Following the artificial molecular pump, the electric molecular motor has achieved a new breakthrough

On January 12, 2023, Beijing time, J. J. Smith, Nobel Prize winner in chemistry at Northwestern University. Professor Fraser Stoddart’s research team, in collaboration with Professor Dean Atamian’s team at the University of Maine and Professor William Goddard at California Polytechnic University, published a new study titled “An electric molecular motor” in the journal Nature.

This study reports the first electric molecular motor based on mechanical interlocking structure, which realizes the unidirectional rotation motion of two small molecular rings around a large molecular ring using electrical energy at the molecular level. This work provides new ideas for the creation of more complex electrically driven molecular machines in the future.

The first and corresponding author of the paper is Dr. Zhang Long, and the co-corresponding author is R. Prof. Dean Astumian, Prof. William A. Goddard III and Prof. J. Professor Fraser Stoddart.

Unlike the macroscopic world, the microscopic and nanoworlds are full of irregular Brownian motion. Until now, how to achieve controllable unidirectional motion at the molecular scale has remained a formidable challenge. To that end, scientists in chemistry, physics, and molecular nanotechnology have been working hard over the past few decades to build molecular machines with such functions but simple structures. Especially after the molecular machine won the 2016 Nobel Prize, such research has been paid more attention. Just as the macro world of electric motors is increasingly influencing modern society (such as the current fast-growing electric vehicle industry), man-made electric molecular motors also have the potential to change people’s lives in the future. Although there are some examples of single-molecule electric motors, they all require harsh operating conditions, such as scanning tunneling microscopes and ultra-high vacuum. In this work, Professor Stoddart’s team designed and synthesized a molecular motor that is easy to prepare and converts electrical energy into unidirectional motion in solution.

The design idea for this work builds on a series of redox-driven artificial molecular pumps (Nat. Nanotechnol. 2015, 10, 547–553; Tetrahedron 2017, 73, 4849–4857; Science 2020, 368, 1247–1253; Chem 2020, 6, 1952–1977). Driven by redox reactions to energy ratchets, these molecular pumps pump molecular loops from solution to a linear collection chain. The electric motor adopts a similar design, using a pump to control the unidirectional movement of two small molecular rings around the large molecular ring, just as two keys slide unidirectionally on the key ring. It is worth noting that when only one small ring is put on the large ring, the small ring cannot achieve unidirectional movement around the ring. It shows that the electrostatic action between the two rings is also a necessary condition for the motor to achieve unidirectional movement. As long as the electrical energy is continuously input, the two small rings can rotate continuously in one direction around the large ring.

Figure 1: Design and working principle of electric molecular motor

The authors characterize the structure of electric molecular motors in different redox states by nuclear magnetic resonance hydrogen spectroscopy, visible/near-infrared spectroscopy, cyclic voltammetry, and single crystal X-ray diffraction, and also show how to power molecular motors with electricity by controlled potential electrolysis.

Figure 2: Structural characterization of electric molecular motors

In order to study the unidirectionality of the molecular motor, the authors used an isotope to label one of the small rings of the deuterated generation so that the position of the two ringlets before and after a redox cycle could be monitored by nuclear magnetic resonance hydrogen spectroscopy, and finally determined that the unidirectionality of the molecular motor was 85%, that is, the two ringlets of the molecular motor in solution completed 180° unidirectional rotation after a redox cycle, and the other 15% of the molecular motor returned to the initial state. During oxidation, the authors observed a metastable state, confirming the interaction between the unidirectional movement sum of the two small rings. In addition, the authors also carried out a detailed study of the operating mechanism of the motor through quantum mechanical calculations.

Figure 3: Unidirectional determination of an electric molecular motor

Figure 4: Metastable state during oxidation

Compared to macro electric motors, electric molecular motors are still in the early stages of development, especially in terms of speed and automation. The operation of this molecular motor requires continuous switching of redox potential within a certain time range, and the cycle time is limited by the diffusion rate of the motor molecules in solution, that is, the rate at which electrons are lost from the electrode surface. Although the molecular motor achieves unidirectional motion, it does not actually do useful work, and all that is done so far is to overcome the resistance of the surrounding solution to its relative internal movement. To address this limitation, the next goal is to attach electric molecular motors to surfaces. Just chemically modify one of the two ringlets so that the ringlets can be anchored to the electrode surface in the electrochemical cell. This not only greatly increases the redox rate, but also further affects the electrode surface, ultimately achieving an effective working output. (Source: Science Network)

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