI recognition sites within the dumbbell 12·2PF₆, and the [2]rotaxaneR·2PF₆, as well as in the model compounds NpI and 9·2PF₆

	E(I)BIPY	E(I)NpI	E(II)BIPY	E(II)NpI
a Determined by cyclic voltammetry (argon-purged DMF, 298 K, referenced to SSCE). All values are in units of voltage. b Potential of the “free” recognition site. c Potential of the site after encirclement by the BDNP38C10 ring.
9·2PF6	−0.46 (−0.42)	—	−0.86 (−0.83)	—
NpI	—	−0.58 (−0.54)	—	−1.07 (−1.04)
12·2PF6	−0.46 (−0.42)	−0.58 (−0.54)	−0.86 (−0.83)	−1.07(−1.04)
R·2PF6	−0.46 (−0.42)^b	−0.60 (−0.57)^b	−0.86 (−0.83)^b	−1.10 (−1.07)^b
	−0.61 (−0.58)^c	−0.86 (−0.83)^c	−0.89 (−0.86)^c	−1.11 (−1.08)^c

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Fig. 4 Cyclic voltammograms (argon-purged DMF; scan rate = 200 mV s⁻¹) of the dumbbell 12·2PF₆, as well as 9·2PF₆ and NpI, which serve as model compounds for the recognition sites. Both the first and second scans for each compound are shown.

The first scan of the CV of the rotaxaneR·2PF₆ showed (Fig. 5) four reversible peaks in the reductive region. On the basis of assignments made with respect to the dumbbell 12·2PF₆, the four reduction peaks have been partially assigned to the first and second monoelectronic reduction processes of the free BIPY²⁺ (−0.46 V and −0.86 V) and the free NpI (−0.60 V and −1.10 V) stations, respectively. As a consequence of the noncovalent bonding interactions of the BIPY²⁺ and NpI stations with the BDNP38C10 ring, the first reduction potentials of these units are known to become shifted and, as a consequence, overlap significantly with the first reduction potential of the free NpI (−0.60 V) and the second reduction potential of the BIPY²⁺ (−0.86 V) stations, respectively. The overlapping of these reduction processes is supported (see ESI†) by differential pulse voltammetry (DPV). The second reduction potentials of both the encircled and the free BIPY²⁺/NpI stations, respectively, were observed to be at approximately the same voltages, with only minor shifts towards more negative potentials. These minor shifts of potentials can be explained as a result of the loss of recognition of the BDNP38C10 ring after the first reduction processes relating to both the BIPY²⁺ and NpI stations. Furthermore, the return anodic scan displays four re-oxidation peaks, indicating that all these processes are completely reversible.

Fig. 5 Cyclic voltammograms (argon-purged DMF; scan rate = 200 mV s⁻¹) of the dumbbell 12·2PF₆ and the [2]rotaxaneR·2PF₆. The first and second scans for both compounds are shown. Changes in the relative intensities between the first (red) and second (black) of the first two reduction processes for the rotaxaneR·2PF₆ are observed, while no such changes are observed in the dumbbell component.

Taking into consideration the overlapping potentials for the reduction of free NpI and encircled BIPY²⁺ stations, we estimate the relative amount of the translational isomers—namely, NpI ⊂ BDNP38C10 and BIPY²⁺ ⊂ BDNP38C10—based on the relative amounts of the free BIPY²⁺ to the encircled BIPY²⁺, to be approximately in a 1 : 1 ratio. Therefore, prior to the application of an electrochemical stimulus, the [2]rotaxaneR·2PF₆ exists at equilibrium as an equimolar ratio of the two co-conformational isomers in DMF at 298 K. This equimolar ratio is consistent with binding constants obtained from host–guest titration studies of the BDNP38C10 ring with model BIPY²⁺ and NpI compounds (see ESI†).

The second scan of the rotaxaneR·2PF₆ shows that the relative intensity of the peak corresponding to the reduction of free BIPY²⁺ (Fig. 5) decreased by a small amount relative to the intensity of the peak assigned to the simultaneous first reductions of the encircled BIPY²⁺ and the free NpI stations, in comparison to the first scan. This change in intensity between the first and second scans was not observed in the case of either the dumbbell or the model compounds. These observations suggest that the generation of the NpI²⁻ dianion induces switching of the BDNP38C10 ring over the speed bump to encircle the neutral BIPY⁰ station on account of the repulsive interaction between the NpI²⁻ dianion and the electron-rich ring. The BDNP38C10 ring becomes trapped upon full re-oxidation of the NpI and BIPY²⁺ stations on account of the central speed bump unit which prevents the relaxation of the BDNP38C10 ring back to the NpI station on the timescale of the CV experiment. In order to verify this hypothesis, a solution of the rotaxaneR·2PF₆ was subjected to a potential of −1.5 V at the glassy carbon working electrode for ten seconds, before recording the CV scanning towards the limit of 0 V and returning back to −1.5 V. Indeed, on the return scan, the reduction peak (−0.46 V), corresponding to the reduction of free BIPY²⁺, was observed (Fig. 6) to decrease dramatically (almost undetectable) relative to the reduction peak (−0.60 V) corresponding to the encircled BIPY²⁺ and free NpI—a peak which was observed to increase. Moreover, consistent with the peak assignments, the reduction peak at −0.86 V, corresponding to the reduction of the encircled NpI, as well as the second reduction of both the free and encircled BIPY²⁺ stations, was observed to decrease. Furthermore, the peak assigned to the second reduction of the free and encircled BIPY²⁺ stations was observed to shift to a more negative potential by 30 mV, while the peak assigned to the reduction of the free and encircled NpI stations was observed to shift to a more positive potential by 30 mV, an observation which is consistent with a larger amount of the BDNP38C10 encircling the BIPY²⁺ prior to the second reductions of these stations. Spectroelectrochemistry was also performed on the rotaxaneR·2PF₆. (For details see SI†).

