Effect of Component Mobility on the Properties of Macromolecular [2]Rotaxanes

Macromolecular [2]rotaxanes comprising a polymer axle and crown ether wheel were synthesized to evaluate the effect of component mobility on the properties of the axle polymer, especially its crystallinity. Living ring‐opening polymerization of δ‐valerolactone with a pseudorotaxane initiator with a hydroxy group at the axle terminus was followed by end‐capping with a bulky isocyanate. This yielded macromolecular [2]rotaxanes (M2Rs) possessing polyester axles of varying molecular weights. The crystallinity of the axle polymers of two series of M2Rs, with either fixed and movable components, was evaluated by differential scanning calorimetry. The results revealed that the effect of component mobility was significant in the fixed and movable M2Rs with a certain axle length, thus suggesting that the properties of the axle polymer depend on the mobility of the polyrotaxane components.     

Sequential O- and N-acylation protocol for high-yield preparation and modification of rotaxanes: synthesis, functionalization, structure, and intercomponent interaction of rotaxanes

A pseudorotaxane consisting of a 24-membered crown ether and secondary ammonium salt with the hydroxy group at the terminus was quantitatively acylated by bulky acid anhydride in the presence of tributylphosphane as catalyst to afford the corresponding rotaxane in high yield. Large-scale synthesis without chromatographic separation was easily achieved. The ammonium group in the resulting rotaxane was quantitatively acylated with excess electrophile in the presence of excess trialkylamine. Various N-functionalized rotaxanes were prepared by this sequential double-acylation protocol. 1H NMR spectra and X-ray crystallographic analyses of the rotaxanes showed that the crown ether component was captured on the ammonium group in ammonium-type rotaxane by strong hydrogen-bonding intercomponent interaction. The conformation around the ammonium group was fixed by the hydrogen-bonding interaction. Meanwhile, the conformation of the amide-type rotaxane was determined by the weak CH/pi interaction between the methylene group in crown ether and the benzene ring of the axle component. The N-acylation of ammonium-type rotaxane is useful for the preparation of both functionalized rotaxanes and weak intercomponent interaction-based rotaxanes.     

Photoswitchable rotaxanes on gold nanoparticles

We studied rotaxanes that consisted of a molecular axle, with a photoactive 9-Aryl-9-methoxy-acridane moiety at one end, and a tetracationic ring of cyclobis(paraquat-p-phenylene) (CBQT(4+)). The aim of the study was to deposit the axle ends onto gold nanoparticles (AuNPs). First, we introduced thioctic acid into the axle molecules. Then, rotaxanes were deposited on AuNPs by two methods: 1) Pseudorotaxanes were deposited on the gold surface by forming rotaxanes with the AuNP as a terminator to prevent unthreading of the ring structure; and 2) a chain containing the thioctic ester was introduced into a complete rotaxane, and then it was deposited on the AuNP with the aid of an exchange process. The photoheterolysis of the acridane unit resulted in formation of the corresponding acridinium methoxide; this, in turn, could thermally react to return to the acridane moiety. Due to the creation of a positive charge, the ring moved from the acridane station to a second, evasive station within the axle. This switching cycle could also take place when deposited on the gold surface. However, on the gold surface, the ring movement associated with the switching process was unidirectional.     

Which One is Bulkier: The 3,5‐Dimethylphenyl or the 2,6‐Dimethylphenyl Group? Development of Size‐Complementary Molecular and Macromolecular [2]Rotaxanes

We developed novel size‐complementary molecular and macromolecular rotaxanes using a 2,6‐dimethylphenyl terminal group as the axle‐end‐cap group in dibenzo‐24‐crown‐8‐ether (DB24C8)‐based rotaxanes, where the 2,6‐dimethylphenyl group was found to be less bulky than the 3,5‐dimethylphenyl group. A series of molecular and macromolecular [2]rotaxanes that bear a 2,6‐dimethylphenyl group as the axle‐end‐cap were synthesized using unsubstituted and fluorine‐substituted DB24C8. Base‐induced decomposition into their constituent components confirmed the occurrence of deslipping, which supports the size‐complementarity of these rotaxanes. The deslipping rate was independent of the axle length but dependent on the DB24C8 substituents. A kinetic study indicated the rate‐determining step was that in which the wheel is getting over the end‐cap group, and deslipping proceeded via a hopping‐over mechanism. Finally, the present deslipping behavior was applied to a stimulus‐degradable polymer as an example for the versatile utility of this concept in the context of stimulus‐responsive materials.     

