Light-driven rotary molecular motors: an ultrafast optical study

Molecular rotary motors, though common in nature, were first synthesized rather recently. One of the most promising categories of light-driven rotary molecular motors which allow for optical control is based on helical overcrowded alkenes. In this category of motors, the rotation of the motor's rotor involves four discrete steps: fast light-induced cis-trans isomerisation is followed by a thermally activated step, the same steps are repeated once more to complete the full 360 degrees cycle. Though the rotation rate of the motors is mainly limited by the duration of thermally activated steps, many important factors, such as efficiency and directionality are determined by the light-induced isomerisation. For the development of ultralast molecular motors it is crucial to understand the mechanism and dynamics of the optically induced isomerisation steps. In this paper we will review recent time-resolved optical pump-probe experiments on chiral molecular motors and discuss the results in terms of potential energy landscapes for the ground and excited states of the motor molecules. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim