Chemists on a high

Molecular motors enter a new dimension.

Nature has readily developed a range of advanced molecular motors, whether the linear motion of myosin and actin in muscle tissue or the rotary motion of bacterial flagella. Synthetic molecular motors are at a much earlier stage of development, but could now be catching up as a result of separate research carried out by two international teams of chemists.

Chemists from the University of California, Los Angeles (UCLA), US, and the University of Bologna, Italy, claim to have created a molecular elevator ¹, while another team of chemists from UCLA, together with researchers from the Hebrew University of Jerusalem, has developed a simple molecular rotary motor ². In both cases, the motors demonstrated clear-cut reversible on-off behaviour and could therefore have a number of useful applications, such as acting as molecular switches or gates.

Both of the newly developed motors represent advances over existing linear molecular motors, in which a shuttle ring compound slides from one end of a rigid rod to another, although the team of US and Italian chemists uses linear motors as the basis of the elevator. The researchers had previously created such a motor, which they formed from a rotaxane containing two different stations - a dialkylammonium centre and a bipyridinium unit encircled by a dibenzo[24]crown-8 (DB24C8) ring component. This ring would normally reside at the dialkylammonium station, but adding a base (a tertiary amine) would alter the bonding and cause the ring to move to the bipyridinium station, where it would stay until an acid was added.

To create the elevator, the researchers fused three DBC24C8 rings together around a triphenylene core and combined them with three rotaxane rods, each with two stations, to create a molecular tripod, before adding 3,5-di-tert-butylbenzyl feet. The three attached BC24C8 rings would then move up and down the rotaxane rods on the addition of a base or an acid, thereby acting like an elevator.

Although scientists have created linear motors, the only synthetic devices to show rotary motion are molecular windmills and turnstiles in which one component is able to rotate freely about an axis. This means that the devices can’t be powered or controlled.

To produce controllable rotary motion, the researchers created a metallacarborane comprising a nickel ion sandwiched between two dicarbollide ions. When exposed to light or electricity, the nickel ion changes configuration, causing the top dicarbollide ion to move with respect to the bottom ion. The molecule is then locked in its new position, although it can be changed back to its original position via a reduction reaction.

Jon Evans