Analogues of mechanical devices that operate on the molecular level, such as shuttles, brakes, ratchets, turnstiles and unidirectional spinning motors, are current targets of both synthetic chemistry and nanotechnology. These structures are designed to restrict the degrees of freedom of submolecular components such that they can only move with respect to each other in a predetermined manner, ideally under the influence of some external stimuli. Alternating-current (a.c.) electric fields are commonly used to probe electronic structure, but can also change the orientation of molecules (a phenomenon exploited in liquid crystal displays), or interact with large-scale molecular motions, such as the backbone fluctuations of semi-rigid polymers. Here we show that modest a.c. fields can be used to monitor and influence the relative motion within certain rotaxanes, molecules comprising a ring that rotates around a linear 'thread' carrying bulky 'stoppers' at each end. We observe strong birefringence at frequencies that correspond to the rate at which the molecular ring pirouettes about the thread, with the frequency of maximum birefringence, and by inference also the rate of ring pirouetting giving rise to it, changing as the electric field strength is varied. Computer simulations and nuclear magnetic resonance spectroscopy show the ring rotation to be the only dynamic process occurring on a timescale corresponding to the frequency of maximum birefringence, thus confirming that mechanical motion within the rotaxanes can be addressed, and to some extent controlled, by oscillating electric fields.

make the hydrophobic inner micronetwork water soluble. Experiments directed at the synthesis of hollow micronetworks, which exhibit two shells of different polarity, are currently in progress. ExperimentalThe micronetworks were prepared, by the base catalyzed procedure, as described previously [17±19]. For the preparation of the hollow spheres (sample HK70) the surfactant benzethonium chloride (3.0 g, Aldrich, M = 448 g mol ±1 ) was dissolved in water (125.0 g) containing NaOH (300 mg, 10 wt.-%). Dimethoxydimethylsilane (11.0 g) was added, with vigorous stirring, at room temperature over 1 h. After stirring for a further 12 h the silanol end groups of the PDMS chains were endcapped using methoxytrimethylsilane (100 mg).A mixture of methytrimethoxysilane (10.0 g) and dimethoxydimethylsilane (4.0 g) was added over 1 h and the resultant mixture stirred for a further 5 h. In order to prevent the remaining reactive silanol groups from undergoing interparticle condensation, the endcapping procedure was repeated twice more, with methoxytrimethylsilane (2.5 g), before the removal of the surfactant. The dispersion was destabilized by the addition of methanol and centrifuged. The precipitate was washed three times with methanol, to remove the surfactant, and then dissolved in toluene.At this stage the endcapping of the silanol groups is not yet quantitative. Therefore, hexamethyldisilazane (5.0 g, Wacker-Chemie GmbH, M = 164 g mol ±1 ) was added to the toluene solution and the reaction mixture was stirred, at room temperature, for 12 h. The micronetworks were again precipitated with methanol, centrifuged and dried. A colorless, sticky residue was obtained.In order to remove the linear PDMS chains from the core of the micronetworks the residue was dissolved in THF and ultrafiltered (Millipore cellulose 30 000) 4±5 times. The solvent was evaporated, the solid residue dissolved in benzene, and freeze-dried overnight. Yield: 3.5 g of a colorless powder. Sample VK70 was prepared as described previously [19].Static and dynamic light scattering measurements were performed with standard equipment utilizing an ALV SP-86 goniometer, an ALV 3000 correlator, and a Spectra Physics 2060-11s Krypton ion laser light source (647.1 nm wavelength, 500 mW power) or an ALV SP-125 goniometer, an ALV 5000 correlator, and a Spectra Physics 2060-04s Argon ion laser light source (514.0 nm, 250 mW). Correlation functions were analyzed by the method of cumulants.All samples described here exhibited normalized second cumulants of m 2 < 0.05, and no angular dependence of the diffusion coefficients was observed. Before conducting any measurements the toluene solution (0.2 g L ±1 < c < 1.5 g L ±1 ) was filtered through a 0.2 mm Millex FGN Teflon filter.The refractive index increments were measured with a special, homebuilt, interferometer utilizing a small laser (Polytec, wavelength 543 nm or uniphase, 632 nm) [20].Atomic force micrographs were recorded with a Nanoscope III instrument (Digital Instruments, St. Barbara, CA) operating in tapping mode at a ...