Self-assembly of an amphiphilic

The template-directed synthesis of a [2]rotaxane, in which a pi-electron deficient ring component-cyclobis(paraquat-p-phenylene)-is assembled around a pi-electron rich asymmetric monopyrrolotetrathiafulvalene unit on the rod section of an amphiphilic dumbbell component that is terminated by a hydrophilic dendritic stopper at one end and a hydrophobic tetraarylmethane stopper at the other end, is reported.     

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.     

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.     

Design of [2]rotaxane through image threshold segmentation of electrostatic potential image

An electrostatic potential (ESP)‐based image segmentation method has been used to estimate the ability of proton donation and acceptance involved in ring‐rod recognition. The relative binding strength of [2]rotaxane has also been further estimated from the difference of the characteristic image‐segmentation derived ESP between proton donor and proton acceptor. The size and electrostatic compatibility criteria are introduced to guide the design of interlocked [2]rotaxane. A library of 75 thermodynamically stable [2]rotaxane candidates has been generated, including 16 experimentally known systems. The theoretical results for 16 experimentally known [2]rotaxanes are in good agreement with both the experimental association constants and density functional theory‐calculated binding energies. Our ESP‐based image segmentation model is also applicable to the tristable [2]rotaxane molecular shuttle as well as [1]rotaxane with self‐inclusion function, indicating this simple method is generic in the field of constructing other supramolecular architectures formed with donor/acceptor molecular recognition. © 2016 Wiley Periodicals, Inc.     

Lemniscular hexaphyrins as examples of aromatic and antiaromatic double-twist Möbius molecules

Two reported [26] and [28]hexaphyrins are analyzed via measured and computed geometries and NMR-shieldings as examples of respectively 4n + 2 pi-electron aromatic and 4n pi-electron antiaromatic double-twist M?bius ring systems, adopting a lemniscular/figure-eight topology with linking number LK = 2pi. Values of local twist (TW) and nonlocal writhe (WR) derived from the relation Lk = Tw + WR appear relatively insensitive to the aromatic/antiaromatic character. The [26]hexaphyrin may adopt differing solution and solid-state conformations.     

A photochemically driven molecular-level abacus

A molecular-level abacus-like system driven by light inputs has been designed in the form of a [2]rotaxane, comprising the pi-electron-donating macrocyclic polyether bis-p-phenylene-34-crown-10 (BPP34C10) and a dumbbell-shaped component that contains 1) a Ru(II) polypyridine complex as one of its stoppers in the form of a photoactive unit, 2) a p-terphenyl-type ring system as a rigid spacer, 3) a 4,4'-bipyridinium unit and a 3,3'-dimethyl-4,4'-bipyridinium unit as pi-electron-accepting stations, and 4) a tetraarylmethane group as the second stopper. The synthesis of the [2]rotaxane was accomplished in four successive stages. First of all, the dumbbell-shaped component of the [2]rotaxane was constructed by using conventional synthetic methodology to make 1) the so-called "west-side" comprised of the Ru(II) polypyridine complex linked by a bismethylene spacer to the p-terphenyl-type ring system terminated by a benzylic bromomethyl function and 2) the so-called "east-side" comprised of the tetraarylmethane group, attached by a polyether linkage to the bipyridinium unit, itself joined in turn by a trismethylene spacer to an incipient 3,3'-dimethyl-4,4'-bipyridinium unit. Next, 3) the "west-side" and "east-side" were fused together by means of an alkylation to give the dumbbell-shaped compound, which was 4) finally subjected to a thermodynamically driven slippage reaction, with BPP34C10 as the ring, to afford the [2]rotaxane. The structure of this interlocked molecular compound was characterized by mass spectrometry and NMR spectroscopy, which also established, along with cyclic voltammetry, the co-conformational behavior of the molecular shuttle. The stable translational isomer is the one in which the BPP34C10 component encircles the 4,4'-bipyridinium unit, in keeping with the fact that this station is a better pi-electron acceptor than the other station. This observation raises the question- can the BPP34C10 macrocycle be made to shuttle between the two stations by a sequence of photoinduced electron transfer processes? In order to find an answer to this question, the electrochemical, photophysical, and photochemical (under continuous and pulsed excitation) properties of the [2]rotaxane, its dumbbell-shaped component, and some model compounds containing electro- and photoactive units have been investigated. In an attempt to obtain the photoinduced abacus-like movement of the BPP34C10 macrocycle between the two stations, two strategies have been employed-one was based fully on processes that involved only the rotaxane components (intramolecular mechanism), while the other one required the help of external reactants (sacrificial mechanism). Both mechanisms imply a sequence of four steps (destabilization of the stable translational isomer, macrocyclic ring displacement, electronic reset, and nuclear reset) that have to compete with energy-wasteful steps. The results have demonstrated that photochemically driven switching can be performed successfully by the sacrificial mechanism, whereas, in the case of the intramolecular mechanism, it would appear that the electronic reset of the system is faster than the ring displacement.     

