Figure 2 - Examples of non-directionally controlled molecular rotors.

a, A molecular rotor (left) where hindrance to rotation of the central phenyl ring (the rotor) is removed by the upper and lower bulky molecular end units (the stators). Sufficient spacing for the phenyl rings is generated to allow fast rotation in the solid state as illustrated for two rotor units (right). Reproduced with permission from ref. 32. Copyright (2005) National Academy of Sciences, USA. b, An electrochemically driven rotor where the upper and lower carborane (polyhedral clusters comprising boron and carbon atoms) moieties are bound as ligands to a nickel ion functioning as a 'ball-bearing'. Oxidation and reduction of the nickel centre (the Ni³⁺ / Ni⁴⁺ redox cycle) leads to rotation of the upper ligand relative to the lower ligand, changing the relative position of the alkyl groups (R^1–4) attached to the carborane ligands. Reprinted with permission from ref. 36. Copyright (2004) American Association of the Advancement of Science.