Efficient End‐Capping Synthesis of Neutral Donor–Acceptor [2]Rotaxanes Under Additive‐Free and Mild Conditions

Efficient end‐capping synthesis of neutral donor–acceptor (D –A) [2]rotaxanes without loading any catalysts or activating agents was achieved by utilizing high reactivity of a pentacoordinated hydrosilane toward salicylic acid derivatives. As components of [2]rotaxanes, an electron‐deficient naphthalenediimide‐containing axle with a salicylic acid terminus and several electron‐rich bis(naphthocrown) ether macrocycles were employed. End‐capping reactions with the pentacoordinated hydrosilane underwent smoothly even at low temperature to afford the corresponding [2]rotaxanes in good yields. A [2]rotaxane containing bis‐1,5‐(dinaphtho)‐38‐crown‐10 ether as a wheel molecule was synthesized and isolated in 84 % yield by the end‐capping at ?10 °C, presenting the highest yield ever reported for the end‐capping synthesis of a neutral D –A [2]rotaxane. It was found that the yields of the [2]rotaxanes in the end‐capping reactions were almost parallel to the formation ratios of the corresponding pseudo[2]rotaxanes estimated by utilizing model systems. These results indicate that the end‐capping reaction using the pentacoordinated hydrosilane proceeded without perturbing the threading process, and most of the pseudo[2]rotaxanes underwent efficient end‐capping reaction even at low temperature.     

Light-harvesting in multichromophoric rotaxanes

Two rotaxanes with benzyl ether axles and tetralactam wheels were synthesized through an anion template effect. They carry naphthalene chromophores attached to the stopper groups and a pyrene chromophore attached to the wheel. The difference between the two rotaxanes is represented by the connecting unit of the naphthyl chromophore to the rotaxane axle: a triazole or an alkynyl group. Both rotaxanes exhibit excellent light-harvesting properties: excitation of the naphthalene chromophores is followed by energy transfer to the pyrene unit with efficiency higher than 90% in both cases. This represents an example of light-harvesting function among chromophores belonging to mechanically interlocked components, that is, the axle and the wheel of the rotaxanes.     

2]Rotaxanes containing pyridinium-phosphonium axles and 24-crown-8 ether wheels

A triethylphosphonium group attached to a pyridinium ethane moiety can be used as an axle for the self-assembly of [2]pseudorotaxanes and [2]rotaxanes. Although [2]pseudorotaxane formation is limited due to the bulk of the PR4+ group, [2]rotaxanes can be formed utilising 24-crown-8 ether, benzo-24-crown-8 ether and naphtho-24-crown-8 ether. The synthesis of these [2]rotaxanes and the X-ray structure of the [2]rotaxane containing a 24-crown-8 ether wheel are described. When the crown ether contains an aromatic group two possible conformational isomers exist; these are identified at low temperature by 1H and 31P NMR spectroscopy.     

Synthesis of [3]Rotaxanes that Utilize the Catalytic Activity of a Macrocyclic Phenanthroline–Cu Complex: Remarkable Effect of the Length of the Axle Precursor

[3]Rotaxanes, which consist of one macrocyclic phenanthroline compound and two axle components, were prepared by the oxidative dimerization of an alkyne compound with bulky tris[4′‐cyclohexyl‐(1,1′‐biphenyl)‐4‐yl]methyl blocking group. The catalytic activity of a macrocyclic phenanthroline–Cu complex was utilized to thread the two axle components inside the ring. The alkyne compound with chain of 15 or 20 methylene groups gave [3]rotaxanes in high yields, whereas the axle with a chain of six methylene groups afforded a [3]rotaxane in very poor yield. We also examined the effect of the ring size on the synthesis of [3]rotaxanes. [3]Rotaxanes were not isolated when a macrocyclic phenanthroline compound with a smaller ring size was used.     

Unveiling the Hidden Movements in the Shuttling of Rotaxanes

Movements in molecular machines are usually diverse and coupled, but some of them are often implicit and hard to be observed in experiments. In the present work, the two-or three-dimensional free-energy landscapes characterizing the coupled shuttling and other movements of a series of pH-triggered rotaxanes composed of a crown ether and an H-shaped axle with distinct number of phenyl rings(n=1-3) have been explored. The results show that although the calculated free-energy barriers against shuttling in the rotaxanes(n=2 and 3) change slightly, the movements coupled with the shuttling vary significantly with the axle length. At high pH, the shuttling in the rotaxane of n=2 is coupled with the isomerization of the wheel, while the shuttling in the one of n=3 is accompanied by both the isomerization and the rotation of the macrocycle. In addition, the crown ether underwent greater conformational change during shuttling at low pH compared to that at high pH. These results indicate that disentangling the coupled movements is important to reveal the underlying molecular mechanism of the shuttling.     

