Post-assembly processing of [2]rotaxanes

The concept of using [2]rotaxanes that carry one or more surrogate stoppers which can subsequently be converted chemically into other structural units, resulting in the formation of new interlocked molecular compounds, is introduced and exemplified. Starting from simple NH2(+)-centered/crown-ether-based [2]rotaxanes, containing either one or two benzylic triphenylphosphonium stoppers, the well-known Wittig reaction has been employed to make, 1) other [2]rotaxanes, 2) higher order rotaxanes, 3) branched rotaxanes, and 4) molecular shuttles--all isolated as pure compounds, following catalytic hydrogenations of their carbon-carbon double bonds, obtained when aromatic aldehydes react with the ylides produced when the benzylic triphenylphosphonium derivatives are treated with strong base. The two starting [2]rotaxanes were characterized fully in solution and also in the solid state by X-ray crystallography. The new interlocked molecular compounds that result from carrying out post-assembly Wittig reactions on two [2]rotaxanes were characterized by (dynamic) 1H NMR spectroscopy. In the case of a molecular shuttle in which the crown ether component is dibenzo[24]-crown-8 (DB24C8), shuttling is slow on the 1H NMR timescale, even at high temperatures. However, when DB24C8 is replaced by benzometaphenylene[25]-crown-8 as the ring component in the molecular shuttle, the frequency of the shuttling is observed to be around 100 Hz in [D4]methanol at 63 degrees C.     

Molecular brake systems controlled by light and heat

Machines at a molecular level are in perpetual Brownian motion even at an ambient temperature. One of the representative issues of researches on molecular machines is a development of technology, which can control Brownian motion. This review presents our efforts to achieve the first rationally designed molecular brake systems of threading/dethreading motions, a shuttling motion, and a rocking motion that work reversibly and quantitatively in response to external stimuli without producing any chemical wastes. These molecular brake systems were constructed from a dumbbell shaped secondary ammonium axle and a ring component having photo and thermally reactive moiety.     

An Interlocked Figure-of-Eight Molecular Shuttle

Here is reported the synthesis and characterization of an interlocked figure-of-eight rotaxane molecular shuttle from a dibenzo-24-crown-8 (DB24C8) derivative. This latter bears two molecular chains, whose extremities are able to react together by click chemistry. One of the two substituting chain holds an ammonium function aimed at driving the self-entanglement through the complexation of the DB24C8 moiety. In the targeted figure-of-eight rotaxane, shuttling of the DB24C8 along the threaded axle from the best ammonium station to the weaker binding site triazolium was performed through deprotonation or deprotonation-then-carbamoylation of the ammonium. This way, two discrete co-conformational states were obtained, in which the folding and size of the two loops could be changed.     

Shuttling dynamics in an acid-base-switchable [2]rotaxane.

Molecular shuttles are an intriguing class of rotaxanes which constitute prototypes of mechanical molecular machines and motors. By using stopped-flow spectroscopic techniques in acetonitrile solution, we investigated the kinetics of the shuttling process of a dibenzo[24]crown-8 ether (DB24C8) macrocycle between two recognition sites or "stations"--a secondary ammonium (-NH2+-)/amine (-NH-) center and a 4,4'-bipyridinium (bipy2+) unit--located on the dumbbell component in a [2]rotaxane. The affinity for DB24C8 decreases in the order -NH2+- > bipy2+ > -NH-. Hence, shuttling of the DB24C8 macrocycle can be obtained by deprotonation and reprotonation of the ammonium station, reactions which are easily accomplished by addition of base and acid to the solution. The rate constants were measured as a function of temperature in the range 277-303 K, and activation parameters for the shuttling motion in both directions were determined. The effect of different counterions on the shuttling rates was examined. The shuttling from the -NH2+- to the bipy2+ station, induced by the deprotonation of the ammonium site, is considerably slower than the shuttling in the reverse direction, which is, in turn, activated by reprotonation of the amine site. The results show that the dynamics of the shuttling processes are related to the change in the intercomponent interactions and structural features of the two mutually interlocked molecular components. Our observations also indicate that the counterions of the cationic rotaxane constitute an important contribution to the activation barrier for shuttling.     

