Smallest molecular machine: Reversible sliding motion in ammonium-linked ferrocene

What makes ferrocene so attractive is its unique property: the electronic state of the Fe ion changes from Fe⁺² to iron⁺³_, Rotate its two carbons about 36° around the central molecular axis. Controlling this electronic state through external electrical signals enables precisely controlled molecular rotation. However, a major obstacle to its practical application is that when adsorbed to substrate surfaces, especially flat noble metal substrates, it can easily decompose near room temperature or even under ultra-high vacuum conditions. To date, no established method has been found for anchoring isolated ferrocene molecules on surfaces without decomposing.

In a groundbreaking study, a research team led by Associate Professor Toyo Kazu Yamada of Chiba University Graduate School of Engineering, including Professor Peter Krüger of Chiba University School of Engineering, Professor Satoshi Kera of the Institute of Molecular Science, Japan and Taiwan Professor Masaki Horie of National Tsing Hua University finally overcame this challenge. They succeeded in building the world’s smallest electronically controlled molecular machine. “In this study, we successfully stabilized and adsorbed ferrocene molecules onto the noble metal surface by pre-coating a two-dimensional crown ether molecular film on the noble metal surface. This is based on the molecular motion of ferrocene at the atomic scale. The first direct experimental evidence,” Professor Yamada commented. Their research results were published in the magazine “Small” on November 30, 2024.

In order to stabilize the ferrocene molecule, the research team first modified it by adding ammonium salt to form ferrocene ammonium salt (Fc-amm). This increases durability and ensures that the molecules can be firmly anchored to the substrate surface. These new molecules were then immobilized on a single layer of thin film composed of crown ether cyclic molecules and placed on a flat copper substrate. Crown ether cyclic molecules have a unique structure, and their central ring can accommodate a variety of atoms, molecules and ions. Professor Yamada explained: “Previously, we found that crown ether cyclic molecules can form a monolayer film on a flat metal substrate. This monolayer traps the ammonium ions of the Fc-amm molecule in the central ring of the crown ether molecule, thereby Prevents the decomposition of ferrocene by acting as a shield to the metal substrate.

Next, the research team placed a scanning tunneling microscope (STM) probe on top of the Fc-amm molecule and applied a voltage, causing the molecule to slide laterally. Specifically, when a voltage of -1.3 volts is applied, holes (vacancies left by electrons) enter the electronic structure of the Fe ion, removing it from the Fe ion.²⁺ to iron³⁺ state. This triggers a rotation of the carbon ring, accompanied by lateral sliding of the molecule. Density functional theory calculations indicate that this lateral sliding motion occurs due to Coulomb repulsion between positively charged Fc-amm ions. Importantly, when the voltage is removed, the molecules return to their original positions, indicating that the motion is reversible and can be precisely controlled using electrical signals.

“This research opens up exciting possibilities for ferrocene-based molecular machines. Their ability to perform specialized tasks at the molecular level could lead to revolutionary innovations in many scientific and industrial fields, including precision medicine, smart materials and Advanced manufacturing,” the professor said. Yamada highlighted the potential applications of his technology.