A redox-switchable alpha-cyclodextrin-based [2]rotaxane

A bistable [2]rotaxane comprising an alpha-cyclodextrin (alpha-CD) ring and a dumbbell component containing a redox-active tetrathiafulvalene (TTF) ring system within its rod section has been synthesized using the Cu(I)-catalyzed azide-alkyne cycloaddition, and the redox-driven movements of the alpha-CD ring between the TTF and newly formed triazole ring systems have been elucidated. Microcalorimetric titrations on model complexes suggested that the alpha-CD ring prefers to reside on the TTF rather than on the triazole ring system by at least an order of magnitude. The fact that this situation does pertain in the bistable [2]rotaxane has not only been established quantitatively by electrochemical experiments and backed up by spectroscopic and chiroptical measurements but also been confirmed semiquantitatively by the recording of numerous cyclic voltammograms which point, along with the use of redox-active chemical reagents, to a mechanism of switching that involves the oxidation of the neutral TTF ring system to either its radical cationic (TTF*+) or dicationic (TTF2+) counterparts, whereupon the alpha-CD ring, moves along the dumbbell to encircle the triazole ring system. Since redox control by both chemical and electrochemical means is reversible, the switching by the bistable [2]rotaxane can be reversed on reduction of the TTF*+ or TTF2+ back to being a neutral TTF.     

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.     

Organogel formation by a cholesterol-stoppered bistable [2]rotaxane and its dumbbell precursor

The switching properties, gelation behavior, and self-organization of a cholesterol-stoppered bistable [2]rotaxane containing a cyclobis(paraquat-p-phenylene) ring and tetrathiafulvalene/1,5-dioxynaphthalene recognition units situated in the rod portion of the dumbbell component have been investigated by electrochemical, spectroscopic, and microscopic means. The cyclobis(paraquat-p-phenylene) ring in the [2]rotaxane can be switched between the tetrathiafulvalene and 1,5-dioxynaphthalene recognition units by addressing the redox properties of the tetrathiafulvalene unit. The organogels can be prepared by dissolving the [2]rotaxane and its dumbbell precursor in a CH2Cl2/MeOH (3:2) mixed solvent and liquified by adding the oxidant Fe(ClO4)3. Direct evidence for the self-organization was obtained from AFM investigations which have shown that both of the [2]rotaxane and its dumbbell precursor form linear superstructures which we propose are helical in nature.     

Synthesis of a mechanically linked oligo[2]rotaxane

A bifunctional [2]rotaxane, bearing two free functional groups each in the ring and axial parts, was synthesized, followed by its polycondensation with methylene diphenyl diisocyanate leading to a mechanically linked oligo[2]rotaxane.     

Efficient Intramolecular Energy Transfer between Two Fluorophores in a Bis‐Branched [3]Rotaxane

A bis‐branched [3]rotaxane, with two [2]rotaxane arms separated by an oligo(para‐phenylenevinylene) (OPV) fluorophore, was designed and investigated. Each [2]rotaxane arm employed a difluoroboradiaza‐s‐indacene (BODIPY) dye‐functionalized dibenzo[24]crown‐8 macrocycle interlocked onto a dibenzylammonium in the rod part. The chemical structure of the [3]rotaxane was confirmed and characterized by ¹H and ¹³C NMR spectroscopy and high‐resolution ESI mass spectrometry. The photophysical properties of [3]rotaxane and its reference systems were investigated through UV/Vis absorption, fluorescence, and time‐resolved fluorescence spectroscopy. An efficient energy‐transfer process in [3]rotaxane occurred from the OPV donor to the BODIPY acceptor because of the large overlap between the absorption spectrum of the BODIPY moiety and the emission spectrum of the OPV fluorophore; this shows the important potential of this system for designing functional molecular systems.     

