Host-[2]rotaxane: advantage of converging functional groups for guest recognition

A host-[2]rotaxane was constructed by converting a diaminophenylcalix[4]arene into a [2]rotaxane using the DCC-rotaxane method (Zehnder, D.; Smithrud, D. B. Org. Lett. 2001, 16, 2485-2486). N-Ac-Arg groups were attached to the dibenzo-24-crown-8 ring of the rotaxane to provide a convergent functional group. To demonstrate the advantage provided by the rotaxane architecture for recognition of guests that contain a variety of functional groups, association constants (K(A)) for N-Ac-Trp, indole, N-Ac-Gly, fluorescein, 1-(dimethylamino)-5-naphthalenesulfonate, and pyrene bound to the [2]rotaxane were determined by performing (1)H NMR and fluorescence spectroscopic experiments. The host-[2]rotaxane had the highest affinity for fluorescein with a K(A) = 4.6 x 10(6) M(-)(1) in a 98/2 buffer (1 mM phosphate, pH 7)/DMSO solution. A comparison of K(A) values demonstrates that both the aromatic pocket and ring of the host-[2]rotaxane contribute binding free energy for complexation. Association constants were also derived for the same guests bound to the diaminophenylcalix[4]arene and to a diphenylcalix[4]arene that contained arginine residues displayed in a nonconvergent fashion. The host-[2]rotaxane provides higher affinity and specificity for most guests than the host with divergent N-Ac-Arg groups of the one that only has an aromatic pocket. For example, the K(A) for the complex of the host-[2]rotaxane and fluorescein in the DMSO/water mixture is more than 2 orders of magnitude greater than association constants derived for the other hosts.     

A photoswitchable rotaxane with a folded molecular thread

Novel [2]rotaxanes containing the tetracationic cyclophane cyclobis(paraquat-4,4-biphenylene) and a dumbbell-shaped molecular thread incorporating a photoactive diarylcycloheptatriene station as well as a photoinactive anisol station have been synthesized with yields of nearly 50 % by the alkylative endcapping method. The rotaxane was transformed into the related rotaxane incorporating a diaryl tropylium unit by electrochemical oxidation. The precursor of the cycloheptatrienyl rotaxane, the related pseudorotaxane, and the rotaxanes incorporating the diarylcycloheptatriene and the corresponding tropylium unit were characterized by (1)HNMR spectroscopy and UV/Vis spectroscopy. According to the NMR spectra, both the cycloheptatriene and the tropylium rotaxane possess a folded conformation enabling the tetracationic cyclophane to interact with two stations. The diarylcycloheptatriene station is incorporated inside the cavity of the cyclophane and the anisol station resides alongside the bipyridinium unit of the cyclophane. In contrast, the anisol station is inside the cyclophane in the tropylium rotaxane. The exchange between both conformations can be achieved by introducing the methoxy leaving group into the cycloheptatriene ring; the tropylium rotaxane is generated by photoheterolysis of this methoxy-substituted rotaxane, which reacts thermally back to the cycloheptatriene rotaxane, thus closing the switching cycle. These induced conformational changes achieve a so-called molecular machine.     

Modular Synthesis and Dynamics of a Variety of Donor–Acceptor Interlocked Compounds Prepared by Click Chemistry

A series of donor–acceptor [2]‐, [3]‐, and [4]rotaxanes and self‐complexes ([1]rotaxanes) have been synthesized by a threading‐followed‐by‐stoppering approach, in which the precursor pseudorotaxanes are fixed by using Cu^I‐catalyzed Huisgen 1,3‐dipolar cycloaddition to attach the required stoppers. This alternative approach to forming rotaxanes of the donor–acceptor type, in which the donor is a 1,5‐dioxynaphthalene unit and the acceptor is the tetracationic cyclophane cyclobis(paraquat‐p‐phenylene), proceeds with enhanced yields relative to the tried and tested synthetic strategies, which involve the clipping of the cyclophane around a preformed dumbbell containing π‐electron‐donating recognition sites. The new synthetic approach is amenable to application to highly convergent sequences. To extend the scope of this reaction, we constructed [2]rotaxanes in which one of the phenylene rings of the tetracationic cyclophane is perfluorinated, a feature which significantly weakens its association with π‐electron‐rich guests. The activation barrier for the shuttling of the cyclophane over a spacer containing two triazole rings was determined to be (15.5±0.1) kcal mol^?1 for a degenerate two‐station [2]rotaxane, a value similar to that previously measured for analogous degenerate compounds containing aromatic or ethylene glycol spacers. The triazole rings do not seem to perturb the shuttling process significantly; this property bodes well for their future incorporation into bistable molecular switches.     

