Towards Synthetic Molecular Muscles: Contraction and Stretching of a Linear Rotaxane Dimer We are grateful to M. Jean-Daniel Sauer and Dr. Roland Graff for high-field NMR experiments, Hélène Nierengarten and Raymond Hubert for mass determinations. M.C.J. thanks the European Commission for a Grant (TMR Contract No. ERBFMBICT972547). We also thank the CNRS for financial support (Programme Physique et Chimie du Vivant)

Synthesis of a Bis-macrocycle Containing Two Back-to-Back Rigidly Connected 1,10-Phenanthroline Units as a Central Core and its Incorporation in a Handcuff-Like Catenane

A bis-macrocycle containing two back-to-back connected 1,10-phenanthroline chelates has been prepared. The synthetic strategy involves the preparation of a monocyclic precursor consisting of a 1,10-phenanthroline-5,6-dione fragment incorporated in a 30-membered ring. This important intermediate has been prepared via two distinct routes, both strategies relying on the use of a ketal as a 1,2-dione protective group. A four-component condensation reaction between two molecules of the macrocyclic dione and two equivalents of ammonia (used in large excess) in the presence of a reducing agent (Na(2)S(2)O(4)) leads to the desired bis-ring in good yield. The most direct synthetic route allows preparation of the bis-macrocycle in seven steps from 1,10-phenanthroline in an overall yield of 14 %. Using the now well-established "gathering and threading" effect of copper(I), a doubly threaded species could be obtained in quantitative yield, in which each ring of the bis-macrocycle is threaded by a "molecular string". These fragments bear terminal allylic groups, which are used to prepare the final catenane by performing a double ring-closing metathesis reaction. This final cyclisation reaction is high yielding and affords the desired catenane consisting of a bis-macrocycle of which the two cyclic units are threaded by a large ring. The compound has been fully characterised by classical techniques. Electronic spectroscopy and electrochemical measurements suggest that the two copper complex subunits do not interact electronically, in spite of the aromatic nature of the bridging ligand between the two metal centres.

Multirotations of (Anilinium)([18]Crown-6) Supramolecular Cation Structure in Magnetic Salt of [Ni(dmit)2]−

A solid-state dynamic supramolecular structure consisting of (anilinium)([18]crown-6) was arranged as the cation in a salt of [Ni(dmit)2]- (dmit = 2-thioxo-1,3-dithiole-4,5-dithiolate). With the ammonium moiety of anilinium located within the cavity of [18]crown-6, a hydrogen-bonded supramolecular structure is formed, with an orthogonal arrangement between the pi plane of anilinium and the mean O6 plane of [18]crown-6. In this supramolecular cation, both anilinium and [18]crown-6 act as dynamic units with different rotational modes in the solid state. The uniform stacks of cations form an antiparallel arrangement, thus producing a layer structure. Sufficient space for the 180 degree flip-flop motion of the phenyl ring and the rotation of [18]crown-6 was observed in the cation layer. Thermally activated 180 degree flip-flop motions, with a frequency of 6 MHz at room temperature and an activation energy of 31 kJ mol(-1), were confirmed by temperature-dependent 2H NMR spectra of ([D5]anilinium)-([18]crown-6)[Ni(dmit)2]. A double-minimum potential for the molecular rotation of anilinium, with a barrier of approximately 40 kJ mol(-1), was indicated by ab initio calculations. The wide-line 1H NMR spectra indicated a thermally activated rotation of [18]crown-6 at temperatures above 250 K. Therefore, multiple molecular motions of the 180 degree flip-flop motion of the phenyl ring and the rotation of [18]crown-6 occur simultaneously in the solid state. The temperature-dependent dielectric constants revealed that the molecular motion of [18]crown-6, other than the flip-flop motion, dominates the dielectric response in the measured temperature and frequency range.

