The detailed structure and chemistry of the
various systems used in the design and synthesis of molecularcomponents is beyond the scope of this text. However, we will provide
a general overview of a few molecular structures that are instrumentalin the design of mobile, functional molecules. With the introduction
of each molecular mechanical component, we will provide comparisonswith its macroscopic counterpart in order to clarify the functionality
of each system.
Bearings
Bearings are structures that function to reduce
energy loss to friction during various processes. Bearings are foundin almost every rotating part of your car and facilitate smooth
rotation of parts from the wheel up to the transmission. Researchershave investigated various systems to replicate this function on the
molecular level. Here we take a look at the molecular bearing designedby scientists working in Japan. A monolayer of tightly packed C60
molecules was sandwiched between two single sheets of graphite to forma molecular bearing. The structure resulted in an ultra-lubricated
system with zero frictional forces when the graphite sheets were movedalong the rotating C60 molecules(figure 3).
Molecular Bearings
Left: The C60
molecular bearings consist of a single layer of tightly packedC60 molecules to create a frictionless system of
sliding-translational motion of the graphite sheets. Therotating C60 moleculesallow for smooth movement of the
graphite sheets.
Right: A ball bearing
that is used to facilitate reduced friction rotation ofwheels. The rotating metallic balls allow for smooth rotation
of the outside ring surrounding the balls.
Axles
Axles function to transfer mechanical energy
to turn a specific object such as a wheel. An effective axle ischaracterized by two functions: 1) it must be able to rotate freely
and 2) must be in a fixed, linear position. The axle must be able torotate freely because its function is dependent on its ability to
transfer mechanical energy to rotate an attached structure. In thecase of an automobile, the axle functions to transfer energy generated
by the engine to rotate the wheels of the vehicle. An axle must be ina fixed, linear position because it must provide enough support to
withstand forces placed on it, such as the weight of a chassis. On themolecular scale, the two functions of an effective rotor are
encompassed in the structure of a triple bond, as opposed to single ordouble bonds (figure 4). A single bond is able to rotate freely, but
is not in a fixed linear position. On the other hand, double bonds arein a fixed position, but are unable to rotate.
Molecular Axles: Here, bonds between benzene
molecules are used to illustrate the differences betweensingle, triple, and double bonds. The differences in the
characteristics of the three types covalent bonds describedabove differentiate their functionality as molecular axles. An
axle must be fixed in a linear structure and be able to rotatealong the axis of the bond. A single bond (left) is able to
rotate, but does not provide a fixed angle and position. Adouble bond (right) is fixed in a 180°angle, but is inhibited
from rotational motion. A triple bond (center) is both fixedin a linear position and capable of rotating freely, therefore
the most viable option for a molecular axle.