Synthesis and characterization of a metallocycle-based molecular shuttle

Kinetically stable metallocycle-based molecular shuttles of [2]rotaxanes 4a and 4b, along with [3]rotaxanes 5a and 5b, have been prepared using the rhenium(I)-bridged metallocycle 2 and the dumbbell components containing two stations, 3a and 3b. The rotaxanes were self-assembled by hydrogen bonding interactions upon heating a Cl(2)CHCHCl(2) solution containing their components at 70 degrees C. Each rotaxane was isolated in pure form by silica gel chromatography under ordinary laboratory conditions and fully characterized by elemental analysis and various spectroscopic methods. The (1)H NMR signals for the amide NH and the methylene -(CH(2))(4)- of the station were considerably changed when occupied by the metallocycle. In [2]rotaxane 4b, which has a larger naphthyl spacer, the occupied and unoccupied stations gave widely separated signals in the (1)H NMR spectroscopy at room temperature, but averaged signals of two stations were observed in [2]rotaxane 4a, which has a smaller phenyl spacer. This is attributed to the shuttling of the metallocycle between two stations. The coalescence temperature experiment gave a shuttling rate of approximately 670 s(-)(1) at 19 degrees C in CDCl(3), corresponding to an activation free energy (DeltaG()) of 13.3 kcal/mol. With respect to the relative position of the chloride in the rhenium(I) center, two diastereomers are possible in the [2]rotaxane and three diastereomers are possible in the [3]rotaxane. In fact, the rotaxanes exist as diastereomeric mixtures in nearly equal amounts of all possible diastereomers on the basis of the amide NH signals of the station in the (1)H NMR spectroscopy.     

Synthesis of thiophene-capped [2]rotaxanes

Two types of thiophene-capped [2]rotaxanes, i.e., bithienyl (2T)- and bis(3,4-ethylenedioxythiophene)-yl (BEDOT)-capped [2]rotaxanes, were synthesized. The electron-deficient cyclophane of cyclobis(paraquat-p-phenylene) (CBPQT⁴⁺) was used as a macrocycle. Association constants for inclusion complexation of 2T- and BEDOT-derivatives with CBPQT⁴⁺ were obtained by ¹H NMR titration. Due to the donor-acceptor charge transfer absorption band, 2T- and BEDOT-capped [2]rotaxanes have red and green colors, respectively. On the basis of electrochemical analysis, we confirmed that only BEDOT-capped [2]rotaxane is a promising candidate for [3]rotaxane synthesis through oxidation coupling of the thiophene unit.     

Self-assembly and dynamics of [2]- and [3]rotaxanes with a dinuclear macrocycle containing reversible Os-N coordinate bonds

With a dinuclear macrocycle 2 that contains weak reversible OsVI-N coordinate bonds, self-assembly and equilibrium dynamics of [2]- and [3]rotaxanes have been investigated. When the macrocycle 2 was mixed together with threads 4a-e, which all contain an adipamide station but different sizes of end groups, [2]pseudorotaxane- and rotaxane-like complexes were immediately formed with large association constants of >7 x 103M(-1) in CDCl3 at 298 K. Exchange dynamics, explored by 2D-EXSY experiments, suggest that assembly and disassembly of complexes occur through two distinct pathways, slipping or clipping, and this depends on the size of the end groups. The slipping pathway is predominant with smaller end groups that give pseudorotaxane-like complexes, while the clipping pathway is observed with larger end groups that yield rotaxane-like complexes. Under the same conditions, exchange barriers (deltaG++) were 14.3 kcalmol(-1) for 4a and 16.7 kcalmol(-1) for 4d, and indicate that the slipping process is at least one order of magnitude faster than the clipping process. Using threads 13a and 13b that contain two adipamide groups, more complicated systems have been investigated in which [2]rotaxane, [3]rotaxane, and free components are in equilibrium. Concentration- and temperature-dependent 1H NMR spectroscopic studies allowed the identification of all possible elements and the determination of their relative distributions in solution. For example, the relative distribution of the free components, [2]rotaxane, and [3]rotaxane are 30, 45, and 25 %, respectively, in a mixture of 2 (2mM) and 13a (2mM) in CDCl3 at 10 degrees C. However, [3]rotaxane exists nearly quantitatively in a mixture of 2 (4 mM) and 13 a (2 mM) in CDCl3 at a low temperature - 10 degrees C.     