Synthesis of lipophilic crown ethers with pendant carboxylic acid groups

Synthetic routes to thirteen highly lipophilic crown ether carboxylic acids are described. Seven contain 12–15-membered crown ether units with four ring oxygens and are designed for lithium ion complexation. Three others possess large ring 24-crown-8, 27-crown-9, and 30-crown-10 units. Six new hydroxymethyl crown ethers are prepared as synthetic intermediates.     

Controlled photophysical behaviors between dibenzo-24-crown-8 bearing terpyridine moiety and fullerene-containing ammonium salt

A novel [2]pseudorotaxane was successfully constructed by the complexation of dibenzo[24]-crown-8 (DB24C8) derivative bearing terpyridine moiety (1) with lanthanide ion (Tb(3+)) and fullerene-containing ammonium salt (2), exhibiting the controlled photophysical behaviors as a reversible luminescent lanthanide switch in the presence of K(+) or 18-crown-6 (18C6).     

Self-assembly of novel [3]- and [2]rotaxanes with two different ring components: donor-acceptor and hydrogen bonding interactions and molecular-shuttling behavior

Three of the first kind of hetero[3]rotaxanes, which comprise one linear component and one neutral and one tetracationic ring component, have been assembled by using the intermolecular hydrogen bonding and donor-acceptor interactions. Three neutral [2]rotaxanes and three tetracationic [2]rotaxanes have also been synthesized as intermediate products or for the sake of property comparison. The linear molecules are incorporated with two glycine subunits, for templating the formation of the neutral tetraamide cyclophane, and one or two hydroquinone subunits, for inducing the formation of the tetracationic cyclophane. Variable-temperature (1)H NMR investigation reveals that the shuttling behavior of the tetracationic ring component along the linear component is substantially influenced by the existence of the neutral ring component. The spatial repelling interaction of the neutral ring on the electron-deficient tetracationic ring simultaneously weakens the latter's "positioning" tendency at both electron-rich hydroquinone sites of the linear component. As a result, the activation energy associated with the shuttling process of the tetracationic ring between the two hydroquinone sites is remarkably reduced in comparison to that of the shuttling process of the corresponding neutral ring-free [2]rotaxanes. For the first time, the rotation of the dipyridinium subunit around the axis formed by the two methylene groups connecting them within the tetracationic cyclophane has been investigated by variable-temperature (1)H NMR spectroscopy and the associated kinetic data have also been successfully obtained. Furthermore, the UV-vis and fluorescent properties of the new [2]- and [3]rotaxanes have been studied. The results demonstrate that [3]rotaxanes with different ring components possess unique kinetic features that are not available in [3]rotaxanes with identical ring components.     

Synthesis of a bistable[3]rotaxane and its pH-controlled intramolecular charge-transfer behavior

A[3]rotaxane 1 involving two naphtho-21-crown-7(N21C7) rings and a dumbbell-shaped component 4 was synthesized.The dumbbell-shape molecule 4 contains one viologen nucleus,two secondary alky] ammonium sites and two phenyl stoppers.After threading the N21C7 ring with the thread-like ammonium guest 3,the copper(1)-catalyzed Huisgen alkyne-azide 1,3-dipolar cycloaddition(CuAAC "click" reaction),was performed to connect the pseudorotaxanes with viologen unit 2 and generate 1. Through treating the[3]rotaxane by the base and acid circularly,the two N21C7 rings can make shuttling motion along the axle.Meanwhile the distance between the electron-deficient viologen unit and the electron-rich naphthol group can be adjusted precisely along with a remarkable intramolecular charge-transfer (CT) behavior.     

