Molecular devices
There are lots of molecular devices bearing resemblance to macroscopic machinery. For example chemical compounds behaving as bevel gears and propellers (Triptycyl and amide ring systems) have been reported. Similarly a molecular propeller can be formed when two or three bulky rings such as the aryl rings are connected to one central focal atom. Clockwise rotation of one ring induces a counterclockwise rotation of the opposite ring about the bond connecting it to the central atom.
‘Molecular Turnstiles’ which are rotating plates inside a macro cycle have been created, but their rotations were not controllable. But rotation of a molecular ring about a bond could be controlled by chemical stimuli in a device like a molecular brake.
A propeller-like rotation of a 9-triptycyl ring system connected to a 2, 2’-bipyridine unit could be controlled by the addition and subsequent removal of a metal to stop and release the free rotations along single bonds at will.
Molecular switch triggered by light
Another type of molecular switch is the ‘chiroptical molecular switch’ which is switchable between its two stable right and left handed isomeric forms stimulated by light. Depending on the frequency of light bombarded on it the trans conformations of specific compound can be interconnected.
A rotation can be achieved around a carbon-carbon double bond in a helical alkene when ultraviolet light or the change in temperature triggers a rotation involving four isomerization steps. A second generation motor along with 8 other motors from the same material is now operational. This redesigned motor has distinct upper and lower portions and at a higher speed and provides controlled motion at the molecular level. The light-driven motors when inside liquid crystal (LC) films can produce a color change by inducing a reorganization of mesogenic molecules.
The Catenane molecular motor
The catenanes are also molecular machines of special type of two interlocked ring-like components structures held together without any valence forces and both rings with similar recognition sites can have one of the rings rotating inside the other with the conformations stabilized by non covalent interactions.
When there are different recognition sites within the macro cycles, they can be controlled independently through their own specific stimuli. The stereo-electronic property of one recognition site within a macro cycle can be varied such that at one point it has more affinity to the sites on the other ring. Depending on the stimulus affinity it can be turned on or off, or even reversed. Catenanes also can be designed for chemical, photochemical or electrochemical control.
Molecular ball bearings
These molecular machines work a little like a pair of pliers. But opening one end of the structure's central X-shape does not widen the other end. Instead, the two prongs at that end twist around until they are 90° from their original position. This contortion is hinged on a pair of iron-based molecules that act as molecular ball bearings.
The mechanics of molecular machines is extraordinarily complex and depends on the dynamics of chemical bonds and nanoscale forces, as apposed to the relatively straight-forward engineering principles at work in large-scale mechanical devices.