Recognition through self-assembly. A quadruply-hydrogen-bonded, strapped porphyrin cleft that binds dipyridyl molecules and a [2]rotaxane

Quadruply-hydrogen-bonded porphyrin homodimer Zn1.Zn1 has been designed, assembled, and evaluated as a supramolecular cleft-featured receptor for its ability to bind dipyridyl guests in chloroform-d. Monomer Zn1 consists of a 2-ureidopyrimidin-4(1H)-one unit, which was initially reported by Meijer et al., and a zinc porphyrin unit. The zinc porphyrin is strapped with an additional aliphatic chain for controlling the atropisomerization of porphyrin. The 2-ureidopyrimidin-4(1H)-one unit dimerizes exclusively in chloroform even at the dilute concentration of 10(-)(4) M, while the two "strapped" zinc porphyrin units of the homodimer provide additional binding sites for selective guest recognition. (1)H NMR studies indicate that the new homodimer Zn1.Zn1 adopts an S-type conformation due to strong donor-acceptor interaction between the electron-rich porphyrin units and the electron-deficient 2-ureidopyrimidin-4(1H)-one unit. (1)H NMR, UV-vis, and vapor pressure osmometry investigations reveal that Zn1.Zn1 could function as a new generation of assembled supramolecular cleft, to be able to not only efficiently bind linear dipyridyl molecules 14-17, resulting in the formation of stable termolecular complexes, with K(aasoc) values ranging from 3.8 x 10(6) to 8.9 x 10(7) M(-)(1), but also strongly complex a hydrogen-bond-assembled [2]rotaxane, 18, which consists of a rigid fumaramide thread and a pyridine-incorporated tetraamide cyclophane, with K(aasoc) = 1.2 x 10(4) M(-)(1). (1)H NMR competition experiments reveal that complexation to the dipyriyl guests also promotes the stability of the quadruply-hydrogen-bonded dimeric receptor.     

2]Pseudorotaxanes, [2]rotaxanes and metal-organic rotaxane frameworks containing tetra-substituted dibenzo[24]crown-8 wheels

[2]Pseudorotaxanes, [2]rotaxanes and metal-organic rotaxane framework materials that utilise DB24C8 as the wheel component are well known and structural variations based on changing the axle component are common. Studies in which the DB24C8 wheel is structurally modified are much more limited. Herein, is described the synthesis of symmetrical DB24C8 analogues containing four CH(2)OR (R = CH(2)CH(2)CH(3), CH(2)(C(6)H(5)), C(6)H(5) and C(6)H(4)(4-COOEt)) substituents on the 4 and 5 positions of the aromatic rings. The effect of these molecular appendages on the stability and structures of the interpenetrated and interlocked molecules derived from these new wheels is described. The major effects are an increase in association constants for the formation of [2]pseudorotaxanes relative to DB24C8, the crystal packing of [2]rotaxanes and a change on the internal structure of a 2D MORF (R = C(6)H(5)) compared to DB24C8.     

High-yield diastereoselective synthesis of planar chiral [2]- and [3]rotaxanes constructed from per-ethylated pillar[5]arene and pyridinium derivatives

Planar chiral [2]- and [3]rotaxanes constructed from pillar[5]arenes as wheels and pyridinium derivatives as axles were obtained in high yield using click reactions. The process of rotaxane formation was diastereoselective; the obtained [2]rotaxane was a racemic mixture consisting of (pS, pS, pS, pS, pS) and (pR, pR, pR, pR, pR) forms of the per-ethylated pillar[5]arene (C2) wheel, and other possible types of the [2]rotaxane did not form. Isolation of the enantiopure [2]rotaxanes with one axle through (pS, pS, pS, pS, pS)-C2 or (pR, pR, pR, pR, pR)-C2 wheels was accomplished. Furthermore, pillar[5]arene-based [3]rotaxane was successfully synthesized by attachment of two pseudo [2]rotaxanes onto a bifunctional linker. [3]Rotaxane formed in a 1:2:1 mixture with one axle threaded through two (pS, pS, pS, pS, pS)-C2, one (pS, pS, pS, pS, pS)-C2 and one (pR, pR, pR, pR, pR)-C2 (meso form), or two (pR, pR, pR, pR, pR)-C2 wheels. The [3]rotaxane enantiomers and the meso form were successfully isolated using appropriate chiral HPLC column chromatography. The procedure developed in this study is the starting point for the creation of pillar[5]arene-based interlocked molecules.     

Reversing a rotaxane recognition motif: threading oligoethylene glycol derivatives through a dicationic cyclophane

An already well-established recognition motif-namely one in which the NH2+ centers in the rod sections of the dumbbell components of rotaxanes are encircled by macrocyclic polyether components-has been turned simultaneously outside-in and inside-out, a fact that has been proved beyond any doubt by the stoppering of both ends of a [2]pseudorotaxane to give a stable [2]rotaxane. The [2]pseudorotaxane is formed in nitromethane when a benzylic dibromide, obtained after reacting an excess of 1,4-bis(bromomethyl)benzene with hexaethylene glycol, is added to an equimolar amount of a dicationic cyclophane in which two -CH2OCH2- chains link a pair of dibenzylammonium ions through the para positions on their phenyl rings. When the [2]pseudorotaxane is reacted in nitromethane with triphenylphosphine, a [2]rotaxane and the corresponding free dumbbell compound are isolated in 58 and 31% yields, respectively. The structure of the [2]rotaxane is established by using mass spectrometry (FABMS and ESMS) and NMR (1H and 13C) spectroscopy in nitromethane-d3. The [2]rotaxane exhibits quite dramatic changes in the 1H chemical shifts of the signals for its CH2N+ and CH2O protons compared with those in the free dumbbell compound. The 1H NMR spectrum of the [2]pseudorotaxane shows many similar features. Titration experiments with three of the six different CH2O probes give an average Ka value of 2900 +/- 750 M-1 in nitromethane-d3. The new recognition motif for the template-directed synthesis of rotaxanes can now be exploited at both the molecular and macromolecular levels of structure with numerous potential applications in sight.     

Seven-coordinate iron and manganese complexes with acyclic and rigid pentadentate chelates and their superoxide dismutase activity

The reactions of seven-coordinate [Fe(III)(dapsox)(H(2)O)(2)]ClO(4).H(2)O (1), [Fe(II)(H(2)dapsox)(H(2)O)(2)](NO(3))(2).H(2)O (2), and [Mn(II)(H(2)dapsox)(CH(3)OH)(H(2)O)](ClO4)2(H2O) (3) complexes of the acyclic and rigid pentadentate H(2)dapsox ligand [H2dapsox = 2,6-diacetylpyridinebis(semioxamazide)] with superoxide have been studied spectrophotometrically, electrochemically, and by a submillisecond mixing UV/vis stopped-flow in dimethyl sulfoxide (DMSO). The same studies were performed on the seven-coordinate [Mn(II)(Me(2)[15]pyridinaneN(5))(H(2)O)(2)]Cl(2).H(2)O (4) complex with the flexible macrocyclic Me(2)[15]pyridinaneN(5) ligand (Me(2)[15]pyridinaneN(5) = trans-2,13-dimethyl-3,6,9,12,18-pentaazabicyclo[12.3.1]octadeca-1(18),14,16-triene), which belongs to the class of proven superoxide dismutase (SOD) mimetics. The X-ray crystal structures of 2-4 were determined. All complexes possess pentagonal-bipyramidal geometry with the pentadentate ligand in the equatorial plane and solvent molecules in the axial positions. The stopped-flow experiments in DMSO (0.06% of water) reveal that all four metal complexes catalyze the fast disproportionation of superoxide under the applied experimental conditions, and the catalytic rate constants are found to be (3.7 +/- 0.5) x 10(6), (3.9 +/- 0.5) x 10(6), (1.2 +/- 0.3) x 10(7), and (5.3 +/- 0.8) x 10(6) M(-1) s(-1) for 1-4, respectively. The cytochrome c McCord-Fridovich (McCF) assay in an aqueous solution at pH = 7.8 resulted in the IC(50) values (and corresponding kMcCF constants) for 3 and 4, 0.013 +/- 0.001 microM (1.9 +/- 0.2 x 10(8) M(-1) s(-1)) and 0.024 +/- 0.001 microM (1.1 +/- 0.3 x 10(8) M(-1) s(-1)), respectively. IC(50) values from a nitroblue tetrazolium assay are found to be 6.45 +/- 0.02 and 1.36 +/- 0.03 microM for 1 and 4, respectively. The data have been compared with those obtained by direct stopped-flow measurements and discussed in terms of the side reactions that occur under the conditions of indirect assays.     