A [3]rotaxane with three stable states that responds to multiple-inputs and displays dual fluorescence addresses

A [3]rotaxane molecular shuttle containing two alpha-cyclodextrin (alpha-CD) macrocycles, an azobenzene unit, a stilbene unit, and two different fluorescent naphthalimide units has been investigated. The azobenzene unit and the stilbene unit can be E/Z-photoisomerized separately by light excited at different wavelengths. Irradiation at 380 nm resulted in the photoisomerization of the azobenzene unit, leading to the formation of one stable state of the [3]rotaxane (Z1-NNAS-2CD); irradiation at 313 nm resulted in the photoisomerization of the stilbene unit, leading to the formation of another stable state of the [3]rotaxane (Z2-NNAS-2CD). The reversible conversion of the Z1 and Z2 isomers back to the E isomer by irradiation at 450 nm and 280 nm, respectively, is accompanied by recovery of the absorption and fluorescence spectra of the [3]rotaxane. The E isomer and the two Z isomers have been characterized by 1H NMR spectroscopy and by two-dimensional NMR spectroscopy. The light stimuli can induce shuttling motions of the two alpha-CD macrocycles on the molecular thread; concomitantly, the absorption and fluorescence spectra of the [3]rotaxane change in a regular way. When the alpha-CD macrocycle stays close to the fluorescent moiety, the fluorescence of the moiety become stronger due to the rigidity of the alpha-CD ring. As the photoisomerization processes are fully reversible, the photo-induced shuttling motions of the alpha-CD rings can be repeated, accompanied by dual reversible fluorescence signal outputs. The potential application of such light-induced mechanical motions at the molecular level could provide some insight into the workings of a molecular machine with entirely optical signals, and could provide a cheap, convenient interface for communication between micro- and macroworlds.     

Controlling the rate of shuttling motions in [2]rotaxanes by electrostatic interactions: a cation as solvent-tunable brake

A series of rotaxanes, with phenolic axle centerpieces and tetralactam macrocycles as the wheels, has been prepared in good yields. The threaded rotaxane structure is confirmed in the gas phase by tandem mass spectrometric experiments through a detailed fragmentation pattern analysis, in solution by NMR spectroscopy, and in the solid state through X-ray crystallography. A close inspection of the 1H,1H NOESY and 1H,1H ROESY NMR data reveals the wheel to travel along the axle between two degenerate diamide "stations" close to the two stoppers. By deprotonation of a phenolic OH group in the axle centerpiece with Schwesinger's P1 base, surprisingly no additional shuttling station is generated at the axle center, although the wheel could form rather strong hydrogen bonds with the phenolate. Instead, the wheel continues to travel between the two diamide stations. Experimental data from 1H,1H NOESY spectra, together with theoretical calculations, show that strong electrostatic interactions between the phenolate moiety and the P1 cation displace the wheel from the "phenolate station". The cation acts as a "brake" for the shuttling movement. Instead of suppressing the shuttling motion completely, as observed in other rotaxanes, our rotaxane is the first system in which electrostatic interactions modulate the speed of the mechanical motion between a fast and a slow motion state as a response to a reversible external stimulus. By tuning these electrostatic interactions through solvent effects, the rate of movement can be influenced significantly, when for example different amounts of DMSO are added to dichloromethane. Besides the shuttling motion, circumrotation of the wheel around the axle is observed and analyzed by variable temperature NMR spectroscopy. Force field and AM1 calculations are in good agreement with the experimental findings.     

Sterically crowded azulene-based dication salts as novel guests: synthesis and complexation studies with crown ethers and calixarenes in solution and in the gas phase

