Advancement in Ferrocene-Based Molecular Machines
Artificial molecular machines, which operate by converting external stimuli into mechanical motion at the molecular scale, hold significant promise for fields like catalysis, molecular electronics, medicine, and quantum materials. These machines have the potential to revolutionize how we approach tasks at the molecular level. Ferrocene, a drum-shaped molecule made of an iron (Fe) atom sandwiched between two five-membered carbon rings, is particularly promising as a foundational component for molecular machinery. Since its discovery, which earned the Nobel Prize in Chemistry in 1973, ferrocene has been a cornerstone in the study of molecular machines.
The unique characteristic of ferrocene is that a shift in the electronic state of the Fe ion, from Fe+2 to Fe+3, causes its two carbon rings to rotate by approximately 36° around the central molecular axis. This property offers the potential for precise control over molecular rotation through external electrical signals. However, a significant challenge in utilizing ferrocene for practical applications is its tendency to decompose when adsorbed onto the surface of substrates, especially flat noble metal substrates, near room temperature, even under ultra-high vacuum conditions. Until now, no method has been found to anchor ferrocene molecules on a surface without causing decomposition.
In a groundbreaking study, a research team led by Associate Professor Toyo Kazu Yamada from Chiba University, Japan, along with Professor Peter Krüger from Chiba University, Professor Satoshi Kera of the Institute for Molecular Science, Japan, and Professor Masaki Horie of National Tsing Hua University, Taiwan, has successfully overcome this obstacle. The team has created the world’s smallest electrically controlled molecular machine. “In this study, we successfully stabilized and adsorbed ferrocene molecules onto a noble metal surface by pre-coating it with a two-dimensional crown ether molecular film. This is the first direct experimental evidence of ferrocene-based molecular motion at the atomic scale,” said Prof. Yamada. Their findings were published in Small on November 30, 2024.
To stabilize the ferrocene molecules, the team modified them by adding ammonium salts, forming ferrocene ammonium salts (Fc-amm), which enhanced their durability and secured their fixation to the substrate. These modified molecules were then anchored to a monolayer film made of crown ether cyclic molecules on a flat copper substrate. Crown ether molecules have a central ring that can hold atoms, molecules, and ions, effectively shielding the ferrocene from decomposition. Prof. Yamada explained, “Previously, we found that crown ether cyclic molecules can form a monolayer film on flat metal substrates. This monolayer traps the ammonium ions of Fc-amm molecules in the central ring, preventing decomposition.”
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Next, the researchers used a scanning tunneling microscope (STM) probe to apply an electrical voltage to the Fc-amm molecule. The voltage caused a hole to enter the electronic structure of the Fe ion, switching it from Fe2+ to Fe3+ and triggering the rotation of the carbon rings, accompanied by lateral sliding of the molecule. Density functional theory calculations showed that this sliding motion occurred due to Coulomb repulsion between the positively charged Fc-amm ions. Crucially, when the voltage was removed, the molecule returned to its original position, demonstrating the reversibility of the motion and its precise control through electrical signals.
“This study opens exciting possibilities for ferrocene-based molecular machinery. Their ability to perform specialized tasks at the molecular level can lead to revolutionary innovations in fields such as precision medicine, smart materials, and advanced manufacturing,” said Prof. Yamada, emphasizing the potential applications of this breakthrough.