Dynamic covalent approach to [2]- and [3]roxtanes by utilizing a reversible thiol-disulfide interchange reaction

A dynamic covalent approach to disulfide-containing [2]- and [3]rotaxanes is described. Symmetrical dumbbell-shaped compounds with two secondary ammonium centers and a central located disulfide bond were synthesized as components of rotaxanes. The rotaxanes were synthesized from the dumbbell-shaped compounds and dibenzo-[24]crown-8 (DB24C8) with catalysis by benzenethiol. The yields of isolated rotaxanes reached about 90 % under optimized conditions. A kinetic study on the reaction forming [2]rotaxane 2 a and [3]rotaxane 3 a suggested a plausible reaction mechanism comprising several steps, including 1) initiation, 2) [2]rotaxane formation, and 3) [3]rotaxane formation. The whole reaction was found to be reversible in the presence of thiols, and thermodynamic control over product distribution was thus possible by varying the temperature, solvent, initial ratio of substrates, and concentration. The steric bulk of the end-capping groups had almost no influence on rotaxane yields, but the structure of the thiol was crucial for reaction rates. Amines and phosphines were also effective as catalysts. The structural characterization of the rotaxanes included an X-ray crystallographic study on [3]rotaxane 3 a.     

Stable pillar [5] arene-based pseudo [1] rotaxanes formed in polar solution

Mono-alkyl-functionalized pillar[5] arenes P1,P2,and P3 were synthesized by click reaction,which exhibited different self-assembly behavior in polar solvent DMSO.Stable pseudo [1] rotaxane was formed by the self-complexation from P1 or P2,whereas,concentration-dependent pseudorotaxane structures were generated by P3 which bearing more flexible side chain.Interestingly,the obtained pseudo[1] rotaxanes exhibited a dynamic fast assembly process upon adding NaBF₄,resulting in the formation of Na⁺-induced pseudorotaxanes.     

Utilization of a Crown Ether/Amine‐Type Rotaxane as a Probe for the Versatile Detection of Anions and Acids by Thin‐Layer Chromatography

A crown ether/amine‐type [2]rotaxane was synthesized and utilized as a probe for the detection of acids and anions. The addition of acids to the amine‐type [2]rotaxane solution generated corresponding crown ether/ammonium‐type [2]rotaxanes, which were purified by silica gel column chromatography as ammonium salts. The isolated yields of the [2]rotaxanes, possessing a variety of anions, depended on the acidity and polarity of the counter anions. The behaviours of the ammonium‐type [2]rotaxanes on thin‐layer chromatography (TLC) silica gel reflected the properties of the counter anions. The treatment of the amine‐type [2]rotaxane with acids afforded the corresponding ammonium‐type [2]rotaxanes bearing several different anions. The ammonium‐type [2]rotaxanes behaved similarly to the purified [2]rotaxanes on the TLC silica gel. Furthermore, we succeeded in the analysis of anions using mixtures of the amine‐type [2]rotaxane and salts in an appropriate solvent. We demonstrated the detection of anions by the combination of TLC and the utilization of the [2]rotaxane probe.     