Increased Halide Recognition Strength by Enhanced Intercomponent Preorganisation in Triazolium Containing [2]Rotaxanes

Three triazolium‐based [2]rotaxanes containing different sized axle and macrocycle components were synthesised in good yields (40–57 %) through chloride anion templation. The anion recognition properties of the interlocked receptor systems were investigated using ¹H NMR titration experiments: all three rotaxanes display impressive selectivities for halide anions over the more basic oxoanion acetate. The rotaxanes incorporating shorter, more rigid axle components with aryl‐substituted triazolium groups display substantially higher anion binding affinities than those with longer, bis‐alkyl‐substituted heterocycles, which is attributed to the increased intercomponent preorganisation afforded by the smaller axle component. Computational DFT and molecular dynamics simulations composed of unconstrained and umbrella sampling simulations corroborate the experimental observations.     

Molecular shuttles based on tetrathiafulvalene units and 1,5-dioxynaphthalene ring systems

Six different degenerate [2]rotaxanes were synthesized and characterized. The rotaxanes contained either two tetrathiafulvalene (TTF) units or two 1,5-dioxynaphthalene (DNP) ring systems, both of which serve as recognition sites for a cyclobis(paraquat-p-phenylene) (CBPQT4+) ring. Three different spacer units were incorporated into the dumbbell components of the [2]rotaxanes between the recognition sites. They include a polyether chain, a terphenyl unit, and a diphenyl ether linker, all of which were investigated in order to probe the effect of the spacers on the rate of the shuttling process. Data from dynamic 1H NMR spectroscopy revealed a relatively small difference in the DeltaG++ values for the shuttling process in the [2]rotaxanes containing the three different spacers, in contrast to a large difference between the TTF-containing rotaxanes (18 kcal mol(-1)) and the DNP-containing rotaxanes (15 kcal mol(-1)). This 3 kcal mol(-1) difference is predominantly a result of a ground-state effect, reflecting the much stronger binding of TTF units to the CBPQT4+ ring in comparison with DNP ring systems. An examination of the enthalpic (DeltaH++) and entropic (DeltaS++) components for the shuttling process in the DNP-containing rotaxanes revealed significant differences between the three spacers, a property which could be important in designing new molecules for incorporation into molecular electronic and nanoelectromechanical (NEMs) devices.     

Separated and aligned molecular fibres in solid state self-assemblies of cyclodextrin [2]rotaxanes

The conformations of two [2]rotaxanes, each comprising alpha-cyclodextrin as the rotor, a stilbene as the axle and 2,4,6-trinitrophenyl substituents as the capping groups, have been examined in solution and in the solid state, using (1)H NMR spectroscopy and X-ray crystallography, respectively. In solution, introducing substituents onto the stilbene prevents the cyclodextrin from being localized over one end of the axle. Instead the cyclodextrin moves back and forth along the substituted stilbene. In the solid state, the axles of the rotaxanes form extended molecular fibres that are separated from each other and aligned along a single axis. The molecular fibres are strikingly similar to those formed by the axle component of one of the rotaxanes in the absence of the cyclodextrin, but in the latter case they are neither separated nor all aligned.     

Synthesis and photoinduced electron transfer processes of rotaxanes bearing [60]fullerene and zinc porphyrin: effects of interlocked structure and length of axle with porphyrins

Three rotaxanes, with axles with two zinc porphyrins (ZnPs) at both ends penetrating into a necklace pending a C60 moiety, were synthesized with varying interlocked structures and axle lengths. The intra-rotaxane photoinduced electron transfer processes between the spatially positioned C60 and ZnP in rotaxanes were investigated. Charge-separated (CS) states (ZnP*+, C60*-)rotaxane are formed via the excited singlet state of ZnP (1ZnP*) to the C60 moiety in solvents such as benzonitrile, THF, and toluene. The rate constants and quantum yields of charge separation via 1ZnP decrease with axle length, but they are insensitive to solvent polarity. When the axle becomes long, charge separation takes place via the excited triplet state of ZnP (3ZnP*). The lifetime of the CS state increases with axle length from 180 to 650 ns at room temperature. The small activation energies of charge recombination were evaluated by temperature dependence of electron-transfer rate constants, probably reflecting through-space electron transfer in the rotaxane structures.     

Structural Analysis and Inclusion Mechanism of Native and Permethylated α‐Cyclodextrin‐Based Rotaxanes Containing Alkylene Axles

Native α‐cyclodextrin‐ (α‐CD) and permethylated α‐CD (PMeCD)‐based rotaxanes with various short alkylene chains as axles can be synthesized through a urea end‐capping method. Native α‐CD tends to form [3]‐ or [5]pseudorotaxanes and not [2]‐ or [4]pseudorotaxanes, which indicates that the coupled CDs act as a single fragment. End‐capping reactions of the pseudorotaxanes with C18 and C24 axle lengths do not occur because the axle termini are covered by the densely stacked CDs. The number of PMeCDs on the pseudorotaxane is flexible and mainly depends on the axle length. Peracetylated α‐CD (PAcCD)‐based rotaxanes are synthesized through O‐acetylation of the α‐CD‐based rotaxanes without any decomposition of the rotaxanated structures. The structures of PMeCD‐based [3]‐ and [4]rotaxanes, and the molecular dynamics calculations on [3]pseudorotaxanes, indicate that the tail face of PMeCDs is regularly directed toward the axle termini. On the basis of the results obtained, it can be concluded that the directions and numbers of CDs in rotaxanes containing short alkylene chains depend on 1) the interactions between CDs, 2) the length of the alkylene axle, and 3) the interactions between the axle end and tail face of the CD.     

