Molecular Machines

Molecular Machines

A molecular machine is a multicomponent system in which the reversible movement of the components can be controlled by an external stimulus (S). Known examples of molecular machines includes nondegenerate rotaxanes (Type I), nondegenerate catenanes (Type II), as well as pseudorotaxanes (Type III). Control of the motions of the components in those systems has been introduced, which results in a change in properties which produce a signal that allows the operation of the machine to be monitored. The outside stimuli can be photons, electrons, or chemical species, to generate photochemically-, electrochemically- and chemically-driven molecular machines, respectively.

Molecular Shuttles

In the two-station [2]rotaxane-based molecular shuttle, a macrocyclic bead (red square) moves back and forth, like a shuttle, between two molecular recognition sites (S) with which it interacts through noncovalent bonds. Bulky stoppering end-groups (green circles) ensure that the macrocyclic bead is permanently entrapped along the linear rod containing the two recognition sites. Since the year 1991 when the Stoddart group established the first degenerate molecular shuttle, much effort has been exerted on constructing controllable molecular shuttles with high efficiency, which contain different recognition sites on the linear rod for selective positioning of the macrocyle. Not only the efficiency has been improved close to “all-or-nothing”, but also various form of stimulus (voltage, light, chemical, thermal energy, etc.) has been employed in switching of a number of molecular shuttles. Recently the molecular shuttles are demonstrated to maintain their switching property when attached onto different solid support through covalent chemisorption (Au-S bond formation) or noncovalent physisorption (Langmuir-Blodgett technique), a result that opens the window for their application in novel nanoelectromechanical systems and molecular electronic devices.

Molecular Muscles

The rotation and shuttling motions inherent in switchable catenanes and rotaxanes offer the potential to have the switching harnessed in wholly mechanical devices such as actuators and molecular valves. Taking a lesson from biomolecular motors such as myosin and actin in muscle fiber, the production of mechanical force can be extracted from artificial molecular machines that are self-organized on surfaces in order for their cooperative mechanical movements to be amplified and harnessed in nanoelectromechanical systems (NEMS). Switchable palindromic [3]rotaxanes have been designed and synthesized that are reminiscent of a “molecular muscle” in order to mimic the contraction and extension of skeletal muscle. The design takes advantage of well-established donor-acceptor recognition chemistry to control the localization within the [3]rotaxane of its two tetracationic cyclophanes — cyclobis(paraquat-p-phenylene) (CBPQT⁴⁺) rings — on its two tetrathiafulvalene (TTF) stations, as opposed to on its two naphthalene stations (NP). A prototype [3]rotaxane was shown to generate reversible contraction and extension motions in solution when chemical and electrochemical stimuli were applied. Disulfide tethers have been attached covalently to both of the ring components in the [3]rotaxane for the purpose of self-assembling it onto a gold surface. An array of microcantilever beams, coated with a self-assembled monolayer of palindromically-constituted [3]rotaxane molecules, undergoes controllable and reversible bending when it is exposed to chemical oxidants and reductants. This observation supports the hypothesis that the cumulative nanoscale movements within surface-bound “molecular muscles” can be harnessed to perform larger-scale mechanical work. A logical development, by replacing the chemically-driven redox process with direct electrical or optical stimulation, would establish a technological basis for the production of a new class of multi-scale NEMS devices based upon molecular mechanical motion in interlocked molecules.