Determining the intracellular transport mechanism of a cleft-[2]rotaxane

Rotaxanes are a class of interlocked compounds that have been extensively investigated for their potential utility as switches or sensors. We recently demonstrated that rotaxanes have further application as agents that transport material into cells. This novel finding prompted our investigation into the mechanism by which rotaxanes are involved in transmembrane transport. Two-dimensional NMR analysis showed that a cleft-containing rotaxane exists in two dominant conformations ("closed" and "open"). To determine the importance of conformational flexibility on the ability of the rotaxanes to bind guests and transport material into cells, the rotaxane was chemically modified to lock it in the closed conformation. Charged guests interact less favorably with the locked rotaxane, as compared to the unmodified rotaxane, both in an aqueous solution and in DMSO. In a chloroform solution, both rotaxanes bind the guests with similar affinities. The locked rotaxane exhibited a reduced capacity to transport a fluoresceinated peptide into cells, whereas the unmodified rotaxane efficiently delivers the peptide. Flow cytometry experiments demonstrated that a high percentage of the cells contained the delivered peptide (89-98%), the level of delivery is concentration dependent, and the rotaxanes and peptide have low toxicity. Cellular uptake of the peptide was largely temperature and ATP independent, suggesting that the rotaxane-peptide complex passes through the cellular membrane without requiring active cell-mediated processes. The results show that the sliding motion of the wheel is necessary for the delivery of materials into cells and can enhance the association of guests. These studies demonstrate the potential for rotaxanes as a new class of mechanical devices that deliver a variety of therapeutic agents into targeted cell populations.     

Template-directed synthesis of pyridazine-containing tetracationic cyclophane for construction of [2]rotaxane

Benefiting from its bent molecular structure, 3,6-pyridazinyl contained tetracationic cyclophane (1) is synthesized by template-directed method with high isolated yield up to 92%. This template-directed strategy is further utilized to efficiently construct [2]rotaxane.     

Synthesis of [1]rotaxane via covalent bond formation and its unique fluorescent response by energy transfer in the presence of lithium ion

Although there have been a lot of reports on the synthesis and properties of [n]rotaxanes (mainly n = 2), only a few reports on the synthesis of [1]rotaxane has been published by V?gtle's group and others (see ref 5). Generally speaking, [1]rotaxane might be expected to exhibit properties different from other rotaxanes, because the rotor and the axle in the [1]rotaxane is bound covalently and closely. We report on a novel method to make [1]rotaxanes via covalent bond formation from a macrocyclic compound. That is, we first prepared a bicyclic compound from macrocycle and then proceeded to [1]rotaxane by aminolysis. This is the first synthetic example of preparation of [1]rotaxane via covalent bond formation, not utilizing weak interactions such as hydrogen bonding, charge transfer, via metal complexation, etc. This method might provide a powerful and new tool for construction of [1]rotaxane as a new supramolecular system. In addition, we investigated energy transfer from rotor to axle using [1]rotaxane that we prepared. Energy transfer occurred perfectly from the naphthalene ring of the rotor to the anthracene ring of the axle. We found also that only lithium ion among alkali ions can drastically enhance the fluorescence intensity. This finding could be applicable to ion-sensing systems, switching devices, and so on.     

Self-assembly of novel [3]- and [2]rotaxanes based on donor-acceptor and hydrogen-bonding interactions. Intensified inter-ring repulsion interaction and shuttling behavior

Three novel hetero[3]rotaxanes, which comprise one neutral tetraamide cyclophane, one tetracationic cyclophane, and one linear component, have been assembled by utilizing hydrogen-bonding and donor-acceptor interactions, through three neutral [2]rotaxanes as intermediates. Three tetracationic [2]rotaxanes are also prepared for property comparison. For all three linear components, diamide subunits, the hydrogen-bonding templating moieties, are introduced at the center of the molecules, while the electron-rich hydrogquinone subunits, the donor-acceptor interaction templates, are incorporated between the diamides and the triphenylmethyl stoppers. Compared with the reported [3]rotaxanes, the novel hetero[3]rotaxanes exhibit remarkably intensified spatial interaction between the two ring components, which had been proved by (1)H NMR and UV study. For the first time, inter-ring NOEs are observed for interlocked [3]rotaxanes.     