Quantifying the working stroke of tetrathiafulvalene-based electrochemically-driven linear motor-molecules

A highly constrained [2]rotaxane, constructed in such a way that the tetracationic cyclobis(paraquat-p-phenylene) ring is restricted to reside on a monopyrrolotetrathiafulvalene unit, has been synthesised and characterised. This design allows the deslipping free energy barrier for the tetracationic ring in all three redox states of the rotaxane to be determined.     

Toward Daisy Chain Polymers: "Wittig Exchange" of Stoppers in

Two ammonium ion/crown ether-based [2]rotaxane monomers-each incorporating (i) a dumbbell-shaped component, possessing an exchangeable benzylic triphenylphosphonium stopper, and (ii) a ring component, bearing an aldehyde function-undergo a sequence of Wittig reactions in which the surrogate triphenylphosphonium stopper is exchanged for a ring component either (i) in the same rotaxane molecule to give cyclic daisy chains by an intramolecular, chain-terminating reaction or (ii) in another rotaxane molecule to give acyclic daisy chains by an intermolecular chain-propagating reaction.     

Synthesis of a [2]rotaxane incorporating a Ni(II)-salen moiety: evidence of ring-opening-and-closing protocol

[reaction: see text] We have synthesized a [2]rotaxane from a crown-ether-like macrocycle that undergoes ring opening and closing through cleavage and formation of imino bonds of a salen moiety; the self-assembly of this macrocycle and a dumbbell-shaped rodlike component, followed by addition of nickel acetate, afforded, after counterion exchange, a [2]rotaxane that is stabilized through coordination of the Ni ion to the macrocycle's salen moiety.     

A neutral redox-switchable [2]rotaxane

A limited range of redox-active, rotaxane-based, molecular switches exist, despite numerous potential applications for them as components of nanoscale devices. We have designed and synthesised a neutral, redox-active [2]rotaxane, which incorporates an electron-deficient pyromellitic diimide (PmI)-containing ring encircling two electron-rich recognition sites in the form of dioxynaphthalene (DNP) and tetrathiafulvalene (TTF) units positioned along the rod section of its dumbbell component. Molecular modeling using MacroModel guided the design of the mechanically interlocked molecular switch. The binding affinities in CH(2)Cl(2) at 298 K between the free ring and two electron-rich guests--one (K(a) = 5.8 × 10(2) M(-1)) containing a DNP unit and the other (K(a) = 6.3 × 10(3) M(-1)) containing a TTF unit--are strong: the one order of magnitude difference in their affinities favouring the TTF unit suggested to us the feasibility of integrating these three building blocks into a bistable [2]rotaxane switch. The [2]rotaxane was obtained in 34% yield by relying on neutral donor-acceptor templation and a double copper-catalysed azide-alkyne cycloaddition (CuAAC). Cyclic voltammetry (CV) and spectroelectrochemistry (SEC) were employed to stimulate and observe switching by this neutral bistable rotaxane in solution at 298 K, while (1)H NMR spectroscopy was enlisted to investigate switching upon chemical oxidation. The neutral [2]rotaxane is a chemically robust and functional switch with potential for applications in device settings.     

Evidence of strong hydration and significant tilt of amphiphilic [2]rotaxane molecules in Langmuir films studied by synchrotron X-ray reflectivity

Surface sensitive X-ray techniques have been used to elucidate the structures of amphiphilic [2]rotaxane and dumbbell monolayers at the air/water interface. The [2]rotaxanes were found to adopt highly hydrated tilted and/or folded conformations on the water surface largely due to the hydrophilic nature of their tetracationic ring component. This conformation was less pronounced in monolayers of the dumbbell precursors. Increasing the surface pressure resulted in an expansion of [2]rotaxane monolayers in the vertical direction and decreased hydration.     