The 1,4-bis(3-guaiazulenylmethylium)benzene and 1,4-bis[1-(4,6,8-trimethylazulenylmethylium)]benzene dication salts were synthesized via an acid-catalyzed condensation/dehydration protocol with guaiazulene-terephthalaldehyde (2 : 1 ratio), and 4,6,8-trimethylazulene-terephthalaldehyde (2 : 1 ratio) respectively in one-pot processes. A similar condensation reaction with the parent azulene led to an insoluble oligomer that was shown by MALDI-TOF-MS to contain 1,4-bis[(diazulenyl)methylium]benzene as a repeating unit. Dication salts and were fully characterized by 2D NMR and NOE techniques and by electrospray-MS (ES-MS) and MALDI-TOF-MS. NMR studies confirm that the dications are best represented as bis-tropylium species. A delicate balance of electronic (inductive stabilization) and steric influence of the alkyl groups on the seven-membered ring seems to influence the chemo-/regioselectivity of the co-condensation process. NMR titration and T(1) measurements established that, despite its highly crowded structure, dication forms host-guest HG complexes with dibenzo-30-crown-10 (DB30C10) and dibenzo-24-crown-8 (DB24C8) in solution, but fails to complex with the smaller dibenzo-18-crown-6 (DB18C6). The corresponding HG cation-molecule cluster ions were also detected in the gas phase by ES-MS, showing the formation of both dication-crown 1 : 1 and 1 : 2 complexes. Similar complexation of dication salt with DB30C10 was observed via NMR titration and T(1) measurements in solution and by ES-MS in the gas phase. Although solution complexation studies (NMR titration) did not indicate stable complex formation between and p-tert-butyl-methoxycalix[8]arene, their [HG](2+) and [H(2)G](2+) clusters were detectable by ES-MS. Solution decomplexation experiments (HG(2+) --> H + G(2+)) were performed on -crown complex by addition of DMSO, acetone, silver tosylate, and tropylium cation salt. Complexation of with DB30C10 was also studied by microcalorimetric titration.     

Liquid-liquid phase transfer of uronium salts via complexation by (di)benzo crown ethers : X-ray structure of the benzo-27-crown-9 uronium perchlorate (1:1) complex

The complexation of uronium perchlorate with crown ethers of different ringsizes (18–33 ring atoms) has been studied by using two-phase extraction experiments. Crown ethers with 27 or more ring atoms are the best hosts to transfer uronium salts from an aqueous phase to an organic phase. The amount of uronium perchlorate transferred was measured by coulometric titration of the stoichiometric amount of ammonia produced by enzymatic degradation of urea. The crystal and molecular structure of the 1:1 complex of uronium perchlorate with benzo-27-crown-9 has been determined by X-ray crystallography. The uronium cation is encapsulated in the crown ether cavity with all its hydrogen atoms bonded to the macrocyclic host. A 1:1 complex of uronium picrate with benzo-21-crown-7 was isolated and is assumed to be a perching complex.     

An Operative Electrostatic Slipping Mechanism along Macrocycle Flexibility Accelerates Guest Sliding during pseudo-Rotaxane Formation

A pseudo-rotaxane is a host−guest complex composed of a linear molecule encircled by a macrocyclic ring. These complexes can be assembled by sliding the host over the guest terminal groups. If there is a close match between the molecular volume of the flanking groups on the guest and the cavity size of the macrocycle, the slipping might occur slowly or even become completely hindered. We have previously shown that it is possible to overcome the restraints imposed by steric effects on the sliding process by integrating electrostatic attractive interactions during the slipping step. In this work, we extend our electrostatically assisted slipping approach (EASA) to a new host−guest system featuring a flexible macrocyclic ring and a series of asymmetric guests containing a cyclic tertiary ammonium group. Compelling evidence for pseudo-rotaxane formation is presented, along with thermodynamic and kinetic data. Experimental results suggests that the higher conformational flexibility of 24-crown-8 significantly increases the sliding rate, compared with the more rigid dibenzo-24-crown-8, without affecting complex stability. Furthermore, by combining the EASA and macrocycle flexibility, we were capable to slip a large eight-membered cyclic group across the 24-crown-8 annulus, setting a new limit on the ring molecular size that can pass through a 24-membered crown ether.     

