Rotaxane

Abstract: Cyclodextrins have been used as a cyclic component in the construction of supramolecular architectures. Recently they have been studied as a component in the construction of rotaxanes and catenanes. A cyclodextrin ring can translocate in some rotaxane and catenane structures. Therefore, much attention has been given to cyclodextrins as a component of molecular shuttles, motors, and machines. Attempts to design and synthesize molecular-level machines using cyclodextrins as a cyclic component are described.

Abstract: THE developing field of nanotechnology has generated wide interest across a broad range of scientific disciplines1. In particular, the realization of nanoscale switching devices might have far-reaching implications for computing and biomimetic engineering2–4. But miniaturization of existing semiconductor technology may not be the best approach to the fabrication of structures whose dimensions are smaller than the wavelength of the radiation used in optical lithography and etching techniques5. The approach observed in the natural world, whereby nanostructures are built up through the self-assembly6–9 of smaller molecular entities, holds substantial promise. Nature abounds with molecular switching devices which perform a variety of functions, such as the transport of metabolites across cell membranes or the signalling of nerve impulses. These processes are commonly controlled by stimuli such as changes in ion concentrations and electrical potentials. Here we report the synthesis of a supramolecular structure (compound 1-[PF6]4, Fig. 1A) that can be reversibly switched between two states by proton concentration changes or by electrochemical means. The super-molecule is a rotaxane comprising a molecular ring threaded on an axle containing two ‘docking points’. We can effect controlled switching of the ring from one of these positions to the other. We use 1H NMR and ultra violet/visible spectroscopy to characterize the dynamics of the bead's movement along the thread before and after switching.

Abstract: In his classic thought experiment of 1867, James Clerk Maxwell imagined a tiny demon guarding a trapdoor separating two gas-filled compartments. By allowing only fast-moving molecules to pass from left to right, and slowmoving molecules right to left, the demon induces heating in the right compartment and cooling in the left. Such shifts from equilibrium violate the second law of thermodynamics. A team from the School of Chemistry at the University of Edinburgh — a cab ride from Maxwell's birthplace — has now developed a molecular 'machine' that mimics the battle of Maxwell's demon against equilibrium. The new demon is a specially designed rotaxane — a molecular ring threaded onto a central axle with binding sites where the ring can attach. Previous rotaxane machines were activated by disturbing the ring binding pattern; the ring shuttled between binding sites and moved the system back towards equilibrium. In the new rotaxane, information about ring location is used to move the system away from equilibrium. But the second law survives the assault, as it costs energy (provided as light) to gather and transfer molecular information. Motor proteins and other biological machines are highly efficient at converting energy into directed motion and driving chemical systems away from thermodynamic equilibrium1. But even though these biological structures have inspired the design of many molecules that mimic aspects of their behaviour2,3,4,5,6,7,8,9,10,11,12,13,14,15, artificial nanomachine systems operate almost exclusively by moving towards thermodynamic equilibrium, not away from it. Here we show that information about the location of a macrocycle in a rotaxane—a molecular ring threaded onto a molecular axle—can be used, on the input of light energy, to alter the kinetics of the shuttling of the macrocycle between two compartments on the axle. For an ensemble of such molecular machines, the macrocycle distribution is directionally driven away from its equilibrium value without ever changing the relative binding affinities of the ring for the different parts of the axle. The selective transport of particles between two compartments by brownian motion in this way bears similarities to the hypothetical task performed without an energy input by a ‘demon’ in Maxwell’s famous thought experiment16,17,18,19. Our observations demonstrate that synthetic molecular machines can operate by an information ratchet mechanism20,21,22, in which knowledge of a particle’s position is used to control its transport away from equilibrium.