Although researchers have known about the existence of catenane and rotaxane molecules for a couple of decades, it wasn't until the early 1990s that they were first considered viable building blocks for molecular machines and motors. Now, scientists have identified one such molecule that, when driven solely by light, mimics the motion of a simple piston. Applications that may someday benefit from the use of such a tool include molecular machines designed to mimic biological motions, such as muscle contraction and stretching.
Early attempts to harness catenane and rotaxane molecules faced several roadblocks, including an inability to create large enough volumes of them. With the relatively recent resolution of this problem, these molecules have once again moved to the front burner of many a chemist's R&D efforts, primarily because of their unique properties, which include transition metal complexation, photochemistry, and photoinduced electron transfer and luminescence. Perhaps the latest development is news that one kind of rotaxane in particular shows promise as a linear molecular motor. Unlike prior efforts to develop molecular-level devices that move, this motor stands out in that it is fueled by light, instead of electrochemical, chemical, or photochemical methods.
A rotaxane is essentially a molecule with a macrocyclic bead-like structure locked onto linear thread. After being hit by a nanosecond laser pulse, the molecule produces movement of this bead in much the same way a piston moves within a cylinder (see figure). The light induces a reaction that changes the relative binding potentials of both the home station on the thread and a destination station, causing the bead to "shuttle" from one to the other (similar to a power stroke in a simple piston). With another laser-induced reaction, the bead would return home (a recovery stroke).
More specifically, the research efforts by scientists at the University of Amsterdam (Amsterdam, the Netherlands), the University of Warwick (Warwick, England) and the Università di Bologna (Bologna, Italy), focused on an asymmetrical rotaxane based on hydrogen bonding. At room temperature in acetonitrile, the photoinduced movement of a macrocycle in this molecule takes about 1 µs. After a charge-recombination process lasting roughly 100 µs, this bead then shuttles back to its original position on the thread in what is essentially a continuously reversible process.
According to scientists involved in the project, tuning of the binding properties of the macrocycle, stations, and even the photophysical properties of the active chromophore (the molecule under laser induction), may allow the use of light at a longer wavelength, as well as faster switching times.
REFERENCE
- A. Brouwer et al., Science 291, 2124 (March 16, 2001).