Synthesis of a Structure‐Definite α‐Cyclodextrin‐Based Macromolecular [3]Rotaxane Using a Size‐Complementary Method

The challenging synthesis of an α‐cyclodextrin (CD)‐based macromolecular rotaxane with definite structure was fulfilled using a size‐complementary method. A new peracetylated (PAc) α‐CD‐based size‐complementary [3]rotaxane was prepared and its thermal dissociation kinetics studied. The de‐slippage mechanism was found to be different from that of the native α‐CD‐based system. PAcα‐CD‐based size‐complementary [3]rotaxanes were employed as initiators for a ring‐opening polymerization of ?‐caprolactone to obtain the macromolecular [3]rotaxanes. Detailed investigation of component dissociation showed the highly movable character of the wheel on the polymer main chain. A general method for controlling the movement of wheels in rotaxane frameworks, even in polymer systems, was established. This will enable the development of new supramolecular architectures and molecular machines.     

Translational isomerism in a [3]catenane and a [3]rotaxane

[structure: see text] Post-assembly covalent modification using Wittig chemistry of [2]rotaxane ylides, wherein NH(2)(+) centers in the dumbbell-shaped components are recognized by dibenzo[24]crown-8 (DB24C8) rings, has afforded a [3]catenane and a [3]rotaxane with a precise and synthetically prescribed shortage of DB24C8 rings. The nondegenerate pairs of translational isomers present in both of these interlocked molecular compounds provide the fundamental platform on which to construct sensory devices and nanochemomechanical systems.     

Host-[2]rotaxanes as cellular transport agents

Host-[2]rotaxanes, containing a diarginine-derivatized dibenzo-24-crown-8 (DB24C8) ether as the ring and a cyclophane pocket or an aromatic cleft as one blocking group, are cell transport agents. These hosts strongly associate with a variety of amino acids, dipeptides, and fluorophores in water (1 mM phosphate buffer, pH 7.0), DMSO, and a 75/25 (v/v) buffer to DMSO solution. All peptidic guests in all solvent systems have association constants (K(A)'s) in the range of 1 x 10(4) to 5 x 10(4) M(-)(1), whereas the K(A) range for the fluorophores is 1 x 10(4) to 9 x 10(5) M(-)(1). Association constants for the cyclophane itself, cyclophane 3, are smaller. These values are in the 1 x 10(3) to 5 x 10(3) M(-)(1) range, which shows that the rotaxane architecture is advantageous for guest binding. Cyclophane-[2]rotaxane 1 efficiently transports fluorescein and a fluorescein-protein kinase C (PKC) inhibitor into eukaryotic COS-7 cells, including the nucleus. Interestingly, cleft-[2]rotaxane 2 does not transport fluorescein as efficiently, even though the results from the fluorescence assays show that both [2]rotaxanes bind fluorescein with the same ability.     

Linear artificial molecular muscles

Two switchable, palindromically constituted bistable [3]rotaxanes have been designed and synthesized with a pair of mechanically mobile rings encircling a single dumbbell. These designs are reminiscent of a "molecular muscle" for the purposes of amplifying and harnessing molecular mechanical motions. The location of the two cyclobis(paraquat-p-phenylene) (CBPQT(4+)) rings can be controlled to be on either tetrathiafulvalene (TTF) or naphthalene (NP) stations, either chemically ((1)H NMR spectroscopy) or electrochemically (cyclic voltammetry), such that switching of inter-ring distances from 4.2 to 1.4 nm mimics the contraction and extension of skeletal muscle, albeit on a shorter length scale. Fast scan-rate cyclic voltammetry at low temperatures reveals stepwise oxidations and movements of one-half of the [3]rotaxane and then of the other, a process that appears to be concerted at room temperature. The active form of the bistable [3]rotaxane bears disulfide tethers attached covalently to both of the CBPQT(4+) ring components for the purpose of its self-assembly onto a gold surface. An array of flexible microcantilever beams, each coated on one side with a monolayer of 6 billion of the active bistable [3]rotaxane molecules, undergoes controllable and reversible bending up and down when it is exposed to the synchronous addition of aqueous chemical oxidants and reductants. The beam bending is correlated with flexing of the surface-bound molecular muscles, whereas a monolayer of the dumbbell alone is inactive under the same conditions. This observation supports the hypothesis that the cumulative nanoscale movements within surface-bound "molecular muscles" can be harnessed to perform larger-scale mechanical work.