Light-driven monodirectional molecular rotor

Attempts to fabricate mechanical devices on the molecular level(1,2) have y ielded analogues of rotors(3), gears(4) switches(5), shuttles(6,7), turnsti les(8) and ratchets(9). Molecular motors, however, have not yet been made, even though they are common in biological systems(10) Rotary motion as such has been induced in interlocked systems(11-13) and directly visualized for single molecules(14), but the controlled conversion of energy into unidire ctional rotary motion has remained difficult to achieve. Here we report rep etitive, monodirectional rotation around a central carbon-carbon double bon d in a chiral, helical alkene, with each 360 degrees rotation involving fou r discrete isomerization steps activated by ultraviolet light or a change i n the temperature of the system. We find that axial chirality and the prese nce of two chiral centres are essential for the observed monodirectional be haviour of the molecular motor. Two light-induced cis-trans isomerizations are each associated with a 180 degrees rotation around the carbon-carbon do uble bond and are each followed by thermally controlled helicity inversions , which effectively block reverse rotation and thus ensure that the four in dividual steps add up to one full rotation in one direction only. As the en ergy barriers of the helicity inversion steps can be adjusted by structural modifications, chiral alkenes based on our system may find use as basic co mponents for 'molecular machinery' driven by light.