Supramolecular Systems

Novel Artificial Molecular Machines—Motion Transmission in a Molecular Friction Clutch

Hendrik V. Schröder1, Amel Mekic1, Henrik Hupatz1, Sebastian Sobottka2, Felix Witte1, Marius Gaedke1, Biprajit Sarkar2, Beate Paulus1 and Christoph A. Schalley1

1Institut für Chemie und Biochemie, Freie Universität Berlin, Takustr. 3, 14195, Berlin, GER

2Institut für Chemie und Biochemie, Freie Universität Berlin, Fabeckstraße 34/36, 14195, Berlin, GER

Inspired by nature’s protein machinery, the construction of artificial molecular machines (AMMs) is one of the most promising emergent fields in nanotechnology. Probably the most worthwhile goal in this field is to control mechanical motion and transport processes at the molecular level by external stimuli. In the last three decades, researchers have constructed a plethora of different AMMs such as molecular shuttles, transporters, motors, and assemblers. Due to the enormously promising developments in the field, the Nobel Prize in chemistry 2016 was awarded for “for the design and synthesis of molecular machines”. Here, we present a new type of AMM, a molecular friction clutch. In the macroscopic world, a friction clutch can be found in almost every motor vehicle. It transmits rotational motion, for example from a motor to an engine, by sliding friction between two rotatable discs. The motion transmission can be switched on and off by “clutching” or “declutching” the two discs. Our molecular analogue is based on a [3]rotaxane which consist of an dumbbell-shaped axle molecule with two threaded ring molecules. The wheels are trapped on the axle by sterically demanding stopper groups. Furthermore, the wheels are decorated with tetrathiafulvalene units, a switchable organosulfur compound which can be reversibly oxidized to its corresponding mono- or dication. By a combination of spectrocopical and electrochemical methods, we show that our molecular friction clutch is nicely stable in four different redox-states. Oxidation of the clutch generates attractive interactions between the tetrathiafulvalene units and, thus, synchronizes the rotational motion of the wheels around the axle. Therefore, the wheels’ pirouetting motions—and also the motion transmission—can be externally operated by a simple electrochemical stimulus. This straightforward mechanism to control the transmission of rotation at the nanoscale is very promising for the construction of more complex molecular machines and for the application in “smart materials”.

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Machine-like operation of a rotaxane-based “molecular friction clutch” by electrochemistry.