Syntheses of cis- and trans-dibenzo-30-crown-10 derivatives via regioselective routes and their complexations with paraquat and diquat

cis-Dibenzo-30-crown-10 (cis-DB30C10) diester and trans-dibenzo-30-crown-10 (trans-DB30C10) diester were synthesized regioselectively with reasonable yields. These two isomers were further reduced to cis-dibenzo-30-crown-10 diol (1) and trans-DB30C10 diol (2), respectively. The complexations of cis- and trans-DB30C10 diols with paraquat (3) and diquat (4) were investigated by (1)H NMR, mass spectrometry, UV-vis spectroscopy, and single-crystal X-ray analysis. The reversible control of complexations of 1 x 3 and 2 x 3 by adding small molecules (KPF 6 and dibenzo-18-crown-6) was demonstrated by (1)H NMR. The addition of 2 molar equiv of KPF 6 is enough to dissociate 2 x 3 and 1 x 3 completely while the subsequent addition of 2 molar equiv of DB18C6 allows the two complexes to reform. However, 2 molar equiv of KPF6 cannot dissociate 1 x 4 and 2 x 4 completely. Because the DB30C10 cavity has a better geometry fit with paraquat 3 than with diquat 4, 4-based complexes have much higher association constants than the corresponding 3-based complexes. In the crystal structure of 1 x 4, the two hydroxymethyl groups of the crown ether 1 were joined by a "water bridge" to form a "supramolecular cryptand" while this kind of supramolecular cryptand structure was not observed in the crystal structure of 2 x 4. This is a possible reason for the increase in association constant from 2 x 4 (3.3 x 10(4) M(-1)) to 1 x 4 (5.0 x 10(4) M(-1)).     

Structural analysis of linear/branched ethylene block copolymers

Chain shuttling polymerization has provided new pathway for introduction of different architectures in a single chain. Unlike the commercially available ethylene/1‐octene block copolymers, synthesis and microstructure of linear/branched polyethylene with blocky nature is not extensively studied. In this work, such block copolymers are synthesized based on reversible transfer of growing chains between an ansa metallocene and α‐diimine catalysts, forming linear and branched structures from ethylene, respectively. Investigation of thermal properties reveals that application of 550 equivalent of chain shuttling agent makes blocky structures that show the most deviation from the longstanding relationship between melting temperature and crystallinity or density, alongside with turning broad molecular distribution into unimodal. Thermal fractionation by successive self‐annealing demonstrates formation of broad distribution of linear blocks, as comprehended through appearance of uniform melting peaks at lower temperatures. Corresponding dynamic mechanical properties and crystalline structures reveal soft elastomeric properties, specifically at temperatures around −50°C, opposed to the purely linear chains or linear/branched blends. Correspondingly, blend samples demonstrate significant morphological change upon treatment with a suitable solvent for the branched fraction, contrary to the blocky microstructures.     

Induction of Motion in a Synthetic Molecular Machine: Effect of Tuning the Driving Force

Rotaxane molecular shuttles were studied in which a tetralactam macrocyclic ring moves between a succinamide station and a second station in which the structure is varied. Station 2 in all cases is an aromatic imide, which is a poor hydrogen‐bond acceptor in the neutral form, but a strong one when reduced with one or two electrons. When the charge density on the hydrogen‐bond‐accepting carbonyl groups in station 2 is reduced by changing a naphthalimide into a naphthalene diimide radical anion, the shuttling rate changes only slightly. When station 2 is a pyromellitimide radical anion, however, the shuttling rate is significantly reduced. This implies that the shuttling rate is not only determined by the initial unbinding of the ring from the first station, as previously supposed. An alternative reaction mechanism is proposed in which the ring binds to both stations in the transition state.     

Electron distribution in 1-organo-1H-tetrazole-5-thiols. Crystal and molecular structure of 1-methyl-1H-tetrazole-5-thiol and its potassium (18-crown-6) salt

The crystal and molecular structures of both neutral and anionic 1-methyl-1H-tetrazole-5-thiol, as its potassium(18-crown-6) salt, are reported. In the solid state, the molecular thiotetrazole adopts a planar, dimeric arrangement, in which two neighboring molecules are hydrogen bridged. Each monomeric unit exhibits considerable π electron delocalization over the CN₂S fragment. The anionic form displays extensive, but not uniform, π electron delocalization within the ring, which also extends to the exocyclic carbon–sulfur bond, the structure being best described as a hybrid. The potassium cation is coordinated to the macrocyclic 18-crown-6 ether as expected, but it also interacts with the NCS fragment of the tetrazolethiolate ring.     