Quantitative conformational study of redox-active [2]rotaxanes, part 2: Switching in flexible and rigid bistable [2]rotaxanes

Translational movement of the macrocycle in two structurally similar bistable [2]rotaxanes, which is induced by a four-step electrochemical process in solution, has been investigated by using a methodology developed in the preceding article (Chem. Eur. J. 2008, 14, 1107-1116). Both [2]rotaxanes contain a crown ether that can be accommodated by either of two interconnected viologen recognition sites. These sites are substantially different in terms of their affinity towards the crown ether and they possess considerably different electrochemical reduction potentials. The two [2]rotaxanes differ in the length and the rigidity of a bridge that links these sites. A combination of molecular mechanics modelling and NOE spectroscopy data provides information about the conformations of both [2]rotaxanes in the parent oxidation state when the crown ether exclusively populates the strong recognition site. To determine the population of the recognition sites at subsequent stages of reduction, a paramagnetic NMR technique and cyclic voltammetry were used. The key finding is that the flexibility of the connecting bridge element between the recognition sites interferes with shuttling of the crown ether in [2]rotaxanes. It can be demonstrated that the more flexible trimethylene bridge is folded, thus limiting the propensity of the crown ether to shuttle. Consequently, the crown ether populates the original site even in the second reduced state of the flexible [2]rotaxane. On the contrary, in the [2]rotaxane in which two viologen sites are connected by a larger and more rigid p-terphenylene bridge, the predominant location of the crown ether at the weak recognition site is achieved after just one single electron reduction.     

Branched [n]rotaxanes (n = 2-4) from multiple dibenzo-24-crown-8 ether wheels and 1,2-bis(4,4'-dipyridinium)ethane axles

To investigate the possibility of incorporating the 1,2-bis(pyridinium)ethane[subset or is implied by]24C8 [2]pseudorotaxane motif into dendrimer like macromolecules, a series of branched [n]rotaxanes were prepared employing multiple dibenzo-24-membered crown ether wheels with various aromatic core structures and the 1,2-bis(4,4'-dipyridinium)ethane axle. Yields of branched [2]-, [3]- and [4]rotaxanes were dependent on the size of the core and the relative proximity of the crown ethers arranged around the core unit.     

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.     

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.     

A self-threaded "molecular 8"

A new type of [1]rotaxanes containing two aliphatic bridges between axle and wheel is obtained in 39% yield in a one-step synthesis starting from a [2]rotaxane which contained one sulfonamide group each in both the wheel and the axle. Temperature controlled chemoselective substitution reactions first at these sulfonamide nitrogens and then subsequently at the various other carboxamide nitrogens in the wheel and axle give rise to the formation of an isomeric mixture of three double-bridged [1]rotaxanes which could be separated by HPLC. Structure determination of the main product 3a was possible by NMR experiments supported by molecular modeling calculations. Using different reaction conditions, a double-substituted but not yet bridged [2]rotaxane 4 could be isolated as an intermediate giving further evidence for the assigned structure of 3a and the way of its formation. The shape of this double-bridged [1]rotaxane 3a reminds of a self-intertwining chiral "molecular 8", in which any possible racemization due to deslipping is hindered by the two stoppers originating from the former rotaxane axle. Hence, to the best of our knowledge this is the first example of a molecule in which both concepts, cycloenantiomerism and helical chirality, are realised in one structure. Enantiomer separation of the main product was possible by further HPLC using chiral stationary phases. The Cotton effects of the circular dichrograms are different to those of the already synthesized [1]rotaxanes bearing just one aliphatic bridge between axle and wheel.     

Organometallic Rotaxanes with a Triazole Group in the Axle Component and Their Behavior as Ligands of PtII Complexes

Two ferrocenylmethyl ammonium salts were used as axle components of pseudorotaxanes with dibenzo[24]crown‐8. The pseudorotaxane with an alkyne terminal group in the axle component underwent a Cu‐catalyzed Huisgen coupling reaction (click reaction) with an alkyl azide to afford cationic [2]rotaxanes with a triazole group in the axle molecule. The rotaxane reacted with Ac₂O to produce neutral rotaxanes with an amide group in the axle component. Both cationic and neutral rotaxanes were treated with K[PtCl₃(CH₂?CH₂)] to form the Pt^II‐containing rotaxanes.