This lecture will describe some attempts to make and drive molecular machines. It will describe how a number of nanometre-scale machines have been synthesised using a template-directed strategy from simple building blocks that, because of noncovalent bonding interactions between them, are able to self-assemble into potential molecular abacuses.
In all the cases that will be described, a pi-electron deficient tetracationic cyclophane - namely, cyclobis (paraquat-p-phenylene) - serves as a ring component which is constrained around a dumbbell-shaped component consisting of a linear polyether chain intercepted by at least two, if not three, pi-electron rich units and terminated at each end by bulky groups. It will be demonstrated that the development of an approach towards constructing these molecular abacuses in which the ring component shuttles back and forth along the dumbbell component began with the self-assembly of a molecular shuttle(1) consisting of two hydroquinone rings symmetrically positioned within a polyether chain which is terminated by triisopropylsilyl groups. The tetracarionic cyclophane oscillates in degenerate fashion between the two pi-electron rich hydroquinone rings in the molecular shuttle. Next, it will be shown(2) that replacement of one of the hydroquinone rings by a less good pi-electron donor - e.g. a p-xylyl unit - raises the prospect of translational isomerism, a phenomenon which relates to the relative positions of the ring component with respect to the donor sites on the dumbbell component. By selective oxidation of the hydroquinone site, it had been hoped to effect electrochemical control upon this very simple molecular shuttle. The performance did not live up to expectation. Further development, using much better pi-electron donors, which are also redox active, led(3) to the incorporation of an indole unit as a replacement for one of the hydroquinone rings in the prototype molecular shuttle. However, steric factors intervened and prevented the ring component from residing on the indole unit. These particular problems were redressed(4) by the introduction of a tetrathiafulvalene residue between the two hydroquinone rings in the dumbbell component of the [2] rotaxane. Indeed, this particular molecular shuttle can be subjected to electrochemical control. However, an even better molecular shuttle has been demonstrated(5) based on a dual donor system incorporating 4,4'-biphenol and benzidine units into the dumbbell component. In this particular[2] rotaxane, the tetracationic cyclophane prefers - as expected - to reside on the benzidine unit. However, when the benzidine is protonated, the ring component - a tetracation - migrates to the 4.4'-biphenol unit. Alternatively, when base is added to return this system to neutrality, the ring component migrates preferentially back to the bensidine unit. This molecular shuttle can also be controlled electrochemically.
The lecture will also outline the strategy - and first steps - towards the design of photochemically-driven molecular machines.(6,7)