Polyether macrocycles from intramolecular cyclopropanation and ylide formation. Effect of catalyst and coordination

The use of catalytic metal carbene methodology with diazoacetates for the construction in high yield of polyether macrocycles having ring sizes greater than 25 has been achieved by preventing access to gamma-C-H positions for intramolecular insertion. Cyclopropanation is the exclusive outcome of reactions performed with dirhodium(II) catalysts, and product yields of greater than 70% are obtained without resorting to high dilution with solvents. With copper(I) catalysts having multiple sites for polyether coordination, intramolecular oxonium ylide formation occurs at the terminal oxygen, followed by [2,3]-sigmatropic rearrangement of the pendant allyl group, in competition with cyclopropanation. Sodium ion coordination with the reactant diazo compound inhibits oxonium ylide formation in copper-catalyzed reactions. The composite results are consistent with copper serving as a template for the substrate as well as the site in the ether complex for diazo decomposition and subsequent metal carbene reactions.

Formation of RACK- and grid-type metallosupramolecular architectures and generation of molecular motion by reversible uncoiling of helical ligand strands

The interaction of appropriate metal ions (Pb(II), Zn(II)) with helical ligand strands, obtained by hydrazone polycondensation, generates polymetallic supramolecular architectures of rack and grid types, by uncoiling of the ligand. The interconversion between the helical free ligand and the linearly extended ligand in the complexes produces reversible ion-induced, nanomechanical molecular motions of large amplitude. It has been integrated in an acid-base neutralisation fuelled process, which links the extension/contraction of the ligand strands to alternating changes in pH.

Hydrogen and copper ion-induced molecular reorganizations in scorpionand-like ligands. A potentiometric, mechanistic, and solid-state study

Two aza scorpionand-like macrocycles (L2 and L3) have been prepared. L2 consists of a tren amine with two of its arms cyclizized with a 2,6-bis(bromomethyl)pyridine. In L3, the remaining pendant arm has been further functionalized with a fluorophoric naphthalene group. X-ray data on the compounds [H(L3)]ClO4.H2O (1) and [H3(L3)](H2PO4)3.H2O (2) as well as solution studies (pH-metry, UV-vis, and fluorescence data) show the movement of the pendant arm as a result of the protonation degree of the macrocycles and of the formation of intramolecular hydrogen bonds. X-ray data on the complexes [Cu(L2)](ClO4)2]2.H2O (3) and [Cu(L3)](ClO4)2 (4) and solution studies on Cu2+ coordination show the implication of the nitrogen of the arm in the binding to the metal ion. Kinetic studies on the decomposition and formation of the Cu2+ complexes provide additional information about the pH-dependent molecular reorganizations. Moreover, the obtained information suggests that the kinetics of the tail on/off process is essentially independent of the lability of the metal center.

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 pi-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 pi-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.

A Molecular Plug−Socket Connector

A monocationic plug-socket connector that is composed, at the molecular level, of three components, (1) a secondary dialkylammonium center (CH2NH2+CH2), which can play the role of a plug toward dibenzo[24]crown-8 (DB24C8), (2) a rigid and conducting biphenyl spacer, and (3) 1,4-benzo-1,5-naphtho[36]crown-10 (BN36C10), capable of playing the role of a socket toward a 4,4'-bipyridinium dicationic plug, was synthesized and displays the ability to act as a plug-socket connector. The fluorescent signal changes associated with the 1,5-dioxynaphthalene unit of its BN36C10 portion were monitored to investigate the association of this plug-socket connector with the complementary socket and plug compounds. The results indicate that (1) the CH2NH2+CH2 part of the molecular connector can thread DB24C8 in a trivial manner and (2) the BN36C10 ring of the connector can be threaded by a 1,1'-dioctyl-4,4'-bipyridinium ion only after the CH2NH2+CH2 site is occupied by a DB24C8 ring. The two connections of the three-component assembly are shown to be controlled reversibly by acid/base and red/ox external inputs, respectively. The results obtained represent a key step for the design and construction of a self-assembling supramolecular system in which the molecular electron source can be connected to the molecular electron drain by a molecular elongation cable.