1,2,5]Thiadiazolo[3,4-c][1,2,5]thiadiazolidyl: a long-lived radical anion and its stable salts

[1,2,5]Thiadiazolo[3,4-c][1,2,5]thiadiazole (1) is synthesized in 62% yield by fluoride ion-induced condensation of 3,4-difluoro-1,2,5-thiadiazole with (Me(3)SiN=)(2)S. The reversible electrochemical reduction of 1 leads to the long-lived [1,2,5]thiadiazolo[3,4-c][1,2,5]thiadiazolidyl radical anion (2) and further to the dianion (3). The radical anion 2 is also obtained by the chemical reduction of the precursor 1 with t-BuOK in MeCN. The radical anion 2 is characterized by ESR spectroscopy in solution and in the crystalline state. The stable salts [K(18-crown-6)][2] and [K(18-crown-6)][2].MeCN (8 and 9, respectively) are isolated from the spontaneous decomposition of the [K(18-crown-6)][PhXNSN] (6, X = S; 7, X = Se) salts in MeCN solution followed by XRD characterization. The radical anion 2 acts as a bridging ligand in 8 and as chelating ligand in 9. The structural changes observed by XRD in going from 1 to 2 are explained by means of DFT/(U)B3LYP/6-311+G calculations.     

Electrostatic barriers in rotaxanes and pseudorotaxanes

The ability to control the kinetic barriers governing the relative motions of the components in mechanically interlocked molecules is important for future applications of these compounds in molecular electronic devices. In this Full Paper, we demonstrate that bipyridinium (BIPY²⁺) dications fulfill the role as effective electrostatic barriers for controlling the shuttling and threading behavior for rotaxanes and pseudorotaxanes in aqueous environments. A degenerate [2]rotaxane, composed of two 1,5‐dioxynaphthalene (DNP) units flanking a central BIPY²⁺ unit in the dumbbell component and encircled by the cyclobis(paraquat‐p‐phenylene) (CBPQT⁴⁺) tetracationic cyclophane, has been synthesized employing a threading‐followed‐by‐stoppering approach. Variable‐temperature ¹H NMR spectroscopy reveals that the barrier to shuttling of the CBPQT⁴⁺ ring over the central BIPY²⁺ unit is in excess of 17 kcal mol^?1 at 343 K. Further information about the nature of the BIPY²⁺ unit as an electrostatic barrier was gleaned from related supramolecular systems, utilizing two threads composed of either two DNP units flanking a central BIPY²⁺ moiety or a central DNP unit flanked by a BIPY²⁺ moiety. The threading and dethreading processes of the CBPQT⁴⁺ ring with these compounds, which were investigated by spectrophotometric techniques, reveal that the BIPY²⁺ unit is responsible for affecting both the thermodynamics and kinetics of pseudorotaxane formation by means of an intramolecular self‐folding (through donor–acceptor interactions with the DNP unit), in addition to Coulombic repulsion. In particular, the free energy barrier to threading (Δ${G{{{\ne}\hfill \atop {\rm f}\hfill}}}The ability to control the kinetic barriers governing the relative motions of the components in mechanically interlocked molecules is important for future applications of these compounds in molecular electronic devices. In this Full Paper, we demonstrate that bipyridinium (BIPY(2+)) dications fulfill the role as effective electrostatic barriers for controlling the shuttling and threading behavior for rotaxanes and pseudorotaxanes in aqueous environments. A degenerate [2]rotaxane, composed of two 1,5-dioxynaphthalene (DNP) units flanking a central BIPY(2+) unit in the dumbbell component and encircled by the cyclobis(paraquat-p-phenylene) (CBPQT(4+)) tetracationic cyclophane, has been synthesized employing a threading-followed-by-stoppering approach. Variable-temperature (1)H?NMR spectroscopy reveals that the barrier to shuttling of the CBPQT(4+) ring over the central BIPY(2+) unit is in excess of 17 kcal mol(-1) at 343 K. Further information about the nature of the BIPY(2+) unit as an electrostatic barrier was gleaned from related supramolecular systems, utilizing two threads composed of either two DNP units flanking a central BIPY(2+) moiety or a central DNP unit flanked by a BIPY(2+) moiety. The threading and dethreading processes of the CBPQT(4+) ring with these compounds, which were investigated by spectrophotometric techniques, reveal that the BIPY(2+) unit is responsible for affecting both the thermodynamics and kinetics of pseudorotaxane formation by means of an intramolecular self-folding (through donor-acceptor interactions with the DNP unit), in addition to Coulombic repulsion. In particular, the free energy barrier to threading (ΔG(f)(++)) of the CBPQT(4+) for the case of the thread composed of a DNP flanked by two BIPY(2+) units was found to be as high as 21.7 kcal mol(-1) at room temperature. These results demonstrate that we can effectively employ the BIPY(2+) unit to serve as electrostatic barriers in water in order to gain control over the motions of the CBPQT(4+) ring in both mechanically interlocked and supramolecular systems.     