Fig. 6 (a) CV of the [2]rotaxaneR·2PF₆ recorded after applying a potential of −1.5 V for a 10 s equilibration time before recording the scan to 0 V and back (red), and overlay of the CV of the first scan of the rotaxane recorded starting at 0 V, scanning to −1.5 V and back (dotted line). Relative increases and decreases in the reduction peak intensities between the two different scans are shown (scan rate = 200 mV s⁻¹). (b) Proposed switching mechanism of the [2]rotaxaneR·2PF₆ deduced from the CV data.

In order to further verify that the switching is a result of generation of the NpI²⁻ dianion and not the NpI˙⁻radical anion, a CV of the rotaxane was recorded (see ESI†), initially from 0 V to the scan limit of −0.95 V. We did not observe any significant change in the relative intensity of the peak corresponding to the reduction of the free BIPY²⁺ on the second scan of the CV cycle. This observation indicates that the NpI˙⁻radical anion does not readily induce the switching of the BDNP38C10 ring over the speed bump to the neutral BIPY⁰ on the time scale of the CV experiment. These results strengthen the conclusion that the BDNP38C10 ring can be readily switched over the central speed bump onto the neutral BIPY⁰ unit through the generation of the NpI²⁻ dianion, even although the neutral BIPY⁰ presumably has no affinity for the BDNP38C10 ring. Upon full re-oxidation of the [2]rotaxaneR·2PF₆, the translational isomer BIPY²⁺ ⊂ BDNP38C10 predominates, and probably represents a metastable intermediate state of R·2PF₆, since the ground state distribution corresponds to the 1 : 1 ratio of the two translational isomers.

Conclusions

In summary, a reverse donor–acceptor bistable [2]rotaxane, comprised of the electron-deficient recognition units NpI and BIPY²⁺, the electron-rich BDNP38C10 ring, and a speed bump between the two recognition units, has been designed and synthesized. The speed bump in this bistable [2]rotaxane is sterically of just the right size to allow the shuttling of BDNP38C10 between the two stations. Replacing the tert-butyl groups¹⁸ with ethyl groups on the central tetraarylmethane speed bump core decreases the free energy of activation for the shuttling quite dramatically from 28.5 to 23.8 kcal mol⁻¹ in MeCN. These free energies of activation correspond to half-life times towards equilibration of 21 h (at 343 K) and 3.4 h, respectively. The [2]rotaxane was shown to be readily switchable electrochemically upon generation of the NpI²⁻ dianion in DMF. The scope of electrochemical switching in this reverse donor–acceptor bistable [2]rotaxane validates its candidature for application in molecular (non-volatile) memory devices and provides valuable information for the design and synthesis of mechanically interlocked molecules based on these recognition motifs. Future work on these systems will reside in the development of principles that will make it possible to tune the distribution at equilibrium between the two different co-conformations, thus producing efficient molecular switches for memory applications.

Acknowledgements

The research described in this full paper was sponsored in part by the National Center for Nano Technology Research at the King Abdulaziz City for Science and Technology (KACST) in Saudi Arabia. The authors thank Dr Turki M. Al-Saud and Dr Soliman H. Alkhowaiter at KACST for their generous support of this program of research at Northwestern University. The results are also based upon research sponsored by the Air Force Office of Scientific Research (AFOSR) under agreement number FA9550-07-1-0534. We also thank the National Science Foundation (NSF) for the award of a Graduate Research Fellowship to A. C. F. We wish to acknowledge that G. B. was supported as part of the Non-Equilibrium Research Center (NERC), which is an Energy Frontier Research Center (EFRC) funded by the U. S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences under Award No. DE-SC0000989. He thanks the International Center for Diffraction Data for the award of a 2011 Ludo Frevel Crystallography Scholarship.

Notes and references

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22. Variable temperature ¹H NMR spectrsocopy on the rotaxaneR·2PF₆ in d₇-DMF was also performed in order to demonstrate that the rate of shuttling of the BDNP38C10 ring is much faster in this solvent compared to that in CD₃CN. This faster rate of shuttling can be explained by the decrease in affinity of the BDNP38C10 ring for both the BIPY²⁺ and NpI stations. UV/vis spectroscopic titration data of the BDNP38C10 ring with model compounds for BIPY²⁺ and NpI in DMF at 298 K reveal that the binding constants of these compounds with the ring are approximately 30 M⁻¹. For details see ESI†.
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Footnote

† Electronic supplementary information (ESI) available: Synthetic methods, 1D and 2D NMR spectra, cyclic voltammetry and the kinetic data based of ¹H NMR experiments. See DOI: 10.1039/c0sc00586j

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