Suprapowered nanomachines

    The 2016 Nobel Prize in Chemistry celebrates the development of molecular machinery and highlights the importance of fundamental and curiosity-driven research for furthering science.

    This year's Nobel Prize in Chemistry has been awarded jointly to Jean-Pierre Sauvage (University of Strasbourg), Fraser Stoddart (Northwestern University) and Ben Feringa (University of Groningen) “for the design and synthesis of molecular machines” and there has been broad recognition within the chemistry community that this is an award well deserved. This approval was also without any of the strong debate of recent years on whether the topic awarded should be strictly considered as chemistry, and the more unfair practice of judging new laureates based on the formal chemistry content of their academic backgrounds. Instead, this was an award that should certainly please purists, recognizing chemistry at its most fundamental.

    Any stereotypical machine will invariably consist of moving parts, and, chemically speaking, moving parts can be created in a single molecule by harnessing mechanical bonds. A mechanical bond is itself a deceptively simple concept. Indeed, during the Nobel announcement it took representatives of the Royal Swedish Academy of Sciences only a few moments with a selection of bakery items to introduce the idea of a catenane — interlocked molecular rings — with surprising clarity.

    Representations of catenane (top), rotaxane (middle) and rotor (bottom) molecules.

    Over 30 years ago, and with considerably more effort, Sauvage demonstrated a simple and efficient metal-directed synthesis of such a mechanical bond (pictured, top) in a single molecule1. Stoddart soon after showed how a mechanical bond could also be created by threading a macrocycle onto a dumbbell-shaped linear axle — a rotaxane2 (pictured, middle). Form then inspired function, with both groups achieving controllable rotational and linear movements in their catenane and rotaxane systems. Another stereotypical part of any machine would be a motor, and in a parallel milestone, Feringa demonstrated unidirectional 360° rotary motion in a single molecule using photochemical isomerization in conformationally strained alkenes3 (pictured, bottom), and like Sauvage and Stoddart, he too was quick to test the mechanical potential of his findings. Needless to say, these early developments were inspirational, and to survey the field that has developed since then presents a vast number of new and intricate examples of shuttles, muscles, ratchets, hinges and rotors (to name only a few) with increasing mechanical complexity at the molecular-scale.

    To consider, then, what now lies ahead for the field, it is clear that one popular observation — the lack of practical applications of these systems — will serve as food for thought, and the challenge of translating single-molecule mechanical movements into useful macroscopic functions should not be underestimated. Some interesting early progress has been made to determine whether the operation of molecular machines can be made practically appealing either at a single-molecule level, or by somehow integrating many mechanical molecules into larger systems with enhanced output capabilities. Arrays of bistable rotaxane shuttles have been used as switchable components in molecular memory devices4, with efforts underway to make these delicate materials more robust through integration with porous frameworks5. Enzyme-sensitive rotaxanes6 and nanocontainers using mechanized molecular valves7 are both examples of how molecular machines can be used for on-demand drug delivery. The mechanical movement of molecular rotors can be applied to achieve dynamic stereochemistry during chemical reactions8, and for the light-driven reorganization of liquid-crystal films to rotate microscale objects9. Feringa has also famously shown how functional four-wheel-drive nanocars incorporating multiple rotor units can display directional motion10, and such single-molecule vehicles will soon be put to the test in the first NanoCar Race along a track of less than 100 nm in length (http://nanocar-race.cnrs.fr). While these preliminary practical investigations show promise, molecular machines are still largely a technology in search of a truly viable function, and so many will continue to explore the chemical space for new and exciting mechanical molecules in the hope that their discoveries will one day help to achieve grander practical ambitions.

    But beyond the science, it is fitting that a field built around molecular cooperation might also now serve as a timely testament to the potential rewards of successful academic cooperation between diverse scientific disciplines. In their first statements following the official prize announcement the recipients were quick to acknowledge each other's efforts, as well their colleagues and international collaborators. There were also clear words of caution against recent political developments which could limit or prevent such collaborations in future, as this could put beyond reach significant new discoveries waiting to be unearthed, and may also limit future opportunities for the Nobel committee to recognize other areas of fundamental chemistry. This is clearly a situation to be avoided.

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    Suprapowered nanomachines. Nature Mater 15, 1223 (2016). https://doi.org/10.1038/nmat4820

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