Molecular machines are molecules that mimic life-size machines and can be found in abundance in the shape of molecular switches, molecular propellers, molecular tweezers or even molecular motors. They appear to exist solely for the entertainment of chemists.

In this field molecular hinges are molecules (as the name hinge implies) with a central axis connecting two molecular fragments. A certain degree of rotation around this axis allows variation of the angle between the two sections and in a smart system the hinging process can be controlled. The hinge motif features in many biochemical processes and synthetic hinges can potentially be applied as molecular sensor.

The variety in synthetic hinges is large with each rotation mechanism based on a specific physical chemistry concept. Light driven hinges exist with azobenzene groups as part of the hinge ( DOI ) . Other hinges rely on free rotation around the axis and special attractive forces between the adjoining molecular segments for instance pi stacking ( DOI) Bundled in a third group are hinges that can open or close when an additive breaks or forms a simultaneous bond with two actives sites on de side-group for instance a copper ion ( DOI) or a simple proton ( DOI DOI DOI) .

Two examples of the 2008 harvest of molecular hinges. In one novel system (Sankararaman et al. DOI), free rotation around the axis is provided by two alkyne groups and the two perylene side-groups can interact through pi-stacking. On crystallization from hexane two different crystals are obtained from each of the open or closed conformers.

In another system the side-groups do not only interact through pi-stacking but switching is also made possible by complexation of a metal ion between two pyridine units (Bosch et al. DOI). Surprisingly in light of the Sankararaman result, switching dynamics based on the stacking interaction alone were not investigated.