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 surfacebound 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.

We present a direct measurement of self-diffusion of a single-component glass-forming liquid at the glass transition temperature. Forward recoil spectrometry is used to measure the concentration profiles of deuterio and protio 1,3-bis-(1-naphthyl)-5-(2-naphthyl)benzene (TNB) following annealing-induced diffusion in a vapor-deposited bilayer. These experiments extend the range of measured diffusion coefficients in TNB by 6 orders of magnitude. The results indicate a decoupling of translational diffusion coefficients from viscosity or rotation. At T(g), D(T) is 400 times larger than expected from the Stokes-Einstein equation.

An array of microcantilever beams, coated with a self-assembled monolayer of bistable, redox-controllable [3]rotaxane molecules, undergoes controllable and reversible bending when it is exposed to chemical oxidants and reductants. Conversely, beams that are coated with a redox-active but mechanically inert control compound do not display the same bending. A series of control experiments and rational assessments preclude the influence of heat, photothermal effects, and pH variation as potential mechanisms of beam bending. Along with a simple calculation from a force balance diagram, these observations support the hypothesis that the cumulative nanoscale movements within surface-bound “molecular muscles” can be harnessed to perform larger-scale mechanical work.

Irradiation (λ > 300 nm) of 1-naphthyldiazomethane (11) or 1-naphthyldiazirine (12) isolated in an argon matrix at 10 K produces 4,5-benzobicyclo[4.

Through its exploitation of noncovalent bonding interactions and self-assembly processes, [1] supramolecular assistance to covalent synthesis [2] has established itself as an efficient means of creating molecules with nanoscale dimensions. For two decades, researchers have harnessed the power of postassembly covalent modification [2] to produce an array of mechanically interlocked molecular compounds, [3] some of which have been shown to behave as molecular machines [4] and switches [5] on surfaces and at interfaces, respectively. We have developed a template-directed [6] protocol for the construction of [2]catenanes [7] composed of a crown ether containing p-electron-rich aromatic ring systems and a tetracationic cyclophane comprised of two p-electron-deficient bipyridinium units. The kinetically controlled protocol relies on employing the crown ether as the template, around which the cyclophane is formed [8] from reaction of a dicationic salt with para-xylylene dibromide. If the crown ether is covalently tethered to this second molecule, then the resulting cyclization(s) could occur either intramolecularly and generate a pretzelane [9] or intermolecularly and generate cyclic or linear oligo/polycatenanes (Figure 1).Herein, we report the synthesis of two para-xylylene dibromide derivatives, which have the same crown ether component [10] tethered by different linkers, and describe the outcome of their reactions with the dicationic salt. It transpires that, when the dibromide contains a longer-and more flexible-linker, a pretzelane is obtained in good yields, as suggested by (dynamic) 1 H NMR spectroscopic analyses in solution and confirmed by X-ray crystallographic studies in the solid state. By contrast, when the dibromide contains a shorter-and less flexible-linker, a cyclic bis[2]catenane [11] is obtained as the major product, along with lesser amounts of a pretzelane.The synthesis of the pretzelanes 7·4 PF 6 and 10·4 PF 6 and the cyclic bis[2]catenane 11·8 PF 6 are outlined in Schemes 1 and 2. Reaction of 1, [12] which contains a symmetrically positioned carboxyl group, with an excess of 2 gave the alcohol 3; subsequent esterification of this alcohol with another carboxylic acid derivative 4[13] afforded the dibromide Communications