One of the challenges of modern
synthetic chemistry is the construction of
rotating
molecular motors. These act as
drives for structural elements with
dimensions on the nanometer scale, just a few millionths of a millimeter.
One molecule that can now fulfill these requirements consists of a fixed
fastening unit and a movable rotating part. External activation, by an
alternating electrical field, for example, should make it possible to
control the rotor.
Chemists from Erlangen,
Germany, have now used a relatively simple method to
produce a novel compound that can do justice to these claims. The
researchers drew their inspiration from the construction of a classic
children's toy: the top. As in the playroom version, their molecular top
contains a rotating axis that spins within a fixed housing consisting of
three circular spokes. Two
phosphorus atoms serve as attachment points for
the axis. At the center is an
iron atom connected to carbonyl groups (-CO)
oriented perpendicular to the axis. These act as rotor blades. In addition,
three longer carbohydrate chains are attached to the
phosphorus atoms. The
other ends of these chains are bound together to form the three rings that
make the fixed cage around the rotor.
This basic top-like structure can be varied any number of ways: changing the
length of the carbohydrate chains makes it possible to change the size of
the cage to accommodate different
rotors.
Replacement of one of the three
carbonyl
rotors by a nitrosyl group (-NO) results in a dipole moment, an
electrical asymmetry. The rotor then correspondingly lines up with an
applied electrical field, making it externally controllable. If the rotor is
removed altogether, the researchers obtain ring-shaped
molecules that are
usually only attainable by means of very complex synthetic procedures.