1. Using molecular shape to control molecular motion

Researchers specializing in chemistry work with molecules every day. Molecules such as water and carbon dioxide have relatively simple structures because they consist of only a small number of atoms. However, the molecular world also has molecules with much more complex structures, including some whose forms resemble artificial objects such as motors and wheels. The unique structures of these molecules can be used as molecular machines that perform mechanical movements such as rotation and translation. Research in this field was pioneered by Professor Jean-Pierre Sauvage, Professor J. Fraser Stoddart, and Professor Ben L. Feringa, who were awarded the 2016 Nobel Prize in Chemistry for the design and synthesis of molecular machines. Their achievement was an invaluable starting point for research aimed at utilizing molecular shape to create a wide range of functions, including molecular transporters and molecular muscles.

2. The growth of molecular machine research

Research on molecular machines has grown significantly from the idea of viewing molecules as moving components. In the 1980s, Sauvage and his colleagues succeeded in synthesizing catenanes, in which two ring-shaped structures are interlocked like links in a chain. In this way, the researchers created molecules with the mechanical bonds that would later form the basis of molecular machines^[1]. The two rings of a catenane are mechanically interlocked with each other, but unlike ordinary molecules, they are not connected by chemical bonds. This made it possible to create assembled molecules with freely moving components. Stoddart and his colleagues subsequently developed rotaxanes. These are molecular assemblies in which one or more ring-shaped molecules are threaded onto a rod-shaped molecule and capped at both ends with bulky molecular groups which prevent the rings from slipping off. Using these structures, they created a molecular shuttle in which a ring-shaped molecule moves back and forth between two recognition sites on the rod^[2]. This demonstrated that molecular shape could be used to make a molecular component undergo translational motion. Then, in 1999, Feringa and his colleagues succeeded in creating a light-driven molecular motor capable of continuous unidirectional rotation by using light as an external energy source^[3]. I will discuss molecular motors in greater detail in the next section.

3. A molecular motor that rotates in one direction in response to light

A light-driven molecular motor can be considered a molecular machine because, unlike motion that occurs randomly at thermal equilibrium, it can be designed to repeatedly drive molecular structural changes in a specific direction by supplying energy from an external source, such as light. This light-driven molecular motor consists of two aromatic moieties connected by a sterically hindered alkene at the center. The rotation of the molecule can be driven in one direction by combining photoisomerization of the central carbon-carbon double bond with a subsequent thermally induced structural change. In the next section, I will introduce the nanocar, which was developed by utilizing this molecular motor.

4. Molecular wheels and the nanocar

By combining the molecular motors described above, it is possible to create a molecular unit capable of moving as a whole. In 2011, Professor Tibor Kudernac and his colleagues developed a four-wheeled molecule equipped with four molecular motors serving as its "wheels" and succeeded in moving the molecule across a copper surface through electrical stimulation^[4]. When electrical stimulation is applied to the wheels from the fine tip of a scanning tunneling microscope, the motion of the four rotating molecular motors causes the entire four-wheeled molecule to move across the surface. The significance of this research lies not simply in creating a molecule that resembles a car. Rather, it lies in combining multiple molecular components, including motors, wheels, and a body, according to a specific design, and harnessing the motion of each component to produce mechanical movement of the molecule as a whole. In this sense, the research demonstrates how the fundamental principles of conventional machines operating on the macroscopic scale can be reproduced at the extremely small scale of individual molecules.

5. Molecular muscle that contracts and expands in response to stimuli

Alongside Feringa's research on molecular motors, researchers have also explored molecular motion involving contraction and expansion, rather than rotational motion alone. One notable example is the molecular muscle. In 2000, Sauvage and his colleagues reported a molecular muscle that could expand and contract in response to chemical stimuli by using a dimer of the aforementioned rotaxanes^[5]. The molecule as a whole contracts and expands as the components of the rotaxane move relative to one another. This is thought to reproduce, using a molecular machine, a type of motion similar to the mechanism by which biological muscles contract as filaments slide past one another.

6. Chemistry as the ultimate in miniature engineering

Research on molecular machines has demonstrated that molecules are not merely tiny substances; rather, their shapes give rise to their functions. By making even slight changes to molecular shape, we can create machines that rotate, expand and contract, or transport objects. From this perspective, chemistry can also be seen as the ultimate in miniature engineering; that is, assembling atoms and molecules to create mechanisms that move as intended. The molecular world is normally invisible to us. However, when we consider how molecular shape gives rise to function, we may come to see chemistry in a somewhat different light. I hope this will provide an opportunity to develop a new interest in molecules through their shapes and to appreciate the depth and fascination of chemistry.

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[Reference]
[1] C. O. Dietrich-Buchecker et al., J. Am. Chem. Soc. 1984, 106, 3043-3045.
[2] P. L. Anelli et al., J. Am. Chem. Soc. 1991, 113, 5131-5133.
[3] N. Koumura et al., Nature 1999, 401, 152-155.
[4] T. Kudernac et al., Nature 2011, 479, 208-211.
[5] M. C. Jiménez et al., Angew. Chem. Int. Ed. 2000, 39, 3284-3287.

Yuko Ueno／Professor, School of Science and Engineering, Chuo University
Areas of Specialization: Analytical Chemistry and Structural Chemistry

Yuko Ueno was born in Hiroshima City in 1972.
She graduated from the Department of Chemistry, School of Science, The University of Tokyo, in 1995.
In 1997, she completed the Master’s Program in Chemistry at the Graduate School of Science, The University of Tokyo, and joined Nippon Telegraph and Telephone Corporation (NTT) in the same year.
She earned a Doctor of Science in Chemistry from the Graduate School of Science, The University of Tokyo, in 2002 based on her dissertation. After serving as a Senior Research Scientist, Special Research Scientist, and Group Leader at NTT Basic Research Laboratories, she assumed her current position in 2020.
Her current research interests include spatiotemporal chemistry and biosensing using nanocarbon materials and molecular recognition elements.
Her honors include the Seiyama Award from the Japan Association of Chemical Sensors, Electrochemical Society of Japan (2022), the PROSE Awards from the Association of American Publishers (2020), the Female Analyst Award from the Japan Society for Analytical Chemistry (2019), and the Incentive Award from the Japan Society for Analytical Chemistry (2008).

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