Developing molecular systems with functions analogous to those of macroscopic machine components, such as rotors1,2, gyroscopes3 and valves4, is a long-standing goal of nanotechnology. However, macroscopic analogies go only so far in predicting function in nanoscale environments, where friction dominates over inertia5,6. In some instances, ratchet mechanisms have been used to bias the ever-present random, thermally driven (Brownian) motion and drive molecular diffusion in desired directions7. Here, we visualize the motions of surface-bound molecular rotors using defocused fluorescence imaging, and observe the transition from hindered to free Brownian rotation by tuning medium viscosity. We show that the otherwise random rotations can be biased by the polarization of the excitation light field, even though the associated optical torque is insufficient to overcome thermal fluctuations. The biased rotation is attributed instead to a fluctuating-friction mechanism8,9 in which photoexcitation of the rotor strongly inhibits its diffusion rate.
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The research leading to these results received funding from the European Research Council under the European Union's Seventh Framework Programme (FP7/2007-2013/ERC grant agreement 291593 FLUOROCODE), from the Flemish government in the form of a long-term structural funding ‘Methusalem’ grant (METH/08/04 CASAS), from the ‘Fonds voor Wetenschapplijk Onderzoek Vlaanderen’ (FWO grants G0413.10, G0697.11 and G0197.11), from the Hercules Foundation (HER/08/021) and from the Federal Science Policy of Belgium (IAP-PAI P7/05 ‘Functional Supramolecular Systems’) and the UNIK research initiative of the Danish Ministry of Science, Technology and Innovation (grant 09-065274).
The authors declare no competing financial interests.
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Hutchison, J., Uji-i, H., Deres, A. et al. A surface-bound molecule that undergoes optically biased Brownian rotation. Nature Nanotech 9, 131–136 (2014). https://doi.org/10.1038/nnano.2013.285
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