Synthetic molecular motors and mechanical machines

The widespread use of controlled molecular-level motion in key natural processes suggests that great rewards could come from bridging the gap between the present generation of synthetic molecular systems, which by and large rely upon electronic and chemical effects to carry out their functions, and the machines of the macroscopic world, which utilize the synchronized movements of smaller parts to perform specific tasks. This is a scientific area of great contemporary interest and extraordinary recent growth, yet the notion of molecular-level machines dates back to a time when the ideas surrounding the statistical nature of matter and the laws of thermodynamics were first being formulated. Here we outline the exciting successes in taming molecular-level movement thus far, the underlying principles that all experimental designs must follow, and the early progress made towards utilizing synthetic molecular structures to perform tasks using mechanical motion. We also highlight some of the issues and challenges that still need to be overcome.

Reversible operation of chiral molecular scissors by redox and UV light

Upon changing the oxidation state, a reversible open-close motion of chiral molecular scissors, composed of a redox-active ferrocene pivot and an isomerizable azobenzene strap, can be realized only by UV light.

Chemically controlled self-assembly of [2]pseudorotaxanes based on 1,2-bis(benzimidazolium)ethane cations and 24-crown-8 macrocycles

Cations derived from 1,2-bis(benzimidazolium)ethane can penetrate the cavity of dibenzo-24-crown-8 macrocycles to produce a new family of [2]pseudorotaxanes. These supramolecular structures are held together by a series of charge-assisted hydrogen bonds (+)N-H[dot dot dot]O, ion-dipole and pi-stacking interactions. These new adducts were fully characterised by NMR spectroscopy, ESI mass spectrometry and single-crystal X-ray diffraction. The effect of electron-donating and electron-withdrawing groups on the association constants was also analyzed. Chemical control of the threading/unthreading process was acheived by the alternate addition of acid and base.

Photoresponsive double-stranded helices composed of complementary strands

A photoresponsive single-handed double helical supramolecule composed of complementary strands bearing azobenzene moieties underwent a reversible trans-cis-isomerization regulated by photoirradiation, resulting in a change in its molecular length.

Transition metal complexes as molecular machine prototypes

The field of molecular machines, i.e. multicomponent systems able to undergo large amplitude motions under the action of an external signal, has experienced a spectacular development since the beginning of the 1990s. Transition metal complexes have played an important role in this context, often as components of catenanes and rotaxanes. The present tutorial review will discuss a few systems of this type, taken from the contributions of our group or from others. The stimulus responsible for the controlled motion of the machine can be chemical, electrochemical, or photochemical. Examples of these three categories will be considered.

Adsorption of Fumaramide [2]Rotaxane and Its Components on a Solid Substrate: A Coverage-Dependent Study

The coverage-dependent adsorption on Au(111) of a fumaramide [2]rotaxane and its components, a benzylic amide macrocycle and a fumaramide thread, is studied using high-resolution electron energy loss spectroscopy (HREELS). Up to monolayer coverage, the relative intensity of out-of-plane to in-plane phenyl ring vibrational modes indicates that the macrocycle adopts an orientation with the phenyl rings largely parallel to the surface. The formation of a chemisorption bond is evidenced by the presence of a Au-O stretching vibration. In contrast, the thread shows no evidence of chemisorption or a preferential orientation. The introduction of the thread into the macrocycle partly disrupts the film order so that the resulting chemisorbed rotaxane shows intermediate behavior with a preferential orientation up to 0.5 ML coverage. A decrease in film order and the absence of a preferred molecular orientation is observed for all three molecules at multilayer coverages. The spectral differences are addressed by molecular dynamics simulations in terms of the mobility of the phenyls of the three molecules on Au(111).

Microenvironment Control of Methyl Rotation Induced by Proton Transfer

Methyl rotation induced by proton transfer was found for cis-N-methylacetamide (NMA). More interestingly, it was found that the microenvironment could control the methyl rotation. The atom-centered density matrix propagation (ADMP) method, a recently developed ab initio molecular dynamics, was further carried out to depict the trajectories for methyl rotation of NMA. Moreover, trajectories for methyl rotation of NMA complexed with water molecules were also calculated, and water molecules at the two different sites of NMA were found to reverse or cease the rotational direction of the methyl groups of NMA. This finding that microenvironment can not only control rotational direction of methyl groups but can also cease the rotation may be of significant importance for the control of molecular machines.

Synthesis and Photochemistry of a Two-Position Ru(terpy)(phen)(L)2+ Scorpionate Complex

A dissymmetric 1,10-phenanthroline chelate (N-phen-S) bearing two polyether chains terminated by two monodentate ligands of the benzonitrile (N) and dialkylesulfoxide (S) types was synthesized, characterized, and coordinated to ruthenium. The corresponding Ru(terpy)(N-phen-S)2+ complexes (terpy = 4'-(3,5-ditertiobutylphenyl)-2,2';6',2' '-terpyridine) were fully characterized as being two coordination isomers of the scorpionate type with one of the two tails occupying the sixth position on the coordination sphere. Photoexpulsion of the coordinated tail led to opening of the ruthena-macrocycle and subsequent rearrangement of the bidentate chelate. This rearrangement consisted of a 90 degrees rotation of the phenanthroline around the ruthenium atom. Selective irradiation of one isomer in a mixture of the two was undertaken using band-pass filters; this resulted in an enrichment of the nonirradiated isomer in the mixture. Thermal back-coordination of the tail was investigated in the dark. It took place quantitatively from the corresponding ruthenium chloride complex by trapping of the anion with silver salts.

Fine Tuning of the Rotary Motion by Structural Modification in Light-Driven Unidirectional Molecular Motors

The introduction of bulky substituents at the stereogenic center of light-driven second-generation molecular motors results in an acceleration of the speed of rotation. This is due to a more strained structure with elongated C=C bonds and a higher energy level of the ground state relative to the transition state for the rate-limiting thermal isomerization step. Understanding the profound influence that variation of the substituent at the stereogenic center holds over the rotational speed of the light-driven molecular motor has enabled the development of the fastest molecular motor reported thus far.

Linear oligomers composed of a photochromically contractible and extendable Janus [2]rotaxane

The synthesis and characterization of N,N'-p-xylylene-linked oligo-Janus [2]rotaxanes based on a permethylated alpha-cyclodextrin and their contractible and extendable nature coupled with photochromism are described.

A Comparison of Shuttling Mechanisms in Two Constitutionally Isomeric Bistable Rotaxane-Based Sunlight-Powered Nanomotors

To find out how best to optimize shuttling of the macrocycle in a particular class of photochemically driven molecular abacus, which has the molecular structure of BR-I6+ in its Mark I prototype (Ashton et al., Chem. Eur. J. 2000, 6, 3558), we have synthesized and characterized a Mark II version of this kind of two-station rotaxane comprised of six molecular modules, namely (a) a bisparaphenylene[34]crown-10 electron donor macrocycle M and its dumbbell-shaped component which contains (b) a Ru(ii)-polypyridine photoactive unit P2+ as one of its stoppers, (c) a p-terphenyl-type ring system as a rigid spacer S, (d) 4,4′-bipyridinium (A12+) and (e) 3,3′-dimethyl-4,4′-bipyridinium (A22+) electron acceptor units that can play the role of stations for the macrocycle M, and (f) a tetraarylmethane group T as the second stopper. This Mark II version is identical with BR-I6+ in the Mark I series that works as a sunlight-powered nanomotor (Balzani et al., Proc. Natl. Acad. Sci. USA 2006, 103, 1178), except for the swapping of the two stations A12+ and A22+ along the dumbbell-shaped component, i.e. the Mark I and II bistable rotaxanes are constitutionally isomeric. We have found the closer the juxtaposition of the electron transfer photosensitizer P2+ to the better (A12+) of the two electron acceptors, namely the situation in BR-II6+ compared with that in BR-I6+ results in an increase in the rate — and hence the efficiency — of the photoinduced electron-transfer step. The rate of the back electron transfer, however, also increases. As a consequence, BR-II6+ performs better than BR-I6+ in the fuel-assisted system, but much worse when it is powered by visible light (e.g. sunlight) alone. By contrast, when shuttling is electrochemically driven, the only difference between the two bistable rotaxanes in the Mark I and Mark II series is that the macrocycle M moves in opposite directions.

