Stabilized ferrocene molecules create the world’s smallest electrically controlled molecular machine

Artificial molecular machines are nanoscale machines made up of a few molecules and have the potential to transform fields such as catalysis, molecular electronics, medicine, and quantum materials. These machines work by converting external stimuli, such as electrical signals, into mechanical movement at the molecular level.

Ferrocene is a special drum-shaped molecule with an iron (Fe) atom sandwiched between two five-membered carbocycles that holds promise as a building block for molecular machines. That discovery won the Nobel Prize in Chemistry in 1973 and has been the cornerstone of molecular machine research ever since.

What makes ferrocene so attractive are its unique properties. When the electronic state of Fe ion changes from Fe2+ to Fe3+, its two carbon rings rotate about 36 degrees around the central axis of the molecule. If this electronic state is controlled by external electrical signals, it may be possible to precisely control the rotation of molecules.

However, a major obstacle to its practical application is that when adsorbed onto the surface of a substrate, especially a flat noble metal substrate, it easily decomposes near room temperature or even under ultra-high vacuum conditions. To date, no definitive method has been found to immobilize isolated ferrocene molecules on surfaces without decomposition.

A research team led by Associate Professor Toyokazu Yamada of the Chiba University Graduate School of Engineering, Professor Peter Krueger of the Chiba University School of Engineering, Professor Satoshi Kera of the Institute for Molecular Science, and Professor Masaki Horie of the National Tsinghua University in Japan and Taiwan, finally succeeded in achieving this goal. Overcame challenges. They succeeded in creating the world’s smallest electrically controlled molecular machine.

“In this study, we successfully stabilized and adsorbed ferrocene molecules onto the noble metal surface by pre-coating the noble metal surface with a two-dimensional crown ether molecular film. “This is the first direct experimental evidence of movement,” said Professor Yamada.

Their findings were published in the journal Small on November 30, 2024.

To stabilize the ferrocene molecule, the research team first modified it by adding an ammonium salt to form ferrocene ammonium salt (Fc-amm). This improves durability and allows molecules to be reliably immobilized on the substrate surface.

These new molecules were immobilized on a monolayer film consisting of crown ether cyclic molecules placed on a flat copper substrate. Crown ether cyclic molecules have a unique structure with a central ring that can hold a variety of atoms, molecules, and ions.

“So far, we have discovered that crown ether cyclic molecules can form a monolayer on a flat metal substrate. This monolayer traps the ammonium ion of the Fc-amm molecule in the central ring of the crown ether molecule, and It prevents decomposition,” Professor Yamada explains. By acting as a shield against metal substrates. ”

The researchers then placed a scanning tunneling microscopy (STM) probe over the Fc-amm molecule and applied a voltage, causing the molecule to move laterally. Specifically, when a voltage of -1.3 volts is applied, a hole (the empty space left by an electron) enters the electronic structure of the Fe ion, switching it from the Fe2+ state to the Fe3+ state.

This caused rotation of the carbon ring accompanied by lateral sliding motion of the molecule. Density functional theory calculations showed that this lateral sliding motion is caused by Coulomb repulsion between positively charged Fc-amm ions.

Importantly, when the voltage is removed, the molecules return to their original positions, showing that this movement is reversible and can be precisely controlled using electrical signals.

“This research opens up exciting possibilities for ferrocene-based molecular machines. The ability of molecular machines to perform specialized tasks at the molecular level could be used in many scientific and This could lead to breakthrough innovations across industrial sectors,” said the professor. Mr. Yamada.