Functionally rigid bistable [2]rotaxanes

Two-station [2]rotaxanes in the shape of a degenerate naphthalene (NP) shuttle and a nondegenerate monopyrrolotetrathiafulvalene (MPTTF)/NP redox-controllable switch have been synthesized and characterized in solution. Their dumbbell-shaped components are composed of polyether chains interrupted along their lengths by (i) two pi-electron-rich stations-two NP moieties or a MPTTF unit and a NP moiety-with (ii) a rigid arylethynyl or butadiynyl spacer situated between the two stations and terminated by (iii) flexibly tethered hydrophobic stoppers at each end of the dumbbells. This modification was investigated as a means to simplify both molecular structure and switching function previously observed in related bistable [2]rotaxanes with flexible spacers between their stations and incorporating a cyclobis(paraquat-p-phenylene) (CBPQT4+) ring. The nondegenerate MPTTF-NP switch was isolated as near isomer-free bistable [2]rotaxane. Utilization of MPTTF removes the cis/trans isomerization that characterizes the tetrathiafulvalene (TTF) parent core structure. Furthermore, only one translational isomer is observed (> 95 < 5), surprisingly across a wide temperature range (198-323 K), meaning that the CBPQT4+ ring component resides, to all intents and purposes, predominantly on the MPTTF unit in the ground state. As a consequence of these two effects, the assignment of NMR and UV-vis data is more simplified as compared to previous donor-acceptor bistable [2]rotaxanes. This development has not only allowed for much better control over the position of the ring component in the ground state but also for control over the location of the CBPQT4+ ring during solution-state switching experiments, triggered either chemically (1H NMR) or electrochemically (cyclic voltammetry). In this instance, the use of the rigid spacer defines an unambiguous distance of 1.5 nm over which the ring moves between the MPTTF and NP units. The degenerate NP/NP [2]rotaxane was used to investigate the shuttling barrier by dynamic 1H NMR spectroscopy for the movement of the CBPQT4+ ring across the new rigid spacer. It is evident from these measurements that the rigid spacer poses a much lower barrier to the 1.0 nm movement of the CBPQT4+ ring from one station to another as compared with previous systems-a finding that is thought to be a result of the combination of fewer favorable interactions between the spacer and the CBPQT4+ ring and a relatively unimpeded path between the two NP stations. This example augers well for exploiting rigidity during the development of well-defined bistable [2]rotaxanes, which are unencumbered by the excesses of structural conformations that have characterized the first generations of molecular switches based on the donor-acceptor recognition motif.