Topologically novel multiple rotaxanes and catenanes based on tetraurea calix[4]arenes

Calix[4]arenes bearing at their wide rim four urea residues easily form hydrogen bonded dimeric capsules. This has been used to preorganise alkenyl functions attached to these urea groups for their controlled connection via metathesis reaction. Multimacrocyclic tetraurea derivatives are thus obtained in excellent yields via heterodimers which are formed exclusively with tetratosylurea derivatives. Heterodimerisation of such bis- and tetraloop tetraureas leads analogously to multicatenanes, or to rotaxanes by stoppering. Huge macrocycles are detached from tetraloop derivatives by cleavage of the urea function.

Artificial Molecular Motors Powered by Light

The bottom-up construction and operation of machines and motors of molecular size is a topic of great interest in nanoscience, and a fascinating challenge of nanotechnology. The problem of the energy supply to make molecular motors work is of the greatest importance. Research in the last ten years has demonstrated that light energy can indeed be used to power artificial nanomotors by exploiting photochemical processes in appropriately designed systems. More recently, it has become clear that under many aspects light is the best choice to power molecular motors; for example, systems that show autonomous operation and do not generate waste products can be obtained. This review is intended to discuss the design principles at the basis of light-driven artificial nanomotors, and provide an up-to-date overview on the prototype systems that have been developed.

A Fast-Moving [2]Rotaxane Whose Stoppers Are Remote from the Copper Complex Core

[reaction: see text] A new copper-complexed rotaxane is described. It consists of a two-coordination site ring threaded by a sterically non-hindering 2,2'-bipyridine derivative. An electrochemical signal (oxidation or reduction of the copper center, Cu(I) or Cu(II)) induces rearrangement of the system. By using long and flexible linkers between the stoppers and the central complex, ligand exchange is fast, which leads to short response times (on the millisecond time scale and even below).

Controlling the speed of rotation in molecular motors. Dramatic acceleration of the rotary motion by structural modification

Substitution of a 6-membered by a 5-membered ring upper half in the light driven second generation molecular motors resulted in a dramatic increase of the speed of rotation.

Temperature-Dependent and Friction-Controlled Electrochemically Induced Shuttling Along Molecular Strings Associated with Electrodes

The temperature and solvent composition dependence of the electrochemically stimulated rate of shuttling of the redox-active cyclophane, cyclobis(paraquat-p-phenylene), on a molecular string has been studied. The molecular string includes a pi-donor diiminebenzene-site that is associated on one side with an electrode, and stoppered on the other side with an adamantane unit. The cyclophane rests on the pi-donor site, owing to stabilizing pi-donor-acceptor interactions. Electrochemical reduction of the cyclophane units, to the bis-radical cation cyclophane, results in the shuttling of the reduced cyclophane towards the electrode, a process that is driven by the removal of the stabilizing donor-acceptor interactions, and the electrostatic attraction of the reduced product by the electrode. The latter process is energetically downhill, and is temperature-independent. Upon oxidation of the reduced cyclophane that is associated with the electrode, the energetically uphill shuttling of the oxidized cyclophane to the pi-donor site proceeds. The rate of this translocation process has been found to be temperature-dependent, and controlled by the solvent composition. The experimental results have been theoretically analyzed in terms of Kramers' molecular friction model. The theoretical fitting of the experimental results, using solutions of variable composition, reveals that the rate-constants for the uphill reaction in a pure aqueous solution follow the temperature-dependence of the viscosity of water. The results demonstrate the significance of friction phenomena in shuttling processes within molecular machines.

9-(Cycloheptatrienylidene)-fluorene Derivative: Remarkable Ratiometric pH Sensor and Computing Switch with NOR Logic Gate

2,7-Bis(4-dimethylaminophenyl)-9-(cycloheptatrienylidene)fluorene (1) was synthesized and characterized. 1 could be used as a fluorescent sensor either for pH or for pyridinium halide. Integrated with the ratiometric method and an NOR logic gate, a tunable two-input/multi-output system was presented on the basis of this single molecule. [structure: see text]