Redox-controllable amphiphilic [2]rotaxanes

With the fabrication of molecular electronic devices (MEDs) and the construction of nanoelectromechanical systems (NEMSs) as incentives, two constitutionally isomeric, redox-controllable [2]rotaxanes have been synthesized and characterized in solution. Therein, they both behave as near-perfect molecular switches, that is, to all intents and purposes, these two rotaxanes can be switched precisely by applying appropriate redox stimuli between two distinct chemomechanical states. Their dumbbell-shaped components are composed of polyether chains interrupted along their lengths by i) two pi-electron rich recognition sites-a tetrathiafulvalene (TTF) unit and a 1,5-dioxynaphthalene (DNP) moiety-with ii) a rigid terphenylene spacer placed between the two recognition sites, and then terminated by iii) a hydrophobic tetraarylmethane stopper at one end and a hydrophilic dendritic stopper at the other end of the dumbbells, thus conferring amphiphilicity upon these molecules. A template-directed protocol produces a means to introduce the tetracationic cyclophane, cyclobis(paraquat-p-phenylene) (CBPQT(4+)), which contains two pi-electron accepting bipyridinium units, mechanically interlocked around the dumbbell-shaped components. Both the TTF unit and the DNP moiety are potential stations for CBPQT(4+), since they can establish charge-transfer and hydrogen bonding interactions with the bipyridinium units of the cyclophane, thereby introducing bistability into the [2]rotaxanes. In both constitutional isomers, (1)H NMR and absorption spectroscopies, together with electrochemical investigations, reveal that the CBPQT(4+) ring is predominantly located on the TTF unit, leading to the existence of a single translational isomer (co-conformation) in both cases. In addition, a model [2]rotaxane, incorporating hydrophobic tetraarylmethane stoppers at both ends of its dumbbell-shaped component, has also been synthesized as a point of reference. Molecular synthetic approaches were used to construct convergently the dumbbell-shaped compounds by assembling progressively smaller building blocks in the shape of the rigid spacer, the TTF unit and the DNP moiety, and the hydrophobic and hydrophilic stoppers. The two amphiphilic bistable [2]rotaxanes are constitutional isomers in the sense that, in one constitution, the TTF unit is adjacent to the hydrophobic stopper, whereas in the other, it is next to the hydrophilic stopper. All three bistable [2]rotaxanes have been isolated as green solids. Electrospray and fast atom bombardment mass spectra support the gross structural assignments given to all three of these mechanically interlocked compounds. Their photophysical and electrochemical properties have been investigated in acetonitrile. The results obtained from these investigations confirm that, in all three [2]rotaxanes, i) the CBPQT(4+) cyclophane encircles the TTF unit, ii) the CBPQT(4+) cyclophane shuttles between the TTF and DNP stations upon electrochemical or chemical oxidation/reduction of the TTF unit, and iii) folded conformations are present in which the CBPQT(4+) cyclophane, while encircling the TTF unit, interacts through its pi-accepting bipyridinium exteriors with other pi-donating components of the dumbbells, especially those located within the stoppers.     

Pentafluorophenyl Esters as Exchangeable Stoppers for the Construction of Photoactive [2]Rotaxanes

Stable pillar[5]arene-containing [2]rotaxane building blocks with pentafluorophenyl ester stoppers have been efficiently prepared on a multi-gram scale. Reaction of these building blocks with various nucleophiles gave access to a wide range of [2]rotaxanes with amide, ester or thioester stoppers in good to excellent yields. The rotaxane structure is fully preserved during these chemical transformations. Actually, the addition-elimination mechanism at work during these transformations totally prevents the unthreading of the axle moiety of the mechanically interlocked system. The stopper exchange reactions were optimized both in solution and under mechanochemical solvent-free conditions. While amide formation is more efficient in solution, the solvent-free conditions are more powerful for the transesterification reactions. Starting from a fullerene-functionalized pillar[5]arene derivative, this new strategy gave easy access to a photoactive [2]rotaxane incorporating a C₆₀ moiety and two Bodipy stoppers. Despite the absence of covalent connectivity between the Bodipy and the fullerene moieties in this photoactive molecular device, efficient through-space excited state interactions have been evidenced in this rotaxane.